BIOLOGY

HOW
LIFE
WORKS
this page left intentionally blank
BIOLO GY
HOW
LIFE
WORKS
SECOND EDITION

James Morris
BRANDEIS UNIVERSIT Y

Daniel Hartl
H A R VA R D U N I V E RS I T Y

Andrew Knoll
H A R VA R D U N I V E RS I T Y

Robert Lue
H A R VA R D U N I V E RS I T Y

Melissa Michael
U N I V E R S I T Y O F I L L I N O I S AT U R B A N A - C H A M PA I G N

ANDREW BERRY, ANDREW BIEWENER,

BRIAN FARRELL, N. MICHELE HOLBROOK

H A R VA R D U N I V E RS I T Y
PUBLISHER Kate Ahr Parker ART AND MEDIA DIRECTOR Robert Lue, Harvard University
ACQUISITIONS EDITOR Beth Cole EXECUTIVE MEDIA EDITOR Amanda Nietzel
LEAD DEVELOPMENTAL EDITOR Lisa Samols SENIOR DEVELOPMENT EDITOR FOR TEACHING & LEARNING STRATEGIES Elaine Palucki
SENIOR DEVELOPMENTAL EDITOR Susan Moran SENIOR MEDIA PRODUCER Keri DiManigold
DEVELOPMENTAL EDITOR Erica Champion PROJECT EDITOR Robert Errera
EDITORIAL ASSISTANTS Jane Taylor, Alexandra Garrett, Abigail Fagan MANUSCRIPT EDITOR Nancy Brooks

REVIEW COORDINATOR Donna Brodman DIRECTOR OF DESIGN, CONTENT MANAGEMENT Diana Blume
PROJECT MANAGER Karen Misler DESIGN Tom Carling, Carling Design Inc.
DIRECTOR OF MAR KET DEVELOPMENT Lindsey Jaroszewicz ART MANAGER Matt McAdams
EXECUTIVE MAR KETING MANAGER Will Moore ILLUSTRATIONS Imagineering

CREATIVE DIRECTOR Mark Mykytiuk, Imagineering

PHOTO EDITOR Christine Buese

PHOTO RESEARCHERS Lisa Passmore and Richard Fox

PRODUCTION MANAGER Paul Rohloff

COMPOSITION Sheridan Sellers

PRINTING AND BINDING RR Donnelley

COVER IMAGE: Thorsten Henn/Getty Images

Library of Congress Control Number: 2015950976

ISBN-13: 978-1-4641-2609-3
ISBN-10: 1-4641-2609-7

©2016, 2013 by W. H. Freeman and Company

All rights reserved.

Printed in the United States of America

First printing

Macmillan Learning
W. H. Freeman and Company
One New York Plaza
Suite 4500
New York, NY 10004-1562
www.macmillanhighered.com
DEDICATION
To all who are curious about life and how it works
ABOUT THE AUTHORS
JAMES R. MORRIS is Professor of Biology at Brandeis University. He ROBERT A. LUE is Professor of Molecular and Cellular Biology at
teaches a wide variety of courses for majors and non-majors, including Harvard University and the Richard L. Menschel Faculty Director
introductory biology, evolution, genetics and genomics, epigenetics, of the Derek Bok Center for Teaching and Learning. Dr. Lue has a
comparative vertebrate anatomy, and a first-year seminar on Darwin’s longstanding commitment to interdisciplinary teaching and research,
On the Origin of Species. He is the recipient of numerous teaching and chaired the faculty committee that developed the first integrated
awards from Brandeis and Harvard. His research focuses on the rapidly science foundation in the country to serve science majors as well as
growing field of epigenetics, making use of the fruit fly Drosophila pre-medical students. The founding director of Life Sciences Education
melanogaster as a model organism. He currently pursues this research at Harvard, Dr. Lue led a complete redesign of the introductory
with undergraduates in order to give them the opportunity to do curriculum, redefining how the university can more effectively foster
genuine, laboratory-based research early in their scientific careers. new generations of scientists as well as science-literate citizens.
Dr. Morris received a PhD in genetics from Harvard University and Dr. Lue has also developed award-winning multimedia, including
an MD from Harvard Medical School. He was a Junior Fellow in the the animation “The Inner Life of the Cell.” He has coauthored
Society of Fellows at Harvard University, and a National Academies undergraduate biology textbooks and chaired education conferences
Education Fellow and Mentor in the Life Sciences. He also writes short on college biology for the National Academies and the National
essays on science, medicine, and teaching at his Science Whys blog Science Foundation and on diversity in science for the Howard Hughes
(http://blogs.brandeis.edu/sciencewhys). Medical Institute and the National Institutes of Health. In 2012, Dr.
Lue’s extensive work on using technology to enhance learning took
a new direction when he became faculty director of university-wide
online education initiative HarvardX; he now helps to shape Harvard’s
DANIEL L. HARTL is Higgins Professor of Biology in the Department
engagement in online learning to reinforce its commitment to
of Organismic and Evolutionary Biology at Harvard University and
teaching excellence. Dr. Lue earned his PhD from Harvard University.
Professor of Immunology and Infectious Diseases at the Harvard
Chan School of Public Health. He has taught highly popular courses
in genetics and evolution at both the introductory and advanced
levels. His lab studies molecular evolutionary genetics and population MELISSA MICHAEL is Director for Core Curriculum and Assistant
genetics and genomics. Dr. Hartl is the recipient of the Samuel Director for Undergraduate Instruction for the School of Molecular
Weiner Outstanding Scholar Award as well as the Gold Medal of the and Cellular Biology at the University of Illinois at Urbana-Champaign.
Stazione Zoologica Anton Dohrn, Naples. He is a member of the A cell biologist, she primarily focuses on the continuing development
National Academy of Sciences and the American Academy of Arts of the School’s undergraduate curricula. She is currently engaged in
and Sciences. He has served as President of the Genetics Society several projects aimed at improving instruction and assessment at
of America and President of the Society for Molecular Biology and the course and program levels. Her research focuses primarily on how
Evolution. Dr. Hartl’s PhD is from the University of Wisconsin, and creative assessment strategies affect student learning outcomes, and
he did postdoctoral studies at the University of California, Berkeley. how outcomes in large-enrollment courses can be improved through
Before joining the Harvard faculty, he served on the faculties of the use of formative assessment in active classrooms.
the University of Minnesota, Purdue University, and Washington
University Medical School. In addition to publishing more than 400
scientific articles, Dr. Hartl has authored or coauthored 30 books.
ANDREW BERRY is Lecturer in the Department of Organismic and
Evolutionary Biology and an undergraduate advisor in the Life Sciences
at Harvard University. With research interests in evolutionary biology
ANDREW H. KNOLL is Fisher Professor of Natural History in the and history of science, he teaches courses that either focus on one
Department of Organismic and Evolutionary Biology at Harvard of the areas or combine the two. He has written two books: Infinite
University. He is also Professor of Earth and Planetary Sciences. Dr. Knoll Tropics, a collection of the writings of Alfred Russel Wallace, and, with
teaches introductory courses in both departments. His research focuses James D. Watson, DNA: The Secret of Life, which is part history, part
on the early evolution of life, Precambrian environmental history, and exploration of the controversies surrounding DNA-based technologies.
the interconnections between the two. He has also worked extensively
on the early evolution of animals, mass extinction, and plant evolution.
He currently serves on the science team for NASA’s mission to Mars.
ANDREW A. BIEWENER is Charles P. Lyman Professor of Biology in
Dr. Knoll received the Phi Beta Kappa Book Award in Science for Life on
the Department of Organismic and Evolutionary Biology at Harvard
a Young Planet. Other honors include the Paleontological Society Medal
University and Director of the Concord Field Station. He teaches
and Wollaston Medal of the Geological Society, London. He is a member
both introductory and advanced courses in anatomy, physiology, and
of the National Academy of Sciences and a foreign member of the Royal
biomechanics. His research focuses on the comparative biomechanics
Society of London. He received his PhD from Harvard University and
and neuromuscular control of mammalian and avian locomotion, with
then taught at Oberlin College before returning to Harvard.
relevance to biorobotics. He is currently Deputy Editor-in-Chief for
the Journal of Experimental Biology. He also served as President of the
American Society of Biomechanics.

vi
BRIAN D. FARRELL is Director of the David Rockefeller Center for students receive general education credit on the sixteen campuses of
Latin American Studies and Professor of Organismic and Evolutionary the University of North Carolina system.
Biology and Curator in Entomology at the Museum of Comparative
Zoology at Harvard University. He is an authority on coevolution
between insects and plants and a specialist on the biology of beetles.
JOHN MERRILL is Director of the Biological Sciences Program in the
He is the author of many scientific papers and book chapters on the
College of Natural Science at Michigan State University. This program
evolution of ecological interactions between plants, beetles, and
administers the core biology course sequence required for all science
other insects in the tropics and temperate zone. Professor Farrell also
majors. He is a National Academies Education Mentor in the Life
spearheads initiatives to repatriate digital information from scientific
Sciences. In recent years he has focused his research on teaching and
specimens of insects in museums to their tropical countries of origin.
learning with emphasis on classroom interventions and enhanced
In 2011–2012, he was a Fulbright Scholar to the Universidad Autónoma
assessment. A particularly active area is the NSF-funded development
de Santo Domingo in the Dominican Republic. Professor Farrell
of computer tools for automatic scoring of students’ open-ended
received a BA degree in Zoology and Botany from the University of
responses to conceptual assessment questions, with the goal of making
Vermont and MS and PhD degrees from the University of Maryland.
it feasible to use open-response questions in large-enrollment classes.

N. MICHELE HOLBROOK is Charles Bullard Professor of Forestry in RANDALL PHILLIS is Associate Professor of Biology at the University
the Department of Organismic and Evolutionary Biology at Harvard of Massachusetts Amherst. He has taught in the majors introductory
University. She teaches an introductory course on biodiversity as biology course at this institution for 19 years and is a National
well as advanced courses in plant biology. She studies the physics and Academies Education Mentor in the Life Sciences. With help from
physiology of vascular transport in plants with the goal of understanding the Pew Center for Academic Transformation (1999), he has been
how constraints on the movement of water and solutes between soil and instrumental in transforming the introductory biology course to an
leaves influences ecological and evolutionary processes. active learning format that makes use of classroom communication
systems. He also participates in an NSF-funded project to design model-
based reasoning assessment tools for use in class and on exams. These
ASSESSMENT AUTHORS tools are being designed to develop and evaluate student scientific
reasoning skills, with a focus on topics in introductory biology.
JEAN HEITZ is a Distinguished Faculty Associate at the University of
Wisconsin in Madison, WI. She has worked with the two-semester
introductory sequence for biological sciences majors for over 30 years. DEBRA PIRES is an Academic Administrator at the University of
Her primary roles include developing both interactive discussion/ California, Los Angeles. She teaches the introductory courses in the Life
recitation activities designed to uncover and modify misconceptions Sciences Core Curriculum. She is also the Instructional Consultant for
in biology and open-ended process-oriented labs designed to give the Center for Education Innovation & Learning in the Sciences (CEILS).
students a more authentic experience with science. The lab experience Many of her efforts are focused on curricular redesign of introductory
includes engaging all second-semester students in independent biology courses. Through her work with CEILS, she coordinates faculty
research, either mentored research or a library-based meta-analysis of development workshops across several departments to facilitate
an open question in the literature. She is also the advisor to the Peer pedagogical changes associated with curricular developments. Her
Learning Association and is actively involved in TA training. She has current research focuses on what impact the experience of active
taught a graduate course in “Teaching College Biology,” has presented learning pedagogies in lower division courses may have on student
active-learning workshops at a number of national and international performance and concept retention in upper division courses.
meetings, and has published a variety of lab modules, workbooks, and
articles related to biology education.

ASSESSMENT CONTRIBUTORS
MARK HENS is Associate Professor of Biology at the University of
ELENA R. LOZOVSKY, Principal Staff Scientist, Department of
North Carolina Greensboro, where he has taught introductory biology
Organismic and Evolutionary Biology, Harvard University
since 1996. He is a National Academies Education Mentor in the Life
Sciences and is the director of his department’s Introductory Biology FULTON ROCKWELL, Research Associate, Department of Organismic
Program. In this role, he guided the development of a comprehensive and Evolutionary Biology, Harvard University
set of assessable student learning outcomes for the two-semester
introductory biology course required of all science majors at UNCG. In
various leadership roles in general education, both on his campus and
statewide, he was instrumental in crafting a common set of assessable
student learning outcomes for all natural science courses for which

vii
 VISION AND STORY OF
BIOLOGY: HOW LIFE WORKS
Dear students and instructors,
One of the most frequent questions we get about the second edition of Biology: How Life
Works is, “Has science really changed that much in three years?”

O
ngoing discoveries in biology mean that a new edition Instructors have told us that they especially like the activities
of an introductory biology textbook will certainly have that can be used in class to foster active learning among
some new science content. But, more importantly, our students. In response, the second edition includes a rich set of
second edition is new in the sense that we had the opportunity, activities across the introductory biology curriculum. Some are
for the first time, to listen to students and instructors who used short, taking just a couple of minutes to explore a specific topic
HLW in the classroom. The second edition is responsive to this or concept. Others are longer, spanning several class periods and
group and their input has proven invaluable. exploring topics and concepts across many chapters.

What we heard from this community is that the philosophy of Although students and instructors appreciated a streamlined
HLW resonates with students and instructors. They appreciate text, we also heard that more attention was needed in ecology.
a streamlined text that rigorously focuses on introductory In response, we added a chapter that focuses on physical
biology, an emphasis on integration, a modern treatment of processes and global ecology. This new chapter also had a ripple
biology, and equal attention to text, assessment, and media. effect throughout the later chapters of Part 2, giving us more
These elements haven’t changed—they are the threads that space to explore other ecological concepts more deeply as well.
connect the first and second editions. In fact, all of the
changes of the second edition are integrated within the The media in HLW is many layered, so that a static visual
framework of the first edition; they are not simply add-ons. synthesis on the page becomes animated online — and even
interactive — through visual synthesis maps and simulations,
We are particularly excited about the work we’ve done in using a consistent visual language and supported by assessment.
assessment. In the first edition, we worked with a creative and Media resources for this new edition have been expanded to
dedicated team of assessment authors to create something reflect its increased emphasis on global ecology — for example,
wholly new: not a standard test bank, but a thoughtful, there is a new Visual Synthesis figure and online map on the
curated, well-aligned set of questions that can be used for flow of matter and energy in ecosystems. We also developed
teaching as well as testing. These questions are written at a additional media resources that focus on viruses, cells, and
variety of cognitive levels. In addition, they can be used in a tissues.
variety of contexts, including pre-class, in-class, homework,
and exam, providing a learning path for students. We feel that this edition is a wonderful opportunity for us to
continue to develop an integrated set of resources to support
Our approach was so well received that we took it a step further instructor teaching and student learning in introductory biology.
in the second edition. The HLW team is excited to have Melissa Thank you for taking the time to use it in the classroom.
Michael, our lead assessment author in the first edition, join us
as a lead author in the second edition. Her new role allows her Sincerely,
to work more closely with the text and media, which makes for The Biology: How Life Works Author Team
an even tighter alignment among these various components.

viii
RETHINKING The Biology: How Life Works team set out to create a resource for today’s
biology students that would reimagine how content is created and

BIOLOGY delivered. With this second edition, we’ve refined that vision using
feedback from the many dedicated instructors and students who have
become a part of the How Life Works community.

We remain committed to the philosophy of How Life Works: a streamlined,

integrated, and modern approach to introductory biology.

Thematic Selective
We wrote How Life Works with six themes in mind. We used It is unrealistic to expect the majors course to cover every-
these themes as a guide to make decisions about which thing. We envision How Life Works not as a reference for all of
concepts to include and how to organize them. The themes biology, but as a resource focused on foundational concepts,
provide a framework that helps students see biology as a set of terms, and experiments. We explain fundamental topics care-
connected concepts rather than disparate facts. fully, with an appropriate amount of supporting detail, so that
students leave an introductory biology class with a framework
• The scientific method is a deliberate way of asking and on which to build.
answering questions about the natural world.
• Life works according to fundamental principles of
chemistry and physics. Integrated
• The fundamental unit of life is the cell.
How Life Works moves away from minimally related chapters
• Evolution explains the features that organisms share and to provide guidance on how concepts connect to one another
those that set them apart. and the bigger picture. Across the book, key concepts such
• Organisms interact with one another and with their as chemistry are presented in context and Cases and Visual
physical environment, shaping ecological systems that Synthesis figures throughout the text provide a framework
sustain life. for connecting and assimilating information.
• In the 21st century, humans have become major agents in
ecology and evolution.

WHAT'S NEW IN THE SECOND EDITION?

Expanded ecology coverage on physical processes and Lead Author Melissa Michael guides our assessment team
global ecology provides additional emphasis on ecological in refining and expanding our collection of thoughtful,
concepts, while ensuring that content is integrated into the well-curated assessments. Dr. Michael’s role ensures a tight
larger theme of evolution. Learn more about the expanded alignment between the assessments and the media and text.
ecology coverage on page xviii. Learn more on page xvi.

Visual Synthesis Figures and Online Maps on the A rich collection of in-class activities provides active learn-
Flow of Matter and Energy through Ecosystems, Cellular ing materials for instructors to use in a variety of settings.
Communities, and Viruses allow students to explore Learn more about the new in-class activities on page xvii.
connections between concepts through dynamic
visualizations. Learn more about the new media on page xix. Improved LaunchPad functionality makes it easier
to search and filter within our expansive collection of
assessment questions. Learn more about new LaunchPad
functionality on page xii.

ix
TABLE OF CONTENTS
The table of contents is arranged in a familiar way to allow its easy use in a range of EVOLUTION COVERAGE: Chapter 1 introduces
introductory biology courses. On closer look, there are significant differences that evolution as a major theme of the book before
discussing gene expression in Chapters 3 and 4 as a
aim to help biology teachers incorporate the outlooks and research of biology today.
foundation for later discussions of the conservation
Key differences are identified by ˜ and unique chapters by . of metabolic pathways and enzyme structure
(Chapters 6–8) and genetic and phenotypic
variation (Chapters 14 and 15). After the chapters
on the mechanisms and patterns of evolution
(Chapters 21–24), we discuss the diversity of all
organisms in terms of adaptations and comparative
CHAPTER 1 Life: Chemical, Cellular, and Evolutionary Foundations ˜ features, culminating in ecology as the ultimate
illustration of evolution in action.
CASE 1 THE FIRST CELL: LIFE’S ORIGINS

CHEMISTRY: Chemistry is taught in the context

CHAPTER 2 The Molecules of Life ˜ of biological processes, emphasizing the key
principle that structure determines function.
CHAPTER 3 Nucleic Acids and Transcription
CHAPTER 4 Translation and Protein Structure
CHAPTER 5 Organizing Principles: Lipids, Membranes, and Cell Compartments
CHAPTER 6 Making Life Work: Capturing and Using Energy THE CELL: The first set of chapters emphasizes
CHAPTER 7 Cellular Respiration: Harvesting Energy from Carbohydrates and three key aspects of a cell—information flow
Other Fuel Molecules
˜ (Chapters 3 and 4), actively maintaining a
constant internal environment (Chapter 5), and
CHAPTER 8 Photosynthesis: Using Sunlight to Build Carbohydrates harnessing energy (Chapters 6–8). Placing these
basic points at the start of the textbook gives
them emphasis and helps students build their
CASE 2 CANCER: WHEN GOOD CELLS GO BAD
knowledge of biology naturally.

CHAPTER 9 Cell Signaling

CHAPTER 10 Cell and Tissue Architecture: Cytoskeleton, Cell Junctions, and
Extracellular Matrix
CHAPTER 11 Cell Division: Variations, Regulation, and Cancer CASE STUDIES: Biology is best understood when
presented using real and engaging examples as a
framework for synthesizing information. Eight
CASE 3 YOU, FROM A TO T: YOUR PERSONAL GENOME ˜ carefully positioned Cases help provide this
framework. For example, the Case about your
personal genome is introduced before the set
CHAPTER 12 DNA Replication and Manipulation
of chapters on genetics and is revisited in each
CHAPTER 13 Genomes of these chapters where it serves to reinforce
important concepts.
CHAPTER 14 Mutation and DNA Repair
CHAPTER 15 Genetic Variation
CHAPTER 16 Mendelian Inheritance
CHAPTER 17 Inheritance of Sex Chromosomes, Linked Genes, and Organelles GENETICS: The genetics chapters start with
genomes and move to inheritance to provide a
CHAPTER 18 The Genetic and Environmental Basis of Complex Traits
˜
modern, molecular look at genetic variation and
CHAPTER 19 Genetic and Epigenetic Regulation how traits are transmitted.

CHAPTER 20 Genes and Development

CASE 4 MALARIA: COEVOLUTION OF HUMANS AND A PARASITE UNIQUE CHAPTERS: Biology: How Life Works,
Second Edition includes chapters that don’t
traditionally appear in introductory biology texts,
CHAPTER 21 Evolution: How Genotypes and Phenotypes Change over Time one in almost every major subject area. These
novel chapters represent shifts toward a more
CHAPTER 22 Species and Speciation
modern conception of certain topics in biology
CHAPTER 23 Evolutionary Patterns: Phylogeny and Fossils and are identified by .
CHAPTER 24 Human Origins and Evolution

x
“The approach to teaching is something my colleagues and I had To hear the authors talk about the table
been waiting for in a textbook. However, the text is flexible of contents in more depth, visit
enough to accommodate a traditional teaching style.” biologyhowlifeworks.com

– Steve Uyeda, Pima Community College

BIOGEOCHEMICAL CYCLES: We present the

CHAPTER 25 Cycling Carbon ˜ carbon cycle as a bridge between the molecular
and organismal parts of the book, showing how
different kinds of organisms use the biochemical
CASE 5 THE HUMAN MICROBIOME: DIVERSITY WITHIN processes discussed in the first half of the book to
create a cycle that drives life on Earth and creates
CHAPTER 26 Bacteria and Archaea ecosystems. The carbon cycle along with other
biogeochemical cycles—sulfur and nitrogen—
CHAPTER 27 Eukaryotic Cells: Origins and Diversity provides the conceptual backbone around which
CHAPTER 28 Being Multicellular prokaryotic diversity is organized.

CASE 6 AGRICULTURE: FEEDING A GROWING POPULATION

CHAPTER 29 Plant Structure and Function: Moving Photosynthesis onto Land

CHAPTER 30 Plant Reproduction: Finding Mates and Dispersing Young
CHAPTER 31 Plant Growth and Development PLANT DEFENSE: The chapter on plant defense
provides a strong ecological and case-based
CHAPTER 32 Plant Defense: Keeping the World Green ˜ perspective on the strategies plants use to survive
their exploitation by pathogens and herbivores.
CHAPTER 33 Plant Diversity
CHAPTER 34 Fungi: Structure, Function, and Diversity

CASE 7 PREDATOR-PREY: A GAME OF LIFE AND DEATH

DIVERSITY AND PHYSIOLOGY: Diversity

CHAPTER 35 Animal Nervous Systems
follows physiology in order to provide a basis for
CHAPTER 36 Animal Sensory Systems and Brain Function understanding the groupings of organisms and
˜ to avoid presenting diversity as a list of names
CHAPTER 37 Animal Movement: Muscles and Skeletons
to memorize. When students understand how
CHAPTER 38 Animal Endocrine Systems organisms function, they can understand the
different groups in depth and organize them
CHAPTER 39 Animal Cardiovascular and Respiratory Systems
intuitively. To give instructors maximum
CHAPTER 40 Animal Metabolism, Nutrition, and Digestion flexibility, brief descriptions of unfamiliar
organisms and the major groups of organisms
CHAPTER 41 Animal Renal Systems: Water and Waste
have been layered in the physiology chapters, and
CHAPTER 42 Animal Reproduction and Development the diversity chapters include a brief review of
organismal form and function.
CHAPTER 43 Animal Immune Systems

CASE 8 BIODIVERSITY HOT SPOTS: RAIN FORESTS AND CORAL REEFS

CHAPTER 44 Animal Diversity

CHAPTER 45 Animal Behavior
CHAPTER 46 Population Ecology
NEW CHAPTER 48: BIOMES AND GLOBAL
CHAPTER 47 Species Interactions, Communities, and Ecosystems ECOLOGY is part of the greatly expanded ecology
CHAPTER 48 Biomes and Global Ecology ˜ coverage on physical processes and global ecology.
The new coverage broadens connections between
CHAPTER 49 The Anthropocene: Humans as a Planetary Force ecological concepts, and is carefully integrated into
the larger theme of evolution.

xi
RETHINKING THE TEXTBOOK
THROUGH LAUNCHPAD
O
rdinarily, textbooks are developed by first writing The text, visuals, and assessments come together most
chapters, then making decisions about art and effectively through LaunchPad, Macmillan’s integrated
images, and finally assembling a test bank and learning management system. In LaunchPad, students and
ancillary media. Biology: How Life Works develops the text, instructors can access all components of Biology: How Life Works.
visual program, and assessment at the same time. These
three threads are tied to the same set of core concepts, LaunchPad resources for How Life Works are flexible and aligned.
share a common language, and use the same visual palette, Instructors have the ability to select the visuals, assessments, and
which ensures a seamless learning experience for students activities that best suit their classroom and students.
throughout the course. All resources are aligned to one another as well as to the text
to ensure effectiveness in helping students build skills and develop
knowledge necessary for a foundation in biology.

NEW IN LAUNCHPAD
FOR BIOLOGY: HOW
LIFE WORKS, SECOND
EDITION
Functionality to search the question
database and filter questions for a
number of variables including Core
Concept, difficulty level, Bloom’s level,
and class setting allows instructors to
make best use of the robust assessment
assets of How Life Works.

New question types include sequenced

questions and multiple true-false.
Learn more on page xviii.

Metadata tags for each question show

at a glance information, including
instructional guidance for select
questions.

xii
LAUNCHPAD LAUNCHPADWORKS.COM
Where content counts.
Where service matters. Powerful, Simple, and Inviting
Where students learn. LaunchPad for How Life Works includes: e-Book to encourage students to use the
resources at hand.
The complete Biology: How Life
Works interactive e-Book
Pre-built units that are easy to adapt
Carefully curated multimedia visuals and augment to fit your course.
and assessments, assignable by the
instructor and easily accessible by A Gradebook that provides clear
students. feedback to students and instructors on
performance in the course as a whole
LearningCurve adaptive quizzing and individual assignment.
that puts “testing to learn” into action,
with individualized question sets and LMS integration allows LaunchPad
feedback for each student based on his to be easily integrated into your school’s
or her correct and incorrect responses. learning management system so your
All the questions are tied back to the Gradebook and roster are always in sync.

LEARNINGCURVE LEARNINGCURVEWORKS.COM

Students agree that LearningCurve is

extremely helpful in their studies.
Macmillan’s LearningCurve adaptive quizzing is part of almost every LaunchPad and
has been enthusiastically embraced by students and instructors alike. LearningCurve
provides specific feedback for every question and includes links to relevant sections
of the e-Book. Questions are written exclusively for the LaunchPad in which they are
offered and the question banks are robust and varied.

LearningCurve helped students

Prepare better for class

Improve knowledge of
course material Recommend LearningCurve
to other students
Earn better grades

Percentage of students

Strongly agree Agree Somewhat agree

xiii
 RETHINKING The art in the text of How Life Works and the associated
media in LaunchPad were developed in coordination with

THE VISUAL the text and assessments to present an integrated and

engaging visual experience for students.
PROGRAM Two of the biggest challenges introductory biology students face are
connecting concepts across chapters and building a contextual picture,
or visual framework, of a complex process. To help students think like
biologists, we provide Visual Synthesis figures at twelve key points in the
book. These figures bring together multiple images students have already
seen into a visual summary, helping students see how individual concepts
connect to tell a single story.

This Visual Synthesis figure on Speciation brings together multiple concepts from the chapters on evolution.

xiv
Online, the Visual Synthesis figures turn
into zoomable, dynamic Visual Synthesis
Maps where students can explore both
the big picture and the details.

Many of the Visual Synthesis Maps also

include 3D animations (for concepts that
are best learned by seeing them in action)
and simulations (for concepts best
learned by doing).

Assessment questions accompany all

Visual Synthesis Maps, 3D animation
activities, and simulations to help students
work through a core concept in a variety
of ways.

xv
 RETHINKING Well-designed assessment is a tremendous tool for instructors in gauging
student understanding, actively teaching students, and preparing students

ASSESSMENT for exams. The Biology: How Life Works assessment author team applied
decades of experience researching and implementing assessment practices
to create a variety of questions and activities for pre-class, in-class,
homework, and exam settings. All assessment items are carefully aligned
with the text and media and have the flexibility to meet the needs of
instructors with any experience level, classroom size, or teaching style.

Alignment Flexibility
If the questions, exercises, and activities in the course aren’t The How Life Works assessment authors teach in a variety
aligned with course objectives and materials, practice with of classroom sizes and styles, and recognize that there is a
these resources may not help students succeed in their exam wide diversity of course goals and circumstances. Each set of
or in future biology courses. Each How Life Works assessment materials, from in-class activities to exam questions, includes
item is carefully aligned to the goals and content of the text, a spectrum of options for instructors. All the assessment items
and to the assessment items used in other parts of the course. are housed in the LaunchPad platform, which is designed to
Students are guided through a learning path that provides allow instructors to assign and organize assessment items to
them with repeated and increasingly challenging practice suit the unique needs of their course and their students.
with the important concepts illustrated in the text and media.

Is the reaction illustrated by the solid line endergonic or exergonic?

Sample assessment
questions, including a.) endergonic b.) exergonic
a sequenced question
Is the reaction illustrated by the dashed line endergonic or exergonic?
based on a graph.
a.) endergonic b.) exergonic

Which of the following reactions would you predict could be coupled to ATP
synthesis from ADP + Pi? Select all that apply.
a.) creatine phosphate + H2O b creatine + Pi, ∆G -10.3 kcal/mol
b.) phosphoenolpyruvate + H2O b pyruvate + Pi, ∆G -14.8 kcal/mol
c.) glucose 6-phosphate + H2O b glucose + Pi, ∆G -3.3 kcal/mol
d.) glucose 1-phosphate + H2O b glucose + Pi, ∆G -5.0 kcal/mol
Answer the following questions about the reactions
e.) glutamic acid + NH3 b glutamine, ∆G +3.4 kcal/mol
shown in the graph.

Which arrow indicates the activation energy of the catalyzed reaction?

The emperor penguins of Antarctica live on a diet of fish and crustaceans
❑A | B | ❑C | ❑D | ❑E
obtained from the cold Antarctic seawaters. During their annual breeding
Which arrow indicates the activation energy of the uncatalyzed reaction? cycle, however, they migrate across the frozen continent to their breeding
❑A | ❑B | C | ❑D | ❑E grounds 50 miles away from the sea (and 50 miles away from their source of
food). For over two months the male emperor penguins care for and incubate
Which arrow represents the free energy of the substrate? the eggs while the females return to the sea to feed. During this time a male
A | ❑B | ❑C | ❑D | ❑E penguin can lose up to 50% of its biomass (by dry weight). Where did this
biomass go?
Which arrow indicates the free energy of the products?
a.) It was converted to CO2 and H2O and then released.
❑A | ❑B | ❑C | D | ❑E
b.) It was converted to heat and then released.
Which arrow indicates the change in free energy (∆G) of the reaction? c.) It was converted to ATP molecules.
❑A | ❑B | ❑C | ❑D | E

xvi
RETHINKING Active learning exercises are an important component of the learning
pathway and provide students with hands-on exploration of challenging

ACTIVITIES topics and misconceptions. The second edition of Biology: How Life Works
includes a new collection of over 40 in-class activities crafted to address the
concepts that students find most challenging.

T
he activities collection was designed to cover a range of
classroom sizes and complexity levels, and many can be
easily adapted to suit the available time and preferred
teaching style. Each activity includes a detailed activity guide
for instructors. The activity guide introduces the activity,
outlines learning objectives, and provides guidance on how to
implement and customize the activity.

Many of the assessment questions and activities throughout

How Life Works incorporate experimental thinking and
data analysis. In addition, two activity types round out the
assessment collection by providing applied practice with the
data sets and examples from the text. Through Working with
Data activities, students explore and analyze the experiment
from a How Do We Know? figure from the text. Mirror
Experiment activities introduce students to a new scientific
study that relates to, or “mirrors,” one of the How Do We
Know? experiments and ask them to apply what they have
learned about data analysis to this new scenario.

Excerpts of activities from Chapters 11 and 36

xvii
 WHAT’S NEW IN
THE SECOND EDITION?
F
rom the start, Biology: How Life Works was envisioned not as a reference book for all of biology, but as a resource
focused on foundational concepts, terms, and experiments, all placed in a framework that motivates student
interest through a coherent and authentic presentation of current science. In preparing this edition, we
carefully considered the latest breakthroughs and incremental, but nevertheless significant, changes across the fields
of biology. We also reached out to adopters, instructors not using our book, and primary literature to determine what
concepts and details are relevant, important, and necessary additions. Our integrated approach to text, media, and
assessment means that all changes are carefully reflected in each of these areas.

MAJOR CHANGES AND UPDATES

We’ve greatly expanded the coverage of the carbon cycle, habitat loss and its based on figures or data, and questions that
ecology in the second edition of Biology: effects on biodiversity, and the ask students to consider how perturbing
How Life Works. overexploitation of resources and its a system would affect outcomes. As in
effects on community ecology. The the first edition, questions are carefully
A new ecology chapter, Chapter 48: chapter ends with a new section on aligned with core concepts from the text.
Biomes and Global Ecology, takes a conservation biology that explores how New and revised assessment questions
broad look at ecology on the largest scale. conservationists are working to preserve also accompany Visual Synthesis Maps,
It begins with how and why climates are natural habitats. simulations, animations, and other visual
distributed as they are around the world and media, to more effectively probe student
introduces Earth’s major biomes. Biomes A new Visual Synthesis figure on the understanding of the media tools they’ve
crystallize the relationships among ecology, flow of matter and energy through explored.
evolution, and physical environment— ecosystems illustrates, explores, and
landscapes look different in different parts physically situates the relationships among The second edition also includes several
of the world because of the morphological concepts from Chapters 25, 26, 47, 48, and new question types. Sequenced
and physiological adaptations that plants 49. In LaunchPad, students and instructors questions ask students several,
and animals have made to different can interact with an accompanying individually scored questions about a
climates. The chapter is distinguished dynamic, zoomable, and interactive Visual single scenario or system. These questions
by extensive discussion of biomes in the Synthesis Map based on this figure. often build on one another to guide
aquatic realm, especially in the oceans. students from lower-order thinking to
Our new collection of over 40 in-class higher-order thinking about the same
Chapter 47: Species Interactions, activities provides tools for instructors to concept. Multiple true–false questions
Communities, and Ecosystems includes engage their students in active learning. ask students several, individually scored
expanded coverage of the ways In-class activities are designed to address true–false questions about a single
species interact with one another difficult concepts, and can be used with scenario or system.
in communities. This chapter now has a variety of classroom sizes and teaching
more detail on facilitation, herbivory, and styles. Each activity includes a detailed Improved functionality in LaunchPad
biodiversity. activity guide for instructors. allows instructors to search the question
database and filter questions by a number of
Chapter 49: The Anthropocene: Humans We’ve expanded our collection of variables, including core concept, difficulty
as a Planetary Force includes new high-quality assessment questions level, Bloom’s level, and class setting.
discussions exploring how human by adding over 1000 new questions. New Metadata tags for each question show
activities affect ecology. The chapter questions are particularly focused on additional information at a glance, including
now examines fracking and its effects on higher-order thinking, including questions instructional guidance for select questions.

xviii
NEW MEDIA
Cell Communities Visual Synthesis Map to accompany
the printed Visual Synthesis figure
New Animations
Virus Visual Synthesis Map to accompany the printed Chapter 9: Basic Principles of Cell Signaling  Chapter 10: Dynamic Nature of Actin Filaments
Visual Synthesis figure
Chapter 9: G protein-coupled Receptor Signaling Chapter 19: Lac Operon
New Visual Synthesis figure and map on the Flow of
Matter and Energy in Ecosystems Chapter 9: Signal Amplification Chapter 20: ABC Model of Floral Development
Virus Video featuring author Rob Lue
Chapter 10: Dynamic Nature of Microtubules Chapter 40: Glucose Absorption in the Small Intestine
Cell Membrane simulation
Chapter 10: Motor Proteins Chapter 42: Gastrulation

NEW TOPICS AND The following is a detailed list of content changes in this edition. These range from the very small
(nucleotides shown at physiological pH) to quite substantial (an entire new chapter in the ecology

OTHER REVISIONS section). Especially important changes are indicated with an asterisk ( ).

New coverage of functional groups (Chapter 2) New branching order of the eukaryote tree to A new introduction to the immune system (Chapter 43)
reflect new research in the past three years
Nucleotides now shown at physiological pH (Chapter 3) (Chapter 27 and onward) A new discussion of nematodes (Chapter 44)

Amino acids now shown at physiological pH (Chapter 4) A new paragraph on ciliates (Chapter 27) Introduction of a newly discovered species,
Dendrogramma enigmatica (Chapter 44)
The story of the evolution of photosynthesis now A new explanation of protist diversity (Chapter 27)
brought together in a single major section at the end A simplified population growth equation (Chapter 46)
of Chapter 8 (section 8.5) A new discussion of plant nutrients with a table
(Chapter 29) A new discussion of facilitation (Chapter 47)
Chapters 9 and 10 streamlined to better match our
An enhanced discussion of seeds, including the An expanded discussion of herbivory (Chapter 47)
mission statement
development of the embryo and dispersal structures
(Chapter 30) A new example of microbial symbionts (Chapter 47)
A new discussion of cellular response and what
determines it (Chapter 9) New coverage of the genetic advantages of alternation A new discussion of biodiversity and its importance
of generations, and how it allows inbreeding (Chapter (Chapter 47)
New inclusion of the trombone model of DNA 30)
replication (Chapter 12) An entirely new chapter on physical processes that
Addition of apomixis (Chapter 30) underlie different biomes (Chapter 48)
Addition of CRISPR technology (Chapter 12)
• Differential solar energy around the globe
The section on the role of plant sensory systems in and seasonality
Expanded coverage of retrotransposons and reverse the timing of plant reproduction moved from Chapter
• Wind and ocean currents
transcriptase (Chapter 13) 30 to Chapter 31
• Effects of circulation and topography on rainfall
A new How Do We Know? figure explaining Mendel’s Completely revised explanation of the basis for • Expanded discussion of terrestrial biomes
experimental results (Chapter 16) angiosperm diversity (Chapter 33) • Freshwater and marine biomes
• Integration of concepts of biogeochemical cycles
New coverage of the mechanism of X-inactivation Brief descriptions of unfamiliar organisms and the from Chapters 25 and 26 with ecological concepts
(Chapter 19) major groups of organisms layered in the animal phys- • Global patterns of primary production
iology chapters to make it easier to teach physiology
• Global biodiversity
before diversity (Chapters 35-42)
An expanded discussion of non-random mating
and inbreeding depression (Chapter 21)
Brief review of organismal form and function in the A new exploration of the effect of fracking on the
plant and animal diversity chapters (Chapters 33 carbon cycle (Chapter 49)
Addition of the effect of mass extinctions on and 44), allowing these chapters to be used on their
species diversity (Chapter 23) own or before the physiology chapters and giving New coverage of habitat loss and biodiversity
instructors maximum flexibility (Chapter 49)
Updated discussion of the relationship between
Neanderthals and Homo sapiens, as well as A new section on the composition of blood New coverage of overexploitation of resources and its
Denisovans (Chapter 24) (Chapter 39) effects on community ecology (Chapter 49)

Significantly revised link between the carbon cycle, New diagrams of hormone feedback loops in the A new Core Concept and discussion of conservation
biodiversity, and ecology (Chapter 25) menstrual cycle (Chapter 42) biology (Chapter 49)

xix
 PRAISE FOR
HOW LIFE WORKS
I have taught botany and then Biology II for over We have all seen an improvement in our students’
20 years and have been very frustrated when I have understanding of the material this year, the first year
realized how little knowledge students retained. Since that we used the Morris text.
we have gone to this textbook, I find that the questions – ANUPAMA SESHAN, Emmanuel College
students are asking in class are much more probing
than those in the past, and the students seem much
more engaged in the topics. I am hopeful that this I like the figures, especially the 3D ones — we focus
approach will help our students be deeper thinkers and on “perceptual ability” training in our classes and
better scientists. figures that encourage students to think about cells in
3D are excellent!
– GLORIA CADDELL, University of Central Oklahoma
– KIRKWOOD LAND, University of the Pacific

One of the things that really sold me on this text

was the LaunchPad system: easy to use; intuitive These chapters all seem to draw students through
navigation; really good questions that match the the course by referencing what they have learned
sophistication of the text; love the LearningCurve previously and then adding new information. This
activities; use most of the animations in my lectures! makes the course seem like a complete story instead of a
– SARA CARLSON, University of Akron series of encyclopedia entries to be learned in isolation.
– TIM KROFT, Auburn University at Montgomery

This is the best set of questions I’ve ever seen in a

textbook. They are thorough and the right mix of We used this book last year and overall felt that it
challenging the student with requiring memorization represented a major improvement from our previous
of important details. text.
– KURT ELLIOT, Northwest Vista College – PETER ARMBRUSTER, Georgetown University

xx
Good questions are just as important as a good I think HLW does a better job of presenting
textbook. The available variety of assessment tools introductory material than our current text, which
was very important for our adoption of this text. tends to overwhelm students.

– MATTHEW BREWER, Georgia State University – LAURA HILL, University of Vermont

If the whole book reads like this I would love to use it! With the quick checks and the experiments the first
This is the way I like to teach! I want students to chapter already has the learners thinking about
understand rather than memorize and this chapter experiments and critical analysis.
seems aimed at this. – JOHN KOONTZ, University of Tennessee Knoxville

– JENNIFER SCHRAMM, Chemeketa Community College

This book moves teaching away from merely

The artwork seems very clear-cut and geared to giving understanding all of the bold terms in a textbook in
the students a very specific piece of information with a order to spit them back on a multiple-choice test.
very simple example. This should greatly help students I can use this text in order to prepare my students
with forming a visual image of the various subjects. to understand and learn the general principles and
concepts in biology and how those concepts translate
– CHRIS PETRIE, Eastern Florida State College
across different levels of biology. I would not trade
this textbook for any other book on the market.

The writing style is excellent, it makes a great – PAUL MOORE, Bowling Green State University

narrative and incorporates key scientific experiments

into the explanation of photosynthesis.
– DIANNE JENNING, Virginia Commonwealth University

xxi
this page left intentionally blank
ACKNOWLEDGMENTS

Biology: How Life Works is not only a book. Instead, it is an integrated and cover designer, and Sheridan Sellers, our compositor. Together,
set of resources to support student learning and instructor teaching they managed the look and feel of the book, coming up with creative
in introductory biology. As a result, we work closely with an entire solutions for page layout.
community of authors, publishers, instructors, reviewers, and
students. We would like to thank this dedicated group. In digital media, we thank Amanda Nietzel for her editorial insight in
making pedagogically useful media tools, and Keri deManigold and Chris
First and foremost, we thank the thousands of students we have Efstratiou for managing and coordinating the media and websites. They
collectively taught. Their curiosity, intelligence, and enthusiasm have each took on this project with dedication, persistence, enthusiasm,
been sources of motivation for all of us. and attention to detail that we deeply appreciate.

Our teachers and mentors have provided us with models of patience, We are extremely grateful to Elaine Palucki for her insight into
creativity, and inquisitiveness that we strive to bring into our own teaching and learning strategies, Donna Brodman for coordinating the
teaching and research. They encourage us to be life-long learners, many reviewers, and Jane Taylor, Alexandra Garrett, and Abigail Fagan
teachers, and scholars. for their consistent and tireless support.

We feel very lucky to be a partner with W. H. Freeman and Macmillan Imagineering under the patient and intelligent guidance of Mark
Learning. From the start, they have embraced our project, giving us the Mykytuik provided creative, insightful art to complement, support,
space and room to achieve something unique, while at the same time and reinforce the text. We also thank our illustration coordinator, Matt
providing guidance, support, and input from the broader community of McAdams, for skillfully guiding our collaboration with Imagineering.
instructors and students. Christine Buese, our photo editor, and Lisa Passmore and Richard Fox,
our photo researchers, provided us with a steady stream of stunning
Beth Cole, our acquisitions editor, deserves thanks for taking on the photos, and never gave up on those hard-to-find shots. Paul Rohloff,
second edition and becoming our leader. She keeps a watchful eye our production manager, ensured that the journey from manuscript to
on important trends in science, education, and technology, carefully printing was seamless.
listens to what we want to do, and helps us put our aspirations in a
larger context. We would also like to acknowledge Kate Parker, Publisher of Sciences,
Chuck Linsmeier, Vice President of Editorial, Susan Winslow,
Lead developmental editor Lisa Samols continues to have just the Managing Director, and Ken Michaels, Chief Executive Officer, for
right touch — the ability to listen as well as offer intelligent their support of How Life Works and our unique approach.
suggestions, serious with a touch of humor, quiet but persistent. Senior
developmental editor Susan Moran has an eye for detail and the uncanny We also sincerely thank Erin Betters, Jere A. Boudell, Donna Koslowsky,
ability to read the manuscript like a student. Developmental editor Erica and Jon Stoltzfus for thoughtful and insightful contributions to the
Champion brings intelligence and thoughtfulness to her edits. assessment materials.

Karen Misler kept us all on schedule in a clear and firm but always We are extremely grateful for all of the hard work and expertise of the
understanding and compassionate way. sales representatives, regional managers, and regional sales specialists. We
have enjoyed meeting and working with this dedicated sales staff, who
Lindsey Jaroszewicz, our market development manager, is remarkable are the ones that ultimately put the book in the hands of instructors.
for her energy and enthusiasm, her attention to detail, and her
creativity in ways to reach out to instructors and students. Will Moore, Countless reviewers made invaluable contributions to this book and
our marketing manager, refined the story of How Life Works 2e and deserve special thanks. From catching mistakes to suggesting new and
works tirelessly with our sales teams to bring the second edition to innovative ways to organize the content, they provided substantial
instructors and students everywhere. input to the book. They brought their collective years of teaching to
the project, and their suggestions are tangible in every chapter.
We thank Robert Errera for coordinating the move from manuscript
to the page, and Nancy Brooks for helping to even out the prose. We Finally, we would like to thank our families. None of this would have
also thank Diana Blume, our design director, Tom Carling, our text been possible without their support, inspiration, and encouragement.

xxiii
Contributors, First Edition
Thank you to all the instructors who worked in collaboration with the authors and assessment authors to write Biology: How
Life Works assessments, activities, and exercises.

Allison Alvarado, University of California, Los Angeles Kerry Kilburn, Old Dominion University
Peter Armbruster, Georgetown University Jo Kurdziel, University of Michigan
Zane Barlow-Coleman, formerly of University of David Lampe, Duquesne University
Massachusetts, Amherst Brenda Leady, University of Toledo
James Bottesch, Brevard Community College Sara Marlatt, Yale University*
Jessamina Blum, Yale University Kelly McLaughlin, Tufts University
Jere Boudell, Clayton State University Brad Mehrtens, University of Illinois at Urbana-
David Bos, Purdue University Champaign
Laura Ciaccia West, Yale University* Nancy Morvillo, Florida Southern College
Laura DiCaprio, Ohio University Jennifer Nauen, University of Delaware
Tod Duncan, University of Colorado, Denver Kavita Oommen, Georgia State University
Cindy Giffen, University of Wisconsin, Madison Patricia Phelps, Austin Community College
Paul Greenwood, Colby College Melissa Reedy, University of Illinois at Urbana-
Stanley Guffey, The University of Tennessee, Knoxville Champaign
Alison Hill, Duke University Lindsay Rush, Yale University*
Meg Horton, University of North Carolina at Sukanya Subramanian, Collin College
Greensboro Michelle Withers, West Virginia University

*Graduate student, Yale University Scientific Teaching Fellow

Reviewers, Class Testers, and Focus Group Participants

Thank you to all the instructors who reviewed and/or class tested chapters, art, assessment questions, and other Biology: How
Life Works materials.

First Edition Michael Angilletta, Arizona State Charles Baer, University of Florida
Thomas Abbott, University of University Brian Bagatto, University of
Connecticut Jonathan Armbruster, Auburn Akron
Tamarah Adair, Baylor University University Alan L. Baker, University of New
Sandra Adams, Montclair State Jessica Armenta, Lone Star Hampshire
University College System Ellen Baker, Santa Monica College
Jonathon Akin, University of Brian Ashburner, University of Mitchell Balish, Miami University
Connecticut Toledo Teri Balser, University of Florida
Eddie Alford, Arizona State Andrea Aspbury, Texas State Paul Bates, University of
University University Minnesota, Duluth
Chris Allen, College of the Nevin Aspinwall, Saint Louis Michel Baudry, University of
Mainland University Southern California
Sylvester Allred, Northern Felicitas Avendano, Grand View Jerome Baudry, The University of
Arizona University University Tennessee, Knoxville
Shivanthi Anandan, Drexel Yael Avissar, Rhode Island College Mike Beach, Southern
University Ricardo Azpiroz, Richland College Polytechnic State University
Andrew Andres, University of Jessica Baack, Southwestern Andrew Beall, University of North
Nevada, Las Vegas Illinois College Florida

xxiv
Gregory Beaulieu, University Heather Bruns, Ball State Catharina Coenen, Allegheny
of Victoria University College
John Bell, Brigham Young Jill Buettner, Richland Mary Colavito, Santa Monica
University College College
Michael Bell, Richland Stephen Burnett, Clayton State Craig Coleman, Brigham Young
College University University
Rebecca Bellone, University Steve Bush, Coastal Carolina Alex Collier, Armstrong Atlantic
of Tampa University State University
Anne Bergey, Truman State David Byres, Florida State College Sharon Collinge, University of
University at Jacksonville Colorado, Boulder
Laura Bermingham, University of James Campanella, Montclair Jay Comeaux, McNeese State
Vermont State University University
Aimee Bernard, University of Darlene Campbell, Cornell Reid Compton, University of
Colorado, Denver University Maryland
Annalisa Berta, San Diego State Jennifer Campbell, North Ronald Cooper, University of
University Carolina State University California, Los Angeles
Joydeep Bhattacharjee, University John Campbell, Northwest Victoria Corbin, University of
of Louisiana, Monroe College Kansas
Arlene Billock, University of David Canning, Murray State Asaph Cousins, Washington State
Louisiana, Lafayette University University
Daniel Blackburn, Trinity Richard Cardullo, University of Will Crampton, University of
College California, Riverside Central Florida
Mark Blackmore, Valdosta State Sara Carlson, University of Kathryn Craven, Armstrong
University Akron Atlantic State University
Justin Blau, New York University Jeff Carmichael, University of Scott Crousillac, Louisiana State
Andrew Blaustein, Oregon State North Dakota University
University Dale Casamatta, University of Kelly Cude, College of the
Mary Bober, Santa Monica North Florida Canyons
College Anne Casper, Eastern Michigan Stanley Cunningham, Arizona
Robert Bohanan, University of University State University
Wisconsin, Madison David Champlin, University of Karen Curto, University of
Jim Bonacum, University of Southern Maine Pittsburgh
Illinois at Springfield Rebekah Chapman, Georgia State Bruce Cushing, The University of
Laurie Bonneau, Trinity University Akron
College Samantha Chapman, Villanova Rebekka Darner, University of
David Bos, Purdue University University Florida
James Bottesch, Brevard Mark Chappell, University of James Dawson, Pittsburg State
Community College California, Riverside University
Jere Boudell, Clayton State P. Bryant Chase, Florida State Elizabeth De Stasio, Lawrence
University University University
Nancy Boury, Iowa State Young Cho, Eastern New Mexico Jennifer Dechaine, Central
University University Washington University
Matthew Brewer, Georgia State Tim Christensen, East Carolina James Demastes, University of
University University Northern Iowa
Mirjana Brockett, Georgia Steven Clark, University of D. Michael Denbow, Virginia
Institute of Technology Michigan Polytechnic Institute and State
Andrew Brower, Middle Ethan Clotfelter, Amherst University
Tennessee State University College Joseph Dent, McGill University

xxv
Terry Derting, Murray State John Elder, Valdosta State J. Yvette Gardner, Clayton State
University University University
Jean DeSaix, University of North William Eldred, Boston Gillian Gass, Dalhousie University
Carolina at Chapel Hill University Jason Gee, East Carolina
Donald Deters, Bowling Green David Eldridge, Baylor University University
State University Inge Eley, Hudson Valley Topher Gee, University of North
Hudson DeYoe, The University of Community College Carolina at Charlotte
Texas, Pan American Lisa Elfring, University of Arizona Vaughn Gehle, Southwest
Leif Deyrup, University of the Richard Elinson, Duquesne Minnesota State University
Cumberlands University Tom Gehring, Central Michigan
Laura DiCaprio, Ohio University Kurt Elliott, Northwest Vista University
Jesse Dillon, California State College John Geiser, Western Michigan
University, Long Beach Miles Engell, North Carolina State University
Frank Dirrigl, The University of University Alex Georgakilas, East Carolina
Texas, Pan American Susan Erster, Stony Brook University
Kevin Dixon, Florida State University Peter Germroth, Hillsborough
University Joseph Esdin, University of Community College
Elaine Dodge Lynch, Memorial California, Los Angeles Arundhati Ghosh, University of
University of Newfoundland Jean Everett, College of Pittsburgh
Hartmut Doebel, George Charleston Carol Gibbons Kroeker,
Washington University Brent Ewers, University of University of Calgary
Jennifer Doll, Loyola University, Wyoming Phil Gibson, University of
Chicago Melanie Fierro, Florida State Oklahoma
Logan Donaldson, York College at Jacksonville Cindee Giffen, University of
University Michael Fine, Virginia Wisconsin, Madison
Blaise Dondji, Central Commonwealth University Matthew Gilg, University of
Washington University Jonathan Fingerut, St. Joseph’s North Florida
Christine Donmoyer, Allegheny University Sharon Gillies, University of the
College Ryan Fisher, Salem State Fraser Valley
James Dooley, Adelphi University University Leonard Ginsberg, Western
Jennifer Doudna, University of David Fitch, New York University Michigan University
California, Berkeley Paul Fitzgerald, Northern Virginia Florence Gleason, University of
John DuBois, Middle Tennessee Community College Minnesota
State University Jason Flores, University of North Russ Goddard, Valdosta State
Richard Duhrkopf, Baylor Carolina at Charlotte University
University Matthias Foellmer, Adelphi Miriam Golbert, College of the
Kamal Dulai, University of University Canyons
California, Merced Barbara Frase, Bradley University Jessica Goldstein, Barnard
Arthur Dunham, University of Caitlin Gabor, Texas State College, Columbia University
Pennsylvania University Steven Gorsich, Central Michigan
Mary Durant, Lone Star College Michael Gaines, University of University
System Miami Sandra Grebe, Lone Star College
Roland Dute, Auburn University Jane Gallagher, The City College System
Andy Dyer, University of South of New York, The City Robert Greene, Niagara
Carolina, Aiken University of New York University
William Edwards, Niagara Kathryn Gardner, Boston Ann Grens, Indiana University,
University University South Bend

xxvi
Theresa Grove, Valdosta State David Hicks, The University of Jonghoon Kang, Valdosta State
University Texas at Brownsville University
Stanley Guffey, The University of Karen Hicks, Kenyon College George Karleskint, St. Louis
Tennessee, Knoxville Alison Hill, Duke University Community College at
Nancy Guild, University of Kendra Hill, South Dakota State Meramec
Colorado, Boulder University David Karowe, Western Michigan
Lonnie Guralnick, Roger Williams Jay Hodgson, Armstrong Atlantic University
University State University Judy Kaufman, Monroe
Laura Hake, Boston College John Hoffman, Arcadia University Community College
Kimberly Hammond, University Jill Holliday, University of Florida Nancy Kaufmann, University of
of California, Riverside Sara Hoot, University of Pittsburgh
Paul Hapeman, University of Wisconsin, Milwaukee John Kauwe, Brigham Young
Florida Margaret Horton, University of University
Luke Harmon, University of North Carolina at Greensboro Elena Keeling, California
Idaho Lynne Houck, Oregon State Polytechnic State University
Sally Harmych, University of University Jill Keeney, Juniata College
Toledo Kelly Howe, University of New Tamara Kelly, York University
Jacob Harney, University of Mexico Chris Kennedy, Simon Fraser
Hartford William Huddleston, University University
Sherry Harrel, Eastern Kentucky of Calgary Bretton Kent, University of
University Jodi Huggenvik, Southern Illinois Maryland
Dale Harrington, Caldwell University Jake Kerby, University of South
Community College and Melissa Hughes, College of Dakota
Technical Institute Charleston Jeffrey Kiggins, Monroe
J. Scott Harrison, Georgia Randy Hunt, Indiana University Community College
Southern University Southeast Scott Kight, Montclair State
Diane Hartman, Baylor University Tony Huntley, Saddleback University
Mary Haskins, Rockhurst College Stephen Kilpatrick, University of
University Brian Hyatt, Bethel College Pittsburgh, Johnstown
Bernard Hauser, University of Jeba Inbarasu, Metropolitan Kelly Kissane, University of
Florida Community College Nevada, Reno
David Haymer, University of Colin Jackson, The University of David Kittlesen, University of
Hawaii Mississippi Virginia
David Hearn, Towson University Eric Jellen, Brigham Young Jennifer Kneafsey, Tulsa
Marshal Hedin, San Diego State University Community College
University Dianne Jennings, Virginia Jennifer Knight, University of
Paul Heideman, College of Commonwealth University Colorado, Boulder
William and Mary Scott Johnson, Wake Technical Ross Koning, Eastern
Gary Heisermann, Salem State Community College Connecticut State University
University Mark Johnston, Dalhousie David Kooyman, Brigham Young
Brian Helmuth, University of University University
South Carolina Susan Jorstad, University of Olga Kopp, Utah Valley
Christopher Herlihy, Middle Arizona University
Tennessee State University Stephen Juris, Central Michigan Anna Koshy, Houston
Albert Herrera, University of University Community College
Southern California Julie Kang, University of Todd Kostman, University of
Brad Hersh, Allegheny College Northern Iowa Wisconsin, Oshkosh

xxvii
Peter Kourtev, Central Michigan Janet Loxterman, Idaho State Richard Merritt, Houston
University University Community College
William Kroll, Loyola University, Ford Lux, Metropolitan State Jennifer Metzler, Ball State
Chicago College of Denver University
Dave Kubien, University of New José-Luis Machado, Swarthmore James Mickle, North Carolina
Brunswick College State University
Allen Kurta, Eastern Michigan C. Smoot Major, University of Brian Miller, Middle Tennessee
University South Alabama State University
Ellen Lamb, University of North Charles Mallery, University of Allison Miller, Saint Louis
Carolina at Greensboro Miami University
Troy Ladine, East Texas Baptist Mark Maloney, Spelman Yuko Miyamoto, Elon University
University College Ivona Mladenovic, Simon Fraser
David Lampe, Duquesne Carroll Mann, Florida State University
University College at Jacksonville Marcie Moehnke, Baylor
Evan Lampert, Gainesville State Carol Mapes, Kutztown University
College University of Pennsylvania Chad Montgomery, Truman State
James Langeland, Kalamazoo Nilo Marin, Broward College University
College Diane Marshall, University of Jennifer Mook, Gainesville State
John Latto, University of New Mexico College
California, Santa Barbara Heather Masonjones, University Daniel Moon, University of North
Brenda Leady, University of of Tampa Florida
Toledo Scott Mateer, Armstrong Atlantic Jamie Moon, University of North
Jennifer Leavey, Georgia Institute State University Florida
of Technology Luciano Matzkin, The University Jeanelle Morgan, Gainesville State
Hugh Lefcort, Gonzaga University of Alabama in Huntsville College
Brenda Leicht, University of Iowa Robert Maxwell, Georgia State David Morgan, University of West
Craig Lending, The College at University Georgia
Brockport, The State University Meghan May, Towson University Julie Morris, Armstrong Atlantic
of New York Michael McGinnis, Spelman State University
Nathan Lents, John Jay College of College Becky Morrow, Duquesne
Criminal Justice, The City Kathleen McGuire, San Diego University
University of New York State University Mark Mort, University of Kansas
Michael Leonardo, Coe College Maureen McHale, Truman State Nancy Morvillo, Florida Southern
Army Lester, Kennesaw State University College
University Shannon McQuaig, St. Petersburg Anthony Moss, Auburn University
Cynthia Littlejohn, University of College Mario Mota, University of Central
Southern Mississippi Susan McRae, East Carolina Florida
Zhiming Liu, Eastern New Mexico University Alexander Motten, Duke
University Lori McRae, University of Tampa University
Jonathan Lochamy, Georgia Mark Meade, Jacksonville State Tim Mulkey, Indiana State
Perimeter College University University
Suzanne Long, Monroe Brad Mehrtens, University of John Mull, Weber State
Community College Illinois at Urbana-Champaign University
Julia Loreth, University of North Michael Meighan, University of Michael Muller, University of
Carolina at Greensboro California, Berkeley Illinois at Chicago
Jennifer Louten, Southern Douglas Meikle, Miami Beth Mullin, The University of
Polytechnic State University University Tennessee, Knoxville

xxviii
Paul Narguizian, California State John Peters, College of Peggy Rolfsen, Cincinnati State
University, Los Angeles Charleston Technical and Community
Jennifer Nauen, University of Chris Petrie, Brevard Community College
Delaware College Mike Rosenzweig, Virginia
Paul Nealen, Indiana University of Patricia Phelps, Austin Polytechnic Institute and State
Pennsylvania Community College University
Diana Nemergut, University of Steven Phelps, The University Doug Rouse, University of
Colorado, Boulder of Texas at Austin Wisconsin, Madison
Kathryn Nette, Cuyamaca College Kristin Picardo, St. John Fisher Yelena Rudayeva, Palm Beach
Jacalyn Newman, University of College State College
Pittsburgh Aaron Pierce, Nicholls State Ann Rushing, Baylor
James Nienow, Valdosta State University University
University Debra Pires, University of Shereen Sabet, La Sierra
Alexey Nikitin, Grand Valley State California, Los Angeles University
University Thomas Pitzer, Florida Rebecca Safran, University of
Tanya Noel, York University International University Colorado
Fran Norflus, Clayton State Nicola Plowes, Arizona State Peter Sakaris, Southern
University University Polytechnic State University
Celia Norman, Arapahoe Crima Pogge, City College of Thomas Sasek, University of
Community College San Francisco Louisiana, Monroe
Eric Norstrom, DePaul University Darren Pollock, Eastern New Udo Savalli, Arizona State
Jorge Obeso, Miami Dade College Mexico University University
Kavita Oommen, Georgia State Kenneth Pruitt, The University H. Jochen Schenk, California State
University of Texas at Brownsville University, Fullerton
David Oppenheimer, University Sonja Pyott, University of North Gregory Schmaltz, University of
of Florida Carolina at Wilmington the Fraser Valley
Joseph Orkwiszewski, Villanova Rajinder Ranu, Colorado State Jean Schmidt, University of
University University Pittsburgh
Rebecca Orr, Collin College Philip Rea, University of Andrew Schnabel, Indiana
Don Padgett, Bridgewater State Pennsylvania University, South Bend
College Amy Reber, Georgia State Roxann Schroeder, Humboldt
Joanna Padolina, Virginia University State University
Commonwealth University Ahnya Redman, West Virginia David Schultz, University of
One Pagan, West Chester University Missouri, Columbia
University Melissa Reedy, University of Andrea Schwarzbach, The
Kathleen Page, Bucknell Illinois at Urbana-Champaign University of Texas at
University Brian Ring, Valdosta State Brownsville
Daniel Papaj, University of University Erik Scully, Towson University
Arizona David Rintoul, Kansas State Robert Seagull, Hofstra University
Pamela Pape-Lindstrom, Everett University Pramila Sen, Houston
Community College Michael Rischbieter, Presbyterian Community College
Bruce Patterson, University of College Alice Sessions, Austin
Arizona, Tucson Laurel Roberts, University of Community College
Shelley Penrod, Lone Star College Pittsburgh Vijay Setaluri, University of
System George Robinson, The University Wisconsin
Roger Persell, Hunter College, The at Albany, The State University Jyotsna Sharma, The University of
City University of New York of New York Texas at San Antonio

xxix
Elizabeth Sharpe-Aparicio, Blinn Barbara Stegenga, University of Terry Trier, Grand Valley State
College North Carolina, Chapel Hill University
Patty Shields, University of Patricia Steinke, San Jacinto Stephen Trumble, Baylor
Maryland College, Central Campus University
Cara Shillington, Eastern Asha Stephens, College of the Jan Trybula, The State University
Michigan University Mainland of New York at Potsdam
James Shinkle, Trinity Robert Steven, University of Alexa Tullis, University of Puget
University Toledo Sound
Rebecca Shipe, University of Eric Strauss, University of Marsha Turell, Houston
California, Los Angeles Wisconsin, La Crosse Community College
Marcia Shofner, University of Sukanya Subramanian, Collin Mary Tyler, University of Maine
Maryland College Marcel van Tuinen, University
Laurie Shornick, Saint Louis Mark Sugalski, Southern of North Carolina at
University Polytechnic State University Wilmington
Jill Sible, Virginia Polytechnic Brad Swanson, Central Michigan Dirk Vanderklein, Montclair State
Institute and State University University University
Allison Silveus, Tarrant County Ken Sweat, Arizona State Jorge Vasquez-Kool, Wake
College University Technical Community College
Kristin Simokat, University of David Tam, University of North William Velhagen, New York
Idaho Texas University
Sue Simon-Westendorf, Ohio Ignatius Tan, New York University Dennis Venema, Trinity Western
University William Taylor, University of University
Sedonia Sipes, Southern Illinois Toledo Laura Vogel, North Carolina State
University, Carbondale Christine Terry, Lynchburg University
John Skillman, California State College Jyoti Wagle, Houston Community
University, San Bernardino Sharon Thoma, University of College
Marek Sliwinski, University of Wisconsin, Madison Jeff Walker, University of
Northern Iowa Pamela Thomas, University of Southern Maine
Felisa Smith, University of New Central Florida Gary Walker, Appalachian State
Mexico Carol Thornber, University of University
John Sollinger, Southern Oregon Rhode Island Andrea Ward, Adelphi University
University Patrick Thorpe, Grand Valley State Fred Wasserman, Boston
Scott Solomon, Rice University University University
Morvarid Soltani-Bejnood, The Briana Timmerman, University of Elizabeth Waters, San Diego State
University of Tennessee South Carolina University
Vladimir Spiegelman, University Chris Todd, University of Douglas Watson, The University
of Wisconsin, Madison Saskatchewan of Alabama at Birmingham
Chrissy Spencer, Georgia Institute Gail Tompkins, Wake Technical Matthew Weand, Southern
of Technology Community College Polytechnic State University
Kathryn Spilios, Boston Martin Tracey, Florida Michael Weber, Carleton
University International University University
Ashley Spring, Brevard Randall Tracy, Worcester State Cindy Wedig, The University of
Community College University Texas, Pan American
Bruce Stallsmith, The University James Traniello, Boston Brad Wetherbee, University of
of Alabama in Huntsville University Rhode Island
Jennifer Stanford, Drexel Bibit Traut, City College of San Debbie Wheeler, University of the
University Francisco Fraser Valley

xxx
Clay White, Lone Star College Kelly Young, California State David Baum, University of
System University, Long Beach Wisconsin, Madison
Lisa Whitenack, Allegheny James Yount, Brevard Community Kevin S. Beach, The University of
College College Tampa
Maggie Whitson, Northern Min Zhong, Auburn University Philip Becraft, Iowa State
Kentucky University University
Stacey Wild East, Tennessee State Second Edition Alexandra Bely, University of
University Barbara J. Abraham, Hampton Maryland
Herbert Wildey, Arizona State University Lauryn Benedict, University of
University and Phoenix Jason Adams, College of DuPage Northern Colorado
College Sandra D. Adams, Montclair State Anne Bergey, Truman State
David Wilkes, Indiana University, University University
South Bend Richard Adler, University of Joydeep Bhattacharjee,
Lisa Williams, Northern Virginia Michigan, Dearborn University of Louisiana,
Community College Nancy Aguilar-Roca, University of Monroe
Elizabeth Willott, University of California, Irvine Todd Bishop, Dalhousie
Arizona Shivanthi Anandan, Drexel University
Mark Wilson, Humboldt State University Catherine Black, Idaho State
University Lynn Anderson-Carpenter, University
Ken Wilson, University of University of Michigan Andrew R. Blaustein, Oregon
Saskatchewan Christine Andrews, The State University
Bob Winning, Eastern Michigan University of Chicago James Bolton, Georgia Gwinnett
University Peter Armbruster, Georgetown College
Candace Winstead, California University Jim Bonacum, University of
Polytechnic State University Jessica Armenta, Austin Illinois at Springfield
Robert Wise, University of Community College Laurie J. Bonneau, Trinity College
Wisconsin, Oshkosh Brian Ashburner, University of James Bottesch, Eastern Florida
D. Reid Wiseman, College of Toledo State College
Charleston Ann J. Auman, Pacific Lutheran Lisa Boucher, The University of
MaryJo Witz, Monroe University Texas at Austin
Community College Nicanor Austriaco, Providence Nicole Bournias-Vardiabasis,
David Wolfe, American River College California State University,
College Felicitas Avendano, Grand View San Bernardino
Kevin Woo, University of Central University Nancy Boury, Iowa State
Florida J. P. Avery, University of North University
Denise Woodward, Penn State Florida Matthew Brewer, Georgia State
Shawn Wright, Central New Jim Bader, Case Western Reserve University
Mexico Community College University Christoper G. Brown, Georgia
Grace Wyngaard, James Madison Ellen Baker, Santa Monica College Gwinnett College
University Andrew S. Baldwin, Mesa Jill Buettner, Richland College
Aimee Wyrick, Pacific Union Community College Sharon K. Bullock, University of
College Stephen Baron, Bridgewater North Carolina at Charlotte
Joanna Wysocka-Diller, Auburn College Lisa Burgess, Broward College
University Paul W. Bates, University of Jorge Busciglio, University of
Ken Yasukawa, Beloit College Minnesota, Duluth California, Irvine
John Yoder, The University of Janet Batzli, University of Stephen Bush, Coastal Carolina
Alabama Wisconsin, Madison University

xxxi
David Byres, Florida State College Janice Countaway, University of Meghan Duffy, University of
at Jacksonville Central Oklahoma Michigan
Gloria Caddell, University of Joseph A. Covi, University of Richard E. Duhrkopf, Baylor
Central Oklahoma North Carolina at Wilmington University
Guy A. Caldwell, The University Will Crampton, University of Jacquelyn Duke, Baylor
of Alabama Central Florida University
Kim A. Caldwell, The University Kathryn Craven, Armstrong State Kamal Dulai, University of
of Alabama University California, Merced
John S. Campbell, Northwest Lorelei Crerar, George Mason Rebecca K. Dunn, Boston
College University College
Jennifer Capers, Indian River Kerry Cresawn, James Madison Jacob Egge, Pacific Lutheran
State College University University
Joel Carlin, Gustavus Adolphus Richard J. Cristiano, Houston Kurt J. Elliott, Northwest Vista
College Community College Northwest College
Sara G. Carlson, University of Cynthia K. Damer, Central Miles Dean Engell, North
Akron Michigan University Carolina State University
Dale Casamatta, University of David Dansereau, Saint Mary’s Susan Erster, Stony Brook
North Florida University University
Merri Lynn Casem, California Mark Davis, Macalester College Barbara I. Evans, Lake Superior
State University, Fullerton Elizabeth A. De Stasio, Lawrence State University
Anne Casper, Eastern Michigan University Lisa Felzien, Rockhurst
University Tracy Deem, Bridgewater College University
David Champlin, University of Kimberley Dej, McMaster Ralph Feuer, San Diego State
Southern Maine University University
Rebekah Chapman, Georgia State Terrence Delaney, University of Ginger R. Fisher, University of
University Vermont Northern Colorado
P. Bryant Chase, Florida State Tracie Delgado, Northwest John Flaspohler, Concordia
University University College
Thomas T. Chen, Santa Monica Mark S. Demarest, University of Sam Flaxman, University of
College North Texas Colorado, Boulder
Young Cho, Eastern New Mexico D. Michael Denbow, Virginia Nancy Flood, Thompson Rivers
University Polytechnic Institute and State University
Sunita Chowrira, University of University Arthur Frampton, University of
British Columbia Jonathan Dennis, Florida State North Carolina at Wilmington
Tim W. Christensen, East Carolina University Caitlin Gabor, Texas State
University Brandon S. Diamond, University University
Steven Clark, University of of Miami Tracy Galarowicz, Central
Michigan AnnMarie DiLorenzo, Montclair Michigan University
Beth Cliffel, Triton College State University Raul Galvan, South Texas College
Liane Cochran-Stafira, Saint Frank J. Dirrigl, Jr., University of Deborah Garrity, Colorado State
Xavier University Texas, Pan American University
John G. Cogan, The Ohio State Christine Donmoyer, Allegheny Jason Mitchell Gee, East Carolina
University College University
Reid Compton, University of Samuel Douglas, Angelina College T.M. Gehring, Central Michigan
Maryland John D. DuBois, Middle University
Ronald H. Cooper, University of Tennessee State University John Geiser, Western Michigan
California, Los Angeles Janet Duerr, Ohio University University

xxxii
Carol A. Gibbons Kroeker, David Haymer, University of John S. K. Kauwe, Brigham Young
Ambrose University Hawaii at Manoa University
Susan A. Gibson, South Dakota Chris Haynes, Shelton State Lori J. Kayes, Oregon State
State University Community College University
Cynthia J. Giffen, University of Christiane Healey, University of Todd Kelson, Brigham Young
Michigan Massachusetts, Amherst University, Idaho
Matthew Gilg, University of David Hearn, Towson University Christopher Kennedy, Simon
North Florida Marshal Hedin, San Diego State Fraser University
Sharon L. Gillies, University of the University Jacob Kerby, University of South
Fraser Valley Triscia Hendrickson, Morehouse Dakota
Leslie Goertzen, Auburn College Stephen T. Kilpatrick, University
University Albert A. Herrera, University of of Pittsburgh, Johnstown
Marla Gomez, Nicholls State Southern California Mary Kimble, Northeastern
University Bradley Hersh, Allegheny College Illinois University
Steven Gorsich, Central Michigan Anna Hiatt, East Tennessee State Denice D. King, Cleveland State
University University Community College
Daniel Graetzer, Northwest Laura Hill, University of Vermont David Kittlesen, University of
University Jay Hodgson, Armstrong State Virginia
James Grant, Concordia University Ann Kleinschmidt, Allegheny
University James Horwitz, Palm Beach State College
Linda E. Green, Georgia Institute College Kathryn Kleppinger-Sparace,
of Technology Sarah Hosch, Oakland University Tri-County Technical College
Sara Gremillion, Armstrong State Kelly Howe, University of New Daniel Klionsky, University of
University Mexico Michigan
Ann Grens, Indiana University, Kimberly Hruska, Langara Ned Knight, Reed College
South Bend College Benedict Kolber, Duquesne
John L. Griffis, Joliet Junior William Huddleston, University University
College of Calgary Ross Koning, Eastern
Nancy A. Guild, University of Carol Hurney, James Madison Connecticut State University
Colorado, Boulder University John Koontz, The University of
Lonnie Guralnick, Roger Williams Brian A. Hyatt, Bethel University Tennessee, Knoxville
University Bradley C. Hyman, University of Peter Kourtev, Central Michigan
Valerie K. Haftel, Morehouse California, Riverside University
College Anne Jacobs, Allegheny College Elizabeth Kovar, The University
Margaret Hanes, Eastern Robert C. Jadin, Northeastern of Chicago
Michigan University Illinois University Nadine Kriska, University of
Sally E. Harmych, University of Rick Jellen, Brigham Young Wisconsin, Whitewater
Toledo University Tim L. Kroft, Auburn University at
Sherry Harrel, Eastern Kentucky Dianne Jennings, Virginia Montgomery
University Commonwealth University William Kroll, Loyola University
J. Scott Harrison, Georgia L. Scott Johnson, Towson of Chicago
Southern University University Dave Kubien, University of New
Pat Harrison, University of the Russell Johnson, Colby College Brunswick
Fraser Valley Susan Jorstad, University of Jason Kuehner, Emmanuel
Diane Hartman, Baylor University Arizona College
Wayne Hatch, Utah State Matthew Julius, St. Cloud State Josephine Kurdziel, University of
University Eastern University Michigan

xxxiii
Troy A. Ladine, East Texas Baptist Peter B. McIntyre, University of Jennifer S. O’Neil, Houston
University Wisconsin, Madison Community College
Diane M. Lahaise, Georgia Iain McKinnell, Carleton Kavita Oommen, Georgia State
Perimeter College University University
Janice Lai, Austin Community Krystle McLaughlin, Lehigh Nathan Opolot Okia, Auburn
College, Cypress Creek University University at Montgomery
Kirk Land, University of the Susan B. McRae, East Carolina Robin O’Quinn, Eastern
Pacific University Washington University
James Langeland, Kalamazoo Mark Meade, Jacksonville State Sarah A. Orlofske, Northeastern
College University Illinois University
Neva Laurie-Berry, Pacific Richard Merritt, Houston Don Padgett, Bridgewater State
Lutheran University Community College Northwest University
Brenda Leady, University of James E. Mickle, North Carolina Lisa Parks, North Carolina State
Toledo State University University
Adrienne Lee, University of Chad Montgomery, Truman State Nilay Patel, California State
California, Fullerton University University, Fullerton
Chris Levesque, John Abbott Scott M. Moody, Ohio University Markus Pauly, University of
College Daniel Moon, University of California, Berkeley
Bai-Lian Larry Li, University of North Florida Daniel M. Pavuk, Bowling Green
California, Riverside Jamie Moon, University of North State University
Cynthia Littlejohn, University of Florida Marc Perkins, Orange Coast
Southern Mississippi Jonathan Moore, Virginia College
Jason Locklin, Temple College Commonwealth University Beverly Perry, Houston
Xu Lu, University of Findlay Paul A. Moore, Bowling Green Community College
Patrice Ludwig, James Madison State University John S. Peters, College of
University Tsafrir Mor, Arizona State Charleston
Ford Lux, Metropolitan State University Chris Petrie, Eastern Florida State
University of Denver Jeanelle Morgan, University of College
Morris F. Maduro, University of North Georgia John M. Pleasants, Iowa State
California, Riverside Mark Mort, University of Kansas University
C. Smoot Major, University of Anthony Moss, Auburn Michael Plotkin, Mt. San Jacinto
South Alabama University College
Barry Margulies, Towson Karen Neal, Reynolds Community Mary Poffenroth, San Jose State
University College University
Nilo Marin, Broward College Kimberlyn Nelson, Pennsylvania Dan Porter, Amarillo College
Michael Martin, John Carroll State University Sonja Pyott, University of North
University Hao Nguyen, California State Carolina at Wilmington
Heather D. Masonjones, University, Sacramento Mirwais Qaderi, Mount Saint
University of Tampa John Niedzwiecki, Belmont Vincent University
Scott C. Mateer, Armstrong State University Nick Reeves, Mt. San Jacinto
University Alexey G. Nikitin, Grand Valley College
Robert Maxwell, Georgia State State University Adam J. Reinhart, Wayland
University Matthew Nusnbaum, Georgia Baptist University
Joseph McCormick, Duquesne State University Stephanie Richards, Bates
University Robert Okazaki, Weber State College
Lori L. McGrew, Belmont University David A. Rintoul, Kansas State
University Tiffany Oliver, Spelman College University

xxxiv
Trevor Rivers, University of Pramila Sen, Houston Bruce Stallsmith, The University
Kansas Community College of Alabama in Huntsville
Laurel Roberts, University of Anupama Seshan, Emmanuel Maria L. Stanko, New Jersey
Pittsburgh College Institute of Technology
Casey Roehrig, Harvard University Alice Sessions, Austin Nancy Staub, Gonzaga University
Jennifer Rose, University of North Community College Barbara Stegenga, University of
Georgia Vijay Setaluri, University of North Carolina
Michael S. Rosenzweig, Virginia Wisconsin Robert Steven, University of
Polytechnic Institute and State Timothy E. Shannon, Francis Toledo
University Marion University Lori Stevens, University of
Caleb M. Rounds, University of Wallace Sharif, Morehouse Vermont
Massachusetts, Amherst College Mark Sturtevant, Oakland
Yelena Rudayeva, Palm Beach Mark Sherrard, University of University
State College Northern Iowa Elizabeth B. Sudduth, Georgia
James E. Russell, Georgia Cara Shillington, Eastern Gwinnett College
Gwinnett College Michigan University Mark Sugalski, Kennesaw State
Donald Sakaguchi, Iowa State Amy Siegesmund, Pacific University, Southern
University Lutheran University Polytechnic State University
Thomas Sasek, University of Christine Simmons, Southern Fengjie Sun, Georgia Gwinnett
Louisiana at Monroe Illinois University, Edwardsville College
Leslie J. Saucedo, University of S. D. Sipes, Southern Illinois Bradley J. Swanson, Central
Puget Sound University, Carbondale Michigan University
Udo M. Savalli, Arizona State John Skillman, California State Brook O. Swanson, Gonzaga
University West Campus University, San Bernardino University
Smita Savant, Houston Daryl Smith, Langara College Ken Gunter Sweat, Arizona
Community College Southwest Julie Smith, Pacific Lutheran State University West
Leena Sawant, Houston University Campus
Community College Southwest Karen Smith, University of British Annette Tavares, University of
H. Jochen Schenk, California State Columbia Ontario Institute of
University, Fullerton Leo Smith, University of Kansas Technology
Aaron E. Schirmer, Northeastern Ramona Smith-Burrell, Eastern William R. Taylor, University of
Illinois University Florida State College Toledo
Mark Schlueter, Georgia Joel Snodgrass, Towson University Samantha Terris Parks, Georgia
Gwinnett College Alan J. Snow, University of Akron– State University
Gregory Schmaltz, University of Wayne College Jessica Theodor, University of
the Fraser Valley Judith Solti, Houston Community Calgary
Jennifer Schramm, Chemeketa College, Spring Branch Sharon Thoma, University of
Community College Ann Song, University of Wisconsin, Madison
Roxann Schroeder, Humboldt California, Fullerton Sue Thomson, Auburn University
State University C. Kay Song, Georgia State at Montgomery
Tim Schuh, St. Cloud State University Mark Tiemeier, Cincinnati State
University Chrissy Spencer, Georgia Institute Technical and Community
Kevin G. E. Scott, University of of Technology College
Manitoba Rachel Spicer, Connecticut Candace Timpte, Georgia
Erik P. Scully, Towson University College Gwinnett College
Sarah B. Selke, Three Rivers Ashley Spring, Eastern Florida Nicholas Tippery, University of
Community College State College Wisconsin, Whitewater

xxxv
Chris Todd, University of William Velhagen, Caldwell Charles Welsh, Duquesne
Saskatchewan University University
Kurt A. Toenjes, Montana State Sara Via, University of Naomi L. B. Wernick,
University, Billings Maryland University of Massachusetts,
Jeffrey L. Travis, The University at Christopher Vitek, University of Lowell
Albany, The State University of Texas, Pan American Mary E. White, Southeastern
New York Neal J. Voelz, St. Cloud State Louisiana University
Stephen J. Trumble, Baylor University David Wilkes, Indiana University,
University Mindy Walker, Rockhurst South Bend
Jan Trybula, The State University University Frank Williams, Langara College
of New York at Potsdam Andrea Ward, Adelphi Kathy S. Williams, San Diego
Cathy Tugmon, Georgia Regents University State University
University Jennifer Ward, University of Lisa D. Williams, Northern
Alexa Tullis, University of Puget North Carolina at Asheville Virginia Community College
Sound Pauline Ward, Houston Christina Wills, Rockhurst
Marsha Turell, Houston Community College University
Community College Alan Wasmoen, Metropolitan Brian Wisenden, Minnesota State
Pat Uelmen Huey, Georgia Community College University, Moorhead
Gwinnett College Elizabeth R. Waters, San Diego David Wolfe, American River
Steven M. Uyeda, Pima State University College
Community College Matthew Weand, Southern Ramakrishna Wusirika, Michigan
Rani Vajravelu, University of Polytechnic State Technological University
Central Florida University G. Wyngaard, James Madison
Moira van Staaden, Bowling K. Derek Weber, Raritan Valley University
Green State University Community College James R. Yount, Eastern Florida
Dirk Vanderklein, Montclair State Andrea Weeks, George Mason State College
University University Min Zhong, Auburn University

xxxvi
BIOLOGY

HOW
LIFE
WORKS
CONTENTS Evolution predicts a nested pattern of relatedness among
species, depicted as a tree. 16
Evolution can be studied by means of experiments. 17
HOW DO WE KNOW? Can evolution be demonstrated in the
laboratory? 18
About the Authors vi
1.5 Ecological Systems 19
Preface viii
Basic features of anatomy, physiology, and behavior shape
Acknowledgments xxiii ecological systems. 19
Ecological interactions play an important role in evolution. 20
PART 1 FROM CELLS TO ORGANISMS
1.6 The Human Footprint 20
CHAPTER 1 LIFE
? CASE 1 The First Cell: Life’s Origins 25
Chemical, Cellular, and Evolutionary Foundations 3

CHAPTER 2 THE MOLECULES OF LIFE 29

1.1 The Scientific Method 4

Observation allows us to draw tentative explanations called
hypotheses. 4 2.1 Properties of Atoms 30
A hypothesis makes predictions that can be tested by Atoms consist of protons, neutrons, and electrons. 30
observation and experiments. 5
Electrons occupy regions of space called orbitals. 30
General explanations of natural phenomena supported by
many experiments and observations are called theories. 6 Elements have recurring, or periodic, chemical
properties. 31
HOW DO WE KNOW? What caused the extinction of the
dinosaurs? 7 2.2 Molecules and Chemical Bonds 32

1.2 Chemical and Physical Principles 8 A covalent bond results when two atoms share electrons. 33

The living and nonliving worlds share the same chemical A polar covalent bond is characterized by unequal sharing of
foundations and obey the same physical laws. 8 electrons. 33

The scientific method shows that living organisms come from An ionic bond forms between oppositely charged ions. 34
other living organisms. 10 A chemical reaction involves breaking and forming chemical
HOW DO WE KNOW? Can living organisms arise from bonds. 35
nonliving matter? 10
2.3 Water: The Medium of Life 35
HOW DO WE KNOW? Can microscopic life arise from
Water is a polar molecule. 35
nonliving matter? 11
A hydrogen bond is an interaction of a hydrogen atom and
1.3 The Cell 12 an electronegative atom. 35
Nucleic acids store and transmit information needed for Hydrogen bonds give water many unusual properties. 36
growth, function, and reproduction. 12
pH is a measure of the concentration of protons in solution. 37
Membranes define cells and spaces within cells. 14
Metabolism converts energy from the environment into a form 2.4 Carbon: Life’s Chemical Backbone 37
that can be used by cells. 14 Carbon atoms form four covalent bonds. 38
A virus is genetic material in need of a cell. 15 Carbon-based molecules are structurally and functionally
diverse. 38
1.4 Evolution 15
Variation in populations provides the raw material for 2.5 Organic Molecules 39
evolution. 15 Functional groups add chemical character to carbon chains. 39

xxxviii
 Proteins are composed of amino acids. 40 The RNA polymerase complex is a molecular machine that
opens, transcribes, and closes duplex DNA. 63
Nucleic acids encode genetic information in their nucleotide
sequence. 40 3.4 Fate of the RNA Primary Transcript 63
Complex carbohydrates are made up of simple sugars. 42 Messenger RNA carries information for the synthesis of a
Lipids are hydrophobic molecules. 43 specific protein. 63
Primary transcripts in eukaryotes undergo several types of
? 2.6 How Did the Molecules of Life Form? 45
chemical modification. 64
The building blocks of life can be generated in the laboratory. 45
Some RNA transcripts are processed differently from protein-
HOW DO WE KNOW? Could the building blocks of organic coding transcripts and have functions of their own. 65
molecules have been generated on the early Earth? 46
Experiments show how life’s building blocks can form
macromolecules. 46 CHAPTER 4 TRANSLATION AND PROTEIN STRUCTURE 69

CHAPTER 3 NUCLEIC ACIDS AND TRANSCRIPTION 49

4.1 Molecular Structure of Proteins 70

Amino acids differ in their side chains. 71
Successive amino acids in proteins are connected by peptide
3.1 Major Biological Functions of DNA 50
bonds. 72
DNA can transfer biological characteristics from one organism
The sequence of amino acids dictates protein folding, which
to another. 50
determines function. 73
HOW DO WE KNOW? What is the nature of the genetic material? 51
Secondary structures result from hydrogen bonding in the
DNA molecules are copied in the process of replication. 51 polypeptide backbone. 73
Genetic information flows from DNA to RNA to protein. 51 HOW DO WE KNOW? What are the shapes of proteins? 74
HOW DO WE KNOW? What is the nature of the genetic material? 52 Tertiary structures result from interactions between amino
acid side chains. 75
3.2 Chemical Composition and Structure of DNA 53
Polypeptide subunits can come together to form quaternary
A DNA strand consists of subunits called nucleotides. 53
structures. 76
DNA is a linear polymer of nucleotides linked by
Chaperones help some proteins fold properly. 76
phosphodiester bonds. 54
Cellular DNA molecules take the form of a double helix. 55 4.2 Translation: How Proteins Are Synthesized 77

The three-dimensional structure of DNA gave important clues Translation uses many molecules found in all cells. 77
about its functions. 56 The genetic code shows the correspondence between codons
Cellular DNA is coiled and packaged with proteins. 58 and amino acids. 79
HOW DO WE KNOW? How was the genetic code
3.3 Retrieval of Genetic Information Stored in DNA:
deciphered? 80
Transcription 58
Translation consists of initiation, elongation, and
? What was the first nucleic acid molecule, and how did it
termination. 81
arise? 59
? How did the genetic code originate? 83
RNA is a polymer of nucleotides in which the 5-carbon sugar
is ribose. 59 VISUAL SYNTHESIS Gene Expression 84

In transcription, DNA is used as a template to make 4.3 Protein Evolution and the Origin of New Proteins 86
complementary RNA. 60
Most proteins are composed of modular folding
Transcription starts at a promoter and ends at a terminator. 60 domains. 86
RNA polymerase adds successive nucleotides to the 3� end of Amino acid sequences evolve through mutation and
the transcript. 62 selection. 86

xxxix
 Mitochondria provide the eukaryotic cell with most of its
CHAPTER 5 ORGANIZING PRINCIPLES
usable energy. 111
Lipids, Membranes, and Cell Compartments 89
Chloroplasts capture energy from sunlight. 111

CHAPTER 6 MAKING LIFE WORK

Capturing and Using Energy 115

5.1 Structure of Cell Membranes 90

Cell membranes are composed of two layers of lipids. 90
? How did the first cell membranes form? 91
Cell membranes are dynamic. 92
Proteins associate with cell membranes in different ways. 93 6.1 An Overview of Metabolism 116
Organisms can be classified according to their energy and
5.2 The Plasma Membrane and Cell Wall 94
carbon sources. 116
HOW DO WE KNOW? Do proteins move in the plane of the
Metabolism is the set of chemical reactions that sustain life. 117
membrane? 95
The plasma membrane maintains homeostasis. 96 6.2 Kinetic and Potential Energy 118

Passive transport involves diffusion. 96 Kinetic and energy potential energy are two forms of energy. 118

Primary active transport uses the energy of ATP. 97 Chemical energy is a form of potential energy. 118

Secondary active transport is driven by an electrochemical ATP is a readily accessible form of cellular energy. 119
gradient. 98
6.3 The Laws of Thermodynamics 119
Many cells maintain size and composition using active
The first law of thermodynamics: Energy is conserved. 119
transport. 99
The second law of thermodynamics: Energy transformation
The cell wall provides another means of maintaining cell
always results in an increase of disorder in the universe. 119
shape. 100
6.4 Chemical Reactions 120
5.3 The Internal Organization of Cells 100
A chemical reaction occurs when molecules interact. 120
Eukaryotes and prokaryotes differ in internal organization. 101
The laws of thermodynamics determine whether a chemical
Prokaryotic cells lack a nucleus and extensive internal
reaction requires or releases energy available to do work. 121
compartmentalization. 101
The hydrolysis of ATP releases energy. 122
Eukaryotic cells have a nucleus and specialized internal
structures. 101 Non-spontaneous reactions are often coupled to
spontaneous reactions. 123
5.4 The Endomembrane System 104
6.5 Enzymes and the Rate of Chemical Reactions 124
The endomembrane system compartmentalizes the cell. 104
The nucleus houses the genome and is the site of RNA Enzymes reduce the activation energy of a chemical reaction. 124
synthesis. 105 Enzymes form a complex with reactants and products. 125
The endoplasmic reticulum is involved in protein and lipid Enzymes are highly specific. 126
synthesis. 105
HOW DO WE KNOW? Do enzymes form complexes with
The Golgi apparatus modifies and sorts proteins and lipids. 105 substrates? 126
Lysosomes degrade macromolecules. 107 Enzyme activity can be influenced by inhibitors and activators. 127
Protein sorting directs proteins to their proper location in or Allosteric enzymes regulate key metabolic pathways. 127
out of the cell. 108
? What naturally occurring elements might have spurred
5.5 Mitochondria and Chloroplasts 111 the first reactions that led to life? 128

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 Fatty acids and proteins are useful sources of energy. 148
CHAPTER 7 CELLULAR RESPIRATION
Harvesting Energy from Carbohydrates and Other Fuel Molecules 131 The intracellular level of ATP is a key regulator of cellular
respiration. 149
Exercise requires several types of fuel molecules and the
coordination of metabolic pathways. 150

CHAPTER 8 PHOTOSYNTHESIS
Using Sunlight to Build Carbohydrates 153

7.1 An Overview of Cellular Respiration 132

Cellular respiration uses chemical energy stored in molecules
such as carbohydrates and lipids to provide ATP. 132
ATP is generated by substrate-level phosphorylation and
oxidative phosphorylation. 133
Redox reactions play a central role in cellular respiration. 133
Cellular respiration occurs in four stages. 135 8.1 Photosynthesis: An Overview 154
Photosynthesis is widely distributed. 154
7.2 Glycolysis: The Splitting of Sugar 135
Photosynthesis is a redox reaction. 155
Glycolysis is the partial breakdown of glucose. 137
The photosynthetic electron transport chain takes place on
7.3 Pyruvate Oxidation 137 specialized membranes. 155
The oxidation of pyruvate connects glycolysis to the citric HOW DO WE KNOW? Does the oxygen released by
acid cycle. 137 photosynthesis come from H2O or CO2? 156

7.4 The Citric Acid Cycle 138 8.2 The Calvin Cycle 157
The citric acid cycle produces ATP and reduced electron The incorporation of CO2 is catalyzed by the enzyme rubisco. 157
carriers. 138 NADPH is the reducing agent of the Calvin cycle. 158
? What were the earliest energy-harnessing reactions? 139 The regeneration of RuBP requires ATP. 158
7.5 The Electron Transport Chain and Oxidative The steps of the Calvin cycle were determined using
Phosphorylation 140 radioactive CO2. 158
The electron transport chain transfers electrons and pumps Carbohydrates are stored in the form of starch. 158
protons. 140 HOW DO WE KNOW? How is CO2 used to synthesize
The proton gradient is a source of potential energy. 142 carbohydrates? 159
ATP synthase converts the energy of the proton gradient 8.3 Capturing Sunlight into Chemical Forms 160
into the energy of ATP. 142
Chlorophyll is the major entry point for light energy in
HOW DO WE KNOW? Can a proton gradient drive the
photosynthesis. 160
synthesis of ATP? 143
Photosystems use light energy to drive the photosynthetic
7.6 Anaerobic Metabolism and the Evolution of Cellular electron transport chain. 161
Respiration 144 The photosynthetic electron transport chain connects two
Fermentation extracts energy from glucose in the absence photosystems. 162
of oxygen. 145 HOW DO WE KNOW? Do chlorophyll molecules operate on
? How did early cells meet their energy requirements? 146 their own or in groups? 163
The accumulation of protons in the thylakoid lumen drives
7.7 Metabolic Integration 147
the synthesis of ATP. 164
Excess glucose is stored as glycogen in animals and starch
Cyclic electron transport increases the production of ATP. 166
in plants. 147
Sugars other than glucose contribute to glycolysis. 147 8.4 Challenges to Photosynthetic Efficiency 166

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 Excess light energy can cause damage. 166 9.5 Receptor Kinases and Long-Term Responses 191
Photorespiration leads to a net loss of energy and carbon. 168 Receptor kinases phosphorylate each other, activate
intercellular signaling pathways, lead to a response, and
Photosynthesis captures just a small percentage of incoming
are terminated. 192
solar energy. 169
? How do cell signaling errors lead to cancer? 192
8.5 The Evolution of Photosynthesis 170
Signaling pathways are integrated to produce a response
? How did early cells use sunlight to meet their energy in a cell. 193
requirements? 170
The ability to use water as an electron donor in photosynthesis
evolved in cyanobacteria. 170 CHAPTER 10 CELL AND TISSUE ARCHITECTURE 197
Eukaryotic organisms are believed to have gained
photosynthesis by endosymbiosis. 171
VISUAL SYNTHESIS Harnessing Energy: Photosynthesis
and Cellular Respiration 172

? CASE 2 Cancer: When Good Cells Go Bad 176

CHAPTER 9 CELL SIGNALING 179

10.1 Tissues and Organs 198
Tissues and organs are communities of cells. 198
The structure of skin relates to its function. 199

10.2 The Cytoskeleton 200

Microtubules and microfilaments are polymers of protein
subunits. 200
Microtubules and microfilaments are dynamic
structures. 201

9.1 Principles of Cell Communication 180 Motor proteins associate with microtubules and micro-
filaments to cause movement. 202
Cells communicate using chemical signals that bind to specific
receptors. 180 Intermediate filaments are polymers of proteins that vary
according to cell type. 204
Signaling involves receptor activation, signal transduction,
response, and termination. 181 The cytoskeleton is an ancient feature of cells. 205

10.3 Cell Junctions 206

9.2 Cell Signaling over Long and Short Distances 182
Cell adhesion molecules allow cells to attach to other cells
Endocrine signaling acts over long distances. 183
and to the extracellular matrix. 206
Signaling can occur over short distances. 183
Anchoring junctions connect adjacent cells and are reinforced
HOW DO WE KNOW? Where do growth factors come from? 184 by the cytoskeleton. 207
Signaling can occur by direct cell–cell contact. 184 Tight junctions prevent the movement of substances through
the space between cells. 208
9.3 Cell-Surface and Intracellular Receptors 185
Communicating junctions allow the passage of molecules
Receptors for polar signaling molecules are on the cell
between cells. 210
surface. 185
Receptors for nonpolar signaling molecules are in the interior 10.4 The Extracellular Matrix 210
of the cell. 186 The extracellular matrix of plants is the cell wall. 211
Cell-surface receptors act like molecular switches. 186 The extracellular matrix is abundant in connective tissues
of animals. 212
9.4 G Protein-Coupled Receptors and Short-Term Responses 187
? How do cancer cells spread throughout the body? 213
The first step in cell signaling is receptor activation. 187
Extracellular matrix proteins influence cell shape and gene
Signals are often amplified in the cytosol. 188 expression. 214
Signals lead to a cellular response. 188 HOW DO WE KNOW? Can extracellular matrix proteins
Signaling pathways are eventually terminated. 190 influence gene expression? 215

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 Tumor suppressors block specific steps in the development
CHAPTER 11 CELL DIVISION
of cancer. 238
Variations, Regulation, and Cancer 219
Most cancers require the accumulation of multiple mutations. 238
VISUAL SYNTHESIS Cellular Communities 240

? CASE 3 You, from A to T: Your Personal Genome 244

CHAPTER 12 DNA REPLICATION AND MANIPULATION 247

11.1 Cell Division 220

Prokaryotic cells reproduce by binary fission. 220
Eukaryotic cells reproduce by mitotic cell division. 221
The cell cycle describes the life cycle of a eukaryotic cell. 221

11.2 Mitotic Cell Division 222

The DNA of eukaryotic cells is organized as chromosomes. 222
Prophase: Chromosomes condense and become visible. 223 12.1 DNA Replication 248

Prometaphase: Chromosomes attach to the mitotic spindle. 224 During DNA replication, the parental strands separate and
new partners are made. 248
Metaphase: Chromosomes align as a result of dynamic
changes in the mitotic spindle. 224 HOW DO WE KNOW? How is DNA replicated? 249

Anaphase: Sister chromatids fully separate. 224 New DNA strands grow by the addition of nucleotides to the
3� end. 250
Telophase: Nuclear envelopes re-form around newly
segregated chromosomes. 225 In replicating DNA, one daughter strand is synthesized
continuously and the other in a series of short pieces. 251
The parent cell divides into two daughter cells by cytokinesis. 225
A small stretch of RNA is needed to begin synthesis of a
11.3 Meiotic Cell Division 226 new DNA strand. 252
Pairing of homologous chromosomes is unique to meiosis. 226 Synthesis of the leading and lagging strands is coordinated. 252
Crossing over between DNA molecules results in exchange DNA polymerase is self-correcting because of its proof-
of genetic material. 227 reading function. 254
The first meiotic division brings about the reduction in
chromosome number. 227 12.2 Replication of Chromosomes 254

The second meiotic division resembles mitosis. 228 Replication of DNA in chromosomes starts at many places
almost simultaneously. 254
Division of the cytoplasm often differs between the sexes. 231
Telomerase restores tips of linear chromosomes shortened
Meiosis is the basis of sexual reproduction. 231
during DNA replication. 255
11.4 Regulation of the Cell Cycle 233
12.3 Isolation, Identification, and Sequencing of DNA
Protein phosphorylation controls passage through the cell Fragments 257
cycle. 233
The polymerase chain reaction selectively amplifies regions
HOW DO WE KNOW? How is progression through the cell of DNA. 257
cycle controlled? 234
Electrophoresis separates DNA fragments by size. 259
Different cyclin–CDK complexes regulate each stage of the
Restriction enzymes cleave DNA at particular short
cell cycle. 235
sequences. 260
Cell cycle progression requires successful passage through
DNA strands can be separated and brought back together
multiple checkpoints. 235
again. 261
? 11.5 What Genes Are Involved in Cancer? 236 DNA sequencing makes use of the principles of DNA
Oncogenes promote cancer. 236 replication. 263

HOW DO WE KNOW? Can a virus cause cancer? 237 ? What new technologies are being developed to sequence
your personal genome? 264
Proto-oncogenes are genes that when mutated may cause
cancer. 238 12.4 Genetic Engineering 264

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 Recombinant DNA combines DNA molecules from two or Organelle DNA forms nucleoids that differ from those in
more sources. 264 bacteria. 285
Recombinant DNA is the basis of genetically modified 13.5 Viruses and Viral Genomes 285
organisms. 266
Viruses can be classified by their genomes. 286
DNA editing can be used to alter gene sequences almost at will. 267
The host range of a virus is determined by viral and host
surface proteins. 287

CHAPTER 13 GENOMES 271 Viruses have diverse sizes and shapes. 287
Viruses are capable of self-assembly. 288

CHAPTER 14 MUTATION AND DNA REPAIR 291

13.1 Genome Sequencing 272

HOW DO WE KNOW? How are whole genomes
sequenced? 272
Complete genome sequences are assembled from smaller 14.1 The Rate and Nature of Mutations 292
pieces. 272
For individual nucleotides, mutation is a rare event. 292
Sequences that are repeated complicate sequence assembly. 273
Across the genome as a whole, mutation is common. 293
? Why sequence your personal genome? 274
Only germ-line mutations are transmitted to progeny. 293
13.2 Genome Annotation 275 ? What can your personal genome tell you about your genetic
Genome annotation identifies various types of sequence. 275 risk factors? 294

Genome annotation includes searching for sequence motifs. 276 Mutations are random with regard to an organism’s needs. 295

Comparison of genomic DNA with messenger RNA reveals HOW DO WE KNOW? Do mutations occur randomly, or are
the intron–exon structure of genes. 276 they directed by the environment? 296

An annotated genome summarizes knowledge, guides 14.2 Small-Scale Mutations 297

research, and reveals evolutionary relationships among
Point mutations are changes in a single nucleotide. 297
organisms. 277
Small insertions and deletions involve several nucleotides. 298
The HIV genome illustrates the utility of genome annotation
and comparison. 277 Some mutations are due to the insertion of a transposable
element. 300
13.3 Gene Number, Genome Size, and Organismal Complexity 278
HOW DO WE KNOW? What causes sectoring in corn
Gene number is not a good predictor of biological kernels? 300
complexity. 278
14.3 Chromosomal Mutations 301
Viruses, bacteria, and archaeons have small, compact genomes. 279
Duplications and deletions result in gain or loss of DNA. 301
Among eukaryotes, there is no relationship between genome
size and organismal complexity. 279 Gene families arise from gene duplication and evolutionary
divergence. 302
About half of the human genome consists of transposable
elements and other types of repetitive DNA. 280 An inversion has a chromosomal region reversed in
orientation. 303
13.4 Organization of Genomes 281
A reciprocal translocation joins segments from
Bacterial cells package their DNA as a nucleoid composed of nonhomologous chromosomes. 303
many loops. 281
14.4 DNA Damage and Repair 303
Eukaryotic cells package their DNA as one molecule per
chromosome. 282 DNA damage can affect both DNA backbone and bases. 303
The human genome consists of 22 pairs of chromosomes and Most DNA damage is corrected by specialized repair
two sex chromosomes. 282 enzymes. 304

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 Early theories of heredity predicted the transmission of
CHAPTER 15 GENETIC VARIATION 309
acquired characteristics. 326
Belief in blending inheritance discouraged studies of
hereditary transmission. 326

16.2 The Foundations of Modern Transmission Genetics 327

Mendel’s experimental organism was the garden pea. 327
In crosses, one of the traits was dominant in the offspring. 328

16.3 Segregation: Mendel’s Key Discovery 330

Genes come in pairs that segregate in the formation of
15.1 Genotype and Phenotype 310 reproductive cells. 330
Genotype is the genetic makeup of a cell or organism; The principle of segregation was tested by predicting the
phenotype is its observed characteristics. 310 outcome of crosses. 331
The effect of a genotype often depends on several factors. 310 A testcross is a mating to an individual with the homozygous
recessive genotype. 332
Some genetic differences are major risk factors for disease. 311
Segregation of alleles reflects the separation of chromosomes
Not all genetic differences are harmful. 312
in meiosis. 332
A few genetic differences are beneficial. 313
Dominance is not universally observed. 332
15.2 Genetic Variation and Individual Uniqueness 314 The principles of transmission genetics are statistical and
Areas of the genome with variable numbers of tandem stated in terms of probabilities. 333
repeats are useful in DNA typing. 314 Mendelian segregation preserves genetic variation. 334
Some polymorphisms add or remove restriction sites in the DNA. 315
16.4 Independent Assortment 335
15.3 Genomewide Studies of Genetic Variation 316
Independent assortment is observed when genes segregate
Single-nucleotide polymorphisms (SNPs) are single-base independently of one another. 335
changes in the genome. 316
HOW DO WE KNOW? How are single-gene traits inherited? 336
? How can genetic risk factors be detected? 317
Independent assortment reflects the random alignment of
Copy-number variation constitutes a significant proportion chromosomes in meiosis. 337
of genetic variation. 318
Phenotypic ratios can be modified by interactions between genes. 338
15.4 Genetic Variation in Chromosomes 318
16.5 Patterns of Inheritance Observed in Family Histories 339
Nondisjunction in meiosis results in extra or missing
Dominant traits appear in every generation. 339
chromosomes. 319
Recessive traits skip generations. 340
Some human disorders result from nondisjunction. 319
Many genes have multiple alleles. 340
HOW DO WE KNOW? What is the genetic basis of Down
syndrome? 320 Incomplete penetrance and variable expression can obscure
inheritance patterns. 341
Extra or missing sex chromosomes have fewer effects than
extra autosomes. 320 ? How do genetic tests identify disease risk factors? 342

Nondisjunction is a major cause of spontaneous abortion. 322

CHAPTER 17 INHERITANCE OF SEX CHROMOSOMES,

CHAPTER 16 MENDELIAN INHERITANCE 325 LINKED GENES, AND ORGANELLES 345

16.1 Early Theories of Inheritance 326 17.1 The X and Y Sex Chromosomes 346

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 In many animals, sex is genetically determined and associated The relative importance of genes and environment can be
with chromosomal differences. 346 determined by differences among individuals. 367
Segregation of the sex chromosomes predicts a 1:1 ratio Genetic and environmental effects can interact in
of females to males. 347 unpredictable ways. 367

17.2 Inheritance of Genes in the X Chromosome 348 18.2 Resemblance Among Relatives 368
X-linked inheritance was discovered through studies of male For complex traits, offspring resemble parents but show
fruit flies with white eyes. 348 regression toward the mean. 369
Genes in the X chromosome exhibit a “crisscross” inheritance Heritability is the proportion of the total variation due to
pattern. 348 genetic differences among individuals. 370
X-linkage provided the first experimental evidence that genes
18.3 Twin Studies 371
are in chromosomes. 350
Twin studies help separate the effects of genes and
Genes in the X chromosome show characteristic patterns in
environment in differences among individuals. 371
human pedigrees. 351
HOW DO WE KNOW? What is the relative importance of
17.3 Genetic Linkage and Recombination 353 genes and of the environment for common traits? 372
Nearby genes in the same chromosome show linkage. 353
18.4 Complex Traits in Health and Disease 373
The frequency of recombination s a measure of the distance
Most common diseases and birth defects are affected by
between linked genes. 355
many genes that each have relatively small effects. 373
Genetic mapping assigns a location to each gene along a
Human height is affected by hundreds of genes. 374
chromosome. 355
HOW DO WE KNOW? Can recombination be used to construct
? Can personalized medicine lead to effective treatments of
common diseases? 375
a genetic map of a chromosome? 356

Genetic risk factors for disease can be localized by genetic

mapping. 356 CHAPTER 19 GENETIC AND EPIGENETIC REGULATION 377

17.4 Inheritance of Genes in the Y Chromosome 357

Y-linked genes are transmitted from father to son to
grandson. 357
? How can the Y chromosome be used to trace ancestry? 358

17.5 Inheritance of Mitochondrial and Chloroplast DNA 359

Mitochondrial and chloroplast genomes often show
uniparental inheritance. 359
Maternal inheritance is characteristic of mitochondrial
19.1 Chromatin to Messenger RNA in Eukaryotes 378
diseases. 360
Gene expression can be influenced by chemical modification
? How can mitochondrial DNA be used to trace ancestry? 360
of DNA or histones. 378
Gene expression can be regulated at the level of an entire
chromosome. 380
CHAPTER 18 THE GENETIC AND ENVIRONMENTAL BASIS
OF COMPLEX TRAITS 363 Transcription is a key control point in gene expression. 382
RNA processing is also important in gene regulation. 385

19.2 Messenger RNA to Phenotype in Eukaryotes 384

Small regulatory RNAs inhibit translation or promote mRNA
degradation. 384
Translational regulation controls the rate, timing, and location
of protein synthesis. 384
Protein structure and chemical modification modulate protein
effects on phenotype. 385

18.1 Heredity and Environment 364 ? How do lifestyle choices affect expression of your personal
genome? 386
Complex traits are affected by the environment. 365
Complex traits are affected by multiple genes. 366 19.3 Transcriptional Regulation in Prokaryotes 386

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 Transcriptional regulation can be positive or negative. 387 20.5 Cell Signaling in Development 415
Lactose utilization in E. coli is the pioneering example of A signaling molecule can cause multiple responses in the cell. 415
transcriptional regulation. 388 Developmental signals are amplified and expanded. 416
HOW DO WE KNOW? How does lactose lead to the
VISUAL SYNTHESIS Genetic Variation and Inheritance 418
production of active β-galactosidase enzyme? 388

The repressor protein binds with the operator and prevents ? CASE 4 Malaria: Coevolution of Humans and a Parasite 422
transcription, but not in the presence of lactose. 389
The function of the lactose operon was revealed by genetic
studies. 390 CHAPTER 21 EVOLUTION
The lactose operon is also positively regulated by How Genotypes and Phenotypes Change over Time 425
CRP–cAMP. 390
Transcriptional regulation determines the outcome of
infection by a bacterial virus. 391
VISUAL SYNTHESIS Virus: A Genome in Need of a Cell 394

CHAPTER 20 GENES AND DEVELOPMENT 399

21.1 Genetic Variation 426

Population genetics is the study of patterns of genetic
variation. 426
Mutation and recombination are the two sources of genetic
variation. 427

21.2 Measuring Genetic Variation 427

To understand patterns of genetic variation, we require
20.1 Genetic Programs of Development 400 information about allele frequencies. 427
The fertilized egg is a totipotent cell. 400 Early population geneticists relied on observable traits and
Cellular differentiation increasingly restricts alternative fates. 401 gel electrophoresis to measure variation. 428

HOW DO WE KNOW? How do stem cells lose their ability to DNA sequencing is the gold standard for measuring genetic
differentiate into any cell type? 402 variation. 428

? Can cells with your personal genome be reprogrammed for HOW DO WE KNOW? How is genetic variation measured? 429
new therapies? 403
21.3 Evolution and the Hardy–Weinberg Equilibrium 430
20.2 Hierarchical Control of Development 404 Evolution is a change in allele or genotype frequency over
Drosophila development proceeds through egg, larval, and time. 430
adult stages. 404 The Hardy–Weinberg equilibrium describes situations in
The egg is a highly polarized cell. 405 which allele and genotype frequencies do not change. 430

Development proceeds by progressive regionalization and The Hardy–Weinberg equilibrium relates allele frequencies
specification. 406 and genotype frequencies. 431

Homeotic genes determine where different body parts The Hardy–Weinberg equilibrium is the starting point for
develop in the organism. 408 population genetic analysis. 432

20.3 Evolutionary Conservation of Key Transcription Factors 21.4 Natural Selection 432
in Development 410 Natural selection brings about adaptations. 432
Animals have evolved a wide variety of eyes. 410 The Modern Synthesis combines Mendelian genetics and
Pax6 is a master regulator of eye development. 410 Darwinian evolution. 434
Natural selection increases the frequency of advantageous
20.4 Combinatorial Control in Development 412 mutations and decreases the frequency of deleterious
Floral differentiation is a model for plant development. 412 mutations. 434
The identity of the floral organs is determined by ? What genetic differences have made some individuals more
combinatorial control. 413 and some less susceptible to malaria? 434

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 Natural selection can be stabilizing, directional, or disruptive. 435 ? How did malaria come to infect humans? 455
HOW DO WE KNOW? How far can artificial selection be Sympatric populations—those not geographically separated—
taken? 436 may undergo speciation. 456
Sexual selection increases an individual’s reproductive Speciation can occur instantaneously. 458
success. 437
22.4 Speciation and Selection 459
21.5 Migration, Mutation, Genetic Drift, and Non-Random
Speciation can occur with or without natural selection. 459
Mating 438
Natural selection can enhance reproductive isolation. 459
Migration reduces genetic variation between populations. 438
VISUAL SYNTHESIS Speciation 460
Mutation increases genetic variation. 438
Genetic drift has a large effect in small populations. 438
Non-random mating alters genotype frequencies without CHAPTER 23 EVOLUTIONARY PATTERNS
affecting allele frequencies. 439
Phylogeny and Fossils 463
21.6 Molecular Evolution 440
The molecular clock relates the amount of sequence difference
between species and the time since the species diverged. 440
The rate of the molecular clock varies. 440

CHAPTER 22 SPECIES AND SPECIATION 445

23.1 Reading a Phylogenetic Tree 464

Phylogenetic trees provide hypotheses of evolutionary
relationships. 464
The search for sister groups lies at the heart of phylogenetics. 465
A monophyletic group consists of a common ancestor and
all its descendants. 466
Taxonomic classifications are information storage and
22.1 The Biological Species Concept 446 retrieval systems. 467

Species are reproductively isolated from other species. 446 23.2 Building a Phylogenetic Tree 468
The BSC is more useful in theory than in practice. 447 Homology is similarity by common descent. 468
The BSC does not apply to asexual or extinct organisms. 447 Shared derived characters enable biologists to reconstruct
evolutionary history. 469
Ring species and hybridization complicate the BSC. 448
The simplest tree is often favored among multiple possible
Ecology and evolution can extend the BSC. 448
trees. 469
22.2 Reproductive Isolation 449 Molecular data complement comparative morphology in
Pre-zygotic isolating factors occur before egg fertilization. 450 reconstructing phylogenetic history. 471

Post-zygotic isolating factors occur after egg fertilization. 450 Phylogenetic trees can help solve practical problems. 472
HOW DO WE KNOW? Did an HIV-positive dentist spread
22.3 Speciation 450
the AIDS virus to his patients? 473
Speciation is a by-product of the genetic divergence of
separated populations. 451 23.3 The Fossil Record 474

Allopatric speciation is speciation that results from the Fossils provide unique information. 474
geographical separation of populations. 451 Fossils provide a selective record of past life. 474
Dispersal and vicariance can isolate populations from Geological data indicate the age and environmental setting
each other. 451 of fossils. 476
HOW DO WE KNOW? Can vicariance cause speciation? 452 Fossils can contain unique combinations of characters. 479
Co-speciation is speciation that occurs in response to HOW DO WE KNOW? Can fossils bridge the evolutionary
speciation in another species. 455 gap between fish and tetrapod vertebrates? 481

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 Rare mass extinctions have altered the course of evolution. 481 Some human differences have likely arisen by natural
selection. 500
23.4 Comparing Evolution’s Two Great Patterns 482
? What human genes are under selection for resistance to
Phylogeny and fossils complement each other. 482 malaria? 501
Agreement between phylogenies and the fossil record
provides strong evidence of evolution. 482 24.5 Culture, Language, and Consciousness 502
Culture changes rapidly. 502
Is culture uniquely human? 503
CHAPTER 24 HUMAN ORIGINS AND EVOLUTION 485 Is language uniquely human? 503
Is consciousness uniquely human? 503

PART 2 FROM ORGANISMS TO THE ENVIRONMENT

CHAPTER 25 CYCLING CARBON 509

24.1 The Great Apes 486

Comparative anatomy shows that the human lineage branches
off the great apes tree. 486
Molecular analysis reveals that our lineage split from the
chimpanzee lineage about 5–7 million years ago. 487
HOW DO WE KNOW? How closely related are humans and
chimpanzees? 488 25.1 The Short-Term Carbon Cycle 510

The fossil record gives us direct information about our Photosynthesis and respiration are key processes in
evolutionary history. 488 short-term carbon cycling. 511
HOW DO WE KNOW? How much CO2 was in the atmosphere
24.2 African Origins 491 1000 years ago? 512
Studies of mitochondrial DNA reveal that modern humans The regular oscillation of CO2 reflects the seasonality of
evolved in Africa relatively recently. 491 photosynthesis in the Northern Hemisphere. 512
HOW DO WE KNOW? When and where did the most recent Human activities play an important role in the modern
common ancestor of all living humans live? 492 carbon cycle. 513
Studies of the Y chromosome provide independent evidence Carbon isotopes show that much of the CO2 added to air
for a recent origin of modern humans. 494 over the past half century comes from burning fossil fuels. 513
Neanderthals disappear from the fossil record as modern
HOW DO WE KNOW? What is the major source of the
humans appear, but have contributed to the modern human
CO2 that has accumulated in Earth’s atmosphere over the
gene pool. 494
past two centuries? 514

24.3 Distinct Features of Our Species 495 25.2 The Long-Term Carbon Cycle 515
Bipedalism was a key innovation. 495 Reservoirs and fluxes are key in long-term carbon cycling. 515
Adult humans share many features with juvenile Physical processes add and remove CO2 from the atmosphere. 516
chimpanzees. 496
Records of atmospheric composition over 400,000 years
Humans have large brains relative to body size. 496 show periodic shifts in CO2 content. 517
The human and chimpanzee genomes help us identify genes Variations in atmospheric CO2 over hundreds of millions
that make us human. 498 of years reflect plate tectonics and evolution. 520

24.4 Human Genetic Variation 498 25.3 The Carbon Cycle: Ecology, Biodiversity, and Evolution 521
The prehistory of our species has had an impact on the Food webs trace the cycling of carbon through communities
distribution of genetic variation. 499 and ecosystems. 521
The recent spread of modern humans means that there are Biological diversity reflects the many ways that organisms
few genetic differences between groups. 500 participate in the carbon cycle. 522

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 The carbon cycle weaves together biological evolution and Thaumarchaeota may be the most abundant cells in the oceans. 546
environmental change through Earth history. 522 HOW DO WE KNOW? How abundant are archaeons in
the oceans? 547
? CASE 5 The Human Microbiome: Diversity Within 525
26.6 The Evolutionary History of Prokaryotes 548
Life originated early in our planet’s history. 548
CHAPTER 26 BACTERIA AND ARCHAEA 529 Prokaryotes have coevolved with eukaryotes. 549
? How do intestinal bacteria influence human health? 550

CHAPTER 27 EUKARYOTIC CELLS

Origins and Diversity 553

26.1 Two Prokaryotic Domains 530

The bacterial cell is small but powerful. 530
Diffusion limits cell size in bacteria. 530
Horizontal gene transfer promotes genetic diversity in bacteria. 531
Archaea form a second prokaryotic domain. 534
27.1 The Eukaryotic Cell: A Review 554
26.2 An Expanded Carbon Cycle 535
Internal protein scaffolding and dynamic membranes
Many photosynthetic bacteria do not produce oxygen. 535 organize the eukaryotic cell. 554
Many bacteria respire without oxygen. 536 In eukaryotic cells, energy metabolism is localized in
Photoheterotrophs obtain energy from light but obtain mitochondria and chloroplasts. 555
carbon from preformed organic molecules. 537 The organization of the eukaryotic genome also helps explain
Chemoautotrophy is a uniquely prokaryotic metabolism. 537 eukaryotic diversity. 555
Sex promotes genetic diversity in eukaryotes and gives rise to
26.3 Other Biogeochemical Cycles 538
distinctive life cycles. 556
Bacteria and archaeons dominate Earth’s sulfur cycle. 538
27.2 Eukaryotic Origins 557
The nitrogen cycle is also driven by bacteria and archaeons. 539
? What role did symbiosis play in the origin of
26.4 The Diversity of Bacteria 540 chloroplasts? 557
Bacterial phylogeny is a work in progress. 540 HOW DO WE KNOW? What is the evolutionary origin of
HOW DO WE KNOW? How many kinds of bacterium live chloroplasts? 558
in the oceans? 541 ? What role did symbiosis play in the origin of
What, if anything, is a bacterial species? 542 mitochondria? 559

Proteobacteria are the most diverse bacteria. 543 ? How did the eukaryotic cell originate? 560

The gram-positive bacteria include organisms that cause How did the eukaryotic cell originate? 560
and cure disease. 543 In the oceans, many single-celled eukaryotes harbor
Photosynthesis is widely distributed on the bacterial tree. 544 symbiotic bacteria. 562

26.5 The Diversity of Archaea 545 27.3 Eukaryotic Diversity 562

The archaeal tree has anaerobic, hyperthermophilic Our own group, the opisthokonts, is the most diverse
organisms near its base. 545 eukaryotic superkingdom. 563

The Archaea include several groups of acid-loving Amoebozoans include slime molds that produce multicellular
microorganisms. 546 structures. 564
Only Archaea produce methane as a by-product of energy Archaeplastids, which include land plants, are photosynthetic
metabolism. 546 organisms. 566
One group of the Euryarchaeota thrives in extremely salty Stramenopiles, alveolates, and rhizarians dominate eukaryotic
environments. 546 diversity in the oceans. 568

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 Photosynthesis spread through eukaryotes by repeated Regulatory genes have played an important role in the
endosymbioses involving eukaryotic algae. 570 evolution of complex multicellular organisms. 592
HOW DO WE KNOW? How did photosynthesis spread through HOW DO WE KNOW? What controls color pattern in
the Eukarya? 571 butterfly wings? 593

27.4 The Fossil Record of Protists 572

? CASE 6 Agriculture: Feeding a Growing Population 595
Fossils show that eukaryotes existed at least 1800 million
years ago. 573
Protists have continued to diversify during the age of animals. 573 CHAPTER 29 PLANT STRUCTURE AND FUNCTION
Moving Photosynthesis onto Land 599

CHAPTER 28 BEING MULTICELLULAR 577

29.1 Plant Structure and Function: An Evolutionary Perspective 600

Land plants are a monophyletic group that includes vascular
28.1 The Phylogenetic Distribution of Multicellular Organisms 578 plants and bryophytes. 600
Simple multicellularity is widespread among eukaryotes. 578
29.2 The Leaf: Acquiring CO2 While Avoiding Dessication 601
Complex multicellularity evolved several times. 579
CO2 uptake results in water loss. 601
28.2 Diffusion and Bulk Flow 580 The cuticle restricts water loss from leaves but inhibits the
uptake of CO2. 602
Diffusion is effective only over short distances. 581
Stomata allow leaves to regulate water loss and carbon
Animals achieve large size by circumventing limits imposed
gain. 603
by diffusion. 581
CAM plants use nocturnal CO2 storage to avoid water
Complex multicellular organisms have structures specialized
loss during the day. 603
for bulk flow. 581
C4 plants suppress photorespiration by concentrating CO2 in
28.3 How to Build a Multicellular Organism 582 bundle-sheath cells 604
Complex multicellularity requires adhesion between cells. 583 HOW DO WE KNOW? How do we know that C4
How did animal cell adhesion originate? 583 photosynthesis suppresses photorespiration? 606

Complex multicellularity requires communication 29.3 The Stem: Transport of Water Through Xylem 606
between cells. 584
Xylem provides a low-resistance pathway for the movement
HOW DO WE KNOW? How do bacteria influence the life of water. 607
cycles of choanoflagellates? 584
HOW DO WE KNOW? How large are the forces that allow
Complex multicellularity requires a genetic program leaves to pull water from the soil? 608
for coordinated growth and cell differentiation. 585
Water is pulled through xylem by an evaporative pump. 609
28.4 Variations on a Theme: Plants versus Animals 587 Xylem transport is at risk of conduit collapse and cavitation. 610
Cell walls shape patterns of growth and development
29.4 The Stem: Transport of Carbohydrates Through Phloem 610
in plants. 587
Phloem transports carbohydrates from sources to sinks. 611
Animal cells can move relative to one another. 588
Carbohydrates are pushed through phloem by an osmotic
28.5 The Evolution of Complex Multicellularity 589 pump. 611
Fossil evidence of complex multicellular organisms is first Phloem feeds both the plant and the rhizosphere. 612
observed in rocks deposited 579 –555 million years ago. 589
29.5 The Root: Uptake of Water and Nutrients from the Soil 612
Oxygen is necessary for complex multicellular life. 590
Plants obtain essential mineral nutrients from the soil. 613
Land plants evolved from green algae that could carry out
photosynthesis on land. 591 Nutrient uptake by roots is highly selective. 614

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 Nutrient uptake requires energy. 614
CHAPTER 31 PLANT GROWTH AND DEVELOPMENT 641
Mycorrhizae enhance nutrient uptake. 615
Symbiotic nitrogen-fixing bacteria supply nitrogen to both
plants and ecosystems. 616

? How has nitrogen availability influenced agricultural

productivity? 616

CHAPTER 30 PLANT REPRODUCTION

Finding Mates and Dispersing Young 619
31.1 Shoot Growth and Development 642
Shoots grow by adding new cells at their tips. 642
Stem elongation occurs just below the apical meristem. 643
The development of new apical meristems allows stems
to branch. 644
The shoot apical meristem controls the production and
arrangement of leaves. 644
Young leaves develop vascular connections to the stem. 646
30.1 Alternation of Generations 620 Flower development terminates the growth of shoot
The algal sister groups of land plants have one multicellular meristems. 646
generation in their life cycle. 620
31.2 Plant Hormones 647
Bryophytes illustrate how the alternation of generations
Hormones affect the growth and differentiation of plant cells. 647
allows the dispersal of spores in the air. 621
Polar transport of auxin guides the placement of leaf primordia
Dispersal enhances reproductive fitness in several ways. 623
and the development of vascular connections with the stem. 647
Spore-dispersing vascular plants have free-living
? What is the developmental basis for the shorter stems of high-
gametophytes and sporophytes. 623
yielding rice and wheat? 649
30.2 Seed Plants 625
Cytokinins, in combination with other hormones, control
The seed plant life cycle is distinguished by four major the growth of axillary buds. 650
steps. 625
31.3 Secondary Growth 650
Pine trees illustrate how the transport of pollen in air
allows fertilization to occur in the absence of external Shoots produce two types of lateral meristem. 651
sources of water. 625 The vascular cambium produces secondary xylem
Seeds enhance the dispersal and establishment of the next and phloem. 651
sporophyte generation. 627 The cork cambium produces an outer protective layer. 652

30.3 Flowering Plants 628 Wood has both mechanical and transport functions. 652

Flowers are reproductive shoots specialized for the 31.4 Root Growth and Development 654
production, transfer, and receipt of pollen. 628
Roots grow by producing new cells at their tips. 654
The diversity of floral morphology is related to modes of
Root elongation and vascular development are coordinated. 654
pollination. 631
The formation of new root apical meristems allows roots
Angiosperms have mechanisms to increase outcrossing. 632 to branch. 655
HOW DO WE KNOW? Are pollinator shifts associated with The structures and functions of root systems are diverse. 655
the formation of new species? 633
31.5 The Environmental Context of Growth and Development 656
Angiosperms delay provisioning their ovules until after
fertilization. 635 Plants orient the growth of their stems and roots by light
and gravity. 656
Fruits enhance the dispersal of seeds. 636
HOW DO WE KNOW? How do plants grow toward light? 657
? How did scientists increase crop yields during the Green
Revolution? 637 HOW DO WE KNOW? How do seeds detect the presence
of plants growing overhead? 659
30.4 Asexual Reproduction 638 Seeds can delay germination if they detect the presence of
Asexually produced plants disperse with and without seeds. 638 plants overhead. 659

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 Plants grow taller and branch less when growing in the Nutrient-rich environments select for plants that allocate
shade of other plants. 660 more resources to growth than to defense. 681
Roots elongate more and branch less when water is scarce. 661 Exposure to multiple threats can lead to trade-offs. 682
Exposure to wind results in shorter and stronger stems. 661
32.4 Defense and Plant Diversity 682
31.6 Timing of Developmental Events 661 The evolution of new defenses may allow plants to diversify. 682
Flowering time is affected by day length. 661 Pathogens, herbivores, and seed predators can increase plant
Plants use their internal circadian clock and photoreceptors to biodiversity. 683
determine day length. 662 ? Can modifying plants genetically protect crops from
Vernalization prevents plants from flowering until winter herbivores and pathogens? 683
has passed. 663
Plants use day length as a cue to prepare for winter. 664
CHAPTER 33 PLANT DIVERSITY 687

CHAPTER 32 PLANT DEFENSE

Keeping the World Green 667

33.1 Plant Diversity: An Evolutionary Overview 688

Four major transformations in life cycle and structure
characterize the evolutionary history of plants. 688
32.1 Protection Against Pathogens 668 Plant diversity has changed over time. 690
Plant pathogens infect and exploit host plants by a variety of
33.2 Bryophytes 690
mechanisms. 668
Bryophytes are small, simple, and tough. 690
Plants are able to detect and respond to pathogens. 670
The small gametophytes and unbranched sporophytes of
Plants respond to infections by isolating infected regions. 671
bryophytes are adaptations for reproducing on land. 691
Mobile signals trigger defenses in uninfected tissues. 671
Bryophytes exhibit several cases of convergent evolution with
HOW DO WE KNOW? Can plants develop immunity to specific the vascular plants. 692
pathogens? 672
Sphagnum moss plays an important role in the global
Plants defend against viral infections by producing siRNA. 673 carbon cycle. 693
A pathogenic bacterium provides a way to modify plant
genomes. 673 33.3 Spore-Dispersing Vascular Plants 693
Rhynie cherts provide a window into the early evolution
32.2 Defense Against Herbivores 674 of vascular plants. 693
Plants use mechanical and chemical defenses to avoid being Lycophytes are the sister group of all other vascular plants. 694
eaten. 674
Ancient lycophytes included giant trees that dominated
Diverse chemical compounds deter herbivores. 675
coal swamps about 320 million years ago. 695
Some plants provide food and shelter for ants, which actively
HOW DO WE KNOW? Did woody plants evolve more
defend them. 677
than once? 696
Grasses can regrow quickly following grazing by mammals. 678
Ferns and horsetails are morphologically and ecologically
32.3 Allocating Resources to Defense 679 diverse. 697

Some defenses are always present, whereas others are Fern diversity has been strongly affected by the evolution
turned on in response to a threat. 679 of angiosperms. 698

Plants can sense and respond to herbivores. 679 An aquatic fern contributes to rice production. 698

HOW DO WE KNOW? Can plants communicate? 680 33.4 Gymnosperms 699

Plants produce volatile signals that attract insects that prey Cycads and ginkgos are the earliest diverging groups of living
upon herbivores. 681 gymnosperms. 699

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 Conifers are woody plants that thrive in dry and cold climates. 700 34.3 Diversity 726
Gnetophytes are gymnosperms that have independently Fungi are highly diverse. 726
evolved xylem vessels and double fertilization. 702 Fungi evolved from aquatic, unicellular, and flagellated
33.5 Angiosperms 702 ancestors. 727

Angiosperms may have originated in the shady understory Zygomycetes produce hyphae undivided by septa. 728
of tropical forests. 702 Glomeromycetes form endomycorrhizae. 728
Angiosperm diversity results from flowers and xylem vessels, The Dikarya produce regular septa during mitosis. 728
among other traits, as well as interactions with animals and
Ascomycetes are the most diverse group of fungi. 729
other organisms. 704
HOW DO WE KNOW? Can a fungus influence the behavior
Monocots are diverse in shape and size despite not forming
of an ant? 731
a vascular cambium. 704
Basidiomycetes include smuts, rusts, and mushrooms. 731
HOW DO WE KNOW? When did grasslands expand over
the land surface? 706 ? How do fungi threaten global wheat production? 733
Eudicots are the most diverse group of angiosperms. 707
? CASE 7 Predator–Prey: A Game of Life and Death 736
? What can be done to protect the genetic diversity of crop
species? 708

VISUAL SYNTHESIS Angiosperms: Structure and Function 710

CHAPTER 35 ANIMAL NERVOUS SYSTEMS 739

CHAPTER 34 FUNGI
Structure, Function, and Diversity 715

35.1 Nervous System Function and Evolution 740

Animal nervous systems have three types of nerve cell. 740
Nervous systems range from simple to complex. 741
? What body features arose as adaptations for successful
34.1 Growth and Nutrition 716
predation? 742
Hyphae permit fungi to explore their environment for food
resources. 716 35.2 Neuron Structure 743
Fungi transport materials within their hyphae. 717 Neurons share a common organization. 743
Not all fungi produce hyphae. 717 Neurons differ in size and shape. 744
Fungi are principal decomposers of plant tissues. 718 Neurons are supported by other types of cell. 744
Fungi are important plant and animal pathogens. 718 35.3 Neuron Function 745
Many fungi form symbiotic associations with plants and
The resting membrane potential is negative and results in
animals. 720
part from the movement of potassium ions. 745
Lichens are symbioses between a fungus and a green alga
Neurons are excitable cells that transmit information by
or a cyanobacterium. 720
action potentials. 746
34.2 Reproduction 722 Neurons propagate action potentials along their axons by
Fungi proliferate and disperse using spores. 722 sequentially opening and closing adjacent Na+ and K+ ion
channels. 748
Multicellular fruiting bodies facilitate the dispersal of sexually
produced spores. 723 Neurons communicate at synapses. 749

The fungal life cycle often includes a stage in which haploid HOW DO WE KNOW? What is the resting membrane

cells fuse, but nuclei do not. 723 potential? How does electrical activity change during
an action potential? 750
HOW DO WE KNOW? What determines the shape of fungal
spores that are ejected into the air? 724 Signals between neurons can be excitatory or inhibitory. 752

Genetically distinct mating types promote outcrossing. 725 35.4 Nervous System Organization 754

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 Nervous systems are organized into peripheral and central The brain is divided into lobes with specialized functions. 778
components. 754 Information is topographically mapped into the vertebrate
Peripheral nervous systems have voluntary and involuntary cerebral cortex. 780
components. 755
36.6 Memory and Cognition 780
The nervous system helps to maintain homeostasis. 756
The brain serves an important role in memory and learning. 780
Simple reflex circuits provide rapid responses to stimuli. 757
Cognition involves brain information processing and decision
making. 781
CHAPTER 36 ANIMAL SENSORY SYSTEMS AND BRAIN
FUNCTION 761
CHAPTER 37 ANIMAL MOVEMENT
Muscles and Skeletons 785

36.1 Animal Sensory Systems 761

Specialized sensory receptors detect diverse stimuli. 762
Chemoreceptors are universally present in animals. 763 37.1 Muscles: Biological Motors That Generate Force
and Produce Movement 786
Mechanoreceptors are a second general class of ancient
sensory receptors. 764 Muscles use chemical energy to produce force and
Electroreceptors sense light, thermoreceptors sense movement. 786
temperature, and nociceptors sense pain. 764 Muscles can be striated or smooth. 786
Stimuli are transmitted by changes in the firing rate of action Skeletal and cardiac muscle fibers are organized into
potentials. 764 repeating contractile units called sarcomeres. 787

36.2 Smell and Taste 766 Muscles contract by the sliding of myosin and actin
filaments. 789
Smell and taste depend on chemoreception of molecules
carried in the environment and in food. 766 Calcium regulates actin–myosin interaction through
excitation–contraction coupling. 791
36.3 Gravity, Movement, and Sound 767
Calmodulin regulates Ca activation and relaxation of
2+

Hair cells sense gravity and motion. 767 smooth muscle. 792
Hair cells detect the physical vibrations of sound. 768
37.2 Muscle Contractile Properties 792
? How have sensory systems evolved in predators
and prey? 771 Muscle length affects actin–myosin overlap and generation
of force. 792
36.4 Vision 771
HOW DO WE KNOW? How does filament overlap affect force
Animals see the world through different types of eyes. 771 generation in muscles? 793
The structure and function of the vertebrate eye underlie Muscle force and shortening velocity are inversely related. 793
image processing. 773
Antagonist pairs of muscles produce reciprocal motions at
Vertebrate photoreceptors are unusual because they
a joint. 794
hyperpolarize in response to light. 774
Muscle force is summed by an increase in stimulation
Color vision detects different wavelengths of light. 774
frequency and the recruitment of motor units. 795
Local sensory processing of light determines basic features
of shape and movement. 775 Skeletal muscles have slow-twitch and fast-twitch fibers. 796

HOW DO WE KNOW? How does the retina process visual ? How do different types of muscle fiber affect the speed
information? 776 of predators and prey? 797

36.5 Brain Organization and Function 777 37.3 Animal Skeletons 797
The brain processes and integrates information received from Hydrostatic skeletons support animals by muscles that act
different sensory systems. 777 on a fluid-filled cavity. 797

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 Exoskeletons provide hard external support and protection. 798 Local chemical signals regulate neighboring target cells. 823
The rigid bones of vertebrate endoskeletons are jointed for Pheromones are chemical compounds released into the
motion and can be repaired if damaged. 799 environment to signal physiological and behavioral changes
in other species members. 823
37.4 Vertebrate Skeletons 800

Vertebrate bones form directly or by forming a cartilage

model first. 801
CHAPTER 39 ANIMAL CARDIOVASCULAR AND
Two main types of bone are compact and spongy bone. 801 RESPIRATORY SYSTEMS 827
Bones grow in length and width, and can be repaired. 802

Joint shape determines range of motion and skeletal muscle

organization. 802
Muscles exert forces by skeletal levers to produce joint
motion. 802

CHAPTER 38 ANIMAL ENDOCRINE SYSTEMS 807

39.1 Delivery of Oxygen and Elimination of Carbon Dioxide 828

Diffusion governs gas exchange over short distances. 828
Bulk flow moves fluid over long distances. 829

39.2 Respiratory Gas Exchange 830

Many aquatic animals breathe through gills. 830
Insects breathe air through tracheae. 832
Most terrestrial vertebrates breathe by tidal ventilation
38.1 An Overview of Endocrine Function 808 of internal lungs. 832
he endocrine system helps to regulate an organism’s response Mammalian lungs are well adapted for gas exchange. 833
to its environment. 808 The structure of bird lungs allows unidirectional airflow for
The endocrine system regulates growth and development. 809 increased oxygen uptake. 834

HOW DO WE KNOW? How are growth and development Voluntary and involuntary mechanisms control breathing. 835
controlled in insects? 810
39.3 Oxygen Transport by Hemoglobin 835
The endocrine system underlies homeostasis. 811
Blood is composed of fluid and several types of cell. 835
38.2 Properties of Hormones 813
HOW DO WE KNOW? What is the molecular structure of
Hormones act specifically on cells that bind the hormone. 813 hemoglobin and myoglobin? 837
Two main classes of hormone are peptides and amines, Hemoglobin is an ancient molecule with diverse roles
and steroid hormones. 813 related to oxygen binding and transport. 837
Hormonal signals are to strengthen their effect. 815 Hemoglobin reversibly binds oxygen. 837
Hormones are evolutionarily conserved molecules with Myoglobin stores oxygen, enhancing delivery to muscle
diverse functions. 818 mitochondria. 838

38.3 The Vertebrate Endocrine System 818 Many factors affect hemoglobin–oxygen binding. 839

The pituitary gland integrates diverse bodily functions 39.4 Circulatory Systems 840
by secreting hormones in response to signals from the
hypothalamus. 819 Circulatory systems have vessels of different sizes to
facilitate bulk flow and diffusion. 841
Many targets of pituitary hormones are endocrine tissues
that also secrete hormones. 820 Arteries are muscular, elastic vessels that carry blood
away from the heart under high pressure. 841
Other endocrine organs have diverse functions. 821
Veins are thin-walled vessels that return blood to the
? How does the endocrine system influence predators heart under low pressure. 843
and prey? 821
Compounds and fluid move across capillary walls by
38.4 Other Forms of Chemical Communication 822 diffusion, filtration, and osmosis. 843

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 ? How do hormones and nerves provide homeostatic 40.4 Digestion and Absorption of Food 864
regulation of blood flow as well as allow an animal to respond The digestive tract has regional specializations. 864
to stress? 843
Digestion begins in the mouth. 864
39.5 The Evolution, Structure, and Function of the Heart 844 Further digestion and storage of nutrients take place in
Fish have two-chambered hearts and a single circulatory the stomach. 865
system. 845 Final digestion and nutrient absorption take place in the
Amphibians and reptiles have more three-chambered hearts small intestine. 866
and partially divided circulations. 845 The large intestine absorbs water and stores waste. 869
Mammals and birds have four-chambered hearts and fully The lining of the digestive tract is composed of distinct layers. 869
divided pulmonary and systemic circulations. 846 Plant-eating animals have specialized digestive tracts that
Cardiac muscle cells are electrically connected to contract in reflect their diets. 870
synchrony. 847

Heart rate and cardiac output are regulated by the autonomic

nervous system. 848 CHAPTER 41 ANIMAL RENAL SYSTEMS
Water and Waste 875

CHAPTER 40 ANIMAL METABOLISM, NUTRITION,

AND DIGESTION 851

41.1 Water and Electrolyte Balance 876

Osmosis governs the movement of water across cell
membranes. 876
Osmoregulation is the control of osmotic pressure inside
40.1 Patterns of Animal Metabolism 852
cells and organisms. 877
Animals rely on anaerobic and aerobic metabolism. 852
Osmoconformers match their internal solute concentration
Metabolic rate varies with activity level. 854 to that of the environment. 878
? Does body temperature limit activity level in predators Osmoregulators have internal solute concentrations that
and prey? 854 differ from that of their environment. 878
Metabolic rate is affected by body size. 855 ? Can the loss of water and electrolytes in exercise be
exploited as a strategy to hunt prey? 880
Metabolic rate is linked to body temperature. 855
HOW DO WE KNOW? How is metabolic rate affected 41.2 Excretion of Wastes 881
by running speed and body size? 856 The excretion of nitrogenous wastes is linked to an animal’s
habitat and evolutionary history. 881
40.2 Animal Nutrition and Diet 857
Excretory organs work by filtration, reabsorption,
Energy balance is a form of homeostasis. 857 and secretion. 882
An animal’s diet must supply nutrients that it cannot synthesize. 857 Animals have diverse excretory organs. 883
VISUAL SYNTHESIS Homeostasis and Thermoregulation 858 Vertebrates filter blood under pressure through paired kidneys. 884

40.3 Adaptations for Feeding 861 41.3 Structure and Function of the Mammalian Kidney 886
Suspension filter feeding is common in many aquatic The mammalian kidney has an outer cortex and inner
animals. 861 medulla. 886
Large aquatic animals apprehend prey by suction feeding Glomerular filtration isolates wastes carried by the blood
and active swimming. 862 along with water and small solutes. 887
Jaws and teeth provide specialized food capture and The proximal convoluted tubule reabsorbs solutes by active
mechanical breakdown of food. 862 transport. 888

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 The loop of Henle acts as a countercurrent multiplier to create a VISUAL SYNTHESIS Reproduction and Development 916
concentration gradient from the cortex to the medulla. 888 Childbirth is initiated by hormonal changes. 918
HOW DO WE KNOW? How does the mammalian kidney
produce concentrated urine? 890
The distal convoluted tubule secretes additional wastes. 891 CHAPTER 43 ANIMAL IMMUNE SYSTEMS 921
The final concentration of urine is determined in the
collecting ducts and is under hormonal control. 891
The kidneys help regulate blood pressure and blood volume. 892

CHAPTER 42 ANIMAL REPRODUCTION AND

DEVELOPMENT 897

43.1 An Overview of the Immune System 922

Pathogens cause disease. 922
The immune system distinguishes self from nonself. 922
The immune system consists of innate and adaptive immunity. 923

43.2 Innate Immunity 923

The skin and mucous membranes provide the first line of
42.1 The Evolutionary History of Reproduction 898 defense against infection. 924
Asexual reproduction produces clones. 898 White blood cells provide a second line of defense against
Sexual reproduction involves the formation and fusion of pathogens. 925
gametes. 899 Phagocytes recognize foreign molecules and send signals
Many species reproduce both sexually and asexually. 900 to other cells. 926

Exclusive asexuality is often an evolutionary dead end. 901 Inflammation is a coordinated response to tissue injury. 926

HOW DO WE KNOW? Do bdelloid rotifers reproduce The complement system participates in the innate and adaptive
only asexually? 902 immune systems. 927

42.2 Movement onto Land and Reproductive Adaptations 903 43.3 Adaptive Immunity: B Cells and Antibodies 928

Fertilization can take place externally or internally. 903 B cells produce antibodies. 929

r-strategists and K-strategists differ in number of offspring Mammals produce five classes of antibody with different
and parental care. 904 biological functions. 929

Animals either lay eggs or give birth to live young. 904 Clonal selection is the basis for antibody specificity. 930
Clonal selection also explains immunological memory. 931
42.3 Human Reproductive Anatomy and Physiology 905
Genomic rearrangement creates antibody diversity. 931
The male reproductive system is specialized for the
production and delivery of sperm. 905 HOW DO WE KNOW? How is antibody diversity generated? 932

The female reproductive system produces eggs and supports 43.4 Adaptive Immunity: T cells and Cell-Mediated Immunity 934
the developing embryo. 907
T cells include helper and cytotoxic cells. 934
Hormones regulate the human reproductive system. 908
T cells have T cell receptors on their surface that recognize an
42.4 Gamete Formation to Birth in Humans 911 antigen in association with MHC proteins. 934

Male and female gametogenesis have both shared and The ability to distinguish between self and nonself is
distinct features. 911 acquired during T cell maturation. 936

Fertilization occurs when a sperm fuses with an oocyte. 912 43.5 Three Pathogens: A Virus, Bacterium, and Eukaryote 936
The first trimester includes cleavage, gastrulation, and The flu virus evades the immune system by antigenic drift
organogenesis. 913 and shift. 937
The second and third trimesters are characterized by Tuberculosis is caused by a slow-growing, intracellular
fetal growth. 915 bacterium. 937

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 The malaria parasite changes surface molecules by antigenic Amniotes evolved terrestrial eggs. 970
variation. 938
44.5 The Evolutionary History of Animals 972
? CASE 8 Biodiversity Hotspots: Rain Forests and Reefs 941 Fossils and phylogeny show that animal forms were initially
simple but rapidly evolved complexity. 972
The animal body plans we see today emerged during
the Cambrian Period. 973
CHAPTER 44 ANIMAL DIVERSITY 945
Tabulations of described fossils show that animal diversity
has been shaped by both radiation and mass extinction over
the part 500 million years. 974
Animals began to colonize the land 420 million years ago. 974

? How have coral reefs changed through time? 975

VISUAL SYNTHESIS Diversity Through Time 976

44.1 A Tree of Life for More than a Million Animal Species 946 CHAPTER 45 ANIMAL BEHAVIOR 981

Phylogenetic trees propose an evolutionary history of

animals. 946
Morphology and development provide clues to animal
phylogeny. 946
Molecular sequence comparisons have confirmed some
relationships and raised new questions. 948

44.2 The Simplest Animals: Sponges, Cnidarians, Ctenophores,

and Placozoans 948
Sponges are simple and widespread in the oceans. 948 45.1 Tinbergen’s Questions 982

Cnidarians are the architects of life’s largest constructions: 45.2 Dissecting Behavior 983
coral reefs. 950
The fixed action pattern is a stereotyped behavior. 983
Ctenophores and placozoans represent the extremes of body The nervous system processes stimuli and evokes behaviors. 984
organization among early-branching animals. 952
Hormones can trigger certain behaviors. 985
The order of early branches on the animal tree remains
Breeding experiments can help determine the degree to
uncertain. 953
which a behavior is genetic. 986
The discovery of new animals with a unique body plan
Molecular techniques provide new ways of testing the role
complicates phylogenetic hypotheses still further. 954
of genes in behavior. 987
44.3 Bilaterian Animals 956 HOW DO WE KNOW? Can genes influence behavior? 988
Lophotrozochoans make up nearly half of all animal phyla,
45.3 Learning 989
including the diverse and ecologically important annelids and
mollusks. 956 Non-associative learning occurs without linking two events. 990

Ecdysozoans include nematodes, the most numerous animals, Associative learning occurs when two events are linked. 990
and arthropods, the most diverse. 959 Learning is an adaptation. 990
HOW DO WE KNOW? How did the diverse feeding HOW DO WE KNOW? To what extent are insects capable
appendages of arthropods arise? 961 of learning? 991
Deuterostomes include humans and other chordates, and 45.4 Orientation, Navigation, and Biological Clocks 992
also acorn worms and sea stars. 963
Orientation involves a directed response to a stimulus. 992
Chordates include vertebrates, cephalochordates,
and tunicates. 964 Navigation is demonstrated by the remarkable ability of
homing in birds. 993
44.4 Vertebrate Diversity 966 Biological clocks provide important time cues for many
Fish are the earliest-branching and most diverse vertebrate behaviors. 993
animals. 967 HOW DO WE KNOW? Does a biological clock play a role
The common ancestor of tetrapods had four limbs. 969 in birds’ ability to orient? 994

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45.5 Communication 995 Patterns of survivorship vary among organisms. 1013
Communication is the transfer of information between Reproductive patterns reflect the predictability of a species’
a sender and receiver. 995 environment. 1014
Some forms of communication are complex and learned The life history of an organism shows trade-offs among
during a sensitive period. 996 physiological functions. 1014

Other forms of communication convey specific information. 997 46.4 Metapopulation Dynamics 1015

45.6 Social Behavior 997 A metapopulation is a group of populations linked by

immigrants. 1015
Group selection is a weak explanation of altruistic behavior. 998
Island biogeography explains species diversity on habitat
Reciprocal altruism is one way that altruism can evolve. 999
islands. 1016
The concept of kin selection is based on the idea that it is
? How do islands promote species diversification? 1019
possible to contribute genetically to future generations by
helping close relatives. 999

CHAPTER 47 SPECIES INTERACTIONS, COMMUNITIES,

AND ECOSYSEMS 1021
CHAPTER 46 POPULATION ECOLOGY 1003

47.1. The Niche 1022

46.1 Populations and Their Properties 1004
The niche is a species’ place in nature. 1022
Populations are all the individuals of a species in a particular
The realized niche of a species is more restricted that
place. 1005
its fundamental niche. 1023
Three key features of a population are its size, range,
Niches are shaped by evolutionary history. 1023
and density. 1005
HOW DO WE KNOW? How many butterflies are there in 47.2 Antagonistic Interactions Between Species 1023
a given population? 1006 Limited resources foster competition. 1023
Ecologists estimate population size and density by Competitive exclusion can prevent two species from
sampling. 1006 occupying the same niche at the same time. 1024

46.2 Population Growth and Decline 1007 ? Can competition drive species diversification? 1025

Population size is affected by birth, death, immigration, Species compete for resources other than food. 1025
and emigration. 1007 Predation, parasitism, and herbivory are interactions in
Exponential growth is characterized by a constant per which one species benefits at the expense of another. 1025
capital growth rate. 1008 HOW DO WE KNOW? Can predators and prey coexist stably
Carrying capacity is the maximum number of individuals in certain environments? 1026
a habitat can support. 1008 Facilitation can occur when two species prey on a third
Logistic growth produces an S-shaped curve and describes species. 1027
the growth of many natural populations. 1009
47.3 Mutualistic Interactions Between Species 1028
Factors that influence population growth can be dependent
Mutualisms are interactions between species that benefit
on or independent of its density. 1009
both participants. 1028
46.3 Age-Structured Population Growth 1010 Mutualisms may evolve increasing interdependence. 1028
Birth and death rates vary with age and environment. 1011 HOW DO WE KNOW? Have aphids and their symbiotic

Survivorship curves record changes in survival probability bacteria coevolved? 1029

over an organism’s life-span. 1011 Digestive symbioses recycle plant material. 1030

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 Mutualisms may be obligate or facultative. 1030 The biological carbon cycle shapes ecological interactions
and reflects evolution. 1060
The costs and benefits of species interactions can change
over time. 1030 The nitrogen cycle also reflects the interplay between
ecology and evolution. 1062
47.4 Ecological Communities 1031
Phosphorus cycles through ecosystems, supporting primary
Species that live in the same place make up communities. 1031 production. 1062
? How is biodiversity measured? 1032 Global patterns of primary production reflect variations in
One species can have a great effect on all other members climate and nutrient availability. 1063
of the community. 1032 HOW DO WE KNOW? Does iron limit primary production
Keystone species have disproportionate effects on in some parts of the oceans? 1065
communities. 1033
48.4 Global Biodiversity 1066
Disturbance can modify community composition. 1034
? Why does biodiversity decrease from the equator toward
Succession describes the community response to new
the poles? 1066
habitats or disturbance. 1035
Evolutionary and ecological history underpins diversity. 1068
47.5 Ecosystems 1036
Species interactions result in food webs that cycle carbon
and other elements through ecosystems. 1036
CHAPTER 49 THE ANTHROPOCENE
Species interactions form trophic pyramids that transfer Humans as a Planetary Force 1071
energy through ecosystems. 1037
Light, water, nutrients, and diversity all influence rates
of primary production. 1038
HOW DO WE KNOW? Does species diversity promote
primary productivity? 1039

VISUAL SYNTHESIS Succession: Ecology in Microcosm 1040

CHAPTER 48 BIOMES AND GLOBAL ECOLOGY 1045

49.1 The Anthropocene Period 1072
Humans are a major force on the planet. 1072

49.2 Human Influence on the Carbon Cycle 1073

As atmospheric carbon dioxide levels have increased, so
has mean surface temperature. 1074
Changing environments affect species distribution and
community composition. 1076
? How has climate change affected coral reefs around the
world? 1077
48.1 The Physical Basis of Climate 1046
HOW DO WE KNOW? What is the effect of increased
The principal control on Earth’s surface temperature is the
atmospheric CO2 and reduced ocean pH on skeleton
angle at which solar radiation strikes the surface. 1046
formation in marine algae? 1080
Heat is transported toward the poles by wind and ocean
What can be done? 1080
currents. 1047
Global circulation patterns determine patterns of rainfall, 49.3 Human Influence on the Nitrogen and Phosphorus
but topography also matters. 1049 Cycles 1082

48.2 Biomes 1049 Nitrogen fertilizer transported to lakes and the sea causes
eutrophication. 1082
Terrestrial biomes reflect the distribution of climate. 1050
Phosphate fertilizer is also used in agriculture, but has
Aquatic biomes reflect climate, and also the availability of
finite sources. 1082
nutrients and oxygen and the depth to which sunlight
penetrates through water. 1052 What can be done? 1083

48.3 Global Ecology: Cycling Bioessential Elements 1060 49.4 Human Influence on Evolution 1084

lxi
lxi
 Human activities have reduced the quality and size of many Conservation biologists have a diverse toolkit for
habitats, decreasing the number of species they can support. 1084 confronting threats to biodiversity. 1092
Overexploitation threatens species and disrupts ecological Global change provides new challenges for conservation
relationships within communities. 1085 biology in the 21st century. 1092
VISUAL SYNTHESIS Flow of Matter and Energy Sustainable development provides a strategy for
Through Ecosystems 1086 conserving biodiversity while meeting the needs of the
human population. 1093
Humans play an important role in the dispersal of species. 1088
Humans have altered the selective landscape for many 49.6 Scientists and Citizens in the 21st Century 1094
pathogens. 1089
Quick Check Answers Q-1
Are amphibians ecology’s “canary in the coal mine”? 1090
Glossary G-1
49.5 Conservation Biology 1091
? What are our conservation priorities? 1091 Index I-1

lxii
lxii
BIOLOGY

HOW
LIFE
WORKS
 PA RT 1 CHAPTER 1

Life
FROM Chemical, Cellular, and

CELLS TO
Evolutionary Foundations

ORGANISMS
” Nothing in biology
makes sense except
in the light of evolution.”
—THEODOSIUS DOBZHANSKY

1
 CH
C HA
APPTTEERR 11

Life
Chemical, Cellular, and
Evolutionary Foundations

Core Concepts
1.1 The scientific method is a
deliberate way of asking and
answering questions about the
natural world.
1.2 Life works according to
fundamental principles of
chemistry and physics.
1.3 The fundamental unit of life is
the cell.
1.4 Evolution explains the features
that organisms share and those
that set them apart.
1.5 Organisms interact with
one another and with their
physical environment, shaping
ecological systems that sustain
life.
1.6 In the 21st century, humans
have become major agents in
ecology and evolution.
Giovanna Griffo/Gallery Stock.

1
4 SECTION 1.1 THE SCIENTIFIC METHOD

Every day, remarkable things happen within and around you. we know about life? And we hope you will develop a basis for
Strolling through a local market, you come across a bin full of crisp making informed decisions about your life, career, and the actions
apples, pick one up, and take a bite. Underlying this unremarkable you take as a citizen.
occurrence is an extraordinary series of events. Your eyes sense the
apple from a distance, and nerves carry that information to your
brain, permitting identification. Biologists call this cognition, an 1.1 THE SCIENTIFIC METHOD
area of biological study. Stimulated by the apple and recognizing
it as ripe and tasty, your brain transmits impulses through nerves How do we go about trying to understand the vastness and
to your muscles. How we respond to external cues motivates complexity of nature? For most scientists, studies of the natural
behavior, another biological discipline. Grabbing the apple requires world involve the complementary processes of observation and
the coordinated activities of dozens of muscles that move your experimentation. Observation is the act of viewing the world
arm and hand to a precise spot. These movements are described around us. Experimentation is a disciplined and controlled way
by biomechanics, yet another area of biological research. And, as of asking and answering questions about the world in an unbiased
you bite down on the apple, glands in your mouth secrete saliva, manner.
starting to convert energy stored in the apple as sugar into energy
that you will use to fuel your own activities. Physiology, like Observation allows us to draw tentative explanations
biomechanics, lies at the heart of biological function. called hypotheses.
The study of cognition, behavior, biomechanics, and physiology Observations allow us to ask focused questions about nature. Let’s
are all ways of approaching biology, the science of how life works. say you observe a hummingbird like the one pictured in Fig. 1.1
Biologists, scientists who study life, have come to understand a hovering near a red flower, occasionally dipping its long beak into
great deal about these and other processes at levels that run from the bloom. What motivates this behavior? Is the bird feeding on
molecular mechanisms within the cell, through the integrated some substance within the flower? Is it drawn to the flower by its
actions of many cells within an organ or body, to the interactions vivid color? What benefit, if any, does the flower derive from this
among different organisms in nature. We don’t know everything busy bird?
about how life works—in fact, it seems as if every discovery raises Observations such as these, and the questions they raise,
new questions. But biology provides us with an organized way of allow us to propose tentative explanations, or hypotheses. We
understanding ourselves and the world around us. might, for example, hypothesize that the hummingbird is carrying
Why study biology? The example of eating an apple was pollen from one flower to the next, facilitating reproduction in
deliberately chosen because it is an everyday occurrence that we the plant. Or we might hypothesize that nectar produced deep
ordinarily wouldn’t think about twice. The scope of modern biology, within the flower provides nutrition for the hummingbird—that
however, is vast, raising questions that can fire our imaginations, the hummingbird’s actions reflect the need to take in food. Both
affect our health, and influence our future. How, for example, will hypotheses provide a reasonable explanation of the behavior we
our understanding of the human genome change the way that observed, but they may or may not be correct. To find out, we have
we fight cancer? How do bacteria in our digestive system help to test them.
determine health and well-being? Will expected increases in the Charles Darwin’s classic book, On the Origin of Species,
temperature and acidity of seawater doom coral reefs? Is there, or published in 1859, beautifully illustrates how we can piece
has there ever been, life on Mars? And, to echo the great storyteller together individual observations to construct a working
Rudyard Kipling, why do leopards have spots, and tigers stripes? hypothesis. In this book, Darwin discussed a wide range of
We can describe six grand themes that connect and unite the observations, from pigeon breeding to fossils and from embryology
many dimensions of life science, from molecules to the biosphere. to the unusual animals and plants found on islands. Darwin noted
These six themes are stated as Core Concepts for this chapter and the success of animal breeders in selecting specific individuals for
are introduced in the following sections. Throughout the book, reproduction and thereby generating new breeds for agriculture or
these themes will be visited again and again. We view them as the show. He appreciated that selective breeding is successful only if
keys to understanding the many details in subsequent chapters specific features of the animals can be passed from one generation
and relating them to one another. Our hope is that by the time to the next by inheritance. Reading economic treatises by the
you finish this book, you will have an understanding of how life English clergyman Thomas Malthus, he understood that limiting
works, from the molecular machines inside cells and the metabolic environmental resources could select among the variety of
pathways that cycle carbon through the biosphere to the process of different individuals in populations in much the way that breeders
evolution, which has shaped the living world that surrounds (and select among cows or pigeons.
includes) us. You will, we hope, see the connections among these Gathering together all these seemingly disparate pieces of
different ways of understanding life, and come away with a greater information, Darwin argued that life has evolved over time by
understanding of how scientists think about and ask questions means of natural selection. Since its formulation, Darwin’s initial
about the natural world. How, in fact, do we know what we think hypothesis has been tested by experiments, many thousands of
 CHAPTER 1 L I F E : C H E M I C A L , C E L LU L A R , A N D E VO L U T I O N A RY F O U N DAT I O N S 5

Once we have a hypothesis, we can

FIG. 1.1 A hummingbird visiting a flower. This simple observation leads to questions: Why test it to see if its predictions are accurate.
do hummingbirds pay so much attention to flowers? Why do they hover near red Returning to the hummingbird and flower,
flowers? Source: Charles J. Smith. we can test the hypothesis that the bird
is transporting pollen from one flower to
the next, enabling the plant to reproduce.
Observation provides one type of test: If
we catch and examine the bird just after it
visits a flower, do we find pollen stuck to
its beak or feathers? If so, our hypothesis
survives the test.
Note, however, that we haven’t proved
the case. Pollen might be stuck on the bird
for a different reason—perhaps it provides
food for the hummingbird. However, if
the birds didn’t carry pollen from flower
to flower, we would reject the hypothesis
that they facilitate pollination. In other
words, a single observation or experiment
can lead us to reject a hypothesis, or it
can support the hypothesis, but it cannot
prove that a hypothesis is correct. To move
forward, then, we might make a second
set of observations. Does pollen that
adheres to the hummingbird rub off when
the bird visits a second flower of the same
species? If so, we have stronger support
for our hypothesis.
We might also use observations to
test a more general hypothesis about birds
them. Our knowledge of many biological phenomena, ranging and flowers. Does red color generally attract birds and so facilitate
from biodiversity to the way the human brain is wired, depends pollination in a wide range of flowers? To answer this question, we
on direct observation followed by careful inferences that lead to might catalog the pollination of many red flowers and ask whether
models of how things work. they are pollinated mainly by birds. Or we might go the opposite
direction and catalog the flowers visited by many different birds—
A hypothesis makes predictions that can be tested by are they more likely to be red than chance alone might predict?
observation and experiments. Finally, we can test the hypothesis that the birds visit the
Not just any idea qualifies as a hypothesis. Two features set flowers primarily to obtain food, spreading pollen as a side effect
hypotheses apart from other ways of attacking problems. First, of their feeding behavior. We can measure the amount of nectar in
a good hypothesis makes predictions about observations not yet the flower before and after the bird visits and calculate how much
made or experiments not yet run. Second, because hypotheses energy has been consumed by the bird during its visit. Continued
make predictions, we can test them. That is, we can devise observations over the course of the day will tell us whether the
an experiment to see whether the predictions made by the birds gain the nutrition they need by drinking nectar, and whether
hypothesis actually occur, or we can go into the field to try the birds have other sources of food.
to make further observations predicted by the hypothesis. A In addition to observations, in many cases we can design
hypothesis, then, is a statement about nature that can be tested experiments to test hypotheses. One of the most powerful types
by experiments or by new observations. Hypotheses are testable of experiment is called a controlled experiment. In a controlled
because, even as they suggest an explanation for observations experiment, the researcher sets up several groups to be tested,
made previously, they make predictions about observations yet to keeping the conditions and setup as similar as possible from one
be made. group to the next. Then, the researcher deliberately introduces
something different, known as a variable, into one group that
j Quick Check 1 Mice that live in sand dunes commonly have he or she hypothesizes might have some sort of an effect. This
light tan fur. Develop a hypothesis to explain this coloration. is called the test group. In another group, the researcher does
6 SECTION 1.1 THE SCIENTIFIC METHOD

not introduce this variable. This is a control group, and the

expectation is that no effect will occur in this group. FIG. 1.2 The scientific method.
Controlled experiments are extremely powerful. By changing
just one variable at a time, the researcher is able to determine Observations
if that variable is important. If many variables were changed at
once, it would be difficult, if not impossible, to draw conclusions
from the experiment because the researcher would not be able to
figure out which variable caused the outcome. The control group
plays a key role as well. Having a group in which no change is Hypothesis
expected ensures that the experiment works as it is supposed to
and provides a baseline against which to compare the results of the
test groups.
For example, we might test the hypothesis that hummingbirds
facilitate pollination by doing a controlled experiment. In this Predictions
case, we could set up groups of red flowers that are all similar to If results are
not consistent,
one another. For one group, we could surround the flowers with a reject or revise
fine mesh that allows small insects access to the plant but keeps the hypothesis.
hummingbirds away. For another group, we would not use a mesh.
Experiments
The variable, then, is the presence of a mesh; the test group is the or new
flowers with the mesh; and the control group is the flowers without observations
the mesh since the variable was not introduced in this group.
If results are consistent
Will the flowers be pollinated? If only the group without over many experiments
the mesh is pollinated, this result lends support to our initial based on many related
hypotheses, the hypotheses
hypothesis. In this case, the hypothesis becomes less tentative and
becomes a theory.
more certain. If both groups are pollinated, our hypothesis is not
supported, in which case we may discard it for another explanation
or change it to account for the new information. THEORY

j Quick Check 2 Design a controlled experiment that tests the

hypothesis that cigarette smoke causes lung cancer.

Using observations to generate a hypothesis and then Working in Italy, the American geologist Walter Alvarez
making predictions based on that hypothesis that can be tested collected samples from the precise point in the rock layers that
experimentally are the first two steps in the scientific method, corresponds to the time of the extinction. Careful chemical
outlined in Fig. 1.2. The scientific method is a deliberate and analysis showed that rocks at this level are unusually enriched in
careful way of asking questions about the unknown. We make the element iridium. Iridium is rare in most rocks on continents
observations, collect field or laboratory samples, and design and and the seafloor, but is relatively common in rocks that fall from
carry out experiments or analyses to make sense of things we space—that is, in meteorites. From these observations, Alvarez
initially do not understand. The scientific method has proved to and his colleagues developed a remarkable hypothesis: 66 million
be spectacularly successful in helping us to understand the years ago, a large (11-km diameter) meteor slammed into Earth,
world around us. We explore several aspects of the scientific and in the resulting environmental havoc, dinosaurs and many
method, including experimental design, data and data other species became extinct. This hypothesis makes specific
presentation, probability and statistics, and scale and predictions, described in Fig. 1.3, which turned out to be supported
approximation on . by further observations. Thus, observational tests support the
To emphasize the power of the scientific method, we turn hypothesis that nearly 150 million years of dinosaur evolution
to a famous riddle drawn from the fossil record (Fig. 1.3). Since were undone in a moment.
the nineteenth century, paleontologists have known that before
mammals expanded to their current ecological importance, other General explanations of natural phenomena
large animals dominated Earth. Dinosaurs evolved about supported by many experiments and observations
210 million years ago and disappeared abruptly 66 million years are called theories.
ago, along with many other species of plants, animals, and As already noted, a hypothesis may initially be tentative.
microscopic organisms. In many cases, the skeletons and shells of Commonly, in fact, it will provide only one of several possible
these creatures were buried in sediment and became fossilized. ways of explaining existing data. With repeated observation and
Layers of sedimentary rock therefore record the history of Earth. experimentation, however, a good hypothesis gathers strength,
FIG. 1.3
HOW DO WE KNOW?

What caused the extinction of the dinosaurs?

BACKGROUND Dinosaurs were diverse and ecologically important for nearly 150 million years but became extinct about
66 million years ago.

OBSERVATION
274

Paleogene
276

Iridium, common in 278

meteorites, was

Depth (m)
discovered in rock layers 280
corresponding to the
time of extinction.
282

Cretaceous
Photo Source: Kirk Johnson, Denver 284
Museum of Nature & Science.
286
0 0.5 1 1.5 2 2.5 3 3.5 4
Amount of Iridium (parts per billion)
HYPOTHESIS The impact of a large meteorite disrupted communities on land and in the sea, causing the extinction of the dinosaurs
and many other species.

PREDICTIONS Independent evidence of a meteor impact should be found in rock layers corresponding to the time of the extinction
and be rare or absent in older and younger beds.

FURTHER OBSERVATIONS

Quartz crystals that form

only at high temperature
and pressure—conditions
met by giant meteors as
they crash into Earth—
occur abundantly in rock
layers dated to the time of
the extinction.

U.S.A.

By 1990, geologists had

Mexico
located the “smoking
gun”— a crater of just the
right age and size in the
Yucatán Peninsula of
Mexico.
Pacific
Photo Sources: (top left) Dr. David Ocean
Kring/Science Source; (right)
Image courtesy of V. L. Sharpton/
Lunar and Planetary Institute.

CONCLUSION A giant meteor struck Earth 66 million years ago, causing the extinction of the dinosaurs and many other species.

FOLLOW-UP WORK Researchers have documented other mass extinctions, but the event that eliminated the dinosaurs appears to be the only
one associated with a meteorite impact.

SOURCE Alvarez, W. 1998. T. rex and the Crater of Doom. New York: Vintage Press.
7
8 SECTION 1.2 C H E M I C A L A N D P H YS I C A L P R I N C I P L E S

and we have more and more confidence in it. When a number

of related hypotheses survive repeated testing and come to be FIG. 1.4 A climber scaling a rock. Living organisms like this
accepted as good bases for explaining what we see in nature, climber contain chemicals that are found in rocks, but only
scientists articulate a broader explanation that accounts for all the living organisms reproduce in a manner that allows for
hypotheses and the results of their tests. We call this statement evolution over time. Source: Scott Hailstone/Getty Images.
a theory, a general explanation of the world supported by a large
body of experiments and observations (see Fig. 1.2).
Note that scientists use the word “theory” in a very particular
way. In general conversation, “theory” is often synonymous
with “hypothesis,” “idea,” or “hunch”—“I’ve got a theory about
that.” But in a scientific context, the word “theory” has a specific
meaning. Scientists speak in terms of theories only if hypotheses
have withstood testing to the point where they provide a
general explanation for many observations and experimental
results. Just as a good hypothesis makes testable predictions, a
good theory both generates good hypotheses and predicts their
outcomes. Thus, scientists talk about the theory of gravity—a
set of hypotheses you test every day by walking down the street
or dropping a fork. Similarly, the theory of evolution is not one
explanation among many for the unity and diversity of life. It is
a set of hypotheses that has been tested for more than a century
and shown to provide an extraordinarily powerful explanation of
biological observations that range from the amino acid sequences
of proteins to the diversity of ants in a rain forest. In fact, as
we discuss throughout this book, evolution is the single most
important theory in all of biology. It provides the most general and
powerful explanation of how life works.

1.2 CHEMICAL AND PHYSICAL

PRINCIPLES
We stated earlier that biology is the study of life. But what exactly
is life? As simple as this question seems, it is frustratingly difficult
to answer. We all recognize life when we see it, but coming up
with a definition is harder than it first appears.
Living organisms are clearly different from nonliving things.
But just how different is an organism from the rock shown in
Fig. 1.4? On one level, the comparison is easy: The rock is much
simpler than any living organism we can think of. It has far fewer
components, and it is largely static, with no apparent response to
environmental change on timescales that are readily tracked.
In contrast, even an organism as relatively simple as a spatial organization on several scales; (2) the ability to change in
bacterium contains many hundreds of different chemical response to the environment; (3) the ability to reproduce; and
compounds organized in a complex manner. The bacterium is also (4) the capacity to evolve. Nevertheless, the living and nonliving
dynamic in that it changes continuously, especially in response to worlds share an important attribute: Both are subject to the basic
the environment. Organisms reproduce, which minerals do not. laws of chemistry and physics.
And organisms do something else that rocks and minerals don’t:
They evolve. Indeed, the molecular biologist Gerald Joyce has The living and nonliving worlds follow the same
defined life as a chemical system capable of undergoing Darwinian chemical rules and obey the same physical laws.
evolution. The chemical elements found in rocks and other nonliving things
From these simple comparisons, we can highlight four key are no different from those found in living organisms. In other
characteristics of living organisms: (1) complexity, with precise words, all the elements that make up living things can be found
 CHAPTER 1 L I F E : C H E M I C A L , C E L LU L A R , A N D E VO LU T I O N A RY F O U N DAT I O N S 9

The first law of thermodynamics states that energy can

FIG. 1.5 Composition of (a) Earth’s crust and (b) cells in the neither be created nor destroyed; it can only be transformed from
human body. The Earth beneath our feet is made up of the one form into another. In other words, the total energy in the
same elements found in our feet, but in strikingly different universe is constant, but the form that energy takes can change.
proportions. Living organisms are energy transformers. They acquire energy
from the environment and transform it into a chemical form that
a. Earth’s crust
cells can use. All organisms obtain energy from the sun or from
Earth’s crust consists mainly chemical compounds. Some of this energy is used to do work—
of oxygen and silicon.
such as moving, reproducing, and building cellular components—
and the rest is dissipated as heat. The energy that is used to do
b. Human body work plus the heat that is generated is the total amount of energy,
Living organisms are made which is the same as the input energy (Fig. 1.6). In other words,
up mostly of oxygen, the total amount of energy remains constant before and after
carbon, and hydrogen.
energy transformation.
The second law of thermodynamics states that the degree
of disorder (or the number of possible positions and motions of
molecules) in the universe tends to increase. Think about a box
Oxygen full of marbles distributed more or less randomly; if you want
49.5% Silicon
25.7%
to line up all the red ones or blue ones in a row, you have to do
work—that is, you have to add energy. In this case, the addition
of energy increases the order of the system, or, put another way,
Carbon
Other Oxygen 18%
decreases its disorder. Physicists quantify the amount of disorder
9.1% 65% (or the number of possible positions and motions of molecules) in
a system as the entropy of the system.
Hydrogen Living organisms are highly organized. As with lining up
Aluminum 7.5% 10%
Iron 4.7% marbles in a row, energy is needed to maintain this organization.
Calcium 3.4%
Other Given the tendency toward greater disorder, the high level of
5.5%
Hydrogen 0.14% organization of even a single cell would appear to violate the
Carbon 0.03% Calcium 1.5% second law. But it does not. The key is that a cell is not an isolated
Iron <0.05% system and therefore cannot be considered on its own; it exists
Aluminum in an environment. So we need to take into account the whole
<0.001%
system, the cell plus the environment that surrounds it. As energy

in the nonliving environment—there is nothing special about

our chemical components when taken individually. That said, the
relative abundances of elements in organisms differ greatly from FIG. 1.6 Energy transformation and the first law of
those in the nonliving world. In the universe as a whole, hydrogen thermodynamics. The first law states that the total amount
and helium make up more than 99% of known matter, while of energy in any system remains the same. Organisms
Earth’s crust contains mostly oxygen and silicon, with significant transform energy from one form to another, but the total
amounts of aluminum, iron, and calcium (Fig. 1.5a). In organisms, energy in any system is constant.
by contrast, oxygen, carbon, and hydrogen are by far the most
abundant elements (Fig. 1.5b). As discussed more fully in Chapter
2, carbon provides the chemical backbone of life. The particular Heat
properties of carbon make possible a wide diversity of molecules Input Conversion
that, in turn, support a wide range of functions within cells. energy process in
organisms
All living organisms are subject to the physical laws of the Work
universe. Physics helps us to understand how animals move and
why trees don’t fall over; it explains how redwoods conduct water
upward through their trunks and how oxygen gets into the cells
Energy in the All organisms convert
that line your lungs. Indeed, two laws of thermodynamics, both form of sunlight input energy into heat
of which describe how energy is transformed in any system, or food is taken and work. No energy is
determine how living organisms are able to do work and maintain up by organisms. created or destroyed in
the process.
their spatial organization.
10 SECTION 1.2 C H E M I C A L A N D P H YS I C A L P R I N C I P L E S

HOW DO WE KNOW?
is harnessed by cells, only some is used to do work; the rest is
FIG. 1.8
dissipated as heat (Fig. 1.6). That is, conversion of energy from
one form to another is never 100% efficient. Heat is a form of
energy, so the total amount of energy is conserved, as dictated by Can living organisms arise
the first law. In addition, heat corresponds to the motion of small
molecules—the greater the heat, the greater the motion, and the from nonliving matter?
greater the motion, the greater the degree of disorder. Therefore,
the release of heat as organisms harness energy means that the
BACKGROUND Until the 1600s, many people believed that
total entropy for the combination of the cell and its surroundings
increases, in keeping with the second law (Fig. 1.7). rotting meat spontaneously generates maggots (fly larvae).

HYPOTHESIS Francesco Redi hypothesized that maggots

The scientific method shows that living organisms come only from flies and are not spontaneously generated.
come from other living organisms.
EXPERIMENT Redi used three jars containing meat. One jar
Life is made up of chemical components that also occur in the
nonliving environment and obey the same laws of chemistry was left open; one was covered with gauze; one was sealed
and physics. Can life spontaneously arise from these nonliving with a cap.
materials? We all know that living organisms come from other RESULTS
living organisms, but it is worth asking how we know this.
Direct observation can be misleading here. For example, raw
meat, if left out on a plate, will rot and become infested with
maggots (fly larvae). It might seem as though the maggots appear Flies
spontaneously. In fact, the question of where maggots come from
Maggots Meat
was a matter of vigorous debate for centuries, until application
Open jar Gauze-covered jar Sealed jar

Maggots No maggots No maggots

appeared in appeared in the appeared in
FIG. 1.7 Energy transformation and the second law of the open jar. gauze-covered jar. the sealed jar.
thermodynamics. The second law states that disorder in
any system tends to increase. Entropy can decrease locally
(inside a cell, for example) because the heat released CONCLUSION The presence of maggots in the open jar
increases disorder in the environment. and the absence of maggots in the gauze-covered and
sealed jars supported the hypothesis that maggots come
from flies and allowed Redi to reject the hypothesis that
maggots are spontaneously generated.

FOLLOW-UP WORK Redi’s experiment argued against

the idea of spontaneous generation for insects. However,
it was unclear whether his results could be extended
to microbes. Applying the scientific method, Louis
Pasteur used a similar approach about 200 years later to
investigate this question (see Fig. 1.9).

As simple
compounds are of the scientific method settled the issue. In the 1600s, the
combined into Italian physician and naturalist Francesco Redi hypothesized that
more complex Some of the energy maggots (and hence flies) in rotting meat come only from other
molecules, the is released as heat,
entropy inside increasing the flies that laid their eggs in the meat.
the cell decreases, entropy (motion of To test his hypothesis, Redi set up an experiment in which
requiring energy. molecules) in the he placed meat in three glass jars (Fig. 1.8). One jar was left open,
environment. The
total entropy of the a second was covered with gauze, and the third was sealed with
cell plus the a cap. The jars were left in a room with flies. Note that in this
environment
increases.
experiment, the three jars were subject to the same conditions—
the only difference was the opening of the jar. The open jar allowed
 HOW DO WE KNOW?
FIG. 1.9

Can microscopic life arise from nonliving matter?

BACKGROUND Educated people in Pasteur’s time knew that microbes grow well in nutrient-rich liquids like broth. It was also known
that boiling would sterilize the broth, killing the microbes.

HYPOTHESIS Pasteur hypothesized that if microbes were generated spontaneously from nonliving matter, they should reappear in
sterilized broth without the addition of microbes.

EXPERIMENT Pasteur used two flasks, one with a straight neck and one with a swan neck. The straight-neck flask allowed dust particles
with microbes to enter. The swan-neck flask did not.

RESULTS

Broth
Broth remains clear
and sterile—no
microbes appear.
Straight- Swan-
neck neck
flask flask Boiling kills all the Dust particles carrying microbes Broth becomes
microbes, thereby enter the straight-neck flask, cloudy because of
sterilizing the broth. but not the swan-neck flask. growth of microbes.

CONCLUSION The presence of microbes in the straight-neck flask and the absence of microbes in the swan-neck flask supported the
hypothesis that microbes come from other microbes and are not spontaneously generated.

DISCUSSION Redi’s and Pasteur’s research illustrate classic attributes of well-designed experiments. Multiple treatments are set up, and
nearly all conditions are the same in them all—they are constant, and therefore cannot be the cause of different outcomes of the experiment.
One key feature—the variable—is changed by the experimenter from one treatment to the next. This is a place to look for explanations of
different experimental outcomes.

for the passage of flies and air; the jar with the gauze allowed for flask. The straight-neck flask allowed airborne dust particles
the passage of air but not flies; and the sealed jar did not allow air carrying microbes to fall into the sterile broth, while the swan-
or flies to enter. Over time, Redi observed that maggots appeared neck flask prevented dust from getting inside. Over time, Pasteur
only on the meat in the open jar. No maggots appeared in the observed that microbes grew in the broth inside the straight-neck
other two jars, which did not allow access to the meat by flies. flask but not in the swan-neck flask. From these observations,
These observations supported Redi’s hypothesis that flies come Pasteur rejected the hypothesis that microbes arise spontaneously
from other flies, and did not provide support for the alternative from sterile broth. Instead, exposure to microbes carried on
hypothesis that maggots arise spontaneously from meat. airborne dust particles is necessary for microbial growth.
Redi demonstrated that living organisms come from other Redi’s and Pasteur’s experiments demonstrated that living
organisms, but some argued that his conclusion might apply only organisms come from other living organisms and are not generated
to larger organisms—microscopic life might be another matter spontaneously from chemical components. But this raises the
entirely. It was not until the nineteenth century that the French question of how life arose in the first place. If life comes from life,
chemist and biologist Louis Pasteur tested the hypothesis that where did the first living organisms come from? Although today
microorganisms can arise by spontaneous generation (Fig. 1.9). all organisms are produced by parental organisms, early in Earth’s
Pasteur filled two glass flasks with broth that had first been history this was not the case. Scientists hypothesize that life
sterilized over heat—one with a straight vertical neck and the initially emerged from chemical compounds about 4 billion years
other with a curved swan neck. As in Redi’s experiments, there ago. That is, chemical systems capable of evolution arose from
was only one variable, in this case the shape of the neck of the chemical reactions that took place on the early Earth. We’ll return

11
12 SECTION 1.3 THE CELL

to the great question of life’s origin in Case 1: The First Cell and in pictured in Fig. 1.11b, extend slender projections known as axons
Chapters 2 through 8. for distances as great as a meter, and the cannonball-size egg of an
ostrich in Fig. 1.11c is a single giant cell.
The types of cell just mentioned—bacteria, yeasts, skin cells,
1.3 THE CELL nerve cells, and an egg—seem very different, but all are organized
along broadly similar lines. In general, all cells contain a stable
The cell is the simplest entity that can exist as an independent blueprint of information in molecular form; they have a discrete
unit of life. Every known living organism is either a single cell boundary that separates the interior of the cell from its external
or an ensemble of a few to many cells (Fig. 1.10). Most bacteria environment; and they have the ability to harness materials and
(like those in Pasteur’s experiment), yeasts, and the tiny algae energy from the environment.
that float in oceans and ponds spend their lives as single cells. In
contrast, plants and animals contain billions to trillions of cells Nucleic acids store and transmit information needed
that function in a coordinated fashion. for growth, function, and reproduction.
Most cells are tiny, their dimensions well below the threshold The first essential feature of a cell is its ability to store and
of detection by the naked eye (Fig. 1.11). The cells that make up transmit information. To accomplish this, cells require a stable
the layers of your skin (Fig. 1.11a) average about 100 microns (μm) archive of information that encodes and helps determine their
or 0.1 mm in diameter, which means that about 10 would fit in a physical attributes. Just as the construction and maintenance of
row across the period at the end of this sentence. Many bacteria a house requires a blueprint that defines the walls, plumbing,
are less than a micron long. Certain specialized cells, however, and electrical wiring, organisms require an accessible and reliable
can be quite large. Some nerve cells in humans, like the ones archive of information that helps determine their structure

FIG. 1.10 Unicellular and multicellular organisms. All living organisms are made up of cells: (a) bacteria; (b) brewer’s yeast; (c) algae;
(d) cheetahs; (e) humans. Sources: a. Steve Gschmeissner/Science Source; b. Steve Gschmeissner/Science Source; c. Michael Abbey/Getty Images;
d. Sven-Olof Lindblad/Science Source; e. Megapress/Alamy.

a b c

d e
 CHAPTER 1 L I F E : C H E M I C A L , C E L LU L A R , A N D E VO LU T I O N A RY F O U N DAT I O N S 13

DNA is a double-stranded helix, with

FIG. 1.11 Cell diversity. Cells vary greatly in size and shape: (a) skin cells; (b) nerve each strand made up of varying sequences
cells; (c) ostrich egg. Sources: a. Biophoto Associates/Science Source; b. Dr. Jonathan Clarke. of four different kinds of molecules
Wellcome Images; c. Hemis/Alamy. connected end to end. It is the arrangement
of these molecular subunits that makes
a b
DNA special; in essence, they provide a
0.1 mm four-letter alphabet that encodes cellular
information. Notably, the information
encoded in DNA directs the formation
of proteins, the key structural and
functional molecules that do the work of
the cell. Virtually every aspect of the cell’s
c existence—its internal architecture, its
shape, its ability to move, and its various
1 mm
chemical reactions—depends on proteins.
How does the information stored in
DNA direct the synthesis of proteins?
First, existing proteins create a copy of the
DNA’s information in the form of a closely
related molecule called ribonucleic acid,
or RNA. The synthesis of RNA from a DNA
template is called transcription, a term that
describes the copying of information from
one form into another. Specialized molecular
structures within the cell then “read” the
RNA molecule to determine what building
blocks to use to create a protein. This process,
and metabolic activities. Another hallmark of life is the ability called translation, converts information stored in the language of
to reproduce. To reproduce, cells must be able to copy their nucleic acids to information in the language of proteins.
archive of information rapidly and accurately. In all organisms, The pathway from DNA to RNA (specifically to a form of
the information archive is a remarkable molecule known as RNA called messenger RNA, or mRNA) to protein is known as
deoxyribonucleic acid, or DNA (Fig. 1.12). the central dogma of molecular biology (Fig. 1.13). The central
dogma describes the basic flow of information in a cell and, while
there are exceptions, it constitutes a fundamental principle in
FIG. 1.12 A molecule of DNA. DNA is a double helix made up of biology. As proteins are ultimately encoded by DNA, we can define
varying sequences of four different subunits. specific stretches or segments of DNA according to the proteins
that they encode. This is the simplest definition of a gene: the
DNA sequence that corresponds to a specific protein product.
DNA has another remarkable feature. In addition to storing
information, it is easily copied, or replicated, allowing genetic

FIG. 1.13 The central dogma

DNA
of molecular biology,
defining the flow of Transcription
information in all living
organisms from DNA to
RNA to protein. RNA

Translation

Protein
14 SECTION 1.3 THE CELL

information to be passed from cell to cell or from an organism contributions from their surroundings, both simple ions and the
to its progeny. Each organism’s DNA archive can be stably and building blocks required to manufacture macromolecules. They also
reliably passed from generation to generation in large part because release waste products into the environment. As discussed more
of its double-stranded helical structure. During replication, each fully in Chapter 5, the plasma membrane controls the movement of
strand of the double helix serves as a template for a new strand. materials into and out of the cell.
Replication is necessarily precise and accurate because mistakes In addition to the plasma membrane, many cells have internal
introduced into the cell’s information archive may be lethal to the membranes that divide the cell into discrete compartments,
cell. That said, errors in DNA can and do occur during the process each specialized for a particular function. A notable example
of replication, and environmental insults can damage DNA as is the nucleus, which houses the cell’s DNA. Like the plasma
well. Such changes are known as mutations; they can spell death membrane, the nuclear membrane selectively controls movement
for the cell, or they can lead to the variations that underlie the of molecules into and out of it. As a result, the nucleus occupies a
diversity of life and the process of evolution. discrete space within the cell, separate from the space outside the
nucleus, called the cytoplasm.
j Quick Check 3 How does the central dogma help us to
Not all cells have a nucleus. In fact, cells can be grouped
understand how mutations in DNA can result in disease?
into two broad classes depending on whether or not they have a
nucleus. Cells without a nucleus are called prokaryotes, and cells
Membranes define cells and spaces within cells. with a nucleus are eukaryotes.
The second essential feature of all cells is a plasma membrane that The first cells that emerged about 4 billion years ago were
separates the living material within the cell from the nonliving prokaryotic. Their descendants include the familiar bacteria, found
environment around it (Fig. 1.14). This boundary between today nearly everywhere that life can persist. Some prokaryotes
inside and outside does not mean that cells are closed systems live in peaceful coexistence with humans, inhabiting our gut and
independent of the environment. On the contrary, there is an active aiding digestion. Others cause disease—salmonella, tuberculosis,
and dynamic interplay between cells and their surroundings that and cholera are familiar examples. The success of these cells
is mediated by the plasma membrane. All cells require sustained depends in part on their small size, their ability to reproduce
rapidly, and their ability to obtain energy and nutrients from
diverse sources. Most prokaryotes live as single-celled organisms,
but some have simple multicellular forms.
FIG. 1.14 The plasma membrane. The plasma membrane
Eukaryotes evolved much later, roughly 2 billion years ago,
surrounds every cell and controls the exchange of
from prokaryotic ancestors. They include familiar groups such as
material with the environment. Sources: (top) Dr. Gopal Murti/
animals, plants, and fungi, along with a wide diversity of single-
SPL/Science Source; (bottom) Don W. Fawcett/Science Source.
celled microorganisms called protists. Eukaryotic organisms
exist as single cells like yeasts or as multicellular organisms
Plasma
membrane Outside of cell like humans. In multicellular organisms, cells may specialize to
perform different functions. For example, in humans, muscle
cells contract; red blood cells carry oxygen to tissues; and skin cells
Cytoplasm
provide an external barrier.
The terms “prokaryotes” and “eukaryotes” are useful in drawing
attention to a fundamental distinction between these two groups
of cells. However, today, biologists recognize three domains of life—
Bacteria, Archaea, and Eukarya (Chapters 26 and 27). Bacteria and
Archaea both lack a nucleus and are therefore prokaryotes, whereas
Eukarya are eukaryotic. Archaea are single-celled microorganisms,
many of which flourish under seemingly hostile conditions, such as
the hot springs of Yellowstone National Park.

Metabolism converts energy from the environment

into a form that can be used by cells.
A third key feature of cells is the ability to harness energy from the
environment. Let’s go back to our introductory example of eating
an apple. The apple contains sugars, which store energy in their
chemical bonds. By breaking down sugar, our cells harness this
energy and convert it into a form that can be used to do the work
of the cell. Energy from the food we eat allows us to grow, move,
Transmission electron micrograph of a cell communicate, and do all the other things that we do.
 CHAPTER 1 L I F E : C H E M I C A L , C E L LU L A R , A N D E VO L U T I O N A RY F O U N DAT I O N S 15

Organisms acquire energy from just two sources—the sun and displays a remarkable degree of diversity. We don’t really know
chemical compounds. The term metabolism describes chemical how many species share our planet, but reasonable estimates run
reactions by which cells convert energy from one form to another to 10 million or more. Both the unity and the diversity of life are
and build and break down molecules. These reactions are required explained by the process of evolution, or change over time.
to sustain life. Regardless of their source of energy, all organisms
use chemical reactions to break down molecules, releasing energy Variation in populations provides the raw material
in the process that is stored in a chemical form called adenosine for evolution.
triphosphate, or ATP. This molecule enables cells to carry out all Described in detail, evolution by natural selection calls on
sorts of work, including growth, division, and moving substances complex mathematical formulations, but at heart its main
into and out of the cell. principles are simple, indeed unavoidable. When there is variation
Many metabolic reactions are highly conserved between within a population of organisms, and when that variation can be
organisms, meaning the same reactions are found in many inherited (that is, when it can be passed from one generation to
different organisms. This observation suggests that the reactions the next), the variants best suited for growth and reproduction
evolved early in the history of life and have been maintained in a given environment will contribute disproportionately to the
for billions of years because of their fundamental importance to next generation. As Darwin recognized, farmers have used this
cellular biochemistry. principle for thousands of years to select for crops with high yield
or improved resistance to drought and disease. It is how people
A virus is genetic material in need of a cell. around the world have developed breeds of dog ranging from
It’s worth taking a moment to consider viruses. A virus is an agent terriers to huskies (Fig. 1.15). And it is why antibiotic resistance
that infects cells. It is smaller and simpler than cells. Why, then, is on the rise in many disease-causing microorganisms. Life has
aren’t viruses the smallest unit of life? We just considered three been shaped by evolution since its origin, and the capacity for
essential features of cells—the capacity to store and transmit Darwinian evolution may be life’s most fundamental property.
information, a membrane that selectively controls movement in
and out, and the ability to harness energy from the environment.
Viruses have a stable archive of genetic information, which can FIG. 1.15 Artificial selection. Selection over many centuries has
be RNA or DNA, surrounded by a protein coat and sometimes resulted in remarkable variations among dogs. Charles
a lipid envelope. But viruses cannot harness energy from the Darwin called this “selection under domestication” and
environment. Therefore, on their own viruses cannot read and use noted that it resembles selection that occurs in nature.
the information contained in their genetic material, nor can they Source: © Rob Brodman 2011.
regulate the passage of substances across their protein coats or lipid
envelopes the way that cells do. To replicate, they require a cell.
A virus infects a cell by binding to the cell’s surface, inserting
its genetic material into the cell, and, in most cases, using the
cellular machinery to produce more viruses. In this way, it is often
said that a virus “hijacks” a cell. The infected cell may produce
more viruses, sometimes by lysis, or breakage, of the cell, and the
new viruses can then infect more cells. In some cases, the genetic
material of the virus integrates into the DNA of the host cell.
We discuss viruses many times throughout the book. Each
species of Bacteria, Archaea, and Eukarya is susceptible to many
types of virus that are specialized to infect its cells. Several hundred
types of virus are known to infect humans, and the catalog is
still incomplete. Useful tools in biological research, viruses have
provided a model system for many problems in biology, including
how genes are turned on and off and how cancer develops.

1.4 EVOLUTION
The themes introduced in the last two sections stress life’s unity:
Cells form the basic unit of all life; DNA, RNA, and proteins
carry out the molecular functions of all cells; and metabolic
reactions build and break down macromolecules. We need only
look around us, however, to recognize that for all its unity, life
16 SECTION 1.4 E VO L U T I O N

The apples in the bin from which you made your choice didn’t some of the flowers of another, enabling the sperm inside pollen
all look alike. Had you picked your apple in an orchard, you would grains to fertilize egg cells within that single flower. All the seeds
have seen that different apples on the same tree looked different— on an apple tree contain shared genes from one parent, the tree
some smaller, some greener, some misshapen, a few damaged by on which they developed. But they contain distinct sets of genes
worms. Such variation is so commonplace that we scarcely pay contributed by sperm transported in pollen from other trees. In
attention to it. Variation is observed among individuals in virtually all sexual organisms, fertilization produces unique combinations
every species of organism. Variation that can be inherited provides of genes, which explains in part why sisters and brothers with the
the raw material on which evolution acts. same parents can be so different from one another.
The causes of variation among individuals within a species Genetic variation arises ultimately from mutations. Mutations
are usually grouped into two broad categories. Variation among arise either from random errors during DNA replication or from
individuals is sometimes due to differences in the environment; this environmental factors such as ultraviolet (UV) radiation, which
is called environmental variation. Among apples on the same tree can damage DNA. If these mutations are not corrected, they are
some may have good exposure to sunlight; some may be hidden in passed on to the next generation. To put a human face on this,
the shade; some were lucky enough to escape the female codling consider that lung cancer can result from an environmental
moth, whose egg develops into a caterpillar that eats its way into the insult, such as cigarette smoking, or from a genetic susceptibility
fruit. These are all examples of environmental variation. inherited from the parents.
The other main cause of variation among individuals is In nature, most mutations that harm growth and reproduction
differences in the genetic material that is transmitted from parents die out after a handful of generations. Those that are neither
to offspring; this is known as genetic variation. Differences among harmful nor beneficial can persist for hundreds or thousands
individuals’ DNA can lead to differences among the individuals’ of generations. And those that are beneficial to growth and
RNA and proteins, which affect the molecular functions of the cell reproduction can gradually become incorporated into the genetic
and ultimately can lead to physical differences that we can observe. makeup of every individual in the species. That is how evolution
Genetic differences among apples produce varieties whose mature works: The genetic makeup of a population changes over time.
fruits differ in taste and color, such as the green Granny Smith, the
yellow Golden Delicious, and the scarlet Red Delicious. Evolution predicts a nested pattern of relatedness
But even on a single tree, each apple contains seeds that are among species, depicted as a tree.
genetically distinct because the apple tree is a sexual organism. Evolutionary theory predicts that new species arise by the
Bees carry pollen from the flowers of one tree and deposit it in divergence of populations through time from a common ancestor.
As a result, closely related species are
likely to resemble each other more
closely than they do more distantly
FIG. 1.16 Phylogenetic relationships among primates. (a) Humans share many features with related species. You know this to be
chimpanzees. (b) Humans and chimpanzees, in turn, share more features with gorillas true from common experience. All of
than they do with other species, and so on down through the evolutionary tree of us recognize the similarity between
primates. Treelike patterns of nested similarities are the predicted result of evolution. a chimpanzee’s face and body and
Source: AP Photo/Bela Szandelszky. our own (Fig. 1.16a), and biologists
have long known that we share more
a. b. features with chimpanzees than we
Lemurs do with any other species.
Tarsiers Humans and chimpanzees,
in turn, share more features with
New World monkeys
gorillas than they do with any other
Old World monkeys species. And humans, chimpanzees,
and gorillas share more features
Gibbons
with orangutans than they do with
Orangutans any other species. And so on. We
can continue to include a widening
Nodes Gorillas
indicate a diversity of species, successively
common Chimpanzees adding monkeys, lemurs, and other
ancestor.
primates, to construct a set of
Humans
evolutionary relationships that can
Time be depicted as a tree (Fig. 1.16b). In
this tree, the tips or branches on the
 CHAPTER 1 L I F E : C H E M I C A L , C E L LU L A R , A N D E VO L U T I O N A RY F O U N DAT I O N S 17

FIG. 1.17 The tree of life.

Most of life’s diversity, and so

most of its deep evolutionary
ARCHAEA
history, is microbial.

BACTERIA
EUKARYOTES
Plants All land plants lie
on this branch.

Animals
Animal diversity
Proposed position of root lies on this branch
Time runs from the of the tree.
root to the branches.

right represent different groups of organisms, nodes (where lines comparisons of DNA sequences and fossils show that the close
split) represent the most recent common ancestor, and time runs similarity between humans and chimpanzees reflects descent
from left to right. from a common ancestor that lived about 6 million years ago.
Evolutionary theory predicts that primates should show a Their differences reflect what Darwin called “descent with
nested pattern of similarity, and this is what morphological and modification”—evolutionary changes that have accumulated
molecular observations reveal. We can continue to add other over time since the two lineages split. For example, as discussed
mammals and then other vertebrate animals to our comparison, in Chapter 24, the flat face of humans, our small teeth, and
in the process generating a pattern of evolutionary relationships our upright posture all evolved within our ancestors after they
that forms a larger tree, with the primates confined to one limb. diverged from the ancestors of chimpanzees. At a broader
And using comparisons of DNA among species, we can generate scale, the fundamental features shared by all organisms reflect
still larger trees, ones that include plants as well as animals and the inheritance from a common ancestor that lived billions of years
full diversity of microscopic organisms. Biologists call the full set of ago. And the differences that characterize the many branches
evolutionary relationships among all organisms the tree of life. on the tree of life have formed through the continuing action of
This tree, illustrated in Fig. 1.17, has three major branches evolution since the time of our earliest ancestors.
representing the three domains mentioned earlier and is made up Four decades ago, the geneticist Theodosius Dobzhansky
mostly of microorganisms. The last common ancestor of all living wrote, “Nothing in biology makes sense except in the light of
organisms, which form a root to the tree, is thought to lie between evolution.” For this reason, evolution permeates discussions
the branch leading to Bacteria and the branch leading to Archaea throughout this book, whether we are explaining the molecular
and Eukarya. The plants and animals so conspicuous in our daily biology of cells, how organisms function and reproduce, how
existence make up only two branches on the eukaryotic limb of species interact in nature, or the remarkable biological diversity of
the tree. our planet.
The tree of life makes predictions for the order of appearance
of different life-forms in the fossil record. For example, Fig. Evolution can be studied by means of experiments.
1.16—one small branch on the greater tree—predicts that humans Both the nested patterns of similarity among living organisms
should appear later than monkeys, and that primates more akin and the succession of fossils in the geologic record fit the
to lemurs and tarsiers should appear even earlier. The greater tree predictions of evolutionary theory. Can we actually capture
also predicts that all records of animal life should be preceded by a evolutionary processes in action? One way to accomplish this is
long interval of microbial evolution. As we will see in subsequent in the laboratory. Bacteria are ideal for these experiments because
chapters, these predictions are confirmed by the geologic record. they reproduce rapidly and can form populations with millions of
Shared features, then, sometimes imply inheritance from individuals. Large population size means that mutations are likely
a common ancestor. In combination, molecular studies such as to form in nearly every generation, even though the probability
18 SECTION 1.4 E VO L U T I O N

that any individual cell will acquire a mutation is small. (In from those of earlier generations—that is, did evolution occur?—
contrast to bacteria, think about trying evolutionary experiments and did the bacteria evolve an improved ability to use succinate?
on elephants!) In fact, the bacteria did evolve an improved ability to use
One such experiment is illustrated in Fig. 1.18. The succinate, demonstrated by the results shown in Fig. 1.18. This
microbiologists Santiago Elena and Richard Lenski grew experiment illustrates how experiments can be used to test
populations of the common intestinal bacterium Escherichia coli hypotheses and it shows evolution in action. Furthermore,
in liquid medium, with the organic acid succinate as the only follow-up studies of the bacterial DNA identified differences in
source of food. In general, E. coli cells feed on succinate poorly if genetic makeup that resulted in the improved ability of E. coli
at all, leading the researchers to hypothesize that any bacterium to use succinate. Many experiments of this general type have
with a mutation that increased its ability to use succinate would been carried out, applying the scientific method to demonstrate
reproduce at a faster rate than other bacteria in the population. how bacteria adapt through mutation and natural selection to any
The key questions were: Did bacteria from later generations differ number of environments.

HOW DO WE KNOW?

FIG. 1.18 t=0 t=1

Can evolution be Ancestral

1:1

demonstrated in the
laboratory? Later

Ancestral
BACKGROUND Escherichia coli is an intestinal bacterium
Later
commonly used in the laboratory. It grows poorly in liquid media
where the only source of food is succinate. Santiago Elena and
Richard Lenski wondered whether E. coli grown for 20,000 Source: Neerja Hajela, Michigan State University.

generations in succinate would evolve in ways that improved their

Over time, the bacteria grown in succinate showed more and more
ability to metabolize this compound.
improvement in growth compared to the starting bacteria.
HYPOTHESIS Any bacterium with a random mutation that
Growth of later strains relative

increases its ability to utilize succinate will reproduce at a faster rate

than other bacteria in the population. Over time, such a mutant will
to ancestral strains

increase in frequency relative to other types of bacteria, thereby Later strains grew more rapidly
demonstrating evolution in a bacterial population. than ancestral strains at each
generation.
EXPERIMENT Cells of E. coli can be frozen in liquid nitrogen,
which keeps them in a sort of suspended animation in which
no biological processes take place, but the cells survive. At the
beginning of the experiment, the researchers froze a large number
of samples of the starting bacteria (“Ancestral”). As the experiment Generation
progressed, they took samples of the bacterial populations at
CONCLUSION Evolution occurred in the population: The bacteria
intervals (“Later”) and grew them together with a thawed sample
evolved an improved ability to metabolize succinate.
of the starting bacteria in succinate. They then compared the rate
of growth of the ancestral bacteria with that of the bacteria taken at FOLLOW-UP WORK Other experimental studies have shown
later time points. that bacteria are able to evolve adaptations to a wide variety of
environmental conditions.
RESULTS At each time interval, the cells from later time
points grew more rapidly than the ancestral cells when the two SOURCE (top) Elena, S. F., and R. E. Lenski. 2003. “Evolution Experiments with
populations were grown together in succinate. Microorganisms.” Nature Review Genetics 4:457– 469.
 CHAPTER 1 L I F E : C H E M I C A L , C E L LU L A R , A N D E VO L U T I O N A RY F O U N DAT I O N S 19

Experiments in laboratory evolution help us to understand tolerances. They are also strongly influenced by interactions with
how life works, and they have an immensely important practical other species, including other plants that compete for the limited
side as well. They allow biologists to develop new and beneficial resources available for growth, animals that feed on them or
strains of microorganisms that, for example, remove toxins from spread their pollen, and microorganisms that infest their tissues.
lakes and rivers. And they show how some of our worst pathogens Ecology is the study of how organisms interact with one another
develop resistance to drugs designed to eliminate them. and with their physical environment in nature.

j Quick Check 4 How might the heavy-handed use of antibiotics

Basic features of anatomy, physiology, and behavior
result in the increase of antibiotic-resistant cells in bacterial
shape ecological systems.
populations?
Apple trees, we noted, reproduce sexually. How is this
accomplished given that the trees have no moving parts? The
answer is that bees carry pollen from one flower to the next,
1.5 ECOLOGICAL SYSTEMS enabling sperm carried within the pollen grain to fuse with an
egg cell protected within the flower (Fig. 1.19). The process is
Watching a movie, we don’t need a narrator to tell us whether much like the one discussed at the beginning of the chapter in
the action is set in a tropical rain forest, the African savanna, or which hummingbirds carry pollen from flower to flower. In both
Arctic tundra. The plants in the scene make it obvious. Plants can examples, the plants complete their life cycles by exploiting close
be linked closely with environment—palm trees with the tropics, interactions with animal species. Birds and bees visit flowers,
for example, or cacti with the desert—because the environmental attracted, as we are, by their color and odor. Neither visits flowers
distributions of different species reflect the sum of their biological with the intent to pollinate; they come in expectation of a meal.
features. Palms cannot tolerate freezing and so are confined to As the birds and bees nestle into flowers to collect nutritious
warmer environments; the cactus can store water in its tissues, nectar, pollen rubs off on their bodies.
enabling it to withstand prolonged drought. But the geographic But if animals help apples to fertilize their eggs, how do
ranges of palms and cacti reflect more than just their physical apple trees disperse from one site to another? Again, the plants
rely on animals—the apple’s flesh attracts
mammals that eat the fruit, seeds and all, and
spread the seeds through defecation. Humans
FIG. 1.19 Bee pollination. Many plants reproduce sexually by exploiting the behavior tend to be finicky—we carefully eat only the
of animals. Here a honeybee pollinates an apple flower. Source: Donald Specker/ sweet outer flesh, discarding the seedy core.
Animals Animals – Earth Scenes. Nonetheless, even as humans have selected
apples for their quality of fruit, we have
dispersed apple trees far beyond their natural
range.
The many ways that organisms interact
in nature reflect their basic functional
requirements. If a plant is to grow and
reproduce, for example, it must have access
to light, carbon dioxide, water, and basic
nutrients. All plants have these requirements,
so one plant’s success in gathering nutrients
may mean that fewer nutrients are available
for its neighbor—the plants may compete for
limited resources needed for growth
(Fig. 1.20a). Animals in the same area require
organic molecules for nutrition, and some of
them may eat the plant’s leaves or bark
(Fig. 1.20b). Predation also benefits some
organisms at the expense of others. In
combination, the physical tolerances of
organisms and the ways that organisms
interact with one another determine the
structure and diversity of communities.
20 SECTION 1.6 T H E H U M A N F O OT P R I N T

FIG. 1.20 Ecological relationships. (a) The trees in a rain forest all require water and nutrients. Neighboring plants compete for the limited
supplies of these materials. (b) Plants provide food for many animals. Leaf-cutter ants cut slices of leaves from tropical plants and
transport them to their nests, where the leaves grow fungi that the ants eat. Sources: a. Louise Murray/Getty Images; b. Gail Shumway/Getty Images.

a b

In short, ecological relationships reflect the biomechanical, avoid predation by synthesizing toxic compounds in their leaves
physiological, and behavioral traits of organisms in nature. Form may gain the upper hand. In each case, interactions between
and function, in turn, arise from molecular processes within cells, organisms lead to the evolution of particular traits.
governed by the expression of genes. To take another example from mammal–plant interactions:
Selection for fleshy fruits improved the dispersal of apples
Ecological interactions play an important role because it increased the attractiveness of these seed-bearing
in evolution. structures to hungry mammals. Tiny yeasts make a meal on
G. Evelyn Hutchinson, one of the founders of modern ecology, sugary fruits as well, in the process producing alcohol that deters
wrote a book called The Ecological Theater and the Evolutionary potential competitors for the food. Already in prehistoric times,
Play (1965). This wonderful title succinctly captures a key humans had learned to harness this physiological capability of
feature of biological relationships. It suggests that ecological yeasts, and for this reason the total abundance and distribution of
communities provide the stage on which the play of evolution Vitis vinifera, the wine grape, has increased dramatically through
takes place. time.
As an example, let’s look again at plants competing for
resources. As a result of this competition, natural selection may
favor plants that have more efficient uptake of nutrients or 1.6 THE HUMAN FOOTPRINT
water. In the example of animals eating plants, natural selection
may favor animals with greater jaw strength or more efficient The story of life has a cast of millions, with humans playing only
extraction of nutrients in the digestive system. In turn, plants that one of many roles in an epic 4 billion years in the making. Our
 CHAPTER 1 L I F E : C H E M I C A L , C E L LU L A R , A N D E VO L U T I O N A RY F O U N DAT I O N S 21

own species, Homo sapiens, has existed for only the most recent in the 21st century human activities have taken on special
1/200 of 1% of life’s history, yet there are compelling reasons to importance because our numbers and technological abilities make
pay special attention to ourselves. We want to understand how our footprint on Earth’s ecology so large. Human activities now
our own bodies work and how humans came to be: Curiosity about emit more carbon dioxide than do volcanoes, through industrial
ourselves is after all a deeply human trait. processes we convert more atmospheric nitrogen to ammonia than
We also want to understand how biology can help us conquer nature does, and we commandeer, either directly or indirectly, as
disease and improve human welfare. Epidemics have decimated much as 25% of all photosynthetic production on land. To chart
human populations throughout history. The Black Death— our environmental future, we need to understand our role in the
bubonic plague caused by the bacterium Yersinia pestis—is Earth system as a whole.
estimated to have killed half the population of Europe in the And, as we have become major players in ecology, humans
fourteenth century. Casualties from the flu pandemic of 1918 have become important agents of evolution. As our population
exceeded those of World War I. Even King Tut, we now know, has expanded, some species have expanded along with us. We’ve
suffered from malaria in his Egyptian palace approximately seen how agriculture has sharply increased the abundance and
3300 years ago. Throughout this book, we discuss how basic distribution of grapes, and the same is true for corn, cows, and
biological principles are helping scientists to prevent and cure apple trees. At the same time, we have inadvertently helped
the great diseases that have persisted since antiquity, as well as other species to expand—the crowded and not always clean
modern ones such as AIDS and Ebola. environments of cities provide excellent habitats for cockroaches
Furthermore, we need to understand the evolutionary and and rats (Fig. 1.21).
ecological consequences of a human population that now exceeds Other species, however, are in decline, their populations
7 billion. All species affect the world around them. However, reduced by hunting and fishing, changes in land use, and other

FIG. 1.21 Species that have benefited from human activity, including (a) corn, (b) rats, and (c) cockroaches.
Sources: a. Fred Dimmick/iStockphoto; b. Arndt Sven-Erik/age footstock; c. Nigel Cattlin/Alamy.

a b

c
22 SECTION 1.6 T H E H U M A N F O OT P R I N T

FIG. 1.22 Extinct and endangered species. Humans have caused many organisms to become extinct, such as (a) the dodo, (b) passenger pigeon,
(c) Bali tiger, and (d) dusky seaside sparrow. Burgeoning human populations have diminished the ranges of many others, including
(e) the white rhinoceros. Sources: a. Science Source; b. G. I. Bernard/Science Source; c. Look and Learn/Bridgeman Images; d. P.W. Sykes/U.S. Fish and Wildlife
Service; e. James Warwick/Science Source.

a b c

d e

human activities. When Europeans first arrived on the Indian Throughout this book, we return to the practical issues of
Ocean island of Mauritius, large flightless birds called dodos were life science. How can we use the principles of biology to improve
plentiful (Fig. 1.22). Within a century, the dodo was extinct. Early human welfare, and how can we live our lives in ways that control
Europeans in North America were greeted by vast populations our impact on the world around us? The answers to the questions
of passenger pigeons, more than a million in a single flock. By critically depend on understanding biology in an integrated fashion.
the early twentieth century, the species was extinct, a victim of While it is tempting to consider molecules, cells, organisms, and
hunting and habitat change through expanding agriculture. Other ecosystems as separate entities, they are inseparable in nature.
organisms both great (the Bali tiger) and small (the dusky seaside To tackle biological problems, whether building an artificial cell,
sparrow) have become extinct in recent decades. Still others are stopping the spread of infectious diseases such as HIV or malaria,
imperiled by human activities; the magnificent white rhinoceros feeding a growing population, or preserving endangered habitats
of Africa is threatened by both habitat destruction and poaching. and species, we need an integrated perspective. In decisively
Whether any rhinos will exist at the end of this century will important ways, our future welfare depends on improving our
depend almost entirely on decisions we make today. knowledge of how life works. •
 CHAPTER 1 L I F E : C H E M I C A L , C E L LU L A R , A N D E VO L U T I O N A RY F O U N DAT I O N S 23

Core Concepts Summary A virus is an infectious agent composed of a genome and

protein coat that uses a host cell to replicate. page 15
1.1 The scientific method is a deliberate way of
asking and answering questions about the natural 1.4 Evolution explains the features that organisms share
world. and those that set them apart.

Observations are used to generate a hypothesis, a tentative When there is variation within a population of organisms, and
explanation that makes predictions that can be tested. when that variation can be inherited, the variants best able to
page 4 grow and reproduce in a particular environment will contribute
disproportionately to the next generation, leading to a change
On the basis of a hypothesis, scientists design experiments
in the population over time, or evolution. page 15
and make additional observations that test the
hypothesis. page 5 Variation can be genetic or environmental. The ultimate source
of genetic variation is mutation. page 16
A controlled experiment typically involves several groups
in which all the conditions are the same and one group Organisms show a nested pattern of similarity, with humans
where a variable is deliberately introduced in order to more similar to primates than other organisms, primates more
determine if that variable has an effect. page 5 similar to mammals, mammals more similar to vertebrates, and
so on. page 16
If a hypothesis is supported through continued
Evolution can be demonstrated by laboratory experiments.
observation and experiments over long periods of time, it
page 18
is elevated to a theory, a sound and broad explanation of
some aspect of the world. page 6
1.5 Organisms interact with one another and with their
physical environment, shaping ecological systems that
1.2 Life works according to fundamental principles
sustain life.
of chemistry and physics.
Ecology is the study of how organisms interact with one another
The living and nonliving worlds follow the same chemical
and with their physical environment in nature. page 19
rules and obey the same physical laws. page 8
These interactions are driven in part by the anatomy,
Experiments by Redi in the 1600s and Pasteur in the 1800s
physiology, and behavior of organisms, that is, the basic
demonstrated that organisms come from other organisms
features of organisms shaped by evolution. page 19
and are not spontaneously generated. page 10

Life originated on Earth about 4 billion years ago, arising 1.6 In the 21st century, humans have become major
from nonliving matter. page 11 agents in ecology and evolution.
Humans have existed for only the most recent 1/200 of 1% of
1.3 The fundamental unit of life is the cell.
life’s 4-billion-year history. page 20
The cell is the simplest biological entity that can exist
In spite of our recent arrival, our growing numbers are leaving
independently. page 12
a large ecological and evolutionary footprint. page 21
Information in a cell is stored in the form of the nucleic Solving biological problems requires an integrated
acid DNA. page 12 understanding of life, with contributions from all the fields
The central dogma describes the usual flow of information of biology, including molecular biology, cell biology, genetics,
in a cell, from DNA to RNA to protein. page 13 organismal biology, and ecology, as well as from chemistry,
physics, and engineering. page 22
The plasma membrane is the boundary that separates the
cell from its environment. page 14

Cells with a nucleus are eukaryotes; cells without a Self-Assessment

nucleus are prokaryotes. page 14 1. Name and summarize the steps in the scientific method.
Metabolism is the set of chemical reactions in cells that 2. Differentiate among a guess, a hypothesis, and a theory.
build and break down macromolecules and harness energy.
page 14 3. Describe the difference between a test group and a control
group, and explain why they are important.
2244 SSEECLTFI-O
ANS S1E.S1S MTEHNET S C I E N T I F I C M E T H O D

4. State the first and second laws of thermodynamics and 8. Name and describe several features that determine the shape
describe how they apply to living organisms. of ecological systems.

5. Describe what it means to say that a cell is life’s functional 9. Name three ways that humans have affected life on Earth.
unit.
10. Summarize the six themes that are discussed in this chapter.
6. Describe the experimental evidence that demonstrates
that living organisms come from other living organisms.
Log in to to check your answers to the Self-
7. Explain how evolution accounts for both the unity and the Assessment questions, and to access additional learning tools.
diversity of life.
 CHAPTER 1 LIFE 25

CASE 1

The First Cell

Life’s Origins

Deep underground, in Mexico’s Cueva de Villa Luz, the have evolved into the millions of different species that
cave walls drip with slime. The rocky surfaces are teeming populate the planet today.
with colonies of mucus-producing bacteria. No sunlight How did the first living cell arise? Scientists generally
reaches these organisms far beneath Earth’s surface. accept that life arose from nonliving materials—a
Instead, the bacteria survive by capturing energy released process called abiogenesis—and thousands of laboratory
as they oxidize hydrogen sulfide gas that exists within experiments performed over the past sixty years provide
the cave. As a by-product of that reaction, the microbes glimpses of how this might have occurred. In our modern
produce sulfuric acid, making the slime that oozes from world, the features that separate life from nonlife are
the cave walls—dubbed “snottites” by researchers—as relatively easy to discern. But Earth’s first organisms were
corrosive as battery acid. almost certainly much less complicated than even the
Snottites might be stomach turning, but they’re simplest bacteria alive today. And before those first truly
intriguing, too. The organisms that produce snottites are living things appeared, molecular systems presumably
called extremophiles because they live in places where existed that hovered somewhere between the living and
humans and most other animals cannot survive. Such the nonliving.
microorganisms may tell us something about life when All cells require an archive of information, a
Earth was young. membrane to separate the inside of the cell from its
From cave-dwelling surroundings, and the ability to gather materials and
All cells require an bacteria to 100-ton blue harness energy from the environment. In modern
whales, the diversity of life organisms, the cell’s information archive is DNA, the
archive of information, on Earth is astounding. Yet double-stranded molecule that contains the instructions
a membrane to separate all of our planet’s organisms, needed for cells to grow, differentiate, and reproduce.
living and extinct, exist on Without that molecular machinery, life as we know it
the inside of the cell branches of the same family would not exist.
from its surroundings, tree. Bacteria that produce DNA is critical, and it’s complex. Among the
snottites, swordfish, humans, organisms alive today, the smallest known genome
and the ability to hydrangeas—all evolved from belongs to the bacterium Carsonella rudii. Even that
gather materials and a single common ancestor. genome contains nearly 160,000 DNA base pairs. How
When and how life could such sophisticated molecular systems have arisen?
harness energy from the originated are some of the The likely answer to that question is: step by step.
environment. biggest questions in biology. Laboratory experiments have shown how precursors to
Earth is nearly 4.6 billion nucleic acids might have come together under chemical
years old. Chemical evidence from 3.5-billion-year-old conditions present on the young Earth. It’s exceedingly
rocks in Australia suggests that biologically driven carbon unlikely that a molecule as complex as DNA was employed
and sulfur cycles existed at the time those rocks were by the very first living cells. As you’ll see in the chapters
formed. In the eons since, the first primitive life-forms that follow, scientists have gathered evidence suggesting

25
 Snottites deep
underground in a
Mexican cave. These
stalactite-like slime
formations are produced by
bacteria that gain energy by
reacting hydrogen sulfide
gas with oxygen. Source:
Kenneth Ingham Photography.

that RNA, rather than DNA,

stored information in early
cells and, indeed, did much
more than that, catalyzing
chemical reactions much as
proteins do today.
While some kind of
information archive was
necessary for life to unfold,
there is more to the story.
Living things must have a
barrier that separates them
from their environment.
All cells, whether found as
single-celled bacteria or by
the trillions in trees or humans, are individually encased an archive of information and enclosed in some kind of
in a cell membrane. primitive membrane—evolved into individual units that
Once again, scientists can only guess at how the first could grow, reproduce, and evolve.
cell membranes came about, but research shows that Such a series of events may sound unlikely. However,
the molecules that make up modern membranes possess some scientists argue that given the chemicals present on
properties that may have led them to form spontaneously the early Earth, it was likely—and even inevitable—that
on the early Earth. At first, the membranes were probably they would come together in such a way that life would
quite simple—straightforward (but leaky) barriers that emerge. Indeed, relatively simple, naturally occurring
kept the contents of early cells separated from the world materials such as metal ions have been shown to play a role
at large. Over time, as chance variations arose, those in key cellular reactions. Billions of years after the first cells
membranes that provided a better barrier were favored by arose, some of those metal ions—such as complexes of iron
natural selection. Moreover, proteins became embedded and sulfur—still play a critical role in cells.
in membranes, providing gates or channels that regulated That’s one reason researchers are so interested
the transport of ions and small molecules into and out of in studying organisms found today in extreme
the cell. environments. The sulfur-hungry snottites in the Cueva
A third essential characteristic of living things is the de Villa Luz help us to understand life 1–2 billion years
ability to harness energy from the environment. Here, ago, when oxygen was less plentiful and hydrogen
too, it’s feasible that a series of natural chemical processes sulfide more abundant. Still earlier, when life arose
led to entities that could achieve this feat. Simple about 4 billion years ago, the planet’s atmosphere
reactions that produced molecular by-products would contained no oxygen—humans couldn’t survive in such
have enabled more complex reactions down the road. a world and neither could snottite bacteria. However, by
Ultimately, that collection of reactions—combined with studying modern extremophiles living in oxygen-free
26
 A hydrothermal vent. Some scientists
think that this type of vent provided a
favorable environment for chemical
reactions that led to the origin of
life. Source: Image courtesy of New Zealand
American Submarine Ring of Fire 2007
Exploration, NOAA Vents Program, the Institute
of Geological & Nuclear Sciences and NOAA-OE.

environments, scientists may uncover clues about how of life’s origins produces many more questions than
Earth’s first cells came together and functioned. answers—and not just for biologists. The mystery of
Did life arise just once? Or could it have started up life spans the fields of biology, chemistry, physics, and
and died out several times before it finally got a foothold? planetary science. Though the questions are vast, our
If, given Earth’s early chemistry, life here was inevitable, understanding of life’s origins is likely to come about the
could it have arisen elsewhere in the universe? The study same way life itself arose: step by step.

? CASE 1 QUESTIONS
Special sections in Chapters 2–8 discuss the following questions related to Case 1.

1. How did the molecules of life form? See page 45.

2. What was the first nucleic acid molecule, and how did it arise? See page 59.
3. How did the genetic code originate? See page 83.
4. How did the first cell membranes form? See page 91.
5. What naturally occurring elements might have spurred the first reactions that led to life? See page 128.
6. What were the earliest energy-harnessing reactions? See page 139.
7. How did early cells meet their energy requirements? See page 146.
8. How did early cells use sunlight to meet their energy requirements? See page 170.
27
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 CHAPTER 2

The Molecules
of Life

Core Concepts
2.1 The atom is the fundamental
unit of matter.
2.2 Atoms can combine to form
molecules linked by chemical
bonds.
2.3 Water is essential for life.
2.4 Carbon is the backbone of
organic molecules.
2.5 Organic molecules include
proteins, nucleic acids,
carbohydrates, and lipids, each
of which is built from simpler
units.
2.6 Life likely originated on Earth
by a set of chemical reactions
that gave rise to the molecules
of life.
Science Photo Library/Alamy.

29
30 SECTION 2.1 P RO P E RT I E S O F ATO M S

When biologists speak of diversity, they commonly point to the

2 million or so species named and described to date, or to the FIG. 2.1 A carbon atom. Each carbon atom has six protons, six
10–100 million living species thought to exist in total. Life’s neutrons, and six electrons. The net charge of any atom is
diversity can also be found at a very different level of observation: neutral because there are as many electrons as protons.
in the molecules that make up each and every cell. Life depends
Nucleus
critically on many essential functions, including storing and (6 protons + 6 neutrons)
transmitting genetic information, establishing a boundary to
separate cells from their surroundings, and harnessing energy
e-
from the environment. These functions ultimately depend on the e-
chemical characteristics of the molecules that make up organisms.
In spite of the diversity of molecules and functions, the
chemistry of life is based on just a few types of molecule, which in e- e- Electron
turn are made up of just a few elements. Of the 100 or so chemical
elements, only about a dozen are found in more than trace
+ + Proton
amounts in living organisms. These elements interact with one +
another in only a limited number of ways. So, the question arises: +
+
How is diversity generated from a limited suite of chemicals and + + Neutron
interactions? The answer lies in some basic features of chemistry.
e-
e-
2.1 PROPERTIES OF ATOMS
Since antiquity, it has been accepted that the materials of nature
are made up of a small number of fundamental substances e-
combined in various ways. Today, we call these substances
elements. From the seventeenth century through the end of the
Carbon atom
nineteenth, elements were defined as pure substances that could
not be broken down further by the methods of chemistry. In time,
it was recognized that each element contains only one type of
atom, the basic unit of matter. By 1850, about 60 elements were can vary, changing its mass. Isotopes are atoms of the same
known, including such common ones as oxygen, copper, gold, element that have different numbers of neutrons. For example,
and sodium. Today, 118 elements are known. Of these, 94 occur carbon has three isotopes: About 99% of carbon atoms have six
naturally and 24 have been created artificially in the laboratory. neutrons and six protons, for an atomic mass of 12; about 1% have
Elements are often indicated by a chemical symbol that consists of seven neutrons and six protons, for an atomic mass of 13; and only
a one- or two-letter abbreviation of the name of the element. For a very small fraction have eight neutrons and six protons, for an
example, carbon is represented by C, hydrogen by H, and helium atomic mass of 14. The atomic mass is sometimes indicated as a
by He. superscript to the left of the chemical symbol. For instance, 12C is
the isotope of carbon with six neutrons and six protons.
Atoms consist of protons, neutrons, and electrons. Typically, an atom has the same number of protons and
Elements are composed of atoms. The atom contains a dense electrons. Because a carbon atom has six protons and six electrons,
central nucleus made up of positively charged particles called the positive and negative charges cancel each other out and the
protons and electrically neutral particles called neutrons. A carbon atom is electrically neutral. Some chemical processes cause
third type of particle, the negatively charged electron, moves an atom to either gain or lose electrons. An atom that has lost an
around the nucleus at some distance from it. Carbon, for example, electron is positively charged, and one that has gained an electron
typically has six protons, six neutrons, and six electrons (Fig. 2.1). is negatively charged. Electrically charged atoms are called ions.
The number of protons, or the atomic number, specifies an atom The charge of an ion is specified as a superscript to the right of the
as a particular element. An atom with one proton is hydrogen, for chemical symbol. Thus, H1 indicates a hydrogen ion that has lost
example, and an atom with six protons is carbon. an electron and is positively charged.
Together, the protons and neutrons determine the atomic
mass, the mass of the atom. Each proton and neutron, by Electrons occupy regions of space called orbitals.
definition, has a mass of 1, whereas an electron has negligible Electrons move around the nucleus, but not in the simplified way
mass. The number of neutrons in atoms of a particular element shown in Fig. 2.1. The exact path that an electron takes cannot
 CHAPTER 2 THE MOLECULES OF LIFE 31

FIG. 2.2 Electron orbitals and energy levels (shells) for hydrogen and carbon. The orbital of an electron can be visualized as a cloud of points
that is more dense where the electron is more likely to be. The hydrogen atom contains a single orbital, in a single energy level (a and c).
The carbon atom has five orbitals, one in the first energy level and four in the second energy level (b and c).

The first electron shell The second electron shell consists of

consists of a single four orbitals: the larger spherical orbital
spherical orbital. and three dumbbell-shaped orbitals.

a. Hydrogen’s b. The orbitals of carbon

0e-
single orbital z z z z z
2e-
1e- 2e-
+
+
+
1e-
+
+ y x x x x + +

1e-
x y y y y
Hydrogen
(1 electron)
Spherical orbitals Dumbbell-shaped orbitals Carbon (6 electrons)

c. Energy levels of hydrogen and carbon

In this simplified diagram, the

electron energy levels (shells) are
depicted as circles and the electrons
that occupy them as dots. The cloud
in the center is the nucleus.
Hydrogen
Carbon

be known, but it is possible to identify a region in space, called dumbbell-shaped orbitals is empty. Because a full orbital contains
an orbital, where an electron is present most of the time. For two electrons, it would take a total of four additional electrons to
example, Fig. 2.2a shows the orbital for hydrogen, which is simply completely fill all of the orbitals at this energy level. Therefore,
a sphere occupied by a single electron. Most of the time, the after the first shell, the maximum number of electrons per energy
electron is found within the space defined by the sphere, although level is eight. Fig. 2.2c shows simplified diagrams of atoms in
its exact location at any instant is unpredictable. which the highest energy level, or shell, of hydrogen, represented
The maximum number of electrons in any orbital is two. Most by the outermost circle, contains one electron, and that of carbon
atoms have more than two electrons and so have several orbitals contains four electrons.
positioned at different distances from the nucleus. These orbitals
j Quick Check 1 In the early 1900s, Ernest Rutherford produced
differ in size and shape. Electrons in orbitals close to the nucleus
a beam of very small positive particles and directed it at a thin
have less energy than do electrons in orbitals farther away, so
piece of gold foil just a few atoms thick. Most of the particles
electrons fill up orbitals close to the nucleus before occupying
passed through the foil without changing their path; very rarely, a
those farther away. Several orbitals can exist at a given energy
particle was deflected. What conclusions can you draw from this
level, or shell. The first shell consists of the spherical orbital
experiment about the structure of an atom?
shown in Fig. 2.2a.
Fig. 2.2b shows electron orbitals for carbon. Of carbon’s six
electrons, two occupy the small spherical orbital representing the Elements have recurring, or periodic, chemical
lowest energy level. The remaining four are distributed among properties.
four possible orbitals at the next highest energy level: One of these The chemical elements are often arranged in a tabular form known
four orbitals is a sphere (larger in diameter than the orbital at the as the periodic table of the elements, shown in Fig. 2.3 and
lowest energy level) and three are dumbbell-shaped. In carbon, generally credited to the nineteenth-century Russian chemist
the outermost spherical orbital has two electrons, two of the Dmitri Mendeleev. The table provides a way to organize all the
dumbbell-shaped orbitals have one electron each, and one of the chemical elements in terms of their chemical properties.
32 SECTION 2.2 MOLECULES AND CHEMICAL BONDS

1 2
FIG. 2.3 The periodic table of the
H Abundance in cells He
elements. Elements are
3 4 5 6 7 8 9 10
Li Be High Low Trace None
B C N O F Ne arranged by increasing
11 12 13 14 15 16 17 18 number of protons, the
Na Mg Al Si P S Cl Ar atomic number. The
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 elements in a column share
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr similar chemical properties.
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
55 56 57-71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86
Cs Ba La-Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
87 88 89-103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118
Fr Ra Ac-Lr Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Uuq Uup Uuh Uus Uuo

57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
89 90 91 92 93 94 95 96 97 98 99 100 101 102 103
Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

In the periodic table, the elements are indicated by their interact with other elements to form a diversity of molecules, as
chemical symbols and arranged in order of increasing atomic we will see in the next section.
number. For example, the second row of the periodic table begins
with lithium (Li) with 3 protons and ends with neon (Ne) with 10
protons. 2.2 MOLECULES AND CHEMICAL
For the first three horizontal rows in the periodic table, BONDS
elements in the same row have the same number of shells, and
so also have the same number and types of orbitals available to Atoms can combine with other atoms to form molecules, which
be filled by electrons. Across a row, therefore, electrons fill the are groups of two or more atoms attached together. that act as
shell until a full complement of electrons is reached on the right- a single unit. When two atoms form a molecule, the individual
hand side of the table. Fig. 2.4 shows the filling of the shells for atoms interact through what is called a chemical bond, a form of
elements in the second row of the periodic table. attraction between atoms that holds them together. The ability of
The elements in a vertical column are called a group or family. atoms to form bonds with other atoms explains in part why just a
Members of a group all have the same number of electrons in their few types of element can come together in many different ways to
outermost shell. For example, carbon (C) and lead (Pb) both have make a variety of molecules that can carry out diverse functions in
four electrons in their outermost shell. The number of electrons a cell. There are several ways in which atoms can interact with one
in the outermost shell determines in large part how elements another, and therefore many different types of chemical bond.

FIG. 2.4 Energy levels (shells) of row 2 of the periodic table. The complete complement of electrons in the outer shell of this row of elements
is eight.

Across the row, electrons are added until the outer shell
contains its complete complement of eight electrons.

Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon

 CHAPTER 2 THE MOLECULES OF LIFE 33

A covalent bond results when two atoms share

electrons. FIG. 2.5 A covalent bond. A covalent bond is formed when two
The ability of atoms to combine with other atoms is determined in atoms share a pair of electrons in a molecular orbital.
large part by the electrons farthest from the nucleus—those in the Chemical formula H2
outermost orbitals of an atom. These electrons are called valence Structural formula H H
electrons, and they are at the highest energy level. When
There is one electron
atoms combine with other atoms to form a molecule, the atoms in the colored area
share valence electrons with each other. Specifically, when the representing an Molecular orbital
outermost orbitals of two atoms come into proximity, two atomic orbital.

orbitals each containing one electron merge into a single orbital

containing a full complement of two electrons. The merged orbital H + H H H
is called a molecular orbital, and each shared pair of electrons
constitutes a covalent bond that holds the atoms together. Hydrogen atom Hydrogen atom Hydrogen
y g g gas
We can represent a specific molecule by its chemical formula, (1 electron) (1 electron) (2 electrons)
which is written as the letter abbreviation for each element
Two atoms of hydrogen
followed by a subscript giving the number of that type of atom combine to form hydrogen
in the molecule. Among the simplest molecules is hydrogen gas, gas by sharing electrons in
a molecular orbital.
illustrated in Fig. 2.5, which consists of two covalently bound
hydrogen atoms indicated by the chemical formula H2. Each
hydrogen atom has a single electron in a spherical orbital. When
the atoms join into a molecule, the two orbitals merge into a single oxygen atoms the number is eight. This simple rule for forming
molecular orbital containing two electrons that are shared by the stable molecules is known as the octet rule and applies to many,
hydrogen atoms. A covalent bond between atoms is denoted by a but not all, elements. For example, as shown in Fig. 2.6, one
single line connecting the two chemical symbols for the atoms, as carbon atom (C, with four valence electrons) combines with four
shown in the structural formula in Fig. 2.5. hydrogen atoms (H, with one valence electron each) to form
CH4 (methane); nitrogen (N, with five valence electrons)
j Quick Check 2 From their positions in the periodic table (see
combines with three H atoms to form NH3 (ammonia); and oxygen
Fig. 2.3), can you predict how many lithium atoms and hydrogen
(O, with six valence electrons) combines with two H atoms to
atoms can combine to form a molecule?
form H2O (water). Interestingly, the elements of the same column
Two adjacent atoms can sometimes share two pairs of in the next row behave similarly. This is just one example of the
electrons, forming a double bond denoted by a double line recurring, or periodic, behavior of the elements.
connecting the two chemical symbols for the atoms. In this case,
four orbitals occupied by a single electron merge to form two A polar covalent bond is characterized by unequal
molecular orbitals. sharing of electrons.
Molecules tend to be most stable when the two atoms In hydrogen gas (H2), the electrons are shared equally by the two
forming a bond share enough electrons to completely occupy the hydrogen atoms. In many bonds, however, the electrons are not
outermost energy level or shell. The outermost shell of a hydrogen shared equally by the two atoms. A notable example is provided
atom can hold two electrons, whereas for carbon, nitrogen, and by the bonds in a water molecule (H2O), which consists of two

FIG. 2.6 Four molecules. Atoms tend to combine in such a way as to complete the complement of electrons in the outer shell.

H H

The electrons in an
H H C H N H O atom’s outermost
shell are the valence
electrons.
H H H H
H

Hydrogen gas, H2 Methane, CH4 Ammonia, NH3 Water, H2O

34 SECTION 2.2 MOLECULES AND CHEMICAL BONDS

to attract electrons, a property known as electronegativity.

FIG. 2.7 A polar covalent bond. In a polar covalent bond, the two Electronegativity tends to increase across a row in the periodic
atoms do not share the electrons equally. table; as the number of protons across a row increases, electrons
are held more tightly to the nucleus. Therefore, oxygen is more
Chemical formula H2O
electronegative than hydrogen and attracts electrons more readily
Structural formula + +
H H The bonds linking the than does hydrogen. In a molecule of water, oxygen has a slight
hydrogen and oxygen
O negative charge, while the two hydrogen atoms have a slight
– atoms are polar, with a
partial positive charge (+) positive charge (Fig. 2.7). When electrons are shared unequally
Space-filling model + + near the hydrogen atoms between the two atoms, the resulting interaction is described as a
H H and a partial negative
charge (–) near the polar covalent bond.
O oxygen atom. A covalent bond between atoms that have the same, or
nearly the same, electronegativity is a nonpolar covalent bond,
–
which means that the atoms share the bonding electron pair
Molecular orbitals
almost equally. Nonpolar covalent bonds include those in gaseous
H H
hydrogen (H2) and oxygen (O2), as well as carbon–carbon (C–C)
O and carbon–hydrogen (C–H) bonds. Molecules held together by
nonpolar covalent bonds are important in cells because they do
not mix well with water.
hydrogen atoms each covalently bound to a single oxygen atom
(Fig. 2.7). An ionic bond forms between oppositely charged ions.
In a molecule of water, the electrons are more likely to In water, the difference in electronegativity between the oxygen
be located near the oxygen atom. The unequal sharing of and hydrogen atoms leads to unequal sharing of electrons. In more
electrons results from a difference in the ability of the atoms extreme cases, when an atom of very high electronegativity is

FIG. 2.8 An ionic bond. (a) Sodium chloride (salt) is formed by the attraction of two ions. (b) In solution, the ions are surrounded by water
molecules.
a.

In sodium chloride (salt), sodium loses The two ions are

an electron and becomes positively attracted to each other.
charged, and chlorine gains an electron
and becomes negatively charged.
 

Sodium atom Chlorine atom Sodium ion Chlorine ion

(Na) (Cl) (Na+) (Cl–)

Sodium chloride (NaCl)

b.
Sodium chloride (NaCl) dissolves in
solution because the sodium ions
(Na+) and chloride ions (Cl–) each
– + –
become surrounded by water
– Na 
molecules.
–

+ +
Cl–
+ +

NaCl H2O
 CHAPTER 2 THE MOLECULES OF LIFE 35

paired with an atom of very low electronegativity, the difference the same time, each oxygen atom forms new covalent bonds with
in electronegativity is so great that the electronegative atom two hydrogen atoms, forming two molecules of water. In fact,
“steals” the electron from its less electronegative partner. In this this reaction is the origin of the name “hydrogen,” which literally
case, the atom with the extra electron has a negative charge and means “water former.” The reaction releases a good deal of energy;
is a negative ion. The atom that has lost an electron has a positive it was used in the main engine of the space shuttle.
charge and is a positive ion. The two ions are not covalently bound, In biological systems, chemical reactions provide a way to
but because opposite charges attract they associate with each build and break down molecules for use by the cell, as well as to
other in what is called an ionic bond. An example of a compound harness energy, which can be held in chemical bonds (Chapter 6).
formed by the attraction of a positive ion and a negative ion is
table salt, or sodium chloride (NaCl) (Fig. 2.8a).
When sodium chloride is placed in water, the salt dissolves to 2.3 WATER: THE MEDIUM OF LIFE
form sodium ions (written as “Na1”) that have lost an electron and
so are positively charged, and chloride ions (Cl2) that have gained On Earth, all life depends on water. Indeed, life originated in water,
an electron and so are negatively charged. In solution, the two and the availability of water strongly influences the environmental
ions are pulled apart and become surrounded by water molecules: distributions of different species. Furthermore, water is the single
The negatively charged ends of water molecules are attracted to most abundant molecule in cells, so water is the medium in which
the positively charged sodium ion, and the positively charged ends the molecules of life interact. So important to life is water that, in
of other water molecules are attracted to the negatively charged the late 1990s, the National Aeronautic and Space Administration
chloride ion (Fig. 2.8b). Only as the water evaporates do the (NASA) announced that the search for extraterrestrial life would be
concentrations of Na1 and Cl2 increase to the point where the ions based on a simple strategy: Follow the water. NASA’s logic makes
join and precipitate as salt crystals. sense because, within our own solar system, Earth stands out both
for its abundance of water and the life it supports. What makes
A chemical reaction involves breaking and forming water so special as the medium of life?
chemical bonds.
The chemical bonds that link atoms in molecules can change in Water is a polar molecule.
a chemical reaction, a process by which atoms or molecules, As we saw earlier, water molecules have polar covalent bonds,
called reactants, are transformed into different molecules, called characterized by an uneven distribution of electrons. A molecule
products. During a chemical reaction, atoms keep their identity like water that has regions of positive and negative charge is
but change which atoms they are bonded to. called a polar molecule. Molecules, or even different regions of
For example, two molecules of hydrogen gas (2H2) and one the same molecule, fall into two general classes, depending on
molecule of oxygen gas (O2) can react to form two molecules of how they interact with water: hydrophilic (“water loving”) and
water (2H2O), as shown in Fig. 2.9. In this reaction, the numbers hydrophobic (“water fearing”).
of each type of atom are conserved, but their arrangement is Hydrophilic compounds are polar; they dissolve readily in
different in the reactants and the products. Specifically, the H–H water. That is, water is a good solvent, capable of dissolving many
bond in hydrogen gas and the O5O bond in oxygen are broken. At substances. Think of what happens when you stir a teaspoon of
sugar into water: The sugar seems to disappear as it dissolves. What
is happening is that the sugar molecules are dispersing through
the water and becoming separated from one another, forming a
FIG. 2.9 A chemical reaction. During a chemical reaction, atoms
solution in the watery, or aqueous, environment.
retain their identity, but their connections change as bonds
By contrast, hydrophobic compounds are nonpolar. Nonpolar
are broken and new bonds are formed.
compounds do not have regions of positive and negative charge. As
+ + + +
H H a result, they arrange themselves to minimize their contact with
H H
O O water. For example, oil molecules are hydrophobic, and when oil
– – and water are mixed, the oil molecules organize themselves into
2H2 + O2 2H2O droplets that limit the oil–water interface. This hydrophobic
Hydrogen gas Oxygen Water
effect, in which polar molecules like water exclude nonpolar
ones, drives such biological processes as the folding of proteins
H H (Chapter 4) and the formation of cell membranes (Chapter 5).
H H O O O
H H
A hydrogen bond is an interaction between a hydrogen
H H atom and an electronegative atom.
O
Because the oxygen and hydrogen atoms have slight charges,
Reactants Products water molecules orient themselves to minimize the repulsion of
36 SECTION 2.3 WAT E R : T H E M E D I U M O F L I F E

case of water, the result of many such interactions is a molecular

FIG. 2.10 Hydrogen bonds in liquid water. Because of thermal network stabilized by hydrogen bonds. Typically, a hydrogen bond is
motion, hydrogen bonds in water are continually breaking depicted by a dotted line, as in Fig. 2.10.
and forming between different pairs of molecules. Hydrogen bonds are much weaker than covalent bonds, but it
is hydrogen bonding that gives water many interesting properties,
+ + + described next. In addition, the presence of many weak hydrogen
H H
H +
O H + O H + bonds can help stabilize biological molecules, as in the case of
+
– O H
– – – nucleic acids and proteins.
O H
+
+
+ H H
+ +H H  A hydrogen bond
Hydrogen bonds give water many unusual properties.
H + + O H  O forms between two
O H – – water molecules Hydrogen bonds influence the structure of both liquid water
– when the partial (Fig. 2.11a) and ice (Fig. 2.11b). When water freezes, most
+ +H positive charge of a
+ + H
H Hydrogen + hydrogen atom is water molecules become hydrogen bonded to four other water
H O O H
–
bonds
–
attracted to the molecules, forming an open crystalline structure we call ice. As
O H + partial negative
– the temperature increases and the ice melts, some of the hydrogen
– + +
charge of an
+ H +H H oxygen atom. bonds are destabilized and break, allowing the water molecules
O H O
O – to pack more closely and explaining why liquid water is denser
H +H  –
+
 than ice. As a result, ice floats on water, and ponds and lakes freeze
from the top down, and therefore do not freeze completely. This
property allows fish and aquatic plants to survive winter in the
like charges so that positive charges are near negative charges. For cold water under the layer of ice.
example, because of its slight positive charge, a hydrogen atom in
j Quick Check 3 Why do containers of water, milk, soda, or other
a water molecule tends to orient toward the slightly negatively
liquids sometimes burst when frozen?
charged oxygen atom in another molecule.
A hydrogen bond is the name given to this interaction between Hydrogen bonds also give water molecules the property of
a hydrogen atom with a slight positive charge and an electronegative cohesion, meaning that they tend to stick to one another. A
atom of another molecule. In fact, any hydrogen atom covalently consequence of cohesion is high surface tension, a measure of the
bound to an electronegative atom (such as oxygen or nitrogen) will difficulty of breaking the surface of a liquid. Cohesion between
have a slight positive charge and can form a hydrogen bond. In the molecules contributes to water movement in plants. As water

FIG. 2.11 Liquid water and ice. Hydrogen bonds create (a) a dense structure in water, and (b) a highly ordered, less dense, crystalline structure
in ice.
a. Liquid water b. Ice
 CHAPTER 2 THE MOLECULES OF LIFE 37

evaporates from leaves, water is pulled upward, sometimes as high pH of blood is slightly basic, with a pH around 7.4. This value is
as 100 meters above the ground in giant sequoia and coast redwood sometimes referred to in medicine as physiological pH. Freshwater
trees, which are among the tallest trees on Earth. lakes, ponds, and rivers tend to be slightly acidic because carbon
The hydrogen bonds of water also influence how water dioxide from the air dissolves in the water and forms carbonic acid
responds to heating. Molecules are in constant motion, and this (Chapter 6).
motion increases as the temperature increases. When water is
heated, some of the energy added by heating is used to break
hydrogen bonds instead of causing more motion among the 2.4 CARBON: LIFE’S CHEMICAL
molecules, so the temperature increases less than if there were no BACKBONE
hydrogen bonding. The abundant hydrogen bonds make water more
resistant to temperature changes than other substances, a property Hydrogen and helium are far and away the most abundant
that is important for living organisms on a variety of scales. In the elements in the universe. In contrast, the solid Earth is dominated
cell, water resists temperature variations that would otherwise by silicon, oxygen, aluminum, iron, and calcium (Chapter 1). In
result from numerous biochemical reactions. On a global scale, other words, Earth is not a typical sample of the universe. Nor
the oceans minimize temperature fluctuations, stabilizing the is the cell a typical sample of the solid Earth. Fig. 2.12 shows
temperature on Earth in a range compatible with life. the relative abundance by mass of chemical elements present
In short, water is clearly the medium of life on Earth, but is in human cells after all the water has been removed. Note that
this because water is uniquely suited for life, or is it because life just four elements—carbon (C), oxygen (O), hydrogen (H), and
on Earth has adapted through time to a watery environment? nitrogen (N)—constitute 94% of the total dry mass, and that the
We don’t know the answer, but probably both explanations are most abundant element is carbon. The elemental composition
partly true. Chemists have proposed that under conditions of of human cells is typical of all cells. Human life, and all life as
high pressure and temperature, other small molecules, among we know it, is based on carbon. Carbon molecules play such
them ammonia (NH3) and some simple carbon-containing an important role in living organisms that carbon-containing
molecules, might display similar characteristics friendly to life. molecules have a special name—they are called organic
However, under the conditions that exist on Earth, water is the molecules. Their central role in life implies that there must be
only molecule uniquely suited to life. Water is a truly remarkable something very special about carbon, and there is. Carbon has
substance, and life on Earth would not be possible without it. the ability to combine with many other elements to form a wide

pH is a measure of the concentration of protons in

solution.
In any solution of water, a small proportion of the water molecules FIG. 2.12 Approximate proportions by dry mass of chemical
exist as protons (H1) and hydroxide ions (OH2). The pH of a elements found in human cells.
solution measures the proton concentration ([H1]), which is
important as the pH influences many chemical reactions and
biological processes. It is calculated by the following formula:

pH 5 –log [H1]
Oxygen (O)
30%
The pH of a solution can range from 0 to 14. Since the pH scale
is logarithmic, a difference of one pH unit corresponds to a tenfold
difference in hydrogen ion concentration. A solution is neutral
Hydrogen (H)
(pH 5 7) when the concentrations of protons (H1) and hydroxide 9%
ions (OH2) are equal. When the concentration of protons is Carbon (C)
higher than that of hydroxide ions, the pH is lower than 7 and 47%
the solution is acidic. When the concentration of protons is
Nitrogen (N)
lower than that of hydroxide ions, the pH is higher than 7 and the 8%
solution is basic. An acid can therefore be described as a molecule
that releases a proton (H1), and a base is a molecule that accepts a Phosphorus (P)
3%
proton in aqueous solution.
Sulfur (S)
Pure water has a pH of 7—that is, it is neutral, with an equal Others Potassium (K)
concentration of protons and hydroxide ions. The pH of most 1% Calcium (Ca)
cells is approximately 7 and is tightly regulated, as most chemical Sodium (Na)
Chlorine (Cl)
reactions can be carried out only in a narrow pH range. Certain Magnesium (Mg)
cellular compartments, however, have a much lower pH. The 2%
38 SECTION 2.4 C A R B O N : L I F E' S C H E M I C A L B AC K B O N E

variety of molecules, each specialized for the functions

it carries out in the cell. FIG. 2.14 Diverse carbon-containing molecules. (a) The molecule ethane
contains one C–C bond. (b) A linear chain of carbon atoms and a ring
Carbon atoms form four covalent bonds. structure that also contains oxygen contain multiple C–C bonds.
One of the special properties of carbon is that, in a.
forming molecular orbitals, a carbon atom behaves C C bond
Chemical C2H6 H H
as if it had four unpaired electrons. This behavior formula Ethane
occurs because one of the electrons in the outermost
spherical orbital moves into the empty dumbbell- H H H H
Structural C C
shaped orbital (see Fig. 2.2). In this process, the single H C C H
formula
large spherical orbital and three dumbbell-shaped H H
orbitals change shape, becoming four equivalent
hybrid orbitals, each with one electron. H H
Fig. 2.13 shows the molecular orbitals that result
b.
when one atom of carbon combines with four atoms
Chemical
of hydrogen to form the gas methane (CH4). Each formula C7H16 C5H10O
of the four valence electrons of carbon shares a new
molecular orbital with the electron of one of the H
hydrogen atoms. These bonds can rotate freely about H H H H C O
their axis. Furthermore, because of the shape of the Structural H H H H H
H C C C C H H
orbitals, the carbon atom lies at the center of a three- formula C C
C C C H H
dimensional structure called a tetrahedron, and the H H H H H H
H H H C C
four molecular orbitals point toward the four corners
H H
of this structure. The ability of carbon to form four
covalent bonds, the spatial orientation of these bonds
O
in the form of a tetrahedron, and the ability of each
bond to rotate freely all contribute importantly to the Simplified
structure
structural diversity of carbon-based molecules.

Carbon-based molecules are structurally

These structures are shown in simplified form, where a carbon
and functionally diverse. atom is at the end of a line or the angle where two lines join;
Carbon has other properties that contribute to its the hydrogen atoms that share any otherwise unshared
ability to form a diversity of molecules. For example, electrons of each carbon atom are assumed but not shown.

FIG. 2.13 A carbon atom with four covalent bonds. One carbon
atom combines with four hydrogen atoms to form carbon atoms can link with each other by covalent bonds to form
methane. long chains. These chains can be branched, or two carbons at the
ends of the chain or within the chain can link to form a ring.
Chemical CH4 Among the simplest chains is ethane, shown in Fig. 2.14a.
formula Methane H
Ethane is formed when two carbon atoms become connected by
a covalent bond. In this case, the orbitals of unpaired electrons
in two carbon atoms form the covalent bond. Each carbon atom
H is also bound to three hydrogen atoms. Some more complex
Structural examples of carbon-containing molecules are shown in Fig. 2.14b,
formula H C H
C
H as both structural formulas and in simplified form.
H
H Two adjacent carbon atoms can also share two pairs of
electrons, forming a double bond, as shown in Fig. 2.15. Note
that each carbon atom has exactly four covalent bonds, but in this
The molecular orbitals of
methane point to the four case two are shared between adjacent carbon atoms. The double
corners of a tetrahedron. H bond is shorter than a single bond and is not free to rotate, so all
of the covalent bonds formed by the carbon atoms connected by
 CHAPTER 2 THE MOLECULES OF LIFE 39

that silicon might provide an alternative to carbon

FIG. 2.15 Molecules containing double bonds between carbon atoms. as a chemical basis for life. However, silicon readily
binds oxygen. On Earth, nearly all of the silicon atoms
Chemical
C2H4 C7H14 C6H6 found in molecules are covalently bound to oxygen.
formula
Studies of Mars and meteorites show that silicon
is tightly bound to oxygen throughout our solar
H H
H
H
system, and that is likely to be true everywhere we
H H
H H H H C C might explore. There are about 1000 different silicate
Structural C
C C H C C C C H C C H
formula C C H minerals on Earth, but this diversity pales before the
H H H C C
H H H millions of known carbon-based molecules. If we ever
H H
H H discover life beyond Earth, very likely its chemistry
will be based on carbon.

a double bond are in the same geometrical plane. As with single

bonds, double bonds can be found in chains of atoms or ring 2.5 ORGANIC MOLECULES
structures.
While the types of atoms making up a molecule help Chemical processes in the cell depend on just a few classes of
characterize the molecule, the spatial arrangement of atoms is carbon-based molecules. Proteins provide structural support and
also important. For example, 6 carbon atoms, 13 hydrogen atoms, act as catalysts that facilitate chemical reactions. Nucleic acids
2 oxygen atoms, and 1 nitrogen atom can join covalently in many encode and transmit genetic information. Carbohydrates provide
different arrangements to produce molecules with different a source of energy and make up the cell wall in bacteria, plants,
structures. Two of these many arrangements are shown in and algae. Lipids make up cell membranes, store energy, and act as
Fig. 2.16. Note that some of the connections between atoms are signaling molecules.
identical in the two molecules (black) and some are different These molecules are all large, consisting of hundreds or
(green), even though the chemical formulas are the same thousands of atoms, and many are polymers, complex molecules
(C6H13O2N1). Molecules that have the same chemical formula but made up of repeated simpler units connected by covalent bonds.
different structures are known as isomers. Proteins are polymers of amino acids, nucleic acids are polymers
We have seen that carbon-containing molecules can adopt of nucleotides, and carbohydrates such as starch are polymers of
a wide range of arrangements, a versatility that helps us to simple sugars. Lipids are a bit different, as we will see, in that they
understand how a limited number of elements can create an are defined by a property rather than by their chemical structure.
astonishing variety of molecules. We might ask whether carbon The lipid membranes that define cell boundaries consist of fatty
is uniquely versatile. Put another way, if we ever discover life on a acids bonded to other organic molecules.
distant planet, will it be carbon based, like us? Silicon, just below Building macromolecules from simple, repeating units
carbon in the periodic table (see Fig. 2.3), is the one other element provides a means of generating virtually limitless chemical
that is both reasonably abundant and characterized by four atomic diversity. Indeed, in macromolecules, the building blocks of
orbitals with one electron each. Some scientists have speculated polymers play a role much like that of the letters in words.
In written language, a change in the content or order of letters
changes the meaning of the word (or renders it meaningless).
For example, by reordering the letters of the word SILENT you
can write LISTEN, a word with a different meaning. Similarly,
FIG. 2.16 The isomers isoleucine and leucine. Isoleucine and rearranging the building blocks that make up macromolecules
leucine are isomers: Their chemical formulas are the provides an important way to make a large number of diverse
same, but their structures differ. macromolecules whose functions differ from one to the next.
In the following sections, we focus on the building blocks
Chemical C6H13O2N1 C6H13O2N1
formula Isoleucine Leucine of these four key molecules of life, reserving a discussion of the
structure and function of the macromolecules for later chapters.
H H
O O
Structural
H2N C C H2N C C
Functional groups add chemical character to
formula
OH OH carbon chains.
CH CH2
The simple repeating units of polymers are often based on a
H2C CH3 CH nonpolar core of carbon atoms. But attached to these carbon atoms
H3C H3C CH3 are functional groups. Functional groups are groups of one or
40 SECTION 2.5 O RG A N I C M O L E C U L E S

more atoms that have particular chemical properties on their own, functions. Since proteins consist of amino acids linked covalently
regardless of what they are attached to. Among the functional to form a chain, we need to examine the chemical features of
groups frequently encountered in biological molecules are amine amino acids to understand the diversity and versatility of proteins.
(5NH), amino (–NH2), carboxyl (–COOH), hydroxyl (–OH), ketone The general structure of an amino acid is shown in Fig. 2.17a.
(5O), phosphate (–O–PO3H2), sulfhydryl (–SH), and methyl Each amino acid contains a central carbon atom, called the
(–CH3). The nitrogen, oxygen, phosphorus, and sulfur atoms in a (alpha) carbon, covalently linked to four groups: an amino
these functional groups are more electronegative than the carbon group (–NH2; blue), a carboxyl group (–COOH; brown), a
atoms, and functional groups containing these atoms are polar. hydrogen atom (H), and an R group, or side chain, (green) that
The methyl group (–CH3), on the other hand, is nonpolar. differs from one amino acid to the next. The identity of each
Because many functional groups are polar, otherwise nonpolar amino acid is determined by the structure and composition of the
molecules containing these groups become polar and so become side chain. The side chain of the amino acid glycine is simply H, for
soluble in the cell’s aqueous environment. In other words, they example, and that of alanine is CH3. In most amino acids, the
disperse in solution throughout the cell. Moreover, because many a carbon is covalently linked to four different groups. Glycine is
functional groups are polar, they are also reactive. Notice in the the exception, since its R group is a hydrogen atom.
following sections that the reactions joining simpler molecules At the pH commonly found in a cell (pH 7.4), the amino and
into polymers usually take place between functional groups. carboxyl groups are ionized (charged), with the amino group
gaining a proton (–NH13 ; blue) and the carboxyl group losing a
Proteins are composed of amino acids. proton (–COO2; brown), as shown in Fig. 2.17b.
Proteins do much of the cell’s work. Some function as catalysts Amino acids are linked in a chain to form a protein
that accelerate the rates of chemical reactions (in which case they (Fig. 2.17c). The carbon atom in the carboxyl group of one amino
are called enzymes), and some act as structural components acid is joined to the nitrogen atom in the amino group of the next
necessary for cell shape and movement. Your body contains many by a covalent linkage called a peptide bond. In Fig. 2.17c, the chain
thousands of distinct types of protein that perform a wide range of of amino acids includes four amino acids, and the peptide bonds are
indicated in red. The formation of a peptide bond involves the loss of
a water molecule since in order to form a C–N bond, the carbon atom
of the carboxyl group must release an oxygen atom and the nitrogen
FIG. 2.17 Amino acids and peptide bonds. (a) An amino acid atom of the amino group must release two hydrogen atoms. These
contains four groups attached to a central carbon atom. can then combine to form a water molecule (H2O). The loss of
(b) In the environment of a cell, the amino group gains a a water molecule also occurs in the linking of subunits to form
proton and the carboxyl group loses a proton. (c) Peptide polymers such as nucleic acids and complex carbohydrates.
bonds link amino acids to form a protein. Cellular proteins are composed of combinations of 20 different
amino acids, each of which can be classified according to the
a. Amino acid
H chemical properties of its R group. The particular sequence, or
O
order, in which amino acids are present in a protein determines
H2N C C Carboxyl
Amino group
how it folds into its three-dimensional structure. The three-
group OH
R dimensional structure, in turn, determines the protein’s function.
 carbon R group In Chapter 4, we examine how the sequence of amino acids in a
particular protein is specified and discuss how proteins fold into
b. Ionized amino acid their three-dimensional shapes.
H
O
H3N+ C C Carboxyl Nucleic acids encode genetic information in their
Amino
group R
O– group nucleotide sequence.
Nucleic acids are examples of informational molecules—that is,
 carbon R group
they are large molecules that carry information in the sequence
of nucleotides that make them up. This molecular information is
c. Polypeptide chain (protein)
much like the information carried by the letters in an alphabet, but
Peptide bond
in the case of nucleic acids, the information is in chemical form.
H O H
O The nucleic acid deoxyribonucleic acid (DNA) is the
O R2 O R4
C N C C N C genetic material in all organisms. It is transmitted from parents
H NH+ C C N C C O– to offspring, and it contains the information needed to specify
R1 H R3 H
H H H H the amino acid sequence of all the proteins synthesized in an
organism. The nucleic acid ribonucleic acid (RNA) has multiple
Amino Amino Amino Amino functions; it is a key player in protein synthesis and the regulation
acid acid acid acid of gene expression.
 CHAPTER 2 THE MOLECULES OF LIFE 41

(OH) group on the second carbon (designated the 2� carbon),

FIG. 2.18 A ribonucleotide and a deoxyribonucleotide, the units whereas deoxyribose has a hydrogen atom at this position
of RNA and DNA. (hence, deoxyribose). (By convention, the carbons in the sugar
Ribonucleotide are numbered with primes—1�, 2�, etc.—to distinguish them from
carbons in the base—1, 2, etc.)
The bases are built from nitrogen-containing rings and are
O-
5’ O of two types. The pyrimidine bases (Fig. 2.19a) have a single
-O P O CH2 Base (A, G, U, or C)
ring and include cytosine (C), thymine (T), and uracil (U). The
O C C purine bases (Fig. 2.19b) have a double-ring structure and include
4’ H H 1’
Phosphate H H guanine (G) and adenine (A). DNA contains the bases A, T, G, and
group 3’ C C
2’ C, and RNA contains the bases A, U, G, and C. Just as the order of
OH OH amino acids provides the information carried in proteins, so, too,
Ribose
does the sequence of nucleotides determine the information in
sugar
DNA and RNA molecules.
In DNA and RNA, each adjacent pair of nucleotides is
Deoxyribonucleotide connected by a phosphodiester bond, which forms when a
phosphate group in one nucleotide is covalently joined to the
O- sugar unit in another nucleotide (Fig. 2.20). As in the formation of
5’ O
-O P O CH2 Base (A, G, T, or C) a peptide bond, the formation of a phosphodiester bond involves
the loss of a water molecule.
O C C
4’ H H 1’ DNA in cells usually consists of two strands of nucleotides
Phosphate H H
3’ C C twisted around each other in the form of a double helix
group 2’
OH H
Deoxyribose
sugar FIG. 2.20 The phosphodiester bond. Phosphodiester bonds link
successive deoxyribonucleotides, forming the backbone
DNA and RNA are long molecules consisting of nucleotides of the DNA strand.
bonded covalently one to the next. Nucleotides, in turn, are
composed of three components: a 5-carbon sugar, a nitrogen-
Phosphate
containing compound called a base, and one or more phosphate group
O-
groups (Fig. 2.18). The sugar in RNA is ribose, and the sugar in
-O P O
DNA is deoxyribose. The sugars differ in that ribose has a hydroxyl
O
O Base
5’ CH2
FIG. 2.19 Pyrimidine bases and purine bases. (a) Pyrimidines
H H H H
have a single-ring structure, and (b) purines have a
3’
double-ring structure. O H
a. Pyrimidine bases -O P O
NH2 O O O
H3C O
N NH NH 5’ CH2 Base

H H H H
N O N O N O
3’
O H
Cytosine (C) Thymine (T) Uracil (U)
Phosphodiester -O P O
bond
b. Purine bases O
O
O NH2 In a nucleic acid, each 5’ CH2 Base
N N base is attached to
NH N either a ribose or a H H
H H
deoxyribose by the
3’
bond indicated in red.
N N NH2 N N OH H
Deoxyribose
Guanine (G) Adenine (A) sugar
42 SECTION 2.5 O RG A N I C M O L E C U L E S

(Fig. 2.21a). The sugar–phosphate backbones of the strands wrap

like a ribbon around the outside of the double helix, and the bases FIG. 2.22 Structural formulas for some 6-carbon aldoses and
point inward. The bases form specific purine–pyrimidine pairs that ketoses.
are complementary: Where one strand carries an A, the other H O H O H
carries a T; and where one strand carries a G, the other carries a C Aldehyde Aldehyde
1C C H C OH
(Fig. 2.21b). Base pairing results from hydrogen bonding between group group
H C OH H C OH C O Ketone
the bases (Fig. 2.21c). 2
group
Genetic information in DNA is contained in the sequence, or HO C H HO C H HO C H
3
order, in which successive nucleotides occur along the molecule. H C OH HO C H H C OH
4
Successive nucleotides along a DNA strand can occur in any order,
H C OH H C OH H C OH
and hence a long molecule could contain any of an immense number 5

of possible nucleotide sequences. This is one reason why DNA is an H C OH H C OH H C OH

6
efficient carrier of genetic information. In Chapter 3, we consider H H H
the structure and function of DNA and RNA in greater detail. Glucose Galactose Fructose
(an aldose) (an aldose) (a ketose)
Complex carbohydrates are made up of simple sugars.
Many of us, when we feel tired, reach for a candy bar for a quick
energy boost. The energy in a candy bar comes from sugars, which The simplest carbohydrates are sugars (also called
are quickly broken down to release energy. Sugars belong to a class saccharides). Simple sugars are linear or, far more commonly,
of molecules called carbohydrates, distinctive molecules composed cyclic molecules containing five or six carbon atoms. All 6-carbon
of C, H, and O atoms, usually in the ratio 1 : 2 : 1. Carbohydrates sugars have the same chemical formula (C6H12O6) and differ
provide a principal source of energy for metabolism. only in configuration. Glucose (the product of photosynthesis),
galactose (found in dairy products), and fructose
(a commercial sweetener) are examples; they
share the same formula (C6H12O6) but differ in the
FIG. 2.21 The structure of DNA. (a) DNA is most commonly in the form arrangement of their atoms (Fig. 2.22).
of a double helix, with the sugar and phosphate groups forming
the backbone and the bases oriented inward. (b) The bases are j Quick Check 4 Take a close look at Fig. 2.22.
complementary, with A paired with T and G paired with C. (c) Base How is glucose different from galactose?
pairing results from hydrogen bonds.

a. b.
A simple sugar is also called a monosaccharide
(mono means “one”), and two simple sugars
Adenine (A) Thymine (T)
linked together by a covalent bond is called a
Guanine (G) Cytosine (C) disaccharide (di means “two”). Sucrose (C12H22O11),
or table sugar, is a disaccharide that combines
one molecule each of glucose and fructose.
c. Simple sugars combine in many ways to form
H polymers called polysaccharides (poly means
N N H O CH3 “many”) that provide long-term energy storage
Base
pairs (starch and glycogen) or structural support
N A N H N T (cellulose in plant cell walls). Long, branched
Deoxyribose
N N chains of monosaccharides are called complex
O carbohydrates.
Deoxyribose
Let’s take a closer look at monosaccharides,
Sugar– H
phosphate the simplest sugars. Monosaccharides are
N
backbone O H N unbranched carbon chains with either an
N
aldehyde (HC5O) or a ketone (C5 O) group
G N H N C (Fig. 2.22). Monosaccharides with an aldehyde
Deoxyribose
N N group are called aldoses and those with a ketone
N H O
Deoxyribose group are known as ketoses. In both types of
H monosaccharide, the other carbons each carry one
Hydrogen bond hydroxyl (–OH) group and one hydrogen (H) atom.
 CHAPTER 2 THE MOLECULES OF LIFE 43

Carbohydrate diversity stems in part from the monosaccharides

FIG. 2.23 Formation of the cyclic form of glucose. that make up carbohydrates, similar to the way that protein and
nucleic acid diversity stems from the sequence of their subunits.
Aldehyde group Some complex carbohydrates are composed of a single type of
6 6
CH2OH CH2OH monosaccharide, while others are a mix of different kinds of
5
C OH
5
C O monosaccharide. Starch, for example, is a sugar storage molecule
H H H H in plants composed completely of glucose molecules, whereas
H H
C 1C C 1C pectin, a component of the cell wall, contains up to five different
4 OH H 4 OH H
O OH monosaccharides.
OH OH
3
C C2 3
C C 2

H OH H OH Lipids are hydrophobic molecules.

Linear glucose Cyclic glucose
Proteins, nucleic acids, and carbohydrates all are polymers made
up of smaller, repeating units with a defined structure. Lipids are
different. Instead of being defined by a chemical structure, they
When the linear structure of a monosaccharide is written with the share a particular property: Lipids are all hydrophobic. Because
aldehyde or ketone group at the top, the carbons are numbered they share a property and not a structure, lipids are a chemically
from top to bottom. diverse group of molecules. They include familiar fats that make
Virtually all of the monosaccharides in cells are in ring form up part of our diet, components of cell membranes, and signaling
(Fig. 2.23), not linear structures. To form a ring, the carbon in molecules. Let’s briefly consider each in turn.
the aldehyde or ketone group forms a covalent bond with the Triacylglycerol is an example of a lipid that is used for energy
oxygen of a hydroxyl group carried by another carbon in the storage. It is the major component of animal fat and vegetable oil.
same molecule. For example, cyclic glucose is formed when the A triacylglycerol molecule is made up of three fatty acids joined
oxygen atom of the hydroxyl group on carbon 5 forms a covalent to glycerol (Fig. 2.25). A fatty acid is a long chain of carbon atoms
bond with carbon 1, which is part of an aldehyde group. The attached to a carboxyl group (–COOH) at one end (Figs. 2.25a
cyclic structure is approximately flat, and you can visualize it and 2.25b). Glycerol is a 3-carbon molecule with OH groups
perpendicular to the plane of the paper with the covalent bonds attached to each carbon (Fig. 2.25c). The carboxyl end of each fatty
indicated by the thick lines in the foreground. The groups attached acid chain attaches to glycerol at one of the OH groups (2.25d),
to any carbon therefore project either above or below the ring. releasing a molecule of water.
When the ring is formed, the aldehyde oxygen becomes a hydroxyl Fatty acids differ in the length of their hydrocarbon chain—
group. The presence of the polar hydroxyl groups through the that is, they differ in the number of carbon atoms in the chain.
sugar ring makes these molecules highly soluble in water. (A hydrocarbon is a molecule composed entirely of carbon and
Monosaccharides, especially 6-carbon sugars, are the building hydrogen atoms.) Most fatty acids in cells contain an even number
blocks of complex carbohydrates. Monosaccharides are attached to of carbon atoms because they are synthesized by the stepwise
each other by covalent bonds called glycosidic bonds (Fig. 2.24). addition of 2-carbon units.
As with peptide bonds, the formation of glycosidic bonds involves Some fatty acids have one or more carbon–carbon double
the loss of a water molecule. A glycosidic bond is formed between bonds; these double bonds can differ in number and location. Fatty
carbon 1 of one monosaccharide and a hydroxyl group carried by a acids that do not contain double bonds are described as saturated.
carbon atom in a different monosaccharide molecule. Because there are no double bonds, the maximum number of

FIG. 2.24 Glycosidic bonds. Glycosidic bonds link carbohydrate molecules together; in this example they link glucose monomers together to
form the polysaccharide starch.
6
CH2OH CH2OH CH2OH CH2OH
5
C O C O C O C O
H H H H H H H H
4 H H H H
C C C C C C C C
OH H 1 OH H OH H OH H
OH
OH O O O
3C C C C C C C C
2
H OH H OH H OH H OH

Glycosidic bonds
44 SECTION 2.5 O RG A N I C M O L E C U L E S

therefore, form oil droplets inside the

FIG. 2.25 Triacylglycerol and its components. cell. Triacylglycerols are an efficient
form of energy storage because, by
a. Palmitic acid (fatty acid) excluding water molecules, a large
Hydrocarbon chain number can be packed into a small
O Carboxyl
group volume.
CH2 CH2 CH2 CH2 CH2 CH2 CH2 C
CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH2 OH Although fatty acid molecules
are uncharged, the constant motion
of electrons leads to regions of slight
b. Palmitoleic acid (fatty acid) positive and slight negative charges
O (Fig. 2.26). These charges in turn
CH CH2 CH2 CH2 C attract or repel electrons in neighboring
HC CH2 CH2 CH2 CH2 OH molecules, setting up areas of positive
CH2 and negative charge in those molecules
H2C as well. The molecules are temporarily
An unsaturated
CH2 fatty acid contains polarized molecules. Such molecules
H2C one or more carbon–
carbon double
weakly bind to one another because
CH2 bonds. of the attraction of opposite charges.
H3C These interactions are known as van
der Waals forces. The van der Waals
forces come into play only when atoms
c. Glycerol
H
are sufficiently close to one another,
CH2 CH2 and they are weaker than hydrogen
HO C OH
bonds, but many van der Waals forces
OH acting together help to stabilize
molecules.
Because of van der Waals forces, the
d. Triacylglycerol
melting points of fatty acids depend on
O
their length and level of saturation. As
CH2 C CH2 CH2 CH2 CH2 CH2
O CH2 CH2 CH2 CH2 CH2 CH3 the length of the hydrocarbon chains
increases, the number of van der Waals
O Triacylglycerols interactions between the chains also
are formed by
CH C CH2 CH2 CH2 CH2 CH2 increases. The melting temperature
O CH2 CH2 CH2 CH2 CH2 CH3 the addition
of three fatty increases because more energy is
O acid chains to
glycerol. needed to break the greater number
CH2 C CH2 CH2 CH2 CH2 CH2 of van der Waals interactions. Kinks
O CH2 CH2 CH2 CH2 CH2 CH3
introduced by double bonds reduce the
tightness of the molecular packing and
thus the number of intermolecular
interactions. As a result, the melting
hydrogen atoms is attached to each carbon atom, so all of the temperature is lower. Therefore, an unsaturated fatty acid has a
carbon atoms are said to be “saturated” with hydrogen atoms lower melting point than a saturated fatty acid of the same length.
(Fig. 2.25a). Fatty acids that contain carbon–carbon double bonds Animal fats such as butter are composed of triacylglycerols with
are unsaturated (Fig. 2.25b). The chains of saturated fatty acids saturated fatty acids and are solid at room temperature, whereas
are straight, while the chains of unsaturated fatty acids have a kink plant fats and fish oils are composed of triacylglycerols with
at each double bond. unsaturated fatty acids and are liquid at room temperature.
Triacylglycerols can contain different types of fatty acids Steroids such as cholesterol are a second type of lipid
attached to the glycerol backbone. The hydrocarbon chains of (Fig. 2.27). Like other steroids, cholesterol has a core composed
fatty acids do not contain polar covalent bonds like those in a of 20 carbon atoms bonded to form four fused rings, and it is
water molecule. Their electrons are distributed uniformly over hydrophobic. Cholesterol is a component of animal cell membranes
the whole molecule and thus these molecules are uncharged. As a (Chapter 5) and serves as a precursor for the synthesis of steroid
consequence, triacylglycerols are all extremely hydrophobic and, hormones such as estrogen and testosterone (Chapter 38).
 CHAPTER 2 THE MOLECULES OF LIFE 45

Phospholipids are a third type of lipid. They are a

FIG. 2.26 Van der Waals forces.Transient asymmetry in the distribution major component of the cell membrane and are described
of electrons along fatty acid chains leads to asymmetry in Chapter 5.
in neighboring molecules, resulting in weak electrostatic
attractions.
O ? CASE 1 THE FIRST CELL: LIFE’S ORIGINS
+ + +
HO
– – – 2.6 HOW DID THE MOLECULES
OF LIFE FORM?
O
In Chapter 1, we considered the similarities and differences
HO O between living and nonliving things. Four billion years
HO ago, however, the differences may not have been so
pronounced. Scientists believe that life originated early in
our planet’s history, formed by a set of chemical processes
that, through time, produced organisms that could be
distinguished from their nonliving surroundings. How
O can we think scientifically about one of biology’s deepest
+ + +
and most difficult problems? One idea is to approach life’s
HO Van der origins experimentally, asking whether chemical reactions
– – –
Waals
O
+ + + force likely to have taken place on the early Earth can generate
the molecules of life. It is important to note that even the
HO
– – – simplest organisms living today are far more complicated
O
than our earliest ancestors. No one suggests that cells as
HO
we know them emerged directly from primordial chemical
reactions. Rather, the quest is to discover simple molecular
systems able to replicate themselves and subject to natural
selection.
A key starting point is the observation, introduced
O
+ + + earlier in this chapter, that the principal macromolecules
HO found in organisms are themselves made of simpler
– – –
O
molecules joined together. Thus, if we want to understand
+ + + how proteins might have emerged on the early Earth,
HO
– – – we should begin with the synthesis of amino acids, and
O if we are interested in nucleic acids, we should focus on
+ + +
nucleotides.
HO
– – –
The building blocks of life can be generated in
the laboratory.
Research into the origins of life was catapulted into the
FIG. 2.27 The chemical structure of cholesterol. experimental age in 1953 with an elegant experiment
carried out by Stanley Miller, then a graduate student
CH3
in the laboratory of Nobel laureate Harold Urey. Miller
CH3
started with gases such as water vapor, methane, and
hydrogen gas, all thought to have been present in the early
CH3
atmosphere. He put these gases into a sealed flask and
CH3 then passed a spark through the mixture (Fig. 2.28). On
the primitive Earth, lightning might have supplied the
CH3 energy needed to drive chemical reactions, and the spark
was meant to simulate its effects. Analysis of the contents
of the flask showed that a number of amino acids were
HO generated.
46 SECTION 1.1 THE SCIENTIFIC METHOD

HOW DO WE KNOW?
Miller and others conducted many variations on his original
FIG. 2.28
experiment, all with similar results. Today, many scientists
doubt that the early atmosphere had the composition found
Could the building blocks in Miller’s experimental apparatus, but amino acids and other
biologically important molecules can form in a variety of
of organic molecules have simulated atmospheric compositions. If oxygen gas (O2) is
absent and hydrogen is more common in the mixture than
been generated on the early carbon, the addition of energy generates diverse amino acids.
The absence of oxygen gas is critical since these types of
Earth? reactions cannot run to completion in modern air or seawater.
Here, however, geology supports the experiments: Chemical
analyses of Earth’s oldest sedimentary rocks indicate that,
BACKGROUND In the 1950s, Earth’s early atmosphere was
for its first 2 billion years, Earth’s surface contained little or
widely believed to have been rich in water vapor, methane,
no oxygen.
ammonia, and hydrogen gas, with no free oxygen.
Later experiments have shown that other chemical
EXPERIMENT Stanley Miller built an apparatus, shown below, reactions can generate simple sugars, the bases found
designed to simulate Earth’s early atmosphere. Then he passed a in nucleotides, and the lipids needed to form primitive
spark through the mixture to simulate lightning. membranes. Independent evidence that simple chemistry can
form the building blocks of life comes from certain meteorites,
which provide samples of the early solar system and contain
diverse amino acids, lipids, and other organic molecules.

Electrodes
Experiments show how life’s building blocks can
form macromolecules.
Spark From the preceding discussion, we have seen that life’s simple
Gas inlet
H2O, CH4, CH4, NH3 building blocks can be generated under conditions likely to
NH3, H2 have been present on the early Earth, but can these simple
Direction of units be linked together to form polymers? Once again, careful
circulation
Cold experiments have shown how polymers could have formed
water Condenser
for cooling in the conditions of the early Earth. Clay minerals that form
from volcanic rocks can bind nucleotides on their surfaces
Sampling (Fig. 2.29a). The clays provide a surface that places the
valve nucleotides near one another, making it possible for them to
join to form chains or simple strands of nucleic acid.
In a classic experiment, biochemist Leslie Orgel placed
H2O
Heating a short nucleic acid sequence into a reaction vessel and
source then added individual chemically modified nucleotides. The
nucleotides spontaneously joined into a polymer, forming the
RESULTS As the experiment proceeded, reddish material sequence complementary to the nucleic acid already present
accumulated on the walls of the flask. Analysis showed that the (Fig. 2.29b).
brown matter included a number of amino acids. Such experiments show that nucleic acids can be
synthesized experimentally from nucleotides, but until
CONCLUSION Amino acids can be generated in conditions that
recently the synthesis of nucleotides themselves presented a
mimic those of the early Earth.
formidable problem for research on the origins of life. Many
FOLLOW-UP WORK Recent analysis of the original extracts, tried to generate nucleotides from their sugar, base, and
saved by Miller, shows that the experiment actually produced phosphate components, but no one succeeded until 2009.
about 20 different amino acids, not all of them found in That year, John Sutherland and his colleagues showed that
organisms. nucleotides can be synthesized under conditions thought to
be like those on the young Earth. These chemists showed how
SOURCES Miller, S. L. 1953. “Production of Amino Acids Under Possible simple organic molecules likely to have formed in abundance
Primitive Earth Conditions.” Science 117:528–529; Johnson, A. P., et al. 2008.
“The Miller Volcanic Spark Discharge Experiment.” Science 322:404.
on the early Earth react in the presence of phosphate
molecules, yielding the long-sought nucleotides.

46
 CHAPTER 2 THE MOLECULES OF LIFE 47

Such humble beginnings eventually

gave rise to the abundant diversity of life FIG. 2.29 Spontaneous polymerization of nucleotides. (a) Clays may have played an
we see around us, described memorably by important role in the origin of life by providing surfaces for nucleotides to form
Charles Darwin in the final paragraph of On nucleic acids. (b) Addition of a short RNA molecule to a flask containing modified
the Origin of Species: nucleotides results in the formation of a complementary RNA strand. Source: a. Ray L
Frost, Professor of Physical Chemistry, Queensland University of Technology. Australia.
It is interesting to contemplate an entangled
a. b.
bank, clothed with many plants of many
5’ 3’
kinds, with birds singing on the bushes, with
C C C G C C C G C C C G C C C Template
various insects flitting about, and with worms strand
C G G
crawling through the damp earth, and to reflect G C
Free
that these elaborately constructed forms, so nucleotide
different from each other, and dependent on monomer
each other in so complex a manner, have all
been produced by laws acting around us. . . . 5’ 3’
There is grandeur in this view of life . . . from C C C G C C C G C C C G C C C
so simple a beginning endless forms most G G C G G G
G 3’ 5’
beautiful and most wonderful have been, and
are being, evolved. •
5’ 3’
C C C G C C C G C C C G C C C
GG G C G G G C G G G C G G G New strand
3’ 5’

Core Concepts Summary

2.1 The atom is the fundamental unit of matter. atom to combine with other atoms to form molecules.
page 33
Atoms consist of positively charged protons and electrically
neutral neutrons in the nucleus, as well as negatively charged A covalent bond results from the sharing of electrons
electrons moving around the nucleus. page 30 between atoms to form a molecular orbital. page 33

The number of protons determines the identity of A polar covalent bond results when two atoms do not
an atom. page 30 share electrons equally as a result of a difference in the
ability of the atoms to attract electrons, a property called
The number of protons and neutrons together determines
electronegativity. page 33
the mass of an atom. page 30
An ionic bond results from the attraction of oppositely
The number of protons versus the number of electrons
charged ions. page 34
determines the charge of an atom. page 30

Negatively charged electrons travel around the nucleus in 2.3 Water is abundant and essential for life.
regions called orbitals. page 30
Water is a polar molecule because shared electrons are
The periodic table of the elements reflects a regular distributed asymmetrically between the oxygen and
and repeating pattern in the chemical behavior of hydrogen atoms. page 35
elements. page 31 Hydrophilic molecules dissolve readily in water, whereas
hydrophobic molecules in water tend to associate with one
2.2 Atoms can combine to form molecules linked another, minimizing their contact with water. page 35
by chemical bonds.
A hydrogen bond results when a hydrogen atom covalently
Valence electrons occupy the outermost energy level bonded to an electronegative atom interacts with an
(shell) of an atom and determine the ability of an electronegative atom of another molecule. page 35
48 SSEECLT
F I-O
ANS S1E.S1S MTEH
NET S C I E N T I F I C M E T H O D

Water forms hydrogen bonds, which help explain its The tight packing of fatty acids in lipids is the result of van
high cohesion, surface tension, and resistance to rapid der Waals forces, a type of weak, noncovalent bond.
temperature change. page 36 page 44

The pH of an aqueous solution is a measure of the acidity of

2.6 Life likely originated on Earth by a set of chemical
the solution. page 37
reactions that gave rise to the molecules of life.
2.4 Carbon is the backbone of organic molecules. In 1953, Stanley Miller and Harold Urey demonstrated that
amino acids can be generated in the laboratory in conditions
A carbon atom can form up to four covalent bonds with
that mimic those of the early Earth. page 45
other atoms. page 38
Other experiments have shown that sugars, bases, and lipids
The geometry of these covalent bonds helps explain the
can be generated in the laboratory. page 46
structural and functional diversity of organic molecules.
page 38 Once the building blocks were synthesized, they could join
together in the presence of clay minerals to form polymers.
2.5 Organic molecules include proteins, nucleic acids, page 46
carbohydrates, and lipids, each of which is built from
simpler units.
Amino acids are linked by covalent bonds to form proteins.
Self-Assessment
page 40 1. Name and describe the components of an atom.
An amino acid consists of a carbon atom (the a carbon) 2. Explain how the periodic table of the elements is organized.
attached to a carboxyl group, an amino group, a hydrogen
3. Differentiate between covalent bonds and generally
atom, and a side chain. page 40
weaker interactions such as polar covalent, hydrogen, and
The side chain determines the properties of an amino acid. ionic bonds.
page 40 4. List three unusual properties of water and explain why
Nucleotides assemble to form nucleic acids, which store these properties make water conducive to life.
and transmit genetic information. page 40 5. List the four most common elements in organic molecules
Nucleotides are composed of a 5-carbon sugar, a nitrogen- and state which common macromolecules always contain all
containing base, and a phosphate group. page 41 four of these elements.
6. List features of carbon that allow it to form diverse
Nucleotides in DNA incorporate the sugar deoxyribose, and
structures.
nucleotides in RNA incorporate the sugar ribose. page 41
7. List essential functions of proteins, nucleic acids,
The bases are pyrimidines (cytosine, thymine, and uracil)
carbohydrates, and lipids.
and purines (guanine and adenine). page 41
8. Describe how diversity is achieved in polymers, using
Sugars are carbohydrates, molecules composed of C, H,
proteins as an example.
and O atoms, usually in the ratio 1:2:1, and are a source of
energy. page 42 9. Sketch the basic structures of amino acids, nucleotides,
monosaccharides, and fatty acids.
Monosaccharides assemble to form disaccharides or longer
polymers called complex carbohydrates. page 42 10. What evidence is there for the hypothesis that life originated
on Earth by the creation and polymerization of small organic
Lipids are hydrophobic. page 43 molecules by natural processes?
Triacylglycerols store energy and are made up of glycerol
and fatty acids. page 43 Log in to to check your answers to the Self-
Fatty acids consist of a linear hydrocarbon chain of variable Assessment questions, and to access additional learning tools.
length with a carboxyl group at one end. page 43

Fatty acids are either saturated (no carbon–carbon double

bonds) or unsaturated (one or more carbon–carbon double
bonds). page 43
 CHAPTER 3

Nucleic
Acids and
Transcription
Core Concepts
3.1 Deoxyribonucleic acid
(DNA) stores and transmits
genetic information.
3.2 DNA is a polymer of
nucleotides and forms a
double helix.
3.3 Transcription is the process
by which RNA is synthesized
from a DNA template.
3.4 The primary transcript
is processed to become
messenger RNA (mRNA).

Pasieka/Science Source.

49
50 SECTION 3.1 M A J O R B I O LO G I C A L F U N C T I O N S O F D N A

So much in biology depends on shape. Take your hand, for example. In this chapter, we examine the structure of DNA in more
You can pick up a pin, text on a smartphone, or touch your pinky to detail and show how its structure is well suited to its biological
your thumb. These activities are made possible by the coordinated function as the carrier and transmitter of genetic information.
movement of dozens of bones, muscles, nerves, and blood vessels
that give shape to your hand. The functional abilities of your hand
emerge from its structure. A causal connection between structure 3.1 MAJOR BIOLOGICAL
and function exists in many molecules, too. Proteins are a good FUNCTIONS OF DNA
example. Composed of long, linear strings of 20 different kinds
of amino acids in various combinations, each protein folds into DNA is the molecule by which hereditary information is
a specific three-dimensional shape due to chemical interactions transmitted from generation to generation. Today the role of DNA
between the amino acids along the chain. The three-dimensional is well known, but at one time hardly any biologist would have bet
structure of the protein determines its functional properties, such on it. Any poll of biologists before about 1950 would have shown
as what other molecules it can bind with, and enables the protein overwhelming support for the idea of proteins as life’s information
to carry out its job in the cell. molecule. Compared with the seemingly monotonous, featureless
Another notable example is the macromolecule structure of DNA, the three-dimensional structures of proteins are
deoxyribonucleic acid (DNA), a linear polymer of four different highly diverse. Proteins carry out most of the essential activities in
subunits. DNA molecules from all cells and organisms have a the life of a cell, and so it seemed logical to assume that they would
very similar three-dimensional structure, reflecting their play a key role in heredity, too. But while proteins do play a role in
shared ancestry. This structure, called a double helix, is heredity, they play a supporting role in looking after the DNA—
composed of two strands coiled around each other to form a rather like the way worker bees are essential in maintaining the
sort of spiral staircase. The banisters of the spiral staircase are queen bee, who alone is able to reproduce.
formed by the linear backbone of the paired strands, and the
steps are formed by the pairing of the subunits at the same level in DNA can transfer biological characteristics from one
each strand. organism to another.
The spiral-staircase structure is common to all cellular DNA The first experiments to demonstrate that molecules can transfer
molecules, and its structure gave immediate clues to its function. genetic information from one organism to another were carried
First, DNA stores information. Some of the information in out in 1928 by Frederick Griffith, working with the bacterium
DNA encodes for proteins that provide structure and do much Streptococcus pneumoniae. This organism causes a variety of
of the work of the cell. Information in DNA is called genetic infections in humans and a deadly form of pneumonia in mice. In
information, and it is organized in the form of genes, as textual the original experiments, illustrated in Fig. 3.1, Griffith studied
information is organized in the form of words. Genes can exist two strains of the bacterium, one a virulent (harmful) strain that
in different forms in different individuals, even within a single caused pneumonia and death when injected into mice, and the
species. Differences in genes can affect the shape of the hand, other a mutant, nonvirulent strain that allowed injected mice to
for example, yielding long or short fingers or extra or missing survive. When the debris of dead virulent cells was mixed with live
fingers. As we will see, it is the order of individual subunits (bases) nonvirulent cells, some of the nonvirulent cells became virulent.
of DNA that accounts for differences in genes. Genes usually Griffith concluded that some type of molecule in the debris carried
have no effect on the organism unless they are “turned on” and the genetic information for virulence, but he did not identify the
their product is made. The turning on of a gene is called gene molecule.
expression. The molecular processes that control whether Experiments carried out in 1944 by Oswald Avery, Colin
gene expression occurs at a given time or in a given cell constitute MacLeod, and Maclyn McCarty showed that the molecule
gene regulation. responsible for the conversion, or transformation, of nonvirulent
Second, DNA transmits genetic information from one cells into virulent cells is DNA (Fig. 3.2). These scientists killed
generation to the next. The transmission of genetic information virulent cells with heat, and then purified the remains to make
from parents to their offspring enables species of organisms to a solution, or preparation. They found that the preparation could
maintain their identity through time. The genetic information carry out transformation. When the preparation was treated
in DNA guides the development of the offspring, ensuring that with enzymes that destroy any trace of protein or RNA, the
parental apple trees give rise to apple seedlings and parental geese transforming ability of the preparation remained. But when the
give rise to goslings. As we will see, determining the double-helical preparation was treated with an enzyme that destroys DNA, the
structure of DNA provided one of the first hints of how genetic transforming ability was lost.
information could be faithfully copied from cell to cell, and from These experiments, along with others, established that DNA
one generation to the next. is the genetic material. Today, transformation is widely used in
HOW DO WE KNOW?
FIG.3.1 and died (Fig. 3.1d). Furthermore, when he isolated bacteria from
the dead mice, they had the appearance of the virulent strain, even

What is the nature of the though he had injected nonvirulent bacteria.

genetic material?
RESULTS
c. d.
Killed Killed
virulent virulent
BACKGROUND In the 1920s, it was not clear what biological molecule bacteria and live
nonvirulent
carries genetic information. Fred Neufeld, a German microbiologist,
bacteria
identified several strains of the bacterium Streptococcus pneumoniae,
one of which was virulent and caused death when injected into mice
(Fig. 3.1a), and another which was nonvirulent and did not cause
illness when injected into mice (Fig. 3.1b).

a. b.
Virulent Nonvirulent
bacteria bacteria

Mouse remains healthy. Mouse dies of pneumonia.

CONCLUSION One strain of bacteria (nonvirulent) can be

transformed into another (virulent) by an unknown molecule from
the virulent cells. In other words, the unknown molecule carries
information that causes virulence.

FOLLOW-UP WORK Griffith’s experiments were followed up by

many researchers, most notably Oswald Avery, Colin MacLeod,
and Maclyn McCarty, who identified DNA as the molecule
Mouse dies of pneumonia. Mouse remains healthy.
responsible for transforming bacteria from one strain to the other
EXPERIMENT Frederick Griffith was also a microbiologist interested (see Fig. 3.2). In addition, the process in which DNA is taken up
in bacterial virulence. He made a puzzling observation. He noted that by cells, called transformation, is now a common technique used in
nonvirulent bacteria do not cause mice to get sick (Fig. 3.1b) and molecular biology.
virulent bacteria that had been killed do not cause mice to get sick SOURCE Griffith, F. 1928. “The Significance of Pneumococcal Types.” Journal of
(Fig. 3.1c), but when the two were mixed, the injected mice got sick Hygiene 27:113–159.

biological research and in the genetic modification of agricultural An unrepaired error in DNA replication results in a mutation,
plants and animals (Chapter 12). which is a change in the genetic information in DNA. A mutation
in DNA causes the genetic difference between virulent and
DNA molecules are copied in the process of nonvirulent Streptococcus pneumoniae. While most mutations in
replication. genes are harmful, rare favorable mutations are essential in the
DNA can serve as the genetic material because it is unique process of evolution because they allow populations of organisms
among cellular molecules in being able to specify exact copies to change through time and adapt to their environment.
of itself. This copying process, known as replication, discussed
in detail in Chapter 12, allows the genetic information from one Genetic information flows from DNA to RNA
DNA molecule to be copied into that of another DNA molecule. to protein.
Faithful replication is critical in that it allows DNA to pass genetic Biologists often say that genetic information in DNA directs the
information from cell to cell and from parent to offspring. The activities in a cell or guides the development of an organism, but
copying must reproduce the sequence of subunits almost exactly these effects of DNA are indirect. Most of the active molecules in
because, as we will see in Chapter 14, mistakes in DNA replication cells and development are proteins, including the enzymes that
that go unrepaired may be harmful to the cell or organism. convert energy into usable forms and the proteins that provide

51
FIG.3.2
H OW DO WE KNOW?
structural support for the cell. DNA acts indirectly
What is the nature of the genetic by specifying the sequence of amino acid subunits of
which each protein is composed, and this sequence in
material? turn determines the three-dimensional structure of
the protein, its chemical properties, and its biological
BACKGROUND Oswald Avery, Colin MacLeod, and Maclyn McCarty also activities.
studied virulence in pneumococcal bacteria. They recognized the significance In specifying the amino acid sequence of proteins,
of Griffith’s experiments (see Fig. 3.1) and wanted to identify the molecule DNA acts through an intermediary molecule
responsible for transforming nonvirulent bacteria into virulent ones. known as ribonucleic acid (RNA), another type of
linear polymer. As we saw in Chapter 1, the flow of
EXPERIMENT Avery, MacLeod, and McCarty prepared an extract from information from DNA to RNA to protein has come
virulent bacteria that could transform nonvirulent bacteria into virulent to be known as the central dogma of molecular
ones. This extract allowed them to perform a series of tests and controlled biology (Fig. 3.3). The central dogma states that
experiments. To identify what caused transformation, they separated the genetic information can be transferred from DNA to
extract into its macromolecular components. The transforming activity RNA to protein. Through the years, some exceptions
remained associated with the DNA; however, the DNA preparation also to this “dogma” have been discovered, including the
contained trace amounts of RNA and protein. They treated this preparation transfer of genetic information from RNA to DNA (as
with enzymes that destroyed one of the three molecules. Their hypothesis was in HIV, which causes AIDS), from RNA to RNA (as in
that transformation would not occur if they destroyed the molecule responsible replication of the genetic material of influenza virus),
for it. and even from protein to protein (in the unusual
Virulent bacteria
RESULTS
(killed) DNA is extracted case of disease-causing molecules called prions).
from heat-killed Nevertheless, the central dogma still conveys the
virulent cells, along
with trace amounts
basic idea that in most cases the flow of information
of RNA and protein. is from DNA to RNA to protein.
The first step in this process is transcription,
in which the genetic information in a molecule of
RNase Protease DNase
DNA is used as a template, or pattern, to generate a
molecule of RNA. The term “transcription” is used
because it emphasizes that both molecules use the
same language of nucleic acids. Transcription is the
first step in gene expression, which is the production
Nonvirulent Nonvirulent Nonvirulent Nonvirulent
bacteria bacteria bacteria bacteria of a functional gene product. The second step in
the readout of genetic information is translation,
in which a molecule of RNA is used as a code for
the sequence of amino acids in a protein. The term
“translation” is used to indicate a change of languages,
Virulent and Virulent and Virulent and Nonvirulent from nucleotides that make up nucleic acids to amino
nonvirulent nonvirulent nonvirulent bacteria only acids that make up proteins.
bacteria bacteria bacteria
The processes of transcription and translation
The untreated Only extracts treated with the enzyme that are regulated, meaning that they do not occur at all
extract can destroys DNA were unable to transform times in all cells, even though all cells in an individual
transform nonvirulent bacteria.
nonvirulent contain the same DNA. Genes are expressed, or
cells into “turned on,” only at certain times and places, and not
virulent cells.
expressed, or “turned off,” at other times and places.
In multicellular organisms, for instance, cells are
CONCLUSION DNA is the molecule responsible for transforming nonvirulent
specialized for certain functions, and these different
bacteria into virulent bacteria. This experiment provided a key piece of
functions depend on which genes are on and which
evidence that DNA is the genetic material.
genes are off in specific cells. Muscle cells express
FOLLOW-UP WORK These experiments were followed up by Alfred Hershey genes that encode for proteins involved in muscle
and Martha Chase, who used a different system to confirm that DNA is the contraction, but these genes are not expressed in
genetic material. skin cells or liver cells, for example. Similarly, during
SOURCE Avery, O., C. MacLeod, and M. McCarty. 1944. “Studies on the Chemical Nature development of a multicellular organism, genes may
of the Substance Inducing Transformation of Pneumococcal Types.” Journal of Experimental
Medicine 79:137–158.

52
 CHAPTER 3 N U C L E I C AC I D S A N D T R A N S C R I P T I O N 53

point in modern biology. The discovery of the structure of DNA

FIG.3.3 The central dogma of molecular biology, which defines opened the door to understanding how genetic information is
the usual flow of information in a cell from DNA to RNA stored, faithfully replicated, and altered by rare mutations.
to protein.
A DNA strand consists of subunits called nucleotides.
In the sixty years since the publication of Watson and Crick’s
DNA paper, we have all become familiar with the iconic double helix
of DNA. The elegant shape of the twisting strands relies on the
structure of DNA’s subunits, called nucleotides. As we saw in
Chapter 2, nucleotides consist of three components: a 5-carbon
Transcription sugar, a base, and one or more phosphate groups (Fig. 3.4).
Each component plays an important role in DNA structure. The
5-carbon sugars and phosphate groups form the backbone of
the molecule, with each sugar linked to the phosphate group of
Protein the neighboring nucleotide. The bases sticking out from the sugar
mRNA give each nucleotide its chemical identity. Each strand of DNA
consists of an enormous number of nucleotides linked one to
Ribosome
the next.
Translation DNA had been discovered 85 years before its three-dimensional
structure was determined, and in the meantime a great deal had
been learned about its chemistry, specifically the chemistry of
Proteins provide structure
and carry out many essential nucleotides. Fig. 3.4 illustrates a nucleotide. In the figure, the
activities in a cell. 5-carbon sugar is indicated by the pentagon, in which four of the
five vertices represent the position of a carbon atom. By convention
the carbon atoms of the sugar ring are numbered clockwise with
be required at certain times but not at others. In this case, the primes (1�, 2�, and so forth, read as “one prime,” “two prime,” and
timing of expression is carefully controlled. so forth). Technically, the sugar in DNA is 2�-deoxyribose because
In prokaryotes, transcription and translation occur in the the chemical group projecting downward from the 2� carbon is a
cytoplasm, but in eukaryotes, the two processes are separated hydrogen atom (–H) rather than a hydroxyl group (–OH), but for
from each other, with transcription occurring in the nucleus our purposes the term deoxyribose will suffice.
and translation in the cytoplasm. The separation of transcription Note in Fig. 3.4 that the phosphate group attached to the
and translation in time and space in eukaryotic cells allows 5� carbon has negative charges on two of its oxygen atoms. These
for additional levels of gene regulation that are not possible in charges are present because at cellular pH (around 7), the free
prokaryotic cells. In spite of this and other differences in the hydroxyl groups attached to the phosphorus atom are ionized by
details of transcription and translation between prokaryotes and the loss of a proton, and hence are negatively charged. It is these
eukaryotes, the processes are sufficiently similar that they must negative charges that make DNA a mild acid, which you will recall
have evolved early in the history of life. from Chapter 2 is a molecule that tends to lose protons to the
aqueous environment.
Each base is attached to the 1� carbon of the sugar and projects
above the sugar ring. A nucleotide normally contains one of four
3.2 CHEMICAL COMPOSITION
AND STRUCTURE OF DNA
FIG. 3.4 Nucleotide structure.
By about 1950, some biologists were convinced by the work of
Griffith, Avery, and others that DNA is the genetic material.
Ionized O-
To serve as the genetic material, DNA would have to be able to hydroxyl 5’ O
replicate itself, undergo rare mutations, replicate mutant forms groups O- P O H2C Base (A, G, T, or C)
as faithfully as the original forms, and direct the synthesis of O 4’ 1’
H H H H
other macromolecules in the cell. How could one molecule do all Phosphate
3’ 2’
this? Part of the answer emerged in 1953, when James D. Watson group
OH H
and Francis H. C. Crick announced a description of the three- Deoxyribose
dimensional structure of DNA. This discovery marked a turning sugar
54 SECTION 3.2 C H E M I C A L CO M P O S I T I O N A N D S T RU C T U R E O F D N A

groups is called a nucleoside monophosphate, diphosphate, or

FIG. 3.5 Bases normally found in DNA. triphosphate, respectively (Fig. 3.6). The nucleoside triphosphates
NH2 O
are particularly important because, as we will see later in this
chapter, they are the molecules that are used to form nucleotide
N N
N NH polymers: DNA and RNA. In addition, nucleoside triphosphates
have other functions in the cell, notably as carriers of chemical
N N NH2
N N energy in the form of ATP and GTP.
Adenine (A) Guanine(G)
Purines DNA is a linear polymer of nucleotides linked by
phosphodiester bonds.
O NH2 Not only were the nucleotide building blocks of DNA known
CH3 before the structure was discovered, it was also known how
NH N they were linked into a polymer. The chemical linkages between
nucleotides in DNA are shown in Fig. 3.7. The characteristic
N O N O
covalent bond that connects one nucleotide to the next is
Thymine (T) Cytosine (C)
Pyrimidines

FIG. 3.7 Nucleotides linked by phosphodiester bonds to form a

kinds of bases, denoted A, G, T, and C (Fig. 3.5). Two of the bases DNA strand.
are double-ring structures known as purines; these are the
bases adenine (A) and guanine (G), shown across the top of the
figure. The other two bases are single-ring structures known as 5’-phosphate
pyrimidines; these are the bases thymine (T) and cytosine (C), group
shown across the bottom. -O
NH2
The combination of sugar and base is known as a nucleoside, -O P O
N
which is shown in simplified form in Fig. 3.6. A nucleoside with O N
A
one or more phosphate groups constitutes a nucleotide. More O
5’ CH2 N N
specifically, a nucleotide with one, two, or three phosphate
H H H H
3’
O H
-O P O O
FIG. 3.6 Nucleosides and nucleotides. A nucleoside is a sugar N
O NH
attached to a base, and nucleotides are nucleosides G
O
with one, two, or three phosphate groups attached. 5’ CH2 N N NH2
A nucleotide is therefore a nucleoside phosphate.
H H H H
HO CH2 3’
Nucleoside Base O H
NH2
-O P O
HO N
O C
O O
P CH2 5’ CH2 N
Nucleoside Base
monophosphate H H
H H
HO 3’
O H
O
Phosphodiester -O P O CH3
P P CH2 bond NH
Nucleoside Base O T
diphosphate O O
5’ CH2 N
HO
H H H H
3’
Nucleoside P P P CH2 OH H
Base
triphosphate 3’-hydroxyl
HO group
 CHAPTER 3 N U C L E I C AC I D S A N D T R A N S C R I P T I O N 55

indicated by the vertical red lines that connect the 3� carbon of one left is the 5� end. Hence, we could say the sequence in Fig. 3.7 is
nucleotide to the 5� carbon of the next nucleotide in line through AGCT, which means 5�-AGCT-3�.
the 5�-phosphate group. This C–O–P–O–C linkage is known as a
phosphodiester bond, which in DNA is a relatively stable bond Cellular DNA molecules take the form of a double
that can withstand stress like heat and substantial changes in pH helix.
that would break weaker bonds. The succession of phosphodiester To the knowledge of the chemical makeup of the nucleotides
bonds traces the backbone of the DNA strand. and their linkages in a DNA strand, Watson and Crick added
The phosphodiester linkages in a DNA strand give it polarity, results from earlier physical studies indicating that DNA is a
which means that one end differs from the other. In Fig. 3.7, the long molecule. They also relied on important information from
nucleotide at the top has a free 5� phosphate, and is known as the X-ray diffraction studies by Rosalind Franklin implying that DNA
5� end of the molecule. The nucleotide at the bottom has a free molecules form a helix with a simple repeating structure. Analysis
3� hydroxyl and is known as the 3� end. The DNA strand in of the pattern of X-rays diffracted from a crystal of a molecule can
Fig. 3.7 has the sequence of bases AGCT from top to bottom, but indicate the arrangement of atoms in the molecules.
because of strand polarity we need to specify which end is which. With these critical pieces of information in hand, Watson
For this strand of DNA, we could say that the base sequence is and Crick set out to build a model of DNA that could account for
5�-AGCT-3� or equivalently 3�-TCGA-5�. When a base sequence is the results of all previous chemical and physical experiments,
stated without specifying the 5� end, by convention the end at the using sheet metal cutouts of the bases and wire ties for the
sugar–phosphate backbone. After many false
starts and much disappointment, they finally
found a structure that worked. They realized
immediately that they had made one of the
FIG. 3.8 Structure of DNA. The DNA double helix can be shown with (a) the atoms as
most important discoveries in all of biology,
solid spheres or (b) the backbones as ribbons.
and that day, February 28, 1953, they lunched
a. b.
at the Eagle, a pub across the street from their
3’ 5’
laboratory, where Crick loudly pronounced,
“We have discovered the secret of life.” The
Sugar–
phosphate Eagle is still there in Cambridge, England, and
backbone sports on its wall a commemorative plaque
marking the table where the two ate.
Why all the fuss (and why the Nobel
Prize nine years later)? First, let’s look at the
structure, and you will see that the structure
itself tells you how DNA carries and transmits
genetic information. The Watson–Crick
34 Å = 3.4 nm Major
(10 base pairs) structure, now often called the double helix,
groove
per complete is shown in Fig. 3.8. Fig. 3.8a is a space-filling
turn
model, in which each atom is represented as
a color-coded sphere. The big surprise of the
Minor structure is that it consists of two DNA strands
Major
groove groove like those in Fig. 3.7, each wrapped around
the other in the form of a helix coiling to the
right, with the sugar–phosphate backbones
winding around the outside of the molecule
Minor and the bases pointing inward. In the double
groove
helix, there are 10 base pairs per complete turn,
and the diameter of the molecule is 2 nm, a
measurement that is hard to relate to everyday
objects, but it might help to know that the cross
section of a bundle of 100,000 DNA molecules
would be about the size of the period at the end
3’ 5’ of this sentence. The outside contours of
the twisted strands form an uneven pair of
Diameter 20 Å = 2 nm
T A G C grooves, called the major groove and the
56 SECTION 3.2 C H E M I C A L CO M P O S I T I O N A N D S T RU C T U R E O F D N A

minor groove. These grooves are important because proteins that

interact with DNA often recognize a particular sequence of bases FIG. 3.9 Base pairing. Adenine pairs with thymine, and guanine
by making contact with the bases via the major or minor groove pairs with cytosine. These base pairs differ in the number
or both. of hydrogen bonds.
Importantly, the individual DNA strands in the double
helix are antiparallel, which means that they run in opposite A and T are held
together by two
directions. That is, the 3� end of one strand is opposite the 5� end of
hydrogen bonds.
the other. In Fig. 3.8a, the strand that starts at the bottom left and
H
coils upward begins with the 3� end and terminates at the top with N
N H O CH3
the 5� end, whereas its partner strand begins with its 5� end at the
bottom and terminates with the 3� end at the top. N N H N
Fig. 3.8b shows a different depiction of double-stranded
Deoxyribose
DNA, called a ribbon model, which clearly shows the sugar– N N
O
phosphate backbones winding around the outside with the bases Deoxyribose
paired between the strands. The ribbon model of the structure Adenine (A) Thymine (T)
closely resembles a spiral staircase, with the backbones forming
the banisters and the base pairs the steps. If the amount of
DNA in a human egg or sperm (3 billion base pairs) were scaled to G and C are held
together by three
the size of a real spiral staircase, it would reach from Earth to hydrogen bonds.
the moon. H
Note that, as shown in Fig. 3.8b, an A in one strand pairs only N
O H N
with a T in the other, and G pairs only with C. Each base pair
contains a purine and a pyrimidine. This precise pairing maintains N N H N
the structure of the double helix since pairing two purines Deoxyribose
N N
would cause the backbones to bulge and pairing two pyrimidines N H O
Deoxyribose
would cause them to narrow. The pairing of one purine with one H
pyrimidine preserves the distance between the backbones along
Guanine (G) Cytosine (C)
the length of the entire molecule.
Because they form specific pairs, the bases A and T are said to
be complementary, as are the bases G and C. The formation of
only A–T and G–C base pairs means that the paired strands in a
double-stranded DNA molecule have different base sequences. The An almost equally important factor contributing to the
strands are paired like this: stability of the double helix is the interactions between bases in
the same strand (Fig. 3.10). This stabilizing force is known as base
5�-AT GC- 3� stacking, and it occurs because the nonpolar, flat surfaces of the
3�- T ACG-5� bases tend to group together away from water molecules, and
hence stack on top of one another as tightly as possible.
Where one strand has the base A, the other strand across the
way has the base T, and where one strand has a G, the other The three-dimensional structure of DNA gave
has a C. In other words, the paired strands are not identical important clues about its functions.
but complementary. Because of the A–T and G–C base pairing, The double helix is a good example of how chemical structure and
knowing the base sequence in one strand immediately tells you biological function come together. The structure of the molecule
the base sequence in its partner strand. itself immediately suggested how genetic information is stored
Why is it that A pairs only with T, and G only with C? Fig. 3.9 in DNA. One of the most important features of DNA structure is
illustrates the answer. The specificity of base pairing is brought that there is no restriction on the sequence of bases along a DNA
about by hydrogen bonds that form between A and T (two strand. Any A, for example, can be followed by another A or a T, C
hydrogen bonds) and between G and C (three hydrogen bonds). A or G. The lack of sequence constraint suggested that the genetic
hydrogen bond in DNA is formed when an electronegative atom information in DNA could be encoded in the sequence of bases
(O or N) in one base shares a hydrogen atom (H) with another along the DNA, much as textual information in a book is stored
electronegative atom in the base across the way. Hydrogen bonds in a sequence of letters of the alphabet. With any of four possible
are relatively weak bonds, typically 5% to 10% of the strength of bases at each nucleotide site, the information-carrying capacity
covalent bonds, and can be disrupted by high pH or heat. However, of a DNA molecule is unimaginable. The number of possible base
in total they contribute to the stability of the DNA double helix. sequences of a DNA molecule only 133 nucleotides in length is
 CHAPTER 3 N U C L E I C AC I D S A N D T R A N S C R I P T I O N 57

what it does, but in the years following it

FIG. 3.10 Interactions stabilizing the double helix. Hydrogen bonds between the bases became clear that the major processes in
in opposite strands and base stacking of bases within a strand contribute to the the readout of genetic information were
stability of the DNA double helix. transcription and translation (see Fig. 3.3).
The sequence of bases along either
5’ strand completely determines that of
HO
3’ the other because wherever one strand
-O P O
H carries an A, the other must carry a T,
O N and wherever one carries a G, the other
O H N OH
O N must carry a C. The complementary base
CH2 G N H N C
N N sequences of the strands means that, in
N H O
CH2 any double-stranded DNA molecule, the
H O
total number of A bases must equal that
O of T, and the total number of G bases
-O P O must equal that of C. These equalities are
O
-O P O O often written in terms of the percent (%)
N
H of each base in double-stranded DNA. In
O N H O CH3
these terms, the base-pairing rules imply
CH2 O N A N H N T that %A 5 %T and %G 5 %C. Interestingly,
N N these equalities were known and described
O
O CH2 by the American biochemist Erwin
Chargaff before the double helix was
Base- O
stacking -O discovered, and they provided important
O attraction P O
clues to the structure of DNA. It was the
-O O
P O double helix that finally showed why they
H
O H 3C O H N
N are observed.

CH2 O T N H N A N j Quick Check 1 The letter R is

N N conventionally used to represent any
O
O CH2 purine base (A or G) and Y to represent any
pyrimidine base (T or C). In double-stranded
O
DNA, what is the relation between %R
-O O
O P and %Y?
-O P O O
H
The complementary, double-stranded
O N H O
N nature of the double helix also suggested
O N
a mechanism by which DNA replication
CH2 C N H N G
N N could take place. In fact, in their 1953
O H N paper describing the structure of DNA,
H O CH2
Watson and Crick wrote, “It has not
OH Hydrogen
O escaped our notice that the specific pairing
bonding
Sugar–phosphate backbone -O P O we have postulated immediately suggests
3’ a possible copying mechanism for the
OH
5’ genetic material.”
A simplified outline of DNA
replication is shown in Fig. 3.11. In brief,
the two strands of a parental double helix
equal to the estimated number of electrons, protons, and neutrons unwind, and as they do each of the parental strands serves as a
in the entire universe! This is the secret of how DNA can carry the template for the synthesis of a complementary daughter strand.
genetic information for so many different types of organisms, and When the process is complete, there are two molecules, each of
how variation in DNA sequence even within a single species can which is identical in sequence to the original molecule, except
underlie genetic differences among individuals. possibly for rare errors (mutations) that cause one base pair to
Watson and Crick’s model still left many questions be replaced with another. Although the process of replication as
unanswered in regard to how the information is read out and depicted in Fig. 3.11 looks exceedingly simple and straightforward,
58 SECTION 3.3 R E T R I E VA L O F G E N E T I C I N F O R M AT I O N S TO R E D I N D N A : T R A N S C R I P T I O N

then relieve the strain and help to preserve the 10 base pairs per
FIG. 3.11 DNA replication.The structure of the double helix gave turn in the double helix.
an important clue to how it replicates. In eukaryotic cells, most DNA molecules in the nucleus are
5’ linear, and each individual molecule forms one chromosome.
There is a packaging problem here, too, which you can appreciate
by considering that the length of the DNA molecule contained in
Parental strands serve
as the templates for a single human chromosome is roughly 6000 times greater than
3’
the daughter strands. the average diameter of the cell nucleus. Double-stranded DNA
molecules in eukaryotes are usually packaged with proteins called
3’
5’ histones, and others, to form a complex of DNA and proteins
referred to as chromatin (Chapter 13).
Parental Daughter
strands strands Histone proteins are found in all eukaryotes, and they interact
5’ 3’ with double-stranded DNA without regard to sequence. The
reason for this ability is that these proteins are evolutionarily
conserved, which means that they are very similar in sequence
5’
from one organism to the next. Conserved DNA, RNA, or protein
sequences indicate that they serve an essential function and
therefore have not changed very much over long stretches of
3’
evolutionary time. The more distantly related two organisms are
that share conserved sequences, the more highly conserved the
sequence is.
there are technical details that make the actual process more
complex. These are discussed in Chapter 12.

Cellular DNA is coiled and packaged with proteins. 3.3 RETRIEVAL OF GENETIC
The DNA molecules inside cells are highly convoluted. They have INFORMATION STORED IN
to be because DNA molecules in cells have a length far greater DNA: TRANSCRIPTION
than the diameter of the cell itself. The DNA of a bacterium
known as Mycoplasma, for example, if stretched to its full linear Although the three-dimensional structure of DNA gave important
extent would be about 1000 times longer than the diameter of clues about how DNA stores and transmits information, it left
the bacterial cell. Many of the double-stranded DNA molecules open many questions about how the genetic information in DNA
in prokaryotic cells are circular and form supercoils in which is read out to control cellular processes. In 1953, when the double
the circular molecule coils upon itself, much like what happens helix was discovered, virtually nothing was known about these
to a rubber band when you twist it between your thumb and processes. Within a few years, however, evidence was already
forefinger (Fig. 3.12). Supercoiling is caused by enzymes called accumulating that DNA carries the genetic information for proteins,
topoisomerases that cleave, partially unwind, and reattach a and that proteins are synthesized on particles called ribosomes.
DNA strand, which puts strain on the DNA double helix. Supercoils But in eukaryotes, DNA is located in the nucleus and ribosomes are

FIG. 3.12 Supercoils. A highly twisted rubber band forms coils of coils (supercoils), much as a circular DNA molecule does when it contains too
many base pairs per helical turn.

Supercoil Coil
 CHAPTER 3 N U C L E I C AC I D S A N D T R A N S C R I P T I O N 59

located in the cytoplasm. There must therefore be an intermediary catalyzed the reaction more efficiently, and after only a few dozen
molecule by which the genetic information is transferred from rounds of the procedure, very efficient RNA catalysts had evolved.
the DNA in the nucleus to ribosomes in the cytoplasm, and some Experiments such as this one suggest that RNA molecules can
researchers began to suspect that this intermediary was another evolve over time and act as catalysts. Therefore, many scientists
type of nucleic acid called ribonucleic acid (RNA). believe that RNA, with its dual functions of information storage
The hypothesis of an RNA intermediary that carries genetic and catalysis, was a key molecule in the very first forms of life.
information from DNA to the ribosomes was supported by a clever If RNA played a key role in the origin of life, why do cells now
experiment carried out in 1961 by Sydney Brenner, François Jacob, use DNA for information storage and proteins to carry out other
and Matthew Meselson. They used the virus T2, which infects cells cellular processes? RNA is much less stable than DNA, and proteins
of the bacterium Escherichia coli and hijacks the cellular machinery are more versatile, so a plausible explanation is that life evolved
to produce viral proteins. The researchers found that while T2 from an RNA-based world to one in which DNA, RNA, and proteins
DNA never associates with bacterial ribosomes, the infected cells are specialized for different functions.
produce a burst of RNA molecules shortly after infection and
before viral proteins are made. This finding and others suggested RNA is a polymer of nucleotides in which the 5-carbon
that RNA retrieves the genetic information stored in DNA for sugar is ribose
use in protein synthesis. The transfer of genetic information RNA is a polymer of nucleotides linked by phosphodiester bonds
from DNA to RNA constitutes the key step of transcription in the similar to those in DNA (see Fig. 3.7). Each RNA strand therefore
central dogma of molecular biology (see Fig. 3.2). In this section, has a polarity determined by which end of the chain carries the
we examine RNA and the process of transcription. 3� hydroxyl (–OH) and which end carries the 5� phosphate. There
are a number of important differences that distinguish RNA from
DNA, however (Fig. 3.13). First, the sugar in RNA is ribose, which
? CASE 1 THE FIRST CELL: LIFE’S ORIGINS carries a hydroxyl group on the 2� carbon (highlighted in pink
What was the first nucleic acid molecule, and how in Fig. 3.13a). Hydroxyls are reactive functional groups, so the
did it arise? additional hydroxyl group on ribose in part explains why RNA is
RNA is a remarkable molecule. Like DNA, it can store information a less stable molecule than DNA. Second, the base uracil in RNA
in its sequence of nucleotides. In addition, some RNA molecules
can actually act as enzymes that facilitate chemical reactions.
Because RNA has properties of both DNA (information storage)
and proteins (enzymes), many scientists think that RNA, not FIG. 3.13 RNA. RNA differs from DNA in that (a) RNA contains
DNA, was the original information-storage molecule in the earliest the sugar ribose rather than deoxyribose and (b) the base
forms of life on Earth. This idea, sometimes called the RNA world uracil rather than thymine.
hypothesis, is supported by other evidence as well. Notably, as we a.
will see, RNA is involved in key cellular processes, including DNA OH OH
replication, transcription, and translation. Many scientists believe O O
5’ CH2 OH 5’ CH2 OH
that this involvement is a remnant of a time when RNA played a 4’ 4’
Ribose 1’ Deoxyribose 1’
more central role in life’s fundamental processes. H H H H H H H H
Ingenious experiments carried out by Jack W. Szostak and 2’ 2’
collaborators show how RNA could have evolved the ability to 3’ 3’
OH OH OH H
catalyze a simple reaction. A strand of RNA was synthesized in
the laboratory and then replicated many times to produce a large Ribose has a hydroxyl (–OH) group
population of identical RNA molecules. Next, the RNA was exposed where deoxyribose has a hydrogen (–H).
to a chemical that induced random changes in the identity of some
of the nucleotides in these molecules. These random changes were
b.
mutations that created a population of diverse RNA molecules,
Uracil has a hydrogen (–H) where
much in the way that mutation builds genetic variation in cells.
thymine has a methyl (–CH3) group.
Next, all of these RNA molecules were placed into a container,
and those RNA variants that successfully catalyzed a simple O O
reaction—cleaving a strand of RNA, for example, or joining two H CH3
strands together—were isolated, and the cycle was repeated. In H N H N
Uracil Thymine
each round of the experiment, the RNA molecules that functioned (U) (T)
best were retained, replicated, subjected to treatments that O O
induced additional mutations, and then tested for the ability N N
to catalyze the same reaction. With each generation, the RNA Ribose Deoxyribose
60 SECTION 3.3 R E T R I E VA L O F G E N E T I C I N F O R M AT I O N S TO R E D I N D N A : T R A N S C R I P T I O N

replaces thymine in DNA (Fig. 3.13b). The groups that participate j Quick Check 2 A segment of one strand of a double-stranded
in hydrogen bonding (highlighted in pink in Fig. 3.13b) are DNA molecule has the sequence 5�-ACTTTCAGCGAT-3�. What
identical so that uracil pairs with adenine (U–A) just as thymine is the sequence of an RNA molecule synthesized from this DNA
pairs with adenine (T–A). Third, while the 5� end of a DNA strand template?
is typically a monophosphate, the 5� end of an RNA molecule is
typically a triphosphate. Transcription starts at a promoter and ends at a
Two other features that distinguish RNA from DNA are terminator.
physical rather than chemical. One is that RNA molecules are A long DNA molecule typically contains thousands of genes, most
usually much shorter than DNA molecules. A typical RNA of them coding for proteins or RNA molecules with specialized
molecule used in protein synthesis consists of a few thousand functions, and hence thousands of different transcripts are
nucleotides, whereas a typical DNA molecule consists of millions produced. For example, the DNA molecule in the bacterium
or tens of millions of nucleotides. The other major distinction is E. coli has about 4 million base pairs and produces about 4000 RNA
that most RNA molecules in the cell are single stranded, whereas transcripts, most of which code for proteins. A typical map of a
DNA molecules, as we saw, are double stranded. Single-stranded small part of a long DNA molecule is shown in Fig. 3.15. Each green
RNA molecules often form complex three-dimensional structures segment indicates the position where a transcription is initiated,
by folding back upon themselves, which enhances their stability. and each purple segment indicates the position where it ends.
The green segments are promoters, regions of typically a
In transcription, DNA is used as a template to make few hundred base pairs where RNA polymerase and associated
complementary RNA. proteins bind to the DNA duplex. Many eukaryotic and archaeal
Conceptually, the process of transcription is straightforward. As a promoters contain a sequence similar to 5�-TATAAA-3�, which
region of the DNA duplex unwinds, one strand is used as a template is known as a TATA box because the TATA sequence is usually
for the synthesis of an RNA transcript that is complementary present. The first nucleotide to be transcribed is usually positioned
in sequence to the template according to the base-pairing rules, about 25 base pairs from the TATA box, and transcription takes
except that the transcript contains U (uracil) where the template place as the RNA polymerase moves along the template strand in
has an A (Fig. 3.14). The transcript is produced by polymerization the 3�-to-5� direction.
of ribonucleoside triphosphates. The enzyme that carries out the Transcription continues until the RNA polymerase encounters
polymerization is known as RNA polymerase, which acts by adding a sequence known as a terminator (shown in purple in Fig. 3.15).
successive nucleotides to the 3� end of the growing transcript Transcription stops at the terminator, and the transcript is released.
(Fig. 3.14). Only the template strand of DNA is transcribed. Its A long DNA molecule contains the genetic information for
partner, called the nontemplate strand, is not transcribed. hundreds or thousands of genes. For any one gene, usually only one
It is important to keep in mind the direction of growth of the DNA strand is transcribed; however, different genes in the same
RNA transcript and the direction that the DNA template is read. double-stranded DNA molecule can be transcribed from opposite
All nucleic acids are synthesized by addition of nucleotides to the strands. Which strand is transcribed depends on the position of
3� end. That is, they grow in a 5�-to-3� direction, also described the promoter. As shown in Fig. 3.15, promoters in opposite strands
simply as the 3� direction. Just like two strands of DNA in a double result in transcription occurring in opposite directions. The DNA
helix, the DNA template and the RNA strand transcribed from it strand that contains the promoter sequence matters because, as
are antiparallel, meaning the DNA template runs in the opposite noted earlier, transcription can proceed only by successive addition
direction from the RNA (Fig. 3.14). of nucleotides to the 3� end of the transcript.
Transcription does not take place indiscriminately
from promoters but is a regulated process. For genes
called housekeeping genes, whose products are
FIG. 3.14 Transcription.The DNA double helix unwinds for transcription, and
needed at all times in all cells, transcription takes place
usually only one strand, the template strand, is transcribed.
continually. But most genes are transcribed only at
Template strand certain times, under certain conditions, or in certain
5’ cell types. In E. coli, for example, the genes that encode
G A A T C C G T C A C G T T G proteins needed to utilize the sugar lactose (milk sugar)
ACG
G T T A UGCC U U A G G C A G U G are transcribed only when lactose is present in the
3’ C G AA U
A C AG 3’ environment. For such genes, regulation of transcription
T AAC GCC A T TC G T
A T T GC GGT AA GC A T G 5’ RNA transcript often depends on whether the RNA polymerase and
T CG
5’ CC associated proteins are able to bind with the promoter
AA
T TGC
CT T AGG CAG TGC AAC (Chapter 19).
3’ In bacteria, promoter recognition is mediated by
Nontemplate strand a protein called sigma factor, which associates with
 CHAPTER 3 N U C L E I C AC I D S A N D T R A N S C R I P T I O N 61

RNA polymerase and facilitates its binding to specific promoters.

FIG. 3.15 Transcription along a stretch of DNA. A DNA molecule One type of sigma factor is used for transcription of housekeeping
usually contains many genes that are transcribed genes and many others, but there are other sigma factors for
individually and at different times, often from opposite genes whose expression is needed under special environmental
strands. conditions such as lack of nutrients or excess heat.
Promoter recognition in eukaryotes is considerably more
Transcription is initiated at a complicated. Transcription requires the combined action of at
promoter sequence and ends at a
terminator sequence. The transcript
least six proteins known as general transcription factors that
is synthesized in a 5’-to-3’ direction. assemble at the promoter of a gene. Assembly of the general
DNA RNA
transcription factors is necessary for transcription to occur, but
Terminator Promoter
3’ 5’ not sufficient. Also needed is the presence of one or more types
5’ 3’ of transcriptional activator protein, each of which binds to
a specific DNA sequence known as an enhancer (Fig. 3.16).
3’ 5’
Transcriptional activator proteins help control when and in which
5’ 3’
cells transcription of a gene will occur. They are able to bind
Which strand is transcribed can with enhancer DNA sequences in or near the gene, and also bind
differ from one gene to the next.
with proteins that allow transcription to begin. The presence of
the transcriptional activator proteins that bind with enhancers

FIG. 3.16 The eukaryotic transcription complex, composed of many different proteins.

Transcriptional
Enhancer sequences start site
Promoter
3’ 5’ DNA
5’ 3’

Transcriptional activator proteins General

1 General transcription factors bind
transcription
factors
to the promoter, and transcriptional
activator proteins bind to enhancers. 3’ 5’
5’ 3’

2 Through looping of DNA,

5’
3’
Enhancer
transcriptional activator proteins,
sequence
mediator complex, RNA Pol II, and
general transcription factors are
brought into close proximity,
allowing transcription to proceed.
DNA Mediator
complex

5’
3’
Promoter

RNA polymerase
complex (Pol II)

Promoter region
62 SECTION 3.3 R E T R I E VA L O F G E N E T I C I N F O R M AT I O N S TO R E D I N D N A : T R A N S C R I P T I O N

controlling the expression of the gene is

therefore required for transcription of FIG. 3.17 Transcription bubble. Within the polymerase, the two strands of DNA separate and the
any eukaryotic gene to begin (Fig. 3.16). growing RNA strand forms a duplex with the DNA template.
Once transcriptional activator
proteins have bound to enhancer DNA RNA
sequences, they can attract, or recruit, transcript
P
a mediator complex of proteins, P
P
RNA polymerase
which in turn recruits the RNA 5’ complex (Pol II)
polymerase complex to the promoter.
RNA–DNA duplex
Because enhancers can be located Template
almost anywhere in or near a gene, the DNA strand
recruitment of the mediator complex and
3’ 5’
the RNA polymerase complex may require
the DNA to loop around as shown in Fig. 5’ 3’ 3’
3.16. Cells have several different types of
RNA polymerase enzymes, but in both
prokaryotes and eukaryotes all protein-
coding genes are transcribed by just one of
them. In eukaryotes, the RNA polymerase
complex responsible for transcription
of protein-coding genes is called Pol II.
Once the mediator complex and Pol II
complex are in place, transcription begins,
and this process is called transcriptional
initiation. known as elongation. Transcription takes place in a sort of bubble in
which the strands of the DNA duplex are separated and the growing
RNA polymerase adds successive nucleotides to the end of the RNA transcript is paired with the template strand, creating
3� end of the transcript. an RNA–DNA duplex (Fig. 3.17). In bacteria, the total length of the
Once transcriptional initiation takes place, successive transcription bubble is about 14 base pairs, and the length of the
ribonucleotides are added to grow the transcript. This step is RNA–DNA duplex in the bubble is about 8 base pairs.
Details of the polymerization reaction are
shown in Fig. 3.18. The incoming ribonucleoside
triphosphate, shown at the bottom right, is
FIG. 3.18 The polymerization reaction that allows the RNA transcript to be elongated. accepted by the RNA polymerase only if it
undergoes proper base pairing with the base in
Template strand the template DNA strand. In Fig. 3.18, there is
3’
a proper match because U pairs with A. At this
OH RNA transcript 1 Incoming ribonucleotides point, the RNA polymerase orients the oxygen in
are accepted if they correctly
5’ base pair with the template the hydroxyl group at the 3� end of the growing
T A P P P DNA. strand into a position from which it can attack
P
the innermost phosphate of the triphosphate
Direction of
G C HO P chain growth of the incoming ribonucleoside, competing for
P
the covalent bond. The bond connecting the
P
2 The 3’–OH of the growing innermost phosphate to the next is a high-energy
A U HO
P strand attacks the high- phosphate bond, which when cleaved provides
energy phosphate bond of
the incoming ribonucleotide, the energy to drive the reaction that creates the
A HO OH
P providing the energy to drive phosphodiester bond attaching the incoming
5’ 3’ the reaction.
nucleotide to the 3� end of the growing chain. The
term “high-energy” here refers to the amount
Hydrogen U P P of energy released when the phosphate bond is
bonding
P P P broken that can be used to drive other chemical
HO 3 The two phosphates of the reactions.
incoming ribonucleotide are The polymerization reaction releases a
OH released as pyrophosphate.
phosphate–phosphate group (pyrophosphate),
 CHAPTER 3 N U C L E I C AC I D S A N D T R A N S C R I P T I O N 63

FIG. 3.19 The RNA polymerase complex in prokaryotes. This 3.4 FATE OF THE RNA
molecular machine has channels for DNA input and PRIMARY TRANSCRIPT
output, nucleotide input, and RNA output, and features
that disrupt the DNA double helix, stabilize the RNA– The RNA transcript that comes off the template DNA strand is
DNA duplex, and allow the DNA double helix to re-form. known as the primary transcript, and it contains the genetic
information of the gene that was transcribed. For protein-coding
genes, this means that the primary transcript includes the
information needed to direct the ribosome to produce the protein
RNA transcript corresponding to the gene (Chapter 4). The RNA molecule that
combines with the ribosome to direct protein synthesis is known
RNA polymerase
as the messenger RNA (mRNA) because it serves to carry the
acts here
genetic “message” (information) from the DNA to the ribosome.
As we will see in this section, there is a major difference between
prokaryotes and eukaryotes in the manner in which the primary
transcript relates to the mRNA. We will also see that some
DNA already
transcribed Incoming RNA genes do not code for proteins, but for RNA molecules that have
nucleotides functions of their own.

RNA displaced and Messenger RNA carries information for the synthesis
DNA strands rejoin DNA still to
DNA strands be transcribed of a specific protein.
separated In prokaryotes, the relation between the primary transcript and
the mRNA is as simple as can be: The primary transcript is the
mRNA. Even as the 3� end of the primary transcript is still being
synthesized, ribosomes bind with special sequences near its
5� end and begin the process of protein synthesis (Fig. 3.20). This
shown at the lower right in Fig. 3.18, which also has a high-energy intimate connection between transcription and translation can
phosphate bond that is cleaved by another enzyme. Cleavage of take place because prokaryotes have no nuclear envelope
the pyrophosphate molecule makes the polymerization reaction to spatially separate transcription from translation; the two
irreversible, and the next ribonucleoside triphosphate that processes are coupled, which means that they are connected in
complements the template is brought into line. space and time.

j Quick Check 3 What is the consequence for a growing RNA

transcript if an abnormal nucleotide with a 3� H is incorporated
rather than a 3� OH? How about a 2� H rather than a 2� OH?
FIG. 3.20 Fate of the primary transcript for protein-coding genes
in prokaryotes.
The RNA polymerase complex is a molecular machine
that opens, transcribes, and closes duplex DNA.
Transcription does not take place spontaneously. It requires
template DNA, a supply of ribonucleoside triphosphates, and RNA 5’
polymerase, a large multiprotein complex in which transcription
3’
occurs. To illustrate RNA polymerase in action, we consider here Prokaryotic cell
bacterial RNA polymerase because of its relative simplicity. Primary transcript = mRNA
The transcription bubble forms and transcription takes place
3’
inside the polymerase (Fig. 3.19). RNA polymerase contains
Template DNA strand
structural features that separate the DNA strands, allow an
RNA–DNA duplex to form, elongate the transcript nucleotide by 3’
nucleotide, release the finished transcript, and restore the original
5’
DNA double helix. Fig. 3.19 shows how structure and function
come together in the bacterial RNA polymerase.
RNA polymerase is a remarkable molecular machine capable In prokaryotes, transcription
5’ and translation are coupled;
of adding thousands of nucleotides to a transcript before translation begins even before
dissociating from the template. It is also very accurate, with only transcription is completed.
Ribosome starting
about 1 incorrect nucleotide incorporated per 10,000 nucleotides. to synthesize protein
64 SECTION 3.4 FAT E O F T H E R N A P R I M A RY T R A N S C R I P T

Primary transcripts for protein-coding genes in

prokaryotes have another feature not shared with those in FIG. 3.21 Fate of the primary transcript for protein-coding genes
eukaryotes: They often contain the genetic information for the in eukaryotes.
synthesis of two or more different proteins, usually proteins that
Eukaryotic cell
code for successive steps in the biochemical reactions that produce
small molecules needed for growth, or successive steps needed
to break down a small molecule used for nutrients or energy.
Molecules of mRNA that code for multiple proteins are known as
polycistronic mRNA because the term “cistron” was once widely
used to refer to a protein-coding sequence in a gene.
Exon Intron

Primary transcripts in eukaryotes undergo several

DNA
types of chemical modification.
In eukaryotes, the nuclear envelope is a barrier between the 5’ cap Poly(A) tail
processes of transcription and translation. Transcription takes
place in the nucleus, and translation in the cytoplasm. The Primary AAAAA
transcript
separation allows for a complex chemical modification of the (RNA)
primary transcript, known as RNA processing, which converts
the primary transcript into the finished mRNA, which can then be The 5’ end is
translated by the ribosome. modified by a Polyadenylation
special nucleotide adds a poly(A)
RNA processing consists of three principal types of chemical called the 5’ cap. tail to the 3’ end.
modification, illustrated in Fig. 3.21. First, the 5ˇ end of the
primary transcript is modified by the addition of a special mRNA AAAAA
nucleotide attached in an unusual linkage. This addition is
Spliced exons
called the 5ˇ cap, and it consists of a modified nucleotide called
7-methylguanosine. An enzyme attaches the modified nucleotide Introns are excised from
to the 5ˇ end of the primary transcript essentially backward: in the RNA strand and exons
a normal linkage between two nucleotides, the phosphodiester are spliced together.

bond forms between the 5ˇ carbon of one and

the 3ˇ-OH group of the next, but here the cap is
linked to the RNA transcript by a triphosphate
bridge between the 5ˇ carbons of both ribose sugars FIG. 3.22 Structure of the 5ˇ cap on eukaryotic messenger RNA.
(Fig. 3.22). The 5ˇ cap is essential for translation
7-Methylguanosine
because in eukaryotes the ribosome recognizes an 5’ end of
mRNA by its 5ˇ cap. Without the cap, the ribosome 5’-to-5’ RNA transcript
OH OH phosphate NH2
would not attach the mRNA and translation would linkage N
not occur. H H H H N
The second major modification of eukaryotic O
A
H2N N N H2C P P P CH2 N
primary transcripts is polyadenylation, the N
O
addition of a string of about 250 consecutive
HN H H H H
A-bearing ribonucleotides to the 3ˇ end, forming a N
poly(A) tail (see Fig. 3.21). Polyadenylation plays O CH3 O OH
an important role in export of the mRNA into The 5’ cap consists of a modified base CH
H2
P Base
the cytoplasm. In addition, both the 5ˇ cap and linked by its 5’ carbon to the 5’ end of the
primary transcript by a bridge composed
poly(A) tail help to stabilize the RNA transcript. of three phosphates.
Single-stranded nucleic acids can be unstable and O OH
OH
are even susceptible to enzymes that break them
P CH2
down. In eukaryotes, the 5ˇ cap and poly(A) tail Base
protect the two ends of the transcript and increase
the stability of the RNA transcript until it is O OH
translated in the cytoplasm.
P
Not every stretch of the RNA transcript ends
up being translated into protein. Transcripts in
 CHAPTER 3 N U C L E I C AC I D S A N D T R A N S C R I P T I O N 65

the lariat is released (Fig. 3.23d). The lariat making up the intron is
FIG. 3.23 RNA splicing. quickly broken down into its constituent nucleotides.
a. About 90% of all human genes contain at least one intron.
Primary Exon Intron Although most genes contain 6 to 9 introns, the largest number is
transcript 147, found in a muscle gene. Most introns are just a few thousand
nucleotides in length, but about 10% are longer than 10,000
Spliceosome components nucleotides. The presence of multiple introns in most genes allows
b.
for a process known as alternative splicing, in which primary
transcripts from the same gene can be spliced in different ways to
5’ 3’ yield different mRNAs and therefore different protein products
A site within the intron (Fig. 3.24). More than 80% of human genes are alternatively
attacks the 5’ splice site. spliced. In most cases, the alternatively spliced forms differ in
whether a particular exon is or is not removed from the primary
c. transcript along with its flanking introns. Alternative splicing
allows the same transcript to be processed in diverse ways to
produce mRNA molecules with different combinations of exons
coding for different proteins.

5’ 3’ 3’ j Quick Check 4 When a region of DNA that contains the

genetic information for a protein is isolated from a bacterial cell
The cleaved 5’ splice site attacks and inserted into a eukaryotic cell in a proper position between a
the 3’ splice site. promoter and a terminator, the resulting cell usually produces the
correct protein. But when the experiment is done in the reverse
d. direction (eukaryotic DNA into a bacterial cell), the correct protein
is often not produced. Can you suggest an explanation?

Some RNA transcripts are processed differently

3’ 5’ 3’ from protein-coding transcripts and have functions
of their own.
Lariat quickly breaks down Spliced exons
into individual nucleotides. are adjacent in
Not all primary transcripts are processed into mRNA. Some
the processed RNA transcripts have functions of their own, and many of these
RNA. transcripts are produced by RNA polymerases other than

eukaryotes often contain regions of protein-coding sequence, FIG. 3.24 Alternative splicing. A single primary transcript can be
called exons, interspersed with noncoding regions called introns. spliced in different ways.
The third type of modification of the primary transcript is the
removal of the noncoding introns (see Fig. 3.21). The process of Intron Exon
Gene
intron removal is known as RNA splicing, which is catalyzed by a
complex of RNA and protein known as the spliceosome.
The mechanism of splicing is outlined in Fig. 3.23. In the first
step, the spliceosome brings a specific sequence within the intron Primary
transcription E1 E2 E3 E4
into proximity with the 5� end of the intron, at a site known as
the 5� splice site (Fig. 3.23a). The proximity enables a reaction that
cuts the RNA at the 5� splice site, and the cleaved end of the intron Alternative splicing
connects back on itself forming a loop and tail called a lariat
(Fig. 3.23b). In the next step, the spliceosome brings the E3 E4
mRNA
5� splice site close to the splice site at the 3� end of the intron. The E1 E2 E4 E1 E2 E3
5� splice site attacks the 3� splice site (Fig. 3.23c), cleaving the bond
that holds the lariat on the transcript and attaching the ends of the
exons to each other. The result is that the exons are connected and Protein A Protein B
66 CO R E CO N C E P T S S U M M A RY

Pol II. These primary transcripts undergo different types of RNA * Small, regulatory RNA molecules that can inhibit translation
processing, and their processed forms include such important or cause destruction of an RNA transcript. Two major types
noncoding RNA types as: of small regulatory RNA are known as microRNA (miRNA)
and small interfering RNA (siRNA).
* Ribosomal RNA (rRNA), found in all ribosomes that aid in
translation. In eukaryotic cells, the genes and transcripts
By far, the most abundant transcripts in mammalian cells
for ribosomal RNA are concentrated in the nucleolus, a
are those for ribosomal RNA and transfer RNA. In a typical
distinct, dense, non–membrane-bound spherical structure
mammalian cell, about 80% of all of the RNA consists of ribosomal
observed within the nucleus.
RNA, and another approximately 10% consists of transfer RNA.
* Transfer RNA (tRNA) that carries individual amino acids for Why are these types of RNA so abundant? The answer is that they
use in translation. are needed in large amounts to synthesize the proteins encoded
in the messenger RNA. The roles of mRNA, tRNA, and rRNA in
* Small nuclear RNA (snRNA), found in eukaryotes and
involved in splicing, polyadenylation, and other processes in
protein synthesis are discussed in the next chapter. •
the nucleus.

Core Concepts Summary Cellular DNA molecules consist of a helical spiral of two
paired, antiparallel strands called a double helix. page 55
3.1 Deoxyribonucleic acid (DNA) stores and
In a DNA double helix, A pairs with T, and G pairs with C.
transmits genetic information.
page 56
Experiments carried out by Griffith in 1928 demonstrated
The structure of DNA relates to its function. Information is
that bacteria can transmit genetic information from one
coded in the sequence of bases, and the structure suggests
strain to another. page50
a mechanism for replication, in which each parental strand
Experiments performed by Avery, MacLeod, and McCarty serves as a template for a daughter strand. page 56
in 1944 showed that DNA is the molecule that transmits
DNA in eukaryotic cells is packaged with evolutionarily
genetic information. page 50
conserved proteins called histones. page 58
DNA is copied in the process of replication. page 51

Ribonucleic acid (RNA) is synthesized from a DNA

3.3 Transcription is the process by which RNA is
template. page 51
synthesized from a DNA template.
RNA, like DNA, is a polymer of nucleotides linked by
The central dogma of molecular biology states that the
phosphodiester bonds. page 59
usual flow of genetic information is from DNA to RNA to
protein. DNA is transcribed to RNA, and RNA is translated Some types of RNA can store genetic information and
to protein. page 51 other types can catalyze chemical reactions. These
characteristics have led to the RNA world hypothesis, the
3.2 DNA is a polymer of nucleotides and forms a idea that RNA played a critical role in the early evolution of
double helix. life on Earth. page 59
A nucleotide consists of a 5-carbon sugar, a phosphate Unlike DNA, RNA incorporates the sugar ribose instead of
group, and a base. page 53 deoxyribose and the base uracil instead of thymine.
page 59
The four bases of DNA are adenine, guanine, cytosine, and
thymine. page 53 RNA is synthesized from one of the two strands of DNA,
called the template strand. page 60
Successive nucleotides are linked by phosphodiester bonds
to form a linear DNA molecule. page 54 RNA is synthesized by RNA polymerase in a 5�-to-
3� direction, starting at a promoter and ending at a
DNA strands have polarity, with a 5�-phosphate group at
terminator in the DNA template. page 60
one end and a 3� hydroxyl group at the other end.
page 54
 CHAPTER 3 N U C L E I C AC I D S A N D T R A N S C R I P T I O N S 6677

RNA polymerase separates the two DNA strands, allows

an RNA–DNA duplex to form, elongates the transcript,
Self-Assessment
releases the transcript, and restores the DNA duplex. 1. Explain how the functions of DNA emerge from the
page 62 structure of its monomers and its antiparallel, double-
helical, three-dimensional structure.
3.4 The primary transcript is processed to become
2. Describe how DNA molecules are replicated.
messenger RNA (mRNA).
3. Explain how the sequence of a molecule of DNA, made
In prokaryotes, the primary transcript is immediately
up of many monomers of only four possible nucleotides,
translated into protein. page 63
can encode the enormous amount of genetic information
In eukaryotes, transcription and translation are separated stored in the chromosomes of living organisms.
in time and space, with transcription occurring in the
4. Describe the usual flow of genetic information in a cell.
nucleus and then translation in the cytoplasm. page 64
5. Name four differences between the structure of DNA and
In eukaryotes, there are three major types of modification
RNA.
to the primary transcript—the addition of a 5� cap,
polyadenylation, and splicing. page 64 6. Describe how a molecule of RNA is synthesized using a
DNA molecule as a template.
The 5� cap is a 7-methylguanosine added to the 5� end of
the transcript. page 64 7. Explain the relationship between RNA structure and
function.
The poly(A) tail is a stretch of adenines added to the 3� end
of the transcript. page 64 8. Name and describe three mechanisms of RNA processing
in eukaryotes, and explain their importance to the cell.
Splicing is the excision of introns from the transcript,
bringing exons together. page 65 9. List three types of noncoding RNA and describe their
functions.
Alternative splicing is a process in which primary
transcripts from the same gene are spliced in different
ways to yield different protein products. page 65
Log in to to check your answers to the Self-
Some RNAs, called noncoding RNAs, do not code for Assessment questions, and to access additional learning tools.
proteins, but instead have functions of their own.
page 65
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 CHAPTER 4

Translation
and Protein
Structure
Core Concepts
4.1 Proteins are linear polymers of
amino acids that form three-
dimensional structures with
specific functions.
4.2 Translation is the process in
which the sequence of bases in
messenger RNA specifies the
order of successive amino acids
in a newly synthesized protein.
4.3 Proteins evolve through
mutation and selection and by
combining functional units.

Pasieka/SPL/Getty Images.

69
70 SECTION 4.1 M O L E C U L A R S T RU C T U R E O F P ROT E I N S

Hardly anything happens in the life of a cell that does not require Amino acids differ in their side chains.
proteins. They are the most versatile of macromolecules, each The general structure of an amino acid was discussed in Chapter 2
with its own built-in ability to carry out a cellular function. Some and is shown again in Fig. 4.1. It consists of a central carbon atom,
proteins aggregate to form relatively stiff filaments that help define called the a (alpha) carbon, connected by covalent bonds to four
the cell’s shape and hold organelles in position. Others span the different chemical groups (Fig. 4.1a): an amino group (–NH2,
cell membrane and form channels or pores through which ions and shown in dark blue), a carboxyl group (–COOH, shown in
small molecules can move. Many others are enzymes that catalyze brown), a hydrogen atom (–H, shown in light blue), and a variable
the thousands of chemical reactions needed to maintain life. Still side chain or R group (shown in green). In the environment of a
others are signaling proteins that enable cells to coordinate their cell, where the pH is in the range 7.35–7.45 (called physiological
internal activities or to communicate with other cells. pH), the amino group gains a proton to become –NH13 and the
Want to see some proteins? Look at the white of an egg. Apart carboxyl group loses a proton to become –COO2 (Fig. 4.1b). The
from the 90% or so that is water, most of what you see is protein. four covalent bonds from the a carbon are at equal angles. As a
The predominant type of protein is ovalbumin. Easy to obtain in result, an amino acid forms a tetrahedron, a pyramid with four
large quantities, ovalbumin was one of the first proteins studied triangular faces (Fig. 4.1c).
by the scientific method (Chapter 1). In the 1830s, ovalbumin The R groups of the amino acids differ from one amino acid
was shown to consist largely of carbon, hydrogen, nitrogen, and to the next. They are what make the “letters” of the amino acid
oxygen. Each molecule of ovalbumin was estimated to contain at “alphabet” distinct from one another. Just as letters differ in their
least 400 carbon atoms. Leading chemists of the time scoffed at shapes and sounds—vowels like E, I, and O and hard consonants
this number, believing that no organic molecule could possibly like B, P, and T—amino acids differ in their chemical and physical
be so large. Little did they know: The number of carbon atoms in properties.
ovalbumin is actually closer to 2000 than to 400! The chemical structures of the 20 amino acids commonly
The reason that proteins can be such large organic molecules found in proteins are shown in Fig. 4.2. The R groups (shown
began to become clear only about a hundred years ago, when in green) are chemically diverse and are grouped according to
scientists hypothesized that proteins are polymers (large their properties, with a particular emphasis on whether they
molecules made up of repeated subunits). Now we know that are hydrophobic or hydrophilic, or have special characteristics
proteins are linear polymers of any combination of 20 amino that might affect a protein’s structure. These properties strongly
acids, each of which differs from the others in its chemical
characteristics. In size, ovalbumin is actually an average protein,
consisting of a chain of 385 amino acids.
In Chapter 3, we discussed how genetic information flows FIG. 4.1 Structure of an amino acid. The central carbon atom is
from the sequence of bases of DNA into the sequence of bases attached to an amino group, a carboxyl group, an R group
in a transcript of RNA. For protein-coding genes, the transcript or side chain, and a hydrogen atom.
is processed into messenger RNA (mRNA). In this chapter, we
a. H b. H
examine how amino acid polymers are assembled by ribosomes
by means of a template of messenger RNA and transfer RNAs.
 carbon
We also discuss how the amino acid sequences of proteins
C C
help determine their three-dimensional structures and diverse Amino +
HN H3N
group 2 COOH COO–
chemical activities, as well as how proteins change through
evolutionary time. Carboxyl
R R
group

c.
4.1 MOLECULAR STRUCTURE
OF PROTEINS
If you think of a protein as analogous to a word in the English
language, then the amino acids are like letters. The comparison
is not altogether fanciful, as there are about as many amino acids
in proteins as letters in the alphabet, and the order of both amino
acids and letters is important. For example, the word PROTEIN has
the same letters as POINTER, but the two words have completely The corners of this structure
different meanings. Similarly, the exact order of amino acids in a form a tetrahedral shape
around the  carbon.
protein determines the protein’s shape and function.
 CHAPTER 4 T R A N S L AT I O N A N D P ROT E I N S T RU C T U R E 71

FIG. 4.2 Structures of the 20 amino acids commonly found in proteins.

HYDROPHOBIC AMINO ACIDS HYDROPHILIC AMINO ACIDS

Polar
– – +
+H N
3 CH COO +H N
3 CH COO H3N CH COO– +H
3N CH COO
– +H N
3 CH COO
–

CH3 CH CH2 CH2 CH2

H3C CH3 CH C NH2 CH2
Alanine Valine H3C CH3
(Ala, A) (Val, V) O C NH2
Leucine O
(Leu, L)
Asparagine Glutamine
(Asn, N) (Gln, Q)
+H N
3 CH COO– +H N
3 CH COO– +H N
3 CH COO–
COO–
+H +H N –
3N CH 3 CH COO
CH CH2 CH2
H2C CH3 CH2 H C CH3
CH2
H3C OH OH
S CH3
Serine Threonine
Isoleucine Methionine Phenylalanine (Ser, S) (Thr, T)
(Ile, I) (Met, M) (Phe, F)
Basic
+H N – +H N CH COO
– +H N
3 CH COO– +H N
3 CH COO– +H
3N CH COO–
3 CH COO 3

CH2 CH2 CH2 CH2

CH2
CH2 CH2 NH+
NH
CH2 +H N CH2 HN
2
+H
OH 3N CH2 H 2N C N H
Tryptophan Tyrosine Lysine Arginine Histidine
(Trp, W) (Tyr, Y) (Lys, K) (Arg, R) (His, H)

Acidic
COO–
+H N +H N –
CH CH COO
SPECIAL AMINO ACIDS 3 3

CH2 CH2
COO– –
+H N
3 CH HN CH COO +H N
3 CH COO–
C CH2
H H2C CH2 CH2 –
O O
C
CH2
SH –
O O
Glycine Proline Cysteine Aspartic acid Glutamic acid
(Gly, G) (Pro, P) (Cys, C) (Asp, D) (Glu, E)

influence how a polypeptide folds, and hence the three- create temporary charges in the interacting molecules,
dimensional shape of the protein. which are then attracted to each other. The tendency for
Hydrophobic amino acids are those that do not readily interact hydrophilic water molecules to interact with each other and for
with water or form hydrogen bonds. Most hydrophobic amino hydrophobic molecules to interact with each other is the very
acids have nonpolar R groups composed of hydrocarbon chains or same tendency that leads to the formation of oil droplets in water.
uncharged carbon rings. Because water molecules in the cell form This is also the reason why most hydrophobic amino acids tend to
hydrogen bonds with each other instead of with the hydrophobic be buried in the interior of folded proteins, where they are kept
R groups, the hydrophobic R groups tend to aggregate with each away from water.
other. Their aggregation is also stabilized by weak van der Waals Amino acids with polar R groups have a permanent charge
forces (Chapter 2), in which asymmetries in electron distribution separation, in which one end of the R group is slightly more
72 SECTION 4.1 M O L E C U L A R S T RU C T U R E O F P ROT E I N S

negatively charged than the other. As we saw in Chapter 2, polar

molecules are hydrophilic, and they tend to form hydrogen bonds FIG. 4.3 Formation of a peptide bond. A peptide bond forms
with each other or with water molecules. between the carboxyl group of one amino acid and the
The R groups of the basic and acidic amino acids are strongly amino group of another.
polar. At the pH of a cell, the R groups of the basic amino acids Amino
gain a proton and become positively charged, whereas those of the group
Carboxyl H
acidic amino acids lose a proton and become negatively charged. NH3+
group
Because the R groups of these amino acids are charged, they are
H
usually located on the outside surface of the folded molecule. C COO– + NH3+ C
R
The charged groups can also form ionic bonds with each other
 carbon
and with other charged molecules in the environment, in which COO–
R
a negatively charged group or molecule bonds with a positively
charged group or molecule. This ability to bind another molecule
of opposite charge is one important way in which proteins can Amino acid Amino acid
associate with each other or with other macromolecules such H2O
as DNA. Amino
The properties of several amino acids are noteworthy because end
NH3+ O
of their effect on protein structure. These amino acids include
glycine, proline, and cysteine. Glycine is different from the other
H H
amino acids because its R group is hydrogen, exactly like the C C Peptide
bond
hydrogen on the other side of the a carbon, and therefore it is
not asymmetric. All of the other amino acids have four different N C
R R
groups attached to the a carbon and are asymmetric. In
addition, glycine is nonpolar and small enough to tuck into COO–
spaces where other R groups would not fit. The small size of H
Carboxyl
glycine’s R group also allows for freer rotation around the C–N end
bond since its R group does not get in the way of the R groups of
neighboring amino acids. Thus, glycine increases the flexibility of Peptide
the polypeptide backbone, which can be important in the folding
of the protein.
Proline is also distinctive, but for a different reason. Note how
its R group is linked back to the amino group. This linkage creates a acid in line, and a molecule of water is released. Note that, in
kink or bend in the polypeptide chain and restricts rotation of the the resulting molecule, the R groups of each amino acid point in
C–N bond, thereby imposing constraints on protein folding in its different directions.
vicinity, an effect the very opposite of glycine’s. The C5O group in the peptide bond is known as a carbonyl
Cysteine makes a special contribution to protein folding group, and the N–H group is an amide group. Note in Fig. 4.3
through its –SH group. When two cysteine side chains in the same that these two groups are on either side of the peptide bond. The
or different polypeptides come into proximity, they can react to electrons of the peptide bond are more attracted to the C5O group
form an S–S disulfide bond, which covalently joins the side chains. than to the NH group because of the greater electronegativity of
Such disulfide bonds are stronger than the ionic interactions of the oxygen atom. The result is that the peptide bond has some
other pairs of amino acid, and form cross-bridges that can connect of the characteristics of a double bond. The peptide bond is shorter
different parts of the same protein or even different proteins. This than a single bond, for example, and it is not free to rotate like
property contributes to the overall structure of single proteins or a single bond. The other bonds are free to rotate around their
combinations of proteins. central axes.
Polymers of amino acids ranging from as few as two to many
Successive amino acids in proteins are connected by hundreds share a chemical feature common to individual amino
peptide bonds. acids: namely, that the ends are chemically distinct from each
Amino acids are linked together to form proteins. Fig. 4.3 shows other. One end, shown at the left in Fig. 4.3, has a free amino
how amino acids in a protein are bonded together. The bond group; this is the amino end of the molecule. The other end
formed between the two amino acids is a peptide bond, shown has a free carboxyl group, which is the carboxyl end of the
in red in Fig. 4.3. In forming the peptide bond, the carboxyl group molecule. More generally, a polymer of amino acids connected by
of one amino acid reacts with the amino group of the next amino peptide bonds is known as a polypeptide. Typical polypeptides
 CHAPTER 4 T R A N S L AT I O N A N D P ROT E I N S T RU C T U R E 73

produced in cells consist of a few hundred amino acids. In human

cells, the shortest polypeptides are about 100 amino acids in FIG. 4.4 Levels of protein structure. The manner in which a
length; the longest is the muscle protein titin, with 34,350 polypeptide folds determines its shape and function.
amino acids. The term protein is often used as a synonym for
polypeptide, especially when the polypeptide chain has folded
The primary
into a stable, three-dimensional conformation. Amino acids that structure is the
are incorporated into a protein are often referred to as amino acid sequence of
residues. In a polypeptide chain at physiological pH, the amino amino acids.
and carboxyl ends are in their charged states of NH ⫹3 and COO–,
respectively. However, for simplicity, we denote the ends as NH2
and COOH.
␣ helix

The sequence of amino acids dictates protein folding,

which determines function. ␤ sheet
Up to this point, we have considered the sequence of amino acids
that make up a protein. This is the first of several levels of protein
structure, illustrated in Fig. 4.4. The sequence of amino acids in
a protein is its primary structure. The sequence of amino acids
ultimately determines how a protein folds. Interactions between
stretches of amino acids in a protein form local secondary The secondary
structures. Longer-range interactions between these secondary structure results
from interactions of
structures in turn support the overall three-dimensional shape nearby amino acids.
of the polypeptide, which is its tertiary structure. Finally,
some proteins are made up of several individual polypeptides
that interact with each other, and the resulting ensemble is the
quaternary structure.
Proteins have a remarkably wide range of functions in the
cell, from serving as structural elements to communicating with
the external environment to accelerating the rate of chemical
The tertiary
reactions. No matter what the function of a protein is, the ability structure is the
to carry out this function depends on the three-dimensional shape three-dimensional
shape of a
of the protein. When fully folded, some proteins contain pockets polypeptide.
with positively or negatively charged side chains at just the right
positions to trap small molecules; others have surfaces that can
bind another protein or a sequence of nucleotides in DNA or RNA;
some form rigid rods for structural support; and still others keep
their hydrophobic side chains away from water molecules by
inserting into the cell membrane.
The sequence of amino acids in a protein (its primary
structure) is usually represented by a series of three-letter or
one-letter abbreviations for the amino acids (abbreviations for
the 20 common amino acids are given in Fig. 4.2). By convention,
the amino acids in a protein are listed in order from left to right,
starting at the amino end and proceeding to the carboxyl end.
The amino and the carboxyl ends are different, so the order
matters. Just as TIPS is not the same word as SPIT, the sequence The quaternary
Thr–Ile–Pro–Ser is not the same polypeptide as Ser–Pro–Ile–Thr. structure results from
interactions of
polypeptide subunits.
Secondary structures result from hydrogen bonding
in the polypeptide backbone.
Hydrogen bonds can form between the carbonyl group in one
peptide bond and the amide group in another, thus allowing
HOW DO WE KNOW?

FIG. 4.5 the proteins. A film or other detector records the pattern as a series
of spots, which is known as a diffraction pattern. The locations
What are the shapes of and intensities of these spots can be used to infer the position and
arrangement of the atoms in the molecule.
proteins? RESULTS The X-ray diffraction pattern for hemoglobin looks like
this:
BACKGROUND The three-dimensional shapes of proteins can be
Source: Courtesy
determined by X-ray crystallography. One of the pioneers in this of William E. From this two-
Royer, University of dimensional pattern,
field was Dorothy Crowfoot Hodgkin, who used this technique Massachusetts Medical researchers can use
to define the structures of cholesterol, vitamin B12, penicillin, and School and Vukica mathematical methods
Srajer, BioCARS, to determine the three-
insulin. She was awarded the Nobel Prize in Chemistry in 1964 for Center for Advanced dimensional shape of
her early work. Max Perutz and John Kendrew shared the Nobel Radiation Sources, The
the protein.
University of Chicago.
Prize in Chemistry in 1962 for defining the structures of myoglobin
and hemoglobin using this method.
FOLLOW-UP WORK Linus Pauling and Robert Corey used X-ray
METHOD X-ray crystallography can be used to determine the
crystallography to determine two types of secondary structures
shape of proteins, as well as other types of molecules. The first
commonly found in proteins—the a helix and the b sheet. Today,
step, which can be challenging, is to make a crystal of the protein
this technique is a common method for determining the shape of
molecules. A crystal is a solid structure in which the atoms of a
proteins.
protein (or any other molecule) are in an ordered and repeating
pattern in three dimensions. Then X-rays are aimed at the crystal
SOURCES Crowfoot, D. 1935. “X-Ray Single Crystal Photographs of Insulin.”
while it is rotated. Some X-rays pass through the crystal, while Nature 135:591–592; Kendrew, J. C., et al. 1958. “A Three-Dimensional Model of
others are scattered in different directions when they hit atoms of the Myoglobin Molecule Obtained by X-Ray Analysis.” Nature 181:662–666.

localized regions of the polypeptide chain to fold. This localized positioned in the folded protein, and how it might interact with
folding is a major contributor to the secondary structure of the other molecules.
protein. In the early 1950s, American structural biologists Linus The other secondary structure that Pauling and Corey found is
Pauling and Robert Corey used X-ray crystallography to study the the b sheet, depicted in Fig. 4.7. In a b sheet, the polypeptide folds
structure of proteins. This technique was pioneered by British back and forth on itself, forming a pleated sheet that is stabilized by
biochemists Dorothy Crowfoot Hodgkin, Max Perutz, and John hydrogen bonds between carbonyl groups in one chain and amide
Kendrew, among others (Fig. 4.5). Pauling and Corey studied groups in the other chain across the way (dashed lines). The R groups
crystals of highly purified proteins and discovered that two types project alternately above and below the plane of the b sheet. b sheets
of secondary structure are found in many different proteins. typically consist of 4 to 10 polypeptide chains aligned side by side,
These are the a (alpha) helix and the b (beta) sheet. Both these with the amides in each chain hydrogen-bonded to the carbonyls on
secondary structures are stabilized by hydrogen bonding along the either side (except for those at the ends of each strand).
polypeptide backbone. b sheets are typically denoted by broad arrows, where the
In a helices, like the one shown in Fig. 4.6, the polypeptide direction of the arrow runs from the amino end of the polypeptide
backbone is twisted tightly in a right-handed coil with 3.6 amino segment to the carboxyl end. In Fig. 4.7, the arrows run in
acids per complete turn. The helix is stabilized by hydrogen bonds opposite directions, and the polypeptide chains are said to be
that form between each amino acid’s carbonyl group (C5O) and antiparallel. b sheets can also be formed by hydrogen bonding
the amide group (N–H) four residues ahead in the sequence, as between polypeptide chains that are parallel (pointing in the same
indicated by the dashed lines in Fig. 4.6. Note that the R groups direction). However, the antiparallel configuration is more stable
project outward from the a helix. The chemical properties of because the carbonyl and amide groups are more favorably aligned
the projecting R groups largely determine where the a helix is for hydrogen bonding.

74
 CHAPTER 4 T R A N S L AT I O N A N D P ROT E I N S T RU C T U R E 75

Tertiary structures result from interactions between

FIG. 4.6 An α helix. Hydrogen bonding between carbonyl and amino acid side chains.
amide groups in the backbone stabilize the helix. The tertiary structure of a protein is the three-dimensional
conformation of a single polypeptide chain, usually made up of
1 several secondary structure elements. The shape of a protein
is defined largely by interactions between the amino acid R
Each carbonyl
groups. By contrast, the formation of secondary structures relies
group in the on interactions in the polypeptide backbone and is relatively
2 backbone forms independent of the R groups. Tertiary structure is determined by
3
a hydrogen bond
O with an amide the spatial distribution of hydrophilic and hydrophobic R groups
group four along the molecule, as well as by different types of chemical bonds
residues away.
H
and interactions (ionic, hydrogen, and van der Waals) that form
4
between various R groups. The amino acids whose R groups form
5
6
bonds with each other may be far apart in the polypeptide chain, but
can end up near each other in the folded protein. Hence, the tertiary
structure usually includes loops or turns in the backbone that allow
7 these R groups to sit near each other in space and for bonds to form.
8
The three-dimensional shapes of proteins can be illustrated
␣ carbon in different ways, as shown in Fig. 4.8. A ball-and-stick model
Carbonyl group (C=O) (Fig. 4.8a) draws attention to the atoms in the amino acid chain.
Amide group (N–H)
A ribbon model (Fig. 4.8b) emphasizes secondary structures, with
R group (side chain)
α helices depicted as twisted ribbons and β sheets as broad arrows.
9
Finally, a space-filling model (Fig. 4.8c) shows the overall shape
10 H atom
and contour of the folded protein.
Single bond
Remember that the folding of a polypeptide chain is
Peptide bond
11 determined by the sequence of amino acids. The primary
Hydrogen bond
structure determines the secondary and tertiary structures.
Furthermore, tertiary structure determines function because it is
the three-dimensional shape of the molecule—the contours and
distribution of charges on the outside of the molecule and the
presence of pockets that might bind with smaller molecules on

FIG. 4.7 A β sheet. Hydrogen bonds between neighboring strands stabilize the structure.

Hydrogen bonds form between

carbonyl groups in one polypeptide
and amide groups in a different part
of the polypeptide.
Adjacent strands can run in
the same direction (parallel),
or in opposite directions
(antiparallel), as shown here.

␣ carbon
H O Carbonyl group (C=O)
Amide group (N–H)
O H R group (side chain)
H atom
Single bond
Peptide bond
Hydrogen bond
76 SECTION 4.1 M O L E C U L A R S T RU C T U R E O F P ROT E I N S

FIG. 4.8 Three ways of showing the structure of the protein tubulin: (a) ball-and-stick model; (b) ribbon model; (c) space-filling model.

a. b. c.

the inside—that enables the protein to serve as structural support, Polypeptide subunits can come together to form
membrane channel, enzyme, or signaling molecule. Fig. 4.9 shows quaternary structures.
the tertiary structure of a bacterial protein that contains a pocket Although many proteins are complete and fully functional as a
in the center in which certain R groups can form hydrogen bonds single polypeptide chain with a tertiary structure, there are many
with a specific small molecule and hold it in place. other proteins that are composed of two or more polypeptide
The principle that structure determines function can be chains or subunits with a tertiary structure that come together
demonstrated by many observations. For example, most proteins to form a higher-order quaternary structure. In the case of a
can be unfolded, or denatured, by chemical treatment or high multi-subunit protein, the activity of the complex depends on the
temperature that disrupts the hydrogen and ionic bonds holding quaternary structure formed by the combination of the various
the tertiary structure together. Under these conditions, the tertiary structures.
proteins lose their functional activity. Similarly, mutant proteins In a protein with quaternary structure, the polypeptide
containing an amino acid that prevents proper folding are often subunits may be identical or different (Fig. 4.10). Fig. 4.10a
inactive or don’t function properly. shows an example of a protein produced by HIV that consists of
two identical polypeptide subunits. By contrast, many proteins,
j Quick Check 1 A mutation leads to a change in one amino acid in
such as hemoglobin (shown in Fig. 410b), are composed of
a protein. The result is that the protein no longer functions properly.
different subunits. In either case, the subunits can influence each
How is this possible?
other in subtle ways and influence their function. For example,
the hemoglobin in red blood cells that carries oxygen has four
subunits. When one of these binds oxygen, a slight change in its
structure is transmitted to the other subunits, making it easier for
FIG. 4.9 Tertiary structure determines function. This bacterial
them to take up oxygen. In this way, oxygen transport from the
protein has a cavity that can bind with a small molecule
lungs to the tissues is improved.
(shown as a ball-and-stick model in the center).

Chaperones help some proteins fold properly.

The amino acid sequence (primary structure) of a protein
determines how it forms its secondary, tertiary, and quaternary
structures. For about 75% of proteins, the folding process takes
place within milliseconds as the molecule is synthesized. Some
proteins fold more slowly, however, and for these molecules
folding is a dangerous business. The longer these polypeptides
remain in a denatured (unfolded) state, the longer their
hydrophobic groups are exposed to other macromolecules in the
crowded cytoplasm. The hydrophobic effect, along with van der
Waals interactions, tends to bring the exposed hydrophobic groups
together, and their inappropriate aggregation may prevent proper
folding. Correctly folded proteins can sometimes unfold because
of elevated temperature, for example, and in the denatured state
they are subject to the same risks of aggregation.
 CHAPTER 4 T R A N S L AT I O N A N D P ROT E I N S T RU C T U R E 77

FIG. 4.10 Quaternary structure. Polypeptide units of proteins may be identical, as in (a) an enzyme from HIV, or different, as in (b) hemoglobin.

a. b.

This enzyme consists of two identical Hemoglobin is made up of four

polypeptide subunits, shown in light subunits: two copies of the
green and dark green. polypeptide depicted in magenta
and two copies of the polypeptide
depicted in blue.

Cells have evolved proteins called chaperones that help shown in Fig. 4.11. What are these needed components? First,
protect slow-folding or denatured proteins until they can attain the cell needs ribosomes, which are complex structures of RNA
their proper three-dimensional structure. Chaperones bind with and protein that bind with mRNA and are the site of translation.
hydrophobic groups and nonpolar R groups to shield them from In prokaryotes, translation occurs as soon as the mRNA comes off
inappropriate aggregation, and in repeated cycles of binding and the DNA template. In eukaryotes, the processes of transcription
release they give the polypeptide time to find its correct shape. and translation are physically separated: Transcription takes
place in the nucleus, and translation takes place in the cytoplasm.
In both eukaryotes and prokaryotes, the ribosome consists
of a small subunit and a large subunit, each composed of 1 to
4.2 TRANSLATION: HOW PROTEINS 3 types of ribosomal RNA and 20 to 50 types of ribosomal protein.
ARE SYNTHESIZED
The three-dimensional structure of a protein determines what
it can do and how it works, and the immense diversity in the
tertiary and quaternary structures among proteins explains FIG. 4.11 The central dogma, showing how information flows
their wide range of functions in cellular processes. Yet it is the from DNA to RNA to protein. Note the large number of
sequence of amino acids along a polypeptide chain—its primary cellular components required for translation.
structure—that governs how the molecule folds into a stable
Messenger RNA
three-dimensional configuration. How is the sequence of amino (mRNA)
DNA
acids specified? It is specified by the sequence of nucleotides in the
DNA, in coded form. The decoding of the information takes Initiation factors
Transcription
place according to the central dogma of molecular biology, Elongation factors
which defines information flow in a cell from DNA to RNA to
Release factors
protein (Fig. 4.11). In transcription, the sequence of bases along RNA
part of a DNA strand is used as a template in the synthesis of Aminoacyl
a complementary sequence of bases in a molecule of RNA, as Translation tRNA
synthetases
described in Chapter 3. In translation, the sequence of bases in Met
Arg

an RNA molecule known as messenger RNA (mRNA) is used to Transfer

Protein RNA
Va
l

specify the order in which successive amino acids are added to a (tRNA) U
U A C C
A
UC U

newly synthesized polypeptide chain.

Ribosome
(ribosomal RNA +
Translation uses many molecules found in all cells. ribosomal
Translation requires many components. Well over 100 genes proteins)
encode components needed for translation, some of which are
78 SECTION 4.2 T R A N S L AT I O N : H O W P ROT E I N S A R E S Y N T H E S I Z E D

While the ribosome establishes the correct reading frame for

FIG. 4.12 Simplified structure of ribosomes in prokaryotes and the codons, the actual translation of each codon in the mRNA
eukaryotes. into one amino acid in the polypeptide is carried out by means
of transfer RNA (tRNA). Transfer RNAs are small RNA molecules
Large subunit
of 70 to 90 nucleotides (Fig. 4.13). Each has a characteristic
self-pairing structure that can be drawn as a cloverleaf, as in Fig.
E P A 4.13a, in which the letters indicate the bases common to all tRNA
molecules, but the actual structure is more like that in Fig. 4.13b.
Three bases in the anticodon loop make up the anticodon; these
are the three nucleotides that undergo base pairing with the
Small subunit
corresponding codon.

Exit Peptidyl Aminoacyl

site site site

The large subunit includes FIG. 4.13 Transfer RNA structure depicted in (a) a cloverleaf
three binding sites for
tRNAs.
configuration and (b) a more realistic three-
dimensional structure.

a. 3’ end
Amino acid
5’ end A attachment
Eukaryotic ribosomes are larger than prokaryotic ribosomes. C
C site
As indicated in Fig. 4.12, the large subunit of the ribosome
A
includes three binding sites for molecules of transfer RNA, which GC
CG
are called the A (aminoacyl) site, the P (peptidyl) site, and the GC
GU T⌿C loop
E (exit) site. AU
DHU loop UA
A major role of the ribosome is to ensure that, when the
UA
mRNA is in place on the ribosome, the sequence in the mRNA U
GACAC
coding for amino acids is read in successive, non-overlapping CUC A UGUG
GAGC
groups of three nucleotides, much as you would read the sentence

THEBIGBOYSAWTHEBADMANRUN CG
CG
AU
Each non-overlapping group of three adjacent nucleotides (like G
A
THE or BIG or BOY in our sentence analogy) constitutes a codon,
and each codon in the mRNA codes for a single amino acid in the Anticodon loop
polypeptide chain.
In the example above, it is clear that the sentence begins with
THE. However, in a long linear mRNA molecule, the ribosome Anticodon
could begin at any nucleotide. As an analogy, if we knew that the
letters THE were the start of the phrase, then we would know b.

immediately how to read

C CA Amino acid
ZWTHEBIGBOYSAWTHEBADMANRUN attachment
site
However, without knowing where to start reading this string
of letters, we could find three ways to break the sentence into
three-letter words:

ZWT HEB IGB OYS AWT HEB ADM ANR UN

Z WTH EBI GBO YSA WTH EBA DMA NRU N
ZW THE BIG BOY SAW THE BAD MAN RUN

The different ways of parsing the string into three-letter words

are known as reading frames. The protein-coding sequence in an
mRNA consists of its sequence of bases, and just as in our sentence
analogy, it can be translated into the correct protein only if it is
Anticodon
translated in the proper reading frame.
 CHAPTER 4 T R A N S L AT I O N A N D P ROT E I N S T RU C T U R E 79

Each tRNA has the nucleotide sequence CCA at its 3� end

(Fig. 4.13), and the 3� hydroxyl of the A is the attachment site for FIG. 4.15 Role of base pairing. Base pairing determines the
the amino acid corresponding to the anticodon. Enzymes called relationship between a three-base triplet in double-
aminoacyl tRNA synthetases connect specific amino acids to stranded DNA to a codon in mRNA, and between a codon
specific tRNA molecules (Fig. 4.14). Therefore, these enzymes are in mRNA to an anticodon in tRNA.
directly responsible for actually translating the codon sequence Nontemplate strand
in a nucleic acid to a specific amino acid in a polypeptide chain. 5’ 3’
Most organisms have one aminoacyl tRNA synthetase for each A T G DNA
T A C
amino acid. The enzyme binds to multiple sites on any tRNA that
3’ 5’
has an anticodon corresponding to the amino acid, and it catalyzes Template strand
formation of the covalent bond between the amino acid and tRNA.
A tRNA that has no amino acid attached is said to be uncharged, Transcription M
and one with its amino acid attached is said to be charged. Amino et
U
acid tRNA synthetases are very accurate and attach the wrong A
C
amino acid far less often than 1 time in 10,000.
Codon
Although the specificity for attaching an amino acid to the A UG
correct tRNA is a property of aminoacyl tRNA synthetase, the 5’ 3’
specificity of DNA–RNA and codon–anticodon interactions result Translation mRNA
from base pairing. Fig. 4.15 shows the relationships for one codon Met Amino acid
in double-stranded DNA, in the corresponding mRNA, and in the Note that the first base
in the codon (5’ end)
codon–anticodon pairing between the mRNA and the tRNA. Note pairs with the last base Charged tRNA
that the first (5�) base in the codon in mRNA pairs with the last (3�) (3’ end) in the anticodon. 3’ 5’ Codon–anticodon
U A C pairing
A UG
5’ 3’

FIG. 4.14 Function of aminoacyl tRNA synthetase enzymes. Aminoacyl tRNA synthetases
attach specific amino acids to tRNAs and are therefore responsible for translating base in the anticodon because, as noted in
the codon sequence in an mRNA into an amino acid sequence in a protein. Chapter 3, nucleic acid strands that undergo
Aminoacyl
base pairing must be antiparallel.
Val
tRNA
Gln Ile synthetase The genetic code shows the
Free amino acids
Each aminoacyl correspondence between codons
tRNA synthetase and amino acids.
Ile binds to one Fig. 4.15 shows how the codon AUG
uncharged tRNA
and its specifies the amino acid methionine (Met)
corresponding by base pairing with the anticodon of a
C A amino acid.
C charged tRNA, denoted tRNAMet. Most
U A G
GUU G codons specify an amino acid according to a
A
Uncharged tRNA U genetic code. This code is sometimes called
the “standard” genetic code because, while
it is used by almost all cells, some minor
Gln
differences are found in a few organisms as
Val There is a specific
enzyme for each well as in mitochondria.
amino acid. The codon at which translation begins is
GUU called the initiation codon, and it is coded by
AUG, which specifies Met. The polypeptide
C A C
is synthesized from the amino end to the
Ile
carboxyl end, and so Met forms the amino
The enzyme attaches end of any polypeptide being synthesized;
the amino acid to the
3’ end of the tRNA. however, in many cases the Met is
U A G cleaved off by an enzyme after synthesis
Charged tRNA is complete. The AUG codon is also used
80 SECTION 4.2 T R A N S L AT I O N : H O W P ROT E I N S A R E S Y N T H E S I Z E D

to specify the incorporation of Met at internal sites within the codon, and the amino acid on that tRNA is attached to the
polypeptide chain. growing chain to become the new carboxyl end of the polypeptide
As is apparent in Fig. 4.15, the AUG codon that initiated chain. This process continues until one of three “stop” codons is
translation is preceded by a region in the mRNA that is not encountered: UAA, UAG, or UGA. (The stop codons are also called
translated. The position of the initiator AUG codon in the termination codons or sometimes nonsense codons.) At this point,
mRNA establishes the reading frame that determines how the the polypeptide is finished and released into the cytosol.
downstream codons (those following the AUG) are to be read. The standard genetic code was deciphered in the 1960s by a
Once the initial Met creates the amino end of a new combination of techniques, but among the most ingenious were
polypeptide chain, the downstream codons are read one by one in chemical methods for making synthetic RNAs of known sequence
non-overlapping groups of three bases. At each step, the ribosome by American biochemist Har Gobind Khorana and his colleagues.
binds to a tRNA with an anticodon that can base pair with the This experiment is illustrated in Fig. 4.16.

HOW DO WE KNOW?
CHAPTER 3 N U C L E I C AC I D S A N D T R A N S C R I P T I O N 8 0
FIG. 4.16

How was the genetic code b.

5’ – U C U C U C U C U C U C U C U C U C U C U C U C U C U C U C – 3’

deciphered? Ser Leu Ser Leu Ser Leu Ser Leu Ser Leu

BACKGROUND The genetic code is the correspondence between CONCLUSION Here again there are three reading frames, but each

three-letter nucleotide codons in RNA and amino acids in a of them has alternating UCU and CUC codons. The researchers
protein. American biochemist Har Gobind Khorana performed key could not deduce from this result whether UCU corresponds to Ser
experiments that helped to crack the code. For this work he shared and CUC to Leu or the other way around; the correct assignment
the Nobel Prize in Physiology or Medicine in 1968 with Robert W. came from experiments using other synthetic mRNA molecules.
Holley and Marshall E. Nirenberg. EXPERIMENT 3 AND RESULTS When a synthetic mRNA

METHOD Khorana and his group made RNAs of known sequence. with repeating UCA was used, three different polypeptides
They then added these synthetic RNAs to a solution containing all were produced—polyserine (Ser), polyhistidine (His), and
of the other components needed for translation. By adjusting the polyisoleucine (Ile).
concentration of magnesium and other factors, the researchers c.
could get the ribosome to initiate synthesis with any codon, even if 5’ – U C A U C A U C A U C A U C A U C A U C A U C A U C A U C A – 3’

not AUG.
Ser Ser Ser Ser Ser Ser Ser Ser Ser Ser
EXPERIMENT 1 AND RESULTS When a synthetic poly(U)
was used as the mRNA, the resulting polypeptide was 5’ – U C A U C A U C A U C A U C A U C A U C A U C A U C A U C A – 3’
polyphenylalanine (Phe–Phe–Phe…):
His His His His His His His His His
a.
5’ – U U U U U U U U U U U U U U U U U U U U U U U U U U U U U U – 3’
5’ – U C A U C A U C A U C A U C A U C A U C A U C A U C A U C A – 3’

Phe Phe Phe Phe Phe Phe Phe Phe Phe Phe
Ile Ile Ile Ile Ile Ile Ile Ile Ile

CONCLUSION The codon UUU corresponds to Phe. The poly(U)

mRNA can be translated in three possible reading frames, CONCLUSION The results do not reveal which of the three reading
depending on which U is the 5� end of the start codon, but in each frames corresponds to which amino acid, but this was sorted out by
of them, all the codons are UUU. studies of other synthetic polymers.
EXPERIMENT 2 AND RESULTS When a synthetic mRNA with SOURCE Khorana, H. G. 1972. “Nucleic Acid Synthesis in the Study of the Genetic

alternating U and C was used, the resulting polypeptide had Code.” In Nobel Lectures, Physiology or Medicine 1963–1970. Amsterdam:
Elsevier.
alternating serine (Ser) and leucine (Leu):
 CHAPTER 4 T R A N S L AT I O N A N D P ROT E I N S T RU C T U R E 81

TABLE 4.1 The standard genetic code.

First Second Third

position position position
(5’ end) (3’ end)

U C A G

UUU Phe UCU Ser UAU Tyr UGU Cys U

F Y C
U UUC Phe UCC Ser UAC Tyr UGC Cys C
UUA Leu UCA Ser S UAA Stop UGA Stop A
UUG Leu L UCG Ser UAG Stop UGG Trp W G

CUU Leu CCU Pro CAU His CGU Arg U

H
CUC Leu CCC Pro CAC His CGC Arg C
C L P R
CUA Leu CCA Pro CAA Gln CGA Arg A
CUG Leu CCG Pro CAG Gln Q CGG Arg G

AUU Ile ACU Thr AAU Asn AGU Ser S U

I N
AUC Ile ACC Thr AAC Asn AGC Ser C
A T
AUA Ile ACA Thr AAA Lys AGA Arg A
K R
AUG Met M ACG Thr AAG Lys AGG Arg G

GUU Val GCU Ala GAU Asp GGU Gly U

D
GUC Val GCC Ala GAC Asp GGC Gly G C
G V A
GUA Val GCA Ala GAA Glu GGA Gly A
GUG Val GCG Ala GAG Glu E GGG Gly G

Nonpolar Polar Basic Acidic Stop codon

The standard genetic code shown in Table 4.1 has 20 amino j Quick Check 2 What polypeptide sequences would you expect
acids specified by 64 codons. Many amino acids are therefore to result from a synthetic mRNA with the repeating sequence
specified by more than one codon, and hence the genetic code is 5�-UUUGGGUUUGGGUUUGGG-3�.
redundant, or degenerate. The redundancy has strong patterns,
however: Translation consists of initiation, elongation,
and termination.
• The redundancy results almost exclusively from the third Translation is usually divided into three separate processes. The
codon position. first is initiation, in which the initiator AUG codon is recognized
and Met is established as the first amino acid in the new
• When an amino acid is specified by two codons, they differ
polypeptide chain. The second process is elongation, in which
either in whether the third position is a U or a C (both
successive amino acids are added one by one to the growing chain.
pyrimidine bases), or in whether the third position is an A or
And the third process is termination, in which the addition of
a G (both purine bases).
amino acids stops and the completed polypeptide chain is released
• When an amino acid is specified by four codons, the identity from the ribosome.
of the third codon position does not matter; it could be U, C, Initiation of translation (Fig. 4.17) requires a number of
A, or G. protein initiation factors that bind to the mRNA. In eukaryotes,
one group of initiation factors binds to the 5� cap that is added to
The chemical basis of these patterns results from two features the mRNA during processing. These recruit a small subunit of the
of translation. First, in many tRNA anticodons the 5� base that ribosome, and other initiation factors bring up a transfer
pairs with the 3� (third) base in the codon is chemically modified RNA charged with Met (Fig. 4.17a). The initiation complex then
into a form that can pair with two or more bases at the third moves along the mRNA until it encounters the first AUG triplet.
position in the codon. Second, in the ribosome, there is less than The position of this AUG establishes the translational reading
perfect alignment between the third position of the codon and frame.
the base that pairs with it in the anticodon, so the requirements When the first AUG codon is encountered, a large ribosomal
for base pairing are somewhat relaxed; this feature of the codon– subunit joins the complex, the initiation factors are released, and
anticodon interaction is referred to as wobble. the next tRNA is ready to join the ribosome (Fig. 4.17b). Note
82 SECTION 4.2 T R A N S L AT I O N : H O W P ROT E I N S A R E S Y N T H E S I Z E D

in Fig. 4.17b that the tRNAMet binds with

FIG. 4.17 Translational initiation and elongation. In eukaryotes, the codon used for the P (peptidyl) site in the ribosome and
initiation is the AUG codon nearest the 5� end of the mRNA. Note that the that the next tRNA in line comes in at the
growing polypeptide chain remains attached to a tRNA throughout translation. A (aminoacyl) site. Once the new tRNA is in
place, a reaction takes place in which the bond
a.
connecting the Met to its tRNA is transferred
Met
to the amino group of the next amino acid in
Initiation factors line, forming a peptide bond between the two
recruit the small amino acids (Fig. 4.17c). An RNA in the large
UA C ribosomal subunit
A UG and tRNAMet and subunit is the actual catalyst for this reaction.
scan the mRNA for The new polypeptide is now attached to the
an AUG codon.
tRNA in the A site. The ribosome then shifts
one codon to the right (Fig. 4.17d). With this
b. move, the uncharged tRNAMet shifts to the
Val E site and is released into the cytoplasm, and
When the complex
Met reaches an AUG, the peptide-bearing tRNA shifts to the P site;
CA
the large ribosomal the movement of the ribosome also empties the
E P A U subunit joins, the A site, making it available for the next charged
initiation factors are
UA C released, and a tRNA tRNA in line to come into place.
AU GG U A complementary to At this point, steps (d) and (e) in Fig. 4.17
the next codon binds
to the A site. are repeated over and over again in the process
of elongation, during which the polypeptide
chain grows in length by the addition of
c.
successive amino acids. Ribosome movement
Me
t along the mRNA and formation of the peptide
Val bonds require energy, which is obtained
A reaction transfers
E the Met to the amino with the help of proteins called elongation
acid on the tRNA factors. Elongation factors are bound to GTP
UA C C A U in the A site, forming
AUG GU A a peptide bond. molecules, and break their high-energy bonds
to provide energy for the elongation of the
polypeptide.
The elongation process shown in
d.
Fig. 4.17 continues until one of the stop codons
The ribosome moves (UAA, UAG, or UGA) is encountered; these
down one codon,
Me Arg which puts the amino codons signal termination of polypeptide
t
acid carrying the synthesis. Termination takes place because
Val polypeptide into the
C the stop codons do not have corresponding
UA E A UC
U
P site and the now-
uncharged tRNA into tRNA molecules. Rather, when a stop codon is
the E site, where it is encountered, a protein release factor binds to
C A U ejected. A new tRNA
A UGGU A A G A the A site of the ribosome. The release factor
complementary to the
next codon binds to the causes the bond connecting the polypeptide
A site.
to the tRNA to break, creating the carboxyl
terminus of the polypeptide and completing
the chain. Once the finished polypeptide is
e. Me released, the small and large ribosomal subunits
t
Va
l disassociate from the mRNA and from each
Ar
g The polypeptide
transfers to the amino other.
acid on the tRNA in Although elongation and termination are
the A site. The very similar in prokaryotes and in eukaryotes,
UG G C UU polypeptide is
AUG GUA elongated by translation initiation differs between the
repeating steps (d) two (Fig. 4.18). In eukaryotes, the initiation
and (e).
complex forms at the 5� cap and scans along
the mRNA until the first AUG is encountered
 CHAPTER 4 T R A N S L AT I O N A N D P ROT E I N S T RU C T U R E 83

FIG. 4.18 Initiation in eukaryotes and in prokaryotes. (a) In eukaryotes, translation is initiated only at the 5� cap. (b) In prokaryotes, initiation
takes place at any Shine–Dalgarno sequence; this mechanism allows a single mRNA to include coding sequences for multiple
polypeptides.

a. Monocistronic mRNA b. Polycistronic mRNA

Initiation in eukaryotes Initiation in prokaryotes is at any Shine–Dalgarno

is at the 5’ cap, and the first sequence; the mRNA can therefore be a polycistronic
AUG is the start codon. mRNA that codes for several polypeptides.
Shine–Dalgarno sequence
(5’-AGGAGGU-3’)
AUG codon Stop codon AUG codon Stop codon
5’ cap 3’ 5’ 3’

NH2 COOH NH2 COOH NH2 COOH NH2 COOH

Peptide

(Fig. 4.18a). In prokaryotes, the mRNA molecules have no 5� cap. ? CASE 1 THE FIRST CELL: LIFE’S ORIGINS
Instead, the initiation complex is formed at one or more internal
How did the genetic code originate?
sequences present in the mRNA known as a Shine–Dalgarno
During transcription and translation, proteins and nucleic
sequence (Figure 4.18b). In E. coli, the Shine–Dalgarno sequence
acids work together to convert the information stored in DNA
is 5�-AGGAGGU-3�, and it is followed by an AUG codon eight
into proteins. If we think about how such a system might
nucleotides farther downstream that serves as an initiation codon
have originated, however, we immediately confront a chicken-
for translation. The ability to initiate translation internally allows
and-egg problem: Cells need nucleic acids to make proteins, but
prokaryotic mRNAs to code for more than one protein. Such an
proteins are required to make nucleic acids. Which came first? In
mRNA is known as a polycistronic mRNA. In Fig. 4.18b, the
Chapter 3, we discussed the special features that make RNA an
polycistronic mRNA codes for three different polypeptide chains,
attractive candidate for both information storage and catalysis
each with its own AUG initiation codon preceded eight nucleotides
in early life. Early in evolutionary history, then, proteins had to
upstream by its own Shine–Dalgarno sequence. Each Shine–
be added to the mix. No one fully understands how they were
Dalgarno sequence can serve as an initiation site for translation, and
incorporated, but researchers are looking closely at tRNA, the
so all three polypeptides can be translated.
molecule involved in the “translating” step of translation.
A polycistronic mRNA results from transcription of a group of
In modern cells, tRNA shuttles amino acids to the ribosome,
functionally related genes located in tandem along the DNA and
but an innovative hypothesis suggests that in early life tRNA-
transcribed as a single unit from one promoter. This type of gene
like molecules might have served a different function. This
organization is known as an operon. Prokaryotes have many of
proposal holds that the early precursors of the ribosome were
their genes organized into operons because the production of a
RNA molecules that facilitated the replication of other RNAs,
polycistronic mRNA allows all the protein products to be expressed
not proteins. In this version of an RNA world, precursors to
together whenever they are needed. Typically, the genes organized
tRNA would have shuttled nucleotides to growing RNA strands.
into operons are those whose products are needed either for
Researchers hypothesize that tRNAs bound to amino acids
successive steps in the synthesis of an essential small molecule,
may have acted as simple catalysts, facilitating more accurate
such as an amino acid, or else for successive steps in the breakdown
RNA synthesis. Through time, amino acids brought into close
of a source of energy, such as a complex carbohydrate.
proximity in the process of building RNA molecules might have
j Quick Check 3 Bacterial DNA containing an operon encoding polymerized to form polypeptide chains. From there, natural
three enzymes is introduced into chromosomal DNA in yeast (a selection would favor the formation of polypeptides that
eukaryote) in such a way that it is properly flanked by a promoter enhanced replication of RNA molecules, bringing proteins into
and a transcriptional terminator. The bacterial DNA is transcribed the chemistry of life.
and the RNA correctly processed, but only the protein nearest the All of the steps in gene expression, including transcription
promoter is produced. Can you suggest why? and translation, are summarized in Fig. 4.19 on the next pages.
V I S UA L S Y N T H E S I S Gene Expression
FIG. 4.19 Integrating concepts from Chapters 3 and 4

Transcription RNA processing

During transcription, RNA polymerase In eukaryotes, the primary RNA transcript
reads a DNA sequence and produces a is modified in the nucleus, creating
complementary, antiparallel strand of RNA. mature mRNA and noncoding RNA.
DNA RNA mRNA

Noncoding RNAs do not encode proteins and

include tRNA and rRNA. These RNAs are
Nucleolus transcribed from DNA found in the nucleolus.

tRNA

DNA During mRNA processing, introns are excised from the

strand while exons are spliced together. The mRNA receives
Gene a 5’ cap and a poly(A) tail, then travels out of the nucleus.

Exon Intron AA
AAA

5’ cap
Primary Spliceosome
RNA
transcript
polymerase
(RNA)

RNA transcription begins at

AAA
a promoter region and AA
continues until a terminator Intron
sequence is encountered. lariat

AA
AAA
RNA
transcript
5’
RNA–DNA Template
mplate
Promoter duplex DNA
A Terminator
region sequence AAAA
A
3’ 5’
3’
5’ 3’

Gene

Polymerase
movement

Nucleus

84
Transport Translation
mRNA travels through Ribosomes translate mRNA in the cytosol,
nuclear pores into the producing polypeptide chains. The resulting
cytoplasm. proteins carry out vital cellular functions.
mRNA Protein

Met
Glu Glu Proteins have many different

p
Pr roles in the cell. They

As

u
o

Le
provide structural support,
Ser

Val
Thr Arg Growing act as enzymes that facilitate
polypeptide chemical reactions, and are

Phe
chain involved in cell signaling and
communication.
During elongation, the

Ala
appropriate tRNA anticodon
matches with the available
Me codon on the mRNA, bringing
t
Gly the next amino acid to the
Va polypeptide chain.
l

Ar
g Growing
Glu actin
tRNA Thr
filament
U Incoming
UC E P A tRNA charged
CU with amino acid
site site U
Ribosome UGG Anticodon
AGA A C C GA A
5’ 3’
Codon mRNA

Ribosome moves
along mRNA in a
5’ to 3’ direction.
Actin
protein

A ribosome attaches to the mRNA

and scans the transcript for an AUG
start codon. The ribosome initiates
translation at that point and
synthesizes a polypeptide chain
based on the sequence of the
mRNA until it reaches a stop codon.

Ribosomes

Cytosol Extracellular fluid

85
86 SECTION 4.3 P ROT E I N E VO L U T I O N A N D T H E O R I G I N O F N E W P ROT E I N S

folding domain called a TIM barrel (Fig. 4.20a) is named after the
4.3 PROTEIN EVOLUTION AND THE enzyme triose phosphate isomerase, in which it is a prominent
ORIGIN OF NEW PROTEINS feature. The TIM barrel consists of alternating a helices and
parallel b sheets connected by loops. In many enzymes with a
The amino acid sequences of more than a million proteins are TIM barrel, the active site is formed by the loops at the carboxyl
known, and the particular three-dimensional structure has been ends of the sheets. Fig. 4.20b is a b barrel formed from antiparallel
determined for each of more than 10,000 proteins. While few of b sheets. b barrel structures occur in proteins in some types of
the sequences and structures are identical, many are sufficiently bacteria, usually in proteins that span the cell membrane, where
similar that the proteins can be grouped into about 25,000 the b barrel provides a channel that binds hydrophilic molecules.
protein families. A protein family is a group of structurally and The number of known folding domains is only about 2500,
functionally related proteins as a result of shared evolutionary which is far fewer than the number of protein families. The reason
history. for the discrepancy is that different protein families contain
Why are there not more types of proteins? The number different combinations of folding domains. Modern protein
of possible sequences is unimaginably large. For example, for families are composed of different combinations of a number of
a polypeptide of only 62 amino acids, there are 2062 possible folding domains, each of which contributes some structural or
sequences (because each of the 62 positions could be occupied by functional feature of the protein. Different types of protein folds
any of the 20 amino acids). The number 2062 equals approximately occur again and again in different contexts and combinations. The
1080; this number is also the estimated total number of electrons, earliest proteins may have been little more than single folding
protons, and neutrons in the entire universe! So why are there so domains that could aggregate to form more complex functional
few protein families? The most likely answer is that the chance units. As life evolved, the proteins became longer by joining the
that any random sequence of amino acids would fold into a stable DNA coding for the individual folding units together into a single
configuration and carry out some useful function in the cell is very molecule.
close to zero. For example, human tissue plasminogen activator, a protein
that is used in treating strokes and heart attacks because it
Most proteins are composed of modular folding dissolves blood clots, contains domains shared with cell-surface
domains. receptors, a domain shared with cellular growth factors, and a
If functional proteins are so unlikely, how could life have evolved? domain that folds into large loops facilitating protein–protein
The answer is that the earliest proteins were probably much interactions. Hence, novel proteins do not always evolve from
shorter than modern proteins and needed only a trace of function. random combinations of amino acids; instead, they often
Only as proteins evolved through billions of years did they become evolve by joining already functional folding domains into novel
progressively longer and more specialized in their functions. Many combinations.
protein families that exist today exhibit small regions of three-
dimensional structure in which the protein folding is similar. Amino acid sequences evolve through mutation and
These regions range in length from 25 to 100 or more amino selection.
acids. A region of a protein that folds in a similar way relatively Another important reason that complex proteins can evolve
independently of the rest of the protein is known as a folding against seemingly long odds is that evolution proceeds stepwise
domain. through the processes of mutation and selection. A mutation
Two examples of folding domains are illustrated in Fig. 4.20. is a change in the sequence of a gene. The process of mutation
Many folding domains are functional units in themselves. The is discussed in Chapter 14, but for now all you need to know is
that mutations affecting proteins occur at random in regard to
their effects on protein function. In protein-coding genes, some
mutations may affect the amino acid sequence; others might
FIG. 4.20 Examples of folding domains: (a) TIM barrel; change the level of protein expression or the time in development
(b) b barrel. or type of cell in which the protein is produced. Here, we will
a. b.
consider only those mutations that change the amino acid
sequence.
By way of analogy, we can use a simple word game. The object
of the game is to change an ordinary English word into another
meaningful English word by changing exactly one letter. Consider
the word GONE. To illustrate “mutations” of the word that are
random with respect to function (that is, random with respect to
whether the change will yield a meaningful new word), we wrote
a computer program that would choose one letter in GONE at
 CHAPTER 4 T R A N S L AT I O N A N D P ROT E I N S T RU C T U R E 87

random and replace it with a different random letter. The first GONE will be gone. In a similar way that one meaningful word
24 “mutants” of GONE are: may replace another, one amino acid sequence may be replaced
with a different one in the course of evolution.
UONE GNNE GONJ GOZE
A real-world example that mirrors the word game is found
GONH GOLE GFNE XONE
in the evolution of resistance of the malaria parasite to the
NONE GKNE GJNE DONE
drug pyrimethamine. This drug inhibits an enzyme known as
GCNE GONB GOIE GGNE
dihydrofolate reductase, which the parasite needs to survive and
GONI GFNE GPNE GENE
reproduce inside red blood cells. Resistance to pyrimethamine is
BONE GOWE OONE GYNE
known to have evolved through a stepwise sequence of four amino
Most of the mutant words are gibberish, corresponding to acid replacements. In the first replacement, serine (S)
the biological reality that most random amino acid replacements at the 108th amino acid in the polypeptide sequence (position
impair protein function to some extent. On the other hand, 108) was replaced with asparagine (N); then cysteine (C) at
some mutant proteins function just as well as the original, and position 59 was replaced with arginine (R); asparagine (N)
a precious few change function. In the word-game analogy, the at position 51 was then replaced with isoleucine (I); finally,
mutants that can persist correspond to meaningful words, those isoleucine (I) at position 164 was replaced with leucine (L). If we
words shown in red. list the amino acids according to their single-letter abbreviation
In a population of organisms, random mutations are retained in the order of their occurrence in the protein, the evolution of
or eliminated through the process of selection among individuals resistance followed this pathway:
on the basis of their ability to survive and reproduce. This process
was introduced in Chapter 1 and is considered in greater detail in NCSI ➛ NCNI ➛ NRNI ➛ IRNI ➛ IRNL
Chapter 21, but the principle is straightforward. Most mutations where the mutant amino acids are shown in red. Each successive
that impair protein function will be eliminated because, if the amino acid replacement increased the level of resistance so that a
function of the nonmutant protein contributes to survival and greater concentration of drug was needed to treat the disease. The
reproduction, the individuals carrying these mutations will leave quadruple mutant IRNL is resistant to such high levels that the
fewer offspring than others. Mutations that do not impair function drug is no longer useful.
may remain in the population for long periods because their carriers Depicted according to stepwise amino acid replacements, the
survive and reproduce in normal numbers; a mutation of this type analogy between the evolution of pyrimethamine resistance and
has no tendency to either increase or decrease in frequency over the word game is clear. It should also be clear from our earlier
time. In contrast, individuals that carry the occasional mutation discussion that hundreds of other mutations causing amino acid
that improves protein function will reproduce more successfully replacements in the enzyme must have occurred in the parasite
than others. Because of the enhanced reproduction, the mutant during the course of evolution, but only these amino acid changes
gene encoding the improved protein will gradually increase in occurring in this order persisted and increased in frequency
frequency and spread throughout the entire population. because they conferred greater survival and reproduction of the
In the word game, any of the mutants in red may persist in parasite under treatment with the drug.
the population, but suppose that one of them, GENE for example,
is actually superior to GONE (considered more euphonious, j Quick Check 4 What do you think happened to the mutations
perhaps). Then GENE will gradually displace GONE, and eventually that decreased survival or reproduction of the parasites? •

Core Concepts Summary properties of their R groups—hydrophobic, basic, acidic,

polar—and by special structures. page 70
4.1 Proteins are linear polymers of amino acids
Amino acids are connected by peptide bonds to form
that form three-dimensional structures with specific
proteins. page 72
functions.
The primary structure of a protein is its amino acid
An amino acid consists of an a carbon connected by covalent
sequence. The primary structure determines how a protein
bonds to an amino group, a carboxyl group, a hydrogen atom,
folds, which in turn determines how it functions. page 73
and a side chain or R group. page 70
The secondary structure of a protein results from the
There are 20 common amino acids that differ in their
interactions of nearby amino acids. Examples include the
R groups. Amino acids are categorized by the chemical
a helix and b sheet. page 73
88 SELF-ASSESSMENT

The tertiary structure of a protein is its three-dimensional sequence of amino acids will fold properly to carry out a
shape, which results from long-range interactions of amino specific function is very small. page 86
acid R groups. page 75
A region of a protein that folds in a particular way and that
Some proteins are made up of several polypeptide carries out a specific function is called a folding domain.
subunits; this group of subunits is the protein’s quaternary page 86
structure. page 76
Proteins evolve by combining different folding domains.
Chaperones help some proteins fold properly. page 76 page 86

Proteins also evolve by changes in amino acid sequence,

4.2 Translation is the process by which the sequence
which occurs by mutation and selection. page 87
of bases in messenger RNA specifies the order of
successive amino acids in a newly synthesized protein.
Translation requires many cellular components, including Self-Assessment
ribosomes, tRNAs, and proteins. page 77
1. Draw one of the 20 amino acids and label the amino
Ribosomes are composed of a small and a large subunit, group, the carboxyl group, the R group (side chain), and
each consisting of RNA and protein; the large subunit the a carbon.
contains three tRNA-binding sites that play different roles
2. Name four major groups of amino acids, categorized
in translation. page 77
by the properties of their R groups. Explain how the
An mRNA transcript of a gene has three possible reading chemical properties of each group affect protein shape.
frames composed of three-nucleotide codons. page 78
3. Describe how peptide bonds, hydrogen bonds, ionic
tRNAs have an anticodon that base pairs with the codon in bonds, disulfide bridges, and noncovalent interactions
the mRNA and carries a specific amino acid. page 78 (van der Waals forces and the hydrophobic effect) define a
protein’s four levels of structure.
Aminoacyl tRNA synthetases attach specific amino acids
to tRNAs. page 79 4. Explain how the order of amino acids determines the
way in which a protein folds.
The genetic code defines the relationship between the
three-letter codons of nucleic acids and their corresponding 5. Explain the relationship between protein folding and
amino acids. It was deciphered using synthetic RNA protein function.
molecules. page 79
6. Describe the relationship between the template
The genetic code is redundant in that many amino acids strand of DNA, the codons in mRNA, anticodons in tRNA,
are specified by more than one codon. page 81 and amino acids.

Translation consists of three steps: initiation, elongation, 7. Describe the steps of translation initiation,
and termination. page 81 elongation, and termination.

8. Name and describe two ways that proteins can

4.3 Proteins evolve through mutation and selection
acquire new functions in the course of evolution.
and by combining functional units.
A protein family is a group of proteins that are structurally Log in to to check your answers to the Self-
and functionally related. page 86 Assessment questions, and to access additional learning tools.

There are far fewer protein families than the total number
of possible proteins because the probability that a random
 CHAPTER 5

Organizing
Principles
Lipids, Membranes,
and Cell Compartments

Core Concepts
5.1 Cell membranes are
composed of lipids, proteins,
and carbohydrates.
5.2 The plasma membrane is a
selective barrier that controls
the movement of molecules
between the inside and the
outside of the cell.
5.3 Cells can be classified as
prokaryotes or eukaryotes,
which differ in the degree of
internal compartmentalization.
5.4 The endomembrane system is
an interconnected system of
membranes that includes the
nuclear envelope, endoplasmic
reticulum, Golgi apparatus,
lysosomes, vesicles, and plasma
membrane.
5.5 Mitochondria and chloroplasts
are organelles involved in
harnessing energy, and likely
evolved from free-living
prokaryotes.
Dr. Jeremy Burgess/Science Source.

89
90 SECTION 5.1 S T RU C T U R E O F C E L L M E M B R A N E S

“With the discovery of the cell, biologists found their atom.” So in separating a cell from the external environment and defining
stated François Jacob, the French biologist who shared the Nobel structural and functional spaces within cells.
Prize in Physiology or Medicine in 1965. Just as the atom is the
smallest, most basic unit of matter, the cell is the smallest, most
basic unit of living organisms. All organisms, from single-celled 5.1 STRUCTURE OF CELL MEMBRANES
algae to complex multicellular organisms like humans, are made
up of cells. Therefore, essential properties of life, including growth, Cells are defined by membranes. After all, membranes physically
reproduction, and metabolism, must be understood in terms of cell separate cells from their external environment. In addition,
structure and function. membranes define spaces within many cells that allow them to
Cells were first seen sometime around 1665, when the English carry out their diverse functions.
scientist Robert Hooke built a microscope that he used to observe Lipids are the main component of cell membranes. They have
thin sections of dried cork tissue derived from plants. In these properties that allow them to form a barrier in an aqueous (watery)
sections, Hooke observed arrays of small cavities and named environment. Proteins are often embedded in or associated with
them “cells” (Fig. 5.1). Although Hooke was probably looking at the membrane, where they perform important functions such as
the cell walls of empty (rather than living) cells, his observations transporting molecules. Carbohydrates can also be found in cell
nevertheless led to the concept that cells are the fundamental membranes, usually attached to lipids (glycolipids) and proteins
unit of life. Later in the seventeenth century, the Dutch (glycoproteins).
microbiologist Anton van Leeuwenhoek greatly improved the
magnifying power of microscope lenses, enabling him to see and Cell membranes are composed of two layers of lipids.
describe unicellular organisms, including bacteria, protists, and The major types of lipid found in cell membranes are phospholipids,
algae. Today, modern microscopy provides unprecedented detail introduced in Chapter 2. Most phospholipids are made up of a
and a deeper understanding of the inner architecture of cells. glycerol backbone attached to a phosphate group and two fatty
Cells differ in size and shape, but they share many features. acids (Fig. 5.2). The phosphate head group is hydrophilic (“water-
This similarity in the microscopic organization of all living loving”) because it is polar, enabling it to form hydrogen bonds
organisms led to the development in the middle of the nineteenth with water. By contrast, the two fatty acid tails are hydrophobic
century of one of the pillars of modern biology: the cell theory. (“water-fearing”) because they are nonpolar and do not form
Based on the work and ideas of Matthias Schlieden, Theodor hydrogen bonds with water. Molecules with both hydrophilic and
Schwann, Rudolf Virchow, and others, the cell theory states that hydrophobic regions in a single molecule are termed amphipathic.
all organisms are made up of cells, that the cell is the fundamental In an aqueous environment, amphipathic molecules such as
unit of life, and that cells come from preexisting cells. There is no phospholipids behave in an interesting way. They spontaneously
life without cells, and the cell is the smallest unit of life. arrange themselves into various structures in which the polar
This chapter focuses on cells and their internal organization. head groups on the outside interact with water and the nonpolar
We pay particular attention to the key role that membranes play tail groups come together on the inside away from water. This
arrangement results from the tendency of polar
molecules like water to exclude nonpolar molecules
or nonpolar groups of molecules.
FIG. 5.1 The first observation of cells. Robert Hooke used a simple microscope The shape of the structure is determined by
to observe small chambers in a sample of cork tissue that he described as the bulkiness of the head group relative to the
“cells.” Sources: (left) Science Museum/SSPL/The Image Works; (right) Ted Kinsman/ hydrophobic tails. For example, lipids with bulky
Science Source. heads and a single hydrophobic fatty acid tail are
wedge-shaped and pack into spherical structures
Drawing by Hooke Cork tissue
called micelles (Fig. 5.3a). By contrast, lipids with
less bulky head groups and two hydrophobic tails
form a bilayer (Fig. 5.3b). A lipid bilayer is a structure
formed of two layers of lipids in which the hydrophilic
heads are the outside surfaces of the bilayer and the
hydrophobic tails are sandwiched in between, isolated
from contact with the aqueous environment.
The bilayers form closed structures with an inner
space since free edges would expose the hydrophobic
chains to the aqueous environment. This organization
in part explains why bilayers are effective cell
membranes. It also explains why membranes are
self-healing. Small tears in a membrane are rapidly
 CHAPTER 5 O RG A N I Z I N G P R I N C I P L E S : L I P I D S , M E M B R A N E S , A N D C E L L CO M PA RT M E N T S 91

FIG. 5.2 Phospholipid structure. Phospholipids, the major component of cell

? CASE 1 THE FIRST CELL: LIFE’S ORIGINS
membranes, are made up of glycerol attached to a phosphate-containing head How did the first cell membranes
group and two fatty acid tails. They are amphipathic because they have both form?
hydrophilic and hydrophobic domains. What are the consequences of the ability
of phospholipids to form a bilayer when
CH3
ⴙ
placed in water? The bilayer structure forms
H3C N CH3
Choline spontaneously, dependent solely on the
CH2
properties of the phospholipid and without
HYDROPHILIC

CH2
the action of an enzyme, as long as the
O
concentration of free phospholipids is high
O P O ⴚ
Phosphate
enough and the pH of the solution is similar
Polar O
head to that of a cell. The pH is important because
CH2 CH CH2
group
Glycerol
O O
it ensures that the head groups are in their
Phospholipids backbone
have hydrophilic
C O C O
ionized (charged) form and thus suitably
and hydrophobic
components. CH2 CH2
hydrophilic. Thus, if phospholipids are added
CH2 CH2 to a test tube of water at neutral pH, they
CH2 CH2 spontaneously form spherical bilayer structures
CH2 CH2 called liposomes that surround a central
CH2 CH2 space (Fig. 5.3c). As the liposomes form,
CH2 CH2 they may capture macromolecules present in
CH2 CH solution.
HYDROPHOBIC

Fatty
acid
CH2 HC CH2 Such a process may have been at work
chains CH2 H2C CH2 in the early evolution of life on Earth.
CH2 H2C CH2 Experiments show that liposomes can
CH2 H2C CH2
form, break, and re-form in environments
H2C CH3
CH2 like tidal flats that are repeatedly dried and
CH2 flooded with water. The liposomes can even
CH2
grow, incorporating more and more lipids
CH2
from the environment, and capture nucleic
CH2
acids and other molecules in their interiors.
CH3
Depending on their chemical composition,
Phospholipids can be represented in a
variety of ways to emphasize overall early membranes might have been either
structure, different domains, and 3D shape.
leaky or almost impervious to the molecules
of life. Over time, they evolved in such a way
as to allow at least limited molecular traffic
sealed by the spontaneous rearrangement of the lipids surrounding between the environment and cell interior. At some point, new
the damaged region because of the tendency of water to exclude lipids no longer had to be incorporated from the environment.
nonpolar molecules. Instead, proteins guided lipid synthesis within the cell, although

FIG. 5.3 Phospholipid structures. Phospholipids can form (a) micelles, (b) bilayers, or (c) liposomes when placed in water.

a. Micelle b. Bilayer c. Liposome

Polar head group

(hydrophilic)

Nonpolar tails
(hydrophobic)
92 SECTION 5.1 S T RU C T U R E O F C E L L M E M B R A N E S

FIG. 5.4 Saturated and unsaturated fatty acids in phospholipids. The composition of cell membranes affects the tightness of packing.

a. Saturated: stearic acid, CH3(CH2)16COOH b. Unsaturated: oleic acid, CH3(CH2)7CH CH(CH2)7COOH

HO OH
C O O C
H2C CH2
CH2 H2C
H2C CH2
CH2 H2C
H2C Saturated fatty acid chains CH2 Unsaturated fatty acids have
CH2 lack double bonds, H2C one or more double bonds
resulting in phospholipids that introduce kinks in the
H2C with a straight structure CH2 phospholipids, reducing the
CH2 that favors tight packing. HC tightness of packing.
H2C HC
CH2 CH2
H2C
H2C CH2
CH2
H2C H2C CH2
CH2 H2C CH2
H2C
H3C
CH2
H 3C

how this switch to protein-mediated synthesis happened remains mobility. Likewise, saturated fatty acid tails, which have no double
uncertain. bonds, are straight and tightly packed—again reducing mobility
All evidence suggests that membranes formed originally by (Fig. 5.4a). The double bonds in unsaturated fatty acids introduce
straightforward physical processes, but that their composition kinks in the fatty acid tails, reducing the tightness of packing and
and function evolved over time. François Jacob once said that enhancing lipid mobility in the membrane (Fig. 5.4b).
evolution works more like a tinkerer than an engineer, modifying
j Quick Check 1 Most animal fats are solid at room temperature,
already existing materials rather than designing systems from
whereas plant and fish oils tend to be liquid. Both contain fatty
scratch. It seems that the evolution of membranes is no exception
acids. Can you predict which type of fat contains saturated fatty
to this pattern.
acids, and which type contains unsaturated fatty acids?

Cell membranes are dynamic. In addition to phospholipids, cell membranes often contain
Lipids freely associate with one another because of extensive other types of lipid, and these can also influence membrane
van der Waals forces between their fatty acid tails (Chapter 2). fluidity. For example, cholesterol is a major component of
These weak interactions are easily broken and re-formed, so lipid animal cell membranes, representing about 30% by mass of the
molecules are able to move within the plane of the membrane, membrane lipids. Like phospholipids, cholesterol is amphipathic,
sometimes very rapidly: A single phospholipid can move across with both hydrophilic and hydrophobic groups in the same
the entire length of a bacterial cell in less than a second. Lipids can molecule. In cholesterol, the hydrophilic region is simply a
also rapidly rotate around their vertical axis, and individual fatty hydroxyl group (–OH) and the hydrophobic region consists of four
acid chains are able to flex, or bend. As a result, membranes are interconnected carbon rings with an attached hydrocarbon chain
dynamic: they are continually moving, forming, and re-forming (Fig. 5.5). This structure allows cholesterol to insert into the lipid
during the lifetime of a cell. bilayer so that its head group interacts with the hydrophilic head
Because membrane lipids are able to move in the plane of group of phospholipids, while the ring structure participates in
the membrane, the membrane is said to be fluid. The degree of van der Waals interactions with the fatty acid chains.
membrane fluidity depends on which types of lipid make up the Cholesterol increases or decreases membrane fluidity
membrane. In a single layer of the lipid bilayer, the strength of the depending on temperature. At temperatures typically found
van der Waals interactions between the lipids’ tails depends on in a cell, cholesterol decreases membrane fluidity because
the length of the fatty acid tails and the presence of double bonds the interaction of the rigid ring structure of cholesterol with
between neighboring carbon atoms. The longer the fatty acid the phospholipid fatty acid tails reduces the mobility of the
tails, the more surface is available to participate in van der Waals phospholipids. However, at low temperatures, cholesterol
interactions. The tighter packing that results tends to reduce lipid increases membrane fluidity because it prevents phospholipids
 CHAPTER 5 O RG A N I Z I N G P R I N C I P L E S : L I P I D S , M E M B R A N E S , A N D C E L L CO M PA RT M E N T S 93

FIG. 5.5 Cholesterol in the lipid bilayer. Cholesterol molecules embedded in the lipid bilayer affect the fluidity of the membrane.

Hydrophilic OH
head group

Polar
head CH3
The amphipathic groups
structure of cholesterol Hydrophobic,
allows it to pack tightly rigid planar
with phospholipids. group of rings CH3
CH3

Hydrophobic
hydrocarbon
tail

H3C CH3

Cholesterol

from packing tightly with other phospholipids. Thus, cholesterol flip-flop requires the hydrophilic head group to pass through
helps maintain a consistent state of membrane fluidity by the hydrophobic interior of the membrane. As a result, there
preventing dramatic transitions from a fluid to solid state. is little exchange of components between the two layers of
For many decades, it was thought that the various types of lipid the membrane, which in turn allows the two layers to differ in
found in the membrane were randomly distributed throughout composition. In fact, in many membranes, different types of lipid
the bilayer. However, more recent studies show that specific types are present primarily in one layer or the other.
of lipid, such as sphingolipids, sometimes assemble into defined
patches called lipid rafts. Cholesterol and other membrane Proteins associate with cell membranes in
components such as proteins also appear to accumulate in some different ways.
of these regions. Thus, membranes are not always a uniform fluid Most membranes contain proteins as well as lipids. For example,
bilayer, but instead can contain regions with discrete components. proteins represent as much as 50% by mass of the membrane of a
Although lipids are free to move in the plane of the membrane, red blood cell. Membrane proteins serve different functions
the spontaneous transfer of a lipid between layers of the bilayer, (Fig. 5.6). Some act as transporters, moving ions or other
known as lipid flip-flop, is very rare. This is not surprising since molecules across the membrane. Other membrane proteins

FIG. 5.6 Functions of membrane proteins.

Extracellular
fluid

Transporter Receptor Enzyme

Cytoplasm Anchor
94 SECTION 5.2 T H E P L A S M A M E M B R A N E A N D C E L L WA L L

FIG. 5.7 Integral and peripheral membrane proteins. Integral membrane proteins are permanently associated with the membrane. Peripheral
membrane proteins are temporarily associated with one or other of the two lipid bilayers or with an integral membrane protein.

Extracellular Integral membrane Fatty

fluid proteins acid

Integral membrane Peripheral membrane

proteins include proteins are temporarily
transmembrane associated with either the
proteins that span internal or the external
the entire membrane. side of the membrane.

Integral membrane proteins

Peripheral membrane proteins
Cytoplasm

act as receptors that allow the cell to receive signals from the proteins to move within the membrane and assist proteins in
environment. Still others are enzymes that catalyze chemical clustering in lipid rafts.
reactions or anchors that attach to other proteins and help to Proteins, like lipids, are free to move in the membrane. How do
maintain cell structure and shape. we know this? The mobility of proteins in the cell membrane can
These various membrane proteins can be classified into two be demonstrated using an elegant experimental technique called
groups depending on how they associate with the membrane fluorescence recovery after photobleaching, or FRAP (Fig. 5.8).
(Fig. 5.7). Integral membrane proteins are permanently In this technique, proteins embedded in the cell membrane are
associated with cell membranes and cannot be separated from the labeled with fluorescent dye molecules. Labeling all the proteins
membrane experimentally without destroying the membrane in a membrane creates a fluorescent cell that can be visualized
itself. Peripheral membrane proteins are temporarily associated with a fluorescence microscope. A laser is then used to bleach the
with the lipid bilayer or with integral membrane proteins through fluorescent dye molecules in a small area of the cell membrane,
weak noncovalent interactions. They are easily separated from leaving a nonfluorescent spot on the surface of the cell. If
the membrane by simple experimental procedures that leave the proteins in the cell membrane were not capable of movement,
structure of the membrane intact. the bleached area would remain nonfluorescent. However, over
Most integral membrane proteins are transmembrane time, fluorescence appears in the bleached area, telling us that
proteins that span the entire lipid bilayer, as shown in Fig. 5.7. fluorescent proteins that were not bleached moved into the
These proteins are composed of three regions: two hydrophilic bleached area.
regions, one protruding from each face of the membrane, and a The idea that lipids and proteins coexist in the membrane,
connecting hydrophobic region that spans the membrane. This and that both are able to move in the plane of the membrane,
structure allows for separate functions and capabilities of each end led American biologists S. Jonathan Singer and Garth Nicolson to
of the protein. For example, the hydrophilic region on the external propose the fluid mosaic model in 1972. According to this model,
side of a receptor can interact with signaling molecules, whereas the lipid bilayer is a fluid structure within which molecules move
the hydrophilic region on the internal side of the membrane often laterally, and is a mosaic (a mixture) of two types of molecules,
interacts with other proteins in the cytoplasm of the cell to pass lipids and proteins.
along the message.
Peripheral membrane proteins may be associated with either
the internal or external side of the membrane (Fig. 5.7). These 5.2 THE PLASMA MEMBRANE
proteins interact either with the polar heads of lipids or with AND CELL WALL
integral membrane proteins by weak noncovalent interactions
such as hydrogen bonds. Peripheral membrane proteins are only Phospholipids with embedded proteins make up the membrane
transiently associated with the membrane and can play a role in surrounding all cells. This membrane, called the plasma
transmitting information received from external signals. Other membrane, is a fundamental, defining feature of all cells. It is the
peripheral membrane proteins limit the ability of transmembrane boundary that defines the space of the cell, separating its internal
95 SECTION 5.1 M
HOW DO WE KNOW?
FIG. 5.8

Do proteins move in the plane of the

membrane?
BACKGROUND Fluorescent recovery after photobleaching (FRAP) is a technique used to measure mobility Photo source: FRAP of cytoplasmic EGFP in living
HeLa cells, performed using an UltraVIEW®
of molecules in the plane of the membrane. A fluorescent dye is attached to proteins embedded in the cell spinning disk confocal system (PerkinElmer Inc.).
membrane in a process called labeling. A laser is then used to bleach a small area of the membrane. HeLa cells were transfected with pEGFP-C1
(Clontech Laboratories, Inc.) using GeneJuice®
HYPOTHESIS If membrane components such as proteins move in the plane of the membrane, the bleached transfection reagent (Novagen®). A region of
interest in the cytoplasm was photobleached
spot should become fluorescent over time as unbleached fluorescent molecules move into the bleached area.
using the UltraVIEW® photokinesis unit, and the
If membrane components do not move, the bleached spot should remain intact. recovery of fluorescence in this region was observed.
(Fluorescence Recovery After Photobleaching
EXPERIMENT AND RESULTS (FRAP) using the UltraVIEW PhotoKinesis accessory,
PerkinElmer Technical Note).
Fluorescent molecules Laser beam Bleached area

The membrane is labeled with Photobleaching Fluorescence gradually

fluorescent molecules, resulting causes a bleached returns to the bleached
in a uniform fluorescence over area where there is area because of the
the entire membrane. no fluorescence. movement of fluorescent
proteins into the area.
Fluorescence (in bleached area)

Before bleaching

Laser Recovery
bleaching

Time
CONCLUSION The gradual recovery of fluorescence in the bleached area indicates that proteins move in the plane of the
membrane.

SOURCE Peters, R., et al. 1974. “A Microfluorimetric Study of Translational Diffusion in Erythrocyte Membranes.” Biochim Biophys Acta 367:282–294.

95
96 SECTION 5.2 T H E P L A S M A M E M B R A N E A N D C E L L WA L L

contents from the surrounding environment. But the plasma

membrane is not simply a passive boundary or wall. Instead, FIG. 5.9 Diffusion, the movement of molecules due to random
it serves an active and important function. The environment motion. Net movement of molecules results only when
outside the cell is changing all the time. In contrast, the internal there are concentration differences.
environment of a cell operates within a narrow window of
conditions, such as a particular pH range or salt concentration.
It is the plasma membrane that actively maintains intracellular
conditions compatible with life.
In addition, the cells of many organisms have a cell wall
external to the plasma membrane. The cell wall plays an important Time

role in maintaining the shape of these cells. In this section, we

consider the functions performed by these two key structures.
Permeable
The plasma membrane maintains homeostasis. membrane
The active maintenance of a constant environment is known as
homeostasis, and it is a critical attribute of cells and of life itself. Higher solute Lower solute
Chemical reactions and protein folding, for example, are carried concentration concentration
out efficiently only within a narrow range of conditions. How does There is net movement of There is no net movement of
the plasma membrane maintain homeostasis? The answer is that solute from the area of higher solute but diffusion continues.
solute concentration to the area
it is selectively permeable (or semipermeable). This means that of lower solute concentration.
the plasma membrane lets some molecules in and out freely; it lets
others in and out only under certain conditions; and it prevents
other molecules from passing through at all.
The membrane’s ability to act as a selective barrier is the result Diffusion leads to a net movement of the substance from one
of the combination of lipids and embedded proteins of which it is region to another when there is a concentration gradient in the
composed. The hydrophobic interior of the lipid bilayer prevents distribution of a molecule, meaning that there are areas of higher
ions as well as charged or polar molecules from diffusing freely and lower concentrations. In this case, diffusion results in net
across the plasma membrane. Furthermore, many macromolecules movement of the molecule from an area of higher concentration
such as proteins and polysaccharides are too large to cross the to an area of lower concentration (Fig. 5.9).
plasma membrane on their own. By contrast, gases, lipids, and Some molecules diffuse freely across the plasma membrane
small polar molecules can freely move across the lipid bilayer. as a result of differences in concentrations between the inside
Protein transporters in the membrane allow the export and import and the outside of a cell. Oxygen and carbon dioxide, for example,
of molecules, including certain ions and nutrients, that cannot move into and out of the cell in this way. Certain hydrophobic
cross the cell membrane on their own. molecules, such as triacylglycerols (Chapter 2), are also able to
The identity and abundance of these membrane-associated diffuse through the cell membrane, which is not surprising since
proteins vary among cell types, reflecting the specific functions of the lipid bilayer is likewise hydrophobic.
different cells. For example, cells in your gut contain membrane Some molecules that cannot move across the lipid bilayer
transporters that specialize in the uptake of glucose, whereas directly can move passively toward a region of lower concentration
nerve cells have different types of ion channel that are involved in through protein transporters. When a molecule moves by
electrical signaling. diffusion through a membrane protein and bypasses the lipid
bilayer, the process is called facilitated diffusion. Diffusion
Passive transport involves diffusion. and facilitated diffusion both result from the random motion of
The simplest form of movement into and out of cells is passive molecules, and net movement of the substance occurs when there
transport. Passive transport works by diffusion, which is the are concentration differences (Fig. 5.10). In the case of facilitated
random movement of molecules. Molecules are always moving diffusion, the molecule moves through a membrane transporter,
in their environments. For example, molecules in water at room whereas in the case of simple diffusion, the molecule moves
temperature move around at about 500 m/sec, which means that directly through the lipid bilayer.
they can move only about 3 molecular diameters before they Membrane transporters are of two types. The first type is
run into another molecule, leading to about 5 trillion collisions a channel, which provides an opening between the inside and
per second. The frequency with which molecules collide have outside of the cell within which certain molecules can pass,
important consequences for chemical reactions, which depend on depending on their shape and charge. Some membrane channels
the interaction of molecules (Chapter 6). are gated, which means that they open in response to some sort
 CHAPTER 5 O RG A N I Z I N G P R I N C I P L E S : L I P I D S , M E M B R A N E S , A N D C E L L CO M PA RT M E N T S 97

transport. Although the plasma membrane is hydrophobic,

FIG. 5.10 Simple diffusion and facilitated diffusion through the water molecules are small enough to move passively through the
cell membrane. membrane to a limited extent by simple diffusion. In addition,
Extracellular many cells have specific protein channels, known as aquaporins,
fluid for transporting water molecules. These channels allow water to
move more readily across the plasma membrane by facilitated
diffusion than is possible by simple diffusion.
The net movement of a solvent such as water across a
selectively permeable membrane such as the plasma membrane is

Concentration gradient
known as osmosis. As in any form of diffusion, water moves from
regions of higher water concentration to regions of lower water
concentration (Fig. 5.11). Because water is a solvent within which
nutrients such as glucose or ions such as sodium or potassium are
dissolved, water concentration drops as solute concentration rises.
Therefore, it is sometimes easier to think about water moving from
regions of lower solute concentration toward regions of higher solute
Simple Channel concentration. Either way, the direction of water movement is the
diffusion Carrier same. During osmosis, the net movement of water toward the side
Cytoplasm Facilitated diffusion of the membrane with higher solute concentration continues until
it is opposed by another force. This force could be pressure due to
gravity (in the case of Fig. 5.11) or the cell wall (in the case of plants,
fungi, and bacteria, as described below).

of signal, which may be chemical or electrical (Chapter 9). The j Quick Check 2 A container is divided into two compartments
second type of transporter is a carrier, which binds to and then by a membrane that is fully permeable to water and small ions.
transports specific molecules. Membrane carriers exist in two Water is added to one side of the membrane (side A), and a 5%
conformations, one that is open to one side of the cell, and another solution of sodium chloride (NaCl) is added to the other (side B). In
that is open to the other side of the cell. Binding of the transported which direction will water molecules move? In which direction will
molecule induces a conformational change in the membrane sodium and chloride ions move? When the concentration is equal
protein, allowing the molecule to be transported across the lipid on both sides, will diffusion stop?
bilayer, as shown on the right in Fig. 5.10.
Up to this point, we have focused our attention on the Primary active transport uses the energy of ATP.
movement of molecules (the solutes) in water (the solvent). Passive transport works to the cell’s advantage only if the
We can take a different perspective and focus instead on water concentration gradient is in the right direction, from higher
movement. Water itself also moves into and out of cells by passive on the outside to lower on the inside for nutrients that the cell

FIG. 5.11 Osmosis. Osmosis is the net movement

of a solvent such as water across a selectively
permeable membrane from an area of lower
solute concentration to an area of higher
solute concentration.
Time
Solute

Osmosis Water
Selectively permeable
membrane

Higher solute Lower solute Membrane allows passage

concentration, concentration, of water but not solute.
lower water higher water
concentration concentration
98 SECTION 5.2 T H E P L A S M A M E M B R A N E A N D C E L L WA L L

FIG. 5.12 Primary active transport. The sodium-potassium pump is a membrane protein that uses the energy stored in ATP to move sodium and
potassium ions against their concentration gradients.

Extracellular
fluid
Na
Na Concentration gradient

Higher Na K movement Lower K

K Concentration gradient
Na 1 2 3 4 K
binding binding
site site

Na movement
Lower Na  Higher K
ATP ADP
K

Cytoplasm

needs to take in, and from higher on the inside and lower on the molecules in the same direction, and are referred to as symporters
outside for wastes that the cell needs to export. However, many or cotransporters.
of the molecules that cells require are not highly concentrated
in the environment. Although some of these molecules can Secondary active transport is driven by an
be synthesized by the cell, others must be taken up from the electrochemical gradient.
environment. In other words, cells have to move these substances Active transport can also work in another way. Because small
from areas of lower concentration to areas of higher concentration. ions cannot cross the lipid bilayer, many cells use a transport
The “uphill” movement of substances against a concentration protein to build up the concentration of a small ion on one side
gradient, called active transport, requires energy. The transport of the membrane. The resulting concentration gradient stores
of many kinds of molecules across membranes requires energy, potential energy that can be harnessed to drive the movement
either directly or indirectly. In fact, most of the energy used of other substances across the membrane against their
by a cell goes into keeping the inside of the cell different from concentration gradient.
the outside, a function carried out by proteins in the plasma For example, some cells actively pump protons (H1) across the
membrane. cell membrane using ATP (Fig. 5.13a). As a result, in these cells
During active transport, cells move substances through the concentration of protons is higher on one side of the
transport proteins embedded in the cell membrane. Some of these membrane and lower on the other side. In other words, the pump
proteins act as pumps, using energy directly to move a substance generates a concentration gradient, also called a chemical gradient
into or out of a cell. A good example is the sodium-potassium because the entity forming the gradient is a chemical (Fig. 5.13b).
pump (Fig. 5.12). Within cells, sodium is kept at concentrations We have already seen that concentration differences favor the
much lower than in the exterior environment; the opposite is true movement of protons back to the other side of the membrane.
of potassium. Therefore, both sodium and potassium have to be However, the lipid bilayer blocks the movement of protons to the
moved against a concentration gradient. The sodium-potassium other side and therefore stores potential energy, just like a dam
pump actively moves sodium out of the cell and potassium into or battery.
the cell. This movement of ions takes energy, which comes In addition to the chemical gradient, another force favors the
from the chemical energy stored in ATP. Active transport that movement of protons back across the membrane: a difference in
uses energy directly in this manner is called primary active charge. Because protons carry a positive charge, the side of the
transport. Note that the sodium ions and potassium ions move membrane with more protons is more positive than the other side.
in opposite directions. Protein transporters that work in this This difference in charge is called an electrical gradient. Protons
way are referred to as antiporters. Other transporters move two (and other ions) move from areas of like charge to areas of unlike
 CHAPTER 5 O RG A N I Z I N G P R I N C I P L E S : L I P I D S , M E M B R A N E S , A N D C E L L CO M PA RT M E N T S 99

a. b. c.
FIG. 5.13 Secondary active transport.
Extracellular     
   Protons are pumped across
fluid     a membrane by (a) primary
   

  active transport, resulting
  
Proton 
 in (b) an electrochemical
gradient, which drives

Proton concentration gradient

+
 + 
 + 
+ 
+ 
+ 
+ +
 (c) the movement of

another molecule against

Electrical gradient

1 2 3  its concentration gradient.

–
 – 
 – 
– 
– 
– 
– –

 ADP

 
ATP  



  
Cytoplasm  


Protons are pumped across The proton pump generates An antiporter uses the proton
membrane by primary an electrochemical gradient, electrochemical gradient to move
active transport. with a higher concentration a different molecule out of the cell
of protons outside the cell against its concentration gradient.
and a lower concentration
of protons inside the cell.

charge, driven by an electrical gradient. A gradient that has both solutions (Fig. 5.14). If a red blood cell is placed in a hypertonic
charge and chemical components is known as an electrochemical solution (one with a higher solute concentration than that
gradient (Fig. 5.13b). inside the cell), water leaves the cell by osmosis and the cell
If protons are then allowed to pass through the cell membrane shrinks. By contrast, if a red blood cell is placed in a hypotonic
by a transport protein, they will move down their electrochemical solution (one with a lower solute concentration than that inside
gradient toward the region of lower proton concentration. These the cell), water moves into the cell by osmosis and the cell lyses,
transport proteins can use the movement of protons to drive the or bursts. Animal cells solve the problem of water movement
movement of other molecules against their concentration gradient in part by keeping the intracellular fluid isotonic (that is, at the
(Fig. 5.13c). The movement of protons is always from regions same solute concentration) as the extracellular fluid. Cells use
of higher to lower concentration, whereas the movement of the the active transport of ions to maintain equal concentrations
coupled molecule is from regions of lower to higher concentration.
Because the movement of the coupled molecule is driven by
the movement of protons and not by ATP directly, this form of
transport is called secondary active transport. Secondary active FIG. 5.14 Changes in red blood cell shape due to osmosis. Red
transport uses the potential energy of an electrochemical gradient
blood cells shrink, swell, or burst because of net water
to drive the movement of molecules; by contrast, primary active
movement driven by differences in solute concentration
transport uses the chemical energy of ATP directly.
between the inside and the outside of the cell.
The use of an electrochemical gradient as a temporary energy
source is a common cellular strategy. For example, cells use a sodium
Shrunk Normal Swollen Lysed
electrochemical gradient generated by the sodium-potassium pump
to transport glucose and amino acids into cells. In addition, cells use H2O H2O H2O H 2O
a proton electrochemical gradient to move other molecules and, as
we discuss below and in Chapter 7, to synthesize ATP.

Many cells maintain size and composition using

active transport. Hypertonic Isotonic Hypotonic Very hypotonic
Many cells use active transport to maintain their size.
Solute concentration in extracellular space
Consider human red blood cells placed in a variety of different
100 SECTION 5.3 T H E I N T E R N A L O RG A N I Z AT I O N O F C E L L S

inside and out, and the sodium-potassium pump plays an

important role in keeping the inside of the cell isotonic with FIG. 5.15 The plant cell wall and vacuoles. The plant cell wall is
the extracellular fluid. a rigid structure that maintains the shape of the cell. The
pressure exerted by water absorbed by vacuoles also
j Quick Check 3 In the absence of the sodium-potassium pump, provides support. Source: Biophoto Associates/Getty Images.
the extracellular solution becomes hypotonic relative to the inside
of the cell. Poisons such as the snake venom ouabain can interfere
with the action of the sodium-potassium pump. What are the
consequences for the cell?

Human red blood cells avoid shrinking or bursting by

maintaining an intracellular environment isotonic with the
extracellular environment, the blood. But what about a single-celled
Vacuole
organism, like Paramecium, swimming in a freshwater lake? In this
case, the extracellular environment is hypotonic compared with the
concentration in the cell’s interior. As a result, Paracemium faces the
risk of bursting from water moving in by osmosis. Paramecium and
some other single-celled organisms contain contractile vacuoles
that solve this problem. Contractile vacuoles are compartments that
take up excess water from inside the cell and then, by contraction, Cell wall
expel it into the external environment. The mechanism by which
water moves into the contractile vacuoles differs depending on
the organism. The contractile vacuoles of some organisms take in
water through aquaporins, while the contractile vacuoles of other
organisms first take in protons through proton pumps, with water reduces turgor pressure and the cells can no longer maintain
following by osmosis. their shape within the cell wall. Plant vacuoles have many other
functions and are often the most conspicuous feature of plant
The cell wall provides another means of maintaining cells. They also explain in part why plant cells are typically larger
cell shape. than animal cells and can store water, as well as nutrients, ions,
As we have seen, animals often maintain cell size by using active and wastes.
transport to keep the inside and outside of the cell isotonic. How The cell wall is made up of many different components,
do organisms such as plants, fungi, and bacteria maintain cell including carbohydrates and proteins. The specific components
size and shape? A key feature of these organisms is the cell wall differ depending on the organism. The plant cell wall is composed
surrounding the plasma membrane (Fig. 5.15). The cell wall plays of polysaccharides, one of the best known of which is cellulose,
a critical role in the maintenance of cell size and shape. When a polymer of the sugar glucose. Cellulose is the most abundant
Hooke looked at cork through his microscope, what he saw was biological material in nature. Many types of algae have cell walls
not living cells but the remains of cell walls. made up of cellulose, as in plants, but others have cell walls made
The cell wall provides structural support and protection to the of silicon or calcium carbonate. Fungi have cell walls made of
cell. Because the cell wall resists expansion, it allows pressure to chitin, another polymer based on sugars. In bacteria, the cell wall
build up in a cell when it absorbs water. Recall that when an animal is made up primarily of peptidoglycan, a mixture of amino acids
cell, like a red blood cell, is placed in pure water, it swells until it and sugars.
bursts. By contrast, when a plant cell is placed in pure water, water
enters the cell by osmosis until the pressure created by the cell
wall’s resisting further expansion opposes the driving force for the 5.3 THE INTERNAL
water to enter. ORGANIZATION OF CELLS
The force exerted by water pressing against an object is called
hydrostatic pressure, or turgor pressure. The pressure exerted by In addition to the plasma membrane, many cells contain
water inside the cell on the cell wall provides structural support membrane-bound regions within which specific functions are
for many organisms that is similar in function to the support carried out. Such a cell can be compared to a large factory with
provided by animals’ skeletons. In addition, plant and fungal cells many rooms and different departments. Each department has a
have another structure, called the vacuole (different from the specific function and internal organization that contribute to the
contractile vacuole), that can absorb water and also contribute to overall “life” of the factory. In this section, we give an overview of
turgor pressure (Fig. 5.15). It is therefore easy to understand why two broad classes of cells that can be distinguished by the presence
plants wilt when dehydrated—the loss of water from the vacuoles or absence of these membrane-enclosed compartments.
 CHAPTER 5 O RG A N I Z I N G P R I N C I P L E S : L I P I D S , M E M B R A N E S , A N D C E L L CO M PA RT M E N T S 101

Eukaryotes and prokaryotes differ in internal

organization. FIG. 5.16 Prokaryotic and eukaryotic cells. Prokaryotic cells lack
All cells have a plasma membrane and contain genetic material. a nucleus and extensive internal compartmentalization.
In some cells, the genetic material is housed in a membrane- Eukaryotic cells have a nucleus and extensive internal
bound space called the nucleus. Cells can be divided into two compartmentalization.
classes based on the absence or presence of a nucleus (Chapter 1).
Prokaryotes, including bacteria and archaeons, lack a nucleus;
eukaryotes, including animals, plants, fungi, and protists, have a
nucleus (Fig. 5.16). The presence of a nucleus in eukaryotes allows
for the processes of transcription and translation to be separated
in time and space. This separation in turn allows for more complex
ways to regulate gene expression than are possible in prokaryotes
(Chapter 19).
There are other differences between prokaryotes and
eukaryotes. For example, we saw in Chapter 3 that promoter
recognition during transcription is different in prokaryotes and
eukaryotes. In addition, there are differences in the types of lipid
that make up their cell membranes. In mammals, as we have seen,
cholesterol is present in cell membranes. Cholesterol belongs
to a group of chemical compounds known as sterols, which are Nucleus
molecules containing a hydroxyl group attached to a four-ringed Plasma
structure. In eukaryotes other than mammals, diverse sterols are membrane
synthesized and present in cell membranes. Most prokaryotes Nucleoid
do not synthesize sterols, but some synthesize compounds
called hopanoids. These five-ringed structures are thought to
serve a function similar to that of cholesterol in mammalian cell
membranes. Prokaryotic cell Eukaryotic cell
1 µm
In spite of all of these and other differences between
prokaryotes and eukaryotes, it is the absence or presence
of a nucleus that defines the two groups. However, from an
evolutionary perspective, archaeons and eukaryotes are more eukaryotic cells are commonly much larger, on the order of
closely related to each other than either are to bacteria. For 10 times larger in diameter and 1000 times larger in volume. The
example, archaeons share with eukaryotes many genes involved small size of prokaryotic cells means that they have a relatively
in transcription and translation, and DNA is packaged with high ratio of surface area to volume, which makes sense for
histones in both groups. an organism that absorbs nutrients from the environment. In
other words, there is a large amount of membrane surface area
Prokaryotic cells lack a nucleus and extensive internal available for absorption relative to the volume of the cell that it
compartmentalization. serves. In addition, most prokaryotes lack the extensive internal
Prokaryotes do not have a nucleus—that is, there is no physical organization characteristic of eukaryotes.
barrier separating the genetic material from the rest of the cell.
Instead, the DNA is concentrated in a discrete region of the cell Eukaryotic cells have a nucleus and specialized
interior known as the nucleoid. Bacteria often contain additional internal structures.
small circular molecules of DNA known as plasmids that carry a Eukaryotes are defined by the presence of a nucleus, which
few genes. Plasmids are commonly transferred between bacteria houses the vast majority of the cell’s DNA. The nuclear membrane
through the action of threadlike structures known as pili (singular, allows for more complex regulation of gene expression than is
pilus), which extend from one cell to another. Genes for antibiotic possible in prokaryotic cells (Chapters 3 and 19). In eukaryotes,
resistance are commonly transferred in this way, which accounts DNA is transcribed to RNA inside the nucleus, but the RNA
for the quick spread of antibiotic resistance among bacterial molecules carrying the genetic message travel from inside to
populations. outside the nucleus, where they instruct the synthesis of
Although the absence of a nucleus is a defining feature proteins.
of prokaryotes, other features also stand out. For example, Eukaryotes have a remarkable internal array of membranes.
prokaryotes are small, typically just 1–2 microns (a micron is These membranes define compartments, called organelles, that
1/1,000,000 of a meter) in diameter or smaller. By contrast, divide the cell contents into smaller spaces specialized for different
102 SECTION 5.3 T H E I N T E R N A L O RG A N I Z AT I O N O F C E L L S

functions. Fig. 5.17a shows a macrophage, a type of animal cell, such as proteins, nucleic acids, lipids, and complex carbohydrates.
with various organelles. The endoplasmic reticulum (ER) is the Peroxisomes also contain many different enzymes and are
organelle in which proteins and lipids are synthesized. The Golgi involved in important metabolic reactions, including the breakdown
apparatus modifies proteins and lipids produced by the ER and of fatty acids and the synthesis of certain types of phospholipid.
acts as a sorting station as they move to their final destinations. Mitochondria are specialized organelles that harness energy for
Lysosomes contain enzymes that break down macromolecules the cell. Many cell membranes that define these organelles are

FIG. 5.17 An animal cell and a plant cell. Animal and plant cells have many cell components in common.

a. Typical features of an animal cell

The endoplasmic reticulum

is involved in both protein
and lipid synthesis.
The nucleus is the
storehouse for the
cell’s genetic
information and the Lysosomes degrade
site for RNA synthesis. macromolecules.

The Golgi apparatus

modifies and sorts
proteins and lipids as
they move to their final
destinations in or out of
Mitochondria the cell.
produce most of
the ATP that serves
as the energy
currency of the cell.

Peroxisomes break down

The plasma membrane is specific organic molecules,
composed of phospholipids such as fatty acids, and
and proteins, and regulates synthesize other organic
the passage of materials molecules, such as
into and out of the cell. cholesterol and some
types of phospholipids.
The cytoskeleton is a
network of protein filaments
and other associated proteins
that provide the cell with an
internal structural framework.

2 μm
 CHAPTER 5 O RG A N I Z I N G P R I N C I P L E S : L I P I D S , M E M B R A N E S , A N D C E L L CO M PA RT M E N T S 103

associated with a protein scaffold called the cytoskeleton that and chloroplasts that convert energy of sunlight into chemical
helps cells to maintain their shape and serves as a network of tracks energy.
for the movement of substances within cells (Chapter 10). The entire contents of a cell other than the nucleus make up
Fig. 5.17b shows a typical plant cell. In addition to the the cytoplasm. The jelly-like internal environment of the cell that
organelles described above, plant cells have a cell wall outside the surrounds the organelles inside the plasma membrane is referred
plasma membrane, vacuoles specialized for water uptake, to as the cytosol.

b. Typical features of a plant cell

The plant cell wall is a Vacuoles are organelles that

rigid barrier composed contribute to the structural
of polysaccharides. rigidity of plants by
maintaining turgor pressure
against cell walls.

Plasma membrane

Mitochondria

Chloroplasts enable Nucleus

plant cells to harness
the energy of sunlight
to synthesize sugars.

Endoplasmic
reticulum

Plasmodesmata connect Golgi apparatus

neighboring plant cells.

Cytoskeleton
104 SECTION 5.4 T H E E N D O M E M B R A N E S YS T E M S

In the next two sections, we consider these organelles in more with another organelle or the plasma membrane, re-forming a
detail, focusing on the role of membranes in forming distinct continuous membrane and unloading their contents.
compartments within the cell. In total, these interconnected membranes make up the
endomembrane system. The endomembrane system includes
the nuclear envelope, endoplasmic reticulum, Golgi apparatus,
5.4 THE ENDOMEMBRANE SYSTEM lysosomes, the plasma membrane, and the vesicles that move
between them (Fig. 5.18). In plants, the endomembrane system is
In eukaryotes, the total surface area of intracellular membranes actually continuous between cells through connecting pores called
is about tenfold greater than that of the plasma membrane. This plasmodesmata (see Fig. 5.17; Chapters 10 and 28).
high ratio of internal membrane area to plasma membrane area Extensive internal membranes are not common in
underscores the significant degree to which a eukaryotic cell prokaryotic cells. However, photosynthetic bacteria have internal
is divided into internal compartments. Many of the organelles membranes that are specialized for harnessing light energy
inside cells are not isolated entities, but instead communicate (Chapters 8 and 26).
with one another. In fact, the membranes of these organelles are
either physically connected by membrane “bridges” or they are The endomembrane system compartmentalizes
transiently connected by vesicles, small membrane-enclosed the cell.
sacs that transport substances within a cell or from the interior Because many types of molecules are unable to cross cell
to the exterior of the cell. These vesicles form by budding off an membranes on their own, the endomembrane system divides the
organelle, taking with them a piece of the membrane and internal interior of a cell into two distinct “worlds,” one inside the spaces
contents of the organelle from which they derive. They then fuse defined by these membranes and one outside these spaces. A
molecule within the interior of the ER
can stay in the ER or end up in the interior
of the Golgi apparatus or even outside
FIG. 5.18 The endomembrane system. The endomembrane system is a series of
the cell by the budding off and fusing
interconnected membrane-bound compartments in eukaryotic cells.
of a vesicle between these organelles.
Similarly, a molecule associated with
the ER membrane can move to the Golgi
membrane or the plasma membrane
by vesicle transport. Molecules in
the cytosol are in a different physical
space, separated by membranes of the
endomembrane system. This physical
separation allows specific functions to
take place within the spaces defined
by the membranes and also within the
membrane itself.
Nuclear envelope In spite of forming a continuous
and interconnected system, the various
Endoplasmic compartments have unique properties
reticulum and maintain distinct identities
determined in part by which lipids and
proteins are present in their membranes.
Golgi apparatus
Vesicles not only bud off from and
fuse with organelles but also with the
Vesicle plasma membrane. When a vesicle fuses
Lysosome with the plasma membrane, the process
Plasma membrane is called exocytosis. It provides a way
Exocytosis for a vesicle to empty its contents to the
extracellular space or to deliver proteins
Endocytosis embedded in the vesicle membrane to
the plasma membrane (Fig. 5.18). The
process also works in reverse: A vesicle
 CHAPTER 5 O RG A N I Z I N G P R I N C I P L E S : L I P I D S , M E M B R A N E S , A N D C E L L CO M PA RT M E N T S 105

can bud off from the plasma membrane, enclosing material from the sites of protein synthesis, in which amino acids are assembled
outside the cell and bringing it into the cell interior. This process is into polypeptides guided by the information stored in mRNA
called endocytosis. Together, exocytosis and endocytosis provide (Chapter 4). In this way, the nuclear envelope and its associated
a way to move material into and out of cells without passing protein pores regulate which molecules move into and out of
through the cell membrane. the nucleus.

The nucleus houses the genome and is the site The endoplasmic reticulum is involved in protein
of RNA synthesis. and lipid synthesis.
The innermost organelle of the endomembrane system is the The outer membrane of the nuclear envelope is physically
nucleus, which stores DNA, the genetic material that encodes continuous with the endoplasmic reticulum (ER), an organelle
the information for all the activities and structures of the cell. bounded by a single membrane (Fig. 5.20). The ER is a
The nuclear envelope defines the boundary of the nucleus conspicuous feature of many eukaryotic cells, accounting in some
(Fig. 5.19). It actually consists of two membranes, the inner and cases for as much as half of the total amount of membrane. The
outer membranes, and each is a lipid bilayer with associated proteins. ER produces and transports many of the lipids and proteins used
These two membranes are continuous with each other at inside and outside the cell, including all transmembrane proteins,
openings called nuclear pores. These pores are large protein as well as proteins destined for the Golgi apparatus, lysosomes, or
complexes that allow molecules to move into and out of the export out of the cell. The ER is also the site of production of most
nucleus, and thus are essential for the nucleus to communicate of the lipids that make up the various internal and external cell
with the rest of the cell. For example, some proteins that are membranes.
synthesized in the cytosol, such as transcription factors, move Unlike the nucleus, which is a single spherical structure in
through nuclear pores to enter the nucleus, where they control the cell, the ER consists of a complex network of interconnected
how and when genetic information is expressed. tubules and flattened sacs. The interior of the ER is continuous
In addition, the transfer of information encoded by DNA throughout and is called the lumen. As shown in Fig. 5.20,
depends on the movement of mRNA (messenger RNA) molecules the ER has an almost mazelike appearance when sliced and
out of the nucleus through these pores. After exiting the nucleus, viewed in cross section. Its membrane is extensively convoluted,
mRNA binds to free ribosomes in the cytosol or ribosomes allowing a large amount of membrane surface area to fit within
associated with the endoplasmic reticulum (ER). Ribosomes are the cell.
When viewed through an electron microscope, ER membranes
have two different appearances (Fig. 5.20). Some look rough
because they are studded with ribosomes. This portion of the ER is
FIG. 5.19 A surface view of the nuclear envelope. The nucleus is referred to as rough endoplasmic reticulum (RER). The rough
surrounded by a double membrane and houses the cell’s ER synthesizes transmembrane proteins, proteins that end up in
DNA. Source: Don W. Fawcett/Science Source. the interior of organelles, and proteins destined for secretion. As a
result, cells that secrete large quantities of protein have extensive
Nucleus
rough ER, including cells of the gut that secrete digestive enzymes
Nuclear and cells of the pancreas that produce insulin. All cells have at
pores
least some rough ER for the production of transmembrane and
organelle proteins.
There is a small amount of ER membrane in most cells that
appears smooth because it lacks ribosomes. This portion of the
ER is therefore called smooth endoplasmic reticulum (SER)
(Fig. 5.20). Smooth ER is the site of fatty acid and phospholipid
biosynthesis. Thus, this type of ER predominates in cells
specialized for the production of lipids. For example, cells that
synthesize steroid hormones have a well-developed SER that
1 µm produces large quantities of cholesterol. Enzymes within the SER
convert cholesterol into steroid hormones.
The nuclear envelope is perforated by
membrane protein openings called nuclear The Golgi apparatus modifies and sorts proteins
pores. Small molecules and ions can passively and lipids.
diffuse through the pores, but large proteins
and RNA require active transport.
Although it is not physically continuous with the ER, the Golgi
apparatus is often the next stop for vesicles that bud off the ER.
106 SECTION 5.4 T H E E N D O M E M B R A N E S YS T E M S

FIG. 5.20 The endoplasmic reticulum (ER). The ER is a major site for lipid and protein synthesis. (Proteins are also synthesized in the
cytoplasm.) Sources: (top left) Biophoto Associates/Science Source; (bottom left) David M. Phillips/Science Source.

Smooth endoplasmic reticulum

The rough ER is associated with ribosomes.
Many proteins, including those that are Rough endoplasmic reticulum
destined for secretion, are synthesized by
ribosomes associated with the rough ER.
Ribosome
bound to
rough ER

Protein

The smooth ER lacks ribosomes and is the

primary site of lipid synthesis.

Ribosome
free in
cytoplasm

Protein

These vesicles carry lipids and proteins, either within the vesicle plasma membrane or other organelles. Vesicles are therefore the
interior or embedded in their membranes. The movement of these primary means by which proteins and lipids move through the
vesicles from the ER to the Golgi apparatus and then to the rest Golgi apparatus to their final destinations.
of the cell is part of a biosynthetic pathway in which lipids and Enzymes within the Golgi apparatus chemically modify
proteins are sequentially modified and delivered to their final proteins and lipids as they pass through it. These modifications
destinations. The Golgi apparatus has three primary roles: take place in a sequence of steps, each performed in a different
(1) It further modifies proteins and lipids produced by the ER; region of the Golgi apparatus, since each region contains a
(2) it acts as a sorting station as these proteins and lipids move to different set of enzymes that catalyzes specific reactions.
their final destinations; and (3) it is the site of synthesis of most of As a result, there is a general movement of vesicles from the
the cell’s carbohydrates. ER through the Golgi apparatus and then to their final
Under the microscope, the Golgi apparatus looks like stacks destinations.
of flattened membrane sacs, called cisternae, surrounded by An example of a chemical modification that occurs
many small vesicles (Fig. 5.21). These vesicles transport proteins predominantly in the Golgi apparatus is glycosylation, in which
and lipids from the ER to the Golgi apparatus, and then between sugars are covalently linked to lipids or specific amino acids of
the various cisternae, and finally from the Golgi apparatus to the proteins. As these lipids and proteins move through the Golgi
 CHAPTER 5 O RG A N I Z I N G P R I N C I P L E S : L I P I D S , M E M B R A N E S , A N D C E L L CO M PA RT M E N T S 107

FIG. 5.21 The Golgi apparatus. The Golgi apparatus sorts proteins and lipids to other organelles, the plasma membrane, or the cell exterior.
Source: Biophoto Associates/Science Source.

Vesicle

Cisternae

Golgi
apparatus

The Golgi apparatus

receives proteins and lipids
from the ER and sorts them
to other organelles, the
plasma membrane, or the
cell exterior.

Transmembrane
Soluble
protein
protein
Carbohydrate

apparatus, they encounter different enzymes in each region that Lysosomes degrade macromolecules.
add or trim sugars. Glycoproteins are important components of The ability of the Golgi apparatus to sort and dispatch proteins to
the eukaryotic cell surface. The sugars attached to the protein can particular destinations is dramatically illustrated by lysosomes.
protect the protein from enzyme digestion by blocking access Lysosomes are specialized vesicles derived from the Golgi
to the peptide chain. As a result, glycoproteins form a relatively apparatus that degrade damaged or unneeded macromolecules
flexible and protective coating over the plasma membrane. The (Fig. 5.22). They contain a variety of enzymes that break down
distinctive shapes that sugars contribute to glycoproteins and macromolecules such as proteins, nucleic acids, lipids, and
glycolipids also allow them to be recognized specifically by other complex carbohydrates. Macromolecules destined for degradation
cells and molecules in the external environment. For example, are packaged by the Golgi apparatus into vesicles. The vesicles then
human blood types (A, B, AB, and O) are defined by the particular fuse with lysosomes, delivering their contents to the lysosome
sugars that are linked to proteins and lipids on the surface of red interior.
blood cells. The formation of lysosomes also illustrates the ability of
While traffic usually travels from the ER to the Golgi the Golgi apparatus to sort key proteins. The enzymes inside
apparatus, a small amount of traffic moves in the reverse the lysosomes are synthesized in the RER, sorted in the Golgi
direction, from the Golgi apparatus to the ER. This reverse apparatus, and then packaged into lysosomes. In addition, the
pathway is important to retrieve proteins in the ER or Golgi Golgi apparatus sorts and delivers specialized proteins that become
that were accidentally moved forward and to recycle membrane embedded in lysosomal membranes. These include proton pumps
components. that keep the internal environment at an acidic pH of about 5,
108 SECTION 5.4 T H E E N D O M E M B R A N E S YS T E M S

FIG. 5.22 Lysosomes. Lysosomes are vesicles

that degrade macromolecules. Source:
Don W. Fawcett/Getty Images.

Enzymes

The Golgi apparatus

delivers enzymes
that break down
macromolecules to a
lysosome. A vesicle
with macromolecules
Lysosomes also merges with the
lysosome.

Proton pump Transporter

Protons Broken-down
macromolecules

Proton pumps help maintain an acidic

pH, while broken-down macromolecules
are transported out of the lysosome by
transporters.

the optimum pH for the activity of the enzymes inside. Other membrane. Protein sorting is the process by which proteins
proteins in the lysosomal membranes transport the breakdown end up where they need to be to perform their function. Protein
products of macromolecules, such as amino acids and simple sorting directs proteins to the cytosol, the lumen of organelles, the
sugars, across the membrane to the cytosol for use by the cell. membranes of the endomembrane system, or even out of the cell
The function of lysosomes underscores the importance entirely.
of having separate compartments within the cell bounded by Recall that proteins are produced in two places: free ribosomes
selectively permeable membranes. Lysosomal enzymes cannot in the cytosol and membrane-bound ribosomes on the rough ER.
function in the normal cellular environment, which has a pH of Proteins produced on free ribosomes are sorted after they are
about 7, and many of a cell’s enzymes and proteins would unfold translated. These proteins often contain amino acid sequences,
and degrade if the entire cell were at the pH of the inside of a called signal sequences, that allow them to be recognized and
lysosome. By restricting the activity of these enzymes to the sorted. As shown in Fig. 5.23, there are several types of signal
lysosome, the cell protects proteins and organelles in the cytosol sequences that direct proteins synthesized on free ribosomes
from degradation. to different cellular compartments. Proteins with no signal
sequence remain in the cytosol (Fig. 5.23a). Proteins destined for
Protein sorting directs proteins to their proper mitochondria or chloroplasts often have a signal sequence at their
location in or out of the cell. amino ends (Fig. 5.23b). Proteins targeted to the nucleus usually
As we have seen, eukaryotic cells have many compartments, and have signal sequences located internally (Fig. 5.23c). These
different proteins function in different places, such as enzymes nuclear signal sequences, called nuclear localization signals,
in lysosomes or transmembrane proteins embedded in the plasma enable proteins to move through pores in the nuclear envelope.
 CHAPTER 5 O RG A N I Z I N G P R I N C I P L E S : L I P I D S , M E M B R A N E S , A N D C E L L CO M PA RT M E N T S 109

FIG. 5.23 Signal sequences on proteins synthesized by free FIG. 5.24 Pathways for proteins destined (left) to be secreted
ribosomes. (a) Most proteins with no signal peptide or (right) transported to the plasma membrane.
remain in the cytosol. Other signal sequences direct
proteins to (b) mitochondria and chloroplasts or to
(c) the nucleus.

a. No signal peptide

To cyt
osol Rough
endoplasmic
b. Amino-terminal signal reticulum

To chloroplast
To mitochondrion

c. Internal signal
us
cle
nu
To Golgi apparatus

Proteins produced on the rough ER, as we have seen, end up

in the lumen of the endomembrane system, secreted out of the
cell, or as transmembrane proteins (Fig. 5.24). These proteins
are sorted as they are translated. They begin translation on free
ribosomes, but a specific signal sequence at their amino terminal
end directs the ribosome to the rough ER and into a membrane
channel that leads into the ER lumen. If the polypeptide contains
no other signal sequence, it continues into the lumen. If it
contains a second sequence, called a signal-anchor sequence,
it does not continue all the way into the lumen and ends up in Cytoplasm
the membrane. Let’s consider in more detail how sorting of these
proteins happens.
Proteins destined for the ER lumen or secretion have an
amino-terminal signal sequence, shown in Fig. 5.25. As a free
ribosome translates the protein, this sequence is recognized by an
RNA–protein complex known as a signal-recognition particle Plasma
membrane
(SRP). The SRP binds to both the signal sequence and the free
ribosome, and brings about a pause in translation (Fig. 5.25a). The
SRP then binds with a receptor on the RER so that the ribosome is
now associated with the RER (Fig. 5.25b). The SRP receptor brings
Extracellular fluid
the ribosome to a channel in the membrane of the RER. The SRP
then dissociates and translation continues, allowing the growing
polypeptide chain to be threaded through the channel (Fig. 5.25c).
A specific protease cleaves the signal sequence as it emerges in
the lumen of the ER (Fig. 5.25d). Some proteins are retained in
the interior of the ER and others are transported in vesicles to the
interior of the Golgi apparatus. Some of these proteins are secreted
by exocytosis.
110 SECTION 5.4 T H E E N D O M E M B R A N E S YS T E M S

FIG. 5.25 Interaction of a signal sequence, signal-recognition particle (SRP), and SRP receptor. Binding of a signal sequence with a signal-
recognition particle (SRP) halts translation, followed by docking of the ribosome on the ER membrane, release of the SRP, and
continuation of translation.

a. b. c. d.

The signal-recognition particle The SRP The SRP receptor brings the The protein ends up in the
(SRP) binds to a signal sequence binds to the ribosome to a transmembrane lumen of the ER, where it may
in the amino-terminal end of the SRP receptor channel; the SRP dissociates; remain, be transported to the
growing polypeptide and halts on the ER protein synthesis resumes; and lumen of another organelle, or
translation. membrane. the growing polypeptide chain be secreted out of the cell.
is threaded through the channel.

Cytosol

Ribosome
SRP

Signal
sequence

Rough Channel in SRP receptor Cleaved

ER lumen rough ER signal sequence

Proteins destined for cell membranes contain a signal-anchor ER membrane until the signal-anchor sequence is encountered
sequence in addition to the amino-terminal signal sequence (Fig. 5.26a). The signal-anchor sequence is hydrophobic and is
(Fig. 5.26). After the growing polypeptide chain and its ribosome therefore able to diffuse laterally in the lipid bilayer (Fig. 5.26b).
are brought to the ER, it is threaded through the channel in the At this point, the ribosome dissociates from the channel while

a. b. c.

FIG. 5.26 Targeting of a transmembrane

COO⫺ protein by means of a
hydrophobic signal-anchor
Cytosol sequence. Proteins with a
signal-anchor sequence end up
embedded in the membrane.

Channel in
rough ER
Signal-anchor
sequence
Rough NH⫹
3 NH⫹ NH⫹
3
3
ER lumen

Proteins with signal-anchor The ER channel When translation is

sequences are threaded releases the protein completed, the protein
through a channel in the into the membrane. remains in the membrane.
ER membrane until the
signal-anchor sequence is
encountered.
 CHAPTER 5 O RG A N I Z I N G P R I N C I P L E S : L I P I D S , M E M B R A N E S , A N D C E L L CO M PA RT M E N T S 111

translation continues. When translation is completed, the carboxyl Mitochondria provide the eukaryotic cell with most
end of the chain remains on the cytosolic side of the ER membrane, of its usable energy.
the amino end is in the ER lumen, and the region between them Mitochondria are organelles that harness energy from chemical
resides in the membrane (Fig. 5.26c). Transmembrane proteins such compounds like sugars and convert it into ATP, which serves as the
as these may stay in the membrane of the ER or end up in other universal energy currency of the cell. ATP is able to drive the many
internal membranes or the plasma membrane, where they serve as chemical reactions in the cell. Mitochondria are present in nearly
transporters, pumps, receptors, or enzymes. all eukaryotic cells.
Mitochondria are rod-shaped organelles with an outer
membrane and a highly convoluted inner membrane whose folds
5.5 MITOCHONDRIA project into the interior (Fig. 5.27). A proton electrochemical
AND CHLOROPLASTS gradient is generated across the inner mitochondrial membrane,
and the energy stored in the gradient is used to synthesize ATP
The membranes of two organelles, mitochondria and for use by the cell. In the process of breaking down sugar and
chloroplasts, are not part of the endomembrane system. Both synthesizing ATP, oxygen is consumed and carbon dioxide is
of these organelles are specialized to harness energy for the cell. released. Does this process sound familiar? It also describes your
Interestingly, they are both semi-autonomous organelles that own breathing, or respiration. Mitochondria are the site of cellular
grow and multiply independently of the other membrane-bound respiration, and the oxygen that you take in with each breath
compartments, and they contain their own genomes. As we is used by mitochondria to produce ATP. Cellular respiration is
discuss in Chapter 27, the similarities between the DNA of these discussed in greater detail in Chapter 7.
organelles and the DNA of certain bacteria have led scientists to
conclude that these organelles originated as bacteria that were Chloroplasts capture energy from sunlight.
captured by a eukaryotic cell and, over time, evolved to their Both animal and plant cells have mitochondria to provide
current function. them with life-sustaining ATP. In addition, plant cells and

FIG. 5.27 Mitochondria. Mitochondria synthesize most of the ATP required to meet the cell’s energy needs. Source: Keith R. Porter/Science Source.

Mitochondria have a
double membrane, Outer membrane
consisting of an inner Inner membrane
and outer membrane
and an aqueous
compartment in between.
112 CO R E CO N C E P T S S U M M A RY

FIG. 5.28 Chloroplasts. Chloroplasts capture energy from sunlight and use it to synthesize sugars. Source: Dr. Jeremy Burgess/Science Source.

Chloroplasts are
Outer membrane
surrounded by a
double membrane Inner membrane
like mitochondria
and in addition have Thylakoid
a third membrane in membrane
the interior, the
thylakoid membrane.

green algae have organelles called chloroplasts that capture molecules called pigments, of which chlorophyll is the most
the energy of sunlight to synthesize simple sugars (Fig. 5.28). important. The green color of chlorophyll explains why so many
This process, called photosynthesis, results in the release of plants have green leaves.
oxygen as a waste product. Like the nucleus and mitochondria, Chlorophyll plays a key role in the chloroplast’s ability to
chloroplasts are surrounded by a double membrane. They also capture energy from sunlight. Using the light energy collected
have a third, internal membrane. This membrane defines a by this pigment, enzymes present in the chloroplast use
separate internal compartment called the thylakoid. The carbon dioxide as a carbon source to produce carbohydrates.
thylakoid membrane contains specialized light-collecting Photosynthesis is discussed in greater detail in Chapter 8. •

Core Concepts Summary

5.1 Cell membranes are composed of lipids, proteins, Membrane fluidity is influenced by length of fatty acid
and carbohydrates. chains, presence of carbon–carbon double bonds in fatty acid
chains, and amount of cholesterol. page 92
Phospholipids have both hydrophilic and hydrophobic
regions. As a result, they spontaneously form structures Many membranes also contain proteins that span the
such as micelles and bilayers when placed in an aqueous membrane (transmembrane proteins) or are temporarily
environment. page 90 associated with one or other layer of the lipid bilayer
(peripheral proteins). page 93
Membranes are fluid, meaning that membrane
components are able to move laterally in the plane of the
membrane. page 92
 CHAPTER 5 O RG A N I Z I N G P R I N C I P L E S : L I P I D S , M E M B R A N E S , A N D C E L L CO M PA RT M E N T S 113

5.2 The plasma membrane is a selective barrier that The endoplasmic reticulum is continuous with the outer
controls the movement of molecules between the inside nuclear envelope and manufactures proteins and lipids for
and the outside of the cell. use by the cell or for export out of the cell. page 105

Selective permeability results from the combination of The Golgi apparatus communicates with the endoplasmic
lipids and proteins that makes up cell membranes. reticulum by transport vesicles. It receives proteins and
page 96 lipids from the endoplasmic reticulum and directs them
to their final destinations. page 105
Passive transport is the movement of molecules by
diffusion, the random movement of molecules. There Lysosomes break down macromolecules like proteins
is a net movement of molecules from regions of higher to simpler compounds that can be used by the cell. page 107
concentration to regions of lower concentration.
page 96
Protein sorting directs proteins to their final destinations
in or out of the cell. page 108
Passive transport can occur by the diffusion of molecules
directly through the plasma membrane (simple diffusion) Proteins synthesized on free ribosomes are sorted after
or be aided by protein transporters (facilitated translation and proteins synthesized on ribosomes associated
diffusion). page 96 with the rough endoplasmic reticulum are sorted during
translation. page 108
Active transport moves molecules from regions of lower
concentration to regions of higher concentration and Proteins synthesized on free ribososomes are often
requires energy. page 97 sorted by means of a signal sequence and are destined for
the cytosol, mitochondria, chloroplasts, or nucleus.
Primary active transport uses energy stored in ATP;
page 108
secondary active transport uses the energy stored in an
electrochemical gradient. page 98 Proteins synthesized on ribosomes on the rough endoplasmic
reticulum have a signal sequence that is recognized by a
Animal cells often maintain size and shape by protein
signal-recognition particle. These proteins end up as
pumps that actively move ions in and out of the cell.
transmembrane proteins, in the interior of various organelles,
page 99
or secreted. page 109
Plants, fungi, and bacteria have a cell wall outside the
plasma membrane that maintains cell size and shape. 5.5 Mitochondria and chloroplasts are organelles
page 100 involved in harnessing energy, and likely evolved from
free-living prokaryotes.
5.3 Cells can be classified as prokaryotes or Mitochondria harness energy from chemical compounds for
eukaryotes, which differ in the degree of internal use by both animal and plant cells. page 111
compartmentalization.
Chloroplasts harness the energy of sunlight to build
Prokaryotic cells lack a nucleus and other internal sugars. page 111
membrane-enclosed compartments. page 101

Prokaryotic cells include bacteria and archaeons and are

much smaller than eukaryotes. page 101 Self-Assessment
Eukaryotic cells have a nucleus and other internal 1. Describe how lipids with hydrophilic and hydrophobic regions
compartments called organelles. page 101 behave in an aqueous environment.

Eukaryotes include animals, plants, fungi, and protists. 2. Describe two types of association between proteins and
page 102 membranes.

3. Describe an experiment that demonstrates that proteins

5.4 The endomembrane system is an interconnected
move in membranes.
system of membranes that includes the nuclear
envelope, endoplasmic reticulum, Golgi apparatus, 4. Name three parameters that need to be stably maintained
lysosomes, vesicles, and plasma membrane. inside a cell.

The nucleus, which is enclosed by a double membrane 5. Explain the role of lipids and proteins in maintaining the
called the nuclear envelope, houses the genome. page 105 selective permeability of membranes.
114 SELF-ASSESSMENT

6. Distinguish between passive and active transport 10. Explain how a protein ends up free in the cytosol,
mechanisms across cell membranes. embedded in the plasma membrane, or secreted from the cell.

7. Describe three different ways in which cells maintain

size and shape.
Log in to to check your answers to the Self-
8. Compare the organization, degree of Assessment questions, and to access additional learning tools.
compartmentalization, and size of prokaryotic and
eukaryotic cells.

9. Name the major organelles in eukaryotic cells and

describe their functions.
 CHAPTER 6

Making Life
Work
Capturing and Using Energy

Core Concepts
6.1 Metabolism is the set of
biochemical reactions that
transforms biomolecules and
transfers energy.
6.2 Kinetic energy is energy of
motion, and potential energy is
stored energy.
6.3 The laws of thermodynamics
govern energy flow in
biological systems.
6.4 Chemical reactions involve the
breaking and forming of bonds.
6.5 The rate of biochemical
reactions is increased by
protein catalysts called
Purestock/Getty Images. enzymes.

115
116 SECTION 6.1 A N OV E RV I E W O F M E TA B O L I S M

We have seen that cells require a way to encode and transmit chemical compounds (Fig. 6.1). Organisms that capture energy
information and a membrane to separate inside from out. The from sunlight are called phototrophs. Plants are the most familiar
third requirement of a cell is energy, which cells need to do work. example. Plants use the energy of sunlight to convert carbon
Cells grow and divide, move, change shape, pump ions in and out, dioxide and water into sugar and oxgen (Chapter 8). Sugars, such
transport vesicles, and synthesize macromolecules such as DNA, as glucose, contain energy in their chemical bonds that can be used
RNA, proteins, and complex carbohydrates. All of these activities to synthesize ATP, which in turn can power the work of the cell.
are considered work, and they therefore require energy. Other organisms derive their energy directly from chemical
We are all familiar with different forms of energy—the sun compounds. These organisms are called chemotrophs, and animals
and wind provide sources of energy, as do fossil fuels such as oil are familiar examples. Animals ingest other organisms, obtaining
and natural gas. We have learned to harness the energy from organic molecules such as glucose that they break down in the
these sources and convert it to other forms, such as electricity, to presence of oxygen to produce carbon dioxide and water. In this
provide needed power to our homes and cities. process, the energy in the chemical bonds of the organic molecule
Cells are faced with similar challenges. They must harness is converted to energy carried in the bonds of ATP (Chapter 7).
energy from the environment and convert it to a form that allows In drawing this distinction between plants and animals, we have
them to do the work necessary to sustain life. Cells harness to be careful. Although sunlight provides the energy that plants
energy from the sun and from chemical compounds, including use to synthesize glucose, plants still power most cellular processes
carbohydrates, lipids, and proteins. Although the source of energy by breaking down the sugar they make, just as animals do. And
may differ among cells, all cells convert energy to a form that can although chemotrophs harness energy from organic molecules, the
be easily used to drive cellular processes. All cells use energy in the energy in these organic molecules is generally derived from the sun.
form of a molecule called adenosine triphosphate (ATP). Bearing these two points in mind, we use the terms “phototroph”
ATP is often called the universal “currency” of cellular energy to and “chemotroph” because they call our attention to the flow of
indicate that ATP provides energy in a form that all cells can readily energy from the sun to organisms and then from one organism to
use to perform the work of the cell. Although “currency” provides the next (discussed more fully in Chapter 25).
a useful analogy for the role that ATP plays, keep in mind an Organisms can also be classified in terms of where they get their
important distinction between actual currency, such as a dollar bill, carbon (Fig. 6.1). Some organisms are able to convert carbon dioxide
and ATP. A dollar bill represents a certain value but does not in fact (an inorganic form of carbon) into glucose (an organic form of
have any value in itself. By contrast, ATP does not represent energy; carbon). These organisms are autotrophs, or “self feeders,” because
it actually contains energy in its chemical bonds. Nevertheless, the they make their own organic carbon using inorganic carbon as the
analogy is useful because ATP, like a dollar bill, engages in a broad starting material. Plants again are an example, so plants are both
range of energy “transactions” in the cell, as we discuss below. phototrophs and autotrophs, or photoautotrophs.
In this chapter, we consider energy in the context of cells. Other organisms do not have the ability to convert carbon
What exactly is energy? What principles govern its flow in dioxide into organic forms of carbon. Instead, they obtain their
biological systems? And how do cells make use of it? carbon from organic molecules synthesized by other organisms,
called preformed organic molecules. In other words, these
organisms eat other organisms or molecules derived from other
6.1 AN OVERVIEW OF METABOLISM organisms. Such organisms are heterotrophs, or “other feeders,”
as they rely on other organisms for their organic forms of carbon.
When considering a cell’s use of energy, it is helpful to also consider Animals obtain carbon in this way, and so animals are both
the cell’s sources of carbon because carbon is the backbone of chemotrophs and heterotrophs, or chemoheterotrophs. In fact,
the organic molecules that make up cells and because cells often animals get their energy and carbon from the same molecule. That
use carbon-based compounds as a stable form of energy storage. is, a molecule of glucose supplies both energy and carbon.
How organisms obtain the energy and carbon needed for growth As we move away from the familiar examples of plants
and other vital functions is so fundamental that it is sometimes and animals and begin in Part 2 to explore the diversity of life,
used to provide a metabolic classification of life (Fig. 6.1). Simply we will see that not all organisms fit into the two categories
put, organisms have two ways of harvesting energy from their of photoautotrophs and chemoheterotrophs (Fig. 6.1). Some
environment and two sources of carbon. Together, this means that microorganisms gain energy from sunlight but obtain their carbon
there are four principal ways in which organisms acquire the energy from preformed organic molecules; such organisms are called
and materials needed to grow, function, and reproduce. photoheterotrophs. Other microorganisms extract energy from
inorganic sources but build their own organic molecules; these
Organisms can be classified according to their energy organisms are called chemoautotrophs. They are often found in
and carbon sources. extreme environments, such as deep-sea vents, where sunlight
Organisms have two ways of harvesting energy from their is absent and inorganic compounds such as hydrogen sulfide
environment: They can obtain energy either from the sun or from are plentiful.
 CHAPTER 6 M A K I N G L I F E W O R K : C A P T U R I N G A N D U S I N G E N E RG Y 117

FIG. 6.1 A metabolic classification of organisms. Organisms can be classified according to their energy and carbon sources. Photo sources:
(clockwise from top-left) Dr. Tony Brain/Science Source; ScienceFoto.DE/Dr. Andre Kemp/Getty Images; Copyright 1997 Microbial Diversity, Rolf Schauder; Martin
Oeggerli/Science Source; DNY59/iStockphoto; David J. Patterson; EM image by Manfred Rohde, Helmholtz Centre for Infection Research, Braunschweig, Germany;
Anna Omelchenko/Dreamstime.com.

All organisms

Energy Phototrophs Chemotrophs

source
Energy from sunlight Energy from chemical compounds

Carbon Autotrophs Heterotrophs Autotrophs Heterotrophs

source
Carbon from inorganic Carbon from Carbon from inorganic Carbon from
sources (such as CO2) organic compounds sources (such as CO2) organic compounds

Cyanobacteria Heliobacteria Sulfur-oxidizing bacteria Most bacteria

Examples

Vascular plants Most green Hydrogen bacteria Animals

non-sulfur bacteria

Metabolism is the set of chemical reactions that

sustain life. FIG. 6.2 Catabolism and anabolism. The energy harvested as ATP
The building and breaking down of sugars such as glucose andthe during the breakdown of molecules in catabolism can be
harnessing and release of energy in the process are driven by used to synthesize molecules in anabolism.
chemical reactions in the cell. The term metabolism encompasses
the entire set of these chemical reactions that convert molecules Macromolecules
Carbohydrates
into other molecules and transfer energy in living organisms. Proteins
These chemical reactions are occurring all the time in your cells. Fats
Nucleic acids
Many of these reactions are linked, in that the products of one are
the reactants of the next, forming long pathways and intersecting ADP ADP
+ +
networks.
Catabolism

An a b o l i s m

Metabolism is divided into two branches: Catabolism is the

set of chemical reactions that break down molecules into smaller ATP + H2O ATP + H2O
units and, in the process, produce ATP, and anabolism is the set
of chemical reactions that build molecules from smaller units and
Subunits
require an input of energy, usually in the form of ATP (Fig. 6.2). Sugars
For example, carbohydrates can be broken down, or catabolized, Amino acids
Fatty acids
into sugars, fats into fatty acids and glycerol, and proteins into Nucleotides
amino acids. These initial products can be broken down further to
118 SECTION 6.2 K I N E T I C A N D P OT E N T I A L E N E RG Y

release energy stored in their chemical bonds. The synthesis of than at the bottom (Fig. 6.3). If it were not blocked by the floor,
macromolecules such as carbohydrates and proteins, by contrast, it would move from the position of higher potential energy (the
is anabolic. top of the stairs) to the position of lower potential energy (the
bottom of the stairs) because of the force of gravity. Similarly, an
electrochemical gradient of molecules across a cell membrane is a
6.2 KINETIC AND POTENTIAL ENERGY form of potential energy. Given a pathway through the membrane,
the molecules move down their concentration and electrical
There are many different sources of energy, including sunlight, gradients from higher to lower potential energy (Chapter 5).
wind, electricity, and fossil fuels. The food we eat also contains Energy can be converted from one form to another. The ball
energy. Energy can be defined as the capacity to do work. For a at the top of the stairs has a certain amount of potential energy
cell, work involves processes we discussed in earlier chapters, such because of its position. As it rolls down the stairs, this potential
as synthesizing DNA, RNA, and proteins, pumping substances energy is converted to kinetic energy associated with movement
across the plasma membrane, and moving vesicles between of the ball and the surrounding air. When the ball reaches the
various compartments of a cell. bottom of the stairs, the remaining energy is stored as potential
Although there are many different sources of energy, energy energy. Conversely, it takes an input of energy to move the ball
comes in just two major forms. In this section, we consider these back to the top of the stairs, and this input of energy is stored as
two forms of energy. In addition, we focus on how energy is held in potential energy.
chemical bonds of molecules such as glucose and ATP.
Chemical energy is a form of potential energy.
Kinetic energy and potential energy are two forms We obtain the energy we need from the food we eat, which
of energy. contains chemical energy. Chemical energy is a form of potential
Energy can be classified as one of two forms: kinetic energy or energy held in the chemical bonds between pairs of atoms in a
potential energy (Fig. 6.3). Kinetic energy is the energy of motion, molecule. Recall from Chapter 2 that a covalent bond results from
and it is perhaps the most familiar form of energy. A moving the sharing of electrons between two atoms. Covalent bonds form
object, such as a ball bouncing down a set of stairs, possesses kinetic when the sharing of electrons between two atoms results in a
energy. Kinetic energy is associated with any kind of movement, more stable configuration than if the orbitals of the two atoms did
such as a person running or a muscle contracting. Similarly, light not overlap.
is associated with the movement of photons, electricity with the The more stable configuration will always be the one with
movement of electrons, and thermal energy (perceived as heat) lower potential energy. As a result, energy is required to break a
with the movement of molecules, so these, too, are forms of covalent bond because going from a lower energy state to a higher
kinetic energy. one requires an input of energy. Conversely, energy is released
Energy is not always associated with motion. An immobile when a covalent bond forms.
object can still possess a form of energy called potential energy, Some bonds are stronger than other ones. A strong bond
or stored energy. Potential energy depends on the structure of the is hard to break because the arrangement of orbitals in these
object or its position relative to its surroundings, and it is released molecules is much more stable than the two atoms would be on
by a change in the object’s structure or position. For example, the their own. As a result, strong bonds do not contain very much
potential energy of a ball is higher at the top of a flight of stairs chemical energy, similar to the potential energy of a ball at
the bottom of a flight of stairs (Fig. 6.3). This may at first seem
counterintuitive, but makes sense when you consider that strong
FIG. 6.3 Potential and kinetic energy. Some of the high potential bonds have a very stable arrangement of orbitals and therefore do
energy of a ball at the top of a set of stairs is transformed to not require a lot of energy to remain intact. Examples of molecules
kinetic energy as the ball rolls down the stairs. with strong covalent bonds that contain relatively little chemical
energy are carbon dioxide (CO2) and water (H2O).
Conversely, some covalent bonds are relatively weak. These
bonds are easily broken because the arrangement of orbitals in
these molecules is only somewhat more stable than if the two
atoms did not share any electrons. As a result, weak covalent
bonds require a lot of energy to stay intact and contain a lot
of chemical energy, similar to the potential energy of a ball at
the top of a flight of stairs (Fig. 6.3). Organic molecules such
as carbohydrates, lipids, and proteins contain relatively weak
High potential energy Kinetic energy Low potential energy covalent bonds, including many carbon–carbon (C–C) bonds and
 CHAPTER 6 M A K I N G L I F E W O R K : C A P T U R I N G A N D U S I N G E N E RG Y 119

(section 6.4). The released energy in turn can be harnessed to

FIG. 6.4 The structure of ATP. power the work of the cell.

The bonds linking

these phosphate
groups have high NH2 6.3 THE LAWS OF THERMODYNAMICS
potential energy.
N
N Energy from the sun can be converted by plants and other
O- O- O-
O phototrophs into chemical potential energy held in the bonds of
-O P O P O P O CH2 N N molecules. Chemical reactions can transfer this chemical energy
O O O Adenine between different molecules. And energy from these molecules
H H
Phosphate groups H H can be used by the cell to do work. In all these instances, energy
is subject to the laws of thermodynamics. In fact, all processes
OH OH in a cell, from diffusion to osmosis to the pumping of ions to
Ribose
cell movement, are subject to these laws. There are four laws of
Adenosine thermodynamics, but two of them are particularly relevant to
biological processes and are discussed here.
Adenosine–monophosphate (AMP)
The first law of thermodynamics: Energy is conserved.
Adenosine–diphosphate (ADP) The first law of thermodynamics is the law of conservation of
energy, which states that the universe contains a constant amount
Adenosine–triphosphate (ATP) of energy. Therefore, energy is neither created nor destroyed.
New energy is never formed, and energy is never lost. Instead,
energy changes from one form to another. For example, kinetic
carbon–hydrogen (C–H) bonds. Therefore, organic molecules are energy can change to potential energy and vice versa, but the total
rich sources of chemical energy and are called fuel molecules. amount of energy always remains the same (Fig. 6.5).
In the simple example of a ball rolling down a set of stairs
ATP is a readily accessible form of cellular energy. (see Fig. 6.3), the potential energy stored at the top of the stairs is
The chemical energy carried in carbohydrates, lipids, and proteins converted to kinetic energy associated with the movement of the
is harnessed by cells to do work. But cells do not use this energy ball and the surrounding air. The total amount of energy in this
all at once. Instead, through the series of chemical reactions transformation is constant—that is, the difference in potential
described in Chapter 7, they package this energy into a chemical energy at the top and the bottom of the stairs is equal to the total
form that is readily accessible to the cell. One form of chemical amount of kinetic energy associated with the movement of the
energy is adenosine triphosphate, or ATP, shown in Fig. 6.4. The ball and air molecules.
chemical energy in the bonds of ATP is used in turn to drive many
cellular processes, such as muscle contraction, cell movement, and The second law of thermodynamics: Energy
membrane pumps. In this way, ATP serves as a go-between, acting transformations always result in an increase in
as an intermediary between fuel molecules that store a large disorder in the universe.
amount of potential energy in their bonds and the activities of the When energy changes forms, the total amount of energy remains
cell that require an input of energy. constant. However, in going from one form of energy to another,
ATP is composed in part of adenosine, which is made up of the
base adenine and the five-carbon sugar ribose. The ribose is attached
to triphosphate, or three phosphate groups (Fig. 6.4). Its chemical
relatives are ADP (adenosine diphosphate) and AMP (adenosine FIG. 6.5 The first law of thermodynamics. When energy is
monophosphate), with two phosphate groups and one phosphate transformed from one form to another, the total amount of
group, respectively. The use of ATP as an energy source in nearly all energy remains the same.
cells reflects its use early in the evolution of life on Earth.
The chemical energy of ATP is held in the bonds connecting TRANSFORMATION
the phosphate groups. At physiological pH, these phosphate
groups are negatively charged and have a tendency to repel each
Total energy before = Total energy after
other. The chemical bonds connecting the phosphate groups
therefore store chemical energy. This energy is released when new,
Unit of energy
more stable bonds are formed that contain less chemical energy
120 SECTION 6.4 C H E M I C A L R E AC T I O N S

(movement), and the rest is dissipated as thermal energy (which

FIG. 6.6 The second law of thermodynamics. Because the amount is why your muscles warm as you exercise). The amount of
of disorder increases when energy is transformed from chemical potential energy expended is equal to the amount of
one form to another, some of the total energy is not kinetic energy plus the amount of thermal energy, consistent
available to do work. with the first law of thermodynamics. In addition, not all of
Energy, often in
the form of heat, the chemical energy stored in molecules is converted to kinetic
used to increase energy, as thermal energy flowing as heat is a necessary by-product
entropy of the energy transformation, consistent with the second law of
thermodynamics.
TRANSFORMATION
Energy available In living organisms, catabolic reactions result in an increase
to do work of entropy as a single ordered biomolecule is broken down into
several smaller ones with more freedom to move around. Anabolic
Total energy = Total energy reactions, by contrast, might seem to decrease entropy because
before after
they use individual building blocks to synthesize more ordered
biomolecules such as proteins or nucleic acids. Do these anabolic
Unit of energy
reactions violate the second law of thermodynamics? No, because
the second law of thermodynamics always applies to the universe
the energy available to do work decreases (Fig. 6.6). How can the as a whole, not to a chemical reaction in isolation. A local decrease
total energy remain the same, but the energy available to do in entropy is always accompanied by an even higher increase in the
work decrease? The answer is that some of the energy is available entropy of the surroundings. The production of heat in a chemical
to do work, and some is not available to do work. So, energy reaction increases entropy because heat is associated with the
transformations are never 100% efficient since the amount of motion of molecules. The combination of the decrease in entropy
energy available to do work decreases every time energy changes associated with the building of a macromolecule and the increase
forms. in entropy associated with heat always results in a net increase
The energy that is not available to do work takes the form in entropy.
of an increase in disorder. Thus, there is a universal price to pay The key point here is that the maintenance of the high degree
in transforming energy from one form to another. For example, of function and organization of a single cell or a multicellular
when kinetic energy is changed into potential energy, the amount organism requires a constant input of energy. This input, as we
of disorder always increases. This principle is summarized by have seen, comes either from the sun or from the energy stored in
the second law of thermodynamics, which states that the chemical compounds. This energy allows molecules to be built and
transformation of energy is associated with an increase in disorder other work to be carried out, but also leads to greater entropy in
of the universe. The degree of disorder is called entropy. the surroundings.
The degree of disorder is just one way to describe entropy.
j Quick Check 1 Cold air has less entropy than hot air. The second
Another way to think about entropy is to consider the number of
law of thermodynamics states that entropy always increases. Do air
possible positions and motions (collectively called microstates)
conditioners violate this law?
a molecule can take on in a given system. As entropy increases,
so does the number of positions and motions available to the
molecule. Consider the expansion of a gas after the lid is removed
from its container. Given more space, the molecules of the gas are 6.4 CHEMICAL REACTIONS
less constrained and able to move about more freely. They have
more positions available to them and move about at a larger range Living organisms build and break down molecules, pump ions
of speeds, so they have more entropy. across membranes, move, and in general function largely through
In chemical reactions, most of the entropy increase occurs chemical reactions. As we have seen, organisms break down food
through the transformation of various forms of energy into thermal molecules, such as glucose, storing energy in the bonds of ATP,
energy, which we experience as heat. Thermal energy is a type of which then powers chemical reactions that sustain life. Chemical
kinetic energy corresponding to the random motion of molecules reactions are therefore central to life processes. In this section,
and results in a given temperature. The higher the temperature, the we describe what a chemical reaction is and how the laws of
more rapidly the molecules move, and the higher the disorder. thermodynamics apply.
Consider the contraction of a muscle, a form of kinetic energy
associated with the shortening of muscle cells. The contraction of A chemical reaction occurs when molecules interact.
muscle is powered by chemical potential energy in fuel molecules. As we saw in Chapter 2, a chemical reaction is the process by
Some of this potential energy is transformed into kinetic energy which molecules, called reactants, are transformed into other
 CHAPTER 6 M A K I N G L I F E W O R K : C A P T U R I N G A N D U S I N G E N E RG Y 121

The laws of thermodynamics determine whether a

FIG. 6.7 A chemical reaction. Atoms retain their identity during a chemical reaction requires or releases energy available
chemical reaction as bonds are broken and new bonds are to do work.
formed to yield new molecules. We have seen that cells use chemical reactions to perform much
of the work of the cell. The amount of energy available to do work
A single covalent bond A covalent bond is formed is called Gibbs free energy (G). In a chemical reaction, we can
connecting carbon and oxygen between the carbon of CO2
in CO2 and a single covalent and the oxygen of H2O, and compare the free energy of the reactants and products to determine
bond connecting hydrogen and between one oxygen of CO2 whether the reaction releases energy that is available to do work.
oxygen in H2O are broken. and one hydrogen of H2O.
The difference between two values is denoted by the Greek letter
O H delta (∆). In this case, ∆G is the free energy of the products minus
O C O + H O H O C New the free energy of the reactants (Fig. 6.8). If the products of a
O H covalent
bonds reaction have more free energy than the reactants, then ∆G is
positive and a net input of energy is required to drive the reaction
CO2 + H 2O H2CO3 forward (Fig. 6.8a). By contrast, if the products of a reaction have
Carbon Water Carbonic less free energy than the reactants, ∆G is negative and energy is
dioxide acid released and available to do work (Fig. 6.8b).
Reactions with a negative ∆G that release energy and proceed
spontaneously are called exergonic, and reactions with a positive
molecules, called products. During a chemical reaction, atoms ∆G that require an input of energy and are not spontaneous are
keep their identity, but the bonds linking the atoms change. called endergonic. Note that the term “spontaneous” does not
For example, carbon dioxide (CO2) and water (H2O) can react to imply instantaneous or even rapid. “Spontaneous” in this context
produce carbonic acid (H2CO3), as shown in Fig. 6.7. means that a reaction releases energy; “non-spontaneous” means
This reaction is common in nature. For example, it occurs that a reaction requires a sustained input of energy.
when carbon dioxide in the air enters ocean waters, and explains Recall that the total amount of energy is equal to the energy
why the oceans are becoming more acidic with increasing carbon available to do work plus the energy that is not available to
dioxide levels in the atmosphere. This reaction also takes place in do work because of the increase in entropy. We can write this
the bloodstream. When cells break down glucose, they produce relationship as an equation in which the total amount of energy is
carbon dioxide. In animals, this carbon dioxide diffuses out of cells enthalpy (H), the energy available to do work is Gibbs free energy
and into the blood. Rather than remaining dissolved in solution, (G), and the degree of disorder is entropy (S) multiplied by the
most of the carbon dioxide is converted into carbonic acid by the absolute temperature (T ) (measured in degrees Kelvin), since
preceding reaction. In an aqueous (watery) environment like temperature influences the movement of molecules (and hence
the ocean or the blood, carbonic acid exists as bicarbonate ions the degree of disorder). Therefore,
(HCO32) and protons (H1).
Most chemical reactions in cells are readily reversible: The Total amount of energy (H) 5
products can react to form the reactants. For example, carbon energy available to do work (G) 1 energy lost to entropy (TS)
dioxide and water react to form carbonic acid, and carbonic acid
can dissociate to produce carbon dioxide and water. This reverse
reaction also occurs in the blood, allowing carbon dioxide to be
removed from the lungs or gills (Chapter 39). The reversibility of FIG. 6.8 Endergonic and exergonic reactions. An endergonic
the reaction is indicated by a double arrow (Fig. 6.7). reaction requires an input of energy; an exergonic reaction
The way the reaction is written defines forward and reverse releases energy.
reactions: A forward reaction proceeds from left to right and
a. Endergonic reaction b. Exergonic reaction
the reactants are located on the left side of the arrow; a reverse (Non-spontaneous) (Spontaneous)
reaction proceeds from right to left and the reactants are located
Products Reactants
on the right side of the arrow.
The direction of a reaction can be influenced by the
Free energy

Free energy

Amount of Amount of
concentrations of reactants and products. For example, increasing
energy energy
the concentration of the reactants or decreasing the concentration required (+∆G) released (–∆G)
of the products favors the forward reaction. This effect explains
how many reactions in metabolic pathways proceed: The products Reactants Products
of many reactions are quickly consumed by the next reaction,
helping to drive the first reaction forward. Course of reaction Course of reaction
122 SECTION 6.4 C H E M I C A L R E AC T I O N S

We are interested in the energy available for a cell to do

work, or G. Therefore, we express G in terms of the other two FIG. 6.9 Energy in catabolism and anabolism. Catabolic reactions
parameters, H and TS, as follows: have a negative ∆G and release energy, often in the form of
ATP. Anabolic reactions have a positive ∆G and require an
G 5 H 2 TS input of energy, often in the form of ATP.
In a chemical reaction, we can compare the total energy and
entropy of the reactants with the total energy and entropy of the Less disorder (–∆S),
more chemical energy
products to see if there is energy available to do work. As a result, in bonds (+∆H).
we get

∆G 5 ∆H 2 T∆S

This equation is a useful way to see if a chemical reaction takes

place spontaneously, what direction the reaction proceeds in,
and whether net energy is required or released. Let’s take a step
ADP ADP
back and make sure it makes sense intuitively. The value of ∆G + +

Catabolism

An a bo l i s m
depends on both the change in enthalpy and the change in disorder.
–∆G +∆G
Catabolic reactions are those in which the products have less
chemical energy (lower enthalpy) in their bonds than the reactants ATP + H2O ATP + H2O
have, and the products are more disordered (higher entropy) than
the reactants are. In other words, such reactions have a negative
value of ∆H and a positive value of ∆S. Notice that, in the equation
∆G 5 ∆H 2 T∆S, there is a negative sign in front of ∆S, so both the
negative ∆H and positive ∆S contribute to a negative ∆G. Therefore,
these reactions proceed spontaneously (Fig. 6.9).
The reverse is also true: Increasing chemical energy (positive More disorder (+∆S),
less chemical energy
∆H) and decreasing disorder (negative ∆S), as in the synthesis of in bonds (–∆H).
proteins from individual amino acids and other anabolic reactions,
results in a positive value of ∆G and requires a net input of energy
(Fig. 6.9).
There are also cases in which the change in enthalpy and the
The reaction of ATP with water is an exergonic reaction
change in entropy are both positive or both negative. In these
because there is less free energy in the products compared to the
cases, the absolute value of these parameters determines whether
reactants. The free energy difference can be explained by referring
∆G is positive or negative and therefore whether a reaction is
to the formula we derived in the last section. Recall that the
spontaneous or not.
phosphate groups of ATP are negatively charged at physiological
j Quick Check 2 How does increasing the temperature affect the pH and repel each other. ATP has three phosphate groups, and ADP
change in free energy (∆G ) of a chemical reaction? has two. Therefore, ADP is more stable (contains less chemical
energy in its bonds) than ATP, resulting in a negative value of ∆H.
In addition, a single molecule of ATP is broken down into two
The hydrolysis of ATP is an exergonic reaction. molecules, ADP and Pi. Therefore, the reaction is also associated
Let’s apply these concepts to a specific chemical reaction.
with an increase in entropy, or a positive value of ∆S. Since
Earlier, we introduced ATP, the molecule that drives many
∆G 5 ∆ H 2 T∆ S, ∆ G is negative and the reaction is a spontaneous
cellular processes using the chemical potential energy in its
one that releases energy available to do work.
chemical bonds. ATP reacts with water to form ADP and inorganic
The free energy difference for ATP hydrolysis is approximately
phosphate, Pi (HPO422), as shown here and in Fig. 6.10:
–7.3 kcal per mole (kcal/mol) of ATP. This value is influenced
ATP 1 H2O → ADP 1 Pi by several factors, including the concentration of reactants and
products, the pH of the solution in which the reaction occurs,
This is an example of a hydrolysis reaction, a chemical reaction in and the temperature and pressure. The value –7.3 kcal/mol is
which a water molecule is split into a proton (H1) and a hydroxyl the value under standard laboratory conditions in which the
group (OH2). Hydrolysis reactions often break down polymers into concentrations of reactants and products are equal and pressure
their subunits, and in the process one product gains a proton and is held constant. In a cell, it is likely higher, on the order of
the other gains a hydroxyl group. –12 kcal/mol.
 CHAPTER 6 M A K I N G L I F E W O R K : C A P T U R I N G A N D U S I N G E N E RG Y 123

FIG. 6.10 ATP hydrolysis. ATP hydrolysis is an exergonic reaction that releases free energy.

NH2 Free energy NH2

N N
O- O- O- N O- O- N O-
-O O O O -O
P O P O P O CH2 N N + HO P O P O CH2 N N + P OH
H H
O O O O O O
H H H H

OH OH OH OH

ATP + H2O ADP + Pi

Adenosine Water Adenosine Inorganic

triphosphate diphosphate phosphate

Keep in mind that the release of free energy during ATP for the two reactions is negative. So ATP hydrolysis provides the
hydrolysis comes from breaking weaker bonds (with more thermodynamic driving force for the non-spontaneous reaction,
chemical energy) in the reactants and forming more stable bonds and the shared phosphate group couples the two reactions
(with less chemical energy) in the products. The release of free together.
energy then drives chemical reactions and other processes that Following ATP hydrolysis, the cell needs to replenish its
require a net input of energy, as we discuss next. ATP so that it can carry out additional chemical reactions. The
synthesis of ATP from ADP and Pi is an endergonic reaction with a
Non-spontaneous reactions are positive ∆G, requiring an input of energy. In some cases, exergonic
often coupled to spontaneous reactions. reactions can drive the synthesis of ATP by energetic coupling
If the conversion of reactant A into product B is spontaneous, the (Fig. 6.11b). The sum of the ∆G’s of the two reactions is negative
reverse reaction converting reactant B into product A is not. The and the reactions share a phosphate group, allowing the two
∆ G’s for the forward and reverse reactions have the same absolute reactions to proceed.
value but opposite signs. You might expect that the direction Like ATP, other phosphorylated molecules can be hydrolyzed,
of the reaction would always be from A to B. However, in living releasing free energy. Hydrolysis reactions can be ranked by their
organisms, not all chemical reactions
are spontaneous. Anabolic reactions are
a good example; they require an input
of energy to drive them in the right FIG. 6.11 Energetic coupling. A spontaneous (exergonic) reaction drives a non-spontaneous
direction. This raises the question: What (endergonic) reaction. (a) The hydrolysis of ATP drives the formation of glucose
drives non-spontaneous reactions? 6-phosphate from glucose. (b) The hydrolysis of phosphoenolpyruvate drives the
Energetic coupling is a process in synthesis of ATP.
which a spontaneous reaction (negative The coupled reaction
∆ G) drives a non-spontaneous reaction proceeds because ∆G is
(positive ∆ G). It requires that the net negative and Pi is shared
between the two reactions.
∆ G of the two reactions be negative. In
a.
addition, the two reactions must occur
ATP + H2O ADP + Pi ∆G1 = –7.3 kcal/mol Exergonic reaction
together. In some cases, this coupling
Glucose + Pi Glucose + H2O ∆G2 = +3.3 kcal/mol Endergonic reaction
can be achieved if the two reactions
6-phosphate
share an intermediate.
For example, ATP hydrolysis can Glucose + ATP Glucose + ADP ∆G = –4 kcal/mol Coupled reaction
6-phosphate
be used to drive a non-spontaneous
reaction, as shown in Fig. 6.11a. In this b.
case, the phosphate group released Phosphoenolpyruvate + H2O Pyruvate + Pi ∆G1 = –14.8 kcal/mol Exergonic reaction
during ATP hydrolysis is transferred
ADP + Pi ATP + H2O ∆G2 = +7.3 kcal/mol Endergonic reaction
to glucose to produce glucose
6-phosphate. In addition, the net ∆G Phosphoenolpyruvate + ADP Pyruvate + ATP ∆G = –7.5 kcal/mol Coupled reaction
124 SECTION 6.5 E N Z Y M E S A N D T H E R AT E O F C H E M I C A L R E AC T I O N S

possible reactions that could occur in

FIG. 6.12 ∆G of common hydrolysis reactions in a cell. ATP hydrolysis has an intermediate a cell. In this section, we discuss how
value of ∆G compared to other common hydrolysis reactions. enzymes increase the rate of chemical
reactions and how this ability gives
them a central role in metabolism.
–14.8 Phosphoenolpyruvate + H2O Pyruvate + Pi
∆G of hydrolysis reaction

Enzymes reduce the activation

(in kcal/mol)

∆G for ATP hydrolysis is intermediate energy of a chemical reaction.

compared to ∆G of hydrolysis of common
–7.3 ATP + H2O ADP + Pi
phosphorylated molecules, allowing ATP to Earlier, we saw that exergonic reactions
drive reactions as well as be replenished. release free energy and endergonic
reactions require free energy.
–3.3 Glucose 6-phosphate + H2O Glucose + Pi Nevertheless, all chemical reactions
require an input of energy to proceed,
even exergonic reactions that release
energy. For an exergonic reaction, the
energy released is more than the initial
free energy differences (∆G). Fig. 6.12 shows that ATP hydrolysis input of energy, so there is a net release of energy.
has an intermediate free energy difference compared with the Why is an input of energy needed for all chemical reactions?
free energy difference for the hydrolysis of other common As a chemical reaction proceeds, existing chemical bonds break
phosphorylated molecules. Those reactions that have a ∆G more and new ones form. For an extremely brief period of time, a
negative than that of ATP hydrolysis transfer a phosphate group compound is formed in which the old bonds are breaking and the
to ADP by energetic coupling, and those reactions that have a new ones are forming. This intermediate stage between reactants
∆G less negative than that of ATP hydrolysis receive a phosphate and products is called the transition state. It is highly unstable
group from ATP by energetic coupling. Therefore, ADP is an energy and therefore has a large amount of free energy.
acceptor and ATP an energy donor. The ATP–ADP system is at In all chemical reactions, reactants adopt at least one
the core of energetic coupling between catabolic and anabolic transition state before their conversion into products. The graph
reactions. in Fig. 6.13 represents the free energy levels of the reactant,
transition state, and product. This reaction is spontaneous since
the free energy of the reactant is higher than the free energy of
6.5 ENZYMES AND THE RATE the product and ∆G is negative. However, the highest free energy
OF CHEMICAL REACTIONS value corresponds to the transition state.
To reach the transition state, the reactant must absorb energy
Up to this point, we have focused on the spontaneity and direction from its surroundings (the uphill portion of the curve in Fig. 6.13).
of chemical reactions. Now we address their rate. As mentioned As a result, all chemical reactions, even spontaneous ones that
earlier, a spontaneous reaction is not necessarily a fast one. For release energy, require an input of energy that we can think of
example, the breakdown of glucose into carbon dioxide and water as an “energy barrier.” The energy input necessary to reach the
is spontaneous with a negative ∆G, but the rate of the reaction transition state is called the activation energy (EA). Once the
is close to zero and the breakdown of glucose is imperceptible. transition state is reached, the reaction proceeds, products are
However, glucose is readily broken down inside cells all the time. formed, and energy is released into the surroundings (the downhill
How is this possible? The answer is that chemical reactions in a cell portion of the curve in Fig. 6.13). There is an inverse correlation
are accelerated by chemical catalysts. between the rate of a reaction and the height of the energy barrier:
The rate of a chemical reaction is defined as the amount of the lower the energy barrier, the faster the reaction; the higher the
product formed (or reactant consumed) per unit of time. Catalysts barrier, the slower the reaction.
are substances that increase the rate of chemical reactions without In chemical reactions that take place in the laboratory, heat is
themselves being consumed. In biological systems, the catalysts are a common source of energy used to overcome the energy barrier.
usually proteins called enzymes, although, as we saw in Chapter In living organisms, reactions are accelerated by the action of
2, some RNA molecules have catalytic activity as well. Enzymes enzymes. However, enzymes do not act by supplying heat. Instead,
can increase the rate of chemical reactions dramatically. Moreover, they reduce the activation energy by stabilizing the transition
because they are highly specific, acting only on certain reactants state and decreasing its free energy (the red curve in
and catalyzing only some reactions, enzymes play a critical role Fig. 6.13). As the activation energy decreases, the speed of the
in determining which chemical reactions take place from all the reaction increases.
 CHAPTER 6 M A K I N G L I F E W O R K : C A P T U R I N G A N D U S I N G E N E RG Y 125

(enzyme–product, or EP). Finally, the complex dissociates,

FIG. 6.13 An enzyme-catalyzed reaction. An enzyme accelerates a releasing the enzyme and product. Therefore, a reaction catalyzed
reaction by lowering the activation energy, EA. by an enzyme can be described as

S1E ES EP E1P
All reactions require an input of
energy, called activation energy
(EA), to proceed. The formation of this complex is critical for accelerating the
rate of a chemical reaction. Recall from Chapter 4 that proteins
Transition state adopt three-dimensional shapes and that the shape of a protein is
An enzyme linked to its function. Enzymes are folded into three-dimensional
accelerates
EA the reaction shapes that bring particular amino acids into close proximity to
Free energy

by reducing form an active site. The active site of the enzyme is the portion
EA E A.
of the enzyme that binds substrate and catalyzes its conversion
Reactants to the product (Fig. 6.14). In the active site, the enzyme and
∆G is the same substrate form transient covalent bonds and/or weak noncovalent
∆G with and
Uncatalyzed reaction without an interactions. Together, these interactions stabilize the transition
Catalyzed reaction enzyme. state and decrease the activation energy.
Products
Enzymes also reduce the energy of activation by positioning
Course of reaction two substrates to react. The formation of the enzyme–substrate
complex promotes the reaction between two substrates by
aligning their reactive chemical groups and limiting their motion
relative to each other.
Although an enzyme accelerates a reaction by reducing The size of the active site is extremely small compared with
the activation energy, the difference in free energy between the size of the enzyme. If only a small fraction of the enzyme
reactants and products (∆G) does not change. In other words, an is necessary for the catalysis of a reaction, why are enzymes so
enzyme changes the path of the reaction between reactants and large? Of the many amino acids that form the active site, only a
products, but not the starting or end point (Fig. 6.13). Consider few actively contribute to catalysis. Each of these amino acids has
the breakdown of glucose into carbon dioxide and water. The ∆G to occupy a very specific spatial position to align with the correct
of the reaction is the same whether it proceeds by combustion reactive group on the substrate. If the few essential amino acids
or by the action of multiple enzymes in a metabolic pathway in were part of a short peptide, the alignment of chemical groups
a cell. between the peptide and the substrate would be difficult or
even impossible because the length of the bonds and the bond
j Quick Check 3 Which of the following do enzymes change? ∆G ;
angles in the peptide would constrain its three-dimensional
reaction rate; types of product generated; activation energy; the
structure. In fact, in many cases the catalytic amino acids are spaced
laws of thermodynamics.
far apart in the primary structure of the enzyme, but brought close

Enzymes form a complex with reactants and products.

An important characteristic of enzymes is that, as catalysts, they
participate in a chemical reaction but are not consumed in the FIG. 6.14 The active site of the enzyme catalase, shown
process. In other words, they emerge unchanged from a chemical schematically and as a surface model. Substrate binds
reaction, ready to catalyze the same reaction again. How do and is converted to product at the active site.
enzymes increase the reaction rate without being consumed? The Active site Active site
answer is that enzymes form a complex with the reactants and Substrate Substrate
products. H2O2
In a chemical reaction catalyzed by an enzyme, the reactant is
often referred to as the substrate. In such a reaction, the substrate
(S) is converted to a product (P):

S P

In the presence of an enzyme (E), the substrate first forms a

complex with the enzyme (enzyme–substrate, or ES). While Schematic model Molecular model
still part of the complex, the substrate is converted to product of an enzyme of catalase
 126 SECTION 6.5 E N Z Y M E S A N D T H E R AT E O F C H E M I C A L R E AC T I O N S

The formation of a complex between enzymes and substrates

FIG. 6.15 Formation of the active site of an enzyme by protein can be demonstrated experimentally, as illustrated in Fig. 6.16.
folding.
Enzymes are highly specific.
Unfolded
enzyme
Protein folding brings Enzymes are remarkably specific both for the substrate and the
COOH specific amino acids reaction that is catalyzed. In general, enzymes recognize either
Amino acids close to each other to
form the active site. a unique substrate or a class of substrates that share common
chemical structures. In addition, enzymes catalyze only one
COOH
Folded reaction or a very limited number of reactions.
The amino acids that
form the active site enzyme For example, the enzyme succinate dehydrogenase acts only
are often far apart in on succinate, and the enzyme b-(beta-)galactosidase acts only
the linear sequence of Bound
the unfolded enzyme. substrate on a specific class of molecules. The enzyme b-galactosidase
catalyzes the cleavage of the glycosidic bond that links galactose
to glucose in the disaccharide lactose, as well as any glycosidic
NH2
NH2 bond that links galactose to one of several types of molecule. In
this case, the enzyme does not recognize the whole substrate, but
a particular structural motif within it, and very small differences
in the structure of this motif affect the activity of the enzyme.
together in the formation of the active site by protein folding For example, b-galactosidase cleaves b-galactoside but does not
(Fig. 6.15). In other words, the large size of many enzymes is cleave a-(alpha-)galactoside, which differs from b-galactoside only
required at least in part to bring the catalytic amino acids into very in the orientation of the glycosidic bond. Hence, the enzyme is
specific positions in the active site of the folded enzyme. able to discriminate between two identical bonds with different

126 SECTION 5.1 M

HOW DO WE KNOW?

FIG. 6.16

Do enzymes form complexes enzyme b-galactosidase binds b-thiogalactoside but cannot cleave
or release it.

with substrates? CH2OH CH2OH

HO O HO O
BACKGROUND The idea that enzymes form complexes with H O R H S R
OH H OH H
substrates to catalyze a chemical reaction was first proposed in
H H H H
1888 by the Swedish chemist Svante Arrhenius. One of the earliest
experiments that supported this idea was performed by American H OH H OH
β-galactoside β-thiogalactoside
chemist Kurt Stern in the 1930s. He studied an enzyme called
catalase, which is very abundant in animal and plant tissues. In the
conclusion of his paper, he wrote, “It remains to be seen to which METHOD A container is separated into two compartments by a

extent the findings of this study apply to enzyme action in general.” selectively permeable membrane. The membrane is permeable
Therefore, another experiment that analyzes a different enzyme and to b-galactoside and b-thiogalactoside, but not permeable to the
technique are described here. enzyme.

The enzyme b-galactosidase catalyzes the cleavage of the glycosidic

bond that links galactose to glucose in the disaccharide lactose.
1 2
Lactose belongs to a family of molecules called b-galactosides since
it contains a galactose unit attached to the rest of the molecule by
Selectively
a glycosidic bond. In a related compound called b-thiogalactoside, permeable membrane
the oxygen atom in the glycosidic bond is replaced by sulfur. The
 CHAPTER 6 M A K I N G L I F E W O R K : C A P T U R I N G A N D U S I N G E N E RG Y 127

orientations within a molecule. The specificity of enzymes can Inhibitors can act in many different ways, two of which are
be attributed to the structure of their active sites. The enzyme shown in Fig. 6.17. In some cases, an inhibitor is similar in structure
active site interacts only with substrates having a precise three- to the substrate and therefore is able to bind to the active site of the
dimensional structure. enzyme (Fig. 6.17a). Binding of the inhibitor prevents the binding
of the substrate. In other words, the inhibitor competes with the
Enzyme activity can be influenced by inhibitors and substrate for the active site of the enzyme. These types of inhibitors
activators. can often be overcome by increasing the concentration of substrate.
The activity of enzymes can be influenced by inhibitors and Other inhibitors bind to a site other than the active site of the
activators. Inhibitors decrease the activity of enzymes, whereas enzyme, but still inhibit the activity of the enzyme (Fig. 6.17b). In
activators increase the activity of enzymes. Enzyme inhibitors are this case, binding of the inhibitor changes the shape and activity
quite common. They are synthesized naturally by many plants and of the enzyme. This type of inhibitor usually has a structure very
animals as a defense against predators. Similarly, pesticides and different from that of the substrate.
herbicides often target enzymes to inactivate them. Many drugs Enzymes that are regulated by molecules that bind at sites
used in medicine are enzyme inhibitors, including drugs used to other than their active sites are called allosteric enzymes. The
treat infections as well as drugs used to treat cancer and other activity of allosteric enzymes can be influenced by both inhibitors
diseases. Given the importance of chemical reactions and the and activators. They play a key role in metabolic pathways, as we
role of enzymes in metabolism, it is not surprising that enzyme discuss next.
inhibitors have such widespread applications.
There are two classes of inhibitors. Irreversible inhibitors Allosteric enzymes regulate key metabolic pathways.
usually form covalent bonds with enzymes and irreversibly Enzyme activators and inhibitors are sometimes important in the
inactivate them. Reversible inhibitors form weak bonds with normal operation of a cell—for example, to regulate a metabolic
enzymes and therefore easily dissociate from them. pathway. Consider the synthesis of isoleucine from threonine,

EXPERIMENT 1 Radioactively labeled b-thiogalactoside (S) is RESULT Over time, the level of radioactivity is greater in
added to compartment 1 and the movement of S is followed by compartment 2 than in compartment 1.
measuring the level of radioactivity in the two compartments.

RESULT Over time, the level of radioactivity is the same in the

two compartments. Enzyme
Time

Substrate Time
CONCLUSION These results can be explained if the substrate
diffuses from compartment 1 to compartment 2, forms a complex
with the enzyme, and is not released because the enzyme cannot
catalyze the conversion of substrate to product. In other words,
E and S form a complex.
EXPERIMENT 2 Radioactively labeled b-thiogalactoside (S) is
added to compartment 1, enzyme (E) is added to compartment SOURCES Adapted from Doherty, D. G., and F. Vaslow. 1952. “Thermodynamic

2, and the movement of S is followed by measuring the level of Study of an Enzyme–Substrate Complex of Chymotrypsin.” J. Am. Chem. Soc. 74:
931–936. Stern, K. G. 1936. “On the Mechanism of Enzyme Action: A Study
radioactivity in the two compartments. of the Decomposition of Monoethyl Hydrogen Peroxide by Catalase and of an
Intermediate Enzyme–Substrate Compound.” J. Biol. Chem. 114:473–494.
128 SECTION 6.5 E N Z Y M E S A N D T H E R AT E O F C H E M I C A L R E AC T I O N S

FIG. 6.17 Two mechanisms of inhibitor function. (a) Some inhibitors bind to the active site of the enzyme and (b) other inhibitors bind to a site that
is different from the active site. Both types of inhibitor reduce the activity of an enzyme and therefore decrease the rate of the reaction.

a. b.
Substrate Product Substrate
Product

Active site

Enzyme
Enzyme
Inhibitor site In this case, the inhibitor binds to a site
In this case, the inhibitor binds to the other than the active site, changing the
active site of the enzyme, competing shape of the enzyme and reducing the
Inhibitor with the substrate and reducing the rate of the reaction.
rate of the reaction. Inhibitor

a pathway found in some bacteria. This conversion requires five of the five reactions. Isoleucine binds to the first enzyme in the
reactions, each catalyzed by a different enzyme (Fig. 6.18). pathway, threonine dehydratase, at a site distinct from the active
Once the bacterium has enough isoleucine for its needs, it site. As a result, threonine dehydratase is an example of an allosteric
would be a waste of energy to continue synthesizing the amino acid. enzyme. The binding of isoleucine changes the shape of the enzyme
To shut down the pathway once it is no longer needed, the cell relies and in this way inhibits its function.
on an enzyme inhibitor. The inhibitor is isoleucine, the final product The isoleucine pathway also provides an example of negative
feedback, in which the final product inhibits the first step of the
reaction. This is a common mechanism used widely in organisms
FIG. 6.18 Regulation of threonine dehdyratase, an allosteric to maintain homeostasis, that is, the active maintenance of stable
enzyme. The enzyme is inhibited by the final product, conditions or steady levels of a substance (Chapter 5).
isoleucine, which binds to a site distinct from the Threonine dehydratase can also adjust the rate of the
active site. reaction, depending on the concentration of substrate. At a low
concentration of threonine, the rate of the reaction is very slow.
Threonine As the concentration of threonine increases, the activity of
the enzyme increases. At a particular threshold, a small increase
Threonine in threonine concentration results in a large increase in reaction
dehydratase rate. Finally, when there is excess substrate, the reaction rate
P1
slows down.
In the next two chapters, we examine more closely two key
metabolic processes, cellular respiration and photosynthesis. Both
of these processes require many chemical reactions acting in a
coordinated fashion. Allosteric enzymes catalyze key reactions
P2
in these and other metabolic pathways. These enzymes are
usually found at or near the start of a metabolic pathway or at the
crossroads between two metabolic pathways. Allosteric enzymes
– Allosteric
inhibitor are one way that the cell coordinates the activity of multiple
P3
metabolic pathways.

? CASE 1 THE FIRST CELL: LIFE’S ORIGINS

P4
What naturally occurring elements might have spurred
the first reactions that led to life?
Many enzymes are not just made up of protein, but also contain
metal ions. These ions are one type of cofactor, a substance that
Isoleucine
associates with an enzyme and plays a key role in its function.
 CHAPTER 6 M A K I N G L I F E W O R K : C A P T U R I N G A N D U S I N G E N E RG Y 129

Metallic cofactors, especially iron, magnesium, manganese, cobalt, mid-ocean hydrothermal vent systems and other environments
copper, zinc, and molybdenum, bind to diverse proteins, including where oxygen is absent. It has been proposed that reactions now
enzymes used in DNA synthesis and nitrogen metabolism. With carried out in cells by iron–sulfur proteins are the evolutionary
this in mind, scientists have asked whether metal ions might, by descendants of chemical reactions that took place spontaneously
themselves, catalyze chemical reactions thought to have played on the early Earth.
a role in the origin of life. They do. For example, magnesium The idea that your cells preserve an evolutionary memory
and zinc ions added to solutions can accelerate the linking of of ancient hydrothermal environments may seem like science
nucleotides to form RNA and DNA molecules. fiction, but it finds support in laboratory experiments. For
Metallic cofactors also bind to enzymes used in the transport example, the reaction of H2S and FeS to form pyrite has been
of electrons for cellular respiration and photosynthesis, processes shown to catalyze a number of plausibly pre-biotic chemical
that are discussed in the next two chapters. Enzymes that contain reactions, including the formation of pyruvate (a key intermediate
iron and sulfur clustered together are particularly important in in energy metabolism discussed in Chapter 7). Thus, the metals in
the transport of electrons within cells. Iron–sulfur minerals, enzymes help connect the chemistry of life to the chemistry of
especially pyrite (or fool’s gold, FeS2), form commonly in Earth. •

Core Concepts Summary The second law of thermodynamics states that there is an
increase in entropy in the universe over time. page 120
6.1 Metabolism is the set of biochemical reactions
that transforms biomolecules and transfers energy. 6.4 Chemical reactions involve the breaking and
forming of bonds.
Organisms can be grouped according to their source of
energy: Phototrophs obtain energy from sunlight and In a chemical reaction, atoms themselves do not change,
chemotrophs obtain energy from chemical compounds. but which atoms are linked to each other changes, forming
page 116 new molecules. page 121

Organisms can also be grouped according to the source of The direction of a chemical reaction is influenced by the
carbon they use to build organic molecules: Heterotrophs concentration of reactants and products. page 121
obtain carbon from organic molecules, and autotrophs
Gibbs free energy (G) is the amount of energy available to
obtain carbon from inorganic sources, such as carbon
do work. page 121
dioxide. page 116
Three thermodynamic parameters define a chemical
Catabolism is the set of reactions that break down
reaction: Gibbs free energy (G), enthalpy (H), and
molecules and release energy, and anabolism is the set of
entropy (S). page 121
reactions that build molecules and require energy.
page 117 Exergonic reactions are spontaneous (ΔG < 0) and release
energy. page 121
6.2 Kinetic energy is energy of motion and potential Endergonic reactions are non-spontaneous (ΔG > 0) and
energy is stored energy. require energy. page 121
Kinetic energy is due to motion. page 118
The change of free energy in a chemical reaction is
Potential energy depends on the structure of an object or described by ΔG 5 ΔH – TΔS. page 122
its position relative to its surroundings. page 118
The hydrolysis of ATP is an exergonic reaction that drives
Chemical energy is a form of potential energy held in the many endergonic reactions in a cell. page 122
bonds of molecules. page 118
In living systems, non-spontaneous reactions are often
coupled to spontaneous ones. page 123
6.3 The laws of thermodynamics govern energy flow
in biological systems.
The first law of thermodynamics states that energy cannot
be created or destroyed. page 119
130 S E LCFT-IAOSNS E5S. S
1M EMN T

6.5 The rate of biochemical reactions is increased by

protein catalysts called enzymes.
Self-Assessment
1. Name and describe ways that organisms obtain energy and
Enzymes reduce the free energy level of the transition
carbon from the environment.
state between reactants and products, thereby reducing the
energy input, or activation energy, required for a chemical 2. Distinguish between catabolism and anabolism.
reaction to proceed. page 124
3. Name and describe the two forms of energy and
During catalysis, the substrate and product form a provide an example of each.
complex with the enzyme. Transient
C H A P T covalent
ER 5 bonds
O RG ANIZING P R I N C I P L E S : L I P I D S , M E M B R A N E S , A N D C E L L CO M PA RT M E N T S 130
4. Explain the relationship between strength of a covalent
and/or weak noncovalent interactions stabilize the
bond and the amount of chemical energy it contains.
complex. page 125
5. Draw the structure of ATP, indicating the bonds that
The size of the active site of an enzyme is small
are broken during hydrolysis.
compared to the size of the enzyme as a whole and the
active site amino acids occupy a very specific spatial 6. Describe the first and second laws of thermodynamics
arrangement. page 125 and how they relate to chemical reactions.

An enzyme is highly specific for its substrate and for the 7. If the difference between the enthalpy of the products
types of reaction it catalyzes. page 126 and that of the reactants is positive and the difference
between the entropy of the products and reactants is
Inhibitors reduce the activity of enzymes and can act
negative, predict whether the reaction is spontaneous
irreversibly or reversibly. page 127
or not.
Activators increase the activity of enzymes. page 127
8. Describe how the hydrolysis of ATP can drive non-
Allosteric enzymes bind activators and inhibitors at sites spontaneous reactions in a cell.
other than the active site, resulting in a change in their
9. Give three characteristics of enzymes and describe
shape and activity. page 127
how they permit chemical reactions to occur in cells.
Allosteric enzymes are often found at or near the start
10. Explain how protein folding allows for enzyme specificity.
of a metabolic pathway or at the crossroads of multiple
pathways. page 128
Log in to to check your answers to the Self-
Assessment questions, and to access additional learning tools.
 CHAPTER 7

Cellular
Respiration
Harvesting Energy from
Carbohydrates and Other
Fuel Molecules

Core Concepts
7.1 Cellular respiration is a series
of catabolic reactions that
convert the energy in fuel
molecules into ATP.
7.2 Glycolysis is the partial
oxidation of glucose and results
in the production of pyruvate,
as well as ATP and reduced
electron carriers.
7.3 Pyruvate is oxidized to acetyl-
CoA, connecting glycolysis to
the citric acid cycle.
7.4 The citric acid cycle results in
the complete oxidation of fuel
molecules and the generation
of ATP and reduced electron
carriers.
7.5 The electron transport
chain transfers electrons
from electron carriers to
oxygen, using the energy
released to pump protons and
synthesize ATP by oxidative
phosphorylation.
7.6 Glucose can be broken down
in the absence of oxygen by
fermentation, producing a
modest amount of ATP.
7.7 Metabolic pathways are
integrated, allowing control of
Image Source/Getty Images.

the energy level of cells.

131
132 SECTION 7.1 A N OV E RV I E W O F C E L LU L A R R E S P I R AT I O N

The ability to harness energy from the environment is a key as glucose, fatty acids, and proteins are catabolized into smaller
attribute of life. We have seen that energy is needed for all kinds units, releasing the energy stored in their chemical bonds to power
of tasks—among them cell movement and division, muscle the work of the cell.
contraction, growth and development, and the synthesis of
macromolecules. Organic molecules such as carbohydrates, lipids, Cellular respiration uses chemical energy stored
and proteins are good sources of chemical energy. Some organisms, in molecules such as carbohydrates and lipids to
like humans and other heterotrophs, obtain organic molecules produce ATP.
by consuming them in their diet. Others, like plants and other Cellular respiration is a series of catabolic reactions that converts
autotrophs, synthesize these molecules from inorganic molecules the energy stored in food molecules, such as glucose, into the
like carbon dioxide, as we saw in Chapter 6. Regardless of how energy stored in ATP, and produces carbon dioxide as a waste or
organic molecules are obtained, nearly all organisms—animals, by-product. It can occur in the presence of oxygen (termed
plants, fungi, and microbes—break them down in the process aerobic respiration) or in the absence of oxygen (termed anaerobic
of cellular respiration, releasing energy that can be used to do respiration). Most organisms that you are familiar with are capable
the work of the cell. Cellular respiration is a series of chemical of aerobic respiration; some bacteria respire anaerobically
reactions that convert the chemical energy in fuel molecules into (Chapter 26). Here we focus on aerobic respiration. Oxygen is
the chemical energy of adenosine triphosphate (ATP), which can consumed in aerobic respiration, and carbon dioxide and water are
be readily used by cells. produced, as follows:
It is tempting to think that organic molecules are converted
into energy in this process, but this is not the case. Recall from C6H12O6 1 6O2 → 6CO2 1 6H2O 1 energy
Chapter 6 that the first law of thermodynamics (the law of Glucose Oxygen Carbon Water
conservation of energy) states that energy cannot be created or dioxide
destroyed. Biological processes, like all processes, are subject to
the laws of thermodynamics. As a result, the process of cellular In Chapter 6, we saw that molecules such as carbohydrates and
respiration converts the chemical potential energy stored in lipids have a large amount of potential energy in their chemical
organic molecules to chemical potential energy that is useful to bonds. In contrast, molecules like carbon dioxide and water have
cells: the chemical potential energy in ATP. ATP is the universal less potential energy in their bonds. Cellular respiration releases a
energy currency for all cells. large amount of energy because the sum of the potential energy in
It is also easy to forget that organisms other than animals, all of the chemical bonds of the reactants (glucose and oxygen) is
such as plants, use cellular respiration. If plants use sunlight as a higher than that of the products (carbon dioxide and water). The
source of energy, why would they need cellular respiration? As we maximum amount of free energy—energy available to do work—
will see in the next chapter, plants use the energy of sunlight to released during cellular respiration is –686 kcal per mole of glucose.
make carbohydrates. Plants then break down these carbohydrates Recall from Chapter 6 that when ∆G < 0, energy is released.
in the process of cellular respiration to produce ATP. The overall reaction for cellular respiration helps us focus
In this chapter, we discuss the metabolic pathways that on the starting reactants, final products, and release of energy.
supply the energy needs of a cell: the breakdown, storage, and However, it misses the many intermediate steps that take place
mobilization of sugars such as glucose, the synthesis of ATP, and as the cell catabolizes glucose. Tossing a match into the gas tank
the coordination and regulation of these metabolic pathways. of a car releases a tremendous amount of energy in the form of
an explosion, but this energy is not used to do work. Instead, it
is released as light and heat. Similarly, if all the energy stored in
glucose were released at once, most of it would be released as heat
7.1 AN OVERVIEW OF CELLULAR and the cell would not be able to harness it to do work.
RESPIRATION In cellular respiration, energy is released gradually in a series
of chemical reactions (Fig. 7.1). This allows some of this energy to
In the last chapter, we saw that catabolism describes the set of be used to form ATP. On average, 32 molecules of ATP are produced
chemical reactions that break down molecules into smaller units. from the aerobic respiration of a single molecule of glucose. The
In the process, these reactions release chemical energy that can energy needed (∆G of the reaction) to form one mole of ATP from
be stored in molecules of ATP. Anabolism, by contrast, is the set ADP and Pi is at least 7.3 kcal. Thus, cellular respiration harnesses
of chemical reactions that build molecules from smaller units. at least 32  7.3 5 233.6 kcal of energy in ATP for every mole of
Anabolic reactions require an input of energy, usually in the form glucose that is broken down in the presence of oxygen.
of ATP. About 34% of the total energy released by aerobic respiration
Cellular respiration is one of the major sets of catabolic is harnessed in the form of ATP (233.6/686 5 34%), with the
reactions in a cell. During cellular respiration, fuel molecules such remainder of the energy given off as heat. This degree of efficiency
 CHAPTER 7 C E L L U L A R R E S P I R AT I O N : H A RV E S T I N G E N E RG Y F RO M C A R B O H Y D R AT E S A N D OT H E R F U E L M O L E C U L E S 133

How electron movement through the electron transport chain

FIG. 7.1 An overview of cellular respiration. In most organisms, is coupled to ATP synthesis is central to the energy economy of
cellular respiration consumes oxygen and produces most cells and is discussed later in the chapter. Electron transport
ATP by substrate-level phosphorylation and oxidative chains are used in respiration to harness energy from fuel
phosphorylation, as well as carbon dioxide and water. molecules such as glucose and in photosynthesis to harness energy
CO2 from sunlight (Chapter 8).
Catabolism
Carbohydrate
Redox reactions play a central role in cellular
respiration.
Substrate-level
phosphorylation Chemical reactions in which electrons are transferred from
one atom or molecule to another are referred to as oxidation–
Electron carriers
reduction reactions (“redox reactions” for short). Oxidation is
Oxidative the loss of electrons, and reduction is the gain of electrons. The loss
Electron phosphorylation
transport ATP and gain of electrons always occur together in a coupled oxidation–
chain reduction reaction: Electrons are transferred from one molecule
to another so that one molecule loses electrons and one molecule
gains those electrons. The molecule that loses electrons is oxidized
and the molecule that gains electrons is reduced.
O2 H2O We can illustrate oxidation–reduction reactions by looking
at the role played by electron carriers in cellular respiration. Two
compares favorably with that of a gasoline engine, which operates important electron carriers are nicotinamide adenine dinucleotide
with an efficiency of about 25%. and flavin adenine dinucleotide. These electron carriers exist in
two forms—an oxidized form (NAD1 and FAD) and a reduced
ATP is generated by substrate-level phosphorylation form (NADH and FADH2). When fuel molecules such as glucose
and oxidative phosphorylation. are catabolized, some of the steps are oxidation reactions. These
In cellular respiration, the chemical energy stored in a molecule oxidation reactions are coupled with the reduction of electron
of glucose is used to produce ATP in two different ways (Fig. 7.1). carrier molecules. The reduction reactions in which electrons are
In the first, a phosphorylated organic molecule directly transfers transferred to electron carriers can be written as:
a phosphate group to ADP, as we saw in Chapter 6. In this case,
there are two coupled reactions carried out by a single enzyme: NAD1 1 2e2 1 H1 → NADH
the hydrolysis of a phosphorylated organic molecule and the FAD 1 2e2 1 2H1 → FADH2
addition of a phosphate group to ADP. The hydrolysis reaction
Here, NAD1 and FAD accept electrons, resulting in the
releases enough free energy to drive the synthesis of ATP. This way
production of their reduced forms, NADH and FADH2. Note that in
of generating ATP is called substrate-level phosphorylation
redox reactions involving organic molecules such as NAD1 or FAD,
because a phosphate group is transferred to ADP from an enzyme
the gain (or loss) of electrons is often accompanied by the gain (or
substrate, in this case an organic molecule.
loss) of protons (H1). As a result, reduced molecules can easily be
Substrate-level phosphorylation produces only a small amount
recognized by an increase in C–H bonds, and the corresponding
of the total ATP generated in cellular respiration, about 12% if the
oxidized molecules by a decrease in C–H bonds.
fuel molecule is glucose. Most of the ATP produced during cellular
In their reduced forms, NADH and FADH2 can donate
respiration (the remaining 88%) is produced in a different way
electrons. The oxidation of NADH and FADH2 allows electrons
(Fig. 7.1). In this case, the chemical energy of organic molecules is
(and energy) to be transferred to the electron transport chain:
transferred first to electron carriers. The role of these electron
carriers is exactly what their name suggests—they carry electrons NADH → NAD1 1 2e2 1 H1
(and energy) from one set of reactions to another. In cellular FADH2 → FAD 1 2e2 1 2H1
respiration, electron carriers transport electrons released during
the catabolism of organic molecules to the respiratory electron In addition, these reactions produce NAD1 and FAD, which
transport chain. Electron transport chains in turn transfer can then accept electrons from the breakdown of fuel molecules.
electrons along a series of membrane-associated proteins to a In this way, electron carriers act as shuttles, transferring electrons
final electron acceptor and in the process harness the energy derived from the oxidation of fuel molecules such as glucose to the
released to produce ATP. In aerobic respiration, oxygen is the final electron transport chain.
electron acceptor, resulting in the formation of water. This way of Cellular respiration can itself be understood as a redox
generating ATP is called oxidative phosphorylation. reaction, even though it consists of many steps. In aerobic
134 SECTION 7.1 A N OV E RV I E W O F C E L LU L A R R E S P I R AT I O N

FIG. 7.2 Oxidation and reduction in the overall reaction for FIG. 7.3 The four stages of cellular respiration. Cellular
cellular respiration. Electrons are partially lost or gained respiration consists of glycolysis, pyruvate oxidation, the
in the formation of polar covalent bonds. citric acid cycle, and oxidative phosphorylation.
a. Oxidation
Glucose Amino Fatty Glucose
CH2OH acids acids Stage 1
C O Glycolysis Glycolysis
H OH
H
C C Carbon dioxide
OH H ATP Stages 1 & 2:
HO H Fuel molecules
C C O C O are partially
H OH broken down,
producing ATP
Dark Pyruvate and electron
shading carriers.
represents
increased
electron CO2 Stage 2
C C C O density.
Pyruvate
Acetyl-CoA oxidation
Carbon atoms share Carbon atom has partially lost
electrons equally. electrons (and is oxidized) because the
oxygen atom is more electronegative Stage 3
than the carbon atom.
Citric acid
cycle
b. Reduction Citric acid
Oxygen Water cycle
Stage 3:
H H CO2 Fuel molecules
O O
O are fully broken
down, producing
ATP and electron
ATP carriers.

Electron
O O H O carriers Stage 4

Oxidative
e- phosphorylation
Oxygen atoms share Oxygen atom has partially gained
electrons equally. electrons (and is reduced) because the
oxygen atom is more electronegative O2 Stage 4:
than the hydrogen atom. Electron transport chain Electron carriers
donate electrons
H2O to the electron
transport chain,
leading to the
respiration, glucose is oxidized, releasing carbon dioxide, and at synthesis of ATP.
the same time oxygen is reduced, forming water: ATP

Oxidation

C6H12O6 1 6O2 → 6CO2 1 6H2O 1 energy oxygen atom is more electronegative than the carbon atom, so the
electrons are more likely to be found near the oxygen atom.
Reduction As a result, carbon has partially lost electrons to oxygen and is
oxidized.
Let’s first consider the oxidation reaction (Fig. 7.2a). In Let’s now consider the reduction reaction (Fig. 7.2b). In
glucose, there are many C–C and C–H covalent bonds, in which oxygen gas, electrons are shared equally between two oxygen
electrons are shared about equally between the two atoms. By atoms. In water, the electrons that are shared between hydrogen
contrast, in carbon dioxide, electrons are not shared equally. The and oxygen are more likely to be found near oxygen because
 CHAPTER 7 C E L L U L A R R E S P I R AT I O N : H A RV E S T I N G E N E RG Y F RO M C A R B O H Y D R AT E S A N D OT H E R F U E L M O L E C U L E S 135

oxygen is more electronegative than hydrogen. As

a result, oxygen has partially gained electrons and FIG. 7.4 Change in free energy in cellular respiration. Glucose is oxidized through a
is reduced. series of chemical reactions, releasing energy in the form of ATP and reduced
Glucose is a good electron donor because electron carriers.
its oxidation to carbon dioxide releases a lot of
Pyruvate Citric acid
energy, whereas oxygen is a good electron acceptor Glycolysis
oxidation cycle
because it has a high affinity for electrons. In
cellular respiration, glucose is not oxidized all ATP
at once to carbon dioxide. Instead, it is oxidized ATP
slowly in a series of reactions, some of which are
NADH Energy released is stored as
redox reactions. In this way, energy is released in Glucose 0 ATP and electron carriers.
a controlled manner. Some of this energy can be ATP
used to synthesize ATP directly and some of it is ATP
–100
stored temporarily in reduced electron carriers
and then used to generate ATP by oxidative

Change in free energy, ΔG (kcal/mol)

Pyruvate NADH
phosphorylation. –200

j Quick Check 1 For each of the following pairs

of molecules, indicate which member of the –300 Acetyl-CoA NADH
pair is reduced and which is oxidized, and which
has more chemical energy and which has less NADH
–400
chemical energy: NAD1/NADH; FAD/FADH2;
CO2/C6H12O6. ATP
–500
FADH2
Cellular respiration occurs in four stages.
Up to this point, we have focused on the overall –600 NADH
reaction of cellular respiration and the chemistry
of redox reactions. Let’s now look at the entire
process, starting with glucose. Cellular respiration –700

occurs in four stages (Fig. 7.3).

In stage 1, glucose is partially broken down In some bacteria, these reactions take place in the cytoplasm,
to make pyruvate and energy is transferred to ATP and reduced and the electron transport chain is located in the plasma
electron carriers, a process known as glycolysis. membrane.
In stage 2, pyruvate is oxidized to another molecule called Fig. 7.4 shows the change of free energy at each step in
acetyl-coenzyme A (acetyl-CoA), producing reduced electron catabolism of glucose. As you can see, the individual reactions
carriers and releasing carbon dioxide. allow the initial chemical energy present in a molecule of glucose
Acetyl-CoA enters stage 3, the citric acid cycle, also called to be “packaged” into molecules of ATP and reduced electron
the tricarboxylic (TCA) cycle or the Krebs cycle. In this series carriers. Note that the change in free energy is much greater for
of chemical reactions, the acetyl group is completely oxidized the steps that generate reduced electron carriers compared to
to carbon dioxide and energy is transferred to ATP and reduced those that produce ATP directly.
electron carriers. The amount of energy transferred to ATP and
reduced electron carriers in this stage is nearly twice that of stages
1 and 2 combined. 7.2 GLYCOLYSIS: THE
Stage 4 is oxidative phosphorylation. In this series of SPLITTING OF SUGAR
reactions, reduced electron carriers generated in stages 1–3 donate
electrons to the electron transport chain and a large amount of Glucose is the most common fuel molecule in animals, plants,
ATP is produced. and microbes. It is the starting molecule for glycolysis, which
In eukaryotes, glycolysis takes place in the cytoplasm, results in the partial oxidation of glucose and the synthesis of a
and pyruvate oxidation, the citric acid cycle, and oxidative relatively small amount of both ATP and reduced electron carriers.
phosphorylation all take place in mitochondria. The electron Glycolysis literally means “splitting sugar,” an apt name because
transport chain is made up of proteins and small molecules in glycolysis a 6-carbon sugar (glucose) is split in two, yielding two
associated with the inner mitochondrial membrane (Chapter 5). 3-carbon molecules. The process is anaerobic because oxygen is
136 SECTION 7.2 G LYCO LYS I S : T H E S P L I T T I N G O F S U G A R

FIG. 7.5 Glycolysis. Glucose is partially oxidized to pyruvate, with the net production of 2 ATP and 2 NADH.

Amino Fatty Glucose

HO CH2
acids acids

Glucose C O
Glycolysis
H H
ATP H
C C
OH H
ATP
HO OH
First
1
Pyruvate
C C
ATP-consuming
CO2
reaction ADP H OH
Acetyl-CoA Phase 1

Glucose 6-phosphate Preparatory phase,

Citric acid
with consumption
cycle of 2 ATP
CO2

ATP
2
Electron
carriers
Fructose 6-phosphate
e-

Electron transport chain

O2 ATP
Second
H2O ATP-consuming 3
reaction ADP
ATP P O CH2 O CH2 O P
Fructose 1,6-bisphosphate C C
H HO
Cleavage of
H OH
6-carbon sugar
to two 3-carbon
4 C C

molecules OH H
Phase 2
Glyceraldehyde 3-phosphate
+ Cleavage phase
Dihydroxyacetone phosphate

5 O
Glyceraldehyde 3-phosphate Glyceraldehyde 3-phosphate P O CH2 CH C
Pi Pi H
OH
NADH- NAD+ NAD+
producing
reaction
6 6
NADH  H NADH  H

1,3-Bisphosphoglycerate 1,3-Bisphosphoglycerate
First ADP ADP
ATP-producing 7 7
reaction ATP ATP
Phase 3
3-Phosphoglycerate 3-Phosphoglycerate
Payoff phase,
8 8 with production of
4 ATP and 2 NADH
2-Phosphoglycerate 2-Phosphoglycerate

9 9
Phosphoenolpyruvate Phosphoenolpyruvate
ADP ADP
Second
ATP-producing 10 10
reaction ATP ATP O
CH3 C C
Pyruvate Pyruvate
O O
 CHAPTER 7 C E L L U L A R R E S P I R AT I O N : H A RV E S T I N G E N E RG Y F RO M C A R B O H Y D R AT E S A N D OT H E R F U E L M O L E C U L E S 137

not consumed. Glycolysis evolved very early in the evolution of

life, when oxygen was not present in Earth’s atmosphere. It occurs FIG. 7.6 Mitochondrial membranes and compartments.
in nearly all living organisms and is therefore probably the most
widespread metabolic pathway among organisms.
Outer membrane
Glycolysis is the partial breakdown of glucose.
The space between the inner
Glycolysis begins with a molecule of glucose and produces two Intermembrane and outer membranes is the
space
3-carbon molecules of pyruvate and a net total of two molecules intermembrane space.
of ATP and two molecules of the electron carrier NADH. ATP is
produced directly by substrate-level phosphorylation. Inner membrane
Glycolysis is a series of 10 chemical reactions (Fig. 7.5). Mitochondrial
matrix The space inside the
These reactions can be divided into three phases. The first phase
inner membrane is
prepares glucose for the next two phases by the addition of two the mitochondrial
phosphate groups to glucose. This phase requires an input of matrix.
energy. To supply that energy and provide the phosphate groups,
two molecules of ATP are hydrolyzed per molecule of glucose. In
other words, the first phase of glycolysis is an endergonic process.
The phosphorylation of glucose has two important consequences.
Whereas glucose enters and exits cells through specific membrane
transporters, phosphorylated glucose is trapped inside the cell. In
addition, the presence of two negatively charged phosphate groups
in proximity destabilizes the molecule so that it can be broken
apart in the second phase of glycolysis.
The second phase is the cleavage phase, in which the 6-carbon
molecule is split into two 3-carbon molecules. For each molecule of
glucose entering glycolysis, two 3-carbon molecules enter the third key step that links glycolysis to the citric acid cycle. In eukaryotes,
phase of glycolysis. this is the first step that takes place inside the mitochondria.
The third and final phase of glycolysis is sometimes called
the payoff phase because ATP and the electron carrier NADH The oxidation of pyruvate connects glycolysis to the
are produced. Later, NADH will contribute to the synthesis of citric acid cycle.
ATP during oxidative phosphorylation. This phase ends with the The end product of glycolysis is pyruvate, which can be
production of two molecules of pyruvate. transported into mitochondria. Mitochondria are rod-shaped
In summary, glycolysis begins with a single molecule of glucose organelles surrounded by a double membrane (Fig. 7.6;
(six carbons) and produces two molecules of pyruvate (three Chapter 5). The inner and outer mitochondrial membranes are not
carbons each). These reactions yield four molecules of ATP and two close to each other in all areas because the inner membrane has
molecules of NADH. However, two ATP molecules are consumed folds that project inward. These membranes define two spaces.
during the initial phase of glycolysis, resulting in a net gain of two The space between the inner and outer membranes is called the
ATP molecules and two molecules of NADH (Fig. 7.5). intermembrane space, and the space enclosed by the inner
j Quick Check 2 At the end of glycolysis, but before the membrane is called the mitochondrial matrix.
subsequent steps in cellular respiration, which molecules contain Pyruvate is transported into the mitochondrial matrix,
some of the energy held in the original glucose molecule? where it is converted into acetyl-CoA (Fig. 7.7). First, part of
the pyruvate molecule is oxidized and splits off to form carbon
dioxide, the most oxidized (and therefore the least energetic) form
7.3 PYRUVATE OXIDATION of carbon. The electrons lost in this process are donated to NAD1,
which is reduced to NADH. The remaining part of the pyruvate
Glycolysis occurs in almost all living organisms, but it does not molecule—an acetyl group (COCH3)—still contains a large
generate very much energy in the form of ATP. The end product, amount of potential energy that can be harnessed. It is transferred
pyruvate, still contains a good deal of chemical potential energy in its to coenzyme A (CoA), a molecule that carries the acetyl group to
bonds. In the presence of oxygen, pyruvate can be further oxidized the next set of reactions.
to release more energy, first to acetyl-CoA and then even further in Overall, the synthesis of one molecule of acetyl-CoA from
the series of reactions in the citric acid cycle. Pyruvate oxidation is a pyruvate results in the formation of one molecule of carbon
138 SECTION 7.4 T H E C I T R I C AC I D C YC L E

FIG. 7.7 Pyruvate oxidation. Pyruvate is oxidized in the mitochondrial matrix, forming
acetyl-CoA, the first substrate in the citric acid cycle.
Amino Fatty Glucose
acids acids

Glycolysis

ATP

Pyruvate

CO2

Acetyl-CoA
Cytosol Intermembrane space Matrix

Citric acid
cycle NAD+ NADH  H
CO2

Pyruvate Acetyl-CoA
ATP O
CH3 C C CH3 C CoA
Electron
carriers
O O O
e-
Coenzyme A
O2
Electron transport chain
CO2
H2O

ATP

dioxide and one molecule of NADH. Recall, however, that a the variant names “citric acid cycle” and “tricarboxylic acid
single molecule of glucose forms two molecules of pyruvate cycle”). The molecule of citric acid is then oxidized in a series of
during glycolysis. Therefore, two molecules of carbon dioxide, reactions. The last reaction of the cycle regenerates a molecule of
two molecules of NADH, and two molecules of acetyl-CoA are oxaloacetate, joining to a new acetyl group and allowing the cycle
produced from a single starting glucose molecule in this stage of to continue.
cellular respiration. Acetyl-CoA is the substrate of the first step in The citric acid cycle results in the complete oxidation of
the citric acid cycle. the acetyl group of acetyl-CoA. Since the first reaction creates
a molecule with six carbons and the last reaction regenerates a
4-carbon molecule, two carbons are eliminated during the cycle.
7.4 THE CITRIC ACID CYCLE These carbons are released as carbon dioxide. Along with the
release of carbon dioxide from pyruvate during pyruvate oxidation,
The citric acid cycle is the stage in cellular respiration in which these reactions are the sources of carbon dioxide released during
fuel molecules are completely oxidized. Specifically, the acetyl cellular respiration and therefore the sources of the carbon dioxide
group of acetyl-CoA is completely oxidized to carbon dioxide that we exhale when we breathe.
and the chemical energy is transferred to ATP by substrate-level The oxidation reactions that produce carbon dioxide are
phosphorylation and to the reduced electron carriers NADH and coupled with the reduction of the electron carrier NAD1 to NADH
FADH2. In this way, the citric acid cycle supplies electrons to the (Fig. 7.8). In this way, energy released in the oxidation reactions is
electron transport chain, leading to the production of much more transferred to NADH. More reduced electron carriers (NADH and
energy in the form of ATP than is obtained by glycolysis alone. FADH2) are produced in two additional redox reactions. In fact,
the citric acid cycle produces a large quantity of reduced electron
The citric acid cycle produces ATP and reduced carriers: three molecules of NADH and one molecule of FADH2 per
electron carriers. turn of the cycle. These electron carriers donate electrons to the
Like the synthesis of acetyl-CoA, the citric acid cycle takes place electron transport chain, which leads to the production of ATP by
in the mitochondrial matrix. It is composed of eight reactions and oxidative phosphorylation.
is called a cycle because the starting molecule, oxaloacetate, is One of the reactions of the citric acid cycle is a substrate-level
regenerated at the end (Fig. 7.8). phosphorylation reaction that generates a molecule of GTP
In the first reaction, the 2-carbon acetyl group of acetyl-CoA (Fig. 7.8). GTP can transfer its terminal phosphate to a molecule of
is transferred to a 4-carbon molecule of oxaloacetate to form ADP to form ATP. This is the only substrate-level phosphorylation
the 6-carbon molecule citric acid or tricarboxylic acid (hence in the citric acid cycle.
 CHAPTER 7 C E L L U L A R R E S P I R AT I O N : H A RV E S T I N G E N E RG Y F RO M C A R B O H Y D R AT E S A N D OT H E R F U E L M O L E C U L E S 139

FIG. 7.8 The citric acid cycle. The acetyl group of acetyl-CoA is completely oxidized, with the net production of one ATP, three NADH,
and one FADH2.

Amino Fatty Glucose

acids acids

Glycolysis
Acetyl-coenzyme A
2C
ATP
CH3 C CoA CoA SH
Pyruvate O
H2O
CO2

Acetyl-CoA
H  NADH
O C COO CH2 COO

CH2 COO HO C COO

NAD+
Citric acid
cycle
CO2 Oxaloacetate Citrate CH2 COO
4C 6C
ATP

Electron
carriers

e- Malate
O2
4C Isocitrate
Electron transport chain
6C
H2O

ATP
NAD+
Citric acid
cycle NADH  H

H2O
CO2

Fumarate
4C -Ketoglutarate
5C

NAD+
Succinate
4C Succinyl-CoA
FADH2 FAD CoA SH
4C
NADH  H

GDP  Pi
CO2

CoA SH GTP

ADP  Pi ATP

Overall, two molecules of acetyl-CoA produced from a single ? CASE 1 THE FIRST CELL: LIFE’S ORIGINS
molecule of glucose yield two molecules of ATP, six molecules of
What were the earliest energy-harnessing reactions?
NADH, and two molecules of FADH2 in the citric acid cycle.
Some bacteria run the citric acid cycle in reverse, incorporating
j Quick Check 3 At the end of the citric acid cycle, but before the carbon dioxide into organic molecules instead of liberating it.
subsequent steps of cellular respiration, which molecules contain Running the citric acid cycle in reverse requires energy, which is
the energy held in the original glucose molecule? supplied by sunlight (Chapter 8) or chemical reactions (Chapter 26).
140 SECTION 7.5 T H E E L E C T RO N T R A N S P O RT C H A I N A N D O X I DAT I V E P H O S P H O RY L AT I O N

through the extension and modification of this deeply rooted

FIG. 7.9 Molecules produced by the citric acid cycle. Many cycle. As typical of evolution, new cellular capabilities arose by the
organic molecules can be synthesized from citric acid cycle modification of simpler, more general sets of reactions.
intermediates.
Sugars
Alanine 7.5 THE ELECTRON TRANSPORT
Pyruvate
CHAIN AND OXIDATIVE
Lipids
PHOSPHORYLATION
The complete oxidation of glucose during the first three stages
Other amino
Acetate of cellular respiration results in the production of two kinds of
acids, Acetyl-CoA reduced electron carriers: NADH and FADH2. We are now going
pyrimidines to see how the energy stored in these electron carriers is used to
synthesize ATP.
Purines
Aspartate The energy in these electron carriers is released in a series of
redox reactions that occur as electrons pass through a chain of
Oxaloacetate Citrate
Other amino protein complexes in the inner mitochondrial membrane to the
acids final electron acceptor, oxygen, which is reduced to water. The
energy released by these redox reactions is not converted directly
Citric acid
cycle Glutamate
into the chemical energy of ATP, however. Instead, the passage
of electrons is coupled to the transfer of protons (H1) across
-Ketoglutarate
the inner mitochondrial membrane, creating a concentration
and charge gradient (Chapter 5). This electrochemical gradient
provides a source of potential energy that is then used to drive the
Succinyl-CoA synthesis of ATP.
We next explore the properties of the electron transport
Heme,
chain, the proton gradient, and the synthesis of ATP.
chlorophyll
The electron transport chain transfers electrons and
pumps protons.
Why would an organism run the citric acid cycle backward? Electrons donated by NADH and FADH2 are transported along
The answer is that running the cycle in reverse allows an organism a series of four large protein complexes that form the electron
to build, rather than break down, organic molecules. Whether the transport chain (complexes I to IV). These are shown in Fig. 7.10.
cycle is run in the reverse or forward direction, the intermediates These membrane proteins are embedded in the mitochondrial
generated step by step as the cycle turns provide the building inner membrane (see Fig. 7.6). The inner mitochondrial membrane
blocks for synthesizing the cell’s key biomolecules (Fig. 7.9). contains one of the highest concentrations of proteins found in
Pyruvate, for example, is the starting point for the synthesis of eukaryotic membranes.
sugars and the amino acid alanine; acetate is the starting point Electrons enter the electron transport chain at either complex
for the synthesis of the cell’s lipids; oxaloacetate is modified to I or II. Electrons donated by NADH enter through complex I, and
form different amino acids and pyrimidine bases; and a-(alpha-) electrons donated by FADH2 enter through complex II. (Complex
ketoglutarate is modified to form other amino acids. As a result, II is the same enzyme that catalyzes step 6 in the citric acid cycle.)
for organisms that run the cycle in the forward direction, the citric These electrons are transported through either complex I or II to
acid cycle is used to generate both energy-storing molecules (ATP complex III and then through complex IV.
and reduced electron carriers) and intermediates in the synthesis Within each protein complex of the electron transport chain,
of other molecules. For organisms that run the cycle in reverse, electrons are passed from electron donors to electron acceptors.
it is used to generate intermediates in the synthesis of other Each donor and acceptor is a redox couple, consisting of an
molecules and also to incorporate carbon into organic molecules. oxidized and a reduced form of a molecule. The electron transport
The centrality of the citric acid cycle to both the synthesis of chain contains many of these redox couples. When oxygen accepts
biomolecules and to meeting the energy requirements of cells electrons at the end of the electron transport chain, it is reduced
suggests that this cycle evolved early, appearing in some of the to form water:
first cells to feature metabolism. This early appearance of the citric
O2 1 4e2 1 4H1 → 2H2O
acid cycle, in turn, implies that the great variety of biosynthetic
and energy-yielding pathways found in modern cells evolved This reaction is catalyzed by complex IV.
 CHAPTER 7 C E L L U L A R R E S P I R AT I O N : H A RV E S T I N G E N E RG Y F RO M C A R B O H Y D R AT E S A N D OT H E R F U E L M O L E C U L E S 141

FIG. 7.10 The electron transport chain. (a) The electron transport chain consists of four complexes (I to IV) in the inner mitochondrial membrane.
(b) Electrons flow from electron carriers to oxygen, the final electron acceptor. (c) The proton gradient formed from the electron
transport chain has potential energy that is used to synthesize ATP.

a. The electron transport chain in cellular respiration

H⫹ Mitochondrial matrix
NADH
4 H⫹
H⫹ ADP + Pi ATP
NAD+ + 2 H 2O
Complex I
2H⫹
FADH2 FAD +
H⫹ ATP synthase H⫹
O2
⫹
H H⫹
H
Complex III
e-
e-
Complex
Co
Com
omp
om lex II
ple
le I

e- e-

Inner CoQ CoQ

FPO
e- Complex IV
membrane Cytochrome c
H⫹
H⫹ ⫹
H Intermembrane space
H⫹

b. Electron transport

NADH Complexes I and II

harvest electrons from 4 H⫹
H⫹
NAD+ + NADH and FADH2. 2 H 2O
Complex I 2H⫹
FADH2 FAD
AD + Cytochrome c moves
to complex IV where ATP synthase
oxygen is reduced to O2
form water.
Complex III
e-
e-
Co
Com
ommp
plex
p
Complexx IIII
e-
e-
e- Complex IV
CoQ CoQ
Cytochrome c
Coenzyme
C Q is
i reduced
d d tto C QH2 and
o CoQH d transfers
t f
electrons from complexe
complexes
es I and II to complex III.
ATP synthase uses the
c. Proton transport and ATP synthesis electrochemical proton
gradient to drive the
H⫹ synthesis of ATP.
The transport of electrons in complexes I, III, and
IV is coupled with the transport of protons across ADP + Pi ATP
the inner membrane, from the mitochondrial
Complex I matrix to the intermembrane space.

H⫹ ATP synthase H⫹

H⫹ H⫹
Complex III

Co
Complex
omp x IIII
Complex

CoQ CoQ Complex IV

Cytochrome c
H⫹
H⫹ ⫹
H
H⫹
142 SECTION 7.5 T H E E L E C T RO N T R A N S P O RT C H A I N A N D O X I DAT I V E P H O S P H O RY L AT I O N

Electrons also must be transported between the four

complexes (Fig. 7.10). Coenzyme Q (CoQ), also called ubiquinone, FIG. 7.11 ATP synthase. ATP synthase drives the synthesis of ATP
accepts electrons from both complexes I and II. In this reaction, by means of an electrochemical proton gradient.
two electrons and two protons are transferred to CoQ from the
Matrix
mitochondrial matrix, forming CoQH2. Once CoQH2 is formed, ADP + Pi ATP
it diffuses in the inner membrane to complex III. In complex III,
electrons are transferred from CoQH2 to cytochrome c
and protons are released into the intermembrane space.
2 The F1 subunit then
uses this rotational
When it accepts an electron, cytochrome c is reduced, diffuses energy to catalyze the F1
H⫹
in the intermembrane space, and passes the electron to synthesis of ATP.
complex IV.
These electron transfer steps are each associated with the
release of energy as electrons are passed from the reduced electron
1 The Fo subunit forms
F
a channel that rotates as o
carriers NADH and FADH2 to the final electron acceptor, oxygen. protons pass through it.
Some of this energy is used to reduce the next carrier in the
chain, but in complexes I, III, and IV, some of it is used to pump
protons (H) across the inner mitochondrial membrane, from the
mitochondrial matrix to the intermembrane space (Fig. 7.10).
Thus, the transfer of electrons through complexes I, III, and
IV is coupled with the pumping of protons. The result is an Intermembrane space

accumulation of protons in the intermembrane space.

ATP synthase is a molecular machine that
j Quick Check 4 Animals breathe in air that contains more oxygen is composed of two subunits, Fo and F1.
than the air they breathe out. Where is oxygen consumed?

see shortly, the resulting movement of the protons through the

The proton gradient is a source of potential energy. membrane can be used to perform work.
Like all membranes, the inner mitochondrial membrane is In sum, the oxidation of the electron carriers NADH and
selectively permeable: Protons cannot passively diffuse across this FADH2 formed during glycolysis, pyruvate oxidation, and the citric
membrane, and the movement of other molecules is controlled acid cycle leads to the generation of a proton electrochemical
by transporters and channels (Chapter 5). We have just seen gradient, which is a source of potential energy. This source of
that the movement of electrons through membrane-embedded potential energy is used to synthesize ATP.
protein complexes is coupled with the pumping of protons
from the mitochondrial matrix into the intermembrane space. ATP synthase converts the energy of the proton
The consequence is a proton gradient, a difference in proton gradient into the energy of ATP.
concentration across the inner membrane. In 1961, Peter Mitchell proposed a hypothesis to explain how
The proton gradient has two components: a chemical gradient the energy stored in the proton electrochemical gradient is used
due to the difference in concentration and an electrical gradient to synthesize ATP. In 1978, he was awarded the Nobel Prize in
due to the difference in charge between the two sides of the Chemistry for work that fundamentally changed the way we
membrane. To reflect the dual contribution of the concentration understand how energy is harnessed by a cell.
gradient and the electrical gradient, the proton gradient is also According to Mitchell’s hypothesis, the gradient of protons
called an electrochemical gradient. provides a source of potential energy that is converted into
The proton gradient is a source of potential energy, as chemical energy stored in ATP. First, for the potential energy of the
discussed in Chapters 5 and 6. It stores energy much in the proton gradient to be released, there must be an opening in the
same way that a battery or a dam does. Through the actions of membrane for the protons to flow through. Mitchell suggested that
the electron transport chain, protons have a high concentration protons in the intermembrane space diffuse down their electrical
in the intermembrane space and a low concentration in the and concentration gradients through a transmembrane protein
mitochondrial matrix. As a result, there is a tendency for protons channel into the mitochondrial matrix. Second, the movement
to diffuse back to the mitochondrial matrix, driven by a difference of protons through the enzyme must be coupled with the
in concentration and charge on the two sides of the membrane. synthesis of ATP. This coupling is made possible by ATP synthase,
This movement, however, is blocked by the membrane, so the a remarkable enzyme composed of two distinct subunits called Fo
gradient stores potential energy. That energy can be harnessed if and F1 (Fig. 7.11). Fo forms the channel in the inner mitochondrial
a pathway is opened through the membrane because, as we will membrane through which protons flow; F1 is the catalytic unit that
HOW DO WE KNOW?

FIG. 7.12

RESULTS In the presence of light, the concentration of protons

Can a proton gradient drive increased inside the vesicles, suggesting that protons were taken up
by the vesicles.
the synthesis of ATP?

Calculated proton
BACKGROUND Peter Mitchell’s hypothesis that a proton gradient

inside vesicles
concentration
can drive the synthesis of ATP was met with skepticism because it
was proposed before experimental evidence supported it. In the
1970s, biochemist Efraim Racker and his collaborator Walther
Stoeckenius tested the hypothesis.

EXPERIMENT Racker and Stoeckenius built an artificial system

Light on Light off
consisting of a membrane, a bacterial proton pump activated by
light, and ATP synthase. In the dark, the concentration of protons returned to the starting
level. ATP was generated in the light, but not in the dark.
ADP + Pi
Condition Relative level of ATP
Bacterial ATP
proton pump
Light on Light 594

Dark 23

INTERPRETATION In the presence of light, the proton pump was

ATP synthase activated and protons were pumped to one side of the membrane,
leading to the formation of a proton gradient. The proton gradient,
H⫹ in turn, powered synthesis of ATP by ATP synthase.

CONCLUSION A membrane, proton gradient, and ATP synthase

are sufficient to synthesize ATP. This result provided experimental
evidence for Mitchell’s hypothesis.

SOURCES Mitchell, P. 1961. “Coupling of Phosphorylation to Electron and Hydrogen

They measured the concentration of protons in the external Transfer by a Chemiosmotic Type of Mechanism.” Nature 191:144–148; Racker, E.,
and W. Stoeckenius. 1974. “Reconstitution of Purple Membrane Vesicles Catalyzing
medium and the amount of ATP produced in the presence and Light-Driven Proton Uptake and Adenosine Triphosphate Formation.” J. Biol. Chem.
absence of light. 249:662–663.

synthesizes ATP. Proton flow through the channel (Fo) makes it Direct experimental evidence for Mitchell’s idea, called the
possible for the enzyme (F1) to synthesize ATP. chemiosmotic hypothesis, did not come for over a decade.
Proton flow through the Fo channel causes it to rotate, One of the key experiments that provided support for his idea is
converting the energy of the proton gradient into mechanical illustrated in Fig. 7.12.
rotational energy, a form of kinetic energy. The rotation of the Fo
subunit leads to rotation of the F1 subunit in the mitochondrial j Quick Check 5 Uncoupling agents are proteins spanning
matrix (Fig. 7.11). The rotation of the F1 subunit in turn causes the inner mitochondrial membrane that allow protons to pass
conformational changes that allow it to catalyze the synthesis of through the membrane and bypass the channel of ATP synthase.
ATP from ADP and Pi. In this way, mechanical rotational energy is Describe the consequences to the proton gradient and ATP
converted into the chemical energy of ATP. production.

143
 144 SECTION 7.6 A N A E RO B I C M E TA B O L I S M A N D T H E E VO L U T I O N O F C E L L U L A R R E S P I R AT I O N

TABLE 7.1 Approximate Total ATP Yield in Cellular Respiration. FIG. 7.13 The flow of energy in cellular respiration. A
single glucose molecule yields 32 ATP molecules.
SUBSTRATE-LEVEL OXIDATIVE TOTAL
PATHWAY PHOSPHORYLATION PHOSPHORYLATION ATP Cell membrane

Glycolysis 2 ATP 2 NADH = 5 ATP 7

(glucose → 2 pyruvate) Cytosol
Glucose
Pyruvate oxidation 0 ATP 2 NADH = 5 ATP 5
(2 pyruvate → 2 acetyl-
CoA)
Citric acid cycle 2 ATP 6 NADH = 15 ATP Electron carriers
(2 turns, 1 for each produced ATP generated
acetyl-CoA) 2 FADH2 = 3 ATP 20
Total 4 ATP 28 ATP 32 2 NADH Glycolysis 2 ATP

Approximately 2.5 molecules of ATP are produced for each Pyruvate

NADH that donates electrons to the chain and 1.5 molecules of
ATP for each FADH2. Therefore, overall, the complete oxidation of
glucose yields about 32 molecules of ATP from glycolysis, pyruvate 2 NADH
Pyruvate
oxidation, the citric acid cycle, and oxidative phosphorylation oxidation
(Table 7.1). Some of the energy held in the bonds of glucose is now
available in a molecule that can be readily used by cells.
It is worth taking a moment to follow the flow of energy in
cellular respiration, illustrated in its full form in Fig. 7.13. We Acetyl-CoA
began with glucose and noted that it holds chemical potential
Mitochondrial
energy in its covalent bonds. This energy is released in a series of matrix
reactions and captured in chemical form. Some of these reactions 6 NADH
generate ATP directly by substrate-level phosphorylation. Others Citric
are redox reactions that transfer energy to the electron carriers acid 2 ATP
NADH and FADH2. These electron carriers donate electrons to cycle
the electron transport chain, which uses the energy stored in the 2 FADH2
electron carriers to pump protons across the inner membrane
of the mitochondria. In other words, the energy of the reduced
Electron
electron carriers is transformed into energy stored in a proton
transport chain
electrochemical gradient. ATP synthase then converts the energy (ETC) and 28 ATP
of the proton gradient to rotational energy, which drives the oxidative
synthesis of ATP. The cell now has a form of energy that it can use phosphorylation
in many ways to perform work.

Total: 32 ATP

7.6 ANAEROBIC METABOLISM

AND THE EVOLUTION OF
Outer mitochondrial Intermembrane Inner mitochondrial
CELLULAR RESPIRATION membrane space membrane

Up to this point, we have followed a single metabolic path: the

breakdown of glucose in the presence of oxygen to produce
carbon dioxide and water. However, metabolic pathways more
often resemble intersecting roads rather than a single, linear
 CHAPTER 7 C E L L U L A R R E S P I R AT I O N : H A RV E S T I N G E N E RG Y F RO M C A R B O H Y D R AT E S A N D OT H E R F U E L M O L E C U L E S 145

path. We saw this earlier in the discussion of the citric acid cycle,
where intermediates in the cycle often feed into other metabolic FIG. 7.14 Lactic acid and ethanol fermentation pathways.
pathways.
One of the major forks in the metabolic road occurs at a. Lactic acid fermentation
pyruvate, the end product of glycolysis (section 10.2). When HO CH2

oxygen is present, it is converted to acetyl-CoA, which then enters C O

H H
the citric acid cycle, resulting in the production of ATP and reduced H
C C
electron carriers to fuel the electron transport chain, as we saw. OH H H O
When oxygen is not present, however, pyruvate is metabolized HO
C C
OH
CH3 C C
along a number of different pathways. These pathways occur in OH
H OH OH
many living organisms today and played an important role in the
early evolution of life on Earth. Glucose 2 Lactic acid

Glycolysis Lactic acid

Fermentation extracts energy from glucose in the 2 NAD+ fermentation
absence of oxygen.
2 ADP + 2 Pi
Pyruvate has many possible fates in the cell. In the absence of
NAD+
oxygen, it can be broken down by fermentation, which does not produced in
rely on oxygen or any other electron acceptor. Fermentation is 2 ATP fermentation
2 NADH  2H is used in
accomplished through a wide variety of metabolic pathways that
glycolysis.
extract energy from fuel molecules such as glucose. Fermentation
O
pathways are important for anaerobic organisms that live
CH3 C C
without oxygen, as well as some organisms such as yeast that
O O
favor fermentation over oxidative phosphorylation, even in the
presence of oxygen. It is also sometimes used in aerobic organisms 2 Pyruvate
when oxygen cannot be delivered fast enough to meet the cell’s
metabolic needs, as in exercising muscle.
Recall that during glycolysis, glucose is oxidized to form
pyruvate, and NAD1 is reduced to form NADH. For glycolysis b. Ethanol fermentation
to continue, NADH must be oxidized to NAD1. If that did not HO CH2
happen, glycolysis would grind to a halt. In the presence of C O
H H
oxygen, NAD1 is regenerated when NADH donates its electrons H
to the electron transport chain. In the absence of oxygen during C C
OH H
fermentation, NADH is oxidized to NAD1 when pyruvate or a HO OH
C C
derivative of pyruvate is reduced.
H OH CH3 CH2 OH
There are many fermentation pathways, especially in bacteria.
Two of the major pathways are lactic acid fermentation and Glucose 2 Ethanol
ethanol fermentation (Fig. 7.14). Lactic acid fermentation
occurs in animals and bacteria. During lactic acid fermentation, Glycolysis Ethanol
electrons from NADH are transferred to pyruvate to produce 2 NAD+ fermentation
lactic acid and NAD1 (Fig. 7.14a). The overall chemical reaction is 2 ADP + 2 Pi
NAD+
written as follows: produced in
fermentation
2 ATP is used in
Glucose 1 2 ADP 1 2 Pi → 2 lactic acid 1 2 ATP 1 2H2O 2 NADH  2H glycolysis.

Ethanol fermentation occurs in plants and fungi. During O

ethanol fermentation, pyruvate releases carbon dioxide to CH3 C C Acetaldehyde
form acetaldehyde, and electrons from NADH are transferred O O
to acetaldehyde to produce ethanol and NAD1 (Fig. 7.14b). The
2 Pyruvate
overall chemical reaction is written as follows:

Glucose 1 2 ADP 1 2 Pi → 2 ethanol 1 2CO2 1 2 ATP 1 2H2O 2

CO2
 146 SECTION 7.6 A N A E RO B I C M E TA B O L I S M A N D T H E E VO LU T I O N O F C E L LU L A R R E S P I R AT I O N

In both fermentation pathways, NADH is oxidized to NAD.

However, NADH and NAD do not appear in the overall chemical FIG. 7.15 The possible evolution of the electron transport chain
equations because there is no net production or loss of either and oxidative phosphorylation.
molecule. NAD molecules that are reduced during glycolysis are a.
oxidized when lactic acid or ethanol is formed. H⫹
The breakdown of a molecule of glucose by fermentation H⫹
yields only two molecules of ATP. The energetic gain is relatively
small compared with the total yield of aerobic respiration because
the end products, lactic acid and ethanol, are not fully oxidized
and still contain a large amount of chemical energy in their bonds. e-

The modest yield explains why organisms that produce ATP by

fermentation must consume a large quantity of fuel molecules to
power the cell. ATP ADP + Pi

j Quick Check 6 Bread making involves ethanol fermentation

and typically uses yeast, sugar, flour, and water. Why are yeast and
sugar used?
Early cells evolved mechanisms to pump protons out

? CASE 1 THE FIRST CELL: LIFE’S ORIGINS

of the cell, powered by ATP and electron transport.

How did early cells meet their energy requirements?

The four stages of cellular respiration lead to the full oxidation of
glucose, resulting in the release of a large amount of energy stored
in its chemical bonds. The first stage, glycolysis, results in only b.
H⫹
the partial oxidation of glucose, so just some of the energy held in H⫹ H⫹

its chemical bonds is released. Nearly all organisms are capable of

partially breaking down glucose, suggesting that glycolysis evolved
very early in the history of life.
e- e-
Life first evolved about 4 billion years ago in the absence
of atmospheric oxygen. The earliest organisms probably used
one of the fermentation pathways to generate the ATP necessary to
ADP + Pi ATP
power cellular processes because fermentation does not
require atmospheric oxygen. Fermentation occurs in the cytoplasm
and does not require proteins embedded in specialized membranes.
As we have seen, cellular respiration involves an electron
transport chain, composed of proteins embedded in a membrane Eventually, electron-transport-powered pumps became
and capable of transferring electrons from one protein to the efficient enough to run the ATP-driven pump in reverse.
next and pumping protons. The resulting proton gradient powers
the synthesis of ATP. Like fermentation, cellular respiration can
occur in the absence of oxygen, but in that case molecules other
than oxygen, such as sulfate and nitrate, are the final electron
acceptor (Chapter 26). This form of respiration is known as Organisms capable of producing oxygen, the cyanobacteria,
anaerobic respiration and occurs in some present-day bacteria. The did not evolve until about 2.5 billion years ago, maybe earlier.
electron transport chain in these bacteria is located in the plasma The evolution of this new form of life introduced oxygen into
membrane, not in an internal membrane. Earth’s atmosphere. This dramatic change led to the evolution
How might such a system have evolved? An intriguing of new life-forms with new possibilities for extracting energy
possibility is that early prokaryotes evolved pumps to drive from fuel molecules such as glucose. Aerobic respiration, in
protons out of the cell in response to an increasingly acidic which oxygen serves as the final electron acceptor in the electron
environment (Fig. 7.15). Some pumps might have used the energy transport chain, generates much more energy than does anaerobic
of ATP to pump protons, while others used electron transport respiration or fermentation.
proteins to pump protons (Fig. 7.15a). At some point, proton The evolution of cellular respiration illustrates that evolution
pumps powered by electron transport might have generated a often works in a stepwise fashion, building on what is already
large enough electrochemical gradient that the protons could pass present. In this case, aerobic respiration picked up where
back through the ATP-driven pumps, running them in reverse to anaerobic respiration left off, making it possible to harness more
synthesize ATP (Fig. 7.15b). energy from organic molecules to power the work of the cell.
 CHAPTER 7 C E L L U L A R R E S P I R AT I O N : H A RV E S T I N G E N E RG Y F RO M C A R B O H Y D R AT E S A N D OT H E R F U E L M O L E C U L E S 147

the whole body, able to release glucose into the bloodstream when
7.7 METABOLIC INTEGRATION it is needed elsewhere. Glycogen provides a source of glucose
6-phosphate to feed glycolysis when the level of blood glucose is
In this chapter, we have focused on the breakdown of glucose. low. Glucose molecules located at the end of glycogen chains can
What happens if there is more glucose than is needed by the cell? be cleaved one by one, and they are released in the form of glucose
As well as glucose, you probably consume diverse carbohydrates, 1-phosphate. Glucose 1-phosphate is then converted into glucose
lipids, and proteins. How are these broken down? And how 6-phosphate, an intermediate in glycolysis. One glucose molecule
are these various metabolic pathways coordinated so that the cleaved offa glycogen chain produces three and not two molecules
intracellular level of ATP is maintained in a narrow range? In of ATP by glycolysis because the ATP-consuming step 1 of glycolysis
this final section, we consider how the cell responds to these is bypassed.
challenges.
Sugars other than glucose contribute to glycolysis.
Excess glucose is stored as glycogen in animals and The carbohydrates in your diet are digested to produce a
starch in plants. variety of sugars (Fig. 7.17). Some of these are disaccharides
Glucose is a readily available form of energy in organisms, but (maltose, lactose, and sucrose) with two sugar units; others are
it is not always broken down immediately. Excess glucose can monosaccharides (fructose, mannose, and galactose) with a single
be stored in cells and then mobilized—that is, broken down— sugar unit. The disaccharides are hydrolyzed into monosaccharides,
when necessary. Glucose can be stored in two major forms: as which are transported into cells.
glycogen in animals and as starch in plants (Fig. 7.16). Both these The hydrolysis of some disaccharides produces glucose
molecules are large branched polymers of glucose. molecules that directly enter glycolysis. What happens to other
Carbohydrates that are consumed by animals are broken down monosaccharides? They, too, enter glycolysis, although not
into simple sugars and circulate in the blood. The level of glucose as glucose. Instead, they are converted into intermediates of
in the blood is tightly regulated. When the blood glucose level is glycolysis that come later in the pathway. For example, fructose
high, as it is after a meal, glucose molecules that are not consumed is produced by the hydrolysis of sucrose (table sugar) and receives
by glycolysis are linked together to form glycogen in liver and a phosphate group to form either fructose 6-phosphate or
muscle. Glycogen stored in muscle is used to provide ATP for fructose 1-phosphate. In the liver, fructose 1-phosphate is cleaved
muscle contraction. By contrast, the liver does not store glycogen and converted into glyceraldehyde 3-phosphate, which enters
primarily for its own use, but is a central glycogen storehouse for glycolysis at reaction 6 (see Fig. 7.5).

FIG. 7.16 Storage forms of glucose. (a) Glycogen is a storage form of glucose in animal cells, and (b) starch is a storage form of glucose in
plant cells.
a. Glycogen b. Starch

Starch is a large, branched chain of

glucose molecules found in plants
such as potatoes, wheat, and corn. It
is stored in granules inside cells.

Glycogen is a large, branched chain of glucose

molecules attached to a central protein. Glycogen
can be made up of thousands of glucose molecules.
148 SECTION 7.7 M E TA B O L I C I N T E G R AT I O N

FIG. 7.17 Common sugars in your diet.

Disaccharides Monosaccharides

CH2OH
Lactose OH Galactose
O
OH
CH2OH CH2OH
Maltose HO O O HO O OH Fructose
CH2OH CH2OH OH OH OH
O O CH2OH O OH
OH OH OH OH HO

HO O OH CH2OH
Milk
OH OH OH
Mannose
Sucrose
CH2OH CH2OH
H O H CH2OH O H O
OH H H HO Barley OH HO

HO O CH2OH OH OH
H OH OH H

Candy Apple

Fatty acids and proteins are useful sources

of energy. FIG. 7.18 b-oxidation of fatty acids. The fatty acids from lipids are broken down
In addition to carbohydrates, lipids are also a good to produce NADH and FADH2, as well as acetyl-CoA.
source of energy. We know this from common
H 3C CH2 CH2 CH2 CH2 CH2 CH2 CH2 O
experience. Butter, oils, ice cream, and the like all
CH2 CH2 CH2 CH2 CH2 CH2 CH2 C
contain lipids and are high in calories, which are units
Palmitoyl CoA
of energy. We can also infer that lipids are a good S
FAD
source of energy from their chemical structure. Recall CoA
from Chapter 2 that a type of fat called triacylglycerol
is composed of three fatty acid molecules bound to a FADH2
glycerol backbone. These fatty acid molecules are rich Electron
NAD+ transport
in carbon–carbon and carbon–hydrogen bonds, which,
chain
as we saw earlier, carry chemical potential energy.
Following a meal, the small intestine very quickly NADH
absorbs triacylglycerols, which are then transported
by the bloodstream and either consumed or stored in
H3C CH2 CH2 CH2 CH2 CH2 CH2 O H3C O
fat (adipose) tissue. Triacylglycerols are broken down CH2 CH2 CH2 CH2 CH2 CH2 C C
inside cells to glycerol and fatty acids. Then, the fatty
S + S
acids themselves are shortened by a series of reactions
that sequentially remove two carbon units from CoA CoA
their ends (Fig. 7.18). This process is called b-(beta-) Acetyl-CoA
oxidation. It does not produce ATP, but releases a
large number of NADH and FADH2 molecules that
provide electrons for the synthesis of ATP by oxidative
phosphorylation. In addition, the end product of the
Citric acid
reaction is acetyl-CoA, which feeds the citric acid cycle cycle
 CHAPTER 7 C E L L U L A R R E S P I R AT I O N : H A RV E S T I N G E N E RG Y F RO M C A R B O H Y D R AT E S A N D OT H E R F U E L M O L E C U L E S 149

and leads to the production of an even larger quantity of reduced

electron carriers. FIG. 7.19 Regulation of cellular respiration. Cellular respiration is
The oxidation of fatty acids produces a large amount of ATP. inhibited by its products, including ATP and NADH, and
For example, the complete oxidation of a molecule of palmitic activated by its substrates, including ADP and NAD+.
acid, a fatty acid containing 16 carbons, yields about 106 molecules
Cell membrane
of ATP. By contrast, glycolysis yields just 2 molecules of ATP, and
the complete oxidation of a glucose molecule produces about
32 molecules of ATP (Table 7.1). Fatty acids therefore are a useful Cytosol
and efficient source of energy, but they cannot be used by all
tissues of the body. Notably, the brain and red blood cells depend High concentrations High concentrations of
primarily on glucose for energy. of substrates such as products such as ATP
Proteins, like fatty acids, are a source of chemical energy that ADP and NAD+ and NADH indicate a
indicate a low energy high energy state in the
can be broken down, if necessary, to power the cell. Proteins are state in the cell and cell and inhibit the
Glucose
typically first broken down to amino acids, some of which can stimulate the respiratory pathways.
respiratory pathways.
then enter glycolysis and others the citric acid cycle.

The intracellular level of ATP is a key regulator + Glycolysis –

of cellular respiration.
ATP is the key end product of cellular respiration, holding in its
bonds energy that can be used for all kinds of cellular processes.
Pyruvate
ATP is constantly being turned over in a cell, broken down to ADP
and Pi to supply the cell’s energy needs, and re-synthesized by
fermentation and cellular respiration. The level of ATP inside a
cell can therefore be an indicator of how much energy a cell has Pyruvate
+ oxidation –
available. When ATP levels are high, the cell has a high amount
of free energy and is poised to carry out cellular processes. In
this case, pathways that generate ATP are slowed, or down-
regulated. By contrast, when ATP levels are low, the cell activates,
or up-regulates, pathways that lead to ATP synthesis. Other Acetyl-CoA
intermediates of cellular respiration, such as NADH, have a similar
effect in that high NAD1 levels stimulate cellular respiration, NAD+
whereas high NADH levels inhibit it (Fig. 7.19).
How is this kind of coordinated response of the cell possible? Citric
The cell uses several mechanisms, one of which is the regulation + acid –
of enzymes that control key steps of the pathway. One of these cycle
key reactions is reaction 3 of glycolysis. In this reaction, fructose
6-phosphate is converted to fructose 1,6-bisphosphate, and a NADH
molecule of ATP is consumed. This is a key step in glycolysis
because it is highly endergonic and irreversible. As a result, it is Electron transport chain
considered a “committed” step and is subject to tight control. + and
oxidative phosphorylation
–
This reaction is catalyzed by the enzyme phosphofructokinase-1
(PFK-1), which can be thought of as a metabolic valve that
ADP + Pi ATP
regulates the rate of glycolysis.
PFK-1 is an allosteric enzyme with many activators and
ATP
inhibitors (Fig. 7.20). Recall from Chapter 6 that an allosteric
synthesis
enzyme changes its shape and activity in response to the binding
H
of molecules at a site other than the active site. ADP and AMP are
allosteric activators of PFK-1. When ADP and AMP are abundant,
Mitochondrion
one or the other binds to the enzyme and causes the enzyme’s
shape to change. The shape change activates the enzyme,
increasing the rate of glycolysis and the synthesis of ATP. When
ATP is in abundance, it binds to the same site on the enzyme as
ADP and AMP, but in this case binding inhibits the enzyme’s
150 SECTION 7.7 M E TA B O L I C I N T E G R AT I O N

to a familiar example: exercise.

FIG. 7.20 Regulation of PFK-1.The regulation of the glycolytic enzyme phosphofructokinase-1 Exercise such as running, walking,
(PFK-1) is an example of integrated metabolic control. and swimming is a form of kinetic
energy, powered by ATP in muscle
Activation Inhibition cells. Where does this ATP come
ATP levels are low. ATP or citrate levels are high.
from?
Muscle cells, like all cells, do not
ADP Citrate contain a lot of ATP, and stored ATP
ATP
is depleted by exercise in a matter
AMP
of seconds. As a result, muscle cells
+ – rely on fuel molecules to generate
ATP. For a short sprint or a burst
of activity, muscle can convert
Phosphofructokinase-1 stored glycogen to glucose, and then
break down glucose anaerobically
to pyruvate and lactic acid by lactic
acid fermentation. This pathway is
rapid, but it does not generate a lot
of ATP. In addition, it is limited by
Fructose 6-phosphate Fructose 1,6- Fructose 6-phosphate Fructose 1,6- the production of lactic acid, which
bisphosphate bisphosphate
lowers the pH of the blood.
When ATP levels are low, When ATP or citrate levels For longer, more sustained
PFK-1 is activated, allowing Glycolysis are high, PFK-1 is inhibited, G olysis
Glyco
glycolysis to continue. and glycolysis slows. exercise, other metabolic pathways
come into play. Muscle cells contain
many mitochondria, which produce
ATP by aerobic respiration. The
energy yield of aerobic respiration
is much greater than that of
catalytic activity. As a result, glycolysis and the rate of ATP fermentation, but the process is slower. This slower production
production slow down. of ATP by aerobic respiration in part explains why runners cannot
PFK-1 is also regulated by one of its downstream products, maintain the pace of a sprint for longer runs.
citrate, an intermediate in the citric acid cycle (see Fig. 7.8). For even longer exercise, liver glycogen supplements muscle
Citrate acts as an allosteric inhibitor of the enzyme, slowing its glycogen: The liver releases glucose into the blood that is taken
activity. High levels of citrate indicate that is not being consumed up by muscle cells and oxidized to produce ATP. In addition, fatty
by the citric acid cycle and glucose breakdown can be slowed. The acids are released from adipose tissue and taken up by muscle cells,
role of citrate in controlling glycolysis illustrates the coordinated where they are broken down by b-oxidation. b-oxidation yields
regulation of glycolysis and the citric acid cycle. even more ATP than does the complete oxidation of glucose, but
the process is again slower. Storage forms of energy molecules,
Exercise requires several types of fuel molecules and such as fatty acids and glycogen, contain large reservoirs of
the coordination of metabolic pathways. energy, but are slow to mobilize. Thus, exercise takes coordination
In the last two chapters, we considered what energy is and how between different cells, tissues, and metabolic pathways to ensure
it is harnessed by cells. Let’s apply the concepts we discussed adequate ATP to meet the needs of working muscle. •
 CHAPTER 7 C E L L U L A R R E S P I R AT I O N : H A RV E S T I N G E N E RG Y F RO M C A R B O H Y D R AT E S A N D OT H E R F U E L M O L E C U L E S 151

Core Concepts Summary

7.1 Cellular respiration is a series of catabolic Pyruvate oxidation occurs in the mitochondrial
reactions that convert the energy in fuel molecules matrix. page 137
into ATP.
7.4 The citric acid cycle results in the complete
During cellular respiration, sugar molecules like glucose are
oxidation of fuel molecules and the generation of ATP
broken down in the presence of oxygen to produce carbon
and reduced electron carriers.
dioxide and water. page 132
The citric acid cycle takes place in the mitochondrial matrix.
Cellular respiration releases energy because the potential
page 138
energy of the reactants is greater than that of the
products. page 132 The acetyl group of acetyl-CoA is completely oxidized in
the citric acid cycle. page 138
ATP is generated in two ways during cellular respiration:
substrate-level phosphorylation and oxidative The citric acid cycle is a cycle because the acetyl group of
phosphorylation. page 133 acetyl-CoA combines with oxaloacetate, and then a series
Cellular respiration is an oxidation–reduction reaction. of reactions regenerates oxaloacetate. page 138
page 133 A complete turn of the citric acid cycle results in the
In oxidation–reduction reactions, electrons are transferred production of one molecule of GTP (which is converted
from one molecule to another. Oxidation is the loss of to ATP), three molecules of NADH, and one molecule of
electrons, and reduction is the gain of electrons. page 133 FADH2. page 138

Electron carriers transfer electrons to an electron transport Citric acid cycle intermediates are starting points for
chain, which harnesses the energy of these electrons to the synthesis of many different organic molecules.
generate ATP. page 133 page 140

Cellular respiration is a four-stage process that includes

7.5 The electron transport chain transfers electrons
(1) glycolysis; (2) pyruvate oxidation; (3) the citric acid
from electron carriers to oxygen, using the energy
cycle; and (4) oxidative phosphorylation. page 135
to pump protons and synthesize ATP by oxidative
7.2 Glycolysis is the partial oxidation of glucose and phosphorylation.
results in the production of pyruvate, as well as ATP NADH and FADH2 donate electrons to the electron transport
and reduced electron carriers. chain. page 140
Glycolysis takes place in the cytoplasm. page 135 In the electron transport chain, electrons move from one
Glycolysis is a series of 10 reactions in which glucose is redox couple to the next. page 140
oxidized to pyruvate. page 137 The electron transport chain is made up of four complexes.
Glycolysis consists of preparatory, cleavage, and payoff Complexes I and II accept electrons from NADH and FADH2,
phases. page 137 respectively. The electrons are transferred from these two
complexes to coenzyme Q. page 140
For each molecule of glucose broken down during glycolysis,
a net gain of two molecules of ATP and two molecules of Reduced coenzyme Q transfers electrons to complex III
NADH is produced. page 137 and cytochrome c transfers electrons to complex IV.
Complex IV reduces oxygen to water. page 142
The synthesis of ATP in glycolysis results from the direct
transfer of a phosphate group from a substrate to ADP, a The transfer of electrons through the electron transport
process called substrate-level phosphorylation. page 137 chain is coupled with the movement of protons across the
inner mitochondrial membrane into the intermembrane
7.3 Pyruvate is oxidized to acetyl-CoA, connecting space. page 142
glycolysis to the citric acid cycle. The buildup of protons in the intermembrane space results
The conversion of pyruvate to acetyl-CoA results in the in a proton electrochemical gradient, which stores potential
production of one molecule of NADH and one molecule of energy. page 142
carbon dioxide. page 137
152 SELF-ASSESSMENT

The movement of protons back into the mitochondrial Phosphofructokinase-1 controls a key step in glycolysis. It
matrix through the Fo subunit of ATP synthase is coupled has many allosteric activators, including ADP and AMP, and
with the formation of ATP, a reaction catalyzed by the F1 allosteric inhibitors, including ATP and citrate. page 149
subunit of ATP synthase. page 142
The ATP in muscle cells used to power exercise is generated
by lactic acid fermentation, aerobic respiration, and
7.6 Glucose can be broken down in the absence of
b-oxidation. page 150
oxygen by fermentation, producing a modest amount
of ATP.
Pyruvate, the end product of glycolysis, is processed Self-Assessment
differently in the presence and the absence of oxygen.
1. Name and describe the four major stages of cellular
page 145
respiration.
In the absence of oxygen, pyruvate enters one of several
2. Explain what an oxidation–reduction reaction is and why
fermentation pathways. page145
the breakdown of glucose in the presence of oxygen to
In lactic acid fermentation, pyruvate is reduced to lactic produce carbon dioxide and water is an example of an
acid. page 145 oxidation–reduction reaction.

In ethanol fermentation, pyruvate is converted to 3. Describe two different ways in which ATP is generated in
acetaldehyde, which is reduced to ethanol. page 145 cellular respiration.

During fermentation, NADH is oxidized to NAD1, allowing 4. Write the overall chemical equation for glycolysis, noting
glycolysis to proceed. page 145 the starting and ending products and highlighting the
energy-storing molecules that are produced.
Glycolysis and fermentation are ancient biochemical
pathways and were likely used in the common ancestor of 5. Describe two different metabolic pathways that pyruvate
all organisms living today. page 146 can enter.

6. Name the products of the citric acid cycle.

7.7 Metabolic pathways are integrated, allowing
control of the energy level of cells. 7. Describe how the movement of electrons along the electron
transport chain leads to the generation of a proton gradient.
Excess glucose molecules are linked together and stored in
polymers called glycogen (in animals) and starch (in plants). 8. Describe how a proton gradient is used to generate ATP.
page 147
9. Explain how muscle tissue generates ATP during short-term
Other monosaccharides derived from the digestion of and long-term exercise.
dietary carbohydrates are converted into intermediates of
glycolysis. page 147
Log in to to check your answers to the Self-
Fatty acids contained in triacylglycerols are an important Assessment questions, and to access additional learning tools.
form of energy storage in cells. The breakdown of fatty
acids is called b-oxidation. page 148
 CHAPTER 8

Photosynthesis
Using Sunlight to Build
Carbohydrates

Core Concepts
8.1 Photosynthesis is the major
pathway by which energy and
carbon are incorporated into
carbohydrates.
8.2 The Calvin cycle is a three-step
process that uses carbon dioxide
to synthesize carbohydrates.
8.3 The light-harvesting reactions
use sunlight to produce the ATP
and NADPH required by the
Calvin cycle.
8.4 Challenges to the efficiency of
photosynthesis include excess
light energy and the oxygenase
activity of rubisco.
8.5 The evolution of photosynthesis
had a profound impact on life on
Earth.
Imagewerks Japan/Getty Images .

153
154 SECTION 8.1 P H OTO S Y N T H E S I S : A N OV E RV I E W

Walk through a forest and you will be struck, literally if you aren’t direct consumption of plant material and its indirect consumption
careful, by the substantial nature of trees. Where does the material as meat. It is also the source of all the oxygen that we breathe, as
to construct these massive organisms come from? Because trees well as fuels for heating and transportation. Fossil fuels are the
grow upward from a firm base in the ground, a reasonable first legacy of ancient photosynthesis: Oil has its origin in the bodies
guess is the soil. In the first recorded experiment on this question, of marine algae and the organisms that graze on them, while
the Flemish chemist and physiologist Jan Baptist van Helmont coal represents the geologic remains of terrestrial (land) plants.
(1580–1644) found that the 200 pounds of dry soil into which he Thus, one motivation to understand photosynthesis is the sheer
had planted a small willow tree decreased by only 2 ounces over a magnitude and importance of this process for life on Earth. Before
5-year period. During this same period, the tree gained 164 pounds. exploring the details of how photosynthesis actually occurs, let’s
Van Helmont concluded that water must be responsible for the look at what types of organism carry out photosynthesis and
tree’s growth. He was, in fact, half right: A tree is roughly half liquid where they live, as well as what structural components are needed
water. But what he missed completely is that the other half of his to allow cells to capture energy in this remarkable way.
tree had been created almost entirely out of thin air.
The process that allowed Van Helmont’s tree to increase in Photosynthesis is widely distributed.
mass using material pulled from the air is called photosynthesis. The photosynthesis that is most evident to us is carried out by
Photosynthesis is a biochemical process for building carbohydrates plants on land. Trees, grasses, and shrubs are all examples of
using energy from sunlight and carbon dioxide (CO2) taken from photosynthetic organisms. However, photosynthesis occurs among
the air. These carbohydrates are used both as starting points for the prokaryotic as well as eukaryotic organisms, on land as well as in
synthesis of other molecules and as a means of storing energy that the sea. Approximately 60% of global photosynthesis is carried out
can be converted into ATP through cellular respiration. by terrestrial organisms, with the remaining 40% taking place in
the ocean. The majority of photosynthetic organisms in marine
environments are unicellular. About half of oceanic photosynthesis
8.1 PHOTOSYNTHESIS: AN OVERVIEW is carried out by single-celled marine eukaryotes, while the other
half is carried out by photosynthetic bacteria.
Photosynthesis is the major entry point for energy into biological Photosynthesis takes place almost everywhere sunlight
systems. It is the source of all of the food we eat, both through the is available to serve as a source of energy. In the ocean,

FIG. 8.1 Photosynthesis in extreme environments. (a) Desert crust in the Colorado Plateau formed by photosynthetic bacteria and algae.
(b) A hot spring in Yellowstone National Park. The yellow color is due to photosynthetic bacteria. (c) The surface of a permanent snow
pack. The red color is due to photosynthetic algae. Sources: a. Jayne Belnap/USGS; b. f11photo/Shutterstock; c. Shattil & Rozinski/Naturepl.com.

a c

b
 CHAPTER 8 P H OTO S Y N T H E S I S : U S I N G S U N L I G H T TO B U I L D C A R B O H Y D R AT E S 155

photosynthesis occurs in the surface layer extending to about

100 m deep, called the photic zone, through which enough FIG. 8.2 Overview of photosynthesis. In photosynthesis, energy
sunlight penetrates to enable photosynthesis. On land, from sunlight is used to synthesize carbohydrates from
photosynthesis occurs most readily in environments that are both CO2.
moist and warm. Tropical rain forests have high photosynthetic
productivity, as do grasslands and forests in the temperate zone.
However, photosynthetic organisms have evolved adaptations that
allow them to tolerate a wide range of environmental conditions, Oxidation
of water, with
like those illustrated in Fig. 8.1. In very dry regions, a combination O2 produced H2O O2
of photosynthetic bacteria and unicellular algae forms an easily as a by-product
disturbed layer on the surface of the soil known as desert crust.
Photosynthetic bacteria are also found in the hot springs of
Yellowstone National Park at temperatures up to 75˚C. At the other
extreme, unicellular algae can grow on the surfaces of glaciers,
causing the surface of the snow to appear red. In section 8.4, we
e-
discuss why photosynthetic organisms growing in such extreme
environments often appear red or yellow rather than green. Photosynthetic electron
transport chain
Photosynthesis is a redox reaction.
Carbohydrates are synthesized from CO2 molecules during
ATP
photosynthesis, yet they have more energy stored in their
chemical bonds than is contained in the bonds of CO2 molecules.
NADPH
Therefore, to build carbohydrates using CO2 requires an input of
energy. This energy comes from sunlight.
How does energy from sunlight become incorporated into
chemical bonds? The answer to this question arises from the Reduction
fact that the synthesis of carbohydrates from CO2 and water is a of CO2 Calvin
reduction–oxidation, or redox, reaction. In Chapter 7, we saw that to form cycle
carbohydrates CO2
Carbohydrate
reduction reactions are reactions in which a molecule acquires
electrons and gains energy, whereas oxidation reactions are
reactions in which a molecule loses electrons and releases
energy. During photosynthesis, CO2 molecules are reduced to
form higher-energy carbohydrate molecules. This requires both The oxidation of water is linked with the reduction of
an input of energy from ATP and the transfer of electrons from an CO2 through a series of redox reactions in which electrons are
electron donor. In photosynthesis, energy from sunlight is used passed from one compound to another. This series of reactions
to produce ATP and electron donor molecules capable of reducing constitutes the photosynthetic electron transport chain. The
CO2 (Fig. 8.2). process begins with the absorption of sunlight by protein–pigment
Where do the electrons used to reduce CO2 come from? In complexes. The absorbed sunlight provides the energy that drives
photosynthesis carried out by plants and many algae, the ultimate electrons through the photosynthetic electron transport chain. In
electron donor is water. The oxidation of water results in the turn, the movement of electrons through this transport chain is
production of electrons, protons, and O2. Thus, oxygen is used to produce ATP and NADPH. And finally, ATP and NADPH are
formed in photosynthesis as a by-product of water’s role as a the energy sources needed to synthesize carbohydrates using CO2
source of electrons. We can demonstrate that water is the source in a process called the Calvin cycle (see Fig. 8.2).
of the oxygen released during photosynthesis using isotopes,
j Quick Check 1 If you want to produce carbohydrates containing
molecules that can be distinguished on the basis of their molecular
the heavy oxygen (18O) isotope, should you water your plants with
mass (Fig. 8.3).
H218O or inject C18O2 into the air?
Overall, then, the equation for photosynthesis leading to the
synthesis of glucose (C6H12O6) can be described as follows:
The photosynthetic electron transport chain takes
Oxidation
place on specialized membranes.
Electron transport chains play a key role in both photosynthesis
6CO2 + 6H2O C6H12O6 + O2
and respiration (Chapter 7). In both cases, electrons move within
and between large protein complexes embedded in specialized
Reduction membranes. In photosynthetic bacteria, the photosynthetic
156 SECTION 8.1 M

HOW DO WE KNOW?
FIG. 8.3
electron transport chain is located in

Does the oxygen released by membranes within the cytoplasm or, in some
cases, directly in the plasma membrane. In

photosynthesis come from H2O or CO2? eukaryotic cells, photosynthesis takes place in
chloroplasts. In the center of the chloroplast is
BACKGROUND The reactants in photosynthesis are water and carbon dioxide. Both the highly folded thylakoid membrane
contain oxygen, so it is unclear which one is the source of the oxygen that is produced in the (Fig. 8.4). The photosynthetic electron
reaction. transport chain is located in the thylakoid
membrane.
METHOD Most of the oxygen in the atmosphere is 16O, a stable isotope containing The name “thylakoid” is derived from
8 protons and 8 neutrons. A small amount (0.2%) is 18O, a stable isotope with 8 protons thylakois, the Greek word for “sac.” Thylakoid
and 10 neutrons. The relative abundance of molecules containing 16O versus 18O can be membranes form structures that resemble
measured using a mass spectrometer. H2O and CO2 containing a high percentage of 18O can flattened sacs, and these sacs are grouped into
be used to determine whether the oxygen produced in photosynthesis comes from water or structures called grana (singular, granum) that
carbon dioxide. look like stacks of interlinked pancakes. Grana
EXPERIMENT are connected to one another by membrane
bridges in such a way that the thylakoid
membrane encloses a single interconnected
compartment called the lumen. The region
surrounding the thylakoid membrane is called
Source: Sinclair
Stammers/ the stroma. Carbohydrate synthesis takes place
Science Source.
in the stroma, whereas sunlight is captured
1 Place
Chlorella, a and transformed into chemical energy by the
green alga, into photosynthetic electron transport chain in the
two test tubes. thylakoid membrane.
Initial Although photosynthetic organisms are
correctly described as autotrophs because
This test tube has This test tube has they can form carbohydrates from CO2, they
H218O and CO2. H2O and C18O2. also require a constant supply of ATP to meet
each cell’s energy requirements. Although
ATP is produced within chloroplasts, only
2 Wait 2 hours
carbohydrates (and not ATP) are exported
to allow from chloroplasts to the cytosol. This explains
Final
photosynthesis why cells that have chloroplasts also contain
to occur.
mitochondria. In mitochondria, carbohydrates
are broken down to generate ATP (Chapter 7).
RESULTS Cellular respiration is therefore one of several
1.2 features that heterotrophic organisms like
1.0 3 ourselves share with photosynthetic organisms.
Percent 18O2

0.8 Measure
0.6
percentage of We now consider the underlying
dissolved 18O2
0.4 biochemistry of photosynthesis. Though in this
in test tubes.
0.2 chapter we focus on photosynthesis as carried
Initial Final Initial Final out by eukaryotic cells, there is a remarkable
diversity in the way photosynthesis is carried
out in bacteria. We discuss this diversity in
CONCLUSION The percentage of 18O increases only when water contains 18O, but not Chapter 26. Because carbohydrates are the major
when carbon dioxide contains 18O. This finding indicates that the oxygen produced in product of photosynthesis, we first examine
photosynthesis comes from water, not carbon dioxide. the Calvin cycle, the biochemical pathway used
FOLLOW-UP WORK Carbon also has several isotopes, and their measurements have been
in photosynthesis to synthesize carbohydrates
used to determine the source of increased CO2 in the atmosphere today (Chapter 25). from CO2. Once we understand what energy
forms are needed to drive this autotrophic
SOURCE Adapted from Ruben, S., M. Randall, M. Kamen, and J. L. Hyde. 1941. “Heavy Oxygen (O18) as a pathway, we turn our attention to how energy is
Tracer in the Study of Photosynthesis.” Journal of the American Chemical Society 63:877–879.
captured from sunlight. We then examine some

156
 CHAPTER 8 P H OTO S Y N T H E S I S : U S I N G S U N L I G H T TO B U I L D C A R B O H Y D R AT E S 157

of the challenges of coordinating these two metabolic

FIG. 8.4 Chloroplast structure. Chloroplasts contain highly folded stages and consider ways in which photosynthetic organisms
thylakoid membranes. Photo source: Biophoto Associates/Science Source. cope with the inherent biochemical challenges of photosynthesis.
Finally, we provide a short overview of the evolutionary history
of photosynthesis.

8.2 THE CALVIN CYCLE

The Calvin cycle consists of 15 chemical reactions that synthesize
carbohydrates from CO2. These reactions can be grouped into
three main steps: (1) carboxylation, in which CO2 is added to
a 5-carbon molecule; (2) reduction, in which energy and
electrons are transferred to the compounds formed in step 1;
Outer membrane
and (3) regeneration of the 5-carbon molecule needed for
Inner membrane carboxylation (Fig. 8.5).
Thylakoid
membrane The incorporation of CO2 is catalyzed by the
Grana enzyme rubisco.
In the first step of the Calvin cycle (Fig. 8.5), CO2 is added to
Lumen a 5-carbon sugar called ribulose 1,5-bisphosphate (RuBP).
This step is catalyzed by the enzyme ribulose bisphosphate
Stroma carboxylase oxygenase, or rubisco for short. An enzyme that

FIG. 8.5 The Calvin cycle. CO2 is the input and triose phosphate is the output of the Calvin Cycle.

1 Carboxylation: 2 Reduction: NADPH transfers

The addition of CO2 to the two electrons and one proton;
5-carbon compound, RuBP, is inorganic phosphate (Pi) is released.
catalyzed by the enzyme rubisco. ATP ADP
NADPH NADP+

Carboxylation 3-Phosphoglycerate Reduction

CO2 (3-PGA)
input:
Carbon enters CO2
the Calvin Carbohydrate
cycle as CO2. output:
Ribulose-1,5- Triose Carbohydrates exit
bisphosphate phosphate as 3-carbon
(RuBP) compounds.

Regeneration

ADP ATP
+

3 Regeneration of RuBP: In this multistep

process, 3-carbon compounds are reorganized
and combined to produce RuBP.
158 SECTION 8.2 T H E C A LV I N C YC L E

adds CO2 to another molecule is called a carboxylase, explaining A large number of reactions is needed to rearrange the carbon
part of rubisco’s long name. atoms from five 3-carbon triose phosphate molecules into three
Before rubisco can act as a carboxylase, RuBP and CO2 must 5-carbon RuBP molecules. ATP is required for the regeneration
diffuse into its active site. Once the active site is occupied, the of RuBP, raising the Calvin cycle’s total energy requirements to
addition of CO2 to RuBP proceeds spontaneously in the sense two molecules of NADPH and three molecules of ATP for each
that no addition of energy is required. The product is a 6-carbon molecule of CO2 incorporated by rubisco.
compound that immediately breaks into two molecules of The Calvin cycle does not use sunlight directly. For this reason,
3-phosphoglycerate (3-PGA). These 3-carbon molecules are the this pathway is sometimes referred to as the light-independent
first stable products of the Calvin cycle. or even the “dark” reactions of photosynthesis. However, this
pathway cannot operate without the energy input provided
NADPH is the reducing agent of the Calvin cycle. by a steady supply of NADPH and ATP. Both are supplied by
Rubisco is responsible for the addition of the carbon atoms the photosynthetic electron transport chain, in which light is
needed for the formation of carbohydrates, but by itself rubisco captured and transformed into chemical energy. In addition,
does not increase the amount of energy stored within the newly several Calvin cycle enzymes are regulated by cofactors that must
formed bonds. For this energy increase to take place, the carbon be activated by the photosynthetic electron transport chain. Thus,
compounds formed by rubisco must be reduced. Nicotinamide in a photosynthetic cell, the Calvin cycle occurs only in the light.
adenine dinucleotide phosphate (NADPH) is the reducing
j Quick Check 2 The Calvin cycle requires both ATP and
agent used in the Calvin cycle. NADPH transfers the electrons that
NADPH. Which of these molecules provides the major input of
allow carbohydrates to be synthesized from CO2 (Fig. 8.5).
energy needed to synthesize carbohydrates?
Like all components of the Calvin cycle, NADPH can move
freely within the stroma of the chloroplast. Although NADPH is
a powerful reducing agent, energy and electrons are transferred The steps of the Calvin cycle were determined using
from NADPH only under the catalysis of a specific enzyme, thus radioactive CO2.
providing a high degree of control over the fate of these electrons. In a series of experiments conducted between 1948 and 1954, the
In the Calvin cycle, the reduction of 3-PGA involves two steps: American chemist Melvin Calvin and colleagues identified the
(1) ATP donates a phosphate group to 3-PGA, and (2) NADPH carbon compounds produced during photosynthesis (Fig. 8.6). They
transfers two electrons plus one proton (H1) to the phosphorylated supplied radioactively labeled CO2 (14CO2) to the unicellular green
compound, which releases one phosphate group (Pi). Because two alga Chlorella and then plunged the cells into boiling alcohol, thereby
molecules of 3-PGA are formed each time rubisco catalyzes the halting all enzymatic reactions. The carbon compounds produced
incorporation of one molecule of CO2 , two ATP and two NADPH during photosynthesis were thus radioactively labeled and could be
are required for each molecule of CO2 incorporated by rubisco. identified by their radioactivity (Experiment 1 in Fig. 8.6).
NADPH provides most of the energy incorporated in the bonds Figuring out the chemical reactions that connected these
of the carbohydrate molecules produced by the Calvin cycle. labeled compounds, however, required additional experiments.
Nevertheless, ATP plays an essential role in preparing 3-PGA for the For example, by using a very short exposure to 14CO2, Calvin and
addition of energy and electrons from NADPH. colleagues determined that 3-PGA was the first stable product of
These energy transfer steps result in the formation of 3-carbon the Calvin cycle (Experiment 2 in Fig. 8.6).
carbohydrate molecules known as triose phosphates. Triose To determine how 3-PGA is formed, they first supplied
phosphates are the true products of the Calvin cycle and they 14
CO2 so that all of the molecules in the Calvin cycle became
are the principal form in which carbohydrates are exported from radioactively labeled. When they then cut off the supply of CO2,
the chloroplast during photosynthesis. Larger sugars, such as the amount of RuBP increased relative to the amount seen in the
glucose and sucrose, are assembled from triose phosphates in the first experiment. Based on this buildup of RuBP, they concluded
cytoplasm. that the first step in the Calvin cycle was the addition of CO2 to
If every triose phosphate molecule produced by the Calvin RuBP (Experiment 3 in Fig. 8.6).
cycle were exported from the chloroplast, RuBP could not
be regenerated and the Calvin cycle would grind to a halt. In Carbohydrates are stored in the form of starch.
fact, most of the triose phosphate molecules must be used to The Calvin cycle is capable of producing more carbohydrates than
regenerate RuBP. For every six triose phosphate molecules that are the cell needs or, in a multicellular organism, more than the cell is
produced, only one can be withdrawn from the Calvin cycle. able to export. If carbohydrates accumulated in the cell, they would
cause water to enter the cell by osmosis, perhaps damaging the cell.
The regeneration of RuBP requires ATP. Instead, excess carbohydrates are converted to starch, a storage
Of the 15 chemical reactions that make up the Calvin cycle, form of carbohydrates discussed in Chapter 2. Because starch
12 occur in the last step, the regeneration of RuBP (Fig. 8.5). molecules are not soluble, they provide a means of carbohydrate
HOW DO WE KNOW?

FIG. 8.6

How is CO2 used to synthesize carbohydrates?

BACKGROUND In the 1940s, radioactive 14CO2 became available in quantities that allowed experiments. Melvin Calvin and Andrew Benson
used 14CO2 to follow the incorporation of CO2 into carbohydrates.

EXPERIMENTS AND RESULTS

1 Inject 14CO2 into 2 After a specified amount of 3 Identify radioactively

a flask containing time, drop cells into boiling labeled compounds using
Chlorella. alcohol to halt the reactions. 2D paper chromatography.

14CO
Experiment 1: 2
Use 14CO2 to label all
the products of the RuBP
Calvin cycle.
Triose phosphates
3-PGA

2D paper chromatography
separates molecules according
to factors such as size and
charge that affect their mobility.

Experiment 2:
Shorten exposure to
14CO enough so that Experiment 3:
2
only one compound Following exposure to
14CO as in Experiment 1,
is labeled. 2
3-PGA prevent carboxylation by RuBP
withholding CO2 and see Triose phosphates
which labeled product
increases and which 3-PGA
decreases.
Thus, 3-PGA is the initial
product of carboxylation.
Because RuBP increases, while
3-PGA decreases relative to the
amounts in Experiment 1, RuBP is
the other substrate involved in
the carboxylation step.

CONCLUSION The initial step in the Calvin cycle unites the 5-carbon RuBP with CO2, resulting in the production of two molecules of 3-PGA.

FOLLOW-UP WORK In the 1950s, Marshall Hatch and colleagues showed that some plants, including corn and sugarcane, accumulate
a 4-carbon compound as the first product in photosynthesis. In Chapter 29, we explore how C4 photosynthesis allows plants to avoid the
oxygenase reaction of rubisco.
SOURCE Calvin, M., and H. Benson. 1949. “The Path of Carbon in Photosynthesis IV: The Identity and Sequence of the Intermediates in Sucrose Synthesis.” Science 109:140–142.

159
160 SECTION 8.3 C A P T U R I N G S U N L I G H T I N TO C H E M I C A L F O R M S

all stars, produces a broad spectrum of electromagnetic radiation

FIG. 8.7 A chloroplast containing starch granules, shown here in ranging from gamma rays to radio waves. Each point along the
yellow. Source: Biophoto Associates/Science Source. electromagnetic spectrum has a different energy level and a
corresponding wavelength. Visible light is the portion of the
electromagnetic spectrum apparent to our eyes, and it includes
the range of wavelengths used in photosynthesis. The wavelengths
of visible light range from 400 nm to 700 nm. Approximately 40%
of the sun’s energy that reaches Earth’s surface is in this range.
Pigments are molecules that absorb some wavelengths of
visible light (Fig. 8.8). Pigments look colored because they
reflect light enriched in the wavelengths that they do not absorb.
Chlorophyll is the major photosynthetic pigment; it appears
green because it is poor at absorbing green wavelengths. The
chlorophyll molecule consists of a large, light-absorbing “head”
containing a magnesium atom at its center and a long hydrocarbon
“tail” (Fig. 8.9). The large number of alternating single and double
bonds in the head region explains why chlorophyll is so efficient at
absorbing visible light.
Chlorophyll molecules are bound by their tail region to
integral membrane proteins in the thylakoid membrane. These
protein–pigment complexes, referred to as photosystems, are the
functional and structural units that absorb light energy and use it
to drive electron transport.

FIG. 8.8 Light absorbed by leaves. The graph shows the extent
to which wavelengths of visible light are absorbed by
pigments in an intact leaf.

Electromagnetic spectrum
High energy Low energy

Gamma Ultra- Visible Infra- Micro- Radio

storage that does not lead to osmosis. The formation of starch X-rays
rays violet light red waves waves
during the day provides photosynthetic cells with a source of
carbohydrates that they can use during the night (Fig. 8.7).

8.3 CAPTURING SUNLIGHT

INTO CHEMICAL FORMS Visible light is only 400 nm 700 nm
one part of the
electromagnetic
To use sunlight to power the Calvin cycle, the cell must be Shorter Longer
spectrum.
able to use light energy to produce both NADPH and ATP. In wavelengths wavelengths
photosynthesis, light energy absorbed by pigment molecules
Leaves efficiently
drives the flow of electrons through the photosynthetic electron
absorb light across
transport chain. The movement of electrons through the
Absorption

most of the visible

photosynthetic electron chain leads to the formation of both spectrum.
NADPH and ATP.

Chlorophyll is the major entry point for light energy

in photosynthesis.
To understand how light energy is captured and stored by
photosynthesis, we need to know a little about light. The sun, like
 CHAPTER 8 P H OTO S Y N T H E S I S : U S I N G S U N L I G H T TO B U I L D C A R B O H Y D R AT E S 161

Photosystems contain pigments other than chlorophyll, called play an important role in protecting the photosynthetic electron
accessory pigments. The most notable are the orange-yellow transport chain from damage.
carotenoids, which can absorb light from regions of the visible
spectrum that are poorly absorbed by chlorophyll. Thus, the Photosystems use light energy to drive the
presence of these accessory pigments allows photosynthetic cells to photosynthetic electron transport chain.
absorb a broader range of visible light than would be possible with When visible light is absorbed by a chlorophyll molecule, one of
just chlorophyll alone. As we will see in section 8.4, carotenoids its electrons is elevated to a higher energy state (Fig. 8.10). For

FIG. 8.9 Chemical structure of chlorophyll. Shown is FIG. 8.10 Absorption of light energy by chlorophyll. Absorption of
chlorophyll a, found in all photosynthetic eukaryotes and light energy by (a) an isolated chlorophyll molecule in the
cyanobacteria. lab and (b) an antenna chlorophyll molecule.

CH2 a. In solution in the lab

CH CH3
H
C C C
C C Light Heat Fluorescence
H3C C C CH2CH3
Light-absorbing C N N C
region HC Mg CH
Chlorophyll e- e- e-
molecule
C N N C
H2C C C C C CH3 Ground-state Excited-state Ground-state
H electron electron electron
CH C C

CH2 HC C O

CH2 C O
C O O
O CH3
CH2
CH
Energy

C CH3
CH2 Absorption of visible light by an
CH2 isolated chlorophyll molecule
results in the release of heat and
CH2 fluorescence when the electron
Hydrocarbon returns to its ground energy state.
side chain HC CH3
b. In the plant cell
CH2
CH2
CH2 Light

HC CH3
e- e- e-
CH2
CH2
Energy
CH2 transfer
CH
H3C CH3
e- e-

Ground-state Excited-state
electron electron

Absorption of visible light by an

antenna chlorophyll results in the
transfer of energy to an electron in a
neighboring chlorophyll molecule.
162 SECTION 8.3 C A P T U R I N G S U N L I G H T I N TO C H E M I C A L F O R M S

chlorophyll molecules that have been extracted from chloroplasts Without the antennae to gather light energy, reaction centers
in the laboratory, this absorbed light energy is rapidly released, would sit idle much of the time, even in bright sunlight.
allowing the electron to return to its initial “ground” energy state The reaction center chlorophylls have a configuration
(Fig. 8.10a). Most of the energy (>95%) is converted into heat; a distinct from that of the antenna chlorophylls. As a result, when
small amount is reemitted as light (fluorescence). excited, the reaction center transfers an electron to an adjacent
By contrast, for chlorophyll molecules within an intact molecule that acts as an electron acceptor (Fig. 8.11a). When the
chloroplast, energy can be transferred to an adjacent chlorophyll transfer takes place, the reaction center becomes oxidized and
molecule instead of being lost as heat (Fig. 8.10b). When this the adjacent electron-acceptor molecule is reduced. The result is
happens, the energy released as an excited electron returns to its the conversion of light energy into a chemical form. This electron
ground state raises the energy level of an electron in an adjacent transfer initiates a light-driven chain of redox reactions that leads
chlorophyll molecule. This mode of energy transfer is extremely ultimately to the formation of NADPH.
efficient (that is, very little energy is lost as heat), allowing Once the reaction center has lost an electron, it can no longer
energy initially absorbed from sunlight to be transferred from one absorb light or contribute additional electrons. Thus, for the
chlorophyll molecule to another and then on to another. photosynthetic electron transport chain to continue, another
Most of the chlorophyll molecules in the thylakoid electron must be delivered to take the place of the one that has
membrane function as an antenna: Energy is transferred between entered the transport chain (Fig. 8.11b). As we will see below,
chlorophyll molecules until it is finally transferred to a specially these replacement electrons ultimately come from water.
configured pair of chlorophyll molecules known as the reaction
j Quick Check 3 How do antenna chlorophylls differ from
center (Fig. 8.11).
reaction center chlorophylls?
The reaction center is where light energy is converted into
chemical energy as a result of the excited electron’s transfer to
an adjacent molecule. This division of labor among chlorophyll The photosynthetic electron transport chain connects
molecules was discovered in the 1940s in a series of experiments two photosystems.
by the American biophysicists Robert Emerson and William Arnold, In many ways, water is an ideal source of electrons for photo-
who showed that only a small fraction of chlorophyll molecules synthesis. Water is so abundant within cells that it is always
are directly involved in electron transport (Fig. 8.12). We now available to serve as an electron donor in photosynthesis. In
know that several hundred antenna chlorophyll molecules transfer addition, O2, the by-product of pulling electrons from water,
energy to each reaction center. The antenna chlorophylls allow diffuses readily away rather than accumulates. However, from an
the photosynthetic electron transport chain to operate efficiently. energy perspective, water is a challenging electron donor: It takes

FIG. 8.11 The reaction center. (a) Antenna chlorophylls deliver absorbed light energy to the reaction center, allowing electrons to be transferred
to an electron acceptor molecule. (b) After the reaction center has lost an electron, it is reduced by gaining an electron, so it is ready to
absorb additional light energy.

a. Oxidation of the reaction center b. Reduction of the reaction center

Photosystem II Photosystem II

Light

Antenna
chlorophylls

Electron Electron
e- e-
acceptor donor

Reaction center Reaction center

HOW DO WE KNOW?

FIG. 8.12 RESULTS

Measurement 1 (rate of
Do chlorophyll molecules O2 production):

operate on their own or in

groups? Oxygen
electrode
1.57  104 mol O2
per flash per mm3
BACKGROUND By about 1915, scientists knew that chlorophyll
of cells in solution

was the pigment responsible for absorbing light energy in Measurement 2 (chlorophyll concentration):
photosynthesis. However, it was unclear how these pigments
2
contributed to the reduction of CO2. The American physiologists Solution 1 3

Robert Emerson and William Arnold set out to determine of cells 0

7
4
10
containing
the nature of the “photochemical unit” by quantifying how chlorophyll 3.9  107 mol
many chlorophyll molecules were needed to produce one molecule chlorophyll
per mm3 of cells
of O2. in solution
Spectrophotometer
EXPERIMENT Emerson and Arnold exposed flasks of the green
alga Chlorella to flashes of light of such short duration (10 –5 s)
CONCLUSION Because the amount of chlorophyll in their flask
and high intensity (several times that of full sunlight) that each
was much greater than O2 production per flash, Emerson and
chlorophyll molecule would be “excited” only once. They also
Arnold concluded that each photochemical unit contains many
made sure that the time between flashes was long enough to
chlorophyll molecules.
allow the reactions resulting from each flash to run to completion.
They then measured O2 production per flash of high intensity FOLLOW-UP WORK Emerson and Arnold’s work was followed
light, and determined the concentration of chlorophyll present by studies that demonstrated that the photosynthetic electron
in their solution of cells. Finally, they divided the number of transport chain contains two photosystems arranged in series.
chlorophyll molecules per mm3 of solution by O2 production per SOURCE Emerson, R., and W. Arnold. 1932. “The Photochemical Reaction in
flash per mm3 of cells in solution. Photosynthesis.” Journal of General Physiology 16:191–205.

a great deal of energy to pull electrons from water. The amount transport chain. To run these reactions in the opposite direction
of energy that a single photosystem can capture from sunlight is would require an input of energy. Because the overall energy
not enough both to pull an electron from water and produce an trajectory has an up-down-up configuration resembling a “Z,” the
electron donor capable of reducing NADP1. The solution is to use photosynthetic electron transport chain is sometimes referred to
two photosystems arranged in series. The energy supplied by the as the Z scheme.
first photosystem allows electrons to be pulled from water, and the For the two photosystems to work together to move electrons
energy supplied by the second photosystem step allows electrons from water to NADPH, they must have distinct chemical
to be transferred to NADP1. properties. Photosystem II supplies electrons to the beginning
If you follow the flow of electrons from water through both of the electron transport chain. When photosystem II loses
photosystems and on to NADP1, as shown in Fig. 8.13, you can an electron (that is, when it is itself oxidized), it is able to pull
see a large increase in energy as the electrons pass through each electrons from water. In contrast, photosystem I energizes
of the two photosystems. You can also see that at every other electrons with a second input of light energy so they can be used
step along the photosynthetic electron transport chain there to reduce NADP1. The key point here is that photosystem I when
is a small decrease in energy. This decrease in energy indicates oxidized is not a sufficiently strong oxidant to split water, whereas
that these are exergonic reactions (Chapter 6) and thus explains photosystem II is not a strong enough reductant to form NADPH.
why electrons move in one “direction” through the series of The major protein complexes of the photosynthetic electron
redox reactions that make up the photosynthetic electron transport chain include the two photosystems as well as the
163
164 SECTION 8.3 C A P T U R I N G S U N L I G H T I N TO C H E M I C A L F O R M S

membrane-associated protein called ferredoxin

FIG. 8.13 The Z scheme. The use of water as an electron donor requires input of light (Fd) (Fig. 8.14b). The enzyme ferredoxin–NADP
energy at two places in the photosynthetic electron transport chain. reductase then catalyzes the formation of
NADPH by transferring two electrons from two
Photosystem I molecules of reduced ferredoxin to NADP as
Photosystem II
well as a proton from the surrounding solution:

NADP  2e  H → NADPH

j Quick Check 4 Why are two

photosystems needed if H2O is used as an
electron donor?

The accumulation of protons in the

thylakoid lumen drives the synthesis
H2O
NADP+  H of ATP.
So far, we have considered only how the
e- e- e- e-
photosynthetic electron transport chain leads
to the formation of NADPH. However, we
NADPH
O 2  H know that the Calvin cycle also requires ATP.
In chloroplasts, as in mitochondria, ATP is
synthesized by ATP synthase, a transmembrane
protein powered by a proton gradient
(Chapter 7). In chloroplasts, the ATP synthase
is oriented such that the synthesis of ATP is the
result of the movement of protons from the
thylakoid lumen to the stroma, as shown in
Energy

Fig. 8.14c.
How do protons accumulate in the thylakoid
Absorption of light energy A second input of light lumen? Two features of the photosynthetic
by PS II allows electrons energy by PS I produces
electron transport chain are responsible for
pulled from water to enter electron donor molecules
the photosynthetic capable of reducing the buildup of protons in the thylakoid lumen
electron transport chain. NADP+. (Fig. 8.14c). First, the oxidation of water
releases protons and O2 into the lumen. Second,
the cytochrome–b6 f complex, the protein
complex situated between photosystem II and
cytochrome–b6 f complex (cyt), through which electrons pass photosystem I, and plastoquinone together function as a proton
between photosystem II and photosystem I (Fig. 8.14). Small, pump that is functionally and evolutionarily related to proton
relatively mobile compounds convey electrons between these pumping in the electron transport chain of cellular respiration
protein complexes. Plastoquinone (Pq), a lipid-soluble compound (Chapter 7).
similar in structure to coenzyme Q (Chapter 7), carries electrons In photosynthesis, the proton pump involves: (1) the transport
from photosystem II to the cytochrome–b6 f complex by diffusing of two electrons and two protons, by the diffusion of
through the membrane, while plastocyanin (Pc), a water-soluble plastoquinone, from the stroma side of photosystem II to the
protein, carries electrons from the cytochrome–b6 f complex to lumen side of the cytochrome–b6 f complex and (2) the transfer
photosystem I by diffusing through the thylakoid lumen. of electrons within the cytochrome–b6 f complex to a different
Water donates electrons to one end of the photosynthetic molecule of plastoquinone, which results in additional protons
electron transport chain, whereas NADP accepts electrons at the being picked up from the stroma and subsequently released into
other end. The enzyme that pulls electrons from water, releasing the lumen.
both H and O2, is located on the lumen side of photosystem Together, these mechanisms are quite powerful. When the
II. The mechanism by which water splitting occurs is not photosynthetic electron transport chain is operating at full
known, despite the considerable industrial value of developing capacity, the concentration of protons in the lumen can be more
a way to use sunlight to generate hydrogen gas (H2). NADPH than 1000 times greater than their concentration in the stroma
is formed when electrons are passed from photosystem I to a (equivalent to a difference of 3 pH units). This accumulation of
 CHAPTER 8 P H OTO S Y N T H E S I S : U S I N G S U N L I G H T TO B U I L D C A R B O H Y D R AT E S 165

FIG. 8.14 The photosynthetic electron transport chain. (a) An overview of the production of NADPH and ATP. (b) The linear flow of electrons
from H2O to NADPH. (c) The use of a proton electrochemical gradient to synthesize ATP.

a. The production of NADPH and ATP by photosynthesis

NADPH
Stroma
NADP+  H ADP  Pi ATP
Light Light

H e-
Fd N
NADP + ATP synthase H
e- d ase
reductase
red e
PS
SI
Cy
Cyt
y
PS II
e- e-
e-
Pq H

e- H
O2 Pc
H2O

H
Lumen H

b. Electron transport and the synthesis of NADPH

NADPH
Stroma
NADP+ 
The absorption of light energy
H
drives the linear flow of
Light electrons from water to NADPH. Light

e-
Fd NADP+
e- reductase
PS I
Cyt
PS II
e- e- e-
Pq

e-
H2O Pc

Lumen

c. Proton transport and ATP synthesis

Protons flow back into the stroma
Stroma through the ATP synthase, driving
During electron transport, the synthesis of ATP. ADP  Pi ATP
Light protons accumulate in the Light
thylakoid lumen.

H ATP synthase
H

PS II Cyt

Pq H

O2 H
H2O

H
Lumen H
166 SECTION 8.4 C H A L L E N G E S TO P H OTO S Y N T H E T I C E F F I C I E N C Y

FIG. 8.15 Cyclic electron transport. To increase ATP production, some electrons from photosystem I cycle back into the electron transport chain.

NADPH

ADP  Pi ATP
NADP+  H
Stroma Light

H e-
Fd H
NADP+
e- ATP synthase
reductase
PS I
Cyt
e- e-
Pq H

e-
Pc H

Lumen H

protons on one side of the thylakoid membrane can then be used to the lumen. As a result, there are more protons in the lumen that
to power the synthesis of ATP by oxidative phosphorylation as can be used to drive the synthesis of ATP.
described in Chapter 7.

Cyclic electron transport increases the production 8.4 CHALLENGES TO

of ATP. PHOTOSYNTHETIC EFFICIENCY
The Calvin cycle requires two molecules of NADH and three
molecules of ATP for each CO2 incorporated into carbohydrates. The efficient functioning of photosynthesis faces two major
However, the transport of four electrons through the challenges. The first is that if more light energy is absorbed
photosynthetic electron transport chain, needed to reduce than the Calvin cycle can use, excess energy can damage the cell.
two NADP molecules, does not transport enough protons into The second challenge stems from a property of rubisco:
the lumen to produce the required three ATPs. An additional This enzyme can catalyze the addition of either carbon dioxide
pathway for electrons is thus needed to increase the production or oxygen to RuBP. The addition of oxygen instead of carbon
of ATP. dioxide can substantially reduce the amount of carbohydrate
In cyclic electron transport, electrons from photosystem I produced.
are redirected from ferredoxin back into the electron transport
chain (Fig. 8.15). These electrons reenter the photosynthetic Excess light energy can cause damage.
electron transport chain by plastoquinone. Because these Photosynthesis is an inherently dangerous enterprise. Unless
electrons eventually return to photosystem I, this alternative the photosynthetic reactions are carefully controlled, molecules
pathway is cyclic in contrast to the linear movement of electrons will be formed that can damage cells through the indiscriminate
from water to NADPH. oxidization of lipids, proteins, and nucleic acids (Fig. 8.16).
How does cyclic electron transport lead to the production Under normal conditions, the photosynthetic electron
of ATP? As the electrons from ferredoxin are picked up by transport chain proceeds in an orderly fashion from the absorption
plastoquinone, additional protons are transported from the stroma of light to the formation of NADPH. However, when NADP is
 CHAPTER 8 P H OTO S Y N T H E S I S : U S I N G S U N L I G H T TO B U I L D C A R B O H Y D R AT E S 167

FIG. 8.16 Defenses against reactive oxygen species. (a) Reactive oxygen species are generated when light energy or electrons are transferred to
oxygen. (b) Defenses against the reactive oxygen species include antioxidants that neutralize reactive oxygen species and xanthophylls
that convert excess light energy into heat.

a. The problem: Harmful reactive oxygen species are generated.

O2
When not enough ...electrons can be
NADP+ is available, used to reduce O2.
O2
antennae chlorophyll
can transfer energy Cyclic
Light Light
to O2 or... electron
transport
O2*
Fd
d
H e-
PS I
Cyt
PS II
e- e- e-
Pq
H

H 2O e-
Pc
O2

H H

Antioxidants detoxify
reactive oxygen species.
b. The solution: Xanthophylls and antioxidants reduce the amount of reactive oxygen species. O2 Reactive
oxygen  antioxidants
species H2O
Cyclic
Light Fluorescence Heat Light electron
transport

Fd
H e-
PS I
Cyt
PS II
e- e- e-
Pq
H

H 2O e-
Pc
Xanthophylls convert excess O2
light energy to heat,
reducing the rate at which H H
electrons enter the electron
transport chain.

in short supply, the electron transport chain “backs up,” greatly directly to O2 or by the transfer of an electron, forming O2. Both
increasing the probability of creating highly reactive forms of forms of O2 can cause substantial damage to the cell.
oxygen known collectively as reactive oxygen species NADP is returned to the photosynthetic electron transport
(Fig. 8.16a). These highly reactive molecules can be formed either chain by the Calvin cycle’s use of NADPH. Thus, any factor that
by the transfer of absorbed light energy from antenna chlorophyll causes the rate of NADPH use to fall behind the rate of light-driven
168 SECTION 8.4 C H A L L E N G E S TO P H OTO S Y N T H E T I C E F F I C I E N C Y

electron transport can potentially lead to damage. Such an molecules are brightly colored, like the red pigments found
imbalance is likely to occur, for example, in the middle of the day in algae that live on snow shown in Fig. 8.1c. The presence of
when light intensity is highest. Could the cell right the balance antioxidant compounds is one of the many reasons that eating
by supplying NADP1 more quickly? Photosynthetic cells could green, leafy vegetables is good for your health.
speed up the resupply of NADP1 by synthesizing more Calvin A second line of defense is to prevent reactive oxygen species
cycle enzymes. This strategy, however, would be energetically from forming in the first place. Xanthophylls are yellow-orange
expensive. When light levels are low, such as in the morning and pigments that slow the formation of reactive oxygen species by
late afternoon, Calvin cycle enzymes would sit idle. An alternative reducing excess light energy. These pigments accept absorbed light
strategy of reducing the amount of chlorophyll in the leaf runs energy directly from chlorophyll and then convert this energy to
into similar problems. Thus, excess light energy is an everyday heat (Fig. 8.16b). Photosynthetic organisms that live in extreme
event for photosynthetic cells, rather than something that occurs environments often appear brown or yellow because they contain
only in extreme environments. high levels of xanthophyll pigments, as seen in Figs. 8.1a and
The rate at which the Calvin cycle can make use of NADPH 8.1b. Plants that lack xanthophylls grow poorly when exposed to
is also influenced by a number of factors that are independent of moderate light levels and die in full sunlight.
light intensity. For example, cold temperatures cause the enzymes Converting absorbed light energy into heat is beneficial at high
of the Calvin cycle to function more slowly, but they have little light levels, but at low light levels it would decrease the production
impact on the absorption of light energy. On a cold, sunny day, of carbohydrates. Therefore, this capability is switched on only
more light energy is absorbed than can be used by the Calvin cycle. when the photosynthetic electron transport chain is working at
Photosynthetic organisms employ two major lines of defense high capacity.
to avoid the stresses that occur when the Calvin cycle cannot
keep up with light harvesting (Fig. 8.16b). First among these Photorespiration leads to a net loss of energy
are chemicals that detoxify reactive oxygen species. Ascorbate and carbon.
(vitamin C), b-(beta-)carotene, and other antioxidants are able A second challenge to photosynthetic efficiency is the fact that
to neutralize reactive oxygen species. These compounds exist in rubisco can use both CO2 and O2 as substrates. If O2 instead of
high concentration in chloroplasts. Some of these antioxidant CO2 diffuses into the active site of rubisco, the reaction can

FIG. 8.17 Photorespiration. Carbon and energy are lost when rubisco acts as an oxygenase in photorespiration.

Oxygenation: CO2 loss:

Rubisco adds O2 to RuBP, 2-Phosphoglycolate cannot be
resulting in one molecule used by the Calvin cycle. The
of 3-phosphoglycerate conversion of 2-phosphoglycolate
and one molecule of into 3-PGA results in the net loss
2-phosphoglycolate. of reduced carbon.
ATP ADP

NADPH
Oxygenation 3-Phosphoglycerate Reduction
O2 input: (3-PGA)
Rubisco can NADP+
function as an O2 2-Phosphoglycolate
oxygenase.

Ribulose-1,5- Triose Carbohydrate

bisphosphate ATP CO2 phosphate output:
(RuBP)
Fewer carbohydrates
ADP exit as 3-carbon
+ compounds.

Regeneration

ADP ATP
+
 CHAPTER 8 P H OTO S Y N T H E S I S : U S I N G S U N L I G H T TO B U I L D C A R B O H Y D R AT E S 169

still proceed, although O2 is added to RuBP in place of CO2. An catalytic rate means that photosynthetic cells must produce huge
enzyme that adds O2 to another molecule is called an oxygenase. amounts of this enzyme; as much as 50% of the total protein
Recall that rubisco is shorthand for RuBP carboxylase oxygenase, within a leaf is rubisco, and it is estimated to be the most abundant
reflecting rubisco’s ability to catalyze two different reactions. protein on Earth. At the same time, because O2 is approximately
When rubisco adds O2 instead of CO2 to RuBP, the result is 500 times more abundant in the atmosphere than CO2, as much as
one molecule with three carbon atoms (3-PGA) and one molecule one-quarter of the reduced carbon formed in photosynthesis can
with only two carbon atoms (2-phosphoglycolate). The production be lost through photorespiration.
of 2-phosphoglycolate creates a serious problem because this
j Quick Check 6 Why does rubisco have such a low catalytic rate
molecule cannot be used by the Calvin cycle either to produce
(that is, why is it so slow)?
triose phosphate or to regenerate RuBP.
A metabolic pathway to recycle 2-phosphoglycolate is
Photosynthesis captures just a small percentage of
present in photosynthetic cells. A portion of the carbon atoms
incoming solar energy.
in 2-phosphoglycolate are converted into 3-PGA, which can
Typically, only 1% to 2% of the sun’s energy that lands on a leaf
reenter the Calvin cycle. However, this pathway is not able to
ends up in carbohydrates. Does this mean that photosynthesis
return all of the carbon atoms in 2-phosphoglycolate to the
is incredibly wasteful? Or is this process, the product of billions
Calvin cycle; some are released as CO2. Because the overall effect
of years of evolution, surprisingly efficient? This is not an idle
is the consumption of O2 and release of CO2 in the presence of
question. Photosynthesis is relevant to solving several pressing
light, this process is referred to as photorespiration (Fig. 8.17).
global issues: the effects of rising CO2 concentrations on Earth’s
However, whereas respiration produces ATP, photorespiration
climate, the search for a renewable, carbon-neutral fuel to power
consumes ATP. In photorespiration, ATP drives the reactions that
our transportation sector, and the agricultural demands of our
recycle 2-phosphoglycolate into 3-PGA. Thus, photorespiration
skyrocketing human population.
represents a net energy drain on two accounts: First, it results in
Photosynthetic efficiency is typically calculated relative to
the oxidation and loss, in the form of CO2, of carbon atoms that
the total energy output of the sun (Fig. 8.18). However, only
had previously been incorporated and reduced by the Calvin cycle,
visible light has the appropriate energy levels to raise the energy
and second, it consumes ATP.
state of electrons in chlorophyll. Most of the sun’s output
j Quick Check 5 In what ways is photorespiration similar to cellular
respiration (Chapter 7) and in what ways does it differ?

The Calvin cycle originated long before the accumulation of

FIG. 8.18 Photosynthetic efficiency. Maximum photosynthetic
oxygen in Earth’s atmosphere, providing an explanation why an
efficiency is theoretically about 4% of incoming solar
enzyme with these properties might have initially evolved. Still,
energy, but actual yields are closer to 1% to 2%.
why would photorespiration persist in the face of what must be
strong evolutionary pressure to reduce or eliminate the unwanted
reaction with oxygen? 60% of the sun’s output
The difficulty is that for rubisco to favor the addition of CO2 is outside the visible
range and unavailable
over O2 requires that the enzyme be highly selective, and the
for photosynthesis.
price of high selectivity is speed. CO2 and O2 are similar in size
and chemical structure, and for this reason selectivity can only be
Visible
achieved by rubisco binding more tightly with the transition state light
(Chapter 6) of the carboxylation reaction. As a result, the better 8% 8%
rubisco is at discriminating between CO2 and O2, the slower its reflected or converted
transmitted into heat
catalytic rate.
Nowhere is this trade-off more evident than in land plants,
whose photosynthetic cells acquire CO2 from an O2-rich and
CO2-poor atmosphere. The rubiscos of land plants are highly
selective: If they are exposed to equal concentrations of CO2 and
O2, the ratio of CO2 addition to O2 addition is approximately 80 : 1.
As a result, rubisco is a very slow enzyme, with catalytic rates
on the order of three reactions per second. To put this rate in
perspective, it is not uncommon for metabolic enzymes to achieve
a catalytic rate of tens of thousands of reactions per second. 20% Maximum
lost during 4% yield
This trade-off between selectivity and speed is a key constraint carbohydrate synthesis in the form
for photosynthetic organisms. For land plants, rubisco’s low including photorespiration of carbohydrates
 170 SECTION 8.5 T H E E VO L U T I O N O F P H OTO S Y N T H E S I S

(~60%) is not absorbed by chlorophyll and thus cannot be used in produced chemical variants of these UV-absorbing molecules.
photosynthesis. In addition, leaves are not perfect at absorbing One or more of these variant compounds might have been capable
visible light—about 8% is either reflected or passes through the of using sunlight to meet the energy needs of the cell—perhaps
leaf. Finally, even under optimal conditions, not all of the light by transferring electrons to another molecule as a present-day
energy absorbed by chlorophyll can be transferred to the reaction reaction center does.
center and instead is given off as heat (also ~8%). As we have seen, The earliest reaction centers may have used light energy to
when light levels are high, excess light is actively converted into drive the movement of electrons from an electron donor outside
heat by xanthophyll pigments. the cell in the surrounding medium to an electron-acceptor
The photosynthetic electron transport chain therefore molecule within the cell. In this way, energy from sunlight could
captures at most about 24% of the sun’s usable energy arriving have been used to synthesize carbohydrates. The first electron
at the surface of a leaf (100% – 60% – 8% – 8% 5 24%). While donor could have been a soluble inorganic ion like reduced iron,
this number may appear low, it is on a par with the efficiency of Fe21, which is thought to have been abundant in the early ocean.
high-performance photovoltaic cells in solar panels, which convert Alternatively, the first forms of light-driven electron transport
sunlight into electricity. This comparison is even more impressive may have been cyclic and thus not required an electron donor.
when you consider that photosynthetic organisms must build and In either configuration, light-driven electron transport could
maintain all their biochemical machinery. However, energy is lost have been coupled to the net movement of protons across the
at a later step as well. The incorporation of CO2 into carbohydrates membrane, allowing for the synthesis of ATP.
results in considerable loss in free energy, equivalent to ~20% of Similarly, it is unlikely that these first photosynthetic
the total incoming solar radiation. Some of this loss in free energy organisms employed chlorophyll as a means of absorbing sunlight
is due to photorespiration. for the simple reason that the biosynthetic pathway for chlorophyll
In total, therefore, the maximum energy conversion efficiency is complex, consisting of at least 17 enzymatic steps. Yet some of
of photosynthesis is calculated to be around 4% (24% – 20%). the intermediate compounds leading to chlorophyll are themselves
Efficiencies achieved by real plants growing in nature, however, capable of absorbing light. Perhaps each of these now-intermediate
are typically much lower, on the order of 1% to 2%. In Chapter 29, compounds was, at one time, a functional end product used as a
we explore the many factors that can constrain the photosynthetic pigment by an early photosynthetic organism. The biosynthetic
output of land plants, and see how some plants have evolved pathway may have gained steps as chemical variants, produced by
ways to minimize losses in productivity due to drought and random mutations, were selected because they were more efficient
photorespiration. or able to absorb new portions of the visible spectrum. Selection
would have eventually resulted in the chlorophyll pigments that
are used by photosynthetic organisms today.
8.5 THE EVOLUTION OF
PHOTOSYNTHESIS The ability to use water as an electron donor in
photosynthesis evolved in cyanobacteria.
The evolution of photosynthesis had a profound impact on the The most ancient forms of photosynthesis have only a single
history of life on Earth. Not only did photosynthesis provide photosystem in their photosynthetic electron transport chains.
organisms with a new source of energy, but it also released However, as we have seen, a single photosystem cannot capture
oxygen into the atmosphere. As discussed in the previous chapter, enough energy from sunlight both to pull electrons from water
evolution often works in a stepwise fashion, building on what and also raise their energy level enough that they can be used
is already present. Here we consider hypotheses for how the to reduce CO2. Thus, photosynthetic organisms with a single
photosynthetic pathways that are the dominant entry point for photosystem must use more easily oxidized compounds, such
energy into the biosphere today may have evolved. as H2S, as electron donors. These organisms can exist only in
environments where the electron-donor molecules are abundant.
Because these organisms do not use water as an electron donor,
? CASE 1 THE FIRST CELL: LIFE’S ORIGINS
they do not produce O2 during photosynthesis.
How did early cells use sunlight to meet their energy A major event in the history of life was the evolution of
requirements? photosynthetic electron transport chains that use water as an
Sunlight is valuable as a source of energy, but it can also cause electron donor. The first organisms to accomplish this feat were
damage. This is particularly true of ultraviolet wavelengths, the cyanobacteria. These photosynthetic bacteria incorporated
which can damage DNA and other macromolecules. Thus, the two different photosystems into a single photosynthetic electron
earliest interactions with sunlight may have been the evolution transport chain, one to pull electrons from water molecules and
of UV-absorbing compounds that could shield cells from the one to raise the energy level of the electrons so that they can be
sun’s damaging rays. Over time, random mutations could have used to reduce CO2.
 CHAPTER 8 P H OTO S Y N T H E S I S : U S I N G S U N L I G H T TO B U I L D C A R B O H Y D R AT E S 171

How did cyanobacteria end up with two photosystems? We

cannot say for sure, but one relevant piece of information is that FIG. 8.19 The evolutionary history of photosynthesis.
each of the two photosystems present in cyanobacteria is similar (a) Hypotheses for the origin of the two photosystems
in structure to photosystems found in groups of photosynthetic in cyanobacteria. (b) Later, by endosymbiosis, free-
bacteria that contain only a single photosystem. Thus, it is living cyanobacteria became the chloroplasts of
highly unlikely that the photosystems in cyanobacteria evolved eukaryotic cells.
independently. One hypothesis is that the genetic material
PS II
associated with one photosystem was transferred to a bacterium
that already had the other photosystem, resulting in a single
bacterium with the genetic material to produce both types of Transfer of
photosystems (shown on the left in Fig. 8.19). The mechanisms genetic material
Gene
by which genetic material is transferred between bacteria are duplication
PS II PS I PS II
discussed more fully in Chapter 26. Another hypothesis is that
the genetic material associated with one photosystem underwent Prokaryotic
duplication. Over time, one of the two photosystems diverged cells
slightly in sequence and function through mutation and selection,
Genetic divergence
giving rise to two distinct but related photosystems (shown on
the right in Fig. 8.19). This mechanism, called duplication and PS II PS I
divergence, is discussed more fully in Chapter 14. Cyanobacterium
The ability to use water as an electron donor in photosynthesis
had two major impacts on life on Earth. First, it meant that
photosynthesis could occur anywhere there was both sunlight Endosymbiosis
and sufficient water for cells to survive. Second, using water as
an electron donor results in the release of oxygen. Before the PS I
evolution of “oxygenic” photosynthesis, there was little or no PS II
free oxygen in Earth’s atmosphere. All the oxygen in Earth’s
Thylakoid
atmosphere results from photosynthesis by organisms containing
membrane
two photosystems.
Eukaryotic
cell
Eukaryotic organisms are believed to have gained Chloroplast
photosynthesis by endosymbiosis.
Photosynthesis is hypothesized to have gained a foothold among Nucleus Mitochondrion
eukaryotic organisms when a free-living cyanobacterium took
up residence inside a eukaryotic cell (Fig. 8.19). Over time,
the cyanobacterium lost its ability to survive outside its host
cell and evolved into the chloroplast. The outer membrane of
the chloroplast is thought to have originated from the plasma
membrane of the ancestral eukaryotic cell, which surrounded
the ancestral cyanobacterium as it became incorporated into the chloroplasts and mitochondria (Chapter 7) arose in this way is
cytoplasm of the eukaryotic cell. The inner chloroplast membrane called the endosymbiotic hypothesis. It is discussed in Chapter 27.
is thought to correspond to the plasma membrane of the ancestral Cellular respiration and photosynthesis are complementary
free-living cyanobacterium. The thylakoid membrane then metabolic processes. Cellular respiration breaks down
corresponds to the internal photosynthetic membrane found in carbohydrates in the presence of oxygen to supply the energy
cyanobacteria. Finally, the stroma corresponds to the cytoplasm of needs of the cell, producing carbon dioxide and water as
the ancestral cyanobacterium. byproducts, while photosynthesis uses carbon dioxide and water in
The process in which one cell takes up residence inside of the presence of sunlight to build carbohydrates, releasing oxygen
another cell is called endosymbiosis. Therefore, the idea that as a byproduct. We summarize the two processes in Fig. 8.20. •
V I S UA L S Y N T H E S I S Harnessing Energy: Photosynthesis
FIG. 8.20 and Cellular Respiration
Integrating concepts from Chapters 6–8

Photosynthesis
Energy captured from sunlight is stored in carbohydrates.

Photosynthesis requires light energy, CO2, and H2O, and produces carbohydrates and Carbon dioxide is reduced by NADPH and, using the energy
O2. The photosynthetic electron transport chain uses energy from sunlight to drive the provided by ATP, is incorporated into a carbohydrate molecule
movement of electrons from water to NADP⫹ and to produce ATP. in the Calvin cycle.

NADPH NADPH
Stroma
ATP ATP
Light Light NADP+ ADP NADP+
ADP

H⫹ PS I e-
PS II Cyt e-
CO2
e- e- e- Carbohydrate
ATP
e- synthase
ATP

Lumen ADP

Photosynthetic electron transport chain Calvin cycle

H2O CO2
Inputs: Light Light

ADP ATP

Chlorophyll Cytochrome Chlorophyll

NADPH Carbohydrate
in PSII b6f in PSI
e-

ATP ADP

Outputs:
O2 H⫹

172
Aerobic respiration
Energy released from the oxidation of carbohydrates is used to make ATP.

Aerobic respiration requires The citric acid cycle The energy released as electrons are passed down the electron transport chain
carbohydrates and O2 and produces completes the oxidation of generates a proton gradient that powers ATP production by ATP synthase.
CO2, H2O, and ATP. Glycolysis splits organic molecules.
glucose into two molecules of
pyruvate and begins the oxidation of H⫹ H2O
organic molecules. FADH2

NADH NADH NADH NAD+ FAD

Mitochondrial O2 ATP
matrix
NAD+ NAD+ FAD NAD+ NADH FADH2 ADP

e-
e- e-
Acetyl-
Carbohydrate Pyruvate e-
CoA e-
ATP
Complex I Complex II
synthase
ATP CO2 ATP
CO2
Intermembrane Complex III Complex IV
space

Glycolysis Citric acid cycle Electron transport chain

O2 H⫹
Inputs: ADP ADP ADP

NADH Complex I
Acetyl-
Carbohydrate Pyruvate
CoA
III IV e-
FADH2 Complex II

Outputs: ATP ATP ATP

CO2 CO2 H2O

173
174 CO R E CO N C E P T S S U M M A RY

Core Concepts Summary

8.1 Photosynthesis is the major pathway by Reaction centers are located within pigment–protein
which energy and carbon are incorporated into complexes known as photosystems. Special chlorophyll
carbohydrates. molecules in the reaction center transfer excited-state
electrons to an electron-acceptor molecule, thus initiating
In photosynthesis, water is oxidized, releasing oxygen, and
the photosynthetic electron transport chain. page 162
carbon dioxide is reduced, forming carbohydrates. page 155
The electron transport chain consists of a series of electron
Photosynthesis consists of two sets of reactions: (1) the
transfer or redox reactions that take place within both
Calvin cycle, in which carbon dioxide is reduced to form
protein complexes and diffusible compounds. Water is the
carbohydrates, and (2) light-harvesting reactions, in
electron donor and NADP1 is the final electron acceptor.
which ATP and NADPH are generated to drive the Calvin
page 163
cycle. page 155
The linear transport of electrons from water to NADPH
In eukaryotes, photosynthesis takes place in chloroplasts:
requires the energy input of two photosystems. page 163
The Calvin cycle takes place in the stroma, and the
light-harvesting reactions take place in the thylakoid Photosystem II pulls electrons from water, resulting in the
membrane. page 156 production of oxygen and protons on the lumen side of
the membrane. Photosystem I passes electrons to NADP1,
8.2 The Calvin cycle is a three-step process that uses producing NADPH for use in the Calvin cycle. page 163
carbon dioxide to synthesize carbohydrates. The buildup of protons in the lumen drives the production
The three steps of the Calvin cycle are (1) addition of of ATP by oxidative phosphorylation. The ATP synthase is
CO2 (carboxylation); (2) reduction; and (3) regeneration. oriented such that ATP is produced on the stroma side of the
page 157
membrane. page 164

Cyclic electron transport involves the redirection of

The first step is the addition of CO2 to the 5 carbon sugar
electrons from ferredoxin back into the electron transport
RuBP. This step is catalyzed by the enzyme rubisco,
chain and increases ATP production. page 166
considered the most abundant protein on Earth. The
resulting 6-carbon compound immediately breaks down
into two 3-carbon compounds. page 157
8.4 Photosynthesis faces several challenges to its
efficiency.
The second step is the donation of a phosphate group to
An imbalance between the light-harvesting reactions and
the 3 carbon compounds by ATP followed by reduction by
the Calvin cycle can lead to the formation of reactive oxygen
NADPH to produce 3 carbon triose phosphate molecules.
species. page 166
Some of these triose phosphates are exported from the
chloroplast to the cytosol, where they are used to build Protection from excess light energy includes antioxidant
larger sugars. page 158 molecules that neutralize reactive oxygen species and
xanthophyll pigments that dissipate excess light energy as
The third step is the regeneration of RuBP from five
heat. page 168
3-carbon triose phosphates. page 158
Rubisco can act catalytically on oxygen as well as on carbon
Starch formation provides chloroplasts with a way of storing
dioxide. When it acts on oxygen, there is a loss of energy
carbohydrates that will not cause water to enter the cell by
and of reduced carbon from the Calvin cycle. page 168
osmosis. page 158
Rubisco has evolved to favor carbon dioxide over oxygen,
8.3 The light-harvesting reactions use sunlight to but the cost of this selectivity is reduced speed. page 169
produce the ATP and NADPH required by the Calvin
The synthesis of carbohydrates through the Calvin cycle
cycle.
results in significant energy losses, which are due in part to
Visible light is absorbed by chlorophyll. page 160 photorespiration. page 169

Antenna chlorophyll molecules transfer absorbed light The maximum theoretical efficiency of photosynthesis is
energy to the reaction center. page 162 approximately 4% of total incident solar energy. page 170
 CHAPTER 8 P H OTO S Y N T H E S I S : U S I N G S U N L I G H T TO B U I L D C A R B O H Y D R AT E S 175

8.5 The evolution of photosynthesis had a profound 5. Contrast what happens when antenna chlorophylls
impact on life on Earth. absorb light energy with what happens when the reaction
center absorbs light energy. Why are antenna chlorophylls
The ability to use water as an electron donor in
so important in photosynthesis?
photosynthesis evolved in cyanobacteria. page 170
6. Explain why using water as an electron donor
Cyanobacteria evolved two photosystems either by the
requires a photosynthetic electron transport chain with
transfer of genetic material, or by gene duplication and
two photosystems.
divergence. page 171
7. Show in a diagram how energy from sunlight is used
Photosynthesis in eukaryotes likely evolved by
to produce ATP.
endosymbiosis. page 171
8. List the products of linear electron transport and
All of the oxygen in Earth’s atmosphere results from
cyclic electron transport, and describe the role of cyclic
photosynthesis by organisms containing two photosystems.
electron transport.
page 171
9. Describe two strategies that plants use to limit the
formation and effects of reactive oxygen species.
Self-Assessment 10. Explain the trade-off that rubisco faces in terms of
1. Write the overall photosynthetic reaction and identify selectivity and enzymatic speed.
which molecules are oxidized and which molecules are
11. Estimate the overall efficiency of photosynthesis and
reduced.
describe where in the pathway energy is dissipated.
2. Compare the overall reactions of photosynthesis and
12. List three major steps that are hypothesized to have
cellular respiration.
occurred in the evolutionary history of photosynthesis.
3. Name the major inputs and outputs of the Calvin
cycle.
Log in to to check your answers to the Self-
4. Describe the three major steps in the Calvin cycle Assessment questions, and to access additional learning tools.
and the role of the key enzyme rubisco.
 CHAPTER 1 LIFE 176

CASE 2

Cancer
When Good Cells Go Bad

Imagine a simple vaccine that could prevent about aren’t fought off by the immune system can permanently
500,000 cases of cancer worldwide every year. You’d integrate their own eight-gene DNA sequence into the
think such a discovery would be hailed as a miracle. Not host cell’s DNA.
quite. A vaccine to prevent cervical cancer, which affects Once integrated into the DNA of human epithelial
about half a million women and kills as many as 275,000 cells, two viral genes, E6 and E7, are expressed and
annually, has been available since 2006. However, in produce two proteins. These viral proteins inhibit
the United States, this vaccine (and others) has stirred the products of key tumor suppressor genes. Tumor
controversy. suppressor genes code for proteins called tumor
Cancer is uncontrolled cell division. Cell division is suppressors that keep cell division in check by slowing
a normal process that occurs during development of a down cell division, repairing DNA replication errors,
multicellular organism and subsequently as part of the or instructing defective cells to die. When tumor
maintenance and repair of adult tissues. It is carefully suppressors are prevented from doing these jobs, cancer
regulated so that it occurs only at the right time and place. can result.
But sometimes, this careful regulation can be disrupted. HPV strikes in two ways. The viral E6 protein inhibits
When the normal checks on cell division become derailed, a protein called p53, an important tumor suppressor that,
cancer can result. among other functions, prevents cell division in healthy
Most cancers are caused cells when there is DNA damage. However, when bound
When the normal by inherited or acquired by E6, p53 becomes essentially inactive. Meanwhile,
mutations, but some are the viral E7 protein inhibits a protein called Rb, which
checks on cell division caused by a virus. In fact, normally blocks transcription factors that promote cell
become derailed, nearly all cases of cervical division. Without Rb and p53 to put the brakes on cell
cancer are caused by a virus division, cervical cells divide uncontrollably.
cancer can result. called human papillomavirus As they grow and multiply, the abnormal cells push
(HPV). There are hundreds through the basal lamina, a thin layer that separates the
of strains of HPV. Some of these strains cause minor epithelial cells lining the cervix from the connective
problems such as common warts and plantar warts. tissue beneath. Untreated, invasive cancer can spread
Other strains are sexually transmitted. Some can cause to other organs. Once cancer travels beyond its primary
genital warts but aren’t associated with cancer. However, location, the situation is grim. Most cancers cannot be
a handful of “high-risk” HPV strains are strongly tied to cured after they have spread.
cancer of the cervix. More than 99% of cervical cancer The U.S. Food and Drug Administration (FDA) has
cases are believed to arise from HPV infections. approved two vaccines for HPV, both of which protect
HPV infects epithelial cells, a type of cell that lines against the high-risk strains responsible for cervical
the body cavities and covers the outer surface of the body. cancer. Medical groups such as the American Academy
Once inside a cell, the virus hijacks the cellular machinery of Pediatrics and government organizations such as
to produce new viruses. The high-risk strains of HPV that the Centers for Disease Control and Prevention (CDC)
infect the cervix go one step further: Those viruses that recommend vaccinating girls at age 11 or 12, before
176
 Human papillomavirus (HPV) and cervical cancer.
Certain strains of HPV can infect epithelial cells of the cervix
and lead to cancer, which can spread to the rest of the body.

Once inside the cell, HPV

produces viral proteins
HPV that inhibit tumor
suppressors, cellular
proteins that normally
keep cell division in
check. Without tumor
suppressor activity,
cancer cells can replicate.

Epithelium

Basal lamina

Blood Cancer cells break

vessel through the basal
lamina and spread
by blood vessels in
Connective the connective
tissue tissue to the rest of
the body.
Progress of cancer

Precancerous state Cancer develops Metastasis

they become sexually active. In addition, recent CDC However, there are critics of these recommendations
guidelines also recommend vaccinating boys because and mandates. Some say the government shouldn’t be
they can transmit the virus to women when they become making medical decisions. Others argue that the use of
sexually active. On the basis of these recommendations, the vaccine condones sexual activity among young girls
many states are considering legislation that would require and boys. Still others have expressed concern over the
middle-school girls to receive the vaccine. vaccine’s safety.
177
 Studies have found the HPV vaccine to be safe, a At the Mayo Clinic in Rochester, Minnesota,
conclusion backed by a 2010 report from the Institute of researchers are working to design vaccines to prevent
Medicine, an independent nonprofit organization that breast and ovarian cancers from recurring in women who
advises the U.S government on issues of health. In spite have been treated for these diseases. The researchers
of these findings, the HPV vaccine has not been widely have zeroed in on proteins on the surface of cancerous
accepted. The CDC found that by 2010, a full 5 years cells. Cells communicate with one another by releasing
after the vaccine was introduced, only 32% of teenage signaling molecules that are picked up by receptor
girls had been fully vaccinated. In 2013, the percentage molecules on another cell’s surface, much the way a radio
increased to 38%, but remained low. antenna picks up a signal. Cellular communication is
Proponents of HPV vaccination say those numbers critical for a functioning organism. Sometimes, though,
should cause concern. HPV is common: A 2007 study the signaling process goes awry.
found that nearly 27% of American women ages 14–59 The Mayo Clinic team is focusing on two cell-surface
were infected with the virus. Among 20- to 24-year- receptors that, when malfunctioning, lead to cancer.
olds, the infection rate was nearly 45%. Certainly, not One, Her2/neu, promotes the growth of aggressive
everyone who contracts HPV will develop cancer. Most breast cancer cells. The other, folate receptor a (alpha)
people manage to clear the virus from their bodies protein, is frequently overexpressed in breast and
within 2 years of infection. But in some cases, the virus ovarian tumors. The researchers hope to train patients’
hangs on and cancer results. immune systems to generate antibodies that recognize
Because cancer is often difficult to treat, especially these proteins and then destroy the cancerous cells. The
in its later stages, many researchers focus on early approach successfully prevented tumors in mice. Now the
detection or prevention. Early detection of cervical researchers are testing the vaccines in humans.
cancer can be accomplished by routine Pap smears, in Even if these new therapies are successful, cancer
which cells from the cervix are collected and observed researchers have much more work to do. Cancer is not
under a microscope. As we have seen, cervical cancer is one disease but many, and most of them are caused by a
also an obvious target for prevention since it is caused complex interplay of genetic and environmental factors.
by a virus that can be averted with a conventional Any number of things can go wrong as cells communicate,
vaccine. However, most cancers are not caused by grow, and divide. But as researchers learn more about the
viruses, and developing vaccines to prevent those cellular processes involved, they can step in to prevent
cancers is a trickier proposition—but researchers are or treat cancer. As the HPV vaccine shows, there is
pushing ahead. significant progress to be made.

? CASE 2 QUESTIONS
Special sections in Chapters 9–11 discuss the following questions related to Case 2.

1. How do cell signaling errors lead to cancer? See page 192.

2. How do cancer cells spread throughout the body? See page 213.
3. What genes are involved in cancer? See page 236.

178
 CHAPTER 9

Cell Signaling
Core Concepts
9.1 Cells communicate primarily by
sending and receiving chemical
signals.
9.2 Cells can communicate over
long and short distances.
9.3 Signaling molecules bind to
and activate cell-surface and
intracellular receptors.
9.4 G protein-coupled receptors
are a large, conserved family
of receptors that often lead to
short-term responses.
9.5 Receptor kinases are
widespread and often lead to
long-term responses.

Quest/Science Source.

179
180 SECTION 9.1 P R I N C I P L E S O F C E L L CO M M U N I C AT I O N

Up to this point, we have considered how life works by looking

mainly at what happens inside individual cells. We have seen FIG. 9.1 Four elements required for cellular communication:
how a cell uses the information coded in genes to synthesize the a signaling cell, signaling molecule, receptor protein,
proteins necessary to carry out diverse functions. We have also and responding cell.
seen how the plasma membrane actively keeps the environment Signaling cell Responding cell
inside the cell different from the environment outside it. Finally, Signaling Receptor protein
we have explored how a cell harvests and uses energy from the molecule
environment.
In the next three chapters, we zoom out from individual cells
and consider cells in context. Cells always exist in a particular
environment and their behavior is often strongly influenced by
this environment. For example, the environment of unicellular
organisms consists of their physical surroundings and other
cells nearby. The ability of cells to sense and respond to this The signaling cell The responding cell
environment is critical for survival. A unicellular organism senses releases signaling has receptor proteins
molecules. that bind to the
information that signals when to feed, when to move, and when
signaling molecule.
to divide.
In multicellular organisms, cells do not exist by themselves but
as part of cellular communities. These cells may physically adhere to their environment, and also as a way to influence the behavior
to one another, forming tissues and organs. Often multicellular and activity of other cells. As result, the basic principles apply to all
organisms are highly complex, made up of cells of many types cells, both prokaryotic and eukaryotic, and to both unicellular and
with specialized functions. These cells respond to signals, which multicellular organisms.
may come from the outside environment or from other cells of the
organism. Cell communication helps to coordinate the activities Cells communicate using chemical signals that bind to
of the thousands, millions, and trillions of cells that make up the specific receptors.
organism. These cells all come about through the process of cell Cellular communication consists of four essential elements: a
division. Cells divide as part of growth and development, as well signaling cell, a signaling molecule, a receptor protein,
as tissue maintenance and repair. Cell division is an example of and a responding cell (Fig. 9.1). The signaling cell is the source
a cellular activity that is often regulated by signals that tell a cell of a signaling molecule. Signaling molecules vary immensely
when to start dividing and when to stop dividing. and include peptides, lipids, and gases. In all cases, the signaling
In Chapter 9, we look at how cells send, receive, and respond to molecule carries information from one cell to the next. The
signals. In Chapter 10, we focus on how cells maintain their shape signaling molecule binds to a receptor protein on or in the
and, in some cases, physically adhere to one another. As we saw responding cell. As a result, there is a change in activity or
in the case of molecules, form is inextricably linked to function. behavior of the responding cell.
Finally, in Chapter 11, we consider how one cell becomes two Let’s consider a simple example that we are all familiar with.
through the process of cell division. You’re startled or scared, and you experience a strange feeling in
the pit of your stomach and your heart beats faster. Collectively,
these physiological changes are known as the “fight or flight”
9.1 PRINCIPLES OF CELL response. For these changes to occur, a signal goes from signaling
COMMUNICATION cells to responding cells in the stomach, heart, and other organs.
This signal is the hormone adrenaline (also called epinephrine). In
The activities of virtually all cells are influenced by their response to stress, adrenaline is released from the adrenal glands,
surroundings. Cells receive large amounts of information from located above the kidneys. Adrenaline circulates through the body
numerous sources in their surroundings. One source of information and acts on many types of cell, including the cells of your heart,
is the physical environment. Another key source of information causing it to beat more strongly and quickly. When the heart
is other cells—neighboring cells, nearby cells, and even very beats more strongly and quickly, it is able to deliver oxygen more
distant cells. Both unicellular and multicellular organisms sense effectively to the rest of the body.
information from their surroundings and respond by changing their This example illustrates the four basic elements involved in
activity or even dividing. cellular communication. In this case, the signaling cells are specific
In this section, we take a look at general principles of cell cells within the adrenal glands. The signaling molecule is the
communication, including how cells send and receive signals and hormone adrenaline released by these cells. Adrenaline is carried
how a cell responds after it receives a signal. These mechanisms in the bloodstream and interacts with receptor proteins located on
first evolved in unicellular organisms as a way to sense and respond the surface of responding cells, like those of the heart.
 CHAPTER 9 CELL SIGNALING 181

In the 1990s, scientists discovered a short peptide consisting

FIG. 9.2 Communication among bacterial cells. The binding of of 17 amino acids that is continuously synthesized and released by
receptors by the signaling molecule stimulates uptake of DNA. pneumococcal cells. Not long after, a receptor for this peptide was
a. Low cell density discovered on the surface of the pneumococcal cells. The binding
of this peptide to its receptor causes a bacterium to express the
At low population density, genes required for DNA uptake. When the bacteria are at low
the concentration of the
signaling peptide is too low density, the peptide is at too low a concentration to bind to the
to bind to the receptors receptor (Fig. 9.2a). As the population density increases, so does
and stimulate DNA uptake.
the concentration of the peptide, until it reaches a level high
enough to bind to enough receptors to cause the cells to turn on
Receptor
genes necessary for DNA uptake (Fig. 9.2b).
protein This is an example of quorum sensing, a process by which
Bacterial bacteria are able to determine whether they are at low or high
cell population density and then turn on specific genes across the
Signaling entire community. It is used to control and coordinate many
b. High cell density molecule
different types of bacterial behaviors, such as bioluminescence,
At high population antibiotic production, and biofilm formation.
density, the concentration
of the signaling peptide is This example also illustrates the four essential elements
high enough to bind to involved in communication between cells. In this case, however,
the receptors and each bacterial cell is both a signaling cell and a responding cell
stimulate DNA uptake.
because each cell makes the signaling molecule and its receptor.
Nevertheless, the general idea that cells are able to communicate
by sending a signaling molecule that binds to a receptor on
a responding cell is universal among both prokaryotes and
eukaryotes. These four elements act in very much the same way in
DNA
molecule diverse types of cellular communication.

Signaling involves receptor activation, signal

j Quick Check 1 If a hormone is released into the bloodstream
transduction, response, and termination.
and therefore comes into contact with many cells, what determines
What happens when a signaling molecule binds to a receptor on a
which cells in the body respond to the hormone?
responding cell? The first step is receptor activation. On binding
Let’s look at a second example, this time the signal, the receptor is turned on, or activated (Fig. 9.3). The
involving communication among bacteria signal usually activates the receptor by causing a conformational
(Fig. 9.2). Streptococcus pneumoniae (also known
as pneumococcus) is a disease-causing bacterium
that is associated with pneumonia, meningitis, and
FIG. 9.3 Steps in cell signaling: receptor activation, signal transduction, response,
some kinds of arthritis. Many bacteria, including
and termination.
pneumococcus, are able to take up DNA from the
environment and incorporate it into their own Plasma membrane
genome (Chapter 26). By this means, individual
Extracellular Cytoplasm
bacterial cells can acquire genes with advantageous
fluid
properties, including antibiotic resistance.
In the 1960s, it was observed that the rate Receptor activation Signal transduction
of DNA uptake by pneumococcal cells increased
sharply once the bacterial population reached Response Termination
a certain density. Scientists concluded that the
bacteria were able to coordinate DNA uptake across
the population so that uptake occurred only at high
bacterial population density. How is it possible that
The signal binds The signal is transmitted The cell responds, for The response
these bacteria “know” how many other bacteria are to a receptor, to the interior of the cell example by activating is terminated
present? It turns out that these bacteria are able which is then by a signal transduction an enzyme or turning so that new
to communicate this information to one another activated. pathway. on transcription of a signals can be
gene. received.
through the release of a small peptide.
182 SECTION 9.2 C E L L S I G N A L I N G OV E R LO N G A N D S H O RT D I S TA N C E S

change in the receptor. As a result, some receptors bind to and environment. Eventually, when the density of bacteria is low, the
activate other proteins located inside the cell. Other receptors are initiating signal falls below a critical threshold and gene expression
themselves enzymes, and binding of the signal changes the shape is turned back off.
and activity of the enzyme. Still other receptors are channels that This example is relatively simple. Remarkably, however, other
open or close in response to binding a signaling molecule. more complex signaling pathways in a wide range of organisms
Once activated, the receptor often triggers a series of involve the same four elements and steps, and in many cases involve
downstream events in a process called signal transduction. During similar signaling molecules, receptors, and signal transduction
signal transduction, one molecule activates the next molecule, systems that have been evolutionarily conserved over long periods
which activates the next, and so on. In this way, signal transduction of time.
can be thought of as a chain reaction or cascade of biochemical
events set off by the binding and activation of the receptor. An
important aspect of signal transduction is that the signal is often 9.2 CELL SIGNALING OVER LONG
amplified at each step in the pathway. As a result, a low signal AND SHORT DISTANCES
concentration can have a large effect on the responding cell.
Next, there is a cellular response, which can take different In prokaryotes and unicellular eukaryotes, cell communication
forms depending on the nature of the signal and the type of occurs between individual organisms. In complex multicellular
responding cell. For example, signaling pathways can activate eukaryotes, cell signaling involves communication between cells
enzymes involved in metabolic pathways, or turn on genes that within the same organism. The same principles apply in both
cause the cell to divide, change shape, or signal other cells. instances, but there are important differences. In multicellular
The last step is termination, in which the cellular response organisms, the distance between communicating cells varies
is stopped. The response can be terminated at any point along the considerably (Fig. 9.4). When the two cells are far apart, the
signaling pathway. Termination protects the cell from overreacting signaling molecule is transported by the circulatory system. When
to existing signals and therefore helps the cell to have an appropriate they are close, the signaling molecule simply moves by diffusion.
level of response. It also allows the cell to respond to new signals. In addition, many cells in multicellular organisms are physically
In the case of pneumococcal cells, the peptide binds to and attached to one another; in this case, the signaling molecule
activates a receptor on the cell surface. When enough receptors is not released from the signaling cell at all. In this section, we
are bound by the signaling molecule, the message is relayed by explore communication over long and short distances, as well as
signal transduction pathways to the nucleoid. There, genes are communication between cells that are physically associated with
turned on that express proteins involved in DNA uptake from the one another.

FIG. 9.4 Signaling distances. Cell communication can be classified according to the distance between the signaling and responding cells.
a. Endocrine signaling

Signaling
molecule
c. Autocrine signaling

Signal travels through

the circulatory system. Receptor
protein

b. Paracrine signaling d. Contact-dependent signaling

 CHAPTER 9 CELL SIGNALING 183

Endocrine signaling acts over long distances. type of signaling molecule that causes the responding cell to grow,
Signaling molecules released by a cell may have to travel great divide, or differentiate. One of the first growth factors discovered
distances to reach receptor cells in the body. In this case, they was found by scientists who were attempting to understand how
are often carried in the circulatory system. Signaling by means of to maintain cultures of cells in the laboratory. For decades, medical
molecules that travel through the bloodstream is called endocrine research has worked with cells maintained in culture. Initially,
signaling (Fig. 9.4a; Chapter 38). these cultured cells had limited use because they failed to divide
Adrenaline (section 9.1) provides a good example of endocrine outside the body unless they were supplied with unidentified
signaling. Adrenaline, which is produced in the adrenal glands, factors from mammalian blood serum. In 1974, American
is carried by the bloodstream to target cells that are far from the scientists Nancy Kohler and Allan Lipton discovered that one of
signaling cells. Other examples of endocrine signaling involve these factors is secreted by platelets (Fig. 9.5). Consequently,
the mammalian steroid hormones estradiol (an estrogen) and the growth factor was named “platelet-derived growth factor,”
testosterone (an androgen). These hormones travel from the ovaries or PDGF. We now know of scores of molecules secreted by cells
and the testes, respectively (although there are other minor sources that function as growth factors, and in most cases their effects are
of these hormones), through the bloodstream, to target cells in confined to neighboring cells.
various tissues throughout the body. The increased amount of Growth factors secreted by cells in an embryo work over
estrogen in girls during puberty causes the development of breast short distances to influence the kind of cells their neighbors will
tissue and the beginning of menstrual cycles. The increased amount become. In this way, they help shape the structure of the adult’s
of testosterone in boys during puberty causes the growth of muscle tissues, organs, and limbs. For example, in developing vertebrates,
cells, deepening of the voice, and growth of facial hair (Chapter 42). paracrine signaling by the growth factor Sonic Hedgehog (yes,
it’s named after a video game character) ensures that the motor
Signaling can occur over short distances. neurons in your spinal cord are located properly, that the bones of
Signaling can also occur between two cells that are close to each your vertebral column form correctly, and that your thumb and
other (Fig. 9.4b). When cells are close to each other, they do not pinky fingers are on the correct sides of your hands.
require a circulatory system to deliver the signaling molecule. A specialized form of short-range signaling is the
Instead, the signaling molecule can simply move by diffusion communication between neurons (nerve cells), or between
between the two cells. This form of signaling is called paracrine neurons and muscle cells (Chapters 35 and 37). Neurotransmitters
signaling. In this case, signaling molecules travel distances of are a type of signaling molecule released from a neuron. After
about 20 cell diameters, or a few hundred micrometers. release, they diffuse across a small space, called a synapse, between
In paracrine signaling, the signal is usually a small, water- the signaling cell and the responding cell. If the adjacent cell is a
soluble molecule such as a growth factor. A growth factor is a neuron, it often responds by transmitting a nerve impulse further

HOW DO WE KNOW?

FIG. 9.5 component of blood, but it is collected from blood that has not
clotted. American biologists Nancy Kohler and Allan Lipton were

Where do growth factors interested in identifying the source of the factor in blood serum
that allows cells to survive in culture.

come from? HYPOTHESIS Since Kohler and Lipton knew that clotting
depends on the release of substances from platelets, they
hypothesized that a growth-promoting factor was introduced into
BACKGROUND Cells can be grown outside the body in culture.
the blood by platelets during clotting.
However, they survive and grow well only under certain conditions.
Researchers hypothesized that there are substances that are EXPERIMENT 1 The investigators first confirmed earlier
required for cells to grow in culture, but the identity and source of observations using cells that are easily grown in culture called
these substances were for a long time unknown. A key insight came fibroblasts, which they obtained from mice. They cultured two
from the observation that chicken cells grow much better if they are sets of fibroblasts in small plastic dishes. To one of the cultures
cultured in the presence of blood serum rather than blood plasma. they added serum; to the other culture they added plasma. Then
Blood serum is the liquid component of blood that is collected they monitored the rate of cell division in both culture dishes
after blood has been allowed to clot. Blood plasma is also the liquid over time.
(continued on following page)
 184 SECTION 9.2 C E L L S I G N A L I N G OV E R LO N G A N D S H O RT D I S TA N C E S

184 SECTION 5.1 M

HOW DO WE KNOW?

(continued from previous page)

RESULTS 1 They observed that the rate of cell division in the fibroblasts EXPERIMENT 2 To see if the factor is released directly from
cultured in serum was far greater than that of the cells cultured in platelets, they prepared a solution of proteins made from purified
plasma, as expected based on earlier experiments (Fig. 9.5a). platelets, added these proteins to cultured fibroblasts, and
measured cell growth.
a.
RESULTS 2 They found that the solution of platelet proteins also
Unclotted blood Clotted blood
caused the growth of fibroblasts to increase compared to the
growth of fibroblasts in plasma (Fig. 9.5b).
Plasma Serum

b.
Fibroblast 6.0
5.5

Number of cells  105 per dish

5.0 Cultured with platelet proteins
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5 Cultured with plasma
0
1 2 3 4 5 6
Day

9 CONCLUSION Kohler and Lipton concluded that the growth-

Number of cells  105 per dish

8 promoting factor is a protein that is released by platelets on clot

Cultured with serum
7 formation, and therefore normally present in serum.
6
5 FOLLOW-UP WORK Over the next few years, these and other
4 Cultured with plasma scientists purified and characterized the growth factor we know
3 today as platelet-derived growth factor, or PDGF.
2
1 SOURCE Kohler, N., and A. Lipton. 1974. “Platelets as a Source of Fibroblast
0 Growth-Promoting Activity.” Experimental Cell Research 87:297–301.
1 2 3 4 5 6
Day

and then releasing additional neurotransmitters. If the responding developmental decision. In addition, autocrine signaling can be
cell is a muscle cell, it often responds by contracting. used by cancer cells to promote cell division.
In some cases, signaling molecules may be released by a cell
and then bind to receptors on the very same individual cell. Such Signaling can occur by direct cell–cell contact.
cases, where signaling cell and responding cell are one and the In some cases, a cell communicates with another cell through
same, are examples of autocrine signaling (Fig. 9.4c). Autocrine direct contact, without diffusion or circulation of the signaling
signaling is especially important to multicellular organisms molecule. This form of signaling requires that the two
during the development of the embryo (Chapters 20 and 42). communicating cells be in physical contact with each other. A
For example, once a cell differentiates into a specialized cell transmembrane protein on the surface of one cell acts as the
type, autocrine signaling is sometimes used to maintain this signaling molecule, and a transmembrane protein on the surface
 CHAPTER 9 CELL SIGNALING 185

of an adjacent cell acts as the receptor (Fig. 9.4d). In this case, ways, this change in receptor shape is similar to the change that
the signaling molecule is not released from the cell, but instead occurs when a substrate binds to the active site of an enzyme
remains associated with the plasma membrane of the signaling cell. (Chapter 6). The conformational change in the receptor
This form of signaling is important during embryonic ultimately triggers chemical reactions or other changes in the
development. As an example, let’s look at the development of the cytosol, and is therefore a crucial step in the reception and
central nervous system of vertebrate animals. In the brain and interpretation of communications from other cells.
spinal cord, neurons transmit information in the form of electrical
signals that travel from one part of the body to another. The Receptors for polar signaling molecules are on
neurons in the central nervous system are greatly outnumbered by the cell surface.
supporting cells, called glial cells, which nourish and insulate the The location of a particular receptor in a cell depends largely on
neurons. Both the neurons and the glial cells start out as similar whether the signaling molecule is polar or nonpolar (Fig. 9.6).
cells in the embryo, but some of these undifferentiated cells Many signaling molecules, such as the growth factors we just
become neurons and many more become glial cells. discussed, are small, polar proteins that cannot pass through the
During brain development, the amount of a transmembrane
protein called Delta dramatically increases on the surface of some
of these undifferentiated cells. These cells will become neurons.
Delta proteins on each new neuron bind to transmembrane
proteins called Notch on the surface of adjacent, undifferentiated FIG. 9.6 Cell-surface and intracellular receptors. (a) Cell-surface
cells. In this case, the signaling cell is the cell with elevated receptors interact with polar signaling molecules that
levels of Delta protein. The Delta protein in turn is the signaling cannot cross the plasma membrane. (b) Intracellular
molecule, and Notch is its receptor. Cells with activated Notch receptors interact with nonpolar signaling molecules that
receptors become glial cells and not neurons. Because one can cross the plasma membrane.
signaling cell sends this same message to all the cells it contacts,
a. Cell-surface receptor
it is easy to understand how there can be so many more glial cells
than neurons in the central nervous system. Polar Ligand-binding site
signaling
As you can see from these examples, the same fundamental molecule Activated receptor
principles are at work when signaling guides a developing embryo, Extracellular
allows neurons to communicate with other neurons or muscles, domain
triggers DNA uptake by pneumococcal cells, or allows your body Trans-
to respond to stress. All these forms of communication are based membrane
domain Polar signaling
on signals that are sent from a signaling cell to a responding
molecules
cell. These signaling molecules are the language of cellular Cytoplasmic cannot cross
communication. domain the plasma
membrane
Plasma and rely on
membrane cell-surface
9.3 CELL-SURFACE AND receptors.
INTRACELLULAR RECEPTORS
Receptors are proteins that receive and interpret information
carried by signaling molecules. Regardless of the distance between
communicating cells, a message is received by a responding cell b. Intracellular receptor
when the signaling molecule binds to a receptor protein on or in Small,
the responding cell. For this reason, the signaling molecule is often nonpolar
signaling Activated receptor
referred to as a ligand (from the Latin ligare, which means “to bind”). molecule
The signaling molecule binds to a specific part of the receptor protein Small
nonpolar
called the ligand-binding site. The bond is noncovalent and highly signaling
specific: The signaling molecule binds only to a receptor with a molecules
ligand-binding site that recognizes the molecule. can freely
pass
Almost without exception, the binding of a signaling through the
molecule to the ligand-binding site of a receptor causes a Nucleus plasma
membrane
conformational change in the receptor. We say that the DNA
and activate
conformational change “activates” the receptor because it is cytoplasmic
through this change that the receptor passes the message from receptors.
the signaling molecule to the interior of the cell. In many
186 SECTION 9.3 C E L L-S U R FAC E A N D I N T R AC E L LU L A R R E C E P TO R S

FIG. 9.7 Three types of cell-surface receptor. Receptors act like (a) a light switch. (b) G protein-coupled receptors, (c) receptor kinases, and
(d) ligand-gated ion channels can be either “off” (inactive) or “on” (active).

a. A light switch

= =

Off On

b. G protein-coupled receptor c. Receptor kinase d. Ligand-gated ion channel

Ligand
Ions
Receptor G protein

Phosphate Inactive Active

group
Inactive Active
Inactive Active

hydrophobic core of the plasma membrane. The receptor proteins often already bound to DNA and need only to bind their steroid
for these signals are on the outside surface of the responding cell counterpart to turn on gene expression.
(Fig. 9.6a). There are many examples of steroid hormones, including sex
Receptor proteins for growth factors and other polar hormones, glucocorticoids (which raise blood glucose levels), and
ligands are transmembrane proteins with an extracellular ecdysone (involved in insect molting). However, since much of
domain, a transmembrane domain, and a cytoplasmic domain. the information received by cells is transmitted across the plasma
When a signaling molecule binds to the ligand-binding site in membrane through transmembrane receptors, we focus our
the extracellular domain, the entire molecule, including the attention here on the sequence of events that takes place when
cytoplasmic domain of the receptor, undergoes a conformational receptors on the surface of cells bind their ligands.
change, and as a result the molecule is activated. In this way, the
receptor acts as a bridge between the inside and outside of the Cell-surface receptors act like molecular
responding cell that carries the message of the hydrophilic signal switches.
across the hydrophobic core of the plasma membrane. As we saw earlier, a receptor is activated after a signaling molecule
binds to its ligand-binding site. Many receptors act as binary
Receptors for nonpolar signaling molecules are molecular switches, existing in two alternative states, either “on”
in the interior of the cell. or “off” (Fig. 9.7). In this way, receptors behave similarly to a light
Some nonpolar signaling molecules, such as the steroid hormones switch (Fig. 9.7a). When bound to their signaling molecule, the
involved in endocrine signaling, don’t need a receptor on the molecular switch is turned on. When the signaling molecule is no
cell surface in order to relay information to the interior of the longer bound, the switch is turned off.
cell. Since steroids are hydrophobic, they pass easily through the There are thousands of different receptor proteins on the
hydrophobic core of the phospholipid bilayer and into the target surface of any given cell. Most of them can be placed into one
cell. Once inside, steroid hormones bind to receptor proteins of three groups on the basis of their structures and what occurs
located in the cytosol or in the nucleus to form receptor–steroid immediately after the receptor binds its ligand. One type of cell-
complexes (Fig. 9.6b). Steroid–receptor complexes formed in the surface receptor is called a G protein-coupled receptor (Fig. 9.7b;
cytosol enter the nucleus, where they act to control the expression section 9.4). When a ligand binds to a G protein-coupled receptor,
of specific genes. Steroid receptors located in the nucleus are the receptor couples to, or associates with, a G protein, as its name
 CHAPTER 9 CELL SIGNALING 187

suggests. G protein-coupled receptors are evolutionary conserved channels are closed. However, when a signaling molecule binds to
and all have a similar molecular structure (section 9.4) the extracellular portion of a ligand-gated ion channel, the channel
A second group of cell-surface receptors are themselves undergoes a conformational change that opens it and allows ions
enzymes, which are activated when the receptor binds its ligand. to flow in and out. This type of signaling is especially important
Most of these are receptor kinases (Fig. 9.7c and section 9.5). for nerve and muscle cells since their functions depend on a rapid
A kinase is an enzyme that catalyzes the transfer of a phosphate change in ion flow across the plasma membrane.
group from ATP to a substrate. To catalyze this reaction, it binds What happens after a signaling molecule binds to its receptor
both ATP and the substrate. This process is called phosphorylation and flips a molecular switch? Following receptor activation,
(Fig. 9.8). Phosphorylation is important because it affects the signaling pathways transmit the signal to targets in the interior of
activity of the substrate: When a protein is phosphorylated by the cell, the cell responds, and eventually the signal is terminated.
a kinase, it typically becomes active and is switched on. The In the next two sections, we examine the signaling pathways
addition of a phosphate group to a protein can activate it by activated by G protein-coupled receptors and receptor kinases.
altering its shape or providing a new site for other proteins to
bind. Phosphatases remove a phosphate group, a process called
dephosphorylation (Fig. 9.8). When a protein is dephosphorylated 9.4 G PROTEIN-COUPLED
by a phosphatase, it typically becomes inactive and is switched off. RECEPTORS AND SHORT-
Receptors in the third group, ion channels, alter the flow of TERM RESPONSES
ions across the plasma membrane. These channels can be opened
in different ways. Some open in response to changes in voltage G protein-coupled receptors, introduced in section 9.3, are a very
across the membrane; these are called voltage-gated ion channels large family of cell-surface molecules. They are found in virtually
and are discussed in Chapter 35. Other ion channels open when every eukaryotic organism. In humans, for example, approximately
bound by their ligand; these are called ligand-gated ion channels 800 different G protein-coupled receptors have been found. They
(Fig. 9.7d) and are discussed in Chapter 37. Recall from Chapter 5 all have two characteristics in common. First, they have a similar
that channel proteins help ions and other molecules diffuse into structure, consisting of a single polypeptide chain that has seven
and out of the cell by providing a hydrophilic pathway through the transmembrane spanning regions, with the ligand-binding site on
hydrophobic core of the plasma membrane. Most of the time, the the outside of the cell and the portion that binds to the G protein
on the inside of the cell. Second, when activated, they associate
with a G protein. In this way, they are able to transmit the signal
from the outside to the inside of the cell. These characteristics
FIG. 9.8 Phosphorylation and dephosphorylation. A kinase result from their shared evolutionary history.
transfers a posphate group from ATP to a protein, typically In spite of their similarity, different G protein-coupled
activating the protein. A phosphatase removes a phosphate receptors are able to respond to a diverse set of different signaling
group from a protein, typically deactivating the protein. molecules, including hormones, neurotransmitters, and small
molecules. In addition, their effects are quite diverse. For example,
signaling through these receptors is responsible for our senses
of sight, smell, and taste (Chapter 36). In fact, it is thought
Inactive protein that G protein-coupled receptors used for cell communication
in multicellular organisms evolved from sensory receptors in
unicellular eukaryotes.

ATP Pi The first step in cell signaling is receptor activation.

As we saw in section 9.3, G protein-coupled receptors are
Phosphorylation Kinase Dephosphorylation
Phosphatase transmembrane proteins that bind signaling molecules. When a
ligand binds to a G protein-coupled receptor, it is on, or active. In
ADP its active state, it couples to (that is, it binds to) a G protein, which
P is located on the cytoplasmic side of the plasma membrane.
The G protein also has two states, on or off. A G protein is able
to bind to the guanine nucleotides GTP and GDP. When it is bound
to GTP, it is on, or active, and when it is bound to GDP, it is off, or
Active protein
inactive.
When a ligand binds to a G protein-coupled receptor, the
receptor in turn binds to and activates a G protein by causing it
188 SECTION 9.4 G P ROT E I N - CO U P L E D R E C E P TO R S A N D S H O RT-T E R M R E S P O N S E S

FIG. 9.9 Activation of a G protein by a G protein-coupled FIG. 9.10 Adrenaline signaling in heart muscle. Adrenaline binds
receptor. A G protein-coupled receptor is activated when to a G protein-coupled receptor, leading to production
it binds a signaling molecule, which leads to exchange of of the second messenger cAMP and activation of protein
GDP for GTP on the G protein’s a subunit, separation of kinase A. The cell’s response is increased heart rate.
the a subunit, and downstream effects.
Activated
Activated adenylyl
Ligand Inactive target receptor cyclase
protein
Receptor G protein Activated
adenylyl cyclase
When the  subunit   converts ATP into
is bound to GDP, 
the second
  the three subunits GTP ATP
 messenger cAMP,
are joined together
which in turn
GDP and the G protein is Active cAMP activates protein
inactive. G protein kinase A.

When the G protein Activated protein

is associated with Protein kinase A
an activated kinase A phosphorylates
receptor, the GDP Inactive Active proteins in the
bound to the  heart muscle,
  causing the heart
 subunit is replaced
by GTP, which leads rate to increase.
GTP
to separation of the
 subunit from the
GDP  and  subunits.
binds to a G protein-coupled receptor. When adrenaline binds
Active target
protein to its G protein-coupled receptor on cardiac muscle cells, GDP in
the G protein is replaced by GTP and the G protein is activated.
The GTP-bound a subunit then binds to and activates an
enzyme in the cell membrane called adenylyl cyclase (Fig. 9.10).
  Response Adenylyl cyclase converts the nucleotide ATP into cyclic AMP

GTP (cAMP). Cyclic AMP is known as a second messenger. The
term “second messenger” was coined to differentiate it from
signaling molecules like adrenaline and growth factors, which
Activated  subunit binds to are considered first messengers. Second messengers are signaling
and activates target protein.
molecules found inside cells that relay information to the next
target in the signal transduction pathway. In the case of cAMP, it
binds to and activates another molecule, a kinase called protein
kinase A (PKA).
to release GDP and bind GTP. As long as the G protein is bound to A little adrenaline goes a long way as a result of signal
GTP, it is in the on position. The activated G protein goes on to amplification (Fig. 9.11). First, a single receptor bound to
activate additional proteins in the signaling pathway. adrenaline can activate several G protein molecules. Second, each
Some G proteins are composed of three subunits, called the molecule of adenylyl cyclase catalyzes the production of large
a (alpha), b (beta), and γ (gamma) subunits (Fig. 9.9). The amounts of the second messenger cAMP. And third, each PKA
a subunit is the part of the G protein that binds to either GDP or molecule activates multiple protein targets by phosphorylation.
GTP. When GDP bound to the a subunit is replaced by GTP, the These sequential molecular changes in the cytosol amplify the
a subunit separates from the b and γ subunits. In most cases, it is signal so that a very small amount of signaling molecule has a large
the isolated GTP-bound a subunit that is now active and able to effect on a responding cell.
bind to target proteins in the cell.

j Quick Check 2 Is the term “G protein” just a shorter name for a Signals lead to a cellular response.
G protein-coupled receptor? Cells are typically exposed to many different types of signaling
molecules. What determines the response of the cell? In part, that
Signals are often amplified in the cytosol. depends on the types of receptor present on the surface of the cell.
Because different G protein-coupled receptors have different These receptors determine which signals the cell is able to respond
effects in different cells, we will follow the steps in cell signaling to. The response of the cell also depends on the set of proteins that is
by following a specific example. Adrenaline, discussed earlier, found in it, as different cell types have different sets of intracellular
 CHAPTER 9 CELL SIGNALING 189

FIG. 9.11 Amplification of G protein-coupled signaling. Signaling through G protein-coupled receptors is amplified at several places, so a small
amount of signal can produce a large response in the cell.

Adrenaline
Activated
receptor

Each activated receptor activates

multiple G proteins which in turn
activate adenylyl cyclase enzymes.
Amplification

Activated
adenylyl
cyclase

  
GTP ATP GTP ATP GTP ATP

cAMP cAMP
Active 
Amplification subunit of cAMP
G protein
Each adenylyl cyclase enzyme
produces large amounts of the second
messenger cAMP, which activates
many molecules of protein kinase A.

Inactivated
protein
kinase A

Activated
protein
kinase A

ATP
Each protein kinase A enzyme
Amplification phosphorylates and activates
multiple protein targets.
ADP

Protein
targets
Phosphate group

proteins and signaling pathways. As a result, the same signaling resulting influx of calcium ions results in shorter intervals
molecule can have different effects in different types of cells. between muscle contractions and thus a faster heart rate. As long
In the case of the heart, activated PKA leads to the opening as adrenaline is bound to its receptor, the heart rate remains rapid.
of calcium channels that are present in heart muscle cells. The This increase in the heart rate in turn results in increased blood
190 SECTION 9.4 G P ROT E I N - CO U P L E D R E C E P TO R S A N D S H O RT-T E R M R E S P O N S E S

flow to the brain and skeletal muscles to deal with the stress that Once adrenaline leaves the receptor, the receptor reverts to
set off the signal in the first place. its inactive conformation and no longer activates G proteins
This example is typical of signaling through G protein-coupled (Fig. 9.12).
receptors. These receptors tend to activate downstream enzymes Even when a receptor is turned off, a signal will continue to be
or, in some cases, open ion channels. Because they often modify transmitted unless the other components of the signaling pathway
proteins that are already synthesized in the cell, their effects tend are also inactivated. A second place where the signal is terminated
to be rapid, short-lived, and easily reversible, as we will see next. is at the G protein itself (Fig. 9.12). G proteins can catalyze the
hydrolysis of GTP to GDP and inorganic phosphate. This means that
Signaling pathways are eventually terminated. an active, GTP-bound a subunit in the “on” position automatically
After a good scare, we eventually calm down and our heartbeat turns itself “off” by converting GTP to GDP. In fact, the a subunit
returns to normal. This change means that the signaling pathway converts GTP to GDP almost as soon as a molecule of GTP binds to
initiated by adrenaline has been terminated. How does this it. Thus, a G protein is able to activate adenylyl cyclase, and adenylyl
happen? First, most ligands, including adrenaline, do not bind cyclase is able to make cAMP, only during the very short time it
to their receptors permanently. The length of time a signaling takes the a subunit to convert GTP to GDP. Without an active
molecule remains bound to its receptor depends on how tightly receptor to generate more active G protein a subunits, transmission
the receptor holds on to it, a property called binding affinity. of the signal quickly comes to a halt.

FIG. 9.12 Termination of a G protein-coupled signal. G protein-coupled signaling is terminated at several places, allowing the cell to respond
to new signals.
Activated
adenylyl
cyclase

 

GTP ATP

cAMP
Protein
kinase A

Inactive Active

The signal molecule

adrenaline detaches
from the receptor after
a certain amount of Enzymes in the cytosol specifically
time, inactivating the degrade cAMP which stops the
receptor so that it can   phosphorylation and activation of
no longer bind to and 
target proteins by PKA.
activate the G protein. GDP
Within a very short
time, an activated G cAMP AMP
protein deactivates
itself by converting
GTP to GDP. Phosphate group

Active Inactive
Phosphatase
Phosphatases remove phosphate
groups from proteins, causing
them to become inactive.
 CHAPTER 9 CELL SIGNALING 191

Farther down the pathway, an enzyme converts the second on the surface of cells at the site of the wound, where it triggers
messenger cAMP to AMP, which no longer activates protein cell division necessary to repair the wound.
kinase A. Phosphatases remove the phosphate groups added by The cellular responses that result from receptor kinase
PKA, inactivating PKA’s target proteins (Fig. 9.12). In fact, most activation tend to involve changes in gene expression, which in
signaling pathways are counteracted at one or more points as a turn allow cells to grow, divide, differentiate, or change shape. In
means of decreasing or terminating the response of the cell to contrast, the activation of G protein-coupled receptors typically
the signal. leads to shorter-term changes in the cell, like activating enzymes
or opening ion channels.
j Quick Check 3 Name four ways in which the adrenaline signal to Signaling through receptor kinases takes place in most
the heart is terminated. eukaryotic organisms, and the structure and function of these
receptors have been conserved as organisms have evolved over
hundreds of millions of years. A well-studied receptor kinase called
9.5 RECEPTOR KINASES AND Kit provides an example. In vertebrates, signaling through the
LONG-TERM RESPONSES Kit receptor kinase is important for the production of pigment
in skin, feathers, scales, and hair. The conserved function of this
Like the communication that takes place through G protein- receptor can be seen in individuals with mutations in the kit gene,
coupled receptors, signaling through receptor kinases causes as shown in Fig. 9.13. (By convention, the name of a protein,
cells to respond in many ways. During embryonic development, like Kit, is capitalized and in roman type. The name of the gene
receptor kinase signaling is responsible for the formation and that encodes the protein, like kit, is lower case and italicized.)
elongation of limb buds that eventually become our arms and legs. As you can see from Fig. 9.13, mammals, reptiles, birds, and fish
When we cut a finger, platelet-derived growth factor (PDGF) is with a mutation in the kit gene have a similar appearance. This
released from platelets in the blood and binds to its receptor kinase observation indicates that the function of this gene has remained

FIG. 9.13 Mutations in the Kit receptor kinase. Similar patterns of incomplete pigmentation are present in mammals, reptiles, birds, and fish
that have this mutated receptor. Sources: (clockwise) Mark Boulton/Science Source; Mark Smith/Science Source; © Phillip Colla/Oceanlight.com; Werner
Bollmann/Oxford Scientific/Getty Images.
192 SECTION 9.5 R E C E P TO R K I N A S E S A N D LO N G -T E R M R E S P O N S E S

FIG. 9.14 Receptor kinase activation and signaling. Receptor kinases bind signaling molecules, dimerize, phosphorylate each other, and activate
intracellular signal molecules.

Signaling molecule Each member of the The phosphate groups

receptor pair attaches provide binding sites for
phosphate groups to intracellular signaling
the other member. proteins.

Cytoplasmic
ATP Phosphate signaling
ADP group proteins
Inactive receptor Dimerization Active receptor Active receptor

fairly constant since the appearance of the last common ancestor when Ras is activated by a receptor kinase, it exchanges GDP for
of these groups, more than 500 million years ago. GTP. Activated GTP-bound Ras triggers the activation of a protein
kinase that is the first in a series of kinases that are activated in
Receptor kinases phosphorylate each other, activate turn, as each kinase phosphorylates the next in the series. The
intracellular signaling pathways, lead to a response, series of kinases collectively are called the mitogen-activated
and are terminated. protein kinase pathway, or MAP kinase pathway (Fig. 9.15). The
Signaling through receptor kinases follows the same basic final activated kinase in the series enters the nucleus, where it
sequence of events that we saw in signaling though G protein- phosphorylates target proteins. Some of these proteins include
coupled receptors, including receptor activation, signal transcription factors that turn on genes needed for cell division so
transduction, cellular response, and termination. Let’s consider that your cut can heal.
an example. Think about the last time you got a cut. The cut The signals received by receptor kinases are amplified as the
likely bled for a minute or two, and then the bleeding stopped. signal is passed from kinase to kinase. Each phosphorylated kinase
The signaling molecule platelet-derived growth factor (PDGF) in the series activates multiple molecules of the downstream
helps the healing process get started. When platelets in the blood kinase, and the downstream kinase in turn activates many
encounter damaged tissue, they release a number of proteins, molecules of another kinase still farther downstream. In this way,
including PDGF. PDGF is the signaling molecule that binds to a very small amount of signaling molecule (PDGF in our example)
PDGF-specific receptor kinases on the surface of cells at the site of can cause a large-scale response in the cell.
a wound. Receptor kinase signaling is terminated by the same basic
Receptor kinases have an extracellular portion that binds the mechanisms that are at work in G protein-coupled receptor
signaling molecule and an intracellular portion that is a kinase, pathways. For example, protein phosphatases inactivate
an enzyme that transfers a phosphate group from ATP to another receptor kinases and other enzymes of the MAP kinase pathway.
molecule. A single molecule of PDGF binds to the extracellular Furthermore, Ras hydrolyzes GTP to GDP, just like the G protein
portion of two receptors, causing the receptors to partner with a subunit. Shortly after Ras binds to GTP and becomes active, Ras
each other. This partnering of two similar or identical molecules is converts GTP to GDP and becomes inactive. Without an active
called dimerization. Dimerization activates the cytoplasmic kinase receptor kinase to generate more active Ras, activation of the
domains of the paired receptors, causing them to phosphorylate MAP kinase pathway stops.
each other at multiple sites on their cytoplasmic tails (Fig. 9.14).
The addition of these phosphate groups provides places on the ? CASE 2 CANCER: WHEN GOOD CELLS GO BAD
receptor where other proteins bind and become active. How do cell signaling errors lead to cancer?
One of the downstream targets of an activated receptor kinase Many cancers arise when something goes wrong with the way a
is Ras. Ras is a G protein, but in contrast to the three-subunit cell responds to a signal that leads to cell division or, in some cases,
G proteins discussed earlier, Ras consists of a single subunit, when a cell behaves as if it has received a signal for cell division
similar to the a subunit of the three-subunit G proteins. In the when in fact it hasn’t. Defects in cell signaling that lead to cancer
absence of a signal, Ras is bound to GDP and is inactive. However, can take place at just about every step in the cell signaling process.
 CHAPTER 9 CELL SIGNALING 193

(EGF). As a result, the cell is more responsive to the normally low

FIG. 9.15 The MAP kinase pathway. Some receptor kinases signal levels of EGF in the body. The EGF receptor is a receptor kinase,
through Ras, which in turn activates the MAP kinase like the PDGF receptor. Under normal conditions, binding of EGF
pathway, leading to changes in gene expression. to its receptor leads to the controlled division of cells. However, in
cancer, the presence of excess receptors heightens the response of
Signaling Activated Ras
signaling protein the signaling pathway, leading to abnormally high gene expression
molecule
and excess cell division. This is the case, for example, in certain
breast cancers, in which an EGF receptor called HER2/neu is
overexpressed.
Farther down the pathway, mutant forms of the Ras protein
GDP GTP are often present in cancers. One especially harmful mutation
prevents Ras from converting its bound GTP to GDP. The protein
Inactive receptor Active receptor
remains locked in the active GTP-bound state, causing the
sustained activation of the MAP kinase pathway. More than 30%
of all human cancers involve abnormal Ras activity as a result of
one or more mutations in the ras gene.

Signaling pathways are integrated to produce a

Inactive MAP kinase response in a cell.
GTP
In this chapter, we have focused on individual signaling pathways to
Active MAP kinase illustrate the general principles of communication between cells. In
each case, we saw that a cell releases a signaling molecule or in some
ATP cases retains the signaling molecule attached to its surface. The
ADP signaling molecule binds to a cell-surface or intracellular receptor.
A small amount of
signal received by Inactive kinase 2 Binding of the signaling molecule induces a conformational change
receptor kinase is in the receptor, causing it to become active. The activated receptor
amplified when Active kinase 2 in turn causes changes in the interior of the cell, frequently turning
the signal is passed
from kinase to on a signal transduction cascade that is amplified and leads to a
ATP
kinase as each is cellular response. Eventually, the response terminates, and the cell
phosphorylated. ADP
is poised to receive new signals.
Inactive kinase 3 Focusing on each pathway one at a time allowed us to
Active kinase 3 understand how each pathway operates. But in the context of an
entire organism or even a single cell, cell signaling can be quite
complex, with multiple pathways acting at once and interacting
Cytoplasm
with one another.
Multiple types of signaling molecules can bind to a single cell
and activate several signaling pathways simultaneously. In this
case, a cell’s final response depends on how the pathways intersect
with one another. The integration of different signals gives cells a
Changes
in gene wide range of possible responses to their environment. Receiving
expression two different signals may enhance a particular response, such
Transcription
as cell growth, or one signal may inhibit the signaling pathway
factors Nucleus triggered by the other signal, weakening the response.
For example, studies have shown that enzymes in the MAP
kinase pathway can be inhibited by active PKA. Recall that PKA is
activated by elevated cAMP levels associated with the G protein-
In some cases, a tumor may form as the result of the coupled receptor pathway. The regulation of the MAP kinase
overproduction of a signaling molecule or the production of an pathway by PKA illustrates how one signaling pathway can inhibit
altered form of a signaling molecule. In other cases, the source of another.
the problem is the receptor. For example, individuals with some Researchers have started to make use of this molecular
forms of cancer have from 10 to 100 times the normal number of crosstalk. Many patients with breast cancer have elevated MAP
receptors for a signaling molecule called epidermal growth factor kinase activity in their tumor cells. When human breast cancer
194 CO R E CO N C E P T S S U M M A RY

cells are genetically modified to express an activated G protein through Ras to the MAP kinase pathway. As we better understand
a subunit, their growth after transplantation into mice is how different signaling pathways integrate in specific cell types,
significantly inhibited. In addition, in cell culture, elevated cAMP we have a chance to alter the activity of particular pathways and
levels block cells from responding to growth factors that signal ultimately the response of the cell. •

Core Concepts Summary

9.1 Cells communicate primarily by sending and G protein-coupled receptors associate with G proteins,
receiving chemical signals. which relay the signal to the interior of the cell. page 186

There are four essential players in communication between Receptor kinases phosphorylate each other and activate
two cells: a signaling cell, a signaling molecule, a receptor target proteins. page 187
protein, and a responding cell. page 180
Ligand-gated ion channels open in response to a signal,
The signaling molecule binds to its receptor on the allowing the movement of ions across the plasma
responding cell, leading to receptor activation, signal membrane. page 187
transduction and amplification, a cellular response, and
eventually termination of the response. page 181 9.4 G protein-coupled receptors are a large,
conserved family of receptors that often lead to short-
9.2 Cells can communicate over long and short term responses.
distances.
G protein-coupled receptors bound to signaling molecules
Endocrine signaling takes place over long distances and associate with G proteins. page 187
often relies on the circulatory system for transport of
G proteins are active when bound to GTP and inactive
signaling molecules. page 183
when bound to GDP. page 187
Paracrine signaling takes place over short distances
Some G proteins are composed of three subunits, denoted
between neighboring cells and relies on diffusion.
a, b, and γ. When a G protein encounters an activated
page 183
receptor, the a subunit exchanges GDP for GTP, dissociates
Autocrine signaling occurs when a cell signals itself. from the b and γ subunits, and becomes active. page 188
page 184
Signal transduction cascades are amplified in the
Some forms of cell communication depend on direct contact cytosol. page 188
between two cells. page 184
Intracellular, cytosolic signals are short-lived before they
are terminated. page 189
9.3 Signaling molecules bind to and activate cell-
surface and intracellular receptors.
9.5 Receptor kinases are widespread and often lead
A signaling molecule, or ligand, binds specifically to the to long-term responses.
ligand-binding site of the receptor. Binding causes the
Ligand binding to receptor kinases causes them to dimerize.
receptor to undergo a conformational change that activates
The paired receptor kinases phosphorylate each other’s
the receptor. page 185
cytoplasmic domains, leading to activation of intracellular
Receptors for polar signaling molecules, including growth signaling pathways. page 192
factors, are located on the plasma membrane. page 186
The phosphorylated receptors bind other proteins, which in
Receptors for nonpolar signaling molecules, such as turn activate other cytosolic signaling molecules, such
steroid hormones, are located in the cytosol or in the as Ras. page 192
nucleus. page 186
When Ras is activated, the GDP to which it is bound is
There are three major types of cell-surface receptor: released and is replaced by GTP. The active, GTP-bound Ras
G protein-coupled receptors, receptor kinases, and ion binds to and activates the first in a series of kinases.
channels. All act as molecular switches. page 186 page 192
 CHAPTER 9 CELL SIGNALING 195

Cancer can be caused by errors in any step of the signaling 5. Explain how signals can specifically target only some cells,
pathways involved in cell division. page 192 even if they are released into the bloodstream and come
into contact with many cells.
Mutations in receptor kinases and Ras are frequently
associated with human cancers. page 193 6. Describe three different responses of a cell-surface
receptor on binding a signaling molecule and undergoing a
Several signaling pathways can take place simultaneously
conformational change.
in a single cell, and the cellular response depends on the
integration of these pathways. page 193 7. List several responses a cell might have to a signaling
molecule.

Self-Assessment 8. Compare and contrast receptors associated with polar and

nonpolar signaling molecules.
1. Name the four essential elements in cell communication.
9. List three ways in which a signal is amplified in a G
2. Name the steps that occur when a signal binds to a protein-coupled receptor signaling pathway.
receptor on a responding cell.
10. Describe three ways in which the response of a cell to a
3. Describe one way in which endocrine, paracrine, autocrine, signal can be terminated.
and contact-dependent signaling pathways are similar to
one another and one way in which they are different from
one another.
Log in to to check your answers to the Self-
4. Explain how cells respond to external signals, even when Assessment questions, and to access additional learning tools.
those signals cannot enter the cell.
this page left intentionally blank
 CHAPTER 10

Cell and Tissue

Architecture
Cytoskeleton, Cell Junctions,
and Extracellular Matrix

Core Concepts
10.1 Tissues and organs are
communities of cells that
perform specific functions.
10.2 The cytoskeleton is
composed of microtubules,
microfilaments, and
intermediate filaments that
help maintain cell shape.
10.3 Cell junctions connect cells
to one another to form
tissues.
10.4 The extracellular matrix
provides structural support
and informational cues.
Science Source.

197
198 SECTION 10.1 T I S S U E S A N D O RG A N S

A recurring theme in this book, and in all of biology, is that form

and function are inseparably linked. This is true at every level of FIG. 10.1 Diverse cell types. Cells differ in shape and are well
structural organization, from molecules to organelles to cells and adapted for their various functions. Sources: a. Cheryl Power/
to organisms themselves. We saw in Chapter 4 that the function Science Source; b. PROFESSORS P. MOTTA & T. NAGURO/Science

of proteins depends on their shape. In Chapters 7 and 8, we saw Source; c. Innerspace Imaging/Science Source; d. Biophoto Associates/

that the function of organelles like mitochondria and chloroplasts Science Source; e. Don W. Fawcett/Science Source.

depends in part on the large internal surface areas of the inner a

membrane of mitochondria and the thylakoid membrane of
The biconcave shape
chloroplasts. of a red blood cell
The functions of different cell types are also reflected in their maximizes its surface
area for gas exchange
shape and internal structural features (Fig. 10.1). Consider a red and allows it to deform
blood cell. It is shaped like a disk that is slightly indented in the as it passes through the
circulatory system.
middle, and it lacks a nucleus and other organelles (Fig. 10.1a).
This unusual shape and internal organization allow it to deform
readily, so it can pass through blood vessels with diameters smaller
than that of the red blood cell itself. Spherical cells in the liver b
(Fig. 10.1b) that synthesize proteins and glycogen look and
function very differently from long, slender muscle cells
(Fig. 10.1c) that contract to exert force. A neuron (Fig. 10.1d), Hepatocytes (liver cells)
contain large amounts
with its long and extensively branched extensions that of rough ER, shown
communicate with other cells, is structurally and functionally here surrounding the
central nucleus, needed
quite distinct from a cell lining the intestine (Fig. 10.1e) that for protein synthesis.
absorbs nutrients.
In multicellular organisms, cells often exist in communities,
forming tissues and organs. Again, form and function are
intimately linked. Think of the branching structure and large c
surface area of the mammalian lung, which is well adapted for
gas exchange, or the muscular heart, adapted for pumping blood Long, multinucleated
muscle cells have a
through large circulatory systems. striped appearance
In this chapter, we look at what determines cell shape. We also and are specialized for
contraction.
examine the different ways cells connect to one another to build
tissues and organs in multicellular organisms. Finally, we discuss
the physical environment outside cells that is synthesized by cells
and influences their behavior.
d

10.1 TISSUES AND ORGANS Long, slender

extensions of the
Multicellular organisms consist of communities of cells with plasma membrane
allow neurons to
several notable features (see below and Chapter 28). First, and most communicate with
obviously, cells adhere to one another. Most cells in multicellular other cells.
organisms are physically attached to other cells or to substances in
their surroundings. Second, cells communicate with one another.
As we saw in Chapter 9, cells respond to signals from neighboring
cells and the physical environment. In addition, as we will see, cells e
in tissues have pathways for the movement of molecules from one
cell to another. Third, cells are specialized to carry out different Thin, comb-like
functions as a result of development and differentiation. The projections, called
microvilli, on intestinal
different shapes of cells often reflect these specialized functions. cells increase their
absorptive surface
area.
Tissues and organs are communities of cells.
A biological tissue is a collection of cells that work together to
perform a specific function. Animals and plants have tissues that
 CHAPTER 10 C E L L A N D T I S S U E A RC H I T E C T U R E : C Y TO S K E L E TO N , C E L L J U N C T I O N S , A N D E X T R AC E L L U L A R M AT R I X 199

FIG. 10.2 The skin. The skin is a community of cells organized into
two layers—the epidermis and dermis—that together
provide protection for the underlying tissues of the body.
Photo source: Jose Luis Pelaez Inc/Blend Images/Getty Images.

Epidermis
Basal lamina

Dermis

Epidermis

Artery Nerve Vein

Sweat
gland
Keratinocytes Basal
lamina

Melanocyte

allow them to carry out the various processes necessary to sustain The outer layer of skin, the epidermis, serves as a water-resistant,
them. In animals, for example, four types of tissue—epithelial, protective barrier. The layer beneath the epidermis is the dermis.
connective, nervous, and muscle—combine to make up all the This layer of the skin supports the epidermis, both physically
organs of the body. Two or more tissues often combine and and by supplying it with nutrients. It also provides a cushion
function together as an organ, such as a heart or lung. surrounding the body.
Tissues and organs have distinctive shapes that reflect how they As you can see in Fig. 10.2, the epidermis is several cell layers
work and what they do. In the same way, the different cell types thick. Cells arranged in one or more layers are called epithelial
that make up these organs have distinctive shapes based on what cells and together make up a type of animal tissue called epithelial
they do in the organ. In animals, the shape of cells is determined and tissue. Epithelial tissue covers the outside of the body and
maintained by structural protein networks in the cytoplasm called lines many internal structures, such as the digestive tract and
the cytoskeleton (section 10.2). The shape and structural integrity vertebrate blood vessels. The epidermal layer of skin is primarily
of tissues and organs depend on the ability of cells to connect to one composed of epithelial cells called keratinocytes. The epidermis
another. In turn, the connection of cells to one another depends on also contains melanocytes that produce pigment that gives skin its
structures called cell junctions (section 10.3). Equally important coloration.
to a strong, properly shaped tissue or organ is the ability of cells to Keratinocytes in the epidermis are specialized to protect
adhere to a meshwork of proteins and polysaccharides outside the underlying tissues and organs. They are able to perform this
cell called the extracellular matrix (section 10.4). function in part because of their elaborate system of cytoskeletal
filaments. These filaments are often connected to the cell
The structure of skin relates to its function. junctions that hold adjacent keratinocytes together. Cell junctions
To start our investigation of the cytoskeleton, cell junctions, and also connect the bottom layer of keratinocytes to a specialized
the extracellular matrix, let’s consider a community of cells very form of extracellular matrix called the basal lamina (also called
familiar to all of us—our own skin (Fig. 10.2). The structure of the basement membrane, although it is not in fact a membrane),
mammalian skin is tied to its function. Skin has two main layers. which underlies and supports all epithelial tissues.
200 SECTION 10.2 T H E C Y TO S K E L E TO N

The second layer of skin, the dermis, is made up mostly of Microtubules and microfilaments are polymers
connective tissue, a type of tissue characterized by few cells and of protein subunits.
substantial amounts of extracellular matrix. The main type of cell Microtubules are hollow tubelike structures with the largest
in the dermis is the fibroblast, which synthesizes the extracellular diameter of the three cytoskeletal elements, about 25 nm
matrix. The dermis is strong and flexible because it is composed of (Fig. 10.3a). They are polymers of protein dimers. Each dimer is
tough protein fibers of the extracellular matrix. The dermis also made up of two slightly different tubulin proteins, called
has many blood vessels and nerve endings. ␣ (alpha) and ␤ (beta) tubulin. One ␣ tubulin and one ␤ tubulin
We will come back to the skin several times in this chapter combine to make a tubulin dimer and the tubulin dimers are
to show more precisely how these multiple connections among assembled to form the microtubule.
cytoskeleton, cell junctions, and the extracellular matrix make the Microtubules help maintain cell shape and the cell’s internal
skin a watertight and strong protective barrier. structure. In animal cells, microtubules radiate outward to the
cell periphery from a microtubule organizing center called the
centrosome. This spokelike arrangement of microtubules
10.2 THE CYTOSKELETON helps cells withstand compression and thereby maintain their
shape. Many organelles are tethered to microtubules, and thus
Just as the bones of vertebrate skeletons provide internal microtubules guide the arrangement of organelles in the cell.
support for the body, the protein fibers of the cytoskeleton Microfilaments are polymers of actin monomers, arranged to
provide internal support for cells (Fig. 10.3). All eukaryotic form a helix. They are the thinnest of the three cytoskeletal fibers,
cells have at least two cytoskeletal elements, microtubules about 7 nm in diameter, and are present in various locations in the
and microfilaments. Animal cells have a third element, cytoplasm (Fig. 10.3b). They are relatively short and extensively
intermediate filaments. All three of these cytoskeletal elements branched in the cell cortex, the area of the cytoplasm just beneath
are long chains, or polymers, made up of protein subunits. In the plasma membrane. At the cortex, microfilaments reinforce the
addition to providing structural support, microtubules and plasma membrane and organize proteins associated with it.
microfilaments enable the movement of substances within cells as These cortical microfilaments are also important in part
well as changes in cell shape. for maintaining the shape of a cell, such as the biconcave shape

FIG. 10.3 Three types of cytoskeletal element. (a) Microtubule; (b) microfilament; and (c) intermediate filament.

a. Microtubule b. Microfilament c. Intermediate filament

␣-tubulin ␤-tubulin
A microtubule is a A microfilament is An intermediate filament is
hollow tube formed a double helix of a strong fiber composed of
from tubulin dimers. actin monomers. intermediate filament
Tubulin dimer protein subunits. Protein
subunits

Actin
monomer

Microvilli

Centrosome
 CHAPTER 10 C E L L A N D T I S S U E A RC H I T E C T U R E : C Y TO S K E L E TO N , C E L L J U N C T I O N S , A N D E X T R AC E L L U L A R M AT R I X 201

FIG. 10.4 Microfilaments in intestinal microvilli. Actin microfilaments help to maintain the structure of microvilli in the intestine. Photo source:
©1982, Rockefeller University Press. Originally published in The Journal of Cell Biology. 94:425–443.

Band of microfilaments
around the circumference Microvillus
of the cell

Plasma
membrane

Cross-linking
actin-binding
proteins

Actin
microfilaments

Intestinal epithelial cells

of red blood cells discussed earlier (see Fig. 10.1a). The shape of
absorptive epithelial cells such as those in the small intestine FIG. 10.5 Plus and minus ends of microtubules and microfilaments.
is also maintained with the help of microfilaments. In these The two ends assemble at different rates.
cells, bundles of microfilaments are found in microvilli, hairlike Microtubule
projections that extend from the surface of the cell, and longer
Microfilament
bundles of microfilaments form a band that extends around the
circumference of epithelial cells (Fig. 10.4).

Microtubules and microfilaments are dynamic

structures.
When we think of the parts of a skeleton, we are inclined to think
of our bones. Unlike our bones, which undergo remodeling but do The plus

not change rapidly, microtubules and microfilaments are dynamic. + + end

assembles
They become longer by the addition of subunits to their ends, and quickly.
become shorter by the loss of subunits.
The rate at which protein subunits are added depends on the
concentrations of tubulin and actin in that region of the cell. At
high concentrations of subunits, microtubules and microfilaments
can become longer at both ends, although the subunits are The minus
assembled more quickly on one end than the other. The faster- – – end
assembling end is called the plus end and the slower-assembling assembles
slowly.
end is called the minus end (Fig. 10.5).
The dynamic nature of microfilaments and microtubules is
important for some of their functions. For example, some forms
202 SECTION 10.2 T H E C Y TO S K E L E TO N

FIG. 10.6 Dynamic instability. The plus ends of microtubules undergo cycles of rapid disassembly followed by slow assembly.

Centrosome

Polymer-
ization

Depolymerization

of cell movement, such as a single-celled amoeba foraging for food away from the plasma membrane toward the minus end, located
or a mammalian white blood cell chasing down foreign bacteria, at the centrosome in the interior of the cell. Movement along
depend on actin polymerization and depolymerization. It is also microtubules by kinesin and dynein is driven by conformational
required when a single cell divides in two during cytokinesis changes in the motor proteins and is powered by energy harvested
(Chapter 11). from ATP.
Microtubules make up the spindles that attach to Let’s look at an especially striking example of this system at
chromosomes during cell division. In this case, the ability of work in the specialized skin cells called melanophores present in
spindle microtubules to “explore” the space of the cell and some vertebrates. Melanophores are similar to the melanocytes
encounter chromosomes is driven by a unique property of in our own skin that produce the pigment melanin. However,
microtubules: Their plus ends undergo seemingly random cycles of rather than hand off their melanin to other cells as in humans,
rapid depolymerization followed by slower polymerization. These melanophores keep their pigment granules and move them
cycles of depolymerization and polymerization are called dynamic around the cell in response to hormones or neuronal signals. This
instability (Fig. 10.6). They allow spindle microtubules to quickly redistribution of melanin within the cell allows animals such
find and attach to chromosomes during cell division. as fish or amphibians to change color. For example, at night the
melanin granules in the skin of a zebrafish embryo are dispersed
Motor proteins associate with microtubules throughout the melanophores, making it darkly colored. As
and microfilaments to cause movement. morning comes and the day brightens, the pigment granules
A motor is a device that imparts motion. We saw that microtubules aggregate at the center of the cell around the centrosome, causing
and microfilaments have some capacity to lengthen and shorten the embryo’s color to lighten (Fig. 10.8).
by polymerization and depolymerization. However, when joined The melanin granules in the melanophores move back and
by small accessory proteins called motor proteins, microtubules forth along microtubules, transported by kinesin and dynein.
and microfilaments are capable of causing amazing movements. Kinesin moves the granules out toward the plus end of the
For example, microtubules function as tracks for transport microtubule during dispersal, and dynein moves them back toward
within the cell. Two motor proteins that associate with these the minus end during aggregation. The daytime and nighttime
microtubule tracks are kinesin and dynein. Kinesin transports camouflage provided by this color change helps prevent young,
cargo toward the plus end of microtubules, located at the developing organisms from being spotted by hungry predators
periphery of the cell (Fig. 10.7). By contrast, dynein carries its load lurking below.
 CHAPTER 10 C E L L A N D T I S S U E A RC H I T E C T U R E : C Y TO S K E L E TO N , C E L L J U N C T I O N S , A N D E X T R AC E L L U L A R M AT R I X 203

FIG. 10.7 Intracellular transport. The motor protein kinesin FIG. 10.8 Color change in zebrafish embryos, driven by motor
interacts with microtubules to move vesicles in the cell. proteins kinesin and dynein. Melanin granules are
redistributed along microtubules in the melanophores of
the skin. Photo sources: (left and right) Jane Bradbury, “Small Fish, Big
Kinesin carries cargo, such as vesicles, toward the
plus ends of microtubules. Science”, PLOS Biology, 2, (5), May 2004, pg. 0569, Image courtesy of
Adam Amsterdam, Massachusetts Institute of Technology, Boston, MA;
(bottom) Courtesy Darren Logan, Wellcome Trust Sanger Institute.

Kinesin Microtubule

– +

ATP

ATP

Night (dark) Day (light)

ATP

In the dark, melanin granules are In the light, melanin granules are
dispersed outward by kinesin, aggregated toward the center by
causing the embryo to be darkly dynein, causing the embryo to be
ATP colored. lightly colored.
204 SECTION 10.2 T H E C Y TO S K E L E TO N

FIG. 10.9 Cilia and flagella in diverse cell types. Cilia and flagella, composed of microtubules, move cells or allow cells to propel substances.
Sources: (from left to right) SPL/Science Source; Eye of Science/Science Source; Andrew Syred/Science Source; Juergen Berger/Science Source.

Coordinated beating of the The cilia in these human airway These unicellular algae are Some cells of multicellular
cilia that cover the epithelial cells propel mucus propelled by two flagella. organisms, including these
paramecium moves the cell containing debris out of the lungs. sperm cells, swim by
through its environment. movement of a flagellum.

j Quick Check 1 Would a defect in dynein or in kinesin cause a We have seen that different cell types use the same tubulin
zebrafish embryo to remain darkly colored after daybreak? dimers to form microtubules and the same actin monomers
to form microfilaments. By contrast, the proteins making up
In addition to providing tracks for the transport of material
intermediate filaments differ from one cell type to another. For
within the cell, microtubules are found in cilia and flagella,
example, in epithelial cells, these protein subunits are keratins;
fiberlike organelles that propel the movement of cells or
in fibroblasts, they are vimentins; and in neurons, they are
substances surrounding the cell (Fig. 10.9). In these organelles,
neurofilaments. Some intermediate filaments, called lamins, are
microtubules associate with the motor protein dynein, which
even found inside the nucleus, where they provide support for the
causes movement. Many single-celled eukaryotic organisms that
nuclear envelope (Fig. 10.10). There are well over 100 different
live in aquatic environments propel themselves through the water
kinds of intermediate filaments.
by means of the motion of cilia (which are short) or flagella (which
Once assembled, many intermediate filaments become
are long). Some cells of multicellular organisms also have cilia or
attached to cell junctions at their cytoplasmic side, providing
flagella. For example, the sperm cells of algae, some plants, and
many animals are propelled by one or more flagella. Epithelial cells
in a number of animal tissues, such as the lining of the trachea and
the upper respiratory tract, have cilia that move substances along FIG. 10.10 Intermediate filaments in the cytoplasm and the
the surface of the cell layer. nucleus of a cell. Source: Courtesy of R. D. Goldman.
Like microtubules, microfilaments also associate with motor
proteins to produce movement. Actin microfilaments associate
with myosin to transport various types of cellular cargo, such
Intermediate filaments
as vesicles, inside of cells. Furthermore, microfilaments are composed of keratins
responsible for changes in the shape of many types of cell. One of in the cytoplasm are
the most dramatic examples of cell shape change is the shortening stained red.

(contraction) of a muscle cell. Muscle contraction depends on the

interaction of myosin with microfilaments, and is powered by ATP Intermediate
filaments composed
(Chapter 37). of lamins in the
nucleus are stained
Intermediate filaments are polymers of proteins that blue.

vary according to cell type.

The intermediate filaments of animal cells have a diameter
that is intermediate between those of microtubules and
microfilaments, about 10 nm (see Fig. 10.3c). They are polymers
of intermediate filament proteins that combine to form strong,
cable-like structures in the cell. As a result, they provide cells with
mechanical strength.
 CHAPTER 10 C E L L A N D T I S S U E A RC H I T E C T U R E : C Y TO S K E L E TO N , C E L L J U N C T I O N S , A N D E X T R AC E L L U L A R M AT R I X 205

FIG. 10.11 Intermediate filaments in the epidermis of the skin. Intermediate filaments bind to cell junctions called desmosomes, forming
a strong, interconnected network. Epidermolysis bullosa is a group of blistering diseases of the skin that can be caused by a defect in
intermediate filaments. Source: Helen Osler/Northscot/Rex USA.

Epidermis

Keratinocytes Basal
lamina

Melanocyte

Intermediate
filaments

Cell junction

Intermediate filaments
linked to cell junctions
in keratinocytes provide
a great deal of structural Defects in intermediate
support to the skin. filaments make the skin far less
resistant to physical stress.

strong support for the cells (Fig. 10.11). In the case of epithelial section is sometimes recommended in cases where the disease is
cells, this anchoring results in structural continuity from one cell diagnosed during pregnancy.
to another that greatly strengthens the entire epithelial tissue.
This is especially important for tissues that are regularly subject to The cytoskeleton is an ancient feature of cells.
physical stress, such as the skin and the lining of the intestine. Actin and tubulin are found in all eukaryotic cells, and their
Genetic defects that disrupt the intermediate filament structure and function have remained relatively unchanged
network can have severe consequences. For example, some throughout the course of evolution. The amino acid sequences
individuals with epidermolysis bullosa, a group of rare genetic of yeast tubulin and human tubulin are 75% identical. Similar
diseases, have defective keratin genes. Intermediate filaments comparisons of actin from amoebas and animals show that they
do not polymerize properly in these individuals, thus forming are 80% identical after close to a billion years of evolution. In
weaker connections between the layers of cells that make up the fact, a mixture of yeast and human actin monomers forms hybrid
epidermis. As a consequence, the outer layers can detach, resulting microfilaments able to function normally in the cell.
in extremely fragile skin that blisters in response to the slightest Not long ago, it was believed that cytoskeletal proteins were
trauma (Fig. 10.11). The sensitivity to physical stress is so extreme present only in eukaryotic cells. However, a number of studies
that infants with epidermolysis bullosa often suffer significant have shown that many prokaryotes also have a system of proteins
damage to the skin during childbirth. Therefore, Caesarean similar in structure to the cytoskeletal elements of eukaryotic
206 SECTION 10.3 CELL JUNCTIONS

TABLE 10.1 Major Functions of Cytoskelatal Elements.

CYTOSKELETAL ELEMENT SUBUNITS MAJOR FUNCTIONS

Microtubules Tubulin dimers Cell shape and support

Cell movement (by cilia, flagella)
Cell division (chromosome segregation)
Vesicle transport
Organelle arrangement

Microfilaments Actin monomers Cell shape and support

Cell movement (by crawling)
Cell division (cytokinesis)
Vesicle transport
Muscle contraction

Intermediate filaments Diverse Cell shape and support

cells and are involved in similar processes, including the separation more closely at the roles of the various types of cell junction in
of daughter cells during cell division. Interestingly, at least one tissues, as well as their interaction with the cytoskeleton.
of these prokaryotic cytoskeleton-like proteins is expressed in
the chloroplasts and mitochondria of some eukaryotic cells. The Cell adhesion molecules allow cells to attach to other
presence of this protein in these organelles lends support to the cells and to the extracellular matrix.
theory that chloroplasts and mitochondria were once independent In 1907, American embryologist H. V. Wilson discovered that if
prokaryotic cells that developed a symbiotic relationship with he pressed a live sponge through fine cloth he could break up the
another cell. This idea is called the endosymbiotic theory and it is sponge into individual cells. Then if he swirled the cells together,
discussed in Chapter 27. they would coalesce back into a group resembling a sponge. If he
The major functions of microtubules, microfilaments, and swirled the cells from sponges of two different species together, he
intermediate filaments are summarized in Table 10.1. observed that the cells sorted themselves out—that is, cells from
one species of sponge associated only with cells from that same
species (Fig. 10.12a). Fifty years later, German-born embryologist
10.3 CELL JUNCTIONS Johannes Holtfreter observed that if he took neuronal cells and
skin cells from an amphibian embryo and treated them the same
The cell is the fundamental unit of living organisms (Chapter 1). way that Wilson had treated sponge cells, the embryonic cells
Some estimates place the number of cells in an adult human being would sort themselves according to tissue type (Fig. 10.12b).
at between 50 and 75 trillion, whereas others place the number Cells are able to sort themselves because of the presence of
at well above 100 trillion. Whichever estimate is more accurate, various proteins on their surface called cell adhesion molecules
humans, as well as all complex multicellular organisms, are made that attach cells to one another or to the extracellular matrix.
up of a lot of cells! What then keeps us (or any other multicellular While a number of cell adhesion molecules are now known, the
organism) from slumping into a pile of cells? And what keeps cells cadherins (calcium-dependent adherence proteins) are especially
organized into tissues, and tissues into organs? important in the adhesion of cells to other cells. There are many
Tissues are held together and function as a unit because of cell different kinds of cadherin, and a given cadherin may bind only
junctions. Cell junctions physically connect one cell to the next to another cadherin of the same type. This property explains
and anchor cells to the extracellular matrix. Some tissues have cell Holtfreter’s observations of the cells from amphibian embryos.
junctions that perform roles other than adhesion. For example, cell E-cadherin (for “epidermal cadherin”) is present on the surface of
junctions in the outer layer of the skin and the lining of intestine embryonic epidermal cells, and N-cadherin (for “neural cadherin”)
provide a seal so that the epithelial sheet can act as a selective is present on neuronal cells. The epidermal cells adhered to one
barrier. Other cell junctions allow communication between adjacent another through E-cadherin, and the neuronal cells adhered to
cells so that they work together as a unit. In this section, we look each other through N-cadherin.
 CHAPTER 10 C E L L A N D T I S S U E A RC H I T E C T U R E : C Y TO S K E L E TO N , C E L L J U N C T I O N S , A N D E X T R AC E L L U L A R M AT R I X 207

FIG. 10.12 Cell type–specific cell adhesion. Experiments showed that cells from (a) sponges and from (b) amphibian embryos are in each
case able to adhere to one another in a specific manner. Photo sources: a. (top) Borut Furlan/WaterFrame/age fotostock; (bottom) Franco Banfi/
WaterFrame/age fotostock.

a. Sponge

Sponges were Individual cells from The cells from each

broken up into both sponges were sponge sorted
individual cells. swirled together. themselves out.

b. Amphibian embryo

Tissues were Individual cells from The cells from each tissue
broken up into both tissues were sorted themselves out
individual cells. mixed together. and adhered to form new
tissue.

Epidermal
cells
Neural plate Neural
cells cells
Cross
section
Early
epidermal
cells

Outside

Cadherins are transmembrane proteins (Chapter 5). The proteins, and their cytoplasmic domain is linked to microfilaments
extracellular domain of a cadherin molecule binds to the or intermediate filaments (Fig. 10.13b). Also like the cadherins,
extracellular domain of a cadherin of the same type on an integrins are of many different types, each binding to a specific
adjacent cell. The cytoplasmic portion of the protein is extracellular matrix protein. Integrins are present on the
linked to the cytoskeleton, including microfilaments and surface of virtually every animal cell. In addition to their role
intermediate filaments (Fig. 10.13a). This arrangement provides in adhesion, integrins also act as receptors that communicate
structural continuity from the cytoskeleton of one cell to information about the extracellular matrix to the interior of the
the cytoskeleton of another, increasing the strength of tissues cell (section 10.4)
and organs.
As well as being stably connected to other cells, cells also Anchoring junctions connect adjacent cells and are
attach to proteins of the extracellular matrix. Cell adhesion reinforced by the cytoskeleton.
molecules that enable cells to adhere to the extracellular matrix Cadherins and integrins are often organized into cell junctions,
are called integrins. Like cadherins, integrins are transmembrane complex structures in the plasma membrane that allow cells to
208 SECTION 10.3 CELL JUNCTIONS

the desmosomes of adjacent cells.

FIG. 10.13 Cell adhesion by cadherins and integrins. (a) Cadherins are transmembrane proteins The cytoplasmic domain of these
that connect cells to other cells; (b) integrins are transmembrane proteins that connect cadherins connects to intermediate
cells to the extracellular matrix. filaments in the cytoskeleton. This
second type of physical connection
among neighboring cells greatly
Cell 1 enhances the structural integrity of
epithelial cell layers.
Epithelial cells are not only
Cell 2 attached to one another, but also to
the underlying extracellular matrix
(specifically, the basal lamina). In this
case, the cells are firmly anchored to
the extracellular matrix by a type of
desmosome called a hemidesmosome
a. Cadherins b. Integrins
(Fig. 10.14). Integrins are the
Microfilament
Cytosol Cytosol prominent cell adhesion molecules in
Microfilament hemidesmosomes. The extracellular
Cell 1
membrane Intracellular domains bind extracellular matrix
domain proteins, and the cytoplasmic domains
Extracellular
domain Cell connect to intermediate filaments.
Integrin membrane These intermediate filaments connect
Transmembrane
Cadherin domain
dimer to desmosomes in other parts of the
Extracellular
domain plasma membrane. The result is a
Cell 2 firmly anchored and reinforced layer
membrane Intermediate
filament
of cells.
Intracellular Extracellular
Cytosol domain matrix j Quick Check 2 Adherens
junctions and desmosomes both
attach cells to other cells and are
made up of cadherins. How, then,
are they different?

adhere to one another. These anchoring cell junctions are of two Tight junctions prevent the movement of substances
types: adherens junctions and desmosomes (Fig. 10.14). through the space between cells.
In our earlier discussion of microfilaments (section 10.2), we Epithelial cells form sheets or boundaries that line tissues and
saw that a long bundle of actin microfilaments forms a band that organs, including the digestive tract, respiratory tract, and outer
extends around the circumference of epithelial cells, such as the layer of the skin. Like any effective boundary, a layer of epithelial
epithelial cells that line the intestine. This band of actin is attached cells must limit or control the passage of material across it.
to the plasma membrane by cadherins in a beltlike structure called Adherens junctions and desmosomes provide strong adhesion
an adherens junction (Fig. 10.14). The cadherins in the adherens between cells, but they do not prevent materials from passing
junction of one cell attach to the cadherins in the adherens freely through the spaces between the cells. This function is
junctions of adjacent cells. This arrangement establishes a physical provided by a different type of cell junction. In vertebrates, these
connection among the actin cytoskeletons of all cells present in an are called tight junctions (Fig. 10.14). Tight junctions establish
epithelial layer of cells. a seal between cells so that the only way a substance can travel
Like adherens junctions, desmosomes are cell junctions that from one side of a sheet of epithelial cells to the other is by
allow cells to adhere to one another. However, unlike adherens moving through the cells by means of one of the cellular transport
junctions that form a belt around the circumference of cells, mechanisms discussed in Chapter 5.
desmosomes are buttonlike points of adhesion (Fig. 10.14). A tight junction is a band of interconnected strands of integral
Cadherins are at work here, too, strengthening the connection membrane proteins, particularly proteins called claudins and
between cells in a manner similar to adherens junctions. occludins. Like adherens junctions, tight junctions encircle the
Cadherins in the desmosome of one cell bind to cadherins in epithelial cell. The proteins forming the tight junction in one cell
 CHAPTER 10 C E L L A N D T I S S U E A RC H I T E C T U R E : C Y TO S K E L E TO N , C E L L J U N C T I O N S , A N D E X T R AC E L L U L A R M AT R I X 209

FIG. 10.14 Cell junctions. Cell junctions connect cells to other cells or to the basal lamina and are reinforced by the cytoskeleton.

Tight junction
Cell 1
membrane
Cell 2
membrane
Extracellular
space
Tight
junction
protein

Adherens junction

Cadherin
proteins

Microfilaments

Desmosome

Intermediate
filaments

Cadherin
proteins

Gap junction

Gap junction
channel

Molecule
passing
from one cell
to another

Hemidesmosome

Epithelial
Intermediate
cells
filaments
Cell 1
Integrin
Cell 2 Basal lamina Basal
lamina
Connective
tissue Connective
tissue
210 SECTION 10.4 T H E E X T R AC E L L U L A R M AT R I X

TABLE 10.2 Types and Functions of Cell Junctions.

CELL JUNCTION MAJOR COMPONENT CYTOSKELETAL ATTACHMENT PRIMARY FUNCTION

Anchoring
Adherens junction Cadherins Microfilaments Cell–cell adhesion
Desmosome Cadherins Intermediate filaments Cell–cell adhesion
Hemidesmosome Integrins Intermediate filaments Cell–extracellular matrix adhesion

Barrier
Tight junction Claudins, occludins Epithelial boundary

Communicating
Gap junction Connexins Communication between animal cells
Plasmodesma Cell membrane Communication between plant cells

bind to the proteins forming the tight junctions in adjacent cells. Plasmodesmata (the singular form is plasmodesma) are
Also like adherens junctions, tight junctions connect to actin passages through the cell walls of adjacent plant cells. They are
microfilaments. similar to gap junctions in that they allow cells to exchange ions
Cells that have tight junctions have two sides because the tight and small molecules directly, but the similarity ends there. In
junction divides the plasma membrane into two distinct regions plasmodesmata, the plasma membranes of the two connected cells
(Fig. 10.14). The portion of the plasma membrane in contact with are actually continuous. The size of the opening is considerably
the lumen, or the inside of any tubelike structure like the gut, is larger than that of gap junctions, large enough for cells to transfer
called the apical membrane. The apical membrane defines the “top” RNA molecules and proteins, an ability that is especially important
side of the cell. The rest of the plasma membrane is the basolateral during embryonic development. Plasmodesmata allow plant cells
membrane, which defines the bottom (“baso”) and sides (“lateral”) to send signals to one another despite being enclosed within rigid
of the cell. These two regions of the plasma membrane are of cell walls.
different composition because the tight junction prevents lipids and From this discussion, we see that cell junctions interact to
proteins in the membrane on one side of the junction from diffusing create stable communities of cells in the form of tissues and
to the other side. As a result, the apical and basolateral membranes organs. These cell junctions are important for the functions of
of a cell are likely to have different integral membrane proteins, tissues, allowing cells to adhere to each other and the extracellular
which causes them to be functionally different as well. In the small matrix, act as a barrier, and communicate rapidly. The types and
intestine, for example, glucose is transported from the lumen into functions of the cell junctions are summarized in Table 10.2.
intestinal epithelial cells by transport proteins on the apical side of
the cells, and is transported out of the cells into the circulation by j Quick Check 3 Which type(s) of cell junction prevent(s)
facilitated diffusion through a different type of glucose transporter substances from moving through the space between cells? Which
restricted to the basolateral sides of the cells (Chapter 40). type(s) of cell junction attach(es) cells to one another?

Communicating junctions allow the passage of

molecules between cells. 10.4 THE EXTRACELLULAR MATRIX
Not all cell junctions are involved in the adhesion of cells to each
other or in sealing a layer of cells. Gap junctions (Fig. 10.14) of Up to this point, we have looked at how the cytoskeleton
animal cells and plasmodesmata of plant cells (see Fig. 5.17b) maintains the shape of cells. We have also seen how the stable
permit materials to pass directly from the cytoplasm of one cell association of animal cells with one another and with the
to the cytoplasm of another. Gap junctions are a complex of extracellular matrix is made possible by cell junctions, and that
integral membrane proteins called connexins arranged in a these junctions are reinforced by the cytoskeleton. As important
ring. The ring of connexin proteins connects to a similar ring of as the cytoskeleton and cell junctions are to the structure of
proteins in the membrane of an adjacent cell. Ions and signaling cells and tissues, it is the extracellular matrix that provides the
molecules pass through these junctions, allowing cells to molecular framework that ultimately determines the structural
communicate. In the heart, for example, ions pass though gap architecture of plants and animals.
junctions connecting cardiac muscle cells. This rapid electrical The extracellular matrix is an insoluble meshwork composed
communication allows the muscle cells to beat in a coordinated of proteins and polysaccharides. Its components are synthesized,
fashion (Chapter 39). secreted, and modified by many different cell types. There
 CHAPTER 10 C E L L A N D T I S S U E A RC H I T E C T U R E : C Y TO S K E L E TO N , C E L L J U N C T I O N S , A N D E X T R AC E L L U L A R M AT R I X 211

are many different forms of extracellular matrix, which differ middle lamella is synthesized first, during the late stages of cell
in the amount, type, and organization of the proteins and division. It is composed of a gluelike complex carbohydrate, and
polysaccharides that make them up. In both plants and animals, it is the main mechanism by which plant cells adhere to one
the extracellular matrix not only contributes structural support another. The primary cell wall is formed next and consists mainly
but also provides informational cues that determine the activity of of cellulose, but it also contains a number of other molecules,
the cells that are in contact with it. including pectin. The primary cell wall is laid down while the cells
are still growing. It is assembled by enzymes on the surface of the
The extracellular matrix of plants is the cell wall. cell and remains thin and flexible. Once cell growth has stopped,
The paper we write on, the cotton fibers in the clothes we wear, the secondary cell wall is constructed in many, but not all, plant
the wood in the chairs we sit on are, in fact, the extracellular cells. It also is made largely of cellulose but in addition contains
matrix of plants. In plants, the extracellular matrix forms the a substance called lignin. Lignin hardens the cell wall and makes
cell wall, and the main component of the plant cell wall is the it water resistant. In woody plants, the cell wall can be up to 25%
polysaccharide cellulose (Chapters 2 and 5). Its presence in the cell lignin. The rigid secondary cell wall permits woody plants to grow
wall of every plant makes cellulose the most widespread organic to tremendous heights. Giant sequoia trees grow to more than
macromolecule on Earth. 300 feet and are supported entirely by the lignin-reinforced
The plant cell wall represents possibly one of the most cellulose fibers of the interconnected cell walls.
complex examples of an extracellular matrix. It is certainly one of As a plant cell grows, additional cell wall components must be
the most diverse in the functions it performs. Cell walls maintain synthesized to expand the area of the wall. Unlike the extracellular
the shape and turgor pressure of plant cells and act as a barrier matrix components that are secreted by animal cells, the cellulose
that prevents foreign materials and pathogens from reaching the polymer is assembled outside the cell, on the extracellular surface
plasma membrane. In many plants, cell walls collectively serve as a of the plasma membrane. Both the glucose monomers that form
skeletal support structure for the entire plant. the polymer and the enzymes that attach them are delivered to
The plant cell wall is composed of as many as three layers: the the cell surface by arrays of microtubules. Here is yet another
outermost middle lamella, the primary cell wall, and the secondary example of how the cytoskeleton plays an indispensable role in
cell wall, located closest to the plasma membrane (Fig. 10.15). The regulating the shape of a cell.

FIG. 10.15 The three layers of the plant cell wall: middle lamella, primary cell wall, and secondary cell wall. The major component of the plant
cell wall is cellulose, a polymer of glucose. Photo source: Biophoto Associates/Science Source.
Primary cell wall

Cellulose Secondary cell wall

Plasma
Lignin membrane

Middle lamella
Primary cell
wall
Secondary cell
wall
Plasma
membrane
212 SECTION 10.4 T H E E X T R AC E L LU L A R M AT R I X

The main type of cell in the dermis is the fibroblast. Fibroblasts

FIG. 10.16 Animal connective tissue. Connective tissue is synthesize most of the extracellular matrix proteins.
composed of protein fibers in a gel-like polysaccharide Connective tissue is unusual compared to other tissue types
matrix. Photo source: Biophoto Associates/Science Source. in that it is dominated by the extracellular matrix and has a low
cell density. Consequently, the extracellular matrix determines
Cells in the connective tissue of
the properties of different types of connective tissue. Other
animals reside in a complex meshwork
of extracellular matrix fibers. more specialized types of animal connective tissue include bone,
cartilage, and tendon.
Collagen is the most abundant protein in the extracellular matrix
Elastin of animals. There are more than 20 different forms of collagen, and in
fibers humans collagen accounts for almost a quarter of the protein present
Collagen in the body. Over 90% of this collagen is type I collagen, which is
fibers present in the dermis of your skin, where it provides strong, durable
Fibroblast support for the overlying epidermis. The tendons that connect your
nuclei muscles to bones and the ligaments that connect your bones to
Elastin
other bones are able to withstand the physical stress placed on them
fibers because they are made up primarily of collagen.
Collagen’s strength is related to its structure. Like a rope or
Collagen
fibers a cable, collagen is composed of intertwined fibers that make it

Fibroblast
cell
FIG. 10.17 Collagen. Type I collagen molecules are organized in a
Fibroblast
nucleus triple helix and grouped into bundles called fibrils, which
in turn are grouped into bundles called fibers. This type
Gel-like
matrix of arrangement, seen in fibers and steel cables, imparts
tremendous strength. Photo sources: (top to bottom) Tom Grundy/
Alamy; Egon Bömsch/imageBROKER/age fotostock; Eye of Science/
The extracellular matrix is abundant in Science Source.
connective tissues of animals.
The extracellular matrix of animals, like
that of plants, is a mixture of proteins and
polysaccharides secreted by cells. The animal
extracellular matrix is composed of large
fibrous proteins, including collagen, elastin, Collagen
and laminin, which impart tremendous tensile molecules
(triple helices)
strength. These fibrous proteins are embedded
in a gel-like polysaccharide matrix. The matrix
is negatively charged, attracting positively
charged ions and water molecules that provide
Collagen
protection against compression and other fibrils
physical stress. Collagen
Collagen molecules are
The extracellular matrix can be found in bundled like steel
fiber
abundance in animal connective tissue (Fig. wires in a cable.
10.16). Connective tissue structurally integrates
and supports various parts of the body and,
in this way, is necessary for multicellularity.
All animals express similar connective tissue
proteins, highlighting their importance and
evolutionarily conserved function. Connective
tissue underlies all epithelial tissues, as we have
seen (section 10.1). For example, the dermis of
the skin is connective tissue. It provides support
and nutrients to the overlying epidermis.
 CHAPTER 10 C E L L A N D T I S S U E A RC H I T E C T U R E : C Y TO S K E L E TO N , C E L L J U N C T I O N S , A N D E X T R AC E L L U L A R M AT R I X 213

much stronger than if it were a single fiber of the same diameter. for a cell to metastasize, it must enter and leave the bloodstream
A collagen molecule consists of three polypeptides wound around through capillaries or other vessels. Since all blood vessels,
one another in a triple helix. A bundle of collagen molecules forms including capillaries, have a basal lamina, a metastatic tumor cell
a fibril, and the fibrils are assembled into fibers (Fig. 10.17). Once needs to cross a basal lamina at least twice—once on the way into
multiple collagen fibers are assembled into a ligament or tendon, the bloodstream and again on the way out (Fig. 10.19). Since cells
the final structure is incredibly strong. attach to basal lamina proteins by means of integrins, many studies
The basal lamina is a specialized layer of extracellular have compared the integrins in metastatic and non-metastatic cells
matrix that is present beneath all epithelial tissues, including in the search for potential targets for treatment.
the lining of the digestive tract, epidermis of the skin, and
endothelial cells that line the blood vessels of vertebrates
(Fig. 10.18). The role of the basal lamina is to provide a structural
foundation for these epithelial tissues. The basal lamina is made FIG. 10.19 Metastatic cancer cells. Some cancer cells spread from
of several proteins, including a special type of collagen. The triple- the original site of cancer formation to the bloodstream
helical structure of collagen provides flexible support to and then to distant organs of the body.
the epithelial sheet.
Tumor within normal tissue

? CASE 2 CANCER: WHEN GOOD CELLS GO BAD

How do cancer cells spread throughout the body?
Nonmalignant, or benign, tumors are encapsulated masses of cells
that divide continuously because regulation of cell division has gone
awry (Chapter 11). As the tumor grows, its border pushes outward
against adjacent tissues. Benign tumors are rarely life threatening
unless the tumor interferes with the function of a vital organ.
Malignant tumors are more dangerous. They contain some Tumor cells
cells that can metastasize, that is, break away from the main tumor
Metastatic cancer cells
and colonize distant sites in the body. Metastatic tumor cells have cross the capillary basal
an enhanced ability to adhere to extracellular matrix proteins, lamina twice, once to enter
Basal lamina
the circulatory system
especially those in the basal lamina. This is significant because and again to leave it.

FIG. 10.18 The basal lamina. The basal lamina, found beneath
epithelial tissue, is a specialized form of extracellular
matrix. Photo source: ISM/Phototake. Metastatic cancer
cells form new
tumors at distant
sites in the body.

Epithelial
tissue
Extracellular
matrix proteins
in basal lamina
Integrins
Metastatic
cancer cell

Basal
lamina Some metastatic
cancer cells cross
the basal lamina by
means of specific
Connective types of integrins.
tissue
214 SECTION 10.4 T H E E X T R AC E L LU L A R M AT R I X

In some types of cancer, the number of specific integrins on

the cell surface is an indicator of metastatic potential. Melanoma FIG. 10.21 Cell shape determined by composition of the
provides an example. A specific type of integrin is present in high extracellular matrix. Neurons adopt different shapes
amounts on metastatic melanoma cells but is absent on non- depending on whether or not they are cultured with
metastatic cells from the same tumor. In laboratory tests, blocking the extracellular matrix protein laminin. Source: Courtesy
these integrins eliminates the melanoma cell’s ability to cross an Motoyoshi Nomizu.

artificial basal lamina. Drugs targeting this integrin protein are

currently in clinical trials.

Extracellular matrix proteins influence cell shape

and gene expression.
Cells continue to interact with the extracellular matrix long after
they have synthesized it or moved into it, and these interactions
can have profound effects on cell shape and gene expression. Some
of these cellular responses are the result of interactions between Neurons maintained in the Neurons cultured on a
the extracellular matrix and integrins on the cell surface. The absence of laminin attach to surface coated with laminin
the surface but do not take on develop axon-like extensions
integrins act as receptors that relay the signal to the interior of the
the appearance of a nerve cell. of the plasma membrane.
cell as the first step in a signal transduction pathway, like those
discussed in Chapter 9. Biologists have observed the results of
these interactions in experiments conducted with cells grown in
culture in the laboratory.
two-dimensional surface coated with extracellular matrix proteins
An example shows how the structure of the extracellular
attach to the matrix and flatten out as they maximize their
matrix can influence the shape of cells. Fibroblasts cultured on a
adhesion to the matrix. By contrast, the same cells cultured in a
three-dimensional gel of extracellular matrix look and behave like
the spindle-shaped, highly migratory fibroblasts present in living
FIG. 10.20 Cell shape determined by the structure of the
connective tissue (Fig. 10.20).
extracellular matrix. Fibroblasts adopt different shapes
A second example shows how the composition of the
depending on whether they are grown on a two-
extracellular matrix can influence cell shape. When nerve cells are
dimensional or a three-dimensional matrix. Photo source:
grown in culture on a plastic surface, they attach to the surface
F. Tao, S. Chaudry, B. Tolloczko, J. G. Martin, and S. M. Kelly Modulation
of the dish but do not take on a neuron-like shape. However,
of smooth muscle phenotype in vitro by homologous cell substrate Am J
when these cells are grown on the same surface coated with the
Physiol Cell Physiol June 1, 2003 284:(6) C1531-C1541; published
extracellular matrix protein laminin, they develop long extensions
ahead of print March 5, 2003, doi:10.1152/ajp.
that resemble the axons and dendrites of normal nerve cells
(Fig. 10.21).
Fibroblasts grown on a two-
dimensional matrix attach In addition to influencing cell shape, the structure and
and become flattened. composition of the extracellular matrix can influence gene
expression of the cells that are grown in it. When stimulated by
the milk-inducing hormone prolactin, mouse mammary epithelial
cells express the genes for milk proteins. These cells grown in
culture on plain glass coated with collagen remain alive and
apparently healthy, and even make and secrete a small amount
On glass
of milk proteins, including β-casein. However, if the mammary
Fibroblasts grown in a epithelial cells are grown in three-dimensional collagen gels, they
three-dimensional matrix synthesize and secrete up to 10 times more β-casein.
are spindle shaped and
look as they do in vivo. j Quick Check 4 Do you think cadherins or integrins are
responsible for the dependence of mammary cells on the
extracellular matrix for their ability to produce milk proteins? Why?

An experiment that explores the importance of the

composition of the extracellular matrix proteins in the regulation
of gene expression is described in Fig. 10.22. This experiment, and
the others described here, demonstrate that there is a dynamic
interplay between the extracellular matrix and the cells that
In suspension synthesize it. •
HOW DO WE KNOW?

FIG. 10.22
1.00

Relative level of albumin mRNA

On collagen
Can extracellular matrix 0.80
On collagen with
matrix proteins

proteins influence gene 0.60

expression? 0.40

0.20
BACKGROUND The adhesion of cells to the extracellular
matrix is required for cell division, DNA synthesis, and proper 48 72 96 120 144 166
cell shape. Research by Iranian-American cell biologist Mina Time (h)
Bissell and colleagues indicated that a cell’s interaction with
extracellular matrix proteins influences gene expression. Bissell
100
discovered that mammary cells synthesize and secrete high levels
of the milk protein b-casein when grown in a three-dimensional

(µg/ml medium/24 h)
80

Albumin secretion
collagen matrix but not in a two-dimensional collagen matrix.
American cell biologist Joan Caron followed up these studies 60
using liver cells (hepatocytes).
40
HYPOTHESIS Caron hypothesized that a specific protein in the
extracellular matrix is necessary for the expression of the protein 20
albumin from hepatocytes grown in culture. Albumin is a major
product of hepatocytes. 48 72 96 120 144 166
EXPERIMENT Caron cultured hepatocytes on a thin layer of Time (h)
type I collagen, which does not induce albumin synthesis. Next,
she added a mixture of several different extracellular matrix 1.00 72 h after plating
proteins to the culture and looked for changes in albumin gene 120 h after plating
expression and protein secretion into the media. She then
Relative level of albumin mRNA

0.80
tested individual extracellular matrix proteins from the mixture
to see which one was responsible for the increase in albumin
gene expression. 0.60

RESULTS Caron found that when she cultured cells on

collagen with a combination of three extracellular matrix 0.40
proteins—laminin, type IV collagen, and heparin sulfate
proteoglycan (HSPG)—the cells synthesized albumin mRNA
0.20
and secreted albumin protein for several weeks, but if she
cultured the cells on collagen alone, they did not (top and
middle graphs). In addition, when she tested individual
No matrix Laminin, Laminin Type IV HSPG
extracellular matrix proteins, she found that laminin, but not proteins type IV alone collagen alone
any of the other proteins, caused an increase in albumin gene collagen, alone
expression (bottom graph). and HSPG
Added components
CONCLUSION Caron’s hypothesis was supported by the
SOURCES Lee, E. Y., et al. 1985. “Interaction of Mouse Mammary Epithelial Cells with
experiments. A specific extracellular matrix protein, laminin,
Collagen Substrata: Regulation of Casein Gene Expression and Secretion.” Proceedings of
influences the expression of albumin by hepatocytes. the National Academy of Sciences, USA 82:1419–1423; Caron, J. M. 1990. “Induction of
Albumin Gene Transcription in Hepatocytes by Extracellular Matrix Proteins.” Molecular
FOLLOW-UP WORK Bissell continued her work with and Cellular Biology 10:1239–1243.

mammary cells and found that the expression of the b-casein

gene was also increased by laminin in the same way as the
albumin gene in hepatocytes.

215
216 CO R E CO N C E P T S S U M M A RY

Core Concepts Summary

10.1 Tissues and organs are communities of cells that Anchoring cell junctions include adherens junctions and
perform specific functions. desmosomes. page 208

A tissue is a collection of cells that work together to perform Adherens junctions form a belt around the circumference
a specific function. page 198 of a cell. They are composed of cell adhesion molecules
called cadherins and connect to microfilaments. page 208
Two or more tissues often work together to form an
organ. page 199 Desmosomes are button-like points of adhesion between
cells. They are composed of cadherins and connect to
Cytoskeletal elements determine the shape of the
intermediate filaments. page 208
cell. page 199
Cell junctions connect cells to one another and to Hemidesmosomes are composed of cell adhesion
the extracellular matrix, a meshwork of proteins and molecules called integrins and connect cells to the
polysaccharides outside the cell. page 199 extracellular matrix and intermediate filaments.
page 208
10.2 The cytoskeleton is composed of microtubules, Tight junctions prevent the passage of substances through
microfilaments, and intermediate filaments that help the space between cells and divide the plasma membrane
maintain cell shape. into apical and basolateral regions. page 208
All eukaryotic cells have microtubules and microfilaments. Gap junctions (in animals) and plasmodesmata (in
Animal cells also have intermediate filaments. page 200 plants) allow cells to communicate rapidly with one
Microtubules are hollow polymers of tubulin dimers, and another. page 210
microfilaments are helical polymers of actin monomers.
Both microtubules and microfilaments provide structural 10.4 The extracellular matrix provides structural
support to the cell. page 200 support and informational cues.

Microtubules and microfilaments are dynamic structures The extracellular matrix is an insoluble meshwork of
and can assemble and disassemble rapidly. page 201 proteins and polysaccharides secreted by the cells it
surrounds. It provides structural support to cells, tissues,
Microtubules go through rounds of assembly and rapid and organs. page 210
disassembly called dynamic instability. page 202
In plants, the extracellular matrix is found in the cell
Microtubules associate with the motor proteins dynein and wall, and the main component of the plant cell wall is the
kinesin to transport substances in the cell. page 202 polysaccharide cellulose. page 211
Microfilaments associate with the motor protein myosin to In animals, the extracellular matrix is found in abundance
transport vesicles in the cell and to cause cell shape changes, in connective tissue. page 212
such as muscle contraction. page 204
Collagen is the primary component of connective tissues
Intermediate filaments are polymers of proteins that differ in animals and is exceptionally strong. page 212
depending on cell type. They provide stable structural
support for many types of cells. page 204 A specialized extracellular matrix called the basal lamina is
present under all epithelial cell layers. page 213
Some prokaryotic cells have protein polymer-like elements
that function similarly to microtubules and microfilaments. In addition to providing structural support for cells, the
page 205 extracellular matrix can influence cell shape and gene
expression. page 214
10.3 Cell junctions connect cells to one another to
form tissues.
Self-Assessment
Cell junctions anchor cells to each other and to the
extracellular matrix, allow sheets of cells to act as a barrier, 1. Name three types of cytoskeletal element, the subunits
and permit communication between cells in tissues. they are composed of, their relative sizes, and the major
page 206 functions of each type.
 CHAPTER 10 C E L L A N D T I S S U E A RC H I T E C T U R E : C Y TO S K E L E TO N , C E L L J U N C T I O N S , A N D E X T R AC E L L U L A R M AT R I X 217

2. Explain how the dynamic nature of microtubules and 6. Predict the effects of interfering with the function of
microfilaments is important for their functions. cadherins and integrins.

3. Describe the functions of the three major motor 7. Name two places where the extracellular matrix can
proteins and state which cytoskeletal element each be found in plants and animals.
interacts with.
8. Describe two effects that the extracellular matrix can
4. Describe three major types and functions of cell have on the cells that synthesize it.
junctions.
Log in to to check your answers to the Self-
5. Identify the cytoskeletal element that interacts with Assessment questions, and to access additional learning tools.
adherens junctions, desmosomes, and hemidesmosomes.
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 CHAPTER 11

Cell Division
Variations, Regulation,
and Cancer

Core Concepts
11.1 During cell division, a single
parental cell divides into two
daughter cells.
11.2 Mitotic cell division is the basis
of asexual reproduction in
unicellular eukaryotes and the
process by which cells divide in
multicellular eukaryotes.
11.3 Meiotic cell division is essential
for sexual reproduction, the
production of offspring that
combine genetic material from
two parents.
11.4 The cell cycle is regulated so
that cell division occurs only at
appropriate times and places.
11.5 Cancer is uncontrolled cell
division that results from
mutations in genes that control
cell division.
Dr. Torsten Wittmann/Science Source.

219
220 SECTION 11.1 CELL DIVISION

Cells come from preexisting cells. This is one of the fundamental genetic material (DNA) present in the single parent cell. Second,
principles of biology and a key component of the cell theory, which the parent cell must be large enough to divide in two and still
was introduced in Chapter 1. Cell division is the process by which contribute sufficient cytoplasmic components such as proteins,
cells make more cells. Multicellular organisms begin life as a single lipids, and other macromolecules to each daughter cell. Satisfying
cell, and then cell division produces the millions, billions, or in the these requirements means that key cellular components must
case of humans, trillions of cells that make up the fully developed be duplicated before cell division takes place. This duplication of
organism. Even after a multicellular organism has achieved its adult material is achieved in a series of steps that constitutes the life cycle
size, cell division continues. In plants, cell division is essential for of every cell. When you think of a life cycle, you might think of
continued growth. In many animals, cell division replaces worn-out various stages beginning with birth and ending with death. In the
blood cells, skin cells, and cells that line much of the digestive tract. case of a single cell, the life cycle begins and ends with cell division.
If you fall and scrape your knee, the cells at the site of the wound In this section, we explore the different mechanisms by which
begin dividing to replace the damaged cells and heal the scrape. prokaryotic and eukaryotic cells divide. Prokaryotic cells divide
Cell division is also important in reproduction. In bacteria, for by binary fission. When eukaryotic cells divide, they first divide
example, a new generation is produced when the parent cell divides the nucleus by mitosis, and then divide the cytoplasm into two
and forms two daughter cells. The parent cell first makes identical daughter cells by cytokinesis. As we discuss, it is likely that
copies of its genetic material so that each of the two daughter mitosis evolved from binary fission.
cells has the same genetic material as the parent cell. The type of
reproduction that occurs when offspring receive genetic material Prokaryotic cells reproduce by binary fission.
from a single parent is called asexual reproduction. Because DNA The majority of prokaryotic cells, namely bacteria and archaeons,
replication is not completely error free, the daughter cells may carry divide by binary fission. In this form of cell division, a cell
small genetic differences or mutations compared to the parent cell. replicates its DNA, increases in size, and divides into two daughter
By contrast, sexual reproduction results in offspring that cells. Each daughter cell receives one copy of the replicated
receive genetic material from two parents. Half the genetic parental DNA. The molecular mechanisms that drive binary fission
material is supplied by the female parent and is present in the have been studied most extensively in bacteria. The process of
egg and the other half is supplied by the male parent and is binary fission is similar in archaeons, as well as in chloroplasts and
contributed by the male’s sperm. Eggs and sperm are specialized mitochondria, organelles within plant, fungal, and animal cells
cells called gametes. A female gamete and a male gamete merge that evolved from free-living prokaryotic cells (Chapters 5 and 27).
during fertilization to form a new organism (Chapter 42). If the Let’s consider the process of binary fission in the intestinal
egg and the sperm each contain the complete genetic material bacterium Escherichia coli (Fig. 11.1). The circular genome of E. coli
from a parent, what prevents the offspring from having twice as is attached by proteins to the inside of the plasma membrane. DNA
many chromosomes as each parent? A unique feature of gametes replication is initiated at a specific location on the circular DNA
is that they contain half the number of chromosomes as the molecule, called the origin of replication, and proceeds in opposite
other cells in the parent organism. So when fertilization occurs, directions around the circle. The result is two DNA molecules,
the combination of genetic material from the egg and the sperm each of which is attached to the plasma membrane at a different
results in a new organism with the same number of chromosomes site. The two attachment sites are initially close together. The cell
as the parents. The production of gametes comes about by a form then elongates and, as it does so, the two DNA attachment sites
of cell division that results in daughter cells with half the number move apart. When the cell is about twice its original size and the
of chromosomes as the parent cell. As we will see, the products of DNA molecules are well separated, a constriction forms at the
this cell division are not genetically identical to the parent. midpoint of the cell. Eventually, new membrane and cell wall are
What determines when cells divide and, importantly, when synthesized at the site of the constriction, dividing the single cell
they should not? And what determines which cells divide? To into two. The result is two daughter cells, each having the same
answer these questions, we must understand the process of cell genetic material as the parent cell.
division and how it is controlled. This discussion will lay the Like most cellular processes, binary fission requires the
groundwork for exploring how cancer results from a loss of control coordination of many components in both time and space. Recent
of cell division. research has identified several genes whose products play a key
role in bacterial cell division. One of these genes, called FtsZ, has
been especially well studied. Many copies of the protein it encodes
11.1 CELL DIVISION assemble and form a ring at the site of constriction where the new
cell wall forms between the two daughter cells. FtsZ is present in
Cell division is the process by which a single cell becomes two the genomes of diverse bacteria and archaeons, suggesting that it
daughter cells. While this process may seem simple, successful plays a fundamental role in prokaryotic cell division. Interestingly,
cell division must satisfy several important requirements. First, it appears to be evolutionarily related to tubulin, which you will
the two daughter cells must each receive the full complement of recall from Chapter 10 makes up the dynamic microtubules found
 CHAPTER 11 C E L L D I V I S I O N : VA R I AT I O N S , R E G U L AT I O N , A N D C A N C E R 221

into daughter cells. And whereas the DNA of prokaryotes is

FIG. 11.1 Binary fission. Cell division in bacteria and archaeons attached to the inside of the plasma membrane, allowing
occurs by binary fission. replicated DNA to be separated into daughter cells by cell growth,
the DNA of eukaryotes is located in the nucleus. As a result,
1 The circular bacterial Site of eukaryotic cell division requires first the breakdown and then the
attachment
DNA molecule is attached
of DNA to
re-formation of the nuclear envelope, as well as mechanisms other
by proteins to the inner
membrane than cell growth to separate replicated DNA. As we saw in Chapter
membrane (red).
DNA
10 and discuss in more detail in section 11.2, chromosomes of
dividing eukaryotic cells attach to the mitotic spindle, which
2 DNA replication begins
separates them into daughter cells.
at a specific location and Interestingly, some unicellular eukaryotes exhibit forms of
proceeds bidirectionally cell division that have characteristics of binary fission and mitosis.
around the circle. For example, dinoflagellates, like all eukaryotes, have a nucleus
and linear chromosomes. However, unlike most eukaryotes, the
3 The newly synthesized nuclear envelope does not break down but stays intact during
DNA molecule is also Newly cell division. Furthermore, the replicated DNA is attached to the
attached to the inner synthesized
membrane, near the nuclear envelope. The nucleus then grows and divides in a manner
DNA
attachment site of the reminiscent of binary fission. These and other observations
initial molecule.
of intermediate forms of cell division in additional organisms
strongly suggest that mitosis evolved from binary fission.
4 As replication proceeds,
the cell elongates
symmetrically around the
The cell cycle describes the life cycle of a eukaryotic
midpoint, separating the cell.
DNA attachment sites. Cell division in eukaryotic cells proceeds through a number
of steps that make up the cell cycle (Fig. 11.2). The cell cycle
5 Cell division begins consists of two distinct stages: M phase and interphase. During
with the synthesis of new M phase, the parent cell divides into two daughter cells. M phase
membrane and wall
material at the midpoint.

6 Continued synthesis
FIG. 11.2 The cell cycle. The eukaryotic cell cycle consists of
completes the constriction M phase (mitosis and cytokinesis) and interphase.
and separates the daughter
cells. G0 phase
(cells that are
not actively
dividing)
M phase
in eukaryotic cells that are important in intracellular transport, (mitosis and
cell movement, and cell division. cytokinesis)

j Quick Check 1 What do you predict would be the consequence

of a mutation in FtsZ that disrupts the function of the protein it
G2 phase
encodes? (Gap 2)

Eukaryotic cells reproduce by mitotic cell division.

G1 phase
The basic steps of binary fission that we just saw—replication of
(Gap 1)
DNA, segregation of replicated DNA to daughter cells, and division
of one cell into two—occur in all forms of cell division. However,
cell division in eukaryotes (by mitosis) is more complicated than
cell division in prokaryotes (by binary fission). Compared with
the single, relatively small, circular DNA molecule that is the Interphase
genome of prokaryotic cells, the genome of eukaryotic cells is S phase
(DNA synthesis)
typically much larger and is organized into one or more linear
chromosomes, each of which must be replicated and separated
222 SECTION 11.2 M I TOT I C C E L L D I V I S I O N

consists of two different events: (1) mitosis, the separation of the of preparations for DNA synthesis (Fig. 11.2). Liver cells remain in
chromosomes into two nuclei, and (2) cytokinesis, the division of G0 for as much as a year. Other cells such as nerve cells and those
the cell itself into two separate cells. Usually, these two processes that form the lens of the eye enter G0 permanently; these cells
go hand in hand, with cytokinesis typically beginning even before are nondividing. Thus, many brain cells lost to disease or damage
mitosis is complete. In most mammalian cells, M phase lasts about cannot be replaced. Although cells in G0 have exited the cell
an hour. cycle, they are active in other ways—in particular, cells in G0 still
The second stage of the cell cycle, called interphase, is the perform their specialized functions. For example, liver cells in G0
time between two successive M phases (Fig. 11.2). For many still carry out metabolism and detoxification.
years, it was thought that the relatively long period of interphase
is uneventful. Today, we know that during this stage the cell
makes many preparations for division. These preparations include 11.2 MITOTIC CELL DIVISION
replication of the DNA in the nucleus so that each daughter cell
receives a copy of the genome, and an increase in cell size so that Mitotic cell division (mitosis followed by cytokinesis) is the normal
each daughter cell receives sufficient amounts of cytoplasmic and mode of asexual reproduction in unicellular eukaryotes, and it is
membrane components to allow it to survive on its own. the means by which an organism’s cells, tissues, and organs develop
Interphase can be divided into three phases, as shown in and are maintained in multicellular eukaryotes. During mitosis
Fig. 11.2. Among the many preparations that the cell must make and cytokinesis, the parental cell’s DNA is divided and passed on to
during interphase, one particularly important task is the replication two daughter cells. This process is continuous, but is divided into
of the entire DNA content of the nucleus. Since replication discrete steps marked by dramatic changes in the cytoskeleton and
involves the synthesis of DNA, this stage is called S phase (“S” for in the packaging and movement of the chromosomes.
“synthesis”).
In most cells, S phase does not immediately precede or The DNA of eukaryotic cells is organized as
follow mitosis but is separated from it by two gap phases: chromosomes.
G1 phase between the end of M phase and the start of S phase, and One of the key challenges faced by a dividing eukaryotic cell is
G2 phase between the end of S phase and the start of M phase. ensuring that both daughter cells receive an equal and complete
Many essential processes occur during both “gap” phases, despite set of chromosomes. The length of DNA contained in the nucleus
the name. For example, during the G1 phase, specific regulatory of an average eukaryotic cell is on the order of 1 to 2 m, well
proteins are made and activated. Once active, the regulatory beyond the diameter of a cell. The DNA therefore needs to be
proteins, many of which are kinases, then promote the activity condensed to fit into the nucleus, and then further condensed
of enzymes that synthesize DNA. In the G2 phase, both the during cell division so that it does not become tangled as it
size and protein content of the cell increase in preparation for segregates into daughter cells.
division. Thus, G1 is a time of preparation for S-phase DNA synthesis, In eukaryotic cells, DNA is organized with histones and other
and G2 is a time of preparation for M-phase mitosis and cytokinesis. proteins into chromatin, which can be looped and packaged to form
How long does a cell take to pass through the cell cycle? the structures we know as chromosomes (Chapters 3 and 13). One
That depends on the type of cell and the organism’s stage of of the earliest events in mitosis is the condensing of chromosomes
development. Actively dividing cells in some human tissues such from long, thin, threadlike structures typical of interphase to short,
as the intestine and skin require frequent replenishing. It usually dense forms that are identifiable under the microscope during
takes cells in these tissues about 12 hours to complete the cell M phase.
cycle. Most other actively dividing cells in your body take about Every species is characterized by a specific number of
24 hours to complete the cycle. A unicellular eukaryote like yeast chromosomes, and each chromosome contains a single molecule
can complete an entire cell cycle in just 90 minutes. Champions of DNA carrying a specific set of genes. When chromosomes
in the race through the cell cycle are the embryonic cells of some condense and become visible during mitosis, they adopt
frog species. Early cell divisions divide the cytoplasm of the large characteristic shapes and sizes that allow each chromosome to be
frog egg cell into many smaller cells and so no growth period is identified by its appearance in the microscope. The portrait formed
needed between cell divisions. Consequently, there are virtually by the number and shapes of chromosomes representative of a
no G1 and G2 phases, and as little as 30 minutes pass between species is called its karyotype. Most of the cells in the human
cell divisions. body, with the exception of the gametes, contain 46 chromosomes
Not all the cells in your body are actively dividing since not all (Fig. 11.3). In contrast, cells from horses have 64 chromosomes,
tissues require the rapid replenishing of cells. Instead, many cells and cells from corn have 20.
pause in the cell cycle somewhere between M phase and S phase In a normal human karyotype, the 46 chromosomes can be
for periods ranging from days to more than a year. This period is arranged into 23 pairs, 22 pairs of homologous chromosomes
called the G0 phase and is distinguished from G1 by the absence numbered 1 to 22 from the longest to the shortest chromosome
 CHAPTER 11 C E L L D I V I S I O N : VA R I AT I O N S , R E G U L AT I O N , A N D C A N C E R 223

FIG. 11.3 A human karyotype. This karyotype shows 22 pairs FIG. 11.4 Homologous chromosomes and sister chromatids. Sister
of chromosomes plus 2 sex chromosomes, or chromatids result from the duplication of chromosomes.
46 chromosomes in total. Source: ISM/Phototake. They are held together at the centromere.
Homologous
chromosomes Homologous Homologous
chromosomes chromosomes

1 2 3 4 5 S phase

6 7 8 9 10 11 12 Centromeres Sister chromatids

13 14 15 16 17 18 cell receives the same number of chromosomes as present in the

parent cell, as we describe now.

j Quick Check 2 Which DNA sequences are more alike: a pair of

sister chromatids or a pair of homologous chromosomes?
19 20 21 22 X

Sex chromosomes Prophase: Chromosomes condense and become

visible.
Mitosis takes place in five stages, each of which is easily identified
and 1 pair of sex chromosomes (Fig. 11.3). Each pair of by events that can be observed in the microscope (Fig. 11.5).
homologous chromosomes represents two of the same type of When you look in a microscope at a cell in interphase, specific
chromosome (both carrying the same set of genes), one of which chromosomes cannot be distinguished because they are long and
was received from the mother and the other from the father. The thin. As the cell moves from G2 phase to the start of mitosis, the
sex chromosomes are the X and Y chromosomes. Individuals with chromosomes condense and become visible in the nucleus. The
two X chromosomes are female, and those with an X and a first stage of mitosis is known as prophase and is characterized by
Y chromosome are male. the appearance of visible chromosomes.
The number of complete sets of chromosomes in a cell is Outside the nucleus, in the cytosol, the cell begins to assemble
known as its ploidy. A cell with one complete set of chromosomes the mitotic spindle, a structure made up predominantly of
is haploid, and a cell with two complete sets of chromosomes microtubules that pull the chromosomes into separate daughter
is diploid. Some organisms, such as plants, can have four or cells. Recall from Chapter 10 that the centrosome is a compact
sometimes more complete sets of chromosomes. Such cells are structure that is the microtubule organizing center for animal
polyploid. cells. The centrosome is thus the structure from which the
In order for cell division to proceed normally, every spindles radiate. Plant cells also have microtubule-based mitotic
chromosome in the parent cell must be duplicated so that each spindles, but they lack centrosomes.
daughter cell receives a full set of chromosomes. This duplication As part of the preparation for mitosis during S phase in
occurs during S phase. Even though the DNA in each chromosome animal cells, the centrosome duplicates and each one begins
duplicates, the two identical copies, called sister chromatids, do to migrate around the nucleus, the two ultimately halting at
not separate. They stay side by side, physically held together at a opposite poles in the cell at the start of prophase. The final
constriction called the centromere. At the beginning of mitosis, locations of the centrosomes define the opposite ends of the
the nucleus of a human cell contains 46 chromosomes, each of cell that will eventually be separated into two daughter cells. As
which is a pair of identical sister chromatids linked together at the the centrosomes make their way to the poles of the cell, tubulin
centromere (Fig. 11.4). Thus, counting chromosomes is simply a dimers assemble around them, forming microtubules that radiate
matter of counting centromeres. from each centrosome. These radiating filaments form the
During mitosis, the sister chromatids separate from each mitotic spindle and later serve as the guide wires for chromosome
other and go to opposite ends of the cell, so that each daughter movement.
224 SECTION 11.2 M I TOT I C C E L L D I V I S I O N

Prometaphase: Chromosomes attach

FIG. 11.5 Mitosis, or nuclear division. Mitosis can be divided into separate steps, but
to the mitotic spindle.
the process is continuous. Source: Jennifer Waters/Science Source.
In the next stage of mitosis, known as
prometaphase, the nuclear envelope breaks down
1 Prophase: Chromosomes condense. and the microtubules of the mitotic spindle attach
Centrosomes radiate microtubules and
migrate to opposite poles. to chromosomes (Fig. 11.5). The microtubules
Mitotic spindles radiating from the centrosomes grow and shrink
as they explore the region of the cell where the
nucleus once was. This process of growing and
shrinking depends on the dynamic instability of
microtubules, discussed in Chapter 10.
As the ends of the microtubules encounter
chromosomes, they attach to the chromosomes
at their centromeres. Associated with the
centromere of each chromosome are two protein
Chromatin Nuclear Centrosome
fibers envelope complexes called kinetochores, one located on
each side of the constriction (Fig. 11.6). Each
kinetochore is associated with one of the two
Chromosome
sister chromatids and forms the site of attachment
2 Prometaphase: for a single spindle microtubule. This arrangement
Microtubules of the ensures that each sister chromatid is attached to
mitotic spindle attach a spindle microtubule radiating from one of the
to chromosomes.
poles of the cell. The symmetrical tethering of
Nuclear envelope each chromosome to the two poles of the cell is
starts to break down. Sister
chromatids
essential for proper chromosome segregation.

Metaphase: Chromosomes align as a

result of dynamic changes in the mitotic
spindle.
3 Metaphase: Once each chromosome is attached to the
Chromosomes align
in center of cell. mitotic spindles from both poles of the cell, the
microtubules of the mitotic spindle lengthen or
shorten to move the chromosomes into position
in the middle of the cell. There the chromosomes
Spindle pole
are lined up in a single plane that is roughly
equidistant from both poles of the cell. This
stage of mitosis, when the chromosomes are
4 Anaphase: Sister aligned in the middle of the dividing cell, is called
chromatids (which metaphase (see Fig. 11.5). It is one of the most
become individual
chromosomes when
visually distinctive stages under the microscope.
the centromere splits)
separate and travel to Anaphase: Sister chromatids fully
opposite poles.
separate.
In the next stage of mitosis, called anaphase,
the sister chromatids separate (see Fig. 11.5).
The centromere holding a pair of sister chromatids
5 together splits, allowing the two sister chromatids
Telophase:
Nuclear envelope to separate from each other. After separation,
re-forms and each chromatid is considered to be a full-fledged
chromosomes
decondense. chromosome. The spindle microtubules attached
to the kinetochores gradually shorten, pulling the
newly separated chromosomes to the opposite poles
of the cell.
 CHAPTER 11 C E L L D I V I S I O N : VA R I AT I O N S , R E G U L AT I O N , A N D C A N C E R 225

it in two. This process is similar to what occurs in binary fission,

FIG. 11.6 Kinetochores. Kinetochores are sites of spindle though in the case of binary fission, the process is driven by FtsZ
attachment and are located on both sides of the protein, a homolog of tubulin, not by actin. The constriction of the
centromere. contractile ring is driven by motor proteins that slide bundles of
actin filaments in opposite directions. Successful division results
in two daughter cells, each with its own nucleus. The daughter
cells are now free to enter G1 phase and start the process anew.
Sister chromatids
For the most part, mitosis is similar in animal and in plant cells,
Centromere but cytokinesis is different (Fig. 11.7b). Since plant cells have a
cell wall, the division of the cell is achieved by constructing a new
Kinetochore
cell wall. During telophase, dividing plant cells form a structure
Spindle called the phragmoplast in the middle of the cell. The phragmoplast

FIG. 11.7 Cytokinesis, or cytoplasmic division. (a) In animal cells,

cytokinesis involves a contractile ring made of actin. (b) In
plant cells, it involves the growth of a new cell wall called a
cell plate. Sources: a. Dr. Paul Andrews, University of Dundee/Science
Source; b. Carolina Biological Supply Company/Phototake.
a.

This event is the basis for the equal segregation of Centrosome

chromosomes between the two daughter cells. During S phase in a
human cell, each of the 46 chromosomes is duplicated to yield Decondensing
46 pairs of identical sister chromatids. Thus, when the chromatids chromosomes
are separated at anaphase, an identical set of 46 chromosomes
arrives at each spindle pole, the complete genetic material for one
Contractile
of the daughter cells. ring

Telophase: Nuclear envelopes re-form around newly Re-forming

segregated chromosomes. nucleus
Once a complete set of chromosomes arrives at a pole, the
chromosomes have entered the area that will form the cytosol of a Spindles
new daughter cell. This event marks the beginning of telophase,
during which the cell prepares for its division into two new cells
(see Fig. 11.5). The microtubules of the mitotic spindle break down
and disappear, while a nuclear envelope reforms around each set b.
of chromosomes, creating two new nuclei. As the nuclei become
increasingly distinct in the cell, the chromosomes contained
within them decondense, becoming less visible in the microscope.
This stage marks the end of mitosis. Decondensing
chromosomes

The parent cell divides into two daughter cells by

cytokinesis. Cell plate
Usually, as mitosis is nearing its end, cytokinesis begins and the
parent cell divides into two daughter cells (Fig. 11.7). In animal Re-forming
nucleus
cells, this stage begins when a ring of actin filaments, called
the contractile ring, forms against the inner face of the cell
membrane at the equator of the cell perpendicular to the axis of
Spindles
what was the spindle (Fig. 11.7a). As if pulled by a drawstring, the
ring contracts, pinching the cytoplasm of the cell and dividing
226 SECTION 11.3 M E I OT I C C E L L D I V I S I O N

consists of overlapping microtubules that guide vesicles containing chromosomes by half. During meiosis II, sister chromatids
cell wall components to the middle of the cell. During late separate, as in mitosis.
anaphase and telophase, these vesicles fuse to form a new cell wall, Meiosis I begins with prophase I, illustrated in Fig. 11.8. The
called the cell plate, in the middle of the dividing cell. Once this beginning of prophase I marks the earliest visible manifestation
developing cell wall is large enough, it fuses with the original cell
wall at the perimeter of the cell. Cytokinesis is then complete and
the plant cell has divided into two daughter cells.
FIG. 11.8 Prophase I of meiosis. Chromosomes condense and
j Quick Check 3 What would be the consequence if a cell homologous chromosomes pair.
underwent mitosis but not cytokinesis?
Centrosome
1 Chromosomes Nuclear
first become visible envelope
11.3 MEIOTIC CELL DIVISION as thin threads. DNA
replication is already Chromosomes
complete.
As we discussed, mitotic cell division is important in the
development of a multicellular organism and in the maintenance
and repair of tissues and organs. Mitotic cell division is also the
basis of asexual reproduction in unicellular eukaryotes. We now
turn to the basis of sexual reproduction. In sexual reproduction, Microtubule
gametes fuse during fertilization to form a new organism. A new
organism produced by sexual reproduction has the same number
2 Homologous Pair of
chromosomes continue homologous
of chromosomes as its parents because the egg and sperm each to condense and chromosomes
contain half the number of chromosomes as each diploid parent. undergo synapsis
(gene-for-gene pairing).
Gametes are produced by meiotic cell division, a form of cell
division that includes two rounds of nuclear division. By producing
haploid gametes, meiotic cell division makes sexual reproduction
possible. Bivalent
There are several major differences between meiotic cell
division and mitotic cell division. First, meiotic cell division results 3 When synapsis is Paternal homolog
in four daughter cells instead of two. Second, each of the four complete, each pair of
homologous chromo-
daughter cells contains half the number of chromosomes as the somes forms a bivalent. Maternal homolog
parent cell. (The word “meiosis” is from the Greek for “diminish” Each chromosome
or “lessen.”) Third, the four daughter cells are each genetically consists of two sister
chromatids. Non-sister chromatids
unique. In other words, they are genetically different from each
other and from the parental cell. Sister chromatids
In multicellular animals, the cells produced by meiosis are the
haploid eggs and sperm that fuse in sexual reproduction. In other
organisms, such as fungi, the products are spores, and in some
unicellular eukaryotes, the products are new organisms. In this
4 The chromosomes
continue to shorten
section, we consider the steps by which meiosis occurs, its role in and thicken and the
sexual reproduction, and how it likely evolved. chiasmata between
non-sister chromatids
become apparent.
Pairing of homologous chromosomes is unique
to meiosis.
Like mitotic cell division, meiotic cell division follows one round
of DNA synthesis, but, unlike mitotic cell division, meiotic cell
division consists of two successive cell divisions. The two cell
divisions are called meiosis I and meiosis II, and they occur one
after the other. Each cell division results in two cells, so that by 5 The nuclear envelope
the end of meiotic cell division a single parent cell has produced begins to break down.
four daughter cells. During meiosis I, homologous chromosomes
separate from each other, reducing the total number of
 CHAPTER 11 C E L L D I V I S I O N : VA R I AT I O N S , R E G U L AT I O N , A N D C A N C E R 227

of chromosome condensation. The chromosomes first appear of a crossover, the physical breakage and reunion between non-
as long, thin threads present throughout the nucleus. By sister chromatids.
this time, DNA replication has already taken place, so each Through the process of crossing over, homologous
chromosome has become two sister chromatids held together at chromosomes of maternal origin and paternal origin exchange DNA
the centromere. segments. The positions of these exchanges along the chromosome
What happens next is an event of enormous importance, and are essentially random, and therefore each chromosome that
it is unique to meiosis. The homologous chromosomes pair with emerges from meiosis is unique, containing some DNA segments
each other, coming together to lie side by side, gene for gene, in from the maternal chromosome and others from the paternal
a process known as synapsis. Even the X and Y chromosomes chromosome. The process is very precise: Usually, no nucleotides
pair, but only at the tip where their DNA sequences are nearly are gained or lost as homologous chromosomes exchange material.
identical. Because one of each pair of homologs is maternal Occasionally, the exchange is imprecise and portions of the
in origin and the other is paternal in origin, chromosome chromatids may be gained or lost, resulting in loss or duplication
pairing provides an opportunity for the maternal and paternal of material. Note the results of crossing over as shown in Fig. 11.9:
chromosomes to exchange genetic information, as described in The recombinant chromatids are those that carry partly paternal
the next section. and partly maternal segments. In this way, crossing over increases
Because each homologous chromosome is a pair of sister genetic diversity.
chromatids attached to a single centromere, a pair of synapsed The number of chiasmata that are formed during meiosis
chromosomes creates a four-stranded structure: two pairs of depends on the species. In humans, the usual range is 50–60
sister chromatids aligned along their length. The whole unit chiasmata per meiosis. Most bivalents have at least one chiasma.
is called a bivalent, and the chromatids attached to different Even the X and Y chromosomes are joined by a chiasma in
centromeres are called non-sister chromatids (Fig. 11.8). the small region where they are paired. In addition to their
Non-sister chromatids result from the replication of homologous role in exchanging genetic material, the chiasmata also play a
chromosomes (one is maternal and the other is paternal in mechanical role in meiosis by holding the bivalents together while
origin), so they have the same set of genes in the same order, they become properly oriented in the center of the cell during
but are not genetically identical. By contrast, sister chromatids metaphase, the stage we turn to next.
result from replication of a single chromosome, so are genetically
identical. The first meiotic division brings about the reduction in
chromosome number.
j Quick Check 4 In a human cell at the end of prophase I, how
At the end of prophase I, the chromosomes are fully condensed
many chromatids, centromeres, and bivalents are present?
and have formed chiasmata, the nuclear envelope has begun to
disappear, and the meiotic spindle is forming. We are now ready
Crossing over between DNA molecules results in
to move through the remaining stages of meiosis I, which are
exchange of genetic material.
illustrated in Fig. 11.10.
Within the bivalents are cross-like structures, each called a
In prometaphase I, the nuclear envelope breaks down and
chiasma (from the Greek meaning a “cross piece”; the plural is
the meiotic spindles attach to kinetochores on chromosomes.
“chiasmata”) (Fig. 11.9). Each chiasma is a visible manifestation
In metaphase I, the bivalents move so that they come to lie on
an imaginary plane cutting transversely across the spindle. Each
bivalent lines up so that its two centromeres lie on opposite
sides of this plane, pointing toward opposite poles of the cell.
FIG. 11.9 Chiasmata. Crossing over at chiasmata between non- Importantly, the orientation of these bivalents is random
sister chromatids results in recombinant chromatids. with respect to each other. For some, the maternal homolog is
Homologous Recombinant attached to the spindle radiating from one pole and the paternal
chromosomes Bivalent chromatids homolog is attached to the spindle originating from the other
pole. For others, the orientation is reversed. As a result, when the
homologous chromosomes separate from each other in the next
Paternal
homolog
step, a complete set of chromosomes moves toward each pole, and
that chromosome set is a random mix of maternal and paternal
Maternal
homolog homologs. The random alignment of chromosomes on the spindle
in metaphase I further increases genetic diversity in the products
of meiosis.
Non-sister Sister Chiasma At the beginning of anaphase I, the two homologous
chromatids chromatids chromosomes of each bivalent separate as they are pulled in
228 SECTION 11.3 M E I OT I C C E L L D I V I S I O N

of a given chromosome during prometaphase I. Anaphase I is thus

FIG. 11.10 Meiosis I. Meiosis I is the reductional division: The very different from anaphase of mitosis, in which the centromeres
number of chromosomes is halved. split and each pair of chromatids separates.
The end of anaphase I coincides with the arrival of the
chromosomes at the poles of the cell. Only one of the two
1 Prophase I (later
homologous chromosomes goes to each pole, so in human cells
stage): Chiasmata there are 23 chromosomes at each pole at the end of meiosis I,
present. which is the haploid number of chromosomes. Each of these
chromosomes consists of two chromatids attached to a single
centromere. Meiosis I is sometimes called the reductional
division since it reduces the number of chromosomes in daughter
cells by half.
In telophase I, the chromosomes may uncoil slightly and a
nuclear envelope briefly reappears. In many species (including
2 Prometaphase I:
humans), the cytoplasm divides, producing two separate cells.
Spindles attach to The chromosomes do not completely decondense, however, and
kinetochores on so telophase I blends into the first step of the second meiotic
chromosomes.
division. Importantly, there is no DNA synthesis between the two
meiotic divisions.

j Quick Check 5 List three ways in which meiosis I differs from

mitosis.

3 The second meiotic division resembles mitosis.

Metaphase I: Homolo-
gous pairs line up in center Now let’s turn to the second meiotic division, meiosis II, shown
of cell, with bivalents in Fig. 11.11. In this division, sister chromatids separate, creating
oriented randomly with
respect to each other.
haploid daughter cells. Starting with prophase II, the second
meiotic division is in many respects like a normal mitotic division,
except that the nuclei in prophase II have the haploid number
of chromosomes, not the diploid number. In prophase II, the
chromosomes recondense to their maximum extent. Toward the
end of prophase II, the nuclear envelope begins to disappear (in
4 Anaphase I:
those species in which it has formed), and the spindle begins to be
Homologous chromo- set up.
somes separate, but In prometaphase II, spindles attach to kinetochores and, in
sister chromatids do
not separate. metaphase II, the chromosomes line up so that their centromeres
lie on an imaginary plane cutting across the spindle.
In anaphase II, the centromere of each chromosome splits.
The separated chromatids, now each regarded as a full-fledged
chromosome, are pulled toward opposite poles of the spindle. In
5 Telophase I this sense, anaphase II resembles anaphase of mitosis.
and cytokinesis: Finally, in telophase II, the chromosomes uncoil and
Daughter cells become decondensed and a nuclear envelope re-forms around
are ready to
move into each set of chromosomes. The nucleus of each cell resulting from
prophase II. telophase II has the haploid number of chromosomes. Because
cells in meiosis II have the same number of chromosomes at the
beginning and at the end of the process, meiosis II is often called
the equational division. Telophase II is followed by the division
of the cytoplasm in many species.
opposite directions. The key feature of anaphase I is that the
centromeres do not split and the two chromatids that make up j Quick Check 6 The genetic constitution of each cell after
each chromosome remain together. This occurs because spindle telophase II is different from the others. What two processes during
microtubules from one pole of the cell attach to both kinetochores meiosis result in these differences?
 CHAPTER 11 C E L L D I V I S I O N : VA R I AT I O N S , R E G U L AT I O N , A N D C A N C E R 229

FIG. 11.11 Meiosis II. Meiosis II is the equational division: The number of chromosomes stays the same.

1 Prophase II:
Cells from meiosis I

The nuclear
envelope breaks
down and the
chromosomes
condense.

2 Prometaphase II:
Spindles attach to
kinetochores on
chromosomes.

3 Metaphase II:
Chromosomes align
in center of cell.

4 Anaphase II:
Sister chromatids
separate.

5 Telophase II
and cytokinesis:
The nuclear
envelope
re-forms and
the cytoplasm
divides.
230 SECTION 11.3 M E I OT I C C E L L D I V I S I O N

FIG. 11.12 Comparison of mitosis and meiosis.

MEIOSIS I

Parent cell Prophase I

(Diploid) (Duplicated
chromosomes)

Metaphase I

Anaphase I

Telophase I and
Cytokinesis

MITOSIS MEIOSIS II

Prophase Prophase II
(Duplicated
chromosomes)

Metaphase Metaphase II

Anaphase Anaphase II

Telophase and Telophase II

Cytokinesis and
(Diploid) Cytokinesis
(Haploid)
 CHAPTER 11 C E L L D I V I S I O N : VA R I AT I O N S , R E G U L AT I O N , A N D C A N C E R 231

TABLE 11.1 Comparison of Mitosis and Meiosis.

MITOSIS MEIOSIS

Function Asexual reproduction in unicellular eukaryotes Sexual reproduction

Development in multicellular eukaryotes Production of gametes and spores
Tissue regeneration and repair in multicellular
eukaryotes
Organisms All eukaryotes Most eukaryotes

Number of rounds of DNA synthesis 1 1

Number of cell divisions 1 2

Number of daughter cells 2 4

Chromosome complement of
daughter cell compared with parent cell Same Half

Pairing of homologous chromosomes No Meiosis I—Yes

Meiosis II—No

Crossing over No Meiosis I—Yes

Meiosis II—No

Separation of homologous chromosomes No Meiosis I—Yes

Meiosis II—No

Centromere splitting Yes Meiosis I—No

Meiosis II—Yes

Separation of sister chromatids Yes Meiosis I—No

Meiosis II—Yes

A comparison of mitosis and meiosis gives us hints about how the functional egg cell, and the other meiotic products receive only
meiosis might have evolved (Fig. 11.12 and Table 11.1). During small amounts of cytoplasm. These smaller cells are called polar
meiosis I, maternal and paternal homologs separate from each bodies. In male mammals (Fig. 11.13b), the cytoplasm divides
other, whereas during meiosis II, sister chromatids separate from about equally in both meiotic divisions, and each of the resulting
each other, similar to mitosis. The similarity of meiosis II and meiotic products goes on to form a functional sperm. During the
mitosis suggests that meiosis likely evolved from mitosis. Mitosis development of the sperm, most of the cytoplasm is eliminated, and
occurs in all eukaryotes and was certainly present in the common what is left is essentially a nucleus in the sperm head equipped with
ancestor of all living eukaryotes. Meiosis is present in most, but a long whiplike flagellum to help propel it toward the egg.
not all, eukaryotes. Because the steps of meiosis are the same in
all eukaryotes, meiosis is thought to have evolved in the common Meiosis is the basis of sexual reproduction.
ancestor of all eukaryotes and has been subsequently lost in Sexual reproduction involves two processes: meiotic cell division
some groups. and fertilization. Meiotic cell division, as we just saw, produces
cells with half the number of chromosomes present in the
Division of the cytoplasm often differs between parent cell. In multicellular animals, the products of meiotic cell
the sexes. division are gametes: An egg cell is a gamete and a sperm cell
In multicellular organisms, division of the cytoplasm in meiotic cell is a gamete. Each gamete is haploid, containing a single set of
division differs between the sexes. In female mammals chromosomes. In humans, meiosis takes place in the ovaries of
(Fig. 11.13a), the cytoplasm is divided very unequally in both the female and the testes of the male, and each resulting gamete
meiotic divisions. Most of the cytoplasm is retained in one meiotic contains 23 chromosomes, including one each of the 22 numbered
product, a very large cell called the oocyte, which can develop into chromosomes plus either an X or a Y chromosome.
232 SECTION 11.3 M E I OT I C C E L L D I V I S I O N

FIG. 11.13 Cytoplasmic division in females and in males. (a) In females, cytoplasmic division results in one oocyte and three polar bodies;
(b) in males, it results in four sperm cells.

a. Female b. Male

DNA replication

First meiotic division

Second meiotic division

Polar bodies

Oocyte Sperm cells

During fertilization, these gametes fuse to form a single cell chromosomes), and from fertilization (different gametes are
called a zygote. The zygote is diploid, having two complete sets combined to produce a new, unique individual). The increase in
of chromosomes, one from each parent. Therefore, fertilization genetic diversity made possible by sexual reproduction allows
restores the original chromosome number. organisms to evolve and adapt more quickly to their environment
As we discuss further in Chapters 16 and 42, sexual than is possible with asexual reproduction.
reproduction plays a key role in increasing genetic diversity. In the life cycle of multicellular animals like humans, then, the
Genetic diversity results from meiotic cell division (the cells that diploid organism produces single-celled haploid gametes that fuse
are produced are each genetically different from one another as a to make a diploid zygote. In this case, the only haploid cells in the
result of crossing over and the random segregation of homologous life cycle are the gametes, and the products of meiotic cell division
 CHAPTER 11 C E L L D I V I S I O N : VA R I AT I O N S , R E G U L AT I O N , A N D C A N C E R 233

do not undergo mitotic cell division but instead fuse to become a of animal embryos revealed two interesting patterns. First, as the
diploid zygote. However, there are a number of life cycles in other cells undergo this rapid series of divisions, several proteins appear
organisms that differ in the timing of meiotic cell division and and disappear in a cyclical fashion. Researchers interpreted this
fertilization, discussed more fully in Chapter 27. Some organisms, observation to mean that these proteins might play a role in
like most fungi, are haploid. These haploid cells can fuse to the control of the progression through the cell cycle. Second,
produce a diploid zygote, but this cell immediately undergoes several enzymes become active and inactive in cycles. These
meiotic cell division to produce haploid cells, so that the only enzymes are kinases, proteins that phosphorylate other proteins
diploid cell in the life cycle is the zygote (Chapter 34). Other (Chapter 9). The timing of kinase activity is delayed slightly
organisms, like plants, have both multicellular haploid and diploid relative to the appearance of the cyclical proteins.
phases (Chapter 30). In this case, meiotic cell division produces These and many other observations led to the following view
haploid spores that divide by mitotic cell division to produce a of cell cycle control: Proteins are synthesized that activate the
multicellular haploid phase; subsequently, haploid cells fuse to kinases. These regulatory proteins are called cyclins because their
form a diploid zygote that also divides by mitotic cell division to levels rise and fall with each turn of the cell cycle. Once activated
produce a multicellular diploid phase. by cyclins, the kinases phosphorylate target proteins involved
in promoting cell division (Fig. 11.14). These kinases, cyclin-
dependent kinases, or CDKs, are always present within the cell
11.4 REGULATION OF THE CELL CYCLE but are active only when bound to the appropriate cyclin. It is the
kinase activity of the cyclin–CDK complexes that triggers the
Both mitotic and meiotic cell division must occur only at certain
times and places. Mitotic cell division, for example, occurs during
growth of a multicellular organism, wound healing, or in the
maintenance of actively dividing tissues such as the skin or lining FIG. 11.14 Cyclins and cyclin-dependent kinases (CDKs). Cyclins
of the intestine. Similarly, meiotic cell division occurs only at and CDKs control progression through the cell cycle.
certain times during development. Even for unicellular organisms,
cell division must be regulated so that it takes place only when
Cyclin 1 Cyclins bind to and
conditions are favorable—for example, when enough nutrients activate cyclin-
are present in the environment. Thus, a cell may have to receive a dependent kinases to
control progression
signal before it will divide. In Chapter 9, we saw how cells respond through the cell cycle.
to signals. Growth factors, for example, bind to cell-surface receptors
and activate intracellular signaling pathways that lead to cell division.
Even when a cell receives a signal to divide, it does not divide
until it is ready. Has all of the DNA been replicated during S phase?
Has the cell grown to a size sufficient to support division into Cyclin-dependent Cyclin-CDK
kinase (CDK)
viable daughter cells? If these and other preparations have not been complex
accomplished, the cell halts its progression through the cell cycle.
So, cells have regulatory mechanisms that initiate cell
division, as well as mechanisms for spotting faulty or incomplete
preparations and arresting cell division. When these mechanisms
fail—for example, dividing in the absence of a signal or when
the cell is not ready—the result is uncontrolled cell division, a
hallmark of cancer. In this section, we consider how cells control
their passage through the cell cycle. Our focus is on control of
mitotic cell division. However, many of the same factors also Target
regulate meiotic cell division, a reminder of the close evolutionary protein
connection between these two forms of cell division. Cyclin
degrades 2 Cyclin-CDK
complexes
Protein phosphorylation controls passage through phosphorylate
the cell cycle. target proteins
P that promote
Early animal embryos, such as those of frogs and sea urchins, are cell division.
useful models for studying cell cycle control because they are
large and undergo many rapid mitotic cell divisions following
fertilization. During these rapid cell divisions, mitosis and S phase Phosphorylated
alternate with virtually no G1 and G2 phases in between. Studies target protein
234 SECTION 11.4 R E G U L AT I O N O F T H E C E L L C YC L E

required cell cycle events. Therefore, the cyclical change in cyclin– These cell cycle proteins, including cyclins and CDKs, are widely
CDK activity depends on the cyclical levels of the cyclins. Cyclins conserved across eukaryotes, reflecting their fundamental role in
in turn may be synthesized in response to signaling pathways that controlling cell cycle progression, and have been extensively studied
promote cell division. in yeast, sea urchins, mice, and humans (Fig. 11.15).

234 SECTION 11.1 P

HOW DO WE KNOW?

FIG. 11.15
Level of protein increases

How is progression through

75 and decreases, and then
increases and decreases
again as the embryo

the cell cycle controlled? divides by mitosis.

50

Cyclin level
BACKGROUND The cell
cycle is characterized by
cyclical changes in many 25
components of the cell:
Chromosomes condense
and decondense, the
mitotic spindle forms and 0 1 2
breaks down, the nuclear Source: SeaPics.com. Time (hours)
envelope breaks down and Interphase – Mitosis – Interphase – Mitosis – Interphase
re-forms. How these regular and cyclical changes are controlled
is the subject of active research. First, clues came from studies in
yeast in which mutations in certain genes blocked progression
through the cell cycle, suggesting that these genes encode proteins
that play a role in cell cycle progression. Additional clues came Source: Biology Pics/Science Source.
from studies in embryos of the sea urchin Arbacia punctulata.
These embryos are large and divide rapidly by mitosis, making
CONCLUSION Hunt and colleagues called this new protein
them a good model system for the study of the control of cell
“cyclin.” Although they did not know its function, its fluctuating
division. In the early 1980s, it was known that inhibition of protein
level suggested it might play a role in the control of the cell cycle.
synthesis blocks key steps of cell division in sea urchins. It was also
However, more work was needed to figure out if cyclins actually
known that an enzyme called MPF, or M-phase promotion factor,
cause progression through the cell cycle or whether their levels
is important for the transition from G2 to M phase.
oscillate in response to progression through the cell cycle.
EXPERIMENT To better understand events in the cell cycle,
FOLLOW-UP WORK Hunt and colleagues found a similar
English biochemist Tim Hunt and colleagues measured protein
rise and fall in the levels of certain proteins in the sea urchin
levels in sea urchin embryos as they divide rapidly by mitosis. He
Lytechinus pictus and surf clam Spisula solidissima. MPF was
added radioactive methionine (an amino acid) to eggs, which
found to consist of a cyclin protein and a cyclin-dependent
became incorporated into any newly synthesized proteins. The
kinase that play a key role in the G2–M transition. In other
eggs were then fertilized and allowed to develop. Samples of the
words, cyclins do in fact control progression through the cell
rapidly dividing embryos were taken every 10 minutes and run on
cycle by their interactions with cyclin-dependent kinases. Hunt
a gel to visualize the levels of different proteins.
shared the Nobel Prize in Physiology or Medicine in 2001 for
RESULTS Most protein bands became darker as cell division his work on cyclins.
proceeded, indicating more and more protein synthesis. However,
SOURCE T. Evans et al. 1983. “Cyclin: A Protein Specified by Maternal
the level of one protein band oscillated, increasing in intensity and mRNA in Sea Urchin Eggs That Is Destroyed at Each Cleavage Division.”
then decreasing with each cell cycle, as shown in the graph. Cell 33:389–396.
 CHAPTER 11 C E L L D I V I S I O N : VA R I AT I O N S , R E G U L AT I O N , A N D C A N C E R 235

The M cyclin–CDK complex

FIG. 11.16 Three cyclin–CDK complexes. Each one is active at different times in the initiates multiple events associated with
cell cycle. mitosis. For example, M cyclin–CDK
phosphorylates structural proteins in the
M cyclin–CDK helps
prepare the cell for
nucleus, triggering the breakdown of the
mitosis. nuclear envelope in prometaphase.
M cyclin–CDK also phosphorylates
proteins that regulate the assembly of
tubulin into microtubules, promoting the
M formation of the mitotic spindle.

Cell cycle progression requires

successful passage through multiple
checkpoints.
G2 Cyclins and CDKs not only allow a cell to
progress through the cell cycle, but also
give the cell opportunities to halt the
G1 cell cycle should something go wrong. In
other words, if the preparations for the
next stage of the cell cycle are incomplete
or if there is some kind of damage, there
are mechanisms that block the cyclin–
CDK activity required for the next step,
S cyclin–CDK Interphase pausing cell division until preparations are
helps initiate
S complete or the damage is repaired. Each of
DNA synthesis.
these mechanisms is called a checkpoint.
Cells have many cell cycle checkpoints.
Three major checkpoints that have been
Time when
well studied are illustrated in Fig. 11.17. The
G1/S cyclin–CDK presence of damaged DNA arrests the cell
cyclin–CDK
complex prepares cell
complex is at the end of G1 before DNA synthesis; the
for DNA replication.
active
presence of unreplicated DNA arrests the
cell at the end of G2 before the cell enters
mitosis; and abnormalities in chromosome
attachment to the spindle arrest the
Different cyclin–CDK complexes regulate each stage cell in early mitosis. By way of illustration, we focus on the key
of the cell cycle. checkpoint that occurs at the end of G1 in response to the presence
In mammals, there are several different cyclins and CDKs that act at of damaged DNA.
specific steps of the cell cycle. Three steps in particular are subject to DNA can be damaged by environmental insults such as
cyclin–CDK regulation in all eukaryotes (Fig. 11.16). ultraviolet radiation or chemical agents. Typically, damage
The G1 /S cyclin–CDK complex, which is active at the end takes the form of double-stranded breaks in the DNA. If the cell
of the G1 phase, is necessary for the cell to enter S phase. For progresses through mitosis with DNA damage, the damage might
example, this cyclin–CDK complex activates a protein that be inherited by the daughter cells, or the chromosomes might not
promotes the expression of histone proteins needed for packaging segregate normally. Some checkpoints delay progression through
the newly replicated DNA strands. the cell cycle until DNA damage is repaired. An important one
The S cyclin–CDK complex is necessary for the cell to initiate occurs in late G1. This DNA damage checkpoint depends on several
DNA synthesis. It activates enzymes and other proteins necessary regulatory proteins, some of which recognize damaged DNA
for DNA replication. Once replication has begun at a particular while others arrest cell cycle progression before S phase
place on the DNA, S cyclin–CDK activity prevents the replication (Fig. 11.17).
proteins from reassembling at the same place and re-replicating When DNA is damaged by radiation, a specific protein kinase is
the same DNA sequence. Synthesizing the same sequence activated that phosphorylates a protein called p53. Phosphorylated
repeatedly would be dangerous because cells with too much DNA p53 binds to DNA, where it turns on the expression of several genes.
could die or become cancerous. One of these genes codes for a protein that binds to and blocks the
 236 SECTION 11.5 W H AT G E N E S A R E I N VO LV E D I N C A N C E R ?

DNA damage, p53 is sometimes called the “guardian of the

FIG. 11.17 Cell cycle checkpoints. Cell cycle checkpoints monitor key steps in genome.” Mutations in p53 are common in cancer, a topic we
the cell cycle. turn to next.
Spindle assembly checkpoint (before anaphase): j Quick Check 7 Can you think of two ways in which the
Are all chromosomes attached to the spindle?
function of p53 can be disrupted?

? M ? CASE 2 WHEN GOOD CELLS GO BAD

DNA replication
checkpoint ? 11.5 WHAT GENES ARE
(at the end of G2):
Is all DNA
INVOLVED IN CANCER?
replicated?
G2
As we have just seen, cells have evolved mechanisms
that promote passage through the cell cycle, such as the
cyclin–CDK complexes, as well as mechanisms that halt the
G1 cell cycle when the cell is not ready to divide, such as the
DNA damage checkpoint that depends on the p53 protein.
When cellular mechanisms that promote cell division are
inappropriately activated or the normal checks on cell
division are lost, cells may divide uncontrollably. The result
can be cancer, a group of diseases characterized by rapid,
Interphase
uncontrolled cell division.
S In this section, we explore various ways in which cell
? cycle control can fail and lead to cancer. We begin with a
discussion of a cancer caused by a virus. Although most
cancers are not caused by viruses, the study of cancers caused
activity of the G1/S cyclin–CDK complex DNA damage checkpoint by viruses helped us to understand how cancers develop.
(Fig. 11.18). In so doing, p53 arrests the cell (before entering S phase):
Is DNA damaged?
at the G1 /S transition, giving the cell time to Oncogenes promote cancer.
repair the damaged DNA. The p53 protein Our understanding of cancer is based partly on early
therefore is a good example of protein observations of cancers in animals. In the first decade of
involved in halting the cell cycle when the cell is not ready to divide. the twentieth century, Peyton Rous studied cancers called
Because of its role in protecting the genome from accumulating sarcomas in chickens (Fig. 11.19). His work and that of

FIG. 11.18 DNA damage checkpoint controlled by p53. M

Gene G2
turned
Nucleus DNA ON G1
damage
P

P
S
p53 P
P
P
–
p53
Inhibiting the cell
Cytoplasm cycle gives the cell
time to repair the
p53 is a protein DNA damage activates protein Phosphorylated p53 acts as a damaged DNA.
found in the nucleus. kinases that phosphorylate p53. transcription factor that turns on
genes that inhibit the cell cycle.
 HOW DO WE KNOW?

FIG. 11.19 into a healthy chicken. In a second experiment, he passed the

extract through a filter to remove all cells, including cancer cells
Can a virus cause cancer? and bacterial cells, and then injected this cell-free extract into a
healthy chicken.
BACKGROUND In the early 1900s, little was known about the RESULTS In both cases, healthy chickens injected with the
cause of cancer or the nature of viruses. Peyton Rous, an American extract from the sarcoma developed cancer at the site of
pathologist, studied a form of cancer called a sarcoma in chickens. injection. Microscopic examination of the cancer showed it to be
First, he moved a cancer tumor from a diseased chicken to a the same type of cancer as the original one.
healthy chicken, and found that the cancer could be transplanted.
Then, he tried to isolate the factor that causes the cancer in CONCLUSIONS AND FOLLOW-UP WORK Rous concluded,

chickens. He made an extract of the tumor, filtered it to remove all contrary to his hypothesis, that a small agent—a virus or
the cells, and injected the extract into a healthy chicken to see if it chemical—is capable of causing cancer. Later experiments
could induce cancer. confirmed that the cause of the cancer is a virus. This result was
surprising, controversial, and dismissed at the time. A second
HYPOTHESIS Experiments in other organisms, such as mice, rats, cancer-causing virus was not found until the 1930s. Although
and dogs, showed that an extract free of cells from a tumor does most cancers are not caused by viruses, work with cancer-
not cause cancer. Therefore, Rous hypothesized that a cell-free causing viruses helped to identify cellular genes that, when
extract of the chicken sarcoma would not induce cancer in healthy mutant, can lead to cancer. In 1966, Rous shared the Nobel Prize
chickens. in Physiology or Medicine for his discovery.
EXPERIMENT Rous took a sample of the sarcoma from a chicken,
SOURCES Rous, P. 1910. “A Transmissable Avian Neoplasm (Sarcoma of the
ground it up, suspended it in solution, and centrifuged it to Common Fowl).” J. Exp. Med. 12:696–705; Rous, P. 1911. “Transmission of a
remove the debris. In one experiment, he injected this extract malignant new growth by means of a cell-free filtrate.” JAMA 56:198.

The chicken
A sample of sarcoma is The tumor is ground The extract is injected develops a tumor at
obtained from a chicken. up, suspended, and into a healthy chicken. the site of injection.
centrifuged to create
an extract.

The extract is passed

through a filter to remove
cells and bacteria.

The filtered cell-free The chicken

Source: Biology Pics/Science Source. extract is injected into develops a tumor at
a healthy chicken. the site of injection.

237
238 SECTION 11.5 W H AT G E N E S A R E I N VO LV E D I N C A N C E R ?

others led to the discovery of the first virus known to cause cancer promotes cell division by binding to and dimerizing a receptor
in animals, named the Rous sarcoma virus. kinase in the membrane of the target cell. Dimerization of the
Viruses are assemblages of protein surrounding a core of receptor leads to the activation of several signaling pathways
either RNA or DNA. They multiply by infecting cells and using and the promotion of cell division. In one type of leukemia (a
the biochemical machinery of their host to synthesize proteins cancer of blood cells), a mutation in the gene that encodes the
encoded in their genome and to make more copies of themselves. PDGF receptor results in an altered receptor that is missing
Viruses typically carry only a handful of genes, making it relatively the extracellular portion needed to bind the growth factor. The
easy to identify which of those genes is involved in cancer. The mutant receptor dimerizes on its own, independent of PDGF
investigation of cancer-causing viruses therefore provided major binding, and is therefore always turned on. The overactive PDGF
insights into our understanding of cancer. receptor activates too many target proteins over too long a time
As discussed in Chapter 9, growth factors activate several period, leading to uncontrolled proliferation of blood cells.
types of proteins inside the cell that promote cell division. The
gene from the Rous sarcoma virus that promotes uncontrolled Tumor suppressors block specific steps in the
cell division encodes an overactive protein kinase similar to the development of cancer.
kinases in the cell that function as signaling proteins. This viral Up to this point, we have considered what happens when
gene is named v-src, for viral-src (pronounced “sarc” and short for mechanisms that promote cell division are inappropriately
“sarcoma,” the type of cancer it causes). activated. Now let’s consider what happens when mechanisms
The v-src gene is one of several examples of an oncogene, that usually prevent cell cycle progression are removed.
or cancer-causing gene, found in viruses. A real surprise was the Earlier, we discussed cell cycle checkpoints that halt the cell
discovery that the v-src oncogene is found not just in the Rous cycle until the cell is ready to divide. One of these checkpoints
sarcoma virus. It is an altered version of a gene normally found depends on the p53 protein, which normally arrests cell division in
in the host animal cell, known as c-src (cellular-src). The c-src gene response to DNA damage. When the p53 protein is mutated or its
plays a role in the normal control of cell division during embryonic function is inhibited, the cell can divide before the DNA damage is
development. repaired. The result is that cells continue to divide in the presence
of damaged DNA, leading to the accumulation of mutations that
Proto-oncogenes are genes that when mutated may promote cell division. The p53 protein is mutated in many types
cause cancer. of human cancer, highlighting its critical role in regulating the
The discovery that the v-src oncogene has a normal counterpart cell cycle.
in the host cell was an important step toward determining the The p53 protein is one example of a tumor suppressor.
cellular genes that participate in cell growth and division. These Tumor suppressors are proteins whose normal activities inhibit
normal cellular genes are called proto-oncogenes. They are cell division. Some tumor suppressors participate in cell cycle
involved in cell division, but do not themselves cause cancer. Only checkpoints, as is the case for p53. Other tumor suppressors
when they are mutated do they have the potential to cause cancer. repress the expression of genes that promote cell division, while
Today, we know of scores of proto-oncogenes, most of which were still others trigger cell death.
identified through the study of cancer-causing viruses in chickens, Tumor suppressors act in opposition to proto-oncogenes.
mice, and cats. Therefore, whether a cell divides or not depends on the activities
Oncogenes also play a major role in human cancers. Most of both proto-oncogenes and tumor suppressors: Proto-oncogenes
human cancers are not caused by viruses. Instead, human proto- must be turned on and tumor suppressors must be turned
oncogenes can be mutated into cancer-causing oncogenes by off. Given the importance of controlling cell division, it is not
environmental agents such as chemical pollutants. For example, surprising that cells have two counterbalancing systems that must
organic chemicals called aromatic amines present in cigarette be in agreement before cell division takes place.
smoke can enter cells and damage DNA, resulting in mutations
j Quick Check 8 How do oncogenes differ from tumor suppressor
that can convert a proto-oncogene into an oncogene.
genes?
What types of functions are performed by the products of
proto-oncogenes? Nearly every protein that performs a key step in
a signaling cascade that promotes cell division can be the product Most cancers require the accumulation of multiple
of a proto-oncogene. These include growth factors, cell-surface mutations.
receptors, G proteins, and protein kinases. Each of these can be Most human cancers require more than the overactivation of
mutated to become oncogenes. one oncogene or the inactivation of a single tumor suppressor.
Let’s consider an example of a proto-oncogene. In Chapter 9, Given the multitude of different tumor suppressor proteins that
we discussed platelet-derived growth factor, or PDGF. This protein are produced in the cell, it is likely that one will compensate for
 CHAPTER 11 C E L L D I V I S I O N : VA R I AT I O N S , R E G U L AT I O N , A N D C A N C E R 239

even the complete loss of another. Cells have evolved a redundant

FIG. 11.20 Multiple-mutation model for the development of arrangement of these control mechanisms to ensure that cell
cancer. Cells that become cancerous typically carry division is properly regulated.
mutations in several different genes. When several different cell cycle regulators fail, leading
to both the overactivation of oncogenes and the loss of tumor
Inactivation of first suppressor activity, cancer will likely develop. The cancer may be
tumor suppressor gene benign, which means that it is relatively slow growing and does
not invade the surrounding tissue, or it may be malignant, which
means that it grows rapidly and invades surrounding tissues. In
many cases of malignant colon cancer, for example, tumor cells
contain at least one overactive oncogene and several inactive
tumor suppressor genes (Fig. 11.20). The gradual accumulation
Normal cells
of these mutations over a period of years can be correlated with
the stepwise progression of the cancer from a benign form to full
malignancy.
Taken together, we can now define some of the key
Activation of oncogene characteristics that make a cell cancerous. Uncontrolled
cell division is certainly important, but given the communities
of cells and extracellular matrix we have discussed over the last
few chapters, additional characteristics should also be considered.
In 2000, American biologists Douglas Hanahan and Robert
Weinberg wrote a paper in the journal Cell (Cell 100: 57–70) called
Benign cancer “Hallmarks of Cancer” in which they highlighted key features of
cancer cells. These include the ability to divide on their own in
the absence of growth signals; resistance to signals that inhibit
cell division or promote cell death; the ability to invade local
Inactivation of second and distant tissues (metastasis); and the production of signals to
tumor suppressor gene promote new blood vessel growth for nutrients to support cell
division.
Considered in this light, a cancer cell is one that no longer
plays by the rules of a normal cellular community. Cancer
therefore serves to remind us of the normal controls and processes
that are required to allow cells to exist in a community. These
Malignant cancer
processes, which work together, are summarized in Fig. 11.21 on
the following pages. •
Inactivation of third
tumor suppressor gene

Metastatic
cancer

Metastasis
at a new site
 V I S UA L S Y N T H E S I S Cellular Communities
FIG. 11.21 Integrating concepts from Chapters 9–11

Cell division 3 Metaphase:

Mitotic cell division is the process by
which a single cell divides into two cells
2 Prometaphase:
Chromosomes align in
center of cell.
Spindles attach to
that, except for rare mutations, are
kinetochores on
genetically identical to the parent cell. chromosomes.

4 Anaphase: Sister
chromatids (individual
1 Prophase: Chromosomes
chromosomes when the
centromere splits)
condense. Centrosomes
separate and travel to
radiate microtubules and
opposite poles.
migrate to opposite poles.

5 Telophase and
cytokinesis: Nuclear
envelope re-forms,
chromosomes
M phase condense, and the
G0 phase cytoplasm divides.
(mitosis and (cells that are
cytokinesis) not actively
dividing)

G2 phase
(Gap 2)

G1 phase
(Gap 1)

Interphase
S phase
(DNA synthesis)

Epidermis

Dermis

Source: Jose Luis Pelaez Inc/Blend

Images/Getty Images.

240
 Cell signaling
Cells communicate with each other and respond to
their environment using signaling molecules that
bind to receptors, leading to responses inside the cell.

Termination

Cell adhesion
Tissues are communities of cells that are held together
and communicate by various types of cellular junctions.
Other junctions connect cells with protein networks
in the extracellular environment.
Response

Signal
transduction
and
amplification

Receptor
Tight junction activation

Adherens
junction

Hemidesmosome
Desmosome Signaling
molecules

Gap junction

Basal lamina

Connective tissue Blood vessel

241
242 CO R E CO N C E P T S S U M M A RY

Core Concepts Summary

11.1 During cell division, a single parental cell Meiotic cell division is a form of cell division that
divides into two daughter cells. reduces the number of chromosomes by half to produce
haploid gametes or spores that have one copy of each
Prokaryotic cells divide by binary fission, in which a cell
chromosome. page 226
replicates its DNA, segregates its DNA, and divides into two
cells. page 220 Fertilization involves the fusion of haploid gametes to
produce a diploid cell. page 226
Eukaryotic cells divide by mitosis (nuclear division) and
cytokinesis (cytoplasmic division). Together, mitosis and Meiosis consists of two successive cell divisions: The first is
cytokinesis are known as mitotic cell division. page 222 reductional (the chromosome number is halved), and the
second is equational (the chromosome number stays the
M phase (mitosis and cytokinesis) alternates with
same). Each division consists of prophase, prometaphase,
interphase, which consists of G1 , S (synthesis), and
metaphase, anaphase, and telophase. page 226
G2 phases. These four stages together constitute the cell
cycle. page 222 In meiosis I, homologous chromosomes pair and exchange
genetic material at chiasmata, or regions of crossing over.
Cells that do not need to divide exit the cell cycle and are
In contrast to mitosis, centromeres do not split and sister
in G0. page 222
chromatids do not separate. page 227

11.2 Mitotic cell division is the basis of asexual Genetic diversity is generated by crossing over and random
reproduction in unicellular eukaryotes and the process alignment and subsequent segregation of maternal and
by which cells divide in multicellular eukaryotes. paternal homologs on the metaphase plate in meiosis I.
page 227
DNA in a eukaryotic cell is packaged as linear chromosomes.
page 222 Meiosis II is similar to mitosis, in which chromosomes
align on the metaphase plate, centromeres split, and sister
Humans have 46 chromosomes: 22 pairs of homologous chromatids separate from each other. page 228
chromosomes and 1 pair of sex chromosomes. Each parent
contributes one complete set of 23 chromosomes at The similarity of meiosis II and mitosis suggests that
fertilization. page 222 meiosis evolved from mitosis. page 231

During S phase, chromosomes replicate, resulting in The division of the cytoplasm differs between the
the formation of sister chromatids held together at the sexes: Male meiotic cell division produces four functional
centromere. page 223 sperm cells, whereas female meiotic cell division produces
a single functional egg cell and three polar bodies.
Mitosis involves five steps following DNA replication: page 231
(1) prophase—the chromosomes condense and become
visible under the light microscope; (2) prometaphase—
11.4 The cell cycle is regulated so that cell division
the spindles attach to the centromeres; (3) metaphase—
occurs only at appropriate times and places.
the chromosomes line up in the middle of the cell;
(4) anaphase—the centromeres split and the chromosomes Cells have regulatory mechanisms for promoting and
move to opposite poles; and (5) telophase—the nuclear preventing cell division. page 233
envelope re-forms and chromosomes decondense. page 223
Levels of proteins called cyclins increase and decrease
Mitosis is followed by cytokinesis, in which one cell divides during the cell cycle. page 233
into two. In animals, a contractile ring of actin pinches the
cell in two. In plants, a new cell wall, called the cell plate, is Cyclins form complexes with cyclin-dependent kinases
synthesized between the daughter cells. page 225 (CDKs), activating the CDKs to phosphorylate target
proteins involved in cell division. page 233

11.3 Meiotic cell division is essential for sexual These complexes are activated by signals that promote cell
reproduction, the production of offspring that combine division. page 234
genetic material from two parents.
Different cyclin–CDK complexes control progression
Sexual reproduction involves meiosis and fertilization, both of through the cell cycle at key steps, including G1 /S phase,
which are important in increasing genetic diversity. page 226 S phase, and M phase. page 235
 CCHHAAPPTTEERR1111 CCEELLLLDDI V
I VI SI SI O
I ONN: :VA
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I ATI O
I ONNSS, ,RREEGGUULLAT
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I ONN, ,AANNDDCCAANNCCEERR 224433

The cell also has checkpoints that halt progression through Self-Assessment
the cell cycle if something is not right. page 235
1. Compare and contrast the ways in which prokaryotic cells
The p53 protein is an example of a checkpoint protein; it and eukaryotic cells divide.
prevents cell division in the presence of DNA damage.
page 235 2. Describe three situations in which mitotic cell
division occurs.
11.5 Cancer is uncontrolled cell division that results 3. Name the five steps of mitosis, and draw the
from mutations in genes that control cell division. structure and position of the chromosomes at each step.
Cancer results when mechanisms that promote cell division 4. Describe how chromosomes behave in meiosis. State when
are inappropriately activated or the normal checks on cell chromosomes are duplicated (forming sister chromatids)
division are lost. page 236 and when they are not duplicated.
Cancers can be caused by certain viruses carrying oncogenes 5. Compare and contrast mitotic cell division and
that promote uncontrolled cell division. page 238 meiotic cell division in terms of number of products,
number of cell divisions, and the processes unique to each.
Viral oncogenes have cellular counterparts called proto-
oncogenes that play normal roles in cell growth and division 6. Name two ways in which meiotic cell division creates
and that, when mutated, can cause cancer. page 238 genetic diversity, and explain how each occurs.

Oncogenes and proto-oncogenes often encode proteins 7. Explain how cytokinesis differs between animal and
involved in signaling pathways that promote cell division. plant cells.
page 238 8. Describe the roles of cyclins and cyclin-dependent
Tumor suppressors encode proteins, such as p53, that block kinases in the cell cycle.
cell division and inhibit cancers. page 238 9. Give three examples of checkpoints that the cell
monitors before proceeding through the cell cycle.
Cancers usually result from several mutations in proto-
oncogenes and tumor suppressor genes that have 10. Describe what an oncogene, a proto-oncogene, and a
accumulated over time within the same cell. page 238 tumor suppressor gene do.

Log in to to check your answers to the Self-

Assessment questions, and to access additional learning tools.
244 SECTION 12.1 P

CASE 3

You, from A to T
Your Personal Genome

As an American Studies painstaking work, scientists published the first draft of the
major at Georgetown complete human genome. The human genome consists of
University, Claudia the DNA in one complete set of 23 chromosomes. It contains
Gilmore had plenty about 3 billion base pairs, the structural units of DNA. In
of experience taking the years that followed, much attention has been placed on
exams. But at the age understanding the genetic differences between individuals.
of 21, she faced an In reality, there isn’t one single human genome.
altogether different Everyone on Earth (with the exception of identical twins)
kind of test—and no has his or her own unique genetic sequence. Your personal
amount of studying genome, consisting of two sets of chromosomes, is the
could have prepared blueprint that codes for your hair color, the length of
Answers in the genetic her for the result. your nose, and your susceptibility to certain diseases. On
sequence. Claudia Gilmore Gilmore had average, any two human genomes are 99.9% identical,
found that her genome contains decided to be tested meaning that they differ at about 3 million base pairs.
a mutation that can lead to breast for a mutation in a Oftentimes, those differences have no impact on
cancer. Source: Photo by Dominic gene known as BRCA1. health. In some cases, however, a particular genetic
Gutierrez, courtesy Claudia Gilmore. Specific mutations in signature is associated with disease. Sometimes a gene
the BRCA1 and BRCA2 mutation makes a given illness inevitable. Certain
genes are associated with an increased risk of breast and mutations in a gene called HTT, for instance, always result
ovarian cancers. in Huntington’s disease, a degenerative brain condition that
Gilmore’s grandmother had battled breast cancer and usually appears in middle age.
later died from ovarian cancer. Before her death, she had The link between mutations and disease isn’t always so
tested positive for the BRCA1 mutation. Her son, Gilmore’s clear-cut, however. Most genetic diseases are complex in
father, was tested origin, and it may take multiple genetic mutations, as well
Mutations that increase and discovered he as other non-genetic factors, for the disease to develop.
the risk of developing a had inherited the Mutations that increase the risk of developing a particular
mutation. After disease are called genetic risk factors.
particular disease are talking with a Certain mutations in the BRCA1 and BRCA2 genes are
called genetic risk factors. genetic counselor to known genetic risk factors for breast and ovarian cancers,
help her understand for instance. But not everyone with these mutations devel-
the implications of the test, Gilmore gave a sample of her ops cancer. According to the National Cancer Institute,
own blood and crossed her fingers. about 60% of women with a harmful BRCA1 mutation will
“I had a fifty-fifty chance of inheriting the mutation. I develop breast cancer in her lifetime, compared with about
knew there was a great possibility it would be a part of my 12% of women in the general population. And 15% to 40%
future,” she says. “But I was 21, I was healthy. A part of me of women with a BRCA1 mutation will be diagnosed with
also thought this could never really happen to me.” ovarian cancer, compared with just 1.4% of women without
Unfortunately, it could. Two weeks after her blood was that genetic change.
drawn, she learned that she, too, carried the mutated gene. The BRCA1 mutations are just some of the thousands of
Genetic testing is becoming increasingly common— harmful genetic changes that geneticists have identified so
in some cases, even routine. In 2003, after 13 years of far. Other common mutations have been shown to elevate

244
 The BRCA1 and BRCA2 What does your genome say about you? Researchers have identified genes that, when
genes are associated mutated, contribute to disease, as well as differences that help reveal who you are and
BRCA2 with an increased risk
of breast and ovarian where you come from. Sources: (left to right) Willatt/Science Source; ISM/Phototake; ISM/Phototake.
gene
cancer.
Chromosome 13
A mutation in the HTT
gene on chromosome
BRCA1 3 causes Huntington’s
gene HTT disease.
gene The particular pattern
Chromosome 17 of mutations inherited
on the Y chromosome
can reveal a person’s
ancestry.
Chromosome 3
Distinct
pattern
of SNPs

Y chromosome

one’s risk of developing heart disease, diabetes, various the shape of your fingernails to the shade of your skin—
cancers, and numerous other common illnesses. Often, that don’t affect your health but make you the person you
these mutations involve changes to just a single base are. Some DTC testing services aim to tell customers about
pair of DNA. These common single-letter mutations are their history by screening genes to identify variations that
known as single nucleotide polymorphisms, or SNPs. are more common in certain geographical regions or among
Genome sequencing technology has improved members of certain ethnic groups. One such company offers
significantly over the last decade, making it easier and genetic tests to African-Americans to determine in what part
less expensive to scan an individual’s DNA for potentially of the African continent their ancestors originated.
harmful SNPs. A number of companies now offer genetic Other popular DTC tests inspect DNA samples for
tests directly to the public. Unlike tests such as the SNPs associated with certain diseases and physical traits—
BRCA1 blood test that Gilmore was given, these direct-to- everything from Parkinson’s disease and age-related macular
consumer (DTC) tests are offered to customers without degeneration to earwax type and propensity for baldness.
any involvement of a medical professional, at a cost of a Advocates of the tests say the technology puts the power
few hundred dollars. of genetic information in the hands of consumers. Critics, on
Some of these tests aren’t related to health at all. Your the other hand, argue that the information provided by DTC
personal genome contains many unique features—from tests isn’t always very meaningful. The presence of some SNPs

245
246 SECTION 11.1 P

might raise the risk of a rare disease by just 2% or 3%, for author Angelina Jolie, who recounted her experience
example. In many cases, the precise link between mutation in the New York Times, writing, “Once I knew that
and disease is still being sorted out. Also, without input from this [BRCA1 mutation] was my reality, I decided to be
a genetic counselor or medical professional, consumers may proactive and to minimize the risk as much I could. I
not know how to interpret the information revealed by the made a decision to have a preventive double mastectomy.
tests. The American Medical Association has recommended I started with the breasts, as my risk of breast cancer
that a doctor always be involved when any genetic testing is is higher than my risk of ovarian cancer, and the
performed. surgery is more complex. . . . I am writing about it now
Moreover, knowing your genetic risk factors isn’t the because I hope that other women can benefit from my
whole story. When it comes to your health and well-being, experience.” 1
the environment also plays a significant role. Someone For now, it’s still too costly to sequence every
might override a genetic predisposition for skin cancer by individual’s entire genome. But each year, many more
using sunscreen faithfully every day. On the other hand, a genetic tests hit the market. Already, doctors are
person might have a relatively low genetic risk for type 2 beginning to design medical treatments based on a
diabetes, but still boost the odds of developing the disease patient’s personal genome. People with a certain genetic
by eating a poor diet and getting little physical exercise. profile, for example, are less likely than others to benefit
Claudia Gilmore is especially careful to exercise from statins, medications prescribed to lower cholesterol.
regularly and eat a healthy diet. Still, she can only control Doctors are also choosing which cancer drugs to prescribe
her environment to a degree. She knew that, given her based on the unique genetic signatures of patients and
genetic status, her risk of breast cancer remained high. their tumors.
She made the extraordinary decision to eliminate that risk We’ve only just entered the era of personal genomics.
by undergoing a mastectomy at the age of 23. It wasn’t While there’s much left to decipher, it’s clear that each
an easy decision, she says, but she feels privileged to have of our individual genomes contains a wealth of biological
been able to take proactive steps to protect her health. “I’ll information. And, as Gilmore says, “I’ve always been
be a ‘previvor’ instead of a survivor,” she says. taught that knowledge is power.”
Two years later the same medical choice was made 1
Angelina Jolie, “My Medical Choice,” New York Times, A25, May 14,
by the American actress, film director, screenwriter, and 2013.

?C A S E 3 Q U E S T I O N S
Special sections in Chapters 12–20 discuss the following questions related to Case 3.

1. What new technologies are being developed to sequence your personal genome? See page 264.
2. Why sequence your personal genome? See page 274.
3. What can your personal genome tell you about your genetic risk factors? See page 294.
4. How can genetic risk factors be detected? See page 317.
5. How do genetic tests identify disease risk factors? See page 342.
6. How can the Y chromosome be used to trace ancestry? See page 358.
7. How can mitochondrial DNA be used to trace ancestry? See page 360.
8. Can personalized medicine lead to effective treatments of common diseases? See page 375.
9. How do lifestyle choices affect expression of your personal genome? See page 386.
10. Can cells with your personal genome be reprogrammed for new therapies? See page 403.

246
 CHAPTER 12

DNA Replication
and Manipulation

Core Concepts
12.1 In DNA replication, a single
parental molecule of DNA
produces two daughter
molecules.
12.2 The replication of linear
chromosomal DNA requires
mechanisms that ensure
efficient and complete
replication.
12.3 Techniques for manipulating
DNA follow from the basics
of DNA structure and
replication.
12.4 Genetic engineering allows
researchers to alter DNA
sequences.
Dr. Gopal Murti/Science Source.

247
248 SECTION 12.1 D N A R E P L I C AT I O N

One of the overarching themes of biology discussed in Chapter 1 is the template strand determines the order of the complementary
that the functional unit of life is the cell, a theme that rests in turn bases added to the daughter strand. For example, the sequence
on the fundamental concept that all cells come from preexisting 5�-ATGC-3� in the template strand specifies the sequence 3�-TACG-5�
cells. In Chapter 11, we considered the mechanics of cell division in the daughter strand because A pairs with T and G pairs with C.
and how it is regulated. Prokaryotic cells multiply by binary fission, (The designations 3� and 5� convey the antiparallel orientation of
whereas eukaryotic cells multiply by mitosis and cytokinesis. These the strands.)
processes ensure that cellular reproduction results in daughter A key prediction of the model shown in Fig. 12.1 is
cells that are like the parental cell. In other words, in cell division, semiconservative replication. That is, after replication, each
like begets like. The molecular basis for the resemblance between new DNA duplex consists of one strand that was originally part
parental cells and daughter cells is that a double-stranded DNA of the parental duplex and one newly synthesized strand. An
molecule in the parental cell duplicates and gives rise to two alternative model is conservative replication, which proposes that
double-stranded daughter DNA molecules that are identical to each the original DNA duplex remains intact and the daughter DNA
other except for rare mutations that may have taken place. duplex is completely new. Which model is correct? If there were
The process of duplicating a DNA molecule is called DNA a way to distinguish newly synthesized daughter DNA strands
replication, and it occurs in virtually the same way in all (“new strands”) from previously synthesized parental strands
organisms, reflecting its evolution very early in life’s history. The (“old strands”), the products of replication could be observed and
process is conceptually simple—the parental strands separate and the mode of replication determined.
new complementary partner strands are made—but the molecular American molecular biologists Matthew S. Meselson and
details are more complicated. Once scientists understood the Franklin W. Stahl carried out an experiment to determine how
molecular mechanisms of DNA replication, they could devise DNA replicates. This experiment, described in Fig. 12.2, has been
improved methods for manipulating and studying DNA. An called “the most beautiful experiment in biology” because it so
understanding of DNA replication is therefore fundamental not elegantly demonstrated the scientific method (Chapter 1) of
only to understanding how cells and organisms produce offspring hypothesis, prediction, and experimental test. They distinguished
like themselves, but also to understanding some of the key “old” from “new” DNA strands by labeling them with
experimental techniques in modern biology. nonradioactive isotopes of nitrogen that have different densities:
the normal form of nitrogen, 14N, and a heavier form with an extra
neutron, denoted 15N.
12.1 DNA REPLICATION Meselson and Stahl found that DNA in fact replicates
semiconservatively (Fig. 12.2). This finding also predicted the
You may recall from Chapter 3 that double-stranded DNA consists results when cells are allowed to undergo two rounds of replication
of a pair of deoxyribonucleotide polymers wound around each in a medium containing only light 14N nitrogen. The heavy strand
other in antiparallel helical coils in such a way that a purine and light strand each serve as templates for a new light daughter
base (A or G) in one strand is paired with a pyrimidine base (T strand. The result is that half of the DNA molecules will have one
or C, respectively) in the other strand. To say that the strands
are antiparallel means that they run in opposite directions: One
strand runs in the 5�-to-3� direction and the other runs in the
3�-to-5� direction. These key elements of DNA structure are the FIG. 12.1 DNA replication. During DNA replication, each
only essential pieces of information needed to understand the parental strand serves as a template for the synthesis of a
mechanism of DNA replication. complementary daughter strand.
5’

During DNA replication, the parental strands

separate and new partners are made. During replication, the two
strands of the parental
When Watson and Crick published their paper describing duplex separate. 3’
the structure of DNA, they also coyly laid claim to
another discovery: “It has not escaped our notice that 3’ Each parental strand
5’ serves as a template
the specific pairing we have postulated [A with T, and for the synthesis of a
Replication
G with C] immediately suggests a copying mechanism new daughter strand,
fork
for the genetic material.” The copying mechanism they 5’ using the base-pairing
3’ rules of A with T and G
had in mind is exquisitely simple. The two strands of the with C.
parental duplex molecule separate (Fig. 12.1), and each The bases are paired
in the parental duplex,
individual parental strand serves as a model, or template either A–T or G–C. 5’
strand, for the synthesis of a daughter strand. As each
daughter strand is synthesized, the order of the bases in
3’
HOW DO WE KNOW?

FIG.12.2

How is DNA replicated? 1 At the beginning

of the experiment,
both strands are
BACKGROUND Watson and Crick’s discovery of the structure
labeled with heavy
of DNA in 1953 suggested a mechanism by which DNA is nitrogen (15N). The
replicated. Experimental evidence came from research by molecules form a
band with the density
American molecular biologists Matthew Meselson and Franklin of “heavy” DNA.
Stahl in 1958.

HYPOTHESIS DNA replicates in a semiconservative manner,

meaning that each new DNA molecule consists of one parental
1.722 2 After one round of
strand and one newly synthesized strand. Density replication in the
of DNA absence of heavy
ALTERNATIVE HYPOTHESIS DNA replicates in a (gm/cm3) nitrogen, the parental
conservative manner, meaning that one DNA molecule consists strand still contains
the heavy nitrogen
of two parental strands, and the other consists of two newly (15N), but the
synthesized strands. daughter strand
contains light
METHOD Meselson and Stahl distinguished parental strands nitrogen (14N). The
daughter DNA
(“old”) from newly synthesized strands (“new”) using two molecules form a
1.715
isotopes of nitrogen atoms. Old strands were labeled with a band with an
heavy form of nitrogen with an extra neutron (15N), and new intermediate density.
strands were labeled with the normal, lighter form of nitrogen
(14N). 3 After two rounds
of replication, half of
EXPERIMENT The researchers first grew bacterial cells on the duplex DNA
medium containing only the heavy 15N form of nitrogen. As the molecules have one
strand with 15N and
cells grew, 15N was incorporated into the DNA bases, resulting, one with 14N, and the
after several generations, in DNA containing only 15N. They other half have both
then transferred the cells into medium containing only light strands with 14N. The
daughter DNA
14
N nitrogen. After one round of replication in this medium, 1.708 molecules therefore
cell replication was halted. The researchers could not observe form two bands, one
1.715
intermediate in
the DNA directly, but instead they measured the density of density and the other
the DNA by spinning it in a high-speed centrifuge in tubes light.
containing a solution of cesium chloride.

PREDICTION If DNA replicates conservatively, half of the

DNA in the cells should be composed of two heavy parental composed only of 14N would have a density of 1.708 gm/cm3.
strands containing 15N, and half should be composed of two After two rounds of replication, half the molecules exhibited
light daughter strands containing 14N. If DNA replicates a density of 1.715 gm/cm3, indicating a duplex molecule
semiconservatively, the daughter DNA molecules should each with one heavy strand and one light strand, and the other half
consist of one heavy strand and one light strand. exhibited a density of 1.708 gm/cm3, indicating a duplex
molecule containing two light strands.
RESULTS When the fully 15N-labeled parental DNA was
spun, it concentrated in a single heavy band with a density CONCLUSION DNA replicates semiconservatively, supporting
of 1.722 gm/cm3. After one round of replication, the DNA the first hypothesis.
formed a band at a density of 1.715 gm/cm3, which is the
density expected of a duplex molecule containing one heavy SOURCE Meselson, M., and F. W. Stahl. 1958. “The Replication of DNA in
(15N-labeled) strand and one light (14N-labeled) strand. DNA Escherichia coli.” PNAS 44:671–82.

249
250 SECTION 12.1 D N A R E P L I C AT I O N

heavy old strand and one light new strand and an intermediate
density, and half of the DNA molecules will have one light old FIG. 12.4 DNA synthesis by nucleotide addition to the 3� end of a
strand and one light new strand and a low density. This is precisely growing DNA strand.
what they observed (Fig. 12.2). Template Daughter
3’
j Quick Check 1 Suppose Meselson and Stahl had done their
OH
experiment the other way around, starting with cells fully labeled 5’
with 14N light DNA and then transferring them to medium Direction of
T A P
containing only 15N heavy DNA. What density of DNA molecule P chain growth
would you predict after one and two rounds of replication?
G C P
P
Important as it was in demonstrating semiconservative 1 Incoming
replication in bacteria, the Meselson–Stahl experiment left A T P nucleotides are
P accepted if they
open the possibility that DNA replication in eukaryotes might correctly base pair
be different. It was only some years after the Meselson–Stahl C
OH with the template.
P
experiment that methods for labeling DNA with fluorescent 3’
nucleotides were developed. These methods allowed researchers
T G
to visualize entire strands of eukaryotic DNA and follow each P 2 The 3’ OH of the
strand through replication. Fig. 12.3 shows a human chromosome P P P growing strand
with unlabeled DNA that subsequently underwent two rounds of P G attacks the high-
energy phosphate
replication in medium containing a fluorescent nucleotide. The 5’ OH bond of the
chromosome was photographed at metaphase of the second round incoming nucleotide
P P to initiate the
of mitosis, after chromosome duplication but before the separation
synthesis reaction.
of the chromatids into the daughter cells. Notice that one chromatid Pyrophosphate
contains hybrid DNA with one labeled strand and one unlabeled
strand, which fluoresces faintly (light); the other chromatid
contains two strands of labeled DNA, which fluoresces strongly implies that both daughter strands should grow in length by the
(dark). This result is conceptually the same as what was seen by addition of nucleotides near the site where the parental strands
Meselson and Stahl after two rounds of replication and exactly as separate, a site called the replication fork. As more and more
predicted by the semiconservative replication model. The result also parental DNA is unwound and the replication fork moves forward
demonstrates that each eukaryotic chromosome contains a single (to the left in Fig. 12.1), both new strands would also grow in the
DNA molecule that runs continuously all along its length. direction of replication fork movement. But it turns out that this
scenario is impossible.
New DNA strands grow by the addition of nucleotides We have seen that the two DNA strands in a double helix run in
to the 3’ end. an antiparallel fashion: One of the template strands (the bottom one
Although replication is semiconservative, this model alone does in Fig. 12.1) has a left-to-right 5�-to-3� orientation, whereas the other
not tell us the details of replication. For example, the model template strand (the top one in Fig. 12.1) has a left-to-right 3�-to-5�
orientation. Therefore, the new daughter strands also have opposite
orientations, so that near the replication fork the daughter strand in
FIG. 12.3 Eukaryotic DNA replication. Further evidence that DNA the bottom duplex terminates in a 3� hydroxyl, whereas that in the
replication is semiconservative came from observing top duplex terminates in a 5� phosphate. There’s the rub: The strand
the uptake of fluorescent nucleotides into chromosomal that terminates in the 5� phosphate cannot grow in the direction of
DNA. Source: Daniel Hartl. the replication fork because new DNA strands can grow only by the
addition of successive nucleotides to the 3�end. That is, DNA always
Half-labeled Fully labeled
After two rounds of DNA
grows in the 5�-to-3� direction.
replication in a labeled DNA polymerization occurs only in the 5�-to-3� direction
medium, one daughter because of the chemistry of nucleic acid synthesis, discussed in
molecule is half-labeled,
and the other is fully labeled Chapter 3. The building blocks of DNA (Chapter 2) are nucleotides,
(compare with Figure 12.2). each consisting of a deoxyribose sugar with three phosphate
groups attached to the 5� carbon, a nitrogenous base (A, T, G, or
C) attached to the 1� carbon, and a free hydroxyl (–OH) group
attached to the 3� carbon. DNA polymerization occurs when the
 CHAPTER 12 D N A R E P L I C AT I O N A N D M A N I P U L AT I O N 251

3� hydroxyl at the growing end of the polynucleotide chain attacks polymerase enzymes, each specialized for a particular situation.
the triphosphate group at the 5� end of an incoming nucleotide But all DNA polymerases share the same basic function in that
(Fig. 12.4). Each of these incoming nucleotides is a nucleotide they synthesize a new DNA strand from an existing template.
triphosphate (three phosphate groups attached to the 5� carbon Most, but not all, also correct mistakes in replication, as we will
of the deoxyribose). As the incoming nucleotide triphosphate is see. DNA polymerases have many practical applications in the
added to the growing DNA strand, one of the nucleotide’s high- laboratory, which we will discuss later in this chapter.
energy phosphate bonds is broken to release the outermost two
phosphates (called pyrophosphate), and immediately the high- In replicating DNA, one daughter strand is synthesized
energy phosphate bond in the pyrophosphate is cleaved to drive continuously and the other in a series of short pieces.
the polymerization reaction forward and make it irreversible. Because a new DNA strand can be elongated only at the 3� end, the
The polymerization reaction is catalyzed by DNA polymerase, two daughter strands are synthesized in quite different ways
an enzyme that is a critical component of a large protein complex (Fig. 12.5). The daughter strand shown at the bottom of Fig. 12.5 has
that carries out DNA replication. DNA polymerases exist in all its 3� end pointed toward the replication fork, so that as the parental
organisms and are highly conserved, meaning that they vary double helix unwinds, nucleotides can be added onto the 3� end and
little from one species to another because they carry out an this daughter strand can be synthesized as one long, continuous
essential function. A cell typically contains several different DNA polymer. This daughter strand is called the leading strand.

FIG. 12.5 Leading and lagging strand synthesis. In DNA replication, because the template strand is made of two antiparallel strands, one daughter
strand (the leading strand) is synthesized continuously and the other (the lagging strand) is synthesized in smaller pieces.

Newly
synthesized
Template DNA
strands 5’
3’
Unwinding of the
3’ 5’ DNA duplex results
3’ RNA primer
5’ in a replication fork.
Unwinding 5’
3’

5’ Replication always
3’ occurs in the 5’ to
5’ 3’ direction. The
3’ daughter strand
Elongation
5’ 3’ on top elongates
from left to right,
5’ that on the bottom
3’ from right to left.

5’ Replication of the
3’ top strand is
3’ 5’ discontinuous
3’ 5’
3’ (fragmented),
5’
whereas that of the
5’ bottom strand is
3’ continuous.

Lagging strand
5’ Primers are
Discontinuous
3’ removed and
DNA synthesis
replaced with DNA,
3’ 5’ Primer replacement and the fragments
5’ 3’ and ligation of the discontinuous
(lagging) strand are
Leading strand 5’ ligated where they
Continuous 3’ meet.
DNA synthesis
252 SECTION 12.1 D N A R E P L I C AT I O N

The situation is different for the daughter strand shown at

the top in Fig. 12.5. Its 5� end is pointed toward the replication FIG. 12.6 Synthesis and removal of RNA primers in the lagging
fork, but the strand cannot grow in that direction. Instead, as the strand.
replication fork unwinds, it grows away from the fork and forms a RNA RNA
primase primer
stretch of single-stranded DNA of a few hundred to a few thousand
Template
nucleotides, depending on the species. Then, as the parental DNA 5’
strand 3’
duplex unwinds further, a new daughter strand is initiated with
3’
its 5� end near the replication fork, and this strand is elongated at 3’ 5’ RNA primase lays
5’ down an RNA
the 3� end as usual. The result is that the daughter strand shown at primer.
the top in Fig. 12.5 is actually synthesized in short, discontinuous
pieces. As the parental double helix unwinds, a new piece is 3
3’

initiated at intervals, and each new piece is elongated at its 3� end

DNA
until it reaches the piece in front of it. This daughter strand is polymerase 5’
called the lagging strand. The short pieces in the lagging strand 3’ 3’
are sometimes called Okazaki fragments after their discoverer, 3’ DNA polymerase
5’
Japanese molecular biologist Reiji Okazaki. 5’ extends the RNA
primer.
The presence of leading and lagging strands during DNA
replication is a consequence of the antiparallel nature of the two 3
3’
strands in a DNA double helix, and the fact that DNA polymerase RNA DNA
primer polymerase
can synthesize DNA in only one direction. removed
5’
3’ A different DNA
A small stretch of RNA is needed to begin synthesis of 3’ 5’ polymerase
3’ 5’ removes the
a new DNA strand. 5’ primer and
Each new DNA strand must begin with a short stretch of RNA replaces it with
that serves as a primer, or starter, for DNA synthesis. The DNA.
3’
primer is needed because the DNA polymerase complex cannot DNA
begin a new strand on its own; it can only elongate the end of ligase
an existing piece of DNA or RNA. The primer is made by an RNA 5’
3’
polymerase called RNA primase, which synthesizes a short DNA ligase
piece of RNA complementary to the DNA template and does not 3’ 5’ forms a bond
5’
require a primer. Once the primer has been synthesized, the DNA joining the two
DNA fragments.
polymerase takes over and elongates the primer, adding successive
3’
DNA nucleotides to the 3� end of the growing strand.
Because the DNA polymerase complex extends an RNA primer,
all new DNA strands have a short stretch of RNA at their 5� end.
For the lagging strand, there are many such primers, one for complex that replaces the RNA primer with DNA, and the DNA
each of the discontinuous fragments of newly synthesized DNA. ligase that joins the DNA fragments in the lagging strand. Many
As each of these fragments is elongated by DNA polymerase, it other proteins and enzymes work at the same time to ensure
grows toward the primer of the fragment in front of it. When the accurate and efficient synthesis of the daughter strands. One of
growing fragment comes into contact with the primer, a different these, topoisomerase II, works upstream from the replication
DNA polymerase complex takes over, removing the RNA primer fork to relieve the stress on the double helix that results from
and extending the growing fragment with DNA nucleotides its unwinding at the replication fork. In the meantime, helicase
to fill the space left by removal of the RNA primer. When the separates the strands of the parental double helix at the replication
replacement is completed, the adjacent fragments are joined, or fork. Then, single-strand binding protein binds to these single-
ligated, by an enzyme called DNA ligase. This process is illustrated stranded regions to prevent the template strands from coming
in Fig. 12.6. back together. Although many evolutionarily conserved proteins
are required for DNA replication, the underlying process is quite
Synthesis of the leading and lagging strands is simple and the same in all organisms. The two strands of the
coordinated. parental DNA duplex separate, and each serves as a template for
Fig. 12.7a shows an overview of DNA replication including the the synthesis of a daughter strand according to the base-pairing
DNA polymerase complex that elongates each strand at the 3� end, rules of A–T and G–C, with each successive nucleotide being added
the RNA primase that makes the primer, the DNA polymerase to the 3� end of the growing strand.
 CHAPTER 12 D N A R E P L I C AT I O N A N D M A N I P U L AT I O N 253

FIG. 12.7 The replication fork. (a) Many proteins play different roles in replication, including separating and stabilizing the strands of the double
helix. (b) In the cell, proteins involved in replication come together to form a complex that replicates both strands at the same pace.
a.
3’
DNA polymerase

DNA polymerase extends

an RNA primer (red).

Lagging
strand
Helicase unwinds the
parental DNA strands. 5’

The DNA polymerase complex

acts at the site of the growing
chain to increase the chain
length one DNA subunit at a
3’ time, checking for errors.

Topoisomerase II relieves
the stress of unwinding.

Single-strand binding Leading

protein stabilizes single strand
strands of DNA.

5’
b.
5’

5’
Lagging
strand
3’

3’

5’
3’
Leading
strand

As each new primer for an Okazaki fragment is 5’

synthesized, the lagging strand forms a loop that
persists until the new lagging strand encounters 3’
the previous Okazaki fragment.

While Figure 12.7a makes it clear how synthesis of the leading complexes for each strand stay in contact with each other, with the
strand and the lagging strand take place, it does not show how lagging strand’s polymerase releasing and retrieving the lagging
synthesis of these strands occurs at the same time and rate. To strand for the synthesis of each new RNA primer. The positioning
help ensure that both strands of the double helix are replicated at of the polymerases is such that both the leading strand and the
nearly the same rate, synthesis of the leading and lagging strands lagging strand pass through in the same direction, which requires
is coordinated as shown in Fig. 12.7b. The DNA polymerase that the lagging strand be looped around as shown in Fig. 12.7b. In
254 SECTION 12.2 R E P L I C AT I O N S O F C H RO M O S O M E S

of the previous Okazaki fragment is encountered, and then takes

FIG. 12.8 Proofreading, the process by which an incorrect place again when a new lagging-strand primer is formed. The
nucleotide is removed immediately after it is lagging-strand loop is sometimes called a “trombone loop.”
incorporated.
DNA polymerase is self-correcting because of its
proofreading function.
Most DNA polymerases can correct their own errors in a process
called proofreading, which is a separate enzymatic activity from
Replication bubble strand elongation (synthesis) (Fig. 12.8).
When each new nucleotide comes into line in preparation
for attachment to the growing DNA strand, the nucleotide is
DNA temporarily held in place by hydrogen bonds that form between
polymerase
Parent the base in the new nucleotide and the base across the way in the
strand template strand. The strand being synthesized and the template
Very rarely, an strand therefore have complementary bases—A paired with T, or
3’ incorrect nucleotide
is added. G paired with C. However, on rare occasions, improper hydrogen
5’ bonds form, with the result that an incorrect nucleotide is attached
Daughter
strand Incorrect to the new DNA strand. DNA polymerase can correct errors because
nucleotide it detects mispairing between the template and the most recently
added nucleotide. Mispairing between a base in the parental strand
and a newly added base in the daughter strand activates a DNA-
When this happens, cleavage function of DNA polymerase that removes the incorrect
the proofreading nucleotide and inserts the correct one in its place.
function of DNA
3’ polymerase removes Mutations resulting from errors in nucleotide incorporation
the incorrect still occur, but proofreading reduces their number. In the
5’ nucleotide. bacterium E. coli, for example, about 99% of the incorrect
nucleotides that are incorporated during replication are removed
and repaired by the proofreading function of DNA polymerase.
Those that slip past proofreading and other repair systems
Then the correct (Chapter 14) lead to mutations, which are then faithfully copied
nucleotide is added and passed on to daughter cells. Some of these mutations may be
3’ to replace the harmful, but others are neutral and a rare few may be beneficial.
incorrect one.
5’ These mutations are the ultimate source of genetic variation that
we see among individuals of the same species and among species,
as we explore further in Chapter 15.

12.2 REPLICATION OF CHROMOSOMES

3’ The steps involved in DNA replication are universal, suggesting
5’ that they evolved in the common ancestor of all living organisms.
In addition, they are the same whether they occur in a test tube or
in a cell, or whether the segment of DNA being replicated is short
or long. However, the replication of an entire linear chromosome
this configuration, the 3ˇ end of the lagging strand and the 3ˇ end poses particular challenges. Here, we consider two such challenges
of the leading strand are elongated together, which ensures that encountered by cells in replicating their chromosomes: how
neither strand outpaces the other in its rate of synthesis. replication starts and how it ends.
As we will see in Chapter 14, when DNA damage occurs during
replication, the rate of synthesis slows down so that the DNA can Replication of DNA in chromosomes starts at many
be repaired. If synthesis of one strand slows down to repair DNA places almost simultaneously.
damage, synthesis of the other strand slows down, too. The pairing DNA replication is relatively slow. In eukaryotes, it occurs at a
of the replication complexes is disrupted when the RNA primer rate of about 50 nucleotides per second. At this rate, replication
 CHAPTER 12 D N A R E P L I C AT I O N A N D M A N I P U L AT I O N 255

from end to end of the DNA molecule in the largest human

chromosome would take almost two months. In fact, it takes FIG. 12.10 Replication of a circular bacterial chromosome.
only a few hours. This fast pace is possible because in a long DNA
molecule, replication begins almost simultaneously at many Replication starts at the
origin and moves around
places. Each point at which DNA synthesis is initiated is called an Circular Origin of the circular chromosome
origin of replication. The opening of the double helix at each chromosome replication in both directions.
origin of replication forms a replication bubble with a replication
fork on each side, each with a leading strand and a lagging strand
Replication
with topoisomerase II, helicase, and single-strand binding protein forks
playing their respective roles (Fig. 12.9). DNA synthesis takes
place at each replication fork, and as the replication forks move
in opposite directions the replication bubble increases in size.
When two replication bubbles meet, they fuse to form one larger
replication bubble.
Note in Fig. 12.9 that within a single replication bubble, the
same daughter strand is the leading strand at one replication fork
and the lagging strand at the other replication fork. This situation
results from the fact that the replication forks in each replication
bubble move away from each other. When two replication bubbles Some DNA molecules, including most of the DNA molecules
fuse and the leading strand from one meets the lagging strand in bacterial cells and the DNA in mitochondria and chloroplasts
from the other, the ends of the strands that meet are joined by (Chapter 3) are small circles, not long linear molecules. Such
DNA ligase, just as happens when the discontinuous fragments circular DNA molecules typically have only one origin of
within the lagging strand meet (see Fig. 12.6). replication (Fig. 12.10). Replication takes place at both replication
forks, and the replication forks proceed in opposite directions
around the circle until they meet and fuse on the opposite side,
completing one round of replication.
FIG. 12.9 Origins of replication. The replication of eukaryotic
chromosomal DNA is initiated at many different places on
Telomerase restores tips of linear chromosomes
the chromosome.
shortened during DNA replication.
A circular DNA molecule can be replicated completely because it
Replication can begin at any origin of Each replication bubble
replication. Eukaryotic chromosomes has two replication has no ends, and the replication forks can move completely around
have many origins of replication, whereas forks that move in the circle (Fig. 12.10). Linear DNA molecules have ends, however,
prokaryotic chromosomes have one. opposite directions.
and at each round of DNA replication the ends become slightly
Origins of replication shorter. The reason for the shortening is illustrated in Fig. 12.11.
Recall that each fragment of newly synthesized DNA starts
3’ 5’
5’ 3’ with an RNA primer. On the leading strand, the only primer
required is at the origin of replication when synthesis begins. The
leading strand is elongated in the same direction as the moving
Replication At each replication fork, the new replication fork and is able to replicate the template strand all the
bubbles grow strand with the free 3’ end is the way to the end. But on the lagging strand, which grows away from
as replication leading strand and that with the
continues. free 5’ end is the lagging strand. the replication fork, many primers are required, and the final RNA
primer is synthesized about 100 nucleotides from the 3� end of
3’ 5’ the template. Because this primer initiates synthesis of the final
5’ 3’
Okazaki fragment of the lagging strand, there is no other Okazaki
fragment to synthesize the missing 100 base pairs and remove the
primer. Therefore, when DNA replication is complete, the new
When two replication bubbles daughter DNA strand (light blue in Fig. 12.11) will be missing about
meet, they fuse to make one 100 base pairs from the tip (plus a few more owing to the length
larger bubble.
of the primer). When this daughter strand is itself replicated, the
newly synthesized strand must terminate at the shortened end of
3’ 5’
5’ 3’ the template strand, and so the new duplex molecule is shortened
by about 100 base pairs from the original parental molecule. The
256 SECTION 12.2 R E P L I C AT I O N S O F C H RO M O S O M E S

strand shortening in each round of DNA replication is a problem

FIG. 12.11 Shortening of linear DNA at the ends. Because an because without some mechanism to restore the tips, the DNA in
RNA primer cannot be synthesized precisely at the 3� the chromosome would eventually be nibbled away to nothing.
end of a DNA strand, its partner strand becomes a little Fig. 12.12 illustrates the mechanism that eukaryotic
shorter in each round of replication. organisms have evolved to solve the problem of shortened ends.
Each end of a eukaryotic chromosome is capped by a repeating
sequence called the telomere. The repeating sequence that
Ends of template Replication fork
DNA strands constitutes the telomere differs from one group of organisms to
the next, but in the chromosomes of humans and other vertebrate
it consists of the sequence 3�-GGGATT-5� repeated over and over
again in about 1500–3000 copies. For simplicity, just four copies
of this telomere sequence are shown in Fig. 12.12. The telomere
The leading strand Lagging strand is slightly shortened in each round of DNA replication, as shown
replicates the whole in Fig. 12.11, but the shortened end is quickly restored by an
template strand. Leading strand enzyme known as telomerase, which contains an RNA molecule
complementary to the telomere sequence.
As shown in Fig. 12.12, the telomerase replaces the lost
telomere repeats using its RNA molecule as a guide to add
The last RNA primer Template strand successive DNA nucleotides to the 3� end of the template strand.
on the lagging 3’
strand sits near the Once the 3� end of the template strand has been elongated, an
5’
end of the template
RNA primer Lagging strand
RNA primer and the complementary DNA strand are synthesized.
strand.
In this way the original telomere is completely restored. Because
there are no genes in the telomere, the slight shortening
Unreplicated and subsequent restoration that take place have no harmful
template DNA consequences.
The RNA primer is Telomerase activity differs from one cell type to the next. It is
removed, and a 3’
5’
fully active in germ cells, which produce sperm or eggs, and also
section of template
DNA remains in stem cells, which are undifferentiated cells that can undergo
unreplicated. an unlimited number of mitotic divisions and can differentiate
into any of a large number of specialized cell types. Stem cells are
found in embryos, where they differentiate into all the various cell
Daughter strand of types. Stem cells are also found in some tissues of the body after
In the next round of next generation
replication, the 3’
embryonic development, where they replenish cells that have a
shortened template high rate of turnover, such as blood and intestinal cells, and play a
results in a shorter 5’
chromosome. Shorter template role in tissue repair.
strand of next In contrast to the high activity of telomerase in germ cells
generation
and stem cells, telomerase is almost inactive in most cells in the
adult body. In these cells, the telomeres are actually shortened by
about 100 base pairs in each mitotic division. Telomere shortening
limits the number of mitotic divisions that the cells can undergo
because human cells stop dividing when their chromosomes
If this pattern were have telomeres with fewer than about 100 copies of the telomere
allowed to persist, the
chromosomes would be
repeat. Adult somatic cells can therefore undergo only about
severely shortened after 50 mitotic divisions until the telomeres are so short that the cells
several generations. stop dividing.
Many biologists believe that the limit on the number of cell
divisions explains in part why our tissues become less youthful and
wounds heal more slowly with age. The telomere hypothesis of aging
is still controversial, but increasing evidence suggests that it is one of
several factors that lead to aging. The flip side of the coin is observed
in cancer cells, in which telomerase is reactivated and helps support
the uncontrolled growth and division of abnormal cells.
 CHAPTER 12 D N A R E P L I C AT I O N A N D M A N I P U L AT I O N 257

FIG. 12.12 Telomerase. Telomerase prevents successive shortening of linear chromosomes. 12.3 ISOLATION,
IDENTIFICATION,
Ends of template AND SEQUENCING
Lagging strand
DNA strands OF DNA FRAGMENTS
Leading strand
Watson and Crick’s discovery of the
The terminal part of the structure of DNA and knowledge of the
telomere in the template mechanism of replication allowed biologists
DNA strand remains
unreplicated.
not only to understand some of life’s
Template strand central processes, but also to create tools
3’
to study how life works. Biologists often
GGGA T T GGGA T T GGGA T T GGGA T T GGG
T AACCC TAACCC TAACCC need to isolate, identify, and determine
5’ the nucleotide sequence of particular DNA
Lagging strand fragments. Such procedures can determine
whether a genetic risk factor for diabetes
has been inherited, blood at a crime scene
matches that of a suspect, a variety of rice
Telomerase
G A or wheat carries a genetic factor for insect
G G
T
3’ resistance, or two species of organisms are
The telomerase enzyme T
GGGA T T GGGA T T GGGA T T GGGA T T GGG closely related. Many of the experimental
contains an RNA template CCCUAACCC T AACCC TAACCC TAACCC procedures for the isolation, identification,
that allows the shortened
3’ end of the template 5’ 3’ 5’ and sequencing of DNA are based on
strand to be restored by knowledge of DNA structure and physical
the addition of more RNA template
telomere repeats. properties of DNA. Others make use of the
principles of DNA replication.
Telomere repeats In this section, we discuss how particular
3’ fragments of double-stranded DNA can be
GGGA T TGGGA T T GGGA T T GGGA T T GGGA T T GGGA T T GGG produced, how DNA fragments of different
CCCUAACCC T AACCC TAACCC TAACCC sizes can be physically separated, and how
5’ 3’ 5’
the nucleotide sequence of a piece of DNA
can be determined.

The polymerase chain reaction

DNA selectively amplifies regions of
polymerase
Telomere repeats Template strand DNA.
3’ DNA that exists in the nuclei of your cells
GGGA T TGGGA T TGGGA T T GGGA T T GGGA T T GGGA T T GGGA T T GGG is present in two copies per cell, except for
CCCUAACCC T AACCC T AACCC T AA T AACCC TAACCC TAACCC
DNA in the X and Y chromosomes in males,
5’ 3’ 5’
which are each present in only one copy
RNA primer
per cell in males. In the laboratory, it is very
A new segment of lagging strand difficult to manipulate or visualize a sample
can then be formed so that the containing just one or two copies of a DNA
original telomere in the template molecule. Instead, researchers typically
strand is completely restored.
work with many identical copies of the DNA
molecule they are interested in. A common
method for making copies of a piece of DNA
is the polymerase chain reaction (PCR),
Human chromosomes contain
thousands of telomere repeats. which allows a targeted region of a DNA
molecule to be replicated (or amplified)
into as many copies as desired. PCR is both
selective and highly sensitive, so it is used to
258 SECTION 12.3 I S O L AT I O N , I D E N T I F I C AT I O N , A N D S E Q U E N C I N G O F D N A F R AG M E N T S

FIG. 12.13 The polymerase chain reaction (PCR). PCR results in amplification of the DNA sequence flanked by the two primers.

a. Each cycle of amplification includes three steps.

Denaturation Annealing Extension
A solution containing double-stranded When the solution is cooled, the two primers DNA polymerase synthesizes new DNA strands
DNA (the template duplex) is heated to anneal to their complementary sequence on (complementary to the template dupex
separate the DNA into two individual the strands of the template duplex. strands) by extending primers in a 5’ to 3’
strands. direction.
3’ 5’ 3’ 5’ 3’
Hydrogen bonds (base pairs) 3’ 5’ 3’ 5’
Target sequence
(Strand A) DNA DNA
Amplified DNA
primer B primer A
Target sequence
(Strand B) 5’ 3’ 5’ 3’
Hydrogen bonds (base pairs)
5’ 3’ 5’ 3’ 5’

b. PCR repeats the cycle of amplification.

Target sequence

1 The template duplex is often longer than 5’ 3’ Template duplex

the amplified region. 3’ 5’

2 Primers for PCR are typically 20–30

nucleotides in length. Their sequences are used
for amplification. The primers are chosen to
base pair with one or the other strand of the
template duplex, with their 3’ ends oriented
toward each other. The primers are added to
the reaction mixture in great excess to ensure
pairing with any complementary sequence.

Cycle 1 product:
2 copies

3 Each round of amplification

doubles the number of molecules
that have the same sequence as
the template duplex.

Cycle 2 product:
4 copies

4 After n cycles of
amplification there
are 2n copies of the Cycle n product:
target sequence. 2n copies
When n = 30, there are
230  109 copies.
 CHAPTER 12 D N A R E P L I C AT I O N A N D M A N I P U L AT I O N 259

amplify and detect small quantities of nucleic acids, such as HIV After sufficient time to allow new DNA synthesis, the solution
in blood-bank supplies, or to study DNA samples as minuscule is heated again, and the cycle of denaturation, annealing, and
as those left by a smoker’s lips on a cigarette butt dropped at the extension is repeated over and over, as indicated in Fig. 12.13b,
scene of a crime. The starting sample can be as small as a single usually for 25–35 cycles. In each cycle, the number of copies of
molecule of DNA. the targeted fragment is doubled. The first round of PCR amplifies
The principles of PCR are illustrated in Fig. 12.13. Because the the targeted region into 2 copies, the next into 4, the next into
PCR reaction is essentially a DNA synthesis reaction, it requires 8, then 16, 32, 64, 128, 256, 512, 1024, and so forth. By the third
the same basic components used by the cell to replicate its DNA. round of amplification, the process begins to produce molecules
In this case, the procedure takes place in a small plastic tube that are only as long as the region of the template duplex flanked
containing a solution that includes four essential components: by the sequences complementary to the primers. After several
more rounds of replication the majority of molecules are of this
1. Template DNA. At least one molecule of double-stranded
type. The doubling in the number of amplified fragments in each
DNA containing the region to be amplified serves as the
cycle justifies the term “chain reaction.”
template for amplification.
Although PCR is elegant in its simplicity, the DNA polymerase
2. DNA polymerase. The enzyme DNA polymerase is used to enzymes from many species (including humans) irreversibly lose
replicate the DNA. both structure and function at the high temperature required to
separate the DNA strands. At each cycle, you would have to open
3. All four deoxynucleoside triphosphates. Deoxynucleoside
the tube and add fresh DNA polymerase. This is possible, and
triphosphates with the bases A, T, G, or C are needed as
in fact it was how PCR was done when the technique was first
building blocks for the synthesis of new DNA strands.
developed, but the procedure is time consuming and tedious. To
4. Two primers. Two short sequences of single-stranded DNA solve this problem, we now use DNA polymerase enzymes that
are required for the DNA polymerase to start synthesis. are heat-stable, such as those from the bacterial species Thermus
Enough primer is added so that the number of primer DNA aquaticus, which lives at the near–boiling point of water in
molecules is much greater than the number of template natural hot springs, including those at Yellowstone National Park.
DNA molecules. This polymerase, called Taq polymerase, remains active at high
temperatures. Once the reaction mixtures are set up, the entire
The primer sequences are oligonucleotides (oligos is the procedure is carried out in a fully automated machine. The time
Greek word for “few”) produced by chemical synthesis and are of each cycle, temperatures, number of cycles, and other variables
typically 20–30 nucleotides long. Their base sequences are chosen can all be programmed. The fact that DNA polymerase from a
to be complementary to the ends of the region of DNA to be bacterium that lives in hot springs can be used to amplify DNA
amplified. In other words, the primers flank the specific region of from any organism reminds us again of the conserved function and
DNA to be amplified. The 3� end of each primer must be oriented evolution early in the history of life of this enzyme.
toward the region to be amplified so that, when DNA polymerase
extends the primer, it creates a new DNA strand complementary Electrophoresis separates DNA fragments by size.
to the targeted region. Because the 3� ends of the primers both PCR amplification does not always work as you might expect.
point toward the targeted region, one of the primers pairs with Sometimes the primers have the wrong sequence and so fail to
one strand of the template DNA and the other pairs with the other anneal properly; sometimes they anneal to multiple sites and
strand of the template DNA. several different fragments are amplified. To determine whether
PCR creates new DNA fragments in a cycle of three or not PCR has yielded the expected product, a researcher must
steps, as shown in Fig. 12.13a. The first step, denaturation, determine the size of the amplified DNA molecules. Usually,
involves heating the solution in a plastic tube to a temperature the researcher knows what the size of the correctly amplified
just short of boiling so that the individual DNA strands of the fragment should be, making it possible to compare the expected
template separate (or “denature”) as a result of the breaking of size to the actual size.
hydrogen bonds between the complementary bases. The second One way to determine the actual size of a DNA fragment
step, annealing, begins as the solution is cooled. Because of is by gel electrophoresis (Fig. 12.14), a procedure in which
the great excess of primer molecules, the two primers bind (or DNA samples are inserted into slots or wells near the edge of a
“anneal”) to their complementary sequence on the DNA (rather rectangular slab of porous material resembling solidified agar
than two strands of the template duplex coming back together). (the “gel”), which is composed of a tangle of polymers that
In the final step, extension, the solution is heated to the make it difficult for large molecules to pass through. The gel is
optimal temperature for DNA polymerase and the polymerase then inserted into an apparatus and immersed in a solution that
elongates (or “extends”) each primer with the deoxynucleoside allows an electric current to be passed through it (Fig. 12.14a).
triphosphates. Since fragments of double-stranded DNA are negatively charged
260 SECTION 12.3 I S O L AT I O N , I D E N T I F I C AT I O N , A N D S E Q U E N C I N G O F D N A F R AG M E N T S

Quick Check 2 You do a PCR reaction,

FIG. 12.14 Gel electrophoresis. (a) A typical electrophoresis apparatus consists of a plastic run a sample of the product on a gel,
tray, gel, and solution. (b) DNA bands can be visualized after staining with a dye and use a dye to visualize the bands. You
that fluoresces under ultraviolet light. Photo source: Guy Tear/Wellcome Images. expect a single product of 300 bp, which
a. b. you observe, but you also see a band of
about 550 bp. Can you suggest a reason
DNA samples are why? Does the size of the unexpected
inserted into wells at
one edge of the gel. band indicate anything about your target
sequence?
⫺
Gel electrophoresis can separate
DNA fragments DNA fragments produced by any
move toward the
positive pole means, not only PCR. Genomic DNA
according to their can be cut with certain enzymes and
size. Smaller
fragments move
the resulting fragments separated by
faster and larger gel electrophoresis, as described in
ones move more the next section. Similar procedures
slowly.
can also be used to separate protein
⫹ molecules.

Restriction enzymes cleave DNA

at particular short sequences.
A band represents DNA The bands in the gel become visible In addition to amplifying segments
of a particular size. when viewed in fluorescent light.
of DNA, researchers often also make
use of techniques that cut DNA at
specific sites. Cutting DNA molecules
because of the ionized phosphate groups along the backbone, the allows pieces from the same or different organisms to be brought
molecules move toward the positive pole of the electric field. together in recombinant DNA technology, which is discussed
The DNA molecules move according to their size. Short in the next section. It also is a way to determine whether or not
fragments pass through the pores of the gel more readily than specific sequences are present in a segment of DNA, as techniques
large fragments, and so in a given interval of time short fragments for cutting DNA depend on specific DNA sequences. Finally,
move a greater distance in the gel than large fragments. The rate cutting DNA allows whole genomes to be broken up into smaller
of migration is dependent only on size, not on sequence, and so pieces for further analysis, such as DNA sequencing.
all fragments of a given size move together at the same rate in The method for cutting DNA makes use of a class of enzyme
a discrete band, which can be made visible by dyes that bind to that recognizes specific, short nucleotide sequences in double-
DNA and fluoresce under ultraviolet light (Fig. 12.14b). A solution stranded DNA and cleaves the DNA at these sites. The enzymes are
of DNA fragments of known sizes is usually placed in one of the known as restriction enzymes, of which about 1000 different
wells, resulting in a series of bands, called a ladder, which can be kinds have been isolated from bacteria and other microorganisms.
used for size comparison. The recognition sequences the enzymes cleave, often called
Consider a PCR experiment with 25 base-pair (bp) primers restriction sites, are typically four or six base pairs long. Most
flanking a 250-bp length of DNA. PCR amplifies this region, restriction enzymes cleave double-stranded DNA at or near the
generating many millions of copies of a 300-bp DNA fragment restriction site. For example, the enzyme EcoRI has the following
consisting of the 250-bp target sequence and the primer sequences restriction site:
on both ends. To determine if the experiment worked as expected, ↓
a sample of the product is checked by gel electrophoresis. This 5⬘-GAATTC-3⬘
sample is loaded into the well of the gel, a current is applied for a 3⬘-CTTAAG-5⬘
↑
short period of time, and DNA fragments migrate to positions in
the gel corresponding to their size, creating bands of DNA that are Wherever the enzyme finds this site in a DNA molecule,
visualized with a dye. If a single band of 300 bp is seen on the gel, it cleaves each strand exactly at the position indicated by the
then the experiment worked as expected. Sometimes, however, vertical arrows. Note that the EcoRI restriction site is symmetrical:
the reaction might yield no bands, a single band of the incorrect Reading from the 5ˇ end to the 3ˇ end, the sequence of the top
size, or multiple bands. The researcher would then need to go back strand is exactly the same as the sequence of the bottom strand.
and investigate why the experiment did not work as expected. This kind of symmetry is palindromic (it reads the same in both
 CHAPTER 12 D N A R E P L I C AT I O N A N D M A N I P U L AT I O N 261

be extracted from the gel for further analysis or manipulation. For

TABLE 12.1 Examples of Restriction Enzymes with Six-Base
instance, the extracted fragments can be sequenced or ligated to
Cleavage Sites
other fragments.
But most genomes are too large to give such a simple picture.
↓ ↓
5⬘-GGATCC-3⬘ 5⬘-AAGCTT-3⬘ For example, the human genome is cleaved into more than a
BamHI Hind III million different fragments by EcoRI, and these fragments appear
3⬘-CCTAGG-5⬘ 3⬘-TTCGAA-5⬘
↑ ↑ as one big smear in a gel rather than as a series of discrete bands.
In the next section, we discuss how individual bands containing a
↓ ↓
5⬘-GGTACC-3⬘ 5⬘-GAGCTC-3⬘ sequence of interest can be detected in such a gel.
KpnI SstI
3⬘-CCATGG-5⬘ 3⬘-CTCGAG-5⬘
↑ ↑ DNA strands can be separated and brought back
↓ ↓ together again.
5⬘-GTTAAC-3⬘ 5⬘-CCCGGG-3⬘ A researcher may wish to know whether a gene in one species
HpaI SmaI
3⬘-CAATTG-5⬘ 3⬘-GGGCCC-5⬘ is present in the DNA of a related species, but the nucleotide
↑ ↑
sequence of the gene is unknown. In such a case, techniques
like PCR that require knowledge of the target DNA sequence
cannot be used. Instead, the researcher can determine whether
a DNA strand containing the gene in one species can base pair
directions) and is typical of restriction sites. Note also that the site with the complement of a similar gene in a DNA strand from the
of cleavage is not in the center of the recognition sequence. The other species. The base pairing of complementary single-stranded
cleaved double-stranded molecules therefore each terminate in a nucleic acids is known as renaturation or hybridization, and the
short single-stranded overhang. In this case the overhang is at the process is the opposite of denaturation. In denaturation, the DNA
5ˇ end, as shown below: strands in a duplex molecule are separated, and in renaturation,
complementary strands come together again. Denaturation and
↓ ↓
renaturation are also opposites in regard to the experimental
5⬘-G -3⬘ 5⬘- AATTC-3⬘
+ conditions in which they occur. When a solution containing DNA
3⬘-CTTAA -5⬘ 3⬘- G-5⬘
↑ ↑ molecules is gradually heated to a sufficiently high temperature,
the strands denature. When the solution is allowed to cool, the
Table 12.1 shows more examples of restriction enzymes. complementary strands renature.
Some cleave their restriction site to produce a 5ˇ overhang, others Denatured DNA strands from one source can renature with
produce a 3ˇ overhang, and still others cleave in the middle of DNA strands from a different source if their sequences are
their restriction site and leave blunt ends with no overhang. precisely or mostly complementary. Two very closely related
The standard symbols for restriction enzymes include both sequences will have more perfectly matched bases and thus more
italic and roman letters. The italic letters stand for the species hydrogen bonds holding them together than two sequences that
from which the enzyme is derived (E. coli in the case of EcoRI, are less closely related. The degree of base pairing between two
Bacillus amyloliquefaciens in the case
of BamHI), and the Roman letters
designate the particular restriction
FIG. 12.15 EcoRI restriction fragments (a) on a circular DNA plasmid and (b) separated on
enzyme isolated from that species.
a gel.
When a particular restriction a. b.
enzyme is used to break up a whole 5.5 kb
⫺
genome into smaller fragments, the
specificity of restriction enzymes
22 kb
ensures that the DNA from each cell in 22 kb 4 kb
an individual organism yields the same Gel electrophoresis
separates the resulting 7.5 kb
set of fragments and any particular Each tick mark DNA fragments according 6 kb
DNA sequence present in the cells is represents a to their size (in kb, or 5.5 kb
contained in a fragment of the same cleavage site 7.5 kb thousands of base pairs). 5 kb
for EcoRI.
size. If the genome is small enough that 4 kb
the number and sizes of the fragments
⫹
are limited, the individual fragments can
be visualized directly in a gel 5 kb
(Fig. 12.15). These fragments can then 6 kb
262 SECTION 12.3 I S O L AT I O N , I D E N T I F I C AT I O N , A N D S E Q U E N C I N G O F D N A F R AG M E N T S

FIG. 12.16 A Southern blot. Photo source: Reproduced with permission from Da’dara, A.A., Walter, R. D. 1998 Biochem J., 336(Pt 3): 545–550. © The Biochemical Society.

Labeled Sealable
Filter paper probe plastic bag

Gel

1 A 2 The resulting 3 After electrophoresis, 4 After 5 The filter paper is placed 6 To visualize the
restriction fragments are the DNA fragments in blotting, the into a plastic bag containing positions of the bands
enzyme is separated by size by the gel are denatured filter paper is a solution with the where the probe
added to a gel electrophoresis. and then transferred removed. single-stranded, labeled hybridized, the filter
solution (blotted) onto filter probe, and the probe sticks paper is exposed to
containing paper. to complementary fragments X-ray film, and either
the DNA of by hybridization. (The bands light or radioactive
interest. are not visible at this stage.) emissions darken the
film over each band.

sequences affects the temperature at which they renature: Very (Fig. 12.16) after its inventor, the British molecular biologist
closely related sequences renature at a higher temperature than Edwin M. Southern.
less closely related sequences. Evolutionary biologists have used In a Southern blot, single-stranded DNA molecules that
this principle to estimate the proportion of perfectly matched have resolved into bands on a gel are transferred to a filter paper,
bases present in the DNA of different species, which is used as such that they are on the same spot on the paper as they were
a measure of how closely those species are related. In general, on the gel. The filter paper is washed with a solution containing
the more closely two species are related, the more
similar their DNA sequences. Knowledge of species
relatedness is important in many applications,
FIG. 12.17 (a) A deoxynucleotide and (b) a dideoxynucleotide. Incorporation
including conservation, identifying endangered
of a dideoxynucleotide prevents strand elongation.
species, and tracing the evolutionary history of
organisms. a. b.
Renaturation makes it possible to use a small O O
DNA fragment as a probe. This fragment is usually -O P O -O P O
attached to a light-emitting or radioactive chemical
O O
that serves as a label. The probe can be used to
O O
determine whether or not a sample of double- 5’ CH2 OH 5’ CH2 OH
stranded DNA molecules contains sequences that 4’ 1’ 4’ 1’
H H H H H H H H
are complementary to it. Any DNA fragment can 3’ 2’ 3’ 2’
be used as a probe, and a probe can be obtained in O H H H
any number of ways, such as by chemical isolation -O P O
of a DNA fragment, amplification by PCR, or A dideoxynucleotide lacks the 3’ hydroxyl
O group, and it cannot be elongated because
synthesizing a nucleic acid from free nucleotides.
O there is no hydroxyl group to attack an
Let’s say you are interested in determining the 5’ CH2 OH incoming nucleotide triphosphate.
number of copies of a particular DNA sequence in 4’ 1’
H H H H
a genome or determining whether a given gene in 3’ 2’
human genomic DNA is intact. Recall that human H
and other genomic DNA cut with a restriction HO
enzyme yield a large number of DNA fragments that
look like a smear following gel electrophoresis. A normal deoxynucleotide has
A labeled probe can be used to determine the a hydroxyl (–OH) group on the
3’ carbon, allowing this end to
size and number of a DNA sequence of interest in be elongated.
a gel. The method is known as a Southern blot
 CHAPTER 12 D N A R E P L I C AT I O N A N D M A N I P U L AT I O N 263

single-stranded probes that hybridize to whichever pieces of DNA

on the filter paper are complementary. The paper is then exposed FIG. 12.18 Sanger sequencing. (a) The incorporation of an
to X-ray film (in the case of radioactively labeled probes) so the incoming dideoxynucleotide stops the elongation of a
bands can be visualized. new strand. (b) Dideoxynucleotides terminate strands at
The presence of a band or bands on the film therefore indicates different points in the template sequence. (c) Separation
the number and size of DNA fragments that are complementary of interrupted daughter strands by size shows where each
to the probe. This information in turn tells you the sizes and the terminator was incorporated and hence the identity of
number of copies of a particular DNA sequence present in the the corresponding nucleotide in the template strand.
a.
starting sample. 3’ 5’
Interrupted
T A daughter strand
DNA sequencing makes use of the principles of DNA G C The sequence of the template
replication. A T strand is unknown. Elongation
T A of the daughter strands stops
The ability to determine the nucleotide sequence of DNA G C whenever a dideoxynucleotide
molecules has given a tremendous boost to biological research. G C Dideoxynucleotide terminator is incorporated at
C 3’ terminator with the 3’ end.
Techniques used to sequence DNA follow from our understanding T flourescent tag
of DNA replication. Consider a solution containing identical C
A Template strand
single-stranded molecules of DNA, each being used as the
5’
template for the synthesis of a complementary daughter
strand originating at a short primer sequence. The problem is
b.
to determine the nucleotide sequence of the template strand.
5’ In this case, each of the
A brilliant answer to this problem was developed by the English 3’ 5’
? 3’ possible dideoxy-
geneticist Frederick Sanger, an achievement rewarded with a ? ? nucleotides is labeled with
? T ? ?
share in the Nobel Prize in Chemistry in 1980. (It was his second ? ? a different fluorescent dye
? 3’ 3’
Nobel; he had also been honored with the award in 1958 for his ? 5’ ? ?
? ? ? ? ? 3’
discovery of a method for determining the sequence of amino ? ? ? ? ? 5’
? ?
acids in a polypeptide chain.) ? ? ? C ? ?
? ? A ? 3’ ? ?
Recall that a free 3� hydroxyl group is essential for each step ? ? ? ? ?
? ? 3’ ? ?
in elongation because that is where the incoming nucleotide is ? ? ?
5’ ? ? ?
attached (Fig. 12.17a). Making use of this fact, Sanger synthesized ? ?
? ?
? 5’ ?
dideoxynucleotides, in which the 3� hydroxyl group on the ? G
?
?
sugar ring is absent (Fig. 12.17b). Whenever a dideoxynucleotide 5’ 3’
5’
is incorporated into a growing daughter strand, there is no
hydroxyl group to attack the incoming nucleotide, and strand
c.
growth is stopped dead in its tracks (Fig. 12.18a). For this reason, a
dideoxynucleotide is known as a chain terminator. By including After the interrupted
daughter strands are
a small amount of each of the chain terminators in a reaction tube separated by size, a trace
along with larger quantities of all four normal nucleotides, a DNA of the fluorescent tags
reveals the identities of
primer, a DNA template, and DNA polymerase, Sanger was able to
the successive dideoxy-
produce a series of interrupted daughter strands, each terminating ribonucleotides that
at the site at which a dideoxynucleotide was incorporated. terminate strand
elongation, and allows the
Fig. 12.18b shows how the interrupted daughter strands help template sequence to be
us to determine the DNA sequence by the procedure now called deduced.
Sanger sequencing. In a tube containing dideoxy-A and all the 3’ 5’ 5’ 3’
other elements required for many rounds of DNA replication, a Once the
? A ? ? ?? ? ? ? ? ? A T
strand of DNA is synthesized complementary to the template ? C ? ?? ? ? ? ? ? C daughter G
? T ?? ? ? ? ? ? T strand A
until, when it reaches a T in the template strand, it incorporates
? A? ? ? ? ? ? A sequence T
an A. Only a small fraction of the A nucleotides in the sequencing ? ? ? C ? ? ? C is known... G
reaction are in the dideoxy form, so only a fraction of the daughter ? ? ? C ? ? C G
? G ? ? ? G ...the sequence C
strands incorporate a dideoxy-A at that point, resulting in A
? Because there are many A ? ? of the template T
termination. The rest of the strands incorporate a normal deoxy-A ? more normal nucleotides G ? G strand can be C
? T T determined. A
and continue synthesis, although most of these will be stopped than dideoxynucleotides,
5’ each strand can terminate 3’ 3’ 5’
at some point farther along the line when a T is reached again. at a different place.
Similarly, in a reaction containing dideoxy-C, DNA fragments will
 264 SECTION 12.4 GENETIC ENGINEERING

be produced whose sizes correspond to the positions of the Cs, and approach passes individual DNA molecules through a pore in a
likewise for dideoxy-T and dideoxy-G. charged membrane; as each nucleotide passes through the pore it
Each of the four dideoxynucleotides is chemically labeled with is identified by means of the tiny difference in charge that occurs.
a different fluorescent dye, as indicated by the different colors These miniaturized and automated devices carry out what is often
of A, C, T, and G in Fig. 12.18b, and so all four terminators can be called massively parallel sequencing, which can determine the
present in a single reaction and still be distinguished. After DNA sequence of hundreds of millions of base pairs in a few hours.
synthesis is complete, the daughter strands are separated by size The goal of technology development was captured in the
with gel electrophoresis. The smallest daughter molecules migrate catchphrase “the $1000 genome,” a largely symbolic target
most quickly and therefore are the first to reach the bottom indicating a reduction in sequencing cost from about $1 million
of the gel, followed by the others in order of increasing size. A per megabase to less than $1 per megabase. For all practical
fluorescence detector at the bottom of the gel “reads” the colors purposes, the $1000-genome target has been achieved, but $1000
of the fragments as they exit the gel. What the scientist sees is a is still too costly to make genome sequencing a routine diagnostic
trace (or graph) of the fluorescence intensities, such as the one procedure. The technologies are still evolving and the costs
shown in Fig. 12.18c. The differently colored peaks, from left to decreasing, and so it is a fair bet that in the coming years your own
right, represent the order of fluorescently tagged DNA fragments personal genome can be sequenced quickly and cheaply.
emerging from the gel. Thus, a trace showing peaks colored green-
purple-red-green-purple-purple-blue-green-blue-red corresponds
to a daughter strand having the sequence 5�-ACTACCGAGT-3� 12.4 GENETIC ENGINEERING
(Fig. 12.18c). In the Sanger sequencing method, each sequencing
reaction can determine the sequence of about 1000 nucleotides in Along with methods to manipulate DNA fragments came the
the template DNA molecule. capability of isolating genes from one species and introducing
them into another. This type of genetic engineering is called
j Quick Check 3 You have determined that the newly synthesized
recombinant DNA technology because it literally recombines
strand of DNA in your sequencing reaction has the sequence
DNA molecules from two (or more) different sources into a single
5�-ACTACCGAGT-3�. What is the sequence of the template strand?
molecule. Recombinant DNA technology involves cutting DNA
by restriction enzymes, isolating them by gel electrophoresis,
? CASE 3 YOU, FROM A TO T: YOUR PERSONAL GENOME and ligating them with enzymes used in DNA replication. This
What new technologies are being developed to technology is possible because the DNA of all organisms is the
sequence your personal genome? same, differing only in sequence but not in chemical or physical
One of the high points of modern biology has been the structure. When DNA fragments from different sources are
determination of the complete nucleotide sequence of the DNA in combined into a single molecule and incorporated into a cell, they
a large number of species, including ours. The human genome and are replicated and transcribed just like any other DNA molecule.
many others were sequenced by Sanger sequencing. This technique Recombinant DNA technology can combine DNA from
works well and is still the gold standard for accuracy, but it takes any two sources, including different species. DNA from one
time and is expensive for large genomes like the human genome. species of bacteria can be combined with another, or a human
There have been many improvements in Sanger sequencing since it gene can be combined with bacterial DNA, or the DNA from
was first described, including the use of four-color fluorescent dyes a plant and a fungus can be combined into a single molecule.
to label the DNA fragments, capillary electrophoresis to separate the These new sequences may be unlike any found in nature, raising
fragments, photocells to read the fluorescent signals automatically questions about their possible effects on human health and the
as the products run off the gel, and highly efficient enzymes. environment.
Together, these methods have increased the speed and decreased This section discusses one of the basic methods for producing
the cost of DNA sequencing considerably. recombinant DNA, some of the important applications of
However, being able to sequence everyone’s genome, including recombinant DNA technology such as genetically modified
yours, will require new technologies to bring down the cost organisms, and a new method that can be used to change the DNA
further and to increase the speed of sequencing. The first human sequence of any organism into any other desired sequence.
genome sequence, completed in 2003 at a cost of approximately
$2.7 billion, stimulated great interest in large-scale sequencing Recombinant DNA combines DNA molecules from two
and the development of devices that increased scale and decreased or more sources.
cost. As in the development of computer hardware, emphasis was The first application of recombinant DNA technology was the
on making the sequencing devices smaller while increasing their introduction of foreign DNA fragments into the cells of bacteria
capacity through automation. in the early 1970s. The method is simple and straightforward, and
Modern methods of sequencing include the use of fluorescent remains one of the mainstays of modern molecular research. It
nucleotides or light-detection devices that reveal the identity of can be used to generate a large quantity of a protein for study or
each base as it is added to the growing end of a DNA strand. Another therapeutic use.
 CHAPTER 12 D N A R E P L I C AT I O N A N D M A N I P U L AT I O N 265

The method requires a fragment of double-stranded DNA

that serves as the donor. The donor fragment may be a protein- FIG. 12.19 Recombinant DNA. Donor DNA fragments are joined
coding gene, a regulatory part of a gene, or any DNA segment with a vector molecule that can replicate in bacterial cells.
of interest. If you are interested in generating bacteria that
could produce human insulin, you might use the coding region Donor DNA and vector DNA
of the human insulin gene as your donor DNA molecule. Also are both cleaved with the
same restriction enzyme, in
required is a vector sequence into which the donor fragment is this case EcoRI.
to be inserted. The vector is the carrier of the donor fragment, Vector DNA
and it must have the ability to be maintained in bacterial cells. Donor DNA (chromosome) (plasmid)
A frequently used vector is a bacterial plasmid, a small circular
molecule of DNA found naturally in certain bacteria that can
replicate when the bacterial genomic DNA replicates and be
Restriction
transmitted to the daughter bacterial cells when the parental cell enzyme
divides. Many naturally occurring plasmids have been modified by
genetic engineering to make them suitable for use as vectors in
recombinant DNA technology.
A common method for producing recombinant DNA is shown
in Fig. 12.19. In order to make sure donor DNA can be fused
with vector DNA, both pieces are cut with the same restriction
enzyme so they both have the same overhangs. In the example
shown in Fig. 12.19, the donor DNA is a fragment produced by
digestion of genomic DNA with the restriction enzyme EcoRI,
resulting in four-nucleotide 5� overhangs at the fragment ends.
The vector is a circular plasmid that contains a single EcoRI

CT

G
G
cleavage site, so digestion of the vector with EcoRI opens the G CTT A A

C
TA

T
+ T
circle with a single cut that also has four-nucleotide 5� overhangs. A ATT C G A
A

A
Note that the overhangs on the donor fragment and the vector
are complementary, allowing the ends of the donor fragment
to renature with the ends of the opened vector when the two
The resulting
molecules are mixed. Once this renaturation has taken place, the fragments are mixed
ends of the donor fragment and the vector are covalently joined by together and joined
DNA ligase. The joining of the donor DNA to the vector creates the by DNA ligase. The
genomic and vector Recombinant DNA
recombinant DNA molecule. fragments have
The next step in the procedure is transformation, in which complementary
ends.
the recombinant DNA is mixed with bacteria that have been DNA
chemically coaxed into a physiological state in which they take ligase
up DNA from outside the cell. Having taken up the recombinant
DNA, the bacterial cells are transferred into growth medium,
The recombinant
where they multiply. Since the vector part of the recombinant DNA molecule is
DNA molecule contains all the DNA sequences needed for its Bacterial DNA introduced into
replication and partition into the daughter cells, the recombinant a bacterial cell
by means of
DNA multiplies as the bacterial cell multiplies. If the recombinant transformation.
DNA functions inside the bacterial cell, then new genetic
characteristics may be expressed by the bacteria. For example,
the recombinant DNA may allow the bacterial cells to produce a
human protein, such as insulin or growth hormone.

j Quick Check 4 In making recombinant DNA molecules

that combine restriction fragments from different organisms,
researchers usually prefer restriction enzymes like BamHI or
HindIII that generate fragments with “sticky ends” (ends with
overhangs) rather than enzymes like HpaI or SmaI (Table 12.1) As the bacterial DNA replicates and divides, the recombinant DNA is
that generate fragments with “blunt ends” (ends without also replicated and transferred to the daughter cells. The recombinant
DNA therefore multiplies along with the bacterial DNA.
overhangs). Can you think of a reason for this preference?
266 SECTION 12.4 GENETIC ENGINEERING

Recombinant DNA is the basis of genetically modified and many others have been engineered to resist insect pests, and
organisms. other engineered products are rice with a high content of vitamin
Applications of recombinant DNA have gone far beyond genetically A, tomatoes with delayed fruit softening, potatoes with waxy
engineered bacteria. Using methods that are conceptually similar to starch, and sugarcane with increased sugar content. To model
those described for bacteria but differing in many details, scientists disease, researchers have used recombinant DNA to produce
have been able to produce varieties of genetically engineered organisms such as laboratory mice that have been engineered to
viruses and bacteria, laboratory organisms, agricultural crops, and develop heart disease and diabetes. By studying these organisms,
domesticated animals (Fig. 12.20). Examples include sheep that researchers can better understand human diseases and begin to
produce a human protein in their milk used to treat emphysema, find new treatments for them.
chickens that produce eggs containing human antibodies to Genetically engineered organisms are known as transgenic
help fight harmful bacteria, and salmon with increased growth organisms or genetically modified organisms (GMOs).
hormone for rapid growth. Plants such as corn, canola, cotton, Transgenic laboratory organisms are indispensable in the study

FIG. 12.20 Genetically modified organisms (GMOs). Organisms can be genetically modified so that (a) wheat resists weed-killing herbicides,
(b) soybeans resist insects and have improved oil quality, (c) sheep produce more healthy fats, (d) chickens are unable to spread bird
flu, (e) salmon have faster growth, and (f) pigs digest plant phosphorus more efficiently. Sources: a. Adam Hart-Davis/Science Source; b. Steve
Percival/Science Source; c. meirion matthias/Shutterstock; d. John Daniels/Ardea; e. AquaBounty Technologies; f. Carla Gottgens/Bloomberg via Getty Images.

a b c

e
 CHAPTER 12 D N A R E P L I C AT I O N A N D M A N I P U L AT I O N 267

FIG. 12.21 DNA editing. In this example, CRISPR RNA and its associated protein are used to cleave target DNA, and double-stranded template
DNA is used in DNA repair to alter its sequence.

Cell with target DNA

to be edited

Plasmid DNA with Plasmid containing editing

sequences for CRISPR template DNA
RNA and Cas9

a. CRISPR b. c. d. e.
RNA Target Editing
DNA template
DNA

Hairpin
Edited
target
Exonuclease DNA

Cleavage

Region CRISPR
complementary associated
to target DNA protein
(Cas9)

CRISPR RNA CRISPR RNA guides An exonuclease The editing The result is an
combines with Cas9 to the target widens gap in template is used edited DNA with
the Cas9 protein. DNA and the the target DNA. to repair the gap altered sequence.
target is cleaved. in the target DNA.

of gene function and regulation and to identify genetic risk One of the newest and most exciting ways to edit DNA
factors for disease. In crop plants and domesticated animals, goes by the acronym CRISPR (clustered regularly interspaced
GMOs promise enhanced resistance to disease, faster growth and short palindromic repeats), and it was discovered in an
higher yields, more efficient utilization of fertilizer or nutrients, unexpected way. Researchers noted that about half of all species
and improved taste and quality. But there are concerns about of bacteria and most species of Archaea contain small segments
unexpected effects on human health or the environment, the of DNA of about 20–50 base pairs derived from plasmids or
increasing power and influence of agribusiness conglomerates, viruses, but their function was at first a mystery. Later, it was
and ethical objections to tampering with the genetic makeup of discovered that they play a role in bacterial defense. When a
animals and plants. Nevertheless, more than 250 million acres of bacterium is infected by a virus for the first time, it makes a copy
GMO crops are grown annually in more than 20 countries. The of part of the viral genome and incorporates it into its genome.
majority of this acreage is in the United States and South America. On subsequent infection by the same virus, the DNA copy of
Resistance to the use of GMOs in Europe remains strong and vocal. the viral genome is transcribed to RNA that combines with a
protein that has a DNA-cleaving function. The RNA serves as
DNA editing can be used to alter gene sequences a guide to identify target DNA in the virus by complementary
almost at will. base pairing, and the protein cleaves the target DNA. In this way,
Recombinant DNA technology combines existing DNA from two bacteria “remember” and defend themselves from past infections.
or more different sources. Its usefulness in research, medicine, The phrase “clustered regularly interspaced short palindromic
and agriculture, however, is limited by the fact that it can make repeats” describes the organization of the viral DNA segments in
use only of existing DNA sequences. Therefore, scientists have the bacterial genome.
also developed many different techniques to alter the nucleotide In modern genetic engineering, the CRISPR mechanism is
sequence of almost any gene in a deliberate, targeted fashion. put to practical use to alter the nucleotide sequence of almost any
In essence, these techniques allow researchers to “rewrite” the gene in any kind of cell. One method is outlined in Fig. 12.21. The
nucleotide sequence so that specific mutations can be introduced first step is to transform a cell with a plasmid containing sequences
into genes to better understand their function, or mutant versions that code for a CRISPR RNA as well as the CRISPR-associated
of genes can be corrected to restore normal function. Collectively, protein Cas9. The RNA contains a region that can form a hairpin-
these techniques are known as DNA editing. shaped structure, as well as a region engineered to have bases
268 CO R E CO N C E P T S S U M M A RY

complementary to any DNA molecule in the cell to be altered, with the complementary ends of the editing template, and DNA
known as the target DNA (Fig. 12.21a). When the RNA undergoes synthesis elongates the target DNA strands and closes the gap
base pairing with the target DNA, Cas9 cleaves the target DNA (Fig. 12.21d). The result is that the target DNA is restored, but its
(Fig. 12.21b). Exonucleases in the cell then expand the gap (Fig. sequence has been altered according to the sequence present in
12.21c). The gap can be repaired using another DNA molecule that the editing template (Fig. 12.21e).
serves as a template for editing the target DNA (Fig. 12.21d). This DNA editing by CRISPR is technically straightforward and
editing template DNA is introduced to the cell by a plasmid and highly efficient. The method has generated great interest because
contains a sequence of interest to replace the degraded sequence of its potential to correct genetic disorders of the blood, immune
of the target DNA, flanked by sequences complementary to the system, or other tissues and organs in which only a subset of cells
target. The strands of the gapped target DNA undergo base pairing with restored function can alleviate symptoms. •

Core Concepts Summary

12.1 In DNA replication, a single parental molecule 12.3 Techniques for manipulating DNA follow from
of DNA produces two daughter molecules. the basics of DNA structure and replication.
DNA replication involves the separation of the two strands The polymerase chain reaction (PCR) is a technique for
of the double helix at a replication fork and the use of these amplifying a segment of DNA. page 257
strands as templates to direct the synthesis of new strands.
PCR requires a DNA template, DNA polymerase, the four
page 248
nucleoside triphosphates, and two primers. It is a repeated
DNA replication is semiconservative, meaning that each cycle of denaturation, annealing, and extension. page 259
daughter DNA molecule consists of a newly synthesized
Gel electrophoresis allows DNA fragments to be separated
strand and a strand that was present in the parental DNA
according to size, with small fragments migrating farther
molecule. page 248
than big fragments in a gel. page 259
Nucleotides are added to the 3� end of the growing strand.
Restriction enzymes cut DNA at specific recognition
Therefore, synthesis proceeds in a 5�-to-3� direction.
sequences called restriction sites, which cut DNA leaving a
page 250
single-stranded overhang or a blunt end. page 260
At the replication fork, one new strand is synthesized
In DNA denaturation, the two strands of a single DNA
continuously (the leading strand) and the other is
molecule separate from each other. In DNA renaturation
synthesized in small pieces that are ligated together (the
or hybridization, two complementary strands come back
lagging strand). page 251
together again. page 261
DNA polymerase requires RNA primers for DNA
A Southern blot involves using a filter paper as a substrate
synthesis. page 252
for DNA fragments that have been cut up by restriction
DNA polymerase can correct its own mistakes by detecting enzymes and hybridized to a probe. page 262
a pairing mismatch between a template base and an
In Sanger sequencing of DNA, dideoxynucleotide chain
incorrect new base. page 254
terminators are used to stop the DNA synthesis reaction
and produce a series of short DNA fragments from which
12.2 The replication of linear chromosomal DNA
the DNA sequence can be determined. page 263
requires mechanisms that ensure efficient and
complete replication. New DNA sequencing technologies are being developed
to increase the speed and decrease the cost of sequencing,
Chromosomal DNA has many origins of replication, and
perhaps making it possible to sequence everyone’s
replication proceeds from all of these almost simultaneously.
personal genomes. page 264
page 254

Telomerase prevents chromosomes from shortening after

each round of replication by adding a short stretch of DNA
to the ends of chromosomes. page 255
 CHAPTER 12 D N A R E P L I C AT I O N A N D M A N I P U L AT I O N 269

12.4 Genetic engineering allows researchers to alter DNA strands and how this affects the direction of DNA
DNA Sequences. synthesis.

A recombinant DNA molecule can be made by cutting DNA 3. List the differences and similarities in the way the two
from two organisms with the same restriction enzyme and daughter strands of DNA are synthesized at a replication
then using DNA ligase to join them. page 264 fork.

Recombinant DNA is the basis for genetically modified 4. Explain why replicating the tips of linear chromosomes
organisms (GMOs), which offer both potential benefits is problematic and how the cell overcomes this challenge.
and risks. page 266
5. Describe what PCR does. Name and explain its three
Almost any DNA sequence in an organism can be altered steps and give at least two uses for the PCR technique.
by means of a form of DNA editing called CRISPR, which
6. Explain how the properties of DNA determine how
uses modified forms of molecules found in bacteria and
it moves through a gel, is cut by restriction enzymes, and
archaeons that can cleave double-stranded DNA at a
hybridizes to other DNA strands.
specific site. page 267
7. Describe how DNA molecules are sequenced.

Self-Assessment 8. Describe how recombinant DNA techniques can be

used to express a mammalian gene in bacteria.
1. Explain how DNA structure itself suggests a mechanism of
DNA replication.
Log in to to check your answers to the Self-
2. Explain how the chemical structure of Assessment questions, and to access additional learning tools.
deoxynucleotides determines the orientation of the
this page left intentionally blank
 CHAPTER 13

Genomes
Core Concepts
13.1 A genome is the genetic
material of a cell, organism,
organelle, or virus, and its
sequence is the order of
bases along the DNA or
(in some viruses) RNA.
13.2 Researchers annotate
genome sequences to
identify genes and other
functional elements.
13.3 The number of genes in a
genome and the size of a
genome do not correlate
well with the complexity of
an organism.
13.4 The orderly packaging
of DNA allows it to carry
out its functions and fit inside
the cell.
13.5 Viruses have diverse
genomes, but all require a
host cell to replicate.
SSPL/Getty Images.

271
272 SECTION 13.1 GENOME SEQUENCING
HOW DO WE KNOW?
In Chapter 12, we saw how small pieces of DNA are isolated,
identified, and sequenced. This technology has advanced to the point FIG.13.1
where the complete genome sequences for thousands of species have
been determined, including those of humans and our closest primate How are whole genomes
relatives, as well as dozens of other mammals. The term genome
refers to the genetic material of a specified organism, cell, nucleus, sequenced?
organelle, or virus. Some genomes, like that of HIV, are small,
whereas others, like the human genome, are large. Technically, the
BACKGROUND DNA sequencing technologies can only
human genome refers to the DNA in the chromosomes present in
a sperm or egg. However, the term is often used informally to mean determine the sequence of DNA fragments far smaller than the
all of the genetic material in an organism. So your personal genome genome itself. How can the sequences of these small fragments
consists of the DNA in two sets of chromosomes, one inherited from be used to determine the sequence of an entire genome? In the
each parent, plus a much smaller mitochondrial genome inherited early years of genome sequencing, many researchers thought
from your mother. The human genome sequence is so long that that it would be necessary to know first where in the genome
printing it in the size of the type used in this book would require each fragment originated before sequencing it. A group at Celera
1.5 million pages. As we will see, however, the human genome is far Genomics reasoned that if so many fragments were sequenced
from the largest among organisms. that the ends of one would almost always overlap with those of
The sequence of a genome is merely a long string of A’s, T’s, G’s, others, then a computer program with sufficient power might be
and C’s, which represent the order of bases present in successive able to assemble the short sequences to reveal the sequence of
nucleotides along the DNA molecules in the genome. But a genome the entire genome.
sequence, on its own, is not very useful to scientists. Additional HYPOTHESIS A genome sequence can be determined by
research is required to understand what proteins and other sequencing small, randomly generated DNA fragments and
molecules are encoded in the genome sequence, and to learn when assembling them into a complete sequence by matching regions of
these molecules are produced during an organism’s lifetime and overlap between the fragments.
what they do.
EXPERIMENT Hundreds of millions of short sequences from
In this chapter, we discuss how the sequence of a genome is
determined and analyzed to reveal its key biological features, such the genome of the fruit fly, Drosophila melanogaster, were
as the protein-coding genes. We also examine what other kinds of sequenced. Fig. 13.1a shows examples of overlapping fragments,
DNA sequences are present in genomes and how these sequences are using a sentence from Watson and Crick’s original paper on the
organized, with special emphasis on the human genome. Finally, we chemical structure of DNA as an analogy.
explore the diversity of genome types present in viruses.

13.1 GENOME SEQUENCING

In this first section, we focus on how genomes are sequenced,
What exactly is a genome? Originally, the term referred to the building on the DNA sequencing technology introduced in
complete set of chromosomes present in a reproductive cell, like a Chapter 12.
sperm or an egg, which in the human genome is 23 chromosomes.
The word “genome” is almost as old as the word “gene,” and it was Complete genome sequences are assembled from
coined at a time when chromosomes were thought to consist of smaller pieces.
densely packed genes lined up one after another. In Chapter 12, we discussed a method of DNA sequencing known
We know now, however, that chromosomes consist primarily as Sanger sequencing. Sequencing methods are now automated to
of DNA and associated proteins, and that the genetic information the point where specialized machines can determine the sequence
in the chromosomes resides in the DNA. One might therefore of billions of DNA nucleotides in a single day. However, even
define the genome as the DNA molecules that are transmitted from with recent advances, the data are obtained in the form of short
parents to offspring. This definition has the advantage of including sequences, typically less than a few hundred nucleotides long.
the DNA in organisms that lack true chromosomes, such as bacteria If you are interested in sequencing a short DNA fragment, these
and archaeons, as well as eukaryotic organelles that contain their technologies work well. However, let’s say you are interested in
own DNA, such as mitochondria and chloroplasts. But a definition the sequence of human chromosome 1, which is a DNA molecule
restricted to DNA is too narrow because it excludes viruses like HIV, approximately 250 million nucleotides long. How can you
whose genetic material consists of RNA. Defining a genome as the sequence a DNA molecule as long as that?
genetic material transmitted from parent to offspring therefore In one approach, the single long DNA molecule is first broken
embraces all known cellular forms of life, all known organelles, and up into small fragments, each of which is short enough to be
all viruses. sequenced by existing technologies. Even though the sequence
 CHAPTER 13 GENOMES 273

RESULTS The computer program the group had written to assemble the fragments worked. The researchers were able to sequence the
entire Drosophila genome by piecing together the fragments according to their overlaps. In the sentence analogy, the fragments (Fig.
13.1a) can be assembled into the complete sentence (Fig. 13.1b) by matching the overlaps between the fragments.

a.
1 “It has not escaped our no
2 specific pairing we have post The sequences of DNA fragments are
analogous to sentence fragments.
3 suggests a plausible copying m
4 d our notice that the specifi
5 ve postulated immediately suggests
6 pying mechanism for the genetic material.”

b.
1 4 2 5 3 6
The sentence fragments can be assembled in the
“It has not escaped our no
correct order according to their overlaps and
d our notice that the specifi
the original complete sentence reconstructed.
specific pairing we have post
ve postulated immediately suggests
suggests a plausible copying m
pying mechanism for the genetic material.”

CONCLUSION The hypothesis was supported: Celera Genomics could determine the entire genomic sequence of an organism by sequencing
small, random fragments and piecing them together at their overlapping ends.

FOLLOW-UP WORK Today, the computer assembly method is routinely used to determine genome sequences. This method is also used to
infer the genome sequences of hundreds of bacterial species simultaneously—for example, in bacterial communities sampled from seawater or
from the human gut.

SOURCE Adams, M. D., et al. 2000. “The Genome Sequence of Drosophila melanogaster.” Science 287:2185–2195.

data obtained from a small DNA fragment is only a minuscule is called shotgun sequencing because the sequenced fragments
fraction of the length of the DNA molecules in most genomes, do not originate from a particular gene or region but from sites
each run of an automated sequencing machine yields hundreds scattered randomly across the chromosome.
of millions of these short sequences from random locations
j Quick Check 1 DNA sequencing technology has been around
throughout the genome. To sequence a whole genome, researchers
since the late 1970s. Why did sequencing whole genomes
typically sequence such a large number of random DNA fragments
present a challenge?
that, on average, any particular small region of the genome is
sequenced 10–50 times. This redundancy is necessary to minimize
both the number of errors present in the final genome sequence Sequences that are repeated complicate sequence
and the number and size of gaps where the genome sequence is assembly.
incomplete. Sequence assembly is not quite as straightforward as Fig. 13.1
When the sequences of a sufficient number of short stretches suggests. Real sequences are composed of the nucleotides A, T,
of the genome have been obtained, the next step is sequence G, and C, and any given short sequence could come from either
assembly: The short sequences are put together in the correct strand of the double-stranded DNA molecule. Therefore, the
order to generate the long, continuous sequence of nucleotides in overlaps between fragments must be long enough both to ensure
the DNA molecule present in each chromosome. that the assembly is correct and to determine from which strand
Assembly is accomplished by complex computer programs, of DNA the short sequence originated.
but the principle is simple. The short sequences are assembled Some features of genomes present additional challenges to
according to their overlaps, as illustrated in Fig. 13.1, which uses sequence assembly, and the limitations of the computer programs
a sentence to represent the nucleotide sequence. This approach for handling such features require hands-on assembly. Chief
273
274 SECTION 13.1 GENOME SEQUENCING

among these complicating features is the problem of repeated of this kind are troublesome for sequencing machines because
sequences known collectively as repetitive DNA. any single-stranded fragment consisting of alternating AT can fold
There are a variety of types of repeated sequence in eukaryotic back upon itself to form a double-stranded structure in which A is
genomes, and some are shown in Fig. 13.2. The repeated sequence paired with T. Such structures are more stable than the unfolded
may be several thousand nucleotides long and present in multiple single-stranded structures and are not easily sequenced. It is
identical or nearly identical copies. These long repeated sequences quite easy to see that the sequence 5�-AAAAAATTTTTT-3� can
may be dispersed throughout the genome (Fig. 13.2a), or they may fold back itself to form a hairpin because the 5�-AAAAAA-3� is
be tandem, meaning that they are next to each other (Fig. 13.2b). complementary to the 3�- TTTTTT-5�. Any sequence that contains
The difficulty with long repeated sequences is that they internal complementarity can form a foldback loop, including the
typically are much longer than the short fragments sequenced by sequence 5�-…ATATATAT…-3� in Fig. 13.2c.
automated sequencing. As a result, the repeat may not be detected
j Quick Check 2 Let’s say that a stretch of repeated AT is
at all. And if the repeat is detected, there is no easy way of knowing
successfully sequenced. From what you know of the difficulties of
the number of copies of the repeat, that is, whether the DNA
sequencing long repeated sequences, what other problems might
molecule includes two, three, four, or any number of copies of the
you encounter in assembling these fragments?
repeat. Sometimes, researchers can use the ends of repeats, where
the fragments overlap with an adjacent, nonrepeating sequence, as
a guide to the position and number of repeats. ? CASE 3 YOU, FROM A TO T: YOUR PERSONAL GENOME
To illustrate the assembly problems caused by repeated Why sequence your personal genome?
sequences, let’s consider an analogy. In Shakespeare’s play Hamlet, The goal of the Human Genome Project, which began in 1990, was
the word “Hamlet” occurs about 500 times scattered throughout to sequence the human genome as well as the genomes of certain
the text, like a dispersed repeat (Fig. 13.2a). If you chose short key organisms used as models in genetic research. The model
sequences of letters from the play at random and found the organisms chosen are the mainstays of laboratory biology—a
sequence “Hamlet” or “amlet” or “Haml,” you would have no way species each of bacteria, yeast, nematode worm, fruit fly, and
of knowing which of the 500 “Hamlets” these letters came from. mouse. By 2003, the genome sequences of these model organisms
Only if you found sequences that contained “Hamlet” overlapping had been completed, as well as that of the human genome. By
with adjacent unique text could you identify their origin. A similar then, the cost of sequencing had become so low and the sequence
problem occurs in the case of tandem repeats. output so high that many more genomes were sequenced than
In another type of repeat the repeating sequence is short, even originally planned. Genome sequencing is even cheaper today, and
as short as two nucleotides, such as AT, repeated over and over soon you can choose to have your personal genome sequenced.
again in a stretch of DNA (Fig. 13.2c). Short repeating sequences Why sequence more genomes? And if the human genome is
sequenced, why sequence yours? As we saw in Case 3: You, from A
to T, there is really no such thing as the human genome, any more
than there is the fruit fly genome or the mouse genome. With the
FIG. 13.2 Principal types of sequence repeats found in
exception of identical twins, every person’s genome is unique,
eukaryotic genomes. Repeats often pose problems in
the product of a fusion of a unique egg with a unique sperm.
DNA sequencing. (Repeats are not drawn to scale.)
The sequence that is called “the human genome” is actually a
a.
composite of sequences from different individuals. This sequence
Dispersed repeats is nevertheless useful because most of us share the same genes
and regulatory regions, organized the same way on chromosomes.
Detailed knowledge of your own personal genome can be valuable.
Our individual DNA sequences differ at millions of nucleotide sites
from one person to the next. Some of these differences account
b.
Tandem repeats in part for the physical differences we see among us; others have
the potential to predict susceptibility to disease and response to
medication. For Claudia Gilmore, knowledge of the sequence of
her BRCA1 gene had a significant impact on her life.
Determining these differences is a step toward personalized
c.
medicine, in which an individual’s genome sequence, by revealing
Simple-sequence repeats
his or her disease susceptibilities and drug sensitivities, allows
treatments to be tailored to that individual. There may come
a time, perhaps within your lifetime, when personal genome
sequencing becomes part of routine medical testing. Information
… A T A T A T A T A T A T A T A T A T A T A T A T A T A T A T A T AT …
about a patient’s genome will bring benefits but also raises ethical
… TATATATATATATATATATATATATATATATATA… concerns and poses risks to confidentiality and insurability.
 CHAPTER 13 GENOMES 275

13.2 GENOME ANNOTATION FIG. 13.3 Genome annotation. Given the DNA sequence of a
genome, researchers can pinpoint locations of various
A goal of biology is to identify all the macromolecules in biological types of sequence.
systems and understand their individual functions and the ways
in which they interact. This research has practical applications:
Increased understanding of the molecular and cellular basis of Noncoding RNA
Single-copy gene
disease, for example, can lead to improved diagnosis and treatment. Dispersed repeat
The value of genome sequencing in identifying Tandem repeat
Simple-sequence repeat
macromolecules is that the genome sequence contains, in coded
form, the nucleotide sequence of all RNA molecules transcribed Double-stranded DNA
from the DNA as well as the amino acid sequence of all proteins.
There is a catch, however. A genome sequence is merely an
extremely long list of A’s, T’s, G’s, and C’s that represent the
order in which nucleotides occur along the DNA in one strand
of the double helix. (Because of complementarity, knowing the
sequence of one strand specifies the other.) The catch is that
in multicellular organisms, not all the DNA is transcribed into
RNA, and not all the RNA that is transcribed is translated into
protein. Therefore, genome sequencing is just the first step in
understanding the function of any particular DNA sequence.
Following genome sequencing, the next step is to identify the
locations and functions of the various types of sequence present
in the genome. that control transcription, the intron–exon boundaries in the
gene, and any known or predicted alternative forms in which
Genome annotation identifies various types the introns and exons are spliced (Chapter 3). Each single-copy
of sequence. gene is given a unique name and its protein product identified.
Genomes contain many different types of sequence, among Note in Fig. 13.3 that single-copy genes can differ in size from
them protein-coding genes. Protein-coding genes are themselves one gene to the next. The annotations in Fig. 13.3 also specify the
composed of different regions, including regulatory elements that locations of sequences that encode RNAs that are not translated
specify when and where an RNA transcript is produced, noncoding into proteins, as well as various types of repeated sequence. Most
introns that are removed from the RNA transcript during RNA genomes also contain some genes that are present in multiple
processing (Chapter 3), and protein-coding exons that contain copies that originate from gene duplication; however these are
the codons that specify the amino acid sequence of a polypeptide usually identifiable in the same way as single-copy genes because
chain (Chapter 4). Genomes also contain coding sequences for the copies often differ enough in sequence that they can be
RNAs that are not translated into protein (noncoding RNAs), distinguished.
such as ribosomal RNA, transfer RNA, and other types of small Small genomes such as that of HIV and other viruses can
RNA molecule (Chapter 3). Finally, while much of the DNA in the be annotated by hand, but for large genomes like the human
genomes of multicellular organisms is transcribed at least in some genome, computers are essential. In the human genome, some
cell types, the functions of a large portion of these transcripts in protein-coding genes extend for more than a million nucleotides.
metabolism, physiology, development, or behavior are unknown. Roughly speaking, if the sequence of the approximately 3 billion
Genome annotation is the process by which researchers nucleotides in a human egg or sperm was printed in normal-sized
identify the various types of sequence present in genomes. type like this, the length of the ribbon would stretch 4000 miles
Genome annotation is essentially an exercise in adding (6440 kilometers), about the distance from Fairbanks, Alaska,
commentary to a genome sequence that identifies which types to Miami, Florida. By contrast, for the approximately 10,000
of sequence are present and where they are located. It can be nucleotides in the HIV genome, the ribbon would extend a mere
thought of as a form of pattern recognition, where the patterns are 70 feet (21 meters).
regularities in sequence that are characteristic of protein-coding Genome annotation is an ongoing process because, as
genes or other types of sequence. macromolecules and their functions and interactions become
An example of genome annotation is shown in Fig. 13.3. better understood, the annotations to the genome must be
Genes present in one copy per genome are indicated in orange. updated. A sequence that is annotated as nonfunctional today
Most single-copy genes are protein-coding genes. In some may be found to have a function tomorrow. For this reason, the
computer programs that analyze genomes, the annotation of annotation of certain genomes—including the human genome—
a single-copy gene also specifies any nearby regulatory regions will certainly continue to change.
276 SECTION 13.2 G E N O M E A N N OTAT I O N

Genome annotation includes searching for sequence by itself is enough to annotate the DNA segment as potentially
motifs. protein coding. The qualifier “potentially” is necessary because
Because genome annotation is essentially pattern recognition, it ORFs identified in a DNA sequence do not necessarily code for
begins with the identification of patterns called sequence motifs, protein. For this reason they are often called putative ORFs. A
telltale sequences of nucleotides that indicate what types of function region containing a putative ORF may exist merely by chance (even
(or absence of function) may be encoded in a particular region of a random sequence of nucleotides will contain ORFs averaging
the genome (Fig. 13.4). Sequence motifs can be found in the DNA 21 codons in length); or a putative ORF may not be transcribed; or if
itself or in the RNA sequence inferred from the DNA sequence. Once a putative ORF is transcribed, it might be in a noncoding RNA or an
identified, sequence motifs are typically confirmed by experimental intron of a protein-coding RNA. In the next section, we discuss how
methods. One sequence motif we have already encountered in the analysis of messenger RNA sequences can determine whether a
Chapter 3 is a promoter, a sequence where RNA polymerase and putative ORF is an actual ORF in DNA coding for protein.
associated proteins bind to the DNA to initiate transcription. Fig. 13.4b shows another type of sequence motif, this one
Another example of a sequence motif is an open reading also present in a hypothetical RNA transcript inferred from the
frame (ORF) (Fig. 13.4a). The motif for an open reading frame is DNA sequence. The nucleotide sequence at one end of the RNA
a long string of nucleotides that, if transcribed and processed into is complementary to that at the other end, so the single-stranded
messenger RNA, would result in a set of codons for amino acids molecule is able to fold back on itself and undergo base pairing
that does not contain a stop codon. The presence of an ORF motif to form a hairpin-shaped structure. Such hairpin structures
are characteristic of certain types of RNA that function in
gene regulation (Chapter 19). The DNA from which this RNA is
FIG. 13.4 Some common sequence motifs useful in genome transcribed has complementary sequences on either end as well.
annotation. (a) An open reading frame; (b) a noncoding Some sequence motifs are detected directly in the double-
RNA molecule; (c) transcription factor binding sites. stranded DNA. Fig. 13.4c shows two copies of a short sequence
that is a known binding site for DNA-binding proteins called
transcription factors (Chapter 3), which initiate transcription.
a.
RNA from a protein-coding region Transcription factor binding sites are often present in multiple
contains an open reading frame
consisting of triplets of nucleotides
copies and in either strand of the DNA. Sometimes they are
that can specify amino acids. located near the region of a gene where transcription is initiated
because the transcription factor helps determine when the gene
5’ 3’
will be transcribed. However, they can also be located far upstream
A U G G C C C G A U A C C U G G C A A A U G A U
of the gene, downstream of the gene, or in introns, and so their
identification is difficult.
MET ALA ARG TYR LEU ALA ASN ASP
Comparison of genomic DNA with messenger RNA
reveals the intron–exon structure of genes.
b.
In annotating an entire genome, researchers typically make use of
Regions of some RNA molecules, such as information outside the genome sequence itself. This information
those in transfer RNA, form hairpin may include sequences of messenger RNA molecules that are
structures in which the molecule folds
back on itself. isolated from various tissues or various stages of development of
AG UC the organism. Recall from Chapter 3 that messenger RNA (mRNA)
5’ A A
C molecules undergo processing and are therefore usually simpler than
UG U U A A C G C G A U G A C A U G G
U the DNA sequences from which they are transcribed—for example,
A C A A U U G C G C U A C U G U A G A
C C
introns are removed and exons are spliced together. The resulting
3’ A
AU mature mRNA therefore contains a long sequence of codons
AC
uninterrupted by a stop codon—in other words, an ORF. The ORF in
an mRNA is the region that is actually translated into protein.
c. One aspect of genome annotation is the determination of
DNA sequences that bind transcription which portions of the genome sequence correspond to sequences
factors are often short sequences present in in mRNA transcripts. An example is shown in Fig. 13.5, which
multiple copies near a protein-coding gene.
compares the DNA and mRNA for the beta (b) chain of hemoglobin,
the oxygen-carrying protein in red blood cells. Note that the
genomic DNA contains some sequences present in the mRNA,
5’ 3’
which correspond to exons, and some sequences that are not
A A T G T A C T A C A T T G A A T G T A T G G C
T T A C A T G A T G T A A C T T A C A T A C C G present in the mRNA, which correspond to introns. Comparison
3’ 5’ of mRNA with genomic DNA therefore reveals the intron–exon
 CHAPTER 13 GENOMES 277

that is conserved is likely to be important, even if its function is

FIG. 13.5 Identification of exons and introns by comparison of unknown, since it has changed very little over evolutionary time.
genomic DNA with mRNA sequence.

Exon 1 Intron Exon 2 Intron Exon 3

The HIV genome illustrates the utility of genome
annotation and comparison.
a. Genomic
DNA HIV and related viruses provide an example of how genomes are
annotated and how comparisons among genome sequences can
reveal evolutionary relationships. A virus is a small infectious agent
that contains a nucleic acid genome packaged inside a protein coat
b. Observed
mRNA called a capsid. Viruses can bind to receptor proteins on cells of the
host organism, enabling the viral genome to enter the cell. Infection
5’ untranslated Open reading frame 3’ untranslated
region region of a host cell is essential to viral reproduction because viruses use
cellular ATP and hijack cellular machinery to replicate, transcribe,
and translate their genome in order to make more viruses. Later in
structure of protein-coding genes. In fact, introns were first this chapter we will see that viruses can differ in the types of nucleic
discovered by comparing b-globin mRNA with genomic DNA. acid that make up their genome and how their messenger RNA is
produced, but here we consider only the genome of HIV.
An annotated genome summarizes knowledge, guides Whereas the genome of all cells consists of double-stranded
research, and reveals evolutionary relationships DNA, the genome of HIV is single-stranded RNA. The sequence
among organisms. of the HIV genome identifies it as a retrovirus that replicates by a
Genome annotation, which aims to identify all the functional and DNA intermediate that can be incorporated into the host genome.
repeat sequences present in the genome, is an imperfect science. More narrowly, the sequence of the HIV genome groups it among
Even in a well-annotated genome, some protein-coding sequences the mammalian lentiviruses, so named because of the long lag
or other important features may be overlooked, and occasionally between the initial time of infection and the appearance of
the annotation of a sequence motif is incorrect. symptoms (lenti- means “slow”).
Because researchers often have to rely on sequence motifs Fig. 13.6 shows the evolutionary relationships among a
alone and not experimental data, their descriptions may be sample of lentiviruses, grouped according to the similarity of
vague. For example, a common annotation in large genomes is their genome sequences. The evolutionary tree shows that closely
“hypothetical protein.” In some cases, such as the genome of related viruses have closely related hosts. For example, simian
the malaria parasite, this type of annotation accounts for about
50% of the possible protein-coding genes. There is no hint of
what a hypothetical protein may do or even whether it is actually
produced, since it is determined solely by the presence of a FIG. 13.6 Evolutionary relationships among viruses related to
putative ORF in the genomic sequence and not by the presence HIV. Data from A. Katzourakis, M. Tristem, O. G. Pybus, and
of actual mRNA or protein. Other annotations might be R. J. Gifford, 2007, “Discovery and Analysis of the First Endogenous
“DNA-binding protein,” “possible hairpin RNA,” or “tyrosine Lentivirus,” Proceedings of the National Academy of Sciences USA
kinase”—with no additional detail about these motifs’ functions 104(15): 6261– 6265.
or relationships to other sequences. In short, although some Horse lentiviruses
genome annotations summarize experimentally verified facts,
many others are hypotheses and guides to future research. Sheep lentiviruses
Genome sequences contain information about ancestry
Cow lentiviruses
and evolution, and so comparisons among genomes can reveal
how different species are related. For example, the sequence of Cat lentiviruses
the human genome is significantly more similar to that of the
chimpanzee than to that of the gorilla, indicating a more recent
Human and simian
common ancestry of humans and chimpanzees (Chapter 1). lentiviruses
Analysis of the similarities and differences in protein-coding SIV hoest
genes and other types of sequence in the genomes of different SIV syke
species is an area of study called comparative genomics. Such SIV tal
studies help us understand how genes and genomes evolve. HIV2
They can also guide genome annotation because the sequences
HIV1 types
of important functional elements are often very similar among
genomes of different organisms. Sequences that are similar in SIV mod
different organisms are said to be conserved. A sequence motif
278 SECTION 13.3 G E N E N U M B E R , G E N O M E S I Z E , A N D O RG A N I S M A L CO M P L E X I T Y

Gene number is not a good

FIG. 13.7 The annotated genome of HIV. predictor of biological complexity.
tat rev
The complete genome sequences of many
organisms allow us to make comparisons
gag pol vif vpr env among them (Table 13.1). As different
nef genomes were sequenced and annotated,
vpu it came as something of a surprise to find
Components Proteins required Surface proteins that humans have about the same number
of capsid for reverse of protein-coding genes as many organisms
transcription and with much smaller genomes. For example,
integration into host DNA
the small flowering plant Arabidopsis
thaliana has a genome only about 5% as
• Matrix • Protease • Surface large as that of humans, but the plant has
• Capsid • Reverse glycoprotein
at least as many protein-coding genes.
• Nucleocapsid transcriptase • Transmembrane
• Integrase glycoprotein And the nematode worm Caenorhabditis
• Vpr-binding
protein elegans contains only 959 cells, of which
302 are nerve cells that form the worm’s
lentiviruses are more closely related to one another than they brain. Humans have 100 trillion cells, of which 100 billion are nerve
are to cat lentiviruses. This observation implies that the genomes cells. Despite having 100 million times as many cells as the worm,
of the viruses evolve along with the genomes of their hosts. A we have roughly the same number of protein-coding genes. Another
second feature shown by the evolutionary tree is that human HIV example of the disconnect between level of complexity and genome
originated from at least two separate simian viruses that switched size is the observation that the genome size of the mountain
hosts from simians (most likely chimpanzees) to humans. grasshopper Podisma pedestris is more than 100 times that of the
The annotated sequence of the HIV genome tells us a lot fruit fly Drosophila melanogaster. While there is no agreed-upon
about the biology of this virus (Fig. 13.7). Many details are left definition of organismal complexity, it would be hard to make a case
out here, but the main point is to show the functional elements that grasshoppers are any more or less complex than fruit flies.
of HIV in the form of an annotated genome. The open reading On the other hand, some organisms are able to do more things
frame denoted gag encodes protein components of the capsid, with the genes they have than other organisms. For example,
pol encodes proteins needed for reverse transcription of the viral the expression of protein-coding genes can be regulated in many
RNA into DNA and incorporation into the host genome, and env subtle ways, causing different gene products to be made in
encodes proteins that are embedded in the lipid envelope. The different amounts in different cells at different times (Chapters
annotation in Fig. 13.7 also includes the genes tat and rev, encoding 19 and 20). Differential gene expression allows the same protein-
proteins essential for the HIV life cycle, as well as the genes vif, coding genes to be deployed in different combinations to yield
vpr, vpu, and nef, which encode proteins that enhance virulence in a variety of distinct cell types. In addition, proteins can interact
organisms. Identification of the genes necessary to complete the
HIV cycle is the first step to finding drugs that can interfere with
the cycle and prevent infection. TABLE 13.1 Gene Numbers of Several Organisms. Humans have
j Quick Check 3 Fig. 13.6 shows that closely related lentiviruses more genes than fruit flies and nematode worms, but not as many as
have closely related host organisms. What does this observation might be expected on the basis of complexity. Data from “Sequence Composition
suggest about the ability of lentiviruses to infect a wide variety of the Human Genome” in International Genome Sequencing Consortium, 2001, “Initial

of hosts? Sequencing and Analysis of the Human Genome” (PDF). Nature 409(6822): 860–921,
doi:10.1038/35057062, PMID 11237011.

APPROXIMATE
13.3 GENE NUMBER, GENOME SIZE, NUMBER OF PROTEIN-
AND ORGANISMAL COMPLEXITY COMMON NAME SPECIES NAME CODING GENES

Mustard plant Arabidopsis thaliana 27,000

Before whole-genome sequencing, molecular biology and evolution
Human Homo sapiens 25,000
research tended to focus on single genes. Researchers studied
Nematode worm Caenorhabditis elegans 22,000
how individual genes are turned on and off, and how a gene in one
organism is related to a gene in another organism. Now, with the Fruit fly Drosophila melanogaster 17,000
availability of genome sequences from multiple species, it is possible Baker’s yeast Saccharomyces cerevisiae 6000
to make comparisons across full genomes. Some of the results have Gut bacterium Escherichia coli 4000
been striking.
 CHAPTER 13 GENOMES 279

with one another so that, even though there are relatively few
types of protein, they are capable of combining in many different TABLE 13.2 Genome Sizes of Several Organisms. Genome size
ways to perform different functions. We also have seen that varies tremendously among eukaryotic organisms, and there is no
a single gene may yield multiple proteins, either because of correlation between genome size and the complexity of an organism.
alternative splicing (different exons are spliced together to make Data from “Sequence Composition of the Human Genome” in International Human

different proteins) or posttranslational modification (proteins Genome Sequencing Consortium, 2001, “Initial Sequencing and Analysis of the Human

undergo biochemical changes after they have been translated). Genome” (PDF). Nature 409(6822): 860–921, doi:10.1038/35057062, PMID

Overall, the data in Table 13.1 pose many tantalizing 11237011.

evolutionary questions and suggest that major evolutionary

changes can be accomplished not only by the acquisition of APPROXIMATE
COMMON NAME SPECIES NAME GENOME SIZE (Mb)
whole new genes, but also by modifying existing genes and their
regulation in subtle ways. Fruit fly Drosophila melanogaster 180
Fugu fish Fugu rubripes 400
Viruses, bacteria, and archaeons have small,
Boa constrictor Boa constrictor 2100
compact genomes.
Human Homo sapiens 3100
As well as comparing numbers of genes, we can also compare sizes of
genomes in different organisms. Before making such comparisons, Locust Schistocerca gregaria 9300
we need to understand how genome size is measured. Genomes are Onion Allium cepa 18,000
measured in numbers of base pairs, and the yardsticks of genome Newt Amphiuma means 84,000
size are a thousand base pairs (a kilobase, kb), a million base pairs (a Lungfish Protopterus aethiopicus 140,000
megabase, Mb), and a billion base pairs (a gigabase, Gb). Fern Ophioglossum petiolatum 160,000
Most viral genomes range in size from 3 kb to 300 kb, but a Amoeba Amoeba dubia 670,000
few are very large. The largest viral genome, found in a virus that
infects the amoeba Acanthamoeba polyphaga, is 1.2 Mb. This viral
genome contains almost 1000 protein-coding genes, including
some for sugar, lipid, and amino acid metabolism not found in any among similar organisms (Fig. 13.8). For comparison, Fig. 13.8 also
other viruses. shows the range of genome sizes among bacteria and archaeons.
The largest viral genome is twice as large as that of the The largest eukaryotic genome exceeds the size of the smallest
bacterium Mycoplasma genitalium. At 580 kb and encoding only by a factor of more than 500,000—and both the smallest and the
471 genes, the genome of M. genitalium is the smallest known largest are found among protozoa. The range among flowering plants
among free-living bacteria, those capable of living entirely on (angiosperms) is about three orders of magnitude, and the range
their own. The complete sequence of small bacterial genomes has among animals is about seven orders of magnitude. One species
allowed researchers to define the smallest, or minimal, genome of lungfish has a genome size more than 45 times larger than
(and therefore the minimal set of proteins) necessary to sustain the human genome (Table 13.2). Clearly, there is no relationship
life. Current findings suggest that the small M. genitalium is about between the size of the genome and the complexity of the organism.
two times larger than the minimal genome size thought to be The disconnect between genome size and organismal
necessary to encode all the functions essential to life. complexity is called the C-value paradox. The C-value is the amount
The genomes of bacteria and archaeons are information dense, of DNA in a reproductive cell, and the paradox is the apparent
meaning that most of the genome has a defined function. Roughly contradiction between genome size and organismal complexity,
speaking, 90% or more of their genomes consist of protein-coding leading to the difficulty of predicting one from the other.
genes (although in many cases the protein has an unknown
j Quick Check 4 Given our knowledge of genome sizes in
function). Bacterial genomes range in size from 0.5 to 10 Mb.
different organisms, would you predict that Homo sapiens or the
The bigger genomes have more genes, allowing these bacteria to
two-toed salamander (Amphiuma means) has the larger genome?
synthesize small molecules that other bacteria have to scrounge
for, or to use chemical energy in the covalent bonds of substances Why are some eukaryotic genomes so large? One reason is
that other bacteria cannot. Archaeons, whose genomes range in polyploidy, or having more than two sets of chromosomes in the
size from 0.5 to 5.7 Mb, have similar capabilities. genome. Polyploidy is especially prominent in many groups of
plants. Humans have two sets of 23 chromosomes, giving us
Among eukaryotes, there is no relationship between 46 chromosomes in total. But the polyploid bread wheat Triticum
genome size and organismal complexity. aestivum, for example, has six sets of seven chromosomes.
In eukaryotes, just as the number of genes does not correlate well Polyploidy has played an important role in plant evolution.
with organismal complexity, the size of the genome is unrelated Many agricultural crops are polyploid, including wheat, potatoes,
to the metabolic, developmental, and behavioral complexity of the olives, bananas, sugarcane, and coffee. Among flowering plants, it
organism (Table 13.2). The range of genome sizes is huge, even is estimated that 30% to 80% of existing species have polyploidy
280 SECTION 13.3 G E N E N U M B E R , G E N O M E S I Z E , A N D O RG A N I S M A L CO M P L E X I T Y

organisms. It came as a great

FIG. 13.8 Ranges of genome size. Even within a single group, genome sizes of different species surprise to learn that in the
can vary by several factors of 10. After Fig. 1, p. 186 in T. R. Gregory, 2004, “Macroevolution, human genome only about 2.5%
Hierarchy Theory, and the C-Value Enigma,” Paleobiology 30:179–202. of the genome actually
codes for proteins. The
other 97.5% includes
sequences we have
encountered earlier,
including introns,
noncoding DNA, and
Mammals
Birds various other types of
Reptiles repetitive sequences.
Frogs In Fig. 13.10, we see the
Salamanders composition of the human
Lungfishes
genome, including the principal
Teleosts
Chondrichthyes types of repeated sequences.
Non-vertebrate chordates Earlier we saw how repeated
Crustaceans sequences could be classified as
Insects dispersed or tandem according
Arachnids
to their organization in the
Myriapods
Mollusks genome. In Fig. 13.10 the repeated
Annelids sequences are classified according
Echinoderms to their function. Among the
Flatworms
repeated sequences is alpha (a)
Rotifers
Angiosperms
satellite DNA, which consists
Gymnosperms of tandem copies of a 171-bp
Pteridophytes sequence repeated near
Bryophytes each centromere an average
Nematodes
of 18,000 times. The a satellite
Sponges
Fungi DNA is essential for attachment
Algae of spindle fibers to the
Protozoa centromeres during cell division
Bacteria (Chapter 11).
Archaeons
Fig. 13.10 also shows the
105 106 107 108 109 1010 1011 1012 proportions of several types of
Base pairs repeated sequence collectively
known as transposable
in their evolutionary history, either because of the duplication of elements (abbreviated TE ; they are also called transposons),
the complete set of chromosomes in a single species, or because which are DNA sequences that can replicate and insert themselves
of hybridization, or crossing, between related species followed by into new positions in the genome. As a result, they have the
duplication of the chromosome sets in the hybrid (Fig. 13.9). Some potential to increase their copy number in the genome over time.
ferns take polyploidy to an extreme: One species has 84 copies of a Transposable elements are sometimes referred to as “selfish”
set of 15 chromosomes—1260 chromosomes altogether. DNA because it seems that their only function is to duplicate
However, the principal reason for large genomes among some themselves and proliferate in the genome, making them the
eukaryotes is that their genomes contain large amounts of DNA ultimate parasite.
that do not code for proteins, such as introns and DNA sequences Transposable elements make up about 45% of the DNA in
that are present in many copies. These repeated sequences are the human genome. They can be grouped into two major classes
discussed next. based on the way they replicate. One class consists of DNA
transposons, which replicate and transpose by DNA replication
About half of the human genome consists of trans- and repair. The other class consists of elements that transpose
posable elements and other types of repetitive DNA. by means of an RNA intermediate. These are sometimes called
Complete genome sequencing has allowed the different types retrotransposons because their RNA is used as a template to
of noncoding DNA to be specified more precisely in a variety of synthesize complementary strands of DNA, a process that reverses
 CHAPTER 13 GENOMES 281

of the accumulation of transposable

FIG. 13.9 Polyploidy in plants. A type of crocus known as Golden Yellow was formed by elements (Fig. 13.10). In some genomes
hybridization between two different species, and it contains a full set of chromosomes repetitive DNA is maintained over long
from each parent. The parental chromosomes in the hybrid are indicated in orange periods of time even as new species
and blue in the diagram on the right. Photo source: McPHOTO/age fotostock.. evolve and diversify, whereas in other
genomes the amount of repetitive DNA
is held in check because of deletion and
other processes. The result is that the
genomes of different species can contain
vastly differing amounts of repetitive
DNA, and this accounts in large part for
the C-value paradox.

13.4 ORGANIZATION
OF GENOMES
Regardless of whether an organism has a
large or a small genome, the genomes of
all organisms are large relative to the size
of the cell. For example, if the circular
genome of the intestinal bacterium
Escherichia coli were fully extended,
its length would be 200 times greater
the usual flow of genetic information from DNA into RNA (retro- than the diameter of the cell itself. The fully extended length
means “backward”). More than 40% of the human genome of DNA in human chromosome 1, our longest chromosome,
consists of various types of retrotransposons, whereas only about would be 10,000 times greater than the diameter of the average
3% of human DNA consists of DNA transposons. human cell. There is consequently a need to package an enormous
Over the course of evolutionary time, the amount of repetitive length of DNA into a form that will fit inside the cell while still
DNA in a genome can change drastically, in large part because allowing the DNA to replicate and carry out its coding functions.
The mechanism of packaging differs substantially in bacteria,
archaeons, and eukaryotes. We focus here on bacteria and
FIG. 13.10 Sequence composition of the human genome. eukaryotes, primarily because less is known about how DNA is
packaged in archaeons.
Miscellaneous category:
unrecognized, ambiguously
annotated, or nonfunctional Bacterial cells package their DNA as a nucleoid
gene duplicates Untranslated composed of many loops.
5’ ends of mRNA Bacterial genomes are circular, and the DNA double helix is
Untranslated underwound, which means that it makes fewer turns in going
3’ ends of mRNA
around the circle than would allow every base in one strand to
pair with its partner base in the other strand. Underwinding is

sa Other caused by an enzyme, topoisomerase II, that breaks the double
te
llit ding
DN es Co helix, rotates the ends to unwind the helix, and then seals the
A tr
ans break. Underwinding creates strain on the DNA molecule, which
pos
ons
is relieved by the formation of supercoils, in which the DNA
Protein- molecule coils on itself. Supercoiling allows all the base pairs to
Introns coding
genes form, even though the molecule is underwound. (You can make
(about your own supercoil by stretching and twirling the ends of a rubber
Retrotransposons 25,000) band, then relaxing the stress slightly to allow the twisted part to
form coils around itself. See Fig. 3.12.) Supercoils that result from
underwinding are called negative supercoils, and those that result
from overwinding are positive supercoils. In most organisms, DNA
is negatively supercoiled.
282 SECTION 13.4 O RG A N I Z AT I O N O F G E N O M E S

In bacteria, the supercoils of DNA form a structure with packaging. First, eukaryotic DNA is wrapped twice around a group
multiple loops called a nucleoid (Fig. 13.11). The supercoil loops of histone proteins called a nucleosome. A nucleosome is made
are bound together by proteins. In E. coli, the nucleoid has about up of eight histone proteins: two each of histones H2A, H2B,
100 loops, each containing about 50 kb of DNA. The protein H3, and H4. The histone proteins are rich in the amino acids
binding that forms the loops as well as the negative supercoiling of lysine and arginine, whose positive charges are attracted to the
the DNA compress the molecule into a compact volume. negative charges of the phosphates along the backbone of each
DNA strand.
Eukaryotic cells package their DNA as one molecule This first level of packaging of the DNA is sometimes referred
per chromosome. to as “beads on a string,” with the nucleosomes the beads and
In eukaryotic cells, DNA in the nucleus is packaged differently the DNA the string. It is also called a 10-nm fiber in reference to
from DNA in bacteria. As discussed in Chapter 3, eukaryotic DNA its diameter, which is about five times the diameter of the DNA
is linear and each DNA molecule forms a single chromosome. In double helix (Fig. 13.12). In general, these are areas of the genome
a chromosome, DNA is packaged with proteins to form a DNA- that are transcriptionally active.
protein complex called chromatin. There are several levels of The next level of packaging occurs when the chromatin is
more tightly coiled, forming a 30-nm fiber (Fig. 13.12). As the
chromosomes in the nucleus condense in preparation for cell
FIG. 13.11 Bacterial nucleoid. The circular bacterial chromosome division, each chromosome becomes progressively shorter and
twists on itself to form supercoils, which are anchored thicker as the 30-nm fiber coils onto itself to form a 300-nm coil, a
by proteins. Source. Dr. Klaus Boller/Science Source. 700-nm coiled coil, and finally a 1400-nm condensed chromosome
Nucleoid in a manner that is still not fully understood. The progressive
packaging constitutes chromosome condensation, an active,
energy-consuming process requiring the participation of several
types of proteins.
Greater detail of the structure of a fully condensed chromosome
is revealed when the histones are chemically removed (Fig. 13.13).
Without histones, the DNA spreads out in loops around a sup-
porting protein structure called the chromosome scaffold. Each
loop of relaxed DNA is 30 to 90 kb long and anchored to the scaffold
Bacterium at its base. Before removal of the histones, the loops are compact
and supercoiled. Each human chromosome contains 2000–8000
such loops, depending on its size.
In E. coli, the nucleoid Core region Protein Despite intriguing similarities between the nucleoid model
consists of about 100 in Fig. 13.11 and the chromosome scaffold model in Fig. 13.13, the
loops, each with about Supercoiled DNA
50 kb of negatively structures evolved independently and make use of different types
supercoiled DNA. of protein to bind the DNA and form the folded structure of DNA
and protein. Furthermore, the size of the eukaryotic chromosome
is vastly greater than the size of the bacterial nucleoid. To
appreciate the difference in scale, keep in mind that the volume of
a fully condensed human chromosome is five times larger than the
volume of a bacterial cell.

The human genome consists of 22 pairs of

chromosomes and two sex chromosomes.
As emphasized in Chapter 11, the orderly process of meiosis is
possible because chromosomes occur in pairs. The pairs usually
match in size, general appearance, and position of the centromere,
but there are exceptions, such as the X and Y sex chromosomes.
The pairs of chromosomes that match in size and appearance are
When the DNA in a
loop is nicked, the called homologous chromosomes. The members of each pair
supercoils in that of homologous chromosomes have the same genes arranged in
loop unwind and the same order along their length. If the DNA duplexes in each
the DNA duplex
forms a relaxed pair of homologs were denatured, each DNA strand could form a
double helix. duplex with its complementary strand from the other homolog.
There would be some differences in DNA sequence due to
 CHAPTER 13 GENOMES 283

FIG. 13.12 Levels of chromosome condensation. DNA is wrapped around nucleosomes to form a 10-nm fiber, which can become coiled into a
30-nm fiber and higher-order structures.

6 Condensed chromatid
1400 nm in diameter

5 Coiled coil
700 nm in
diameter

4 Coiled chromatin fiber

300 nm in diameter

3 Chromatin fiber
30 nm in diameter

Histone
proteins
Nucleosome

2 Nucleosome fiber
10 nm in diameter
1 DNA duplex
2 nm in diameter
284 SECTION 13.4 O RG A N I Z AT I O N O F G E N O M E S

FIG. 13.13 (a) A chromosome with histones and (b) a FIG. 13.14 A chromosome paint of chromosomes from a
chromosome depleted of histones, showing the human male. (a) Condensed chromosomes are
underlying scaffold. Source: Courtesy of Ulrich Laemmli. “painted” with fluorescent dyes. (b) Chromosomes
are arranged in the standard form of a karyotype.
Relaxed DNA after
DNA with histones histones removed Source: NHGRI, www.genome.gov.

Chromosome Scaffold
b

genetic variation, but not so many differences as to prevent DNA

hybridization.
Chromosome painting illustrates the nearly identical nature
of the DNA molecules in each pair of homologs. In this technique,
individual chromosomes are isolated from cells in metaphase
of mitosis. Metaphase of mitosis is the easiest stage in which to
isolate chromosomes because of the availability of chemicals that
prevent the spindle from forming. These chemicals block the cell
cycle at metaphase, so cells progress to metaphase and then stop.
Once the chromosomes have been isolated, the DNA from each
chromosome is fragmented, denatured, and labeled with a unique
combination of fluorescent dyes. Under fluorescent light, the dyes chromosomes show the same pattern of fluorescent color. This
give the DNA in each type of chromosome a different color. The means that a particular labeled DNA fragment hybridized only
fluorescently labeled DNA fragments are then mixed with and with the two homologs of one chromosome. Hence, the DNA in
hybridized to intact metaphase chromosomes from another cell. each pair of homologous chromosomes is different from that in
Each labeled fragment hybridizes to its complementary sequence any other pair of homologous chromosomes.
in the metaphase chromosomes, “painting” each metaphase Higher resolution of human chromosomes can be obtained by
chromosome with dye-labeled DNA fragments. the use of stains that bind preferentially to certain chromosomal
A chromosome paint of the chromosomes in a human male regions and produce a visible pattern of bands, or crosswise
is shown in Fig. 13.14. Fig. 13.14a shows the chromosomes in the striations, in the chromosomes. One such stain is the Giemsa
random orientation in which they were found in the metaphase stain; a Giemsa-stained karyotype is shown in Fig. 13.15. Note
cell, and Fig. 13.14b shows them arranged in a standard form called that each chromosome has a unique pattern of bands and that
a karyotype (Chapter 11). To make a karyotype, the images of the homologous chromosomes can readily be identified by their
homologous chromosomes are arranged in pairs from longest to identical banding patterns. Procedures using the Giemsa stain
shortest, with the sex chromosomes placed at the lower right. yield about 300 bands that are used as landmarks for describing
In this case, the sex chromosomes are XY (one X chromosome the location of genes along the chromosome. Such landmarks are
and one Y chromosome), indicating that the individual is male; also important in identifying specific chromosomal abnormalities
in a female the sex chromosomes would be XX (a pair of associated with diseases, including some childhood leukemias.
X chromosomes). Including the sex chromosomes, humans have Every species of eukaryote has its characteristic number of
23 pairs of chromosomes. chromosomes. The number differs from one species to the next,
An important observation from the chromosome paint shown with little relation between chromosome number and genome
in Fig. 13.14 is that the two members of each pair of homologous size. Despite variation in number, the rule that chromosomes
 CHAPTER 13 GENOMES 285

of the organelles became smaller during the course of evolution

FIG. 13.15 Giemsa bands in a karyotype of a human male. because most of the genes were transferred to the DNA in the
Each dashed line marks the position of the centromere. nucleus. If the products of these transferred genes are needed in
Source: Darryl Leja, NHGRI. www.genome.gov. the organelles, they are synthesized in the cytoplasm and targeted
for entry into the organelles by signal sequences (Chapter 5).
DNA in mitochondria and chloroplasts is usually circular
and exists in multiple copies per organelle. Among animals,
the size of a mitochondrial DNA molecule ranges from 14 to 18
1 2 3 4 5 kb. Mitochondrial DNA in plants is generally much larger, up
to 100 kb. Plant chloroplast DNA is more uniform in size than
mitochondrial DNA, with a range of 130 to 200 kb.
On the basis of size alone, some kind of packaging of organelle
6 7 8 9 10 11 12 DNA is necessary. For example, the mitochondrial DNA in human
cells is a circular molecule of 16 kb. Fully extended, it would have
a circumference about as large as that of the mitochodrion itself.
From their bacterial origin, you might expect organelle DNA to be
13 14 15 16 17 18 packaged as a nucleoid rather than as a chromosome, and indeed
it is. But the structures of the nucleoid in mitochondria and
chloroplasts differ from each other, and also differ from those in
19 20 21 22 X/Y free-living bacteria.

come in pairs holds up pretty well, with the exception of the 13.5 VIRUSES AND VIRAL GENOMES
sex chromosomes. Polyploids, too, are an exception, but even
in naturally occurring polyploids the number of copies of each Now we come to viral genomes—but not because they are
homologous chromosome is usually an even number, so there are particularly complex. Viral genomes are actually rather small and
pairs of homologs after all. compact with little or no repetitive DNA, and their sequences are
The occurrence of chromosomes in pairs allows eukaryotes to relatively easy to assemble, as we have seen in Fig. 13.7 for HIV.
reproduce sexually. When reproductive cells are formed during What sets viral genomes apart from those of cellular organisms
meiotic cell division, each cell receives one and only one copy of are the types of nucleic acid that make up their genomes and the
each of the pairs of homologous chromosomes (Chapter 11). When manner in which these genomes are replicated in the infected
reproductive cells from two individuals fuse to form an offspring cells. In their nucleic-acid composition and manner of replication,
cell, the chromosome number characteristic of the species is viral genomes are far more diverse than the genomes of cellular
reconstituted. organisms: All cellular organisms have genomes of double-stranded
DNA that replicate by means of the processes described in Chapter
Organelle DNA forms nucleoids that differ from 12. Not so with viruses. Furthermore, whereas biologists classify
those in bacteria. cellular organisms according to their degree of evolutionary
Most eukaryotic cells contain mitochondria, and many relatedness, they classify viruses based on their type of genome
contain chloroplasts. Each type of organelle has its own DNA, and mode of replication.
meaning that eukaryotic cells have multiple genomes. Each We mostly think of viruses as causing disease, and indeed
eukaryotic cell has a nuclear genome consisting of the DNA they do. Familiar examples include influenza (“flu”), polio, and
in the chromosomes. Cells with mitochondria also have a HIV. In some cases, they cause cancer (Case 2: Cancer). In fact, the
mitochondrial genome, and those with chloroplasts also have a term “virus” comes from the Latin for “poison.” But viruses play
chloroplast genome. other roles as well. Some viruses transfer genetic material from
Because the genome organization and mechanisms of protein one cell to another. This process is called horizontal gene transfer
synthesis in these organelles resemble those of bacteria, most to distinguish it from parent-to-offspring (vertical) gene transfer.
biologists subscribe to the theory that the organelles originated Horizontal gene transfer has played a major role in the evolution
as free-living bacterial cells that were engulfed by primitive of bacterial and archaeal genomes, as well as in the origin and
eukaryotic cells billions of years ago (Chapter 27). In Chapters 7 spread of antibiotic-resistance genes (Chapter 26). Molecular
and 8, we saw that the likely ancestor of mitochondria resembled biologists have learned to make use of this ability of viruses to
a group of today’s non-photosynthetic bacteria (a group that deliver genes into cells.
includes E. coli), and the likely ancestor of chloroplasts resembled In this section, we focus on viral diversity with an emphasis
today’s photosynthetic cyanobacteria. In both cases, the DNA on viral genomes. Thousands of viruses have been described in
286 SECTION 13.5 V I RU S E S A N D V I R A L G E N O M E S

detail, and probably millions more have yet to be discovered. It has and partially single-stranded, or single-stranded RNA or DNA with
been estimated that life on Earth is host to 1031 virus particles— a positive (1) or negative (2) sense. The sense of a nucleic acid
ten hundred thousand million more virus particles than grains molecule is positive if its sequence is the same as the sequence of
of sand! Most of these viruses infect bacteria, archaeons, and the mRNA that is used for protein synthesis, and negative if it is
unicellular eukaryotes, and they are they are especially abundant the complementary sequence. For example, a (1)RNA strand
in the ocean (1011 viruses per liter). Viruses are small, consisting has the same nucleotide sequence as the mRNA, whereas a (2)
of little more than a genome in a package, and they can reproduce RNA strand has the complementary sequence. Similarly, a (1)
only by hijacking host-cell functions, but as a group they have had DNA strand has the same sequence as the mRNA, and a (2)DNA
amazing evolutionary success. strand has the complementary sequence (except that U in RNA
is replaced with T in DNA). Because mRNA is synthesized from
Viruses can be classified by their genomes. a DNA template, it is the (2)DNA strand that is used for mRNA
The genomes of viruses are diverse. Some genomes are composed synthesis (Chapter 3).
of RNA and others of DNA. Some are single stranded, others are Two groups synthesize mRNA by the enzyme reverse
double stranded, and still others have both single- and double- transcriptase, and therefore are placed into their own groups
stranded regions. Some are circular and others are made up of a (VI and VII). Reverse transcriptase is an RNA-dependent DNA
single piece or multiple linear pieces of DNA (called linear and polymerase that uses a single-stranded RNA as a template to
segmented genomes, respectively). synthesize a DNA strand that is complementary in sequence to
Unlike forms of cellular life, there is no evidence that all the RNA (Fig. 13.16). The reverse transcriptase then displaces
viruses share a single common ancestor. Different types of the RNA template and replicates the DNA strand to produce a
virus may have evolved independently more than once. Since double-stranded DNA molecule that can be incorporated into
classification of viruses based on evolutionary relatedness is not the host genome. In synthesizing DNA from an RNA template,
possible, other criteria are necessary. One of the most useful the enzyme reverses the usual flow of genetic information from
classifications is the type of nucleic acid the virus contains DNA to RNA. This capability is so unusual and was so unexpected
and how the messenger RNA, which produces viral proteins, is that many molecular biologists at first doubted whether such
synthesized. The classification is called the Baltimore system after an enzyme could exist. Finally the enzyme was purified and its
David Baltimore, who devised it. properties verified, for which its discoverers, Howard Temin and
According to the Baltimore system, there are seven major David Baltimore, were awarded the Nobel Prize in Physiology or
groups of viruses, designated I–VII, as shown in Fig. 13.16. These Medicine in 1975, shared with Renato Dulbecco.
groups are largely based on whether their nucleic acid is double- As we saw earlier, genome size varies greatly among different
stranded DNA, double-stranded RNA, partially double-stranded viruses. RNA viral genomes tend to be smaller than DNA viral

FIG. 13.16 The Baltimore system of virus classification. This system classifies viruses by type of nucleic acid and the way mRNA is produced.

Group VII Group VI and VII viruses 3’ Group II

Incomplete double- use reverse transcriptase Single-stranded DNA,
stranded DNA, 3’ to produce double- 5’ () sense
Reverse transcriptase stranded DNA. ()DNA (Example: Canine
(Example: Hepatitis B) 5’
parvovirus)
Group VI
Single-stranded RNA, 5’ 5’ Group I
() sense, Double-stranded DNA
3’ 3’
Reverse transcriptase ()RNA ()DNA (Example: Adenovirus)
(Example: HIV)

Group IV 5’ 5’ 5’ Group III

Single-stranded RNA, Double-stranded RNA
3’ 3’ 3’
() sense ()RNA ()RNA mRNA() (Example: Rotavirus)
(Example: Polio)

Group IV and V viruses Group V viruses

use RNA-dependent 5’ Group V must synthesize
RNA polymerase to Single-tranded RNA, mRNA with a
produce complementary 3’ () sense () sense in order
RNA strands. ()RNA
(Examples: Influenza, to produce viral
Ebola) proteins.
 CHAPTER 13 GENOMES 287

genomes. Most eukaryotic viruses that have RNA genomes and

most plant viruses, including tobacco mosaic virus, are in group FIG. 13.17 A plant infected by tobacco mosaic virus. Courtesy
IV. Among bacterial and archaeal viruses, most genomes consist of H.D. Shew; Reproduced, by permission, from Shew, H.D., and Lucas,

double-stranded DNA. G.B., eds. 1991. Compendium of Tobacco Diseases. American

Phytopathological Society, St. Paul, MN.

The host range of a virus is determined by viral

and host surface proteins.
Although viruses show great diversity in their genomic nucleic-
acid composition and manner of replication, they all consist
of some type of nucleic acid, a capsid, and sometimes a lipid
envelope (Chapter 1). Viruses can reproduce only by infecting
living cells and exploiting the cell’s metabolism and protein
synthesis to produce more viruses. Most viruses are tiny, some
hardly larger than a ribosome, 25–30 nm in diameter. Roughly
speaking, the average size of a virus, relative to that of the host
cell it infects, may be compared to the size of an average person
relative to that of a commercial airliner.
All known cells and organisms are susceptible to viral
infection, including bacteria, archaeons, and eukaryotes. A cell in
which viral reproduction occurs is called a host cell. Some viruses
kill the host cell; others do not. Although viruses can infect all range in the early 1990s, leading to the infection and death of
types of living organism, a given virus can infect only certain lions in Tanzania.
species or types of cell. At one extreme, a virus can infect just a
single species. Smallpox, which infects only humans, is a good Viruses have diverse sizes and shapes.
example. In cases like this, we say that the virus has a narrow host Viruses show a wide variety of shapes and sizes, which are
range. For other viruses, the host range can be broad. For example, determined in some cases by the type of genome they carry.
rabies infects many different types of mammal, including Three examples are shown in Fig. 13.18. The T4 virus (Fig. 13.18a)
squirrels, dogs, and humans. Similarly, tobacco mosaic virus, a infects cells of the bacterium Escherichia coli. Viruses that infect
plant virus, infects more than 100 different species of plants bacterial cells are called bacteriophages, which literally means
(Fig. 13.17). No matter how broad the host range, however, plant “bacteria eaters.” The T4 bacteriophage has a complex structure
viruses cannot infect bacteria or animals, and bacterial viruses that includes a head composed of protein surrounding a molecule
cannot infect plants or animals. of double-stranded DNA, a tail, and tail fibers. In infecting a host
Host specificity results from the way that viruses gain entry cell, the T4 tail fibers attach to the surface, and the DNA and some
into cells. Proteins on the surface of the capsid or envelope (if proteins are injected into the cell through the tail.
present) bind to proteins on the surface of host cells. These Most viruses are not structurally so complex. Consider the
proteins interact in a specific manner, so the presence of the host tobacco mosaic virus, which infects plants (see Fig. 13.17). It has
protein on the cell surface determines which cells a virus can a helical shape formed by the arrangement of protein subunits
infect. For example, a protein on the surface of the HIV envelope entwined with a molecule of single-stranded RNA (Fig. 13.18b).
called gp120 (encoded by the env gene; see Fig. 13.7) binds to Tobacco mosaic virus was the first virus discovered, revealed in
a protein called CD4 on the surface of certain immune cells experiments showing that the infectious agent causing brown
(Chapter 43), so HIV infects these cells but not other cells. spots and discoloration of tobacco leaves was so small that it could
Following attachment, the virus gains entry into the cell, pass through the pores of filters that could trap even the smallest
where it can replicate by using the host cell machinery to make bacterial cells.
new viruses. In some cases, the viral genome integrates into the Many viruses have an approximately spherical shape formed
host cell genome (Chapter 19). from polygons of protein subunits that come together at their
The host range of a virus can change because of mutation edges to form a polyhedral capsid. Among the most common
and other mechanisms (Chapter 43). For example, avian, or bird, polyhedral shapes is an icosahedron, which has 20 identical
flu is a type of influenza virus that was once restricted primarily triangular faces. The example in Fig. 13.18c is adenovirus, a
to birds but now can infect humans. The first reported case in common cause of upper respiratory infections in humans. Many
humans was in 1996, and the disease has since spread widely. viruses that infect eukaryotic cells, such as adenovirus, are
Similarly, HIV once infected only nonhuman primates, but its surrounded by a glycoprotein envelope composed of a lipid bilayer
host range expanded in the twentieth century to include humans. with embedded proteins and glycoproteins that recognize and
Canine distemper virus, which infects dogs, expanded its host attach to host cell receptors.
288 SECTION 13.5 V I RU S E S A N D V I R A L G E N O M E S

FIG. 13.18 Virus structures. Viruses come in a variety of shapes, including (a) the head-and-tail structure of a bacteriophage, (b) the helical
shape of tobacco mosaic virus, and (c) the icosahedral shape of adenovirus. Sources: a. Dept. of Microbiology, Biozentrum/Getty Images; b. Biology
Pics/Science Source; c. BSIP/Science Source.

a b c

Viruses are capable of self-assembly.

The genome of a virus contains the genetic information needed FIG. 13.19 Tobacco mosaic virus. The protein and RNA
to specify all the structural components of the virus. Progeny components assemble themselves inside a cell.
virus particles are formed according to molecular self-assembly: RNA
When the viral components are present in the proper relative
Protein
amounts and under the right conditions, the components interact
spontaneously to assemble themselves into the mature virus
particle.
Tobacco mosaic virus illustrates the process of self-assembly.
In the earliest stages, the coat-protein monomers assemble into
two circular layers forming a cylindrical disk. This disk binds with
the RNA genome, and the combined structure forms the substrate
for polymerization of all the other protein monomers into a helical
filament that incorporates the rest of the RNA as the filament
grows (Fig. 13.19). The mature virus particle consists of 2130
protein monomers and 1 single-stranded RNA molecule of 6400
ribonucleotides. •
 CHAPTER 13 GENOMES 289

Core Concepts Summary Eukaryotic cells package their DNA into linear
chromosomes. page 282
13.1 A genome is the genetic material of a cell,
DNA in eukaryotes is wound around groups of histone
organism, organelle, or virus, and its sequence is the
proteins called nucleosomes to form a 10-nm fiber, which
order of bases along the DNA or (in some viruses)
in turn coils to form higher-order structures, such as the
RNA.
30-nm fiber. page 282
The sequence of an organism’s genome can be determined by
Diploid organisms have two copies of each chromosome,
breaking up the genome into small fragments, sequencing
called homologous chromosomes. page 284
these fragments, and then putting the sequences together at
their overlaps. page 272 Humans have 23 pairs of chromosomes, including the X
and Y sex-chromosome pair. Females are XX and males
Sequences that are repeated in the genome can make
are XY. page 284
sequence assembly difficult. page 273
The genomes of mitochondria and chloroplasts are
13.2 Researchers annotate genome sequences to organized into nucleoids that resemble, but are distinct
identify genes and other functional elements. from, those of bacteria. page 285

Genome annotation is the process by which the types

13.5 Viruses have diverse genomes, but all
and locations of the different kinds of sequences, such as
require a host cell to replicate.
protein-coding genes, are identified. page 275
Viruses can be classified by the Baltimore system, which
Genome annotation sometimes involves scanning the
defines seven groups on the basis of type of nucleic acid and
DNA sequence for characteristic sequence motifs.
the way the mRNA is synthesized. page 286
page 276
Viruses can infect all types of organism, but a given virus
Comparison of DNA sequences with messenger RNA
can infect only some types of cell. page 287
sequences reveals the intron–exon structure of protein-
coding genes. page 276 The host range of a virus is determined by proteins on viral
and host cells. page 287
By comparing annotated genomes of different organisms,
we can gain insight into their ancestry and evolution. Viruses have diverse shapes, including head-and-tail,
page 277 helical, and icosahedral. page 287

The annotated HIV genome shows it is a retrovirus, and it Viruses are capable of molecular self-assembly under the
contains the genes gag, pol, and env. page 277 appropriate conditions. page 288

13.3 The number of genes in a genome and the size

of a genome do not correlate well with the complexity Self-Assessment
of an organism.
1. Describe the shotgun method for determining the
The C-value paradox describes the observation that the size complete genome sequence of an organism.
of a genome (measured by its C-value) does not correlate
2. Repeated sequences can be classified according to their
with an organism’s complexity. page 279
organization in the genome as well as according to their
In eukaryotes, the C-value paradox can be explained by function. Give at least two examples of each.
differences in the amount of noncoding DNA, including
3. Explain the purpose of genome annotation.
repetitive sequences and transposons. Some genomes have
a lot of noncoding DNA; others do not. page 280 4. Describe how the comparison of genomic DNA to
messenger RNA can identify the exons and introns in a
13.4 The orderly packaging of DNA allows it to carry gene.
out its functions and fit inside the cell.
5. Explain how comparing the sequences of two genomes can
Bacteria package their circular DNA in a structure called a help to infer evolutionary relationships.
nucleoid. page 281
290 SELF-ASSESSMENT

6. What are some reasons why, in multicellular eukaryotes, 10. Describe the steps necessary to synthesize mRNA from
genome size is not necessarily related to number of each of the following: double-stranded DNA, single-
protein-coding genes or organismal complexity? stranded (1)DNA, single-stranded (2)DNA, single-
stranded (1)RNA, and single-stranded (2)RNA.
7. Compare and contrast the mechanisms by which bacterial
cells and eukaryotic cells package their DNA.

8. Draw a nucleosome, indicating the positions of DNA and Log in to to check your answers to the Self-
proteins. Assessment questions, and to access additional learning tools.

9. Define “homologous chromosomes” and describe a

technique that you could use to show their similarity.
 CHAPTER 14

Mutation and
DNA Repair

Core Concepts
14.1 Mutations are very rare for
any given nucleotide and
occur randomly without
regard to the needs of an
organism.
14.2 Small-scale mutations
include point mutations,
insertions and deletions,
and movement of
transposable elements.
14.3 Chromosomal mutations
involve large regions of one
or more chromosomes.
14.4 DNA can be damaged by
mutagens, but most DNA
damage is repaired.

Tetra Images/Getty Images.

291
292 SECTION 14.1 T H E R AT E A N D N AT U R E O F M U TAT I O N S

The sequencing of the human genome was a great step forward.

However, as we saw in the previous chapter, it is a simplification FIG. 14.1 Rate of mutation per nucleotide per replication. Data
to speak of the human genome. Virtually all species have abundant from: J. W. Drake, B. Charlesworth, D.Charlesworth, J. F. Crow, 1998,

amounts of genetic variation, that is, different nucleotides at the “Rates of Spontaneous Mutation,” Genetics 148:1667–1686.

same site in the genome. The reference human genome sequence 10–3
is actually a composite of several genomes, displaying the most
RNA viruses
common nucleotide at most sites. 10–4 and retroviruses
Variation among different individuals’ genomes arise from

Rate of mutation per nucleotide pair per

mutations. Any heritable change in the genetic material is a 10–5 HIV
mutation, and by “heritable” we mean that the mutation is
stable and therefore passed on through cell division (meiotic

round of replication
10–6
cell division, mitotic cell division, or binary fission). The process
by which mutations occur is fundamental in biology because 10–7
mutation is the ultimate source of genetic variation. Genetic
DNA viruses
variation accounts in part for the physical differences we see
10–8
among individuals, such as differences in hair color, eye color, and
height. And, on a much larger scale, genetic variation results in the
10–9 Bacteria
diversity of organisms on this planet, from bacteria to blue whales. Fruit fly
Mouse
There are many different types of mutation, from small Yeast Nematode
10–10
changes affecting a single base to larger alterations, such as the
Bread mold Human
duplication or deletion of a segment of a chromosome. In this
10–11
chapter, we examine some of the basic principles of mutation: 104 105 106 107 108 109 1010
the different types of mutation, how and when they occur, and Genome size (base pairs)
how they are repaired. In Chapter 15, we look at common types
of mutation, or genetic variation, present in populations, and, in
Chapters 16 and 17, at how this variation is inherited from one
generation to the next. of one nucleotide pair for a different nucleotide pair. Nucleotide-
substitution mutations are nevertheless relatively rare, and their
rate of occurrence differs among organisms. Fig. 14.1 compares
14.1 THE RATE AND NATURE the rates of newly arising mutations in a given base pair in a single
OF MUTATIONS round of replication in viruses and several types of organisms.
Most of these mutations are due to errors in replication. As seen in
Mutations result from mistakes in DNA replication or from the graph, the mutation rates for different organisms range across
unrepaired damage to DNA. The damage may be caused by reactive almost eight orders of magnitude.
molecules produced in the normal course of metabolism, by The highest rates of mutation per nucleotide per replication
chemicals in the environment, or by radiation of various types, are found among RNA viruses and retroviruses, including HIV.
including X-rays and ultraviolet light. Most genomes also contain Lower rates occur in DNA viruses, and even lower rates in
DNA sequences that can “jump” from one position to another in unicellular organisms such as bacteria and yeast. The rates of
the genome, and their insertion into or near genes is a source of mutation per nucleotide per DNA replication are nearly the same
mutation. Yet another source of mutation is incorrectly repaired for all multicellular animals, including mice and humans.
chromosome breaks caused by reactive chemicals or radiation. Mutation rates vary over a large range for a number of
Most mutations are spontaneous, occurring by chance reasons. RNA viruses and retroviruses have a relatively high rate
in the absence of any assignable cause. They occur randomly, of mutation because RNA is a less stable molecule than DNA,
unconnected to an organism’s needs—it makes no difference but more importantly because the replication of these genomes
whether or not a given mutation would benefit the organism. lacks a proofreading function. For the other genomes, the rate of
Whether a favorable mutation does or does not occur is purely mutation per nucleotide per DNA replication reflects differences
a matter of chance. This key principle, that mutations are in fidelity of replication and the efficiency of proofreading and
spontaneous and random, is the focus of this section. other mechanisms of DNA repair.
For the cellular organisms plotted in Fig. 14.1, the average
For individual nucleotides, mutation is a rare event. mutation rate is about 10210, which means that, on average,
We’ll see later in this chapter that there are several different types only 1 nucleotide in every 10 billion is mistakenly substituted
of mutation, but the most common mutation is the substitution for another.
 CHAPTER 14 M U TAT I O N A N D D N A R E PA I R 293

But averaging conceals many details. First, certain nucleotides per replication (Fig. 14.1), humans also have the largest rate of
are especially prone to mutation and can exhibit rates of mutation mutation per genome per generation (Fig. 14.2). This seeming
that are greater than the average by a factor of 10 or more. Sites paradox arises because humans have a large genome and undergo
in the genome that are especially mutable are called hotspots. many cell divisions per generation.
Second, in some multicellular organisms, the rates of mutation In humans, the average number of newly arising nucleotide-
differ between the sexes. In humans, for example, the rate of substitution mutations per genome in one generation is about
mutation is substantially greater in males than in females. Finally, 30, or about 60 per diploid zygote. However, about 80% of the
the rates for the multicellular animals plotted in Fig. 14.1 depend newly arising mutations in a zygote come from the father. This
on the type of cell: A distinction must be made between mutations is because, in a human male at age 30, the diploid germ cells
that occur in germ cells (haploid gametes and the diploid cells have gone through about 400 cycles of DNA replication and cell
that give rise to them) and mutations that occur in somatic cells division before meiosis takes place, as compared with about
(the other cells of the body). In mammals, the rate of mutation 30 cycles in females. Moreover, while the number of newly arising
per nucleotide per replication is greater in somatic cells than in mutations from the mother remains approximately constant
germ cells. with mother’s age, the number of newly arising mutations from
the father increases with age. Sperm from men of age 40 contain
Across the genome as a whole, mutation is common. about twice as many newly arising mutations as sperm from men
We can also look at mutation rate at another level of scale. of age 20.
Instead of considering the rate per nucleotide per replication, Such a large number of new mutations as occurs in the
we can examine the rate of mutation across an entire genome human genome in one generation would be intolerable in
in one generation. While it is clear from Fig. 14.1 that the rate of organisms with a high density of protein-coding genes, such as
mutation per nucleotide per replication in most multicellular bacteria or fungi. It is tolerable in humans and other mammals
organisms is low, the rate of mutation per genome per generation only because more than 90% of the nucleotides in the genome
depends on the size of the genome and the number of cell seem free to vary without deleterious consequences for the
divisions per generation. Taking into account genome size and organism, and a mere 2.5% of the genome code for protein
cell divisions per generation, the rate of new mutations across the (Chapter 13). The vast majority of newly arising mutations
whole genome per generation is shown in Fig. 14.2. Note that, therefore occur in noncoding DNA and are likely neutral or very
while humans have the smallest rate of mutation per nucleotide nearly neutral in their effects. It is these mutations, accumulated
through many generations and shuffled by recombination and
independent assortment (Chapter 11), that account for the
genetic diversity of most populations and the genetic uniqueness
FIG. 14.2 Rate of mutation per genome per generation. Data of each individual.
from: J. W. Drake, B. Charlesworth, D.Charlesworth, J. F. Crow, 1998,
“Rates of Spontaneous Mutation,” Genetics 148:1667–1686. Only germ-line mutations are transmitted to progeny.
100 Which is more important—the rate of mutation per nucleotide
per replication or the rate per genome per generation? That
Rate of mutation per genome per generation

30 Human depends on context. Mutations can take place in any type of

Mouse cell. Those that occur in eggs and sperm and the cells that give
10
rise to them are called germ-line mutations, and those in
3 nonreproductive cells are called somatic mutations. This
distinction is important because somatic mutations affect only
1 RNA viruses Fruit fly
and retroviruses the individual in which they occur—they are not transmitted
to future generations. In contrast, germ-line mutations are
0.3
transmitted to future generations because they occur in
HIV Nematode
0.1 reproductive cells.
For germ-line mutations, it is the rate of mutation per genome
0.03
per generation that matters more. Germ-line mutations are
0.01 important to the evolutionary process because, as they are passed
DNA viruses Bread from one generation of organism to the next, they may eventually
Bacteria mold
0.003 come to be present in many individuals descended from the
Yeast
original carrier.
104 105 106 107 108 109 1010 For somatic mutations, the mutation rate that matters is
Genome size (base pairs) the rate of mutation per nucleotide per replication. Although
294 SECTION 14.1 T H E R AT E A N D N AT U R E O F M U TAT I O N S

somatic mutations are not transmitted to future generations,

they are transmitted to daughter cells in mitotic cell divisions FIG. 14.4 Three somatic mutations implicated in the origin of
(Chapter 11). Hence, a somatic mutation affects not only the invasive colon cancer. These mutations must occur in
cell in which it occurs, but also all the cells that descend from it. the same cell lineage for cancer to develop. Source: Kathleen
The areas of different color or pattern that appear in “sectored” R. Cho, University of Michigan Medical School.

flowers, valued as ornamental plants (Fig. 14.3), are usually

due to somatic mutations in flower-color genes. A mutation in
a flower-color gene occurs in one cell, and as the cell replicates
during development of the flower, all its descendants in the cell
lineage—the generations of cells that originate from a single Normal
cells Normal colon tissue
ancestral cell—carry that mutation, producing a sector with
altered coloration. Mutation in
APC gene
Most cancers result from mutations in somatic cells
(Chapter 11). In some cases, the mutation increases the activity
of a gene that promotes cell growth and division, while in other
cases, it decreases the activity of a gene that restrains cell growth Cells mutant
in APC gene Small benign polyp
and division. In either event, the mutant cell and its descendants
escape from one of the normal control processes. Fortunately, a Mutation in
Ras gene
single somatic mutation is usually not sufficient to cause cancer—
usually two or three or more mutations in different genes are
required to derail control of normal cell division so extensively
that cancer results. Cells mutant in Large benign polyp
To cause cancer, the mutations must occur sequentially in APC and Ras genes
Mutation
a single cell line. Fig. 14.4 shows three key mutations that have in p53
been implicated in the origin of invasive colon cancer: p53, Ras, gene
and APC. Each mutation occurs randomly, but if, by chance, a
mutation in the Ras gene occurs in a cell that is derived from one
in which the APC gene has been mutated, that cell’s progeny forms Cells mutant in APC, Ras, Malignant colon cancer
and p53 genes
The accumulation
of three successive
FIG. 14.3 Somatic mutation. Somatic mutations in the Japanese mutations in a
morning glory (Ipomoea nil) in cell lineages that differ lineage of colon
cells results in
in their ability to make purple pigment cause sectors of malignant colon
different pigmentation in the flower. Source: Courtesy Atsushi cancer.
Hoshino, National Institute for Basic Biology, Japan.

a polyp. Another chance mutation in the same cell line, which now
carries mutations in both the APC and Ras genes, could lead to
malignant cancer. Normally, the occurrence of multiple mutations
in a single cell lineage is rare, but in people exposed to chemicals
that cause mutations or who carry mutations in DNA repair
processes, multiple mutations in a single cell lineage are more
likely to occur and so the risk of cancer is increased.

? CASE 3 YOU, FROM A TO T: YOUR PERSONAL GENOME

What can your personal genome tell you about your
genetic risk factors?
Cancer is usually due to a series of mutations that occur
sequentially in a single lineage of somatic cells, as illustrated
for colon cancer in Fig. 14.4. Each type of cancer is caused by its
 CHAPTER 14 M U TAT I O N A N D D N A R E PA I R 295

own particular sequence of mutations, although mutations in alcohol. While we may not be able to do much about the genetic
some genes are implicated in several different types of cancer. An risk factors for any of these conditions, knowing that we have
example is the p53 gene, the nonmutant product of which detects them may make us more careful about the lifestyle choices that
DNA damage and slows the cell cycle to allow time for DNA repair we make.
(Chapter 11). Mutations in p53 are one step in the mutational
Mutations are random with regard to an
progression of many different types of cancer, including colon
organism’s needs.
cancer and breast cancer.
How do mutations arise? Consider the following example: If
In most individuals with cancer, all the sequential mutations
an antibiotic is added to a liquid culture of bacterial cells that
that cause the cancer are spontaneous mutations that take place
are growing and dividing, most of the cells are killed, but a few
in somatic cells. They are not transmitted through the germ line,
survivors continue to grow and divide. These survivors are found
and so there is little or no increased risk of cancer in the offspring.
to contain mutations that confer resistance to the antibiotic.
In some families, however, there is a germ-line mutation in one
This simple observation raises a profound question. Does this
of the genes implicated in cancer that is transmitted from parents
experiment reveal the presence of individual bacteria with
to their children. In any child who inherits the mutation, all
mutations that had arisen spontaneously and were already
cells in the body contain the defective gene, and hence the cells
present? Or do the antibiotic-resistant mutants arise in response
already have taken one of the mutational steps that lead to cancer.
to the presence of the antibiotic?
The effect of such a germ-line mutation is therefore to reduce
These alternative hypotheses have deep implications for
the number of additional mutations that would otherwise be
all of biology because they suggest two very different ways in
necessary to produce cancer cells.
which mutations might arise. The first suggests that mutations
Any mutation that increases the risk of disease in an
occur without regard to the needs of an organism. According
individual is known as a genetic risk factor for that disease.
to this hypothesis, the presence of the antibiotic in the
For colon cancer, the major genetic risk factors are mutations
experiment with bacterial cells does not direct or induce
in APC, Ras, and p53. For breast cancer, the major genetic risk
antibiotic resistance in the cells, but instead allows the small
factors are mutations in the BRCA1 and BRCA2 genes (Case 3:
number of preexisting antibiotic-resistant mutants to flourish.
You, from A to T). A risk factor does not cause the disease, but it
The second hypothesis suggests that there is some sort of
makes the disease more likely to occur. For the genes implicated
feedback between the needs of an organism and the process of
in colon cancer, each is a risk factor because when it is mutated,
mutation, and the environment directs specific mutations that
fewer additional mutations are needed to bring about tumor
are beneficial to the organism.
growth.
To distinguish between these two hypotheses, Joshua and
The DNA sequence of each of our personal genomes can reveal
Esther Lederberg in 1952 carried out a now-famous experiment,
the genetic risk factors that each of us carries, not only for cancer
described in Fig. 14.5. Bacterial cells were grown and formed
but for many other diseases as well. Not all genetic risk factors are
colonies on agar plates in the absence of antibiotic. Then, using
known for all diseases, and a great deal of current research aims
replica plating, a technique they invented, the Lederbergs
to identify new ones. But many genetic risk factors are already
transferred these colonies to new plates containing antibiotic.
known for a large number of common diseases, including high
Only bacteria that were resistant to the antibiotic grew on the
blood pressure, diabetes, inflammatory bowel disease, age-related
new plates. Because replica plating preserved the arrangement of
macular degeneration, Alzheimer’s disease, and many forms of
the colonies, the Lederbergs were able to go back to the original
cancer. Therefore, your personal genome can be of great value in
plate and identify the colony that produced the antibiotic-resistant
identifying diseases for which you carry risk factors, as was the
colony on the replica plate. From that original colony, they were
case for Claudia Gilmore (Case 3: You, from A to T).
then able to isolate a pure culture of antibiotic-resistant bacteria.
Our personal genomes can identify only genetic risk
Replica plating allowed the Lederbergs to isolate a pure culture
factors, however. In many cases, disease risk is substantially
of antibiotic-resistant bacteria, even though the original bacteria
increased by environmental risk factors as well, especially
never were exposed to antibiotic. This result supported the first
lifestyle choices. While there are genetic risk factors for lung
hypothesis: Mutations occur randomly, and without regard to
cancer, for example, the single biggest environmental risk factor
the needs of the organisms. The role of the environment is not
is smoking tobacco. For skin cancer, the greatest environmental
to create specific mutations, but instead to select for them. The
risk factor is exposure to the damaging ultraviolet rays in sunlight
principle the Lederbergs demonstrated is true of all organisms so
or in the sunlamps used in tanning beds. For heart disease, it is
far examined.
smoking, lack of physical activity, and obesity. For diabetes, it
is an unhealthy diet. j Quick Check 1 If mutations occur at random with respect to an
For breast cancer, the environmental risk factors include organism’s needs, how does a species become more adapted to its
certain forms of hormone therapy, lack of physical activity, and environment over time?
HOW DO WE KNOW?

FIG. 14.5

Do mutations occur randomly, METHOD To distinguish between these two hypotheses, Joshua
and Esther Lederberg developed replica plating. In this technique,

or are they directed by the bacteria are grown on agar plates, where they form colonies
(Fig. 14.5a). The cells in any one colony result from the division of

environment? a single original cell, and thus they constitute a group of cells that
are genetically identical except for rare mutations that occur in the
course of growth and division. Then a disk of sterilized velvet is
BACKGROUND Researchers have long observed that beneficial pressed onto the plate. Cells from each colony stick to the velvet
mutations tend to persist in environments where they are useful—in disk (in mirror image, but the relative positions of the colonies are
the presence of antibiotic, bacterial populations become antibiotic preserved). The disk is then pressed onto the surface of a fresh
resistant; in the presence of insecticides, insect populations become plate, transferring to the new plate a few cells that originate from
insecticide resistant. each colony on the first agar plate, in their initial positions.

HYPOTHESIS These observations lead to two hypotheses about EXPERIMENT First, the Lederbergs grew bacterial colonies on
how a mutation, such as one that confers antibiotic resistance medium without antibiotic, called a nonselective medium because
on bacteria, might arise. The first suggests that mutations occur all cells are able to grow and form colonies on it. Then, by replica
randomly in bacterial populations and over time become more plating, they transferred some cells from each colony to a plate
common in the population in the presence of antibiotic (which containing antibiotic, so only antibiotic-resistant cells could multiply
destroys those bacteria without the mutation). In other words, they and form colonies. (Medium containing antibiotic is a selective
occur randomly with respect to the needs of an organism. The second medium because it “selects” for a particular attribute or element, in
hypothesis suggests that the environment, in this case the application this case antibiotic-resistant cells.) Because replica plating preserves
of antibiotic, induces or directs antibiotic resistance. the arrangement of the colonies, the location of an antibiotic-
a. resistant colony on the selective medium reveals the location of
its parental colony on the nonselective plate (Fig. 14.5b). Finally,
the Lederbergs were able to go back to the parental colony and
demonstrate that it was a pure culture of antibiotic-resistant bacteria
by plating cultures of this colony on selective medium (Fig. 14.5c).

CONCLUSION The Lederbergs’ replica-plating experiments

demonstrated that antibiotic-resistant mutants can arise in the
absence of antibiotic because at no time in the experiments did the
Agar plate 1 Agar plate 2 cells on nonselective medium come into contact with the antibiotic.
Incubation to allow Only the successive generations of daughter cells carried over to
growth of colonies
selective medium by replica plating were exposed to the antibiotic.
Nevertheless, by their procedure
the Lederbergs were able to isolate
pure colonies of antibiotic-resistant
b. c.
cells.

FOLLOW-UP WORK These results

The sterile Mutant colony
velvet identified on have been extended to other types
By chance, template original plate of mutation and other organisms,
this colony picks up both
mutant and suggesting that mutations are
contains a
few mutant nonmutant random and not directed by the
cells. cells.
environment.
Nonselective Nonselective Culture diluted
medium Incubation to allow medium and cells spread SOURCE Lederberg, J., and E. M.
growth of colonies on selective Lederberg. 1952. “Replica Plating and
Only the mutant cells grow medium Indirect Selection of Bacterial Mutants.”
on selective medium; the Journal of Bacteriology 63:399–406.
position of the colony tells
you which colony on the
nonselective medium
contains the mutant cells. Selective medium Pure culture of antibiotic-
resistant bacteria

296
 CHAPTER 14 M U TAT I O N A N D D N A R E PA I R 297

14.2 SMALL-SCALE MUTATIONS FIG. 14.6 A point mutation. A point mutation is a change in a
single nucleotide.
At the molecular level, a mutation is a change in the nucleotide
sequence of a genome. Such changes can be small, affecting one A G is erroneously
incorporated into the
or a few bases, or large, affecting entire chromosomes. We begin
daughter strand
by considering the origin and effects of small-scale changes to the opposite the T and is
DNA sequence. While mutation provides the raw material that not corrected by the
proofreading function. G T C T A A T
allows evolution to take place, it can play this role only because of C A G A T T A
an important feature of living systems: Once a mutation has taken
place in a gene, the mutant genome is replicated as faithfully as the
nonmutant genome. G T C T A A T Second round of
C A G G T T A DNA replication

Point mutations are changes in a single nucleotide.

Most DNA damage or errors in replication are immediately removed G T C T A A T First round of G T C C A A T
or corrected by specialized enzymes in the cell. (Some examples C A G A T T A DNA replication C A GG T T A
of DNA repair will be discussed in section 14.4.) We have already
seen one example of DNA repair, the proofreading function of DNA In the next round of
G T C T A A T
polymerase during the process of replication, which acts to remove an C A G A T T A replication, the G specifies
a C in the opposite strand.
incorrect nucleotide from the 3� end of growing DNA strand (see The new C–G pair is
Fig. 12.7). Damage that is corrected is not regarded as a mutation replicated as faithfully as
because the DNA sequence is immediately restored to its original state. the original T–A pair and
the mutation is now
If a change in DNA is to become stable and subsequently present in this cell lineage.
inherited through mitotic or meiotic cell divisions, it must escape
correction by the DNA repair systems. The example in Fig. 14.6
shows how a mutation incorporated during replication can become On the other hand, mutations in coding sequences do have
a permanent change to the genome. If, during replication, DNA predictable consequences in an organism. Fig. 14.7 shows an
polymerase mistakenly adds a G (instead of a T) to a growing strand example in the human DNA sequence coding for the amino acid
across from an A and its proofreading function does not catch the chain of b-(beta-)globin, a subunit of the protein hemoglobin,
error, the result is a T–G mismatch in the double-stranded DNA. which carries oxygen in red blood cells. Fig. 14.7a shows a
At the next replication the G in the
new template strand specifies a C in the
daughter strand, with the result that the
FIG. 14.7 Synonymous and nonsynonymous mutations. Synonymous mutations do not change
daughter DNA duplex has a perfectly
the amino acid sequence of the resulting protein. Nonsynonymous mutations change
matched C–G base pair. From this point
the amino acid sequence.
forward, the DNA molecule containing
the mutant C–G base pair will replicate as a. Nonmutant b. Synonymous c. Nonsynonymous
faithfully as the original molecule bearing mutation mutation
the nonmutant T–A base pair. A mutation
in which one base pair (in this example, C C T G A GG A G C C T G A AG A G C C T G T GG A G
GG A C T C C T C GG A C T T C T C G G A C A C C T C DNA
T–A) is replaced by a different base pair (in
this example, C–G) is called a nucleotide Transcribed
Transcription
substitution or point mutation. This is strand
the most frequent type of mutation. mRNA
The effect of a point mutation depends C C UG A GG A G C C UG A AG A G C C UGUGG A G

in part on where in the genome it occurs. In Translation

many multicellular eukaryotes, including
PRO GLU
PRO GLU GLUGLU PRO GLU
PRO G U GLU GLU PRO VAL
PRO V L GLUGLU Protein
humans, the vast majority of DNA in the
genome does not code for protein or RNA The nonmutant gene A nucleotide A nucleotide
expresses normal substitution that substitution that
(Chapter 13). Most of the sequences in -globin. does not change changes the amino
noncoding DNA have no known function, the amino acid is acid is a
which may explain why many mutations in a synonymous nonsynonymous
mutation (silent mutation (missense
noncoding DNA have no detectable effects mutation). mutation).
on the organism.
298 SECTION 14.2 S M A L L-S C A L E M U TAT I O N S

small part of the nonmutant DNA sequence, transcription and

translation of which results in incorporation of the amino acids FIG. 14.8 A nonsense mutation. A nonsense mutation can change
Pro–Glu–Glu in the b-globin polypeptide. an amino acid to a stop codon, resulting in a shortened
Fig. 14.7b shows an example of a harmless nucleotide and unstable protein.
substitution in this coding sequence. The mutation consists of a. Nonmutant b. Nonsense mutation
the substitution of an A–T base pair for the normal G–C base
pair. In the mRNA, the mutation changes the normal GAG C C T G A GG A G C C T T A GG A G
codon into the mutant GAA codon. But GAG and GAA both code GG A C T C C T C G G A A T C C T C DNA
for the same amino acid, glutamic acid (Glu). In other words,
Transcribed
they are synonymous codons, and so the resulting amino acid Transcription
strand
sequences are the same: Pro–Glu–Glu. Such mutations are called
mRNA
synonymous (silent) mutations. This example is typical in that C C UG A GG A G C C UUA GG A G
the synonymous codons differ at their third position (the 3� end of Translation
the codon). A quick look at the genetic code (see Table 4.1) shows
that most amino acids can be specified by synonymous codons, and PRO GLU GLU PRO STOP Protein
that in most cases the synonymous codons differ in the identity of
A nucleotide substitution
the nucleotide at the third position. that creates a stop codon
On the other hand, a point mutation in coding sequences can is called a nonsense
mutation.
sometimes have a drastic effect on an organism. Fig. 14.7c shows
such an example. In this case, a point mutation substitutes the first
A–T base pair for a T–A base pair. The result is a change in the mRNA
from GAG, which specifies Glu (glutamic acid), into GUG, which Small insertions and deletions involve several
specifies Val (valine). The resulting protein therefore contains the nucleotides.
amino acids Pro–Val–Glu instead of Pro–Glu–Glu. In other words, Another relatively common type of mutation is the deletion or
the nucleotide substitution results in an amino acid replacement. insertion of a small number of nucleotides. In noncoding DNA,
Point mutations that cause amino acid replacements are called such mutations have little or no effect. In protein-coding regions,
nonsynonymous (missense) mutations. their effects depend on their size. A small deletion or insertion that
Since the complete b-globin chain consists of 146 amino acids, is an exact multiple of three nucleotides results in a polypeptide
it may seem that a change in only one amino acid would have little with as many fewer (in the case of a deletion) or more (in the case
effect. But the change in even a single amino acid can affect the of an insertion) amino acids as there are codons deleted or inserted.
three-dimensional structure of a protein, and therefore change Thus, a deletion of three nucleotides eliminates one amino acid,
its ability to function. Individuals who inherit two copies of the and an insertion of six adds two amino acids.
mutant b-globin gene that specifies the Glu-to-Val replacement The effects of a deletion of three nucleotides can be seen in
have a disease known as sickle-cell anemia. In this condition, the cystic fibrosis. This disease is characterized by the production of
hemoglobin molecules tend to crystallize when exposed to lower abnormal secretions in the lungs, liver, and pancreas and other
than normal levels of oxygen. The crystallization of hemoglobin glands. Its chief symptoms are recurrent respiratory infections,
causes the cell to collapse from its normal ellipsoidal shape malnutrition resulting from incomplete digestion and absorption of
into the shape of a half-moon, or “sickle.” In this form, the red fats and proteins, and liver disease. Patients with cystic fibrosis have
blood cell is unable to carry the normal amount of oxygen. More an accumulation of thick, sticky mucus in their lungs, which often
important, the sickled cells tend to block tiny capillary vessels, leads to respiratory complications, including recurrent bacterial
interrupting the blood supply to vital tissues and organs and infections. Untreated, 95% of affected children die before age 5.
resulting in severe pain and fever. With proper medical care, including regular physical therapy to clear
Fig. 14.8 shows a third way that a point mutation can affect the lungs, antibiotics, pancreatic enzyme supplements, and good
a protein, one that nearly always has severe effects. This is a nutrition, the average life expectancy is currently 30 to 40 years.
nonsense mutation, which creates a stop codon that terminates The mutations responsible for cystic fibrosis are in the gene
translation. In the example in Fig. 14.8, the mutation creates encoding the cystic fibrosis transmembrane conductance regulator
a UAG codon in the mRNA. Because UAG is a translational (CFTR) (Fig. 14.9). The CFTR protein is a chloride channel,
stop codon, the resulting polypeptide terminates after Pro. which acts as a transporter to pump chloride ions out of the cell.
Polypeptides that are truncated are nearly always nonfunctional. Malfunction of CFTR causes ion imbalances that result in abnormal
They are also unstable and are quickly destroyed. Eukaryotic secretions from the many cell types in which the CFTR gene is
cells have mechanisms to destroy mRNA molecules that contain expressed. About 70% of the mutations associated with cystic
premature stop codons. fibrosis have a specific mutation known as ∆508 (delta 508), which
 CHAPTER 14 M U TAT I O N A N D D N A R E PA I R 299

is a deletion of three nucleotides that eliminates a phenylalanine

normally present at position 508 in the protein. The missing FIG. 14.10 A frameshift mutation. A frameshift mutation changes
amino acid results in a CFTR protein that does not fold properly the translational reading frame.
and is degraded before reaching the membrane. Some researchers
a. Nonmutant
hope that future therapy will include drugs that stabilize the
mutant protein or even treatments that repair the defective gene
C C T G A GG A G A A G T C T G C C DNA
in affected cells. GG A C T C C T C T T C A G A C GG
Small deletions or insertions that are not exact multiples of 3
can cause major changes in amino acid sequence because they do Transcription Transcribed
strand
not insert or delete entire codons. The effect of such a mutation
mRNA
can be appreciated by seeing how deletion of a single letter turns
C C UG A GG A G A A GU C UG C C
a perfectly sensible sentence of three-letter words into gibberish.
Translation
Consider the sentence:
PRO GLU GLU LYS SER ALA Protein
THE BIG BOY SAW THE CAT EAT THE BUG

If the red E is deleted, the new reading frame for three-letter

b. Frameshift mutation (insertion)
words is as follows:

THB IGB OYS AWT HEC ATE ATT HEB UG C C T C G A G G A G A A G T C T G C C DNA

GG AG C T C C T C T T C A G A C G G

Transcription
FIG. 14.9 Effect of a single amino acid deletion in CFTR, the mRNA
cystic fibrosis transmembrane conductance regulator. C C U C G A GG A G A A GU C UG C C
Translation
a. Nonmutant CFTR b. Mutant CFTR
PRO ARG GLY GLU VAL CYS Protein
A T C T T T GG T A T T GG T
T A G A A A C C A T A A C C A DNA An insertion or deletion that is not an exact
multiple of three nucleotides changes the
reading frame of translation. Such a mutation
Transcribed
Transcription is called a frameshift mutation.
strand
mRNA
A U C UUUGGU AUUGGU
Translation The result is unintelligible. Similarly, an insertion of a single
nucleotide causes a one-nucleotide shift in the reading frame
ILE PHE GLY ILE GLY Protein
of the mRNA, and it changes all codons following the site of
Missing amino acid (PHE) insertion. For this reason, such mutations are called frameshift
In the most common mutant mutations.
proteins, three nucleotides Fig. 14.10 shows the consequences of a frameshift mutation in
are deleted in the CFTR the b-globin gene. The normal sequence in Fig. 14.10a corresponds
gene, resulting in a missing
amino acid at position 508. to amino acids 5–10. The frameshift mutation in Fig. 14.10b is
Chloride ions caused by the insertion of a C–G base pair. The mRNA transcript
of the DNA therefore also has a single-base insertion. When this
mRNA is translated, the one-nucleotide shift in the reading frame
CFTR results in an amino acid sequence that has no resemblance to
transporter
the original protein. All amino acids downstream of the site of
insertion are changed, resulting in loss of protein function.

j Quick Check 2 The coding sequence of genes that specify

The mutant CFTR
protein is unstable,
proteins with the same function in related species sometimes
The CFTR transporter and is degraded differ from each other by an insertion or deletion of contiguous
pumps chloride ions before reaching the nucleotides. The number of nucleotides that are inserted or deleted
out of the cell. membrane.
is almost always an exact multiple of 3. Why is this expected?
300 SECTION 14.2 S M A L L-S C A L E M U TAT I O N S

Some mutations are due to the insertion of a all organisms contain several types of transposable element, each
transposable element. present in multiple copies per genome.
An important source of new mutations in many organisms is the Transposable elements were discovered by American geneticist
insertion of movable DNA sequences into or near a gene. Such Barbara McClintock in the 1940s (Fig. 14.11). She studied corn
movable DNA sequences are called transposable elements or (maize) because genetic changes that affect pigment formation
transposons. As we saw in Chapter 13, the genomes of virtually can be observed directly in the kernels. The normal color of

HOW DO WE KNOW?

FIG. 14.11

What causes sectoring in corn mutant yellow kernels reverted to normal purple, resulting in purple
sectors in an otherwise yellow kernel. From this observation, she

kernels? inferred that the Ds element had jumped out of the anthocyanin
gene, restoring its function. She also demonstrated that restoration
of the original purple color was associated with mutations
BACKGROUND In the late 1940s, Barbara McClintock discovered elsewhere in the genome. From this observation, she inferred that
what are now called transposable elements, DNA sequences that the Ds element had integrated elsewhere in the genome, where it
can move from one position to any other in the genome. She studied disrupted the function of another gene.
corn (Zea mays). Wild-type corn has purple kernels, resulting
CONCLUSION McClintock’s conclusion is illustrated in Fig.
from expression of purple anthocyanin pigment (Fig. 14.11a). A
14.11c: Transposable elements can be excised from their original
mutant with yellow kernels results from lack of purple anthocyanin
position in the genome and inserted into another position.
pigment. McClintock noticed that streaks of purple pigmentation
could be seen in many yellow kernels (Fig. 14.11b). This observation c.
indicated that the mutation causing yellow color was unstable and Transposition
that the gene could revert to the normal purple color. Transposable
element
Transposable
a b
elements can disrupt
the normal function
of a gene.

Transposable
element excised When a transposable
element is removed,
the function of the
Photo sources: a. photo_journey/Shutterstock; b. Rob Martienssen, Cold Spring Harbor original gene (dark
Laboratory. blue) can be restored.

HYPOTHESIS McClintock hypothesized that the yellow mutant FOLLOW-UP WORK McClintock won the Nobel Prize in
color resulted from a transposable element, which she called Physiology or Medicine in 1983. Later experiments showed
Dissociator (Ds), jumping into a site near or in the anthocyanin gene that Ds is a transposable element that lacks a functional gene for
and disrupting its function. She attributed the purple streaks to cell transposase, the protein needed for the element to move, and
lineages in which the transposable element had jumped out again, Ac is a transposable element that encodes transposase. Presence
restoring the anthocyanin gene. of Ac produces active transposase that allows Ds to move. Much
additional work showed that there are many different types
EXPERIMENT AND RESULTS By a series of genetic crosses,
of transposable element and that they are ubiquitous among
McClintock showed that the genetic instability of Ds was due
organisms.
to something on another chromosome that she called Activator
(Ac). She set up crosses in which she could track the Ac-bearing SOURCE McClintock, B. 1950. “The Origin and Behavior of Mutable Loci in
chromosome. She observed that in the presence of Ac, cells in Maize.” Proceedings of the National Academy of Sciences of the USA. 36:344–355.
 CHAPTER 14 M U TAT I O N A N D D N A R E PA I R 301

maize kernels is purple. Each kernel consists of many cells, and pasted at various sites in the genome, potentially disrupting the
if no mutations affecting pigmentation occur in a kernel, it will function of many different genes.
be uniformly purple. The kernel pigments are synthesized by
several different enzymes in a metabolic pathway, and any of
these enzymes can be rendered nonfunctional by mutations in 14.3 CHROMOSOMAL MUTATIONS
their genes, including the insertion of transposable elements.
Since McClintock’s work, we have learned that most transposable Whereas most mutations involve only one or a few nucleotides,
elements are segments of DNA a thousand or more base pairs long. some affect larger regions extending over hundreds of thousands
When such a large piece of DNA inserts into a gene, it can interfere or millions of nucleotides and have effects on chromosome
with transcription, cause errors in RNA processing, or disrupt the structure that are often large enough to be visible through an
open reading frame. The result in the case of maize is that the cell ordinary optical microscope. Double-stranded breaks in DNA that
is unable to produce pigment, and so the kernels will be yellow. are incorrectly repaired can lead to chromosomal mutations. The
Among mutants affecting kernel pigmentation, McClintock breaks may result from interactions between DNA and reactive
observed certain mutants that resulted in kernels that were molecules produced in metabolism or from reactive chemicals in
mostly yellow but speckled with purple. She suspected that these the environment or by radiation (especially X-rays). Chromosomal
particular mutants might result from a transposable element mutations can also arise from errors in DNA replication,
jumping into and out of a gene. Each colored sector consists of particularly in sequences that are tandemly repeated along the
a lineage of daughter cells from a single ancestral cell in which DNA (Chapter 13).
pigment synthesis had been restored. McClintock realized that, Chromosomal mutations can delete or duplicate regions of a
just as the inability of the kernel cells to produce pigment was chromosome containing several or many genes, and the resulting
caused by a transposable element jumping into a gene, restoration change in gene copy number also changes the amount of the
of that ability could be caused by the transposable element products of these genes in the cell. Chromosomal mutations
jumping out again. Her hypothesis that transposable elements are can also alter the linear order of genes along a chromosome or
responsible for the pigment mutations was confirmed when she interchange the arms of nonhomologous chromosomes. While
found that in cells where pigment had been restored, mutations these types of chromosomal mutations do not change gene copy
affecting other genes had also occurred. She deduced that these number, they do affect chromosome pairing and segregation in
other mutations were due to a transposable element jumping out meiosis. These effects distinguish chromosome abnormalities
of a pigment gene and into a different gene in the same cell. from nucleotide substitutions, small-scale deletions and
The movement, or transposition, of transposable elements duplications, transpositions, and other submicroscopic mutations.
occurs by different mechanisms according to the type of transposon In this section, we briefly consider the major types of chromosome
(Chapter 13). McClintock’s original discovery was of a DNA abnormalities in more detail.
transposon that transposes by a cut-and-paste mechanism in which
the transposon is cleaved from its original location in the genome Duplications and deletions result in gain or loss
by a specific enzyme (transposase) and inserted into a different of DNA.
position. Removal of the transposon from its original position and Among the most common chromosomal abnormalities are those
repair of the cleavage leads to restoration of gene function, which in which a segment of the chromosome is either present in two
in McClintock’s experiment was the ability to produce purple copies or is missing altogether (Fig. 14.12). A chromosome in
pigment. Unstable mutations due to DNA transposons can occur in which a region is present twice instead of once is said to contain a
almost any organism, including the Japanese morning glory, whose duplication (Fig. 14.12a). Although large duplications that include
sectored flowers are shown in Fig. 14.3. hundreds or thousands of genes are usually harmful and quickly
Not all transposons undergo transposition by a cut-and- eliminated from the population, small duplications including only
paste mechanism. As discussed in Chapter 13, retrotransposons
undergo transposition by an RNA intermediate, and when these
types of transposable elements move, the retrotransposon FIG. 14.12 Duplication and deletion. (a) In a duplication, a
used as a template for transcription stays behind in its original segment of chromosome is repeated. (b) In a deletion,
location. This mode of transposition might be called a copy-and- a segment of chromosome is missing.
paste mechanism. It is mediated by two enzymes. One is reverse
transcriptase (Chapter 13), which produces a double-stranded DNA Normal chromosome
copy of the retrotransposon from its RNA transcript, much in the
same way that reverse transcriptase produces double-stranded
DNA from viral RNA genomes. The other enzyme is an integrase,
which cuts genomic DNA and inserts the retrotransposon at the a. Duplication b. Deletion
cut site. By this mechanism, a retrotransposon can be copied and
302 SECTION 14.3 C H RO M O S O M A L M U TAT I O N S

FIG. 14.13 Origin of the b-globin gene family. The b-globin gene family evolved through several episodes of duplication and divergence.
Data from: Y. A. Trusov and P. H. Dear, 1996, “A Molecular Clock Based on the Expansion of Gene Families,” Nucleic Acids Research 24(6):995–999.

DNA on chromosome 11
G A
-globin -globin -globin -globin -globin
(embryo) (fetus) (fetus) (adult) (adult)

20 MY

40 MY
Time (in millions of years)

100 MY
Gene for Gene for
prenatal -globin adult -globin

Duplication
event

200 MY
Original single copy
of globin gene
one or a few genes can be maintained over many generations.
Usually, duplication of a region of the genome is less harmful than New genes can be created by duplication and
divergence. The resulting genes usually remain
deletion of the same region. similar in function, forming a gene family.
An example of a deletion, in which a region of the
chromosome is missing, is shown in Fig. 14.12b. A deletion can
result from an error in replication or from the joining of breaks
in a chromosome that occur on either side of the deleted region. It is worth emphasizing that one rarely observes deletions
Even though a deletion may eliminate a gene that is essential or duplications that include the centromere, the site associated
for survival, the deletion can persist in the population because with attachment of the spindle fibers that move the chromosome
chromosomes usually occur in homologous pairs. If one member during cell division (Chapter 11). The reason is that an abnormal
of a homologous pair has a deletion of an essential gene but the chromosome without a centromere, or one with two centromeres,
gene is present in the other member of the pair, that one copy is usually lost within a few cell divisions because it cannot be
of the gene is often sufficient for survival and reproduction. In directed properly into the daughter cells during cell division.
these cases, the deletion can be transmitted from generation to
generation and persist harmlessly, as long as the chromosome is Gene families arise from gene duplication and
present along with a normal chromosome. evolutionary divergence.
But some deletions decrease the chance of survival or Small duplications play an important role in the origin of new
reproduction of an organism even when the homologous genes in the course of evolution. In most cases, when a gene is
chromosome is normal. In general, the larger the deletion, the duplicated, one of the copies is free to change without causing
smaller the chance of survival. In the fruit fly Drosophila, individuals harm to the organism because the other copy continues to carry
with deletions of more than 100–150 genes rarely survive even out the normal function of the gene. Occasionally, a mutation in
when the homologous chromosome is normal. The interpretation of the “extra” copy of the gene may result in a beneficial effect on
the reduced survival is that organisms are sensitive to the dosage, survival or reproduction, and gradually a new gene is formed from
or number of copies, of each gene. Normal embryonic development the duplicate. These new genes usually have a function similar to
requires that genes be present in a particular dosage. Although small that of the original gene.
deviations from normal gene dosage can be tolerated, as indicated This process of creating new genes from duplicates of old ones
by the survival of individuals containing small duplications or is known as duplication and divergence. The term divergence
deletions, the cumulative effects of large deviations from normal refers to the slow accumulation of differences between duplicate
gene dosage are incompatible with life. It is usually not the total copies of a gene that occurs on an evolutionary time scale.
number of copies of each gene that matters, but rather the number Multiple rounds of duplication and divergence can give rise to a
of copies of each gene relative to other genes. This explains why group of genes with related functions known as a gene family.
some plant populations can include polyploids with multiple The largest gene family in the human genome has about 400 genes
complete sets of chromosomes (Chapter 13), but in the same species and encodes for proteins that detect odors. These proteins are
large duplications and deletions are lethal. structurally very similar, but differ in the region that binds small
 CHAPTER 14 M U TAT I O N A N D D N A R E PA I R 303

odor molecules. It is the diversity of the odorant binding sites that

allows us to identify so many different smells. FIG. 14.15 Reciprocal translocation. A reciprocal translocation
Most gene families are not as large and diverse as that for odor results from an interchange of parts between
detection. Fig. 14.13 shows the evolutionary origin of the family nonhomologous chromosomes.
of globin genes, which in humans are spread across about 50 kb of
chromosome 11. The globin gene family consists of five different
genes that are expressed at various times during development Chromosome
Reciprocal translocation
breaks
(embryo, fetus, or adult). The two γ -(gamma-)globin polypeptides
are nearly identical in amino acid sequence but expressed at
different levels in the fetus, whereas the adult δ-(delta) and
b-polypeptides differ somewhat more and are expressed at very
different levels. The sequences are all similar enough to imply that
the genes arose through duplication and divergence. explains in part why the order of genes along a chromosome can
Globin sequences have changed through evolutionary time differ even among closely related species.
at a relatively constant rate, and so the number of differences
between any two sequences is proportional to the time since they A reciprocal translocation joins segments from
were created by duplication. The relative constancy of rates of nonhomologous chromosomes.
evolutionary change in a DNA nucleotide sequence or a protein A reciprocal translocation (Fig. 14.15) occurs when two
amino acid sequence is known as a molecular clock, and it allows different (nonhomologous) chromosomes undergo an exchange
molecular differences among genomes to be correlated with the of parts. In the formation of a reciprocal translocation, both
fossil record and dated accordingly (Chapter 21). For example, the chromosomes are broken and the terminal segments are
earliest duplication event in the tree in Fig. 14.13, which produced exchanged before the breaks are repaired. In large genomes,
distinct genes for fetal and adult hemoglobin, took place at about the breaks are likely to occur in noncoding DNA, so the breaks
the same time (200 million years ago) that the common ancestor themselves do not usually disrupt gene function.
of today’s placental mammals became a distinct species from the Since reciprocal translocations change only the arrangement of
common ancestor of today’s marsupial mammals. genes and not their number, most reciprocal translocations do not
affect the survival of organisms. Proper gene dosage requires the
An inversion has a chromosomal region reversed presence of both parts of the reciprocal translocation, however, as
in orientation. well as one copy of each of the normal homologous chromosomes.
Chromosomes in which the normal order of a block of genes is Problems can arise in meiosis because both chromosomes involved
reversed contain an inversion (Fig. 14.14). An inversion is typically in the reciprocal translocation may not move together into the
produced when the region between two breaks in a chromosome same daughter cells, resulting in gametes with only one part of the
is flipped in orientation before the breaks are repaired. Especially reciprocal translocation. This inequality does upset gene dosage,
in large genomes, the breaks are likely to occur in noncoding because these gametes have extra copies of genes in one of the
DNA rather than within a gene. Whereas large inversions can chromosomes and are missing copies of genes in the other. In the
cause problems in meiosis, small inversions are common in many next chapter, we will see that such abnormalities are observed in a
populations and play an important role in chromosome evolution. A significant number of human embryos.
small inversion can have almost no effect on the organism because
it still contains all of the genes present in the original chromosome,
merely flipped in their order, and is too small to cause problems in 14.4 DNA DAMAGE AND REPAIR
meiosis. The accumulation of inversions over evolutionary time
DNA is a fragile molecule, prone to damage of many different
kinds. Every day, in each cell in our bodies, the DNA is damaged
in some way at tens of thousands of places along the molecule.
FIG. 14.14 Inversion. A chromosome with an inversion has a Fortunately, cells have evolved mechanisms for repairing various
segment of chromosome present in reverse orientation. types of damage and restoring the DNA to its original condition. In
this section, we examine some of the major types of DNA damage
and some of the ways in which it is repaired.
Normal chromosome
DNA damage can affect both DNA backbone
and bases.
Most mutations are spontaneous and occur naturally. However,
mutations can also be induced by radiation or chemicals.
Inversion Mutagens are agents that increase the probability of mutation.
304 SECTION 14.4 D N A DA M AG E A N D R E PA I R

FIG. 14.16 Major types of DNA damage. Most types of DNA damage can be repaired by specialized enzymes.

Single-stranded Cross-linked Missing Bulky side Double-stranded

break in DNA thymine bases base group attached break in DNA
backbone to a base backbone

The presence of a mutagen can increase the probability of j Quick Check 3 Earlier, we described the Lederbergs’
mutation by a factor of 100 or more. experiment, which demonstrated that mutations are not directed
Some of the most important types of DNA damage induced by the environment. But mutagens, which are environmental, can
by mutagens are illustrated in Fig. 14.16, which shows a highly lead to mutations. What’s the difference?
damaged DNA molecule. Some types of damage affect the
structure of the DNA double helix. These include breaks in the Most DNA damage is corrected by specialized
sugar–phosphate backbone, one of the main mutagenic effects repair enzymes.
of X-rays. Breaks can occur in just one strand of the DNA or Cells contain many specialized DNA-repair enzymes that correct
both. Ultraviolet light can cause cross-links between adjacent specific kinds of damage, and in this section we examine a few
pyrimidine bases, especially thymine, resulting in the formation examples. Perhaps the simplest is the repair of breaks in the sugar–
of thymine dimers. Covalent bonding between adjacent thymines phosphate backbone, which are sealed by DNA ligase, an enzyme
in a DNA strand causes the double helix to become pinched, both that can repair the break by using the energy in ATP to join the
the major groove and the minor groove become wider, and the 3� hydroxyl of one end to the 5� phosphate of the other end. Most
T–A base pairing is weakened. organisms have multiple different types of DNA ligase, some of
Yet another type of structural damage is loss of a base from which participate in DNA replication (Chapter 12) and others in
one of the deoxyribose sugars, resulting in a gap in one strand DNA repair. One type of ligase seals single-stranded breaks in
where no base is present. Spontaneous loss of a purine base is one DNA, and a different type seals double-stranded breaks. Double-
of the most common types of DNA damage, occurring at the rate stranded breaks often result in chromosomal rearrangements
of about 13,000 purines lost per human cell per day. Most of these because they are less likely to be repaired than single-stranded
mutations result from the interaction between DNA and normal breaks. In addition to their importance in DNA replication and
metabolic by-products. The rate increases with age, and it can also repair, ligases are an important tool in research in molecular
be increased by exposure to oxidizing agents such as household biology because they allow DNA molecules from different sources
bleach or hydrogen peroxide. to be joined to produce recombinant DNA (Chapter 12).
Other types of damage affect the bases themselves. Bases that As we saw earlier in this chapter, mispairing of bases during
are chemically damaged tend to mispair. Some bases are damaged DNA replication leads to the incorporation of incorrect nucleotides
spontaneously when reaction with a water molecule replaces and potentially to nucleotide substitutions. About 99% of the
an amino group (2NH2) with an atom of oxygen (5O), which mispaired bases are corrected immediately by the proofreading
interferes with the base’s ability to form hydrogen bonds with a function of DNA polymerase, in which the mispaired nucleotide
complement. Some naturally occurring molecules mimic bases and is removed immediately after incorporation and replaced by the
can be incorporated into DNA and cause nucleotide substitutions. correct nucleotide (Chapter 12).
Caffeine mimics a purine base, for example (although to be Even after proofreading by DNA polymerase, the proportion of
mutagenic, the amount of caffeine required is far more than any mismatched nucleotides in replicated DNA is about 1026, which is
normal person could possibly consume). at least 1000 times greater than the actual frequency of errors in
Chemicals that are highly reactive tend to be mutagenic, often cellular organisms (see Fig. 14.1). What accounts for the difference
because they add bulky side groups to the bases that hinder proper is a second-chance mechanism for catching mismatches that is
base pairing. The main environmental source of such chemicals is known as postreplication mismatch repair (Fig. 14.17).
tobacco smoke. Other chemicals can perturb the DNA replication In the bacterium E. coli, postreplication mismatch repair
complex and cause the insertion or deletion of one or occasionally begins when a protein known as MutS binds to the site of a
several nucleotides. mismatch (Fig. 14.17a) and brings two other proteins (MutL
 CHAPTER 14 M U TAT I O N A N D D N A R E PA I R 305

nondividing cells. One example is base excision repair, which

FIG. 14.17 Postreplication mismatch repair. In postreplication corrects abnormal or damaged bases. In the first step of base
mismatch repair, the segment of a DNA strand excision repair, an abnormal or damaged base is cleaved from the
containing the mismatch is removed and then sugar in the DNA backbone. Then, the baseless sugar is removed
resynthesized. from the backbone, leaving a gap of one nucleotide. Finally, a
a. repair polymerase inserts the correct nucleotide into the gap.
Parental strand An example of base excision repair is shown in Fig. 14.18,
C C T GG T G A G T A G Newly synthesized strand which depicts a molecule in which uracil instead of cytosine is
GG A C C GC T C A T C
present in a DNA strand (Fig. 14.18a). Recall that uracil is normally
MutS recognizes mismatched
bases in DNA and initiates the found in mRNA, but not in DNA. Most organisms have an enzyme
Mismatch repair process. called DNA uracil glycosylase that cleaves the uracil from its
deoxyribose sugar, resulting in a sugar in the DNA backbone
b.
lacking a base (Fig. 14.18b). Such baseless sugars are recognized by
an enzyme called AP endonuclease (Fig. 14.18c), which cleaves the
C C T GG T G A G T A G
GG A C C GC T C A T C backbone on both sides of the sugar that lacks a base. This cleavage
results in a one-nucleotide gap, which is filled by new synthesis
MutL and MutH proteins are using the intact DNA strand as a template. The result is that uracil
recruited, and MutH breaks the is removed and replaced with cytosine. Specialized glycosylases
backbone some distance away. remove other types of abnormal or damaged bases from the DNA,
c.
and these enzymes are also coupled with the AP endonuclease.
C C T GG T G A G T A G
GG A A T C

C C
G T An exonuclease removes
successive nucleotides, including FIG. 14.18 Base excision repair. In base excision repair, an
the one with the mismatched base.
C

improper base in DNA and its deoxyribose sugar are

C

both removed, and the resulting gap is then repaired.

a.

d. C C T G A GG A G T A G Uracil in DNA signals the repair

GG A C T UC T C A T C process.
A DNA polymerase fills in the
C C T GG T G A G T A G missing nucleotides, and a DNA
GG A C C A C T C A T C Uracil in DNA
ligase joins the backbones.

Corrected mismatch
b.

C C T G A GG A G T A G DNA uracil glycosylase cleaves the

and MutH) to the site. MutL determines which DNA backbone GG A C T C T C A T C uracil from the deoxyribose sugar.
will be cleaved, and MutH cleaves the backbone in the vicinity
U
of the mismatch (Fig. 14.17b). An exonuclease degrades the
cleaved strand to a distance beyond the site of the mismatch
(Fig. 14.17c). A DNA polymerase then synthesizes new DNA to
c.
fill the gap, correcting the mismatch, and a DNA ligase seals the
remaining nick in the DNA backbone. Postreplication mismatch AP endonuclease cleaves the
C C T G A GG A G T A G
repair usually, but not always, cleaves the daughter strand, so the GG A C T C T C A T C backbone and removes the sugar.
corrected strand matches the parental template strand.
Eukaryotes have proteins corresponding to those in Fig. 14.17,
and the importance of postreplication mismatch repair is
underlined by the finding that a genetic predisposition to certain
d.
types of colon cancer results from mutations in the human
Other enzymes close the gap by
counterparts of MutS and MutL. new DNA synthesis, using the
C C T G A GG A G T A G
The result of proofreading by DNA polymerase and GG A C T C C T C A T C intact nucleotide opposite the site
postreplication mismatch repair is an increase in the fidelity of as a template.

DNA replication. Cells have evolved many other systems that Uracil replaced
repair different types of DNA damage that can occur even in with cytosine
306 CO R E CO N C E P T S S U M M A RY

Cells have also evolved mechanisms to repair short stretches

FIG. 14.19 Nucleotide excision repair. In nucleotide excision of DNA containing mismatched or damaged bases. One of
repair, a damaged segment of a DNA strand is removed these is known as nucleotide excision repair, which has
and resynthesized using the undamaged partner strand a similar mechanism of action to mismatch repair but uses
as a template. different enzymes (Fig. 14.19). Instead of degrading a DNA
a. strand nucleotide by nucleotide until the mismatch is removed,
nucleotide excision repair removes an entire damaged section of
C C T GG T G A G T A G One or more damaged bases
GG A C C AC T C A T C signals the repair process. a strand at once. Nucleotide excision repair is also used to remove
nucleotides with bulky side groups, as well as thymine dimers
resulting from ultraviolet light.
The importance of excision repair is illustrated by the disease
b. xeroderma pigmentosum (XP), in which nucleotide excision repair
is defective. People with XP are exquisitely sensitive to the UV
C C T GG T G A G T A G radiation in sunlight. Because of the defect in nucleotide excision
GG A C C AC T C A T C repair, damage to DNA resulting from UV light is not corrected,
leading to the accumulation of mutations in skin cells. The result
Enzymes cleave the DNA is high rates of skin cancer. People with XP must minimize their
backbone at sites flanking the
damage. exposure to sunlight and in extreme cases stay out of the sun
c. altogether.
When DNA repair mechanisms function properly, they reduce
C C T GG T G A G T A G the rate of mutation to a level that is compatible with life. But
GG A T C
they are not perfect, and they do not catch all the mistakes. The
A C C AC T C
The region with damaged
bases is removed. •
result is genetic variation, the raw material of evolution.

d.

The gap is filled by new DNA

C C T GG T G A G T A G synthesis, using the ungapped
GG A C C A C T C A T C strand as the template.

Damage repaired

Core Concepts Summary

14.1 Mutations are very rare for any given nucleotide Mutations are random in that they occur without
and occur randomly without regard to the needs of an regard to the needs of an organism. This principle can be
organism. demonstrated by replica plating bacterial colonies in the
absence and presence of antibiotic. page 295
For an individual nucleotide, the frequency of mutation is
very rare, though it differs among species. page 292
14.2 Small-scale mutations include point
Mutations are common across an entire genome and across mutations, insertions and deletions, and movement of
many cell divisions. page 293 transposable elements.
A point mutation, or nucleotide substitution, is a change
Germ-line mutations occur in haploid gametes as well
of one base for another. page 297
as in the diploid cells that give rise to them, and somatic
mutations occur in nonreproductive cells. Only mutations The effect of a point mutation depends on where it
in germ cells are transmitted to progeny and play a role in occurs. If it occurs in noncoding DNA, it will likely have
evolution. page 293 no effect on an organism. If it occurs in a protein-coding
 CHAPTER 14 M U TAT I O N A N D D N A R E PA I R 307

gene, it can result in a change in the amino acid sequence Many specialized DNA repair enzymes can correct DNA
(nonsynonymous mutation), no change in the amino acid damage. page 304
sequence (synonymous mutation), or the introduction of a
DNA ligase seals breaks in DNA and is an important tool
stop codon (nonsense mutation). page 297
for molecular biologists. page 304
Small insertions or deletions in DNA add or remove one
Postreplication mismatch repair provides a backup
base or a few contiguous bases. Their effect depends on
mechanism for mistakes not caught by the proofreading
where in the genome they occur and on their size. An
function of DNA polymerase. page 304
insertion or deletion of a single nucleotide in a protein-
coding gene results in a frameshift mutation, in which all Base excision repair corrects individual nucleotides
the codons downstream of the insertion or deletion are and involves several DNA repair enzymes working
changed. page 298 together. page 305

Transposable elements are DNA sequences that can jump Nucleotide excision repair functions similarly to
from one place in a genome to another. They can affect the postreplication mismatch repair but excises longer
expression of a gene if they insert into or near a gene. stretches of damaged nucleotides. page 306
page 300

14.3 Chromosomal mutations involve large regions Self-Assessment

of one or more chromosomes.
1. Mutations in which types of cell are most likely to
A duplication is a region of the chromosome that is contribute to evolutionary change in a population of
present two times, and a deletion is the loss of part of a organisms? Why?
chromosome. page 301
2. Explain the difference between the mutation rate for a
Duplications and deletions both affect gene dosage, given nucleotide and the mutation rate for a given cell.
which can have important effects on a cell or organism.
3. Explain what it means to say that mutations are random.
page 302
4. Describe the effects of nonsynonymous, synonymous, and
Gene duplication followed by evolutionary divergence
nonsense mutations on a protein and the effects of small
results in gene families made up of genes with related but
insertions or deletions in an open reading frame.
not identical functions. page 302
5. Explain how the location of a small-scale mutation in the
An inversion is a segment of a chromosome in reverse
genome can determine the effect it has on the functions of
orientation. page 303
a cell.
A reciprocal translocation involves the exchange of
6. Which type of chromosomal abnormality—deletion,
a part of one chromosome with another. Reciprocal
duplication, inversion, or translocation—would you expect
translocations do not affect gene dosage, but can lead to
to have the greatest effect on the organisms that carry it,
problems during meiosis. page 303
and why?

14.4 DNA can be damaged by mutagens, but most 7. Explain how a gene family, such as the odorant receptor
DNA damage is repaired. gene family, is thought to have evolved.

Some types of damage alter the structure of DNA, such as 8. What is a mutagen? Name two common mutagens and
single-stranded or double-stranded breaks, cross-linked their effects on DNA.
thymine dimers, or missing bases. page 304
9. Describe a DNA repair mechanism.
Other types of DNA damage, such as changes in the side
groups that form hydrogen bonds or the addition of side
Log in to to check your answers to the Self-
groups that interfere with base pairing, affect the bases Assessment questions, and to access additional learning tools.
themselves. page 304
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 CHAPTER 15

Genetic
Variation
Core Concepts
15.1 Genetic variation describes
common genetic differences
(polymorphisms) among the
individuals in a population at
any given time.
15.2 Human genetic variation can
be detected by DNA typing,
which can uniquely identify
each individual.
15.3 Two common types of
genetic variation are single-
nucleotide polymorphisms
(SNPs) and copy-number
variation (CNV).
15.4 Chromosomal variants can
also occur but are usually
harmful.

Tastyart Ltd Rob White/Getty Images.

309
310 SECTION 15.1 G E N OT Y P E A N D P H E N OT Y P E

In the last chapter, we saw how errors in DNA arise and what one difference per thousand nucleotides across the genome. In
common types of error occur. What is the fate of all these mistakes? other words, we differ from one person to the next in a very large
Most, as we have seen, are corrected by DNA repair mechanisms. number but very small fraction of nucleotides.
In this case, the DNA sequence is not changed. Some errors escape Mutations, as we have seen, are the ultimate source of
these repair mechanisms and are replicated as faithfully as the differences among genotypes. If we consider any present-day
original DNA sequence. If such mutations occur in somatic (body) population of organisms, we find that some mutations are very
cells, they can be passed on in the individual’s cells through mitotic common. Geneticists use the term polymorphism to refer to any
cell divisions, but they will not be passed on to progeny. If they genetic difference among individuals that is sufficiently common
occur in the germ line, they can be passed on to progeny. Over that it would almost certainly be present in a group of
time, through evolution (Chapters 1 and 21), the proportion of 50 randomly chosen individuals. For example, if many people have
individuals in a population carrying these mutations may increase an A–T pair at a particular site in the genome, but many others
or decrease. Therefore, if we look at any present-day population have a G–C pair at the same site, this difference is a polymorphism.
of organisms, such as the human population, we will find that Of course, the polymorphism is the result of a mutation. All
it harbors lots of genetic differences, all of which result from individuals once had the same genotype at this site, but a mutation
mutations that occurred sometime in the past. Genetic variation occurred at this site sometime in the past, and is now commonly
refers to genetic differences that exist among individuals in a found in the population.
population at a particular point in time. Phenotype is an individual’s observable characteristics
In the human population, you can observe the effects of or traits, such as height, weight, eye color, and so forth. The
common genetic differences by looking at the people around you. phenotype may be visible, as in these characteristics, or may
They differ in height, weight, facial features, skin color, eye color, be seen in the development, physiology, or behavior of a cell or
hair color, hair texture, and in many other ways. These traits differ organism. For example, color blindness and lactose intolerance
in part because of genetic variation, and in part because of the are phenotypes. The phenotype results in part from the genotype:
environment. Weight is affected by diet, for example, and skin A genotype with a mutated gene for an enzyme that would
color by exposure to sunlight. normally metabolize lactose can lead to the phenotype of lactose
In this chapter, we consider examples of major types of DNA intolerance. However, the environment also commonly plays
variation and chromosomal variation in populations today and an important role, so it is most accurate to say that a phenotype
examine their consequences for the organism. We also describe results from an interaction between the genotype and the
key molecular techniques that allow genetic variation to be environment. These genotype–environment interactions are
studied directly in DNA molecules. The emphasis here is primarily, discussed in Chapter 18.
but not exclusively, on human populations.
The effect of a genotype often depends on several
factors.
15.1 GENOTYPE AND PHENOTYPE Let’s examine how genotype influences phenotype with an
example of a genetic polymorphism that we introduced in the
We tend to think of mutations as something negative or harmful; previous chapter. Fig. 15.1 shows genetic variation in the gene for
the term “mutant” ordinarily connotes something abnormal. But b-(beta-)globin, one of the subunits of hemoglobin that carries
because mutations result in genetic variation among individuals oxygen in red blood cells. Three forms of the b-globin gene are
and organisms, we are all mutants, different from one another shown in the figure: A, S, and C. These three forms of the gene are
genetically because of mutations, that is, differences in our DNA. relatively common in certain African populations. The different
Whereas some mutations are harmful, some have no effect on an forms of any gene are called alleles, and they correspond to
organism, and some indeed are beneficial. Without mutations, different DNA sequences (polymorphisms) in the genes. In this
evolution would not be possible. Mutations generate the occasional case, the most common allele is the A allele, which has a GAG
favorable variants that allow organisms to evolve and become codon in the position indicated. This codon translates to glutamic
adapted to their environment over time (Chapter 21). acid (Glu) in the resulting polypeptide.
The allele denoted “S” in Fig. 15.1 is associated with sickle-cell
Genotype is the genetic makeup of a cell or organism; anemia (Chapter 14). In this allele, the GAG codon of the A allele
phenotype is its observed characteristics. is instead a GTG codon, with the result that the glutamic acid in
The genetic makeup of a cell or organism constitutes its the protein is replaced with valine (Val). The third allele in Fig. 15.1
genotype. A population with a gene pool that has many variants is the C allele. It has a variation in the same codon as the S allele,
in many different genes will consist of organisms with numerous but in this case the change is from GAG to AAG, with the result
different genotypes. For example, any two human genomes that glutamic acid is replaced by lysine (Lys). Although in each case
are likely to differ at about 3 million nucleotide sites, or about only one amino acid of the b-globin protein is affected, this change
 CHAPTER 15 G E N E T I C VA R I AT I O N 311

Furthermore, in Africa, where malaria is

FIG. 15.1 Three alleles of the gene encoding b-globin, a subunit of hemoglobin. Both widespread, being a heterozygote is actually
the S allele and the C allele are associated with increased resistance to malaria. beneficial because it affords partial protection
A allele S allele C allele against malaria.
Whether the effects of an allele, in this
case the S allele, are beneficial or harmful
DNA G A G G T G A A G
C T C C A C T T C illustrates two important principles about
the connection between genotype and
phenotype. First, the answer often depends
on whether the mutation is homozygous or
RNA
G A G GUG A A G
heterozygous. In areas with malaria, the
S allele is harmful as a homozygous genotype
but beneficial as a heterozygous genotype.
Protein GLU VAL LYS Second, the effect of a particular genotype
may depend on the environment. The S allele,
as a heterozygous genotype, is beneficial only
GLU VAL LYS
GLU VAL LYS in malarial-prone regions, where it offers
protection from the disease that outweighs
its other effects. In areas without malaria, it
is harmful.
HbA HbS HbC What about the C allele? Individuals who
inherit an A allele from one parent and a
C allele from the other parent (genotype AC)
can have a dramatic effect on the function of the protein, since the have the phenotype of partial protection against malaria, and
amino acid sequence determines how a protein folds, and protein those who inherit a C allele from both parents (genotype CC) are
folding in turn determines the protein’s function (Chapter 4). not only more protected from malaria but also have at worst a
An individual who inherits an allele of the same type from mild anemia that usually needs no medical treatment. As with
each parent is said to be homozygous. For the hemoglobin A, S, the S allele, the phenotype of the C allele depends on whether the
and C alleles, there are three possible homozygous genotypes: individual is heterozygous or homozygous for the allele.
AA, SS, and CC. The first letter or symbol in each pair indicates
j Quick Check 1 A mutation arises in a bacterium that
the allele inherited from one parent, and the second indicates the
confers antibiotic resistance. Is this mutation harmful, beneficial,
allele inherited from the other parent. By convention, alleles and
or neutral?
genotypes are designated by italic letters.
By contrast, individuals who inherit different types of allele
from their parents are heterozygous. For the hemoglobin A, S, Some genetic differences are major risk factors
and C alleles, there are three possible heterozygous genotypes: for disease.
AS, AC, and SC. Note that while each individual can have only As we saw in the last chapter, when a mutation occurs in the
two alleles of a gene, many more alleles can exist in an entire coding sequence of a gene, it may have no effect on the amino acid
population. In the hemoglobin example, a person can be sequence, or it may result in a change in the amino acid sequence,
homozygous, with two identical alleles, or heterozygous, with introduce a stop codon, or shift the reading frame. Many of the
two different alleles, but there are three different alleles available mutations that alter amino acid sequence are harmful. Harmful
in the population can mix and match in an individual. How those mutations are often eliminated in one or a few generations
alleles are inherited is explored in Chapter 16. because they decrease the survival and therefore the capacity for
What are the effects of these mutations? Are they beneficial, reproduction of the individuals that carry them.
harmful, or neutral? The short answer is, it depends. Let’s Sometimes, however, harmful mutations persist in a
consider the S allele. When it is inherited from both parents, the population. For example, many polymorphisms increase
individual is homozygous SS, which results in the phenotype of susceptibility to particular diseases. One such polymorphism
sickle-cell anemia. In the absence of proper medical care, patients increases the risk of emphysema, a condition marked by loss of
with sickle-cell anemia usually die before adulthood. However, lung tissue. Emphysema patients suffer from shortness of breath
when the S allele is inherited from one parent and the A allele is even when at rest, a wheezy cough, and increased blood pressure
inherited from the other parent, the individual is heterozygous in the arteries of the lungs. Without proper treatment, the disease
( AS) and the phenotype is only a mild form of sickle-cell disease. progresses to respiratory failure or congestive heart failure, and
312 SECTION 15.1 G E N OT Y P E A N D P H E N OT Y P E

FIG. 15.2 A harmful mutation. (a) The enzyme alpha-1 antitrypsin (α1AT) normally inhibits the activity of elastase, which breaks down elastin in
the lung. (b) Cigarette smoke inhibits α1AT activity and increases the risk of emphysema. (c) A mutation in α1AT called PiZ is harmful
because it reduces α1AT activity and also leads to increased risk of emphysema.

a. Normal
α1AT gene α1AT Inhibited
The α1AT gene encodes elastase Normal lung elasticity
an enzyme that inhibits is maintained by a
the function of elastase, balance between
an enzyme that breaks elastin production and
Expression of
down elastin in the lung. destruction.
α1AT enzyme

b. Smoking
α1AT gene Active
Cigarette smoking Emphysema results
reduces the activity of elastase
from too much
α1AT, decreasing breakdown in the lung,
inhibition of elastase which can result from
and leading to increased Expression of
cigarette smoking…
elastin breakdown. α1AT enzyme

c. Mutant

PiZ allele of α1AT gene

Individuals who are
homozygous for the PiZ
allele produce α1AT with … or homozygous PiZ
reduced activity, leading mutations.
to increased elastin Low levels of
breakdown in the lung. α1AT activity

eventual death. Onset of the disease usually occurs in middle age, PiZ allele, and about 1 in 3000 is homozygous PiZ/PiZ. Individuals
and the lifespan of affected individuals is shortened by 10 to 30 with the homozygous genotype produce α1AT with reduced
years depending on the effect of treatment. activity and hence have reduced elastase inhibition, leading
About 80% of all cases of emphysema are associated with to severe emphysema and death in more than 70% of affected
cigarette smoking, which affects the action of the enzyme alpha-1 individuals (Fig. 15.2c). Smoking markedly increases both the
antitrypsin (α1AT). The main function of α1AT is to inhibit severity of the disease and the rapidity of its progression in PiZ/PiZ
another enzyme, known as elastase, which breaks down the individuals. The life expectancy of PiZ/PiZ nonsmokers is 65 years,
connective-tissue protein elastin in lungs (Fig. 15.2). The elasticity whereas that of PiZ/PiZ smokers is only 40 years.
of the lung, which allows normal breathing, requires a balance The combination of homozygous PiZ and smoking is an
between the production and the breakdown of elastin. Too-rapid example of a genotype-by-environment interaction, in which
breakdown by elastase is normally prevented by the inhibition of a phenotype is the result of an interplay between genes and the
elastase by α1AT (Fig. 15.2a). Cigarette smoke reduces the activity environment. In this case, a particular combination of genetic and
of α1AT, resulting in excessive destruction of elastin, loss of lung environmental risk factors is much worse than either risk factor
elasticity, and emphysema (Fig. 15.2b). acting alone. Chapter 18 includes a more detailed discussion of
While most cases of emphysema are due to smoking, in some genotype-by-environment interactions.
individuals genetics plays a role. An individual’s chance of getting
emphysema is significantly increased by inheriting a mutation in Not all genetic differences are harmful.
the gene that encodes α1AT. Among the many different alleles of Some mutations have no effect on the organism, or have
this gene, a defective allele denoted PiZ is particularly common effects that are not associated with differences in survival or
in populations of European descent. Among Caucasians in the reproduction. Such mutations are considered neutral. Many
United States, about 1 person in 30 is heterozygous for the mutations have neutral effects on organisms because they occur
 CHAPTER 15 G E N E T I C VA R I AT I O N 313

in noncoding DNA. Neutral mutations are therefore especially should still eat your vegetables! But if you find broccoli and its
likely to occur in organisms with large genomes and abundant relatives somewhat bitter, you may well be a taster.
noncoding DNA (Chapter 13).
A few genetic differences are beneficial.
j Quick Check 2 Given what you read about the human genome
While many mutations are neutral, or nearly so, and many
in Chapter 13, would you predict that most mutations in humans
others are harmful, some mutations are beneficial. In human
are harmful, beneficial, or neutral?
populations, beneficial mutations are often discovered through
Sometimes common, seemingly harmless genetic variations their effects in protecting against infectious disease. The most
occur in coding sequences. One example in human populations is widely known example is probably the sickle-cell allele in the gene
the taster phenotype associated with perception of a bitter taste encoding the β chain of hemoglobin, which when heterozygous
from certain chemicals, including phenylthiocarbamide (PTC). The protects against malaria (see Fig. 15.1). In this section, we consider
taster polymorphism was discovered in 1931 when a commercial another example: This one protects against AIDS, which is caused
chemist seeking a new artificial sweetener accidentally released by the human immunodeficiency virus (HIV).
a cloud of fine crystalline PTC and heard his colleague working By means of its surface glycoprotein (a product of the env
nearby complain about its bitter taste. The chemist himself tasted region in the annotated HIV genome shown in Fig. 13.7), HIV
nothing. He began testing his own and other families, and set the combines with a cell-surface receptor called CD4 to gain entry
stage for future genetic studies. into immune cells called T cells. Interaction with CD4 alone,
The minimal concentration for PTC tasting varies among however, does not enable the virus to infect the T cell. The HIV
individuals, but being able to taste a concentration of 0.5 millimolar surface glycoprotein must also interact with another receptor on
or less is often taken as the cutoff between the taster and nontaster the T cell, which is denoted CCR5, in the early stages of infection
phentoypes. In a sample from Utah, the frequency of nontasters is (Fig. 15.3). The normal function of CCR5 is to bind certain small
about 30%. This is typical for people of European descent, but the secreted proteins that promote tissue inflammation in response to
frequency of nontasters differs among populations, from as low as infection. But because CCR5 is also an HIV receptor, cells lacking
3% in West Africa to as high as 40% in India. CCR5 are more difficult to invade.
The ability or inability to taste PTC is due largely, but not A beneficial effect of a particular mutation in the CCR5 gene
exclusively, to alleles of a single gene called TAS2R38 that encodes a was discovered in studies focusing on HIV patients whose infection
taste receptor in the tongue. Nonhuman primates are homozygous had not progressed to full-blown AIDS after 10 years or more. The
for an allele of this gene known as the PAV allele, so called because protective allele is denoted the Δ32 allele because the mutation is
the protein it encodes has the amino acids proline (P), alanine (A), a 32-base-pair deletion in the coding sequence of the CCR5 gene
and valine (V) at specific positions. Humans also have the PAV allele, (Fig. 15.4). Because 32 is not a multiple of 3, the reading frame
and PAV/PAV homozygous genotypes are almost all tasters. for translation is shifted at the site of the deletion, and instead of
The most common allele associated with the nontaster the normal amino acid sequence Ser–Gln–Tyr–Gln–Phe · · ·, the
phenotype is AVI, in which the amino acids in the taste receptor mutant sequence is Ile–Lys–Asp –Ser–His · · · . Not only is the amino
are alanine ( A), valine (V), and isoleucine (I) instead of proline,
alanine, and valine. About 80% of homozygous AVI/AVI genotypes
are nontasters. One copy of the PAV allele is usually sufficient FIG. 15.3 HIV infection of T cells. HIV gains entry into a T cell by
for the taster phenotype; about 98% of PAV/AVI heterozygous interacting with a CD4 protein and a CCR5 receptor on
genotypes are tasters. the surface of T cells.
The factors contributing to the taster phenotype are more HIV surface
complex than a genotype at a single gene, however. The AVI/AVI HIV glycoprotein
genotype tips the balance toward the nontaster phenotype, but
not completely. Other genes and the environment also play a role.
Why is this strange variation present in the human
population? One hypothesis is that an aversion to compounds that
For HIV to invade
contain a thiourea group (N–C⫽S), as PTC does, may discourage a T cell, it must
eating certain plants that produce poisonous defense compounds. interact with a
CD4 receptor
One class of such compounds is the glycosinolates, which are and a CCR5
present in many wild plants and some cultivated vegetables, co-receptor.
including broccoli, watercress, turnip, and horseradish. Sure
CD4 surface protein CCR5 co-receptor
enough, PAV/PAV tasters rate such vegetables as significantly more
bitter than AVI/AVI nontasters, whereas PAV/AVI heterozygous Extracellular
fluid
genotypes are intermediate in their perception of bitterness.
We hasten to add, however, that the low level of glycosinolates
Cytoplasm CD4+ T cell
in broccoli and other cultivated vegetables is nontoxic, so you
314 SECTION 15.2 G E N E T I C VA R I AT I O N A N D I N D I V I D UA L U N I Q U E N E S S

FIG. 15.4 A beneficial mutation in the human population. Mutant CCR5 has a 32-nucleotide deletion that results in defective CCR5 protein and
therefore slows the progression of HIV to AIDS.

a. Nonmutant CCR5 allele

HIS PHE PRO TYR SER GLN TYR GLN PHE TRP LYS ASN PHE GLN THR LEU LYS ILE VAL ILE

. . . C A T T T T C C A T A C AG T C AG T A T C AA T T C T GGAAGAA T T T C C AGA C A T T AAAGA T AG T C A T C . . .

b. ∆32 mutant CCR5 allele

The deletion causes a shift

in the translational reading
frame, and all downstream
amino acids are incorrect.

. . . CA T T T T C CA T ACA T T AAAGA T AG T C A T C . . . Translation continues for another 26 amino

acids, at which point the shifted reading
HIS PHE PRO TYR ILE LYS ASP SER HIS frame encounters a termination codon.

acid sequence different from the nonmutant form, the ribosome important effects on phenotypes, most of the genetic variation
encounters a stop codon a mere 26 amino acids farther along in populations is neutral or has no obvious effects. Much of the
and translation terminates. The mutant protein is 215, not 352, neutral variation consists of differences in noncoding DNA.
amino acids long. The CCR5 protein produced by the ∆32 allele is Nonetheless, this variation can be revealed by direct studies of
completely inactive. DNA that employ many of the techniques for DNA manipulation
The effect of the ∆32 allele is pronounced. In individuals with described in Chapter 12.
the homozygous ∆32/∆32 genotype, HIV progression to AIDS
is rarely observed. There is some protection even in individuals Areas of the genome with variable numbers of tandem
with heterozygous ∆32 genotypes, where progression to AIDS is repeats are useful in DNA typing.
delayed by an average of about 2 years after infection by HIV. At many locations in the human genome, short (10–50 base-pair)
Much has been written about the evolutionary history of the sequences are repeated in multiple tandem copies. At each location
∆32 allele. It is found almost exclusively in European populations, the number of copies of the repeat may differ from chromosome to
where the frequency of heterozygous genotypes ranges from chromosome. This kind of variation, in which chromosomes have
10% to 25%. The narrow geographical distribution was originally different numbers of a repeated sequence at a particular location,
interpreted to mean that the allele was selected over time because is called a variable number tandem repeat, or VNTR. There are
it provided protection against some other infectious agent that about 30,000 different VNTR locations in the human genome.
also interacted with the CCR5 protein. Just as an individual has two copies of each gene, possibly with
Beneficial mutations not only provide protection against two different alleles, so each individual has two copies of each of
disease. Many beneficial mutations permit organisms to become these VNTRs, with copies differing in the number of repeats being
better adapted to their environment. For example, certain birds, analogous to different alleles of a gene. Each VNTR typically has
such as Rüppell’s vulture, commonly fly at altitudes of 20,000 many alleles differing in copy number, so that, except for identical
feet and sometimes much higher. This feat is possible because of twins, an individual’s genotype for 6–8 VNTR locations is usually
mutations in the structure of hemoglobin that allow hemoglobin sufficient to identify the individual uniquely. Differences among
to bind oxygen with high affinity, even at the low pressure of individuals, such as VNTRs, are the basis of DNA typing, in which
oxygen high in the atmosphere. These mutations were selected the analysis of a small quantity of DNA can be as reliable for
and passed on generation after generation, making the birds well identifying individuals as fingerprints. This is why DNA typing is
adapted to flying at high altitudes. often called DNA fingerprinting.
Any of several methods can be used to DNA fingerprint
someone on the basis of that individual’s VNTRs. Amplification by
15.2 GENETIC VARIATION AND the polymerase chain reaction (PCR, Chapter 12) is a convenient
INDIVIDUAL UNIQUENESS and reliable technique because each VNTR in the genome is flanked
by a unique sequence that can be used to design PCR primers that
While examples like sickle-cell anemia, emphysema, and HIV will amplify one and only one VNTR. Although the number of
susceptibility show that genetic variation in some genes has alleles of a particular VNTR can be very large, an example with only
 CHAPTER 15 G E N E T I C VA R I AT I O N 315

FIG. 15.5 Variable number tandem repeats (VNTRs). (a) The number of short, repeated sequences at a given site in a human chromosome can
vary. (b) These differences can be visualized using PCR followed by gel electrophoresis.

a. b.
Evidence (blood or
Allele A1 other tissue from
Position a crime scene)
of PCR Genotype
primers
A1 A2 A3 A4 A5 A1 A1 A1 A1 A2 A2 A2 A3 A3 A4
Allele A2
A1 A2 A3 A4 A5 A2 A3 A4 A5 A3 A4 A5 A4 A5 A5 E

A5
Allele A3 A4
A3

Allele A4 A2
A1

Allele A5
Each of the different genotypes yields
a unique pattern of bands in the gel.

AGTAGTAGTAGTAGT

five alleles is shown in Fig. 15.5, where the repeated sequence is very likely that the samples come from the same individual (or from
shown in green between the flanking unique sequences. identical twins). On the other hand, if any of the polymorphisms
No matter how many alleles exist in the population as a whole, fails to match band for band, then it is almost certain that the
any one diploid individual can have only two alleles, which are samples come from different individuals (barring such technical
present at the same positions in the homologous chromosomes mishaps as samples whose DNA is contaminated, degraded, or
inherited from the mother and father. With five alleles there are mislabeled).
5 possible homozygous genotypes and 10 possible heterozygous One of the advantages of DNA typing is that a large amount
genotypes. These can be visualized after PCR by separating the of DNA is not needed. Even minuscule amounts of DNA can
resulting molecules by size by gel electrophoresis (Chapter 12). be typed, so tiny spots of blood, semen, or saliva are sufficient
The pattern of bands expected from the DNA in each genotype samples. A cotton swab wiped across the inside of your cheek will
formed from the alleles in Fig. 15.5a is shown in Fig. 15.5b. Note contain enough DNA for typing; so will a discarded paper cup or a
that each genotype yields a distinct pattern of bands, and so these cigarette butt. One enterprising researcher in England used PCR
patterns can be used to identify the genotype of any individual for to type the dog feces on his lawn and matched it to the DNA from
a particular region of DNA. hair he obtained from neighborhood dogs, much to the chagrin of
With as few as 10 equally frequent alleles, the likelihood that the guilty dog’s owner.
any two individuals would have the same genotype at a particular
location by chance alone is about 1 in 50, and with 20 equally j Quick Check 3 In typing DNA from a sample found at a crime
frequent alleles the likelihood decreases to about 1 in 200. These scene, how can a DNA mismatch prove that a suspect is not the
numbers suggest why DNA typing can yield a genetic fingerprint source of that sample, whereas a DNA match does not necessarily
that can uniquely match DNA samples. prove that a suspect is the source?
The lane labeled “E” at the far right in Fig. 15.5b shows the
DNA bands for this same polymorphism from biological material Some polymorphisms add or remove restriction sites
collected at the scene of a crime. In this case, the bands in lane E in the DNA.
imply that the source of the evidence is an individual of genotype Restriction enzymes, which cleave double-stranded DNA at
A2 /A4 because this is the only genotype with bands with vertical specific sequences known as restriction sites, typically four or six
positions in the gel that exactly match those of the evidence sample. nucleotides long, were among the first tools used to study genetic
A single polymorphism of the type shown in Fig. 15.5 is not enough variation in DNA. These enzymes are useful in DNA fingerprinting
to establish for sure that two pieces of DNA come from the same because DNA from different individuals can differ in the distance
person, but because the human genome has polymorphic sites between adjacent restriction sites or in the presence or absence
scattered throughout, many polymorphisms can be examined. If six of a particular restriction site at some location in the genome.
or eight of these polymorphisms match between two samples, it is This kind of polymorphism, in which differences in restriction
316 SECTION 15.3 G E N O M E W I D E S T U D I E S O F G E N E T I C VA R I AT I O N

sites result in different lengths of restriction fragments, is called a Just as with VNTRs, any individual can carry at most two
restriction fragment length polymorphism, or RFLP. different alleles of a restriction fragment length polymorphism.
Fig. 15.6 shows an example. Here, a restriction enzyme cleaves As seen in the RFLP in Fig. 15.6b, DNA from homozygous AA
DNA near the sequence GAGGAG. The three sites shown are in and individuals yields only the short and medium fragments, and that
flanking the b-globin gene from the sickle-cell example we explored from homozygous SS individuals yields only the long fragment.
earlier in the chapter. However, in the S allele (see Figure 15.1), the DNA from heterozygous AS individuals yields all three fragments.
restriction site in the middle includes a GAG that is mutated to GTG, Therefore, each of the three genotypes AA, AS, and SS yields a
making the sequence unrecognizable to the restriction enzyme. As unique pattern of bands, allowing each of the possible genotypes
a result, the DNA of the S allele is cleaved at only two sites, whereas to be identified.
the DNA of the A allele is cleaved at all three. How many restriction fragment length polymorphisms
These differences can be visualized by gel electrophoresis are there in the human genome? As in the case of VNTRs, the
(Chapter 12), in which DNA is isolated from different individuals, human genome has many restriction sites, so there are numerous
cut with restriction enzymes, and separated by size on a gel. RFLPs in the human population, making it another type of
In this case, we use a restriction enzyme that recognizes the polymorphism that can be used in DNA typing.
GAGGAG sequence. The A allele produces two fragments: a short
fragment that moves rapidly through the gel and ends up near the j Quick Check 4 When a segment of DNA containing either a
bottom, and a longer fragment that ends up near the middle. The VNTR or an RFLP is analyzed, the result is fragments of DNA of
fragment produced by the S allele is even longer because there is different lengths. How, then, are VNTRs and RFLPs different?
no restriction site in the middle. This fragment ends up forming a
band near the top of the gel.
15.3 GENOMEWIDE STUDIES
OF GENETIC VARIATION
FIG. 15.6 Restriction fragment length polymorphism (RFLP) VNTRs and RFLPs are forms of genetic variation that are still
between the A and S alleles of the b-globin gene. widely used in DNA fingerprinting. But large-scale and relatively
inexpensive DNA sequencing methods are now also used to
a.
GAGGAG GTGG AG identify and study genetic variation. These methods have revealed
two types of genetic variation that are very common in the human
Allele A Allele S genome: variation of individual nucleotides and variation in copy
number of regions of DNA that can include one or more genes. In
A restriction enzyme
this section, we focus on these two types of genetic variation.
Position of cleaves both DNA
restriction samples at all of the sites
site Single-nucleotide polymorphisms (SNPs) are single-
it recognizes, producing
fragments of different base changes in the genome.
lengths. Following One of the most common types of genetic variation is a
electrophoresis, the gels difference in a nucleotide at a specific site. A single-nucleotide
show the position of the
fragments to which the polymorphism (SNP) is a site in the genome where either of two
probe hybridizes. different nucleotide pairs can occur and where each nucleotide

b.
Genotype AA Genotype SS Genotype AS

Number and position of bands indicate the genotype.

 CHAPTER 15 G E N E T I C VA R I AT I O N 317

FIG. 15.7 Single-nucleotide polymorphisms (SNPs). A SNP in the ? CASE 3 YOU, FROM A TO T: YOUR PERSONAL GENOME
region neighboring OCA2 is strongly associated with blue How can genetic risk factors be detected?
eyes. Sources: (left) Amos Morgan/Getty Images; (right) Stockbroker SNPs result from point mutations, the most frequent type of
xtra/age fotostock. mutation, which occurred in the past and then spread through
the population. A point mutation is the substitution of one base
C–G allele T–A allele
G T pair for another in double-stranded DNA (Chapter 14). Each of
Site of single- us is genetically unique partly because of the abundance of SNPs
C nucleotide A
polymorphism in the human genome. There are approximately 3 million SNPs
(SNP) that distinguish any one human genome from any other. SNPs are
abundant in the genomes of most species, and while many have no
effect on the organism, some are thought to be the main source
of evolutionary innovation and others are major contributors
to inherited disease. Practically speaking, SNPs are important
because they can be used to detect the presence of a genetic risk
factor for a disease before the onset of the disease. For example,
the ability to detect the b-globin SNPs shown in Fig. 15.1 allows
prenatal identification of the b-globin genotype of a fetus.
While there is great interest in developing ultrafast DNA
sequencing machines that can determine anyone’s personal
genome quickly at relatively low cost, 99.9% of the nucleotides
pair is common enough in the population to be present in a
between any two genomes are identical. An alternative is to focus
random sample of 50 diploid individuals. The differences among
on genotyping just SNPs. As many as one million SNPs at different
the hemoglobin alleles are SNPs. The A, S, and C alleles differ from
positions in the genome can be genotyped simultaneously, and the
one another at just one nucleotide site (see Fig. 15.1).
genotyping can be carried out on thousands or tens of thousands
Eye color is also associated with a SNP. The blue eye phenotype
of individuals. Such massive genotyping allows any SNP associated
results from reduced expression of a gene called OCA2, which
with a disease to be identified, which is especially important for
encodes a membrane protein involved in the transport of small
complex diseases affected by many different genetic risk factors
molecules, including the amino acid tyrosine, which is a precursor
(Chapter 18).
of the melanin pigment associated with the brown eye phenotype.
What does it mean to say that a given SNP is associated with
Although more than a dozen alleles of OCA2 are known that have
a disease? It means that individuals carrying one of the alleles of
an amino acid replacement in the protein, none of these results in
that SNP are more likely to develop the disease than those carrying
blue eyes.
the other allele. The increased risk depends on the disease and
The SNP implicated in blue eyes is a site that is a C–G base
can differ from one SNP to the next. Sickle-cell anemia affords an
pair in one allele. The other common allele has a T–A at this
example at one extreme of the spectrum of effects. In this case,
position, which is not associated with blue eyes (Fig. 15.7). In one
the T–A base pair in the S allele of the b-globin gene is the SNP
study, the homozygous C–G genotype was found in 94% of 183
that results in the amino acid replacement of glutamic acid with
individuals with blue eyes and in only 2% of 176 individuals with
valine in the protein. Because SS individuals always have sickle-cell
brown eyes. This strong association is quite unexpected because
anemia, it would be fair to say that homozygous S “causes” sickle-
the SNP associated with blue eyes is in an intron of a neighboring
cell anemia.
gene, and is therefore noncoding. The proposed mechanism for
But except for inherited diseases that result from single mutant
the association is that the C–G allele makes the adjacent OCA2
genes, which are usually rare, the vast majority of SNPs implicated
gene less accessible to transcription factors, thereby reducing
in disease increase the risk only moderately as compared with
the amount of the transporter protein and consequently the
individuals lacking the risk factor. We then say that the SNP is
production of melanin in the iris. The proposed mechanism may be
“associated” with the disease since the SNP alone does not cause
right or wrong; this example emphasizes that scientists have much
the disease but only increases the risk. For heart disease, diabetes,
to learn about the possible phenotypic effects of noncoding DNA.
and some other diseases, many SNPS at different places in the
The C–G versus T –A SNP is typical of most SNPs in that only
genome, as well as environmental risk factors, can be associated
two of the four possible base pairs (G–C and A–T in addition to
with the disease. Usually, genetic and environmental risk factors
C–G and T–A) are present in the population at any appreciable
act cumulatively: the more you have, the greater the risk.
frequency.
As emphasized in the case of Claudia Gilmore’s genome,
j Quick Check 5 What’s the difference between a point mutation certain SNPs in the BRCA1 and BRCA2 genes are associated with
and a SNP? an increased risk of breast and ovarian cancers. Women who carry
318 SECTION 15.4 G E N E T I C VA R I AT I O N I N C H RO M O S O M E S

a mutation in either of these genes can minimize their risk by

frequent mammograms and other tests that enable early treatment. FIG. 15.9 Copy number variation in a gene that helps break down
Both genes are large—BRCA1 codes for a protein of 1863 amino starch. Copy numbers of AMY1 vary between societies
acids and BRCA2 for one of 3418 amino acids—and many different with high-starch and low-starch diets. Data from G. H. Perry,
mutations in these genes can predispose to breast or ovarian cancer. N. J. Dominy, K. G. Claw, A. S. Lee, H. Fiegler, R. Redon, J. Werner,

In certain high-risk populations, however, such as Ashkenazi Jews, F. A. Villanea, J. L. Mountain, R. Misra, N. P. Carter, C. Lee, and A. C.

only a few mutations predominate, and the SNPs associated with Stone, 2007, “Diet and the Evolution of Human Amylase Copy Number

these mutations can be detected easily and efficiently. Variation,” Nature Genetics 39:1256–1260.

0.70
Copy-number variation constitutes a significant High-starch diet
proportion of genetic variation. 0.60 Low-starch diet In societies with low-starch

Proportion of chromosomes
diets the average AMY1 copy
In addition to SNPs, another very common form of genetic
number per chromosome 1
variation present in the modern human population is copy- 0.50
is about 2.5, whereas in
number variation (CNV), or differences among individuals in those with high-starch diets
0.40 it is about 3.5.
the number of copies of a region of the genome. In contrast to
the short repeats characteristic of VNTRs, the regions involved in 0.30
CNVs are large and may include one or more genes. An example
is shown in Fig. 15.8. In this case, a region of the genome that is 0.20

normally present in only one copy per chromosome (Fig. 15.8a) 0.10
may in some chromosomes be duplicated (Fig. 15.8b) or deleted
(Fig. 15.8c). The multiple copies of the CNV region are usually 0.00
1 2 3 4 5 6 7
adjacent to one another along the chromosome.
Number of copies of AMY1 in chromosome 1
One of the surprises that emerged from sequencing the human
genome was that CNV is quite common in the human population.
Any two individuals’ genomes differ in copy number at about five
different regions, each with an average length of 200 kb to 300 kb. groups of people: societies with a long history of a high-starch
Across the genome as a whole, about 10% to 15% of the genome is diet, and societies with a long history of a low-starch diet. There
subject to copy-number variation. is a clear tendency for chromosomes from the latter group to
Some CNVs occur in noncoding regions, but others consist have fewer copies of AMY1 than those from the former group. On
of genes that are present in multiple tandem copies along the average, a chromosome 1 from the low-starch group has a copy
chromosome. An example is the human gene AMY1 for the salivary number of about 2.5, whereas one from the high-starch group has
gland enzyme amylase, which aids in the digestion of starch. This an average of about 3.5. Because each individual has two copies of
gene is located in chromosome 1, and the AMY1 copy number chromosome 1, this difference means that the average individual
differs from one chromosome 1 to the next. Fig. 15.9 shows the in the low-starch group has about 5 copies of AMY1, whereas the
distribution of AMY1 copy number along chromosome 1 in two average individual in the high-starch group has about 7 copies. A
plausible hypothesis is that extra copies were selected in groups
with a high-starch diet because of the advantage extra copies
conferred in digesting starch.
FIG. 15.8 Copy-number variation in a region of a chromosome.

a.
One copy of region 15.4 GENETIC VARIATION IN
CHROMOSOMES
Copy-number variation usually involves only one or a small
b.
number of genes, and the size of the duplicated or deleted region
Two copies of region
(duplication) is physically so small that the differences are undetectable
with conventional microscopy. In the human genome, some
common variants involve large regions of the chromosome and
c. In copy-number variation,
are big enough to be visible through a microscope. Most of these
No copies the size of the duplicated or variations involve regions of the genome with an extremely low
(deletion) deleted region can include density of genes, such as regions around the centromere or on the
one or more complete genes.
long arm of the Y chromosome.
 CHAPTER 15 G E N E T I C VA R I AT I O N 319

It may come as a surprise to learn that major chromosomal

FIG. 15.10 Nondisjunction. (a) Chromosomes separate evenly differences occur quite commonly in humans. They are
into gametes in normal meiotic cell division. infrequently observed, however, because most of them are lethal.
(b) Homologous chromosomes fail to separate in first- Nevertheless, a few major chromosomal differences are found
division nondisjunction. (c) Sister chromatids fail to in the general population. Some are common enough that their
separate in second-division nondisjunction. phenotypic effects are familiar. Others are less common, and a few
have no major phenotypic effects at all. In this section, we focus
a. Normal meiosis on the types of difference that are observed, what causes them,
and their phenotypic effects.

Nondisjunction in meiosis results in extra or missing

chromosomes.
Nondisjunction is the failure of a pair of chromosomes to
separate during anaphase of cell division (Chapter 11). The result is
that one daughter cell receives an extra copy of one chromosome,
and the other daughter cell receives no copy of that chromosome.
Nondisjunction can take place in mitosis, and it leads to cell
Metaphase I lineages with extra or missing chromosomes—which are often
observed in cancer cells.
When nondisjunction takes place in meiosis, the result
Metaphase II
is reproductive cells (gametes) that have either an extra or a
Gametes
missing chromosome. Fig. 15.10 illustrates nondisjunction in
meiosis. Fig. 15.10a shows key stages in a normal meiosis, in which
b. First-division nondisjunction chromosome separation (disjunction) takes place in both meiotic
divisions, with the result that each gamete receives one and only
Nondisjunction one copy of each chromosome.
results in
Failure of chromosome separation can occur in either of the
gametes with
an extra two meiotic divisions. Both types of nondisjunction result in
chromosome... gametes with an extra chromosome or a missing chromosome,
1st division but there is an important difference. In first-division
nondisjunction
nondisjunction (Fig. 15.10b), which is much more common,
the homologous chromosomes fail to separate and all of the
... and gametes resulting gametes have an extra or missing chromosome. On the
that are missing other hand, in second-division nondisjunction (Fig. 15.10c),
a chromosome. sister chromatids fail to separate, giving rise to products with
a normal number of chromosomes, an extra chromosome, or a
missing chromosome.

c. Second-division nondisjunction Some human disorders result from nondisjunction.

Approximately 1 out of 155 live-born children (0.6%) has a major
chromosome abnormality of some kind. About half of these have a
major abnormality in chromosome structure, such as an inversion
or translocation (Chapter 14). The others have a chromosome
that is present in an extra copy or a chromosome that is missing
2nd division because of nondisjunction—a parent has produced an egg or a
nondisjunction sperm with an extra or a missing chromosome (Fig. 15.11).
Extra Perhaps the most familiar of the conditions resulting from
chromosome
nondisjunction is Down syndrome, which results from the
presence of an extra copy of chromosome 21. Down syndrome is
Missing also known as trisomy 21 because affected individuals have three
chromosome copies of chromosome 21. The extra chromosome can be seen in
the chromosome layout called a karyotype. French researchers
320 SECTION 15.4 G E N E T I C VA R I AT I O N I N C H RO M O S O M E S

FIG. 15.11 Incidence of extra and missing chromosomes in HOW DO WE KNOW?

human live births. After D. L. Hartl and M. Ruvolo, 2012,
Genetics: Analysis of Genes and Genomes, 8th ed., Burlington, MA:
Jones & Bartlett, Table 8.2, p. 268. FIG.15.12

What is the genetic basis of

47, +21
(Down syndrome)
Down syndrome?
13 individuals out of
10,000 live births 47, +18 BACKGROUND The
2 individuals
detailed images of cells and
47, +13 chromosomes available today
45, X (Turner syndrome) 2 individuals
1 individual are produced by microscopic
47, XXY and imaging techniques not
(Klinefelter 47, XXX available during most of
syndrome) 5 individuals
5 individuals the history of genetics and
47, XYY cell biology. Techniques
5 individuals
for observing human
chromosomes were originally
so unreliable that the correct
Altogether, about 33 individuals per 10,000 human number of human Source: Terry Harris/Rex Features/
live births have extra or missing chromosomes. AP Images.
chromosomes was firmly
established only in the mid-1950s. Mistakes were common,
such as counting two nearby chromosomes as one or including
chromosomes in the count of one nucleus when they actually
Jérôme Lejeune, Marthe Gautier, and Raymond Turpin first saw
belonged to a nearby nucleus. Despite these limitations, in 1959
the extra chromosome when they applied a technique to visualize
French scientists Jérôme Lejeune, Marthe Gautier, and Raymond
a patient’s chromosomes (Fig. 15.12). The shorthand
Turpin decided to investigate whether any common birth
for the condition is 47,121, meaning that the individual has
abnormalities were associated with chromosomal abnormalities.
47 chromosomes with an extra copy of chromosome 21.
They chose Down syndrome, one of the most common birth
The presence of an extra chromosome 21 leads to a set of
abnormalities, because its genetic basis was an enigma.
traits that, taken together, constitute a syndrome. These traits
presumably result from an increase in dosage of the genes and HYPOTHESIS Down syndrome is associated with extra or
other elements present on chromosome 21, although not all the missing chromosomes.
specific genes or genetic elements have been identified. Children
with Down syndrome are mentally disabled to varying degrees;
most are in the mild to moderate range and with special education
and support they can acquire basic communication, self-help, and
social skills. People with Down syndrome are short because of Two other trisomies of autosomes (chromosomes other
delayed maturation of the skeletal system; their muscle tone is than the X and Y chromosomes) are sometimes found in live
low; and they have a characteristic facial appearance. About 40% of births. These are trisomy 13 and trisomy 18. Both are much rarer
children with Down syndrome have major heart defects, and life than Down syndrome and the effects are more severe. In both
expectancy is currently about 55 years. Among adults with Down cases, the developmental abnormalities are so profound that
syndrome, the risk of dementia is about 15% to 50%, which is newborns with these conditions usually do not survive the first
about five times greater than the risk in the general population. year of life. Other trisomies occur, but they are not compatible
Although Down syndrome affects approximately 1 in 750 live- with life.
born children overall, there is a pronounced effect of mother’s age
on the risk of Down syndrome. For mothers of ages 45–50, the risk Extra or missing sex chromosomes have fewer effects
of having a baby with Down syndrome is approximately 50 times than extra autosomes.
greater than it is for mothers of ages 15–20. The increased risk is Children born with an abnormal number of autosomes are rare,
the reason why many physicians recommend tests to determine but extra or missing sex chromosomes ( X or Y) are relatively
abnormalities in the fetus for pregnant women over the age of 35. common among live births. In most mammals, females have two
EXPERIMENT The chromosomes of five boys and four girls with
Down syndrome were studied. A small patch of skin was taken from
each child and placed in growth medium. In this medium, fibroblast
cells, which synthesize the extracellular matrix that provides the
structural framework of the skin, readily undergo DNA synthesis
and divide. After allowing cellular growth and division, the
fibroblast cells were prepared in a way that allowed the researchers
to count the number of chromosomes under a microscope.

RESULTS Chromosomes were counted in 103 cells, yielding counts

of 46 chromosomes (10 cells), 47 chromosomes (84 cells), and
48 chromosomes (9 cells). For each cell examined, the researchers
noted whether the count was “doubtful” or “absolutely certain.”
The researchers marked cells as “doubtful” if the chromosomes or
nuclei were not well separated from one another. They encountered
57 cells of which they felt “absolutely certain,” and in each of these
they counted 47 chromosomes. The extra chromosome was always
one of the smallest chromosomes in the set.

CONCLUSION The researchers concluded that Down syndrome

is the result of a chromosomal abnormality, an extra copy of one of Source: Rex Features via AP Images, Biophoto Associates/Science Source.
the smallest chromosomes. The discovery helped resolve many of Colorization by Mary Martin.

the questions surrounding the genetics of Down syndrome.

Human Genome Project has indicated that genes on chromosome
FOLLOW-UP WORK Improved techniques including chromosome
21 have direct roles in many of the signs and symptoms of Down
banding (shown in Fig. 13.15) later indicated that the extra
syndrome.
chromosome was chromosome 21. The finding stimulated many
other chromosome studies. Today, it is clear that, except for the sex SOURCE J. Lejeune, Gautier, M., and Turpin, R. 1959. “Premier exemple d’aberration
autosomique humaine.” Comptes rendus des séances de l’Académie des Sciences.
chromosomes, the vast majority of fetuses with extra or missing
248:1721–1722. Translated from the French in Landmarks in Medical Genetics, edited by
chromosomes undergo spontaneous abortion. Furthermore, the P. S. Harper. Oxford, New York, 2004.

X chromosomes ( XX), and males have one X and one Y chromosome absence of detectable phenotypic effects in 47, XXX females has a
( XY). The presence of the Y chromosome, not the number of completely different explanation that has to do with the manner
X chromosomes, leads to male development. The female karyotype in which the activity of genes in the X chromosome is regulated.
47, XXX (47 chromosomes, including three X chromosomes) and In the cells of female mammals, all X chromosomes except one are
the male karyotype 47, XYY (47 chromosomes, including one X and inactivated and gene expression is largely repressed. (This process,
two Y chromosomes) are found among healthy females and males. called X-inactivation, is discussed in Chapter 20.) Because of
These persons are in the normal range of physical development and X-inactivation, a 47, XXX female has one active X chromosome per
mental capability, and usually their extra sex chromosome remains cell, the same number as in a 46, XX female.
undiscovered until their chromosomes are examined for some
other reason. j Quick Check 6 A male baby is born with the sex-chromosome
The reason that 47, XYY males show no detectable phenotypic constitution XYY. Both parents have normal sex chromosomes
effects is the unusual nature of the Y chromosome. The Y contains (XY in the father, XX in the mother). In which meiotic division of
only a few functional genes other than the gene that stimulates which parent did the nondisjunction take place that produced the
the embryo to take the male developmental pathway. The XYY baby?
321
 322 SECTION 15.4 G E N E T I C VA R I AT I O N I N C H RO M O S O M E S

Earlier, we saw that in XX females one X chromosome

FIG. 15.13 Symptoms associated with (a) Klinefelter syndrome undergoes inactivation. Why, then, do individuals with just one
and (b) Turner syndrome. X chromosome show any symptoms at all? The explanation is that
a. b. some genes on the inactive X chromosome in XX females are not
47, XXY (Klinefelter) completely inactivated. Evidently, for normal development to take
Tall stature place, both copies of some genes that escape complete inactivation
Slightly feminized 45, X (Turner) must be expressed.
physique A karyotype with 45, X is one of the rarest seen in live-born
Short stature
Poor beard growth Characteristic babies. The reason is that more than 99% of the fetuses that are
facial features 45, X undergo spontaneous abortion—the subject of the next
Tendency to lose
chest hairs Neck-skin section.
webbing
Breast development
Constriction
of the aorta Nondisjunction is a major cause of spontaneous
Female-type Poor breast abortion.
pubic hair pattern development Trisomies and the sex-chromosome abnormalities discussed in the
Widely spaced previous section account for most of the simpler chromosomal
Small testes nipples abnormalities that are found among babies born alive. However,
Underdeveloped these chromosomal abnormalities represent only a minority of
gonadal structures
those that actually occur. In fertilized eggs with extra or missing
No menstruation

FIG. 15.14 Chromosomal abnormalities in spontaneous

abortion. After D. L. Hartl and M. Ruvolo, 2012, Genetics:
Analysis of Genes and Genomes, 8th ed., Burlington, MA: Jones &
Bartlett, Table 8.2, p. 268.
Two other sex chromosome abnormalities do have
Chromosomal
phenotypic effects, especially on sexual development (Fig. 15.13). abnormality: 13
The more common is 47, XXY (47 chromosomes, including two
X chromosomes and one Y chromosome). Individuals with this
18 Spontaneous abortion
karyotype are male because of the presence of the Y chromosome, Live-born
Autosomal
but they have a distinctive group of symptoms known as trisomy 21
Klinefelter syndrome (Fig. 15.13a). Characteristics of the
Other
syndrome are very small testes but normal penis and scrotum. autosomes
Growth and physical development, for the most part, are normal,
although affected individuals tend to be tall for their age. About half X
of the individuals have some degree of mental impairment. When a Among the fetuses
XXY
person with Klinefelter syndrome reaches puberty, the testes fail to Sex in 100,000 recognized
enlarge, the voice remains high pitched, pubic and facial hair remain chromosomes pregnancies, about
XYY 8000 have major
sparse, and there is some enlargement of the breasts. Affected males chromosomal
do not produce sperm and hence are sterile; up to 10% of men who XXX abnormalities. Of
these, 7500 undergo
seek aid at infertility clinics turn out to be XXY. spontaneous
Balanced
Much rarer than all the other sex chromosome abnormalities translocation abortion, and 500 are
Translocations born alive.
is 45, X (45 chromosomes, with just one X chromosome), the Unbalanced
karyotype associated with characteristics known as Turner translocation
syndrome (Fig. 15.13b). Affected females are short and often have
3n
a distinctive webbing of the skin between the neck and shoulders. Polyploidy
There is no sexual maturation. The external sex organs remain 4n
immature, the breasts fail to develop, and pubic hair fails to grow.
Internally, the ovaries are small or absent, and menstruation does Other

not occur. Although the mental abilities of affected females are 0 500 1000 1500 2000 2500 3000 3500
very nearly normal, they have specific defects in spatial abilities Number of fetuses per
and arithmetical skills. 100,000 recognized pregnancies
 CHAPTER 15 G E N E T I C VA R I AT I O N 323

FIG. 15.15 Formation of polyploid organisms. (a) A triploid organism can result from failure of division in meiosis. (b) A tetraploid organism can
result from failure of cell division in mitosis.

a. Formation of triploid organisms b. Formation of tetraploid organisms

Normal haploid gamete DNA
replication
during S No cell
Fertilization Fertilization phase division

No cell
division Triploid Tetraploid
fertilized zygote
egg
Mitosis

Metaphase I Metaphase II
Diploid
gametes

Meiosis

chromosomes, the dosage of genes in these chromosomes is Altogether, about 15% of all recognized pregnancies terminate
unbalanced relative to the rest of the genome, and the embryos with spontaneous abortion of the fetus, and roughly half of these
usually fail to complete development. At some time during are due to major chromosomal abnormalities. This number tells
pregnancy—in some cases very early, in other cases relatively only part of the story because embryos with a missing autosome
late—the chromosomally abnormal embryo or fetus undergoes are not found among spontaneously aborted fetuses. These must
spontaneous abortion. occur at least as frequently as those with an extra autosome
The relative proportions of some of the major chromosomal because both are created by the same event of nondisjunction (see
abnormalities in spontaneous abortion are shown in Fig. 15.14. Fig. 15.12). The explanation seems to be that in fertilized eggs with
The bars in red represent recognized pregnancies that terminate in a missing autosome the abortion occurs shortly after fertilization,
spontaneous abortion, and those in blue represent those in which
the fetus develops to term and is born alive. Note the large number
and most cases are not recognized. •
of autosomal trisomies, none of which (with the exception of
trisomies 13, 18, and 21) permits live births. Even among these,
about 75% of fetuses with trisomy 21 undergo spontaneous FIG. 15.16 (a) A balanced and (b) an unbalanced translocation.
abortion, and the proportions are even greater for trisomies 13 and
18. The 45, X karyotype also very infrequently results in live birth. a. Balanced b. Unbalanced
A surprisingly large number of fetuses that undergo translocation translocation

spontaneous abortion are triploid (with three complete sets of

Inherited from
chromosomes, 69 altogether) or tetraploid (four complete sets, carrier parent
92 altogether). These karyotypes usually result from a defective
spindle apparatus and failure of cell division in anaphase. When Inherited from
this occurs in meiosis, the result is a diploid gamete and a chromosomally
normal parent
triploid fertilized egg (Fig. 15.15a). It can also occur after normal
In an unbalanced
fertilization, when the diploid egg goes through mitosis but the translocation, one large
cells fail to divide after the DNA is replicated. The result segment of a
is a tetraploid (Fig. 15.15b). Many spontaneous abortions result chromosome is present
Offspring in three copies and a
from an unbalanced translocation (Chapter 14), in which large segment of a
only part of a reciprocal translocation (along with one of the different chromosome
is present in only one
nontranslocated chromosomes) is inherited from one of the copy.
parents (Fig. 15.16).
324 SELF-ASSESSMENT

Core Concepts Summary

15.1 Genetic variation describes common genetic 15.4 Chromosomal variants can also occur but are
differences (polymorphisms) among the individuals in usually harmful.
a population at any given time.
Nondisjunction is the failure of a pair of chromosomes to
The genotype of an organism is its genetic makeup. separate during anaphase of cell division. It can occur in
page 310 mitosis or meiosis and results in daughter cells with extra or
missing chromosomes. page 319
The phenotype of an organism is any observable
characteristic of that organism, such as its appearance, Nondisjunction in meiosis leads to gametes with extra or
physiology, or behavior. The phenotype results missing chromosomes. page 319
from a complex interplay of the genotype and the
Most human fetuses with extra or missing chromosomes
environment. page 310
spontaneously abort, but some are viable and exhibit
A person’s genotype can be homozygous, in which he characteristic syndromes. page 319
or she has two alleles of the same type (one from each
Trisomy 21 (Down syndrome) is usually characterized by
parent), or heterozygous, in which he or she has a different
three copies of chromosome 21. page 320
type of allele from each parent. page 311
Extra or missing sex chromosomes (X or Y) are common
Genetic variation can be neutral (no effect on survival or
and result in syndromes such as Klinefelter syndrome
reproduction), harmful (associated with decreased survival
(47, XXY) and Turner syndrome (45, X). page 322
or reproduction), or beneficial (associated with increased
survival or reproduction). page 311

15.2 Human genetic variation can be detected

Self-Assessment
by DNA typing, which can uniquely identify each 1. Explain the terms “genotype” and “phenotype.” What
individual. determines a genotype? What determines a phenotype?

A variable number tandem repeat (VNTR) results from 2. Describe the relationship between a genotype and a
differences in the number of small-sequence repeats in a phenotype: Does the same genotype always result in
given area of the genome. page 314 the same phenotype? Must individuals with the same
phenotype have the same genotype?
DNA typing (DNA fingerprinting) analyzes genetic
polymorphisms at multiple genes with multiple alleles. 3. With regard to mutations, what is meant by the terms
Because there are so many possible genotypes in the “harmful,” “beneficial,” and “neutral”? Why it is
population, this procedure can uniquely identify an sometimes an oversimplification to consider a mutation as
individual with high probability. page 315 either harmful, beneficial, or neutral?

A restriction fragment length polymorphism (RFLP) 4. Describe two types of genetic polymorphism that are
results from small changes in the DNA sequence between useful in DNA typing.
chromosomes, such as point mutations, that create or
5. Define the term “SNP” and explain why researchers are
destroy restriction sites. page 316
interested in detecting SNPs.

15.3 Two common types of genetic polymorphism 6. Diagram how nondisjunction in meiosis I or II can result
are single-nucleotide polymorphisms (SNPs) and in extra or missing chromosomes in reproductive cells
copy-number variation (CNV). (gametes).

An SNP is a common difference in a single nucleotide. The 7. Describe the consequences of an extra copy of
DNA in any two human genomes differs at about 3 million chromosome 21 (Down syndrome).
SNPs. Most SNPs are not associated with any detectable
effect on phenotype. page 316
Log in to to check your answers to the Self-
CNV is a difference in the number of copies of a particular Assessment questions, and to access additional learning tools.
DNA sequence among chromosomes. page 318
 CHAPTER 16

Mendelian
Inheritance
Core Concepts
16.1 Early theories of heredity
incorrectly assumed the
inheritance of acquired
characteristics and blending of
parental traits in the offspring.
16.2 The study of modern
transmission genetics began
with Gregor Mendel, who
used the garden pea as his
experimental organism and
studied traits with contrasting
characteristics.
16.3 Mendel’s first key discovery
was the principle of
segregation, which states
that members of a gene pair
separate equally into gametes.
16.4 Mendel’s second key
finding was the principle of
independent assortment,
which states that different
gene pairs segregate
independently of one another.
16.5 The patterns of inheritance
that Mendel observed in peas
can also be seen in humans.
Koichi Saito/A.collection/Getty Images.

325
326 SECTION 16.1 E A R LY T H E O R I E S O F I N H E R I TA N C E

Except for identical twins, each of us has his or her own personal this theory is that any trait, or characteristic, of an individual
genome, a unique human genome differing from all that have can be transmitted from parent to offspring. Even traits that
existed before and from all that will come after. Genetic variation are acquired during the lifetime of an individual, such as muscle
from one person to the next leads in part to our individuality, from strength or bodily injury, were thought to be heritable because
differences in appearance to differences in the ways our bodies of the substance supposedly passed from each body part to the
work. Examples of genetic variation in the human population reproductive organs.
range from harmless curiosities like the genetic difference in taste The theory that acquired characteristics can be inherited
receptors that determine whether or not you taste broccoli as being was invoked to explain such traits as the webbed feet of ducks,
unpleasantly bitter, to mutant forms of genes resulting in serious which were thought to result from many successive generations
diseases like sickle-cell anemia or emphysema. in which adult ducks stretched the skin between their toes
This chapter focuses on how that genetic variation is inherited. while swimming and passed this trait to offspring. A few
Transmission genetics deals with the manner in which genetic decades later, however, Aristotle (384–322 BCE) emphasized
differences among individuals are passed from generation to several observations that the theory of inheritance of acquired
generation. We are all aware of the effects of genetics. We know characteristics cannot account for:
that children resemble their parents and that there are sometimes
• Traits such as hair color can be inherited, but it is difficult to
uncanny similarities among even distant relatives. But some
see how hair—a nonliving tissue—could send substances to
patterns are more difficult to discern. Traits such as eye color, nose
the reproductive organs.
shape, or risk for a particular disease may be passed down faithfully
generation after generation, but sometimes they are not, and • Traits that are not yet present in an individual can be
sometimes they appear and disappear in seemingly random ways. transmitted to the offspring. For example, a father and his
As a modern science, transmission genetics began with the pea- adult son can both be bald, even though the son was born
breeding experiments carried out by the monk Gregor Mendel in before the father became bald.
the 1860s. However, even before then, people understood enough
• Parts of the body that are lost as a result of surgery or
about inheritance that they were able to select crops and livestock
accident are not missing in the offspring.
with particular characteristics.
From these and other observations, Aristotle concluded that
the process of heredity transmits only the potential for producing
16.1 EARLY THEORIES traits present in the parents, and not the traits themselves.
OF INHERITANCE Nevertheless, Hippocrates’ theory influenced biology until
well into the 1800s. It was incorporated into an early theory of
Thousands of years before Mendel, many societies carried out evolution proposed by the French biologist Jean-Baptiste Lamarck
practical plant and animal breeding. The ancient practices were around 1800. Charles Darwin, however, developed an alternative
based on experience rather than on a full understanding of the theory—the theory of evolution by natural selection (Chapter 21).
rules of genetic transmission, but they were nevertheless highly While traits acquired during the lifetime of a parent are not
successful. In Mesoamerica, for example, Native Americans chose transmitted to the offspring, parental misfortune or misbehavior
corn (maize) plants for cultivation that had the biggest ears and can nevertheless result in impaired fetal development and in some
softest kernels. Over many generations, their cultivated corn came cases permanent damage. For example, maternal malnutrition, drug
to have less and less physical resemblance to its wild ancestral addiction, alcoholism, infectious disease, and other conditions can
species. Similarly, people in the Eurasian steppes selected their severely affect the fetus, but these effects are due to disruption of
horses for a docile temperament suitable for riding or for hitching fetal development and not to changes in the genome.
to carts or sleds. Practical breeding of this kind provided the crops
and livestock from which most of our modern domesticated Belief in blending inheritance discouraged studies of
animals and plants derive. hereditary transmission.
Darwin subscribed to the now-discredited model of blending
Early theories of heredity predicted the transmission inheritance, in which traits in the offspring resemble the average
of acquired characteristics. of those in the parents. For example, this model predicts that the
The first written speculations about mechanisms of heredity offspring of plants with blue flowers and those with red flowers
were made by the ancient Greeks. Hippocrates (460–377 BCE), will have purple flowers. While traits of offspring are sometimes
considered the founder of Western medicine, proposed that each the average of those of the parents (think of certain cases of
part of the body in a sexually mature adult produces a substance human height), the idea of blending inheritance—which implies
that collects in the reproductive organs and that determines the blending of the genetic material—as a general rule presents
the inherited characteristics of the offspring. An implication of problems. For example, it cannot explain the reappearance of a
 CHAPTER 16 M E N D E L I A N I N H E R I TA N C E 327

Although Darwin was convinced that he was right about

FIG. 16.1 Blending inheritance. This model predicts loss of variation natural selection, he was never able to reconcile his theory
over time and blending of genetic material, which is not with the concept of blending inheritance. Unknown to him, the
observed. solution had already been discovered in experiments carried out by
Gregor Mendel (1822–1884), an Augustinian monk in a monastery
Rare Common
in the city of Brno in what is now the Czech Republic. Mendel’s
key discovery was this: It is not traits that are transmitted in
inheritance—it is genes that are transmitted.

Black White
16.2 THE FOUNDATIONS OF MODERN
TRANSMISSION GENETICS
Mendel’s scientific fame rests on the pea-breeding experiments
 that he carried out in the years 1856 to 1864 (Fig. 16.2), in which
he demonstrated the basic principles of transmission genetics. As
Gray White
we will see in Chapter 18, most genetic variation in populations is
due to multiple genes interacting with one another and with the
environment, while traits due to single genes, such as those that
Mendel studied, tend to be rare. Nevertheless, Mendel’s experiments

were so simple and presented with such clarity that they still serve
as examples of the scientific method at its best (Chapter 1).
Less gray White

Mendel’s experimental organism was the garden pea.

For his experimental material, Mendel used several strains of
trait several generations after it apparently “disappeared” in a ordinary garden peas (Pisum sativum) that he obtained from local
family, such as red hair or blue eyes.
Another difficulty with the concept of blending inheritance
is that variation is lost over time. Consider an example where FIG. 16.2 Gregor Mendel and his experimental organism, the
black rabbits are rare and white rabbits are common (Fig. 16.1). pea plant. Sources: Juliette Wade/Getty Images; (inset) Photo by
Black rabbits, being rare, are most likely to mate with the much Authenticated News/Getty Images.
more common white rabbits. If blending inheritance occurs, the
result will be gray rabbits. These will also mate with white rabbits,
producing lighter-gray rabbits. Over time, the population will
end up being all white, or very close to white, and there will be
less variation. The only way that black rabbits will be present is if
they are reintroduced into the population through mutation or
migration.
Whatever the trait may be, blending inheritance predicts that
inheritance will tend to be a homogenizing force, producing in
each generation a blend of the original phenotypes. But we know
from common experience that variation in most populations is
plentiful.
It is ironic that Darwin believed in blending inheritance because
this mechanism of inheritance is incompatible with his theory of
evolution by means of natural selection. The incompatibility was
pointed out by some of Darwin’s contemporaries, and Darwin
himself recognized it as a serious problem. The problem with
blending inheritance is that rare variants, such as the black rabbits
in the above example, will have no opportunity to increase in
frequency, even if they survive and reproduce more than white
rabbits, since they gradually disappear over time.
328 SECTION 16.2 T H E F O U N DAT I O N O F M O D E R N T R A N S M I S S I O N G E N E T I C S

seed suppliers. His experimental approach was similar to that of seven physical features expressed in contrasting fashion among
a few botanists of the eighteenth and nineteenth centuries who the strains: seed color, seed shape, pod color, pod shape, flower
studied the results of hybridization, or interbreeding between color, flower position, and plant height.
two different varieties or species of an organism. Where Mendel The expression of each trait in each of the original strains
differed from his predecessors was in paying close attention that Mendel obtained was true breeding, which means that the
to a small number of easily classified traits with contrasting physical appearance of the offspring in each successive generation
characteristics. For example, where one strain of peas had yellow is identical to the previous one. For example, plants of the strain
seeds, another had green seeds; and where one had round seeds, with yellow seeds produced only yellow seeds, and those of the
another had wrinkled seeds (Fig. 16.3). Altogether Mendel studied strain with green seeds produced only green seeds. Likewise,
plants of the strain with round seeds produced only round seeds,
and those of the strain with wrinkled seeds produced only
wrinkled seeds.
FIG. 16.3 Contrasting traits. Mendel focused on seven contrasting The objective of Mendel’s experiments was simple. By
traits. means of crosses between the true-breeding strains and crosses
among their progeny, Mendel hoped to determine whether
Dominant Recessive
there are statistical patterns in the occurrence of the contrasting
a. Color of seeds characteristics, such as yellow seeds or green seeds. If such
(yellow or green)
patterns could be found, he would seek to devise a hypothesis to
explain them and then use his hypothesis to predict the outcome
b. Shape of seeds of further crosses.
(round or wrinkled)
In designing his experiments, Mendel departed from other
plant hybridizers of the time in three important ways:
c. Color of pod
(green or yellow) 1. Mendel studied true-breeding strains, unlike many other
plant hybridizers, who used complex and poorly defined
d. Shape of pod material.
(smooth or indented)
2. Mendel focused on one trait, or a small number of traits,
e. Color of flower at a time, with characteristics that were easily contrasted
(purple or white) among the true-breeding strains. Other plant hybridizers
crossed strains differing in many traits and tried to follow
all the traits at once. This resulted in amazingly complex
inheritance, and no underlying patterns could be discerned.

f. Position of flowers 3. Mendel counted the progeny of his crosses, looking for
(along stem or at tip) statistical patterns in the offspring of his crosses. Others
typically noted only whether offspring with a particular
characteristic were present or absent, but did not keep
track of and count all the progeny of a particular cross.

In crosses, one of the traits was dominant in

the offspring.
Mendel began his studies with crosses between true-breeding
g. Plant height
(tall or dwarfed) strains that differed in a single contrasting characteristic. Crossing
peas is not as easy as it may sound. Pea flowers include both
female and male reproductive organs enclosed together within
petals (Fig. 16.4). Because of this arrangement, pea plants usually
fertilize themselves (self-fertilize). Performing crosses between
different plants is a tedious and painstaking process. First, the
flower of the designated female parent must be opened at
an early stage and the immature anthers (male reproductive
organs) clipped off and discarded, so the plant cannot self-fertilize.
Then mature pollen from the designated male parent must be
collected and deposited on the stigma (female reproductive organ)
of the female parent, from where it travels to the reproductive
 CHAPTER 16 M E N D E L I A N I N H E R I TA N C E 329

is dominant to green seed, round seed

FIG. 16.4 Crossing pea plants. Crossing plants in a way that isolates either the ovules or the is dominant to wrinkled seed, and
pollen controls what genetic material each parent contributes to the offspring. so forth.
Mendel explained these findings
1 In crossing peas, the 2 Mature pollen is 3 After fertilization, a small by supposing that there is a hereditary
anthers of the female parent collected from another cloth bag is tied around the
are first exposed and then cut flower and deposited on the fertilized flower to prevent factor for yellow seeds and a different
off to prevent self-fertilization. stigma of the female parent. stray pollen from entering. hereditary factor for green seeds,
Anthers likewise a hereditary factor for round
seeds and a different one for wrinkled
seeds, and so on. We now know that
the hereditary factors that result in
contrasting traits are different forms
Stigma of a gene that affect the trait. The
different forms of a gene are called
Ovules
alleles. The alleles of a gene are
Ovary variations of the DNA sequence of a
Petals Flower on
female parent gene that occupies a particular region
Peeled-back petal
along a chromosome. The particular
Flower on combination of alleles present in
male parent

cells in the ovule at the base of the flower. And finally, a cloth
bag must be tied around the female flower to prevent stray FIG. 16.5 The first-generation hybrid (F1). A cross between two
pollen from entering. No wonder Mendel complained that his of Mendel’s true-breeding plants (the parental, or P1,
eyes hurt! generation) yielded first-generation hybrids displaying
For each of the seven pairs of contrasting traits, true-breeding the dominant trait.
strains differing in the trait were crossed. Fig. 16.5 illustrates a
typical result, in this case for a cross between a plant producing
yellow seeds and a plant producing green seeds. In these kinds of
crosses, the parental generation is referred to as the P1 generation,
Plant grown from Plant grown from
and the first offspring, or filial, generation is referred to as the true-breeding strain true-breeding strain
F1 generation. In the cross of P1 yellow × P1 green, Mendel observed with yellow seeds with green seeds
that all the F1 progeny had yellow seeds. This result was shown to be
independent of the seed color, yellow or green, of the pollen donor
because both of the crosses shown below yielded progeny plants
True-breeding
with yellow seeds:  strains that are
crossed constitute
Pollen from strain Ovules from strain Offspring the P1 or parental
 generation.
with yellow seeds with green seeds seeds yellow
P1 generation
Pollen from strain Ovules from strain Offspring
 F1 generation
with green seeds with yellow seeds seeds yellow

Crosses like these, in which the expressions of the trait in the

female and male parents are interchanged, are known as reciprocal
crosses. Mendel showed that reciprocal crosses yielded the same The trait that
appears in the F1
result for each of his seven pairs of contrasting traits. generation (in this
Moreover, for each pair of contrasting traits, only one of the case yellow seeds)
is dominant, and
traits appeared in the F1 generation. For simple crosses involving the other trait
two parents that are true breeding for different traits, the trait (green seeds) is
that appears in the F1 generation is said to be dominant, and the recessive.
contrasting trait that does not appear is said to be recessive. For
each pair of traits illustrated in Fig. 16.3, the dominant trait is
shown on the left, and the recessive on the right. Thus, yellow seed
330 SECTION 16.3 S E G R E G AT I O N : M E N D E L’ S K E Y D I S COV E RY

an individual constitutes its genotype, and the expression of

the trait in the individual constitutes its phenotype. As noted, FIG. 16.6 Self-fertilization of the F1 hybrid, resulting in seeds of
Mendel found that, for each of his traits, one phenotype was the F2 generation. The recessive trait appeared again in
dominant and the other phenotype was recessive. the F2 generation.
In the cross between true-breeding plants that produce yellow
Seeds from F1 plants produced
seeds and true-breeding plants that produce green seeds, the from a cross of true-breeding
genotype of each F1 seed includes an allele for yellow from the yellow-seed and green-seed
plants are yellow because
yellow parent and an allele for green from the green parent. The yellow is dominant and green is
phenotype of each F1 seed is nevertheless yellow, because yellow is recessive in seed color.
dominant to green.
The molecular basis of dominance in this case is the fact that,
in diploid organisms, only one copy of a gene is needed to carry
out its normal function. The yellow color in pea seeds results from
an enzyme that breaks down green chlorophyll, allowing yellow
pigments to show through. Green seeds result from a mutation
F1 generation
in this gene that inactivates the enzyme, and so the green Peas are normally self-fertilizing
and so, if they are left alone, the
chlorophyll is retained. In an F1 hybrid, such as that in Fig. 16.5, pollen produced in each flower
which receives a nonmutant gene from one parent and a mutant fertilizes the ovules, producing
gene from the other, the seeds are yellow because one copy of the a seed pod whose contents
have genotypes that depend on
nonmutant gene produces enough of the enzyme to break down the alleles in that individual
the chlorophyll to yield a yellow seed. flower‘s pollen and ovules.

16.3 SEGREGATION: MENDEL’S

KEY DISCOVERY Because fertilization takes
place at random, any
individual pod can have a
Mendel’s most important discovery was that the F1 progeny of a ratio of dominant: recessive
cross between plants with different traits did not breed true. In that deviates from 3:1.
the F2 generation, produced by allowing the F1 flowers to undergo
self-fertilization, the recessive trait reappeared (Fig. 16.6). Not
only did the recessive trait reappear, it reappeared in a definite F2 generation
numerical proportion. Among a large number of F2 progeny, (3 yellow seeds:1 green seed)
Mendel found that the dominant : recessive ratio was very close to
3 : 1. The results he observed among the F2 progeny for each of the
seven pairs of traits are given in Table 16.1. Across experiments for
all seven traits, the ratio of dominant : recessive F2 offspring was and aa genotypes are said to be homozygous, which means
14,949 : 5010. Although there is variation from one experiment to that both alleles inherited from the parents are the same.
the next, the overall ratio of 14,949 : 5010 equals 2.98 : 1, which is a
j Quick Check 1 What are the genotypes and phenotypes for
very close approximation to 3 : 1.
Mendel’s true-breeding parent plants?

Genes come in pairs that segregate in the formation 2. Each reproductive cell, or gamete, contains only one allele
of reproductive cells. of each gene. In this case, a gamete can contain the A allele
The explanation for the 3 : 1 ratio in the F2 and Mendel’s or the a allele, but not both.
observations in general can be summarized with reference to
3. In the formation of gametes, the two members of a gene
Fig. 16.7:
pair segregate (or separate) equally into gametes so that
1. Except for cells involved in reproduction, each cell of a half the gametes get one allele and half get the other allele.
pea plant contains two alleles of each gene. In each true- This separation of alleles into different gametes defines
breeding strain constituting the P1 generation, the two the principle of segregation. In the case of homozygous
alleles are identical. In Fig. 16.7, we designate the allele plants (such as AA or aa), all the gametes from an individual
associated with yellow seeds as A and that associated with are the same. That is, the homozygous AA strain with
green seeds as a. The genotype of the true-breeding strain yellow seeds produces gametes containing the A allele,
with yellow seeds can therefore be written as AA, and that and the homozygous aa strain with green seeds produces
of the true-breeding strain with green seeds as aa. The AA gametes containing the a allele.
 CHAPTER 16 M E N D E L I A N I N H E R I TA N C E 331

5. When the F1 progeny (genotype Aa) form gametes, by the

TABLE 16.1 Observed F2 Ratios in Mendel‘s Experiments principle of segregation the A and a alleles again separate
equally so that half the gametes contain only the A allele
DOMINANT RECESSIVE
and the other half contain only the a allele (Fig. 16.7).
TRAIT TRAIT TRAIT RATIO
6. In the formation of the F2 generation, the gametes from
Seed color 6,022 2,001 3.01:1
Seed shape 5,474 1,850 2.96:1 the F1 parents again combine at random. The consequences
Pod color 428 152 2.82:1 of random union of gametes can be worked out by means
Pod shape 882 299 2.95:1 of a checkerboard of the sort shown at the bottom of
Flower color 705 224 3.15:1
Fig. 16.7. In this kind of square, known as a Punnett
Flower position 651 207 3.14:1
Plant height 787 277 2.84:1 square after its inventor, the British geneticist Reginald
Punnett, the gametes from each parent, each with its
respective frequency, are arranged along the top and sides
of a grid. Each box in the grid represents the union of the
4. The fertilized egg cell, called the zygote, is formed from
gametes in the corresponding row and column, showing
the random union of two gametes, one from each parent.
all the possible genotypes of offspring that can result from
For the cross AA × aa, the zygote is an F1 hybrid formed
random fertilization.
from the union of an A-bearing gamete with an a-bearing
gamete. Each F1 hybrid therefore has the genotype Aa 7. The boxes of the Punnett square correspond to all of the
(Fig. 16.7). The Aa genotype is heterozygous, which possible offspring genotypes of the F2 generation, with each
means that the two alleles for a given gene inherited from possible genotype’s frequency obtained by multiplication
each parent are different. The seeds produced by the F1 of the gametic frequencies in the corresponding row and
progeny are yellow because yellow is dominant to green. column. For the example illustrated in Fig. 16.7, the expected
Note that each F1 progeny contains an a allele because its genotypes of the offspring are therefore ¼ AA, ½ Aa, and ¼ aa
genotype is Aa, but the phenotype of the seed is yellow and (or 1 : 2 : 1). When there is dominance, however, as there is in
indistinguishable from that of an AA genotype. this case, the AA and Aa genotypes have the same phenotype,
and so the ratio of dominant : recessive phenotypes is 3 : 1. The
j Quick Check 2 Is it possible for two individuals to have the same
Punnett square in Fig. 16.7 illustrates the biological basis of
phenotype but different genotypes? The same genotype, but
the 3 : 1 ratio of phenotypes that Mendel observed in the F2
different phenotypes? How?
generation. Nevertheless, the underlying ratio of AA : Aa : aa
genotypes is 1 : 2 : 1.

j Quick Check 3 What are the expected progeny (genotypes and

FIG. 16.7 The principle of segregation.
phenotypes) from a cross of an AA plant with an Aa plant?
AA  aa
True-breeding
plants are
The principle of segregation was tested by predicting
P1 generation the outcome of crosses.
homozygous, and
each produces one The model of segregation of gene pairs (alleles) and their random
type of gamete.
combination in the formation of a zygote depicted in Fig. 16.7 was
Mendel’s hypothesis, an explanation he put forward to explain an
Aa
observed result. Further experiments were necessary to support or
F1 generation disprove his hypothesis. The true test of a hypothesis is whether
it can predict the results of experiments that have not yet been
carried out (Chapter 1). If the predictions are correct, one’s
confidence in the hypothesis is strengthened. Mendel appreciated
A a
½ ½ this intuitively, even though the scientific method as understood
today had not been formalized.
A Expected ratio of The Punnett square in Fig. 16.7 makes two predictions. The
½ AA Aa AA: Aa:aa genotypes first is that the seeds in the F2 generation showing the recessive
¼ ¼ is 1:2:1.
Aa green seed phenotype should be homozygous aa. If the green
Expected ratio of F2 seeds have the genotype aa, then they should breed true. That is,
F1 a dominant:recessive
generation when the seeds are grown into mature plants and self-fertilization
½ Aa aa phenotypes is 3:1.
is allowed to take place, the self-fertilized aa plants should produce
¼ ¼
only green seeds (aa). This prediction was confirmed by examining
F2 generation seeds actually produced by plants grown from the F2 green seeds.
332 SECTION 16.3 S E G R E G AT I O N : M E N D E L’ S K E Y D I S COV E RY

yields yellow seeds (Fig. 16.8b). In other words, the testcross gives
TABLE 16.2 Genetic Ratios from Self-Fertilization of Plants different results depending on whether the parent is heterozygous
Showing the Dominant Phenotype ( Aa) or homozygous (AA).
Note that in a testcross the phenotypes of the progeny reveal
HOMOZYGOUS HETERO-
TRAIT DOMINANT ZYGOUS RATIO
the alleles present in the gametes from the tested parent. A
testcross with an Aa individual yields ½ Aa (yellow seeds) and
Yellow seeds 166 353 0.94:2 ½ aa (green seeds) since the Aa parent produces ½ A-bearing and
Round seeds 193 372 1.04:2 ½ a-bearing gametes. A testcross with an AA individual yields only
Green pods 40 60 1.33:2
Smooth pods 29 71 0.82:2 Aa (yellow seeds) since the AA parent produces only A-bearing
Purple flowers 36 64 1.13:2 gametes. These results are a direct demonstration of the principle
Flowers along stem 33 67 0.99:2 of segregation since the ratio of the phenotypes of progeny
Tall plants 28 72 0.78:2 reflect the equal segregation of alleles into gametes. Some of
Mendel’s testcross data indicating 1 : 1 segregation in heterozygous
genotypes are shown in Table 16.3.
The second prediction from the Punnett square in Fig. 16.7
is more complex. It has to do with the seeds in the F2 generation Segregation of alleles reflects the separation of
that show the dominant yellow phenotype. Note that although chromosomes in meiosis.
these seeds have the same phenotype, they have two different The principles of transmission genetics have a physical basis in
genotypes ( AA and Aa). Among just the yellow seeds, ⅓ should the process of meiosis (Chapter 11). During meiosis I, maternal
have the genotype AA and ⅔ should have the genotype Aa, for and paternal chromosomes (homologous chromosomes) align on
a ratio of 1 AA : 2 Aa. (The proportions are ⅓ : ⅔ because we the metaphase plate. Then, during anaphase I, the homologous
are considering only the seeds that are yellow.) Plants with the chromosomes separate, and each chromosome goes to a different
AA and Aa genotypes can be distinguished by the types of seed pole. Because gene pairs are carried on homologous chromosomes,
they produce when self-fertilized. The AA plants produce only the segregation of alleles observed by Mendel corresponds to the
seeds with the dominant yellow phenotype (that is, they are separation of chromosomes that takes place in anaphase I.
true breeding), whereas the Aa plants yield dominant yellow Fig. 16.9 illustrates the separation of a pair of homologous
and recessive green seeds in the ratio 3 : 1. Mendel did such chromosomes in anaphase I. In the configuration shown, the
experiments, and the prediction turned out to be correct. His data copies of the A allele (dark blue) separate from the copies of the
confirming the 1 : 2 ratio of AA : Aa among F2 individuals with the a allele (light blue) in anaphase I. The separation of chromosomes
dominant phenotype are shown in Table 16.2. is the physical basis of the segregation of alleles.

A testcross is a mating to an individual with the Dominance is not universally observed.

homozygous recessive genotype. Many traits do not show complete dominance such as Mendel
A more direct test of segregation is to cross the F1 progeny with observed with pea plants. Most traits we see are determined
the true-breeding recessive strain instead of allowing them to by multiple genes or the interaction of genotype and the
self-fertilize. Any cross of an unknown genotype
with a homozygous recessive genotype is known as
a testcross.
FIG. 16.8 A testcross. A cross with a homozygous recessive individual reveals the
The F1 progeny show the dominant yellow seed
genotype of the other parent.
phenotype. A yellow seed phenotype can result
from either of two possible genotypes, Aa or AA. a. Aa  aa b. AA  aa
A testcross with plants of the homozygous Gametes from Gametes from
recessive genotype (aa) can distinguish between homozygous homozygous
these two possibilities (Fig. 16.8). recessive parent recessive parent
Let’s first consider what happens in a testcross a a a a
½ ½ ½ ½
to an Aa individual (Fig. 16.8a). An Aa individual
produces both A-bearing gametes and a-bearing
A A
gametes. As shown by the Punnett square, the ½ ½
Aa Aa Aa Aa
predicted offspring from the testcross are Gametes ¼ ¼ Gametes ¼ ¼
from Aa from AA
½ Aa zygotes, which yield yellow seeds, and ½ aa individual individual
zygotes, which yield green seeds. By contrast, an a A
AA individual produces only A-bearing gametes, ½ aa aa ½ Aa Aa
so all the zygotes have the Aa genotype, which ¼ ¼ ¼ ¼
 CHAPTER 16 M E N D E L I A N I N H E R I TA N C E 333

TABLE 16.3 Phenotype of Progeny from Testcrosses of FIG. 16.10 Incomplete dominance for flower color in
Heterozygotes snapdragons. In incomplete dominance, an
intermediate phenotype is seen.
DOMINANT RECESSIVE
TRAIT TRAIT TRAIT RATIO

Seed color 196 189 1.04:1

Seed shape 193 192 1.01:1 
Flower color 85 81 1.05:1
Plant height 87 79 1.10:1
CRCR CWCW
P1 generation
environment and so do not display the expected 3 : 1 ratio of
phenotypes. But even among traits that are determined by a single
gene, the 3 : 1 phenotype ratio is not always observed. In some
The phenotype of the
cases, the trait shows incomplete dominance, in which the
heterozygous CRCW plant is
phenotype of the heterozygous genotype is intermediate between intermediate, an example
those of the homozygous genotypes. In such cases, the result of of incomplete dominance.
segregation can be observed directly because each genotype has a
CRCW
distinct phenotype. An example is flower color in the snapdragon F1 generation
(Antirrhinum majus), in which the homozygous genotypes
have red (CRCR) or white (CWCW) flowers, and the heterozygous
genotype CRCW has pink flowers (Fig. 16.10). In notating CR CW
½ ½
incomplete dominance, we use superscripts to indicate the alleles,
rather than upper-case and lower-case letters, because neither
allele is dominant to the other. A cross of homozygous CRCR and
CWCW strains results in hybrid F1 progeny that are pink (CRCW), and CR The result of
½ segregation can
when these are crossed the resulting F2 generation consists of be observed
¼ red (CRCR), ½ pink (CRCW), and ¼ white (CWCW). CRCR CRCW directly, because
Note that in this case the genotype ratio and the phenotype ¼ ¼ the ratio of
red : pink :white
ratio are both 1 : 2 : 1, since each genotype has a distinct phenotype. phenotypes is
Such a direct demonstration of segregation makes one wonder 1:2:1, which
reflects the ratio of
whether Mendel’s work might have been appreciated more readily CRCW
CW CR CR : CR CW : CW CW
if his traits had shown incomplete dominance! ½ genotypes.

CRCW CWCW
¼ ¼
FIG. 16.9 Segregation of alleles of a single gene. Homologous
F2 generation
chromosomes separate during meiosis, leading to
segregation of alleles.
The principles of transmission genetics are statistical
Resulting
Segregation reflects the separation
gametes
and are stated in terms of probabilities.
of homologous chromosomes
during anaphase I of meiosis. The element of chance in fertilization implies that the genotype
A of any particular progeny cannot be determined in advance.
However, one can deduce the likelihood, or probability, that a
specified genotype will occur. The probability of occurrence of a
Copies of A allele
A A genotype must always lie between 0 and 1; a probability of 0 means
in replicated
A Meiosis II that the genotype cannot occur, and a probability of 1 means
chromosome
that the occurrence of the genotype is certain. For example, in
a the cross Aa × AA, no offspring can have the genotype aa, so
a Copies of a allele in this mating the probability of aa is 0. Similarly, in the mating
a in replicated
homologous AA × aa, all offspring must have the genotype Aa, so in this mating
chromosome a the probability of Aa is 1.
In many cases, the probability of a particular genotype is
Anaphase I neither 0 nor 1, but some intermediate value. For one gene, the
334 SECTION 16.3 S E G R E G AT I O N : M E N D E L’ S K E Y D I S COV E RY

probabilities for a single individual can be deduced from the one nearest the stem is green and the others yellow. Here,
parental genotypes in the mating and the principle of segregation. the word “and” is a simple indicator that the multiplication
For example, the probability of producing a homozygous recessive rule should be used. Because each seed results from an
individual from the cross Aa × Aa is ¼ (see Fig. 16.7), and that independent fertilization, this probability is given by the
from the cross Aa × aa is ½ (see Fig. 16.8a). product of the probability that the seed nearest the stem is
The genotype and phenotype probabilities for a single aa and the probability that each of the other seeds is either
individual can also be inferred from observed data because the AA or Aa, and hence the probability is ¼ × ¾ × ¾ × ¾ =
overall proportions of two (or more) genotypes among a large 27/256, as shown for the top pod in Fig. 16.11.
number of observations approximates the probability of each of
the genotypes for a single observation. For example, in Mendel’s The addition and multiplication rules are very powerful when
F2 data (see Table 16.1), the overall ratio of dominant : recessive is used in combination. Consider the following question: In the
2.98 : 1, or very nearly 3 : 1. This result implies that the probability mating Aa × Aa, what is the probability that, among four seeds in
that an individual F2 plant has the homozygous recessive a pod, exactly one is green? We have already seen in Fig. 16.11 that
phenotype is very close to ¼, which is the value inferred from the the multiplication rule gives the probability of the seed nearest the
principle of segregation. stem being green as 27/256. As illustrated in Fig. 16.11, there are only
Sometimes it becomes necessary to combine the probabilities four possible ways in which exactly one seed can be green, each of
of two or more possible outcomes of a cross, as in determining the which has a probability of 27/256, and these outcomes are mutually
probability of a genotype occurring based on the probabilities of the exclusive. Therefore, by the addition rule, the probability of there
gametes occurring. In such cases either of two rules are helpful: being exactly one green and three yellow seeds in a pod, occurring
in any order, is given by 27/256 + 27/256 + 27/256 + 27/256 =
1. Addition rule. This principle applies when the possible 108/256, or approximately 42%.
outcomes being considered cannot occur simultaneously.
For example, suppose that a single offspring is chosen at j Quick Check 4 What is the probability that any two peas are
random from the progeny of the mating Aa × Aa, and we green and two are yellow in a pea pod with exactly four seeds?
wish to know the probability that the offspring is either AA
or Aa. The key words here are “either” and “or.” Each of these Mendelian segregation preserves genetic variation.
outcomes is possible, but both cannot occur simultaneously As noted earlier, Darwin was befuddled because blending
in a single individual; the outcomes are mutually exclusive. inheritance would make genetic variation disappear so rapidly that
When the possibilities are mutually exclusive, the addition evolution by means of natural selection could not occur. Although
rule states that the probability of either event occurring is
given by the sum of their individual probabilities. In this
example, the chosen offspring could either have genotype AA FIG. 16.11 Application of the multiplication and addition rules.
(with probability ¼, according to Fig. 16.7), or the offspring
could have genotype Aa (with probability ½). Therefore,
the probability that the chosen individual has either the AA Probability that the seed closest
to the stem is green and the
or the Aa genotype is given by ¼ + ½ = ¾ (see Fig. 16.7). (¼) (¾) (¾) (¾) = 27/256 others are yellow is determined
Alternatively, the ¾ could be interpreted to mean that, by the multiplication rule.
among a large number of offspring from the mating
Aa × Aa, the proportion exhibiting the dominant phenotype
will be very close to ¾. This interpretation is verified by the (¾) (¼) (¾) (¾) = 27/256
data in Table 16.1.

2. Multiplication rule. This principle applies when outcomes

can occur simultaneously, and the occurrence of one has (¾) (¾) (¼) (¾) = 27/256
no effect upon the likelihood of the other. Events that
do not influence one another are independent, and the
multiplication rule states that the probability of two Probability that any seed is
green and the others are
independent events occurring together is the product of (¾) (¾) (¾) (¼) = 27/256
yellow is determined by first
their respective probabilities. This rule is widely used to using the multiplication rule
108/256 = 42% to determine the probability
determine the probabilities of successive offspring of a cross
that a particular seed is
because each event of fertilization is independent of any green, then the addition
other. For example, in the mating Aa × Aa, one may wish to rule to determine that any
seed is green.
determine the probability that, among four peas in a pod, the
 CHAPTER 16 M E N D E L I A N I N H E R I TA N C E 335

Darwin was completely unaware of Mendel’s findings, segregation

was the answer to his problem. FIG. 16.12 Mendel’s crosses with two traits. Plants heterozygous
An important consequence of segregation is that it for two genes affecting different traits produce
demonstrates that the alleles encoding a trait do not alter one offspring with a phenotypic ratio of 9 : 3 : 3 : 1.
another when they are present together in a heterozygous
genotype (except in very rare instances). The recessive trait,
masked in one generation, can appear in the next, looking exactly
as it did in the true-breeding strains. Mendel fully appreciated the
significance of this discovery. In one of his letters, he emphasized Plant grown from Plant grown from
that “the two parental traits appear, separated and unchanged, and true-breeding true-breeding
strain with yellow strain with green
there is nothing to indicate that one of them has either inherited and wrinkled seeds and round seeds
or taken over anything from the other.” In other words, no hint
of any sort of blending between the parental genetic material
takes place and therefore no homogenization of the trait in the
population. Because the individual genes maintain their identity
down through the generations (except for rare mutations), genetic 
variation in a population also tends to be maintained through
time. The maintenance of genetic variation is discussed further
P1 generation
in Chapter 21.
(genotype AA bb) (genotype aa BB)

16.4 INDEPENDENT ASSORTMENT

We have seen that segregation of different alleles of a single gene
results in a 3 : 1 ratio of dominant : recessive phenotypes in the
F2 generation. What happens when the parental strains differ in
Because of dominance, the
two traits, for example when a strain having yellow and wrinkled seeds in the F1 generation
seeds is crossed with a strain having green and round seeds? The are yellow and round.
results of these kinds of experiments constitute Mendel’s second
key discovery, the principle of independent assortment. This
principle states that segregation of one set of alleles of a gene pair
is independent of the segregation of another set of alleles of a
different gene pair. That is, each pair of alleles assorts (segregates)
without affecting or being affected by the assortment of any other F1 generation
(genotype Aa Bb)
pair of alleles.

Independent assortment is observed when genes

segregate independently of one another.
In the cross between a strain with seeds that are yellow and
wrinkled and a strain with seeds that are green and round, the
phenotype of the F1 seeds is easily predicted. Because yellow is The expected ratio of the
dominant to green, and round is dominant to wrinkled, the four types of seeds is
9 yellow, round
F1 seeds are expected to be yellow and round, and in fact they are
3 green, round
(Fig. 16.12). When these seeds are grown and the F1 plants are 3 yellow, wrinkled
allowed to undergo self-fertilization, the result is as shown in 1 green, wrinkled
Fig. 16.12. Among 639 seeds from this cross, Mendel observed the
following:

yellow round 367

F2 generation
green round 122
yellow wrinkled 113
green wrinkled 37
336 SECTION 16.4 I N D E P E N D E N T A S S O RT M E N T
HOW DO WE KNOW?

The ratio of these phenotypes is 9.9 : 3.3 : 3.1 : 1.0, which Mendel FIG. 16.14
realized is close to 9 : 3 : 3 : 1. The latter ratio is that expected if the A
and a alleles for seed color undergo segregation and form gametes
independently of the B and b alleles for seed shape. How did Mendel
How are single-gene traits
come to expect a 9 : 3 : 3 : 1 ratio of phenotypes? For seed color alone
we expect a ratio of ¾ yellow : ¼ green, and for seed shape alone we
inherited?
expect a ratio of ¾ round : ¼ wrinkled. If the traits are independent, BACKGROUND Gregor Mendel’s experiments, carried out in the
then we can use the multiplication rule to predict the outcomes for years 1856–1863, are among the most important in all of biology.
both traits:
EXPERIMENTS Mendel set out to improve upon previous research
yellow round (¾) × (¾) = 9/16
in heredity. He writes that “among all the numerous experiments
green round (¼) × (¾) = 3/16
made, not one has been carried out to such an extent and in such
yellow wrinkled (¾) × (¼) = 3/16
a way as to make it possible to determine the number of different
green wrinkled (¼) × (¼) = 1/16
forms under which the offspring of the hybrids appear, or to arrange
these forms with certainty according to their separate generations, or
Note that 9/16 : 3/16 : 3/16 : 1/16 is equivalent to 9 : 3 : 3 : 1.
definitely to ascertain their statistical relations.” By studying simple
The underlying reason for the 9 : 3 : 3 : 1 ratio of phenotypes
traits across several generations of crosses, Mendel observed how
in the F2 generation is that the alleles for yellow versus green
these traits were inherited.
and those for round versus wrinkled are assorted into gametes
independently of each other. In other words, the hereditary RESULTS Mendel concluded that the “statistical relations” were
transmission of either gene has no effect on the hereditary very clear. Crosses between plants that were hybrids of a single trait
transmission of the other. A Punnett square depicting independent displayed two phenotypes in a ratio of 3:1. Crosses between plants
assortment is shown in Fig. 16.13. The A and a alleles segregate that were hybrids of two traits displayed four different phenotypes in
equally into gametes as ½ A : ½ a, and likewise the B and b a ratio of 9:3:3:1.
alleles segregate equally into gametes as ½ B : ½ b. The result of
HYPOTHESIS From observing these ratios among several different
independent assortment is that the four possible gametic types are
traits, Mendel made two key hypotheses about the inheritance of
produced in equal proportions:
traits, now called Mendel’s laws:

1. The principle of segregation states that individuals inherit two

copies (alleles) of each gene, one from the mother and one from
the father, and when the individual forms reproductive cells, the
FIG. 16.13 Independent assortment of two genes. The Punnett
two copies separate (segregate) equally in the eggs or sperm.
square reveals the underlying phenotypic ratio of
9 : 3 : 3 : 1. Hypothesis: Segregation
Pollen gametes A a
½ ½
AB Ab aB ab
¼ ¼ ¼ ¼
A
½ AA Aa
AB ¼ ¼
¼ AA BB AA Bb Aa BB Aa Bb
1/16 1/16 1/16 1/16
a
½ Aa aa
Ab
¼ ¼
Ovule gametes

¼ AA Bb AA bb Aa Bb Aa bb
1/16 1/16 1/16 1/16

aB
¼ Aa BB Aa Bb aa BB aa Bb
1/16 1/16 1/16 1/16
AB gametes (½) × (½) = ¼
Ab gametes (½) × (½) = ¼
ab
¼ aB gametes (½) × (½) = ¼
Aa Bb Aa bb aa Bb aa bb
1/16 1/16 1/16 1/16
ab gametes (½) × (½) = ¼

There are 9 possible genotypes and 4 possible As the Punnett square in Fig. 16.13 shows, random union of
phenotypes. The ratio of phenotypes is 9:3:3:1.
these gametic types produces the expected ratio of 9 yellow round,
 CHAPTER 16 M E N D E L I A N I N H E R I TA N C E 337

2. The principle of independent assortment states that the two 2. The principle of independent assortment: Although the
copies of each gene segregate into gametes independently of the Punnett square for two pairs of alleles has 16 squares, there are
two copies of another gene. only 9 genotypes, and the hypothesis of independent assortment
predicts that these genotypes should appear in the ratio of
Hypothesis: Independent assortment
1 AA BB : 2 AA Bb : 1 AA bb : 2 Aa BB; 4 Aa Bb : 2 Aa bb : 1 aa BB :
AB Ab aB ab
¼ ¼ ¼ ¼ 2 aa Bb : 1 aa bb. As before, Mendel tested this hypothesis by
self-fertilization of plants grown from the F2 seeds, classifying
AB any that bred true for either trait as homozygous and any that
¼ AA BB AA Bb Aa BB Aa Bb
1/16 1/16 1/16 1/16
segregated to yield both dominant and recessive phenotypes as
heterozygous. In the experimental test, once again the observed
Ab
¼ AA Bb AA bb Aa Bb Aa bb numbers fit the expected values within the margins that would be
1/16 1/16 1/16 1/16
expected by chance.
aB
¼ Aa BB Aa Bb aa BB aa Bb Experimental test
1/16 1/16 1/16 1/16

ab AA BB AA Bb AA bb
¼ Aa Bb Aa bb aa Bb aa bb
1/16 1/16 1/16 1/16
Observed 38 60 28
Expected 33 66 33
ANALYSIS Mendel found support for his two hypotheses in the
statistical analysis of the results of his meticulous crosses.
Aa BB Aa Bb Aa bb
1. The principle of segregation: A prediction of the hypothesis of
Observed 65 138 68
segregation is that, among seeds with the dominant phenotype,
Expected 66 132 66
the ratio of homozygous to heterozygous genotypes should be
1: 2. Mendel tested this prediction in several ways, one of which
was simply to allow plants grown from F2 seeds to self-fertilize. aa BB aa Bb aa bb
Any that were true breeding for the dominant phenotype he
classified as homozygous, and any that segregated to yield both Observed 35 67 30
Expected 33 66 33
dominant and recessive phenotypes he classified as heterozygous.
In the experimental test, the observed numbers fit the expected
values within the margins that would be expected by chance. FOLLOW-UP WORK Mendel’s work was ignored during his
lifetime, and its importance was not recognized until 1900,
Experimental test
16 years after his death. The rediscovery marks the beginning of
the modern science of genetics.

AA Aa SOURCE Mendel’s paper in English is available at http://www.mendelweb.org/

Mendel.html.
Observed 166 353
Expected 173 346

3 green round, 3 yellow wrinkled, and 1 green wrinkled. Fig. 16.14 of homologous chromosomes align randomly on the metaphase
summarizes how Mendel’s experiments led him to formulate his plate in meiosis I. For some pairs of chromosomes, the maternal
two laws. chromosome goes toward one pole during anaphase I, and the
paternal chromosome goes to the other pole, but for other pairs, just
Independent assortment reflects the random the opposite occurs. Because the alignment is random, gene pairs on
alignment of chromosomes in meiosis. different chromosomes assort independently of one another.
Independent assortment of genes on different chromosomes results Fig. 16.15 illustrates two possible alignments that are equally
from the mechanics of meiosis (Chapter 11), in which different pairs likely. In one alignment, the B allele (dark red) goes to the same

337
338 SECTION 16.4 I N D E P E N D E N T A S S O RT M E N T

FIG. 16.15 Independent assortment of genes in different FIG. 16.16 Epistasis, the interaction of genes affecting the
chromosomes. Chromosomes are sorted into daughter same trait. Epistasis can modify the 9 : 3 : 3 : 1 ratio of
cells randomly during meiosis, resulting in independent phenotypes, in this example to 13 : 3.
assortment of genes.
The White Leghorn is white The White Wyandotte is
because the inhibitor allele white because the pigment
Independent assortment of genes in different chromosomes reflects I blocks expression of the allele c does not produce
the fact that nonhomologous chromosomes can orient in either of two pigment allele C. pigment.
ways that are equally likely.

A A b
ⴛ
B
A B A b P1 generation

CC I I cc i i
a b a B
a B
a b
ⴛ
F1 generation
Anaphase I Anaphase I
Cc I i Cc I i

Meiosis II Meiosis II F2 generation

CI Ci cI ci
Resulting gametes Resulting gametes ¼ ¼ ¼ ¼

A A A A CI
¼
B B b b CC I I CC I i Cc I I Cc Ii

a a a a
Ci
¼
b b B B CC Ii CC ii Cc Ii Cc ii

cI
¼
Cc II Cc Ii cc II cc Ii
pole as the A allele (dark blue), and, in the other alignment, the
ci
b allele (light red) goes in the same direction as the A allele. The first ¼
type of alignment results in a 1 : 1 ratio of AB : ab gametes, and the Cc Ii Cc ii cc Ii cc ii
second type of alignment results in a 1 : 1 ratio of Ab : aB gametes.
Because the two orientations are equally likely, the overall ratio Genotypes of the form C– ii have colored
feathers, whereas all other genotypes have
of AB : ab : Ab : aB from a large number of cells undergoing meiosis white feathers. The result is an F2 ratio of
is expected to be 1 : 1 : 1 : 1. This is the principle of independent white : colored of 13:3, which is a modified
assortment for genes located in different chromosomes. form of the expected 9:3:3:1.

Not all genes undergo independent assortment. For example,

Alleles
genes that are sufficiently close together in one chromosome
C Pigment
do not assort independently of one another. Genes in the same c No pigment
chromosome that fail to show independent assortment are said to I Inhibitor
i No inhibitor
be linked and are discussed in Chapter 17.

Phenotypic ratios can be modified by interactions

between genes. dominance, the ratio of phenotypes may be different if the two
The 9 : 3 : 3 : 1 ratio results from independent assortment of two genes affect the same trait. This often happens when the genes
genes when one allele of each gene is dominant and when the code for proteins that act in the same biochemical pathway. In
two genes affect different traits. However, even with complete such cases, the gene products can interact to affect the phenotypic
 CHAPTER 16 M E N D E L I A N I N H E R I TA N C E 339

expression of the genotypes, resulting in a modification of the

expected ratio. Genes that modify the phenotypic expression of FIG. 16.17 Some conventions used in drawing human pedigrees.
other genes are said to show epistasis.
Horizontal line between
There are many types of epistasis leading to different individuals represents mating.
modifications of the 9 : 3 : 3 : 1 ratio. Among the more common Generation
Vertical line leads
modified ratios are 12 : 3 : 1, as well as 9 : 3 : 4 and 13 : 3. Fig. 16.16 I to progeny.
Progeny are
shows one example, in which the F2 generation of a cross arranged
between White Leghorn and White Wyandotte chickens horizontally,
displays the modified ratio 13 : 3. Both breeds are white, but II left to right
in order of
for different genetic reasons. There are two genes involved in birth.
pigment production, each with two alleles. The C gene encodes III
a protein that affects coloration in feathers. The dominant allele
C produces pigment, and the recessive allele c does not produce Double line means
IV mating between
pigment. A different gene, I, codes for an inhibitor protein. The
Identical twins Nonidentical twins relatives.
product of the dominant allele, I, inhibits the expression of C,
whereas the recessive allele, i, does not produce the inhibitor
Female
and so does not inhibit the expression of C. Therefore, the White
Male
Leghorn (genotype CC II) is white because the product of the
dominant allele I inhibits the pigment in the feathers, and the Open symbol means not affected.
White Wyandotte (genotype cc ii) is white because the recessive Darkened symbol means affected.
allele c does not produce feather pigment to begin with. The F1
generation has genotype Cc Ii and is also white. With independent
assortment, only the three C – ii offspring have colored feathers in
how to recognize telltale features of a pedigree that reveal the
the F2 generation (the dash indicates that the second allele could
genotypic nature of a trait.
be either C or c), while the rest have white feathers, and so the
ratio of white : colored is 13 : 3.
Dominant traits appear in every generation.
j Quick Check 5 Why do the F1 chickens with genotype Cc Ii have The pedigree shown in Fig. 16.18 is for a rare dominant trait,
white feathers? brachydactyly, in which the middle long bone in the fingers
fails to grow and therefore the fingers remain very short. This
pedigree, published in 1905, was the first demonstration of
dominant Mendelian inheritance in humans. This particular form
16.5 PATTERNS OF INHERITANCE
of brachydactyly results from a mutation in a gene whose normal
OBSERVED IN FAMILY HISTORIES product is a protein involved in cartilage formation, which is
necessary for bone growth.
Segregation of alleles takes place in human meiosis just as it
These are the features of the pedigree that immediately
does in peas and most other sexual organisms. The results are
suggest dominant inheritance:
not so easily observed as in peas, however, for several reasons.
First, humans do not choose their mating partners for the 1. Affected individuals are equally likely to be females or
convenience of biologists, and so experimental crosses are not males.
possible. Second, the number of children in human families is
2. Most matings that produce affected offspring have only
relatively small, and so the Mendelian ratios are often obscured by
one affected parent. This occurs because the brachydactyly
random fluctuations due to chance. Nevertheless, in many cases
trait is rare, and therefore a mating between two affected
the occurrence of segregation can be observed even in human
individuals is extremely unlikely.
families. The patterns are often easiest to spot for inherited traits
that are rare because in those cases the mutant alleles occur only 3. Among matings in which one parent is affected,
in affected individuals and their close relatives (who may not approximately half the offspring are affected.
themselves be affected).
In studying human families, the record of the ancestral If a dominant trait is rare, then affected individuals will almost
relationships among individuals is summarized in a diagram of always be heterozygous (Aa), not homozyogous (AA). In a mating
family history called a pedigree. Some typical symbols used in in which one parent is heterozygous for the dominant gene (Aa)
pedigrees are shown in Fig. 16.17. The same patterns of dominance and the other is homozygous recessive (aa), half the offspring are
and recessiveness that Mendel observed in his pea plants can be expected to be heterozygous (Aa) and the other half homozygous
seen in some pedigrees. Over the next few sections, we’ll see recessive (aa).
340 SECTION 16.5 PAT T E R N S O F I N H E R I TA N C E O B S E RV E D I N FA M I LY H I S TO R I E S

FIG. 16.18 Pedigree of a trait caused by a dominant allele. This pedigree shows the inheritance of shortened fingers associated with a form of
brachydactyly (inset). Photo source: Stefan Mundlos.

For a rare dominant trait, From matings in which

most matings that produce one parent is affected,
affected offspring have approximately half the
only one affected parent. offspring are affected.

Affected individuals Affected individuals

appear in each are equally likely to be
successive generation. females or males.

Recessive traits skip generations. in Fig. 16.19. For a recessive trait that is sufficiently rare,
Recessive inheritance shows a pedigree pattern very different from virtually all affected individuals have unaffected parents.
that of dominant inheritance. The pedigree shown in Fig. 16.19
4. Affected individuals often result from mating between
pertains to albinism, in which the amount of melanin pigment
relatives, typically first cousins.
in the skin, hair, and eyes is reduced. In most populations, the
frequency of albinism is about 1 in 36,000, but it has a much
Recessive inheritance has these characteristics because
higher frequency—about 1 in 200—among the Hopi and several
recessive alleles can be transmitted from generation to generation
other Native American tribes of the Southwest. (It is not unusual
without manifesting the recessive phenotype. In order for an
for genetic diseases to have elevated frequencies among isolated
affected individual to occur, the recessive allele must be inherited
populations.) This type of albinism is due to a mutation in the gene
from both parents. Mating between relatives often allows rare
OCA2, which encodes a membrane transporter protein thought
recessive alleles to become homozygous because an ancestor
to be important in transport of the amino acid tyrosine, which is
that is shared between the relatives may carry the gene ( Aa). The
used in the synthesis of the melanin pigment responsible for skin,
recessive allele in the common ancestor can be transmitted to both
hair, and eye color.
parents, making them each a carrier of the allele as well. If both
As noted in Chapter 15, another type of mutation affecting
parents are unaffected carriers of the allele, they both have the
expression of this same gene is associated with blue eyes. As shown
genotype Aa, and therefore ¼ of their offspring are expected to be
in Fig. 16.19, double lines represent matings between relatives, and
homozygous aa and affected.
in both cases shown here the mating is between first cousins.
These are the principal pedigree characteristics of recessive
traits: Many genes have multiple alleles.
The examples discussed so far in this chapter involve genes
1. The trait may skip one or more generations. with only two alleles, such as A for yellow seeds and a for green
seeds. Similarly, as noted in Chapter 15, most single-nucleotide
2. Affected individuals are equally likely to be females or
polymorphisms (SNPs) have only two alleles, differing only in
males.
which particular base pair is present at a particular position in
3. Affected individuals may have unaffected parents, as in the genomic DNA. On the other hand, because a gene consists of a
offspring of the second mating in the second generation sequence of nucleotides, any nucleotide or set of nucleotides in
 CHAPTER 16 M E N D E L I A N I N H E R I TA N C E 341

For example, more than 400 different recessive alleles that

FIG. 16.19 Pedigree of a trait caused by a recessive allele. cause phenylketonuria (PKU) have been discovered across the
The photograph is of a group of Hopi males, three with world. PKU is a moderate to severe form of mental retardation
albinism, which is caused by a recessive allele. Source: caused by mutations in the gene encoding the enzyme
BAE GN 02458C 06404400, National Anthropological Archives, phenylalanine hydroxylase. Children affected with PKU are unable
Smithsonian Institution. to break down the excess phenylalanine present in a normal diet,
and the buildup impairs the development of neurons in the brain.
About 1 in 10,000 newborns inherits two mutant alleles, which
could be two copies of the same mutant allele or two different
mutant alleles, and is affected.
The “normal” form of the gene encoding phenylalanine
hydroxylase also exists in the form of multiple alleles, each
of which differs from the others, but nevertheless encodes a
functional form of the enzyme.

j Quick Check 6 How is it possible that there are multiple different

alleles in a population and yet any individual can have only two
alleles?

Incomplete penetrance and variable expression can

obscure inheritance patterns.
A rare recessive trait
may skip one or Many traits with single-gene inheritance demonstrate complications
more generations. that can obscure the expected patterns in pedigrees. Chief among
these are traits with incomplete penetrance, which means that
individuals with a genotype corresponding to a trait do not actually
show the phenotype, either because of environmental effects
or because of interactions with other genes. Penetrance is the
proportion of individuals with a particular genotype that show the
expected trait. If the penetrance is less than 100%, then the trait
shows reduced, or incomplete, penetrance. Familial cancers often
show incomplete penetrance. For example, retinoblastoma caused
Affected individuals Affected individuals Affected individuals by certain mutations in the Rb gene and breast cancer caused by
are equally likely to often result from can have parents certain mutations in the BRCA1 and BRCA2 genes are incompletely
be females or males. mating between who are not affected. penetrant. That is, some people who inherit a mutation in these
relatives.
genes that predisposes to cancer do not in fact develop cancer.
Similarly, some mutations in the apoliprotein E (APOE) gene increase
the risk of developing Alzheimer’s disease. However, not everyone
the gene can undergo mutation. Each of the mutant forms that who inherits these forms of the gene develops Alzheimer’s disease,
exists in a population constitutes a different allele, and hence a so these alleles are also incompletely penetrant.
population of organisms may contain many different alleles of the Another common complication in human pedigrees
same gene, which are called multiple alleles. Some genes have so is variable expressivity, which means that a particular
many alleles that they can be used for individual identification by phenotype is expressed with a different degree of severity in
means of DNA typing, such as the VNTR (variable number tandem different individuals. Don’t confuse variable expressivity with
repeat) sequences discussed in Chapter 15. incomplete penetrance. With variable expressivity, the trait is
In considering genetic diseases, multiple alleles are often always expressed, though the severity varies; with incomplete
grouped into categories such as “mutant” and “normal.” But penetrance, the trait is sometimes expressed and sometimes not.
there are often many different “mutant” and many different Variation among individuals in the expression of a trait can result
“normal” alleles in a population. For the “mutant” alleles, the from the action of other genes, from effects of the environment,
DNA sequences are different from one another, but each produces or both. An example is provided by deficiency of the enzyme
a protein product whose function is impaired under the usual alpha-1 antitrypsin (a1AT) discussed in Chapter 15, which is
environmental conditions. For the “normal” alleles, the DNA associated with loss of lung elasticity and emphysema. Among
sequences are also different, but they all are able to produce individuals with emphysema due to a1AT deficiency, the severity
functional protein. of the symptoms varies dramatically from one patient to the next.
 342 CO R E CO N C E P T S S U M M A RY

In this case, tobacco smoking is an environmental factor that BRCA2 for breast cancer can have frequent mammograms, and those
increases the severity of the disease. with the TCF7L2 risk factor for type 2 diabetes can decrease their
Incomplete penetrance and variable expressivity both provide risk by lifestyle choices that include weight control and exercise.
examples where a given genotype does not always produce the While there are many potential benefits to genetic testing,
same phenotype, since the expression of genes is often influenced there are also some perils. One major concern is maintaining the
by other genes, the environment, or a combination of the two. privacy of those who choose to be tested. With medical records

? CASE 3 YOU, FROM A TO T: YOUR PERSONAL GENOME increasingly going online, who will have access to your test results,
and how will this information be used? Could your test results be
How do genetic tests identify disease risk factors? used to deny you health or life insurance because you have a higher
Your personal genome, as well as that of every human being, than average risk of some medical condition? Or could an employer
contains a unique combination of alleles of thousands of different who got hold of your genetic test results decide to reassign you
genes. Most of these have no detectable effects on health or to another job, or even eliminate your position because of your
longevity, but many are risk factors for genetic diseases. Molecular genetic predispositions? There are some safeguards designed to
studies have discovered particular alleles of genes associated with a protect you from such discrimination. The Genetic Information
large number of such conditions, and the presence of these alleles Nondiscrimination Act (GINA) was signed into law in 2008 and
can be tested. More than a thousand genetic tests have already forbids the use of genetic information in decisions concerning
been deployed, and many more are actively being developed. employment and health insurance. The protection provided by
A genetic test is a method of identifying the genotype of an GINA will, it is hoped, allow for the responsible and productive use
individual. The tests may be carried out on entire populations or of genetic information.
restricted to high-risk individuals. There is also increasing concern about the reliability and
The benefits of genetic testing can be appreciated by an accuracy of genetic tests, especially direct-to-consumer (DTC)
example. Screening of newborns for phenylketonuria identifies genetic tests. DTC tests can be purchased directly without the
babies with high blood levels of phenylalanine. In the absence of intervention of medical professionals. Since the consumer sends a
treatment, 95% of such newborns will progress to moderate or biological sample and DTC tests are carried out by the provider, the
severe mental retardation, whereas virtually all those placed on a tests are not regarded as medical devices and so are unregulated.
special diet with a controlled amount of phenylalanine will have One problem is that some DTC tests are based on flimsy and
mental function within the normal range. For recessive conditions unconfirmed evidence connecting a gene with a disease. Another
like phenylketonuria, tests can be carried out on people with is that the link between genotype and risk may be exaggerated
affected relatives to identify the heterozygous genotypes. Testing for marketing purposes. Yet another is lack of information
can also identify genetic risk factors for disease, and carriers can on quality control in the DTC laboratories. Finally, consumer
take additional precautions. For example, individuals with a1AT misinterpretation may regard genotype as destiny, at one extreme
deficiency can prolong and improve the quality of their lives by descending into depression and despair, and at the other using a
not smoking tobacco, women with genetic risk factors BRCA1 and low-risk genotype to justify an unhealthy lifestyle. •

Core Concepts Summary

16.1 The earliest theories of heredity incorrectly 16.2 The study of modern transmission genetics
assumed the inheritance of acquired characteristics and began with Gregor Mendel, who used the garden pea
blending of parental traits in the offspring. as his experimental organism and studied traits with
contrasting characteristics.
The inheritance of acquired characteristics suggests that
traits that develop during the lifetime of an individual can Mendel started his experiments with true-breeding plants,
be passed on to offspring. With rare exceptions, this mode of ones whose progeny are identical to their parents. He followed
inheritance does not occur. page 326 just one or two traits at a time, allowing him to discern simple
patterns, and he counted all the progeny of his crosses. page 327
Blending inheritance is the incorrect hypothesis that
characteristics in the parents are averaged in the offspring. In crosses of one true-breeding plant with a particular trait
This model predicts the blending of genetic material, and another true-breeding plant with a contrasting trait, just
which does not occur. Different forms of a gene maintain one of the two characteristics appeared in the offspring. The
their separate identities even when present together in the trait that appeared in this generation is dominant, and the
same individual. page 326 trait that is not seen is recessive. page 328
 CHAPTER 16 M E N D E L I A N I N H E R I TA N C E 343

Mendel explained this result by hypothesizing that there orientation of different chromosomes on the meiotic
is a hereditary factor for each trait (now called a gene); that spindle. page 337
each pea plant carries two copies of the gene for each trait;
In some cases, genes interact with each other, modifying the
and that one of two different forms of the gene (alleles) is
expected ratios in crosses. Epistasis is a gene interaction in
dominant to the other one. page 329
which one gene affects the expression of another. page 338

16.3 Mendel’s first key discovery was the principle

16.5 The patterns of inheritance that Mendel
of segregation, which states that members of a gene
observed in peas can also be seen in humans.
pair separate equally into gametes.
In a human pedigree, females are represented by circles
When Mendel allowed the progeny of the first cross to self-
and males as squares; affected individuals are shown as
fertilize, he observed a 3 : 1 ratio of the dominant and recessive
filled symbols and unaffected individuals as open symbols.
traits among the progeny. page 330
Horizontal lines denote matings. page 339
Mendel reasoned that the parent in this generation must
Dominant traits appear in every generation and affect males
have two different forms of the same gene (A and a) and
and females equally. page 339
that these alleles segregate from each other to form gametes
with each gamete getting A or a but not both. When the Recessive traits skip one or more generations and affect males
gametes combine at random, they produce progeny in the and females equally. Affected individuals often result from
genotypic ratio 1 AA : 2 Aa : 1 aa, which yields a phenotypic matings between close relatives. page 340
ratio of 3 : 1 because A is dominant to a. This idea became
Although a given individual has only two alleles of each gene,
known as the principle of segregation. page 330
there can be many alleles of a particular gene in the population
The principle of segregation reflects the separation of as a whole. page 340
homologous chromosomes that occurs in anaphase I of Interpreting pedigrees can be difficult because of incomplete
meiosis. page 332 penetrance and variable expressivity. Penetrance is the
Some traits show incomplete dominance, in which the percentage of individuals with a particular genotype who
phenotype of the heterozygous genotype is intermediate show the expected phenotype, and expressivity is the degree
between those of the two homozygous genotypes. to which a genotype is expressed in the phenotype. page 341
page 333 Genetic testing enables the genotype of an individual to be
The expected frequencies of progeny of crosses can be determined for one or more genes. Such tests carry both
predicted using the addition rule, which states that when benefits and risks. page 342
two possibilities are mutually exclusive, the probability
of either event occurring is the sum of their individual
probabilities; and the multiplication rule, which states Self-Assessment
that when two possibilities occur independently, the 1. In his famous paper, Mendel writes that he set out to
probability of both events occurring is the product of the “determine the number of different forms in which
probabilities of each of the two events. page 333 hybrid progeny appear” and to “ascertain their numerical
interrelationships.” How did his close attention to
16.4 Mendel’s second key finding was the principle numbers lead him to discover segregation and independent
of independent assortment, which states that different assortment?
gene pairs segregate independently of one another.
2. Distinguish among gene, allele, genotype, and
In a cross with two traits, each with two contrasting phenotype.
characteristics, the two traits behave independently of
3. Name and describe Mendel’s two laws.
each other. This idea became known as the principle of
independent assortment. For example, in a self-cross of a 4. Explain how the mechanics of meiosis and the
double heterozygote, the phenotypic ratio of the progeny is movement of homologous chromosomes underlie Mendel's
9 : 3 : 3 : 1, reflecting the independent assortment of two principles of segregation and independent assortment.
3 : 1 ratios. page 335
5. Explain how you can predict the genotypes and
Independent assortment results from chromosome phenotypes of offspring if you know the genotypes of the
behavior during meiosis, specifically from the random parents.
344 SELF-ASSESSMENT

6. Describe an instance in which you would use a 9. Construct a human pedigree for a dominant and a
testcross, and why. recessive trait and explain the patterns of inheritance.

7. Define the multiplication and addition rules, and 10. Discuss the benefits and risks of genetic testing and
explain how these rules can help you predict the outcome personal genomics.
of a cross between parents with known genotypes.

8. What are some reasons why a single trait might not Log in to to check your answers to the Self-
Assessment questions, and to access additional learning tools.
show a 3 : 1 ratio of phenotypes in the F2 generation of a
cross between true-breeding strains, and why a pair of
traits might not show a 9 : 3 : 3 : 1 ratio of phenotypes in the
F2 generation of a cross between true-breeding strains?
 CHAPTER 17

Inheritance of Sex
Chromosomes,
Linked Genes,
and Organelles
Core Concepts
17.1 Many organisms have
a distinctive pair of
chromosomes, often called
the X and Y chromosomes,
that differ between the sexes
and show different patterns
of inheritance in pedigrees
from other chromosomes.
17.2 X-linked genes, which show
a crisscross inheritance
pattern, provided the first
evidence that genes are
present in chromosomes.
17.3 In genetic linkage, two
genes are sufficiently
close together in the same
chromosome that the
particular combination
of alleles present in the
chromosome tends to remain
together in inheritance.
17.4 Most Y-linked genes are
passed from father to son.
17.5 Mitochondria and chloroplast
DNA follow their own
inheritance pattern.
SSPL/Getty Images.

345
346 SECTION 17.1 T H E X A N D Y S E X C H RO M O S O M E S

Mendel’s principles of segregation and independent assortment Reciprocal crosses do not produce the same types and numbers
are the foundation of transmission genetics (Chapter 16). For traits of progeny. In one type of cross, when a color-blind man mates
such as pea color and seed shape that are encoded by single genes with a woman who is not color blind, all of the sons and daughters
and display simple dominance, these principles predict simple have normal color vision. However, in the reciprocal cross, when a
phenotypic ratios in the progeny from self-crosses of heterozygous color-blind woman mates with a man who is not color blind, all of
genotypes. The principle of segregation also defines the inheritance the daughters have normal color vision but all of the sons are color
patterns expected in human pedigrees for traits due to recessive blind. For the X and Y sex chromosomes, reciprocal crosses are not
or dominant mutations, as we saw in the case of albinism and equivalent.
brachydactyly.
However, we also saw in Chapter 16 that not all crosses are as In many animals, sex is genetically determined
simple as those for Mendel’s pea plants. For example, the and associated with chromosomal differences.
3 : 1 phenotypic ratio in progeny of self-crosses of heterozygotes is Most chromosomes come in pairs that match in shape and size.
1 : 2 : 1 in the case of alleles that show incomplete dominance. The The members of each pair are known as homologous chromosomes
9 : 3 : 3 : 1 ratio of phenotypes observed for two genes that show because they have the same genes along their length (Chapter 11).
independent assortment can be altered by epistasis, producing One member of each pair of homologous chromosomes is
ratios such as 12 : 3 : 1 or 13 : 3. None of these is an exception to inherited from the mother and the other from the father.
Mendel’s laws since his laws reflect chromosome movement In many animal species, however, the sex of an individual is
during meiosis (Chapter 11). Instead, they reflect how genes are determined by a distinctive pair of unmatched chromosomes
expressed or how different genes interact to produce a phenotype. known as the sex chromosomes, which are usually designated
This chapter highlights additional patterns of inheritance as the X chromosome and the Y chromosome (Chapter 13).
that Mendel did not observe owing to his choice of experimental Chromosomes other than the sex chromosomes are known as
organism and the traits he studied. None of these patterns autosomes.
undermines or invalidates his insight that alternative alleles of a In humans, a normal female has two copies of the
gene can have different effects on the expression of a phenotype. X chromosome (a sex-chromosome constitution denoted XX), and
Nor do they contradict later discoveries that genes in the nucleus a normal male has one X chromosome and one Y chromosome
are present in homologous chromosomes that pair and segregate ( XY). The sizes of the human X and Y chromosomes are very
in meiosis. Since Mendel’s time, researchers have observed many different from each other (Fig. 17.1). The X chromosome DNA
inheritance patterns that seem to defy one or both of Mendel’s
laws. What such examples reveal is that the location of a gene is
as important to our predictions about the inheritance of a trait as
whether its alleles are dominant or recessive. FIG. 17.1 Human sex chromosomes. The human X and Y sex
In this chapter, we discuss how genes carried in the sex chromosomes differ in size and number of genes. Source:
chromosomes are transmitted differently in males and in females Science Photo Library/Science Source.
and how genes close to each other in the same chromosome do not
undergo independent assortment and therefore violate Mendel’s Almost none of the genes
in the X chromosome
second law. Genes located in the genomes of mitochondria and have counterparts in the
chloroplast appear to defy Mendel’s laws altogether since the Y chromosome.
organelles are inherited differently from the way chromosomes are The tips of the arms of the
X
inherited. Such unique patterns of inheritance expand the types of X and Y chromosomes
ratios and predictions we saw in Chapter 16 and draw our attention share a small region of
Y
homology (red).
to the location and organization of genes—in specific chromosomes,
relative to other genes, or outside the nucleus altogether.

17.1 THE X AND Y SEX

CHROMOSOMES
Mendel concluded from his experiments that reciprocal crosses
yield the same types of progeny in the same proportions. This is in
most cases true, but an important exception occurs with the
X and Y sex chromosomes. For example, red–green color blindness
in humans is due to mutant alleles of a gene in the X chromosome.
 CHAPTER 17 I N H E R I TA N C E O F S E X C H RO M O S O M E S , L I N K E D G E N E S , A N D O RG A N E L L E S 347

molecule is more than 150 Mb long, while the Y chromosome is

only about 50 Mb long. Except for a small region near each tip, the FIG. 17.2 Inheritance of the X and Y chromosomes. Segregation of
gene contents of the X and Y chromosomes are different from each the sex chromosomes in meiosis and random fertilization
other. The X chromosome, which includes more than 1000 genes, result in a 1:1 ratio of female : male embryos.
has a gene density similar to that of most autosomes. The vast
majority of these genes have no counterpart in the Y chromosome. Meiosis in a
In contrast to the X chromosome, the Y chromosome contains female results
in X-bearing
only about 50 protein-coding genes. eggs only.
The regions of homology between the X and Y chromosomes
consist of about 2.7 Mb of DNA near the tip of the short arm ½ ½
and about 0.3 Mb of DNA near the tip of the long arm. These X X
regions of homology allow the chromosomes to pair during Meiosis in a male
meiosis (Chapter 11). In most cells undergoing meiosis, a results in a 1:1
ratio of X-bearing X Eggs X
crossover (physical breakage, exchange of parts, and rejoining and Y-bearing
of the DNA molecules) occurs in the larger of these regions. The sperm.
crossover allows the chromosomes to move as a unit to align
properly at metaphase I so that when their centromeres separate ½
from each other at anaphase I the X and Y chromosomes go to
opposite poles. X X X X X
The relative size and gene content of the X and Y chromosomes Female Female
Sperm
differ greatly among species. In some species of mosquitoes, the
X and Y chromosomes are virtually identical in size and shape, and X Y
the regions of homology include almost the entire chromosome. ½
This situation is unusual, however. In most species, the Y chromo-
some contains many fewer genes than the X chromosome. Some
Y X Y X Y
species, like grasshoppers, have no Y chromosome : Females in Male Male
these species have two X chromosomes, and males have only one
Random fertilization results in an expected
X chromosome. The total number of chromosomes in grasshoppers ratio of ½ XX (female) and ½ XY (male) progeny.
therefore differs between females and males. In birds, moths,
and butterflies, the sex chromosomes are reversed: Females have
two different sex chromosomes and males have two of the same
sex chromosome.
shows a slight excess of males. In the United States, for example,
Segregation of the sex chromosomes predicts a the secondary sex ratio is approximately 100 females : 105 males.
1:1 ratio of females to males. The explanation seems to be that, for reasons that are not
It is ironic that Mendel did not interpret sex as an inherited trait. entirely understood, females are slightly less likely to survive
If he had, he might have realized that sex itself provides one of from conception to birth. On the other hand, males are slightly
the most convincing demonstrations of segregation. In human less likely to survive from birth to reproductive maturity, and so,
males, segregation of the X chromosome from the Y chromosome at the age of reproductive maturity, the sex ratio is very nearly
during anaphase I of meiosis results in half the sperm bearing 1 : 1. Male mortality continues to be greater than that of females
an X chromosome and the other half bearing a Y chromosome throughout life, so that by age 85 and over, the sex ratio is about
(Fig. 17.2). Meiosis in human females results in eggs that each 100 females : 50 males.
contains one X chromosome. With random fertilization, as The sex of each birth appears to be random relative to previous
shown in Fig. 17.2, half of the fertilized eggs are expected to be births—that is, there do not seem to be tendencies for some
chromosomally XX (and therefore female) and half are expected to families to have boys or for other ones to have girls. Many people are
be chromosomally XY (and therefore male). surprised by this fact, for they know of one or more large families
The expected ratio of 1 : 1 of females : males refers to the sex consisting mostly of boys or mostly of girls. But the occasional
ratio at the time of conception. This is the primary sex ratio, and family with children of predominantly one sex is expected simply
it is not easily observed because the earliest stages of human by chance. When one occurs, its unusual sex distribution commands
fertilization and development are inaccessible to large-scale attention out of proportion to the actual numbers of such families.
study. What can be observed is the sex ratio at birth, called the In short, families with unusual sex distributions are not more
secondary sex ratio, which differs among populations but usually frequent than would be expected by chance.
348 SECTION 17.2 I N H E R I TA N C E O F G E N E S I N T H E X C H RO M O S O M E

17.2 INHERITANCE OF GENES IN FIG. 17.3 Morgan’s white-eyed fly. Morgan’s discovery of
THE X CHROMOSOME X-linked genes derived from his crosses of a white-eyed
male in 1910.
Genes in the X chromosome are called X-linked genes. These Parental generation
genes have a unique pattern of inheritance first discovered by
Thomas Hunt Morgan in 1910. Morgan’s pioneering studies of
genetics of the fruit fly Drosophila melanogaster helped bring
Mendelian genetics into the modern era. The discovery of X-linked X
inheritance was not only important in itself, but also provided the
first experimental evidence that chromosomes contain genes. Red-eyed White-eyed
female male
X-linked inheritance was discovered through studies
of male fruit flies with white eyes.
Morgan’s discovery of X-linked genes began when he noticed a F1 generation
white-eyed male in a bottle of fruit flies in which all the others
had normal, or wild-type, red eyes. (The most common phenotype
in a population is often called the wild type.) This was the first All of the progeny have
X red eyes.
mutant he discovered, and finding it was a lucky break. As we saw
in Chapter 16, most mutations are recessive, which means that
when they occur their effect on the organism (the phenotype) is Red-eyed Red-eyed
female male
not observed because of the presence of the nonmutant gene in
the homologous chromosome. Recall that the nonmutant form of
a recessive mutant gene is dominant, and that the different forms F2 generation
of the gene are alleles.
Morgan’s initial crosses are outlined in Fig. 17.3. In the first White eyes reappear in
the next generation,
generation, he crossed the mutant white-eyed male with a wild- but only in males. All
type red-eyed female. All of the progeny (F1) fruit flies had red eyes, of the females have
red eyes, and among
as you would expect from a cross with any recessive mutation. the males, the ratio of
Morgan then carried out matings between brothers and sisters Red-eyed Red-eyed White-eyed red:white eyes is 1:1.
among the F1 generation, and he found that the phenotype of female male male
white eyes reappeared among the progeny. This result, too, was
1:1
expected. However, there was a surprise: Morgan observed that the
white-eye phenotype was associated with the sex of the fly. In the F2
allele of this gene, the recessive mutation will be reflected in the
generation, all the white-eyed fruit flies were male, and the white-
male’s phenotype—in this case, white eyes.
eyed males appeared along with red-eyed males in a ratio of 1 : 1. No
The Punnett square in Fig. 17.4a explains why all the
females with white eyes were observed; all the females had red eyes.
offspring had red eyes when Morgan crossed the white-eyed
male with a red-eyed female in the parental generation. During
Genes in the X chromosome exhibit a “crisscross”
meiosis in the male, the mutant X chromosome segregates from
inheritance pattern.
the Y chromosome, and each type of sperm, X or Y, combines with
When Morgan did his crosses with the white-eyed male, the
a normal X-bearing egg. The result is that the female progeny are
X chromosome had only recently been discovered by microscopic
heterozygous. They have only one copy of the mutant allele, and
examination of the chromosomes in male and female
because the mutant allele is recessive, the heterozygous females
grasshoppers. Morgan was the first to understand that the pattern
do not express the mutant white-eye trait. The male progeny are
of inheritance of the X chromosome would be different from
also red-eyed because they receive their X chromosome from their
that of the autosomes, and he proposed the hypothesis that the
wild-type red-eyed mother.
white-eyed phenotype was due to a mutation in a gene in the
The Punnett square in Fig. 17.4a illustrates two important
X chromosome. This hypothesis could explain the pattern of
principles governing the inheritance of X-linked genes:
inheritance shown in Fig. 17.3.
The key features of X-linked inheritance are shown in Fig. 17.4. 1. The phenotypes of the XX offspring indicate that a male
In Drosophila, as in humans, females are XX and males are XY. transmits his X chromosome only to his daughters. In this
Fig. 17.4a shows an XY male in which the X chromosome contains case, the X chromosome transmitted by the male carries
a recessive mutation. Because the Y chromosome does not carry an the white-eye mutation.
 CHAPTER 17 I N H E R I TA N C E O F S E X C H RO M O S O M E S , L I N K E D G E N E S , A N D O RG A N E L L E S 349

2. The phenotypes of the XY offspring indicate

FIG. 17.4 X-linkage. (a) Cross between a homozygous nonmutant that a male inherits his X chromosome from
female and a male carrying an X-linked recessive allele his mother. In this case, the X chromosome
(red). (b) Cross between a heterozygous female and a transmitted by the mother carries the
nonmutant male. X-linked recessive alleles are expressed nonmutant allele of the gene.
in males because males have only one X chromosome.
a. These principles underlie a pattern often
referred to as crisscross inheritance: An
Most genes in the X X chromosome present in a male in one generation
chromosome have no
counterparts in the Y
must be transmitted to a female in the next
chromosome. Therefore, generation, and in the generation after that can
a recessive mutation in be transmitted back to a male. Therefore, an
an X-linked gene is
½ ½ X chromosome can “crisscross,” or alternate,
expressed in males.
X X between the sexes in successive generations.
Homozygous The mating illustrated in Fig. 17.4b shows
female
another important feature of X-linked inheritance,
Sperm X Eggs X one that explains the results of Morgan’s F1
cross. In this case, the mother is a heterozygous
female. During meiosis, the mutant allele and the
½ nonmutant allele undergo segregation. Half of the
resulting eggs contain an X chromosome with the
A male with an mutant allele, and half contain an X chromosome
X X X X X
X-linked recessive
Heterozygous female Heterozygous female with the nonmutant allele. These combine at
trait will have
X Y heterozygous random with either X-bearing sperm or Y-bearing
(carrier) daughters
Affected male
and normal sons.
sperm. Among the female progeny in the next
generation, half are heterozygous for the mutant
½
allele and the other half are homozygous for the
normal allele. Thus, none of the females exhibits
Y X Y X Y
Normal male Normal male
the white-eye trait because the mutation is
recessive. Among the male progeny, half receive
the mutant allele and have white eyes, whereas the
b.
other half receive the nonmutant allele and have
red eyes. The expected progeny from the cross in
Fig. 17.4b therefore consist of all red-eyed females,
½ ½ and there is a 1 : 1 ratio of red-eyed to white-eyed
X X males, which is what Morgan observed in the
Heterozygous F2 generation of his crosses, as shown in Fig. 17.3.
carrier female Knowing the patterns revealed by the Punnett
Sperm X Eggs X squares, we can now assign genotypes to Morgan’s
original crosses (Fig. 17.5). In Fig. 17.5, the white-
eyed males that Morgan used in his parental
½
generation crosses are given the genotype of w2 Y,
Among progeny and the red-eyed female has a genotype of w1w1.
from a heterozygous The symbol “w2” stands for the recessive white-
X X X X X carrier female, half of
Heterozygous female Homozygous female the daughters are eye mutation in one X chromosome, and the
expected to be symbol “w1” stands for the dominant nonmutant
X Y heterozygous
Normal male carriers and half the
allele (red eyes) in the other X chromosome. The
sons are expected to symbol “Y” stands for the Y chromosome, and it is
½ be affected. important to remember that the Y chromosome
does not contain an allele of the white-eye gene.
Y X Y X Y In the cross between a wild-type red-eyed
Affected male Normal male female and a white-eyed male, illustrated in Fig.
17.5a, the male offspring have red eyes because
350 SECTION 17.2 I N H E R I TA N C E O F G E N E S I N T H E X C H RO M O S O M E

FIG. 17.5 Genotypes of Morgan’s white-eyed fruit fly crosses.

a. b.

X X

Red-eyed White-eyed Red-eyed White-eyed

female male female male
w+w+ w–Y w+w+ w–Y

In this case, red-eyed

X X females are crossed
to white-eyed males.

Red-eyed Red-eyed Red-eyed White-eyed

female male female male
w+w– w+Y w+w– w–Y

In this case,
white eyes
appear in both
sexes in the
ratio red:white
of 1:1.
Red-eyed Red-eyed White-eyed Red-eyed White-eyed Red-eyed White-eyed
female male male female female male male
w+w+ or w+w– w+Y w–Y w+w– w–w– w+Y w–Y

they receive their X chromosome from their mother. The female X chromosome. However, it was one of Morgan’s students who
offspring from this cross receive one of their X chromosomes showed experimentally that the white-eye mutation was actually
from their father, and hence they are heterozygous, w1w2. When a physical part of the X chromosome. Today, it seems obvious that
the male and female progeny are mated together, their offspring genes are in chromosomes because we know that genes consist
consist of all red-eyed females (half of which are heterozygous) and of DNA and that DNA in the nucleus is found in chromosomes.
a 1 : 1 ratio of red-eyed to white-eyed males, exactly as Morgan had But in 1916, when Calvin B. Bridges, who had joined Morgan’s
observed. laboratory as a freshman, was working on his PhD research under
The hypothesis of X-linkage not only explained the original Morgan’s direction, neither the chemical nature of the gene nor
data, but it also predicted the results of other crosses. One the chemical composition of chromosomes was known.
important test is outlined in Fig. 17.5b. Here, the parental cross In one set of experiments, Bridges crossed mutant white-eyed
is the same as that in Fig. 17.5a, but instead of mating the females with wild-type red-eyed males (Fig. 17.6). Usually, the
F1 females to their brothers, they are mated to white-eyed males. progeny consisted of red-eyed females and white-eyed males (Fig.
The prediction is that there should be a 1 : 1 ratio of red-eyed to 17.6a). This is the result expected when the X chromosomes in the
white-eyed females as well as a 1 : 1 ratio of red-eyed to white-eyed mother separate normally at anaphase I in meiosis because all the
males. Again, these were the results observed. By the results of daughters receive a w1-bearing X chromosome from their father and
these crosses and others, Morgan demonstrated that the pattern all the sons receive a w2-bearing X chromosome from their mother.
of inheritance of the white-eye mutation parallels the pattern of But Bridges noted a few rare exceptions among the progeny.
inheritance of the X chromosome. He saw that about 1 offspring in 2000 from the cross was
“exceptional”—either a female with white eyes or a male with red
X-linkage provided the first experimental evidence eyes. The exceptional females were fertile, and the exceptional
that genes are in chromosomes. males were sterile. To explain these exceptional progeny, Bridges
Morgan’s original experiments indicated that the white-eye proposed the hypothesis diagrammed in Fig. 17.6b: The X chromo-
mutation showed a pattern of inheritance like that expected of the somes in a female occasionally fail to separate in anaphase I in
 CHAPTER 17 I N H E R I TA N C E O F S E X C H RO M O S O M E S , L I N K E D G E N E S , A N D O RG A N E L L E S 351

meiosis, and both X chromosomes go to the same pole. Recall

FIG. 17.6 Nondisjunction as evidence that genes are present from Chapter 15 that chromosomes sometimes fail to separate
in chromosomes. (a) Normal chromosome separation normally in meiosis, a process known as nondisjunction.
yields expected progeny. (b) Nondisjunction yields Nondisjunction of X chromosomes results in eggs containing
exceptional progeny. either two X chromosomes or no X chromosome. Figure 17.6b
a. Normal chromosome separation
shows the implications for eye color in the progeny if the
hypothesis is correct. The exceptional white-eyed females would
contain two X chromosomes plus a Y chromosome (genotype
w2/w2/Y), and the exceptional red-eyed males would contain a
X
single X chromosome and no Y chromosome (genotype w1).
Bridges’s hypothesis for the exceptional progeny in Fig. 17.6b
White-eyed Red-eyed
female male was bold, as it assumed that Drosophila males could develop in
w–w– w+Y the absence of a Y chromosome (XO embryos yielding sterile
males, where “O” indicates absence of a chromosome), and that
females could develop in the presence of a Y chromosome (XXY
embryos yielding fertile females). The hypothesis was accurate
Normal chromosome as well as bold. Microscopic examination of the chromosomes
separation results in
each egg containing a in the exceptional fruit flies confirmed that the exceptional
X X
single X chromosome. white-eyed females had XXY sex chromosomes and that the
exceptional sterile red-eyed males had an X but no Y. Because
Most XY males fruit flies with three X chromosomes (XXX) or no X chromosome
receive their X (OY) were never observed, Bridges concluded that embryos with
In a cross with chromosome
white-eyed females, from their these chromosomal constitutions are unable to survive. Bridges
normal chromosome mother. also conducted crosses that showed that nondisjunction can
separation results in take place in males as well as in females. From the phenotypes of
female progeny with
red eyes and male these exceptional fruit flies and their chromosome constitutions,
Red-eyed White-eyed
progeny with white
female male Bridges concluded that the white-eye gene (and by implication any
eyes.
w+w– w–Y other X-linked gene) is physically present in the X chromosome.
Bridges’s demonstration that genes are present in
chromosomes was also the first experimental evidence of
b. Nondisjunction nondisjunction. Drosophila differ from humans in that the
Y chromosome is necessary for male fertility but not for male
development. As we will see later in this chapter, a gene in the
X
Y chromosome itself is the trigger for male development in
humans and other mammals, and so for these organisms, the
White-eyed Red-eyed Y chromosome is needed both for male development and male
female male
w–w– w+Y fertility. Nondisjunction occasionally takes place in meiosis
in humans as well as in fruit flies. When nondisjunction takes
place in the human sex chromosomes, it results in chromosomal
Nondisjunction constitutions such as 47, XXY and 47, XYY males as well as 47,
results in eggs with XXX and 45, X females. Nondisjunction of autosomes can also
either two X occur, resulting in fetuses that have extra copies or missing copies
chromosomes or X
no X chromosome. X O of entire chromosomes. The consequences of nondisjunction of
human chromosomes were examined in Chapter 15.

Rare XO males
receive their X Genes in the X chromosome show characteristic
In a cross with
white-eyed females,
chromosome patterns in human pedigrees.
from their
nondisjunction of the father.
The features of X-linked inheritance can be seen in human
X chromosome pedigrees for traits due to an X-linked recessive mutation.
results in XXY female
progeny with white These are illustrated in Fig. 17.7 for red–green color blindness, a
eyes and XO male White-eyed Red-eyed condition that affects about 1 in 20 males. An individual with red–
progeny with red female male
w+
green color blindness will have difficulty seeing the number in the
eyes. w–w–Y
colored dots in Fig. 17.7.
352 SECTION 17.2 I N H E R I TA N C E O F G E N E S I N T H E X C H RO M O S O M E

The key features of the inheritance

FIG. 17.7 Inheritance of an X-linked recessive mutation. This pedigree shows the inheritance of traits due to rare X-linked recessive
of red–green color blindness, an X-linked recessive mutation. Source: Dorling Kindersley/ alleles, which are noted in the
Getty Images. pedigree, are listed here:

1. Affected individuals are usually

People with red–green
color blindness cannot see males because males need only
the number 5 in this figure. one copy of the mutant gene to be
Unaffected male
affected, whereas females need two
Unaffected female
copies to be affected.
Affected male
2. Affected males have unaffected
For a rare X-linked The heterozygous daughters sons because males transmit their X
recessive trait, most of affected males can have chromosome only to their daughters.
affected individuals affected sons.
are male. 3. A female whose father is affected
can have affected sons because such
The offspring of an affected
male are usually not affected. a female must be a heterozygous
carrier of the recessive mutant allele.

An additional feature worth

mentioning is that the sisters of an
affected male each have a 50% chance
of being a heterozygous carrier because
The sisters of an affected when a brother is affected the
male can have affected sons.
mother must be heterozygous for the
recessive allele.

FIG. 17.8 X-linked hemophilia in European royalty.

Unaffected male
Unaffected female
Affected male
Proven carrier female
Possible carrier female
Albert Victoria

Edward VII

George V Alexandra Nicholas II

Wilhelm II Alfonso XIII

George VI Alexis
GERMANY RUSSIA
Elizabeth II Philip
Juan
Carlos

Diana Charles Felipe

SPAIN

Catherine William Henry

BRITAIN
George Charlotte
 CHAPTER 17 I N H E R I TA N C E O F S E X C H RO M O S O M E S , L I N K E D G E N E S , A N D O RG A N E L L E S 353

j Quick Check 1 Is it possible for an unaffected female to have

female offspring with red–green color blindness? FIG. 17.9 Linkage of the white (w) and crossveinless (cv) genes
in the X chromosome. Genes are written according to
A pedigree for one of the most famous examples of human their order along the chromosome, with a horizontal line
X-linked inheritance is shown in Fig. 17.8. The trait is a form between homologous chromosomes, in this case the X
of hemophilia, which results from a recessive mutation in a and Y. Crossveins are shown in red for clarity.
gene encoding a protein necessary for blood clotting. Affected
individuals bleed excessively from even minor cuts and bruises, Parental generation
and internal bleeding can cause excruciating pain. Affecting about Female Male with both
1 in 7000 males, hemophilia is famous because of its presence homozygous for mutant alleles (for
both nonmutant white eyes and
in many members of European royalty descended from Queen alleles (for red crossveinless
Victoria of England (1819–1901), who was a heterozygous carrier eyes and normal X wings).
of the gene. By the marriages of her carrier granddaughters, the wing veins).
gene was introduced into the royal houses of Germany, Russia,
Red-eyed, White-eyed,
and Spain. The mutant allele is not present in the present royal crossveined crossveinless
family of Britain, however, because this family descends from female male
King Edward VII, one of Victoria’s four sons, who was not himself w+ cv+ w– cv–
affected and therefore passed only a normal X chromosome to his w+ cv+ Y
Crossvein Crossvein
descendants. present absent
The source of Queen Victoria’s hemophilia mutation is not
known. None of her ancestors is reported as having a bleeding
disorder. Quite possibly the mutation was present for a few F1 generation (only males shown)
generations before Victoria was born but remained hidden because
it was passed from heterozygous female to heterozygous female.
Progeny females
are heterozygous
for both mutant X
17.3 GENETIC LINKAGE alleles.
AND RECOMBINATION Red-eyed, Red-eyed,
crossveined crossveined
Mendel was fortunate not only because peas do not possess female male
sex chromosomes, but also because the genes that influenced w+ cv+ w+ cv+
w– cv– Y
the traits he studied, such as round/wrinkled and yellow/green
seeds, are on separate chromosomes or far apart on the same
chromosome. What happens when genes are close to each other in
the same chromosome? We explore the answer to this question in
this section. F2 generation (only males shown)

Nearby genes in the same chromosome show linkage.

Genes that are sufficiently close together in the same chromo-
some are said to be linked. That is, they tend to be transmitted
together in inheritance and do not assort independently of each
other as Mendel observed. Note that linked genes refer to two Red-eyed, White-eyed, Red-eyed, White-eyed,
crossveined crossveinless crossveinless crossveined
genes that are close together in the same chromosome, which may male male male male
be an autosome or sex chromosome. This is not to be confused w+ cv+ w– cv– w+ cv– w– cv+
with an X-linked gene, which is simply one that is present in the Y Y Y Y
X chromosome.
Linkage was discovered in Drosophila by Alfred H. in fruit flies with red eyes and when mutant results in fruit flies
Sturtevant, another of Morgan’s students. Once again, genes in with white eyes. The other recessive mutation is in a gene called
the X chromosome played a key role in the discovery because crossveinless ( cv), which when nonmutant results in fruit flies
the phenotype of the males reveals their genotype as in a with tiny crossveins in the wings and when mutant results in the
testcross with an autosomal recessive. An example is shown in absence of these crossveins.
Fig. 17.9. Sturtevant worked with male fruit flies that have an Sturtevant crossed this doubly mutant male with a female
X chromosome carrying two recessive mutations. One is in the carrying the nonmutant forms of the genes (w1 and cv1) in both
white gene (w) discussed earlier, which when nonmutant results X chromosomes. He saw that the offspring consist of phenotypically
354 SECTION 17.3 G E N E T I C L I N K AG E A N D R E CO M B I N AT I O N

wild-type females that are heterozygous for both genes, and progeny consists of w1cv2 and w2cv1 combinations of alleles.
phenotypically wild-type males. When these are crossed with each These are called recombinants, and they result from a crossover,
other, the female F2 progeny do not tell us anything because they the physical exchange of parts of homologous chromosomes,
are all wild type; each female receives the w1cv1 X chromosome which takes place in prophase I of meiosis (Chapter 11).
from her father and therefore has red eyes and normal crossveins. Crossing over is a key process in meiosis (Chapter 11). Most
In the male F2 progeny, however, the situation is different: Each chromosomes have one or more crossovers that form between
male progeny receives its X chromosome from the mother and its the homologous chromosomes as they pair and undergo meiosis.
Y chromosome from the father, and so the phenotype of each male Human females average about 2.75 crossovers per chromosome
immediately reveals the genetic constitution of the X chromosome pair, and human males average about 2.50 crossovers per
that the male inherited from the mother. chromosome pair. As noted earlier for the X and Y chromosomes,
As shown in Fig. 17.9, the male F2 progeny consist of four types: crossovers between homologous chromosomes are important
mechanically because they help hold the homologs together so
Genotype of F2 Progeny Number of Fruit Flies they can align properly at metaphase I and segregate to opposite
w1cv1/Y (red eyes, normal crossveins) 357 poles at anaphase I.
w2cv2/Y (white eyes, missing crossveins) 341 Fig. 17.10 shows how recombinant chromosomes arise from
crossing over between genes, using the hypothetical genes A and
w1cv2/Y (red eyes, missing crossveins) 52 B. In a cell undergoing meiosis in which a crossover takes place
w cv /Y (white eyes, normal crossveins)
2 1
45 between the genes, the allele combinations are broken up in the
chromatids involved in the exchange, and the resulting gametes
Although all four possible classes of maternal gametes are are AB, Ab, aB, and ab (Fig. 17.10a).
observed in the male progeny, they do not appear in the ratio Note that crossing over does not result only in recombinant
1 : 1 : 1 : 1 expected when gametes contain two independently chromosomes. Fig. 17.10a shows that, even when a crossover
assorting genes (Chapter 16). The lack of independent assortment occurs in the interval between the genes, two of the resulting
means that the genes show linkage. chromosomes contain the nonrecombinant configuration of
The male progeny fall into two groups. One group, alleles. These nonrecombinant configurations occur because
represented by larger numbers of progeny, derives from maternal crossing over occurs at the four-strand stage of meiosis (when
gametes containing either w1cv1 or w2cv2. These are called each homologous chromosome is a pair of sister chromatids),
nonrecombinants because the alleles are present in the same but only two of the four strands (one sister chromatid from each
combination as that in the parent. The other group of male homologous chromosome) are included in any crossover.

FIG. 17.10 Linkage and recombination. (a) Crossing over between genes results in recombination between the alleles of the genes. (b) When
crossing over occurs outside the interval between genes, there is no recombination between the alleles of the genes.
a.

Crossing over between two genes

Crossing over in prophase I of meiosis is a results in two recombinant and two
Centromere physical exchange of chromosome parts. nonrecombinant chromosomes. The overall
frequency of
A B A B A B Nonrecombinant recombinant
for A and B genes chromosomes is
A A b A b a measure of the
B
Recombinant genetic distance
a b for A and B genes
a B a B between the
two genes.
a b a b a b Nonrecombinant
for A and B genes

b.

Crossing over does not If crossing over occurs, but not

always take place between between two genes, the result is four
genes of interest. nonrecombinant chromosomes.

A B A B A B
a b a b
A B
Nonrecombinant
a b for A and B genes
A B A B
a b a b a b
 CHAPTER 17 I N H E R I TA N C E O F S E X C H RO M O S O M E S , L I N K E D G E N E S , A N D O RG A N E L L E S 355

Fig. 17.10b shows a second way in which nonrecombinant In the example with the genes w and cv, the total number of
chromosomes originate. When two genes are close together in a chromosomes observed among the progeny is 357 1 341 1 52 1
chromosome, a crossover may not occur in the interval between 45 5 795, and the number of recombinant chromosomes is
the genes but at some other location along the chromosome. 52 1 45 5 97. The frequency of recombination between w and cv
In this example, the crossover occurs in the region between is therefore 97/ 795 5 0.122, or 12.2%, and this serves as a measure
gene A and the centromere, and therefore the combination of of the genetic distance between the genes. In studies of genetic
alleles A and B remains intact in one pair of chromatids and the linkage, the distance between genes is not measured directly by
combination of alleles a and b remains intact in the homologous physical distance between them, but rather by the frequency of
chromatids. The resulting gametes are equally likely to have either recombination.
AB or ab, both of which are nonrecombinant. The frequency of recombination between any two genes
Nonrecombinant chromosomes can also be the result of two on the same chromosome ranges from 0% (when crossing over
crossover events occurring between two genes, in which the between the genes never takes place) to 50% (when the genes
effects of the first crossover (creating recombinants) is reversed by are so far apart that a crossover between the genes almost
a second crossover (re-creating nonrecombinants). always takes place). Genes that are linked have a recombination
frequency somewhere between 0% and 50%. The maximum
The frequency of recombination is a measure of the frequency of recombination is 50% because, when nonsister
genetic distance between linked genes. chromatids involved in crossovers are chosen at random, any new
When two genes are on separate chromosomes, a ratio of 1 : 1 : 1 : 1 crossover has two equally likely consequences: It can either change
is expected for the nonrecombinant (parental) and recombinant a previously recombinant chromatid into a nonrecombinant
(nonparental) gametic types, as described by the principle of chromatid, or change a previously nonrecombinant chromatid
independent assortment (Chapter 16). For two genes present in into a recombinant chromatid. The result is that however many
the same chromosome, we can consider two extreme situations. crossovers there may be (as long as there is at least one), the
If they are located very far apart from each other, one or more maximum frequency of recombination remains 50%. A frequency
crossovers will almost certainly occur between them, and there of recombination of 50% yields the same ratio of gametic types
will be a 1 : 1 : 1 : 1 ratio of nonrecombinant and recombinant as observed with independent assortment, which means that
gametes (as shown for the case of a single crossover in Fig. 17.10a). genes that are far enough apart in the same chromosome show
At the other extreme, if two genes are so close together that independent assortment.
crossing over never takes place between them, we would expect
j Quick Check 2 Why is the upper limit of recombination
only nonrecombinant chromosomes (Fig. 17.10b).
50% rather than 100%?
What happens in between these extremes? In these cases,
in some cells undergoing meiosis, no crossover takes place Recombination plays an important role in creating new
between the genes, in which case all the resulting chromosomes combinations of alleles in each generation and in ensuring
are nonrecombinant (Fig. 17.10b); and in other cells undergoing the genetic uniqueness of each individual. If there were no
meiosis, a crossover occurs between the genes, in which case half recombination (that is, if all the alleles in each chromosome
the resulting chromosomes are nonrecombinant and half are were completely linked), any individual human would be able
recombinant (Fig. 17.20a). Since the meioses with no crossover to produce only 223 5 8.4 million types of reproductive cells.
between the genes result only in nonrecombinant chromosomes While this is a large number, the average number of sperm per
and those with a crossover result in half nonrecombinant and half ejaculate is much larger—approximately 350 million. Because
recombinant chromosomes, the nonrecombinant chromosomes recombination does occur, and because the crossovers resulting in
in the offspring will be more numerous than the recombinant recombination can occur at any of thousands of different positions
chromosomes. in the genome, each of the 350 million sperm is virtually certain
The actual frequency of recombinants depends on the distance to carry a different combination of alleles.
between the genes. The distance between the genes is important
because whether or not a crossover occurs between the genes Genetic mapping assigns a location to each gene along
is a matter of chance, and the closer the genes are along the a chromosome.
chromosome, the less likely it is that a crossover will take place in With the exception of a few regions, such as the area near the
the interval between them. Because the formation of recombinant centromere, the likelihood of a crossover occurring somewhere
chromosomes requires at least one crossover between the between two points on a chromosome is approximately
genes, genes that are close together (more tightly linked) show proportional to the length of the interval between the points.
less recombination than genes that are far apart. In fact, the Therefore, the frequency of recombination can be used as a
proportion of recombinant chromosomes observed among measure of the physical distance between genes. These distances
the total, which is called the frequency of recombination, are used in the construction of a genetic map, which is a diagram
is a measure of genetic distance between the genes along the showing the relative position of genes along a chromosome.
chromosome. The maps are drawn using a scale in which one unit of distance
 HOW DO WE KNOW?

FIG. 17.11

Can recombination be used EXPERIMENT Taking this idea a step further, Sturtevant reasoned
that if one knew the frequency of recombination between genes

to construct a genetic map of a and b, between b and c, and between a and c, then one should be
able to deduce the order of genes along the chromosome. He also

a chromosome? predicted that, if the order of genes were known to be a–b–c, then
if the genes were close enough, the frequency of recombination
between a and c should equal the sum of the frequencies between
BACKGROUND In 1910, Thomas Hunt Morgan discovered a and b and that between b and c.
X-linkage by studying the white-eye mutation in Drosophila. RESULTS Sturtevant studied the frequencies of recombination
Soon other X-linked mutations were found. Alfred H. Sturtevant, between many pairs of genes along the X chromosome, including
Morgan’s student, decided to test whether mutant genes in the same some of those shown in the illustration shown here.
X chromosome were inherited together, that is, linked. He found
that genes in the X chromosome were linked, but not completely, CONCLUSION The results confirmed Sturtevant’s hypothesis
and that different pairs of genes showed great differences in their and showed that genes could be arranged in the form of a genetic
linkage. Some genes showed almost no recombination, whereas map, depicting their linear order along the chromosome, with the
others underwent so much recombination that they showed distance between any pair of genes proportional to the frequency
independent assortment. of recombination between them. Across sufficiently short regions,
the frequencies of recombination are additive.
HYPHOTHESIS Sturtevant hypothesized that recombination was
due to crossing over between the genes, and that genes farther FOLLOW-UP WORK Genetic mapping remains a cornerstone of
apart in the chromosome would show more recombination. genetic analysis, showing which chromosome contains a mutant
gene and where along the chromosome the gene is located. The
w, white rb, ruby
(white eyes) (ruby eye color) method helped to identify the genes responsible for many single-
y, yellow ec, echinus cv, crossveinless gene inherited disorders, including Huntington’s disease, cystic
(yellow body) (large eyes) (missing wing vein)
fibrosis, and muscular dystrophy.

1.5 4.0 2.0 6.2 SOURCE Sturtevant, A. H. 1913. “The Linear Arrangement of Six Sex-Linked Factors
in Drosophila, as Shown by Their Mode of Association.” Journal of Experimental Zoology
14:43–59.
The distance between adjacent Across sufficiently short
genes is given in map units, distances, the map units are
which is equal to the frequency additive. Thus, the expected
of recombination when frequency of recombination
expressed as a percentage. between w and cv is
4.0 + 2.0 + 6.2 = 12.2%.

(called a map unit) is the distance between genes resulting in 1% However, for two genes that are farther apart than about
recombination. Thus, in a Drosophila genetic map containing the 15 map units, the observed recombination frequency is somewhat
genes w and cv, the distance between the genes is 12.2 map units. smaller than the sum of the map distances between the genes.
Genetic maps are built up step by step as new genes are The reason is that, with greater distances, two or more crossovers
discovered that are genetically linked, as shown in Fig. 17.11. between the genes may occur in the same chromosome, and
Across distances that are less than about 15 map units, the map thus an exchange produced by one crossover may be reversed by
distances are approximately additive, which means that the another crossover farther along the way.
distances between adjacent genes can be added to get the distance
j Quick Check 3 For two genes that show independent
between the genes at the ends. For example, in Fig. 17.11, there are
assortment, what is the frequency of recombination?
two genes between w and cv. The map distance between w and the
next gene, ec, is 4.0, the distance between ec and the next gene, rb,
is 2.0, and the distance between rb and cv is 6.2. The map distance Genetic risk factors for disease can be localized by
between w and cv is therefore 4.0 1 2.0 1 6.2 5 12.2 map units, genetic mapping.
and hence the expected frequency of recombination between The discovery of abundant genetic variation in DNA sequences
these genes is 12.2%, which is the value observed. in human populations, such as single-nucleotide polymorphisms

356
 CHAPTER 17 I N H E R I TA N C E O F S E X C H RO M O S O M E S , L I N K E D G E N E S , A N D O RG A N E L L E S 357

(Chapter 15), made it possible to study genetic linkage in the of the chromosomes that carry the nonmutant allele show the
human genome. At first, the focus was on finding mutations that A–T nucleotide pair. The two chromosomes in which the mutant
cause disease, such as the mutation that causes cystic fibrosis and nonmutant genes are associated with the other SNPs can be
(Chapter 14). The method was to study large families extending attributed to recombination.
over three or more generations in which the disease was present The association in Fig. 17.12a may be contrasted with the
and then to identify the genotypes of each of the individuals pattern in Fig. 17.12b, in which there is no association. In this case,
for thousands of genetic markers (previously discovered DNA each of the alleles of the disease gene is equally likely to carry
polymorphisms) throughout the genome. The goal was to find either form of the SNP. The failure to find an association means
genetic markers that showed a statistical association with the that the SNP is not closely linked to the disease gene, and may in
disease gene, which would indicate genetic linkage and reveal the fact be in a different chromosome. In actual studies, an association
approximate location of the disease gene along the chromosome. is almost never observed, but the lucky find of an association helps
Fig. 17.12 illustrates the underlying concept. It assumes identify the location of the disease gene in the genetic map. Using
100 chromosomes observed among different individuals in a such association methods, hundreds of important disease genes
pedigree, of which 50 carry a mutant allele of a gene and have been located by genetic mapping. Once the location of the
50 carry a nonmutant allele. These chromosomes are tested disease gene is known, the identity and normal function of the
for a marker of known location, in this case a single-nucleotide gene can be determined.
polymorphism (SNP) in which one of the nucleotide pairs in the
DNA is a G–C base pair in some chromosomes and A–T in others.
In Fig. 17.12a, there is clearly an association between the disease 17.4 INHERITANCE OF GENES IN
gene and the SNP. Almost all of the chromosomes that carry the THE Y CHROMOSOME
mutant allele show the G–C nucleotide pair, whereas almost all
Like the X chromosome, the Y chromosome exhibits a particular
pattern of inheritance because of its association with the male
sex. In humans and other mammals, all embryos initially develop
FIG. 17.12 Genetic mapping. SNPs associated with a mutant gene immature internal sexual structures of both females and males.
show where that gene is located in the genetic map. SRY, a gene in the Y chromosome, encodes a protein that is the
trigger for male development. (“SRY” stands for “sex-determining
a. Association b. No association region in the Y chromosome.”) In the presence of SRY, male
Disease Percent Disease Percent structures complete their development and female structures
gene SNP observed gene SNP observed
degenerate. In the absence of SRY, male embryonic structures
G G degenerate and female structures complete their development.
49 25
C C The SRY gene is therefore the male-determining gene in humans
and other mammals. Because they are linked to SRY, most genes
A A in the Y chromosome show a distinctive pattern of inheritance in
1 25 pedigrees, different from the patterns of autosomal genes Mendel
T T
observed in his pea plants, in that they are transmitted only from
Normal
Mendelian father to son.
G G ratios
1 25
C C Y-linked genes are transmitted from father to son
to grandson.
A A Genes that are present in the unique region of the Y chromosome
49 25
T T (the part that cannot cross over with the X) are known as
Y-linked genes, of which there are not many. As well as the
SRY male-determining gene, they include a number of genes in
In an analysis of 100 In this case, there is no which mutations are associated with impaired fertility and low
chromosomes, most association between a
show an association disease gene and a SNP, sperm count.
between a mutant so the location of the The pedigree characteristics of Y-linked inheritance are
allele of a gene and a disease gene in relation
G–C SNP, suggesting to this specific SNP striking (Fig. 17.13):
that the mutant allele cannot be determined.
is located near the SNP. 1. Only males are affected with the trait.
2. Females never inherit or transmit the trait, regardless of
Mutant how many affected male relatives they have.
Nonmutant
3. All sons of affected males are also affected.
 358 SECTION 17.4 I N H E R I TA N C E O F G E N E S I N T H E Y C H RO M O S O M E

Y-chromosome lineage may leave some

FIG. 17.13 Inheritance of Y-linked traits. The pedigree pattern is that of father to son to nonmutant descendants as well as some
grandson to great-grandson, and so forth. mutant descendants, and hence any or all
of the sequences shown may coexist in a
Only males
exhibit the trait. present-day population.
Females do not inherit In human history, the mutations
or transmit the trait. creating new Y-chromosome haplotypes
were occurring at the same time as
populations were migrating and founding
new settlements across the globe, and so
each geographically distinct population
came to have a somewhat different set of
All sons of males with the
trait also show the trait. Y-chromosome haplotypes. The differences
among populations are offset to some
extent by migration among populations,
Because the Y chromosome is always transmitted from father which mixes the geographical locations of various haplotypes.
to son (and never transmitted to daughters), a trait determined Nevertheless, the fact that the mutations accumulate through
by a Y-linked gene will occur in fathers, sons, grandsons, and so time and are completely linked allows the evolutionary history of
forth. Traits resulting from Y-linked genes cannot be present the haplotypes to be reconstructed. It also enables Y chromosomes
in females nor can they be transmitted by females. However, to be traced to their likely ethnic origin.
other than maleness itself and some types of impaired fertility, The worldwide distribution of real Y-chromosome lineages
no physical traits are known that follow a strict Y-linked pattern among human populations is shown in Fig. 17.15. The different
of inheritance. This observation emphasizes the extremely low colors in the pie charts represent different haplotypes.
density of functional genes in the Y chromosome. Neighboring populations tend to have more closely related
Y chromosomes than more distant populations. Four major
? CASE 3 YOU, FROM A TO T: YOUR PERSONAL GENOME clusters of Y-chromosome lineages can be recognized in Fig. 17.15.
One is concentrated in Africa, another in Southeast Asia and
How can the Y chromosome be used to trace ancestry? Australia, a third in Europe and central and western Asia, and the
The example of Claudia Gilmore illustrates how, through tests fourth in North and South America. These clusters correspond
of her personal genome with regard to the BRCA1 and BRCA2 roughly with the spread of human settlements around the globe
mutations, she became aware of her elevated risk of breast cancer. inferred from archaeological evidence.
Your personal genome not only can tell you about your genetic risk
factors for disease, but it also contains important information about
your genetic ancestry. For example, your male ancestors can be
traced through your Y chromosome. The regions at the tips in which FIG. 17.14 Y-chromosome haplotypes. Because Y chromosomes
the X and Y chromosomes share homology is only about 6% of the in a lineage conform to an evolutionary tree, the
entire length of the Y chromosome. This means that 94% of the ancestry of a male’s Y chromosome can be traced.
Y chromosome consists of sequences that are completely linked
with one another because that portion of the chromosome does not ATG C TACG C T

pair with another chromosome and does not undergo crossing over.
Because of this complete linkage, each hereditary lineage of The first G TGC TACG C T Each unique
occurrence of combination
Y chromosomes is separate from every other lineage. As mutations
each red letter of nucleotides
occur along the Y chromosome, they are completely linked to any represents a represents a
past mutations that may be present and also completely linked new mutation different Y
GA G C TACG C T
at that site. chromosome
to any future mutations that may take place. The mutations
Time

haplotype.
therefore accumulate, and this allows the evolutionary history of a
set of sequences to be reconstructed.
GA G TTACG C T
Fig. 17.14 shows an evolutionary tree based on the G AGC TACGCA
accumulation of mutations at a set of nucleotide sites along
the Y chromosome. Each unique combination of nucleotides
GA G TC ACG C T
constitutes a Y-chromosome haplotype, or haploid genotype. GA GC TACGAA
In the figure, the most ancient Y chromosomes are at the top, GAG C TAA GCA
and the accumulation of new mutations as the generations
GA G TCGCG C T
proceed results in the successive creation of new haplotypes. Each GAT TC ACG C T GA GC TAATCA
 CHAPTER 17 I N H E R I TA N C E O F S E X C H RO M O S O M E S , L I N K E D G E N E S , A N D O RG A N E L L E S 359

These organelles contain

FIG. 17.15 Geographical distribution of Y-chromosome haplotypes among native populations. The chlorophyll, a green pigment
evolutionary trees of Y-chromosome haplotypes reflect the origin and movement of different that absorbs light and, in the
Y chromosomes over time. Data from M. A. Jobling and C. Tyler-Smith, 2003, “The Human Y Chromosome: process of photosynthesis
An Evolutionary Marker Comes of Age,” Nature Reviews Genetics 4:598–612. (Chapter 8), produces the
sugars that are essential for
growth of the plant or algal
cells and of the organisms that
eat them. Mitochondria and
chloroplasts have their own
genomes that contain genes
for many of the enzymes
that carry out the organelles’
functions. Genes present in
these genomes move with
the organelle during cell
division, independent of the
segregation of chromosomes
in the nucleus.

Each color in these pie diagrams represents a Mitochondrial and

different Y-chromosome haplotype. Note that chloroplast genomes
pie diagrams with the same predominant often show uniparental
color tend to be clustered geographically.
inheritance.
During sexual reproduction,
organelles do not show the
The implication of Fig. 17.15 is that the haplotype of your regular, highly choreographed movements that chromosomes
Y chromosome (if you have a Y chromosome) contains genetic undergo during Mendelian segregation. Organelles are partitioned
information about its origin. And you can learn what this to the gametes along with other cytoplasmic components, and
information is from genetic testing companies that sell direct-to- therefore their mode of inheritance depends on how the gametes
consumer (DTC) services. Their tests are not regarded as medical are formed, how much cytoplasm is included in the gametes,
devices and so are unregulated, and quality control is sometimes and the fate of the cytoplasm in each parental gamete after
uncertain. Nevertheless, you can send saliva or other biological fertilization.
samples to a DTC provider, which will (for a fee) test your Considering the great diversity in the details of reproduction
Y chromosome and send you a report that details its possible in different groups of organisms, it is not surprising that there is
origin. Of course, because of recombination and independent a diversity of types of inheritance of cytoplasmic organelles. The
assortment of genes in other chromosomes, the ethnic origin three most important types are:
of your Y chromosome may have little or nothing to do with the
ethnic origins of genes in any of your other chromosomes. • Maternal inheritance, in which the organelles in the
offspring cells derive from those in the mother.

• Paternal inheritance, in which the organelles in the

17.5 INHERITANCE OF offspring cells derive from those in the father.

MITOCHONDRIAL AND • Biparental inheritance, in which the organelles in the

CHLOROPLAST DNA offspring cells derive from those in both parents.

Sex chromosomes and linked genes are not the only genes that are In most organisms, either maternal inheritance or paternal
inherited in ways that are unlike the patterns of inheritance that inheritance predominates, but sometimes there is variation
Mendel observed in peas. Genes in mitochondria and chloroplasts from one offspring to the next. For example, transmission of the
also show distinct inheritance patterns. Mitochondria and chloroplasts ranges from strictly paternal in the giant redwood
chloroplasts are ancient organelles of eukaryotic cells originally Sequoia, to strictly maternal in the sunflower Helianthus, to either
acquired by the engulfing of prokaryotic cells (Chapter 5). maternal or paternal (or less frequently biparental) in the fern
Mitochondria generate ATP that cells use for their chemical energy. Scolopendrium, to mostly maternal but sometimes paternal or
Chloroplasts are found only in plant cells and eukaryotic algae. biparental in the snapdragon Antirrhinum.
360 SECTION 17.5 I N H E R I TA N C E O F M I TO C H O N D R I A L A N D C H LO RO P L A S T D N A

There is likewise great diversity among organisms in the 1. Both males and females can show the trait.
inheritance of mitochondrial DNA. Most animals show maternal
2. All offspring from an affected female show the trait.
transmission of the mitochondria, as would be expected from their
large, cytoplasm-rich eggs and the small, cytoplasm-poor sperm. 3. Males do not transmit the trait to their offspring.
Among other organisms, there is again much variation, including
maternal transmission of mitochondria in flowering plants and The pedigree in Fig. 17.16 follows a mitochondrial disease
paternal transmission in the green alga Chlamydomonas. known as MERRF syndrome (the acronym stands for “myoclonic
epilepsy with ragged red fibers”). As its name suggests, the
Maternal inheritance is characteristic of mitochondrial syndrome is characterized by epilepsy, a neurological disease
diseases. characterized by seizures, as well as by the accumulation
In humans and other mammals, mitochondria normally of abnormal mitochondria in muscle fibers. This extremely
show strictly maternal inheritance—the mitochondria in the rare disease is associated with a single point mutation in a
offspring cells derive from those in the mother. Fig. 17.16 shows mitochondrial gene involved in protein synthesis that affects
the characteristic pedigree patterns of a trait encoded by a oxidative phosphorylation (Chapter 7).
mitochondrial genome transmitted through maternal inheritance: More than 40 different diseases show these pedigree
characteristics. They all result from mutations in the
mitochondrial DNA, but the tissues and organs affected as well as
FIG. 17.16 Maternal inheritance. Human mitochondrial DNA is
the severity differ from one to the next. All of the mutations affect
transmitted from a mother to all of her offspring. Source:
energy production in one way or another. In some cases, disease
Courtesy of Dr. Kurenai Tanji, Columbia University Medical Center,
results directly from lack of adequate amounts of ATP, in other
New York, NY.
cases from intermediates in energy production that are toxic to
the cell or that damage the mitochondrial DNA.

? CASE 3 YOU, FROM A TO T: YOUR PERSONAL GENOME

Inherited mitochondrial diseases are How can mitochondrial DNA be used to trace
often associated with muscle
weakness reflecting deficient ancestry?
production of ATP. The red patches Sequencing of mitochondrial DNA reveals mitochondrial
in the microscopic image result from
clumps of defective mitochondria in haplotypes analogous to those in the Y chromosome. As in the
muscle fibers observed in one form Y chromosome, mutations in mitochondrial DNA accumulate
of epilepsy due to mutation in through time, and each mitochondrial haplotype remains intact
mitochondrial DNA.
through successive generations because no recombination
between mitochondrial genomes takes place. This means that
mitochondrial DNA can be used to trace ancestry and population
history, much as described previously for the Y chromosome.
The mitochondrial DNA is actually more informative than
Both males and females Males do not Y-chromosomal DNA because it shows substantially more genetic
exhibit the trait. transmit the trait. variation so its ancestry can be tracked on a finer scale.
The use of mitochondrial DNA to trace human origins and
migration is discussed in greater detail in Chapter 24. As with the
Y chromosome, your personal mitochondrial genome includes
information about its origin. If you want to learn about your
All the offspring mitochondrial DNA, you can send a tissue sample to any of a
of mothers with number of direct-to-consumer genotyping services that will, for a
the trait also show
the trait. fee, analyze the DNA and provide you with a report. •
 CHAPTER 17 I N H E R I TA N C E O F S E X C H RO M O S O M E S , L I N K E D G E N E S , A N D O RG A N E L L E S 361

Core Concepts Summary

17.1 Many organisms have a distinctive pair of In humans, X-linked inheritance shows a pattern in which
chromosomes, often called the X and Y chromosomes, affected individuals are almost always males, affected males
that differ between the sexes and show different have unaffected sons, and a female whose father is affected
patterns of inheritance in pedigrees from other can have affected sons. page 352
chromosomes.
17.3 In genetic linkage, two genes are sufficiently
In humans and other mammals, XX individuals are female
close together in the same chromosome that the
and XY individuals are male. page 346
combination of alleles present in the chromosome
The human X and Y chromosomes are different lengths tends to remain together in inheritance.
and contain different genes, except for small regions of
Genes that are close together in the same chromosome are
homology that allow the two chromosomes to pair in
linked and do not undergo independent assortment.
meiosis. page 347
page 353
Segregation of the X and Y chromosomes during male
Recombinant chromosomes result from crossing over
meiosis results in half of the sperm receiving an
between genes on the same chromosome and show a
X chromosome and half a Y chromosome so that random
nonparental combination of alleles. page 354
union of gametes predicts a 1 : 1 female : male sex ratio at
the time of fertilization. page 347 Nonrecombinant chromosomes have the same
configuration of alleles as one of the parental
17.2 X-linked genes, which show a crisscross chromosomes. page 354
inheritance pattern, provided the first evidence that
In genetic mapping, the observed proportion of
genes are present in chromosomes.
recombinant chromosomes is the frequency of
Thomas Hunt Morgan studied a mutation in the fruit fly recombination and can be used as a measure of distance
Drosophila melanogaster that resulted in fruit flies with along a chromosome. A recombination frequency of
white eyes rather than wild-type red eyes. In this species, as 1% is 1 map unit. page 355
in mammals, females are XX and males are XY. page 348
Gene linkage and mapping are used to identify the
In a cross of a normal red-eyed female with a mutant locations of disease genes in the human genome.
white-eyed male, all of the male and female progeny had page 357
red eyes. When brothers and sisters of this cross were
mated with each other, all of the females had red eyes, but 17.4 Most Y-linked genes are passed from father
males were red-eyed and white-eyed in a 1 : 1 ratio. to son.
page 348
In humans and other mammals, the Y chromosome contains a
This pattern of inheritance is observed because the gene gene called SRY that results in male development. page 357
Morgan studied is located in the X chromosome. The
In Y-linked inheritance, only males are affected and all
nonmutant w1 allele is dominant to the mutant w2 allele,
sons of an affected male are affected. Females are never
and the gene is present only in the X chromosome and not
affected and do not transmit the trait. page 357
in the Y chromosome. page 348
Most Y-linked genes show complete linkage, which allows
X-linked genes show a crisscross inheritance pattern, in
their evolutionary history to be traced. page 358
which the X chromosome with the mutant gene that is
present in males in one generation is present in females in
17.5 Mitochondria and chloroplast DNA follow their
the next generation. page 349
own inheritance pattern.
Calvin Bridges observed rare fruit flies that did not follow
Mitochondria and chloroplasts have their own genomes,
the usual pattern for X-linked inheritance and inferred that
which reflect their evolutionary history as free-living
these exceptional fruit flies resulted from nondisjunction,
prokaryotes. page 359
or failure of homologous chromosomes to segregate, in male
or female meiosis. His observations provided evidence that Mitochondria in humans and other mammals show maternal
genes are carried in chromosomes. page 350 inheritance, in which individuals inherit their mitochondrial
DNA from their mother. page 359
362 SELF-ASSESSMENT

Because mitochondrial DNA does not undergo 5. Describe how recombination frequency can be
recombination and is maternally inherited, it can be used to used to build a genetic map.
trace human ancestry and migration. page 360
6. Describe the pattern of inheritance expected from
a Y-linked gene in a human pedigree.

Self-Assessment 7. Describe the pattern of inheritance expected

from a gene present in mitochondrial DNA in a human
1. Explain how the human X and Y chromosomes can pair
pedigree.
during meiosis even though they are of different lengths
and most of their genes are different. 8. Explain how Y-chromosome and mitochondrial
DNA data can be used to trace ancestry.
2. Describe the biological basis for the 1 : 1 ratio of males
and females at conception in mammals.
Log in to to check your answers to the Self-
3. For a recessive X-linked mutation, such as color
Assessment questions, and to access additional learning tools.
blindness, explain the pattern of inheritance from an
affected male through his daughters into her children.

4. Explain why linked genes do not exhibit independent

assortment.
 CHAPTER 18

The Genetic and

Environmental
Basis of Complex
Traits
Core Concepts
18.1 Complex traits are those
influenced both by the
action of many genes and by
environmental factors.
18.2 Genetic effects on
complex traits are reflected
in resemblance between
relatives.
18.3 Twin studies help separate
the effects of genotype and
environment on variation in
a trait.
18.4 Many common diseases and
birth defects are affected
by multiple genetic and
environmental risk factors.
Ryan McVay/Getty Images.

363
364 SECTION 18.1 H E R E D I T Y A N D E N V I RO N M E N T

Biologists initially had a hard time accepting the principles of single-gene traits, Mendel was able to infer underlying mechanisms
Mendelian inheritance because they seem so at odds with everyday of inheritance based on physical factors we now call genes. By
observations. Common and easily observed traits like height, contrast, complex traits, such as human height, are influenced
weight, hair color, and skin color give no evidence of segregation by multiple genes as well as by the environment. As a result,
in pedigrees, and simple phenotypic ratios like 3 : 1 or 9 : 3 : 3 : 1 are their inheritance patterns are more difficult to follow and simple
not observed for them. The lack of these characteristic ratios raised phenotypic ratios are not observed.
serious doubt whether Mendel’s principles are valid for common In many ways, complex traits are more important than single-
traits. Some biologists concluded that they apply only to seemingly gene Mendelian traits. One reason is their prevalence—complex
trivial traits like round and wrinkled seeds in peas. traits are found in all organisms and include most of the traits we
At about the time that Mendel was studying inheritance in can see around us. By contrast, there are relatively few examples
garden peas, the biologist Francis Galton, a friend and cousin of of common single-gene traits. Complex traits are also important in
Charles Darwin, was studying common traits including human human health and disease. In the most common disorders—among
height. From studies of height and other common traits in parents them heart disease, diabetes, and cancer—single-gene Mendelian
and their offspring, Galton discovered general principles in the inheritance is seldom found. It is therefore important to understand
inheritance of such traits. For example, parents who are tall tend the inheritance of complex traits and common disorders, which
to have offspring who are taller than average but not as tall as are the subjects of this chapter. We begin by describing some of the
themselves. Galton’s principles for the inheritance of common features of complex traits, and then show how these principles are
traits did not invoke genes, segregation, independent assortment, not only compatible with, but are in fact predicted by, Mendelian
or other features of Mendelian inheritance, but they did describe inheritance. Finally, we describe how modern molecular genetics
Galton’s observations. Not only were genes—what Mendel called and genomics have allowed the identification of genes affecting
“hereditary factors”—thought to be unnecessary in Galton’s theory complex traits.
of inheritance, but also many biologists thought that Galton’s
theories and Mendel’s were incompatible.
This, it turned out, was not the case. The traits that Mendel 18.1 HEREDITY AND ENVIRONMENT
studied are now called single-gene traits because each one is
determined by variation at a single gene and the traits for the Complex traits are important not only in humans, but also in
most part are not influenced by the environment. By focusing on agricultural plants and animals. We will examine human height
in some detail because it has been widely
studied, but equally well known complex
FIG. 18.1 Examples of complex traits.(a) Human height, (b) egg number, (c) milk traits are number of eggs laid by hens,
production, and (d) grain yield. Sources: a. Randy Faris/Corbis; b. muratart/Shutterstock; milk production in dairy cows, and yield
c. smereka/Shutterstock; d. Radius Images/Getty Images. per acre of grain (Fig. 18.1).
Many common human diseases,
a b including high blood pressure, obesity,
diabetes, and depression, are complex
traits (Fig. 18.2). High blood pressure,
for example, affects about one-third of
the U.S. population, and obesity another
third. Type 2 diabetes affects around
8% of the U.S. population, and an
estimated 15% will suffer at least one
episode of severe depression in the
course of a lifetime. Taken together,
c d
about 200 million Americans—two-thirds
of the entire population—suffer from
one or more of these common disorders.
None of these traits shows single-gene
Mendelian inheritance.
In many complex traits, the phenotype
of an individual is determined by
measurement: Human height is measured
in inches, milk yield by the gallon, grain
yield by the bushel, egg production by
 CHAPTER 18 T H E G E N E T I C A N D E N V I RO N M E N TA L B A S I S O F CO M P L E X T R A I T S 365

FIG. 18.2 Examples of human diseases that are complex traits. (a) High blood pressure, (b) obesity, (c) diabetes, and (d) depression. Sources:
a. Burwell and Burwell Photography/iStockPhoto; b. Ocean/Corbis; c. Junophoto/Getty Images; d. Imago/ZUMApress.com.

a b c d

the number of eggs, blood pressure by millimeters of mercury, may seem, there are always minor differences from one area to
and blood sugar by millimoles per liter. Because the phenotype the next, and these differences can result in variation in complex
of complex traits such as these is measured along a continuum traits. In the example in Fig. 18.3, the plants in the foreground
with only small intervals between similar individuals, complex are shorter than the others because that corner of the field has
traits like these are often called quantitative traits. By contrast, poorer drainage, and the plants growing there have to compete for
single-gene traits often appear in one of two or more different nutrients with the grass growing nearby.
phenotypes, such as round versus wrinkled seeds, or green versus
yellow seeds.

FIG. 18.3 Phenotypic variation due to variation in the

Complex traits are affected by the environment.
environment. Genetically identical corn plants vary in
Expression of complex traits is notoriously susceptible to lifestyle
height because of variation in exposure to sun and in soil
choices and other environmental factors. Inadequate nutrition is
composition at different locations in the same field.
linked to slow growth rate and short stature in adults. Salt intake is
Source: Bruce Leighty/Getty Images.
associated with an increased likelihood of high blood pressure and is
therefore an environmental risk factor for this common disorder.
An environmental risk factor is a characteristic in a person’s
surroundings that increases the likelihood of developing a particular
disease. For example, a junk-food diet high in fat and carbohydrates
is an environmental risk factor for obesity and diabetes.
Environmental effects are also important in agriculture.
Farmers are well aware that adequate nutrition is essential to
normal growth of chicks, lambs, piglets, and calves, and that
continued high-quality feed is necessary for high egg production
and milk yield when the animals reach adulthood. In crop plants
like grains, adequate soil moisture and nutrients are necessary for
sustained high yields.
Environmental factors not only affect the average phenotype
for complex traits, but they also affect the variation in phenotype
from one individual to the next. For example, most fields of
corn you see planted along the roadside come from seeds that
are genetically identical to one another, yet there is phenotypic
variation in complex traits like plant height (Fig. 18.3). Because
the plants are genetically identical, the differences in phenotype
result from differences in the environment. Some parts of the field
may receive more sunlight than others, and some parts may have
better water drainage. No matter how uniform an environment
366 SECTION 18.1 H E R E D I T Y A N D E N V I RO N M E N T

Environmental effects on complex traits in animals are

evident in true-breeding, homozygous strains like those used by FIG. 18.4 Multiple genes contributing to a complex trait. Three
Mendel in his experiments with pea plants. Such true-breeding unlinked genes, each with two alleles, influence the
strains are called inbred lines, and they are often used for intensity of red coloration of the seed casing in wheat.
research. Even though all animals in any inbred line are genetically
identical and are caged in the same facility and fed the same food, Two plants heterozygous
for each of three unlinked
there is variation from one animal to the next. In one study of
genes affecting the color
cholesterol levels in an inbred line of mice, for example, average of seed casing are crossed.
serum cholesterol was 120 mg/dl (milligrams per deciliter), but
Aa Bb Cc  Aa Bb Cc
the range was 60–180 mg/dl. Because the mice are genetically
identical, the variation in serum cholesterol resulted entirely from
variation in the environment. The genes act cumulatively
to determine the intensity
of red coloration; in this
Complex traits are affected by multiple genes. example, each upper-case
Most complex traits are affected by many genes, in contrast allele in the genotype adds
more red to the phenotype.
to Mendel’s traits, which are primarily affected by only one.
Therefore, in complex traits the familiar phenotypic ratios such ABC abc
3
as 3 : 1 that Mendel saw when he crossed two true-breeding ABc abC
strains with contrasting characters are not observed. For complex 2 4
AbC aBc
traits, the effects of individual genes are obscured by variation in Female aBC
2 3 4
Abc Male
gametes gametes
phenotype that is due to multiple genes affecting the trait and also Abc
2 3 3 4
aBC
due to the environment. The number of genes affecting complex 1 3 3 3 5
aBc AbC
traits is usually so large that different genotypes can have very 1 2 3 3 4 5
abC ABc
similar phenotypes, which also makes it difficult to see the effects
1 2 2 3 4 4 5
of individual genes on a trait. abc ABC
0 2 2 2 4 4 4 6
In a few traits, however, the effects of the environment
1 2 2 3 4 4 5
are minor and the number of genes is small, and in these cases
the genetic basis of the trait can be analyzed. A classic example, 1 2 3 3 4 5
studied by Herman Nilsson-Ehle about a century ago, concerns 1 3 3 3 5
the color of seed casing in wheat (which give the seeds their 2 3 3 4
color), which ranges from nearly white to dark red (Fig. 18.4). His 2 3 4
experiment demonstrated that complex traits are subject to the 2 4
same laws that Mendel worked out for single-gene traits, but that
3
the inheritance patterns are more difficult to see because of the
number of genes involved.
In studying seed color in true-breeding varieties and their The bar graph of color
first-generation and second-generation hybrids, Nilsson-Ehle phenotypes among the
progeny approximates
realized that the relative frequencies of different shades of red a bell-shaped curve.
color could be explained by the effects of three genes that undergo
Relative probability of phenotype

independent assortment, as illustrated by the Punnett square

shown in Fig. 18.4. Each of the genes has two alleles, designated
by combinations of upper-case and lower-case letters. In the
development of seed color, each upper-case allele in a genotype
intensifies the red coloration in an additive fashion. Altogether
there are seven possible phenotypes, ranging from a phenotype
of 0 (nearly colorless, genotype aa bb cc) to 6 (dark red, genotype
AA BB CC).
Nilsson-Ehle first crossed true-breeding dark red plants
( AA BB CC) with true-breeding colorless plants (aa bb cc) to obtain
the hybrids with genotype Aa Bb Cc. He then crossed the hybrids
together, which is the cross shown at the top of Fig. 18.4. Because 0 1 2 3 4 5 6
the three genes show independent assortment (Chapter 16), the Intensity of red coloration
distribution of phenotypes expected in progeny from the cross
 CHAPTER 18 T H E G E N E T I C A N D E N V I RO N M E N TA L B A S I S O F CO M P L E X T R A I T S 367

Aa Bb Cc  Aa Bb Cc is as shown in the Punnett square. The bar to differences in the environment. For other traits, the variation
graph below the Punnett square shows the seed-color phenotypes is due mainly to genetic differences. In the case of human height,
and their relative proportions, from which Nilsson-Ehle inferred roughly 80% of the variation among individuals of the same sex is
three genes with independent assortment. The distribution of due to genetic differences, and the remaining 20% to differences in
seed-color phenotypes is approximated by a bell-shaped curve their environment, primarily differences in nutrition during their
known as a normal distribution. The phenotypes of many years of growth.
complex traits, including human height, conform to the normal
distribution. Nilsson-Ehle’s seed-color case is exceptional in Genetic and environmental effects can interact in
that virtually all complex traits are affected by many more unpredictable ways.
genes than three (in the case of human height, hundreds, as One of the features of complex traits is that genetic and
discussed below). environmental effects may interact, often in unpredictable ways.
When differences in phenotype due to the environment Consider the example shown in Fig. 18.5. In this experiment,
can be ignored, the genetic variation affecting complex traits two strains of corn were each grown in a series of soils in which
can be detected more easily. And when studying inbred lines, the amount of nitrogen had been enriched by the growth of
differences in phenotype due to genotype can be ignored legumes. The yield of strain 1 varies little across these different
because all individuals have the same genotype, and the effects environments. The yield of strain 2, however, increases
of environment can be observed. In most cases, however, both dramatically with soil nitrogen. Each of the lines is known as a
genetic variation and environmental variation among individuals norm of reaction, which for any genotype graphically depicts
are present, and it is difficult to quantify how much variation in how the environment (shown on the x-axis) affects phenotype
phenotype is due to genes and how much is due to environment. (shown on the y-axis) across a range of environments. Note that in
It’s important to point out that complex traits are not really one case in Fig. 18.5, the environment has little to no effect on the
more “complex” than any other biological trait. The term is used phenotype (the norm of reaction is nearly flat), but in the other it
merely to imply that both genetic factors and environmental has a very noticeable effect. In cases like strain 2, it is impossible
factors contribute to variation in phenotype among individuals. to predict the phenotype of a given genotype without knowing
Just as there are environmental factors that affect complex what the environmental conditions are and how the phenotype
traits such as height, so there are genetic factors that affect changes in response to variation in the environment.
height. Similarly, just as an environmental risk factor increases Such variation in the effects of the environment on different
the likelihood of a common disease, so does a genetic risk factor genotypes is known as genotype-by-environment interaction.
predispose an individual to the condition. For example, the This type of interaction is important because it implies that the
human gene ApoE encodes a protein that helps transport fat and effect of a genotype cannot be specified without knowing the
cholesterol. Certain alleles of ApoE are associated with high levels environment, and the other way around. A further implication is
of cholesterol, and one particular allele is a genetic risk factor for that there may be no genotype that is the “best” across a broad
Alzheimer’s disease, the most common serious form of age-related
loss of cognitive ability. Lifestyle environmental factors such as
diet, physical exercise, and mental stimulation are also thought to
play a role in the risk of Alzheimer’s disease. FIG. 18.5 Genotype-by-environment interaction for grain yield
in corn. The effect of soil nitrogen on grain yield is
The relative importance of genes and environment can minimal in strain 1, but dramatic in strain 2. Data courtesy
of J. W. Dudley.
be determined by differences among individuals.
For any one individual, it is impossible to specify the relative roles
of genes and environment in the expression of a complex trait. For 5000 Corn strain 1
Corn strain 2
example, in an individual 66 inches tall it would be meaningless
Grain yield (kg/hectare)

to attribute 33 inches of height to parentage (genes) and 33 inches 4000

of height to nutrition (environment). This kind of partitioning
makes no sense because genes and environment act together so 3000
intimately in each individual that their effects are inseparable. To
attempt to separate them for any individual would be like asking 2000
to what extent it is breathing or oxygen that keeps us alive. Both
are important, and both must take place together if we are to live. 1000
It is nevertheless possible to separate genes and environment
in regard to their effects on the differences, or variation, among
individuals within a particular population. For some traits, the 0 20 40 60 80 100 120
variation seen among individuals is due largely if not exclusively Nitrogen enrichment (kg/hectare)
368 SECTION 18.2 R E S E M B L A N C E A M O N G R E L AT I V E S

FIG. 18.6 Genotype-by-environment interaction for obesity in 18.2 RESEMBLANCE AMONG

mice. When fed a normal diet, both inbred lines grow to RELATIVES
about the same size. When fed a high-fat diet, mouse A
becomes obese but mouse B does not. Source: Phil Jones/ Mendel had many advantages over Galton in his studies of
Georgia Regents University. inheritance. Mendel’s peas were true breeding, produced a new
generation each year, and yielded large numbers of progeny. The
environment had a negligible effect on the traits, and the genetic
effect on each trait was due to alleles of a single gene. Mendel’s

A
FIG. 18.7 Galton’s data showing distribution of height of
offspring of (a) the tallest parents and (b) the shortest
parents. Data from F. Galton, 1888, “Co-Relations and Their
Measurement, Chiefly from Anthropometric Data,” Proceedings of the
B
Royal Society, London 45:135–145, reprinted in K. Pearson, 1920,
“Notes on the History of Correlation,” Biometrika 13:25– 45.
a.
Population Offspring Parental
mean mean mean
When parents are
above average
2.0 height, on average
their offspring are
shorter than the
parents, but taller
Number of offspring

1.5 than the average

of the population.

1.0

range of environments, and likewise no environment that is

0.5
“best” for all genotypes. For complex traits, phenotype depends
on both genotype and environment. For the strains of corn in
Fig. 18.5, for example, which strain is “better” depends on the 0
soil nitrogen. With little nitrogen enrichment, strain 1 is better; 64 65 66 67 68 69 70 71 72 73
at intermediate values, both strains yield about the same; and Height of offspring (inches)
with high nitrogen enrichment, strain 2 is better. In this case,
knowing the norms of reaction, and recognizing the magnitude b.
Parental Offspring Population
and direction of genotype-by-environment interaction, allow each mean mean mean
farmer to use the strains likely to perform best under the available When parents are
below average
conditions of cultivation. height, on average
5
Genotype-by-environment interaction makes the interplay their offspring are
between genes and the environment difficult to predict. An taller than the
4 parents, but shorter
example of genotype-by-environment interaction affecting
Number of offspring

than the average of

obesity is shown in Fig. 18.6. The animals shown are adults of two the population.
inbred lines of mice. When fed a normal diet, both inbred lines 3
grow to about the same size. When fed a high-fat diet, however,
the inbred line A becomes obese, but inbred line B does not. Hence, 2
it is not genotype alone that causes obesity in line A because with
a normal diet, line A does not become obese. Nor is it a high-fat 1
diet alone that causes obesity, because the high-fat diet does not
cause obesity in line B. Rather, the obesity in line A results from
0
a genotype-by-environment interaction (which in this case is a 64 65 66 67 68 69 70 71 72 73
genotype-by-diet interaction). Height of offspring (inches)
 CHAPTER 18 T H E G E N E T I C A N D E N V I RO N M E N TA L B A S I S O F CO M P L E X T R A I T S 369

crosses yielded simple ratios such as 3 : 1 or 1 : 1, which could be greater than the mean height of the whole population of the study
interpreted in terms of segregation of dominant and recessive (68.25 inches) but less than that of the parents. The bar graph in
alleles. Fig. 18.7b is the distribution of height of progeny of the shortest
By contrast, Galton studied variation in such traits as parents, who averaged 66 inches. In this case, the mean height of
height in humans. Humans are obviously not true breeding the progeny is 67 inches, which is less than the mean height of the
and have few offspring. Galton had one big advantage, though: population but greater than that of the parents. Note, however,
Whereas most simple Mendelian traits are relatively uncommon, that some of the offspring of tall parents are taller than their
Galton’s traits are readily observed in everyday life. What did parents, and likewise some of the offspring of short parents are
Galton discover? shorter than their parents. It is only on average that the height of
the offspring is less extreme than that of the parents.
For complex traits, offspring resemble parents but Galton’s results can be plotted on a graph, represented as the
show regression toward the mean. blue data points in Fig. 18.8, in which the average height of each
Galton’s observations are as important in understanding complex pair of parents (called the midparent value) is compared with that
traits as Mendel’s are in understanding single-gene traits. He of their child. For convenience in visualization, midparent heights
studied many complex traits, including human height, strength, are grouped into categories, and the mean of each group is plotted
and various other physical characteristics, including number of along the x-axis. For each group of parents, the height of the
fingerprint ridges. The discovery he regarded as fundamental offspring is also averaged and plotted along the y-axis.
resulted from his data on adult height of parents and their progeny Galton regarded this observation as his most important
(Fig. 18.7). Galton noted that each category of parent (tall or discovery, publishing his results in 1886. Today, we call it regression
short) produced a range of progeny forming a distribution with its toward the mean. The offspring exhibit an average phenotype that
own mean. is closer to the population mean than the phenotype of the parents.
The bar graph in Fig. 18.7a shows the distribution of height In other words, when the average height of the parents is smaller
among the progeny of the tallest parents, whose average height is than the population mean, then the average height of the offspring
72 inches. The mean height of the offspring is 71 inches, which is is greater than that of the parents (but smaller than the population
mean). Likewise, when the average
height of the parents is greater than
the population mean, then the average
FIG. 18.8 Regression toward the mean. Galton’s data on the average adult height of
height of the offspring is smaller than
parents and that of their offspring show that offspring mean height (blue line) falls
that of the parents (but greater than the
between the parental mean (dashed red line) and the population mean (dashed
population mean).
black line).
Regression toward the mean is
This line is expected when observed for two reasons. The first is that
all variation is due to
74 genotype. The offspring during meiotic cell division (Chapter 11),
mean equals the parental segregation and recombination break
0
1. mean. up combinations of genes that result in
=
e
l op extreme phenotypes, such as very tall or
S Galton’s result shows what
72 very short, that are present in the parents.
happens when variation is
0.6 due to genotype and
Mean height of offspring (inches)

= The second reason is that the phenotype

pe
: slo environment. The of the parents results not only from
ta offspring mean shows
’ s da genes but also from the environment.
70 n regression from the
a lto parental mean toward the
G Environmental effects are not inherited,
population mean.
so any effect of the environment on
the parents’ phenotypes is not
This line is expected
68 Slope = 0.0 when all variation is due transmitted to the offspring. For example,
to environment. The if the parents are tall or short purely
offspring mean equals
the population mean. because of an environmental effect like
better or worse nutrition, the average
66
height of the offspring will be equal to the
population mean.

j Quick Check 1 Does regression

64
64 66 68 70 72 74 toward the mean imply that the human
Mean height of parents (inches) population is getting shorter over time?
370 SECTION 18.2 R E S E M B L A N C E A M O N G R E L AT I V E S

Heritability is the proportion of the total variation due among individuals, and specifically to the proportion of
to genetic differences among individuals. the variation among individuals in a population due to differences
How much of the difference in height among individuals is due in genotype.
to genetic differences, and how much is due to environmental Hence, a heritability of 100% does not imply that the
differences? The slope of the line that relates the average phenotype environment cannot affect a trait. As emphasized earlier, the
of parents to the average phenotype of their offspring (in the case environment is always important, just as oxygen is important
of Galton’s data, the blue line in Fig. 18.8) can answer this question to life. What a heritability of 100% means is that variation in the
because it provides a measure of the heritability of the trait. The environment does not contribute to differences among individuals
heritability of a trait in a population of organisms is the proportion in a specific population. For example, if genetically different
of the total variation in the trait that is due to genetic differences strains of chrysanthemums are grown in a greenhouse and
among individuals. For a complex trait, the heritability determines subject to identical environmental conditions, then differences
how closely the mean of the progeny resembles that of the parents. in flowering time have to be due to genetic differences, and the
In Galton’s data, the slope of the line is 0.6 and therefore the heritability of the trait would be 100%. Similarly, a heritability
heritability of height in this population is 60%. of 0% does not imply that genotype cannot affect the trait. A
The red dashed line in Fig. 18.8 presumes a hypothetical ideal heritability of 0% merely means that differences in genotype do
trait in which variation is determined completely by genetic not contribute to the variation in the trait among individuals
differences among individuals and the heritability is 100%. When in a specific population. If genetically identical strains of
heritability is 100%, the slope of the line representing the trait chrysanthemums are grown in different environments, then
is 1. As we do with most complex traits, such as Nilsson-Ehle’s differences in flowering time must be due to the environment, and
wheat seed color, we assume that each of a large number of genes heritability would be 0%.
contributing to the trait has two alleles, one that contributes Heritability therefore is not an intrinsic property of a trait. For
to the trait and one that does not, and the genes undergo chrysanthemum flowering time, the heritability in one case was
independent assortment. The number of alleles in the genotype 100% and in the second 0%. Heritability applies only to the trait in
contributing to the trait determines the phenotype in any a particular population across the range of environments that exist
individual for that trait. (The genetic model is like that shown in at a specific time. Similarly, the heritability depicted by the slope
Fig. 18.4 but with many more genes.) In this ideal case in which of the blue line in Fig. 18.8 applies only to the population studied
heritability is 100%, the average phenotype of the offspring from (205 pairs of British parents and their 930 adult offspring, in the
any pair of parents will equal the average phenotype of the parents late nineteenth century) and may be larger or smaller in different
themselves. In the real world, the ideal is rarely encountered, populations at different times. In particular, the magnitude of the
and deviations from the red line result from complications heritability cannot specify how much of the difference in average
such as dominance, genetic linkage, and epistasis (non-additive phenotype between two populations is due to genotype and how
contributions of the alleles of different genes). much due to environment.
The black dashed line in Fig. 18.8 represents the opposite If the heritability of a trait can change depending on the
extreme, a hypothetical ideal trait in which variation is population and the conditions being studied, why is it useful?
determined completely by differences in the environment Heritability is important in evolution, particularly in studies of
among individuals and the heritability is 0%. When heritability artificial selection, a type of selective breeding in which only
is 0%, the line representing the trait has a slope of 0. As long as certain chosen individuals are allowed to reproduce (Chapter 21).
environmental effects are not transmitted from one generation Practiced over many generations, artificial selection can result in
to the next, the average phenotype of the offspring will be equal considerable changes in morphology or behavior or almost any
to the average of the population as a whole, no matter what the trait that is selected. The large differences among breeds of pigeons
phenotypes of the parents. Although environmental effects are and other domesticated animals prompted Charles Darwin to
usually not transmitted in plant and animal populations, human point to artificial selection as an example of what natural selection
populations are exceptional in showing cultural transmission of could achieve. Heritability is important because this quantity
some environmental effects. For example, the average wealth of determines how rapidly a population can be changed by artificial
the offspring of rich parents is greater than that of the offspring of selection. A trait with a high heritability responds rapidly to
poor parents, and this difference is obviously due to transmission selection, whereas a trait with a low heritability responds slowly
of the parents’ money, not their genes. or not at all.
The term “heritability” is often misinterpreted. The
problem is that, in non-scientific contexts, the word means “the j Quick Check 2 Many people are surprised to learn that,
capability of being inherited or being passed by inheritance.” This while each individual’s fingerprints are unique, the total
definition suggests that heritability has something to do with number of fingerprint ridges is highly heritable, about 90%
the inheritance of a trait. But “heritability” as used for complex heritability in many populations. What does high heritability
traits means no such thing. It refers only to the variation in a trait of this trait mean?
 CHAPTER 18 T H E G E N E T I C A N D E N V I RO N M E N TA L B A S I S O F CO M P L E X T R A I T S 371

In contrast, fraternal twins result when two separate eggs,

18.3 TWIN STUDIES produced by a double ovulation, are fertilized by two different
sperm. Whereas identical twins are genetically identical, fraternal
Galton pioneered studies of twins as a way to separate the effects twins are only as closely related as any other pair of siblings
of genotype and environment on phenotypic differences among (Fig. 18.9b).
individuals. Depending on whether they arise from one or from
two egg cells, twins can be identical (monozygotic twins) or Twin studies help separate the effects of genes and
fraternal (dizygotic twins). Identical twins arise from a single environment in differences among individuals.
fertilized egg (the zygote), which, after several rounds of cell The utility of twins in the study of complex traits derives from the
division, separates into two distinct but genetically identical genetic identity of identical twins. Whereas differences between
embryos. Strikingly similar in overall appearance (Fig. 18.9a), fraternal twins with regard to any trait may arise because of
identical twins have stimulated the imagination since antiquity, genetic or environmental factors (or genotype-by-environment
inspiring stories by the Roman playwright Plautus, William interactions), differences between identical twins must be due
Shakespeare (himself the father of twins), Alexander Dumas, only to environmental factors because the twins are genetically
Mark Twain, and many others. identical. Consequently, if variation in a trait has an important
genetic component, then identical twins will be more similar to
each other than fraternal twins are similar to each other. But if the
genetic contribution to variation is negligible, then identical twins
FIG. 18.9 Identical twins. (a) Identical (monozygotic) twins arise
will not be more similar to each other than fraternal twins are to
from a single fertilized egg and are genetically identical.
each other. One caveat of twin studies is that the environment of
(b) Fraternal (dizygotic) twins arise from two different
identical twins is often more similar than that of fraternal twins.
fertilized eggs and are no more closely related than other
This difficulty can be minimized in part by studying identical
pairs of siblings. Sources: a, AP Photo/Macomb Daily, Craig
twins separated from each other shortly after birth and raised in
Gaffield; b. Wendy Connett/Getty Images.
different environments,
a For complex traits, such as depression or diabetes, the extent to
which twins are alike is measured according to the concordance of
the trait, which is defined as the percentage of cases in which both
members of a pair of twins show the trait when it is known that at
least one member shows it. The relative importance of genetic and
environmental factors in causing differences in phenotype can be
estimated by comparing the concordances of identical and fraternal
twins. A significantly higher concordance rate for a given trait for
identical twins compared to that for fraternal twins suggests that
the trait has a strong genetic component. Similar concordance rates
for identical and fraternal twins suggest that the genetic component
is less important.
In the study of concordance, twin pairs in which neither
member is affected contribute no information, and these twin
pairs must be removed from the analysis. The concordance is based
b solely on twin pairs in which one or both members express the
trait. To take a concrete example, let us symbolize each member of
a twin pair as a closed square (■) if the member shows the trait and
as an open square ( ) if the member does not show the trait. Then,
in any sample of identical twins, there are three possibilities,
illustrated with data for adult-onset diabetes:

■■ 36 identical twin pairs, both members showing adult-onset diabetes

■ 40 identical twin pairs, only one member showing adult-onset diabetes

336 identical twin pairs, neither member showing adult-onset diabetes

In this example, the 336 twin pairs who do not show the trait
provide no information. The concordance is based only on the first
two types of twin pairs, among which in 36 pairs both members
show the trait and in 40 pairs only one shows the trait. In this
372 SECTION 18.3 T WIN STUDIES

case the concordance among identical twins is 36/(36 1 40) 5 (which we now know to include diet and exercise) are also
47%. Among fraternal twins, by contrast, the concordance for important in the risk of adult-onset diabetes.
adult-onset diabetes is only 10%. The difference in concordance Table 18.1 shows identical and fraternal twin concordance
between identical and fraternal twins (47% for identical twins rates for a number of other disorders. Just as with diabetes, the
versus 10% for fraternal twins) implies that differences in the risk marked difference in concordance rates between identical and
of adult-onset diabetes have an important genetic component. fraternal twins for high blood pressure, asthma, rheumatoid
Furthermore, the observation that the concordance for identical arthritis, and epilepsy suggests that all these disorders have an
twins is much less than 100% implies that environmental factors important genetic component. In addition, the fact that the

HOW DO WE KNOW?

FIG. 18.10
For some traits the Some traits show

What is the relative

differences between negligible differences
identical and fraternal between identical
twins are huge. and fraternal twins.

importance of genes and 80

of the environment for

70
Identical (monozygotic)
Fraternal (dizygotic)
60
complex traits?
Twin concordance (%)

50

BACKGROUND Twin studies remain important for assessing 40

the relative importance of “nature” (genotype) and “nurture”
30
(environment) in determining variation among individuals for
complex traits. The idea of using twins to distinguish nature from 20
nurture is usually attributed to Francis Galton because of an article
he wrote about twins in 1875. In fact, the modern twin study does 10
not trace to Galton but to Curtis Merriman in the United States and
0
Hermann Siemens in Germany, who independently hit upon the
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EXPERIMENT The rationale of a twin study is to compare identical
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Cli
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twins with same-sex fraternal twins. In principle, identical twins

nd
tio

differ only because of environment, whereas fraternal twins differ

ten

because of genotype as well as environment. The extent to which

At

fraternal twins differ more from each other than identical twins
differ from each other measures the effects of genotype. Some CONCLUSION The examples shown here indicate very large
twin studies focus on twins reared apart in order to compensate for differences in the importance of genotype versus environment
shared environmental influences that may be stronger for identical among complex traits. These results are typical of most complex
twins than for fraternal twins. traits.

RESULTS The bar graph shows the results of typical twin studies FOLLOW-UP WORK Twin studies must be interpreted in light of
for various traits. The concordance between twins is the fraction of other studies comparing complex traits among individuals with
twin pairs in which both twins show the trait among all those pairs various degrees of genetic relatedness. On the whole, twin studies
in which at least one twin shows the trait. Roughly speaking, the of complex traits have yielded results that are consistent with other
difference in the concordance between identical twins and fraternal available evidence.
twins is a measure of the relative importance of genotype. In the
data shown here, for example, autism and clinical depression both SOURCES Rende, R. D., R. Plomin, and S. G. Vandenberg. 1990. “Who Discovered the
Twin Method?” Behavior Genetics 20:277–285; McGue, M., and T. J. Bouchard, Jr. 1998.
show strong genetic influences, whereas female alcoholism shows “Genetic and Environmental Influences on Human Behavioral Differences.” Annual Review
almost no genetic influence. of Neuroscience 21:1–24.
 CHAPTER 18 T H E G E N E T I C A N D E N V I RO N M E N TA L B A S I S O F CO M P L E X T R A I T S 373

year—collectively more than 35,000—are

TABLE 18.1 Twin Concordance Rates for Several Traits.
indicated in Fig. 18.11. About 20% of these
anomalies are due to an extra chromosome.
CONCORDANCE IN CONCORDANCE IN
DISORDER IDENTICAL TWINS (%) FRATERNAL TWINS (%) Trisomy 21 (Down syndrome) is by far the
most common of these chromosome-number
High blood pressure 25 7 abnormalities. Another roughly 10% are due to
Asthma 47 24 defects in metabolism, many of which—among
Rheumatoid arthritis 34 7 them cystic fibrosis, sickle-cell anemia, and
Epilepsy 37 10 phenylketonuria (the inability to break down
Handedness (left or right) 79 77 the amino acid phenylalanine)—are rare simple
Measles 95 87 Mendelian disorders. Birth anomalies due to
Acute infection leading to death 8 9 extra or missing chromosomes and those due
to single-gene Mendelian inheritance are not
Data from M. McGue and T. J. Bouchard, Jr., 1998, “Genetic and Environmental Influences on Human Behavioral Differences,”
Annual Review of Neuroscience 21:1–24. considered complex traits in terms of their
inheritance. Together these account for about
concordance is not 100%, even for identical twins, implies that 30% of the of birth anomalies in Fig. 18.11. The majority (70%) of
both genes and environment—“nature” and “nurture”—play birth anomalies in Fig. 18.11 are inherited as complex traits that are
important roles in the differences in risk among people. affected by both genetic and environmental risk factors.
Contrast these results with the data in Table 18.1 for
handedness, measles, or death from acute infection. Note that Most common diseases and birth defects are affected
in these cases the concordance rates for identical twins are very by many genes that each have relatively small effects.
similar to those for fraternal twins—there is no significant genetic Because the most common birth anomalies as well as childhood
component to variation in the trait, at least insofar as can be and adult disorders are complex traits, biologists are keenly
determined from twin studies. (Many students are quite surprised interested in identifying the genes that contribute to differences
to learn that handedness does not have a detectable genetic in risk, understanding what these genes do, and translating this
component.) For infectious diseases, exposure to the disease knowledge into prevention or treatment. They have therefore
agents in the environment plays an important role. In the case
of measles, for example, the data imply that, whether twins are
identical or fraternal, when one twin catches measles, the other FIG. 18.11 Incidence of the most common birth anomalies.
will very likely catch it, too. Data from Centers for Disease Control and Prevention; National
Fig. 18.10 shows the identical and fraternal concordance Newborn Screening and Genetics Resource Center; March of Dimes.
rates for a number of common behavioral disorders. For some,
such as schizophrenia, depression, and autism, the difference Numbers in parentheses
between identical and fraternal twins is very large, again implying are the approximate
numbers of babies with
an important role for genetic factors. For other traits, such as the indicated birth
alcoholism in females, the difference is negligible, suggesting an anomaly, out of a total of Congenital eye disorders (863)
important role for environmental and social factors. This example just over 4 million babies Neural tube defects (1345)
born in the United States Congenital
also illustrates that complex traits can have quite different risk each year. gastrointestinal
factors and outcomes in different sexes. defects (2983)

Trisomy
13, 18, 21
18.4 COMPLEX TRAITS IN (7157) Metabolic
HEALTH AND DISEASE disorders
(3104)
Much evidence beyond twin studies implies an important role for
particular alleles as risk factors for diabetes, high blood pressure, Cleft palate and
other deformities Congenital defects
asthma, rheumatoid arthritis, epilepsy, schizophrenia, clinical of the mouth of the skeleton
depression, autism, and many other conditions. These are all and palate and muscles
complex traits, affected by genotype, environment, and genotype- (7012) (6001)
by-environment interactions. Congenital defects
Even the most common birth defects are affected by multiple of the heart and
blood vessels
genetic risk factors. The most common birth anomalies and (6754)
the numbers of affected babies born in the United States each
374 SECTION 18.4 CO M P L E X T R A I T S I N H E A LT H A N D D I S E A S E

begun to apply modern molecular methods—genome sequencing

(Chapter 13), genome annotation (Chapter 13), and genotyping FIG. 18.12 Genes affecting the level of serum cholesterol in
(Chapter 15)—to identifying genes affecting complex traits. the human genome. Data from X. Wang and B. Paigen, 2005,
The identification of genes affecting complex traits is not “Genetics of Variation in HDL Cholesterol in Humans and Mice,”

only a major goal of much current research in human genetics, Circulation Research 96:27– 42.

but also in the genetics of domesticated animals and plants. 1 2 3 4 5 6

Gene identification is also important in model organisms used in
* ** ** *
*
research, especially the laboratory mouse. Much is already known **
about these model organisms and they are easily manipulated,
* *
* * *
making it possible to discover the molecular mechanisms by which *
*
genes affecting complex traits exert their effects. * *
The study of complex traits has reached a stage at which *
patterns are beginning to emerge. Many of these patterns are *
* *
exemplified by the chromosome map in Fig. 18.12, which * *
*
shows the location of genes in the human genome that affect *
cholesterol levels. The first observation is that many genes
contribute to amounts of different types of cholesterol in *
humans—specifically, more than 50 genes contribute to the level
7 8 9 10 11 12
of HDL (high-density lipoproteins, or “good cholesterol”), LDL
(low-density lipoproteins, or “bad cholesterol”), and triglycerides. ** ** *
A second observation is that many of these genes affect two or *
even all three of the types of molecule. (A single gene that has * * *
multiple effects is said to show pleiotropy.) Third, many of the * *
* * *
genes show epistasis, that is, multiple genes act in the same * ** **
pathway to affect a trait (Chapter 16). Fourth, many of the genes
* *
occur in clusters, being physically close together in the same *
*
chromosome. Clustering of genes reflects the fact that many
13 14 15 16 17 18
genes arose through the process of duplication of a single gene
followed by divergence over time, generating a family of genes
*
near one another with related functions (Chapter 14). Finally, * *
the functions of all of the genes that contribute to a complex ** ** *
*
trait are not known. Some of the genes in Fig. 18.12 affect serum
** * *
cholesterol through known metabolic pathways, but many others
work through unknown pathways. Many of the human genes 19 20 21 22 X
shown have counterparts in the mouse genome that have similar **
effects on serum cholesterol, so these genes are open to direct * * *
experimental investigation. ** *
*
A principle that is not evident in Fig. 18.12 is that the
effects of each of the genes on cholesterol levels are very
Genes affecting high-density lipoprotein (HDL)
unequal. Some have relatively large effects, whereas others have Genes affecting triglycerides (TG)
small ones. The distribution of the magnitude of gene effects Genes affecting low-density lipoprotein (LDL)
for complex traits resembles that shown in Fig. 18.13. The axes
* ** Corresponding genes in mice have
similar effects.
are labeled only in relative terms, because the actual numbers
and effect sizes differ from one trait to the next. However, the
main point is that for the majority of genes that contribute to
a complex trait, the magnitude of their individual effects is
typically quite small. The magnitude of the effects of individual identify: those that are numerous with small effects, or those that
genes also often differs between the sexes, which helps explain are few with large effects?
why complex traits so often differ in prevalence or severity
between males and females. Human height is affected by hundreds of genes.
For most complex traits, the genes that have been identified
j Quick Check 3 When genes for complex traits have effects to date account for only a relatively small fraction of the total
that are distributed as shown in Fig. 18.13, which are easier to variation in the trait. One extreme example is adult height. An
 CHAPTER 18 T H E G E N E T I C A N D E N V I RO N M E N TA L B A S I S O F CO M P L E X T R A I T S 375

cancer because of a mutation in the BRCA1 or BRCA2 genes,

FIG. 18.13 Relationship between number of genes and their while another might develop breast cancer because of other
effect on complex traits. genetic risk factors or even environmental ones. Similarly, one
Manyy person might develop emphysema because of a mutation in
Most of the genes the gene that encodes the enzyme alpha-1 antitrypsin (a1AT)
affecting the trait (Chapter 15), and another might have emphysema as a result of
have relatively small
effects. cigarette smoking. Other examples where the same disease can
affecting the trait
Number of genes

be the result of different underlying genetic or environmental

factors include elevated cholesterol levels, high blood pressure,
and depression. Because the underlying genetic basis for the same
disease may be different in different patients, some patients
For a typical complex respond well to certain drugs and others do not.
trait, only a few genes
have a relatively large The traditional strategy for treating diseases is to use the
effect. same medicine for the same disease. However, because we now
Few know that the same disease may have different causes, another
Small Large possibility has emerged: identifying each patient’s genotype for
Magnitude of gene effect
each of the relevant genes, and then matching the treatment to
the genetic risk factors in each patient. The approach is known
enormous amount of data is available for height, not because as personalized medicine. Personalized medicine matches the
height has been studied extensively for its own sake, but because treatment to the patient, not the disease.
in studies of common diseases the height of each individual Personalized medicine not only aims to identify ahead of time
is recorded, and hence data on height are available without medicine that will work effectively, but also to avoid medicines
additional effort or expense. that may lead to harmful side effects, even death. Advertisements
In one analysis, results from 79 separate studies were and enclosures with prescription drugs enumerate long lists of
combined. These studies included more than 250,000 individuals side effects that you may get if you take a particular medicine. At
of European ancestry who were genotyped for common nucleotide present, it is difficult to know in advance who will get one or more
variants (SNPs) at about 3 million nucleotide sites. The analysis of these side effects and who won’t—that is, the side effects of
identified 697 genes affecting height. Some of these genes are taking medicine are themselves complex traits and the result of
known to affect skeletal development, growth hormones, or many underlying genetic and environmental factors. If we could
other growth factors, but most have no obvious connection to identify ahead of time those patients who will respond negatively
the biology of growth. Some of the genes had previously been to a particular medicine and those who won’t, we could minimize
identified by studies of rare mutations that have pronounced potentially harmful effects of medicines on certain individuals.
effects on skeletal growth, either in human families or in Someday, it may be possible to determine each patient’s genome
laboratory mice. An unexpected finding was that a few genes sequence quickly, reliably, and cheaply. At present, personalized
affecting height are known to be associated with bone mineral medicine is restricted to studies of a few key genes known to
density, obesity, and rheumatoid arthritis. These and many other have important effects on treatment outcomes. In the treatment
of the genes identified might affect height indirectly. of asthma, for example, some of the differences in response to
The 697 genes for height identified among the 250,000 albuterol inhalation have been traced to genetic variation in the
individuals account for about 60% of the genetic variation in gene ADRB2, encoding the b-(beta-)2-adrenergic receptor. Similarly,
height, a number that suggests that many more genes with still certain drugs used in the treatment of Alzheimer’s disease are
smaller effect also contribute to variation in height. And this less effective in women with a particular apolipoprotein E (APOE)
analysis does not address at all the effects of the environment on genotype than in other classes of patients. Also, more than half of
human height, nor genotype-by-environment interactions, which the cases of muscle weakness occurring as an adverse effect of drug
likely play an important role as well. Complex traits therefore treatment used to control high cholesterol can be traced to genetic
require sophisticated studies to tease apart all the genetic and variation in the gene SLCO1B1, which encodes a liver transport
environmental factors that play a role. protein. Certain variants of this protein have decreased ability to
transport the drug, resulting in higher concentrations remaining in
? CASE 3 YOU, FROM A TO T: YOUR PERSONAL GENOME the blood and a risk of muscle weakness.
Can personalized medicine lead to effective Other factors in addition to genes are involved in these
treatments of common diseases? disorders, but genes play an important role. Knowledge of a
The multiple genetic and environmental factors affecting complex patient’s genotype can help guide treatment. These are only a few
traits imply that different people can have the same disease for of many examples, but they demonstrate the substantial potential
different reasons. For instance, one person might develop breast benefits of personalized medicine. •
376 SELF-ASSESSMENT

Core Concepts Summary 18.4 Many common diseases and birth defects are
affected by multiple genetic and environmental risk
18.1 Complex traits are those influenced both by the factors.
action of many genes and by environmental factors. Complex traits are often influenced by many genes with
Complex traits that are measured on a continuous scale, like multiple, interacting, and unequal, effects. page 374
human height, are called quantitative traits. page 364 Hundreds of genes affect human height. page 374
It is usually difficult to assess the relative roles of genes Personalized medicine tailors treatment to an individual’s
and the environment (“nature” versus “nurture”) in genetic makeup. page 375
the production of a given trait in an individual, but it is
reasonable to consider the relative roles of genetic and
environmental variation in accounting for differences among Self-Assessment
individuals for a given trait. page 365
1. Explain why some complex traits are also called quantitative
The relative importance of genes and environment in causing traits, and give at least one example.
differences in phenotype among individuals differs among
traits. For some traits (like height), genetic differences are 2. Name several factors that influence variation in
the more important source of variation, whereas for others complex traits.
(such as cancer), environmental differences can be the more 3. Explain why it does not make sense to try to separate
important. page 366 the effects of genes (“nature”) and the environment
Genetic and environmental factors can interact in (“nurture”) in a single individual, while it does make sense
unpredictable ways, resulting in genotype-by-environment to separate genetic and environmental effects on differences
interactions. page 367 among individuals.

4. Explain how you would go about determining the

18.2 Genetic effects on complex traits are reflected in relative importance of genes and the environment for
resemblance between relatives. variation in risk for a complex trait such as type 2 diabetes.
In an analysis of heights of parents and offspring, Galton 5. Graph a trait, like human height, with height on the
observed regression toward the mean, in which the offspring x-axis and number of individuals on the y-axis, and describe
exhibit an average phenotype that is less different from the the shape of the resulting graph.
population mean than that of the parents. page 369
6. Explain why the effect of a genotype on a phenotype
“Heritability” refers to the proportion of the total variation
cannot always be determined without knowing what
in a trait that can be attributed to genetic differences among
the environment is, and why the effect of a particular
individuals. page 370
environment on a phenotype cannot always be determined
The heritability of a given trait can differ among populations without knowing what the genotype is.
because of differences in genotype or environment. page 370
7. Define “regression toward the mean” and explain why
it occurs for complex traits.
18.3 Twin studies help separate the effects of
genotype and environment on variation in a trait. 8. Define the heritability of a trait and explain why the
heritability of a given trait depends on the population being
Monozygotic, or identical, twins result from the fertilization
studied.
of a single egg and are genetically identical. page 371
9. Define twin concordance and explain how twin studies
Dizygotic, or fraternal, twins result from the
can be used to investigate the importance of genetic and
fertilization of two eggs and are genetically related to
environmental factors in the expression of a trait.
each other in the same way that other siblings are
related to each other. page 371 10. For a typical complex trait, describe the relationship between
the number of genes affecting the trait and the magnitude of
“Concordance” refers to the percentage of cases in which
their effects on the trait.
both members of a pair of twins show the trait when it is
known that at least one member shows it. page 371 11. Explain what personalized medicine is and how it relates to
Comparisons of concordance rates of identical twins and complex traits such as human diseases.
concordance rates of fraternal twins can help to determine
to what extent variation in a particular trait has a genetic Log in to to check your answers to the Self-
component. page 371 Assessment questions, and to access additional learning tools.
 CHAPTER 19

Genetic and
Epigenetic
Regulation
Core Concepts
19.1 The regulation of gene
expression in eukaryotes
takes place at many levels,
including DNA packaging in
chromosomes, transcription,
and RNA processing.
19.2 After an mRNA is transcribed
and exported to the
cytoplasm in eukaryotes,
gene expression can be
regulated at the level of
mRNA stability, translation,
and posttranslational
modification of proteins.
19.3 Transcriptional regulation
is illustrated in bacteria by
the control of the production
of proteins needed for the
utilization of lactose, and in
viruses by the control of the
lytic and lysogenic pathways.
Lawrence Berkeley National Laboratory/Science Source.

377
378 SECTION 19.1 C H RO M AT I N TO M E S S E N G E R R N A I N E U K A RYOT E S

In the discussion of complex traits in Chapter 18, we emphasized

the principle that complex traits are not determined by single 19.1 CHROMATIN TO MESSENGER
genes with simple Mendelian inheritance. The traits we usually RNA IN EUKARYOTES
encounter in ourselves or in others, such as height and weight,
diabetes and high blood pressure, are influenced by multiple genes Gene regulation in multicellular eukaryotes leads to cell
that interact with one another and with environmental factors specialization: Different types of cell express different genes. The
such as diet and exercise. The number of genes influencing complex human body contains about 200 major cell types, and although for
traits can be large—for example, hundreds of genes contribute to the most part they share the same genome, they look and function
adult height. The activities of these genes must be coordinated in differently from one another because each type of cell expresses
time (for instance, at a particular point in development) and place different sets of genes. For example, the insulin needed to regulate
(for instance, in a particular type of tissue). For example, growth sugar levels in the blood is produced only by small patches of cells
to normal adult height requires that the production of growth in the pancreas. Every cell in the body contains the genes that
hormone be coordinated in time and place with the production of encode insulin, but only in these patches of pancreatic cells are
growth-hormone receptor. they expressed. In this section, we take a look at gene regulation
In Chapters 3 and 4, we outlined the basic steps of information as it occurs in eukaryotic cells, focusing on regulation at the level
flow in a cell, focusing on transcription and translation. In these of DNA, chromatin, and mRNA.
processes, DNA is a template for the production of messenger RNA
(mRNA), which itself is the template for the synthesis of a protein. Gene expression can be influenced by chemical
For these processes to work in a living organism to produce the modification of DNA or histones.
traits that we see, they must be coordinated so that genes are only Fig. 19.1 shows the major places where gene regulation in
expressed, or turned on, in the right place and time, and in the eukaryotes can take place. The first level of control is at the
right amount. In a multicellular organism, for example, certain chromosome, even before transcription takes place. The manner in
genes are expressed in some cells but not in others. Muscle actin which DNA is packaged in the nucleus in eukaryotes provides an
and myosin are turned on in muscle cells but not in liver or important opportunity for regulating gene expression. Regulation
kidney cells. And even for single-celled organisms like bacteria, at this level of gene expression is determined in part by whether
certain genes are expressed only in response to environmental the proteins necessary for transcription can gain physical access to
signals, such as the availability of nutrients, as we discuss in the genes they transcribe.
section 19.3. DNA in eukaryotes is packaged as chromatin, a complex of
Gene regulation encompasses the ways in which cells control DNA, RNA, and proteins that gives chromosomes their structure
gene expression. It can be thought of as the where? when? and how (Chapters 3 and 13). When chromatin is in its coiled state, the DNA
much? of gene expression. Where (in which cells) are genes turned is not accessible to the proteins that carry out transcription. The
on? When (during development or in response to changes in the chromatin must loosen to allow space for transcriptional enzymes
environment) are they turned on? How much gene product is and proteins to work. This is accomplished through chromatin
made? This chapter provides an overview of the most common ways remodeling, in which the nucleosomes are repositioned to
in which gene expression is regulated. expose different stretches of DNA to the nuclear environment.
One of the important principles we discuss is that gene One way in which chromatin is remodeled is by chemical
regulation can occur at almost any step in the path from DNA modification of the histones around which DNA is wound
to mRNA to protein—at the level of the chromosome itself, by (Fig. 19.2). Modification usually occurs on histone tails, strings
controlling transcription or translation, and, perhaps surprisingly, of amino acids that protrude from the histone proteins in the
even after the protein is made. Each of these steps or levels of gene nucleosome. Individual amino acids in the tails can be modified
expression may be subject to regulation. In other words, each by the addition (or later removal) of different chemical groups,
successive event that takes place in the expression of a gene is a including methyl groups (—CH3) and acetyl groups (—COCH3).
potential control point for gene expression. Most often, methylation or acetylation occurs on the lysine
We begin by discussing gene regulation in eukaryotes, and residues of the histone tails. Some of these modifications tend
then turn to regulation in prokaryotes and viruses. All life, from to activate transcription and others to repress transcription. The
the simplest to the most complex, requires gene regulation. Gene pattern of modifications of the histone tails constitutes a histone
regulation relies on similar processes in all organisms because code that affects chromatin structure and gene transcription.
all life shares common ancestry. Nevertheless, certain features Modification of histones takes place at key times in development
of eukaryotes—the packaging of DNA into chromosomes, to ensure that the proper genes are turned on or off, as well as in
mRNA processing, and the separation in space of transcription response to environmental cues.
and translation—provide additional levels of gene regulation in In many eukaryotic organisms, gene expression is also affected
eukaryotes that are not possible in prokaryotes. by chemical modification of certain bases in the DNA, the most
 CHAPTER 19 G E N E T I C A N D E P I G E N E T I C R E G U L AT I O N 379

FIG. 19.1 Levels of gene regulation. FIG. 19.2 Histone modifications. Typical modifications of the amino
acid lysine observed in the histone tails of nucleosomes
a. Chromatin
include addition of a methyl group (Me), addition of three
methyl groups (Me3), and acetylation (Ac).

Nucleosomes Nucleosome

DNA
b. Transcription

RNA Histone
transcript
Histone tail
of amino acids
c. RNA processing
Exon Intron

O
CH3
CH3 CH3 C CH3
CH3

mRNA AAAAA
Lysine Monomethyl Trimethyl Acetyl
(Me) lysine (Me3) lysine (Ac) lysine

promoter of a gene. Heavy cytosine methylation is associated

with transcriptional repression of the gene near the CpG island.
Nuclear pore Nuclear envelope
The methylation state of a CpG island can change over time or
d. RNA stability
in response to environmental signals, providing a way to turn
mRNA AAAAA genes on or off. Cells sometimes heavily methylate CpG islands of
transposable elements or viral DNA sequences that are integrated
e. Translation
Protein into the genome, thus preventing the expression of genes in the
foreign DNA (Chapter 14).
Ribosome Together, the modification of cytosine bases, changes to
AAAAA histones, and alterations in chromatin structure are often
f. Posttranslational modification termed epigenetic, from the Greek epi- (“over and above”) and
P O “genetic” (“inheritance”). That is, epigenetic mechanisms of gene
CH3
regulation typically involve changes not to the DNA sequence
C
itself but to the manner in which the DNA is packaged. Epigenetic
CH3
modifications can in some cases affect gene expression. They can
Phosphorylation Methylation Acetylation
be inherited through mitotic cell divisions, just as genes are, but
are often reversible and responsive to changes in the environment.
common of which is the addition of a methyl group to the base Furthermore, epigenetic modifications present in sperm or eggs
cytosine (Fig. 19.3). DNA methylation recruits proteins that are sometimes transmitted from parent to offspring, meaning that
affect changes in chromatin structure, histone modification, these chemical modifications can be inherited.
and nucleosome positioning that restrict access of transcription In humans and other mammals, about 100 genes are repressed
factors to gene promoters. Methylation of cytosines often occurs by chemical modification such as DNA methylation in the germ
in cytosine bases that are adjacent to guanosine bases on a DNA line in a sex-specific manner. This sex-specific silencing of gene
strand. Cytosine methylation often occurs in CpG islands, which expression is known as imprinting . For some genes, the allele
are clusters of adjacent CG nucleotides located in or near the of the gene inherited from the mother is imprinted and therefore
380 SECTION 19.1 C H RO M AT I N TO M E S S E N G E R R N A I N E U K A RYOT E S

FIG. 19.3 Methylation states of CpG islands in or near the promoter of a gene.

a. Undermethylated CpG island

NH2

N O
CpG island Undermethylation of the CpG
H island allows transcription.
Normal cytosine

Promoter Transcription can occur.

b. Heavily methylated CpG island

NH2
CH3
N In many organisms,
some cytosine bases
O in DNA are changed
N
CpG island by enzymes into
Heavy methylation of the CpG
H 5-methyl cytosines.
island inhibits transcription.
5-Methyl cytosine

Promoter Transcription does not occur.

silenced, so only the allele inherited from the father is expressed. gene dosage increases the level of expression because each copy of
For other imprinted genes, the allele of the gene inherited from the gene is regulated independently of other copies. For example,
the father is imprinted and therefore silenced, so only the allele as we saw in Chapter 15, the presence of an extra copy of most
inherited from the mother is expressed. The imprinting persists in human chromosomes results in spontaneous abortion because of
somatic cells through the lifetime of the individual but is reset in the increase in the expression of the genes in that chromosome.
its germ line according to the individual’s sex. XX females have twice as many X chromosomes as XY males.
Some of the genes that are imprinted affect the growth rate of For genes located in the X chromosome, the dosage of genes is
the embryo. For example, in matings between lions and tigers that twice as great in females as it is in males. However, the level
take place in captivity, the offspring of a male tiger with a female of expression of X-linked genes is about the same in both sexes.
lion (a “tigon”) is about the same size as its parents, whereas the These observations imply that the regulation of genes in the
offspring of a male lion and a female tiger (a “liger”) is a giant cat— X chromosome is different in females and in males. This
the largest of any big cat that exists. Much of the size difference differential regulation is called dosage compensation.
between a liger and a tigon is thought to result from whether Different species have evolved different mechanisms of
genes for rapid embryonic growth that are subject to imprinting dosage compensation. In the fruit fly Drosophila, males double
are inherited from the mother or the father. the transcription of the single X chromosome to achieve equal
expression compared to the two X chromosomes in females.
Gene expression can be regulated at the level of In the nematode worm Caenorhabditis, transcription of both
an entire chromosome. X chromosomes in females is decreased to one-half the level of
A striking example of an epigenetic form of gene regulation is the the single X chromosome in males. In mammals, including
manner in which mammals equalize the expression of X-linked humans, dosage compensation occurs through the inactivation
genes in XX females and XY males. For most genes, there is a direct of one X chromosome in each cell in females. This process,
relation between the number of copies of the gene (the gene known as X-inactivation, was first proposed by Mary F. Lyon in
dosage) and the level of expression of the gene. An increase in the early 1960s.
 CHAPTER 19 G E N E T I C A N D E P I G E N E T I C R E G U L AT I O N 381

FIG. 19.4 X-inactivation in female mammals. X-inactivation equalizes the expression of most genes in the X chromosome between XX females
and XY males. Source: b. Eyal Nahmias/Alamy.
a. b.

Maternal X
chromosome Paternal X
chromosome
Fertilized egg

Early
divisions

Early
divisions
Random X-inactivation in
the embryo occurs at about
the time of implantation in
Inactivated X the uterine wall.
chromosome

The mosaic black and orange

colors of a calico cat result from
X-chromosome inactivation in
In each cell lineage, a heterozygous female. (The
the inactivated X white patches are due to the
remains inactivated. expression of a different gene.)

Soon after a fertilized egg with two X chromosomes implants

in the mother’s uterine wall, one X chromosome at random is FIG. 19.5 The role of Xist in X-inactivation. The Xist transcript is
inactivated (Fig. 19.4). In Fig. 19.4a, the inactive X chromosome a noncoding RNA that is expressed from the inactive X
is shown in gray. The inactive state persists through cell division, chromosome and coats the entire chromosome, leading
so in each cell lineage, the same X chromosome that was originally to inactivation of most of the genes in the chromosome.
inactivated remains inactive. The result is that a normal female
is a mosaic, or patchwork, of tissue. In some patches, the genes in The Xist gene is
the maternal X chromosome are expressed (and the paternal transcribed and the Xist RNA
transcript spliced, X-chromosome
X chromosome is inactivated), whereas in other patches, the genes and Xist noncoding
in the paternal X chromosome are expressed (and the maternal RNA binds with the
X-chromosome XIC
X chromosome is inactivated). The term “inactive X chromosome” inactivation center (XIC).
is a slight exaggeration since a substantial number of genes are still
transcribed, although usually at a low level.
As one argument for her X-inactivation hypothesis, Lyon Transcription of
Xist continues,
called attention to calico cats, which are nearly always female and the X
(Fig. 19.4b). In calico cats, the orange and black fur colors are due chromosome
becomes coated
to different alleles of a single gene in the X chromosome. In a with Xist RNA.
heterozygous female, X-inactivation predicts discrete patches of
orange and black, and this is exactly what is observed. (The white
Eventually the
patches on a calico cat are due to an autosomal gene.) entire chromosome
How does X-inactivation work? Some of the details are still becomes coated
with Xist RNA.
unknown, but the main features of the process are shown in
Fig. 19.5. A key player is a small region in the X chromosome
Presence of Xist RNA Xist RNA
called the X-chromosome inactivation center (XIC), which
triggers DNA
contains a gene called Xist (X-inactivation specific transcript). The methylation and other
Xist gene is normally transcribed at a very low level, and the RNA is changes associated
with reduced
unstable, but in an X chromosome about to become inactive, Xist transcriptional activity. DNA
transcription markedly increases. The transcript undergoes RNA
382 SECTION 19.1 C H RO M AT I N TO M E S S E N G E R R N A I N E U K A RYOT E S

splicing, but it does not encode a protein. Xist RNA is therefore Transcriptional regulation in eukaryotic cells requires the
an example of a noncoding RNA (Chapter 3). Instead of being coordinated action of many proteins that interact with one
translated, the processed Xist RNA coats the XIC region, and as it another and with DNA sequences near the gene. Let’s first review
accumulates, the coating spreads outward from the XIC until the the basic process of transcription in eukaryotes (Chapter 3). First,
entire chromosome is coated with Xist RNA. The presence of Xist an important group of proteins called general transcription
RNA along the chromosome recruits factors that promote DNA factors are recruited to the gene’s promoter, which is the region
methylation, histone modification, and other changes associated of a gene where transcription is initiated. The transcription
with transcriptional repression. factors are brought there by one of the proteins that bind to a
The Xist gene is both necessary and sufficient for short sequence in the promoter called the TATA box, which is
X-inactivation: If it is deleted, X-inactivation does not occur; usually situated 25–30 nucleotides upstream of the nucleotide
if it is inserted into another chromosome, it inactivates that site where transcription begins. Once bound to the promoter,
chromosome. Using recombinant DNA techniques of the sort the transcription factors recruit the components of the RNA
described in Chapter 12, researchers were able to insert Xist polymerase complex, the enzyme complex that synthesizes the
into one of the three copies of chromosome 21 present in cells RNA transcript complementary to the template strand of DNA.
taken from a patient with Down syndrome. Remarkably, Xist in Where in the many steps of transcription initiation does
its new location in chromosome 21 produced an RNA transcript regulation occur? The first point at which transcription can be
that coated that copy of chromosome 21 and repressed most regulated is in the recruitment of the general transcription factors
transcription of its genes! Whether all genes in chromosome 21 are and components of the RNA polymerase complex (Fig. 19.6).
repressed, and whether they are as fully repressed as those in the Recruitment of these elements is controlled by proteins called
inactive X chromosome, are questions still being investigated, but regulatory transcription factors. Transcription does not occur if
this finding raises clear (but still speculative) ideas about how Xist the regulatory transcription factors do not recruit the components
might be used as a potential treatment for Down syndrome and of the transcription complex to the gene. Some regulatory
various other chromosomal disorders. transcription factors recruit chromatin remodeling proteins that
allow physical access to a gene. Other regulatory transcription
Transcription is a key control point in gene expression. factor have two binding sites, one of which binds with a particular
While access to DNA and appropriate histone modifications DNA sequence in or near a gene known as an enhancer (Fig. 19.6)
are necessary for transcription, they are not sufficient. The and the other of which recruits one or more general transcription
molecular machinery that actually carries out transcription is factors to the promoter region. The general transcription factors
also required once the template DNA is made accessible through then recruit the RNA polymerase complex, and transcription can
chromatin remodeling and histone modification. The mechanisms begin (Chapter 3).
that regulate whether or not transcription occurs are known Hundreds of different regulatory transcription factors control
collectively as transcriptional regulation (see Fig. 19.1b). the transcription of thousands of genes. Some bind with enhancers
and stimulate transcription; others bind
with DNA sequences known as silencers
FIG. 19.6 Protein–DNA and protein–protein interactions in the eukaryotic transcription and repress transcription. Enhancers and
complex. silencers are often in or near the genes
they regulate, but in some cases they
1 Regulatory transcription Enhancer 5’ may be many thousands of nucleotides
factors bind with a DNA sequence 3’
distant from the genes. A typical
sequence called an enhancer.

General
3 The general transcription gene may be regulated by multiple
factors recruit the components enhancers and silencers of different
transcription
DNA factors of the RNA polymerase complex, types, each with one or more regulatory
and transcription takes place.
transcription factors that can bind with
it. Transcription takes place only when
Regulatory Components of the proper combination of regulatory
transcription RNA polymerase
factor complex
transcription factors is present in
5’ the same cell, as shown in Fig. 19.6.
3’ Since transcription of a gene with
Direction of
multiple silencers and enhancers
2 Binding of regulatory TATA box transcription
transcription factors depends on the presence of a particular
recruits the general combination of regulatory transcription
transcription factors to Promoter of gene
factors, this type of regulation is called
the promoter of the gene.
combinatorial control.
 CHAPTER 19 G E N E T I C A N D E P I G E N E T I C R E G U L AT I O N 383

j Quick Check 1 The idea that the expression of some genes is

controlled by the products of other genes was originally criticized FIG. 19.7 Alternative splicing of a mammalian insulin-receptor
in this way: If n genes were to be controlled, then another n genes transcript. Alternative splicing generates different
would be needed to control them, and then another n genes processed mRNAs and different proteins from the same
would be needed to control the controllers, and so on and on. primary transcript.
How does combinatorial control help refute this criticism?
Primary transcript
Exon 9 Exon 10 Exon 11 Exon 12 Exon 13
RNA processing is also important in gene regulation.
A great deal happens in the nucleus after transcription takes place. Intron Intron Intron Intron
The initial transcript, called the primary transcript, undergoes
several types of modification, collectively called RNA processing
mRNA in liver mRNA in skeletal muscle
(Chapter 3), that includes the addition of a nucleotide cap to the
5� end and a string of tens to hundreds of adenosine nucleotides Exon 11 Exon 11
to the 3� end to form the poly(A) tail. These modifications are present missing
necessary for the RNA molecule to be transported to the cytoplasm
9 10 11 12 13 9 10 12 13
and recognized by the translational machinery. The poly(A) tail also
helps to determine how long the RNA will persist in the cytoplasm In the liver, 36 In skeletal muscle,
before being degraded. RNA processing is therefore an important nucleotides alternative splicing
constituting exon excises exon 11
point where gene regulation can occur (see Fig. 19.1c). 11 are included in along with the
In eukaryotes, the primary transcript of many protein-coding the messenger RNA. flanking introns.
genes is far longer than the messenger RNA ultimately used in
protein synthesis. The long primary transcript consists of regions
Receptor with Receptor with
that are retained in the messenger RNA (the exons) interspersed low insulin affinity high insulin affinity
with regions that are excised and degraded (the introns). The
introns are excised during RNA splicing (Chapter 3). The exons
are joined together in their original linear order to form the
processed messenger RNA. amino group (–NH2) from adenosine and converts it to inosine, a
RNA splicing provides an opportunity for regulating base that in translation functions like guanosine. Another enzyme
gene expression because the same primary transcript can be (Fig. 19.8b) removes the amino group from cytosine and converts
spliced in different ways to yield different proteins in a process it to uracil. In the human genome, hundreds if not thousands of
called alternative splicing. This process takes place because what transcripts undergo RNA editing. In many cases, not all copies of
the spliceosome—the splicing machinery—recognizes as an exon the transcript are edited, and some copies may be edited more
in some primary transcripts, it recognizes as part of an intron in
other primary transcripts. The alternative-splice forms may be
produced in the same cells or in different types of cell. Alternative
FIG. 19.8 RNA editing. RNA editing results in chemical
splicing accounts in part for the observation that we produce many
modifications to the bases in mRNA, which can lead to
more proteins than our total number of genes. By some estimates,
changes in the amino acid sequence of the protein.
over 90% of human genes undergo alternative splicing.
a.
Fig. 19.7 shows the primary transcript of a gene encoding an
NH2 O
insulin receptor found in humans and other mammals. During Deamination of adenosine
N N H
RNA splicing in liver cells, exon 11 is included in the messenger N produces inosine. N
RNA, and the insulin receptor produced from this messenger RNA
has low affinity for insulin. In contrast, in cells of skeletal muscle, N N N N
the 36 nucleotides of exon 11 are spliced out of the primary H H
transcript along with the flanking introns. The resulting protein Adenosine Inosine
is 12 amino acids shorter, and this form of the insulin receptor has b.
high affinity for insulin. The different forms of the protein are NH2 O
important: The higher sensitivity of muscle cells to insulin enables Deamination of cytosine H
N produces uracil. N
them to absorb enough glucose to fulfill their energy needs.
Some RNA molecules can become a substrate for enzymes that
N O N O
modify particular bases in the RNA, thereby changing its sequence
and what it codes for. This process is known as RNA editing H H
(Fig. 19.8). One type of editing enzyme (Fig. 19.8a) removes the Cytosine Uracil
384 SECTION 19.1 M E S S E N G E R R N A TO P H E N OT Y P E I N E U K A RYOT E S

migrates to the cytoplasm through one of a few thousand nuclear

FIG. 19.9 Tissue-specific RNA editing of the human pores, large protein complexes that span both layers of the nuclear
apolipoprotein B transcript. Different RNA editing in envelope and regulate the flow of macromolecules in and out
(a) the liver and (b) the intestine results in proteins with of the nucleus. Once the mRNA is in the cytoplasm, there are
different functions. multiple opportunities for gene regulation at the levels of mRNA
a. Processed apolipoprotein mRNA in liver stability, translation, and protein activity (see Figs. 19.1d–19.1f).
5’ 3’
U A U A U G A U A C A A U U U G A U C A G Small regulatory RNAs inhibit translation or promote
mRNA degradation.
Regulatory RNA molecules known as small regulatory RNAs are
among the most exciting recent discoveries in gene regulation.
5’ 3’
They are of exceptional interest to biologists and drug researchers
U A U A U G A U A C A A U U U G A U C A G
because their small size allows easy synthesis in the laboratory,
and researchers can design their sequences to target transcripts of
Tyr Met Ile Gln Phe Asp Gln interest.
Two important types of small regulatory RNAs are known
In the liver, the unedited mRNA encodes a protein of 4563 amino acids
that transports cholesterol in the blood. as siRNA (small interfering RNA) and miRNA (microRNA).
While they have somewhat different functions, they share
similarities in their production (Fig. 19.10). Both types of small
b. Processed apolipoprotein mRNA in intestine RNA are transcribed from DNA and form hairpin structures, or
5’ 3’ stem-and-loops, stabilized by base pairing in the stem. Enzymes in
U A U A U G A U A C A A U U U G A U C A G
the cytoplasm specifically recognize these structures and cleave
In the intestine, a C is edited to U. the stem from the hairpin, then further cut the stem into small,
double-stranded fragments 20–25 base pairs in length.
5’ 3’ One of the two strands from each RNA fragment is
U A U A U G A U A U A A U U U G A U C A G incorporated into a protein complex known as RISC (RNA-
induced silencing complex). The small, single-stranded RNA
Tyr Met Ile Stop codon targets the RISC to specific RNA molecules by base pairing with
short regions in the target. Depending on the type of small
In the intestine, the edited mRNA encodes a protein of 2152 amino acids regulatory RNA, the RNA sequence in the RISC, and the particular
that absorbs lipids from foods.
type of RISC, the small regulatory RNA may result in chromatin
remodeling, degradation of RNA transcripts, or inhibition of mRNA
translation (Fig. 19.10). Other functions have also been reported.
extensively than others. The result is that transcripts from the same Regulation by small RNAs is widespread in eukaryotes, and it
gene can produce multiple types of proteins even in a single cell. is thought to have evolved originally as a defense against viruses
Transcripts from the same gene may undergo different and transposable elements. These molecules have also evolved
editing in different cell types. An example of tissue-specific RNA to have important regulatory roles in their own right. Human
editing is shown in Fig. 19.9. The mRNA fragments show part of chromosomes are thought to encode about 1000 microRNAs, each
the coding sequence for apolipoprotein B. The unedited mRNA of which can target tens or hundreds of mRNA molecules. Half or
in the liver (Fig. 19.9a) is translated into a protein that transports more of human proteins may have their synthesis regulated in part
cholesterol in the blood. In contrast, RNA editing of the message by such small regulatory RNAs.
occurs in the intestine (Fig. 19.9b). The cytosine nucleotide in
j Quick Check 2 How do small regulatory RNAs differ from
codon 2153 is edited to uracil. The edited codon is UAA, which
messenger RNA?
is a stop codon. Translation therefore terminates at this point,
releasing a protein only about half as long as the liver form. This
shorter form of the protein helps the cells of the intestine absorb Translational regulation controls the rate, timing,
lipids from the foods we eat. and location of protein synthesis.
Translation of mRNA into protein provides another level of control
of gene expression. Fig. 19.11 shows the structure of a hypothetical
19.2 MESSENGER RNA TO mRNA molecule in a eukaryotic cell and highlights some of the
PHENOTYPE IN EUKARYOTES features that help regulate its translation (Chapter 4). Not all
mRNA molecules have all the features shown, but almost all mRNA
In eukaryotes, a processed mRNA must exit the nucleus before molecules have a 5� cap, a 5� untranslated region (5� UTR), an open
the translation step of gene expression can occur. The mRNA reading frame (ORF) containing the codons that determine the
 CHAPTER 19 G E N E T I C A N D E P I G E N E T I C R E G U L AT I O N 385

amino acid sequence of the protein, a 3ˇ untranslated

FIG. 19.10 Production of small regulatory RNAs and an overview of their region (3ˇ UTR), and a poly(A) tail. The 5ˇ UTR and 3ˇ
functions. UTR may contain regions that bind with proteins. These
RNA-binding proteins help control mRNA translation
and degradation. The UTRs may also contain binding
sites for small regulatory RNAs. During development,
Precursors of small regulatory RNAs as we will see in Chapter 20, some RNA-binding
may be double-stranded RNA or a proteins interact with molecular motors that transport
single-stranded RNA that forms a hairpin.
the mRNA to particular regions of the cell. In other
cases, the proteins are only in particular locations in
the cell and repress translation of the mRNAs that are
transported there and to which they bind. By either
transport or repression, these proteins cause the mRNA
Precursors are extensively to be translated only in certain places in the cell.
processed in the cytoplasm The cap structure is one of the main recognition
and cleaved into small (20–25 signals for translation initiation, which requires
nucleotide) fragments.
the coordinated action of about 25 proteins. These
proteins are present in most cells in limiting amounts,
RISC complex These fragments are small and so at any one time while some mRNAs from a
regulatory RNAs that are gene are being translated, other mRNAs transcribed
incorporated into a RISC complex
and target complementary mRNA from the same gene may not have a translation
sequences. initiation complex assembled. Upon formation, the
initiation complex moves along the 5ˇ UTR, scanning
Chromatin RNA degradation Translational for an AUG codon (the initiation codon) to allow the
remodeling inhibition complete ribosome to assemble and begin translation.
The 3ˇ UTR and the poly(A) tail are also important
in translation initiation. The efficiency of translation
initiation is greatly increased by physical contact between a protein
that binds the poly(A) tail and one that binds the 5ˇ cap. The physical
contact creates a loop in the mRNA, bringing the 3ˇ end of the mRNA
into proximity with the start site for translation. In fact, most mRNA
FIG. 19.11 Some features of mRNA that affect gene expression.
sequences that regulate translation are present in the 3ˇ UTR.
These include the 5ˇ cap, 5ˇ untranslated region (5ˇ UTR),
Although translation initiation is the principal mode of
3ˇ untranslated region (3ˇ UTR), and poly(A) tail.
translational regulation, not all mRNA molecules are equally
accessible to translation. Among the key variables are the secondary
Proteins can bind to
sequences in the 5’ UTR to (folded) structure of the 5ˇ UTR, the distance from the 5ˇ cap to
transport the mRNA to the AUG initiation codon, and the sequences flanking the AUG
particular locations in the initiation codon.
cell, or can affect translation
initiation directly.
Protein structure and chemical modification modulate
5’ cap 5’ UTR AUG protein effects on phenotype.
Once translation is completed, the resulting protein can alter
Cap ORF the phenotype of the cell or organism by affecting metabolism,
The translation signaling, gene expression, or cell structure. After translation,
machinery proteins are modified in
assembles on
the 5’ cap before
3’ UTR Poly(A) tail multiple ways that regulate
translation is AAAAA their structure and function.
initiated. Collectively, these processes
Translation is often regulated by Translation initiation is are called posttranslational
sequences in the 3’ untranslated enhanced by a poly(A)-binding modification (see Fig. 19.1e).
region (3’ UTR), sometimes by protein that brings the tail into
binding with specific proteins. contact with the initiation Regulation at this level is
machinery on the 5’ cap. essential because some proteins
are downright dangerous. For
example, proteases such as the
 386 SECTION 19.3 T R A N S C R I P T I O N A L R E G U L AT I O N I N P RO K A RYOT E S

digestive enzyme trypsin must be kept inactive until secreted out multiple levels, and because there is much feedback and signaling
of the cell. If they were not, their activity would kill the cell. These back and forth between nucleus and cytoplasm. It is because of
types of protein are often controlled by being translated in inactive these feedback and signaling mechanisms that the effects of
forms that are made active by modification after secretion. lifestyle choices can be propagated up the regulatory hierarchy.
Folding and acquiring stability are key control points for For example, it has been shown that dietary intake of fats and
some proteins (Chapter 4). While many proteins fold properly cholesterol affects not only the activity of enzymes directly
as they come off the ribosome, others require help from other involved in the metabolism of fats and cholesterol, but also the
proteins, called chaperones, which act as folding facilitators. levels of transcription of the genes encoding these enzymes by
Correct folding is important because improperly folded proteins affecting the activity of their regulatory transcription factors.
may form aggregates that are destructive to cell function. Similarly, lifestyles that combine balanced diets with exercise
Many diseases are associated with protein aggregates, including and stress relief have been shown to increase transcription of
Alzheimer’s disease, Huntington’s disease, and the human genes whose products prevent cellular dysfunction and decrease
counterpart of mad cow disease. transcription of genes whose products promote disease.
Posttranslational modification also helps regulate protein So far, we have been talking primarily about complex traits
activity. Many proteins are modified by the addition of one or of the type discussed in Chapter 18, which are affected by
more sugar molecules to the side chains of some amino acids. This multiple genes and by multiple environmental factors as well
modification can alter the protein’s folding and stability, or target as by genotype-by-environment interaction. For example, there
the molecule to particular cellular compartments. Reversible are both genetic and environmental risk factors for breast and
addition of a phosphate group to the side groups of amino acids ovarian cancers, as we have seen. Simple Mendelian traits caused
such as serine, threonine, or tyrosine is a key regulator of protein by mutations in single genes, such as cystic fibrosis and alpha-1
activity (Chapter 9). Introduction of the negatively charged antitrypsin (a-1AT) deficiency (Chapter 17), are less responsive
phosphate group alters the conformation of the protein, in some to lifestyle choices. But even in these cases, lifestyle matters. For
cases switching it from an inactive state to an active state and example, people with a-1AT deficiency should not smoke tobacco
in other cases the reverse. Because the function of a protein and should avoid environments with low air quality.
molecule results from its shape and charge (Chapter 4), a change
in protein conformation affects protein function.
Marking proteins for enzymatic destruction by the addition of 19.3 TRANSCRIPTIONAL REGULATION
chemical groups after translation is also important in controlling IN PROKARYOTES
their activity. For example, we have seen how the destruction of
successive waves of cyclin proteins helps move the cell through its The central message of Fig. 19.1 is that the regulation of gene
division cycle (Chapter 11). expression occurs through a hierarchy of regulatory mechanisms
acting at different levels (and usually at multiple levels) from
DNA to protein. Gene regulation in prokaryotes is simpler
? CASE 3 YOU, FROM A TO T: YOUR PERSONAL GENOME than gene regulation in eukaryotes since DNA is not packaged
How do lifestyle choices affect expression of your into nucleosomes, mRNA is not processed, and transcription
personal genome? and translation are not separated by a nuclear envelope. In
If you examine Fig. 19.1 as a whole and consider the DNA sequence prokaryotes, expression of a protein-coding gene entails
shown as your personal genome, the situation looks pretty grim. transcription of the gene into messenger RNA and translation
You might be led to believe that genes dictate everything, and that of the messenger RNA into protein. Each of these levels of gene
biology is destiny. But if you focus on the lower levels of regulation expression is subject to regulation.
in Fig. 19.1, a different picture emerges. The picture is different Because gene regulation in prokaryotes is simpler than gene
because much of the regulation that occurs after transcription regulation in eukaryotes, prokaryotes have served as model
(regulation of mRNA stability, regulation of translation, organisms for our understanding of how genes are turned on
posttranslational modifications) is determined by the physiological and off. In this section, we consider in more detail how gene
state of your cells, which in turn is strongly influenced by your expression is regulated at the level of transcription in bacteria
lifestyle choices. For example, your cells can synthesize 12 of the and in viruses that infect bacteria. We focus on two well-studied
amino acids in proteins, but if any of these is present in sufficient systems: (1) the regulation of genes in the intestinal bacterium
amounts in your diet, it is absorbed during digestion and not Escherichia coli that allows proteins needed to utilize the sugar
synthesized. The amino acid you ingest blocks the synthetic lactose to be produced only when lactose is present in the
pathway through feedback effects. environment and only when it is the best nutrient available,
The effect of an intervention—genetic or environmental—at and (2) the regulation of genes in a virus that infects E. coli that
any given level can affect regulatory processes at both higher and controls whether the virus integrates its DNA into the bacterial
lower levels. This cascade of regulatory effects in both directions host or lyses (breaks open) the cell. In both cases, specific genes
can occur because the expression of any gene is regulated at are turned on and off in response to environmental conditions.
 CHAPTER 19 G E N E T I C A N D E P I G E N E T I C R E G U L AT I O N 387

Transcriptional regulation can be positive or negative.

In both eukaryotes and prokaryotes, transcription can be positively FIG. 19.13 Role of the repressor in negative transcriptional
or negatively regulated. In positive regulation, a regulatory regulation in prokaryotes. When a repressor protein
molecule (usually a protein) must bind to the DNA at a site near binds to DNA, it prevents transcription of a gene.
the gene in order for transcription to take place. In negative a.
regulation, a regulatory molecule (again, usually a protein) must In negative transcriptional regulation, the native state
bind to the DNA at a site near the gene in order for transcription to of the DNA allows the RNA polymerase complex to be
recruited, and transcription takes place.
be prevented. Most promoters contain one or more short sequences
that function to help recruit the proteins needed for transcription RNA polymerase
when transcription of the gene is activated. In eukaryotes, many complex

promoters contain the sequence 5�-TATAAA-3� (or some variant

of this sequence) about 25–35 base pairs upstream from the
transcription start site, which helps recruit the proteins needed for Polymerase Repressor
binding binding site
transcription. Many prokaryotic promoters contain a pair of such site (promoter)
Transcription can occur.
regulatory sequences located about 10 and 35 base pairs upstream of
the transcription start site. These promoter sequences are called the b.
–10 and –35 sequence motifs.
Fig. 19.12 illustrates positive regulation. The main players
are DNA, the RNA polymerase complex, and a regulatory protein Repressor
protein
called a transcriptional activator. As shown in Fig. 19.12a, when
the activator protein is present in a state that can interact with its
binding site in the DNA, the RNA polymerase complex is recruited
Transcription does not occur.
to the promoter of the gene and transcription takes place. When
the activator is not present, or not able to bind with the DNA If the repressor protein binds to the DNA,
(Fig. 19.12b), the RNA polymerase is not recruited to the promoter it inhibits recruitment of RNA polymerase,
and transcription does not occur.
and transcription does not occur. The binding site for the activator
may be upstream of the promoter, as shown in the figure, be
downstream of the promoter, or even overlap the promoter.

Sometimes, the activator protein combines with a small

molecule in the cell and undergoes a change in shape that alters
FIG. 19.12 Role of the activator in positive transcriptional
its binding affinity for DNA. The change in shape is an example
regulation in prokaryotes. When an activator protein
of an allosteric effect (Chapter 6). In some cases, the activator
binds to DNA, it promotes transcription of a gene.
cannot bind with DNA on its own, but combining with the small
a.
molecule changes its shape and its ability to bind with particular
In positive transcriptional regulation, RNA polymerase sequences in the DNA. The presence of the small molecule in the
can bind to the promoter only if an activator protein
binds to a site near the promoter. cell therefore results in transcription of the gene. Genes subject
to this type of positive control typically encode proteins needed
Activator RNA polymerase
protein complex only when the small molecule is present in the cell. For example,
in E. coli, the genes for the breakdown of the sugar arabinose
are normally repressed in the absence of the sugar because a
Activator Polymerase binding
regulatory protein binds the DNA in such a way as to prevent
binding site site (promoter) transcription. In the presence of arabinose, however, the shape of
b. Transcription can occur. the protein is altered so that it binds a different site in the DNA,
allowing transcription of the genes. For other genes, activator
proteins can bind DNA only when a small molecule is absent
from the cell. These genes typically encode proteins needed for
synthesis of the small molecule. In E. coli, the genes for synthesis
of the amino acid cysteine are regulated in this fashion.
Fig. 19.13 illustrates negative regulation. In this case, the DNA
Transcription does not occur.
in its native state can recruit the RNA polymerase complex, and
If the activator does not bind to the DNA, the transcription takes place at a constant rate unless something turns
RNA polymerase cannot bind and transcription
does not occur.
it off (Fig. 19.13a). What turns it off is binding with a protein called
a repressor (Fig. 19.13b). Again, the binding site for the repressor
388 SECTION 19.3

can be upstream of the promoter, downstream of the promoter, or HOW DO WE KNOW?

overlapping with the promoter.
As with positive control, the ability of the repressor to bind FIG. 19.14
DNA is often determined by an allosteric interaction with a small
molecule. A small molecule that interacts with the repressor
and prevents it from binding DNA is called an inducer. In the
How does lactose lead to
next section, we will look closely at the regulation of genes for
the breakdown of the sugar lactose in E. coli, and we will see that
the production of active
a derivative of lactose binds to and inhibits the repressor and is
therefore an inducer of these genes.
b-galactosidase enzyme?
In other cases, the small molecule changes the conformation
of the repressor so that it can bind to the repressor binding site and BACKGROUND Active b-galactosidase enzyme is observed only in
inhibit transcription. Genes regulated in this way are often needed E. coli cells that are growing in the presence of lactose.
for synthesis of the small molecule. In E. coli, for example, the
HYPOTHESIS One hypothesis is that the enzyme is always being
genes for the synthesis of the amino acid tryptophan are negatively
produced, but is produced in an unstable form that breaks down
regulated by tryptophan. When tryptophan is present in sufficient
rapidly in the absence of lactose. A second hypothesis is that the
amounts, it binds with a regulatory protein to form the functional
enzyme is stable, but is produced only in the presence of lactose.
repressor, and transcription of the genes does not occur. When the
level of tryptophan drops too low to form the repressor, transcription EXPERIMENT François Jacob and Jacques Monod exposed a culture
of the genes is initiated. of growing cells to lactose and later removed it. They measured
j Quick Check 3 Explain how the ability of a large, multisubunit the amount of b-galactosidase present in the culture during the
protein molecule to bind a specific DNA sequence can be altered experiment.
when it binds with a small molecule no larger than a single amino
RESULTS Almost immediately upon addition of lactose,
acid.
b-galactosidase began to accumulate, and its amount steadily
increased. When lactose was removed, the enzyme did not disappear
Lactose utilization in E. coli is the pioneering example of immediately (as would be the case if it were unstable). Instead, the
transcriptional regulation. amount of enzyme remained the same as when lactose was present.
The principle that the product of one gene can regulate transcription This result is expected only if b-galactosidase is a stable enzyme that is
of other genes was first discovered in the 1960s by François Jacob synthesized when lactose is added and stops being synthesized when
and Jacques Monod, who studied how the bacterium E. coli regulates lactose is removed.
production of the proteins needed for utilization of the sugar 4
lactose. Lactose consists of one molecule each of the sugars glucose
β-galactosidase (micrograms)

and galactose covalently linked by a b-(beta-)galactoside bond. An

3
enzyme called b-galactosidase cleaves lactose, releasing glucose and
Lactose
galactose. Both molecules can then be broken down and used as a removed
source of carbon and energy (Chapter 7). 2
Jacob and Monod began their research with the observation
that active b-galactosidase enzyme is observed in cells only in the Lactose
presence of lactose or certain molecules chemically similar to lactose. 1 added
Why is this so? Fig. 19.14 describes and tests two hypotheses.
One is that lactose stabilizes an unstable form of b-galactosidase 0
10 20 30 40 50 60 70 80 90
produced by all cells all the time. The other is that lactose leads to the
Time (minutes)
expression of the gene for b-galactosidase. The experiment shown
in Fig. 19.14 demonstrated that lactose turns on the b-galactosidase CONCLUSION Synthesis of b-galactosidase is turned on when lactose
gene and does not stabilize or activate the enzyme encoded by is added and turned off when lactose is removed, in support of the
the gene. How lactose activates expression of the b-galactosidase second hypothesis.
gene was the subject of additional experiments. These Nobel
FOLLOW-UP WORK These results stimulated pioneering
Prize–winning follow-up studies of Jacob and Monod gave the first
experiments that ultimately led to the discovery of the lactose operon
demonstrated example of transcriptional gene regulation.
and the mechanism of transcriptional regulation by a repressor
To understand how the genes for lactose utilization are regulated,
protein.
you need to know the main players, and these are shown in
Fig. 19.15. The overall situation looks quite complicated, but when SOURCE Monod, J. 1965. “From Enzymatic Adaption to Allosteric Transitions.” Nobel Prize
lecture. http://nobelprize.org/nobel_prizes/medicine/laureates/1965/monod-lecture.htm.
you take it apart and look at the individual pieces, it is in fact quite
 CHAPTER 19 G E N E T I C A N D E P I G E N E T I C R E G U L AT I O N 389

in bacteria. Typically, a group of

FIG. 19.15 Structural and regulatory elements of the lactose operon. The lac operon consists functionally related genes are located
of the promoter, the operator, and all the structural genes that are transcribed into a next to one another along the bacterial
single mRNA called a polycistronic mRNA. DNA and share a promoter, and when
the coding sequences of the structural
lacZ and lacY are called structural genes because they genes are transcribed from the promoter,
code for the primary structure of proteins.
they are transcribed together into a
Promoter
single molecule of messenger RNA.
lacl is the lacI coding (lacP) lacZ coding Such an mRNA is called a polycistronic
structural sequence sequence RNA (“cistron” is an old term for
gene for the CRP–cAMP Operator lacY coding
repressor “coding sequence”). In Fig. 19.15, the
binding site (lacO) sequence
protein. polycistronic RNA includes the coding
sequences for b-galactosidase and lactose
permease. The region of DNA consisting
of the promoter, the operator, and the
coding sequence for the structural genes
is called an operon.
Operons are found in bacteria and
archaeons, whose cells can translate
Repressor polycistronic mRNA molecules correctly
-galactosidase Lactose permease
protein
because their ribosomes can initiate
translation anywhere along an mRNA
that contains a proper ribosome-binding
simple. Let us start with the two coding sequences on the right in site (Chapter 4). In a polycistronic mRNA, each of the coding
the figure: sequences is preceded by a ribosome-binding site, so translation
• lacZ is the gene (coding sequence) for the enzyme can be initiated there.
b-galactosidase, which cleaves the lactose molecule into its
The repressor protein binds with the operator
glucose and galactose constituents. A functional, nonmutant
and prevents transcription, but not in the presence
form of the lacZ gene is denoted lacZ+.
of lactose.
• lacY is the gene (coding sequence) for the protein lactose The lactose operon is negatively regulated by the repressor
permease, which transports lactose from the external protein encoded by the lacI gene. That is, the structural genes
medium into the cell. A functional, nonmutant form of the of the lactose operon are always expressed unless the operon is
lacY gene is denoted lacY+. turned off by a regulatory molecule, in this case the repressor.
Fig. 19.16 shows what the operon looks like in the absence
These genes are called structural genes because they code for
of lactose. The lacI gene, encoding the repressor protein, is
the sequence of amino acids making up the primary structure of each
protein. Bacteria that contain mutations that eliminate function
of the lacZ gene (denoted lacZ2 mutants), or those that eliminate
FIG. 19.16 The lactose operon in the repressed state in the
function of the lacY gene (denoted lacY2 mutants), cannot utilize
absence of lactose.
lactose as a source of energy. Without a functional product from
lacY, lactose cannot enter the cell, and without a functional product RNA polymerase
Promoter
from lacZ, lactose cannot be cleaved into its component sugars. A complex
(lacP)
functional form of b-galactosidase (lacZ+) and of permease (lacY+) are
both essential for the utilization of lactose and for cell growth. lacI coding Operator
sequence (lacO)
Regulation of the lacZ and lacY structural genes is controlled by
the product of another structural gene, called lacI, which encodes
a repressor protein. Located between lacI and lacZ are a series of
regulatory sequences for lacZ and lacY that include a promoter, In the absence of
lactose, the repressor
lacP, whose function is to recruit the RNA polymerase complex and protein binds to the
initiate transcription, and an operator, lacO, which is the binding operator and prevents
transcription from
site for the repressor. Another regulatory region is a binding site for taking place.
a protein called CRP, which is discussed later.
The lacZ and lacY genes share a single set of regulatory elements Repressor
protein
and are transcribed together, a gene organization that is common
390 SECTION 19.3 T R A N S C R I P T I O N A L R E G U L AT I O N I N P RO K A RYOT E S

inferences about how the repressor and operator work were

FIG. 19.17 The lactose operon in the induced state in the originally drawn from studies of mutations (Fig. 19.18). As part
presence of lactose. of their investigation, Jacob and Monod identified bacterial
When the operator is not bound with the repressor,
mutants that always expressed b-galactosidase and permease,
the promoter recruits the RNA polymerase complex even in the absence of lactose. The phenotype of a cell carrying
and transcription of the polycistronic mRNA occurs. such a mutation is said to be constitutive for production of
the proteins. Constitutive expression means that it occurs
RNA polymerase
complex continuously. The most common constitutive phenotype
Operator resulted from a mutation in the lacI gene (lacI2 mutants) that
(lacO) produced a defective repressor protein (Fig. 19.18a).
Jacob and Monod also did experiments in which E. coli
contained not one but two lactose operons (Fig. 19.18b). In
bacterial cells containing one mutant and one nonmutant copy
Promoter of the lacI repressor gene, expression of the structural genes was
(lacP)
no longer constitutive but instead showed normal regulation.
This finding is consistent with the idea that the nonmutant form
of the gene produces a diffusible protein, which implies that the
normal repressor is able to bind to and repress transcription from
both operons, not just the one that it is physically linked to.
A much less common class of constitutive mutants
In the presence of
lactose, the repressor
identified the operator (Fig. 19.18c). Genetic studies of the
protein is unable to mutations in these cells showed that they were not located
bind to the operator. in the lacI gene that encodes the repressor but, instead, closer
to the coding sequence of lacZ. The genetic element in which
the mutations occurred was called the lactose operator (lacO),
expressed constantly at a low level. The repressor protein binds and the constitutive mutations were designated lacO c (“c” for
with the operator ( lacO), the RNA polymerase complex is not “constitutive”). When two different lactose operons, one normal
recruited, and transcription does not take place. and one with lacO c, were in the same cell, the operon carrying
The configuration of the lactose operon in the presence of lacO c was transcribed constitutively, even in the presence of
lactose is shown in Fig. 19.17. When lactose is present in the normal repressor, because the repressor was unable to bind to
cell, the repressor protein is unable to bind to the operator, RNA the mutant operator site in lacOc (Fig. 19.18d). Jacob and Monod
polymerase is recruited, and transcription occurs. In other words, wrote that “to explain this effect, it seems necessary to invoke a
lactose acts as an inducer of the lactose operon since it prevents new type of genetic entity, called an ‘operator,’ which would be:
binding of the repressor protein. The inducer is not actually lactose (a) adjacent to the group of [structural] genes and would control
itself, but rather an isomer of lactose called allolactose, which their [transcriptional] activity; and (b) would be sensitive to the
differs in the way the sugars are linked. Lactose in the cell is always repressor produced by a particular regulatory gene.”
accompanied by a small amount of allolactose, and so induction of
j Quick Check 4 Predict the consequence of a mutation in the
the lactose operon occurs in the presence of lactose.
lac I repressor gene that produces repressor protein that is able to
The binding of the inducer to the repressor results in an
bind to the operator, but not able to bind allolactose.
allosteric change in repressor structure that inhibits the protein’s
ability to bind to the operator. The absence of repressor from the
operator allows the RNA polymerase complex to be recruited to the The lactose operon is also positively regulated
promoter, and the polycistronic mRNA is produced. The resulting by CRP–cAMP.
lactose permease allows lactose to be transported into the cell on Fig. 19.16 and Fig. 19.17 show how the ability of the repressor
a large scale, and b-galactosidase cleaves the molecules to allow to bind with either the operator (in the absence of lactose) or
the constituents to be used as a source of energy and carbon. The with the inducer (in the presence of lactose) provides a simple
lactose operon is therefore an example of negative regulation by a and elegant way for the bacterial cell to transcribe the genes
repressor, whose function is modulated by an inducer. needed for lactose utilization only in the presence of lactose.
The elucidation of these interactions was as far as the research
The function of the lactose operon was revealed by tools used by Jacob and Monod could take them. Since the
genetic studies. original experiments, the lactose operon has been studied in
Although the interactions shown in Fig. 19.16 and Fig. 19.17 much greater detail and additional levels of regulation have been
have since been confirmed by direct biochemical studies, the discovered.
 CHAPTER 19 G E N E T I C A N D E P I G E N E T I C R E G U L AT I O N 391

FIG. 19.18 Lactose operon regulatory mutants. Jacob and Monod found two classes of constitutive mutants, (a) one that affects the repressor
and (c) one that affects the operator. In cells containing both a mutant and a nonmutant lactose operon, (b) those with the repressor
mutant become regulated, and (d) those with the operator mutant remained constitutive.
a. b.
Repressor
lacl mutant promoter
protein
lacl lacP lacO lacZ lacY
Constitutive Regulated
expression expression

A mutation in lacl (repressor) does not produce

functional repressor, so lacZ and lacY are
expressed in the presence and absence of lactose.
In cells containing one Regulated
mutant and one normal expression
copy of lacl (repressor), lacZ
and lacY are expressed only
in the presence of lactose.

c. d.
lacOc mutant

Constitutive Regulated
expression expression

A mutation in lacO (operator) prevents repressor

protein from binding, so lacZ and lacY are
expressed in the presence and absence of lactose.
In cells containing one mutant Constitutive
expression
and one normal copy of lacO
(operator), lacZ and lacY are
expressed in the presence and
absence of lactose.
One of these additional levels involves the CRP binding site
shown in Fig. 19.15, which in Fig. 19.19 is occupied by a protein
called the CRP–cAMP complex. The CRP–cAMP complex is a
positive regulator of the lactose operon, which you will recall is a when lactose is present in the cell (Fig. 19.19a). In this way, cAMP
protein that activates gene expression upon binding DNA. “CRP” is an allosteric activator of CRP binding. However, if lactose is
stands for “cAMP receptor protein.” The role of CRP–cAMP is not present in the cell, the lactose repressor binds to the lactose
to provide another level of control of transcription that is more operator and prevents transcription even in the presence of the
sensitive to the nutritional needs of the cell than the level of cAMP–CRP complex (see Fig. 19.16).
control provided by the presence or absence of lactose. E. coli can In the presence of glucose, cAMP levels are low, and the
utilize many kinds of molecules as sources of energy. When more cAMP–CRP complex does not bind the lactose operon. As a
than one type of energy source is available in the environment, result, even in the presence of lactose, the lactose operon is
certain sources are used before others. For example, glucose is not transcribed to high levels (Fig. 19.19b). In this way, E. coli
preferred to lactose, and lactose is preferred to glycerol. The CRP– preferentially utilizes glucose when both glucose and lactose are
cAMP complex helps regulate which compounds are utilized. present and utilizes lactose only when glucose is depleted.
The concentration of the small molecule cAMP in the cell is
a signal about the nutritional state of the cell. In the absence of Transcriptional regulation determines the outcome
glucose, cAMP levels are high, and cAMP binds to CRP, changing of infection by a bacterial virus.
the shape of CRP so that it can bind at a site near the operator and Transcriptional regulation has also been well studied in
stimulate binding of RNA polymerase to transcribe lacZ and lacY viruses. Bacterial cells are susceptible to infection by a variety
392 SECTION 19.3 T R A N S C R I P T I O N A L R E G U L AT I O N I N P RO K A RYOT E S

FIG. 19.19 The CRP–cAMP complex, a positive regulator of the lactose operon. (a) In the absence of large amounts of glucose, cAMP levels
are high and the CRP–cAMP complex binds to a site near the promoter, where it activates transcription. (b) In the presence of large
amounts of glucose, cAMP levels are low and the CRP–cAMP complex does not bind, so transcription is not induced to high levels,
even in the presence of lactose.

a. Low glucose, high cAMP b. High glucose, low cAMP

cAMP RNA polymerase

complex

CRP
Operator
CRP–cAMP
(lacO)
complex

Promoter
(lacP)

Lactose Repressor
Glucose

of viruses known as bacteriophages (“bacteriophage” literally recombination is reversed, freeing the phage DNA and initiating
means “bacteria-eater,” and is often shortened to just “phage”). the lytic pathway.
Among bacteriophages is a type that can undergo one of two fates At the molecular level, the choice between the lytic and
when infecting a cell. The best known example is bacteriophage lysogenic pathways is determined by the positive and negative
λ (lambda), which infects cells of E. coli. The possible results of regulatory effects of a small number of bacteriophage proteins
λ infection are illustrated in Fig. 19.20. produced soon after infection. Which pathway results depends
Upon infection, the linear DNA of the phage genome is on the outcome of a competition between the production of a
injected into the bacterial cell, and almost immediately the ends protein known as cro and that of another protein known as cI. If
of the molecule join to form a circle. In normal cells growing in the production of cro predominates, the lytic pathway results; if cI
nutrient medium, the usual outcome of infection is the lytic predominates, the lysogenic pathway takes place.
pathway, shown on the left in Fig. 19.20. In the lytic pathway, the Fig. 19.21 shows the small region of the bacteriophage DNA
virus hijacks the cellular machinery to replicate the viral genome in which the key interactions take place. Almost immediately
and produce viral proteins. After about an hour, the infected cell after infection and circularization of the bacteriophage
undergoes lysis and bursts open to release a hundred or more DNA, transcription takes place from the promoters PL and PR.
progeny phage capable of infecting other bacterial cells. Transcription of genes controlled by the PR promoter results in
The alternative to the lytic pathway is lysogeny, shown on a transcript encoding the proteins cro and cII. The cro protein
the right in Fig. 19.20. In lysogeny, the bacteriophage DNA and the represses transcription of a gene controlled by another promoter PM,
bacterial DNA undergo a process of recombination at a specific site which encodes the protein cI. In normal cells growing in nutrient
in both molecules, which results in a bacterial DNA molecule that medium, proteases present in the bacterial cell degrade cII and
now includes the bacteriophage DNA. Lysogeny often takes place prevent its accumulation. With cro protein preventing cI expression
in cells growing in poor conditions. The relative sizes of the DNA and cII protein unable to accumulate, transcription of bacteriophage
molecules in Fig. 19.20 are not to scale. In reality, the length of the genes in the lytic pathway takes place, including those genes needed
bacteriophage DNA is only about 1% of that of the bacterial DNA. for bacteriophage DNA replication, those encoding proteins in the
When the bacteriophage DNA is integrated by lysogeny, the only bacteriophage head and tail, and, finally, those needed for lysis.
bacteriophage gene transcribed and translated is one that represses Alternatively, in bacterial cells growing in poor conditions,
the transcription of other phage genes, preventing entry into the reduced protease activity allows cII protein to accumulate. When
lytic pathway. The bacteriophage DNA is replicated along with the cII protein reaches a high enough level, it stimulates transcription
bacterial DNA and transmitted to the bacterial progeny when the from the promoter PE . The transcript from PE includes the coding
cell divides. Under stress, such as exposure to ultraviolet light, sequence for cI protein, and the cI protein has three functions:
 CHAPTER 19 G E N E T I C A N D E P I G E N E T I C R E G U L AT I O N 393

• It binds with the operator OR and prevents further

FIG. 19.20 Alternative outcomes of infection by bacteriophage λ. expression of cro and cII.
The bacteriophage can enter either the lytic or the
• It stimulates transcription of its own coding sequence from
lysogenic pathway.
the promoter PM , establishing a positive feedback loop that
Mature keeps the level of cI protein high.
λ phage
• It binds with the operator OL and prevents further
Infection
transcription from PL.
The result is that cI production shuts down transcription
of all bacteriophage genes except its own gene, and this is the
regulatory state that produces lysogeny. (The protein needed for
Circularization
recombination between the bacteriophage DNA and the bacterial
DNA is produced by transcription from the PL promoter before it
Recombination is shut down by cI.) When cells that have undergone lysogeny are
Phage DNA Lytic pathway Lysogeny takes place exposed to ultraviolet light or certain other stresses, the cI protein
is replicated; between a
specific site in is degraded. In this case, cro and cII are produced again, and the
phage
proteins are the phage lytic pathway follows.
synthesized. DNA and the Regulation of the lytic and lysogenic pathways works to the
Fragmented bacterial DNA.
bacterial DNA advantage of bacteriophage λ, but the process is not like something
Progeny an engineer might design. That is because biological systems are not
phage are Phage DNA is
replicated with engineered, they evolve. Regulatory mechanisms are built up over
assembled,
cell breaks bacterial DNA time by the selection of successive mutations. Each evolutionary
open, and and transmitted
to bacterial step refines the regulation in such a way as to be better adapted to
mature phage
released. progeny. the environment than it was before. Each successive step occurs
only because it increases survival and reproduction.
We summarize these two alternative outcomes of infection
by a bacterial virus, and the earlier discussions of viral diversity,
replication, host range, and effects on a cell, in Fig. 19.22 on the
following page. •

FIG. 19.21 Transcriptional regulation of cI and cro genes, which determine the lytic versus lysogenic pathway.
cII Reduced
protease
Transcription activity allows
takes place cII to
from accumulate,
promoters PL PL OL cl PM PE PL OL cl PM PE stimulating
and PR, transcription
leading to PR OR cro cII PR OR cro cII from promoter
the synthesis Lysogenic PE and the
of proteins pathway synthesis of cl
cro and cII. protein.
Lytic
pathway

cro cl cl cl protein
cro protein binds with OL
inhibits PM, and OR,
preventing cl preventing
expression. transcription
PL OL cl PM PE PL OL cl PM PE from PL and
Proteases
degrade PR OR cro cII PR OR cro cII expression of
excess cII cro and cII. cl
protein. protein also
increases its
Proteases degrade own
cII protein production by
cII cl
stimulating PM.
 V I S UA L S Y N T H E S I S Virus: A Genome in Need of a Cell
FIG. 19.22 Integrating concepts from Chapters 11, 13, and 19

Diversity
Viruses come in many different shapes and viral genomes include a diverse array of types and structures of
nucleic acids.

Bacteriophage λ Adenovirus Picobirnavirus Rabies Tobacco mosaic virus HIV

(dsDNA) (dsDNA) (dsRNA) (–ssRNA) (+ssRNA) (+ssRNA)

Mature
assembled
viruses are
released.

Fragmented
bacterial DNA

Replicated viral
components
Lysis

Mature
Bacterial λ phage
cell

Circular DNA
Lysogeny

Viral DNA
integrated
into bacterial
DNA

Transcriptional regulation
When bacterial cells are grown in favorable conditions, bacterial proteases degrade a key
viral protein and genes are transcribed that lead to the bursting of the bacterial cell (lysis).
When bacterial cells are grown in poor conditions, the viral protein accumulates and genes
are transcribed that allow the viral genome to remain integrated in host genome (lysogeny).

394
Replication cycle
Viruses are not considered 1 Attachment: HIV attaches to
living organisms because proteins on the surface of the cell and
they cannot replicate on fuses with the plasma membrane.
their own and absolutely
require a cell to complete
their replication cycle.

Human immunodeficiency
virus (HIV)
gp120

Viral RNA
2 Entry:
Co-receptor The virus
enters the
Viral Reverse cytoplasm.
DNA transcriptase
CD4
Host range
A given virus can only infect some
organisms and types of cell, determined
by specific interactions of viral proteins
with cell-surface host proteins.

Host
DNA

3 Replication: The virus

uses the cell to transcribe
the integrated viral DNA
into mRNA which is Effects
translated into protein. Viruses use the resources of the cell to
replicate. They can kill the cell they infect,
leading to disease or death, or cause the
cell to divide uncontrollably, leading to
cancer. In some cases, they can transfer
pieces of DNA from one cell to another.

4 Self-assembly:
The viral components
assemble on their
own inside of the cell.

Mature virion T cell

5 Release: In the case of HIV,

Evolution
new viruses bud from an The host range and effects of a virus can
intact cell. Other kinds of change because of mutation and
viruses can burst (lyse) the recombination. Viruses with segmented
cell to exit, while still others genomes can sometimes exchange
incorporate their viral whole segments. RNA viruses, like HIV
genome into that of the host, and influenza, evolve rapidly because of
but are replicated and the high mutation rate of their genomes.
released at a later time.

395
396 SELF-ASSESSMENT

Core Concepts Summary Gene regulation is influenced by both genetic and

environmental factors. page 386
19.1 The regulation of gene expression in eukaryotes
takes place at many levels, including DNA packaging in 19.3 Transcriptional regulation is illustrated in
chromosomes, transcription, and RNA processing. bacteria by the control of the production of proteins
needed for the utilization of lactose, and in viruses by
Gene expression involves the turning on or turning off of a the control of the lytic and lysogenic pathways.
gene. page 378
Transcriptional regulation can be positive, in which a gene
Gene regulation determines where, when, how much, and is usually off and is turned on in response to the binding to
which gene product is made. page 378 DNA of a regulatory protein called an activator, or negative,
Regulation at the level of chromatin involves chemical in which a gene is usually on and is turned off in response to
modifications of DNA and histones that make a the binding to DNA of a regulatory protein called a repressor.
gene accessible or inaccessible to the transcriptional page 387
machinery. page 378
Jacob and Monod studied the lactose operon in E. coli as a
Dosage compensation is the process by which the expression model for bacterial gene regulation. page 388
of X-linked genes is equalized in XX individuals and XY
When lactose is added to culture of bacteria, the genes
individuals. page 380
for the uptake of lactose (permease, encoded by lacY) and
X-inactivation, the mechanism of dosage compensation for sex cleavage of lactose (b-galactosidase, encoded by lacZ) are
chromosomes in mammals, involves the inactivation of one of expressed. page 388
the two X chromosomes in females. page 381
The lactose operon is negatively regulated by the repressor
X-inactivation occurs by the transcription of a noncoding protein (encoded by lacI), which binds to a DNA sequence
RNA known as Xist, which coats the entire X chromosome, known as the operator. page 389
leading to DNA and histone modifications and transcriptional
When lactose is added to the medium, it induces an
repression. page 381
allosteric change in the repressor protein, preventing it
Transcriptional regulation controls whether or not from binding to the operator and allowing transcription of
transcription of a gene occurs. page 382 lacY and lacZ. In this way, lactose acts as an inducer of the
Transcription can be regulated by regulatory transcription lactose operon. page 390
factors that bind to specific DNA sequences known as An additional level of regulation of the lactose operon is
enhancers that can be near, in, or far from genes. page 382 provided by the CRP– cAMP complex, a positive activator of
Further levels of regulation after a gene is transcribed to mRNA transcription. page 391
include RNA processing, splicing, and editing. page 383 The lytic and lysogenic pathways of bacteriophage λ have also
been well studied as a model of gene regulation. page 392
19.2 After an mRNA is transcribed and exported to
the cytoplasm in eukaryotes, gene expression can be When bacteriophage λ infects E. coli, it can lyse the cell (the
regulated at the level of mRNA stability, translation, and lytic pathway) or its DNA can become integrated into the
posttranslational modification of proteins. bacterial genome (the lysogenic pathway). page 392

Small regulatory RNAs, especially microRNA (miRNA) and In infection of E. coli cells by bacteriophage λ, predominance
small interfering RNA (siRNA), affect gene expression through of cro protein results in the lytic pathway, whereas
their effects on translation or mRNA stability. page 384 predominance of the cI protein results in the lysogenic
pathway. page 392
Translational regulation controls the rate, timing, and
location of protein synthesis. page 384
Translational regulation is determined by many features of an Self-Assessment
mRNA molecule, including the 5� and 3� UTR, the cap, and the
1. Distinguish between gene expression and gene regulation.
poly(A) tail. page 385
2. Explain what is meant by different “levels” of gene
Posttranslational modification comes into play after a protein
regulation and give some examples.
is synthesized, and includes chemical modification of side
groups of amino acids, affecting the structure and activity of a 3. Give two examples of how DNA bases and chromatin
protein. page 385 can be modified to regulate gene expression, and explain
 CHAPTER 19 G E N E T I C A N D E P I G E N E T I C R E G U L AT I O N 397

why these kinds of modifications result in increased or how expression is controlled in the presence and in the
decreased gene expression. absence of lactose.

4. Explain how X-inactivation in female mammals results in 8. Describe the role of the CRP–cAMP complex in positive
patchy coat color in calico cats. regulation of the lactose operon in E. coli.

5. Explain how one protein-coding gene can code for more 9. Describe what is meant by lysis and lysogeny, and explain
than one polypeptide chain. how gene regulation controls these two pathways.

6. Name and describe three ways in which gene expression Log in to to check your answers to the Self-
can be influenced after mRNA is processed and leaves the Assessment questions, and to access additional learning tools.
nucleus.

7. Diagram the lactose operon in E. coli with the proper

order of the elements lacI, lacO, lacY, and lacZ, and explain
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 CHAPTER 20

Genes and
Development
Core Concepts
20.1 In the development
of humans and other
animals, stem cells become
progressively more
restricted in their possible
pathways of cellular
differentiation.
20.2 The genetic control of
development is a hierarchy
in which genes are activated
in groups that in turn
regulate the next set of
genes.
20.3 Many proteins that play key
roles in development are
evolutionarily conserved
but can have dramatically
different effects in different
organisms.
20.4 Combinatorial control
is a developmental
strategy in which cellular
differentiation depends on
the particular combination
of transcription factors
present in a cell.
20.5 Ligand–receptor
interactions activate signal
transduction pathways that
converge on transcription
factors and genes that
determine cell fate.
Perrennou Nuridsany/Science Source.

399
400 SECTION 20.1 G E N E T I C P RO G R A M S O F D E V E LO P M E N T

Altogether, the human body contains about 200 different types the genetic program of development—that is, the genetic
of cell, all of which derive from a single cell, the zygote. Some instructions that lead a single fertilized egg to become a complex
cells derived from the zygote become muscle cells, others nerve multicellular organism.
cells, still others connective tissue. Almost all of these cells have
exactly the same genome: They differ not in their content of The fertilized egg is a totipotent cell.
genes, but instead in the groups of genes that are expressed or In all sexually reproducing organisms, the fertilized egg is
repressed. In other words, these cell types differ as a result of gene special because of its developmental potential. The fertilized
regulation, discussed in the previous chapter. egg is said to be totipotent, which means that it can give rise
Gene regulation is especially important in multicellular to a complete organism. In mammals, the egg also forms the
organisms because it underlies development, the process in membranes that surround and support the developing embryo
which a fertilized egg undergoes multiple rounds of cell division (Chapter 42).
to become an embryo with specialized tissues and organs. During After fertilization, the fertilized egg, or zygote, undergoes
development, cells undergo changes in gene expression as genes successive mitotic cell divisions as it moves along a fallopian tube.
are turned on and off at specific times and places. Gene regulation One cell becomes two, two become four, four become eight, eight
causes cells to become progressively more specialized, a process become sixteen, and so on, with all the cells contained within
known as differentiation. the egg’s outer membrane (Fig. 20.1). Within 4 –5 days after
In this chapter, we focus on the general principles by
which genes control development. We will see that, as cells
differentiate along one pathway, they progressively lose their FIG. 20.1 Early development of a human embryo. The zygote is a
ability to differentiate along other pathways. Yet gene expression totipotent cell because its daughter cells can develop into
can sometimes be reprogrammed to reopen pathways of any cell type and eventually into a complete organism.
differentiation that had previously been shut off, a process
Four-cell stage
that has important implications for therapeutic replacement of
diseased or damaged tissue. From a broader perspective, the study Two-cell stage
of evolutionary changes in developmental processes constitute Morula

the field of evolutionary developmental biology, often called

evo-devo. We will see that, while some of the key molecular
mechanisms of development are used over and over again in
different organisms, they have evolved to yield such differences
in shape and form as to conceal the underlying similarity in Zygote
mechanism. Blastocyst
Gastrula
Inner
20.1 GENETIC PROGRAMS cell
mass
OF DEVELOPMENT
Genetic programs and computer programs have some features
in common. Computer code is written as a linear string of
letters, analogous to the sequence of nucleotides in genomic
DNA. Once a computer program is initiated, it automatically
runs and performs its coded task. Small mistakes in the code,
like mutations, can have big consequences and even cause the
program to crash.
The analogy between genetic programs and computer Germ layers: Ectoderm Mesoderm Endoderm
programs has an important limitation. Computer programs,
designed by humans, are consciously written, whereas genetic
programs evolve. The genetically encoded developmental • Cells in the outer • Cells in the inner layer • Cells lining the
programs of all living organisms emerged over billions of years layer of skin of skin inside of the
• Pigment cells • Muscle cells digestive tract
through mutation and natural selection. These developmental
• Nerve cells in the • Bone cells • Cells lining the
programs changed through time, persisting only if they inside of the lung
brain • Red blood cells
produced organisms that could successfully survive and • Liver cells
reproduce in the existing environment. Here, we explore • Pancreas cells
 CHAPTER 20 G E N E S A N D D E V E LO P M E N T 401

fertilization, the zygote has turned into a ball of cells called the j Quick Check 1 From what you know about embryonic
morula and has traveled from the site of fertilization in one of development, do you think that a cell from the inner cell mass or
the fallopian tubes to the uterus. one from the ectoderm has more developmental potential?
These early cell divisions are different from the mitotic cell
Why do differentiating cells increasingly lose their
divisions that occur later in life because the cells do not grow
developmental potential? One hypothesis focuses on gene
between divisions; they merely replicate their chromosomes
regulation. When cells become committed to a particular
and divide again. The result is that the cytoplasm of the egg is
developmental pathway, genes no longer needed are turned off
partitioned into smaller and smaller packages, with the new
(that is, repressed) and are difficult to turn on again. Another
cells all bunched together inside the membrane that covers the
hypothesis is genome reduction: As cells become differentiated,
developing embryo.
they delete the DNA for genes they no longer need.
Cell division continues in the morula until there are a few
These hypotheses can be distinguished by an experiment
thousand cells. The cells then begin to move relative to one
in which differentiated cells are reprogrammed to mimic
another, pushing against and expanding the membrane that
earlier states. If loss of developmental potential is due to gene
encloses them and rearranging themselves to form a hollow
regulation, then differentiated cells could be reprogrammed to
sphere called a blastocyst (Fig. 20.1). In one region of the inner
become pluripotent or multipotent. If loss of developmental
wall of the blastocyst, there is a group of cells known as the
potential is due to genome reduction, then differentiated
inner cell mass, from which the body of the embryo develops.
cells could not be reprogrammed to become pluripotent or
The wall of the blastocyst forms several membranes that envelop
multipotent.
and support the developing embryo. Once the blastocyst forms,
British developmental biologist John Gurdon carried out
it implants in the uterine wall.
such experiments in the early 1960s (Fig. 20.2). Gurdon used
Once implanted in the uterine wall, the multiplying
a procedure called nuclear transfer, in which a hollow glass
cells of the inner cell mass reorganize to form a gastrula, in
needle is used to insert the nucleus of a cell into the cytoplasm
which the blastula becomes organized into three germ layers
of an egg whose own nucleus has been destroyed or removed.
(Fig. 20.1). Germ layers are sheets of cells that include the
Previous nuclear transfer experiments had been carried
ectoderm, mesoderm, and endoderm and that differentiate
out in the leopard frog, Rana pipiens. Whereas nuclei from
further into specialized cells. Those formed from the ectoderm
pluripotent or multipotent cells could often be reprogrammed
include the outer layer of the skin and nerve cells in the brain;
to develop into normal tadpoles, attempts with nuclei from fully
cells from the mesoderm include cells that make up the inner
differentiated cells failed.
layer of the skin, muscle cells, and red blood cells; and cells
Gurdon tried the experiments in a different organism, the
formed from the endoderm include cells of the lining of the
clawed toad Xenopus laevis, and demonstrated that nuclei from
digestive tract and lung, as well as liver cells and pancreas cells
fully differentiated intestinal cells could be reprogrammed
(Chapter 42).
to support normal development of the tadpole (Fig. 20.2).
Only 10 of 726 experiments succeeded, but this was sufficient
Cellular differentiation increasingly restricts to show that intestinal cell nuclei still contained a complete
alternative fates. Xenopus genome. In other words, his findings supported the first
At each successive stage in development in which the cells hypothesis—all of the same genes are present in intestinal cells
differentiate, they lose the potential to develop into any kind of as in early embryonic cells, but some of the genes are turned off,
cell. The fertilized egg is totipotent because it can differentiate or repressed, during development.
into both the inner cell mass and supporting membranes, and Fig. 20.3 summarizes the results of many nuclear transfer
eventually into an entire organism. The cells of the inner cell experiments carried out in mammals and amphibians. The
mass, called embryonic stem cells, are pluripotent because percentage of reprogramming experiments that fail increases
they can give rise to any of the three germ layers, and therefore as cells differentiate. The best chance of success is to use
to any cell of the body. However, pluripotent cells cannot on pluripotent nuclei from cells in the blastocyst (or its amphibian
their own give rise to an entire organism, as a totipotent cell equivalent, the blastula). However, even some experiments
can. Cells further along in differentiation are multipotent; using nuclei from fully differentiated cells have been successful.
these cells can form a limited number of types of specialized When nuclear transfer succeeds, the result is a clone—an
cell. Cells of the germ layers are multipotent because they can individual that carries an exact copy of the nuclear genome of
give rise only to the cell types specified for each germ layer another individual. In this case, the new individual shares the
in Figure 20.1. Totipotent, pluripotent, and multipotent cells same genome as that of the individual from which the donor
are all stem cells, cells that are capable of differentiating into nucleus was obtained. (The mitochondrial DNA is not from the
different cell types. nuclear donor, but from the donor of the egg cytoplasm.) The
HOW DO WE KNOW?

FIG. 20.2
1 The nucleus of an

How do stem cells egg cell is inactivated

with ultraviolet light.
X
716 experiments:
Development terminated

lose their ability to Unfertilized egg

prior to tadpole stage.

differentiate into
any cell type? Xenopus laevis
tadpole 10 experiments:
Development to tadpole
BACKGROUND During differentiation, cells
2 The nucleus from an stage occurred.
become progressively more specialized and intestinal cell of a tadpole is
restricted in their fates. Early studies left the injected into the egg cell.
mechanisms of differentiation unclear.

HYPOTHESIS One hypothesis is that differentiation occurs as a the cytoplasm of the unfertilized egg are able to support complete
result of changes in gene expression. A second hypothesis is that development of a normal animal. This result allows us to reject the
differentiation occurs as a result of genome reduction, in which genes hypothesis that differentiation occurs by the loss of genes. The first
that are not needed are deleted. hypothesis—that cells become differentiated as a result of changes in
gene expression—was supported. But, because of the small number of
EXPERIMENT John Gurdon carried out experiments in the amphibian successes in reprogramming, additional experiments were needed to
Xenopus laevis to test these hypotheses. He transferred nuclei from validate the conclusions.
differentiated cells into unfertilized eggs whose nuclei had been
inactivated with ultraviolet light. If differentiation is due to changes FOLLOW-UP WORK Gurdon’s work was controversial. Some critics

in gene expression, then the differentiated nucleus should be able to argued that the successful experiments resulted from a small number of
reprogram itself in the egg cytoplasm and differentiate again into all undifferentiated cells present in intestinal epithelium. Others accepted
the cells of a tadpole. If differentiation is accompanied by loss of genes, the conclusion but expressed misgivings about possible applications to
then differentiation is irreversible and development will not proceed. humans. Later experiments that succeeded in cloning mammals from
fully differentiated cells confirmed the original conclusion.
RESULTS The experiment was carried out 726 times. In 716 cases,
development did not occur; in 10, development proceeded normally. SOURCE Gurdon, J. B. 1962. “The Developmental Capacity of Nuclei Taken from Intestinal
Epithelium Cells of Feeding Tadpoles.” Journal of Embryology & Experimental Morphology
CONCLUSION Although the experiment succeeded in only 10 of 10:622–640.

726 attempts, it showed that the nucleus of an intestinal cell and

first mammalian clone was a lamb called Dolly (Fig. 20.4a), born in
Cells that are further along
in differentiation are more 1996. She was produced from the transfer of the nucleus of a cell in
difficult to reprogram. the mammary gland of a sheep to an egg cell with no nucleus, and
50 was the only successful birth among 277 nuclear transfers. Successful
cloning in sheep soon led to cloning in cattle, pigs, and goats.
Percentage of nuclear transplant

Amphibian Mammal The first household pet to be cloned was a kitten named
40
experiments that succeed

CopyCat (Fig. 20.4b), born in 2001 and derived from the nucleus of
a differentiated ovarian cell. CopyCat was the only success among
30 Blastocyst 87 attempts. As shown in Fig. 20.4b, the cat from which the donor
nucleus was obtained was a calico, but CopyCat herself was not,
20

10

FIG. 20.3 Results of nuclear transfer of differentiated cells. Data

0 from: J. B. Gurdon and D. A. Melton, “Nuclear Reprogramming in Cells,”
Blastocyst Adult
Degree of differentiation 2008, Science 322:1811–1815.

402
 CHAPTER 20 G E N E S A N D D E V E LO P M E N T 403

FIG. 20.4 Celebrity clones and their genetic mothers. (a) Dolly; (b) CopyCat. Sources: a. AP Photo/John Chadwick; b. AP Photo/Pat Sullivan.

a b

even though the two cats are clones of each other. The reason for because obtaining embryonic stem cells requires the destruction
their different appearance has to do with X-inactivation, discussed of human blastocysts—that is, early-stage embryos. A major
in Chapter 19. Recall that the mottled orange and black calico breakthrough occurred in 2006 when Japanese scientists
pattern results from random inactivation of one of the two X demonstrated that adult cells can be reprogrammed by activation
chromosomes during development. The lack of a calico pattern of just a handful of genes, most of them encoding transcription
in CopyCat implies that the X chromosomes in the transferred factors or chromatin proteins. The reprogrammed cells were
nucleus did not “reset” as they do in normal embryos. Instead, the pluripotent and were therefore called induced pluripotent stem
inactive X in the donor nucleus remained inactive in all the cells in cells (iPS cells).
the clone. Hence, while CopyCat and her mother share the same The success rate was only about one iPS cell per thousand,
nuclear genome, the genes were not expressed in the same way and the genetic engineering technique required the use of viruses
because of irreversible epigenetic regulation in the donor nucleus. that can sometimes cause cancer. Nevertheless, the result was
regarded as spectacular. Other researchers soon found other genes
j Quick Check 2 X-inactivation can result in two clones of a
that could be used to reprogram adult cells into pluripotent or
cell that differ in the genes they express. Can you think of other
multipotent stem cells, and still other investigators developed
reasons why two genetically identical individuals might look
virus-free methods for delivering the genes. In recent years,
different from each other?
researchers have even discovered small organic molecules that can
reprogram adult cells.
This kind of reprogramming opens the door to personalized
? CASE 3 YOU, FROM A TO T: YOUR PERSONAL GENOME stem cell therapies. The goal is to create stem cells derived from
Can cells with your personal genome be the adult cells of the individual patient. Since these cells contain
reprogrammed for new therapies? the patient’s own genome, problems with tissue rejection are
Stem cells play a prominent role in regenerative medicine, minimized or eliminated (Chapter 43). There remains much
which aims to use the natural processes of cell growth and to learn before therapeutic use of induced stem cells becomes
development to replace diseased or damaged tissues. Stem cells are routine. Researchers will face challenges such as increasing the
already used in bone marrow transplantation and may someday efficiency of reprogramming, verifying that reprogramming
be used to treat Parkinson’s disease, Alzheimer’s disease, heart is complete, making sure that the reprogrammed cells are not
failure, certain types of diabetes, severe burns and wounds, and prone to cancer, and demonstrating that the reprogrammed cells
spinal cord injury. differentiate as they should. Nevertheless, researchers hope that
At first, it seemed as though the use of embryonic stem cells someday soon your own cells containing your personal genome
gave the greatest promise for regenerative medicine because of could be reprogrammed to restore cells or organs damaged by
their pluripotency. This approach proved ethically controversial disease or accident.
404 CHAPTER 20.2 H I E R A RC H I C A L CO N T RO L O F D E V E LO P M E N T

the egg and sperm nuclei fuse (Fig. 20.5a). Unlike in mammalian
20.2 HIERARCHICAL CONTROL development, the early nuclear divisions in the Drosophila embryo
OF DEVELOPMENT occur without cell division, and therefore the embryo consists of a
single cell with many nuclei in the center (Fig. 20.5b). When there
During development of a complex multicellular organism, are roughly 5000 nuclei, they migrate to the periphery (Fig. 20.5c),
many genes are activated and repressed at different times, thus where each nucleus becomes enclosed in its own cell membrane,
restricting cell fates. One of the key principles of development and together they form the cellular blastoderm (Fig. 20.5d).
is that genes expressed early in an organism’s development Then begins the process of gastrulation, in which the cells of
control the activation of other groups of genes that act later in the blastoderm migrate inward, creating layers of cells within the
development. Gene regulation during development is therefore embryo. As in humans and most other animals (section 20.1 and
hierarchical in the sense that genes expressed at each stage in the Chapter 42), gastrulation forms the three germ layers (ectoderm,
process control the expression of genes that act later. mesoderm, and endoderm) that differentiate into different types
of cell. A Drosophila embryo during gastrulation is shown in Fig.
Drosophila development proceeds through egg, larval, 20.5e. At this stage, the embryo already shows an organization
and adult stages. into discrete parts or segments, the formation of which is known
The fruit fly Drosophila melanogaster has played a prominent role as segmentation. There are three cephalic segments, C1–C3 (the
in our understanding of the genetic control of early development, term “cephalic” refers to the head); three thoracic segments, T1–T3
and in particular the hierarchical control of development. (the thorax is the middle region of an insect); and eight abdominal
Researchers have isolated and analyzed a large number of segments (A1–A8). Each of these segments has a different fate in
mutant genes that lead to a variety of defects at different stages development.
in development. These studies have revealed many of the key About one day after fertilization, the embryo hatches from
genes and processes in development, which are the focus of the the egg as a larva (Fig. 20.5f). Over the next eight days, the larva
following sections. grows and replaces its rigid outer shell, or cuticle, twice (Fig. 20.5g
The major events in Drosophila development are illustrated in and 20.5h). After a week of further growth, the cuticle forms a
Fig. 20.5. DNA replication and nuclear division begin soon after casing—called the pupa (Fig. 20.5i)—in which the larva undergoes

FIG. 20.5 Life cycle of the fruit fly Drosophila melanogaster. The life cycle begins with (a) a fertilized egg, followed by (b –e) a developing
embryo, (f–h) larval stages, (i) pupa, and (j) adult.

j. Adult female Gamete

Metamorphosis
a. Fertilized egg
i. Pupa
Posterior
Anterior
10 days
7 days b. Nuclei

15–80 minutes
h.

5 days

c.

3 days 1.5–2.5 hours

g.
2.5–3.5 hours
1 day
7.5–9.5 hours
f.
d. Cellular blastoderm

*Images not drawn to scale. e. Gastrulating embryo

 CHAPTER 20 G E N E S A N D D E V E LO P M E N T 405

A distinguishing feature of bicoid and nanos mutants is that

FIG. 20.6 Normal and mutant Drosophila larvae. (a) Nonmutant the abnormalities in the embryo depend on the genotype of the
larva have anterior, middle, and posterior segments. mother, not the genotype of the embryo. The reason the genotype
(b) Bicoid mutant larva lack anterior segments. (c) Nanos of the mother can affect the phenotype of the developing embryo
mutant larva lack posterior structures. is that successful development requires a functioning oocyte. In
a. Nonmutant larva
Drosophila and many other organisms, the composition of the egg
includes macromolecules (such as RNA and protein) synthesized
Anterior Posterior by cells in the mother and transported into the egg. If the mother
carries mutations in genes involved in the development of the
C1 T1 T2 T3
C2 A1 A2 A3 A4 A5 A6 A7 A8 oocyte, the offspring can be abnormal. Genes such as bicoid and
C3 nanos that are expressed by the mother but affect the phenotype
of the offspring (in this case the developing embryo and larva) are
b. Larva from bicoid mutant called maternal-effect genes.
A normal Drosophila oocyte is highly polarized, meaning that
one end is distinctly different from the other. For example, there
A8
are gradients of macromolecules that define the anterior–posterior
A4 A5 A6 A7 (head-to-tail) axis of the embryo as well as the dorsal–ventral
(back-to-belly) axis. The best known of these gradients are those
c. Larva from nanos mutant of messenger RNAs that are transcribed from the maternal-effect
genes bicoid and nanos. Fig. 20.7a shows the gradient of bicoid
mRNA across the oocyte. The bulk of bicoid mRNA comes from the
C1 T1 T2 T3
A1 A2 A3
mother, and is localized in the anterior of the egg by proteins that
C2
C3 attach them to the cytoskeleton.
The mRNA corresponding to the maternal-effect gene nanos
is also present in a gradient, but most of it is at the posterior
end (Fig. 20.7b). Like bicoid, mRNA for nanos is synthesized
the dramatic developmental changes known as metamorphosis by the mother’s cells and then imported into the oocyte. After
that give rise to the adult fruit fly. fertilization, the zygote produces Bicoid and Nanos proteins from
the localized mRNAs, and they have concentration gradients
The egg is a highly polarized cell. resembling those of the mRNAs in the oocyte (Fig. 20.7).
How genes control development in Drosophila was inferred from
systematic studies of mutants by Christiane Nusslein-Volhard
and Eric F. Wieschaus, work for which they were awarded the
1995 Nobel Prize in Physiology or Medicine. One of their findings FIG. 20.7 Gradients of bicoid and nanos mRNA and protein in
was that development starts even before a zygote is formed, in the developing embryo. (a) The mRNA and protein for
the maturation of the oocyte, the unfertilized egg cell produced bicoid are localized in the anterior end of the egg.
by the mother. The oocyte, which matures under control of (b) The mRNA and protein for nanos are localized in the
the mother’s genes, is also important for normal embryonic posterior end of the egg.
development. This finding applies not only to insects like
Drosophila, but also to many multicellular animals. a. Distribution of bicoid mRNA and protein in the egg
Among the striking mutants Nusslein-Volhard and Wieschaus
generated and investigated were ones that significantly affected
Anterior Posterior
early development (Fig. 20.6). These defects in very early
development can be easily seen by the time the mutants reach
the larval stage. In one class of mutants, called bicoid, larvae are bicoid
missing segments at the anterior end. In another class of mutants,
called nanos, larvae are missing segments at the posterior end. The b. Distribution of nanos mRNA and protein in the egg
mutant larvae are grossly abnormal and do not survive. Nusslein-
Volhard and Wieschaus were able to identify each segment that Anterior Posterior
was missing based on each segment’s distinctive pattern of
hairlike projections. In Fig. 20.6, the patterns are shown as dark
shapes on each segment. nanos
406 CHAPTER 20.2 H I E R A RC H I C A L CO N T RO L O F D E V E LO P M E N T

FIG. 20.8 Caudal and Hunchback gradients in the developing embryo. (a) mRNA levels of hunchback and caudal are uniform across the
embryo. (b) Hunchback and Caudal protein levels are localized to the anterior and posterior ends of the embryo, respectively, because
Bicoid and Nanos control the translation of hunchback and caudal mRNA.

a.

Relative mRNA abundance

1

bicoid
nanos
The gradient of hunchback The gradient of
bicoid mRNA caudal nanos mRNA
peaks near the peaks near the
anterior end of posterior end
the oocyte. of the oocyte.
0
Anterior Posterior
Position in oocyte

b.
Relative protein abundance

Bicoid
1
Nanos
Hunchback
Caudal
Nanos protein
Bicoid protein represses
represses translation of
translation of hunchback
caudal mRNA. mRNA.
0
Anterior Posterior
Position in 2.5-hour embryo

The anterior–posterior axis set up by the gradients of Bicoid genes of the embryo needed for the development of posterior
and Nanos proteins is reinforced by gradients of two transcription structures like genitalia. In this way, maternal genes expressed
factors called Caudal and Hunchback (Fig. 20.8). Like the mRNAs early in development influence the expression of genes of the
for Bicoid and Nanos, the mRNAs for Caudal and Hunchback are embryo that are important in later development. Because the
transcribed from the mother’s genome and transported into the products of the bicoid and nanos mRNA are the ones initially
egg. As shown in Fig. 20.8a, the mRNAs for caudal and hunchback responsible for organizing the anterior and posterior ends of the
are spread uniformly in the cytoplasm of the fertilized egg. embryo, respectively, mothers that are mutant for bicoid have
However, the mRNAs are not translated uniformly in the egg. larvae that lack anterior structures, and mothers that are mutant
Bicoid protein represses translation of caudal, and Nanos protein for nanos have larvae that lack posterior structures.
represses translation of hunchback (Fig. 20.8b). Caudal protein
j Quick Check 3 Would development happen normally if the
is therefore concentrated at the posterior end and Hunchback
mother has normal bicoid function, but the embryo does not? Why
protein is concentrated at the anterior end. The expression of
or why not?
Caudal and Hunchback illustrates gene regulation at the level
of translation, discussed in Chapter 19. Bicoid protein is also a
transcription factor that promotes transcription of the hunchback Development proceeds by progressive regionalization
gene from zygotic nuclei, which reinforces the localization of and specification.
Hunchback protein at the anterior end. Nusslein-Volhard and Wieschaus also discovered mutants of the
The Hunchback and Caudal gradients set the stage for the embryo’s developmental genes. They discovered three classes of
subsequent steps in development. The Hunchback transcription such mutants, and their analysis showed that genes in the embryo
factor targets genes of the embryo needed for the development controlling its development are turned on in groups, and that
of anterior structures like eyes and antennae, and Caudal targets each successive group acts to refine and narrow the pattern of
 CHAPTER 20 G E N E S A N D D E V E LO P M E N T 407

FIG. 20.9 Normal gap-gene expression pattern and mutant FIG. 20.10 Normal pair-rule gene expression pattern and mutant
phenotype. Source: James Langeland, Steve Paddock and Sean phenotype. Source: James Langeland, Steve Paddock and Sean
Carroll, HHMI, University of Wisconsin–Madison. Carroll, HHMI, University of Wisconsin–Madison.

Anterior Posterior Posterior

Anterior

The orange staining The green staining

Nonmutant embryo indicates where the indicates the regions
Nonmutant embryo
gap gene Krüppel is where the pair-rule
normally transcribed; gene hairy is normally
transcription of expressed.
Krüppel requires the
transcription factor
Hunchback, but high
levels (green) repress
transcription.
Nonmutant larva Nonmutant larva
The orange segments The green segments
in the nonmutant depend on the
larva are the segments function of the gene.
C1 T1 T2 T3
A1 A2 A3 A4 A5 A6 A7 A8 that depend on the C1 T1 T2 T3
A1 A2 A3 A4 A5 A6 A7 A8
C2 function of the gene. C2
C3 C3

Mutant larva Mutant larva

Mutant gap gene Mutant pair-rule gene
results in loss of a results in loss of
group of contiguous alternate segments.
C1 T1 T2 T3
A1 A5 A6 A7 A8 C2 T1 T3 A8
segments. A2 A4 A6
C2
C3

differentiation generated by previous groups. This, too, is a general of pair-rule genes (Fig. 20.10). Pair-rule genes receive their
principle of development in many multicellular organisms. name because larvae with these mutations lack alternate body
The anterior–posterior gradient set up by the maternal-effect segments. The example in Fig. 20.10 is hairy, whose mutants lack
genes is first narrowed by genes called gap genes (Fig. 20.9), each the odd-numbered thoracic segments and the even-numbered
of which is expressed in a broad region of the embryo. The name abdominal segments. The pair-rule genes help to establish the
“gap gene” derives from the phenotype of mutant embryos, which uniqueness of each of seven broad stripes across the anterior–
are missing groups of adjoining segments, leaving a gap in the posterior axis.
pattern of segments. Fig. 20.9 shows the expression pattern of the The pair-rule genes in turn help to regulate the next level in
gap gene Krüppel, which is expressed in the middle region of the the segmentation hierarchy, which consists of segment-polarity
embryo. Mutants of Krüppel lack some thoracic and abdominal genes (Fig. 20.11). The segment-polarity genes refine the 7-striped
segments when they reach the larval stage. Krüppel is expressed pattern still further into a 14-striped pattern. Each of the
in the pattern shown in Fig. 20.9 because it is under the control 14 stripes has distinct anterior and posterior ends determined by
of the transcription factor Hunchback, which in this embryo the segment-polarity genes. Embryos with mutations in segment-
is stained in green. High concentrations of Hunchback repress polarity genes lose this anterior–posterior differentiation, with
Krüppel transcription entirely, and low concentrations fail to the result that the anterior and the posterior halves of each
induce Krüppel transcription. Because Hunchback is present segment are mirror images. The example in Fig. 20.11 is engrailed,
in an anterior–posterior gradient (see Fig. 20.8), the pattern of which eliminates the posterior pattern element in each stripe and
Hunchback expression means that Krüppel is transcribed only in replaces it with a mirror image of the anterior pattern element.
the middle region of the embryo where Hunchback is present but
not too abundant. j Quick Check 4 Would the pattern of segment-polarity
The gap genes encode transcription factors that control genes gene expression be normal if one or more gap genes were not
in the next level of the regulatory hierarchy, which consists expressed properly? Why or why not?
408 CHAPTER 20.2 H I E R A RC H I C A L CO N T RO L O F D E V E LO P M E N T

FIG. 20.11 Normal segment-polarity gene expression pattern FIG. 20.12 Homeotic genes and segment identity. (a) Normal
and mutant phenotype. Source: James Langeland, Steve antennae are transformed into legs in an Antennapedia
Paddock and Sean Carroll, HHMI, University of Wisconsin–Madison. mutant. (b) Normal structures in the third thoracic
segment are transformed into wings in a Bithorax
mutant. Sources: a. (left and right) F. Rudolf Turner, Ph.D., Indiana
Anterior Posterior
University; b. (left) Thomas Deerinck, NCMIR/Science Source;
(right) David Scharf/Science Source.

The green staining Normal Mutant

Nonmutant embryo indicates where the
segment-polarity a. Antennapedia
gene engrailed is
expressed.

Nonmutant larva
The green segments
depend on the
function of the gene.
C1 T1 T2 T3
A1 A2 A3 A4 A5 A6 A7 A8
C2
C3 Mutant segment-
polarity gene results
Mutant larva in loss of half of each
segment and
replacement by
mirror image of
C1 T1 T2 T3
A1 A2 A3 A4 A5 A6 A7 A8 remaining half.
C2 b. Bithorax
C3

Homeotic genes determine where different body parts

develop in the organism.
Together, the segment-polarity genes and other genes expressed
earlier in the hierarchy control the pattern of expression of
another set of genes called homeotic (Hox) genes. Originally
discovered in Drosophila, homeotic genes encode some of the
most important transcription factors in animal development. A
homeotic gene is a gene that specifies the identity of a body part or
segment during embryonic development. For example, homeotic
genes instruct the three thoracic segments (T1, T2, and T3) each to
develop a set of legs, and the second thoracic segment (T2) also to
develop wings. throughout the larval body (Fig. 20.13). The development of these
Two classic examples of the consequences of mutations in tissues and their metamorphosis into the adult body parts are
homeotic genes in Drosophila are shown in Fig. 20.12. Fig. 20.12a regulated by the homeotic genes. First expressed at about the same
shows what happens when the homeotic gene Antennapedia, time as the segment-polarity genes (see Fig. 20.11), the homeotic
which specifies the development of the leg, is inappropriately genes continue to be expressed even after the genes that regulate
expressed in anterior segments. The mutation causes legs to early development have shut down. Their continuing activity is
grow where antennae usually would. Similarly, a mutation in the due to the presence of chromatin remodeling proteins that keep
homeotic gene Bithorax results in the transformation of thoracic the chromatin physically accessible to the transcription complex
segment 3 (T3) into thoracic segment 2 (T2), so that the fruit fly (Chapter 19).
has two T2 segments in a row. As shown in the Fig. 20.12b, the Homeotic genes encode transcription factors. The DNA-binding
result is a fruit fly with two complete sets of wings. domain in the homeotic proteins is a sequence of 60 amino acids
In Drosophila, the adult body parts like legs, antennae, and called a homeodomain, whose sequences are very similar from one
wings are formed from organized collections of tissue located homeotic protein to the next across different species.
 CHAPTER 20 G E N E S A N D D E V E LO P M E N T 409

FIG. 20.13 Tissues controlled by homeotic genes. Homeotic FIG. 20.14 Organization of the Hox gene clusters in Drosophila
genes specify the fate of clumps of tissue in the and the body parts that they affect. The order of
Drosophila larva. genes along the chromosome corresponds to their
positions along the anterior–posterior axis in the
developing embryo.
Mouth parts
The Drosophila Hox genes are found in two clusters called
Antenna the Antennapedia complex and the Bithorax complex.
Eye
Leg Antennapedia complex Bithorax complex
Wing
Haltere
lab pb Dfd Scr Antp Ubx AbdA AbdB

Genitalia

Head Thorax Abdomen

The Drosophila genome contains eight Hox genes comprising Because the amino acid sequences of the homeodomains
two distinct clusters, the Antennapedia complex and the of Hox gene products are very similar from one organism to the
Bithorax complex (Fig. 20.14). The genes are arranged along next, Hox gene clusters have been identified in a wide variety of
the chromosome in the same order as their products function animals with bilateral symmetry (organisms in which both sides
in anterior–posterior segments along the embryo. In addition, of the midline are mirror images), from insects to mammals.
the timing of their expression corresponds to their order along Comparison of the number and types of Hox genes in different
the chromosome and location of expression, with genes that are species supports the hypothesis that the ancestral Hox gene
expressed closer to the anterior end turned on earlier than genes cluster had an organization very similar to what we now see
that are expressed closer to the posterior end. The correlation in most organisms with Hox gene clusters. In its evolutionary
among linear order along the chromosome, anterior–posterior history, the vertebrate genome underwent two whole-genome
position in the embryo, and timing of expression is observed in duplications; hence, vertebrates have four copies of the Hox gene
Hox clusters in almost all organisms studied. cluster (Fig. 20.15).

FIG. 20.15 Organization and content of Hox gene clusters in the mouse and the regions of the embryo in which they are expressed.

a.

Inferred Hox gene organization in Gene duplications The mammalian genome contains four
unidentified common ancestor copies of Hox gene clusters. The genes figure
prominently in the regional differentiation of
parts of the brain and vertebral column.

b.
HoxA
a-1 a-2 a-3 a-4 a-5 a-6 a-7 a-9 a-10 a-11 a-13
HoxB
b-1 b-2 b-3 b-4 b-5 b-6 b-7 b-8 b-9
HoxC
c-4 c-5 c-6 c-8 c-9 c-10 c-11 c-12 c-13
HoxD
d-1 d-3 d-4 d-8 d-9 d-10 d-11 d-12 d-13
Gene deletions Anterior
Posterior
410 CHAPTER 20.3 E VO L U T I O N A RY CO N S E RVAT I O N O F K E Y T R A N S C R I P T I O N FAC TO R S I N D E V E LO P M E N T

Unlike the Hox genes in Drosophila, mammalian Hox

genes do not specify limbs but are important in the embryonic FIG. 20.16 Diversity of animal eyes. Sources: a. David M. Dennis/age
development of structures that become parts of the hindbrain, footstock; b. Masa Ushioda/SeaPics.com; c. Reinhard Dirscherl/age

spinal cord, and vertebral column (Fig. 20.15). As in Drosophila, fotostock; d. Andrey Armyagov/iStockphoto; e. Julian Brooks/age

the genes in each cluster are expressed according to their linear fotostock; f. Walter Geiersperger/age footstock.

order along the chromosome, which coincides with the linear a. Planarian b. Jellyfish
order of regions the genes affect in the embryo. Each gene helps
to specify the identity of the region in which it is expressed. Many
of the genes in the mammalian Hox clusters have redundant or
overlapping functions so that learning exactly what each gene
does continues to be a research challenge. The evolutionary and
developmental study of Hox gene conservation and expression is a
good example of recent evo-devo research.

20.3 EVOLUTIONARY CONSERVATION

OF KEY TRANSCRIPTION
FACTORS IN DEVELOPMENT
c. Squid d. Human
As we have seen, Hox genes and the proteins they encode are
very similar in a wide range of organisms. As a result, they can
be identified by their similarity in DNA or amino acid sequence.
Molecules that are similar in sequence among distantly related
organisms are said to be evolutionarily conserved. Their similarity
in sequence suggests that they were present in the most recent
common ancestor and have changed very little over time because
they carry out a vital function. Many transcription factors
e. House fly f. Trilobite
important in development are evolutionarily conserved. An
impressive example is found in the development of animal eyes.

Animals have evolved a wide variety of eyes.

Animal eyes show an amazing diversity in their development and
anatomy (Fig. 20.16). Among the simplest eyes are those of the
planarian flatworm (Fig. 20.16a), which are only pit-shaped cells
containing light-sensitive photoreceptors. The planarian eye has
no lens to focus the light, but the animal can perceive differences
in light intensity. The incorporation of a spherical lens, as in some
jellyfish (Fig. 20.16b), improves the image. (Fig. 20.16f) already had well-formed compound eyes. These
Among the most complex eyes are the camera-type eyes of the differed greatly from the eyes in modern insects, however. For
squid (Fig. 20.16c) and human (Fig. 20.16d), which have a single example, trilobite eyes had hard mineral lenses composed of
lens to focus light onto a light-sensitive tissue, the retina (Chapter calcium carbonate (the world’s first safety glasses, so to speak).
36). Though the single-lens eyes of squids and vertebrates are Altogether, about 96% of living animal species have true eyes
similar in external appearance, they are vastly different in their that produce an image, as opposed to simply being able to detect
development, anatomy, and physiology. differences in light intensity. Despite this common feature,
Some organisms, such as the house fly (Fig. 20.16e) and other the extensive diversity among the eyes of different organisms
insects, have compound eyes, that is, eyes consisting of hundreds encouraged evolutionary biologists in the belief that the ability to
of small lenses arranged on a convex surface pointing in slightly perceive light may have evolved independently about 40 to
different directions. This arrangement of lenses allows a wide 60 times.
viewing angle and detection of rapid movement.
In the history of life, eyes are very ancient. By the time of the Pax6 is a master regulator of eye development.
extraordinary diversification of animals 542 million years ago The diversity of animal eyes suggests that they may have evolved
known as the Cambrian explosion, organisms such as trilobites independently in different organisms. However, more recent
 CHAPTER 20 G E N E S A N D D E V E LO P M E N T 411

evidence suggests an alternative hypothesis—that they evolved

once, very early in evolution, and subsequently diverged over FIG. 20.18 Pax6, a master switch controlling eye
time. One piece of evidence that supports the hypothesis of development. (a) Normal fly antenna; (b) eye
a single origin for light perception is the observation that the tissue induced by expression of fruit fly Pax6 in the
light-sensitive molecule in all light-detecting cells is the same, a antenna; (c) eye tissue induced by expression of
derivative of vitamin A in association with a protein called opsin mouse Pax6 in the antenna. Sources: a. Cheryl Power/
(Chapter 36). The presence of the same light-sensitive molecule Science Source; b. Eye of Science/Science Source; c. Prof. Walter

in diverse eyes argues that it may have been present in the Gehring/Science Source.

common ancestor of all animals with eyes and has been retained a b c
over time.
Another argument against multiple independent origins
of eyes came from studies of eye development. Researchers
identified eyeless, a gene in the fruit fly Drosophila. As its name
implies, the phenotype of eyeless mutants is abnormal eye
development (Fig. 20.17a). When the protein product of the
eyeless gene was identified, it was found to be a transcription factor
called Pax6. Mutant forms of a Pax6 gene were already known to Normal antenna Antennal eye Antennal eye
cause small eyes in the mouse (Fig. 20.17b) and aniridia (absence induced by induced by mouse
of the iris) in humans. Drosophila Pax6 gene Pax6 gene

The mutations in the Pax6 gene that cause the development

defects in the eye in fruit flies, mice, and humans are loss-of-
function mutations. A loss-of-function mutation is just that: a of the Pax6 transcription factor that is not able to carry out its
mutation that inactivates the normal function of a gene. In this function.
case, loss-of-function mutations in Pax6 make a defective version The similarity in phenotypes of Pax6 mutants, along with the
strong conservation of Pax6 in Drosophila and mouse, led Swiss
developmental biologist Walter Gehring to hypothesize that Pax6
might be a master regulator of eye development. In other words,
FIG. 20.17 Effect of Pax6 mutations on eye development. he hypothesized that Pax6 binds to regulatory regions of a set of
(a) Normal and eyeless mutant in Drosophila; genes that turns on a developmental program that induces eye
(b) normal and small eye mutant in mouse. Sources: development. In theory, this means that Pax6 can induce the
a. (left and right) David Scharf/Science Source; b. (left) INSADCO development of an eye in any tissue in which it is expressed.
Photography/Alamy; (right) Jennifer L. Torrance, Photographer, The To test this hypothesis, Gehring and collaborators genetically
Jackson Laboratory. engineered fruit flies that would produce the normal Pax6
transcription factor in the antenna, where it is not normally
Normal Mutant
expressed (Fig. 20.18a). Any mutation in which a gene is
a. Drosophila
expressed in the wrong place or at the wrong time is known as
a gain-of-function mutation. (We saw an example of gain-of-
function mutations when the Hox genes were expressed in the
wrong segments of the fruit fly, producing legs where antennae
normally develop and thus resulting in four-winged fruit flies.)
For genes that control a developmental pathway, loss-of-function
mutations and gain-of-function mutations often have opposite
effects on phenotype.

j Quick Check 5 For genes that control pathways of development,

b. Mouse
loss-of-function mutations are usually recessive whereas gain-
of-function mutations are usually dominant. Can you suggest a
reason why?

Since loss-of-function mutations in Pax6 result in an eyeless

phenotype, a gain-of-function mutation should result in eyes
developing in whatever tissue Pax6 is expressed. In the gain-
of-function mutation, the antenna developed into a miniature
412 CHAPTER 20.4 CO M B I N ATO R I A L CO N T RO L I N D E V E LO P M E N T

compound eye, and electrical recordings demonstrated that in the history of life as a transcription factor able to bind to and
some of these antennal eyes were functional (Fig. 20.18b). The regulate genes involved in early eye development. Over time,
researchers also created other gain-of-function mutations that led different genes in different organisms acquired new Pax6-binding
to eyes on the legs, wings, and other tissues, which the New York cis-regulatory elements by mutation, and if these were beneficial
Times publicized in an article headlined “With New Fly, Science they persisted. The downstream genes that are targets of Pax6
Outdoes Hollywood.” therefore are different in different organisms, but they share two
Gehring and his group then went one step further. They features—they are regulated by Pax6 and they are involved in eye
took the Pax6 gene from mice and expressed it in fruit flies to development. So, the early steps are conserved, but the later ones
see whether the mouse Pax6 gene is similar enough to the fruit are not.
fly version of the gene that it could induce eye development in The Pax6 gene and the Hox genes reveal an important principle
the fruit fly. Specifically, they created transgenic fruit flies that in evo-devo, which is that master regulatory genes that control
expressed the Pax6 gene from mice in the fruit fly antenna. The development are often evolutionarily conserved, whereas the
mouse gene induced a miniature eye in the fly (Fig. 20.18c). downstream genes that they regulate may not be. Downstream
Note, however, that the Pax6 gene from mice induced the genes can evolve new functions, or genes not originally controlled
development of a compound eye of Drosophila, not the single- by the master regulator may evolve to come under its influence,
lens eye of mouse. or genes formerly controlled by the master regulator may evolve
The ability of mouse Pax6 to make an eye in fruit flies suggests to be unresponsive. In this way, a conserved master regulatory
that mouse and fruit fly Pax6 are not only similar in DNA and mechanism may result in distinct developmental outcomes in
amino acid sequence, but also similar in function, and indeed act different organisms. In the case of Pax6, for example, the master
as a master switch that can turn on a developmental program regulatory mechanism for eye development evolved early and is
leading to the formation of an eye. But these observations lead to shared among a wide range of animals, even though the eyes that
another question: Why did the mouse Pax6 gene produce fruit fly are produced are quite diverse.
eyes instead of mouse eyes?
The answer is that the fruit fly genome does not include the
genes needed to make mouse eyes. Mouse Pax6 protein induces 20.4 COMBINATORIAL CONTROL
fly eyes because of the downstream genes affected by Pax6, IN DEVELOPMENT
those that function later in the process of eye development.
Transcription factors like Pax6 interact with their target genes by Most cis-regulatory elements are located near one or more binding
binding with short DNA sequences adjacent to the gene, usually sites for transcription factors, some of which are activators of
at the 5� end, called cis-regulatory elements. These regulatory transcription and others repressors of transcription. The rate
elements are located in the promoter and help determine whether of transcription of any gene in any type of cell is therefore
the adjacent DNA is transcribed. When bound to cis-regulatory determined by the combination of transcription factors that are
elements, some transcription factors act as repressors that prevent present in the cell and by the relative balance of activators and
transcription of the target gene, and others serve as activators repressors. Regulation of gene transcription according to the mix
by recruiting the transcriptional machinery to the target gene of transcription factors in the cell is known as combinatorial
(Chapter 19). A transcription factor can even repress some of its control. Combinatorial control of transcription is another general
target genes and activate others. principle often seen in all multicellular organisms at many stages
In the fruit fly, Pax6 binds to cis-regulatory elements in of development. Here, we discuss flower development as an
many genes, turning some genes on and others off. The products example of this general principle.
of these downstream genes in turn affect the expression of
further downstream genes. The total number of genes that are Floral differentiation is a model for plant development.
downstream of Pax6 and that are needed for eye development is The plant Arabidopsis thaliana, a weed commonly called mouse-ear
estimated at about 2000. Most are not direct targets of cress, is a model organism for developmental studies and presents
Pax6 but are activated indirectly through other transcription a clear case of combinatorial control. As in all plants, Arabidopsis
factors downstream of Pax6. When mouse Pax6 is expressed in has regions of undifferentiated cells, called meristems, where
fruit flies, it is similar enough in sequence to activate the genes growth can take place (Chapter 31). Meristem cells are similar
involved in fruit fly eye development, so it makes sense that to stem cells in animals. They are the growing points where
mouse Pax6 leads to the development of a fruit fly eye, not a shoots, roots, and flowers are formed. Floral meristems consist of
mouse eye. multipotent cells that can differentiate into different structures in
One scenario of how Pax6 became a master switch for eye the flower. In floral meristems of Arabidopsis, the flowers develop
development in a wide range of organisms, but produces a from a pattern of four concentric circles of cells, or whorls, each of
diversity of eyes in these organisms, is that Pax6 evolved early which differentiates into a distinct type of floral structure.
 CHAPTER 20 G E N E S A N D D E V E LO P M E N T 413

• Cells in whorl 3 form the stamens, the male sexual

FIG. 20.19 Development of the Arabidopsis floral organs from structures in which pollen is produced. (The small granules
concentric whorls of cells in the floral meristem. on the stamens in Fig. 20.19b are pollen grains.)
Photo source: Juergen Berger/Max Planck Institute for Developmental
Biology, Tuebingen, Germany.
• Cells in whorl 4 form the carpels, the female reproductive
structures that receive pollen and contain the ovaries.
a. b.

Whorl 4 Carpel The identity of the floral organs is determined by

combinatorial control.
Whorl 3 Stamen
The genetic control of flower development in Arabidopsis was
Whorl 2 Petal discovered by the analysis of mutant plants, an investigation similar
in principle to the way that genetic control of development in fruit
Whorl 1 Sepal
flies was determined. The plant mutants fell into three classes
showing characteristic floral abnormalities (Fig. 20.20). Mutations
in the gene apetala-2 inactivate the gene’s function and affect
whorls 1 and 2 (Fig. 20.20b), those in apetala-3 or pistillata affect
Each whorl of cells in the whorls 2 and 3 (Fig. 20.20c), and those in agamous affect whorls
Cells in the floral meristem floral meristem gives rise 3 and 4 (Fig. 20.20d). (There are different standards for naming
are organized in a pattern of to a different organ of the
four concentric whorls (1-4). mature flower. genes and proteins in different organisms. In Arabidopsis, wild-
type alleles are written in capital letters with italics, and wild-
type proteins in capital letters without italics. Mutant alleles and
From the outermost whorl (whorl 1) of cells to the innermost proteins follow the same rules but are written in lower-case letters.)
whorl (whorl 4), the floral organs are formed as shown in Fig. 20.19: The observation that the mutant phenotypes affect
development of pairs of adjacent whorls already hints at
• Cells in whorl 1 form the green sepals, which are modified
combinatorial control, which researchers incorporated into a model
leaves forming a protective sheath around the petals in the
of floral development called the ABC model. The model invokes
flower bud that unfold like petals when the flower opens.
three activities arbitrarily called A, B, and C. These activities
• Cells in whorl 2 form the petals. represent the function of one or more different proteins that

FIG. 20.20 Phenotypes of the normal flower and floral mutants of Arabidopsis. Source: Courtesy of John Bowman.

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

In the mutant APETALA-3

In a nonmutant flower, In the mutant APETALA-2, or the mutant PISTILLATA, In the mutant AGAMOUS,
whorl 1 makes sepals, whorl 1 makes carpels, whorl 1 makes sepals, whorl 1 makes sepals,
whorl 2 makes petals, whorl 2 makes stamens, whorl 2 makes sepals, whorl 2 makes petals,
whorl 3 makes stamens, whorl 3 makes stamens, whorl 3 makes carpels, whorl 3 makes petals,
whorl 4 makes carpels. whorl 4 makes carpels. whorl 4 makes carpels. whorl 4 makes sepals.
414 CHAPTER 20.4 CO M B I N AT I O N A L CO N T RO L I N D E V E LO P M E N T

the product of AGAMOUS represses the expression of APETALA-2

FIG. 20.21 The ABC model for flower development in in whorls 3 and 4, so APETALA-2 is normally not expressed in
Arabidopsis. whorls 3 and 4.

The combination of A, B, and C activities j Quick Check 6 Predict the phenotype of a flower in which
in each whorl determines which floral APETALA-2 (activity A) and AGAMOUS (activity C) are expressed
organs develop from that whorl.
normally, but APETALA-3 and PISTILLATA (activity B) are
C Whorl 4 Carpel expressed in all four whorls.

C+ B Whorl 3 Stamen The identification of the products of these genes demonstrates

combinatorial control and explains the mutant phenotypes. All of
B+ A Whorl 2 Petal
the genes encode transcription factors, which are called AP2, AP3,
A Whorl 1 Sepal PI, and AG, corresponding to the four genes. The transcription
factors bind to cis-regulatory elements of genes that encode
proteins necessary for each whorl’s development, similar to
what we saw earlier for Pax6 and eye development. AP3 and PI
are both necessary for the B activity because the proteins form a
heterodimer, a protein made up of two different subunits.
Although other transcription factors associated with other
activities that also contribute to flower development were
are hypothesized to be present in cells of each whorl as shown discovered later, the original ABC model is still valid and serves as
in Fig. 20.21. In the course of floral differentiation, the cells of
each whorl become different from those in the other whorls, and
different combinations of genes are activated or repressed in
each whorl. Activity A is present in whorls 1 and 2, activity B in FIG. 20.22 Floral diversity. (a) Colorado blue columbine
whorls 2 and 3, and activity C in whorls 3 and 4. According to the (Aquilegia coaerulea); (b) slipper orchid (Paphiopedilum
ABC model, activity A alone results in the formation of sepals, A and holdenii); (c) ginger flower (Smithatris supraneanae);
B together result in petals, B and C together result in stamens, and C (d) garden rose (hybrid). Sources: a. ljh images/Shutterstock;
b. Cesar Chavez Photography/Big Stock; c. Nonn Panitvong; d. Maria
alone results in carpels. Like any good hypothesis (Chapter 1),
Mosolova/age fotostock.
this one makes specific predictions. Mutants lacking A will have
defects in whorls 1 and 2, mutants lacking B will have defects in a b
whorls 2 and 3, and mutants lacking C will have defects in whorls
3 and 4.
Matching these predictions with the mutants in Fig. 20.20
yields the following correspondence:

• The A activity is encoded by the gene APETALA-2.

• The B activity is encoded jointly by the genes APETALA-3

and PISTILLATA.

• The C activity is encoded by the gene AGAMOUS.

To account for the mutant phenotypes in Fig. 20.20, you need

only to postulate that when A is absent, C is expressed in whorls
c d
1 and 2 in addition to its normal expression in whorls 3 and 4; and
when C is absent, A expression expands into whorls 3 and 4 in
addition to its normal expression in whorls 1 and 2. The domains
of expression expand in this way because, in addition to its role in
activating certain downstream genes, the product of APETALA-2
represses the expression of AGAMOUS in whorls 1 and 2. As a
result, AGAMOUS is normally not expressed in whorls 1 and 2, but
when APETALA-2 is mutant, AGAMOUS is expressed wherever
APETALA-2 normally represses it. The result is activity C in whorls
1 and 2. Similarly, in addition to its role in activating other genes,
 CHAPTER 20 G E N E S A N D D E V E LO P M E N T 415

an elegant example of combinatorial control. As in the case of the

Pax6 gene in the development of animals’ eyes, evolution of the cis- FIG. 20.23 The nematode worm Caenorhabditis elegans.
regulatory sequences in downstream genes has resulted in a wide
variety of floral morphologies in different plant lineages, some of
which are shown in Fig. 20.22.

Tail
Vulva
20.5 CELL SIGNALING IN The vulva is an
DEVELOPMENT opening into
the body cavity
Eggs through which
As we have seen in the discussion of stem cells, differentiated cells fertilized eggs
are laid.
can be reprogrammed by the action of only a few key genes or
small organic molecules. In some cases, the processes that push
differentiation in a forward direction are also quite simple. An
important example is signal transduction, in which an extracellular
molecule acts as a signal to activate a membrane protein that in
Head
turn activates molecules inside the cell that control differentiation
(Chapter 9). The signaling molecule is called the ligand and the
membrane protein that it activates is called the receptor. The
following example shows how a simple ligand–receptor pair can
have profound effects not only on the differentiation of the cell that
carries the receptor, but also on its neighbors.

FIG. 20.24 Vulval development in Caenorhabditis elegans. The

A signaling molecule can cause multiple responses
cells of the vulva differentiate in response to molecular
in the cell.
signals from other cells.
Pioneering experiments on signal transduction in development
were carried out in the soil nematode Caenorhabditis elegans
(Fig. 20.23). This worm is a useful model organism for a.
AC The anchor cell (AC) induces
developmental studies because of its small size (the adult is vulval development.

about 1 mm long), short generation time (2½ days from egg to

The EGF ligand is a protein signal
sexually mature adult), and stereotyped process of development. EGF ligand that induces type 1 differentiation
(A stereotyped process is one that is always the same in each in the nearest cell.
individual.) The adult animal consists of exactly 959 somatic (body) Induction
cells, and the cells’ patterns of division, migration, differentiation, The type 1 cell sends out signals
to the adjacent cells that prevent
and morphogenesis are identical in each individual C. elegans. The Lateral type 1 differentiation (lateral
– –
adult is typically a hermaphrodite that produces both male and inhibition inhibition) and that promote type
2 differentiation.
female reproductive cells. The eggs are fertilized internally by
sperm, and after undergoing five or six cell divisions the eggs are Cell division
laid through an organ known as the vulva (Fig. 20.23).
Vulva development is relatively simple and therefore amenable b.

to experimental studies. The entire structure arises from only three

multipotent cells that undergo either of two types of differentiation
and divide to produce a vulva consisting of exactly 22 cells. Cells 2 2
resulting from type 1 differentiation form the opening of the vulva, 1 1 2
2 2 1 1 2 2 2
and cells resulting from type 2 differentiation form a supporting
structure around the vulval opening. In each of the two types of Two more
differentiation, different genes are activated or repressed, resulting cell divisions
in differentiated cells with different functions.
c.
How each of the three original cells, called progenitor cells,
differentiates is determined by its position in the developing
Two more cell divisions
worm (Fig. 20.24). The fate of the progenitor cells is determined by lead to a 22-cell vulva.
their proximity to another cell, called the anchor cell (“AC” in
416 CHAPTER 20.5 C E L L S I G N A L I N G I N D E V E LO P M E N T

FIG. 20.25 Signal transduction in vulval development in Caenorhabditis elegans.

AC

EGF ligand
Induction

Lateral
inhibition

1 An epidermal growth
factor (EGF) ligand binds EGF ligand
with its receptor on the
cell surface.
EGF receptor EGF receptor

Notch
receptor –

–
2 Stimulation of the EGF
receptor initiates a chain Notch Signal
of protein activation in ligand transduction
the cytoplasm (signal +
transduction).

Notch
receptor

Type 1 Type 2
genes genes

3 Activated proteins enter the nucleus and result in: 4 Stimulation of the Notch receptor results in:
• Type 1 differentiation genes turned ON, • Type 2 differentiation genes turned ON,
• Gene for Notch ligand turned ON, • EGF receptor gene turned OFF.
• Notch receptor gene turned OFF.

Fig. 20.24a), which secretes a protein called epidermal growth not only of the cell itself but also of its neighbors? Fig. 20.25
factor (EGF) that binds to and activates a transmembrane EGF shows the mechanisms in simplified form. On the left is the
receptor (Chapter 9). The progenitor cell closest to the anchor cell progenitor cell nearest the anchor cell, which receives the
receives the most amount of signal, and upon activation of its EGF strongest EGF signal. Activation of the EGF receptor by the ligand
receptor the progenitor cell carries out three functions: initiates a process of signal transduction in the cytoplasm, in
which the signal is transmitted from one protein to the next by
• It activates the genes for differentiation into a type 1 cell
means of proteins at each stage phosphorylating several others,
(Fig. 20.24a).
which amplifies the signal at each stage (Chapter 9). The result
• It prevents the adjacent cells from differentiating as type 1 of signal transduction is that a set of transcription factors is
(a process called lateral inhibition). activated.
In the nucleus, the transcription factors activate transcription
• It induces the adjacent cells to differentiate as type 2 cells.
of genes for type 1 differentiation. The transcription factors also
activate transcription of genes that prevent type 1 differentiation
Developmental signals are amplified and expanded. in neighboring cells, including the genes that produce another
How can a single ligand–receptor pair cause so many changes in type of protein ligand, called Notch. The Notch ligand is a
gene expression that it determines the pathway of differentiation transmembrane protein that activates Notch receptors in the
 CHAPTER 20 G E N E S A N D D E V E LO P M E N T 417

neighboring cells. Activation of the Notch receptor in these cells differentiation and repair of multiple types of cells in the skin;
activates a signal transduction cascade in these cells, which results it is present in all body fluids and helps regulate rapid metabolic
in transcription of the genes for type 2 differentiation. The cascade responses to changing conditions. Human Notch ligands are
started by the binding of Notch also activates transcription of involved in development of the nervous and immune systems as
other genes whose products inhibit the EGF receptor. Inhibiting well as heart, pancreas, and bone. Abnormalities in EGF or Notch
the EGF receptor in type 2 cells prevents EGF from eliciting a type signaling are associated with many different types of cancer.
1 response in the type 2 cell. In addition, at the same time that the Therefore, just as we saw in our discussion of eye and flower
type 1 cell produces the Notch ligand, it produces proteins that development, the molecular players involved in development are
inhibit its own Notch receptors, and this prevents Notch from often evolutionarily conserved across a wide range of organisms.
initiating a type 2 response in the type 1 cell. This is true even of genes that we typically associate with disease,
While vulva development in nematodes is a fairly simple such as BRCA1 and BRCA2 and their link with cancer. These
example of the importance of ligand–receptor signaling in genes not only play a role in cell cycle control in the adult, but
development, EGF and Notch ligands and their receptors are also in early development in many organisms. In fact, although
found in virtually all animals. They are among dozens of ligand– heterozygous mutations in these genes predispose individuals to
receptor pairs that have evolved as signaling mechanisms to breast and ovarian cancers in humans, homozygous mutations are
regulate processes in cellular metabolism and development. lethal in early embryonic stages. In Fig. 20.26, we focus on the
Humans have several distinct but related gene families of EGF BRCA1 gene to summarize key concepts about DNA replication,
and Notch ligands and receptors. Human EGF is important in cell mutation, genetic variation, inheritance, gene regulation, and
survival, proliferation, and differentiation. EGF functions in the development. •
V I S UA L S Y N T H E S I S Genetic Variation and Inheritance
FIG. 20.26 Integrating concepts from Chapters 12–20

Genetic variation:
BRCA1 Genetics Everyone carries two copies of the
The BRCA1 gene on chromosome 17 encodes a protein that repairs double-stranded BRCA1 gene on chromosome 17. Some
breaks in DNA that result from DNA replication or environmental agents. The BRCA1 individuals have an allele of the gene
that increases the risk of breast and
gene is present in all cells, but is expressed, or turned on, in rapidly dividing cells,
ovarian cancer. In some populations,
including breast and ovarian cells. If BRCA1 does not function properly because of such as Ashkenazi Jews, the frequency
a mutation, damaged DNA is not repaired, which in turn increases the risk for of specific mutations in the gene is high.
certain kinds of cancers, particularly breast and ovarian cancers in women.

Mutant Ruth Isaac

cell
Chromosome 17
Tumor
Mutant BRCA1

Healthy Chromosome 17
cell
Wild type allele
Mutant BRCA1

Mutation:
This individual inherits one mutation in the BRCA1
and is heterozygous. In one cell or a few of her
cells, a new mutation is acquired in the normal Susan Ben Sarah
BRCA1 gene, and these cells go on to be
cancerous.

Genotype and phenotype:

For a woman with a mutation in BRCA1, the risk of
breast cancer by age 40 is 37%; by age 55, 66%;
and by age 80, 85%.

Testing and Ethical issues

DNA testing can be used to
determine whether or not a
person carries an allele of a gene
that increases the risk of a disease,
but it also raises questions and
ethical issues. In 2008, the Genetic
Information Nondiscrimination
Act (GINA) was passed to protect
individuals from discrimination by
insurance companies or employers
based on genetic profile.

DNA testing can be used to determine whether any of these

individuals carry a mutation in the BRCA1 gene.
If any individual in this family gets a positive result, everyone
else can figure out their risk of carrying the mutation.
If an individual tests positive, what should he or she do?
What happens if an individual finds a mutation in the BRCA1
gene which is not known to be associated with cancer?

418
? ? ? ?
 Ruth

½ ½

The BRCA1 wild-type and Heterozygous

mutant alleles segregate carrier female
equally into gametes. Mendel’s Laws
Eggs
Mendel’s First Law:
Mendel’s Second Law: Alleles
Alleles of a gene
of different genes segregate
segregate equally
into gametes independently
½ into gametes (eggs
of each other.
Isaac and sperm).

Homozygous wild type Heterozygous

Sperm
Genes and In addition to BRCA1 on
Heterozygous development: chromosome 17, BRCA2 on
carrier male ½ Homozygous mutant chromosome 13 is known to
BRCA1 embryos do increase the risk of breast
not develop normally cancer. Alleles of the BRCA1
Heterozygous Homozygous mutant and are not viable. gene and alleles of the BRCA2
gene segregate into gametes
independently of each other.

Harriet Lily Becky Adam Leah

Genes and the

Chromosome 17
Environment
BRCA 1
Most traits, including
cancer, result from the
interaction of many genes
and the enviromnent.
These two individuals both carry This individual develops Chromosome 13
the same mutation in the BRCA1 breast cancer, but does
gene, but only one goes on to not carry a mutation in BRCA 2
develop breast cancer. the BRCA1 gene.

Unaffected male
Unaffected female
Affected male
Affected female
Photo sources: (left to right) Michael Poehlman/Getty Images; Digital
Vision/Getty Images, Fabrice Lerouge/age fotostock, Piotr Marcinsk/
Dreamstime.com.

419
420 SELF-ASSESSMENT

Core Concepts Summary light perception are evolutionarily conserved suggests that
the ability to perceive light may have evolved once, early in
the evolution of animals. page 410
20.1 In the development of humans and other animals,
stem cells become progressively more restricted in their The Pax6 transcription factor is a master regulator of eye
possible pathways of cellular differentiation. development. Loss-of-function mutations in Pax6 result in
abnormalities in eye development, whereas gain-of-function
The fertilized egg can give rise to a complete organism.
mutations result in eye development in tissues in which eyes
page 400
do not normally form. page 410
At each successive stage in development, cells lose
developmental potential as they differentiate. page 401 20.4 Combinatorial control is a developmental
strategy in which cellular differentiation depends on the
Embryonic stem cells can give rise to any of the three germ
particular combination of transcription factors present in
layers, those further along in differentiation can form only
a cell.
a limited number of specialized cell types, and those still
further along can form only one cell type. page 401 By analyzing mutants that affect flower development in the
plant Arabidopsis, researchers were able to determine the
Stem cells play a prominent role in regenerative medicine, in
genes involved in normal flower development. page 412
which stem cells—in some cases reprogrammed cells from the
patient’s own body—are used to replace diseased or damaged The ABC model of flower development invokes three activities
tissues. page 403 (A, B, and C) present in circular regions (whorls) of the
developing flower, with the specific combination of factors
20.2 The genetic control of development is a hierarchy determining the developmental pathway in each whorl.
in which genes are activated in groups that in turn page 413
regulate the next set of genes.
20.5 Ligand–receptor interactions activate signal
Hierarchical gene control can be seen in fruit fly (Drosophila)
transduction pathways that converge on transcription
development. page 404
factors and genes that determine cell fate.
The oocyte of a fruit fly is highly polarized, with gradients of
Cell signaling involves a ligand, an extracellular molecule
maternal mRNA that set up anterior–posterior and dorsal–
that acts as a signal to activate a membrane receptor protein,
ventral axes. page 405
which in turn activates molecules inside the cell. page 415
These gradients in turn affect the expression of segmentation
Activation of a receptor sets off a pathway of signal
genes in the zygote, including the gap, pair-rule, and segment-
transduction, in which a series of proteins in the cytoplasm
polarity genes, which define specific regions in the developing
become sequentially activated. page 416
embryo. page 407
Because signal transduction can amplify and expand a
The segmentation genes direct the expression of homeotic
developmental signal, a single ligand–receptor pair can cause
genes, key transcription factors that specify the identity of
major changes in gene expression and ultimately determine
each segment of the fly and that are conserved in animal
the pathway of differentiation. page 416
development. page 408
An example of signal transduction in development is
20.3 Many proteins that play key roles in development differentiation of the nematode vulva, which is determined by
are evolutionarily conserved but can have dramatically means of an EGF ligand and receptor. page 416
different effects in different organisms.
Many proteins important in development are similar in Self-Assessment
sequence from one organism to the next. Such proteins are said
to be evolutionarily conserved. page 410 1. Distinguish among totipotent, pluripotent, and multipotent
stem cells, and give an example of where you would find each
The downstream targets of homeotic genes are different
type of cell.
in different animals, allowing homeotic genes to
activate different developmental pathways in different 2. Explain how an individual’s own cells might be used in
organisms. page 410 stem cell therapy.

Although animals exhibit an enormous diversity in eye 3. Draw a diagram to illustrate how a concentration
morphology, the observation that the proteins involved in gradient of a transcription factor along the anterior–posterior
 CHAPTER 20 G E N E S A N D D E V E LO P M E N T 421

axis of a Drosophila embryo can create a region in the middle in 6. Define combinatorial control in the context of the ABC
which transcription of a target gene takes place without being model of floral development.
transcribed in either the anterior or posterior region.
7. Diagram a pathway of signal transduction including a
4. Expression of a homeotic gene in the wrong tissue in ligand, receptor, and ultimately a transcription factor that
Drosophila results in the development of an inappropriate activates a gene that inhibits the receptor.
body part from that tissue. Explain why this happens and
how it shows that homeotic genes are positive regulators of
developmental pathways. Log in to to check your answers to the Self-
Assessment questions, and to access additional learning tools.
5. Explain why master regulatory genes tend to be more
strongly conserved in evolution than are the downstream
genes they regulate.
 CASE 4

Malaria
Coevolution of Humans and a Parasite
1 As it feeds, the mosquito injects
Plasmodium parasites along with its
saliva into the host. The parasites
migrate through the bloodstream
to the host’s liver.
Most people would say the world has more than enough
mosquitoes, but in 2010 scientists at the University
of Arizona conjured up a new variety. The high-tech life cycle. The human part of the cycle begins with a single
bloodsuckers were genetically engineered to resist bite from an infected mosquito. As the insect draws blood,
Plasmodium, the single-celled eukaryote that causes it releases Plasmodium-laden saliva into the bloodstream.
malaria. Once inside their human host, the Plasmodium parasites
Normally, the parasite grows in the mosquito’s gut invade the liver cells. There they undergo cell division for
and is spread to humans by the insect’s bite. By altering a several days, their numbers increasing. Eventually, the
single gene in the mosquito’s genome, the researchers had parasites infect red blood cells, where they continue to
made the insects immune grow and multiply. The mature parasites burst from the
As humans have to the malaria parasite. red blood cells at regular intervals, triggering malaria’s
The accomplishment is a telltale cycle of fever and chills.
tried a succession of noteworthy advance, the Some of the freed Plasmodium parasites go on to infect
weapons to defeat latest in a long line of efforts new red blood cells; others divide to form gametocytes,
to stop Plasmodium in its precursor cells to male and female gametes, which
P. falciparum, the tracks. travel through the victim’s blood vessels. When another
parasite has evolved, Malaria is one of the most mosquito bites the infected individual, it takes up the
devastating diseases on the gametocytes with its blood meal. Inside the insect, the
thwarting their planet. The World Health parasite completes its life cycle. The gametocytes fuse
efforts. Organization estimates that to form zygotes. Those zygotes bore into the mosquito’s
500 million people contract stomach, where they form cells called oocysts that give
malaria annually, primarily in tropical regions. The disease rise to a new generation of parasites. When the infected
is thought to claim about a million lives each year. Of mosquito sets out to feed, the cycle begins again.
those deaths, 85% to 90% occur in sub-Saharan Africa, The battle between malaria and humankind has raged
mostly among children under 5. through the ages. Scientists have recovered Plasmodium
Five species of Plasmodium can cause malaria in DNA from the bodies of 3500-year-old Egyptian
humans. One of them, P. falciparum, is particularly mummies—evidence that those ancient humans were
dangerous, accounting for the vast majority of malaria infected with the malaria parasite. The close connection
fatalities. Over thousands of years, this parasite and its between humans, mosquitoes, and the malaria parasite
human host have played a deadly game of tug-of-war. almost certainly extends back much further.
As humans have tried a succession of weapons to defeat In fact, people who hail from regions where malaria
P. falciparum, the parasite has evolved, thwarting their is endemic are more likely than others to have certain
efforts. And in turn, the tiny organism has helped to shape genetic signatures that offer some degree of protection
human evolution. from the parasite. That indicates that Plasmodium has
Plasmodium is a wily and complicated parasite, been exerting evolutionary pressure on humankind for
requiring both humans and mosquitoes to complete its quite some time.
422
 Meanwhile, we’ve done our best to fight back. For Just as bacteria develop resistance to antibiotics,
centuries, humans fought the infection with quinine, Plasmodium evolves resistance to the antiparasitic drugs
a chemical found in the bark of the South American designed to fight it. In poor, rural areas where malaria is
cinchona tree. In the 1940s, scientists developed a more prevalent, people often can’t follow the recommended
sophisticated drug based on the cinchona compound. protocols for antimalarial treatment. Sick individuals
That drug, chloroquine, was effective, inexpensive, easy may be able to afford only a few pills rather than the full
to administer, and caused few side effects. As a result, recommended dose. The strength and quality of those
it was widely used, and in the late 1950s, P. falciparum pills may be questionable, and the drugs are often taken
began showing signs of resistance to chloroquine. Within without oversight from a medical professional.
20 years, resistance had spread to Africa, and today most Unfortunately, inadequate use of the drugs fuels
strains of P. falciparum have evolved resistance to the resistance. When the pills are altered or the course of
once-potent medication. treatment is abbreviated, not all Plasmodium parasites are

The life cycle of Plasmodium falciparum.

Liver

Liver
Malaria
cell
parasite
6 This mosquito then
leaves and infects

2 5 Gametocytes combine
another human host,
beginning the cycle
Plasmodium incubates in
to form zygotes in the again.
liver cells, maturing into a
mosquito gut. These
form that can invade red
mature into oocysts, and
blood cells.
spread thousands of
parasites through the
mosquito.

Blood vessel

3 The parasite
continues to grow and
multiply by mitosis, 4 Some parasites
bursting from red blood develop into male and
cells to infect other red female gametocytes, Gamete
blood cells. which infect another
mosquito when it takes
Red blood a blood meal from the Zygote
cells host.
Malaria Malaria
parasites parasites
Mature Oocyst
gametocytes

423
 wiped out. Those that survive in the presence of the drug Plasmodium itself. The genetically engineered insects
are likely to evolve resistance to the drug. Because the created by the team in Arizona are a promising step in that
resistant parasites have a survival advantage, the genes for direction.
drug resistance spread quickly through the population. The Arizona researchers set out to alter a cellular
Since chloroquine resistance emerged, pharmaceutical signaling gene that plays a role in the mosquito’s life cycle.
researchers have developed a variety of new medications Mosquitoes normally live 2 to 3 weeks, and Plasmodium
to prevent or treat malaria. Most, however, are far too takes about 2 weeks to mature in the mosquito’s gut.
expensive for people in the poverty-stricken regions The researchers hoped to create mosquitoes that would
where malaria is rampant. And just as was the case die prematurely, before the parasite is mature. The
with chloroquine, almost as quickly as new drugs are genetic modification worked as planned. The engineered
developed, Plasmodium begins evolving resistance. mosquitoes’ life-spans were shortened by 18% to 20%. The
One of the weapons more recently added to the genetic tweak also had a surprising side effect. The altered
drug arsenal is artemisinin, a compound derived from gene completely blocked the development of Plasmodium
the Chinese wormwood tree. It has turned out to be an in the mosquitoes’ guts. The engineered mosquitoes are
effective and relatively inexpensive way to treat malaria incapable of spreading malaria to humans, regardless of
infections. However, pockets of artemisinin resistance how long they live.
have already been uncovered in Southeast Asia. Public While the finding was a laboratory success, it will be
health workers now recommend that artemisinin be much harder to translate the results to the real world.
given in combination with other drugs. Treating infected To create malaria-free mosquitoes in the wild, scientists
patients with multiple drugs is more likely to wipe out would have to release the genetically modified mosquitoes
Plasmodium in their bodies, reducing the chances that and hope that their altered gene spreads through the wild
more drug-resistant strains will emerge. mosquito population. But that gene would be passed on
Given the challenges of developing practical drugs to only if it gave the insects a distinct evolutionary advantage.
prevent or treat malaria, some scientists have turned their The engineered mosquitoes may be malaria free, but so far
attention to other approaches. One goal is to produce a they have no advantage over their wild counterparts.
malaria vaccine. The parasite’s complex life cycle makes It’s clear that slashing malaria rates will not be an
that a complicated endeavor, though. Several vaccines are easy task. Despite decades of research, insecticide-treated
now in various stages of testing, and some show promise. bed nets are still the best method for preventing the
But researchers expect it will be years before a safe, disease. For millennia, the malaria parasite has managed to
effective vaccine for malaria could be available. withstand our efforts to squelch it, yet science continues
Other researchers are focusing their efforts on to push the boundaries. Who will emerge the victor? Stay
the mosquitoes that carry the parasite, rather than on tuned.

?CASE 4 QUESTIONS
Special sections in Chapters 21–24 discuss the following questions related to Case 4.

1. What genetic differences make some individuals more and some less susceptible
to malaria? See page 434.
2. How did malaria come to infect humans? See page 455.
3. What human genes are under selection for resistance to malaria? See page 501.

424
 CHAPTER 21

Evolution
How Genotypes and
Phenotypes Change
over Time

Core Concepts
21.1 Genetic variation refers
to differences in DNA
sequences.
21.2 Patterns of genetic variation
can be described by allele
frequencies.
21.3 Evolution is a change in
the frequency of alleles or
genotypes over time.
21.4 Natural selection leads to
adaptations, which enhance
the fit between an organism
and its environment.
21.5 Migration, mutation,
genetic drift, and non-
random mating are non-
adaptive mechanisms of
evolution.
21.6 Molecular evolution is a
change in DNA or amino
acid sequences over time.
Roc Canals Photography/Getty Images.

425
426 SECTION 21.1 G E N E T I C VA R I AT I O N

Variation is a fact of nature. A walk down any street reveals how color in butterflies. As we saw in Chapter 18, two factors contribute
variable our species is: Skin color and hair color, for example, vary to phenotype: an individual’s genotype, which is the set of alleles
from person to person. Until the publication in 1859 of Charles possessed by the individual, and the environment in which the
Darwin’s On the Origin of Species, scientists tended to view all individual lives. We can take the environment out of the equation
the variation we see in humans and other species as biologically by looking directly at genotypic differences through sequencing
unimportant. According to the traditional view, not only were DNA regions in multiple individuals. We now explore genetic
species individually created in their modern forms by a divine variation directly, in terms of differences at the DNA sequence level.
Creator, but, because the Creator had a specific design in mind
for each species, they were fixed and unchanging. Departures or Population genetics is the study of patterns of genetic
variations from this divinely ordained type were therefore ignored. variation.
Since Darwin, however, we have appreciated that a species Remarkably, in spite of a high degree of phenotypic variation,
does not conform to a type. Rather, a species consists of a range humans actually rank low in terms of overall genetic variation
of variants. In our own species, people may be tall, short, dark- compared with other species. Any two randomly selected humans
haired, fair-haired, and so on. Furthermore, variation is an essential differ from each other on average by one DNA base per thousand
ingredient of Darwin’s theory because the mechanism he proposed (the two genomes are 99.9% identical), while two fruit flies differ
for evolution, natural selection, depends on the differential by ten bases per thousand (the two genomes are 99% identical).
success—in terms of surviving and reproducing—of variants. Even one of the most seemingly uniform species on the planet,
Darwin changed how we view variation. Before Darwin, variation the Adélie penguins seen in Fig. 21.1, is two to three times more
was irrelevant, something to be ignored; after Darwin, it was genetically variable than we are.
recognized as the key to the evolutionary process. As we discuss in Chapter 22, a species consists of individuals
that can exchange genetic material through interbreeding. From
a genetic perspective, a species is therefore a group of individuals
21.1 GENETIC VARIATION capable, through reproduction, of sharing alleles with one another.
Individuals represent different combinations of alleles drawn from
Variation is a major feature of the natural world. We humans the species’ gene pool, that is, from all the alleles present in all
are particularly good at noticing phenotypic variation among individuals in the species. The human gene pool includes alleles
individuals of our own species. As we discussed in Chapter 16, a that cause differences in skin color, hair type, eye color, and so
phenotype is an observable trait, such as human height or wing on. Each one of us has a different set of those alleles—alleles that
cause brown hair and brown
eyes, for example, or black hair
and blue eyes—drawn from that
FIG. 21.1 Genetic diversity in Adélie penguins. Adélie penguins are uniform in appearance but are gene pool.
actually more genetically diverse than humans. Source: Tim Davis/Corbis. Population genetics is the
study of genetic variation in
natural populations, which
are interbreeding groups of
organisms of the same species
living in the same geographical
area. What factors determine
the amount of variation in a
population and in a species?
Why are humans genetically
less variable than penguins?
What factors affect the
distribution of particular
variations? Population genetics
addresses detailed questions
about patterns of variation.
And small differences, given
enough time, can lead to the
major differences we see among
organisms today.
 CHAPTER 21 E VO LU T I O N : H O W G E N OT Y P E S A N D P H E N OT Y P E S C H A N G E OV E R T I M E 427

Mutation and recombination are the two sources of Advantageous mutations, as we will see, can increase in frequency
genetic variation. in a population until eventually they are carried by every member
Genetic variation has two sources: Mutation generates new of a species. These mutations are the ones that result in a species
variation, and recombination followed by segregation of that is adapted to its environment—better able to survive and
homologous chromosomes during meiotic cell division shuffles reproduce in that environment.
mutations to create new combinations. In both cases, new alleles
are formed, as shown in Fig. 21.2.
As we saw in Chapter 14, mutations can be somatic, occurring 21.2 MEASURING GENETIC VARIATION
in the body’s tissues, or germline, occurring in the reproductive
cells and therefore passed on to the next generation. From an Mutations, whether deleterious, neutral, or advantageous, are
evolutionary viewpoint, we are primarily interested in germ-line sources of genetic variation. The goal of population genetics is to
mutations. A somatic mutation affects only the cells descended make inferences about the evolutionary process from patterns of
from the one cell in which the mutation originally arose, and thus genetic variation in nature. The raw information for this comes
affects only that one individual. However, a germ-line mutation from the rates of occurrence of alleles in populations, or allele
appears in every cell of an individual derived from the fertilization frequencies.
involving the mutation-bearing gamete, and thus appears in its
descendants. To understand patterns of genetic variation, we
Mutations can also be classified by their effects on an require information about allele frequencies.
organism (Chapter 15). Mutations occur randomly throughout The allele frequency of an allele x is simply the number of x’s
the genome, and, because most of the genome consists of present in the population divided by the total number of alleles.
noncoding DNA, most mutations are neutral, having little or no Consider, for example, pea color in Mendel’s pea plants. In Chapter
effect on the organism. Most mutations that do occur in protein- 16, we discussed how pea color (yellow or green) results from
coding regions of the genome, however, have a deleterious, or variation at a single gene. Two alleles of this gene are the dominant
harmful, effect on an organism. Rarely, a mutation occurs that A (yellow) allele and the recessive a (green) allele. AA homozygotes
has a beneficial effect. Mutations like these are advantageous if and Aa heterozygotes produce yellow peas, and aa homozygotes
they improve their carriers’ chances of survival or reproduction. produce green peas. Imagine that in a population every pea plant
produces green peas, meaning that only one allele, a, is present:
The allele frequency of a is 100%, and the allele frequency of A is
0%. When a population exhibits only one allele at a particular gene,
we say that the population is fixed for that allele.
FIG. 21.2 Mutation and recombination. The formation of new Now consider another population of 100 pea plants with
alleles occurs by mutation and recombination. genotype frequencies of 50% aa, 25% Aa, and 25% AA. A genotype
frequency is the proportion in a population of each genotype at a
Allele 1
Generation 1 particular gene or set of genes. These genotype frequencies give
T T T T us 50 green-pea pea plants (aa), 25 yellow-pea heterozygotes
Mutation No mutation (Aa), and 25 yellow-pea homozygotes (AA). What is the allele
frequency of a in this population? Each of the 50 aa homozygotes
Allele 2 Allele 1 has two a alleles and each of the 25 heterozygotes has one a allele.
Generation 2
C T T T T T T T Of course, there are no a alleles in AA homozygotes. The total
number of a alleles is thus (2  50) 1 25 5 125. To determine
No mutation Mutation No mutation
the allele frequency of a, we divide the number of a alleles by the
Allele 2 Allele 3 Allele 1 total number of alleles in the population, 200 (because each pea
Generation 3 plant is diploid, meaning that it has two alleles): 125/200 5 62.5%.
C T T T C T G T T T T T
Because we are dealing with only two alleles in this example, the
allele frequency of A is 100% 2 62.5% 5 37.5%
Recombination Thus, the allele frequencies of A and a provide a measure
C T G T shuffles mutations
Recombination to produce new
of genetic variation at one gene in a given population. In this
T T T T sequences. example, we were given the genotype frequencies, and from this
information we determined the allele frequencies. But how are
genotype and allele frequencies measured? We consider three
Allele 4 ways to measure genotype and allele frequencies in populations:
Generation 4
T T G T observable traits, gel electrophoresis, and DNA sequencing.
428 CHAPTER 21.2 M E A S U R I N G G E N E T I C VA R I AT I O N

j Quick Check 1 Using the example of pea color in Mendel’s

pea plants, can you devise equations to determine the allele FIG. 21.3 Single-gene variation. A genetic difference in color in
frequencies of A and a from the genotype frequencies of aa, Aa, the two-spot ladybug, Adalia bipunctata, results from
and AA? variation in a single gene. Sources: (top) © Biopix: G. Drange;
(bottom) Howard Marsh/Shutterstock.

Early population geneticists relied on observable traits

and gel electrophoresis to measure variation.
It would be a simple matter to measure genetic variation in a
population if we could use observable traits. Then we could simply
count the individuals displaying variant forms of a trait and have a
measure of the variation of that trait’s gene. However, as we saw in
Chapter 18, this approach can work only rarely, for two important
reasons. First, many traits are encoded by a large number of genes.
In these cases, it is difficult, if not impossible, to make direct
inferences from a phenotype to the underlying genotype. Even
traits that seem to have a simple set of phenotypes often prove to
have a complicated genetic basis. For instance, human skin color is
determined by at least six different genes. Second, the phenotype
is a product of both the genotype and the environment.
Until the 1960s, there was only one workable solution: to
limit population genetics to the study of phenotypes that are
encoded by a single gene. As these are few, the number of
genes that population geneticists could study was extremely small.
Human blood groups, including the ABO system, provided an early
example of a trait encoded by a single gene with multiple alleles.
At this gene, there are three alleles in the population—
A, B, and O—and therefore six possible genotypes, which result in
four different phenotypes (Table 21.1).

TABLE 21.1 The ABO blood system.

PHENOTYPE GENOTYPE

A AA or AO
B BB or BO
move from one end of the gel to the other is determined by their
AB AB
charge and their size.
O OO
Early studies of protein electrophoresis focused on enzymes
that catalyze reactions that can be induced to produce a dye when
Other instances in which phenotypic variation can be the substrate for the enzyme is added. If we add the substrate, we
readily correlated with genotype include certain markings in can see the locations of the proteins in the gel. The bands in the
invertebrates. For example, the coloring of the two-spot ladybug gel provide a visual picture of genetic variation in the population,
Adalia bipunctata is controlled by a single gene (Fig. 21.3). revealing what alleles are present and what their frequencies are.
However, the genetic basis of most traits is not so simple. Fig. 21.4 shows this sort of experiment.
Single-gene variation became much easier to detect in the
1960s with the application of gel electrophoresis. In Chapter DNA sequencing is the gold standard for measuring
12, we saw how gel electrophoresis separates segments of DNA genetic variation.
according to their size. Before DNA technologies were developed, Protein gel electrophoresis was a leap forward in our ability
the same basic process was applied to proteins to separate to detect genetic variation, but this technique had significant
them according to their electrical charge and their size. In gel limitations. Researchers could study only enzymes because
electrophoresis, the proteins being studied migrate through a gel they needed to be able to stain specifically for enzyme activity
when an electrical charge is applied. The rate at which the proteins and could detect only mutations that resulted in amino acid
HOW DO WE KNOW?

FIG. 21.4 RESULTS The Adh gene has two common alleles, distinguished by a
single amino acid difference that changes the charge of the protein.

How is genetic variation One allele, Fast (F ), accordingly runs faster than the other, Slow
(S). Four individuals are S homozygotes; two are F homozygotes;

measured? and two are FS heterozygotes. Note that the heterozygotes do not
stain as strongly on the gel because each band has half the intensity
of the single band in the homozygote. We can measure the allele
BACKGROUND The introduction of protein gel electrophoresis in frequencies simply by counting the alleles. Each homozygote has two
1966 gave researchers the opportunity to identify differences in of the same allele, and each heterozygote has one of each.
amino acid sequence in proteins both among individuals and, in the
The gel is stained with a biochemical agent
case of heterozygotes, within individuals. Proteins with different
that produces color in the presence of Adh
amino acid sequences run at different rates through a gel in an enzyme.
electric field. Often, a single amino acid difference is enough to
affect the mobility of a protein in a gel.

METHOD Starting with crude tissue—in this case, the ground-up

whole body of a fruit fly—we load the material on a gel and
turn on the current. The rate at which a protein migrates depends
on its size and charge, both of which may be affected by its amino Heterozygote
acid sequence. To visualize the protein at the end of the gel run,
we use a biochemical indicator that produces a stain when the
protein of interest is active. Here, we use an indicator that detects
the activity of alcohol dehydrogenase (Adh). The result is a series
of bands on the gel showing where the Adh proteins in each lane
migrated.
Homozygote Homozygote
for S allele for F allele

Total number of alleles in the population of 8 individuals  8  2  16

The body
of the fly is Number of S in the population  2  (number of S homozygotes) 
Drosophila ground up. (number of heterozygotes)  8  2  10
10 5
Frequency of S  16 = 8

Each well of the gel is loaded with a sample Number of F in the population  2  (number of F homozygotes) 
from one individual, and an electric current
is passed through the gel. The proteins in
(number of heterozygotes)  4  2  6
each sample migrate toward the positive 6 3
electrode according to their charge and size. Frequency of F  16 = 8

⫺ Note that the two allele frequencies add to 1.

CONCLUSION We now have a profile of genetic variation at this gene

for these individuals. Population genetics involves comparing data such
as these with data collected from other populations to determine the
forces shaping patterns of genetic variation.

FOLLOW-UP WORK This technique is seldom used these days because

it is easy now to recover much more detailed genetic information about
genetic variation from DNA sequencing.

SOURCE Lewontin, R. C., and J. L. Hubby. 1966. “A Molecular Approach to the Study of Genic
Heterozygosity in Natural Populations. II. Amount of Variation and Degree of Heterozygosity in
⫹ Natural Populations of Drosophila pseudoobscura.” Genetics 54:595–609.

429
430 CHAPTER 21.3 E VO L U T I O N A N D T H E H A R DY – W E I N B E RG E Q U I L I B R I U M

substitutions that changed a protein’s mobility in the gel. Only genotype frequencies to change. Regardless of which mechanisms
with DNA sequencing did researchers finally have an unambiguous are involved, any change in allele frequencies, genotype
means of detecting all genetic variation in a stretch of DNA, frequencies, or both constitutes evolution.
whether in a coding region or not. The variations studied by
modern population geneticists are differences in DNA sequence, The Hardy–Weinberg equilibrium describes situations
such as a T rather than a G at a specified nucleotide position in a in which allele and genotype frequencies do not
particular gene. change.
Calculating allele frequencies, then, simply involves collecting Allele and genotype frequencies change over time only if specific
a population sample and counting the number of occurrences of forces act on the population. This principle was demonstrated
a given mutation. We can look even closer at the example of the independently in 1908 by the English mathematician G. H. Hardy
Drosophila Adh gene from Fig. 21.4 to focus not on the amino acid and the German physician Wilhelm Weinberg, and has become
difference between the Fast and Slow phenotypes, but on the A known as the Hardy–Weinberg equilibrium. In essence, the
or G nucleotide difference corresponding to the two phenotypes. Hardy–Weinberg equilibrium describes the situation in which
If we sequence the Adh gene from 50 individual flies, we will evolution does not occur. In the absence of evolutionary forces
then have 100 gene sequences from these diploid individuals. We (such as natural selection), allele and genotype frequencies do
find 70 sequences have an A and 30 have a G at the position in not change.
question. Therefore, the allele frequency of A is 70/100 5 0.7 and To determine whether or not evolutionary forces are at work,
the allele frequency of G is 0.3. In general, in a sample of n diploid we need to determine whether or not a population is in Hardy–
individuals, the allele frequency is the number of occurrences of Weinberg equilibrium. The Hardy–Weinberg equilibrium specifies
that allele divided by twice the number of individuals. the relationship between allele frequencies and genotype
frequencies when a number of key conditions are met. In these cases,
j Quick Check 2 Data on genetic variation in populations have
we can conclude that evolutionary forces are not acting on the gene
become ever more precise over time, from phenotypes that are
in the population we are studying. In many ways, then, the Hardy–
determined by a single gene to gel electrophoresis that looks at
Weinberg equilibrium is most interesting when we find instances in
variation among genes that encode for enzymes, to analysis of
which allele or genotype frequencies depart from expectations. This
the DNA sequence. Has this increase in precision resulted in the
finding implies that one or more of the conditions are not met and
uncovering of more genetic variation or less?
that evolutionary mechanisms are at work.
A population that is in Hardy–Weinberg equilibrium meets
these conditions:
21.3 EVOLUTION AND THE HARDY–
WEINBERG EQUILIBRIUM 1. There can be no differences in the survival and
reproductive success of individuals. Let’s examine what
Determining allele frequencies gives us information about genetic happens when this condition is not met. Given two alleles, A
variation. Following and measuring change in that variation over and a, consider what occurs when a, a recessive mutation, is
time is key to understanding the genetic basis of evolution. lethal. All aa individuals die. Therefore, in every generation,
there is a selective elimination of a alleles, meaning that the
Evolution is a change in allele or genotype frequency frequency of a will gradually decline (and the frequency of A
over time. correspondingly increase) over the generations. As we discuss
At the genetic level, evolution is simply a change in the frequency below, we call this differential success of alleles selection.
of an allele or a genotype from one generation to the next. For
2. Populations must not be added to or subtracted from by
example, if there are 200 copies of an allele that causes blue eye
migration. Again, let’s see what happens when this condition
color in a population in generation 1 and there are 300 copies
is not met. Consider a second population adjacent to the one
of that allele in a population of the same size in generation 2,
we used in the preceding example in which all the alleles are
evolution has occurred. In principle, evolution may occur without
A and all individuals have the genotype AA. Then there is a
allele frequencies changing. For instance, even if, in our fruit
sudden influx of individuals from the first population into the
fly example, the A/G allele frequencies stay the same from one
second. The frequency of A in the second population changes
generation to the next, the frequencies of the different genotypes
in proportion to the number of immigrants.
(that is, of AA, AG, and GG) may change. This would be evolution
without allele frequency change. 3. There can be no mutation. If A alleles mutate into a alleles
Evolution is therefore a change in the genetic makeup of a (or other alleles, if the gene has multiple alleles), and vice
population over time. Note an important and often misunderstood versa, then again we see changes in the allele frequencies over
aspect of this definition: Populations evolve, not individuals. the generations. In general, because mutation is so rare, it
Note, too, that this definition does not specify a mechanism for has a very small effect on changing allele frequencies on the
this change. As we will see, many mechanisms can cause allele or timescales studied by population geneticists.
 CHAPTER 21 E VO LU T I O N : H O W G E N OT Y P E S A N D P H E N OT Y P E S C H A N G E OV E R T I M E 431

4. The population must be sufficiently large to prevent we know the frequency of the two alleles, one with A and the
sampling errors. Small samples are likely to be more other with G in the Adh gene. What are the genotype frequencies?
misleading than large ones. Campus-wide, a college’s That is, how many AA homozygotes, AG heterozygotes, and GG
homozygotes do we see? The Hardy–Weinberg equilibrium predicts
sex ratio may be close to 50 : 50, but in a small class of
the expected genotype frequencies from allele frequencies.
8 individuals it is not improbable that we would have 6
women and 2 men (a 75 : 25 ratio). Sample size, in the The logic is simple. Random mating is the equivalent of putting
all the population’s gametes into a single pot and drawing out pairs
form of population size, also affects the Hardy–Weinberg
of them at random to form a zygote, which is the same principle
equilibrium such that it technically holds only for infinitely
we saw in action in the discussion of independent assortment in
large populations. A change in the frequency of an allele
Chapter 16. We therefore put in our 70 A alleles and 30 G alleles
due to the random effects of limited population size is
called genetic drift. and pick pairs at random. What is the probability of picking an AA
homozygote (that is, what is the probability of picking an A allele
5. Individuals must mate at random. For the Hardy–
followed by another A allele)? The probability of picking an A allele
Weinberg equilibrium to hold, mate choice must be made
is its frequency in the population, so the probability of picking
without regard to genotype. For example, an AA homozygote
the first A is 0.7. What is the probability of picking the second A?
when offered a choice of mate from among AA, Aa, or aa
Also 0.7. What then is the probability of picking an A followed by
individuals should choose at random. In contrast, non-
another A? It is the product of the two probabilities: 0.7  0.7 5
random mating occurs when individuals do not mate
0.49. Thus, the frequency of an AA genotype is 0.49. We take the
randomly. For example, AA homozygotes might preferentially
same approach to determine the genotype frequency for the GG
mate with other AA homozygotes. Non-random mating
genotype: Its frequency is 0.3  0.3, or 0.09.
affects genotype frequencies from generation to generation,
What about the frequency of the heterozygote, AG? This is
but does not affect allele frequencies.
the probability of drawing G followed by A, or A followed by G.
There are thus two separate ways in which we can generate the
The Hardy–Weinberg equilibrium relates allele heterozygote. Its frequency is therefore (0.7  0.3) 1 (0.3  0.7)
frequencies and genotype frequencies. 5 0.42.
Now that we have established the conditions required for a As seen in the Punnett square on the left in Fig. 21.5, we can
population to be in Hardy–Weinberg equilibrium, let us explore generalize these calculations algebraically by substituting letters for
the idea in detail. In the Drosophila example we looked at earlier, the numbers we have computed to derive the relation defined by
the Hardy–Weinberg equilibrium.
If the allele frequency of one allele,
A, is p, and the other, G, is q, then
p 1 q 5 1 (because there are no other
FIG. 21.5 Hardy–Weinberg relation. The Hardy–Weinberg relation predicts genotype alleles at this site).
frequencies from allele frequencies, and vice versa.
Genotypes AA AG GG
Eggs Frequencies p2 2pq q2
A a Allele
p q Allele frequency Genotype Genotype
frequency frequency In the graph on the right hand side
AA Aa
A p
(p2) (pq) of aa is q2. of AA is p2. of Fig 21.5, we have replaced the p’s
Sperm

The frequency of 1.0 and q’s with numbers. If no a alleles

Aa aa Genotypes
a q the aa genotype 0.9 are present, then q 5 0 and p 5 1 and
(pq) (q2) under Hardy– aa Aa AA
0.8 the frequency of AA is 1. In this case,
Genotype frequency

Weinberg is the Genotype

0.7 all the genotypes in the population
Allele
Allele frequency

probability of frequency
having both an 0.6 are AA and, accordingly, the blue line
of Aa is 2pq.
a sperm 0.5
(probability q) representing the frequency of AA
0.4
and an a egg in the population is at 1, and the red
(also q): q2. 0.3
line representing the frequency of aa
0.2
0.1 and the purple line representing the
If Hardy–Weinberg conditions are met, This simple
we can compute the frequencies of the relationship
frequency of Aa are both at 0. When
p (A) 0 0.2 0.4 0.6 0.8 1.0 no A alleles are present, all individuals
three possible genotypes: allows us to q (a) 1.0 0.8 0.6 0.4 0.2 0
translate have genotype aa, and the red line is at
Genotypes: AA Aa aa between allele Allele frequency
frequencies 1 and the others at 0.
Genotype and genotype Each line on the graph represents Not only does the Hardy–
frequencies: p2 2pq q2 frequencies. one of the three genotypes.
Weinberg relation predict genotype
432 CHAPTER 21.4 N AT U R A L S E L E C T I O N

frequency from allele frequencies, but it works in reverse, too: deleterious and eliminated by natural selection have no long-term
Genotype frequencies predict allele frequencies. evolutionary impact; ones that are beneficial, however, can result
The graph in Fig. 21.5 also shows how allele and genotype in adaptation to the environment over time.
frequencies are related. We can use the graph to determine
allele frequencies for given genotype frequencies and genotype Natural selection brings about adaptations.
frequencies for given allele frequencies. For example, if the The adaptations we see in the natural world—the exquisite fit of
population we are examining has a 0.5 frequency of heterozygotes, organisms to their environment—were typically taken by pre-
Aa, the purple line in the graph indicates that both p and q are 0.5. Darwinian biologists as evidence of a divine Creator’s existence.
If we know that p 5 0.5 (and therefore q 5 0.5), we can look at the Each species, they argued, was so well adapted—the desert plant
lines to infer that the frequency of heterozygotes is 0.5 and the so physiologically adept at coping with minimal levels of rainfall
frequency of both homozygotes, AA and aa, is the same, 0.25. and the fast-swimming fish so hydrodynamically streamlined—
We can do this mathematically as well. Knowing the genotype that it must have been designed by a Creator.
frequency of AA, for example, permits us to calculate allele With the publication of On the Origin of Species in 1859, Darwin,
frequencies: if, as in our Adh example, p2 is 0.49 (that is, 49% of pictured in Fig. 21.6, overturned the biological convention of his
the population has genotype AA), then p, the allele frequency of A, day on two fronts. First, he showed that species are not unchanging;
is 5 0.7. Because p 1 q 5 1, then q, the allele frequency of a, they have evolved over time. Second, he suggested a mechanism,
is 1 2 0.7 5 0.3. natural selection, that brings about adaptation. Natural selection
Note that these relationships hold only if the Hardy–Weinberg was a brilliant solution to the central problem of biology: how
conditions are met. If not, then allele frequencies can be organisms come to fit so well in their environments. From where
determined only from genotype frequencies, as described earlier in does the woodpecker get its powerful chisel of a bill? And the
section 21.2. hummingbird its long delicate bill for probing the nectar stores

The Hardy–Weinberg equilibrium is the starting point

for population genetic analysis.
Recall our definition of evolution: a change in allele or genotype FIG. 21.6 Charles Darwin. This photograph was taken at about the
frequency from one generation to the next. Given this definition, time Darwin was writing On the Origin of Species. Source:
it might seem odd to be discussing factors necessary for allele akg-images.
frequencies to stay the same. The Hardy–Weinberg equilibrium
not only provides a means of converting between allele and
genotype frequencies, but, critically, it also serves as an indicator
that something interesting is happening in a population when it is
not upheld.
If we find a population whose allele or genotype frequencies
are not in Hardy–Weinberg equilibrium, we can infer that
evolution has occurred. With further study of the population,
we can then consider, for the gene in question, whether the
population is subject to selection, migration, mutation, genetic
drift, or non-random mating. These are the primary mechanisms
of evolution. The Hardy–Weinberg equilibrium gives us a baseline
from which to explore the evolutionary processes affecting
populations. We will start by considering one of the most
important evolutionary mechanisms: natural selection.

j Quick Check 3 When we find a population whose allele

frequencies are not in Hardy–Weinberg equilibrium, what can and
can’t we conclude about that population?

21.4 NATURAL SELECTION

Natural selection results in allele frequencies changing from
generation to generation according to the allele’s impact on the
survival and reproduction of individuals. New mutations that are
 CHAPTER 21 E VO LU T I O N : H O W G E N OT Y P E S A N D P H E N OT Y P E S C H A N G E OV E R T I M E 433

natural selection, its underlying mechanism of adaptation, to

FIG. 21.7 Alfred Russel Wallace. This photograph was taken in public attention. Wallace is only fleetingly mentioned in Darwin’s
Singapore during Wallace’s expedition to Southeast Asia. great work, and Darwin, not Wallace, is now the name associated
Source: A. R. Wallace Memorial Fund & G. W. Beccaloni. with the discovery.
Both Darwin and Wallace recognized their debt to the writings
of a British clergyman, Thomas Malthus. In his Essay on the
Principle of Population, first published in 1798, Malthus pointed
out that natural populations have the potential to increase in
size geometrically, meaning that populations get larger at an
ever-increasing rate. Imagine that human couples can have just
four children (two males and two females), so the population
doubles every generation. Starting with a single couple, by the
twentieth generation, the population will have grown to over a
million—1,048,576, to be precise.
However, this geometric expansion of populations does
not occur. In fact, population sizes are typically stable from
generation to generation. This is because the resources upon
which populations are dependent—food, water, places to live—
are limited. In each generation many fail to survive or reproduce;
there simply is not enough food and other resources to go around.
This implies in turn that individuals within a population must
compete for resources.
Which individuals will win the competition? Darwin and
Wallace suggested that those that are best adapted would most
likely survive and leave more offspring. Genetic variation among
individuals results in some that are more likely than others to
survive and reproduce, passing their genetic material to the next
generation. As a result, the next generation will have a higher
proportion of these same advantageous alleles. Darwin used the
term “natural selection” for the filtering process that acts against
deleterious alleles and in favor of advantageous ones.
Competitive advantage is a function of how well an organism
in flowers? Darwin showed how a simple mechanism, without is adapted to its environment. A desert plant that is more efficient
foresight or intentionality, could result in the extraordinary range of at minimizing water loss than another plant is better adapted to
adaptations that all of life is testimony to. the desert environment. An organism that is better adapted to its
For 20 years after first conceiving the essence of his theory, environment is more fit. Fitness, in this context, is a measure of
Darwin collected supporting evidence. In 1858, however, he was the extent to which the individual’s genotype is represented in the
spurred to begin writing On the Origin of Species by a letter from next generation. We say that the first plant’s fitness is higher than
a little-known naturalist collecting specimens in what is today the second’s if it leaves more surviving offspring either because
Indonesia. By a remarkable coincidence, Alfred Russel Wallace, the plant itself survives for longer, giving it greater opportunity to
shown in Fig. 21.7, had also developed the theory of evolution reproduce, or because it has some other reproductive advantage,
by natural selection. Aware that Darwin was interested in the such as the ability to produce more seeds. Assuming that the
problem but having no idea that Darwin was working on the same trait maintains its advantage, natural selection then acts over
theory, Wallace wrote to Darwin in 1858 to see what he thought generations to increase the overall fitness of a population. A plant
of his idea. population newly arrived in a desert may be poorly adapted to
Suddenly, Darwin was confronted with the prospect of losing its environment, but, over time, alleles that minimize water loss
his claim on the theory that he had been quietly nurturing for increase under natural selection, resulting in a population that is
20 years. But all was not lost. Darwin’s colleagues arranged for the better adapted to the desert.
publication of a joint paper by Wallace and Darwin in 1858. This Such changes in populations take time. Borrowing from the
was done without consulting Wallace, who nonetheless never geologists of his day, Darwin recognized that time was a critical
resented Darwin and afterward was careful to insist that the idea ingredient of his theory. Geologists had put forward a view of
rightly belonged to the older man. It was Darwin’s publication of Earth’s history that argued that large geological changes—like
On the Origin of Species in 1859 that brought both evolution and the carving of the Grand Canyon—can be explained by simple
434 CHAPTER 21.4 N AT U R A L S E L E C T I O N

day-to-day processes operating over vast timescales. Darwin Sometimes, however, natural selection is inefficient in getting rid
applied this worldview to biology. He recognized that small of a deleterious allele. Consider a recessive lethal mutation, b (that
changes, like subtle shifts in the frequencies of alleles, could add is, one that is lethal only as a homozygote, bb, and has no effect as
up to major changes given long enough time periods. What might a heterozygote, Bb). When it first arises, all the other alleles in the
seem to us to be a trivial change over the short term can, over the population are B, which means that the first b allele that appears
long term, result in substantial differences among populations. in the population must be paired with a B allele, resulting in a
Bb heterozygote. Because natural selection does not act against
The Modern Synthesis combines Mendelian genetics heterozygotes in this case, the b allele may increase in frequency
and Darwinian evolution. by chance alone (we discuss below how this happens). Only when
Darwinian evolution involves the change over time of the two b alleles come together to form a bb homozygote does natural
genetic composition of populations and is thus a genetic theory. selection act to rid the population of the allele. Natural selection
Although Mendel published his genetic studies of pea plants in that decreases the frequency of a deleterious allele is called
1866, not long after The Origin, Darwin never saw them, so a key negative selection.
component of the theory was missing. The rediscovery of Mendel’s Many human genetic diseases show this pattern: The
work in 1900 unexpectedly provoked a major controversy among deleterious allele is rare and recessive. Because it is rare,
evolutionary biologists. Some argued that Mendel’s discoveries homozygotes for it are formed only infrequently. Remember that
did not apply to most genetic variation because the traits studied the expected frequency of homozygotes in a population under
by Mendel were discrete, meaning that they had clear alternative the Hardy–Weinberg equilibrium is the square of the frequency
states, such as either yellow or green color in peas. Most of the of the allele in the population. Therefore, if the allele frequency
variation we see in natural populations, in contrast, is continuous, is 0.01, we expect 0.01  0.01, or 1 in every 10,000 individuals, to
meaning that variation occurs across a spectrum (Chapter 18). be homozygous for it. Thus, the genetic disease occurs rarely, and
Human height, for example, does not come in discrete classes. the allele remains in the population because it is recessive and not
People are not either 5 feet tall or 6 feet tall and of no height in expressed as a heterozygote.
between. Instead, they may be any height within a certain range.
How could the factors that controlled Mendel’s discrete traits ? CASE 4 MALARIA: CO-EVOLUTION OF HUMANS AND
account for the continuous variation seen in natural populations? A PARASITE
This question was answered by the English theoretician Ronald What genetic differences have made some individuals
Fisher, who realized that, instead of a single gene contributing more and some less susceptible to malaria?
to a trait like human height, there could be several genes that In addition to allowing alleles to be either eliminated or fixed,
contribute to the trait. He argued that extending Mendel’s theory natural selection can also maintain an allele at some intermediate
to include multiple genes per trait could account for patterns of frequency between 0% and 100%. This form of natural selection is
continuous variation that we see all around us. called balancing selection, and it acts to maintain two or more
Fisher’s insight formed the basis of a synthesis between alleles in a population. A simple case is members of a species that
Darwin’s theory of natural selection and Mendelian genetics face different conditions depending upon where they live. One
that was forged during the middle part of the twentieth century. allele might be favored by natural selection in a dry area, but a
The product of this Modern Synthesis is our current theory different one favored in a wet area. Taking the species as a whole,
of evolution. these alleles are maintained by natural selection at intermediate
frequencies.
Natural selection increases the frequency of Another example of balancing selection occurs when
advantageous mutations and decreases the frequency the heterozygote’s fitness is higher than that of either of the
of deleterious mutations. homozygotes, resulting in selection that ensures that both alleles
Natural selection increases the frequency of advantageous alleles, remain in the population at intermediate frequencies. This form
resulting in adaptation. In some cases, it can promote the fixation of balancing selection is called heterozygote advantage, and it
of advantageous alleles, meaning the allele has a frequency of 1. is exemplified by human populations in Africa, where malaria has
To start with, a new advantageous allele will exist as a single been a long-standing disease. Because the malaria parasite spends
copy in a single individual (that is, as a heterozygote), but, under part of its life cycle in human red blood cells, mutations in the
the influence of natural selection, the advantageous allele can hemoglobin molecule that affect the structure of the red blood
eventually replace all the other alleles in the population. Natural cells have a negative impact on the parasite and can reduce the
selection that increases the frequency of a favorable allele is called severity of malarial attacks.
positive selection. Two alleles of the gene for one of the subunits of hemoglobin
As we have seen, most mutations to functional genes are are A and S (Chapter 15). The A allele codes for normal hemoglobin,
deleterious. In extreme cases, they are lethal to the individuals resulting in fully functional, round red blood cells. The S allele
carrying them and are thus eliminated from the population. encodes a polypeptide that differs from the A allele’s product in
 CHAPTER 21 E VO LU T I O N : H O W G E N OT Y P E S A N D P H E N OT Y P E S C H A N G E OV E R T I M E 435

just a single amino acid, which is enough to cause the molecules to

aggregate end to end, so the red blood cell is distorted into a sickle. FIG. 21.9 Directional selection. Directional selection in a
In regions of the world with malaria, heterozygous individuals population of Galápagos finches caused a shift toward
(SA) have an advantage over homozygous individuals (SS and larger bills in a single drought year, 1977. Data from
AA). SS homozygotes are protected against malaria, but they are Freeman & Herron Evolutionary Analysis 2004.

burdened with severe sickling disease. Sickle-shaped red blood

cells can block capillaries, and therefore people with the SS
genotype are prone to debilitating, painful, and sometimes fatal
episodes resulting from capillary blockage. AA homozygotes lack
sickling disease but are vulnerable to malaria. SA heterozygotes, Finches hatched in 1976, Finches hatched in 1978,
however, do not have severe sickling disease and have some the year before the drought the year after the dought
protection from malaria. As a result, natural selection maintains
both the S and A alleles in the population at intermediate 45
Directional
frequencies. 40 selection shifts the
In areas where there is no malaria, this balance is shifted. population mean
35
Many African-Americans, descended from Africans upon whom

Number of finches
30
natural selection operated in favor of the heterozygote, still carry
25
the S allele, even though the allele is no longer useful to them in
20
their malaria-free environment. If natural selection were to run its
15
course among African-Americans, the S allele would gradually be
eliminated. However, this is a slow process, and many more people 10

will continue to suffer from sickle-cell anemia before it is complete. 5

0
7.3 7.8 8.3 8.8 9.3 9.8 10.3 10.8 11.3
Natural selection can be stabilizing, directional, Beak size (depth in mm)
or disruptive.
Up to this point, we have followed the fate of individual
mutations, which can increase, decrease, or be maintained at an organism. For example, we might track the evolution of height in
intermediate frequency under the influence of natural selection. a population, despite not knowing the specifics of the genetic basis
We can also look at the consequence of natural selection from a of height differences. When we look at natural selection from this
different perspective. Instead of following individual mutations, perspective, we see three types of patterns: stabilizing, directional,
we can look at changes over time of a particular trait of an and disruptive.
Stabilizing selection maintains the status quo and acts
against extremes. A good example is provided by human birth
weight, a trait affected by a number of factors, including many
FIG. 21.8 Stabilizing selection. Stabilizing selection on human
fetal genes (Fig. 21.8). If a baby is too small, then its chances of
birth weight results in selection against babies that are
survival after birth are low. However, if it is too big, there may
either too small or too large. Source: Data from L. L. Cavalli-
be complications during delivery that endanger both mother
Sforza and W. F. Bodmer, 1971, The Genetics of Human Populations,
and baby. Thus, the optimum birth weight is between these two
San Francisco: W. H. Freeman, p. 613.
extremes. In this case, natural selection acts against the extremes.
The vast majority of natural selection is of this kind as deleterious
Mortality is high for The result is that most mutations that cause a departure from the optimal phenotype are
very small babies and babies are born at an
for very large babies.
selected against.
intermediate size.
Whereas stabilizing selection keeps a trait the same over time,
20 directional selection leads to a change in a trait over time. A
100
Percent of babies at a given

well-documented case of directional selection is found in Darwin’s

Percent of population with

70
50
birthweight who die

15 30
finches on the Galápagos Islands (Fig. 21.9). A severe drought in
given birthweight

20 1977 killed a significant amount of the vegetation that provided

10 10 food for one island’s population of seed-eating ground finches,
7
5 Geospiza fortis, reducing that population’s numbers from 750 to
5 3 90. Because plant species that produce big seeds fared better in
2
drought conditions than other plant species did, the average size
0 1 2 3 4 5 of seeds available to the remaining birds increased. Birds with
Birth weight (kg) bigger bills were better at handling the big seeds and therefore
436 CHAPTER 21.4 N AT U R A L S E L E C T I O N

had higher survival rates than birds with smaller bills. Because bill A third mode of selection, known as disruptive selection,
size is genetically determined, the drought resulted in directional operates in favor of extremes and against intermediate forms.
selection for increased bill size. Apple maggot flies of North America, Rhagotletis pomonella,
Artificial selection, which has been practiced by humans provide an example (Fig. 21.11). The larvae of these flies feed on
since at least the dawn of agriculture, is a form of directional the fruit of hawthorn trees. However, with the introduction
selection. Artificial selection is analogous to natural selection, of apples from Europe about 150 years ago, these flies have
but the competitive element is removed. Successful genotypes become pests of apples. Apple trees flower and produce fruit
are selected by the breeder, not through competition. Because it earlier every summer than hawthorns, so disruptive selection
can be carefully controlled by the breeder, artificial selection is has resulted in the production of two genetically distinct groups
astonishingly efficient at generating genetic change. Practiced of flies, one specializing on apple trees and the other on hawthorn
over many generations, artificial selection can create a population trees. Disruptive selection acts against intermediates between
in which the selected phenotype is far removed from that of the the two groups, which miss the peaks of both the apple and
starting population. Fig. 21.10 shows the result of long-continued hawthorn seasons. We explore this mechanism, which can lead to
artificial selection for the oil content in kernels of corn. the evolution of new species, in more detail in the next chapter.

HOW DO WE KNOW?

FIG. 21.10 23
22 Selection for high oil content

How far can artificial

21 Selection for low oil content
20
19
selection be taken? 18
17
16 Note that the
BACKGROUND Begun in the 1890s and continuing to this day at 15 final “high” oil
14 level is more
the University of Illinois as an attempt to manipulate the properties
13 than 4 times
Generation

of corn, this experiment has become one of the longest-running higher than
12
biological experiments in history. the starting
11 point.
HYPOTHESIS Researchers hypothesized that there is a limit to the 10
9
extent to which a population can respond to continued directional
8
selection. 7
EXPERIMENT Corn was artificially selected for either high oil 6
5
content or low oil content: Every generation, researchers bred
4
together just the plants that produced corn with the highest oil 3
content, and did the same for the plants that produced corn with 2
the lowest oil content. Every generation, kernels showed a range of 1
oil levels, but only the 12 kernels with the highest or the lowest oil
0 10 20 30 40 50 60 70 80 90 100
content were used for the next generation. Oil content

RESULTS In the line selected for high oil content, the percentage
of oil more than quadrupled, from about 5% to more than 20%. FOLLOW-UP WORK Genetic analysis of the selected lines

In the line selected for low oil content, the oil content fell so indicates that the differences in oil content are due to the effects of
close to zero that it could no longer be measured accurately, and at least 50 genes.
the selection was terminated. Both selected lines are completely
SOURCE Moose, S. P., J. W. Dudley, and T. R. Rocheford. 2004. “Maize Selection Passes
outside the range of any phenotype observed at the beginning of the Century Mark: A Unique Resource for 21st Century Genomics.” Trends in Plant
the experiment. Science 9:358–364.
 CHAPTER 21 E VO LU T I O N : H O W G E N OT Y P E S A N D P H E N OT Y P E S C H A N G E OV E R T I M E 437

FIG. 21.11 Disruptive selection. Disruptive selection has produced two genetically distinct populations of apple maggot fly, each one
coordinated with fruiting times of two different species of tree. Sources: (photo) Rob Oakleaf (National Science Foundation); Data from Filchak et al.
2000 Nature 407:739– 42.

Before the introduction of

Flies feeding on apples apples, all flies’ life cycles

Percent of total larval population

infesting fruit on a given date
100 Flies feeding on hawthorns would have been
coordinated with hawthorn.
Disruptive selection and
75 differences in the timing of
fruiting have created two
peaks in the distribution
50 —one for apple-specializing
flies, one for hawthorn-
specializing flies—from the
25 single original one.

June 15

July 1

July 15

Aug. 1

Aug. 15

Sept. 1

Sept. 15

Oct. 1

Oct’ 15
Date (1991)

Sexual selection increases an individual’s reproductive selection is indeed acting to reduce the showiness and size of the
success. peacock’s tail, but another form of selection, sexual selection, is
Initially, Darwin was puzzled by features of organisms that acting in the opposite direction. Sexual selection promotes traits
seemed to reduce an individual’s chances of survival. In a letter that increase an individual’s access to reproductive opportunities.
a few months after the publication of The Origin, he wrote, “The Darwin recognized that this could occur in two different ways
sight of a feather in a peacock’s tail, whenever I gaze at it, makes (Fig. 21.12). In one form of sexual selection, members of one sex
me sick!” The tail is metabolically expensive to produce; it is an (usually the males) compete with one another for access to the
advertisement to potential predators; and it is an encumbrance in other sex (usually the females). This form is called intrasexual
any attempt to escape a predator. How could such a feature evolve selection since it focuses on interactions between individuals of
under natural selection? one sex. Because competition typically occurs among males, it is
In his 1871 book, The Descent of Man, and Selection in Relation in males that we see physical traits such as large size and horns and
to Sex, Darwin introduced a solution to this problem. Natural other elaborate weaponry, as well as fighting ability. Larger, more

FIG. 21.12 Sexual selection. (a) Intrasexual selection often involves competition between males, as in this battle between two male elk.
(b) Intersexual selection often involves bright colors and displays by males to attract females, as shown by this male and female
Japanese Red-crowned Crane. Source: (a) Kelly Funk/All Canada Photos/Corbis; (b) Steven Kaufman/Getty Images.

a b
438 CHAPTER 21.5 M I G R AT I O N , M U TAT I O N , G E N E T I C D R I F T , A N D N O N - R A N D O M M AT I N G

powerful males tend to win more fights, hold larger territories, and worse than merely non-adaptive—it may be maladaptive, in that
have access to more females. it causes a decrease in a population’s average fitness. Fair-skinned
Darwin also recognized a second form of sexual selection. people arriving in an equatorial region, for example, are at risk of
Here, males (typically) do not fight with one another, but instead sunburn and skin cancer.
compete for the attention of the female with bright colors
or advertisement displays. In this case, females choose their Mutation increases genetic variation.
mates. This form of selection is called intersexual selection As we saw earlier in this chapter, mutation is a rare event. This
since it focuses on interactions between females and males. means that it is generally not important as an evolutionary
The peacock’s tail is thought to be the product of intersexual mechanism that leads allele frequencies to change. However, as we
selection: Its evolution has been driven by a female preference have also seen, it is the source of new alleles and the raw material
for ever-showier tails. In the absence of sexual selection, natural on which the other forces act. Without mutation, there would be
selection would act to minimize the size of the peacock’s tail. no genetic variation and no evolution.
Presumably, the peacocks’ tails we see are a compromise, a
trade-off between the conflicting demands of reproduction and Genetic drift has a large effect in small populations.
survival. Genetic drift is the random change in allele frequencies from
generation to generation. By “random,” we mean that frequencies
j Quick Check 4 Sexual selection tends to cause bigger size, more
can either go up or down simply by chance. An extreme case is a
elaborate weaponry, or brighter colors in males. Is this an example
population bottleneck, which occurs when an originally large
of stabilizing, directional, or disruptive selection?
population falls to just a few individuals.
Consider a rare allele, A, with a frequency of 1/1000. Habitat
destruction then reduces the population to just one pair of
21.5 MIGRATION, MUTATION, individuals, one of which is carrying A. The frequency of A in this
GENETIC DRIFT, AND NON- new population is 1/4 because each individual has two alleles,
RANDOM MATING giving a total of four alleles. In other words, the bottleneck
resulted in a dramatic change in allele frequencies. It also caused
Selection is evolution’s major driving force, enriching each new a loss of genetic variation as much of the variation present in the
generation for the mutations that best fit organisms to their original population was not present in the surviving pair. That
environments. However, as we have seen, it is not the only is, the surviving pair carries only a few of the alleles that were
evolutionary mechanism. There are other mechanisms that present in their original population. A population of Galápagos
can cause allele and genotype frequencies to change. These are tortoises that has very low levels of genetic diversity probably
migration, mutation, genetic drift, and non-random mating. went through just such a bottleneck about 100,000 years ago
Like natural selection, these mechanisms can cause allele when a volcanic eruption eliminated most of the tortoises’ habitat.
frequencies to change. Unlike natural selection, they do not lead Genetic drift also occurs when a few individuals start a new
to adaptations. population, in what is called a founder event. Such events
occur, for example, when a small number of individuals arrive
Migration reduces genetic variation between on an island and colonize it. Once again, relative to the parent
populations. population, allele frequencies are changed and genetic variation
Migration is the movement of individuals from one population is lost.
to another, resulting in gene flow, the movement of alleles Earlier, we considered the fate of beneficial and harmful
from one population to another. It is relatively simple to see mutations under the influence of natural selection. What about
how movements of individuals and alleles can lead to changes neutral mutations? Natural selection, by definition, does not
in allele frequencies. Consider two isolated island populations govern the fate of neutral mutations. Consider a neutral mutation,
of rabbits, one white and the other black. Now imagine that the m, which occurs in a noncoding region of DNA and therefore has
isolation breaks down—a bridge is built between the islands— no effect on fitness. At first, it is in just a single heterozygous
and migration occurs. Over time, black alleles enter the white individual. What happens if that individual fails to reproduce
population and vice versa, and the allele frequencies of the two (for reasons unrelated to m)? In this case, m is lost from the
populations gradually become the same. population, but not by natural selection (which does not select
The consequence of migration is therefore the homogenizing against m). Alternatively, the m-bearing individual might by
of populations, making them more similar to each other and chance leave many offspring (again for reasons unrelated to m), in
reducing genetic differences between them. Because populations which case the frequency of m increases. In principle, it is possible
are often adapted to their particular local conditions (think of over a long period of time for m to become fixed in the population.
dark-skinned humans in regions of high sunlight versus fair- At the end of the process, every member of the population is
skinned humans in regions of low sunlight), migration may be homozygous mm.
 CHAPTER 21 E VO LU T I O N : H O W G E N OT Y P E S A N D P H E N OT Y P E S C H A N G E OV E R T I M E 439

Like natural selection, genetic drift leads to allele frequency

changes and therefore to evolution. Unlike natural selection, FIG. 21.13 Genetic drift. The fate of neutral mutations is governed
however, it does not lead to adaptations since the alleles whose by genetic drift, the effect of which is more extreme in
frequencies are changing as a result of drift do not affect an small populations than in large populations.
individual’s ability to survive or reproduce. a. Population size = 4
The impact of genetic drift depends on population size 1

Allele frequency
(Fig. 21.13). If m arises in a very small population, its frequency 0.8 A computer can model how the
will change rapidly, as shown in Figs. 21.13a and 21.13b. Imagine m frequency of a given allele might
0.6 randomly change over time
arising in a population of just six individuals (or three pairs).
because of genetic drift. In this
Its initial frequency is 1 in 12, or about 8% (there are a total of 0.4
simulation, the allele drifts from
12 alleles because each individual is diploid). If, by chance, one pair 0.2 a frequency of 0.5 to fixation.
fails to breed and the other two (including the one who is an Mm 0
heterozygote) each produce three offspring, and all three of the 20 40 60 80 100
Mm individual’s offspring happen to inherit the m allele, then the
frequency of m will increase to 3 in 12 (25%) in a single generation. b. Population size = 4
In effect, genetic drift is equivalent to a sampling error. In a small 1

Allele frequency
sample, extreme departures from the expected outcome are 0.8 Each line represents a different
common. simulation of the effect of drift
0.6 on a population of 4 individuals.
On the other hand, if the population is large, as in Figs. 21.13c
All simulations result in fixation
and 21.13d, then changes in allele frequency from generation 0.4
or extinction of the allele in
to generation are much smaller, typically less than 1%. A large 0.2 relatively few generations.
population is analogous to a large sample size, in which we tend 0
not to see large departures from expectation. Toss a coin 1000 20 40 60 80 100
times, and you will end up with approximately 500 heads. Toss a
coin 5 times, and you might well end up with zero heads. In other c. Population size = 40
words, in a small sample of coin tosses, we are much more likely to 1
Allele frequency

see departures from our 50 : 50 expectation than in a large sample. 0.8

The same is true of genetic drift. It is likely to be much more
0.6 Drift is less dramatic in
significant in small populations than in large ones. large populations.
0.4
j Quick Check 5 Why, of all the evolutionary mechanisms, is
0.2
selection the only one that can result in adaptation?
0
20 40 60 80 100

Non-random mating alters genotype frequencies

without affecting allele frequencies. d. Population size = 400
As we saw earlier, another way that a population can evolve is 1
Allele frequency

through non-random mating. In random mating, individuals 0.8

After 100 generations of a
select mates without regard for genotype. In non-random mating, large population, none of
0.6
by contrast, individuals preferentially choose mates according to the simulations result in
0.4 extinction or fixation of
their genotypes. The result is that certain phenotypes increase
the allele.
and others decrease. Because non-random mating just rearranges 0.2
alleles already in the gene pool and, unlike migration or mutation, 0
20 40 60 80 100
does not add new alleles to the population, the genotype
Generations
frequencies change, whereas the allele frequencies do not.
Probably the most evolutionarily significant form of non-
random mating is inbreeding, in which mating occurs between
close relatives. Inbreeding increases the frequency of homozygotes there is inbreeding. A father with one b allele has a son
and decreases the number of heterozygotes in a population and daughter who mate and have offspring. We know that
without affecting allele frequencies. the probability that the brother inherited the allele from his
Consider a rare allele b that is at frequency 0.001 in a father is 0.5, and the same is true for his sister. The probability
population. According to the Hardy–Weinberg equilibrium, the that the siblings’ child is homozygous bb is the probability each
expected frequency of bb homozygotes is 0.0012 5 0.000001. Now sibling inherited a copy from their father (0.5  0.5) multiplied
let’s see what happens to the frequency of bb homozygotes when by the probability that their child inherited b from both of
440 CHAPTER 21.6 M O L E C U L A R E VO LU T I O N

them (0.5  0.5), or (0.5)4 5 0.0625. Needless to say, 0.0625 is a Mutations will occur in one population that will not have arisen in
considerably higher probability than 0.000001. the other population, and vice versa.
If b is a deleterious recessive mutation, it may contribute to A mutation in either population has one of three fates: It
inbreeding depression in the child, a reduction in the child’s goes to fixation (either through genetic drift or through positive
fitness caused by homozygosity of deleterious recessive mutations. selection); it is maintained at intermediate frequencies (by
Inbreeding depression is a major problem in conservation biology, balancing selection); or it is eliminated (either through natural
especially when endangered species are bred in captivity in selection or genetic drift). Different mutations will be fixed in
programs starting with a just a small number of individuals. each population. When we come back thousands of generations
later and sequence the DNA of our original identical individuals’
descendants, we will find that many differences have accumulated.
21.6 MOLECULAR EVOLUTION The populations have diverged genetically. What we are seeing is
evidence of molecular evolution.
How do DNA sequence differences arise among species? Imagine Species are the biological equivalents of islands because
starting with two pairs of identical twins, one pair male and the they, too, are isolated. They are genetically isolated because, by
other female. Now we place one member of each pair together definition, members of one species cannot exchange genetic
on either side of a mountain range (Fig. 21.14). Let’s assume material with members of another (Chapter 22). The amount of
the mountain range completely isolates each couple. What, in time that two species have been isolated from each other is the
genetic terms, will happen over time? The original pairs will time since their most recent common ancestor. Thus, humans and
found populations on each side of the mountain range. The chimpanzees, whose most recent common ancestor lived about
genetic starting point, in each case, is exactly the same, but, over 6–7 million years ago, have been isolated from each other for about
time, differences will accumulate between the two populations. 6–7 million years. Mutations arose and were fixed in the human
lineage over that period; mutations, usually different ones, also
arose and were fixed in the chimpanzee lineage over the same
period. The result is the genetic difference between humans and
FIG. 21.14 Genetic divergence in isolated populations. chimpanzees.

Past The molecular clock relates the amount of sequence

difference between species and the time since the
species diverged.
Parental population The extent of genetic difference, or genetic divergence, between
splits, producing two
two species is a function of the time they have been genetically
“daughter” populations
that are geographically isolated from each other. The longer they have been apart, the
isolated from each other. greater the opportunity for mutation and fixation to occur in
A B
each population. This correlation between the time two species
have been evolutionarily separated and the amount of genetic
divergence between them is known as the molecular clock.
Time

For a clock to function properly, it not only needs to keep time,

but it also needs to be set. We set the clock using dates from the
Different mutations fossil record. For example, in a 1967 study, Vince Sarich and Allan
accumulate
Wilson determined from fossils that the lineages that gave rise
separately in each
population. to the Old and New World monkeys separated about 30 million
years ago. Finding that the amount of genetic divergence between
humans and chimpanzees was about one-fifth of that between
Old and New World monkeys, they concluded that humans and
A B chimpanzees had been separated one-fifth as long, or about 6
million years. Although this is the generally accepted number
today, Sarich and Wilson’s result was revolutionary at the time,
when it was thought that the two species had been separated for as
Present
long as 25 million years.
Genetic difference

The extent of genetic The rate of the molecular clock varies.

divergence is a function Molecular clocks can be useful for dating evolutionary events like
of time since isolation.
the separation of humans and chimpanzees. However, because
 CHAPTER 21 E VO LU T I O N : H O W G E N OT Y P E S A N D P H E N OT Y P E S C H A N G E OV E R T I M E 441

FIG. 21.15 The molecular clock. Different genes evolve at different rates because of differences in the intensity of negative selection.
After Fig. 20-3, p. 733, in A. J. F. Griffiths, S. R. Wessler, S. B. Carroll, and J. Doebley, 2012, Introduction to Genetic Analysis, 10th ed., New York: W. H. Freeman.

Divergence of different groups

Vertebrates
Mammals
Mammals

Lamprey
Reptiles
Reptiles
Reptiles

Insects
Birds

Carp
Separation of

Fish
ancestors of plants
and animals

200

180
Number of amino acid substitutions per 100 residues

160
s
eptide

in
140 lob
og
p

m
He
Fibrino

120

100

80

60
ec
hrom
Cytoc
40

20
Histone gene
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400
Millions of years since divergence
Since vertebrates and insects diverged 600 million years ago,
the amino acid sequences of cytochrome c have acquired 30
differences. Amino acid sequences of hemoglobin have acquired
many more differences in the same time period.

the rates of molecular clocks vary from gene to gene, clock data histone mutation over 2 billion years of evolution. The histone
should be interpreted cautiously. These rate differences can be molecular clock is breathtakingly slow.
attributed largely to differences in intensity of negative selection Other proteins are less subject to such rigorous negative
(which results in the elimination of harmful mutations) among selection. Occasional mutations may therefore become
different genes. The slowest molecular clock on record belongs fixed, either through drift (if they are neutral) or selection (if
to the histone genes, which encode the proteins around which beneficial). The extreme case of a fast molecular clock is that
DNA is wrapped to form chromatin (Chapters 3 and 13). These derived from a pseudogene, a gene that is no longer functional.
proteins are exceptionally similar in all organisms; only 2 amino Because all mutations in a pseudogene are by definition
acids (in a chain of about 100) distinguish plant and animal neutral—there is no function for a mutation to disrupt, so a
histones. Plants and animals last shared a common ancestor more mutation is neither deleterious nor beneficial—we expect to
than 1 billion years ago, which means, because each evolutionary see a pseudogene’s molecular clock tick at a very fast rate. In
lineage is separate, that there have been at least 2 billion years of the histone genes, virtually all mutations are selected against,
evolution since they were in genetic contact. And yet the histones constraining the rate of evolution; in pseudogenes, none is.
have hardly changed at all. Almost any amino acid change fatally Fig. 21.15 shows the varying rates of the molecular clock for
disrupts the histone protein, preventing it from carrying out its different genes.
proper function. Negative selection has thus been extremely In the next chapter, we examine how genetic divergence
effective in eliminating just about every amino acid–changing between populations can lead to the evolution of new species. •
442 CO R E CO N C E P T S S U M M A RY

Core Concepts Summary Independently conceived by Charles Darwin and Alfred

Russel Wallace, natural selection is the differential
reproductive success of genetic variants. page 432
21.1 Genetic variation refers to differences in DNA
sequences. Under natural selection, a harmful allele decreases in
frequency, and a beneficial one increases in frequency. Natural
Visible differences among members of a species (phenotypic
selection does not affect the frequency of neutral mutations.
variation) are the result of differences at the DNA level
page 434
(genetic variation) as well as the influence of the environment.
page 426 Natural selection can maintain alleles at intermediate
frequencies by balancing selection. page 434
Mutation and recombination are the two sources of genetic
variation, but all genetic variation ultimately comes from Changes in phenotype show that natural selection can be
mutation. page 427 stabilizing, directional, or disruptive. page 435
Mutations can be somatic (in body tissues) or germ line (in In artificial selection, a form of directional selection, a
gametes), but germ-line mutations are the only ones that can breeder governs the selection process. page 436
be passed on to the next generation. page 427
Sexual selection involves the evolution of traits that increase
When a mutation occurs in a gene, it creates a new allele. an individual’s access to members of the opposite sex.
Mutations can be deleterious, neutral, or advantageous. page 436
page 427
In intrasexual selection, individuals of the same sex compete
with one another, resulting in traits like large size and
21.2 Patterns of genetic variation can be described by
horns. page 437
allele frequencies.
In intersexual selection, interactions between females
An allele frequency is the number of occurrences of a particular and males result in traits like elaborate plumage in male
allele divided by the total number of occurrences of all alleles birds. page 438
of that gene in a population. page 427
In the past, population geneticists relied on observable traits 21.5 Migration, mutation, genetic drift, and non-
determined by a single gene and protein gel electrophoresis random mating are non-adaptive mechanisms of
to measure genetic variation. page 428 evolution.
DNA sequencing is now the standard technique for measuring Migration involves the movement of alleles between
genetic variation. page 428 populations (gene flow) and tends to have a homogenizing
effect. page 438
21.3 Evolution is a change in the frequency of alleles or Mutation is the ultimate source of variation, but it also can
genotypes over time. change allele frequencies on its own. page 438
The Hardy–Weinberg equilibrium describes situations in which Genetic drift is a kind of sampling error, which acts more
allele frequencies do not change. By seeing if a population is in strongly in small populations than in large ones. page 438
Hardy–Weinberg equilibrium, we can determine whether or
Non-random mating, such as inbreeding, results in an increase
not evolution is occurring in a population. page 430
in homozygotes and a decrease in heterozygotes, but does not
The Hardy–Weinberg equilibrium makes five assumptions. change allele frequencies. page 439
These assumptions are that the population experiences no
selection, no migration, no mutation, no sampling error due 21.6 Molecular evolution is a change in DNA or amino
to small population size, and random mating. page 430 acid sequences over time.
The Hardy–Weinberg equilibrium allows allele frequencies The extent of sequence difference between two species is a
and genotype frequencies to be calculated from each function of the time they have been genetically isolated from
other. page 431 each other. page 440

21.4 Natural selection leads to adaptation, which Correlation between sequence differences among species and
enhances the fit between an organism and its time since common ancestry of those species is known as the
environment. molecular clock. page 440
 CHAPTER 21 E VO LU T I O N : H O W G E N OT Y P E S A N D P H E N OT Y P E S C H A N G E OV E R T I M E 443

The rate of the molecular clock varies among genes because 6. Describe what happens to allele and genotype
some genes are more selectively constrained than others. frequencies under the Hardy–Weinberg equilibrium.
page 441
7. Name the five assumptions of the Hardy–Weinberg
equilibrium and, for each one, explain what happens in a

Self-Assessment population in which that condition is not met.

8. How would you calculate genotype frequencies of

1. Why does the relation between phenotype and genotype
a population in Hardy–Weinberg equilibrium, given the
matter in evolution?
allele frequencies of that trait?
2. What is a neutral mutation and what evolutionary
9. Define natural selection and explain how it is
mechanism causes it to change in frequency over time?
different from other mechanisms of evolution.
3. Define genetic variation and explain how it can be
10. Explain how a molecular clock can be used to determine
measured.
the time of divergence of two species.
4. How would you calculate the allele frequencies for
a two-allele trait in a population if given the genotype
Log in to to check your answers to the Self-
frequencies?
Assessment questions, and to access additional learning tools.
5. Can evolution occur without allele frequency changes?
If not, why not? If so, how?
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 CHAPTER 22

Species and
Speciation
Core Concepts
22.1 Reproductive isolation is
the key to the biological
species concept.
22.2 Reproductive isolation
is caused by barriers to
reproduction before or after
egg fertilization.
22.3 Speciation underlies the
diversity of life on Earth.
22.4 Speciation can occur with or
without natural selection.

Rangzen/Shutterstock.

445
446 SECTION 22.1 T H E B I O LO G I C A L S P E C I E S CO N C E P T

Imagine for a moment a world without speciation, the process

that produces new and distinct forms of life. Life would have FIG. 22.1 Species clusters on the basis of two characteristics.
originated and natural selection would have done its job of Source: Museum of Comparative Zoology, Harvard University.

winnowing advantageous mutations from disadvantageous ones,

but the planet would be inhabited by a single kind of generally
adapted organism. Instead of the staggering biological diversity we
see around us—by current estimates, between 10 and 100 million
species call Earth home—there would be just a single life-form.
From a biological perspective, the planet would be a decidedly
dull place. Evolution is as much about speciation, the engine that

Antenna length
generates this breathtaking biodiversity, as it is about adaptation,
the result of natural selection. If we represent
individuals as dots
and score them for
two characteristics,
22.1 THE BIOLOGICAL we see that
individuals cluster
SPECIES CONCEPT in discrete clouds,
representing
The definition of species has been a long-standing problem in species.
Wing length
biology. Many biologists respond to the problem in the same way
that Darwin himself did. In On the Origin of Species, Darwin wrote,
“No one definition has as yet satisfied all naturalists; yet every
naturalist knows vaguely what he means when he speaks of a members of the same species is not a matter of judgment, but
species.” The difficulty of defining species has come to be called rather a reflection of their ability (or inability) to exchange
the species problem. genetic material by producing fertile offspring. Consider a new
Here is the problem in a nutshell: The species, as an advantageous mutation that appears initially in a single individual.
evolutionary unit, must by definition be fluid and capable of That individual and its offspring inheriting the mutation will have
changing, giving rise through evolution to new species. The whole a competitive advantage over other members of the population,
point of the Darwinian revolution is that species are not fixed. How, and the mutation will increase in frequency until it reaches
then, can we define something that changes over time, and, by the 100%. Migration among populations causes the mutation to
process of speciation, even gives rise to two species from one? spread further until all individuals within the species have it. The
mutation spreads within the species but, with some exceptions,
Species are reproductively isolated from other species. cannot spread beyond it.
We can plainly see biodiversity, but are what we call “species” real Therefore, a species represents a closed gene pool, with alleles
biological entities? Or was the term coined by biologists to simplify being shared among members of that species but usually not with
their description of the natural world? To test whether or not members of others. As a result, it is species that become extinct
species are real, we can examine the natural world, measure some and it is species that, through genetic divergence, give rise to new
characteristic of the different living organisms we see, and then species.
plot these measurements on a graph. Fig. 22.1 shows such a plot, The definition of “species” continues to be debated to this
graphing antenna length and wing length of three different types day. The most widely used and generally accepted definition of a
of butterfly. Note that the dots, representing individual organisms, species is known as the biological species concept (BSC). The
fall into non-overlapping clusters. Today, we can add a molecular BSC was described by the great evolutionary biologist Ernst Mayr
dimension to this kind of analysis. When we compare genomes (1904–2005) as follows:
of multiple organisms, they, too, cluster on the basis of similarity.
Each cluster is a species, and the fact that the clusters are distinct Species are groups of actually or potentially interbreeding
implies that species are biologically real. populations that are reproductively isolated from other such
The distances we see between the dots within a cluster groups.
reflect variation from one individual to the next within a species
(Chapter 21). Humans are highly variable, but overall we are more Let us look at this definition closely. At its heart is the idea
similar to one another than to our most humanlike relative, the of reproductive compatibility. Members of the same species are
chimpanzee. We form a messy cluster, but that cluster does not capable of producing offspring together, whereas members of
overlap with the chimpanzee cluster. different species are incapable of producing offspring together. In
Species, then, are real biological entities, not just a convenient other words, members of different species are reproductively
way to group organisms. Whether or not two individuals are isolated from one another.
 CHAPTER 22 S P E C I E S A N D S P E C I AT I O N 447

the morphospecies concept. Stated simply, the morphospecies

FIG. 22.2 Elephas maximus in Sri Lanka and in India. Sources: (top) concept holds that members of the same species usually look alike.
Louise Morgan/Getty Images; (bottom) Mike Smith/age footstock. For example, the shape, size, and coloration of the bald eagle make
it easy to determine whether or not a bird observed in the wild is a
Indian elephants member of the bald eagle species.
Sri Lankan elephants Today, the morphospecies concept has been extended to
the molecular level: Members of the same species usually
China have similar DNA sequences that are distinct from those of
Pakistan Nepal other species. A remarkable project called the Barcode of Life
has established a database linking DNA sequences to species.
Scientists can therefore identify species from DNA alone by
sequencing the relevant segment of DNA and finding the
India
matching sequence (and therefore the species) in the Barcode of
Bangladesh Life database. Here, at both morphological and molecular levels,
we are returning to those biological clusters of Fig. 22.1 in which
members of a species fall close to one another within a single,
discrete group.
Sri Lanka
Although the morphospecies concept is useful and generally
applicable, it is not infallible. Members of a species may not
always look alike, but instead show different phenotypes called
polymorphisms (Chapter 15)—for example, color difference in
As Mayr and many others realized, however, reproductive some species of birds. Sometimes, males of a species may look
compatibility entails more than just the ability to produce different from females—think of the showy peacock, which
offspring. The offspring must be fertile and therefore capable differs dramatically from the relatively drab peahen. Young can be
of passing their genes on to their own offspring. For example, very different from old: Caterpillars that mature into butterflies
although a horse and a donkey, two different species, can mate are a striking example.
to produce a mule, the mule is infertile. If a hybrid offspring is So members of the same species can look quite different from
infertile, it is a genetic dead end. one another. It is also true that members of different species can
Note, too, the “actually or potentially” part of Mayr’s look quite similar. Some species of butterfly, for example, appear
definition. An Asian elephant living on the island of Sri Lanka and similar but have chromosomal differences that can be observed
one living in nearby India are considered members of the same only with the aid of a microscope (Fig. 22.3). These chromosomal
species, Elephas maximus, even though Sri Lankan and Indian differences prevent the formation of interspecies hybrids, so those
Asian elephants never have a chance to mate with each other in butterflies meet the BSC criterion—they are different species—
nature because they are geographically separated (Fig. 22.2). but they do not meet the morphospecies criterion because they
look so much alike.
The BSC is more useful in theory than in practice. With the application of ever more powerful DNA-based
Species, then, are real biological entities and the BSC is the most approaches to studies of natural populations, scientists frequently
useful way to define them. However, the BSC has shortcomings, uncover what are called cryptic species. These are composed of
the most important of which is that it can be difficult to apply. organisms that had been traditionally considered as belonging to
Imagine you are on a field expedition in a rain forest and you one species because they look similar, but turn out to belong to
find two insects that look reasonably alike. Are they members of two species because of a distinction at the DNA sequence level.
the same species or not? To use the BSC, you would need to test
whether or not they are capable of producing fertile offspring. The BSC does not apply to asexual or extinct
However, in practice, you probably will not have the time and organisms.
resources to perform such a test. And even if you do, it is possible In addition to the difficulties of putting the BSC into practice, a
that members of the same species will not reproduce when you second problem is that it overlooks some organisms. For example,
place them together in a laboratory setting or in your field camp. because it is based on the sexual exchange of genetic information,
The conditions may be too unnatural for the insects to behave in the BSC cannot apply to species that reproduce asexually, such
a normal way. And even if they do mate, you may not have time to as bacteria. Although some asexual species do occasionally
determine whether or not the offspring are fertile. exchange genetic information in a process known as conjugation
Thus, we should consider the BSC as a valid framework for (Chapter 26), true asexual organisms do not fit the BSC.
thinking about species, but one that is difficult to test. On a day- Furthermore, because it depends on reproduction, the BSC
to-day basis, therefore, biologists often use a rule of thumb called obviously cannot be applied to species that have become extinct
448 SECTION 22.1 T H E B I O LO G I C A L S P E C I E S CO N C E P T

populations comes all the way round to the more eastern of the
FIG. 22.3 Three species of Agrodiaetus butterflies. These similar- two Russian populations. Thus, though members of the two
appearing butterflies are identifiable only by differences Russian populations cannot exchange genes directly, they can
in their chromosome numbers. Source: Vladimir Lukhtanov, do so indirectly, with the genetic material passing through many
Zoological Institute of Russian Academy of Sciences, St. Petersburg, Russia. intermediate populations. This situation is more complicated
than anything predicted by the BSC. The two Russian populations
A direct view through are reproductively isolated from each other but they are not
the microscope of
genetically isolated from each other because of gene flow around
chromosomes that
have been stained so the ring.
they can be counted. We also see a complicated situation in some groups of
closely related species of plants. Despite apparently being good
morphospecies that can be distinguished by appearance alone,
many different species of willow (Salix), oak (Quercus), and
dandelion (Taraxacum) are still capable of exchanging genes
with other species in their genera through hybridization, or
interbreeding, between species. By the BSC, these different forms
should be considered one large species because they are able to
reproduce and produce fertile offspring. However, because they
maintain their distinct appearances, natural selection must work
against the hybrid offspring.
This unusual phenomenon seems to occur mainly in plants,
but, with the application of powerful comparative genomic
approaches, we are discovering that the boundaries between
closely related animal species are also not as strictly drawn as
traditionally supposed. As we will see in Chapter 24, for example,
we now know that our own species, Homo sapiens, interbred in the
past with another species, Homo neanderthalensis.

Ecology and evolution can extend the BSC.

Because of these problems, much work has been put into
modifying and improving the BSC and many alternative
definitions of species have been suggested. In general, these
efforts highlight how difficult it is to make all species fit easily into
one definition. The natural world truly defies neat categorization!
and are known only through the fossil record, like different Several useful ideas, however, have come out of this literature.
“species” of trilobites and dinosaurs. One of these is the notion that a species can sometimes be
characterized by its ecological niche, which, as we will discuss
Ring species and hybridization complicate the BSC. further in Chapter 47, is a complete description of the role the
An unusual but interesting geographic pattern shown by ring species plays in its environment—its habitat requirements, its
species, species with populations that are reproductively but nutritional and water needs, and the like. It turns out that it is
not genetically isolated, highlights another shortcoming of the impossible for two species to coexist in the same location if their
BSC. Here, we find that some populations within a species are niches are too similar because competition between them for
reproductively isolated from each other, but others are not. resources will inevitably lead to the extinction of one of them.
For example, populations of the greenish warbler, Phylloscopus This observation has given rise to the ecological species concept
trochiloides, are distributed in a large geographic loop around (ESC), the idea that there is a one-to-one correspondence between
the Himalayas (Fig. 22.4). In Russia to the north, members a species and its niche. Thus, we can determine whether or not
of two neighboring populations do not interbreed. Therefore, asexual bacterial lineages are distinct species on the basis of
according to the BSC, they are different species. However, the differences or similarities in their ecological requirements. If two
more western of the two Russian populations is capable of lineages have very different nutritional needs, for example, we can
reproducing with the population to the south of it, and that infer on ecological grounds that they are separate species.
population is capable of reproducing with the population to the Another species concept is the phylogenetic species
east of it, and so on. Eventually, the loop of genetically exchanging concept (PSC), which is the idea that members of a species all
 CHAPTER 22 S P E C I E S A N D S P E C I AT I O N 449

FIG. 22.4 Ring species. Ring species

contain populations that
The red and blue populations of the warbler Phylloscopus are reproductively isolated
trochiloides cannot interbreed. However, gene flow can
still occur between these populations through a ring of from each other but can still
populations that loop south around the Himalayas. exchange genetic material
through other linking
populations. Source: Courtesy
of Darren E. Irwin.
No interbreeding

Russia

BSC remains our most useful definition

of species, the ESC and PSC broaden and
generalize the concept. For example,
in the case of a normally asexual
Mongolia species that is difficult to study because
conjugation is so infrequent, we can
jointly apply the ESC and PSC. We can
China use the ESC to loosely define the species
Tibetan in terms of its ecological characteristics
Plateau
Gene flow can occur Different colors (for example, its nutritional
around the ring. represent distinct requirements), and we can refine that
populations of
the species. definition by using genetic analyses to
determine whether the group is indeed
India
a species by the PSC’s standard (that is,
that all its members derive from a single
common ancestor).
Despite the shortcomings of the
BSC and the usefulness of alternative
ideas, we stress that the BSC is the
most constructive way to think about
species. In particular, by focusing on
reproductive isolation—the inability of
share a common ancestry and a common fate. It is, after all, species two different species to produce viable, fertile offspring—the BSC
rather than individuals that become extinct. The PSC requires gives us a means of studying and understanding speciation, the
that all members of a species are descended from a single common process by which two populations, originally members of the same
ancestor. It does not specify, however, on what scale this idea species, become distinct.
should be applied. All mammals derive from a single common
j Quick Check 1 Why haven’t we been able to come up with a
ancestor that lived about 200 million years ago, but there are
single, comprehensive, and agreed-upon species concept?
thousands of what we recognize as species of mammals that have
evolved since that long-ago common ancestor. But, under a strict
application of the PSC, would we consider all mammals to be a
single species? Similarly, siblings and cousins are all descended 22.2 REPRODUCTIVE ISOLATION
from a common ancestor, a grandmother, but that is surely not
sufficient grounds to classify more-distant relatives as a distinct Factors that cause reproductive isolation are generally divided into
species. The PSC can be useful when thinking about asexual two categories, depending on when they act. Pre-zygotic isolating
species, but, given the arbitrariness of the decisions involved in factors act before the fertilization of an egg, and post-zygotic
assessing whether or not the descendants of a single ancestor factors come into play after fertilization. In other words, pre-
warrant the term “species,” its utility is limited. zygotic factors prevent fertilization from taking place, whereas
It is worth bearing these ecological and phylogenetic post-zygotic factors result in the failure of the fertilized egg to
considerations in mind when thinking about species. Although the develop into a fertile individual.
450 SECTION 22.3 S P E C I AT I O N

Pre-zygotic isolating factors occur before egg they fit only with the genitalia of females of the same species.
fertilization. Attempts by males of D. melanogaster to copulate with females of
Most species are reproductively isolated by pre-zygotic isolating another species of fruit fly, D. virilis, are prevented by mechanical
factors, which can take many forms. Among animals, species are incompatibility.
often behaviorally isolated, meaning that individuals mate only Both plants and animals may also be pre-zygotically isolated
with other individuals based on specific courtship rituals, songs, in time (temporal isolation). For example, closely related plant
or other behaviors. Chimpanzees may be our closest relative— species may flower at different times of the year, so there is no
and therefore the species we are most likely to confuse with our chance that the pollen of one will come into contact with the
own—but a chimpanzee of the appropriate sex, however attractive flowers of the other. Similarly, members of a nocturnal animal
to a chimpanzee of the opposite sex, fails to provoke even the species simply will not encounter members of a closely related
faintest reproductive impulse in a human. In this case, the pre- species that are active only during the day.
zygotic reproductive isolation of humans and chimpanzees is Plants and animals can also be isolated in space (geographic
behavioral. or ecological isolation). This type of isolation can be subtle. For
Behavior does not play a role in plants, but pre-zygotic factors example, the two Japanese species of ladybug beetle shown in
can still be important in their reproductive isolation. Pre-zygotic Fig. 22.5 can be found living side by side in the same field, but they
isolation in plants can take the form of incompatibility between feed on different plants. Because their life cycles are so intimately
the incoming pollen and the receiving flower, so fertilization fails associated with their host plants (adults even mate on their host
to take place. We see similar forms of isolation between members plants), these two species never breed with each other. This
of marine species, such as abalone, which simply discharge their ecological separation is what leads to their pre-zygotic isolation.
gametes into the water. In these cases, membrane-associated
proteins on the surface of sperm interact specifically with Post-zygotic isolating factors occur after egg
membrane-associated proteins on the surface of eggs of the same fertilization.
species but not with those of different species. These specific Post-zygotic isolating factors involve mechanisms that come into
interactions ensure that a sperm from one abalone species, Haliotis play after fertilization of the egg. Typically, they involve some kind
rufescens, fertilizes only an egg of its own species and not an egg of genetic incompatibility. One example, which we saw earlier
from H. corrugata, a closely related species. Incompatibilities in Fig. 22.3 and will explore later in the chapter, is the case of two
between the gametes of two different species is called gametic organisms with different numbers of chromosomes.
isolation. In some instances, the effect can be extreme. For example,
In some animals, especially insects, incompatibility arises the zygote may fail to develop after fertilization because the two
earlier in the reproductive process. The genitalia of males of the parental genomes are sufficiently different to prevent normal
fruit fly Drosophila melanogaster are configured in such a way that development. In others, the effect is less obvious. Some matings
between different species produce
perfectly viable adults, as in the case of
the horse–donkey hybrid, the mule. As we
FIG. 22.5 Ecological isolation. The ladybugs (a) Henosepilachna yasutomii and (b) H. niponica
have seen, though, all is not well with the
are reproductively isolated from each other because they feed and mate on different
mule from an evolutionary perspective.
host plants. Source: Courtesy Dr. Haruo Katakura.
The horse and donkey genomes are
a. b. different enough to cause the mule to
be infertile. As a general rule, the more
closely related—and therefore genetically
similar (Chapter 21)—a pair of species, the
less extreme the genetic incompatibility
between their genomes.

22.3 SPECIATION
Recognizing that species are groups
of individuals that are reproductively
isolated from other such groups, we
are now in a position to recast the key
question: How does speciation occur?
 CHAPTER 22 S P E C I E S A N D S P E C I AT I O N 451

As Fig. 22.6 shows, speciation is typically a

FIG. 22.6 Speciation. Speciation occurs when two populations that are genetically gradual process. If we try to cross members of
diverging become reproductively isolated from each other. two populations that have genetically diverged
but not diverged far enough for full reproductive
A single interbreeding isolation (that is, speciation) to have arisen,
population is split into
two populations. we may find that the populations are partially
reproductively isolated. They are not yet
separate species, but the genetic differences
A B between them are extensive enough that the
hybrid offspring they produce have reduced
fertility or viability compared to offspring
produced by crosses between individuals within
each population.
Time

Mutations Intermittently, we take individuals

from each population and test Allopatric speciation is speciation that
them to see whether they can still
The two populations
interbreed. results from the geographical separation
diverge genetically
because different
of populations.
Speciation occurs when the two Speciation is the process by which two groups of
mutations occur and
populations are no longer able to
become fixed in organisms become reproductively isolated from
produce viable, fertile offspring.
each population
over time. each other. Because this process requires genetic
isolation between the diverging populations and
because geography is the easiest way to ensure
physical and therefore genetic isolation, many
Genetic difference models of speciation focus on geography.
The process usually begins with the creation
Populations able to interbreed of allopatric (literally, “different place”)
Populations unable to interbreed
populations, populations that are geographically
separated from each other. Clearly, physical
separation will not immediately cause
reproductive isolation. If a single population
Instead of asking, “How do new species arise?” we can ask, “How is split in two by a geographic barrier, and, after a couple of
does reproductive isolation arise between populations?” generations, individuals from each population are allowed to
interbreed again, they will still be capable of producing fertile
Speciation is a by-product of the genetic divergence offspring and therefore still be members of the same species.
of separated populations. What’s also required for speciation to occur is time. Time allows
The key to speciation is the fundamental evolutionary process of for different mutations to become fixed in the two separated
genetic divergence between genetically separated populations. populations so that eventually they become reproductively
As we saw in Chapter 21, if a single population is split into two isolated from each other.
populations that are unable to interbreed, different mutations Because genetic divergence is typically gradual, we often
will appear by chance in the two populations. Like all mutations, find allopatric populations that have yet to evolve even partial
these will be subject to genetic drift or natural selection (or reproductive isolation but which have accumulated a few
both), resulting over time in the genetic divergence of the population-specific traits. This genetic distinctness is sometimes
two populations. Two separate populations that are initially recognized by taxonomists, who call each geographic form a
identical will, over long periods of time, gradually become subspecies by adding a further designation after its species name.
distinct as different mutations are introduced and propagated For example, Sri Lankan Asian elephants, subspecies Elephas
in each population. At some stage in the course of divergence, maximus maximus, are generally larger and darker than Indian
changes occur in one population that lead to its members’ being ones, subspecies Elephas maximus indicus (see Fig. 22.2).
reproductively isolated from members of the other population
(Fig. 22.6). It is this process that results in speciation. Dispersal and vicariance can isolate populations from
Speciation—the development of reproductive isolation each other.
between populations—is, therefore, typically just a by-product of How do populations become allopatric? There are two ways. The
the genetic divergence of separated populations. first is by dispersal, in which some individuals colonize a distant
H5O
4 2WS E CD
T IO
O NW
2 2 .E
1 K
T HN OI OW
E B ?I C A L S P E C I E S CO N C E P T
LO G

FIG. 22.7

Can vicariance cause speciation?

BACKGROUND Three and a half million years ago, the Isthmus EXPERIMENT This study focused on 17 species of snapping
of Panama was not completely formed. Several marine corridors shrimp in the genus Alpheus, a group that is distributed on
remained open, allowing interbreeding between marine populations both sides of the isthmus. The first step was to sequence the
in the Caribbean and the eastern Pacific. Subsequently, the gaps in the same segment of DNA from each species. The next step was to
isthmus were plugged, separating the Caribbean and eastern Pacific compare those sequences in order to reconstruct the phylogenetic
populations and preventing gene flow between them. relationships among the species.

HYPOTHESIS Nancy Knowlton and her colleagues hypothesized

Pacific 6’
that patterns of speciation would reflect the impact of the vicariance The closing of marine Pacific 6
resulting from the closing of the direct marine connections between corridors resulted over time Caribbean 6
the Pacific and the Caribbean. Specifically, they predicted that each in the speciation of Alpheus Pacific 3
into eastern Pacific and
ancestor species (from the time before the formation of the isthmus) Caribbean 3
Caribbean species.
Caribbean 3’
split into two “daughter” species, one in the Caribbean and the other
Pacific 5
in the Pacific. The closest relative of each current Pacific species, then, Caribbean 5
is predicted to be a Caribbean species (and vice versa). Caribbean 4
Pacific 4
Pacific 1
Interbreeding between eastern Pacific and Caribbean 1
Caribbean populations of Alpheus was
Pacific 2
possible via the corridors that existed before
the final formation of the Isthmus of Panama. Caribbean 2
Pacific 7’
3.5 million years ago Pacific 7
Caribbean 7
Caribbean Photo source: Dr. Arthur Anker, NUS, Singapore.
population

RESULTS The phylogeny reveals that species show a distinctly

Land
paired pattern of relatedness: The closest relative of each species is
Shallow one from the other side of the isthmus.
water
CONCLUSION That we see these consistent Pacific/Caribbean
Eastern Pacific sister species pairings strongly supports the hypothesis that the
population
vicariance caused by the formation of the isthmus has driven
speciation in Alpheus. Each Pacific/Caribbean pairing is derived
from a single ancestral species (indicated by a yellow dot) whose
Interbreeding between eastern continuous distribution between the Caribbean and eastern Pacific
Pacific and Caribbean populations was disrupted by the formation of the isthmus. Here, we see striking
is no longer possible because of
the geographic barrier. evidence of the role of vicariance in multiple speciation events.
Today
FOLLOW-UP WORK Speciation is about more than just genetic
Nicaragua
Caribbean differences between isolated populations, as shown here. Knowlton
population
and colleagues also tested the different species for reproductive
Costa
Rica Caribbean Sea isolation and found that there were high levels of isolation between
Caribbean/Pacific pairs. Given that we know that each species pair
has been separated for at least 3 million years, we can then calibrate
Panama
the rate at which new species are produced.

Eastern Pacific Colombia SOURCE Knowlton, N., et al. 1993. “Divergence in Proteins, Mitochondrial DNA,
population Pacific Ocean and Reproductive Compatibility Across the Isthmus of Panama.” Science 260 (5114):
1629–1632.

452
 CHAPTER 22 S P E C I E S A N D S P E C I AT I O N 453

place, such as an island, far from the main source population. disperse to a new location remote from the original population
The second is by vicariance, in which a geographic barrier arises and evolve separately. This may be an intentional act of dispersal,
within a single population, separating it into two or more isolated such as young mammals migrating away from where they were
populations. For example, when sea levels rose at the end of the raised, or it could be an accident brought about by, for example,
most recent ice age, new islands formed along the coastline as an unusual storm that blows migrating birds off their normal
the low-lying land around them was flooded. The populations on route. The result is a distant, isolated island population.
those new islands suddenly found themselves isolated from other “Island” in this case may refer to a true island—like Hawaii—or
populations of their species. This kind of island formation is a may simply refer to a patch of habitat on the mainland that is
vicariance event. appropriate for the species but is geographically remote from
Regardless of how the allopatric populations came about— the mainland population’s habitat area. For a species adapted to
whether through dispersal or vicariance—the outcome is the life on mountaintops, a new island might be another previously
same. The two separated populations will diverge genetically until uninhabited mountaintop. For a rain forest tree species, that
speciation occurs. new island might be a patch of lowland forest on the far side of
Often, vicariance-derived speciation is the easier to study a range of mountains that separates it from its mainland forest
because we can date the time at which the populations were population.
separated if we know when the vicariance occurred. One such The island population is classically small and often in an
event whose history is well known is the formation of the environment that is slightly different from that of the mainland
Isthmus of Panama between Central and South America, shown population. The peripatric speciation model suggests that change
in Fig. 22.7. This event took place about 3.5 million years ago. As a accumulates faster in these peripheral isolated populations than in
result, populations of marine organisms in the western Caribbean the large mainland populations, both because genetic drift is more
and eastern Pacific that had formerly been able to interbreed pronounced in smaller populations than in larger ones and because
freely were separated from each other. After a period of time, the the environment may differ between the mainland and island in a
result was the formation of many distinct species, with each one’s way that results in natural selection driving differences between
closest relative being on the other side of the isthmus. the two populations. These mechanisms cause genetic divergence
Dispersal is important in a specific kind of allopatric of the island population from the mainland one, ultimately leading
speciation known as peripatric speciation (that is, in a to speciation.
peripheral place). In this model, a few individuals from a It is possible to glimpse peripatric differentiation in action
mainland population (the central population of a species) (Fig. 22.8). Studies of a kingfisher, Tanysiptera galatea, in New

FIG. 22.8 Peripatric speciation in action among populations of New Guinea kingfishers. Sources: Data from D. J. Futuyma, 2009, Evolution, 2nd ed.,
Sunderland, MA: Sinauer Associates, Fig. 18.7, p. 484; Photo from C. H. Greenewalt/VIREO.

Australia
Mainland subspecies of Tanysiptera
galatea are similar to one another, but
island subspecies are more distinct,
suggesting faster genetic divergence
on island populations.

New Guinea
The eight
subspecies on New
Guinea and nearby
islands are marked
in different colors.
454 SECTION 22.3 S P E C I AT I O N

Small ground finch FIG. 22.9 Adaptive radiation in

(Geospiza fuliginosa) Darwin’s finches. Today,
there are 13 different
Medium ground finch species of Darwin’s
(G. fortis)
finches. Source: Phylogenetic
Seed-eaters tree data from Fig. 2.1, p.15,
Large ground finch The finches on this branch finch species plot data from
(G. magnirostris) of the tree have bills
adapted for picking and Fig. 2.4, p. 23, in P. R. Grant and
crushing different-sized B. R. Grant, 2008, How and Why
Sharp-billed ground finch seeds.
(G. difficilis) Species Multiply, Princeton, NJ:
Princeton University Press.

Large cactus finch

(G. conirostris)

Cactus finch
(G. scandens)

Bud-eater
Only one finch species
Vegetarian finch evolved with adaptations
(Platyspiza crassirostris) for pulling buds off trees.
Ancestor finch
from South
America
mainland Small tree finch
(Camarhynchus parvulus)

Large tree finch

(C. psittacula)

Medium tree finch

(C. pauper) Insect-eaters
Several species of finch
evolved with differently
Mangrove finch shaped bills adapted to
(C. heliobates) eating different kinds of
insects.
Woodpecker finch
(C. pallidus)

Warbler finch
(Certhidea olivacea)

20
Number of islands
Number of finch species
15
Numbers

10
The number of species
increased as more islands
appeared, suggesting that
allopatric isolation of
5 populations on different
islands was important for
speciation in these birds.

0
5 4 3 2 1 0
Millions of years ago
 CHAPTER 22 S P E C I E S A N D S P E C I AT I O N 455

Guinea and nearby islands show the process under way. There When the medium-billed immigrant finches first arrived on
are eight recognized subspecies of T. galatea, three on mainland the Galápagos, however, no such competition existed. Therefore,
New Guinea (where they exist in large populations separated by natural selection promoted the formation of new species of small-
mountain ranges) and five on nearby islands (where, because the and large-seed specialists from the original medium-seed-eating
islands are small, the populations are correspondingly small). The ancestral stock. With the elimination of the stabilizing selection
mainland subspecies are still quite similar to one another, but the that kept the medium-billed finches medium-billed on the
island subspecies are much more distinct, suggesting that genetic mainland, selection actually operated in the opposite direction,
divergence is occurring faster in the small island populations. If we favoring the large and small extremes of the bill-size spectrum
wait long enough, these subspecies will probably diverge into new because these individuals could take advantage of the abundance
species. of unused resources, small and large seeds. It is this combination
Because dating such dispersal events is tricky—newcomers of emptiness—the availability of ecological opportunity—and the
on an island tend not to leave a record of when they arrived—we potential for allopatric speciation that results in adaptive radiation.
can sometimes use vicariance information to study the timing
j Quick Check 2 Why do we see so many wonderful examples
of peripatric speciation. For instance, we know that the oldest
of adaptive radiation on mid-ocean volcanic archipelagos like the
of the Galápagos Islands were formed 4–5 million years ago by
Galápagos?
volcanic action. Sometime early in the history of the Galápagos,
individuals of a small South American finch species arrived there.
Co-speciation is speciation that occurs in response to
Conditions on the Galápagos are very different from those on the
speciation in another species.
South American continent, where the mainland population of this
As we have seen, physical separation is often a critical ingredient in
ancestral finch lived, and so the isolated island population evolved
speciation. Two populations that are not fully separated from each
to become distinct from its mainland ancestor and eventually
other—that is, there is gene flow between them—will typically
became a new species.
not diverge from each other genetically, because genetic exchange
The finches’ subsequent dispersal among the other islands
homogenizes them. This is why most speciation is allopatric.
of the Galápagos has promoted further peripatric speciation
Allopatric speciation brings to mind populations separated from
(Fig. 22.9). In the graph at the bottom of the figure, we see
each other by stretches of ocean or deserts or mountain ranges.
that the number of finch species is correlated with the number
However, separation can be just as complete even in the absence of
of islands in the archipelago. This is clear indication of the
geographic barriers.
importance of geographic separation (allopatry) in speciation: The
Consider an organism that parasitizes a single host species.
availability of islands provided opportunities for populations to
Suppose that the host undergoes speciation, producing two
become isolated from one another, allowing speciation. The result
daughter species. The original parasite population will also be split
was the evolution of 13 different species of finches, collectively
into two populations, one for each host species. Thus, the two
known today as Darwin’s finches.
new parasite populations are physically separated from each other
The Galápagos finches and their frenzy of speciation illustrate
and will diverge genetically, ultimately undergoing speciation.
the important evolutionary idea of adaptive radiation, a bout
This divergence results in a pattern of coordinated host–parasite
of unusually rapid evolutionary diversification in which natural
speciation called co-speciation, a process in which two groups of
selection accelerates the rates of both speciation and adaptation.
organisms speciate in response to each other and at the same time.
Adaptive radiation occurs when there are many ecological
Phylogenetic analysis of lineages of parasites and their hosts
opportunities available for exploitation. Consider the ancestral
that undergo co-speciation reveals trees that are similar for each
finch immigrants arriving on the Galápagos. A wealth of ecological
group. Each time a branching event—that is, speciation—has
opportunities was open and available. Until the arrival of the
occurred in one lineage, a corresponding branching event occurred
colonizing finches, there were no birds on the islands to eat the
in the other (Fig. 22.10).
plant seeds, or to eat the insects on the plants, and so on. Suppose
that the ancestral finches fed specifically on medium-sized seeds
on the South American mainland (that is, they were medium-seed ? CASE 4 MALARIA: CO-EVOLUTION OF HUMANS AND
specialists, with bills that are the right size for handling medium- A PARASITE
sized seeds). On the mainland, they were constrained to that size How did malaria come to infect humans?
of seed because any attempt to eat larger or smaller ones brought Now let’s look at a human parasite, Plasmodium falciparum, the
them into conflict with other species—a large-seed specialist and a single-celled eukaryote that causes malaria. It had been suggested
small-seed specialist—that already used these resources. In effect, that P. falciparum’s closest relative is another Plasmodium species,
stabilizing selection (Chapter 21) was operating on the mainland P. reichenowi, found in chimpanzees, our closest living relative.
to eliminate the extremes of the bill-size spectrum in the medium- Were P. falciparum and P. reichenowi the products of co-speciation?
seed specialists. When the ancestral population split millions of years ago to give
456 SECTION 22.3 S P E C I AT I O N

FIG. 22.10 Co-speciation. Parasites and their hosts often evolve together, and the result is similar phylogenies. Sources: Data from J. P. Huelsenbeck and
B. Rannala, 1997, “Phylogenetic Methods Come of Age: Testing Hypotheses in an Evolutionary Context,” Science 276:227, doi: 10.1126/science.276.5310.227;
Louse photo by Alex Popinga, courtesy of James Demastes; pocket gopher photo by Richard Ditch @ richditch.com.

Dashed lines represent host–parasite pairs.

Chewing lice (parasites) Pocket gophers (host)

Geomydoecus chapini Orthogeomys hispidus

Geomydoecus setzeri Orthogeomys underwoodi
Geomydoecus panamensis Orthogeomys cavator
Geomydoecus cherriei Orthogeomys cherriei
Geomydoecus costaricensis Orthogeomys heterodus

rise to human and chimpanzee lineages, that population’s parasitic population A can, in principle, appear in population B as members
Plasmodium population could also have been split, ultimately of the two populations interbreed. The new mutation may indeed
yielding P. falciparum and P. reichenowi. have become fixed in population A, but a migrant from population A
Recent studies, however, have disproved this hypothesis. We to population B may introduce the mutation to population B as well.
now know that P. falciparum was introduced to humans relatively Gene flow effectively negates the genetic divergence of populations.
recently from gorillas. Why doesn’t the evolutionary history If there is gene flow, a pair of populations may change over time, but
of Plasmodium follow the classical host–parasite co-speciation they do so together. How, then, can speciation occur if gene flow
pattern? We know that this history is complex and is still being exists? The term we use to describe populations that are in the same
unraveled. However, we also know that the mosquito-borne phase geographic location is sympatric (literally, “same place”). So we
of its life cycle facilitates transfer to new hosts. Malaria parasites can rephrase the question in technical terms: How can speciation
are thus not as inextricably tied to their hosts as the pocket gopher occur sympatrically?
lice in Fig 22.10 and so their evolutionary history is
not as parallel to their hosts.

Sympatric populations—those not FIG. 22.11 Sympatric speciation by disruptive selection. Natural selection
geographically separated—may undergo eliminates individuals in the middle of the spectrum.
speciation.
Can speciation occur without complete
Small-seed specialist Large-seed specialist Natural selection
physical separation of populations? Yes, though
Number of individuals

eliminates medium-
evolutionary biologists are still exploring Natural billed birds in an
selection environment with no
how common this phenomenon is. Recall medium-sized seeds,
how separated populations inevitably diverge and favors small-billed
genetically over time (see Fig. 22.6). If a mutation birds and large-billed
birds in response to
arises in population A after it has separated from the availability of small
population B, that mutation is present only in and large seeds.
population A and may eventually become fixed
(100% frequency) in that population, either
through natural selection, if it is advantageous, or
through drift, if it is neutral. Once the mutation
is fixed in population A, it represents a genetic
Number of individuals

difference between populations A and B. Repeated Disruptive selection, if

independent fixations of different mutations in strong enough and
the two populations result over time in the genetic sustained enough,
may eventually lead to
divergence of separated populations. sympatric speciation.
Now imagine that populations A and B are
not completely separated and there is some gene
Small Large
flow between them. The mutation that arose in Bill size
 CHAPTER 22 S P E C I E S A N D S P E C I AT I O N 457

For speciation to occur sympatrically, natural selection must related species in the same location and argue that they must
act strongly to counteract the homogenizing effect of gene have arisen through sympatric speciation. However, there is an
flow. Consider two sympatric populations of finch-like birds, alternative explanation. One species could have arisen elsewhere
represented in the graph in Fig. 22.11. One population begins to by, for example, peripatric speciation and subsequently moved into
specialize on small seeds and the other on large seeds. If the two the environment of the other one. In other words, the speciation
populations freely interbreed, no genetic differences between the occurred in the past by allopatry, and the two species are only
two will occur and speciation will not take place. Now suppose currently sympatric because migration after speciation occurred.
that the offspring produced by the pairing of a big-seed specialist However, recent studies of plants on an isolated island have
with a small-seed specialist is an individual best adapted to eat provided strong, if not definite, evidence of sympatric speciation.
medium seeds, and there are no medium-sized seeds available in Lord Howe Island is a tiny island (about 10 km long and 2 km
the environment. Natural selection will act against the hybrids, across at its widest) about 600 km east of Australia. Two species of
which will starve to death because there are no medium-sized palm tree that are only found on the island are each other’s closest
seeds for them to eat and they are not well adapted to compete relatives. Because the island is so small, there is little chance that
with the big- or small-seed specialists. Natural selection would the two species could be geographically separated from each other,
then, in effect, eliminate the products of gene flow. So, although meaning that the species are and were sympatric, and, because of
gene flow is occurring, it does not affect the divergence of the the distance of Lord Howe from Australia (or other islands), it is
two populations because the hybrid individuals do not survive extremely likely that they evolved and speciated from a common
to reproduce. As discussed in Chapter 21, this form of natural ancestor on the island.
selection, which operates against the middle of a spectrum of This and other demonstrations show that sympatric speciation
variation, is called disruptive selection. can occur. We must recognize, however, that we do not yet know
However, it turns out to be difficult to find evidence of just how much of all speciation is sympatric and how much is
sympatric speciation in nature, though it may not be especially allopatric. Fig. 22.12 summarizes modes of speciation based on
rare in plants, as we will see. We might find two very closely geography.

FIG. 22.12 Modes of speciation.

Allopatric: Speciation with no gene flow between diverging populations Sympatric: Speciation with gene
flow between diverging populations

Ancestral Ancestral
population population

Vicariance OR Dispersal with large OR Dispersal with small new population:

new population peripatric speciation

In peripatric Disruptive
A B A B A B speciation, selection
Population the ancestral results in two
splits either by population seeds genetically
vicariance or a small peripheral distinct sub-
by dispersal. population (e.g., populations
on an island). within the
River changes course Some individuals population.
to split population. move to new habitat.

Divergence Eventually,
Populations the sub-
occurs over
diverge populations
time, but most
genetically diverge
of the change
over time. sufficiently
accumulates
in the small to become
A B population. two distinct
A B A B species.
458 SECTION 22.3 S P E C I AT I O N

j Quick Check 3 There are hundreds of species of cichlid fish in double the number of chromosomes (that is, the hybrid inherits
Lake Victoria in Africa. Some scientists argue that they evolved a full paired set of chromosomes from each parental species)—a
sympatrically, but recent studies of the lake suggest that it total of 20. In this case, the hybrid has four genomes rather
periodically dried out, leaving a series of small ponds. Why is than the diploid number of two. We call such a double diploid
this observation relevant to evaluating the hypothesis that these a tetraploid. In general, animals cannot sustain this kind of
species arose by sympatric speciation? expansion in chromosome complement, but plants are more
likely to do so. As a result, the formation of new species through
Speciation can occur instantaneously. polyploidy—multiple chromosome sets (Chapter 13)—has been
Although speciation is typically a lengthy process, it can relatively common in plants.
occasionally occur in a single generation, making it sympatric by Polyploids may be allopolyploids, meaning that they are
definition. Typically, cases of such instantaneous speciation produced from hybridization of two different species. For
are caused by hybridization between two species in which the example, related species of Chrysanthemum appear, on the basis
offspring are reproductively isolated from both parents. of their chromosome numbers, to be allopolyploids. Alternatively,
For example, hybridization in the past between two sunflower polyploids may be autopolyploids, meaning that they are derived
species, Helianthus annuus and H. petiolaris (the ancestor of the from an unusual reproductive event between members of a single
cultivated sunflower) has apparently given rise to three new species. In this case, through an error of meiosis in one or both
sunflower species, H. anomalus, H. paradoxus, and H. deserticola parents in which homologous chromosomes fail to separate, a
(Fig. 22.13). H. petiolaris and H. annuus have probably formed gamete may be produced that is not haploid. For example, Anemone
innumerable hybrids in nature, virtually all of them inviable. rivularis, a plant in the buttercup family, has 16 chromosomes,
However, a few—the ones with a workable genetic complement— and its close relative A. quinquefolia has 32. Anemone quinquefolia
are the ones that survived to yield these three daughter species. In appears to be an autopolyploid derived from the joining of A.
this case, each one of these new species has acquired a different mix rivularis gametes with two full sets of chromosomes each.
of parental chromosomes. It is this species-specific chromosome So rampant is speciation by polyploidy in plants that it
complement that makes all three distinct and reproductively affects the pattern of chromosome numbers across all plants. In
isolated from the parent species and from one another. Fig. 22.14, we see the haploid chromosome numbers of thousands
In many cases of hybridization, chromosome numbers may of plant species plotted on a graph. Note that as the numbers
change. Two diploid parent species with 5 pairs of chromosomes, get higher, even numbers tend to predominate, suggesting
for a total of 10 chromosomes each, may produce a hybrid with that the doubling of the total number of chromosomes (a form

Helianthus annuus Helianthus petiolaris

Helianthus anomalus

FIG. 22.13 Speciation by hybridization. In sunflowers,

Helianthus anomalus (bottom) is the product of
natural hybridization between H. annuus (top left)
and H. petiolaris (top right). Source: (top left) Gary A.
Monroe @ USDA-NRCS PLANTS Database, (top right) Jason Rick,
(bottom) Gerald J. Seiler, USDA-ARS.
 CHAPTER 22 S P E C I E S A N D S P E C I AT I O N 459

Natural selection can enhance reproductive isolation.

FIG. 22.14 Plant chromosome numbers. Plant chromosome There is a third way in which natural selection may play an
numbers suggest that polyploidy has played an important important role in speciation. In this case, natural selection
role in plant evolution. Data from Fig. 20-18, p. 752, in A. J. F. contributes directly to the process of speciation (rather than
Griffiths, S. R. Wessler, S. B. Carroll, and J. Doebley, 2012, Introduction contributing indirectly by causing differences among the diverging
to Genetic Analysis, 10th ed., New York: W. H. Freeman. populations), when individuals better at choosing mates from
their own group are selectively favored over those that frequently
8
mate with members of the “wrong” group.
1100 Recall the two hypothetical bird populations, one with large
bills and the other with small bills, whose medium-billed hybrid
1000
offspring are at a disadvantage because of a lack of medium-sized
900 Even numbers are over-
represented, suggesting seeds in their environment. If a large-billed individual cannot
14
800 that genome-doubling distinguish between large- and small-billed individuals as potential
has been an important mates (that is, there is a lack of pre-zygotic isolation between
Number of species

700 mechanism of change in

plant evolution. them), it will frequently make the “wrong” mate choice, picking a
600 small-billed individual and paying a considerable evolutionary cost
16 18
10 of producing poorly adapted hybrid offspring.
500
Now imagine a new mutation in the large-billed group
400 20 that permits individuals carrying it to distinguish between the
24
300 22 two groups of birds and to mate only with other large-billed
individuals. Such a mutation would spread under natural selection
200 28
26 because it would prevent the wasted reproductive effort of
30 32 34 36
100 producing disadvantaged hybrid offspring. This is an example of
38 40
reinforcement of reproductive isolation, or reinforcement
0
1 5 10 20 30 40 50 60 for short. Reinforcement is the process by which diverging
Haploid number of chromosomes populations undergo natural selection in favor of traits that
enhance pre-zygotic isolation, thereby preventing the production
of less fit hybrid offspring. In this case, enhanced pre-zygotic
of polyploidy that will always result in an even number of isolation takes the form of mating discrimination, an increased
chromosomes) is an important factor in plant evolution. ability to recognize and mate with members of one’s own
population.
The best evidence in support of reinforcement comes from
22.4 SPECIATION AND SELECTION a study of related fruit-fly species living either in allopatry
(geographically separated) or sympatry (not geographically
The association of both the origin of species and natural selection separated). Sympatric species evolve pre-zygotic isolating
with Charles Darwin may make it seem that one cannot occur mechanisms more rapidly than allopatric species. Why would
without the other. However, speciation can occur in the presence this be? When the two populations are geographically separated,
or absence of natural selection, and natural selection does not the formation of less fit hybrids is impossible since the two
always lead to speciation. populations do not interbreed, so a mutation that increases mating
discrimination between the two groups would not be favored by
Speciation can occur with or without natural selection. natural selection. In sympatry, however, where the production
Natural selection may or may not play a role in speciation. The of less fit hybrids is a problem, such a mutation provides a fitness
genetic divergence of two populations can be entirely due to benefit, so natural selection favors its spread in the population and
genetic drift, for example, with no role for natural selection. reinforces reproductive isolation between the two groups.
We have also seen two ways in which natural selection can be Fig. 22.15 summarizes the evolutionary mechanisms that
involved in speciation. First, sympatric speciation requires some lead to speciation. Speciation is caused by the accumulation of
form of disruptive natural selection, as when hybrid offspring are genetic differences between populations. Mutation is therefore a
competitively inferior. Second, allopatric speciation (and adaptive key component of the process, but so, too, is the fixation process
radiation) may be facilitated by natural selection. For example, whereby mutations go to 100% in a population (Chapter 21).
when a peripheral population is in a new environment, natural A mutation can go to fixation by selection if it is advantageous
selection will act to promote its adaptation to the new conditions, and by genetic drift. The differences that accumulate between
accelerating in the process the rate of genetic divergence between diverging populations are therefore produced by multiple forces
it and its parent population. acting over long periods on each population. •
 V I S UA L S Y N T H E S I S Speciation
FIG. 22.15 Integrating concepts from Chapters 21 and 22

Genetic Drift
Neutral mutations increase and decrease in frequency in the population because of the random effects of generation-to-generation sampling error.

1 Different mutations arise in each population. Some (black) are deleterious and are Parent population
eliminated by selection. Some (red) are advantageous and swept to fixation (100%
frequency) by selection. Some (blue) are neutral, so their fate is governed by
genetic drift, and they ultimately drift either to extinction or fixation. Past

Change in
pigment Daughter population A
Generations
A neutral mutation that 1
briefly drifts up in frequency 2
3
before drifting to extinction 4
1.0
0.6
0.4
0.2
0.0
Change in
pigment Generations
A neutral mutation that A fixed difference
drifts to fixation occurs when the
e
Tim

1.0 frequency of an allele

reaches 100%. Here, a
fixed difference is
0.6
shown when an entire
row is red or blue.
0.4

0.2 1 fixed difference between

the populations
0.0

Generations

Because neutral mutations, like these that

change fish coloration, do not affect fitness, There are two fixed
their frequencies vary from generation to differences between
generation by chance alone. In small the populations. The
populations, these shifts in frequency can be fish in population B
extreme. If this process runs for long enough, are all larger than the
neutral mutations eventually drift to extinction fish in population A,
(0%) or fixation (100%). and they lack the
distinct pigmentation
of the fish in
population A.

2 fixed differences between

the populations

Present

460
 Natural Selection
Mutations that increase the fitness of individuals become more common, and those that decrease the fitness of individuals become less common over time.

A deleterious
mutation decreases
Less the fitness of the
streamlined organism.
fin shape
A neutral
A deleterious mutation mutation does not
that is eliminated by affect the fitness
natural selection of the organism.
Daughter population B
Generations Change in A beneficial
1
2 pigment mutation increases
3 A neutral mutation that the fitness of an
4
briefly drifts up in frequency organism.
before drifting to extinction
1.0
0.6
0.4
Allopatric 0.2
speciation 0.0
When two populations
are separated by a
geographic boundary, Generations
they diverge genetically
over time and
eventually become
reproductively isolated. Larger
mass
A beneficial mutation driven
to fixation by natural selection
1.0

2 Mutations may be fixed by

0.6
selection or drift in both
populations. Different mutations 0.4
are fixed in each population.
Through time the populations 0.2
diverge genetically.
0.0

Generations

Fish with the mutation are larger

than fish without it. Because large
size is advantageous (a larger fish is
less likely to be eaten by a predator),
larger fish are more likely to survive
and reproduce than smaller fish. The
frequency of the mutation in the
population therefore increases over
time by natural selection. This
process continues until the
mutation is fixed and all the fish in
3 Eventually, the two populations
the population are large.
become so genetically divergent through
the accumulation of fixed differences that
mating between individuals of different
populations fails to produce viable, fertile
offspring. Speciation has occurred.

461
462 SELF-ASSESSMENT

Core Concepts Summary Co-speciation occurs when one species undergoes speciation in
response to speciation in another. In parasites and their hosts,
co-speciation can result in host and parasite phylogenies that
22.1 Reproductive isolation is the key to the biological
have the same branching patterns. page 455
species concept.
Speciation may be sympatric, meaning that there is no
The biological species concept (BSC) states that species are
geographic separation between the diverging populations.
groups of actually or potentially interbreeding populations that
For this type of speciation to occur, natural selection for two
are reproductively isolated from other such groups.
or more different types within the population must act so
page 446
strongly that it overcomes the homogenizing effect of gene
We cannot apply the BSC to asexual or extinct organisms. flow. page 456
page 447

Ring species and hybridization further demonstrate that the 22.4 Speciation can occur with or without natural
BSC is not a comprehensive definition of species. page 448 selection.

The BSC is nevertheless especially useful because it Separated populations can diverge as a result of genetic drift,
emphasizes reproductive isolation. page 449 natural selection, or both. page 459
Natural selection may act on mutations that allow individuals
22.2 Reproductive isolation is caused by barriers to to identify and mate with individuals that are more
reproduction before or after egg fertilization. like themselves. This process is called reinforcement of
Reproductive barriers can be pre-zygotic, occurring before egg reproductive isolation. page 459
fertilization, or post-zygotic, occurring after egg fertilization.
page 449
Self-Assessment
Pre-zygotic isolation may be behavioral, gametic, temporal,
or ecological. page 450 1. Define the term “species.”

In post-zygotic isolation, mating occurs but genetic 2. Given a group of organisms, describe how you would
incompatibilities prevent the development of a viable, fertile test whether they all belong to one species or whether they
offspring. page 450 belong to two separate species.

3. Name two types of organism that do not fit easily into

22.3 Speciation underlies the diversity of life on Earth. the biological species concept. What species concept would
Speciation is typically a by-product of genetic divergence that work best for these organisms?
occurs as a result of the fixation of different mutations in two 4. Explain how ecological and phylogenetic considerations
populations that are not regularly exchanging genes. page 451 can help inform whether or not a group of organisms
If divergence continues long enough, chance differences will represents a single species.
arise that result in reproductive barriers between the two
5. Name four reproductive barriers and indicate whether
populations. page 451
each is pre- or post-zygotic.
Most speciation is thought to be allopatric, involving two 6. Describe how genetic divergence and reproductive
geographically separated populations. page 451 isolation are related to each other.
Geographic separation may be caused by dispersal, resulting
7. Differentiate between allopatric and sympatric speciation,
in the establishment of a new and distant population, or by
and state which is thought to be more common and why.
vicariance, in which the range of a species is split by a change
in the environment. page 451 8. Differentiate between allopatric speciation by dispersal
and by vicariance, and give an example of each.
A special case of allopatric speciation by dispersal is peripatric
speciation, in which the new population is small and outside 9. Describe how genetic drift can result in speciation.
the species’ original range. page 453
10. Describe how natural selection can result in speciation.
Adaptive radiation, in which speciation occurs rapidly to
generate a variety of ecologically diverse forms, is best
Log in to to check your answers to the Self-
documented on oceanic islands after the arrival of a single
Assessment questions, and to access additional learning tools.
ancestral species. page 455
 CHAPTER 23

Evolutionary
Patterns
Phylogeny and Fossils

Core Concepts
23.1 A phylogenetic tree is a
reasoned hypothesis of the
evolutionary relationships
among organisms.
23.2 A phylogenetic tree is built
on the basis of shared derived
characters.
23.3 The fossil record provides
direct evidence of evolutionary
history.
23.4 Phylogeny and fossils provide
independent and corroborating
evidence of evolution.

Dudley Museums Service.

463
464 SECTION 23.1 R E A D I N G A P H Y LO G E N E T I C T R E E

All around us, nature displays nested patterns of similarity among again, giving rise to multiple descendant species. The result is the
species. For example, as noted in Chapter 1, humans are more pattern of nested similarities observed in nature (see Fig. 1.17).
similar to chimpanzees than either humans or chimpanzees are This history of descent with branching is called phylogeny, and is
to monkeys. Humans, chimpanzees, and monkeys, in turn, are much like the genealogy that records our own family histories.
more similar to one another than any one of them is to a mouse. The evolutionary changes inferred from patterns of
And humans, chimpanzees, monkeys, and mice are more similar to relatedness among present-day species make predictions about
one another than any of them is to a catfish. This pattern of nested the historical pattern of evolution we should see in the fossil
similarity was recognized more than 200 years ago and used by the record. For example, groups with features that we infer to have
Swedish naturalist Carolus Linnaeus to classify biological diversity. evolved earlier than others should appear earlier in time as
A century later, Charles Darwin recognized this pattern as the fossils. Paleontological research reveals that the history of life is
expected outcome of a process of “descent with modification,” or indeed laid out in the chronological order predicted on the basis
evolution. of comparative biology. How do we reconstruct the history of life
Evolution produces two distinct but related patterns, both from evolution’s two great patterns, and how do they compare?
evident in nature. First is the nested pattern of similarities found
among species on present-day Earth. The second is the historical
pattern of evolution recorded by fossils. Life, in its simplest form, 23.1 READING A PHYLOGENETIC TREE
originated more than 3.5 billion years ago. Today, an estimated
10 million species inhabit the planet. Short of inventing a time Chapter 22 introduced the concept of speciation, the set of
machine, how can we reconstruct those 3.5 billion years of processes by which physically, physiologically, or ecologically
evolutionary history in order to understand the extraordinary isolated populations diverge from one another to the point where
events that have ultimately resulted in the biological diversity we they can no longer produce fertile offspring. As illustrated in
see around us today? These two great patterns provide the answer. Fig. 23.1, speciation can be thought of as a process of branching.
Darwin recognized that the species he observed must be the Now consider what happens as this process occurs over and over
modified descendants of earlier ones. Distinct populations of an in a group through time. As species proliferate, their evolutionary
ancestral species separate and diverge through time, again and relationships to one another unfold in a treelike pattern, with
individual species at the twig tips and
their closest relatives connected to them
FIG. 23.1 The relationship between speciation and a phylogenetic tree. The phylogenetic tree, at the nearest fork in the branch, called
on the right, depicts the evolutionary relationships that result from the two successive a node. A node thus represents the
speciation events diagrammed on the left. most recent common ancestor of two
descendant species.
Present
Phylogenetic trees provide
3 species

A B C Species A Species B Species C hypotheses of evolutionary

relationships.
Phylogenetics is one of two related
disciplines within systematics, the study
of evolutionary relationships among
organisms. The other is taxonomy, the
Node 2
classification of organisms.
The aim of taxonomy is to recognize
2 species

and name groups of individuals as

Time

A B species, and, subsequently, to group

closely related species into the more
Branch inclusive taxonomic group of the genus,
and so on up through the taxonomic
ranks—species, genus, family, order,
Node 1 class, phylum, kingdom, domain.
Taxonomy, then, provides us with a
The point where a hierarchical classification of species
branch splits is called a
1 species

node. It represents the in groups that are more and more

common ancestor from inclusive, giving us a convenient way
which the descendant to communicate information about the
Past species diverged. Root features each group possesses. So, if we
 CHAPTER 23 E VO LU T I O N A RY PAT T E R N S : P H Y LO G E N Y A N D F O S S I L S 465

want to tell someone about a small animal we have seen with fur,
mammary glands, and extended finger bones that permit it to fly, FIG. 23.2 A phylogeny of vertebrate animals. The branching
we can give them this long description, or we can just say we saw a order constitutes a hypothesis of evolutionary
bat, or a member of Order Chiroptera. All the rest is understood (or relationships within the group.
can be looked up in a reference). Hagfish
Phylogenetics, on the other hand, aims to discover the pattern Jawless fish
of evolutionary relatedness among groups of species or other Lampreys
groups by comparing their anatomical or molecular features, Sharks
Cartilaginous fish
and to depict these relationships as a phylogenetic tree. A and Rays
phylogenetic tree is a hypothesis about the evolutionary history,
Ray-finned fish
or phylogeny, of the species. Phylogenetic trees are hypotheses
because they represent the best model, or explanation, of the Coelacanths Bony fish
relatedness of organisms on the basis of all the existing data. As Lobe-finned
fish
with any model or hypothesis, new data may provide evidence for Lungfish
alternative relationships, leading to changes in the hypothesized
pattern of branching on the tree. Frogs
Many phylogenetic trees explore the relatedness of particular
Salamanders Amphibians
groups of individuals, populations, or species. We may, for
example, want to understand how wheat is related to other, Caecilians
non-commercial grasses, or how disease-causing populations of
Escherichia coli relate to more benign strains of the bacterium. At Lizards
and snakes
a much larger scale, universal similarities of molecular biology Tetrapods
indicate that all living organisms are descended from a single Turtles
Sauropsids
common ancestor. This insight inspires the goal of reconstructing Crocodiles
phylogenetic relationships for all species in order to understand and alligators
how biological diversity has evolved since life originated.
Birds
This universal tree is commonly referred to as the tree of life
(Chapter 1). In Part 2 of this book, we will make use of the tree of Mammals
life and many smaller-scale phylogenetic trees to understand our
planet’s biological diversity.
Fig. 23.2 shows a phylogenetic tree for vertebrate animals.
The informal name at the end of each branch represents a group of earlier. A modern lungfish, for example, is not more primitive or
organisms, many of them familiar. We sometimes find it useful to “less evolved” than an alligator, even though its group branches
refer to groups of species this way (for example, “frogs,” or “Class off the trunk of the vertebrate tree earlier than the alligator
Anura”) rather than name all the individual species or list the group does. After all, both species are the end products of the
characteristics they have in common. It is important, however, to same interval of evolution since their divergence from a common
remember that such named groups represent a number of member ancestor more than 370 million years ago.
species. If, for example, we were able to zoom in on the branch
labeled “Frogs,” we would see that it consists of many smaller The search for sister groups lies at the heart
branches, each representing a distinct species of frog, either living of phylogenetics.
or extinct. Two species, or groups of species, are considered to be closest
This tree provides information about evolutionary relatives if they share a common ancestor not shared by any other
relationships among vertebrates. For example, it proposes that the species or group. In Fig. 23.2, for example, we see that frogs are
closest living relatives of birds are crocodiles and alligators. The more closely related to salamanders than to any other group
tree also proposes that the closest relatives of all tetrapod (four- of organisms because frogs and salamanders share a common
legged) vertebrates are lungfish, which are fish with lobed limbs amphibian ancestor not shared by any other group. Similarly,
and the ability to breathe air. Phylogenetic trees are built from lungfish are more closely related to tetrapods than to any other
careful analyses of the morphological and molecular attributes group. A lungfish may look more like a fish than it does an
of the species or other groups under study. A tree is therefore a amphibian, but lungfish are more closely related to amphibians
hypothesis about the order of branching events in evolution, and it than they are to other fish because lungfish share a common
can be tested by gathering more information about anatomical and ancestor with amphibians (and other tetrapods) that was more
molecular traits. recent than their common ancestor with other fish (Fig. 23.2).
A phylogenetic tree does not in any way imply that more Groups that are more closely related to each other than either
recently evolved groups are more advanced than groups that arose of them is to any other group, like lungfish and tetrapods, are
466 SECTION 23.1 R E A D I N G A P H Y LO G E N E T I C T R E E

called sister groups. Simply put, phylogenetic hypotheses amount The resulting classification emphasizes groups that are
to determining sister-group relationships because the simplest monophyletic, meaning that all members share a single common
phylogenetic question we can ask is which two of any three species ancestor not shared with any other species or group of species.
(or other groups) are more closely related to each other than either In Fig. 23.2, the tetrapods are monophyletic because they all
is to the third. In this light, we can see that a phylogenetic tree is share a common ancestor not shared by any other taxa. Similarly,
simply a set of sister-group relationships; adding a species to the amphibians are monophyletic.
tree entails finding its sister group in the tree. In contrast, consider the group of animals traditionally
Closeness of relationship is then determined by looking to recognized as reptiles, which includes turtles, snakes, lizards,
see how recently two groups share a common ancestor. Shared crocodiles, and alligators (Fig. 23.2). The group “reptiles” excludes
ancestry is indicated by a node, or branch point, on a phylogenetic birds, although they share a common ancestor with the included
tree. Nodes can be rotated without changing the evolutionary animals. Such a group is paraphyletic. A paraphyletic group
relationships of the groups. Fig. 23.3, for example, shows four includes some, but not all, of the descendants of a common
phylogenetic trees depicting evolutionary relationships among ancestor. Early zoologists separated birds from reptiles because
birds, crocodiles and alligators, and turtles. In all four trees, they are so distinctive. However, many features of skeletal
birds are a sister group to crocodiles and alligators because birds, anatomy and DNA sequence strongly support the placement of
crocodiles, and alligators share a common ancestor not shared by birds as a sister group to the crocodiles and alligators.
turtles. The more recent a common ancestor, the more closely There is a simple way to distinguish between monophyletic
related two groups are. Evolutionary relatedness therefore is and paraphyletic groups, illustrated in Fig. 23.4. If in order to
determined by following nodes from the tips to the root of the separate a group from the rest of the phylogenetic tree you need
tree, and is not determined by the order of the tips from the top to only to make one cut, the group is monophyletic. If you need a
bottom of a page. second cut to trim away part of the separated branch, the group is
paraphyletic.
j Quick Check 1 Does either of these two phylogenetic trees indicate
Groupings that do not include the last common ancestor of all
that humans are more closely related to lizards than to mice?
members are called polyphyletic. For example, clustering bats and
birds together as flying tetrapods results in a polyphyletic group
Humans Lizards
(Fig. 23.4).
Identifying monophyletic groups is a main goal of
Mice Humans phylogenetics because monophyletic groups include all
descendants of a common ancestor and only the descendants of
that common ancestor. This means that monophyletic groups
Lizards Mice alone show the evolutionary path a given group has taken since
its origin. Omitting some members of a group, as in the case of
A monophyletic group consists of a common ancestor reptiles and other paraphyletic groups, can provide a misleading
and all its descendants. sense of evolutionary history. By using monophyletic groups in
Up to this point, we have used the word “group” to mean all taxonomic classification, we effectively convey our knowledge of
the species in some taxonomic entity under discussion. A more their evolutionary history.
technical word is taxon (plural, taxa), with taxonomy providing a
formal means of naming groups. Recently, biologists have worked j Quick Check 2 Look at Fig. 23.4. Are fish a monophyletic
to integrate evolutionary history with taxonomic classification. group?

FIG. 23.3 Sister groups. The four trees illustrate the same set of sister-group relationships.

The four trees are equivalent

because nodes can be rotated
without changing evolutionary
relationships.

Crocodiles
Turtles Turtles Birds
and alligators
Crocodiles = = = Crocodiles
Birds Birds
and alligators and alligators
Crocodiles
Birds Turtles Turtles
and alligators
 CHAPTER 23 E VO LU T I O N A RY PAT T E R N S : P H Y LO G E N Y A N D F O S S I L S 467

FIG. 23.4 Monophyletic, paraphyletic, and polyphyletic groups. Only monophyletic groups reflect evolutionary relationships because only they
include all the descendants of a common ancestor.
Hagfish

Lampreys

Sharks and rays

Ray-finned fish

Coelacanths

Lungfish

A monophyletic group Frogs

(blue) includes a
common ancestor and Salamanders Amphibians
all of its descendants.
Caecilians

Lizards and snakes

A paraphyletic group
(green) includes a
common ancestor and Turtles Reptiles
some, but not all, of its
descendants. Crocodiles
and alligators

A polyphyletic group Birds

(red) does not include Flying tetrapods
the common ancestor. Bats

Taxonomic classifications are FIG. 23.5 Classification. Classification reflects our understanding of phylogenetic
information storage relationships, and the taxonomic hierarchy reflects the order of branching.
and retrieval systems.
Genus 1 Species 1
The nested pattern of similarities among
species has been recognized by naturalists Species 2
Family 1
for centuries. In the vocabulary of formal Species 3
classification, closely related species are Order 1 Genus 2 Species 4
grouped into a genus (plural, genera). Species 5
Closely related genera, in turn, belong to Family 2 Genus 3
Species 6
a larger, more inclusive branch of the tree,
Genus 4 Species 7
as a family. Closely related families, in Class 1
Species 8
turn, form an order, orders form a class, Family 3
classes form a phylum (plural, phyla), Species 9
Genus 5
and phyla form a kingdom, each more Species 10
inclusive taxonomic level occupying a Order 2 Species 11
successively larger limb on the tree Genus 6 Species 12
(Fig. 23.5). Biologists today commonly Phylum I Species 13
refer to the three largest limbs of the Family 4
Species 14
entire tree of life as domains (Eukarya, or
Species 15
eukaryotes; Bacteria; and Archaea). Genus 7
The ranks of classification form a Species 16
nested hierarchy, but the boundaries of Species 17
Class 2
ranks above the species level are arbitrary
468 SECTION 23.2 B U I L D I N G A P H Y LO G E N E T I C T R E E

in that there is nothing particular about a group that makes it, for
example, a class rather than an order. A taxonomist examining the FIG. 23.6 Homology and analogy. A homology is a similarity that
17 species included in Fig. 23.5 might decide that species 3 and 4 results from shared ancestry, whereas an analogy is a
are sufficiently distinct from species 5 to warrant placing them into similarity that results from convergent evolution.
a distinct genus. The same taxonomist might then decide that the
Lizards
grouping of the two new genera together should rank as a family. and snakes
For this reason, it is not necessarily true that orders or classes of, for
Turtles
example, birds and ferns are equivalent in any meaningful way. In The amniotic egg Sauropsids
contrast, sister groups are equivalent in several ways—notably, in is a homology of Crocodiles
sauropsids and and alligators
that they diverged from a single ancestor at a single point in time.
mammals because
Therefore, if one branch is a sister group that contains 500 species it evolved once in Birds
and another has 6, the branches have experienced different rates their common
ancestor.
of speciation, extinction, or both since they diverged. In Fig 23.5, Mammals (bats)
Families 1 and 2 are sister groups, but Family 1 has five species while
Family 2 has just one. The presence of wings in birds
and bats is an analogy since wings
evolved independently in birds
and bats, and were not present in
their common ancestor.
23.2 BUILDING A PHYLOGENETIC
TREE
Up to this point, we have focused on how to interpret a Consider two examples. Mammals and birds both produce
phylogenetic tree, a diagram that depicts the evolutionary history of amniotic eggs. Amniotic eggs occur only in groups descended from
organisms. But how do we infer evolutionary history from a group the common ancestor at the node connecting the mammal and
of organisms? That is, how do we actually construct a phylogenetic sauropsid branches of the tree, and so we reason that birds and
tree? Biologists use characteristics of organisms to figure out mammals each inherited this character from a common ancestor
their relationships. Similarities among organisms are particularly in which the trait first evolved (Fig. 23.6). Characters that are
important in that similarities sometimes suggest shared ancestry. similar because of descent from a common ancestor are said to be
However, a key principle of constructing trees is that only some homologous.
similarities are actually useful. Others can in fact be misleading. Not all similarities arise in this way, however. Think of wings,
a character exhibited by both birds and bats. Much evidence
Homology is similarity by common descent. supports the view that wings in these two groups do not reflect
Phylogenetic trees are constructed by comparison of character descent from a common, winged ancestor but rather evolved
states shared among different groups of organisms. Characters independently in the two groups. Similarities due to independent
are the anatomical, physiological, or molecular features that adaptation by different species are said to be analogous. They are
make up organisms. In general, characters have several observed the result of convergent evolution.
conditions, called character states. In the simplest case, a Innumerable examples of convergent evolution less dramatic
character can be present or absent—lungs are present in than wings are known. In some, we even understand the genetic
tetrapods and lungfish, but absent in other vertebrate animals. basis of the convergence. For example, echolocation has evolved
Commonly, however, there are multiple character states. Petals in bats and in dolphins, but not in other mammals. Prestin is
are a character of flowers, for example, and each observed a protein in the hair cells of mammalian ears that is involved
arrangement—petals arranged in a helical pattern, petals arranged in hearing ultrasonic frequencies. Both bats and dolphins
in a whorl, or petals fused into a tube—can be considered a state independently evolved similar changes in their Prestin genes,
of the character of petal arrangement. All species contain some apparently convergent adaptations for echolocation. Similarly,
character states that are shared with other members of their unrelated fish that live in freezing water at the poles, Arctic
group, some that are shared with members of other groups, and and Antarctic, have evolved similar glycoproteins that act as
some that are unique. molecular “antifreeze,” preventing the formation of ice in their
Character states in different species can be similar for one of tissues.
two reasons: The character state (for example, helically arranged In principle, two characters or character states are homologous
petals) was present in the common ancestor of the two groups if they are similar because of descent from a common ancestor
and retained over time (common ancestry), or the character state with the same character or character state; they are analogous if
independently evolved in the two groups as an adaptation to they arose independently because of similar selective pressures.
similar environments (convergent evolution). In practice, to determine if characters observed in two organisms
 CHAPTER 23 E VO LU T I O N A RY PAT T E R N S : P H Y LO G E N Y A N D F O S S I L S 469

are homologous or analogous, we can weigh evidence from where group of tetrapods. Phylogenetic reconstruction on the basis of
other traits place the two organisms on a phylogenetic tree, we can synapomorphies is called cladistics.
look at where on the organisms the trait occurs, and we can look
at the anatomical or genetic details of how the trait is constructed.
The simplest tree is often favored among multiple
Wings in birds and bats are similar in morphological position (both
possible trees.
are modified forelimbs) but differ in details of construction (the
To show how synapomorphies help us chart out evolutionary
bat wing is supported by long fingers), and all other traits of birds
relationships, let’s consider the simple example in Fig. 23.8. We
and bats place them at the tips of different lineages, with many
begin with four species of animals (labeled “A” through “D”) in a
nonwinged species between them and their most recent common
group we wish to study that we will call our ingroup; we believe
ancestor.
the species to be closely related to each other. For comparison, we
have a species that we believe is outside this ingroup—that is, it
j Quick Check 3 Fish and dolphins have many traits in common,
falls on a branch that splits off nearer the root of the tree—and
including a streamlined body and fins. Are these traits homologous
so is called an outgroup (labeled “OG” in Fig. 23.8). Each species
or analogous?
in the ingroup and the outgroup has a different combination of
characters, such as leg number, presence or absence of wings, and
Shared derived characters enable biologists to whether development of young to adult is direct or goes through a
reconstruct evolutionary history. pupal stage (Fig. 23.8a).
Because homologies result from shared ancestry, only We are interested in the relationships among species A–D and so
homologies, and not analogies, are useful in constructing focus on potential synapomorphies, character states shared by some
phylogenetic trees. However, it turns out that only some but not all species within the group. For example, only C and D have
homologies are useful. For example, character
states that are unique to a given species
or other monophyletic group can’t tell us FIG. 23.7 Synapomorphies, or shared derived characters. Homologies that are present in
anything about its sister group. They evolved some, but not all, members of a group help us to construct phylogenetic trees.
after the divergence of the group from its

Jawless fish
sister group and so can be used to characterize Hagfish
a group but not to relate it to other groups.
Similarly, homologies formed in the common Lampreys

Cartilaginous
ancestor of the entire group and therefore
Sharks

fish
present in all its descendants do not help to and rays
identify sister-group relationships among the Sharks,
ray-finned fish,
descendants of that common ancestor. coelacanths, Ray-finned fish
What we need to build phylogenetic lungfish, and

Bony fish
tetrapods have

Lobe-finned
trees are homologies that are shared by Coelacanths
a hinged jaw.
some, but not all, of the members of the group

fish
under consideration. These shared derived Lungfish
characters are called synapomorphies. A Coelacanths and
lungfish have lobed fins. Frogs
derived character state is an evolutionary

Amphibians
innovation (for example, the change from
Salamanders
five toes to a single toe—the hoof—in the Lungfish and
ancestor of horses and donkeys). When such a tetrapods have lungs.
Caecilians
novelty arises in the common ancestor of two
taxa, it is shared by both (thus, the hoof is a Lizards Tetrapods
Tetrapods have and snakes
synapomorphy defining horses and donkeys as walking legs.
Sauropsids

sister groups). Turtles

In Fig. 23.7, we indicate the major Crocodiles
Sauropsids and mammals
synapomorphies that have helped us construct have an amniotic egg. and alligators
the phylogeny of vertebrates. For example,
Birds
the lung is a character present in lungfish and
tetrapods, but absent in other vertebrates. Mammals have hair
and milk glands. Mammals
Thus, the presence of lungs provides one
piece of evidence that lungfish are the sister
470 SECTION 23.2 B U I L D I N G A P H Y LO G E N E T I C T R E E

FIG. 23.8 Constructing a phylogenetic tree from shared derived traits. The strongest hypothesis of evolutionary relationships overall is the tree
with the fewest number of changes because it minimizes the total number of independent origins of character states.

a. Character states

3 pairs More than

3 3+
of legs 3 pairs of legs
OG A B C D
Wings present Wings absent
Legs 3+ 3 3 3 3
Direct
Pupa
Wing development

Development Sucking Grinding

mouthparts mouthparts
Mandibles

b. Possible phylogenetic trees

OG OG OG

A A C
3

3 B B
3 D

C C A

D D B

This phylogeny This phylogeny requires five changes, This phylogeny requires six changes,
requires four changes. including one loss of wings. including one loss of wings and one
reversion to direct development.

c. Phylogeny of Hexapoda (insects)

OG Shrimp

A Silverfish

B Dragonfly Class
Hexapoda
Subclass (910,000
Pterygota species)
C Beetle (900,000
Holometabola
species)
(800,000
species)
D Butterfly

pupae. This character suggests that C and D are more closely related In practice, biologists examine multiple characters and choose the
to each other than either is to the other species. The alternatives are phylogenetic hypothesis that best fits all of the data.
that C and D each evolved pupae independently or that pupae were How do we determine “best fit”? Fig. 23.8b illustrates three
present in the common ancestor of A–D but were lost in A and B. different hypotheses for the relationships among the species based
How do we choose among the alternatives? Studies of the on four characters and their various character states. Each reflects
outgroup show that it does not form pupae, supporting the the sister-group relationship between C and D proposed earlier. The
hypothesis that pupal development evolved within the ingroup. leftmost tree requires exactly four character-state changes during
 CHAPTER 23 E VO LU T I O N A RY PAT T E R N S : P H Y LO G E N Y A N D F O S S I L S 471

the evolution of these species: reduction of leg pairs to three in the be read as admissions of defeat, but instead as problems awaiting
group ABCD, wings in the group BCD, and pupae in group CD, plus a resolution and opportunities for future research.
change of form of the mandible, or jaw, in species D.
Now consider the middle tree in Fig. 23.8b. It groups A and B Molecular data complement comparative morphology
together, and so differs from the tree on the left in requiring either a in reconstructing phylogenetic history.
loss of wings in A, or an additional origin of wings in B, independent Trees can be built using anatomical features, but increasingly tree
of that in the common ancestor of C and D—five changes in all. construction relies on molecular data. The amino acids at particular
The tree on the right groups A and B together, and requires two positions in the primary structure of a protein can be used, as can
extra steps for a total of six steps. No tree that we can construct the nucleotides at specific positions along a strand of DNA.
from species A–D requires fewer than four evolutionary changes, so From genealogy to phylogeny, tracing mutations in DNA or
the left-hand version in Fig. 23.8b is the best available hypothesis RNA sequences has revolutionized the reconstruction of historical
of evolutionary relatedness. In fact, this is the phylogeny for a genetic connections. Whether we are tracing the paternity of the
sample of species from the largest group of animals on Earth, the children of Sally Hemings, mistress of Thomas Jefferson, identifying
Hexapoda—insects and their closest relatives (Fig. 23.8c). the origin of a recent cholera epidemic in Haiti, or placing baleen
In general, trees with fewer character changes are whales near the hippopotamus family in a phylogenetic tree of
preferred to ones that require more because they provide the mammals, molecular data are a rich source of phylogenetic insight.
simplest explanation of the data. This approach is an example There is nothing about molecular data that provides a better
of parsimony, that is, choosing the simpler of two or more record of history than does anatomical data; molecular data simply
hypotheses to account for a given set of observations. When we provide more details because there are more characters that can
use parsimony in phylogenetic reconstruction, we make the vary among the species. A sequence of DNA with hundreds or
implicit assumption that evolutionary change is typically rare. thousands of nucleotides can represent that many characters, as
Over time, most features of organisms stay the same—we have the opposed to the tens of characters usually visible in morphological
same number of ears as our ancestors, the same number of fingers, studies. Indeed, for microbes and viruses there is very little
and so on. Thus, biologists commonly prefer the phylogenetic tree morphology available, so molecular information is critical for
requiring the fewest evolutionary steps. phylogenetic reconstruction. Once a gene or other stretch of DNA
In systematics, parsimony suggests counting character changes or RNA is identified that seems likely (based on previous studies
on a phylogenetic tree to find the simplest tree for the data (the of other species) to vary among the species to be studied,
one with the fewest number of changes). Each change corresponds sequences are obtained and aligned to identify homologous
to a mutation (or mutations) in an ancestral species, and the more nucleotide sites. Analyses of this kind commonly involve
changes or steps we propose, the more independent mutations we comparisons of sequences of about 1000 nucleotides from
must also hypothesize. one or more genes. Increasingly, though, the availability of
Note also that it isn’t necessary to make decisions in advance whole-genome sequences is changing the way we do molecular
about which characters are homologies and which are analogies. We phylogenetics. Rather than comparing the sequences of a few
can construct all possible trees and then choose the one requiring genes, we compare the sequences of entire genomes.
the fewest evolutionary changes. This is a simple matter for the The process of using molecular data is conceptually similar
example in Fig. 23.8 because four species can be arranged into only to the process described earlier for morphological data. Through
15 possible different trees. As the number of groups increases, comparison to an outgroup, we can identify derived and ancestral
however, the number of possible trees connecting them increases molecular characters (whether DNA nucleotides or amino
as well, and dramatically so. There are 105 trees for 5 groups and acids in proteins) and generate the phylogeny on the basis of
945 trees for 6 groups, and there are nearly 2 million possible trees synapomorphies as before.
for 10. For 50 groups the possibilities balloon to 3  1076! Clearly, An alternative method of reconstruction is based on overall
computers are required to sort through all the possibilities. similarity rather than synapomorphies. Here, the premise is
As is true for all hypotheses, phylogenetic hypotheses can be simple: The descendants of a recent common ancestor will have
supported strongly or weakly. Biologists use statistical methods had relatively little time to evolve differences, whereas the
to evaluate a given phylogenetic hypothesis. Available character descendants of an ancient common ancestor have had a lot of time
data may not strongly favor any hypothesis. When support for a to evolve differences. Thus, the extent of similarity (or distance)
specific branching pattern is weak, biologists commonly depict the indicates how recently two groups shared a common ancestor.
relationships as unresolved and show multiple groups diverging Underpinning this approach is the assumption that the rate
from one node, rather than just two. Such branching patterns are of evolution is constant. (Otherwise, a pair of taxa with a recent
not meant to suggest that multiple species diverged simultaneously, common ancestor could be more different than expected because of
but rather to indicate that we lack the data to choose unequivocally an unusually fast rate of evolution.) This rate-constancy assumption
among several different hypotheses of relationship. In Part 2, we is less likely to be violated when we are using molecular data than
show unresolved branches in a number of groups. These shouldn’t when we are using morphological data. Recall from Chapter 21
472 SECTION 23.2 B U I L D I N G A P H Y LO G E N E T I C T R E E

FIG. 23.9 Phylogenetic trees of DNA sequences based on (a) synapomorphies and (b) overall similarity.

DNA sequence for four species a. Phylogenetic tree based on synapomorphies

Nucleotide position OG C B A
8
1 2 3 4 5 6 7 8 9 6
Synapomorphic T C 7 G A
OG A C C T C C G T G C T
reconstruction 9 5 C T Synapomorphy
A T A G G T C A T G G A groups A + B
Taxon 1 A T
B T A G G T T G T G 2 C A
Synapomorphy
C T A G G C C G C A 3 C G groups A, B, + C
The tree shown here is 4 T G
In our experiment, 1000 base pairs of the the most parsimonious
same gene have been sequenced for our four one, meaning that it The nucleotide changes marked on the
species. What is shown are the character incorporates a minimal tree may not be useful in identifying sister
states for each species for the nine nucleotide number of evolutionary groups (for example, 8 T C is a change
sites that vary among the sequences. changes. that is unique to species C) or they may
be synapomorphies, shared between
descendant taxa (for example, the C T
change at 5 pairs species B and A).

Distance
reconstruction
DNA changes: There are 2
differences between A + B
so we assume 1 per branch.

b. Phylogenetic reconstruction based on distance

A B 1 We pair
1 1 the closest
Number of Taxon OG C B A
taxa.
differences between
pairs of taxa A B C OG
Distance
A - 2 4 6
C A B 2 We add reconstruction
2 1 1 the next
B 2 - 4 6 1 closest.
Taxon
C 4 4 - 6

OG 6 6 6 -
OG C B A 3 We add The total distance between two
3 2 1 1 the OG. taxa is the sum of the differences
1 along all the branches that
1 connect the taxa.
Counting up the number of sequence
differences between individuals gives us a
distance matrix. The number of differences
between any one taxon and the others can
be read across any row or down any column.

that the molecular clock is based on the observation of constant than 100 billion observations (mostly nucleotides) collected
accumulation of genetic divergence through time. Fig. 23.9 shows under more than 430,000 taxonomic names. A growing internet
a simple DNA sequence dataset that we can analyze either on the resource is the Encyclopedia of Life, which is gathering additional
basis of synapomorphies (Fig. 23.9a) or on the basis of distance (Fig. biological information about species, including ecology, geographic
23.9b). Note that both give the same result in this case. distributions, photographs, and sounds in pages for individual
Molecular data are often combined with morphological data, species that are easy to navigate. Another web resource, the Tree of
and each can also serve as an independent assessment of the Life, provides information on phylogenetic trees for many groups of
other. Not surprisingly, results from analyses of each kind of data organisms.
are commonly compatible, at least for plants and animals rich in
morphological characters. Phylogenetic trees can help solve practical problems.
The single largest library of taxonomic information is The sequence of changes on a tree from its root to its tips
GenBank, the National Institutes of Health’s genetic data storage documents evolutionary changes that have accumulated through
facility. As of this writing, GenBank gives users access to more time. Trees suggest which groups are older than others, and which
H
4 7O
3W DT O
SEC I O NW
2 2 .E
1 K
THNE O W
BIO LO?
G I C A L S P E C I E S CO N C E P T

FIG. 23.10

Did an HIV-positive dentist spread the AIDS

virus to his patients?
BACKGROUND In the late 1980s, several patients of a Florida dentist contracted AIDS. Molecular analysis showed that the dentist was
HIV-positive.

HYPOTHESIS It was hypothesized that the patients acquired HIV during dental procedures carried out by the infected dentist.

METHOD Researchers obtained two HIV samples each (denoted 1 and

2 in the figure) from several people, including the dentist (Dentist 1 and Dentist-1
Dentist 2), several of his patients (Patients A through G), and other HIV- Patient B-1
positive individuals chosen at random from the local population (LP). In Patient B-2
addition, a strain of HIV from Africa (HIVELI) was included in the analysis.
Patient A-1
RESULTS Biologists constructed a phylogeny based on the nucleotide Patient E-1
sequence of a rapidly evolving gene in the genome of HIV. Because the Patient E-2
gene evolves so quickly, its mutations preserve a record of evolutionary Dentist-2
relatedness on a very fine scale. HIV in some of the infected patients—
Patient C-1
patients A, B, C, E, and G—were more similar to the dentist’s HIV than
Patient C-2
they were to samples from other infected individuals. Some patients’
sequences, however, did not cluster with the dentist’s, suggesting that Patient A-2

these patients, D and F, had acquired their HIV infections from other Patient G-1
sources. Patient G-2

CONCLUSION HIV phylogeny makes it highly likely that the dentist LP02-1

infected several of his patients. The details of how the patients were LP03-1
infected remain unknown, but rigidly observed safety practices make it LP02-2
unlikely that such a tragedy could occur again. Patient D-1

FOLLOW-UP WORK Phylogenies based on molecular sequence characters Patient D-2

are now routinely used to study the origin and spread of infectious diseases, LP03-2
such as swine flu and Ebola. Patient F-1
SOURCE Hillis, D. M., J. P. Huelsenbeck, and C. W. Cunningham. 1994. “Application and Accuracy of Patient F-2
Molecular Phylogenies.” Science 264: 671–677.
HIVELI

traits came first and which followed later. Proper phylogenetic Phylogenetics solved a famous case in which an HIV-positive
placement thus reveals a great deal about evolutionary history, dentist in Florida was accused of infecting his patients (Fig. 23.10).
and it can have practical consequences as well. For example, HIV nucleotide sequences evolve so rapidly that biologists can
oomycetes, microorganisms responsible for potato blight and build phylogenetic trees that trace the spread of specific strains
other important diseases of food crops, were long thought to be from one individual to the next. Phylogenetic study of HIV present
fungi because they look like some fungal species. The discovery, in samples from several infected patients, the dentist, and other
using molecular characters, that oomycetes belong to a very individuals provided evidence that the dentist had, indeed, infected
different group of eukaryotic organisms, has opened up new his patients.
possibilities for understanding and controlling these plant Similarly, phylogenetic studies of influenza virus strains
pathogens. Similarly, in 2006, researchers used DNA sequences to show their origins and subsequent movements among geographic
identify the Malaysian parent population of a species of butterfly regions and individual patients. Today, there is a growing effort to
called lime swallowtails that had become an invasive species in use specific DNA sequences as a kind of fingerprint or barcode for
the Dominican Republic, pinpointing the source populations from tracking biological material. Such information could quickly identify
which natural enemies of this pest can be sought. samples of shipments of meat as being from endangered species, or

473
474 SECTION 23.3 T H E F O S S I L R E CO R D

track newly emerging pests. The Consortium for the Barcode of Life The evolutionary relationship between birds and crocodiles
has already accumulated species-specific DNA barcodes for more highlights a second kind of information provided by fossils. Not
than 100,000 species. Phylogenetic evidence provides a powerful only do fossils record past life, they also provide our only record of
tool for evolutionary analysis and is useful across timescales ranging extinct species. The phylogeny in Fig. 23.7 contains a great deal of
from months to the entire history of life, from the rise of epidemics information, but it is silent about dinosaurs. Fossils demonstrate
to the origins of metabolic diversity. that dinosaurs once roamed Earth, and details of skeletal structure
place birds among the dinosaurs in the vertebrate tree. Indeed,
some remarkable fossils from China show that the dinosaurs most
23.3 THE FOSSIL RECORD closely related to birds had feathers (Fig. 23.11).
A third, and also unique, contribution of fossils is that they
Phylogenies based on living organisms provide hypotheses place evolutionary events in the context of Earth’s dynamic
about evolutionary history. Branches toward the root of the environmental history. Again, dinosaurs illustrate the point. As
tree occurred earlier than those near the tips, and characters discussed in Chapter 1, geologic evidence from several continents
change and accumulate along the path from the root to the suggests that a large meteorite triggered drastic changes in the
tips. Fossils provide direct documentation of ancient life, and global environment 66 million years ago, leading to the extinction
so, in combination, fossils and phylogenies provide strong of dinosaurs (other than birds). In fact, at five times in the past,
complementary insights into evolutionary history. large environmental disturbances sharply decreased Earth’s
biological diversity. These events, called mass extinctions, have
Fossils provide unique information. played a major role in shaping the course of evolution.
Fossils can and do provide evidence for phylogenetic hypotheses,
showing, for example, that groups that branch early in phylogenies Fossils provide a selective record of past life.
appear early in the geologic record. But the fossil record does more Fossils are the remains of once-living organisms, preserved
than this. First, fossils enable us to calibrate phylogenies in terms through time in sedimentary rocks. If we wish to use fossils to
of time. It is one thing to infer that mammals diverged from the complement phylogenies based on modern organisms, we must
common ancestor of birds, crocodiles, turtles, and lizards and snakes understand how fossils form and how the processes of formation
before crocodiles and birds diverged from a common ancestor (see govern what is and is not preserved.
Fig. 23.7), but another matter to state that birds and crocodiles For all its merits, the fossil record should not be thought of as
diverged from each other about 220 million years ago, whereas a complete dictionary of everything that ever walked, crawled, or
the group represented today by mammals branched from other swam across our planet’s surface. Fossilization requires burial, as
vertebrates about 100 million years earlier. As we saw in Chapter 21, when a clam dies on the seafloor and is quickly covered by sand,
estimates of divergence time can be made using molecular sequence or a leaf falls to the forest floor and ensuing floods cover it in mud.
data, but all such estimates must be calibrated using fossils. Through time, accumulating sediments harden into sedimentary

FIG. 23.11 Microraptor gui, a remarkable fossil discovered in approximately 125-million-year-old rocks from China. The structure of its
skeleton identifies M. gui as a dinosaur, yet it had feathers on its arms, tail, and legs. Source: Nature/Xing Xu/Getty Images.
 CHAPTER 23 E VO LU T I O N A RY PAT T E R N S : P H Y LO G E N Y A N D F O S S I L S 475

will be represented in the fossil record.

FIG. 23.12 The Grand Canyon. Erosion has exposed layers of sedimentary rock that record Fortunately, for species that make
Earth history. Source: National Park Service. mineralized skeletons and live on the
shallow seafloor, the fossil record is very
good, preserving a detailed history of
White beds contain fossilized evolution through time.
tracks of vertebrate animals
that lived about 260 million Organisms that lack hard parts
years ago. can leave a fossil record in two other
distinctive ways. Many animals leave
tracks and trails as they move about
or burrow into sediments. These trace
Limestone accumulated in
the oceans about 335
fossils, from dinosaur tracks to the
million years ago, trapping feeding trails of snails and trilobites,
marine animals. preserve a record of both anatomy and
behavior (Fig. 23.13).
Organisms can also contribute
molecular fossils to the rocks. Most
Slopes made of mud laid down biomolecules decay quickly after death.
in a shallow sea about 500 Proteins and DNA, for example, generally
million years ago contain fossils
of early arthropods called break down before they can be preserved,
trilobites. although, remarkably, a sizable fraction of
the Neanderthal genome has been pieced
together from DNA in 40,000-year-old

rocks such as those exposed so dramatically in the walls of the

Grand Canyon (Fig. 23.12). If they are not buried, the remains FIG. 23.13 Trace fossils. These footprints in 150-million-year-old
of organisms are eventually recycled by biological and physical rocks record both the structure and behavior of the
processes, and no fossil forms. In general, the fossil record of dinosaurs that made them. Source: José Antonio Hernaiz/
marine life is more completely sampled than that for land-dwelling age fotostock.

creatures because marine habitats are more likely than those on

land to be places where sediments accumulate and become rock.
Thus, trees and elks living high in the Rocky Mountains have a
low probability of fossilization, whereas clams and corals on the
shallow seafloor are commonly buried and become fossils.
Biological factors contribute to the incompleteness of the fossil
record. Most fossils preserve the hard parts of organisms, those
features that resist decay after death. For animals, this usually
means mineralized skeletons. Clams and snails that secrete shells
of calcium carbonate have excellent fossil records. More than 80%
of the clam species found today along California’s coast also occur
as fossils in sediments deposited during the past million years. In
contrast, nematodes, tiny worms that may be the most abundant
animals on Earth, have no mineralized skeletons and almost no
fossil record. The wood and pollen of plants, which are made in part
of decay-resistant organic compounds, enter the fossil record far
more commonly than do flowers. And, among unicellular organisms,
the skeleton-forming diatoms, radiolarians, and foraminiferans
have exceptionally good fossil records, whereas most amoebas are
unrepresented.
Together, then, the properties of organisms (do they make
skeletons or other features that resist decay after death?) and
environment (did the organisms live in a place where burial
was likely?) determine the probability that an ancient species
476 SECTION 23.3 T H E F O S S I L R E CO R D

bones. Other molecules, especially lipids

like cholesterol, are more resistant to FIG. 23.14 A Burgess Shale fossil. This fossil is Opabinia regalis, an extinct early relative of
decomposition. Sterols, bacterial lipids, and the arthropods, from the 505-million-year-old Burgess Shale. Source: Courtesy of
some pigment molecules can accumulate Smithsonian Institution. Photo by Chip Clark.
in sedimentary rocks, documenting
organisms that rarely form conventional
fossils, especially bacteria and single-celled
eukaryotes.
Rarely, unusual conditions preserve
fossils of unexpected quality, including
animals without shells, delicate flowers or
mushrooms, fragile seaweeds or bacteria,
even the embryos of plants and animals.
For example, 505 million years ago, during
the Cambrian Period, a sedimentary
rock formation called the Burgess Shale
accumulated on a relatively deep seafloor
covering what is now British Columbia.
Waters just above the basin floor contained
little or no oxygen, so that when mud swept
into the basin, entombed animals were sealed off from scavengers, fur and color patterning (Fig. 23.15). Plants were also beautifully
disruptive burrowing activity, and even bacterial preserved, as were insects, some with a striking iridescence still
decay. For this reason, Burgess rocks preserve a remarkable intact. Messel rocks provide a truly outstanding snapshot of life on
sampling of marine life during the initial diversification of land as the age of mammals began.
animals (Fig. 23.14). In general, the fossil record preserves some aspects of biological
The Messel Shale formed more recently—about 50 million history well and others poorly. Fossils provide a good sense of how
years ago—in a lake in what is now Germany. Release of toxic the forms, functions, and diversity of skeletonized animals have
gases from deep within the Messel lake suffocated local animals, changed over the past 500 million years. The same is true for land
and their carcasses settled into oxygen-poor muds on the lake plants and unicellular organisms that form mineralized skeletons.
floor. Fish, birds, mammals, and reptiles are preserved as complete These fossils shed light on major patterns of morphological
and articulated skeletons, and mammals retain impressions of evolution and diversity change through time; their geographic
distributions record the movements of
continents over millions of years; and the
FIG. 23.15 A Messel Shale fossil. Even the furry tail of this extinct squirrel-like mammal is radiations and extinctions they document
preserved in this remarkable 50-million-year-old fossil. show how life responds to environmental
change, both gradual and catastrophic.

Geological data indicate the age and

environmental setting of fossils.
How do we know the age of a fossil?
Beginning in the nineteenth century,
geologists recognized that groups of fossils
change systematically from the bottom of
a sedimentary rock formation to its top. As
more of Earth’s surface was mapped and
studied, it became clear that certain fossils
always occur in layers that lie beneath (and
so are older than) layers that contain other
species. From these patterns, geologists
concluded that fossils mark time in Earth
history. At first, geologists didn’t know why
fossils changed from one bed to the next, but
after Darwin the reason became apparent:
 CHAPTER 23 E VO LU T I O N A RY PAT T E R N S : P H Y LO G E N Y A N D F O S S I L S 477

Fossils record the evolution of life on Earth. They eventually possible with the discovery of radioactive decay. In Chapter 2, we
mapped out the geologic timescale, the series of time divisions discussed isotopes, variants of an element that differ from one
that mark Earth’s long history (Fig. 23.16). another in the number of neutrons they contain. Many isotopes
The layers of fossils in sedimentary rocks can tell us that are unstable and spontaneously break down to form other, more
some rocks are older than others, but they cannot by themselves stable isotopes. In the laboratory, scientists can measure how fast
provide an absolute age. Calibration of the timescale became unstable isotopes decay. Then, by measuring the amounts of the

FIG. 23.16 The geologic timescale, showing major events in the history of life on
Earth. Sources: (top, left to right) Scott Orr/iStockphoto; Hans Steur, The Netherlands;
T. Daeschler/VIREO; Reconstruction artwork: Mark A. Klingler/Carnegie Museum
of Natural History, from the cover of Science, 25 MAY 2001 VOL 292, ISSUE 5521,
Reprinted with permission from AAAS; dimair/Shutterstock; DEA/G. Cigolini/Getty
Images; (bottom, left to right) Eye of Science/Science Source; Andrew Knoll, Harvard
University; Antonio Guillén, Proyecto Agua, Spain; Andrew Knoll, Harvard University.

Age of the dinosaurs

First land
Marine animals vertebrates
diversify. First land First mammals
plant fossils
Homo sapiens
Carboniferous

Quaternary
Paleogene

Neogene
Silurian

Period
Cambrian Ordovician Devonian Permian Triassic Jurassic Cretaceous

Eon Era
Paleozoic Mesozoic Cenozoic

Phanerozoic
541 485 443 419 359 299 252 201 145 66 23 2.5 Present
Millions of years ago

Eon

Hadean Archean Proterozoic Phanerozoic

4567 4000 2500 541 Present

Millions of years ago

Major diversification
Formation First First eukaryotic of eukaryotic
of Earth evidence of life First evidence microfossils microorganisms First animal fossils
of an oxygen-rich
environment
478 SECTION 23.3 T H E F O S S I L R E CO R D

unstable isotope and its stable daughter inside a mineral, they

FIG. 23.17 14
C decay. Scientists can determine the age of relatively can determine when the mineral formed.
young materials such as wood and bone from the Archaeologists commonly use the radioactive decay of the
amount of 14C they contain. isotope carbon-14, or 14C, to date wood and bone, a process
called radiometric dating. As shown in Fig. 23.17, cosmic rays
Cosmic continually generate 14C in the atmosphere, much of which
radiation is incorporated into atmospheric carbon dioxide. Through
photosynthesis, carbon dioxide that contains 14C is incorporated
Neutron
into wood, and animals incorporate small amounts of 14C into
their tissues when they eat plant material. After the organism’s
In the atmosphere, death, the unstable 14C in these tissues begins to break down,
neutrons generated losing an electron to form 14N, a stable isotope of nitrogen.
by cosmic radiation
can collide with 14N Laboratory measurements indicate that half of the 14C in a given
+
+ ++ + +
+
to create 14C. sample will decay to nitrogen in 5730 years, a period called its
+ + + +
+ +
half-life (Fig. 23.17). Armed with this information, scientists
+ +
+

Neutron Nitrogen-14 Carbon-14 Proton can measure the amount of 14C in an archaeological sample
and, by comparing it to the amount of 14C in a sample of known
age—annual rings in trees, for example, or yearly growth of coral
14C that ends up in skeletons—determine the age of the sample.
CO2 carbon dioxide can Because its half-life is so short (by geological standards),
be incorporated in
plants through
14
C is useful only in dating materials younger than 50,000 to
photosynthesis.14C 60,000 years. Beyond that, there is too little 14C left to measure
is next incorporated
accurately. Older geological materials are commonly dated
into animals when
they eat the plants. using the radioactive decay of uranium (U) to lead (Pb): 238U,
incorporated in trace amounts into the minerals of volcanic
rocks, breaks down to 206Pb with a half-life of 4.47 billion
years; 235U decays to 207Pb with a half-life of 704 million years.
After the plant or Calibration of the geologic timescale is based mostly on the ages
animal dies and
is buried, its 14C
of volcanic ash interbedded with sedimentary rocks that contain
decays to the key fossils, as well as volcanic rocks that intrude into (and so are
more stable 14N. younger than) layers of rock containing fossils. In turn, the ages
of fossils provide calibration points for phylogenies.
Half of the 14C in
a fossil turns to The sedimentary rocks that contain fossils also preserve,
14N in 5730 encrypted in their physical features and chemical composition,
years. In another
information about the environment in which they formed.
5730 years, half
of the remaining Sandstone beds, for example, may have rippled surfaces, like
14C decays to
the ripples produced by currents that we see today in the sand
14N, and so on.
of a seashore or lake margin. Pyrite (FeS2), or fool’s gold, forms
+ + when H2S generated by anaerobic bacteria reacts with iron. As
+ + + ++
+ e-
+
+
+ + + + these conditions generally occur where oxygen is absent, pyrite
enrichment in ancient sedimentary rocks can signal oxygen
Carbon-14 Nitrogen-14 Electron
depletion.
100
Percent carbon-14 remaining

We might think our moment in geologic time is

representative of Earth as it has always existed, but nothing
75
could be further from the truth. In the location and sizes of its
continents, ocean chemistry, and atmospheric composition,
50
the Earth we experience is unlike any previous state of the
planet. Today, for example, the continents are distributed
25
widely over the planet’s surface, but 290 million years ago they
were clustered in a supercontinent called Pangaea (Fig. 23.18).
0 5730 11,460 17,190 22,920 28,650 Oxygen gas permeates most surface environments of Earth
Years elapsed today, but 3 billion years ago, there was no O2 anywhere. And,
 CHAPTER 23 E VO LU T I O N A RY PAT T E R N S : P H Y LO G E N Y A N D F O S S I L S 479

FIG. 23.18 Pangaea, 290 million years ago. Plate tectonics has shaped and reshaped Earth’s geography through time. The white area near the
South Pole is glacial ice. Source: © Ron Blakey and Colorado Plateau Geosystems, Inc.

just 20,000 years ago, 2 km of glacial ice stood where Boston lies its braincase, and, especially, its winglike forearms—are distinctly
today. Sedimentary rocks record the changing state of Earth’s birdlike. Spectacularly, the fossils preserve evidence of feathers.
surface over billions of years and show that life and environment Archaeopteryx clearly suggests a close relationship between birds
have changed together through time, each influencing the other. and dinosaurs, and phylogenetic reconstructions that include
information from fossils show that many of the characters
Fossils can contain unique combinations of characters. found today in birds accumulated through time in their dinosaur
Phylogenies hypothesize impressive morphological and ancestors. As noted earlier, even feathers first evolved in dinosaurs
physiological shifts through time—amphibians from fish, for (Fig. 23.19).
example, or land plants from green algae. Do fossils capture a Tiktaalik roseae and other skeletons in rocks deposited 375 to
record of these transitions as they took place? 362 million years ago record an earlier but equally fundamental
Let’s begin with an example introduced earlier in this chapter. transition: the colonization of land by vertebrates. Phylogenies
Phylogenies based on living organisms generally place birds as the show that all land vertebrates, from amphibians to mammals, are
sister group to crocodiles and alligators, but birds and crocodiles descended from fish. As seen in Fig. 23.20, Tiktaalik had fins, gills,
are decidedly different from each other in structure—birds and scales like other fish of its day, but its skull was flattened,
have wings, feathers, toothless bills, and a number of other more like that of a crocodile than a fish, and it had a functional
skeletal features distinct from those of crocodiles. In 1861, just neck and ribs that could support its body—features today found
two years after publication of On the Origin of Species, German only in tetrapods. Along with other fossils, Tiktaalik captures
quarry workers discovered a remarkable fossil that remains key moments in the evolutionary transition from water to land,
paleontology’s most famous example of a transitional form. confirming the predictions of phylogeny.
Archaeopteryx lithographica, now known from 11 specimens
splayed for all time in fine-grained limestone, lived 150 million j Quick Check 4 You have just found a novel vertebrate skeleton
years ago. Its skeleton shares many characters with dromaeosaurs, in 200-million-year-old rocks. How would you integrate this new
a group of small, agile dinosaurs, but several features—its pelvis, fossil species into the phylogenetic tree depicted in Fig. 23.7?
480 SECTION 23.3 T H E F O S S I L R E CO R D

FIG. 23.19 Dinosaurs and birds. A number of dinosaur fossils link birds phylogenetically to their closest living relatives, the crocodiles. The
fossil at the bottom is Archaeopteryx. Data from C. Zimmer, 2009, The Tangled Bank, Greenwood Village, CO: Roberts and Company; (photo) Jason Edwards/
Getty Images.

Crocodiles

Ornithischian
dinosaurs

Eoraptor

Coelophysoids

Allosaurids

Four digits
in hands Compsognathids

Hollow cylindrical feathers Tyrannosaurids

Tufted feathers Oviraptorosaurs

Feathers closed Dromeosaurids

and asymmetrical

Archaeopteryx
Long arms

Living birds
Toothless beak, fused wing
digits, short feathered tail

240 220 200 180 160 140 120 100 80 60 40

Black line segments
Millions of years ago denote first and last
appearance based
on fossils.
4 8 1H O W
SEC T I OD
NO2 2 . 1WTE
H EK
B IN OGW
O LO I C A?
L S P E C I E S CO N C E P T CHAPTER 23 481

FIG. 23.20 Rare mass extinctions have altered the

course of evolution.
Can fossils bridge the evolutionary Beginning in the 1970s, American paleontologist
Jack Sepkoski scoured the paleontological literature,
gap between fish and tetrapod recording the first and last appearances in the
geologic record for every genus of marine animals
vertebrates? he could find. The results of this monumental
effort are shown in Fig. 23.21. Sepkoski’s diagram
BACKGROUND Phylogenies based on both morphological and molecular shows that animal diversity has increased over the
characters indicate that fish are the closest relatives of four-legged land vertebrates. past 540 million years: The biological diversity of
animals in today’s oceans may well be higher than
HYPOTHESIS Land vertebrates evolved from fish by modifications of the skeleton
it has ever been in the past. But it is also clear that
and internal organs that made it possible for them to live on land.
animal evolution is not simply a history of gradual
OBSERVATION Fossil skeletons 390 to 360 million years old show a mix of accumulation.
features seen in living fish and amphibians. Older fossils have fins, fishlike heads, Repeatedly during the past 500 million years,
and gills, and younger fossils have weight-bearing legs, skulls with jaws able to animal diversity in the oceans dropped both rapidly
grab prey, and ribs that help ventilate lungs. Paleontologists predicted that key and substantially, and extinctions also occurred on
intermediate fossils would be preserved in 380–370-million-year-old rocks. In land. Known as mass extinctions, these events
2004, Edward Daeschler, Neil Shubin, and Farish Jenkins discovered Tiktaalik, eliminated ecologically important taxa and thereby
a remarkable fossil that has fins, scales, and gills like fish, but wrist bones and provided evolutionary opportunities for the
fingers, an amphibian-like skull, and a true neck (which fish lack). survivors. In Chapter 22, we explored how species
often radiate on islands where there is little or
Fish Tetrapods no competition or predation. For survivors, mass
extinctions provided ecological opportunities on a
grand scale.
The best-known mass extinction occurred
Eusthenopteron Panderichthys Tiktaalik Acanthostega Ichthyostega 66 million years ago, at the end of the Cretaceous
Period (Fig. 23.21). On land, dinosaurs disappeared
abruptly, following more than 150 million years of
dominance in terrestrial ecosystems. In the oceans,
ammonites, cephalopod mollusks that had long
390 380 370 360 been abundant predators, also became extinct,
Millions of years ago and most skeleton-forming microorganisms in the
oceans disappeared as well. As discussed in Chapter
1, a large body of geologic evidence supports
the hypothesis that this biological catastrophe
was caused by the impact of a giant meteorite.
By eliminating dinosaurs and much more of the
Photo source: Shubin Lab/

biological diversity that had built up over millions

University of Chicago.

of years on land, the mass extinction at the end of

the Cretaceous Period also had the closely related
effect of generating new evolutionary possibilities
for terrestrial animals that survived the event,
including mammals.
Before dinosaurs ever walked on Earth, the
CONCLUSION Fossils confirm the phylogenetic prediction that tetrapod
greatest of all mass extinctions occurred 252 million
vertebrates evolved from fish by the developmental modification of limbs, skulls,
years ago, at the end of the Permian Period, when
and other features.
environmental catastrophe eliminated half of all
FOLLOW-UP WORK Research into the genetics of vertebrate development families in the oceans and about 80% of all genera.
shows that the limbs of fish and amphibians are shaped by similar patterns of gene Estimates of marine species loss run as high as
expression, providing further support for the phylogenetic connection between the 90%. Geologists have hypothesized that this mass
two groups. extinction resulted from the catastrophic effects
SOURCE Daeschler, E. B., N. H. Shubin, and F. A. Jenkins, Jr. 2006. “A Devonian Tetrapod-like Fish and the
Evolution of the Tetrapod Body Plan.” Nature 440:757–763.
 482 SECTION 23.4 CO M PA R I N G E VO L U T I O N ’ S T W O G R E AT PAT T E R N S

When you stroll through a zoo or snorkel above a coral reef,

FIG. 23.21 Mass extinctions. Though they eliminate much of the you are not simply seeing the products of natural selection played
life found on Earth at the time, mass extinctions allow out over Earth history. You are encountering the descendants of
new groups to proliferate and diversify. Source: Sepkoski’s Earth’s biological survivors. Current biological diversity reflects
Online Genus Database. http://strata.geology.wisc.edu/jack/. the interplay through time of natural selection and rare massive
Data from J. J. Sepkoski, Jr., 2002, “A Compendium of Fossil Marine perturbations to ecosystems on land and in the sea. The fossil
Animal Genera,” David Jablonski and Michael Foote (eds.), Bulletin of record provides a silent witness to our planet’s long and complex
American Paleontology 363:1–560.
Tri ian us
evolutionary history.
rm fero

s
De rian n

og ne
Pa eou
Or rian

Ca ian
u ia

e
23.4 COMPARING EVOLUTION’S
Ne ge
en
i
Sil vic

sic
on

Ju sic
n

c
mb

leo
eta
vo
do

ras
rb

as

TWO GREAT PATTERNS

Ca

Pe

Cr

2500
Number of animal genera

The diversity of life we see today is the result of evolutionary

The mass extinction
2000 at the end of the processes playing out over geologic time. Evolutionary process can
Cretaceous Period be studied by experiment, both in the field and in the laboratory,
1500 eliminated
dinosaurs (except but evolutionary history is another matter. There is no experiment
1000 birds) and made we can do to determine why the dinosaurs became extinct—we
way for the age of cannot rerun the events of 66 million years ago, this time without
500 mammals.
the meteorite impact. The history of life must be reconstructed
0 from evolution’s two great patterns: the nested similarity observed
400 200 Present
Millions of years ago in the forms and molecular sequences of living organisms, and the
direct historical archive of the fossil record.
Each significant drop The most devastating mass
in the number of extinction occurred 252
genera represents a million years ago, at the end Phylogeny and fossils complement each other.
mass extinction. of the Permian Period. The great advantage of reconstructing evolutionary history from
living organisms is that we can use a full range of features—
skeletal morphology, cell structure, DNA sequence—to generate
phylogenetic hypotheses. The disadvantage of using comparative
of massive volcanic eruptions. At the end of the Permian Period, biology is that we lack evidence of extinct species, the time
most continents were gathered into the supercontinent Pangaea dimension, and the environmental context. This, of course,
(see Fig. 23.18), and a huge ocean covered more than half of the is where the fossil record comes into play. Fossil evidence has
Earth. Levels of oxygen in the deep waters of this ocean were low, strengths and limitations that complement the evolutionary
the result of sluggish circulation and warm seawater temperatures. information in the living organisms.
Then, a massive outpouring of ash and lava—a million times larger We can use phylogenetic methods based on DNA sequences
than any volcanic eruption experienced by humans—erupted to infer that birds and crocodiles are closely related, but only
across what is now Siberia. Enormous emissions of carbon dioxide fossils can show that the evolutionary link between birds and
and methane from the volcanoes caused global warming (so even crocodiles runs through dinosaurs. And only the geologic record
less oxygen reached deep oceans) and ocean acidification (making can show that mass extinction removed the dinosaurs, paving
it difficult for animals and algae to secrete calcium carbonate the way for the emergence of modern mammals. Paleontologists
skeletons). and biologists work together to understand evolutionary history.
The three-way insult of lack of oxygen, ocean acidification, Biology provides a functional and phylogenetic framework for
and global warming doomed many species on land and in the seas. the interpretation of fossils, and fossils provide a record of life’s
Seascapes dominated for 200 million years by corals and shelled history in the context of continual planetary change.
invertebrates called brachiopods disappeared. As ecosystems
recovered from this mass extinction, they came to be dominated Agreement between phylogenies and the fossil record
by new groups descended from survivors of the extinction. provides strong evidence of evolution.
Bivalves and gastropods diversified, new groups of arthropods Phylogenies based on morphological or molecular comparisons
radiated, including the ancestors of the crabs and shrimps we of living organisms make hypotheses about the timing of
see today, and surviving sea anemones evolved a new capacity to evolutionary changes through Earth history. We humans are
make skeletons of calcium carbonate, resulting in the corals that a case in point. As we discuss in Chapter 24, comparisons of
build modern reefs. In short, mass extinction reset the course of DNA sequences suggest that chimpanzees are our closest living
evolution, much as it did and the end of the Cretaceous Period. relatives. This is hardly surprising, as simple observation shows
 CHAPTER 23 E VO LU T I O N A RY PAT T E R N S : P H Y LO G E N Y A N D F O S S I L S 483

that chimpanzees and humans share many features. However,

among other differences, chimpanzees have smaller stature, FIG. 23.22 Phylogeny and fossils of vertebrates. The branching
smaller brains, long arms that facilitate knuckle-walking (the arms order of the phylogeny corresponds to the order of
help support the body as the chimpanzee moves forward), a more appearance of each group in the fossil record.
prominent snout, and larger teeth. Fish
How did these differences accumulate over the 7 million years
or so since the human and chimpanzee lineages split? Fossils
painstakingly unearthed over the past century show, for example, Amphibians
that the ability to walk upright evolved before the large human
brain. Lucy, a famous specimen of Australopithecus afarensis dating
Sauropsids
to about 3.2 million years ago, was fully bipedal but had a brain the
size of a chimpanzee’s. Fossils like Lucy that are a mix of ancestral
(brain size) and derived (bipedalism) characters provide strong Mammals
support for our phylogenetic tree that shows that humans and
chimpanzees are sister taxa.
The agreement of comparative biology and the fossil record
First fossil First fossil First fossil
can be seen at all scales of observation. Humans form one tip of fish amniote mammal
a larger branch that contains all members of the primate family. First fossil
Primates, in turn, are nested within a larger branch occupied tetrapod
by mammals, and mammals nest within a still larger branch Past Present

550

500

450

400

350

300

250

200
containing all vertebrate animals, which include fish. This
arrangement predicts that the earliest fossil fish should be older Millions of years ago
than the earliest fossil mammals, the earliest fossil mammals older
than the earliest primates, and the earliest primates older than
the earliest humans. This is precisely what the fossil record shows years ago, the earliest tetrapod vertebrates about 360 million
(Fig. 23.22). years ago, the earliest mammals 210 million years ago, the earliest
The agreement between fossils and phylogenies can be seen primates perhaps 55 million years ago, the earliest fossils of our
again and again when we examine different branches of the tree of own species a mere 200,000 years ago. Similarly, if we focus on
life or, for that matter, the tree as a whole. All phylogenies indicate photosynthetic organisms, we find a record of photosynthetic
that microorganisms diverged early in evolutionary history, and bacteria beginning at least 3500 million years ago, algae
mammals, flowering plants, and other large complex organisms 1200 million years ago, simple land plants 470 million years ago,
diverged more recently. The tree’s shape implies that diversity has seed plants 370 million years ago, flowering plants about
accumulated through time, beginning with simple organisms and 140 million years ago, and the earliest grasses 70 million years ago.
later adding complex macroscopic forms. In Part 2, we explore the evolutionary history of life in some
The geologic record shows the same pattern. For nearly detail. Here, it is sufficient to draw the key general conclusion: The
3 billion years of Earth history, microorganisms dominate the fact that comparative biology and fossils, two complementary but
fossil record, with the earliest animals appearing about independent approaches to reconstructing the evolutionary past,
600 million years ago, the earliest vertebrate animals 520 million yield the same history is powerful evidence of evolution. •

Core Concepts Summary Sister groups are more closely related to one another than they
are to any other group. page 465
23.1 A phylogenetic tree is a reasoned hypothesis of the A node is a branching point on a tree, and it can be rotated
evolutionary relationships among organisms. without changing evolutionary relationships. page 466
The nested pattern of similarities seen among organisms is a A monophyletic group includes all the descendants of a common
result of descent with modification and can be represented as a ancestor, and it is considered a natural grouping of organisms
phylogenetic tree. page 465 based on shared ancestry. page 466

The order of branches on a phylogenetic tree indicates the A paraphyletic group includes some, but not all, of the
sequence of events in time. page 465 descendants of a common ancestor. page 466
484 SELF-ASSESSMENT

A polyphyletic group includes organisms from distinct groups The extinction at the end of the Cretaceous Period 66 million
based on shared characters, but it does not include a common years ago led to the extinction of the dinosaurs (other than
ancestor. page 466 birds). page 481

Organisms are classified into domain, kingdom, phylum, The extinction at the end of the Permian Period 252 million
class, order, family, genus, and species. page 467 years ago is the largest documented mass extinction in the
history of Earth. page 481
23.2 A phylogenetic tree is built on the basis of shared
derived characters. 23.4 Phylogeny and fossils provide independent and
corroborating evidence of evolution.
Characters, or traits, existing in different states are used to
build phylogenetic trees. page 468 Phylogeny makes use of living organisms, and the fossil record
supplies a record of species that no longer exist, absolute dates,
Homologies are similarities based on shared ancestry,
and environmental context. page 482
while analogies are similarities based on independent
adaptations. page 468 Data from phylogeny and fossils are often in agreement,
providing strong evidence for evolution. page 482
Homologies can be ancestral, unique to a particular group, or
present in some, but not all, of the descendants of a common
ancestor (shared derived characters). page 468
Self-Assessment
Only shared derived characters, or synapomorphies, are
1. Draw a phylogenetic tree of three groups of organisms and
useful in constructing a phylogenetic tree. page 469
explain how a nested pattern of similarity can be seen in the
Molecular data provide a wealth of characters that tree and how it might arise.
complement other types of information in building 2. Distinguish among monophyletic, paraphyletic, and
phylogenetic trees. page 471 polyphyletic groups, and give an example of each.
Phylogenetic trees can be used to understand evolutionary 3. List the levels of classification, from the least inclusive
relationships of organisms and solve practical problems, such (species) to the most inclusive (domain).
as how viruses evolve over time. page 473
4. Define “homology” and “analogy” and describe two traits
that are homologous and two that are analogous.
23.3 The fossil record provides direct evidence of
evolutionary history. 5. Name a type of homology that is useful in building
phylogenetic trees and explain why this kind of homology, and
Fossils are the remains of organisms preserved in sedimentary
not others, is useful.
rocks. page 474
6. Describe three ways that an organism can leave a record
The fossil record is imperfect because fossilization requires in sedimentary rocks and explain why this means that there
burial in sediment, sediments accumulate episodically and are gaps in the fossil record.
discontinuously, and fossils typically preserve only the hard
7. Explain how the fossil record can be used to determine
parts of organisms. page 474
both the relative and the absolute timescales of past events.
Radioactive decay of certain isotopes of elements provides a
8. Describe the significance of Archeopteryx and Tiktaalik.
means of dating rocks. page 477
9. Describe how mass extinctions have shaped the
Archaeopteryx and Tiktaalik are two fossil organisms that ecological landscape.
document, respectively, the bird–dinosaur transition and
the fish–tetrapod transition. page 479
Log in to to check your answers to the Self-
The history of life is characterized by five mass extinctions Assessment questions, and to access additional learning tools.
that changed the course of evolution. page 481
 CHAPTER 24

Human Origins
and Evolution
Core Concepts
24.1 Anatomical, molecular, and
fossil evidence shows that the
human lineage branches off the
great apes tree.
24.2 Phylogenetic analysis of
mitochondrial DNA and the Y
chromosome shows that our
species arose in Africa.
24.3 During the 5–7 million
years since the most recent
common ancestor of humans
and chimpanzees, our
lineage acquired a number of
distinctive features.
24.4 Human history has had an
important impact on patterns
of genetic variation in our
species.
24.5 Culture is a potent force for
change in modern humans.
Brad Wilson/ Getty Images.

485
486 SECTION 24.1 T H E G R E AT A P E S

Charles Darwin carefully avoided discussing the evolution of our molecular analysis, and through the fossil record. In this section,
own species in On the Origin of Species. Instead, he wrote only we use data from all three sources as we apply the standard
that he saw “open fields for far more important researches,” and methods of phylogenetic reconstruction (Chapter 23) to figure
that “[l]ight will be thrown on the origin of man and his history.” out the evolutionary relationships between humans and other
Darwin, an instinctively cautious man, realized that the ideas mammals.
presented in On the Origin of Species were controversial enough
without his adding humans to the mix. He presented his ideas on Comparative anatomy shows that the human lineage
human evolution to the public only when he published The Descent branches off the great apes tree.
of Man 12 years later, in 1871. There are about 400 species of primates, which include
As it turned out, Darwin’s delicate sidestepping of human prosimians (lemurs, bushbabies), monkeys, and apes (Fig. 24.1).
origins had little effect. The initial print run of The Origin sold out All primates share a number of general features that distinguish
on the day of publication, and the public was perfectly capable them from other mammals: nails rather than claws together
of reading between the lines. The Victorians found themselves with a versatile thumb allow objects to be manipulated more
wrestling with the book’s revolutionary message: that humans are dexterously, and eyes on the front of the face instead of the side
a species of ape. allow stereoscopic (three-dimensional) vision.
Darwin’s conclusions remain controversial to this day among Prosimians are thought to represent a separate primate group
the general public, but they are not controversial among scientists. from the one that gave rise to humans. Lemurs, which today
The evidence that humans are descended from a line of apes whose are confined to the island of Madagascar, are thus only distantly
modern-day representatives include gorillas and chimpanzees is related to humans. Monkeys underwent independent bouts of
compelling. We know now that about 5–7 million years ago the evolutionary change in the Americas and in Africa and Eurasia, so
family tree of the great apes split, one branch ultimately giving the family tree is split along geographic lines into New World and
rise to chimpanzees and the other to our species. It is those Old World monkeys. Though both groups have evolved similar
5–7 million years that hold the key to our humanity. It was over habits, there are basic distinctions. There are differences between
this period—brief by evolutionary standards—that the attributes the teeth of the two groups, and in New World monkeys the
that make our species so remarkable arose. This chapter discusses nostrils tend to be widely spaced, whereas in Old World species
what happened over those 5–7 million years and how we came to they are closer together.
be the way we are. One line of Old World monkeys gave rise to the apes, which
lack a tail and show more sophisticated behaviors than other
monkeys. The apes are split into two groups, the lesser and the
24.1 THE GREAT APES great apes (Fig. 24.2). Lesser apes include the fourteen species
of gibbon, all of which are found in Southeast Asia. The great
We can approach the question of our place in the tree of life in apes include orangutans, gorillas, chimpanzees, and humans.
three different ways: through comparative anatomy, through Taxonomists classify all the descendants of a specified common

FIG. 24.1 The primate family tree. This tree shows the evolutionary relationships of prosimians, monkeys, and apes. Sources: (left to right) George
Holton/Science Source; Penelope Dearman/Getty Images; Daily Mail/Rex/Alamy; Patrick Shyu/Getty Images; Yellow Dog Productions/Getty Images.

Prosimians: Bushbabies and lemurs New World monkeys Old World monkeys Apes

Tail loss

Closely placed nostrils

Opposable thumb
Eyes located at front
of face enable better
depth perception.
 CHAPTER 24 H U M A N O R I G I N S A N D E VO L U T I O N 487

FIG. 24.2 The family tree of the lesser and great apes. The apes consist of two major groups, the lesser apes and great apes. The great apes
group includes humans. Sources: (left to right) Zoonar/K. Jorgensen/age fotostock; S Sailer/A Sailer/age fotostock; J & C Sohns/age fotostock; Michael Dick/
Animals Animals–Earth Scenes; FLPA/Jurgen & Christi/age fotostock; Yellow Dog Productions/Getty Images.

Gibbon (14 species in 4 genera) Orangutan Gorilla Bonobo Chimpanzee Human

Delayed puberty
Earlobes

Robust canine teeth

Prosimians: The characters listed do not represent all the evolutionary

Bushbabies New World Old World changes that occurred. Instead, they are a sample of the
and lemurs monkeys monkeys Apes changes that occurred at each stage.

Lesser apes
Great apes

ancestor as belonging to a monophyletic group (Chapter 23). Just how closely are humans and chimpanzees related?
Thus, humans, blue whales, and hedgehogs are all mammals To answer this question, we need to know the timing of the
because all three are descended from the first mammal, the evolutionary split that led to chimpanzees along one fork and to
original common ancestor of all mammals. Because human humans along the other. As we saw in Chapter 21, DNA sequence
ancestry can be traced to the common ancestor of orangutans, differences accumulate between isolated populations or species,
gorillas, and chimpanzees, humans, too, are a member of the and they do so at a more or less constant rate. As a result, the
monophyletic great ape group. extent of sequence difference between two species is a good
indication of the amount of time they have been separate, that is,
Molecular analysis reveals that the human lineage the amount of time since their last common ancestor.
split from the chimpanzee lineage about 5–7 million The first thorough comparison of DNA molecules between
years ago. humans and chimpanzees was carried out before the advent of
Which great ape is most closely related to humans? That is, which DNA sequencing methods by Mary-Claire King and Allan Wilson
is our sister group? Traditional approaches of reconstructing at the University of California at Berkeley (Fig. 24.3). One of
evolutionary history by comparing anatomical features failed to their methods of measuring molecular differences between
determine which of two candidates, gorillas or chimpanzees, is species relied on DNA–DNA hybridization (Chapter 12). Two
the sister group to humans. It was only with the introduction complementary strands of DNA in a double helix can be separated
of molecular methods of assessing evolutionary relationships— by heating the sample. If the two strands are not perfectly
through the comparison of DNA and amino acid sequences from the complementary, as is the case if there is a base-pair mismatch (for
different species—that we had the answer. Our closest relative is example, a G paired with a T rather than with a C), less heat is
the chimpanzee (Fig. 24.2), or, more accurately, the chimpanzees, required to separate the strands.
plural, because there are two closely related chimpanzee species, King and Wilson used this observation to examine the
the smaller of which is often called the bonobo. differences between a human strand and the corresponding
H
4 8O
8W DT O
SEC I O NW
2 2 .E
1 K
THNE O W
BIO LO?
G I C A L S P E C I E S CO N C E P T

chimpanzee strand. They inferred the extent

FIG. 24.3 of DNA sequence divergence from the melting
temperature and made a striking discovery:
How closely related are humans Human and chimpanzee DNA differ in sequence
by just 1%.
and chimpanzees? King and Wilson’s conclusions have proved
robust as ever more powerful techniques have
BACKGROUND Phylogenetic analysis based on anatomical characteristics had been applied to the comparison of human and
established that chimpanzees are closely related to humans. Mary-Claire King chimpanzee DNA. Today, after sequencing
and Allan Wilson used molecular techniques to determine how closely related entire human and chimpanzee genomes and
the two species are. literally counting the differences between the
two sequences, we find that the figure of 1%
HYPOTHESIS Despite marked anatomical and behavioral differences still approximately holds. Genome sequencing
between the two species, the genetic distance between the two is small, studies give us superbly detailed information on
implying a relatively recent common ancestor. the differences and similarities between the two
genomes, revealing regions that are present in
METHOD When two complementary strands of DNA are heated, the
one species but not the other, and which parts
hydrogen bonds pairing the two helices are broken at around 95°C and the
of the genomes have evolved more rapidly than
double helix denatures, or separates (Chapter 12). Two complementary
others.
strands with a few mismatches separate at a temperature slightly lower than
Because the amount of sequence difference is
95°C because fewer hydrogen bonds are holding the helix together. More
correlated with the length of time the two species
mismatches between the two sequences results in an even lower denaturation
have been isolated, we can convert the divergence
temperature. Using hybrid DNA double helices with one strand contributed by
results into an estimate of the timing of the split
each species—humans and chimpanzees, in this case—and determining their
between the human and chimpanzee lineages. That
denaturation temperature, King and Wilson could infer the genetic distance
split occurred about 5–7 million years ago. All the
(the extent of genetic divergence) between the two species.
extraordinary characteristics that set our species
apart from the rest of the natural world—those
Perfect complementarity: Some mismatch: More mismatch:
95°C denaturation 93°C denaturation 91°C denaturation attributes that are ours and ours alone—arose in
just 5–7 million years.
T C C G T T A A G C T C C T T T C A G C T C A T T T C A G C
A GG C A A T T C G A GG C A A T T C G A GG C A A T T T G j Quick Check 1 Did humans evolve from
chimpanzees? Explain.

RESULTS King and Wilson found that human–chimpanzee DNA molecules

The fossil record gives us direct
separated at a temperature approximately 1°C lower than the temperature at
information about our evolutionary
which human–human DNA molecules separate. This difference could be
history.
calibrated on the basis of studies of other species whose genetic distances
Molecular analysis is a powerful tool for comparing
were known from other methods. The DNA of humans differs from that of
species and populations within species. It allows
chimpanzees by about 1%.
us to compare humans and chimpanzees and
CONCLUSION AND INTERPRETATION King and Wilson noted the look at differences among groups of humans or
discrepancy between the extent of genetic divergence (small) and the extent of groups of chimpanzees. However, for a full picture
anatomical and behavioral divergence (large) between humans and chimpanzees. of human evolution, we must turn to fossils
They suggested that one way in which relatively little genetic change could (Chapter 23).
produce extensive phenotypic change is through differences in gene regulation For the first several million years of human
(Chapter 19). A small genetic change in a control region responsible for switching evolution, all the fossils from the human lineage
a gene on and off might have major consequences for the organism. are found in Africa. This is not surprising. Charles
Darwin himself noted that it was likely that the
FOLLOW-UP WORK The sequences of the chimpanzee and human genomes human lineage originated there, as humans’ two
allow us to compare the two sequences directly, and these data confirm King closest relatives, chimpanzees and gorillas, live
and Wilson’s observations. only in Africa. The fossil material varies in quality,
SOURCE King, M.-C., and A. C. Wilson. 1975. “Evolution at Two Levels in Humans and Chimpanzees.”
and a great deal of ingenuity is often required to
Science 188:107–116. reconstruct the appearance and attributes of an

488
 CHAPTER 24 H U M A N O R I G I N S A N D E VO L U T I O N 489

individual from fragmentary fossil material. It’s hard to determine

whether two fossil specimens with slight differences belong FIG. 24.5 Lucy, a specimen of Australopithecus afarensis. Lucy was
to the same or different species. And, of course, interbreeding, fully bipedal, establishing that the ability to walk upright
the criterion that researchers most often use to define a species evolved at least 3.2 million years ago. Source: The Natural
(Chapter 22), cannot readily be applied to fossils. As a result, History Museum, London/The Image Works.

experts often disagree over the details of the human fossil

record—whether or not, for example, a specimen belongs to a
particular species. Regardless, several robust general conclusions
may be drawn.
The many different species, including modern humans, that
have arisen on the human side of the split since the human/
chimpanzee common ancestor are called hominins. Let’s start
with the earliest known hominin, Sahelanthropus tchadensis
(Fig. 24.4). Discovered in Chad in 2002, the skull of S. tchadensis
combines both modern (human) and ancestral features.
S. tchadensis, which has been dated to about 7 million years ago
and has a chimpanzee-sized brain but hominin-type brow ridges,
probably lived shortly after the split between the hominin and
chimpanzee lineages, although this conclusion is controversial
because of difficulties in interpreting whether a fossil is part of
a newly diverged lineage or just another specimen of the ancestral
lineage.
An important early hominin, dating from about 4.4 million
years ago, is a specimen of Ardipithecus ramidus from Ethiopia.
This individual, known as Ardi, was capable of walking upright,
using two legs on the ground but all four limbs in the trees.
An unusually complete early hominin fossil, Lucy, found
in 1974 at Hadar, Ethiopia, represents the next step in the

FIG. 24.4 The skull of Sahelanthropus tchadensis. This skull, which

is about 7 million years old and was found in Chad, has both
human and chimpanzee features. Source: Richard T. Nowitz/
Science Source.

evolution of hominin gait: She was fully bipedal, habitually

walking upright (Fig. 24.5). Her name comes from the Beatles’
song “Lucy in the Sky with Diamonds,” which was playing in the
paleontologists’ field camp when she was unearthed. This fossil
dates from around 3.2 million years ago. Lucy was a member of
the species Australopithecus afarensis and was much smaller than
modern humans, less than 4 feet tall, and had a considerably
smaller brain (even when the difference in body size is taken
into consideration). In many ways, then, Lucy was similar to the
common ancestor of humans and chimpanzees, except that she
was bipedal.
490 SECTION 24.1 T H E G R E AT A P E S

FIG. 24.6 Hominin lineages. Three main lineages, Ardipithecus, Australopithecus, and Homo existed at various and sometimes overlapping times
in history. The Homo lineage led to modern humans, Homo sapiens. Sources: Data from R. G. Klein, 2009, The Human Career, Chicago: University of
Chicago Press, p. 244; Homo floresiensis skull from R. D. Martin, A. M. MacLarnon, J. L. Phillips, L. Dussubieux, P. R. Williams and W. B. Dobyns, 2006, Comment on
“The Brain of LB1, Homo floresiensis,” Science 312:999.

Ancient DNA reveals that

H. neanderthalensis
interbred with H. sapiens Paranthropus is a
genus of heavy-
boned species
that eventually
0 Homo Homo Homo went extinct
Homo
neanderthalensis sapiens floresiensis without leaving
erectus
descendants.
Homo
heidelbergensis
1
Paranthropus Paranthropus
boisei robustus
Homo
Homo ergaster Homo
(or Kenyanthropus)
2 rudolfensis habilis
Australopithecus
Millions of years ago

garhi Paranthropus
Australopithecus
africanus aethiopicus
p
3
Kenyanthropus Lucy Ardi and
platyops Australopithecus Lucy are
afarensis especially
complete
fossils.
4
Australopithecus
anamensis
Ardi
Ardipithecus
ramidus
5
Ardipithecus
kadabba
Sahelanthropus
tchadensis
6
Orrorin Fossil material
tugenensis
Inferred relationships
among forms
7

Remarkably, there were many different hominin species living The earliest hominin fossils found in Asia are about 2 million years
in Africa at the same time. Fig. 24.6 shows the main hominin old, indicating that at least one group of hominins ventured out of
species and the longevity of each in the fossil record, along with Africa then. The individuals that first left Africa were members of a
suggested evolutionary relationships among the species. Note that species that is sometimes called Homo ergaster and sometimes Homo
all hominins have a common ancestor, but not all of these groups erectus. The confusion stems from the controversies that surround the
lead to modern humans, producing instead other branches of the naming of fossil species. Some researchers contend that H. ergaster
hominin tree that ultimately went extinct. is merely an early form of H. erectus. Here, we designate this first
A number of trends can be seen when we look over the entire hominin Homo ergaster and a later descendant species H. erectus (see
record. Body size increased. Most striking is the increase in size Fig. 24.6). The naming details, however, are relatively unimportant.
of the cranium and therefore, by inference, of the brain, as shown What matters is that some hominins first colonized areas beyond
in Fig. 24.7. Africa about 2 million years ago.
 CHAPTER 24 H U M A N O R I G I N S A N D E VO L U T I O N 491

this event was unclear. Can all modern humans, Homo sapiens,
FIG. 24.7 Increase in brain size (as inferred from fossil cranium date their ancestry to early hominins in Africa that migrated
volume) over 3 million years. Data from T. Deacon, “The out and spread around the world about 2 million years ago? Or
Human Brain,” pp. 115–123, in J. Jones, R. Martin, and D. Pilbeam did the groups that left Africa ultimately go extinct, so that
(eds.), 1992, The Cambridge Encyclopedia of Human Evolution, modern humans evolved from a group of hominins in Africa that
Cambridge, England: Cambridge University Press. migrated out much more recently? Molecular studies helped to
answer this question.
Some of the increase in brain size is due to
increase in body size, but brain size increased
in hominins much faster than body size. Studies of mitochondrial DNA reveal that modern
humans evolved in Africa relatively recently.
Homo sapiens
2000 For a long time, it was argued that modern humans derive
from the Homo ergaster (or H. erectus) populations that spread
Homo erectus
Estimated brain volume (mL)

around the world starting from around the time of the early
1500
Homo habilis emigration from Africa 2 million years ago. This idea is called the
multiregional hypothesis of human origins because it implies
Australopithecus that different Homo ergaster populations throughout Africa
1000
and Eurasia evolved in parallel, with some limited gene flow
among them, each producing modern H. sapiens populations. In
Each point
500 denotes a short, racial differences among humans would have evolved over
measured 2 million years in different geographic locations.
individual.
This idea was overturned in 1987 by another study from
0
3 2 1 0 Allan Wilson’s laboratory, which instead suggested that
Millions of years ago modern humans arose much more recently from Homo ergaster
descendants that remained in Africa (sometimes called Homo
heidelbergensis; see Fig 24.6), and are all descended from an
Another hominin species was Homo neanderthalensis, whose African common ancestor dating from around 200,000 years
fossils appear in Europe and the Middle East. Neanderthals ago. This newer idea is the out-of-Africa hypothesis of human
represent a second hominin exodus from Africa dating from origins.
around 600,000 years ago. Thicker boned than us, and with flatter To test these two hypotheses about human origins, Rebecca
heads that contained brains about the same size as, or slightly Cann, a student in Allan Wilson’s laboratory, analyzed DNA
larger than, ours, Neanderthals disappeared around 30,000 sequences to reconstruct the human family tree. Specifically,
years ago. As we will see, genetic analysis suggests that this she studied sequences of a segment of mitochondrial DNA from
group likely interbred with our own group, Homo sapiens, so this people living around the world (Fig. 24.8).
disappearance was perhaps not as complete as the fossil record Mitochondrial DNA (mtDNA) is a small circle of DNA,
suggests. about 17,000 base pairs long, found in every mitochondrion
Another hominin species became extinct only about (Chapter 17). Cann chose to study mtDNA for several reasons.
12,000 years ago. This was H. floresiensis, known popularly as Although a typical cell contains a single nucleus with just two
the Hobbit. H. floresiensis is peculiar. Limited to the Indonesian copies of nuclear DNA, each cell has many mitochondria, each
island of Flores, adults were only just over 3 feet tall. Some have carrying multiple copies of mtDNA, making mtDNA much
suggested that H. floresiensis is not a genuinely distinct species, more abundant than nuclear DNA and therefore easier to
but, rather, is an aberrant H. sapiens. Plenty of morphological extract. More important, however, is its mode of inheritance.
evidence, however, suggests that H. floresiensis is a distinct All your mtDNA is inherited from your mother in the egg she
species derived from an archaic Homo species, probably H. erectus. produces because sperm do not contribute mitochondria to
Mammals often evolve small body size on islands because of the the zygote. This means that there is no opportunity for genetic
limited availability of food. recombination between different mtDNA molecules, so the
only way in which sequence variation can arise is through
mutation. In nuclear DNA, by contrast, differences between
24.2 AFRICAN ORIGINS two sequences can be introduced through both mutation
and recombination. Recombination obscures genealogical
From the wealth of fossil evidence, it’s clear that modern relationships because it mixes segments of DNA with different
humans first evolved in Africa. But for a while the timing of evolutionary histories.
 HOW DO WE KNOW?

FIG. 24.8

When and where did the most recent common ancestor

of all living humans live?
BACKGROUND With the availability of DNA sequence analysis tools, it became possible to investigate human prehistory by
comparing the DNA of living people from different populations.

HYPOTHESIS The multiregional hypothesis of human origins suggests that our most recent common ancestor was living at the time
Homo ergaster populations first left Africa, about 2 million years ago.

METHOD Rebecca Cann compared mitochondrial DNA (mtDNA) sequences from 147 people from around the world. This approach
required a substantial amount of mtDNA from each individual, which she acquired by collecting placentas from women after
childbirth. Instead of sequencing the mtDNA, Cann inferred differences among sequences by digesting the mtDNA with different
restriction enzymes, each of which cuts DNA at a specific sequence (Chapter 12). If the sequence is present, the enzyme cuts. If any
base in the recognition site of the restriction enzyme has changed in an individual, the enzyme does not cut. The resulting fragments
were then separated by gel electrophoresis. By using 12 different enzymes, Cann was able to assay a reasonable proportion of all the
mtDNA sequence variation present in the sample.
Here, we see a sample dataset for four people and a chimpanzee. There are seven varying restriction sites:

Restriction site number

1 2 3 4 5 6 7

mtDNA (Japan)

mtDNA (S. Africa)

mtDNA (E. Africa)

mtDNA (Native America)

mtDNA (Chimpanzee)

Presence of a restriction site at a specified location

ANALYSIS The data were converted into a family tree by using shared derived characters, described in Chapter 23. By mapping
the pattern of changes in this way, the phylogenetic relationships of the mtDNA sequences can be reconstructed. For example, the
derived “cut” state at restriction site 1 shared by the East African, Japanese, and Native American sequences implies that the three
groups had a common ancestor in which the mutation that created the new restriction site occurred.

Mitochondrial DNA sequences offered Cann a clear advantage. time—are African. All non-Africans are branches off the African
With sequence information from the mtDNA of 147 people from tree. The implication is that Homo sapiens evolved in Africa and
around the world, Cann reconstructed the human family tree only afterward did populations migrate out of Africa and become
(Fig. 24.8). The tree contained two major surprises. First, the two established elsewhere. This finding contradicted the expectations
deepest branches—the ones that come off the tree earliest in of the multiregional theory, which predicted that Homo sapiens

492
 Chimpanzee S. Africa E. Africa Japan Native American
1 2 345 67 1 2 345 67 1 2 34 5 67 1 2 345 67 1 2 345 67

Loss of Gain of
6 7
Loss of
3 Gain of
5 Both Japanese and Native
Gain of American sequences share
2 the gain of restriction site
Gain of 5, implying that the site
1 gain occurred in their
common ancestor.
Loss of
4 Most recent common
ancestor of the four human
sequences analyzed.
Ancestral
The chimpanzee is the 1 2 345 67
phylogenetic outlier
because its common
ancestor with the rest
of the group is the
oldest node on the RESULTS AND CONCLUSION A simplified version of Cann’s phylogenetic tree based on mtDNA is
tree. We can thus treat
it as an outgroup and shown below. Modern humans arose relatively recently in Africa, and their most common ancestor lived
assume that its about 200,000 years ago. The multiregional hypothesis is rejected. Also, the data revealed that all non-
sequence represents Africans derive from within the African family tree, implying that H. sapiens left Africa relatively recently.
the ancestral state.
The Cann study gave rise to the out-of-Africa theory of human origins.

Present
Chimpanzee African Non-African African

200,000 years ago

Time

The common
ancestor of all
living humans

Past

FOLLOW-UP WORK This study set the stage for an explosion in genetic studies of human prehistory
that used the same approach of comparing sequences in order to identify migration patterns. Today, with
molecular tools vastly more powerful than those available to Cann, studies of the distribution of genetic
variation are giving us a detailed pattern, even on relatively local scales, of demographic events.
SOURCE Cann, R. L., M. Stoneking, and A. C. Wilson. 1987. “Mitochondrial DNA and Human Evolution.” Nature 325:31–36.

evolved independently in different locations throughout the occur in mtDNA, Cann was able to estimate the time back to the
Old World. common ancestor of all modern humans as about 200,000 years.
Second, the tree is remarkably shallow. That is, even the Subsequent analyses have somewhat refined this estimate, but the
most distantly related modern humans have a relatively recent key message has not changed: The data contradict the multiregional
common ancestor. By calibrating the rate at which mutations hypothesis, which predicted a number closer to 2 million years.

493
494 SECTION 24.2 AFRICAN ORIGINS

j Quick Check 2 What do the multiregional and out-of-Africa

hypotheses predict about the age of the common ancestor of all FIG. 24.9 (a) Homo sapiens and (b) Homo neanderthalensis.
humans living today? Neanderthals disappeared from Europe about 30,000
years ago, about 10,000 years after a group of modern
humans called Cro-Magnon first appeared in Europe.
Studies of the Y chromosome provide independent Sources: Javier Trueba/MSF/Science Source.
evidence for a recent origin of modern humans.
Two hundred thousand years ago is 10 times more recent than a. Cro-Magnon (modern)
2 million years ago, so this change in dating represents a
revolution in our understanding of human prehistory. But surely
it is risky to make such major claims on the basis of a single study.
Maybe there’s something peculiar about mtDNA, and it doesn’t
offer an accurate picture of the human past.
To find independent evidence for a recent origin of modern
humans in Africa, another dataset was required. One such dataset
comes from studies of the Y chromosome, another segment of
human DNA that does not undergo recombination (Chapter 17).
When an approach similar to Cann’s was used to reconstruct the
human family tree using Y chromosome DNA sequences, the b. Neanderthal
result was completely in agreement with the mtDNA result: The
human family is young, and it arose in Africa.

Neanderthals disappear from the fossil record as

modern humans appear, but have contributed to the
modern human gene pool.
If we exclude H. floresiensis, the last of the nonmodern humans
were the Neanderthals (Fig. 24.9), who lived in Europe and
Western Asia until about 30,000 years ago. What happened?
Were they eliminated by the first population of Homo sapiens Neanderthal brains were slightly larger than ours, but their
to arrive in Europe (a group known as Cro-Magnon for the site skulls were shaped differently. The Neanderthal forehead was
much less pronounced than ours, and the skull in general lower.
in France from which specimens were first described)? Or did they
interbreed with the Cro-Magnons, and, if so, does that mean
that Neanderthals should be included among our direct ancestors?
Indeed, if modern humans interbred with Neanderthals and
our definition of species is based on reproductive isolation A new scenario emerged: As populations of our ancestors headed
(Chapter 22), perhaps we should consider H. sapiens and from Africa into the Middle East, they encountered Neanderthals,
H. neanderthalensis as belonging to the same species. and it was then, before modern humans had spread out across the
In 1997, we thought we had a clear answer. Matthias Krings, world, that interbreeding took place (Fig. 24.10b). Remarkably,
working in Svante Pääbo’s lab in Germany, extracted intact stretches ancient DNA analysis has further revealed that early non-African
of mtDNA from 30,000-year-old Neanderthal bone. If Neanderthals Homo sapiens did not interbreed only with Neanderthals. Another
had interbred with our ancestors, we would expect to see evidence group of Eurasian hominins, relatives of Neanderthals known
of their genetic input in the form of Neanderthal mtDNA in the as Denisovans after the Siberian cave in which their remains
modern human gene pool. However, the Neanderthal sequence were found, also contributed genetically to some modern human
was strikingly different from that of modern humans. That we see populations, especially indigenous people from Australasia.
nothing even close to the Neanderthal mtDNA sequence in modern How can we reconcile the two apparently contradictory results
humans strongly suggested that Neanderthals did not interbreed about the contributions of Neanderthals to the modern human
with our ancestors (Fig. 24.10a). gene pool? The mtDNA study indicates there was no genetic input
This conclusion, however, has been reversed following from Neanderthals in modern humans, but the whole-genome
remarkable technological advances that permitted Pääbo and study suggests that in fact there was. One possible solution lies in
others to sequence the entire Neanderthal genome (rather than the difference in patterns of transmission between mtDNA and
just its mtDNA). Careful population genetic analysis revealed that genomic material (Chapter 17). Recall that mtDNA is maternally
our ancestors did interbreed with Neanderthals, and that 1% to 4% inherited because sperm do not contribute mitochondrial material.
of the genome of every non-African is Neanderthal-derived. Your mtDNA, whether you are male or female, is derived solely
 CHAPTER 24 H U M A N O R I G I N S A N D E VO L U T I O N 495

FIG. 24.10 Evidence for and against early interbreeding between 24.3 DISTINCT FEATURES
Neanderthals and early humans. (a) Studies using only OF OUR SPECIES
mtDNA did not support the idea of interbreeding, but
(b) later studies using whole genomes did. Many extraordinary changes in anatomy and behavior occurred
in the 5–7 million years since our lineage split from the lineage
a. mtDNA phylogeny for Neanderthals and modern humans that gave rise to the chimpanzees. Fossils tell us a great deal about
Homo sapiens those changes, especially when high-quality material such as Lucy
or Ardi is available, but in general this is an area in which fossils
Chimpanzee Neanderthal African Non-African African
are hard to come by and there is a lot of speculation. Speculation
Because is especially common when we try to explain the reasons behind
Neanderthal
mtDNA does not the evolution of a particular trait. Why, for example, did language
fall within the evolve? It is easy enough to think of a scenario in which natural
human family tree
(red), we initially selection favors some ability to communicate—maybe language
believed that arose to facilitate group hunting. There are plenty of plausible
Neanderthals and ideas on the subject, but, in most cases, no evidence, so it is
our ancestors did
not interbreed. impossible to distinguish among competing hypotheses. We can,
b. however, be confident that the events that produced language
occurred in Africa, and, through paleontological studies of past
environments, we can conclude that humans evolved in an
Asia environment similar in many ways to today’s East African savanna.

However, studies of the

Europe full Neanderthal genome
Bipedalism was a key innovation.
revealed that there was The shift from walking on four legs to walking on two was
interbreeding, probably probably one of the first changes in our lineage. Many primates
in the Middle East as
H. sapiens first emerged are partially bipedal—chimpanzees, for example, may be observed
Africa from Africa about 60,000 knuckle-walking—but humans are the only living species
years ago. that is wholly bipedal. As we’ve seen, Lucy, dating from about
3.2 million years ago, was already bipedal, and 4.4 million years
ago, Ardi was partially bipedal. We can further refine our estimate
Initial H. sapiens out-of-Africa migration
of when full bipedalism arose from the evidence of a trace fossil. A
Expansion of H. sapiens populations after
interbreeding with Neanderthals set of 3.5-million-year-old fossil footprints discovered in Laetoli,
Tanzania, reveal a truly upright posture.
Becoming bipedal is not simply a matter of standing up on
from your mother. Therefore, it is possible that the Neanderthal hind legs. The change required substantial shifts in a number of
mtDNA lineage has been lost through genetic drift (Chapter 21). basic anatomical characteristics, described in Fig. 24.11.
An alternative is that the discrepancy in the ancient DNA Why did hominins become bipedal? Maybe it gave them access
stems from a sex-based difference in interbreeding. If Neanderthal to berries and nuts located high on a bush. Maybe it allowed them
females did not interbreed with our ancestors, Neanderthal mtDNA to scan the vicinity for predators. Maybe it made long-distance
would not have entered the modern human gene pool. If only male travel easier. Whatever the reason, bipedalism freed our ancestors’
Neanderthals interbred with our ancestors, we would expect exactly hands. No longer did they need their hands for locomotion, so
the pattern we observe: no Neanderthal mtDNA in modern humans, for the first time there arose the possibility that specialized
but Neanderthal genomic DNA in the modern human gene pool. hand function could evolve. Most primates have some kind of
Regardless of the details of what happened when modern opposable thumb, but the human version is much more dexterous.
humans, Neanderthals, and Denisovans encountered each other, The human thumb has three muscles that are not present in the
one thing is clear: We are forced to reimagine the ancestry of a thumb of chimpanzees, and these allow much finer motor control
large portion of the human population. Every non-African on the of the thumb. Tool use, present but crude in chimpanzees, can be
planet is part Neanderthal, albeit a very small part, and many non- much more subtle and sophisticated with a human hand.
Africans can count Denisovans among their ancestors as well. Bipedalism also made it possible to carry material over long
distances. Hominins could then set up complex foraging strategies
j Quick Check 3 Explain how genetic data are consistent with whereby some individuals supplied others with resources. Exactly
the possibility that only male Neanderthals interbred with our when sophisticated tool use arose in our ancestors is controversial,
ancestors. but it is indisputable that bipedalism contributed. Similarly, the
496 SECTION 24.3 D I S T I N C T F E AT U R E S O F O U R S P E C I E S

ability to manipulate food with the hands, and to carry material

FIG. 24.11 The shift from four to two legs. Anatomical changes using the hands rather than the mouth, likely permitted the
were required for the shift, especially in the structure of evolution of the human jaw—indeed, the entire facial structure—
the skull, spine, legs, and feet. in such a way that language became a possibility.

Adult humans share many features with juvenile

chimpanzees.
King and Wilson’s 1975 discovery that the DNA of humans and
of chimpanzees are 99% identical has recently been confirmed
by DNA sequence comparison of the human and chimpanzee
genomes. It turns out that the two genomes are extraordinarily
similar: All our genes are also present in the chimpanzee and their
sequences in humans and in chimpanzees are extremely similar,
suggesting that the functions of the proteins they code for are the
same. If we are so similar, how can we account for the extensive
differences between the two species?
In their original paper, King and Wilson suggested that much
of the most significant evolution along the hominin lineage came
about through changes in the regulation of genes (Chapter 19).
The foramen magnum, A small change—one that causes a gene to be transcribed at a
the hole in the skull different stage of development, for example—can have a major
through which the
spinal cord passes, is effect. In other words, King and Wilson introduced a model
repositioned so that the whereby small changes in the software could have a major impact
Foramen human skull balances even though the basic hardware is the same.
magnum directly on top of the
vertebral column. One of the gene-regulatory pathways that changed may be
responsible for human neoteny, the long-term evolutionary
process in which the timing of development is altered so that a
sexually mature organism still retains the physical characteristics
The human spine is of the juvenile form. In keeping with King and Wilson’s idea, this
S-shaped so weight is shift in development could conceivably be achieved with relatively
directly over the pelvis.
little genetic change. All that it might take would be a few changes
Spine
to the regulatory switches that control the timing of development.
As early as 1836, French naturalist Étienne Geoffroy Saint-
Hilaire noted that a young orangutan on exhibit in Paris looked
considerably more like a human than the adult of its own species
(Fig. 24.12). Several human attributes support this model,
The pelvis is extensively including heads that are large relative to our bodies (and our
reconfigured for an
correspondingly large brains), a feature of juvenile great apes. A
upright posture, with
internal organs over it. second human attribute is our lack of hair. The juveniles of other
Legs are longer to great apes are not as hairless as humans, but they’re considerably
enable long stride
length for efficient less hairy than adult apes. A third attribute is the position
locomotion and their of the foramen magnum at the base of the skull. In primate
Legs anatomy altered so the development, the foramen magnum starts off in the position it
legs are directly under
the body. occupies in the adult human and then, in nonhuman great apes,
migrates toward the back of the skull. Adult humans have retained
the juvenile great ape foramen magnum position. Finally, it has
been suggested that our mentality, with its questioning and
The foot is narrower and
playfulness, is equivalent in many ways to that of a juvenile ape
has a more developed rather than to that of the comparatively inflexible adult ape.
heel and larger big toe,
which contributes to a
springier foot. Humans have large brains relative to body size.
Feet
In mammals, brain size is typically correlated with body
size. Humans are relatively large-bodied mammals, but our brains
 CHAPTER 24 H U M A N O R I G I N S A N D E VO L U T I O N 497

that natural selection acted in favor of. What are the selective
FIG. 24.12 A juvenile and an adult orangutan. Humans look more factors? Here are some possibilities:
like juvenile orangutans than like adult orangutans,
suggesting that humans may be neotenous great apes. • Tool use. Bipedalism permitted the evolution of manual
Sources: (top to bottom) McPHOTO/W. Layer/age fotostock; Russell
dexterity, which in turn requires a complex nervous
Watkins/Shutterstock.
organization if hands are to be useful.

• Social living. Groups require coordination, and coordination

requires some form of communication and the means of
integrating and acting upon the information conveyed.
One scenario, for example, sees group hunting as critical in
the evolution of the brain: Natural selection favored those
individuals who cooperated best as they pursued large prey.

• Language. Did the evolution of language drive the evolution

of large brains? Or did language arise as a result of having
large brains? Again, we will probably never know, but it is
tempting to speculate that the brain and our extraordinary
powers of communication evolved in concert.

Probably, as with bipedalism, there was no single factor but a

mix of elements that worked together to result in the evolution
of large brain size. We tend to focus on brain size because it is a
convenient stand-in for mental power and because we can measure
it in the fossil record by making the reasonable assumption that the
volume of a fossil’s cranium reflects the size of its brain. However,
it should be emphasized that our minds are not the products solely

FIG. 24.13 Brain size plotted against body size for different
species. Humans have large brains for their body size.
Data from Fig. 2.4, p. 44, in H. J. Jerison, 1973, Evolution of the Brain
and Intelligence, New York: Academic Press.

The solid line Humans are the most

represents the divergent species above
expected brain the line (meaning that they
size for a given have disproportionately
body size. large brains).

10,000 Elephant
5000 Blue
Human
Whale
1000 Chimpanzee
500 Australopithecus Male gorilla
Baboon Lion
100 Sting ray Tiger shark
Brain weight (g)

50 Ostrich
Wolf
Crow
10
are large even for our body size (Fig. 24.13). It is our large 5 Opossum Ostriches are the
brains that have allowed our species’ success, extraordinary Vampire bat most divergent
1 Rat
species below the
technological achievements, and at times destructive dominion 0.50 Mole line (with
over the planet. disproportionately
0.10 small brains).
What factors acted as selective pressure for the evolution 0.05 Goldfish
of the large human brain? Again, speculation is common, but
0.01
because a large brain is metabolically expensive to produce and to 0.001 0.01 0.1 1 10 100 1000 10,000 100,000
maintain, we can conclude that the brain has adaptive features Body weight (kg)
498 SECTION 24.4 H U M A N G E N E T I C VA R I AT I O N

of larger brains. Rather, the reorganization of existing structures

and pathways is the more important product of the evolution of FIG. 24.14 A family tree of mammals showing amino acid
the brain. Not only was the brain expanding over those 7 million changes in FOXP2. The gene is highly conserved, but
years of human evolution, it was also being rewired. two changes are seen in the Neanderthals and modern
The human brain evolved through natural selection. What humans.
we have today is a learning machine capable of generating many
There is only a single amino acid change in
more skills and abilities than just those that directly enhance
FOXP2 between mice and primates, although
evolutionary fitness. For example, let’s assume that the brain their most recent common ancestor lived
evolved to make group hunting more efficient. The result is that about 75 million years ago.
the brain does indeed enhance our ability to hunt together, but
there are numerous by-products of this brain that take its abilities Mouse Rhesus Orangutan Gorilla Chimpanzee Neanderthal Human
beyond just hunting. Making art, for instance, has nothing to do
with group hunting, and yet the brain allows us to do it. A brain
evolved for an essential but mundane task like group hunting lies
at the heart of much that is wonderful about humanity.

The human and chimpanzee genomes help us identify

genes that make us human.
The key genetic differences between humans and chimpanzees In the human lineage, two
changes have occurred since
must lie in the approximately 1% of protein-coding DNA sequence the split with chimpanzees
that differs between the two genomes. What do we see when we 6–7 million years ago.
compare human and chimpanzee genomes? Amino acid
A gene that has attracted a lot of interest is FOXP2, a member change
of a large family of evolutionarily conserved genes that encode in FOXP2

transcription factors that play important roles in development.

Individuals with mutations in FOXP2 often have difficulty
with speech and language. Interestingly, laboratory animals
whose FOXP2 has been knocked out also have communication
impairments. Songbirds are less capable of learning new songs, and
the high-frequency “songs” of mice are disrupted. In addition to 24.4 HUMAN GENETIC VARIATION
its effects on the brain, FOXP2 plays a role in the development of
So far, we have treated humans as all alike, and in many ways,
many tissues.
of course, we are. But there are also many differences from one
Studies of the gene’s amino acid sequence reveal a pattern of
person to the next. Those differences ultimately have two sources:
extreme conservation. The sequences in mice and chimpanzees,
genetic variation and differences in environment (Chapter 18). A
whose most recent common ancestor lived about 75 million
person may be born with dark skin, or a person born with pale skin
years ago, differ by a single amino acid (Fig. 24.14). However, two
may acquire darker skin—a tan—in response to exposure to sun.
amino acids are present in humans but absent in chimpanzees.
The differences we see from one person to the next are
Studies of Neanderthal DNA have shown that Neanderthals
deceptive. Despite appearances, ours is not the most genetically
possessed the modern human version of FOXP2. The presence of
variable species on Earth. While it is certainly true that everyone
the same version of the gene in both species suggests that the
alive today (except for identical twins) is genetically unique,
differences arose in the hominin line before the split between
our species is actually rather low in overall amounts of genetic
our species and Neanderthals, which occurred at least 600,000
variation. Modern estimates based on comparisons of many
years ago.
human DNA sequences indicate that, on average, about 1 in every
FOXP2 gives us a glimpse of the genetic architecture of
1000 base pairs differs among individuals (that is, our level of
the biological traits that are likely to be important in the
DNA variation is 0.1%). That’s about 10 times less genetic variation
determination of “humanness.” Those two differences in amino
than in fruit flies (which nevertheless all look the same to us) and
acid sequence are intriguing, but they do not a human make.
about two to three times less than in Adélie penguins, which look
Substitute the human FOXP2 gene into a mouse and you will not
strikingly similar to one another (see Fig. 21.1).
get a talking mouse. The critical genetic differences are many,
Why, then, are we all so phenotypically different if there is
subtle, and interacting.
so little genetic variation in our species? Given the large size of
j Quick Check 4 The FOXP2 gene is sometimes called the our genome, a level of variation of 0.1% translates into a great
“language gene.” Why is this name inaccurate? many genetic differences. Our genome consists of approximately
 CHAPTER 24 H U M A N O R I G I N S A N D E VO L U T I O N 499

3 billion base pairs, so 0.1% variation means that 3 million bp Middle East (Fig. 24.15). The first phase of colonization took our
differ between any two people chosen at random. Many of those ancestors through Asia and into Australia by about 50,000 years
differences are in noncoding DNA, but some fall in regions of ago. Archaeological evidence indicates that it was not until about
DNA that encode proteins and therefore influence the phenotype. 15,000 years ago that the first modern humans crossed from
When those mutations are reshuffled by recombination, we get Siberia to North America to populate the New World.
the vast array of genetic combinations present in the human Genetic analyses also indicate that other colonizations
population. were even later. Despite its closeness to the African mainland,
the first humans arrived in Madagascar only about 2000 years
The prehistory of our species has had an impact on the ago, and the colonists came from Southeast Asia, not Africa.
distribution of genetic variation. Madagascar populations to this day bear the genetic imprint of
The reasons for our species’ relative lack of genetic variation this surprising Asian input. The Pacific Islands were among the last
compared to other species lie in prehistory, and factors affecting habitable places on Earth to be colonized during the Polynesians’
the geographical distribution of that variation also lie in the past. extraordinary seaborne odyssey from Samoa, which began about
Studies of the human family tree initiated by Rebecca Cann’s 2000 years ago. Hawaii was colonized about 1500 years ago, and
original mtDNA analysis are giving a detailed picture of how our New Zealand only 1000 years ago.
ancestors colonized the planet and how that process affected By evolutionary standards, the beginning of the spread
the distribution of genetic variation across populations today. of modern humans out of Africa about 60,000 years ago is
Detailed analyses of different populations, often using mtDNA very recent. There has therefore been relatively little time for
or Y chromosomes, allow us to reconstruct the history of human differences to accumulate among regional populations, and most
population movements. of the variation in human populations today arose in ancestral
As we have seen, Homo sapiens arose in Africa. Genetic populations before any humans left Africa. When we compare
analyses indicate that perhaps 60,000 years ago, populations levels of variation in a contemporary African population with
started to venture out through the Horn of Africa and into the that in a non-African population, like Europeans or Asians, we

FIG. 24.15 Human migratory routes. Tracking the spread of mitochondrial DNA mutations around the globe allows us to reconstruct the
colonization history of our species. Data from Fig. 6.18, p. 149, in D. J. Futuyma, 2009, Evolution, 2nd ed., Sunderland, MA: Sinauer Associates.

Humans arrived late in the New

World. The precise dates are
disputed, but the first humans
reached North America about
15,000 years ago.

Migratory route 1
(60,000–45,000 years ago)
Migratory route 2
(30,000–20,000 years ago)
Migratory route 3
(15,000?–10,000 years ago)
Migratory route 4
After Homo sapiens evolved in Africa, (<10,000 years ago)
groups moved out, beginning the
colonization of the rest of the planet.
500 SECTION 24.4 H U M A N G E N E T I C VA R I AT I O N

find there is more variation in the African population. This is variants, for example, are subject to genetic drift, and, given the
because the individuals that left Africa 60,000 years ago to found stepwise global colonization history of our species, it is likely that
populations in the Middle East and beyond were a relatively small founder events have played a role as well.
sample of the total amount of genetic variation then present in Selection, too, has been important. It is apparent, for example,
the human population. Non-African populations therefore began that the genetic variants affecting traits we can easily see are an
with less genetic variation, and that initial lower variation is especially prominent feature of the 7% of human genetic variation
reflected in their genetic profiles today. that occurs between so-called races. If we look at other genetic
variants, ones that don’t affect traits that we can see, there is little
The recent spread of modern humans means that there or no racial pattern. An African is as likely to have a particular
are few genetic differences between groups. base-pair mutation in a randomly chosen gene as a European. So
Because the out-of-Africa migration was so recent, the genetic why are visible traits so markedly different among races and other
differences we see among geographical groups—sometimes called traits are not? Given the short amount of time (by evolutionary
races—are minor. This fact is highly counterintuitive. We see standards) since all Homo sapiens were in Africa, it is likely that the
many superficial differences between an African and a Caucasian, differences we see between groups are the product of selection.
such as skin color, facial form, and hair type, and assume that these People with dark skin tend to originate from areas in lower
superficial differences must reflect extensive genetic differences. latitudes with high levels of solar radiation, and people with light
This assumption made sense when the standard theory about the skin tend to originate from areas in higher latitudes with low levels
origin of modern humans was the multiregional one. If European of solar radiation. It is likely that natural selection is responsible
and African populations really had been geographically isolated for the physical differences between these populations. Assuming
from each other for as long as 2 million years, then we would that the ancestors of non-African populations were relatively
expect significant genetic differences among populations. dark-skinned, what selective factors can account for the loss of
We expect isolated populations to diverge genetically over pigmentation?
time as different mutations occur and are fixed in each population. A likely factor is an essential vitamin, vitamin D, which is
The longer two populations have been isolated from each other, particularly important in childhood because it is needed for the
the more genetic differences between them we expect to see production of bone. A deficiency of vitamin D can result in the
(Chapter 21). Isolation lasting 2 million years implies that the skeletal malformation known as rickets. The body can synthesize
differences are extensive, but isolation of just 60,000 years vitamin D, but the process requires ultraviolet radiation. Heavily
suggests they are relatively few. Patterns of genetic variation pigmented skin limits the entry of UV radiation into cells and
among different human populations support the hypothesis that so limits the production of vitamin D. This does not present a
human populations dispersed as recently as 60,000 years ago. difficulty in parts of the world where there is plenty of sunlight,
What we see when we look at genetic markers—variable A’s, T’s, but it can be problematic in regions of low sunlight. Presumably,
G’s, and C’s in human DNA—is that there is indeed very little natural selection favored lighter skin in the ancestors of Eurasian
genetic differentiation by what is sometimes called race. populations because lighter skin promoted the production of
In short, there’s a disconnect: Different groups may look the vitamin.
very different, but, from a genetic perspective, they’re not very Some aspects of body shape and size may also have been
different at all. Any two humans may differ from each other by, influenced by natural selection. In hot climates, where dissipating
on average, only 3 million base pairs, and statistical analyses have body heat is a priority, a tall and skinny body form has evolved.
shown that approximately 85% of that genetic variation occurs Exemplified by East African Masai, this body type maximizes the
within a population (for example, the Yoruba in West Africa); 8% ratio of surface area to volume and thus aids heat loss. In colder
occurs between populations within races (for example, between climates, by contrast, selection has favored a more robust, stockier
Yoruba and Kikuyu, another African group); and the remaining 7% body form, as exemplified by the Inuit, who have a low ratio
occurs between races. The characteristics we use when we assess of surface area to volume that promotes the retention of heat
an individual’s ethnicity, such as skin color, eye type, and hair (Fig. 24.16). In these two cases, these are plausible explanations
form, are encoded by genetic variants that lie in that 7%. If Earth of body form. We should bear in mind, however, that simple one-
were threatened with destruction and only one population— size-fits-all explanations of human difference are almost always
Yoruba, for example—survived, 85% of the total amount of too simplistic. Our species is complex and diverse and often defies
human genetic variation that exists today would still be present in generalizations.
that population. Attempts have been made to identify the adaptive value
of visual differences between races, such as facial features. It’s
Some human differences have likely arisen by natural possible that natural selection played a role in the evolution of
selection. these differences, but an alternative explanation, one originally
Patterns of genetic variation in human populations are shaped suggested by Charles Darwin, is more compelling: sexual selection
by the set of evolutionary forces discussed in Chapter 21. Neutral (Chapters 21 and 45).
 CHAPTER 24 H U M A N O R I G I N S A N D E VO L U T I O N 501

FIG. 24.16 Evolutionary responses of body shape to climate. (a) A heat-adapted Masai in Kenya and (b) a cold-adapted Inuit in Greenland.
Sources: a. Ryan Heffernan/Aurora/Getty Images; b. B&C Alexander/ArcticPhoto.

a b

As we have seen, there is an apparent mismatch between the Homozygotes for the allele encoding normal hemoglobin are also
extent of difference among groups in visible characters, such as at a disadvantage because they are entirely unprotected from the
facial features, and the overall level of genetic difference between parasite.
human groups. Sexual selection can account for this mismatch The S allele is beneficial only in the presence of malaria. If
because it operates solely on characteristics that can readily be there is no malaria in an area, the S allele is disadvantageous, so
seen—think of the peacock’s tail. As we learn more about the natural selection presumably acted rapidly to eliminate it in the
genetic underpinnings of the traits in question, we will be able to ancestors of Europeans when they arrived in malaria-free regions.
investigate directly the factors responsible for the differences we The continued high frequency of the S allele in Africans, some
see among groups. Mediterranean populations, and in populations descended recently
from Africans (such as African-Americans) is, however, a reflection
? CASE 4 MALARIA: CO-EVOLUTION OF HUMANS AND of the response of natural selection to a regional disease.
A PARASITE The hemoglobin genes are not the only genes that are
What human genes are under selection for resistance under selection for resistance to malaria. Glucose-6-phosphate
to malaria? dehydrogenase (G6PD), a gene involved in glucose metabolism,
We see evidence of regional genetic variation in response to is one of several other genes implicated. People who are
local challenges, especially those posed by disease. Malaria, heterozygotes for a mutation in the G6PD gene—and therefore
for example, is largely limited to warm climates because it is have a G6PD enzyme deficiency—can develop severe anemia
transmitted by a species of mosquito that can survive only in when they eat certain foods (most notably fava beans; hence,
these regions. Historically, the disease has been devastating the condition is called favism). People who are heterozygotes
in Africa and the Mediterranean. As we saw in Chapter 21, the for a mutation in the G6PD gene, however, also have increased
sickle allele of hemoglobin, S, has evolved to be present at high resistance to malaria, apparently because they are better at
frequencies in these regions because in heterozygotes it confers clearing infected red blood cells from their bloodstream. In areas
some protection against the disease. But in homozygotes, the S where malaria is common, the advantage of malaria resistance
allele is highly detrimental because it causes sickle-cell anemia. offsets the disadvantage of favism.
502 SECTION 24.5 C U LT U R E , L A N G UAG E , A N D CO N S C I O U S N E S S

Detailed evolutionary analysis of mutations in G6PD shows extraordinarily inhospitable parts of the planet, like the Arctic
that favism has arisen multiple times, each time selectively (Fig. 24.17). The capacity to innovate coupled with the ability to
favored because of its role in the body’s response to the malaria transmit culture is the key to the success of humans.
parasite. As expected, favism, like sickle-cell anemia, is mainly a Culture, of course, changes over time. In many ways, cultural
feature of populations in malarial areas or of populations whose change is responsible for our species’ extraordinary achievements.
evolutionary roots lie in these areas. Genetic evolution is slow because it involves mutation followed
by changes in allele frequencies that take place over many
generations. Cultural change, on the other hand, can occur much
24.5 CULTURE, LANGUAGE, more rapidly. Ten years ago, nobody had heard of smartphones,
AND CONSCIOUSNESS but today, millions of people own them. Or think of the speed at
which a change in clothing style—a shift from flared to straight-
The most remarkable outcome of the evolutionary process leg jeans, for example—spreads through a population. Today’s
described in this chapter is the human brain. This allows us to do human population as a whole is genetically almost identical to the
extraordinary things, like appreciate Bach’s music, read books, population when your grandparents were young. But think of the
and build skyscrapers. But does this wonderful brain make us cultural changes that have occurred in the 50 years or so between
qualitatively different from other organisms? Does it in some way your grandparents’ youth and your own.
take us out of nature? Traditionally, the answer to these questions Despite this clear contrast between biological evolution
would have been a resounding yes. However, research into the and cultural change, we should not necessarily think of the two
capabilities of other species is questioning this conclusion: The processes as independent of each other. Sometimes cultural
human brain is certainly remarkable, but, in essence, what we can change drives biological evolution.
do is merely an extension of what other animals can do. A good example of the interaction between cultural change
and biological evolution is the evolution of lactose tolerance in
Culture changes rapidly. populations for which domesticated animals became an important
Culture is generally defined as a body of learned behavior that is source of dairy product. Most humans are lactose intolerant.
socially transmitted among individuals and passed down from Lactose, a sugar, is a major component of mammalian milk,
one generation to the next. Culture has permitted us in part to including human breast milk. We have an enzyme, lactase, that
transcend our biological limits. To take a simple example, clothing breaks down lactose in the gut, but, typically, the enzyme is
and ingeniously constructed shelters have enabled us to live in produced only in the first years of life, when we are breast-feeding.
Once a child is weaned, lactase production
is turned off. Lactase, however, is clearly a
useful enzyme to have if there is a major
FIG. 24.17 The power of culture. Inventions (such as clothing) have allowed our species to dairy component to your diet.
expand its geographic range. Source: B&C Alexander/ArcticPhoto. Archaeological and genetic analyses
indicate that cattle were domesticated
probably three separate times in the past
10,000 years, in three separate places: in
the Middle East, in East Africa, and in the
Indus Valley. In at least two of these cases,
there has been subsequent human biological
evolution in favor of lactose tolerance, that
is, continued lactase production throughout
life. Analysis of the gene region involved in
switching lactase production on and off has
revealed mutations in European lactose-
tolerant people that are different from those
in African lactose-tolerant people, implying
independent, convergent evolution of this
trait in the two populations. Furthermore, we
see evidence that these changes have evolved
very recently, implying that they arose as
a response to the domestication of cattle.
Here, we see the interaction between cultural
change and biological evolution. Biological
 CHAPTER 24 H U M A N O R I G I N S A N D E VO L U T I O N 503

FIG. 24.18 Nonhuman culture. (a) English blue tits steal cream from a milk bottle on the doorstep. (b) Adult meerkats teach their young how to
handle their prey. (c) Chimpanzees in different populations have devised different ways of using tools to hunt insects. Sources: a. Colin F.
Sargent; b. Dr. Alex Thornton, University of Exeter; c. FLPA/Peter Davey/age fotostock.

a b c

evolution of continued lactase production has resulted from the transmission all benefit from another extraordinary human
change in a cultural practice, namely cattle domestication. attribute, language.

Is culture uniquely human? Is language uniquely human?

Even nonprimates are known to be capable of learning from Language is a complex form of communication involving the
another member of their own species—culture is not uniquely transfer of information encoded in specific ways, such as speech,
human. Bird songs, for instance, may vary regionally because facial expressions, posture, and the like. Despite previous claims
juveniles learn the song from local adults. In a famous case, that language is unique to humans, it is not. The waggle dance of
shown in Fig. 24.18a, small birds called blue tits (Parus caeruleus) a worker bee on its return to the hive communicates information
in Britain learned from each other how to peck through the on the direction, distance, and nature of a food resource (Chapter
aluminum caps of milk bottles left on doorsteps by milkmen 45). Vervet monkeys use warning vocalizations to specify the
to reach the rich cream at the top of the milk. Presumably, one identity of a potential predator: They have one call for “leopard,”
individual discovered accidentally how to peck through a milk for example, and another for “snake.” But nonhuman animal
bottle cap and the others then imitated the action. languages are limited. Attempts to teach even our closest relatives,
Nor is teaching, one way in which culture is transmitted, chimpanzees, to communicate using sign language have met with
limited to humans. In teaching, one individual tailors the only limited success (Fig. 24.19).
information available to another in order to facilitate learning. Grammar provides a set of rules that allow the combination of
Teaching and imitation together make learning highly efficient. words into a virtually infinite array of meanings. Noam Chomsky,
Adult meerkats teach young meerkats how to handle prey father of modern linguistics, has pointed out that all human
by giving them the opportunity to interact with live prey languages are basically similar from a grammatical viewpoint.
(Fig. 24.18b). Humans have what he has called a “universal grammar” that
The most sophisticated example of nonhuman culture is would lead a visiting linguist from another planet to conclude
shown by chimpanzees. Detailed studies of several geographically that all Earth’s languages are dialects of the same basic language.
isolated populations have revealed 39 culturally transmitted That universal grammar is in some way hard wired into the human
behaviors, such as ways of using tools to catch insects, which are brain in such a way that every human infant spontaneously strives
specific to a particular population (Fig. 24.18c). West African to acquire language. The specific attributes of the language depend
Chimpanzees in Guinea and the Ivory Coast both use tools to help on the baby’s environment—a baby in France will learn French,
them harvest the same species of army ants for food (and avoid and one in Japan will learn Japanese—but the basic process is
the painful defensive bites of the ants), but each population has its similar in every case. Chimpanzees, and the entire natural world,
own distinct way of doing this. Chimpanzees, then, are like us in lack the drive toward the acquisition of a grammatical language.
having regional variation in culture.
Despite the cultural achievements of other species, it is clear Is consciousness uniquely human?
that culture in Homo sapiens is very far removed from anything Descartes famously wrote, “Cogito, ergo sum”—“I think,
seen in the natural world. We are amazingly adept at imitation, therefore I am.” Can we legitimately rewrite his statement to
learning, and acquiring culture. Teaching, learning, and cultural declare, “Animals think, therefore they are”? With the growth
504 CO R E CO N C E P T S S U M M A RY

FIG. 24.19 Dr. Susan Savage Rumbaugh with Panbanisha, a female bonobo who learned to
communicate using sign language. Chimpanzees and bonobos are able to learn and
use sign language to express words and simple sentences. Source: Anna Clopet/CORBIS.

of the animal rights movement, particularly in reference to the considerable trouble to bend a hook into the straight wire. She
treatment of animals in factory farms, this question is of more succeeded in getting the food. It is difficult to deny that the crow
than academic interest. We now have many examples of animal thought about the problem and was able to solve it, perhaps in the
thinking from a range of species, including, not surprisingly, same way as we would. Definitions of consciousness are contested,
chimpanzees and gorillas. but, as with language and culture, it seems clear that other species
Perhaps more remarkable are the examples that come from are capable of some form of conscious thought.
animal species that are not closely related to us. In experiments The evolutionary biologist Theodosius Dobzhansky once
carried out by Alex Kacelnik in Oxford, England, a pair of New said, “All species are unique, but humans are uniquest.” Our
Caledonian Crows was presented with two pieces of wire, one “uniquest” status is not derived from having attributes absent in
straight and the other hooked, and offered a food reward that other species, but from the extent to which those attributes are
could be obtained only by using the hooked wire. One member of developed in us. Human language, culture, and consciousness are
the pair, the male, disregarded the experiment and flew off with extraordinary products of our extraordinary brains. Nevertheless,
the hooked wire. The female, however, having discovered that she as Darwin taught us and as we should never forget, we are fully a
could not get the food reward with the straight wire, went to some part of the natural world.•

Core Concepts Summary The great apes include orangutans, gorillas, chimpanzees, and
humans. page 487
24.1 Anatomical, molecular, and fossil evidence shows Analysis of sequence differences between humans and our
that the human lineage branches off the great apes tree. closest relatives, chimpanzees, indicate that our lineage split
from chimpanzees 5–7 million years ago. page 487
Anatomical features indicate that primates are a
monophyletic group that includes prosimians, monkeys, and Lucy, an unusually complete specimen of Australopithecus
apes. The apes in turn include the lesser and great apes. afarensis, demonstrates that our ancestors were bipedal by
page 486 about 3.2 million years ago. page 489
 CHAPTER 24 H U M A N O R I G I N S A N D E VO L U T I O N 505

Hominin fossils occur only in Africa until about 2 million years Humans have very little genetic variation, with only about
ago, when Homo ergaster migrated out of Africa to colonize the 1 in every 1000 base pairs varying among individuals.
Old World. page 490 page 499

Most of the variation among humans occurs within

24.2 Phylogenetic analysis of mitochondrial DNA
populations. As much as 85% of the total amount of genetic
and the Y chromosome shows that our species arose in
variation in humans can be found within a single population.
Africa.
Only about 7% of human genetic variation segregates
Studies of mitochondrial DNA (mtDNA) suggest that the time between groups that are commonly called races. page 500
back to the common ancestor of modern humans is about
Some racial differences, such as skin color and resistance to
200,000 years, implying that modern humans (Homo sapiens)
malaria, have probably arisen by natural selection. page 500
arose in Africa (the out-of-Africa theory). page 491
Other racial differences have probably arisen by sexual
The mtDNA out-of-Africa pattern is supported by
selection. page 501
Y chromosome analysis, which also shows a recent African
origin of modern humans. page 494
24.5 Culture is a potent force for change in modern
Analysis of Neanderthal DNA from 30,000-year-old humans.
material indicates that, as the ancestors of non-African
Cultural evolution and biological evolution may interact, as
humans emigrated from Africa, they interbred with the
in the case of the evolution of lactose tolerance in regions
Neanderthals. page 494
where cattle were domesticated. page 502
Our species originated in Africa and subsequently colonized
Other animals possess simple versions of culture, language,
the rest of the planet, starting about 60,000 years ago.
and even consciousness, but the capabilities of our species in
page 495
all three are truly exceptional. page 503

24.3 During the 5–7 million years since the most recent
common ancestor of humans and chimpanzees, our Self-Assessment
lineage acquired a number of distinctive features.
1. Describe the evidence suggesting that chimpanzees are the
The development of bipedalism involved a wholesale closest living relatives of humans.
restructuring of anatomy. page 495
2. Explain the out-of-Africa theory of human origins and
Neoteny is the process in which the timing of development how studies of mitochondrial DNA and the Y chromosome
is altered over evolution so that a sexually mature organism support it.
retains the physical characteristics of the juvenile form;
3. List four anatomical differences between chimpanzees
humans are neotenous, exhibiting many traits as adults that
and humans, and explain how these changes facilitated
chimpanzees exhibit as juveniles. page 496
walking upright.
There are many possible selective factors that explain the
4. Given the high genetic similarity of humans and
evolution of our large brain, including tool use, social living,
chimpanzees, how can we account for the differences we see
and language. page 497
between the two species?
FOXP2, a transcription factor involved in brain development,
5. Describe three possible selective factors underlying the
may be important in language, as mutations in the gene that
evolution of large brains in our ancestors.
encodes FOXP2 are implicated in speech pathologies.
page 498 6. Explain how differences among different human
populations arose by natural and sexual selection.
24.4 Human history has had an important impact on 7. Provide arguments for and against the idea that culture,
patterns of genetic variation in our species. language, and consciousness are uniquely human.
Because our ancestors left Africa very recently in
evolutionary terms, there has been little chance for genetic Log in to to check your answers to the Self-
differences to accumulate among geographically separated Assessment questions, and to access additional learning tools.
populations. page 499
Gallery Stock.
PA RT 2

FROM
ORGANISMS
TO THE
ENVIRONMENT
” From so simple a beginning
endless forms most beautiful
and most wonderful have been,
and are being, evolved.”
— CHARLES DARWIN
 CHAPTER 25

Cycling Carbon

Core Concepts
25.1 Photosynthesis and respiration
are the key biochemical
pathways for the biological, or
short-term, carbon cycle.
25.2 Physical processes govern the
long-term carbon cycle.
25.3 The carbon cycle can help
us understand ecological
interactions and the evolution
of biological diversity.

Andre Gallant/Getty Images.

509
510 SECTION 25.1 T H E S H O RT-T E R M C A R B O N C YC L E

Let’s imagine that we could tag a carbon atom at its moment of properties of environments. And it provides a basis for assessing the
origin and then follow its odyssey through time and space. Formed role that humans play in our environmental present and future.
in a nuclear blast furnace deep within an ancient star and then
ejected into space as the star died, our atom was eventually swept
up with other materials to form the Earth, a small planet orbiting 25.1 THE SHORT-TERM
the newer star we now call the sun. Volcanoes introduced our CARBON CYCLE
carbon atom into the early atmosphere as carbon dioxide (CO2),
and slowly, over millions of years, this CO2 reacted chemically with In 1958, American chemist Charles David Keeling began a novel
rocks, transferring the carbon to limestone that accumulated on program to monitor the atmosphere. At the time he began
the sea floor. Here, our atom sat for many millions of years, until his study, scientists had only a vague notion of how carbon
earthquakes, erosion, or other geologic activities returned it to dioxide (CO2) behaves in air. Many thought that CO2 might
the atmosphere as, once again, CO2. Slowly but surely, geologic vary unpredictably from time to time, and from place to place.
processes on the early Earth cycled carbon from atmosphere to Keeling decided to find out if in fact it did. From five towers
rocks, and back again. set high on Mauna Loa in Hawaii, 3400 m above sea level, he
Sometime between 4 and 3.5 billion years ago, our carbon sampled the atmosphere every hour and measured its composition
atom began to cycle more rapidly—much more rapidly—as the with an infrared gas analyzer. Within the first few years of his
carbon cycle gained new players. Photosynthetic microorganisms project, a pattern of seasonal oscillation became apparent: CO2
converted CO2 into organic molecules, and respiring concentration in the air reached its annual high point in spring
microorganisms returned CO2 to the environment, completing and then declined by about 6 parts per million (ppm, by volume)
a rapid biological carbon cycle. To this day, the biological carbon to a minimum in early fall (Fig. 25.1). Such regular variation was
cycle endlessly propels our atomic wayfarer from atmosphere and unexpected: What could be causing it?
oceans to cells and back again, while the slow burial of organic Perhaps the pattern was local. Airplane traffic to Hawaii peaked
matter and limestone in sedimentary rocks continues to bring in winter, and maybe that was affecting the air around Mauna
carbon atoms to the solid Earth, returning it to the atmosphere Loa. As measurements continued through a number of years,
only slowly, on geologic timescales. however, the pattern persisted, even as trends in air traffic changed.
In Part 1, we explored the molecular and cellular basis Moreover, monitoring stations in many other parts of the globe gave
of life, culminating in a discussion of how genetic variation similar results. Knowing that the Mauna Loa measurements are
and natural selection shape evolution. Part 2 examines the representative of the atmosphere as a whole allows us to appreciate
products of evolution as it has played out through Earth history: what an annual variation of 6 ppm really means. With calendar-
our emerging sense of the immense biological diversity of Bacteria like regularity, approximately 47 billion metric tons of CO2 were
and Archaea; the myriad microscopic eukaryotes found in oceans, entering and leaving the atmosphere annually—that’s 13 billion
lakes and soils; and the millions of plant, fungal, and animal species metric tons of the element carbon (the rest of the mass is oxygen).
that define our biological landscape. As we will see, function To explain such a pattern, we must consider processes that affect
and diversity underpin the ecological interactions that shape the planet as a whole.
communities, ecosystems, and biomes across our planet. A second pattern is evident in Fig. 25.1. Summertime
We begin with an introduction to the carbon cycle, the removal of CO2 from the atmosphere does not quite balance
intricately linked network of biological and physical processes winter increase, with the result that the amount of CO2 in the
that shuttles carbon among rocks, soil, oceans, air, and organisms. atmosphere on any given date is greater than it was a year earlier.
Why start here? Because the carbon cycle provides a fundamental Atmospheric CO2 levels have therefore increased steadily through
organizing principle for understanding life on Earth. Indeed, it the period of monitoring. For example, mean annual CO2 levels,
lies at the heart of everything else we discuss in Part 2. The about 315 ppm in 1958, reached 404 ppm by late April 2015. This
chemistry of life is, to a first approximation, the chemistry of is an increase of more than 25%, and there is no indication that the
carbon. How organisms move carbon from one species to another, rise will slow or stop any time soon.
and between organisms and their surrounding environment, What causes atmospheric CO2 to vary through the year and
underpins both the efficient functioning of ecosystems and over a timescale of decades? To answer this question, we need to
their persistence over an immense span of time. In no small part, ask what processes introduce CO2 into the atmosphere and what
biological diversity can be understood in terms of the varied processes remove it. Carbon dioxide is added to the atmosphere
ways that organisms obtain the carbon needed for growth and by (1) geologic inputs, mainly from volcanoes and mid-ocean
reproduction. Much of ecology, in turn, concerns the ways that ridges; (2) biological inputs, especially respiration; and (3) human
organisms interact to cycle carbon and transfer the energy stored activities, including deforestation and the burning of fossil fuels.
in organic molecules. The carbon cycle focuses our attention on the Processes that remove CO2 from the atmosphere include
ways that physical and biological processes together determine the (1) geologic removal, especially by chemical weathering in which
 CHAPTER 25 C YC L I N G C A R B O N 511

It is estimated that photosynthetic organisms remove about

FIG. 25.1 The Keeling curve. The Keeling curve provides a record 210 billion metric tons of carbon (in nearly 770 billion metric tons
of atmospheric CO2 concentrations over half a century, of CO2) from the atmosphere each year, about 60% of it by land
measured from an observatory at Mauna Loa, Hawaii. plants and the rest by phytoplankton and seaweeds in the oceans.
Data from Dr. Pieter Tans, NOAA/ESRL (www.esrl.noaa.gov/ This annual photosynthetic removal of CO2 equals about 25%
gmd/ccgg/trends/) and Dr. Ralph Keeling, Scripps Institution of of the total CO2 in the atmosphere. In the absence of processes
Oceanography (scrippsco2.ucsd.edu/). that restore CO2 to the atmosphere, high rates of photosynthesis
would use up the atmospheric reservoir of CO2 in just a few
years. But the Keeling curve tells us that atmospheric CO2 is not

CO2 concentration
CO2 concentrations decreasing rapidly—it is rising. Moreover, the geologic record
regularly cycle up
and down over the indicates that photosynthesis has fueled ecosystems for billions of
course of a year. years without causing CO2 depletion.
What processes return CO2 to the atmosphere, balancing
photosynthetic removal? In Chapter 7, we discussed aerobic
respiration. Humans and many other organisms gain both the
Jan Apr Jul Oct Jan energy and carbon needed for growth from organic molecules
Month (that is, by consuming food). As explained in Chapter 7, aerobic
respiration uses oxygen to oxidize organic molecules to CO2,
400 converting chemical energy in the organic compounds to ATP for
use in cellular processes. Aerobic respiration can be summarized
390
by this chemical equation:
380
CO2 concentration (ppm)

C6H12O6 1 6O2 → 6CO2 1 6H2O

370

360 Plants respire, too, converting the chemical energy in

carbohydrates into useful work. On the present-day Earth, the
350 Atmospheric
levels of CO2 amount of CO2 returned annually to the atmosphere by aerobic
340 show a steady respiration and related processes is about equal to the amount
increase over the
that plants and algae remove by photosynthesis. That is why
330 last 50 years.
photosynthesis does not deplete the CO2 in air.
320 As we discussed in Chapter 8, the equations for oxygenic
310
photosynthesis and aerobic respiration mirror each other.
1960 1970 1980 1990 2000 2010 Photosynthesis continually converts CO2 to organic molecules,
Year using energy from the sun, and respiration harvests the
chemical energy stored in organic molecules, releasing CO2 and
water in the process. Photosynthesis and aerobic respiration are
CO2 in rainwater reacts with exposed rocks, and (2) biological complementary metabolic pathways: Each uses the products
removal, mainly through photosynthesis. As we will see, geologic of the other and generates a new supply of substrates for its
processes play key roles in the carbon cycle on timescales of complement. The result is a cycle (Fig. 25.2).
centuries and longer. Only biology, however, can account for the
annual rhythm of the Keeling curve.

Photosynthesis and respiration are key processes in FIG. 25.2 The short-term carbon cycle. The complementary
short-term carbon cycling. metabolic processes of photosynthesis and respiration
To grow, all organisms require both carbon and energy. As we drive the short-term cycling of carbon through the
saw in Chapter 8, photosynthetic organisms use energy from the biosphere.
sun to form ATP and NADPH, which reduce CO2 to carbohydrates.
As photosynthesis pulls CO2 out of the atmosphere (or water, for Photosynthesis Photosynthesis uses sunlight to
aquatic organisms), the carbon is transferred to carbohydrates, reduce CO2 to carbohydrates,
while oxidizing water to O2.
and oxygen is given off as a by-product. The overall chemical 6CO2 + 6H2O C6H12O6 + 6O2 Respiration runs the same
reaction is shown here: reaction in reverse. Together,
Respiration the processes form a cycle.
6CO2 1 6H2O → C6H12O6 1 6O2
512 SECTION 25.1 T H E S H O RT-T E R M C A R B O N C YC L E

HOW DO WE KNOW?

FIG. 25.3

How much CO2 was in the atmosphere 1000 years ago?

BACKGROUND Direct measurements of the atmosphere show that MEASUREMENTS Identifying chemical indicators that vary on an
CO2 has increased by 25% over the past 50 years. To know whether annual basis in successive layers of ice enabled scientists to assign
or not such a change is unusual, we need to know CO2 levels over a ages to ice samples taken from cores drilled through the Law Dome,
longer time interval. a large ice dome in Antarctica. The youngest ice samples overlap in
age with the first part of the Keeling data, providing a check that air
OBSERVATION Scientists recognized that the snow that
samples trapped in ice have CO2 concentrations similar to those in air
accumulates on glaciers to form new layers of ice is initially full
measured directly over the same interval.
of small spaces that are in contact with the air. Through time, as
snow changes to ice, these spaces become sealed off from the
surrounding environment and form bubbles, preserving a minute 390
sample of air at the time of ice formation.
Mauna Loa (direct measurements of CO2 levels)
370 Law Dome (ice core measurements of CO2 levels)
Atmospheric CO2 (ppm)
350 The fact that ice core
data agree with direct
measurements where
330 both are available gives
confidence in the CO2
310 curve reconstructed
from ice cores.

290

270
1000 1200 1400 1600 1800 2000
Ice core drilled from a Viewed under the Year
glacier in Antarctica microscope, glacial ice Measurements of air samples in glacial ice from
contains small bubbles the Law Dome, Antarctica, show that atmospheric
that trap samples of air. CO2 varied little between 1000 and 1800.
Photo sources: (left) Courtesy of Ted Scambos & Rob
Bauer, National Snow and Ice Data Center, University
of Colorado, Boulder, (right) Courtesy Vin Morgan.

CONCLUSIONS Before the Industrial Revolution, atmospheric CO2 levels had varied little over 1000 years, generally falling between
270 and 280 ppm. From this, scientists have concluded that current changes in atmospheric CO2 are unusual on the timescale of the
past millennium.

SOURCE Etheridge, D. M., et al. 1996. “Natural and Anthropogenic Changes in Atmospheric CO2 over the Last 1000 Years from Air in Antarctic Ice and Firn.” Journal of
Geophysical Research 101:4115–4128.

The regular oscillation of CO2 reflects the a narrow band around the equator, photosynthesis is seasonal,
seasonality of photosynthesis in the Northern with higher rates in the summer and lower rates in the winter.
Hemisphere. Respiration, in contrast, remains more or less constant through
Now that we have identified the major processes in the short- the year.
term (days to decades) carbon cycle, we can examine what Even cursory examination of the Earth shows that land
makes atmospheric CO2 level oscillate seasonally. Except in is distributed asymmetrically, with more land—and hence

512
 CHAPTER 25 C YC L I N G C A R B O N 513

more plants—in the Northern Hemisphere than in the Carbon isotopes show that much of the CO2 added
Southern Hemisphere. For this reason, global atmospheric to air over the past half century comes from burning
CO2 declines through the northern summer, when the ratio fossil fuels.
of photosynthesis to respiration is highest, and then increases We can test the hypothesis that human activities have
through fall and winter, when the ratio is reversed. The result is contributed to the increases in atmospheric CO2 measured at
the seasonal oscillation of atmospheric CO2 levels documented Mauna Loa and in ice cores by making careful measurements of
by Keeling. a chemical detail: the isotopic composition of atmospheric CO2.
In Chapter 2, we saw that many elements have several isotopes,
Human activities play an important role in the atoms of the element that vary in atomic mass because they
modern carbon cycle. have different numbers of neutrons. Carbon has three isotopes:
Now let us ask about the second major pattern evident in the 12
C (with six neutrons, about 99% of all carbon atoms); 13C (with
Keeling curve. Although CO2 levels oscillate on an annual basis, seven neutrons, most of the remaining 1%); and the rare 14C (with
the overall pattern is one of sustained increase. During the eight neutrons, about one part per trillion of atmospheric carbon).
1960s, the observed increase was less than 1 ppm each year. In More than 40 years ago, the Austrian-born chemist Hans
the first decade of the new millennium, it has been closer to Suess began to measure the relative abundances of the three
2 ppm. The current level of atmospheric CO2 is more than 25% isotopes of carbon in atmospheric CO2, a program that continues
higher than it was 50 years ago. Why has CO2 input into the today. He observed that the proportion of 13C in atmospheric
atmosphere outstripped removal for the past half century? CO2 has declined as the total amount of CO2 has increased
Should we consider this pattern unusual? (Fig. 25.4). This subtle change in isotopic composition has
Answering the second question helps us to address the first. occurred because the CO2 being added to the atmosphere has
Conclusions about whether it is unusual for atmospheric CO2 to less 13C than the CO2 already in the air.
increase 25% in 50 years can be answered only if we can compare How does this result influence the hypotheses we
the Keeling curve with longer records of atmospheric CO2 levels. developed to account for increasing atmospheric CO2 levels?
Before Keeling, no one systematically collected air samples. In photosynthesis, CO2 containing the lighter isotope 12C
Fortunately, however, nature did it for us. is incorporated into biomolecules preferentially over CO2
When glacial ice forms, it traps tiny bubbles of air. Each year, containing 13C, and for this reason organic matter generated by
snowfall gives rise to a new layer of ice, and as layers accumulate photosynthesis (and the organic matter of organisms that eat
through time their bubbles preserve a history of the atmosphere. photosynthetic organisms) differs in its proportions of 13C and
The graph in Fig. 25.3 shows the amount of CO2 in air bubbles 12
C from CO2 from volcanic gases and inorganic carbon dissolved
trapped in Antarctic ice that has accumulated over the past in the oceans. Volcanic and dissolved marine carbon do not have
1000 years. Ice samples show that CO2 levels actually began to the right ratio of 13C to 12C to explain the isotopic change observed
increase slowly in the 1800s, the time of major transformations in the atmosphere over time. In contrast, organic matter in
in mining, manufacturing, and transportation we call the living organisms has just the right ratio of 13C to 12C to account
Industrial Revolution. Before that, however, atmospheric CO2 for Suess’s measurements. Therefore, processes that cause a net
levels had varied little since the Middle Ages, staying at 270–280 conversion of organic matter to CO2, for example by burning,
ppm for centuries on end. On a 1000-year timescale, then, big must be adding CO2 to the atmosphere.
changes in CO2 abundance happened only once, in the last 200 Here’s where the other isotope of carbon, 14C, comes in.
years. Atmospheric CO2 inputs and outputs were approximately Measurements also show that the sources of added CO2 are
in balance until the Industrial Revolution. depleted in 14C relative to the CO2 already in the air. Modern
Fig. 25.3 shows that there is a correlation between the organic matter contains too much 14C to account for the
increasing CO2 content of the atmosphere and human activities. observed pattern, but ancient organic matter—the coal,
A correlation simply indicates that two events or processes occur petroleum, and natural gas burned as fossil fuels—is isotopically
together. Can we actually demonstrate that humans have played just right. Chemical analyses, therefore, support the hypothesis
a role in recent atmospheric change? By itself, correlation does that human activities cycle carbon in amounts high enough to
not establish causation, a relationship in which one event leads affect the chemical composition of the atmosphere.
to another. We might propose instead that recent CO2 increase That atmospheric CO2 is increasing is not a hypothesis;
reflects natural processes and only matches the period of it is a measurement. That the burning of fossil fuels plays
industrialization by coincidence. an important role in this increase is also unambiguously
documented by chemical analyses. Current debate focuses not
j Quick Check 1 How does the graph in Fig. 25.3 suggest that on these observations, but instead on the more difficult problem
human activities have influenced CO2 levels in the atmosphere? of understanding how increasing CO2 will affect climate. We
What might be a plausible alternative hypothesis? discuss this debate in Chapter 49. For now, however, we can use
 514 SECTION 25.1 T H E S H O RT-T E R M C A R B O N C YC L E

HOW DO WE KNOW?

FIG. 25.4

What is the major source of the CO2 that has accumulated

in Earth’s atmosphere over the past two centuries?
BACKGROUND Chemical analyses show that CO2 levels in the of plant carbon to CO2 by burning, or it could reflect the burning of
current atmosphere are about 40% higher than they were at the fossil fuels formed by the burial of plant and algal materials in the
time of the American Revolution. This rise coincides with major geologic past.
advances in manufacturing and transportation, which are driven by
FOLLOW-UP WORK Further studies, using 14C, the third, unstable
the burning of fossil fuels. These coincidences in timing suggest that
isotope of C, also show a strong decrease in the proportional
human activities are responsible for increasing CO2 levels.
abundance of 14C in atmospheric CO2 over time. While the burning
HYPOTHESIS The burning of fossil fuels, central to the emergence of vegetation to clear land for agriculture results in CO2 with the
of modern industrial societies, has been the principal cause of the right ratio of 13C to 12C, modern vegetation has far too much 14C to
measured increase of CO2 in the atmosphere. account for observed changes in the 14C of air. Only fossil fuels have
the right ratios of all three carbon isotopes—12C, 13C, and 14C—to
EXPERIMENT AND RESULTS Hans Suess measured the abundance account for the pattern of isotopic change in atmospheric CO2
of the two stable isotopes of carbon, 13C and 12C, in air samples and measured by Suess and others. In fact, 14C data from the high Rocky
demonstrated that the ratio of 13C to 12C in atmospheric CO2 has Mountains indicate that if fossil fuels were the principal source of
decreased over the second half of the twentieth century. Additional added CO2, CO2 levels should have increased by a little less than
measurements of coral skeletons and wood show that the ratio of 13C 2 ppm per year between 2004 and 2008—which is very close to
to 12C in air has been decreasing over the entire period during which the increase actually measured.
atmospheric CO2 levels have been increasing.
Air measurements at Niwot Ridge, Colorado

360 –6.2 80
14C relative to a standard (1000)
(1000, relative to a standard ratio)

350 –6.4

340 The ratio of 13C to –6.6 70

12C has declined.
Atmospheric CO2 (ppm)

320 –6.8
Ratio of 13C to 12C

330 –7.0
60
310 –7.2
The total amount of
300 atmospheric CO2 has –7.4
increased. 50
Amount of

290 –7.6

280 –7.8
40
1700 1750 1800 1850 1900 1950 2000 2002 2004 2006 2008 2010
Year Year

DISCUSSION The ratio of 13C to 12C in CO2 added to the CONCLUSION Fossil fuel burning by industrialized societies has
atmosphere over the past 200 years is lower than that in CO2 that been, and continues to be, a principal source of CO2 buildup in
was already in the air more than 200 years ago. The ratio of Earth’s atmosphere.
13
C to 12C in CO2 emitted by volcanoes is too high to account for the
SOURCES Revelle, R., and H. E. Suess. 1957. “Carbon dioxide exchange between
data, as is the ratio in CO2 released from the oceans. In contrast, atmosphere and ocean and the question of an increase of atmospheric CO2
organic matter formed by photosynthesis has just the right ratio during the past decades.” Tellus 9:18–27; Turnbull, J. C., et al. 2007. “A new
high precision 14CO2 time series for North American continental air.” Journal of
of 13C to 12C to account for Suess’s measurements. By itself, the Geophysical Research: Atmospheres 112, no. D11. Article Number: D11310. doi:
changing abundance of 13C in the air could reflect the conversion 10.1029/2006JD008184.

514
 CHAPTER 25 C YC L I N G C A R B O N 515

is more complicated than a simple exchange of carbon between

FIG. 25.5 Carbon dioxide sources and sinks. Much of the CO2 photosynthesis and respiration. There are additional storehouses
introduced by humans into the atmosphere has not stayed of carbon and additional processes that move carbon from one
there. About half of it has accumulated as dissolved store to another, topics we discuss next.
inorganic carbon in the oceans or as new biomass on the
continents. Data from J. G. Canadell, C. Le Quéré, M. R. Raupach,
C. B. Field, E. T. Buitenhuis, P. Ciais, T. J. Conway, N. P. Gillett, R. A. 25.2 THE LONG-TERM CARBON CYCLE
Houghton, and G. Marland, 2007, “Contributions to Accelerating
Atmospheric CO2 Growth from Economic Activity, Carbon Intensity, and Up to this point, we have considered only the short-term carbon
Efficiency of Natural Sinks,” Proceedings of the National Academy of cycle: exchanges over days, years, and decades driven by the
Sciences USA 104:18866–18870. biological processes of photosynthesis and respiration, and
altered in recent times by human activities. Geologic evidence,
activities (in billion metric tons

During the American however, tells us that on timescales of thousands to millions of

Production of CO2 by human

CO2 Sources
Civil War, human
8 Fossil fuel burning generation of CO2 years, CO2 levels in air have changed even more dramatically.
Deforestation (tropics) came mostly from These longer-term changes mean that we must consider additional
of C per year)

6 Deforestation the clearing of contributions to the carbon cycle: physical processes, including
(outside the tropics) forests for agricul-
ture. Today, about volcanism and climate change. Indeed, the complete carbon
4
80% of annual cycle links Earth’s physical and biological processes, providing a
human additions are
2
due to burning of foundation for understanding the interconnected histories of life
fossil fuels. and environment through our planet’s long history.

Reservoirs and fluxes are key in long-term carbon

The fate of human generated

CO2 Sinks
CO2 (in billion metric tons

8 Ocean Only about half of cycling.

Land the CO2 generated To understand how biological and physical processes interact to
of C per year)

6 Atmospheric CO2 by human activities

ends up in the govern CO2 levels in the air, we must first look at how carbon is
atmosphere. The rest distributed among its various reservoirs, the places where carbon
4 accumulates in the is found on the Earth. As shown in the purple boxes in Fig. 25.6,
oceans or in
2 vegetation and soils. reservoirs of carbon include organisms, the atmosphere, soil, the
oceans, and sedimentary rocks, and both biological and geologic
1850 1900 1950 2000 processes cycle carbon back and forth among these pools.
Year How much carbon is stored in each of Earth’s major
reservoirs? If we add up all the carbon contained in the total
mass of organisms living on land, that amount of carbon is just
some relevant numbers to reveal another important aspect of the a bit smaller than the amount of carbon stored as CO2 in the
carbon cycle. atmosphere. Soil, by comparison, stores as much carbon as do
As shown in the top graph in Fig. 25.5, humans now add land organisms and the atmosphere combined, mostly as slowly
almost 9 billion metric tons of carbon to the atmosphere each decaying organic compounds. In the oceans, the amount of carbon
year, nearly all of it as CO2. Much of this material comes from contained in living organisms is actually very small. Much more
power plants, airplanes, ships, and automobiles, which oxidize resides as inorganic carbon dissolved in the water—most of it
ancient organic matter to CO2 on a grand scale. Further important in the deep oceans as CO2 and bicarbonate and carbonate ions.
inputs come from the conversion of forests to agriculture or Perhaps fortunately for us, the oceans have sopped up some of the
pastureland. Clearing and burning forest trees also oxidizes CO2 generated by human activities.
organic carbon to CO2. The biggest carbon reservoir of all, however, lies beneath our
Interestingly, if we compare our best estimates of human feet, within sediments and sedimentary rocks. Calcium carbonate
CO2 generation since 1958 (not including CO2 added through minerals (CaCO3), which form limestone, and organic matter
respiration) with the measured increase in atmospheric CO2, preserved in sedimentary rocks dwarf all other carbon reservoirs
we find that about half of the CO2 released by human activities combined by three orders of magnitude. Coal, petroleum, and
did not end up in the air. Where did it go? This has been a natural gas make up only a small part of the organic matter stored
major question for years, and it now appears that much of the in sedimentary rocks, but as we have seen, they play an important
“missing carbon” has been stored as inorganic carbon in the role in the modern carbon cycle.
form of CO2 , bicarbonate (HCO3–), and carbonate (CO32–) ions Fluxes are the rates at which carbon flows from one reservoir
dissolved in the oceans. The bottom graph in Fig. 25.5 shows to another. The sensitivity of different reservoirs to change
an estimate of the various places where human-produced CO2 depends on the relative sizes of the reservoir and of the amount
ends up. These observations indicate that Earth’s carbon cycle of movement of material into and out of it. When fluxes are large
516 SECTION 25.2 T H E LO N G -T E R M C A R B O N C YC L E
Marine biological activity:
Photosynthesis and respiration
nearly balance in the big
picture.
FIG. 25.6 Movement of carbon between reservoirs. Atmospheric
reservoir
The numbers for each reservoir are gigatons 830
(a gigaton equals 1 billion metric tons) of carbon,
and those for fluxes are gigatons of carbon per
year. Photo source: Fotosearch Stock Images; Data from
P. Rekacewicz, “The Carbon Cycle,” from Vital Climate
Graphics, UNEP/GRID–Arendal Maps & Graphics Library,
Terrestrial
JPG file, 2005, http://www.grida.no/graphicslib/detail/ organisms
Marine Marine reservoir
the-carbon-cycle_a224.
photosynthesis respiration 450–650
92 90

relative to the size of the reservoir, reservoir size

can change rapidly. As shown by the arrows in Fig.
25.6, the amount of carbon stored as CO2 in air is not Surface water
reservoir
that much larger than the annual fluxes into and 900
out of the atmosphere. For this reason, atmospheric
CO2 abundance can be influenced by a number of
processes at work in the carbon cycle.

Physical processes add and remove CO2 Marine organisms

from the atmosphere. reservoir
Physical processes, like biological ones, are capable 3
of adding and removing CO2 from the atmosphere.
A key process is volcanism: Volcanoes and mid-
ocean ridges release an estimated 0.1 billion metric
tons of carbon (as CO2) into the atmosphere each Deep ocean
year. More CO2, about 0.05 billion metric tons of water reservoir
carbon per year, is released by the slow oxidation 37,100
of coal, oil, and other ancient organic material in
sedimentary rocks exposed at the Earth’s surface.
In nature, bacteria and fungi accomplish most of
this oxidation by respiring old organic molecules. By
burning fossil fuels, humans accelerate this process
dramatically, increasing a hundredfold the rate at
which sedimentary organic carbon is oxidized.
What geologic processes remove carbon from
the atmosphere? About 0.43 billion metric tons of In essence, carbon moves from the atmosphere to the sediments,
CO2 are removed from the atmosphere by chemical where it can be stored for millions of years.
reactions between air and exposed rocks, a process The two linked processes of chemical weathering and mineral
called chemical weathering. To understand how precipitation in the oceans can be summarized by a simple
weathering fits into the carbon cycle, we must first chemical reaction (here, CaSiO3 stands in for rock-forming
understand that CO2 in the air reacts with rainwater minerals as a whole):
to form carbonic acid (H2CO3). This acid slowly reacts
CaSiO3 ⫹ CO2 → CaCO3 ⫹ SiO2
with rock-forming minerals, generating (among
other substances) calcium and bicarbonate (HCO3–) In present-day oceans, the calcium carbonate minerals that
ions that are transported by rivers to the oceans. accumulate on the seafloor do not precipitate directly from
Within the oceans, the calcium and bicarbonate ions seawater, like salt. Instead, they are generated predominantly
react with each other to form calcium carbonate by organisms, as mineralized skeletons and shells. Clams, corals,
(CaCO3) minerals that precipitate out of seawater and and many other organisms form beautiful and functional shells
accumulate on the ocean floor, forming limestone. of CaCO3 (Fig. 25.7). Skeleton formation is an example of
 CHAPTER 25 C YC L I N G C A R B O N 517
Terrestrial biological activity: Human activity:
As in the marine realm, Human inputs of carbon into the
photosynthesis and respiration atmosphere are large relative to the
on land are nearly but not quite net atmospheric exchange photosynthesis is a bit larger than the amount returned to the
in balance. associated with natural biological atmosphere by respiration. The difference isn’t much—about the
processes.
same magnitude as volcanic and weathering fluxes. The “excess”
organic carbon produced by photosynthesis is deposited in
sediments as they accumulate on land or the seafloor, sustaining
a small but constant leak of carbon from the short-term biological
Terrestrial Terrestrial Fossil Change in components of the carbon cycle to long-term geologic reservoirs.
photosynthesis respiration fuels land use
120 119 7.8 0.5 1.5 One final set of geological processes completes the long-term
carbon cycle. Earth’s crust is constantly in motion, propelled by
heat within the Earth’s interior. Plate tectonics is the name
given to this dynamic movement of our planet’s outer layer. New
crust forms at spreading centers, the places where molten rock
rises upward from the underlying mantle. Old crust is destroyed
in subduction zones, where one slab of crust slides beneath
another, returning material to the mantle. Plate tectonics provides
geology’s best explanation for the formation of mountains and
ocean basins, and it plays an important role in the long-term
carbon cycle. The crust that descends into subduction zones carries
with it sediments, including carbonate minerals and organic
matter. Subduction removes carbon from Earth’s surface, but
this carbon will be recycled to the surface as CO2 emitted from
volcanoes and mid-ocean ridges. Fig. 25.8 shows the physical
processes at work in Earth’s long-term carbon cycle.

j Quick Check 2 If plate tectonic processes form a chain of high

Organic matter mountains, would you expect atmospheric CO2 to increase or
in soils reservoir decrease?
1500–2400

Records of atmospheric composition over 400,000

years show periodic shifts in CO2 content.
Earlier in this chapter, we discussed evidence that before the
Industrial Revolution, atmospheric CO2 levels had not changed
Sediments
and sedimentary
rock reservoir
66,000,000–
FIG. 25.7 Clams and corals with skeletons made of calcium
100,000,000
carbonate. Source: Sami Sarkis/Getty Images.

biomineralization, the precipitation of minerals by organisms.

Biomineralization provides yet another link between Earth and
life, and one that has varied through evolutionary history.
The annual carbon fluxes associated with geologic processes
are small relative to the yearly inputs and outputs from
photosynthesis and respiration. As we see in Fig. 25.6, however,
photosynthesis and respiration almost cancel each other out.
In the long-term carbon cycle, it is the net result of these rapid
biological processes—photosynthesis input minus respiratory
output—that matters. Is CO2 produced or consumed when all
biological processes are considered together?
There is some uncertainty in estimates of reservoir and flux
sizes in the present- day carbon cycle, but by most estimates
the amount of carbon incorporated into organic matter by
518 SECTION 25.2 T H E LO N G -T E R M C A R B O N C YC L E

FIG. 25.8 Geological processes that drive the long-term carbon cycle. Numbers are estimates for annual fluxes of carbon, in gigatons.
Source: Data from J. Gaillardet and A. Galy, 2008, “Himalaya—Carbon Sink or Source?” Science 320:1728, doi:10.1126/science.1159279.

Volcanic emissions and

the oxidation of ancient
organic matter (mainly
microorganisms)
introduce CO2 into the
atmosphere.
Volcanism
0.1

Oxidation of coal,
oil, and ancient
organic matter
0.05

Organic
Silicate burial on land
weathering 0.2
0.14 Carbonate
weathering
0.29

Oceanic crust
weathering
0.02

Subduction
Sedimentation 0.083
of carbonate Burial of organic
shells and skeletons matter in sediments
0.22 0.2 Subduction carries CaCO3 and organic
matter down to the mantle, providing
a source of carbon for volcanoes that
will return CO2 to the atmosphere.
Weathering of ancient
rocks effectively removes
The amount of organic matter
CO2 from the atmosphere
buried in sediments effectively
and deposits it in the
reflects the excess of photosynthesis
oceans as CaCO3.
over respiration. Only a small
proportion of this buried organic
matter will become coal or oil.

appreciably for 1000 years or more. Longer-term records, shows that atmospheric CO2 has oscillated between 285 ppm and
however, show that the CO2 levels in air can change substantially 180 ppm for at least 400,000 years. On long timescales, therefore,
through time. the natural variations in the carbon cycle can be large.
At Vostok, high on the Antarctic ice sheet, glacial ice records The bottom graph in Fig. 25.9 shows an estimate of surface
more than 400,000 years of environmental history (Fig. 25.9). temperature obtained by chemical analysis of oxygen isotopes
As shown in the top graph of Fig. 25.9, the youngest samples show in ice from the same glacier and how it differed from the average
about 285 ppm CO2 in the atmosphere, consistent with direct temperature in 1950. Over the last 400,000 years, Antarctic
measurements of air, including the first years of the Keeling curve. temperature has oscillated between peaks of a few degrees warmer
Notice, however, that 20,000 years ago, CO2 levels were much than today and temperatures as much as 6°C to 8ºC colder than
lower—about 180 ppm. In fact, the Vostok ice core in its entirety the present. Interestingly, the temperature and CO2 curves closely
 CHAPTER 25 C YC L I N G C A R B O N 519

FIG. 25.9 Atmospheric CO2 content for the past 400,000 years. These measurements were recorded from air bubbles trapped in glacial ice
at Vostok, Antarctica. Source: Data from P. Rekacewicz, “Temperature and CO2 Concentration in the Atmosphere over the Past 400,000 Years,” from Vital Climate
Graphics, UNEP/GRID–Arendal Maps & Graphics Library, JPG file, 2005, http://www.grida.no/publications/vg/climate/page/3057.aspx. Based on J. R. Petit et al.,
1999, “Climate and Atmospheric History of the Past 420,000 Years from the Vostok Ice Core, Antarctica,” Nature 399:429–436.

Interglacial Glacial

300
CO2 concentration (ppm)

280
260
240
220
200
180
For the past 400,000 years,
160 temperatures have followed
+4 a pattern similar to that of
CO2 levels.
Temperature change
relative to 1950 (C)

+2
0
2
4
6
8
10
400,000 350,000 300,000 250,000 200,000 150,000 100,000 50,000 Present
Years before present

Cold glacial and warm interglacial periods

correlate with minimum and maximum
atmospheric CO2 levels, respectively.

parallel each other. As discussed more fully in Chapter 49, CO2 is The two curves correlate closely with one further
known to be an effective greenhouse gas, meaning that it allows phenomenon, the periodic growth and decay of continental ice
incoming solar radiation to reach Earth’s surface but traps heat sheets. Large glaciers expanded in the Northern and Southern
that is re-emitted from land and sea. Higher concentrations of hemispheres a few million years ago, ushering in an ice age. Today,
CO2 result in warmer temperatures. Therefore, it is not surprising we live in an interglacial interval, when climate is relatively mild,
that climate and atmospheric CO2 levels show the parallel history but 20,000 years ago thick sheets of ice extended far enough away
documented in the figure. from the poles to cover the present site of Boston (Fig. 25.10).

FIG. 25.10 Earth’s most recent ice age. Twenty

thousand years ago, glacial ice
(shown in white) covered much of
North America and Europe, as well as
mountain ranges such as the Andes
and Himalayas. Sea ice (light blue)
expanded markedly in both northern
and southern oceans. Source: Plate tectonic
maps and continental drift animations by C. R.
Scotese, PALEOMAP Project (www.scotese.com).
520 SECTION 25.2 T H E LO N G -T E R M C A R B O N C YC L E

The repeated climatic shifts recorded in ice cores reflect periodic For the past 30 million years or so, atmospheric CO2 levels have
variations in the amount and distribution of solar radiation on probably not exceeded those recorded today at Mauna Loa by much
Earth’s surface, which are caused by oscillating changes in Earth’s (Fig. 25.11). Before that, however, higher levels of CO2—perhaps
orbit around the sun. four to six times the 1958 level—characterized the Mesozoic Era
The temperature and CO2 increases recorded by Vostok ice (252 to 66 million years ago), a generally warm interval commonly
between 20,000 and 10,000 years ago coincide with the last great known as the age of dinosaurs. Levels more like those of today
retreat of continental ice sheets. What processes might explain existed earlier, during much of the late Paleozoic Era (350–252
how atmospheric CO2 could increase by 100 ppm in just a few million years ago), another time of extensive glaciation. And
thousand years, as glaciers began to retreat? Can the short-term before that, about 500 million years ago, early in the Paleozoic Era,
carbon cycle processes of photosynthesis and respiration account atmospheric CO2 may have reached values as high as 15 to 20 times
for this much carbon? Certainly, the amount of forests on Earth’s the present-day level.
surface has varied through the past 500,000 years as ice sheets On timescales of millions of years, atmospheric CO2 and,
grew and decayed, but forests expand as glaciers shrink, so changes hence, climate, are determined in large part by geologic processes:
in forests cannot account for a pattern of increasing atmospheric changes in the rate of organic carbon burial in sediments,
CO2 with the retreat of glaciers. Volcanism and weathering also continental weathering of rocks uplifted into mountains, and
fail to account for the observed pattern. There is no evidence that volcanic gas release. All these processes reflect the action of
volcanic activity has waxed and waned in a pattern that could Earth’s great physical engine: plate tectonics. Seafloor formation
explain observed CO2 variations. And rates of weathering, which and destruction together influence rates of volcanic gas release,
remove CO2 from the atmosphere, should increase as temperature and mountain formation strongly influences rates of weathering
rises, but CO2 levels have actually increased. Something else must and, therefore, the fluxes of sediments that bury organic carbon
be going on. beneath the seafloor.
Scientists continue to debate why atmospheric CO2 oscillates
in parallel with glacial expansion and retreat, but, increasingly,
proposed mechanisms suggest interactions involving the ocean FIG. 25.11 Atmospheric CO2 content for the past 541 million
and its large reservoir of inorganic carbon. For example, it has years. Models and indirect estimates from geological
been hypothesized that during glacial advances, the circulation measurements suggest that CO2 levels have varied
of carbon-rich deep-ocean waters back to the sea surface slows, substantially through Earth’s history. Data from Fig 3.4
causing more inorganic carbon to accumulate in the deep sea. With in K. Wallmann and G. Aloisi, 2012, “The Global Carbon Cycle:
glacial retreat, the oceans circulate more vigorously, returning Geological Processes,” in Fundamentals of Geobiology, A. H. Knoll,
CO2 to the surface and then to the atmosphere. Whatever the D. E. Canfield, and K. O. Konhauser (eds.), Chichester, England:
explanation, the historical record of the past 400,000 years shows Wiley-Blackwell, pp. 20–35.
that climate can and does change without any input from humans,
something we must take into account when considering our Paleozoic Mesozoic Cenozoic
climatic future (Chapter 49). 8000
Estimates of atmospheric CO2
Uncertainty
Atmospheric CO2 (ppm)

Variations in atmospheric CO2 over hundreds of 6000 Estimates from

millions of years reflect plate tectonics and evolution. geological analyses
The atmospheric record trapped in glacial ice extends back in time
4000
less than 1,000,000 years. To estimate atmospheric CO2 levels
for earlier intervals of Earth history, we must rely on computer
models and measurements of chemical or paleontological 2000
features of ancient rocks that are thought to reflect the levels
of atmospheric CO2 at their time of formation. One substitute 0
for direct measurements of ancient CO2 levels is the anatomy of 500 400 300 200 100
Present

fossil leaves: Experiments show that stomata, the small pores on Millions of years ago
leaf surfaces (Chapter 29), decrease in density as atmospheric
CO2 levels increase. The chemistry of ancient soils is also thought The dark blue line Atmospheric CO2 The light blue shading
shows the best levels are thought around the dark blue line
to reflect atmospheric history. Such indirect observations and estimate for CO2 to have been high indicates that actual
analyses come with a large degree of uncertainty, but most Earth through time. before about 350 levels have a 95% chance
scientists accept at least the broad pattern of atmospheric history million years ago. of falling within the
shading.
shown in Fig. 25.11.
 CHAPTER 25 C YC L I N G C A R B O N 521

25.3 THE CARBON FIG. 25.12 A food web. Food webs trace the cycling of carbon through communities.
CYCLE: ECOLOGY,
BIODIVERSITY,
AND EVOLUTION
So far, we have outlined the biological and Secondary
physical pathways that cycle carbon through consumers obtain
Wastes and dead
Earth’s surface environments. How can their carbon by
organisms
eating primary
the carbon cycle provide a framework for consumers.
understanding basic features of ecology,
biodiversity, and evolution?

Food webs trace the cycling of

carbon through communities and
ecosystems.
As you sit quietly by a woodland pond,
the carbon cycle is at work all around you Primary consumers,
(Fig. 25.12). In the forest, plants generate or grazers, obtain
carbohydrates as they photosynthesize, and their carbon by
eating primary
algae and photosynthetic bacteria are doing producers.
the same in the pond. All of the organisms in
a habitat such as a woodland pond constitute
an ecological community. In Chapter 6, we
called photosynthetic organisms autotrophs
(“self feeders”) because they synthesize
the organic molecules needed for growth Primary producers
and reproduction from CO2. Ecologists (plants, algae, and
commonly refer to these organisms as photosynthetic
bacteria) generate
primary producers, reflecting their organic molecules
position within the carbon cycle. Primary by photosynthesis.
producers are the first biological reservoir
for carbon once it has been taken out of the
physical environment.
An insect grazes on leaves, and ducks Bacteria, fungi, protists, and
certain animals consume
feed on both insects and algae. Ducks and
wastes and dead organisms,
insects obtain the carbon they need for making their carbon and other
growth and reproduction from the foods nutrients available once more
to primary producers.
they eat, and they also gain energy by
respiring food molecules. In Chapter 6, we
called such organisms heterotrophs (“other
feeders”), and ecologists label them consumers, again with Secondary consumers, in turn, are predators or scavengers
reference to the carbon cycle. Consumers include all the animals that feed on primary consumers. In particular, animals that eat
in both pond and forest. Beneath our feet, similar interactions play other animals are called carnivores, and these organisms may
out on a microscopic scale, especially among the fungi, protists, provide food for still other carnivores, generating successive tiers
and bacteria that recycle the organic carbon in soil back to CO2. of consumers. Like primary consumers, secondary consumers
Primary consumers consume primary producers. Biologists continue the transfer of carbon from reservoir to reservoir by eating
call these organisms herbivores or grazers. Primary consumers the primary consumers and returning some of the carbon stored in
transfer carbon drawn by the primary producers from the that reservoir to the environment through respiration. Eventually,
environment to a second biological reservoir and, by respiration, having passed from one consumer to another, the carbon originally
return some of the carbon to surrounding air or water. fixed by photosynthesis is returned to the atmosphere by the
522 SECTION 25.3 T H E C A R B O N C YC L E : E CO LO G Y , B I O D I V E R S I T Y , A N D E VO LU T I O N

respiration of fungi, bacteria, and other decomposers that break and maintain diversity by performing comparable metabolic
down dead tissues. The cycle has come full circle. functions in different habitats (Chapter 48). In the forests of
The transfer of carbon from one organism to another is called New England, the tree species that thrive in wet regions differ
a food chain. Because most heterotrophs within a community can from those found on well-drained hillsides. And seasonally dry
consume or be consumed by a number of other species, biologists woodlands in southern California support yet another set of plant
often prefer to speak of food webs, a term that provides a better species. In general, plants of varying size, shape, and physiology
sense of the complexity of biological interactions within the inhabit physically and biologically distinct environments, and the
carbon cycle (Fig. 25.12). Food webs define the interactions among same is true of photosynthetic organisms in lakes and oceans.
organisms in ponds, forests, and many other habitats; we discuss Thus, the immense diversity of photosynthetic organisms
them in detail in Chapter 47. For now, the main point is that food found today does not reflect evolutionary variations in the
webs track the passage of carbon atoms through the biological biochemistry of photosynthesis (although some of that occurs;
carbon cycle. Put another way, the carbon cycle underpins the see Chapter 29) so much as it does structural and physiological
ecological structure of biological communities. adaptations. These adaptations allow the effective gathering of
The complementary metabolic processes of photosynthesis light, nutrients, and—critical to life on land—water, in widely
and respiration cycle carbon through forest and pond varying local environments. Natural selection, acting on local
communities. Furthermore, as we discuss in the next chapter, populations, links the diversity of photosynthetic organisms to the
complementary metabolic processes also cycle nitrogen, sulfur, carbon cycle.
and other elements required for life. By continually recycling If half a million species function as primary producers,
materials, biogeochemical cycles, which involve both biological an estimated 10 million species help to cycle carbon through
and physical processes, sustain life over long intervals. In their respiration. These include the plants, algae, and bacteria that
absence, life could hardly have persisted for 4 billion years. generate carbohydrates by photosynthesis, as well as animals,
fungi, and microorganisms that obtain both carbon and energy
Biological diversity reflects the many ways that from organic compounds in photosynthetic organisms or the
organisms participate in the carbon cycle. consumers that eat them. These organisms are essential to the
Among the most remarkable features of life is its astonishing completion of the short-term carbon cycle, returning carbon
diversity. Biological diversity, or biodiversity, is the product of atoms to the environment as CO2.
evolution, but it is shaped and sustained by ecological interactions Heterotrophic bacteria, amoebas, and humans may use
among organisms and between organisms and the physical essentially the same biochemical pathway to respire organic
environment. molecules, but they differ markedly in how they feed and,
To begin, an estimated 500,000 species of photosynthetic therefore, in what they can eat. Bacteria (and also fungi) absorb
organisms fuel the carbon cycle in all but a few deep-sea and molecules from their environment, but amoebas and many
subterranean environments. In terms of carbon and energy other eukaryotic microorganisms can capture and ingest cells—
metabolism, all these species do pretty much the same thing. Why, they are capable of predation. Animals capture prey, as well,
then, is there such a diversity of photosynthetic organisms? Why but commonly feed on organisms far too large for an amoeba
don’t just a few species dominate the world’s photosynthesis? to eat. As photosynthetic organisms have adapted structurally
The example of the pond and forest helps to make the basis and physiologically to local environments across the globe,
of photosynthetic diversity clear. In the forest community, the consumers have adapted by means of locomotion, mouth and
leaves of several different tree species form a photosynthetic limb specialization, perception, and behavior to obtain their
canopy above the forest floor. Below, shrubs grow, making use of food. So, as you read about bacteria, amoebas, and insects in the
light not absorbed by the leaves above them. And below the shrubs chapters that follow, think about how each operates within the
are grasses, herbs, ferns, and mosses that can grow in the reduced carbon cycle and so contributes to the operation of Earth’s diverse
light levels of the forest floor. In the nearby pond, a few species ecosystems.
of aquatic plants line the water’s edge, but beyond that algae and
photosynthetic bacteria dominate photosynthesis, with some The carbon cycle weaves together biological evolution
species anchored to the pond bottom and others floating in the and environmental change through Earth history.
water column. The world did not always support the biological diversity we see
Locally, then, the different photosynthetic species that today. Indeed, 2 billion years ago, the carbon cycle included mostly
transfer carbon atoms from CO2 to organic molecules subdivide photosynthetic bacteria and microbial heterotrophs because
the forest on the basis of light, water, and nutrient availability—a there were no plants or animals (Fig. 25.13). Subsequently, algae
pattern complicated by grazing and environmental disturbances gained an ecological foothold as nutrients such as nitrate became
such as fire and landslides (Chapter 47). On a larger scale, climate more widely available, and single-celled eukaryotic heterotrophs
and topography vary tremendously from one region to another, expanded their reach by gathering food in ways not possible for
and these features, as well, help to explain how plants can build bacteria, ingesting cells and other types of particulate food. With
 CHAPTER 25 C YC L I N G C A R B O N 523

But life does not just respond passively to

FIG. 25.13 Microbial communities in hot springs in New Zealand. This changing environments—it can drive environmental
environment and others that few eukaryotes tolerate provide a glimpse change through time. For example, the large drop in
of how the carbon cycle worked before algae, amoebas, plants, and atmospheric CO2 suggested for the mid-Paleozoic Era,
animals evolved. Source: Albert Aanensen/Nature Picture Library. 400–350 million years ago (see Fig. 25.11), is thought
to reflect the evolution of a new player in the carbon
cycle: woody plants. The evolution of trees increased
the size of the carbon reservoir on land and ushered
in an important new mechanism for removing carbon
from the air and ultimately to sedimentary rocks. That
mechanism was the burial of plant material on land,
forming peat and, eventually, coal.
And, in the current century, a single animal
species is influencing the carbon cycle in new ways.
That species is us. As noted in our opening discussion
of the Keeling curve, humans are adding CO2 to the
atmosphere at a high rate, warming Earth’s surface by
the greenhouse effect. The consequences of human
activities for Earth and its living organisms are
discussed more fully in Chapter 49. For now, just bear
in mind that modern humans are not simply observers
of the carbon cycle; we are major participants in it.
In the chapters that follow, we examine life’s
diversity in terms of function, phylogeny, and ecology,
the evolution of multicellularity, animals and seaweeds added but don’t be fooled into thinking that these are discrete topics:
further complexity to carbon cycling in the sea. Eventually, They are inextricably intertwined. The remarkable diversity of life
some aquatic green algae evolved the capacity to live on land, on Earth reflects the ecology and evolution of populations that
and animals soon followed, building the carbon cycle on land and
thereby generating unprecedented levels of diversity on our planet.
interact to cycle carbon through the biosphere. •
The history of life is one of accumulating variety and
complexity, based mostly on the great biological components of the
carbon cycle: photosynthesis and respiration. Indeed, the carbon FIG. 25.14 Oxygen levels in Earth’s atmosphere during the
cycle did more than provide a framework for accumulating diversity; history of life.
it changed the very nature of Earth’s surface environments in ways
that enabled new types of organism to evolve.
Earth’s atmosphere contained Oxygen rose to 1% to 10%
When life began, the atmosphere and oceans contained little or
little or no oxygen gas for the of modern levels about 2.4
no oxygen gas. How did our present environment come to be? The first 2 billion years of our billion years ago.
key lies in the formulas for photosynthesis and respiration, shown planet‘s history.
in Fig. 25.2. Photosynthesis and respiration not only cycle carbon,
O2 in the atmosphere

they cycle oxygen (and water) as well. For this reason, the history
(% of modern level)

100
of atmospheric oxygen is closely tied to the workings of the carbon
cycle through time. When oxygen production by photosynthesis 1210
and oxygen consumption by respiration are in balance, oxygen
levels do not change. However, when some of the organic carbon
generated by photosynthesis is buried in sediments, thereby
,,1023
avoiding being returned to the environment through respiration,
4 3 2 1 0
some of the O2 also generated by primary producers can accumulate
Age (billions of years ago)
in the atmosphere and oceans. In other words, because sedimentary Animals
organic matter burial can break the tight coupling depicted in
Oxygen levels did not increase to
Fig. 25.2, it can facilitate an increase in O2 through time. The levels that humans could breathe
geologic history of oxygen shown in Fig. 25.14 results from the until about 580 million years ago,
interactions through time between the carbon (and sulfur) cycle coinciding with the first appearance
of animals in the fossil record.
with plate tectonics. Our oxygen-rich world is the result.
524 SELF-ASSESSMENT

Core Concepts Summary Over time spans of hundreds of millions of years, movements
of Earth’s plates become major influences on the carbon cycle.
page 520
25.1 Photosynthesis and respiration are the key
biochemical pathways of the biological, or short-term,
carbon cycle. 25.3 The carbon cycle can help us understand ecological
interactions and the evolution of biological diversity.
Keeling’s program to monitor CO2 levels in the atmosphere
revealed two patterns: (1) CO2 levels oscillate on a yearly basis, Photosynthetic and other autotrophic organisms are primary
and (2) CO2 levels have increased steadily to the present day producers, converting CO2 into organic molecules. page 521
since measurements began. page 510 Heterotrophic organisms that eat primary producers are called
The first pattern (seasonal oscillation) can be explained by the primary consumers. page 521
global imbalance between photosynthesis and respiration in
Heterotrophs that prey on grazers are called secondary
the summer and in the winter months. page 512
consumers. page 521
The second pattern (steady increase) can be extended further
Interacting networks of species, linked by predator–prey
back in time by measuring the composition of air trapped in
interactions, are called food webs, and they are an important
gas bubbles within polar ice. These measurements indicate
feature of the carbon cycle. page 522
that CO2 levels in the atmosphere remained steady for more
than a thousand years, but then started to increase in the Primary producers are well adapted to obtaining sunlight,
mid-1800s, the time of the Industrial Revolution. page 513 water, and nutrients in different environments. page 522

The ratio of different isotopes of carbon in the atmosphere Primary and secondary consumers are adapted to obtaining
indicates that most of the carbon added to the atmosphere in different sources and sizes of food. page 522
recent decades comes from human activities, particularly the
The evolution of life on Earth can be considered a long history
burning of fossil fuels. page 513
during which organisms have evolved different ways of
25.2 Physical processes govern the long-term obtaining energy and carbon from their environment.
page 523
carbon cycle.
The interactions between biological and geologic processes
Physical processes, which act at slower rates and over longer
at work in the carbon cycle are also responsible for Earth’s
timescales, are the major drivers of the long-term carbon cycle.
oxygen-rich atmosphere and oceans. page 523
page 515

In following carbon around its cycle, it is important to consider

where carbon is stored (reservoirs) and rates at which it is Self-Assessment
added to or removed from these stores (fluxes). page 515
1. Why does the coupling of photosynthesis and respiration
Important reservoirs of carbon include organisms, result in the cycling of carbon?
the atmosphere, soils, the oceans, and sedimentary
rocks. Sedimentary rocks are by far the largest carbon 2. Draw and explain the curve representing changing
reservoir. page 515 atmospheric levels of CO2 over the course of a year.

Physical processes that remove CO2 from the atmosphere 3. Draw and explain the curve representing changing
include the weathering of rocks, and those that add CO2 to the atmospheric levels of CO2 over the past 150 years.
atmosphere include volcanoes and the oxidation of ancient 4. Describe one way by which the source of the modern
organic matter in sedimentary rocks. page 516 increase in CO2 in the atmosphere can be determined.
The short- and long-term carbon cycles are linked by the slow 5. How do geologic processes participate in the long-term
leakage of organic matter from biological communities to carbon cycle?
sediments accumulating on the seafloor. page 516
6. Draw and explain the curve representing changing
Over the last 400,000 years, CO2 levels in the atmosphere have
atmospheric levels of CO2 over the last 400,000 years.
gone through periodic shifts that coincide with repeated cycles
of glaciation. page 518
Log in to to check your answers to the Self-
CO2 levels in the atmosphere 500 million years ago may have Assessment questions, and to access additional learning tools.
been as much as 15 to 20 times higher than present-day levels.
page 520
525 SECTION 12.1 P

CASE 5

The Human Microbiome

Diversity Within
Clostridium difficile doesn’t grab many headlines. Perhaps it for scientists to decipher exactly what role individual species
should. This bacterium is thought to be the most common play in health and in disease. The sheer number of bacteria
cause of hospital-acquired infections in the United States. in your gut only compounds the problem. One international
It sickens as many as 300,000 U.S. hospital patients each team of scientists recently reported that each human gut
year, causing fevers, chills, diarrhea, and severe intestinal contains at least 160 different bacterial species.
pain—and adds more than $1 billion annually to the Despite these challenges, researchers have made strides
nation’s health care costs. Altogether, the infection kills toward understanding the unique bacterial communities
about 29,000 people every year, according to the Centers within our intestinal tract. In 2011, scientists compared all
for Disease Control and Prevention. the DNA sequences from gut microbes of people living in
Now a promising (if stomach-turning) new treatment Europe, America, and Japan. They found that each individual
is gaining traction: fecal transplants. The vast majority of fell into one of three
C. difficile cases occur after patients have taken antibiotics enterotypes, or “gut No one has yet identified
for other infections. The drugs kill the beneficial bacteria types.” Most people
normally found in a healthy gut, allowing C. difficile to had a bacterial mix
the many microbes living
proliferate. Recently, pioneering researchers have begun to dominated by organisms on and within our bodies,
replace the missing microbes by transplanting fecal material from the genus
from healthy donors into patients sick with C. difficile. Ruminococcus. But others
but estimates suggest that
Some doctors have reported that the procedure alleviates had gut types dominated each of us contains 10
symptoms in more than 80% of patients. by the genus Bacteroides
While transplanting fecal bacteria may not yet be or the genus Prevotella.
times as many bacterial
mainstream medicine, there is no question that our gut, as The three different gut cells as human cells.
well as our skin, mouth, nose, and urogenital tract, is home compositions appear
to a diversity of bacterial species. No one has yet identified to produce different vitamins and may even make a person
all of the many microbes living on and within our bodies, more, or less, susceptible to certain diseases.
but estimates suggest that each of us contains 10 times as Surprisingly, the researchers reported that enterotype
many bacterial cells as human cells. For the most part, the was unrelated to gender, body mass index, or nationality. So
relationship between “us and them” is a type of symbiosis, what determines your gut type? A follow-up study suggested
in which both humans and bacteria benefit from the close that enterotypes were associated with long-term dietary
association. Indeed, the microbiome, as our assemblage of patterns. People who eat a diet high in animal protein and
bacterial houseguests is known, affects human health in saturated fats are more likely to contain a gut community
many important ways. dominated by Bacteroides. People who stick to a high-
Some of the most mysterious, and most intriguing, carbohydrate diet rich in plant-based nutrition are more
members of our microbiome live in our gut. Bacteria living likely to host an abundance of Prevotella.
in our intestines help us break down food and synthesize The researchers don’t know yet what a person’s
vitamins. On the other hand, abnormal communities of enterotype means for his or her health. But scientists are
microbes in the gut have been linked to Crohn’s disease, eager to explore the link between microbiome and physical
colon cancer, and nonalcoholic fatty liver disease. well-being. Meanwhile, various researchers are exploring the
Your own personal gut flora is a unique collection microbiome of other organisms—specifically, cows.
of species—even identical twins can have surprisingly As it happens, there are good reasons to peer into
different microbiomes. This variability has made it difficult a cow’s gut. The cow’s rumen (one of its four digestive
525
 The human microbiome. Different groups of bacteria reside in or on each part of
the body. When the Helicocbacter pylori is present in the stomach (H. pylori +), it can
overwhelm other bacterial populations (H. pylori –). Source: Mimi Haddon/Getty Images.

Nostril
Hair

Oral cavity
Esophagus

Skin Stomach
H. pylori (+)

Stomach
H. pylori (–)

Colon

Urogenital
tract

Actinobacteria
Firmicutes
Proteobacteria
Bacteroidetes
Cyanobacteria
Fusobacteria

chambers) contains microbes that break down the cellulose in converting material containing cellulose, such as
in plant material. Without them, cattle wouldn’t be able switchgrass, into biofuels.
to extract adequate nutrition from the grass they graze. As researchers look for more sustainable replacements
Those microbes are the particular focus of scientists at for fossil fuels, many have pinned their hopes on biofuels.
the Department of Energy (DOE), who are interested But the enzymes currently used commercially to break

526
 The bobtail squid and its luminescent bacteria. This squid contains specialized
organs that harbor light-emitting bacteria. Source: Gary Bell/OceanwideImages.com.

down cellulose aren’t efficient or cost effective enough light from the surface as a way to camouflage themselves
to produce biofuels on an industrial scale. That’s where from would-be predators below.
the cows come in. DOE scientists recently analyzed In many cases, animals and their bacterial symbionts
the microbial genes present in the cow rumen. They are so intimately connected that it’s hard to separate one
discovered nearly 28,000 genes for proteins that metabolize from the other. Many human gut bacteria are poorly studied
carbohydrates, and at least 50 new proteins. The hope is that because scientists haven’t been able to grow them outside
some of these newly discovered proteins will help break the body. But some species have taken their intimate
down cellulose in ways that will produce biofuels more bacterial partnership a step further.
efficiently and cheaply. Aphids are small insects that feed on plant sap. They
Mammals such as cows and humans are hardly the rely on the bacterium Buchnera aphidicola to synthesize
only organisms that have symbiotic relationships with certain amino acids for them that aren’t available from
bacteria. All living animals have their own microbiomes. their plant-based diet. The partnership between aphids
Even tiny single-celled eukaryotes can harbor still-tinier and Buchnera dates back as far as 250 million to 150
bacterial guests. million years ago. Over time, the organisms have become
Symbiotic bacteria provide a variety of benefits to completely dependent on one another for survival. The
their hosts. Consider the bobtail squid, Euprymna scolopes, Buchnera bacteria actually live inside the aphid’s cells. In
which possesses a specialized organ to house bacteria. The some ways, the bacteria resemble organelles rather than
bacteria, Vibrio fischeri, are luminescent—that is, capable independent organisms. Intracellular symbionts may not
of generating light. The squid use their Vibrio-filled light be so unusual. In fact, mitochondria and chloroplasts—
organs like spotlights, projecting light downward to match key organelles of eukaryotic cells—are thought to have

527
 originated from symbiotic bacteria. Over time, the Clearly, there are many good reasons to learn more
progenitor bacteria became so intertwined with their host about our bacterial comrades. From evolution to energy to
cells that they developed into an integral part of the cells human health, our microbiomes hold great promise—and
themselves. Studying simple sap-sucking aphids, it turns great mystery. We’ve evolved hand in hand with these
out, may offer insights into the evolutionary history of all bacteria for millions of years. Let’s hope it takes less time to
eukaryotic cells. uncover their secrets.

? CASE 5 QUESTIONS
Special sections in Chapters 26 and 27 discuss the following questions related to Case 5.

1. How do intestinal bacteria influence human health? See page 550.

2. What role did symbiosis play in the origin of chloroplasts? See page 557.
3. What role did symbiosis play in the origin of mitochondria? See page 559.
4. How did the eukaryotic cell originate? See page 560.

528
 CHAPTER 26

Bacteria
and Archaea
Core Concepts
26.1 The tree of life has three main
branches, called domains:
Eukarya, Bacteria, and
Archaea.
26.2 Bacteria and Archaea are
notable for their metabolic
diversity.
26.3 In addition to their key roles
in the carbon cycle, Bacteria
and Archaea are critical to the
biological cycling of sulfur and
nitrogen.
26.4 The extent of bacterial
diversity was recognized when
sequencing technologies could
be applied to non-culturable
bacteria.
26.5 The diversity of Archaea has
only recently been recognized.
26.6 The earliest forms of life
on Earth were Bacteria and
Archaea.
Steve Gschmeissner/SPL/Getty Images.

529
530 SECTION 26.1 T W O P RO K A RYOT I C D O M A I N S

In Chapter 25, we discussed how the cycling of carbon by plants membrane-bounded nuclei, no energy-producing organelles, no
and animals is linked to their production and consumption of sex. This point of view turns out to be more than a little misleading.
oxygen. But can carbon cycle through the deep waters of the Bacteria are the diverse and remarkably successful products of
Black Sea, within black muds beneath swamps and marshlands, nearly 4 billion years of evolution. Today, bacterial cells outnumber
or in other habitats where oxygen is limited or absent? Can eukaryotic cells by several orders of magnitude. Even in your own
biological communities even survive without oxygen? The body, bacteria outnumber human cells 10 to 1.
answer, emphatically, is yes. Indeed, life existed on Earth for more Fig. 26.1 illustrates the bacterial cell, which was briefly
than a billion years before oxygen-rich habitats first appeared on introduced in Chapter 5. The cell’s DNA is present in a single
our planet. circular chromosome, in contrast to the multiple linear
The seemingly alien oxygen-poor environments on today’s chromosomes characteristic of eukaryotic cells. Many bacteria
Earth are populated by organisms that neither produce nor carry additional DNA in the form of plasmids, small circles
consume oxygen, yet are able to cycle carbon. The organisms of DNA that replicate independently of the cell’s circular
responsible for this expanded cycle share one fundamental feature: chromosome. In general, plasmid DNA is not essential for the
They have prokaryotic cell organization. Prokaryotes, which cell’s survival, but it may contain genes that have adaptive value
include bacteria and archaeons, lack a nucleus (Chapter 5). They under specific environmental conditions. No nuclear membrane
inhabit the full range of environments present on Earth, including separates DNA from the surrounding cytoplasm, and so transcribed
those rich in oxygen, but their presence in habitats where plants mRNA is immediately translated into proteins by ribosomes.
and animals cannot live suggests that some of these organisms Bacteria lack the membrane-bounded organelles found in
have biological features quite unlike those of our familiar world. eukaryotic cells. Instead, cell processes such as metabolism are
We’ll look at the carbon cycle again later in this chapter, but first carried out by proteins that float freely in the cytoplasm or are
we discuss Bacteria and Archaea, the prokaryotic domains of embedded in the plasma membrane. A few bacteria, notably the
microscopic organisms so critical to the cycling of carbon and photosynthetic bacteria, contain internal membranes similar
other elements essential to life. We examine their diversity and to those found in chloroplasts and mitochondria. The light
how they have evolved through time. reactions of photosynthetic bacteria take place in association with
membranes distributed within the cytoplasm.
Structural support is provided by a cell wall made of
peptidoglycan, a complex polymer of sugars and amino acids.
26.1 TWO PROKARYOTIC DOMAINS
Some bacteria have thick walls made up of multiple peptidoglycan
Chapter 5 outlined the two distinct ways that cells are organized layers, while others have thin walls surrounded by an outer layer
internally. Eukaryotic cells, which include the cells that make up of lipids. For many years, it was believed that bacteria lacked the
our bodies, have a membrane-bounded nucleus and organelles that cytoskeletal framework that organizes cytoplasm in eukaryotic
form separate compartments for distinct cell functions. Prokaryotic cells. However, careful studies now show that bacteria do have
cells have a simpler organization. No membrane surrounds the an internal scaffolding of proteins that plays an important role in
cell’s DNA, and there is little in the way of cell compartments. determining the shape, polarity, and other spatial properties of
Prokaryotic cell organization is an ancestral character for life as a bacterial cells (Chapter 10).
whole—that is, it is a feature that was present in the last common
ancestor of all organisms alive today. The group defined traditionally Diffusion limits cell size in bacteria.
as the prokaryotes, however, is paraphyletic in that it excludes some Most bacterial cells are tiny: The smallest are only 200–300
descendants of the last common ancestor of all living organisms, nanometers (nm) in diameter, and relatively few are more than
namely eukaryotes (Chapter 23). 1–2 micrometers (µm) long. Why are bacteria so small? The
Bacteria and Archaea are the two domains characterized by answer has to do with diffusion, a critical process introduced in
prokaryotic cell structure. These organisms are present almost Chapter 5. If you could watch the movement of any particular
everywhere on Earth. What they lack in complexity of cell molecule in air or water, you would see that its motion is random,
structure, these tiny cells more than make up for in their dazzling sometimes in one direction and sometimes in another. On
metabolic diversity. As will become clear over the course of this average, however, more molecules move from a region with a
chapter, Bacteria and Archaea underpin the efficient operation of higher concentration of the molecule to a region with a lower
ecosystems on our planet. concentration of the molecule than move in the other direction.
Net movement stops only when the two regions achieve equal
The bacterial cell is small but powerful. concentration, but diffusion continues. Photosynthetic bacteria
Because of their small size and deceptively simple cell organization, gain the carbon dioxide they need by the diffusion of CO2 from the
bacteria were long dismissed as primitive organisms, distinguished environment into the cell, and that is also how respiring bacteria
mostly by the eukaryotic features they lack: They have no take in small organic molecules and oxygen.
 CHAPTER 26 B AC T E R I A A N D A RC H A E A 531

FIG. 26.1 A bacterial cell. The cells of Bacteria (and Archaea) do not have a membrane-bounded nucleus or other organelles.

Chromosomal DNA
The DNA of bacteria is
contained in a circular Cytoplasm
chromosome, folded
into many loops.

Plasmid DNA

Bacteria often have small

circles of additional DNA Ribosome
called plasmids.

Cell wall
Plasma membrane

Bacteria have both a

plasma membrane and
a cell wall.

Flagellum

Diffusion explains why bacterial cells tend to be small. A that of Escherichia coli (Fig. 26.3). But, in one sense, T. namibiensis
small cell has more surface area in proportion to its volume, and cheats: 98% of its volume is taken up by a large vacuole, so the
the interior parts of a small cell are closer to the surrounding metabolically active cytoplasm is restricted to a thin film around the
environment than those of a larger cell. As a consequence, slowly cell’s periphery. Thus, the distance through which nutrients move
diffusing molecules do not have to travel far to reach every part by diffusion is only a few micrometers, as in many other bacteria.
of a small cell’s interior. The surface area of a spherical cell—the Some bacteria are multicellular, forming simple filaments or
area available for taking up molecules from the environment— sheets of cells. More unusual are myxobacteria, which aggregate to
increases as the square of the radius. However, the cell’s volume— form multicellular reproductive structures that are composed of
the amount of cytoplasm that is supported by diffusion—increases several distinct cell types (see Fig. 26.2e).
as the cube of the radius. Therefore, as cell size increases, it
becomes harder to supply the cell with the materials needed for Horizontal gene transfer promotes genetic diversity
growth. For this reason, most bacterial cells are tiny spheres, rods, in bacteria.
spirals, or filaments—small enough for molecules to diffuse into Bacterial genomes are generally smaller than the genomes of
cell interiors (Fig. 26.2). eukaryotes, in part because bacteria lack the large stretches
A few exceptional bacteria exceed 100 µm in maximum of noncoding DNA characteristic of eukaryotic chromosomes
dimension. For example, Thiomargarita namibiensis, a bacterium (Chapter 13). The streamlining of the bacterial genome confers
that lives in oxygen-poor sediments off the coast of southwestern certain benefits. For example, bacteria can reproduce rapidly when
Africa, has a total volume about 100,000,000 times larger than nutrients required for growth are available in the local environment.
532 SECTION 26.1 T W O P RO K A RYOT I C D O M A I N S

FIG. 26.2 Cell shape and size in Bacteria and Archaea. Sources: a. Eye of Science/Science Source; b. Scimat/Getty Images; c. Courtesy Mike Dyall-Smith;
d. David Scharf/Science Source; e. © ANIMA RES.

a. Streptococcus, strings of spheroidal c. Haloquadratum walsbyi, a square

or coccoidal bacteria b. E. coli, bacterial rods archaeon that lives in salt ponds

2 μm 2 μm 2 μm

d. Streptomyces, helical bacteria that

produce antibiotics e. A myxobacterium, a bacterium in which cells aggregate to form fruiting bodies

2 μm 30 μm

Bacteria replicate their DNA from only one or a small number of

FIG. 26.3 Thiomargarita namibiensis, the largest known bacterial initiation sites, so genome size can influence rate of reproduction.
cell. The volume of the cell is mostly taken up by a vacuole, Bacteria do not undergo meiotic cell division and cell fusion.
so the active cytoplasm is only a few micrometers thick. That is, they lack the sexual processes characteristic of eukaryotic
Source: Courtesy Heide Schulz-Vogt, Leibniz Institute for Baltic Sea organisms (Chapters 11 and 42). Despite this, bacterial populations
Research, Department of Biological Oceanography. display remarkable genetic diversity. For example, different strains
of Pseudomonas aeruginosa, a common disease-causing organism,
may differ in genome size by nearly a factor of 2 (3.7 million base
pairs compared to 7.1 million base pairs), yet these strains are quite
similar in function.
How does this variation arise? In eukaryotic organisms, genes
generally pass from parent to offspring. Bacteria also inherit most
of their genes from parental cells. However, bacterial cells can
also obtain new genes from distant relatives in a process called
horizontal gene transfer. This process is a major source of
genetic diversity in bacteria.
How does DNA move from one bacterial cell into another
independently of cell division? Some bacteria synthesize thin
200 µm strands of membrane-bound cytoplasm called pili (Chapter 5)
that connect them to other cells (Fig. 26.4a). After joining to a
 CHAPTER 26 B AC T E R I A A N D A RC H A E A 533

second cell, the pilus contracts, drawing the two a. DNA transfer by conjugation
cells close together. A pore-like opening develops
where the two cells are in close contact, providing a
migration route for the direct cell-to-cell transfer of
DNA. This process, called conjugation, commonly
transfers plasmids from one cell to another, spreading
novel genes throughout a population. Genes that
confer resistance to antibiotics are a well-studied
example of horizontal gene transfer by conjugation.
Genes can also be transferred from one cell to
another without any direct bridge between cells.
DNA released to the environment by cell breakdown
can be taken up by other cells, a process called
transformation (Fig. 26.4b). Transformation was In conjugation, DNA
revealed when experiments showed that harmless (usually a plasmid)
from a donor cell is
strains of the bacteria causing pneumonia could transferred to an
be transformed into virulent strains by exposure adjacent recipient cell.
to media containing dead cells of disease-causing First, a pilus tethers
Pilus the donor to the
strains (Chapter 3). Scientists reasoned that the recipient and brings
transformation occurred because some substance the cells together.
was being taken up by the living bacteria. The
“transforming substance” was later shown to be DNA. Donor Recipient
Today, biologists commonly use transformation in the
laboratory to introduce genes into cells. Once the cells are
closely aligned, the
Viruses provide a third mechanism of horizontal
DNA passes
gene transfer. Recall from Chapter 19 that viruses in through a small
bacterial cells sometimes integrate their DNA into the opening formed
between the cells.
host bacterial DNA. This viral DNA persists within the
cells as they grow and divide. Before the virus leaves
the cell to infect others, the viral DNA removes itself Donor Recipient
from the bacterial genome and is packaged in a protein
b. DNA transfer by transformation
capsule to produce the complete virus particle. This
excision is not always precise, and sometimes genetic
material from the bacterial host is incorporated into In transformation,
the virus. Viruses released from their host cell go on to DNA released into
the environment by
infect others, bringing host-derived genes with them. dead cells is taken
Horizontal gene transfer by means of viruses is called up by a recipient
transduction (Fig. 26.4c). It is common in nature and cell.

is also widely used in the laboratory to introduce novel

genes into bacteria for medical research.
Horizontal gene transfer allows bacterial cells Dead donor Recipient
to gain beneficial genes from organisms distributed
throughout the bacterial domain and beyond. Indeed, c. DNA transfer by transduction
bacteria everywhere are constantly reshaping their
genomes. Bacteria, therefore, are not the poor
In transduction,
DNA is transferred
from a donor to a
recipient cell by a
FIG. 26.4 Horizontal gene transfer in bacteria. DNA, virus.
shown in red in the diagrams, originates from
the donor cell. DNA shown in blue is that of
the recipient cell. Source: Dr. Linda M. Stannard,
University of Cape Town/Science Source. Virus-infected donor Recipient
534 SECTION 26.1 T W O P RO K A RYOT I C D O M A I N S

archaeal cells are prokaryotic—they have no membrane-bounded

FIG. 26.5 A three-branched tree of life, based on sequence nucleus and their genes are arrayed along a single circular
comparisons of the genes for the small subunit of chromosome. Moreover, cell size in Archaea is also limited by
ribosomal RNA. Archaea and Bacteria form distinct diffusion, and genetic diversity is promoted by horizontal gene
branches, although they inherited prokaryotic cell transfer, much as in Bacteria.
organization from a common ancestor. Some data In detail, however, Archaea are quite distinctive (Table 26.1).
suggest that eukaryotes arose within the Archaea, Their membranes are made from lipids different from the fatty
making this domain paraphyletic. acids found in bacterial and eukaryotic membranes. Archaea also
show a diversity of molecules in their cell walls, but none has the
Prokaryotic cell Eukaryotic cell
organization organization peptidoglycan characteristic of Bacteria or the cellulose or chitin
found in most eukaryotic cell walls.
Archaea differ from Bacteria in another intriguing way.
Eukarya
DNA transcription in archaeons employs RNA polymerase and
ribosomes more similar to those of eukaryotes than to bacteria
Archaea (Chapter 3). Moreover, many of the antibiotics that target protein
synthesis in bacteria are ineffective against archaeons, suggesting
Bacteria fundamental differences in translation as well.
Many of the microorganisms first identified as Archaea
have unusual physiological properties or inhabit extreme
environments. For example, some archaeons live in acid mine
cousins of eukaryotes, unable to generate genetic diversity by water at pH 1 (or less!). Others live in water salty enough to
sexual recombination. Instead, bacteria have highly efficient precipitate NaCl (Fig. 26.6), or in deep-sea hydrothermal vents
mechanisms for adding and subtracting genes that permit them where temperatures can exceed 100ºC. We now know, however,
to evolve and adapt rapidly to local conditions. Perhaps the most that many archaeons live under less extreme conditions in soils,
widely discussed and worrisome manifestation of horizontal gene lakes, and the sea. Indeed, they may be the most abundant
transfer is the rapid spread of antibiotic resistance among bacteria organisms throughout much of the ocean.
(Chapter 49).
j Quick Check 2 Why were Archaea originally thought to be
j Quick Check 1 In eukaryotes, sexual reproduction is the main simply unusual forms of Bacteria? What lines of evidence showed
process by which new gene combinations are generated. How this domain to form a distinct branch on the tree of life?
do bacteria generate new gene
combinations in the absence of
sexual reproduction? TABLE 26.1 Principal Differences among Archaea, Bacteria, and Eukarya

CHARACTERISTIC ARCHAEA BACTERIA EUKARYA

Archaea form a second
prokaryotic domain. Cell contains a nucleus and other membrane-bound
For many years, all prokaryotic cells organelles No No Yes
were classified as bacteria. However, DNA occurs in a circular form* Yes Yes No
in 1977, George Fox and Carl Woese Ribosome size 70s 70s 80s
published a revolutionary hypothesis.
Membrane lipids ester-linked †
No Yes Yes
From comparisons of RNA molecules
Photosynthesis with chlorophyll No Yes Yes
from the small subunits of ribosomes
(and later, comparisons of the genes Capable of growth at temperatures greater than 80°C Yes Yes No
for these RNAs), Fox and Woese Histone proteins present in cell Yes No Yes
argued that prokaryotic organisms Operons present in DNA Yes Yes No
actually fall into two distinct groups,
Introns present in most genes No No Yes
as different from each other as either
Capable of methanogenesis Yes No No
is from eukaryotes. If this is true,
the tree of life must have three great Sensitive to the antibiotics chloramphenicol, kanamycin,
and streptomycin No Yes No
branches: Bacteria, Eukarya, and the
limb recognized by Fox and Woese, Capable of nitrogen fixation Yes Yes No
called Archaea (Fig. 26.5). Research Capable of chemoautotrophy Yes Yes No
since the late 1970s confirms this *Eukaryote DNA is linear. †
Archaea membrane lipids are ether-linked.
hypothesis. Like bacterial cells,
 CHAPTER 26 B AC T E R I A A N D A RC H A E A 535

other molecule is available to donate electrons. Nonetheless,

FIG. 26.6 Archaeons in San Francisco Bay. The ponds are flooded where oxygen is limited or absent, other electron donors such as
and then evaporated to precipitate table salt. Salt-loving hydrogen sulfide (H2S) are available for photosynthesis and are
archaeons, made conspicuous by their reddish purple utilized by photosynthetic microorganisms. Similarly, essentially
pigment, thrive here. Source: David Wall/Alamy. all respiration in eukaryotic cells is aerobic, or oxygen utilizing.
Again, this makes good chemical sense because oxidation of
carbohydrates using oxygen generates far more energy than can be
obtained using other oxidants. Once again, however, where oxygen
is absent, other electron acceptors can be used for respiration.
The organisms that can use these alternative electron donors and
electron acceptors are Bacteria and Archaea.
We’ve already noted that Bacteria and Archaea differ from
Eukarya in cell structure and genome organization, but it is the
distinctive ways in which Bacteria and Archaea gather carbon and
harvest energy that make them indispensable parts of nature.
Prokaryotic organisms are thus the real foundation of the carbon
cycle, capable of building and dismantling organic molecules
throughout the full breadth of habitats on Earth.

Many photosynthetic bacteria do not produce oxygen.

In saline bays along the west coast of Australia and many other
low-lying tropical coastlines, carpets of deep blue-green cover
tidal flats along restricted lagoons where high salinity and periodic
exposure limit colonization by algae and animals (Fig. 26.7).
Called microbial mats, the carpets shown in Fig. 26.7a are densely
packed communities of (mostly) bacteria and archaeons that
have a predictable structure based on the depths to which light
and oxygen penetrate into the mat (Figs. 26.7b and 26.7c). In the
upper layer, where both light and oxygen are present, the mat
is dominated by cyanobacteria, bacteria that photosynthesize
like plants and algae, using water as an electron donor and
releasing oxygen gas (the blue-green layer in Fig. 26.7b). It is
26.2 AN EXPANDED CARBON CYCLE the photosynthetic pigments in the cyanobacteria that impart
the blue-green color to microbial mats. The cyanobacteria share
In our discussion of energy metabolism in Chapters 7 and 8, we this layer with heterotrophic bacteria, which get their carbon
emphasized oxidation–reduction (redox) chemistry. In redox from preformed organic molecules. Both cyanobacteria and the
reactions, a pair of molecules reacts, one molecule becoming more heterotrophic bacteria respire aerobically, using O2 to oxidize
oxidized and the other becoming more reduced. If we follow the organic molecules to CO2. Thus, at the mat surface, prokaryotic
transfer of electrons in a redox reaction, we see that the molecule microorganisms carry out a biological carbon cycle much like the
that is reduced gains electrons, whereas the molecule that is one outlined for plants and animals in Chapter 25.
oxidized loses electrons. Reduction therefore requires a source of What happens beneath the mat surface is a different story.
electrons, or an electron donor, and oxidation requires a sink for Note in Fig. 26.7b that a distinct purple layer lies beneath the blue-
electrons, or an electron acceptor. green mat surface. Light penetrates into this layer, but oxygen
In photosynthesis carried out by plants, algae, and cyanobacteria does not because aerobic respiration rapidly consumes O2 within
(Chapter 8), carbon dioxide (CO2) is reduced to form carbohydrates, the mat (Fig. 27.6c). What organisms live where light is present
and water is oxidized to oxygen gas (O2). Water is the electron donor and oxygen absent?
needed to reduce CO2, generating O2 as a by-product. In respiration, Like the cyanobacteria in the surface zone, these bacteria are
as described in Chapter 7, organic molecules are oxidized to CO2 , photosynthetic, but they do not use water as an electron donor
and O2 is reduced to water. In this case, O2 serves as the electron and so do not generate oxygen gas. Called anoxygenic because
acceptor needed to oxidize organic molecules. they do not produce oxygen, these photosynthetic bacteria
In all known photosynthetic eukaryotes, the photosynthetic use electron donors such as hydrogen sulfide (H2S), hydrogen
reaction is oxygenic, or oxygen producing. There is a good reason gas (H2), ferrous iron (Fe21), and even the arsenic compound
for this. Water occurs nearly everywhere and, in the oxygen- arsenite (AsO332). Where O2 is present, these compounds oxidize
rich environments where most eukaryotic organisms thrive, no quickly and so are not available for photosynthesis. Therefore,
 536 SECTION 26.2 A N E X PA N D E D C A R B O N C YC L E

microorganisms that conduct anoxygenic photosynthesis are

FIG. 26.7 Microbial mats. Mats, like the example in (a) from restricted to sunlit habitats where O2 is absent.
Western Australia, cover tidal flats and lagoons where The purple color in the mat layer comes from
salinity or other environmental factors inhibit animals bacteriochlorophyll, a light-harvesting pigment closely
and seaweeds. As shown by the example from (b) Cape related to the chlorophyll found in plants, algae, and
Cod, microbial mats have sharp vertical gradients of cyanobacteria (Fig. 26.8a). Because of its chemical structure,
light, oxygen, and other chemical compounds, and so bacteriochlorophyll absorbs most strongly at wavelengths
support nearly all the energy metabolisms found in (c) different from those absorbed by the cyanobacteria above them
the expanded carbon cycle. Sources: a. Andrew Knoll, Harvard in the mat (Fig. 26.8b).
University; b. Republished with permission of Springer Science+Business
Anoxygenic photosynthesis differs from oxygenic
Media, from The Prokaryotes: Vol. 2: Ecophysiology and Biochemistry
photosynthesis in another important way: It employs only a
2006, The Phototrophic Way of Life Jörg Overmann and Ferrau Garcia-
single photosystem. As discussed in Chapter 8, a single
Pichel, figure 5.
photosystem cannot capture enough energy to oxidize water
a.
and then use those electrons to reduce CO2. Therefore, oxygenic
photosynthesis has two photosystems in series. With only a
single photosystem, bacteria that carry out anoxygenic
photosynthesis use electron donors that donate their electrons
more easily than water.

Many bacteria respire without oxygen.

Light but no oxygen penetrates into the purple layer in Fig. 26.7b,
and neither light nor oxygen penetrates into the black layer
deep within the microbial mat. Yet both layers are inhabited by a
diversity of heterotrophic Bacteria and Archaea that metabolize
organic molecules originally synthesized by photosynthetic
bacteria higher in the mat community.
b. Because there is no oxygen in these layers, these organisms
The blue-green pigment must use other molecules as electron acceptors in cellular
identifies cyanobacteria, the respiration, including oxidized forms of nitrogen (NO32), sulfur
oxygenic photosynthetic (SO422), manganese (Mn41), iron (Fe31), and even arsenic (AsO432).
bacteria that dominate the well-
lit, oxygen-rich mat surface. Such microorganisms thrive in many environments where oxygen
is absent.
The purple pigment marks
Still other types of heterotrophs live in deeper layers of
purple bacteria, anoxygenic microbial mats. Fermentation provides an alternative to cellular
photosynthetic bacteria that respiration as a way of extracting energy from organic molecules.
live where there is light but no
oxygen gas. Whereas cellular respiration is the full oxidation of carbon
compounds to CO2 , fermentation is the partial oxidation of
carbon compounds to molecules that are less oxidized than
In the black subsurface layer,
anaerobic respiration and CO2 (Chapter 7). Fermentation has an advantage over cellular
fermentation support respiration in that it does not require an external electron
microbial populations.
acceptor, such as O2. On the other hand, fermentation yields only a
modest amount of energy.
c.
Fermentation plays an important role in oxygen-poor
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environments that are rich in organic matter, such as landfills and

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me trop
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0 able to metabolize a different intermediate. These cooperating

Oxygenic Aerobic
photosynthesis respiration groups of fermenters enable the breakdown of substances that
2 could not be metabolized by any one organism alone. Groups of
Anoxygenic
photosynthesis fermenters thus form what is essentially an ecological solution
Anaerobic
Chemoautotrophy respiration and
to the metabolically difficult problem of gaining energy from
4
O2 fermentation complex organic molecules. Fermentation is widespread in
H2S Bacteria and Archaea, but is of only minor importance in most
0 Max Eukarya, the exception being yeasts, which thrive in the sugar-rich
 CHAPTER 26 B AC T E R I A A N D A RC H A E A 537

Photoheterotrophs obtain energy from light but

FIG. 26.8 Structures and absorption spectra of obtain carbon from preformed organic molecules.
bacteriochlorophyll a and chlorophyll a. Note that In Chapter 6, we saw that organisms have two sources of energy:
bacteria absorb wavelengths of light that are outside the the sun (phototrophs) and chemical compounds (chemotrophs).
visible spectrum. Organisms also have two sources of the carbon needed for
a. growth and other functions: inorganic molecules like CO2
H2C (autotrophs) and organic molecules like glucose (heterotrophs).
H 3C O H CH CH CH3
3 This means that there are four ways that organisms acquire the
H
CH2CH3 CH2CH3
energy and carbon they need (see Fig. 6.1).
N N N N Up to this point, we have considered only two of these
Mg Mg
N N N N ways. Plants, algae, and cyanobacteria are photoautotrophs,
gaining energy from the sunlight and carbon from CO2 , whereas
animals, fungi, and many prokaryotes are chemoheterotrophs,
C20H39OOC C20H39OOC
CH3OOC O CH3OOC O gaining both energy and carbon from organic molecules taken
Bacteriochlorophyll a Chlorophyll a up from the environment. Among the Bacteria and Archaea,
additional metabolisms are possible. Some use the energy from
The differences in structure (shown in red) between the sunlight to make ATP, just as plants do, but rather than reducing
bacteriochlorophyll used by anoxygenic photosynthetic
bacteria and the chlorophyll used by cyanobacteria and CO2 to make their own organic molecules, they rely on organic
chloroplasts in phyotosynthetic eukaryotes account for the molecules obtained from the environment as the source of
differences in wavelengths absorbed by the two molecules. carbon for growth and other vital functions. These organisms are
photoheterotrophs.
b. Photoheterotrophy can be advantageous in environments
150 rich in dissolved organic compounds. It allows organisms to use
Bacteriochlorophyll a all their absorbed light energy to make ATP while directing all
Chlorophyll a
absorbed organic molecules toward growth and reproduction.
100 Photoheterotrophic bacteria have a single photosystem and
Absorption

so live where oxygen is limited or absent. The purple layer of

a microbial mat very likely contains heliobacteria or green
nonsulfur bacteria that take advantage of the large amounts of
50
organic matter generated by photoautotrophs in the mat.

j Quick Check 3 How do photoautotrophs such as cyanobacteria

differ from photoheterotrophs such as heliobacteria?
0
Ultra-
violet Blue Green Red Infrared
400 500 600 700 800 Chemoautotrophy is a uniquely prokaryotic
Wavelength (nm) metabolism.
On the deep seafloor, animals are relatively uncommon, their
Visible spectrum
abundance limited by the slow descent of organic matter
from surface oceans. However, where hydrothermal springs
punctuate the deep seafloor, animal populations can be
environment of rotting fruit, as well as in breweries throughout remarkably dense (Fig. 26.9). Why are animals so abundant
the world. around hydrothermal vents?
In summary, where O2 is present, many different kinds of The many animals around deep-sea hydrothermal
organisms participate in the carbon cycle. Photosynthetic plants, vents subsist on a thriving community of microorganisms.
algae, and cyanobacteria all transform CO2 into organic molecules, These microorganisms gain carbon by reducing CO2 to form
and animals, fungi, single-celled eukaryotes, bacteria, and carbohydrates. However, they obtain the energy to fuel this
archaeons return CO2 to the environment by aerobic respiration. process not from sunlight but from chemical reactions. Hence,
In Chapter 25, we noted that some of the organic carbon produced these microorganisms are called chemoautotrophs. They
by photosynthesis escapes aerobic respiration and accumulates oxidize molecules and ions such as H2 , H2S, and Fe21 to generate
in oxygen-depleted waters or sediments. Only prokaryotic the ATP and reducing power required to incorporate CO2 into
heterotrophs can complete the recycling of organic carbon in organic molecules. In chemoautotrophic metabolism, the
oxygen-poor environments. Thus, in carrying out anaerobic electron acceptor is most commonly O2 or nitrate (NO32). Thus,
respiration or a diversity of fermentation reactions, prokaryotes chemoautotrophy requires access to both oxidized (O2 and
play a major role in the carbon cycle. nitrate) and reduced (H2 , H2S, and Fe21) substances.
538 SECTION 26.3 OT H E R B I O G E O C H E M I C A L C YC L E S

other heterotrophic bacteria reduce sulfate to H2S. This is

FIG. 26.9 Deep-sea hydrothermal vents. Chemoautotrophs fuel reminiscent of the carbon cycle outlined in Chapter 25, where
the carbon cycle in these environments. Source: Science plants generate oxygen and animals consume it. Metabolisms
Source/Getty Images. at work where oxygen is absent can also complete the carbon
cycle, and they do so by coupling the cycling of carbon to the
cycling of other elements—in this case, sulfur. Once again, we see
complementary metabolic pathways. The expanded carbon cycle of
Bacteria and Archaea promotes the cycling of sulfur, nitrogen, and
other biologically important elements.

Bacteria and archaeons dominate Earth’s sulfur cycle.

Our bodies are mostly carbon, oxygen, and hydrogen, but we also
contain about 0.2% sulfur by weight. Sulfur is a component of the
amino acids cysteine and methionine and therefore is present in
many proteins (Chapter 2). Sulfur is also present in iron–sulfur
clusters that are key components of enzymes fundamental to
electron transfer (Chapter 6) and in some vitamins. Where do we
get the sulfur we need? From the food we eat—protein in steak
or fish provides a rich source of sulfur. Where do cows and fish get
the sulfur they need? Like us, from the food they eat. Food chains
don’t go on forever, so somewhere in the system, some organisms
must take up inorganic sulfur from the environment. Primary
producers, organisms that reduce CO2 to form carbohydrates, fill
this role.
Plants are the dominant primary producers on land. Plants
take up sulfate (SO422) ions from the soil and reduce them within
their cells to hydrogen sulfide (H2S) that can be incorporated
into cysteine and other biomolecules (Fig. 26.10). This process
is called assimilation. Algae and photosynthetic bacteria do

FIG. 26.10 The sulfur cycle. Plants take up sulfur as sulfate ions for
incorporation into proteins and other compounds, and
Oxidized and reduced molecules react readily with each fungi release sulfur during decomposition. However,
other and so are rarely present in the same environment. For the major role in cycling sulfur through the biosphere
this reason, chemoautotrophs tend to live along the interface is played by microbes that oxidize or reduce sulfur
between an environment where oxygen is present and one in compounds to gain energy or carbon.
which oxygen is absent. Chemoautotrophic prokaryotes therefore
thrive where reduced gases from Earth’s interior meet oxygen- SO42–
rich seawater.

j Quick Check 4 How could the biological carbon cycle have Assimilation
Primary producers
worked on the primitive Earth, where oxygen gas was essentially
Energy metabolism
absent from the atmosphere and oceans? Photosynthetic bacteria
Energy metabolism
Anaerobic respiration by Organic S Chemoautotrophic
bacteria and archaeons in proteins bacteria
Chemoautotrophic
26.3 OTHER BIOGEOCHEMICAL archaeons
Decomposition
CYCLES Fungi
Bacteria
Let’s think further about prokaryotic metabolism. We saw that
some photosynthetic and chemosynthetic bacteria oxidize
reduced sulfur compounds like H2S to sulfate (SO422), whereas H2S
 CHAPTER 26 B AC T E R I A A N D A RC H A E A 539

much the same thing in lakes, rivers, and oceans. Why don’t
primary producers take up H2S directly? There are two answers FIG. 26.11 The nitrogen cycle. Unique bacterial and archaeal
to this question. First, H2S is rapidly oxidized in the presence of metabolisms include nitrogen fixation, nitrification and
oxygen and so does not occur in environments where oxygenic denitrification, as well as anammox.
photosynthesis is common. Second, H2S is generally toxic to Nitrogen gas
eukaryotic organisms, so plants and algae do not thrive where it is N2
abundant. (The H2S produced within eukaryotic cells has a short
lifetime and is restricted to intracellular sites distant from those
that are vulnerable to its toxic effects.)
We’ve now seen half the sulfur cycle, the conversion of
sulfate to H2S within cells. How did sulfate molecules get
Denitrification
into the soil in the first place? After death, fungi and bacteria Organic N in
proteins, etc. Nitrogen
decompose cells, returning carbon, sulfur, and other compounds Assimilation fixation
to the environment. Reduced sulfur compounds released from
decomposing cells are oxidized by bacteria and archaeons, Nitrate
Anammox
NO3
completing the cycle. These microbes are chemoautotrophs that Decomposition Bacteria
obtain energy by oxidizing H2S or photosynthesizers that use H2S and
Archaea
as the electron donor. In addition to being taken up by plants, the Nitrification
Assimilation
sulfate produced by these processes is consumed by heterotrophic
Nitrite NO2
bacteria living in oxygen-free environments found in soil,
sediments, and occasionally in lakes and seawater. Sulfate rather
than oxygen is used as the final electron acceptor in respiration Nitrification
and is reduced to H2S. In fact, most of the sulfur cycled biologically
is used to drive energy metabolism in oxygen-poor environments, Ammonia
NH3
not to build proteins.
Note that eukaryotes use neither H2S for photo- (or chemo-)
synthesis nor sulfate in respiration. Thus, the biological sulfur
cycle is completed by bacteria and archaeons alone. nodules that harbor nitrogen-fixing bacteria (Fig. 26.12). That is,
plants like soybean have entered into an intimate partnership with
The nitrogen cycle is also driven by bacteria nitrogen-fixing bacteria to obtain the biologically useful forms of
and archaeons. nitrogen needed for growth (Chapter 29).
The model provided by the sulfur cycle can be extended to other
biologically important elements. Nitrogen is the fourth most
abundant element in the human body, contributing 4% by weight
FIG. 26.12 Roots of a soybean plant. The nodules contain bacteria
as a component of proteins, nucleic acids, and other compounds.
capable of reducing nitrogen gas to ammonia. Source:
As is true of sulfur, primary producers incorporate inorganic forms
Scimat/Getty Images.
of nitrogen into biomolecules, and we get the nitrogen we need
from our food. As we’ll see, however, there is a twist.
Let’s examine the biological nitrogen cycle (Fig. 26.11).
Carbon is a minor component of the atmosphere and sulfur only a
trace constituent, but the air we breathe consists mostly (78%) of
nitrogen gas (N2). Indeed, most of the nitrogen at Earth’s surface
resides in the atmosphere. Although nitrogen is plentiful, the
nitrogen available to primary producers in many environments
is limited. The reason is that plants cannot make use of nitrogen
gas and neither can algae. Only certain bacteria and archaeons
can reduce N2 to ammonia (NH3), a form of nitrogen that can be
incorporated into biomolecules.
The process of converting N2 into a biologically useful form
such as ammonia is called nitrogen fixation, and it is one of the
most important reactions ever evolved by organisms. Farmers
plant soybeans to regenerate nitrogen nutrients in soils, but it is
not the soybean that fixes nitrogen. Rather, soybean roots have
540 SECTION 26.4 T H E D I V E R S I T Y O F B AC T E R I A

The nitrogen cycle, then, begins with nitrogen fixation,

and it continues through many additional metabolisms, most 26.4 THE DIVERSITY OF BACTERIA
of them confined to bacteria and archaeons. Ammonia released
by the decomposition of dead cells can be taken up by primary Traditionally, bacterial taxonomy was a laborious process.
producers, but commonly it is oxidized to nitrate (NO32) by Microbiologists grew bacteria in beakers or on petri dishes,
chemoautotrophic bacteria, a process called nitrification (see carefully separating and enriching individual populations until
Fig. 26.11). This process works efficiently in both soil and water, only one type of bacterium—that is, a pure culture—was present.
and so nitrate is the most common form of nitrogen available to Species were identified on the basis of details of cell structure and
life in both environments. division observed under the microscope, and metabolic capabilities
Anaerobic respiration completes the cycle by returning N2 established through experiments and reactions to chemical stains
to the atmosphere. Some bacteria can use nitrate as an electron or antibiotics. By the 1980s, a few thousand species had been
acceptor in respiration, a process known as denitrification (see described in this way.
Fig. 26.11). Because denitrification is so efficient at returning With molecular sequencing technologies, however, bacterial
nitrogen to the atmosphere as nitrogen gas, nitrogen fixation species are now characterized by their DNA sequences, especially
must continually make nitrogen available for organisms. Without the genes for the small subunit of ribosomal RNA (rRNA). A novel
nitrogen fixation, the nitrogen cycle would slowly wind down, and application of sequencing technology paved the way for modern
the productivity of Earth’s biota—that is, of all its organisms— studies of bacterial diversity and ecology. Microbiologists reasoned
would wind down with it. This is why nitrogen fixation is such an that the same procedures used to extract, amplify, and sequence
important process. genes from pure cultures in the lab could be used to study bacteria
Until the 1990s, the nitrogen cycle was described—completely, in nature. That is, extracting and reading gene sequences directly
it was thought—in terms of the metabolic reactions just discussed. from soil or seawater could reveal the diversity of the microbial
Then, a new and unexpected form of nitrogen metabolism was communities in these environments.
discovered in the oceans. A number of prokaryotic microorganisms The results were stunning. Most bacteria in nature—
function as chemoautotrophs by reacting ammonium ion (NH41) perhaps 99% or more—are species that have not been cultured
with nitrite (NO22) to gain energy. The reaction is shown here: in the lab. Indeed, species well characterized by studies of pure
cultures commonly represent only minor components of natural
NH41 1 NO22 → N2 1 2H2O communities. To give just one illustration, a bacterium called
SAR11 was originally identified on the basis of gene sequences
Note that this is a redox reaction, like other reactions amplified from seawater. SAR11 caught the attention of
involved in energy metabolism. In this case, the ammonium ion microbiologists because it makes up about a third of all sequences
is oxidized to nitrogen gas, and nitrite is reduced to nitrogen identified in the samples from the surface ocean. How could
gas, with water produced as a by-product. The reaction, called we be ignorant of a bacterium that must be among the most
anammox (anaerobic ammonia oxidation), provides only a small abundant organisms on Earth? The answer is simple: No one
amount of energy, but it supports many bacteria and archaeons had cultured it. Recently, SAR11 has been cultured. Formally
in oxygen-depleted parts of the ocean, providing a major route by described as Pelagibacter ubique (ubique is Latin for “everywhere”),
which fixed nitrogen returns to the atmosphere as N2 (see this bacterium is now known to be a heterotroph that plays an
Fig. 26.11). important role in recycling organic molecules dissolved in the sea.
The biological nitrogen cycle, then, begins and ends with More recently, the application of shotgun sequencing
prokaryotic metabolisms. Bacteria and archaeons fix N2 into a techniques (Chapter 13) to the environmental sampling
biologically available form, nitrifying bacteria oxidize ammonia of genomes has allowed microbiologists to begin matching
to nitrate, and still other prokaryotes gain energy from uncultured populations with specific metabolisms. That is, we
denitrification and anammox, returning N2 to the air. Unlike need no longer be content to know from molecular sequences
the carbon and sulfur cycles, the nitrogen cycle does not rely what is living in a given environment. Now we can begin to
at any point on elements stored as minerals in rocks and understand what the different microorganisms are doing there.
sediments. Most of Earth’s nitrogen was released to the For this reason, another revolution in our understanding of the
atmosphere as gas early in our planet’s history, and so movement microbial world lies just around the corner (Fig. 26.13).
into sediments and out of volcanoes contributes relatively little
to the cycle. Humans, however, contribute in important ways, Bacterial phylogeny is a work in progress.
most notably through the industrial synthesis of nitrate fertilizer In Chapter 23, we introduced molecular phylogenies, in which
(Chapter 49). gene sequences are compared to enable us to draw conclusions
about evolutionary relatedness among populations in nature.
j Quick Check 5 How would the nitrogen cycle operate in the Gene sequencing made it possible to test traditional bacterial
absence of bacteria and archaeons? groupings based on form and physiology. Some traditionally defined
HOW DO WE KNOW?

FIG. 26.13

How many kinds of bacterium live in the oceans?

BACKGROUND The oceans are full of bacteria—by some estimates, 35
the sea harbors more than 1029 bacterial cells. How diverse are these
bacterial communities? 30

% of 16S rRNA sequences

METHOD 1 Different types of bacterium can be recognized by 25
the unique nucleotide sequences of their genes. In fact, individual
20
types of bacterium can be identified on the basis of nucleotide
sequence in a relatively small region of the gene for the small 15
subunit of ribosomal RNA (rRNA). This molecular “barcode” can be
sequenced quickly and accurately for samples of DNA drawn from 10
the environment.
5
Using this strategy, scientists associated with the International
Census of Marine Life obtained samples of seawater and seafloor 0
sediment from deep-ocean environments. In the laboratory,

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METHOD 2 In another set of experiments, scientists collected

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samples of seawater from the Sargasso Sea, and from these they

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amplified whole genomes—more than a billion nucleotides’ worth.
They used small-subunit rRNA and other genes to characterize Relative abundance of different bacterial groups in samples from the Sargasso
species richness, and also characterized a large assortment of Sea, based on percent representation of small subunit rRNA sequences. Note the
abundance of SAR11, only recently characterized.
additional sequence data to understand genetic and physiological
diversity.

RESULTS Both surveys found that the bacterial diversity of marine 25,000
environments is much higher than had been thought. The barcode
survey (method 1) found as many as 20,000 distinct types of
20,000
Number of sequence tags

bacteria in a single liter of seawater and estimated that as many as

5–10 million different microbes may live in the world’s oceans.
15,000
Some of these bacteria are abundant and widespread, but most are
rare. The genomic survey (method 2) of the Sargasso Sea found
more than 1800 distinct bacteria in sea-surface samples and, 10,000 The “rare biosphere” is
remarkably, identified more than 1.2 million previously unknown enormous and highly diverse.
genes within these genomes. 5000

CONCLUSION The bacterial diversity of the oceans is huge and

still largely unexplored. Both barcode and genomic surveys are 0
10 20 30 40 50 60
continuing, and promise a more nearly complete accounting of Percent sequence difference from
marine diversity. Together, these surveys will help us understand closest known bacteria
how marine bacteria function and how they sort into communities. Computer-aided comparison of sequences from the world’s oceans shows that
about 75% of all sequences collected are similar to other sequences in the library
of sequence tags. However, 25% are distinctive, forming a rare but enormously
SOURCES Venter, J. C., et al. 2004. “Environmental Genome Shotgun Sequencing
of the Sargasso Sea.” Science 304:66–74; Sogin, M. L., et al. 2006. “Microbial
diverse pool of bacterial types.
Diversity in the Deep Sea and the Underexplored ‘Rare Biosphere’” Proceedings of
the National Academy of Sciences USA 103:12115–12120.

541
542 SECTION 26.4 T H E D I V E R S I T Y O F B AC T E R I A

because they contain genes passed on

FIG. 26.14 Bacterial and archaeal phylogeny. Some scientists argue that the evolution of
by conjugation, transformation, or
prokaryotes should not be viewed as a tree but as a series of branches that diverge
transduction. Statistical analyses of
and then come back together, representing genes that diverge from one another but
complete bacterial genomes suggest
then come to reside together in new organisms because of horizontal gene transfer.
that at least 85% of all genes in bacterial
Source: Adapted from W. F. Doolittle, 2000, “Uprooting the Tree of Life,” Scientific
genomes have been transferred
American 282:90–95.
horizontally at least once. Such
ARCHAEA apparently rampant gene exchange
Crenarchaeota Euryarchaeota might well doom attempts to build a
Animals EUKARYA
BACTERIA Proteobacteria bacterial tree of life, and some biologists
Fungi
Cyanobacteria argue that bacterial evolution is more
Plants properly depicted as a complex bush with
Other
bacteria many connecting branches (Fig. 26.14).
Other
eukaryotes Other microbiologists are more
optimistic. Small-subunit rRNA genes,
for example, show little evidence of
horizontal transfer and so may faithfully
record the evolutionary history of
the organisms in which they occur.
Nonetheless, we need to be cautious
Bacteria that gave rise
to chloroplasts when using bacterial trees to study
the evolution of specific metabolic or
Bacteria that gave rise physiological characteristics. Nitrogen
to mitochondria fixation, photosystems, drug tolerance—
such features depend on genes known to
jump from branch to branch on the tree.
At present, molecular signatures have
been extremely useful in identifying
major groups of bacteria, but less clear
in establishing branching order among
these groups. About 50 major groups
Common ancestral community of primitive cells of bacteria (each one very roughly
equivalent to a eukaryotic phylum) have
been recognized. Nearly half of these
groups—for example, the cyanobacteria— passed the test and stand groups cannot be cultured, but have been identified only by the
today as well-defined bacterial groups. Many, however, did not. cloning of genes from environmental samples. To support our
Molecular sequence comparisons bring enormous new discussion of bacterial diversity, we present a phylogeny based
possibilities to the study of prokaryotic diversity and evolution, on comparisons of whole genomes in Fig. 26.15. As you examine
but they introduce new problems as well. One set of problems this figure, remember that microbiologists continue to debate
arises because major groups of Bacteria and Archaea separated branching order on the bacterial tree.
from one another billions of years ago. Inherited sequence
similarities can be masked by continuing molecular evolution What, if anything, is a bacterial species?
within groups. That is, specific nucleotides in gene sequences may What do we mean when we talk about bacterial species? As we
change not once but multiple times through a long evolutionary saw in Chapter 22, Ernst Mayr defined species as “groups of
history, erasing molecular features that were once the same interbreeding natural populations that are reproductively isolated
because of inheritance from a common ancestor. Also, in some from other such groups.” This widely accepted definition is called
groups rates of sequence evolution appear to be higher than the biological species concept, but a moment’s reflection will show
in others. As a result, these groups have especially divergent that it can’t be applied to the most abundant organisms on Earth—
sequences and so may misleadingly fall toward the bottoms of Bacteria and Archaea. In the absence of sexual reproduction, how
phylogenetic trees. do we think about species in the prokaryotic domains?
Another problem arises because of horizontal gene transfer. Some microbiologists argue that we should abandon the
Phylogenies may falsely group distantly related bacteria concept of species when talking about bacteria, especially since
 CHAPTER 26 B AC T E R I A A N D A RC H A E A 543

process provides a means of recognizing

FIG. 26.15 Evolutionary relationships among bacteria, inferred from data from complete
bacterial species: Populations subject to
genomes. Photosynthetic groups are shown in green (the Acidobacteria have one
the same episodes of periodic selection
known photoheterotrophic member). Other groups discussed in the text are shown
belong to a single species. This is a useful
in red.
way to define bacterial species, but other
Deinococcus/Thermus definitions are possible, and as yet there
Thermotogae is no consensus among microbiologists.
Tenericutes
Heliobacteria
Firmicutes Proteobacteria are the most
Gram-positive bacteria
diverse bacteria.
Green nonsulfur bacteria Proteobacteria are far and away the most
Cyanobacteria
diverse of all bacterial groups (Fig. 26.15).
Actinobacteria
Spirochaetes Defined largely by similarities in rRNA
Planctomycetes sequences, Proteobacteria include many of
Aquificae the organisms that populate our expanded
Green sulfur bacteria
carbon cycle and other biogeochemical
Bacteroidetes
Acidobacteria cycles. For example, they include
anoxygenic photosynthetic bacteria and
chemoautotrophs that oxidize NH3, H2S,
and Fe21, as well as bacteria able to respire
Purple sulfur bacteria
using SO422, NO32, or Fe31.
Many Proteobacteria have evolved
E. coli intimate ecological relationships with
eukaryotic organisms. Some of these
Proteobacteria are mutually beneficial, such as the
nitrogen-fixing bacteria in soybean root
nodules. Other Proteobacteria, however,
are our worst pathogens: the rickettsias
that cause typhus and spotted fever,
Purple nonsulfur bacteria vibrios that trigger cholera, salmonellas
responsible for typhoid fever and food
poisoning, and Pseudomonas aeruginosa
strains that infect burn victims and others
whose resistance is reduced. Escherichia
distantly related bacteria can exchange genes. Nonetheless, nature coli, famous as a laboratory model organism (see Fig. 26.4b), is
is full of microbial populations with well-defined features of form usually a harmless presence in our intestinal tract, but pathogenic
and function. strains can cause serious diarrhea and even death.
In the age of molecular sequence comparisons, bacterial
species have sometimes been defined as populations whose The gram-positive bacteria include organisms that
small-subunit rRNA sequences are more than 97% identical. It cause and cure disease.
would be useful, however, to conceive of bacterial species in terms In 1884, the Danish scientist Hans Christian Gram developed a
of functional processes, much as Ernst Mayr did for eukaryotic diagnostic dye that, after washing, is retained by bacteria with
organisms. In plants, animals, and single-celled eukaryotes, thick peptidoglycan walls, but not by those with thin walls.
interbreeding ensures that populations share the same pool Molecular sequence comparisons confirm that most gram-
of genes, providing a counterforce to mutations and selection positive bacteria (those that retain the dye) form a well-defined
pressures that promote divergence. branch of the bacterial tree (Fig. 26.15). One species in this group,
A similar process exists in bacteria. In early research on the Bacillus subtilis, is the focus of many studies about basic molecular
population genetics of bacteria, biologists observed that genetic function in bacteria. All of us harbor Streptococcus species in our
diversity in laboratory cultures gradually increases through time mouths, and many of us have suffered from strep throat, produced
and then rapidly decreases with the emergence of a successful by streptococcal pathogens. More serious infections, including
variant that outcompetes the rest. The episodic loss of diversity some forms of meningitis, are also associated with streptococci.
is called periodic selection. Some biologists argue that this Staphylococcus bacteria are commonly found on human skin,
544 SECTION 26.4 T H E D I V E R S I T Y O F B AC T E R I A

but only occasionally do we suffer from maladies such as staph unicellular rods and spheroidal cells, as well as multicellular balls
infection, toxic shock syndrome, or food poisoning induced by and filaments (Fig. 26.16). Some filamentous cyanobacteria can
staphylococcal bacteria. even form several different cell types, including specialized cells for
In contrast to these pathogens, gram-positive bacteria called nitrogen fixation and resting cells that provide protection when the
streptomycetes have proved invaluable to medicine because local environment does not favor growth.
of a remarkable property. More than half of the species in this In contrast to the monophyly of cyanobacteria, molecular
group secrete compounds that kill other bacteria and fungi. sequence comparisons show that anoxygenic photosynthesis is
Streptomycetes have provided us with tetracycline, streptomycin, distributed widely on the bacterial tree (see Fig. 26.15). Think of the
erythromycin, and numerous other antibiotics that combat purple layer beneath the surface of many microbial mats (see
infectious disease. Fig. 26.7). The vivid color comes from the light-sensitive
pigments of the aptly named purple bacteria, found within the
Photosynthesis is widely distributed on the bacterial Proteobacteria. Purple bacteria are capable of photoautotrophic
tree. growth using bacteriochlorophyll and a single photosystem. Most,
Molecular studies confirm that all bacteria capable of oxygenic however, show evidence of metabolic diversity and can grow
photosynthesis form a single branch of the bacterial tree: the heterotrophically in the absence of light or appropriate electron
cyanobacteria. Cyanobacteria can be found in environments that donors. A green layer lies beneath the surface of other microbial
range from deserts to the open ocean. Their diverse species include mats, its color caused by light reflected by the pigments of another
group of photosynthetic bacteria, called
green sulfur bacteria because they
commonly gain electrons from hydrogen
sulfide and deposit elemental sulfur
FIG. 26.16 Diversity of form among cyanobacteria. Sources: a. Dr. Ralf Wagner; b. blickwinkel/Alamy;
on their walls. Unlike most groups of
c. Hecker/Sauer/age fotostock; d. Jason Oyadomari.
photosynthetic bacteria, the green sulfur
b. Chroococcus, larger cells bound into a colony by bacteria are intolerant of oxygen gas.
a common extracellular envelope made of Oddly, a species of green sulfur bacteria
a. Aphanothece, small single cells polysaccharides
has been found in hydrothermal rift vents
2 km beneath the surface of the ocean.
Light doesn’t penetrate to these depths,
so if these organisms harvest light, it must
come from incandescent lava as it erupts
on the bottom of the ocean.
Light-harvesting bacteria occur
on several other branches of the
bacterial tree—the photoheterotrophic
heliobacteria, for example, and the
green nonsulfur bacteria often found
in freshwater hot springs. Despite a
10 µm 10 µm century of research, our knowledge of
photosynthesis within the bacterial
tree remains incomplete, as illustrated
d. Anabaena, strings of undifferentiated cells at
either end of the filament, flanking two by the discovery in 2007 of unique
elongated resting cells, and in the middle, light-harvesting bacteria in Yellowstone
c. Oscillatoria, filaments with no cell differentiation a heterocyst specialized for nitrogen fixation National Park. Genomic surveys of hot
springs turned up evidence of a previously
unknown bacteriochlorophyll-containing
microorganism, and gene sequencing
established that this organism belongs
to the phylum Acidobacteria. There is no
evidence for carbon fixation, however,
suggesting that the Yellowstone bacterium
is a photoheterotroph. To date, these
unusual microorganisms have not been
50 µm 20 µm grown in pure culture, so they remain
difficult to study.
 CHAPTER 26 B AC T E R I A A N D A RC H A E A 545

now propose that several of these groups together form a distinct

26.5 THE DIVERSITY OF ARCHAEA clade nicknamed TACK (Thaumarchaeota 1 Aigarchaeota 1
Crenarchaeota 1 Korarchaeota; the upper main branch in
We have already noted that Archaea and Bacteria are prokaryotic, Fig. 26.17). Like the Bacteria, Archaea include forms able to respire
but differ in their membrane and wall composition, molecular aerobically or anaerobically, as well as chemoautotrophs and
mechanisms for transcription and translation, and many other fermenters. There are, however, metabolic differences between
biological features (see Table 26.1). As a result, the two groups tend the two domains. No Archaea carry out photosynthesis using
to thrive in very different environments. Many archaeons tolerate chlorophyll and related pigments, but some Archaea exhibit forms
environmental extremes such as heat and acidity. In fact, for many of energy metabolism unknown in bacteria or eukaryotes.
environmental conditions, archaeons define the known limits of
life. What do these disparate environments have in common? One The archaeal tree has anaerobic, hyperthermophilic
proposal is that archaeons thrive in environments where the energy organisms near its base.
available to fuel cell activities is limited—where there is no light, or Archaeons found on lower branches of the archaeal tree exhibit
a limited abundance of fuels and oxidants for chemoautotrophy or a remarkable ability to grow and reproduce at temperatures of
respiration, or both. That is, archaeons commonly live where energy 80°C or more (Fig. 26.17). Called hyperthermophiles, these
resources are too low to support bacteria and eukaryotes. organisms do not simply tolerate high temperature; they require it.
Bacteria and Archaea diverged from their last common At present, the world record for high temperature growth is held
ancestor more than 3 billion years ago. Just as Bacteria have by Methanopyrus kandleri, which has been shown to grow at 122°C
evolved a phylogenetic diversity that defies easy categorization, among hydrogen- and CO2-rich hydrothermal vents on the seafloor
Archaea also include many distinct types of organism. Many beneath the Gulf of California. Most of these heat-loving archaeons
microbiologists recognize three major divisions of Archaea, called are anaerobic, living as anaerobic respirers or as chemoautotrophs
the Crenarchaeota, Euryarchaeota, and Thaumarchaeota, that obtain energy from the oxidation of hydrogen gas.
but a few poorly known groups may indicate further major Why do anaerobic hyperthermophiles sit on the lowest
branches within the domain (for example, the Korarchaeota and branches of the archaeal tree? Many biologists believe that these
Nanoarchaeota noted in Fig. 26.17). Indeed, some microbiologists features characterized the first Archaea and therefore imply that

30
20
10
10
90
80
70
60
50
40
Crenarchaeota Optimum growth temperature (C)

Methane producer
Salt tolerant
Acid tolerant
Korarchaeota

Thaumarchaeota

FIG. 26.17 Phylogeny of Archaea, showing the

evolution of temperature preference, salt
Euryarchaeota tolerance, acid tolerance, and production of
methane within the domain. Source: Adapted
from M. Groussin and M. Gouy, 2011, “Adaptation to
Environmental Temperature Is a Major Determinant of
Molecular Evolutionary Rates in Archaea,” Molecular
Biology and Evolution 28(9):2667, doi: 10.1093/
molbev/msr098. © The Author 2011. Published by
Oxford University Press on behalf of the Society for
Nanoarchaeota Molecular Biology and Evolution.
546 SECTION 26.5 T H E D I V E R S I T Y O F A RC H A E A

the Archaea evolved in hot environments with no oxygen. As we

discussed in Chapter 23, geologists agree that oxygen gas was not FIG. 26.18 Headwaters of the Rio Tinto, southwestern Spain.
present on the early Earth, consistent with the inference we can Archaeons thrive in these highly acidic waters, pH 1–2.
make from the archaeal tree that early Archaea were anaerobic. A hot Source: Andrew Knoll, Harvard University.

environment for early Archaea can be understood in two ways. Either

the entire ocean was hot—for which there is little evidence—or
Archaea first evolved in hot springs where local supplies of hydrogen
and metals enabled them to live as chemoautotrophs. Because some
versions of the bacterial tree also show hyperthermophilic species on
their lowest branches, it has been suggested that the last common
ancestor of all living organisms lived in a hot environment.

j Quick Check 6 If early branches on the bacterial and archaeal trees

are dominated by hyperthermophilic microorganisms, does this mean
that the early oceans were very hot?

The Archaea include several groups of acid-loving

microorganisms.
In many parts of the globe where humans mine coal or metal ores,
abandoned mines fill up with very acidic water. Called acid mine
drainage, this water has been acidified through the oxidation of
pyrite (FeS2) and other sulfide minerals to produce sulfuric acid
(H2SO4). The pH of these waters is commonly about 1 to 2, and can The red color comes
from iron sulfate ions
actually fall below 0. To us, acid mine drainage is an environmental that form in highly
catastrophe, but a number of archaeons thrive in it (Fig. 26.18). acidic water.
Modified protein and membrane structures enable them to tolerate
their harsh surroundings, making it possible to use sulfur compounds
in the environment for both chemoautotrophic and heterotrophic
growth. What is the limit to growth at low pH? Picrophilus
torridus, a euryarchaeote archaeon, grows optimally at 60°C and in
environments with a pH of 0.7—the pH of battery acid. It is capable One group of the Euryarchaeota thrives in extremely
of growth in environments down to pH 0— a record, as far as we salty environments.
know. In fact, Picrophilus torridus requires an acidic environment. It Methanogens are not the only unique euryarchaeotes. A related
will not grow in rainwater because the pH is too high. group illustrates the limits of tolerance to another environmental
condition—salinity, or saltiness. The Haloarchaea are halophilic (salt-
Only Archaea produce methane as a by-product of loving) photoheterotrophs that use the protein bacteriorhodopsin to
energy metabolism. absorb energy from sunlight (see Fig. 26.6). They live in waters that
Euryarchaeotes include Earth’s only biological source of are salty enough to precipitate table salt (NaCl), maintaining osmotic
natural gas, or methane (CH4). Known as methanogens, they balance by accumulating ions (especially K1) and organic solutes in
generate methane by fermenting acetate (CH3COO2) or by their cytoplasm. Just as extreme acidophiles require acid conditions,
chemoautotrophic pathways in which hydrogen gas (H2) reduces these extreme halophiles require high salt conditions and cannot live
CO2 to CH4. Methanogenic Archaea play an important role in the in dilute water.
carbon cycle, recycling organic molecules in environments where
the supplies of electron acceptors for respiration are limited. Thaumarchaeota may be the most abundant cells
Most methanogens live in soil or sediments, and they are in the deep ocean.
particularly prominent in peats and lake bottoms where sulfate levels Another surprise of environmental sequencing has been the
are low and bacterial sulfate reduction is therefore limited. Some, discovery that the immense volumes of ocean beneath the surface
however, live in the rumens (a specialized chamber in the gut) of cows waters contain vast numbers of archaeons that have not yet been
(Case 5), helping to maximize the energy yield from ingested grass. cultured in the lab. In the deep ocean, these organisms may be
Methane doesn’t last long in the atmosphere—it is quickly oxidized nearly as abundant as the combined number of all bacteria on and
to carbon dioxide. However, the steady supply of methane generated below the surface. Recently segregated as the Thaumarchaeota,
by archaeons contributes significantly to the greenhouse properties these archaeons are chemoautotrophs, deriving energy from the
of the atmosphere. For this reason, biologists are interested in how oxidation of ammonia (Fig. 26.19). Thus, like the discovery of
methanogenic Archaea will respond to 21st-century climate change SAR11, the discovery of marine thaumarchaeotes makes it clear
(Chapter 49). that before the still-emerging age of environmental genetics
HOW DO WE KNOW?

FIG. 26.19

How abundant are archaeons in the oceans?

BACKGROUND Early exploration of microbial diversity in the CONCLUSION Marine thaumarchaeota, unknown in 1990, are
oceans revealed that archaeons are tremendously abundant. Just now known to be among the most abundant organisms in the
how abundant are they, and what metabolisms do they employ to oceans.
obtain energy and carbon?
FOLLOW-UP WORK Archaea are among the most abundant of
METHOD Scientists sampled seawater throughout the depth of the all cells in the world’s oceans. How do these cells live? Microbial
ocean at a test site in the northern Pacific Ocean. To the samples, samples from water known to be sites of nitrification (that is, the
they added molecular tags bound to fluorescent molecules that are conversion of ammonia to nitrite or nitrate) contained an abundance
visible under the microscope. The tags were RNA sequences that of thaumarchaeote cells. The high numbers of thaumarchaeotes
bind to small-subunit ribosomal RNA genes known to be useful in from these communities supported the hypothesis that they are
identifying different types of bacteria and archaeons. In this way, nitrifiers—the higher the abundance of thaumarchaeote cells
separate fluorescent markers were attached to thaumarchaeote (Fig. 26.19b, dark blue line), the higher the amount of nitrite
archaeons, euryarchaeote archaeons, and bacteria. produced (Fig. 26.19b, light blue line). Thaumarchaeotes were
grown in pure culture and shown to grow by consuming ammonia
RESULTS By counting the cells marked by different fluorescent (Fig. 26.19b, red line). In other words, these organisms oxidize
tags in seawater samples, the biologists showed that bacteria ammonia to provide the ATP and reducing power needed to
dominate microbial communities in near-surface seawater, but that incorporate CO2 into organic molecules. Therefore, they play a
thaumarchaeotes are as numerous as bacteria in deeper waters major role in the marine nitrogen cycle, thriving where sources of
(Fig. 26.19a). carbon and energy for other types of metabolism are scarce.

a. b.
0.6 1.50 x 10
Thaumarchaeota
Euryarchaeota 0.5 1.25 x 10
Bacteria NO2 concentration
NH4 concentration
Concentration (mM)

0.4 Cell abundance 1.00 x 10

10
0.3 7.50 x 10

0.2
Depth (m)

5.00 x 10

100 0.1 2.50 x 10

0.0 0.00
2 4 6 8 10 12 14 16 18 20 22 24
Time (d)
1000 Growth of a marine thaumarchaeote in a medium containing ammonium
chloride and bicarbonate as the only sources of energy and carbon,
respectively. As cell number increased, ammonia (the ammonium ion NH41)
was increasingly converted to nitrite (NO22), supporting the hypothesis that
5000
these cells are ammonia-oxidizing chemoautotrophs.
0
20 40 60 80
Percentage of cells belonging
to domain SOURCES Karner, M. B., E. F. DeLong, and D. M. Karl. 2001. “Archaeal
Dominance in the Mesopelagic Zone of the Pacific Ocean.” Nature 409:507–510
Surveys of cell abundance through a depth profile of the Pacific Ocean show (The figure has been modified to reflect more recent nomenclature.); Könneke, K.,
that bacteria dominate cell numbers near the surface, but thaumarchaeotes et al. 2005. “Isolation of an Autotrophic Ammonia-Oxidizing Marine Archaeon.”
make up about 40% of all cells in deeper waters. Nature 437:543–546.

547
548 SECTION 26.6 T H E E VO L U T I O N A RY H I S TO RY O F P RO K A RYOT E S

and genomics, we simply didn’t have a very good idea of which

microorganisms lived where and did what in the oceans. FIG. 26.21 Stromatolites. Modern-day stromatolites at Shark Bay,
Fortunately, that situation is changing rapidly. Western Australia (top) show microbial communities
building the domed structures exhibited by
stromatolites in ancient rocks (bottom). Stromatolites
26.6 THE EVOLUTIONARY are among the earliest records of life on Earth. Sources:
HISTORY OF PROKARYOTES (top) Andrew Knoll, Harvard University; (bottom) François Gohier/
Science Source.

On the tree of life, the plants and animals of our familiar

existence populate only the most recent branches. For most of
Earth’s history, life was entirely microbial. Perhaps surprisingly
given their small size, Bacteria and Archaea have left a fossil
record that allows paleontologists to reconstruct aspects of
life’s early evolution. Sedimentary rocks preserve microscopic
fossils of ancient bacteria, including the photosynthetic
cyanobacteria (Fig. 26.20). Limestones and related rocks also
preserve stromatolites, layered structures that record sediment
accumulation by microbial communities (Fig. 26.21). Some
especially well preserved ancient rocks also contain molecular
fossils—biomolecules, usually lipids, that resist decay and so
preserve a chemical signature of ancient life.

Life originated early in our planet’s history.

Earth is about 4.6 billion years old, but few sedimentary rocks
have survived destruction by geologic processes and remain
to document our planet’s earliest history. Sedimentary rocks
deposited nearly 3.5 billion years ago in Western Australia and
South Africa provide some of our earliest records of Earth’s history.
Remarkably, they preserve a scarce but discernible signature of

FIG. 26.20 Bacterial microfossils. (a) Bacteria that probably

metabolized iron in 1.9-billion-year-old rock from the
Gunflint Formation in northwest Ontario, Canada.
(b) Cyanobacteria preserved in 800-million-year-old
silicified marine carbonates from the Draken Formation
in Spitsbergen, Norway. (c) A short cyanobacterial
filament from silicified tidal flat carbonates of the
1.5-billion-year-old Bilyakh Group in northern Siberia. life. Tiny organic structures preserved in these rocks have been
Source: a–c. Andrew Knoll, Harvard University.
interpreted as microfossils, although this conclusion remains
a b controversial.
More pervasive (and persuasive) evidence of life in early
oceans is provided by stromatolites. Stromatolites in ancient rocks
are strikingly similar to modern structures formed by microbial
communities, and so provide evidence of life that goes back about
10 μm 3.5 billion years (Fig. 26.21). Also, because purely physical processes
cannot explain the isotopic composition of carbon in very old
c limestones and organic matter, we can conclude that a biological
carbon cycle existed 3.5 billion years ago (and, from the evidence of
3.8-billion-year-old rocks in Greenland, possibly earlier).
10 μm 10 μm The chemistry of Earth’s oldest sedimentary rocks also shows
that the early atmosphere and ocean contained little or no free
 CHAPTER 26 B AC T E R I A A N D A RC H A E A 549

oxygen. What would the carbon cycle look like on an oxygen-free

planet? We know the answer from our discussion of prokaryotic FIG. 26.22 A large population of Wolbachia bacteria
metabolism. Photosynthesis can proceed in oxygen-poor waters, (fluorescing green) in the embryo of a fruit fly.
using H2S, H2, or Fe21 as electron donors, and fermenters and Wolbachia can influence the development and
anaerobic respirers can recycle carbon using electron acceptors reproductive biology of their animal hosts. Source:
such as SO422 and Fe31. Geologic evidence suggests that iron Image by Zoe Veneti and Kostas Bourtzis.

played a particularly important role in cycling carbon for the first

billion years of evolutionary history.
The evolution of cyanobacteria, with their capacity for
oxygenic photosynthesis, made possible the accumulation of
oxygen in the atmosphere and oceans. As discussed in Chapter 25,
oxygen began to accumulate in the atmosphere about 2.4 billion
years ago, making possible aerobic respiration and, eventually,
our present-day carbon cycle. Both microfossils and molecular
fossils indicate that cyanobacteria and other photosynthetic
bacteria continued to dominate primary production until about
800 million years ago. Thus, our modern carbon cycle, powered
overwhelmingly by oxygenic photosynthesis and with major
participation by eukaryotic organisms, may have existed for only
the last 20% of Earth’s history.

Prokaryotes have coevolved with eukaryotes.

Even after the rise of eukaryotic cells and, later, plants and
animals, Bacteria and Archaea have remained essential to
the functioning of biogeochemical cycles and hence to the within their tissues. A heterotroph that gains nutrition by
maintenance of habitable environments on Earth. Prokaryotic metabolizing organic compounds supplied by its host, Wolbachia
organisms have also radiated into the many novel environments infects its host’s reproductive tissues and passes from one
made possible by eukaryotes. We noted earlier that bacteria generation to the next in the host’s eggs (Fig. 26.22). Remarkably,
fix nitrogen in nodules formed on soybean roots. Soybeans many Wolbachia strains alter host reproductive cells so that
and nitrogen-fixing bacteria evolved together, a process called the insect host can successfully reproduce only with partners
coevolution. infected by the same Wolbachia strain. Because Wolbachia cells
Coevolutionary relationships between prokaryotic are transferred by eggs and not sperm, they have little use for
microorganisms and eukaryotes are common in nature. As noted males. Consequently, some cause male host embryos to die, thus
earlier, cows metabolize grass with the help of methanogenic increasing the proportion of females. Others cause genetically
archaeons in the rumen, a specialized digestive organ. The male embryos to develop as females, or enable eggs to develop into
microbes enable the cow to gain nutrition from plant materials female embryos without fertilization.
that could otherwise not be digested, and the benefit to the Wolbachia have another effect on their insect hosts that is
microorganisms is a steady supply of food. Similarly, clams that highly relevant to human health: They commonly inhibit infection
live near methane-rich vents in the Gulf of Mexico have evolved by viruses and the eukaryotic parasites that cause malaria. Recent
specialized organs that house methane-oxidizing bacteria, research shows that Wolbachia populations in some fly species
providing them with a reliable source of nutrition. protect their hosts against infection by RNA viruses. Scientists
Bacteria can affect animal behavior as well as nutrition. For introduced these bacteria into mosquitos that transmit dengue
example, some squid species attract mates by emitting light from fever, a tropical disease that infects 50–100 million people each
specialized organs full of bioluminescent bacteria (Case 5). The year. The Wolbachia-infected mosquitos were strongly resistant
bacteria provide the squid with a mechanism for communication to the Dengue virus, which causes dengue fever. Moreover, when
in the dark deep-sea environment, and the squid feeds the introduced into the wild, these virus-resistant forms rapidly
bacteria. Interestingly, many different kinds of bacteria live in expanded to dominate regional mosquito populations. Wolbachia
the waters that surround the squid, but only the bioluminescent parasites present a possible path to controlling dengue fever,
species accumulates in the squid’s light organ. The squid malaria, and other insect-borne diseases.
apparently sends molecular signals that attract just those bacteria. All around us, plants, animals, and microorganisms live in
Bacteria can also influence animal reproduction. Earth intimate association—sometimes to our detriment, but commonly
supports more than a million species of insects, and perhaps in mutually beneficial relationships that influence how we grow,
two-thirds of these harbor the proteobacterial parasite Wolbachia develop, eat, and behave. Humans are no exception.
 550 SECTION 26.6 T H E E VO L U T I O N A RY H I S TO RY O F P RO K A RYOT E S

? CASE 5 THE HUMAN MICROBIOME: DIVERSITY WITHIN A fertilized human egg has no bacteria attached to it, so our
microscopic passengers arrive by colonization—by infection, if
How do intestinal bacteria influence human health? you will. Research on children from Europe and rural Africa clearly
It has been estimated that the bacteria (and, to a much lesser shows the importance of diet in establishing our gut microbiota
extent, archaeons) in and on your body outnumber your own (Fig. 26.24). The African children, raised on a diet low in fat and
cells by as much as 10 to 1. Estimates of the number of microbial animal protein but rich in plant matter, have gut microbiotas
species that inhabit our bodies vary, but about 750 types have been
identified in the mouth alone, and the list remains incomplete
(Fig. 26.23). An equal number resides in the colon, and still more
live on the skin and elsewhere. Some are transients, entering FIG. 26.24 Diet and the intestinal microbiota. Studies of
and leaving the body within a few bacterial generations. Others gut bacteria in children from (a) rural Africa and
are specifically adapted for life within humans, forming complex (b) Western Europe show markedly different
communities of interacting species. communities. Source: Data from C. De Filippo, D. Cavalieri,
At present, much research is focused on the bacteria within M. Di Paola, M. Ramazzotti, J. B. Poullet, S. Massart, S. Collini,
our intestinal tracts. We commonly think of gut bacteria as G. Pieraccini, and P. Lionetti, 2010, “Impact of Diet in Shaping
harmful, but this perception arises from only a small—albeit Gut Microbiota Revealed by a Comparative Study in Children from
devastating—subset of our microbial guests. Illnesses known to be Europe and Rural Africa,” Proceedings of the National Academy of
caused by bacteria within our bodies include cholera, dysentery, Sciences USA 107(33):14693, doi:10.1073/pnas.1005963107.
and tuberculosis. Once the leading causes of death in New York a.
and London, they remain major killers in Africa. Other diseases,
not previously thought to be of microbial origin, are now known
Others
to be caused by bacteria—ulcers, for example, and stomach 15%
cancer, both mediated by the acid-tolerant bacterium Helicobacter Subdoligranulum 4%
pylori. More often than not, however, intestinal bacteria have a Prevotella
Faecalibacterium 4% 53%
beneficial effect. They help to break down food in our digestive
system and secrete vitamins and other biomolecules into the colon Acetitomaculum 4%
Xylanibacter
for absorption into our tissues. Molecular signals from bacteria 20%
guide the proper development of cells that line the interior of our
intestines.

Bacteroidetes The gut biota of the African

Firmicutes children is enriched in two
members of the phylum
FIG. 26.23 The human microbiome. This chart provides a rough Bacteroidetes, both known to
help break down the cellulose
guide to the distribution of the 3000 or more species found in fiber-rich plant foods.
of bacteria found on and within a healthy human
adult, showing the locations of bacteria that had been
sequenced as of 2010. Source: Data from J. Peterson et al., Alistipes 4%
b.
2009, “The NIH Human Microbiome Project,” Genome Research
19:2317–2323; originally published online October 9, 2009.

Blood 1% Others
22% Bacteroides
Urogenital 23%
tract 9%

Subdoligranulum
9%
Airways GI tract
29% Faecalibacterium
14% Roseburia 5%
25%
Acetitomaculum 12%
Skin
21% Bacteroidetes European children have
Mouth
26% Firmicutes intestinal bacteria
enriched in members of
the phylum Firmicutes.
 CHAPTER 26 B AC T E R I A A N D A RC H A E A 551

enriched in species of the phylum Bacteroidetes that are known inflammatory bowel disease, a disabling inflammation of the colon,
to help digest cellulose. European children raised on a typical increases in humans who have been treated with antibiotics.
Western diet rich in sugar and animal fat lack these bacteria, To understand the relationships between the human
harboring instead diverse members of the phylum Firmicutes. We microbiome and human health, we need to know what constitutes
do not understand the full ramifications of these differences, but a healthy gut biota, and this means studying the metabolism,
active research suggests that they may help to explain the relative ecology, and population genetics of the bacteria in our bodies. We
prominence in Western societies of allergies and other disorders need to know which bacteria have evolved to take advantage of
involving the immune system. the environments provided by the human digestive system, and
We also influence our gut biota by ingesting antibiotics. which are “just passing through.” Future visits to the doctor may
Although prescribed for the control of pathogens, most antibiotics include routine genetic fingerprinting of our bacterial biota, as
kill a wide range of bacteria and so can change the balance of our well as treatments designed to keep our microbiome—and, so,
intestinal microbiota. Clinical studies show that the incidence of ourselves—healthy. •

Core Concepts Summary organic molecules and the production of ATP in limited
quantities. page 536
26.1 The tree of life has three main branches, called Some photosynthetic bacteria are photoheterotrophs, obtaining
domains: Eukarya, Bacteria, and Archaea. energy from sunlight but using preformed organic compounds
rather than CO2 as a source of carbon. page 537
Prokaryotic cells are cells that lack a nucleus; they include
Bacteria and Archaea. page 530 Chemoautotrophy, in which chemical energy is used to convert
CO2 to organic molecules, is unique to Bacteria and Archaea.
Bacteria are small and lack membrane-bounded organelles.
page 537
page 530

The bacterial genome is circular. Some bacteria also carry 26.3 In addition to their key roles in the carbon cycle,
smaller circles of DNA called plasmids. page 530 Bacteria and Archaea are critical to the biological cycling
Diffusion limits size in bacterial cells. page 530
of sulfur and nitrogen.

Bacteria can obtain DNA by horizontal gene transfer from Plants and algae can take up sulfur and incorporate it into
organisms that may be distantly related. page 532 proteins, but bacteria and archaeons dominate the sulfur cycle
by means of oxidation and reduction reactions that are linked to
Like Bacteria, Archaea lack a nucleus, but form a second
the carbon cycle. page 538
prokaryotic domain distinct from Bacteria. page 534
Some bacteria and archaeons can reduce nitrogen gas to
Some archaeons are extremophiles, living in extreme
ammonia in a process called nitrogen fixation. page 539
environments characterized by low pH, high salt, or high
temperatures, but others live in less extreme environments like The nitrogen cycle also involves oxidation and reduction
the upper ocean or soil. page 534 reactions by Bacteria and Archaea that are linked to the
carbon cycle. page 540
26.2 Bacteria and Archaea are notable for their
metabolic diversity. 26.4 The extent of bacterial diversity was recognized
when sequencing technologies could be applied to non-
Bacteria are capable of oxygenic photosynthesis, using water
culturable bacteria.
as a source of electrons and producing oxygen as a by-product,
and of anoxygenic photosynthesis, using electron donors other Traditionally, bacterial groups were recognized by morphology,
than water, such as H2S, H2, and Fe21. page 535 physiology, and the ability to take up specific stains in culture.
Some bacteria and archaeons are capable of anaerobic page 540
respiration, in which NO32, SO422, Mn41, and Fe31 serve as the Direct sequencing of ribosomal RNA genes from organisms in
electron acceptor instead of oxygen gas. page 535 soil and seawater samples revealed new groups of bacteria.
Many bacteria and archaeons obtain energy from page 540
fermentation, which involves the partial oxidation of
552 SELF-ASSESSMENT

Proteobacteria are the most diverse group of bacteria and are The early atmosphere and ocean contained little or no free
involved in many of the biogeochemical processes that are linked oxygen. page 548
to the carbon cycle. page 543
Oxygen began to accumulate in the atmosphere and oceans
Gram-positive bacteria include important disease-causing strains about 2.4 billion years ago as a result of the success of
as well as species that are principal sources of antibiotics. cyanobacteria utilizing oxygenic photosynthesis. page 549
page 543
Prokaryotic metabolisms were not only essential in the early
Photosynthetic bacteria are not limited to a single branch of the history of Earth, but are also vital today, as many forms of life
bacterial tree. page 544 depend on biogeochemical cycles and metabolisms unique to
Bacteria and Archaea. page 549
26.5 The diversity of Archaea has only recently been Most animals, including humans, live in intimate association
recognized. with bacteria, which in turn affect health. page 550
Archaeons tend to thrive where energy available for growth is
limited. page 545
Self-Assessment
Archaea are commonly divided into three major groups, the
1. Name and describe the three domains of life.
Crenarchaeota, Thaumarchaeota, and Euryarchaeota.
page 545 2. Describe shared and contrasting features of bacterial and
archaeal cells.
Archaeons at the base of the Crenarchaeota and Euryarchaeota
are hyperthermophiles, meaning that they grow at high 3. Explain how prokaryotic cells obtain nutrients and how this
temperatures. page 545 process puts constraints on their size.

A number of archaeons grow in highly acidic waters, such as 4. Describe how surface area and volume change with size.
those associated with acid mine drainage. page 546
5. Explain how photosynthesis can occur without the
Some archaeons (but no bacteria or eukaryotes) generate production of oxygen, and how respiration can occur without
methane as a by-product of their energy metabolism, requiring oxygen.
contributing in important ways to the carbon cycle. page 546
6. Describe the roles of bacteria and archaeons in the sulfur
Haloarchaea are archaeons that can live only in extremely salty and nitrogen cycles.
environments. page 546 7. Explain how horizontal gene transfer complicates our
Thaumarchaeotes are among the most abundant cells in the understanding of evolutionary relationships among bacteria
oceans. page 546 and archaeons.

8. Name and describe three major groups of Bacteria.

26.6 The earliest forms of life on Earth were Bacteria
9. Name and describe three major groups of Archaea.
and Archaea
10. State the age of Earth and the time when life is thought to
Evidence for the early history of life comes from microfossils,
have first originated.
fossilized structures called stromatolites, and the isotopic
composition of rocks and organic matter. page 548
Log in to to check your answers to the Self-
Fossils indicate that life on Earth originated more than Assessment questions, and to access additional learning tools.
3.5 billion years ago. page 548
 CHAPTER 27

Eukaryotic Cells
Origins and Diversity

Core Concepts
27.1 Eukaryotic cells are defined
by the presence of a nucleus,
but features like a dynamic
cytoskeleton and membrane
system explain their success in
diversifying.
27.2 The endosymbiotic hypothesis
proposes that the chloroplasts
and mitochondria of eukaryotic
cells were originally free-living
bacteria.
27.3 Eukaryotes were formerly
divided into four kingdoms, but
are now divided into at least
seven superkingdoms.
27.4 The fossil record extends our
understanding of eukaryotic
diversity by providing
perspectives on the timing
and environmental context of
eukaryotic evolution.
Lebendkulturen.de/Shutterstock.

553
554 SECTION 27.1 T H E E U K A RYOT I C C E L L : A R E V I E W

Eukaryotic cells have a nucleus—that is their defining

characteristic. As we will see, however, it is quite another set of FIG. 27.1 A eukaryotic cell. Eukaryotic cells have a nucleus and
features that distinguishes eukaryotes in terms of function and extensive internal compartmentalization.
the capacity for evolutionary innovation. Eukaryotic diversity—
In eukaryotes, energy
the animals, plants, fungi, and protists we are familiar with—does metabolism that requires The cytoskeleton
not reflect exceptional versatility in energy metabolism. In fact, membrane stability is enables cells to change
compared with Bacteria and Archaea, the Eukarya obtain carbon confined to organelles. shape by remodeling
quickly.
and energy in only a limited number of ways. For example,
oxygenic photosynthesis is widespread among eukaryotes, Cytoskeleton
but anoxygenic photosynthesis is unknown. Furthermore, Nuclear membrane
aerobic respiration is present almost everywhere, but anaerobic Nucleus
respiration has been reported in only a handful of species. Endoplasmic
The nuclear
Chemoautotrophy has not been documented in eukaryotes at membrane
reticulum
all. Many eukaryotes can ferment organic substrates—yeasts are separates Mitochondrion
particularly important in this regard—but most use fermentation transcription
and translation
only as a supplementary metabolism and lack the diversity of in eukaryotes. Golgi apparatus
reactions known among prokaryotic organisms.
Why then, have eukaryotes been so successful? Eukaryotic
Plasma membrane
success lies in their remarkable capacity to form cells of diverse
shape and size and to produce multicellular structures in which
multiple cell types act in concert. Together, these innovations
Vesicles budding off from A network of membranes is
opened up a range of functional possibilities not found among membranes transport able to change shape and
Bacteria and Archaea. materials into the cell in package molecules and
endocytosis, and release particles for transport within
material from the cell in the cell.
exocytosis.
27.1 THE EUKARYOTIC CELL:
A REVIEW
Chapter 5 introduced the fundamental features of eukaryotic cells.
Here, we revisit those attributes (Fig. 27.1) and consider their the endoplasmic reticulum (ER) and Golgi apparatus, and a plasma
consequences for function and diversity. How do eukaryotic cells membrane that surrounds the cytoplasm. All membranes of the
differ from bacteria and archaeons, and how do these differences endomembrane system are interconnected, either directly or by
explain the roles that eukaryotes play in modern ecosystems? the movement of vesicles. Many are also capable of changing shape
rapidly. In fact, the different membranes are interchangeable in the
Internal protein scaffolding and dynamic membranes sense that material originally added to the endoplasmic reticulum
organize the eukaryotic cell. may in time be transferred to the cell membrane or nuclear
All cells require a mechanism to maintain spatial order in the membrane. Biologists like to say that the membranes of eukaryotic
cytoplasm. As we saw in Chapter 26, bacteria and archaeons rely cells are in dynamic continuity. Membranes are stable, as required
primarily on walls that support the cell from the outside, along for energy metabolism, only in the mitochondria and chloroplasts.
with a framework of proteins within the cytoplasm. Eukaryotes In combination, the dynamic cytoskeleton and membrane
also use an internal scaffolding of proteins to organize the cell, system provide eukaryotes with new possibilities for movement.
mostly microtubules composed of the protein tubulin and For example, amoebas extend finger-like projections that pull
microfilaments of actin. This cytoskeleton, found in all eukaryotic the rest of the cell forward. They also enable eukaryotic cells to
cells, differs in one key property from the rigid protein framework engulf molecules or particles, including other cells, in a process
of bacteria: It can be remodeled quickly, enabling cells to change called endocytosis, which prokaryotic cells simply cannot do.
shape (Chapter 10). Phagocytosis is a specific form of endocytosis in which eukaryotic
Dynamic cytoskeletons require dynamic membranes that cells surround food particles and package them in vesicles that
enable the cell to continue functioning even as it changes shape; bud off from the cell membrane (Fig. 27.2). Thus packaged,
eukaryotes maintain within their cells a remarkably dynamic particles can be transported into the cell interior for digestion. The
network of membranes called the endomembrane system same process also works in reverse: In exocytosis, newly formed
(Chapter 5). This network includes the nuclear envelope, an molecules or cytoplasmic waste are packaged in vesicles and
assembly of membranes that runs through the cytoplasm called moved to the cell surface for removal (Chapter 5).
 CHAPTER 27 E U K A RYOT I C C E L L S : O R I G I N S A N D D I V E R S I T Y 555

FIG. 27.2 Phagocytosis. The flexibility of the eukaryotic cytoskeleton and membrane system enables eukaryotic cells to engulf food particles,
including other cells. This sequence shows an amoeba engulfing a yeast cell. Note how the cell membrane (green) folds inward locally
to form a vesicle around the red food particle. Source: V. Mercanti, S. J. Charette, N. Bennett, J. J. Ryckewaert, F. Letourneur, and P. Cosson, 2006, “Selective
Membrane Exclusion in Phagocytic and Macropinocytic Cups,” Journal of Cell Science 119:4079– 4067.

Intracellular vesicles and the molecules they carry are foodstuffs, including other animals and plants. In consequence,
transported through the cytoplasm by means of molecular motors eukaryotes can exploit sources of food not readily available to
associated with the cytoskeleton (Chapter 10). In this way, both bacterial heterotrophs, which feed on individual molecules. This
nutrients and signaling molecules move through the cell at speeds ability opens up a great new ecological possibility—predation—
much greater than diffusion allows. A major consequence is that increasing the complexity of interactions among organisms.
eukaryotic cells can be much larger than most bacteria. The structural flexibility of eukaryotic cells also allows
The cytoskeleton and membrane system are flexible in another photosynthetic eukaryotes to interact with their environment
way: A change in the expression of a few genes can change their in ways that photosynthetic bacteria cannot. Unicellular algae
shape and organization. This makes possible yet another hallmark (which are eukaryotes) can move effectively through surface
of eukaryotic evolution—complex multicellularity (Chapter 28). waters vertically as well as horizontally and therefore can seek
In complex multicellular organisms, patterns of gene expression and exploit local patches of nutrients. Diatoms, discussed shortly,
can modify the cytoskeleton and membranes of individual cells, go one step further. Large internal vacuoles allow them to store
enabling them to function in different ways. nutrients for later use when nutrient levels in the environment
may become low. Plants have evolved multicellular bodies with
In eukaryotic cells, energy metabolism is localized in many different cell types, allowing them to capture sunlight many
mitochondria and chloroplasts. meters above the ground. That ability gives plants a tremendous
Relative to prokaryotic organisms, eukaryotes are fairly limited in advantage on land, as long as their leaves can obtain water and
the ways they obtain carbon and energy. Moreover, the metabolic nutrients from the soil in which the plants are rooted.
processes that power eukaryotic cells take place only in specific
organelles—aerobic respiration in the mitochondrion (Chapter 7) The organization of the eukaryotic genome also helps
and photosynthesis in the chloroplast (Chapter 8). Only limited explain eukaryotic diversity.
anaerobic processing of food molecules takes place within the Bacteria and Archaea absorb available nutrients quickly, and both
cytoplasm. rapid deployment of metabolic proteins and rapid reproduction
As noted above, many eukaryotic cells engulf food particles are key to the exploitation of patchily distributed nutrients.
and package them inside a vesicle, which is then transported As the majority of a prokaryote’s DNA is arrayed in a single
into the cytoplasm. Within the cytoplasm, enzymes break circular chromosome, speed of replication allows for speed of
down the particles into molecules that can be processed by the reproduction. As a result, selection favors those strains of Bacteria
mitochondria. Many single-celled eukaryotes feed on bacteria and Archaea that retain only the genetic material vital to the
or other eukaryotic cells, and animals, in turn, ingest larger organism.
556 SECTION 27.1 T H E E U K A RYOT I C C E L L : A R E V I E W

Eukaryotes have multiple linear chromosomes and can bdelloid rotifers. The genetic diversity of these organisms is
begin replication from many sites on each one. Eukaryotes are actually high, maintained by high rates of horizontal gene transfer.
thus able to replicate multiple strands of DNA simultaneously Meiotic cell division results in cells with one set of
and rapidly (Chapter 12). This ability relieves the evolutionary chromosomes. Such cells are haploid. Sexual fusion brings two
pressure for streamlining, allowing eukaryotic genomes to build haploid (1n) cells together to produce a diploid (2n) cell that has
up large amounts of DNA that do not code for proteins. Most of two sets of chromosomes. The life cycle of sexually reproducing
this additional DNA was originally considered to have no function eukaryotes, then, necessarily alternates between haploid and
and, indeed, was called “junk DNA.” That view, however, has been diploid states.
modified in recent years, as the complete genome has come to Many single-celled eukaryotes normally exist in the haploid
be better understood (Chapter 13). Eukaryotic genomes appear stage and reproduce asexually by mitotic cell division (Fig. 27.3a).
to contain truly junky DNA, but at least some of the DNA that The green alga Chlamydomonas is one such organism. Under
does not code for proteins functions in gene regulation (Chapter the right conditions, however—typically, starvation or other
19). This regulatory DNA gives eukaryotes the fine control of environmental stress—two cells fuse, forming a diploid cell, or
gene expression required for both multicellular development and zygote. The zygote formed by these single-celled eukaryotes
complex life cycles, two major features of eukaryotic diversity. commonly functions as a resting cell. It covers itself with a
At this point, it can be seen that the evolutionary success of protective wall and then lies dormant until environmental
eukaryotes rests on a combination of features. The innovations conditions improve. In time, further signals from the environment
of dynamic cytoskeletal and membrane systems gave eukaryotes induce meiotic cell division, resulting in four genetically distinct
the structure required for larger cells with complex shapes and haploid cells that emerge from their protective coating to
the ability to ingest other cells. Thus, early unicellular eukaryotes complete the life cycle.
did not gain a foothold in microbial ecosystems by outcompeting As shown in Fig. 27.3b, some single-celled eukaryotes
bacteria and archaeons. Instead, they succeeded by evolving normally exist as diploid cells. An example is provided by the
novel functions. Along with the capacity to remodel cell shape, diatoms, single-celled eukaryotes commonly found in lakes, soils,
eukaryotes evolved complex patterns of gene regulation, which in and the oceans. Diatom cells are mostly diploid and reproduce
turn enabled unicellular eukaryotes to evolve complex life cycles asexually by mitotic cell division to make more diploid cells.
and multicellular eukaryotes to generate multiple, interacting cell Because their mineralized skeletons constrain growth, diatoms
types during growth and development. These abilities opened up become smaller with each asexual division. Once a critical size
still more possibilities for novel functions, which we explore in is reached, meiotic cell division is triggered, producing haploid
this and later chapters. gametes that fuse to regenerate the diploid state as a round, thick-
walled cell. This cell eventually germinates to form an actively
j Quick Check 1 How did the evolutionary expansion of
growing, skeletonized cell. In diatoms, then, short-lived gametes
eukaryotic organisms change the way carbon is cycled through
constitute the only haploid phase of the life cycle.
biological communities?
The two life cycles just introduced, of Chlamydomonas and
diatoms, are similar in many ways. In both, haploid cells fuse to
Sex promotes genetic diversity in eukaryotes and gives form diploid cells, and diploid cells undergo meiotic cell division to
rise to distinctive life cycles. generate haploid cells. Both life cycles also commonly include cells
In Chapter 26, we saw how Bacteria and Archaea generate genetic capable of persisting in a protected form when the environment
diversity by horizontal gene transfer. Horizontal gene transfer becomes stressful. Why some single-celled eukaryotes usually
has been documented in eukaryotic species, but it is relatively occur as haploid cells and others usually occur as diploid cells
uncommon. How then do eukaryotes generate and maintain remains unknown—a good question for continuing research.
genetic diversity within populations? The answer is sex. Sexual reproduction has never been observed in some
Sexual reproduction involves meiosis and the formation eukaryotes, but most species appear to be capable of sex, even
of gametes, and the subsequent fusion of gametes during if they reproduce asexually most of the time. Variations on
fertilization (Chapters 11 and 42). Sex promotes genetic variation the eukaryotic life cycle can be complex, especially in parasitic
in two simple ways. First, meiotic cell division results in microorganisms that have multiple animal hosts. All, however,
gametes or spores that are genetically unique. Each gamete has a have the same fundamental components as the two life cycles
combination of alleles different from the other gametes and from described here.
the parental cell as a result of recombination and independent In the following chapters, we discuss multicellular organisms in
assortment. Second, in fertilization, new combinations of genes detail. Here we note only that animals, plants, and other complex
are brought together by the fusion of gametes. Interestingly, a few multicellular organisms have life cycles with the same features as
eukaryotic groups have lost the capacity for sexual reproduction. those just discussed. The big difference is that in animals, the zygote
The best-studied of these eukaryotes are tiny animals called divides many times to form a multicellular diploid body before a
 CHAPTER 27 E U K A RYOT I C C E L L S : O R I G I N S A N D D I V E R S I T Y 557

FIG. 27.3 Eukaryotic life cycles. (a and b) The life cycles of single-celled eukaryotes differ in the proportion of time spent as haploid (1n) versus
diploid (2n) cells. (c) The life cycles of animals have many mitotic divisions between formation of the zygote and meiosis. (d) Vascular
plants have two multicellular phases.

a. Unicellular eukaryote with prominent haploid phase b. Unicellular eukaryote with prominent diploid phase

1n 1n
Fusion Fusion

Asexual Asexual
reproduction Sexual Sexual
1n 2n 1n 2n reproduction
by mitosis cycle cycle
by mitosis

Meiosis Meiosis
1n 1n

1n 1n
1n 1n

c. Animal d. Vascular plant

1n Sperm 1n
1n
1n 1n 1n multi- Fusion Zygote
Reproductive 2n
2n multi- cellular body
cell Meiosis 1n
cellular body 2n 1n Growth
Gamete by mitosis
Egg Fusion (egg/sperm)

Zygote 2n

Spore
Growth by mitosis Growth 2n multi-
by mitosis 1n cellular body
2n
1n Meiosis Reproductive
cell
1n
small subset of cells within the body undergoes meiotic cell division 1n

to form haploid gametes (eggs and sperm). During fertilization, the

egg and sperm combine sexually to form a zygote (Fig. 27.3c). In
animals, as in diatoms, the only haploid phase of the life cycle is the
gamete. As we will see in Chapter 32, plants have two multicellular ? CASE 5 THE HUMAN MICROBIOME: DIVERSITY WITHIN
phases in their life cycle, one haploid and one diploid (Fig. 27.3d).
Like single-celled eukaryotes, many plants and animals can
What role did symbiosis play in the origin of
reproduce asexually. Humans and other mammals are unusual in
chloroplasts?
The chloroplasts found in plant cells closely resemble certain
that we cannot reproduce asexually.
photosynthetic bacteria, specifically cyanobacteria. The molecular
workings of photosynthesis are nearly identical in the two, and
internal membranes organize the photosynthetic machinery
27.2 EUKARYOTIC ORIGINS
in similar ways (Chapter 8). The Russian botanist Konstantin
In the preceding section, we reviewed the basic features of Sergeevich Merezhkovsky recognized this similarity more than
eukaryotic cells. The distinctive cellular organization of Eukarya a century ago. He also understood that corals and some other
differs markedly from the simpler cells of Bacteria and Archaea. organisms harbor algae within their tissues as symbionts that aid
How did the complex cells of eukaryotes come to exist? In Case the growth of their host. A symbiont is an organism that lives in
5: The Human Microbiome, we saw that symbioses are common, closely evolved association with another species. This association,
even in humans. Is the relationship between humans and bacteria called symbiosis, is discussed in Chapter 47.
unusual, or are symbioses so fundamental that they played key Putting these two observations together, Merezhkovsky came
roles in the evolution of the eukaryotic cell? up with a radical hypothesis. Chloroplasts, he argued, originated as
558 SECTION 27.2 E U K A RYOT I C O R I G I N S

symbiotic cyanobacteria that through time became permanently became clear that the chloroplasts in red and green algae (and in
incorporated into their hosts. Such a symbiosis, in which one land plants) are separated from the cytoplasm that surrounds them
partner lives within the other, is called an endosymbiosis. by two membranes. This is expected if a cyanobacterial cell had been
Merezkhovsky’s hypothesis of chloroplast origin by endosymbiosis engulfed by a eukaryotic cell. The inner membrane corresponds
was difficult to test with the tools available in the early twentieth to the cell membrane of the cyanobacterium, and the outer one is
century, and his idea was dismissed, more neglected than part of the engulfing cell’s membrane system, as shown in Fig. 27.2.
disproved, by most biologists. In addition, the biochemistry of photosynthesis was found to be
In 1967, American biologist Lynn Margulis resurrected the essentially the same in cyanobacteria and chloroplasts. Both use
endosymbiotic hypothesis, supporting her arguments with new two linked photosystems, a common mechanism for extracting
types of data made possible by the then-emerging techniques electrons from water, and the same reactions to reduce carbon
of cell and molecular biology (Fig. 27.4). Transmission electron dioxide (CO2) into organic matter (Chapter 8).
microscopy showed that structural similarities between chloroplasts Such observations kindled renewed interest in the
and cyanobacteria extend to the submicrometer level, for example endosymbiotic hypothesis, but the decisive tests were made
in the organization of the photosynthetic membranes. It also possible by another, and unexpected, discovery. It turns out

HOW DO WE KNOW?

FIG. 27.4

What is the evolutionary origin of

chloroplasts?
HYPOTHESIS Chloroplasts evolved from cyanobacteria living as
endosymbionts within a eukaryotic cell.
1 µm
OBSERVATION Chloroplasts and cyanobacteria have closely similar internal
membranes that organize the light reactions of photosynthesis. Transmission
electron microscopy shows that cyanobacteria (top), red algal chloroplasts
(middle), and plant chloroplasts (bottom) have very similar internal
membranes that organize the light reactions of photosynthesis.

EXPERIMENT Like cyanobacteria, chloroplasts have DNA organized in

a single circular chromosome. Phylogenies based on molecular sequence
comparisons place chloroplasts among the cyanobacteria.

1 µm CONCLUSION Molecular and electron microscope data support the

hypothesis that chloroplasts originated as endosymbiotic cyanobacteria.

SOURCE Giovannoni, S. J., et al. 1988. “Evolutionary Relationships Among Cyanobacteria and
Green Chloroplasts.” Journal of Bacteriology 170:3584–3592. Photos (top to bottom): Dr. Kari
Lounatmaa/Science Source; Biology Pics/Getty Images; Dr. Jeremy Burgess/Science Source.
 CHAPTER 27 E U K A RYOT I C C E L L S : O R I G I N S A N D D I V E R S I T Y 559

that chloroplasts have their own DNA, organized into a single

circular chromosome, like that of bacteria. Armed with this FIG. 27.5 A second example of chloroplast endosymbiosis.
knowledge, investigators could use the tools of molecular The photosynthetic amoeba Paulinella chromatophora
sequence comparison (Chapter 23). The sequences of nucleotides acquired photosynthesis by endosymbiosis
in chloroplast genes closely match those of cyanobacterial genes independently of and more recently than the
but are strikingly different from sequences of nucleotides in genes endosymbiosis that gave rise to chloroplasts in green
within the nuclei of photosynthetic eukaryotes. This finding plants and other eukaryotes. Source: Courtesy of Hwan Su Yoon.
provides strong support for the hypothesis of Merezkhovsky and
Margulis. Chloroplasts are indeed the descendants of symbiotic
cyanobacteria that lived within eukaryotic cells.
Eukaryotes were able to acquire photosynthesis because
they could engulf and retain cyanobacterial cells. But engulfing
another microorganism was just the beginning. The cyanobacteria
most closely related to chloroplasts have 2000 to 3000 genes.
In contrast, among photosynthetic eukaryotes, chloroplast
gene numbers vary from just 60 to 200. Where did the rest of
the cyanobacterial genome go? Some genes may simply have
been lost if similar nuclear genes could supply chloroplast’s
requirements. Many, however, were transported to the nucleus
when chloroplasts broke or by hitching a ride with viruses. The
nuclear genome of the flowering plant Arabidopsis contains several
thousand genes of cyanobacterial origin. Some code for proteins
destined for use within the chloroplast. Mitochondria are also like chloroplasts in having a small
Although foreign to mammals and other vertebrate animals, genome. Indeed, the mitochondrial genome is dramatically
symbiosis between a heterotrophic host and a photosynthetic reduced compared to the ancestral proteobacterial genome, in
partner is common throughout the eukaryotic domain. For most eukaryotes containing only a handful of functioning genes.
example, reef corals harbor photosynthetic cells that live Human mitochondria, for example, code for just 13 proteins and
symbiotically within their tissues. Some corals have even lost the 24 RNAs. Once again, many genes from the original bacterial
capacity to capture food from surrounding waters. Giant clams of endosymbiont migrated to the nucleus, where they still reside.
the genus Tridacna, found in tropical Pacific waters, obtain some Most eukaryotic cells contain mitochondria, but a few single-
or even most of their nutrition from symbiotic algae that live celled eukaryotic organisms found in oxygen-free environments
within their tissues. do not. Biologists earlier hypothesized that these eukaryotes
Until recently, most biologists agreed that chloroplasts evolved before the endosymbiotic event that established
originated from endosymbiotic cyanobacteria only once, in mitochondria in cells having a nucleus, but that proposal turns
a common ancestor of the green algae and red algae. Now, out to be wrong. We know this because of the propensity for
remarkably, a second case of cyanobacterial endosymbiosis has genes to migrate from the endosymbiont to the host’s nucleus.
come to light. Paulinella chromatophora is a photosynthetic Every mitochondria-free eukaryote examined to date has relic
amoeba (Fig. 27.5). Gene sequence comparisons show that its mitochondrial genes in its nuclear genome, providing support for
chloroplast originated in a branch of the cyanobacteria different a second hypothesis, that eukaryotic cells without mitochondria
from the one that gave rise to chloroplasts in other photosynthetic had them once but have lost them.
eukaryotes. About a third to a half of the ancestral genome In fact, many eukaryotes that lack mitochondria contain
remains in P. chromatophora chloroplasts, suggesting that in this small organelles called hydrogenosomes that generate ATP by
organism chloroplast evolution is still in progress. anaerobic processes (Fig. 27.6). These organelles have little or
no DNA, but genes of mitochondrial origin in the cells’ nuclei
? CASE 5 THE HUMAN MICROBIOME: DIVERSITY WITHIN code for proteins that function in the hydrogenosome. Thus,
What role did symbiosis play in the origin of hydrogenosomes appear to be highly altered mitochondria adapted
mitochondria? to life in oxygen-poor environments. Other anaerobic eukaryotes
Like chloroplasts, mitochondria closely resemble free-living contain still smaller organelles called mitosomes that also appear
bacteria in organization and biochemistry. And like chloroplasts, to be remnant mitochondria. Interestingly, mitosomes have
mitochondria contain DNA that confirms their close phylogenetic lost all capacity for metabolism, retaining only genes needed for
relationship to a form of bacteria—in this case proteobacteria assembling the clusters of iron and sulfur found in many proteins.
(Chapter 26). Like chloroplasts, then, mitochondria originated as Why these genes are retained in a modified mitochondrion
endosymbiotic bacteria. remains unknown.
 560 SECTION 27.2 E U K A RYOT I C O R I G I N S

FIG. 27.6 Transmission electron microscope images comparing mitochondria within the aerobic protist Euplotes (left) and Nyctotherus, a
close relative that lives in anoxic environments (right). The organelle in Nyctotherus is halfway between a normal mitochondrion and
a hydrogenosome, supporting the view that these two organelles are descended from a common ancestor. Source: Reprinted by permission
from Macmillan Publishers Ltd: B. Boxma, et al., 2005, “An Anaerobic Mitochondrion That Produces Hydrogen,” Nature 434:74–79. Copyright 2005.

1µm 1µm

Normal mitochondrion in Euplotes Altered mitochondrion in Nyctotherus produces ATP and generates hydrogen.
The small dark body in lower right is an endosymbiotic archaeon.

? CASE 5 THE HUMAN MICROBIOME: DIVERSITY WITHIN recently revealed a previously known archaeon whose genome
How did the eukaryotic cell originate? encodes actin-like proteins and other molecules fundamental
If mitochondria originated as proteobacteria and chloroplasts to cell organization, vesicle trafficking and phagocytosis in
are descended from cyanobacteria, where does the rest of the eukaryotes. Gene sequence comparisons, in fact, place these
eukaryotic cell come from? Analysis of the nuclear genome alone novel archaeons as the closest relatives of eukaryotes. An
provides no clear picture because genes of bacterial, archaeal, archaeal component of the eukaryotic cell seems assured, but
and purely eukaryotic origin are all present. As discussed, many some biologists argue that no eukaryotic cell existed before
nuclear genes originated with the mitochondria and chloroplasts there were mitochondria. Instead, they propose that the
acquired from specific bacteria. However, genes from other eukaryotic cell as a whole began as a symbiotic association
groups of bacteria also reside in the eukaryotic nucleus, between a proteobacterium and an archaeon (Fig. 27.7). The
recording multiple episodes of horizontal gene transfer through proteobacterium became the mitochondrion and provided many
evolutionary history. In contrast, some genes are present only in genes to the nuclear genome. The archaeon provided other
eukaryotes and apparently evolved after the domain originated. genes, including those used to transcribe DNA and translate it
Still others, including the genes that govern DNA transcription into proteins.
and translation, are clearly related to the genes of Archaea. Biologists continue to debate these alternatives. Both
Two starkly different hypotheses have been proposed to hypotheses explain the hybrid nature of the eukaryotic genome,
explain this mix of genes. Some biologists believe that the but neither fully explains the origins and evolution of the
host for mitochondrion-producing endosymbiosis was itself a nucleus, linear chromosomes, the eukaryotic cytoskeleton, or
true eukaryotic cell. This cell had a nucleus, cytoskeleton, and a cytoplasm subdivided by ever-changing membranes. There is
endomembrane system, but only limited ability to derive energy no consensus on the question of eukaryotic origins—it is one
from organic molecules (Fig. 27.7). In this view, nuclear genes of biology’s deepest unanswered questions, awaiting novel
in Eukarya resemble those of Archaea because the primordial observations by a new generation of biologists. Once a dynamic
eukaryotic host cell was closely related to Archaea. cytoskeleton became coupled to a flexible membrane system,
In striking support of this hypothesis, microbiological however, the evolutionary possibilities of eukaryotic form were
exploration of hydrothermal vents beneath the Arctic ocean established.
 CHAPTER 27 E U K A RYOT I C C E L L S : O R I G I N S A N D D I V E R S I T Y 561

FIG. 27.7 Two hypotheses for the origin of the eukaryotic cell. Both hypotheses lead to the evolution of a mitochondrion-bearing protist.

Hypothesis 1 Hypothesis 2
Eukaryotic cells evolved from an ancestral archaeon Eukaryotic cells evolved from a symbiosis
and only later incorporated the proteobacterial cell between an archaeon and a proteobacterium.
that became a mitochondrion. The proteobacterium became a mitochondrion.

DNA Ancestral
archaeon
Cytoplasm
Plasma
membrane Archaeon

Proteobacterium
Infolding
of plasma
membrane

True eukaryotic
cell with nucleus,
cytoplasm, and
endomembrane
system

Archaeon host with

ancestor of mitochondria
as symbiont

Engulfing
of facultatively
aerobic
proteobacterium

Infolding of
plasma membrane
to produce true
eukaryote

Mitochondria Mitochondria

Heterotrophic eukaryote Heterotrophic eukaryote

562 SECTION 27.3 E U K A RYOT I C D I V E R S I T Y

In the oceans, many single-celled eukaryotes harbor substance. Scientists hypothesize that the bacteria benefit from
symbiotic bacteria. this association as well because they get a free ride through the
The evolution of the eukaryotic cell is marked by intimate sediments, enabling these chemoautotrophs to maximize growth
associations between formerly free-living organisms. An by remaining near the boundary between waters that contain
unexpected symbiosis was recently discovered in the Santa Barbara oxygen and those rich in sulfide.
Basin, a local depression in the seafloor off the coast of southern The diversity of these eukaryotic–bacterial symbioses is
California. Sediments accumulating in this basin contain little remarkable—and poorly studied. What has been learned to
oxygen but large amounts of hydrogen sulfide (H2S) generated by date, however, shows that single-celled eukaryotes have evolved
anaerobic bacteria within the sediments. We might predict that numerous symbiotic relationships with chemoautotrophic
eukaryotic cells would be uncommon in these sediments. Not bacteria, associations that feed and protect the eukaryotes,
only is oxygen scarce, but also sulfide actively inhibits respiration enabling them to colonize habitats where most eukaryotes cannot
in mitochondria. It came as a surprise, then, that samples of live. Clearly, then, the types of symbioses that led to mitochondria
sediment from the Santa Barbara Basin contain large populations and chloroplasts on the early Earth were not rare events, but basic
of single-celled eukaryotes. associations between cells that continue to evolve today.
Some of these cells have hydrogenosomes, mitochondria
altered by evolution to generate energy where oxygen is absent.
Others, however, appear to thrive by supporting populations of 27.3 EUKARYOTIC DIVERSITY
symbiotic bacteria on or within their cells. In one case, rod-shaped
bacteria cover the surface of their eukaryotic host (Fig. 27.8). Historically, Domain Eukarya was divided into four kingdoms:
Research has shown that the bacteria metabolize sulfide in the plants, animals, fungi, and protists. Plants, animals, and fungi
local environment, thereby protecting their host from this toxic received special consideration for the obvious reason that
they include large and relatively easily studied species. All
remaining eukaryotes were grouped together as protists, which
were defined as organisms having a nucleus but lacking other
FIG. 27.8 Scanning electron microscope image of bacterial cells features specific to plants, animals, or fungi. The term “protist”
on the surface of a single-celled eukaryote found in is frowned on by some biologists because it doesn’t refer to a
oxygen-depleted sediments in the Santa Barbara Basin, monophyletic grouping of species—that is, an ancestral form
off the coast of California. The bacteria are hypothesized and all its descendants. Nonetheless, as an informal term that
to metabolize the H2S in this environment, thereby draws attention to a group of organisms sharing particular
protecting the eukaryote that they enclose. Source: Reprinted characteristics, “protist” usefully describes the diverse world of
by permission from Macmillan Publishers Ltd: V. P. Edgcomb, S. A. microscopic eukaryotes and seaweeds.
Breglia, N. Yubuki, D. Beaudoin, D. J. Patterson, B. S. Leander, and Two other terms scorned by some biologists but embraced by
J. M. Bernhard, 2011, “Identity of Epibiotic Bacteria on Symbiontid ecologists are “algae” and “protozoa.” Algae are photosynthetic
Euglenozoans in O2-Depleted Marine Sediments: Evidence for Symbiont protists. They may be microscopic single-celled organisms or the
and Host Co-evolution,” The ISME Journal 5:231–243. Copyright 2010. highly visible, multicelled organisms we call seaweed. Protozoa
Courtesy of Naoji Yubuki. are heterotrophic protists. These are almost exclusively single-
celled organisms. “Algae” and “protozoa” in fact are simple and
2 µm useful terms that have little phylogenetic meaning but which
convey a great deal of information about the structure and
function of these organisms.
Protist cells exhibit remarkable diversity. Some have cell
walls, while others do not. Some make skeletons of silica (SiO2)
or calcium carbonate (CaCO3), and others are naked or live within
tests, or “houses,” constructed exclusively of organic molecules.
Some are photosynthetic, while others are heterotrophic. Some
move by beating flagella, and others, like Amoeba, can extend
fingers of cytoplasm, called pseudopodia, to move and capture
food. Most are aerobic, but there are anaerobic protists as well.
Surprisingly, most of these distinctive features have evolved
multiple times and so do not define phylogenetically coherent
groups. As a result, our understanding of evolutionary pattern in
Eukarya had to wait for the molecular age.
 CHAPTER 27 E U K A RYOT I C C E L L S : O R I G I N S A N D D I V E R S I T Y 563

Fig. 27.9 shows a phylogenetic tree of eukaryotes as biologists seem to wander from position to position from one phylogenetic
currently understand it. As in the case of Bacteria, it is easier analysis to the next, sometimes moving together, sometimes
to identify major groups of Eukarya than to establish their separately. Whether the well-resolved superkingdoms constitute
evolutionary relationships to one another. Molecular sequence the only major limbs on the eukaryotic tree is less certain. Many
comparisons consistently recognize seven major groups, called microscopic protists remain unstudied, and we may yet be
superkingdoms, noted in the figure. Animals fall within one surprised by what they show.
of these superkingdoms, the Opisthokonta, whereas plants fall In the following sections, we introduce the major features
within another, the Archaeplastida. Most, but not all, of the of eukaryotic diversity. The species numbers we present indicate
divisions into these major eukaryotic branches are strongly only the taxonomic diversity that is known to us; most protistan
supported by data from genes and genomes. A few groups, notably species remain unknown. Many biologists estimate that only
the cryptophytes, haptophytes, and centrohelid heliozoans, about 1 in 10 protist species has been described so far. As this
is also the case for animals and fungi (but arguably not for land
plants because they are all visible to the naked eye), the relative
FIG. 27.9 The eukaryotic tree of life showing major groups. numbers of species within different superkingdoms may reflect
Branches with photosynthetic species (shown in green) something like the actual distribution of biodiversity among
are distributed widely. Additional branches, including eukaryotes.
animals, foraminiferans, radiolarians, and ciliates, contain
Our own group, the opisthokonts, is the most diverse
species that harbor photosynthetic symbionts. Dashed
eukaryotic superkingdom.
lines indicate a high degree of uncertainty.
Over the past 250 years, biologists have described about
Animals 1.8 million species. Of these, 75% or so fall within the
Choanoflagellates Opisthokonts Opisthokonta (Fig. 27.10), a group that encompasses animals,
fungi, and some protists. The name for this superkingdom is
Fungi
derived from Greek words meaning “posterior pole,” calling
Dictyostelid slime molds
attention to the fact that cell movement within this group is
Plasmodial slime molds propelled by a single flagellum attached to the posterior end of the
Entamoebas Amoebozoans cell. Not all cells in this superkingdom move around. In humans,
Flabellinea for example, the flagellum is limited to sperm. Opisthokonts are
Tubulinea heterotrophic, although some species harbor photosynthetic
symbionts. Animals are the most diverse and conspicuous
Glaucocystophytes
opisthokonts. More than 1.3 million animal species, mostly insects
Red algae Archaeplastids
and their relatives, have been described. Fungi are also diverse,
Green algae (land plants) with more than 75,000 described species, including the visually
Diatoms arresting mushrooms (Fig. 27.10a). Here, however, we focus on
Brown algae Stramenopiles opisthokont protists, somewhat poorly known microorganisms
that hold clues to the origins of complex multicellularity in this
Oomycetes
superkingdom (Chapter 28).
Dinoflagellates It isn’t easy to find morphological characters that unite all
Apicomplexans Alveolates opisthokonts. One reason is that animals and fungi have diverged
Ciliates so strikingly from what must have been the ancestral condition
Cercozoans of the group. However, as molecular sequence comparisons began
to reshape our understanding of eukaryotic phylogeny, it soon
Foraminifera Rhizarians
became clear that fungi and animals are closely related.
Radiolaria
Even more closely related to the animals are the
Cryptophytes choanoflagellates, a group of mostly unicellular protists
Haptophytes characterized by a ring of microvilli, fingerlike projections that
Centrohelid heliozoans form a collar around the cell’s single flagellum (Fig. 27.10b). About
150 choanoflagellate species have been described from marine
Euglenids
and freshwater environments, where they prey on bacteria. As
Trypanosomes
Excavates early as 1841, the close similarity between choanoflagellates
Diplomonads and the collared feeding cells of sponges suggested that these
Parabasalids minute organisms might be our closest protistan relatives. This
564 SECTION 27.3 E U K A RYOT I C D I V E R S I T Y

FIG. 27.10 Opisthokonts. This superkingdom includes animals and fungi, as well as several protistan groups, including choanoflagellates, the
closest protistan relatives of animals. Photo source: Nicole King, Steve Paddock, and Sean Carroll, HHMI, University of Wisconsin.

a. b. Choanoflagellates
Opisthokonts Most diverse groups

Amoebozoans Groups with complex

multicellularity
Archaeplastids

Stramenopiles

Alveolates

Rhizarians

Excavates Animals

10 µm
Choanoflagellates
Filastereans
Ichthyosporids
Nucleariids
Microsporidians Cell Microvilli Flagellum
body
Fungi

view gained widespread popularity with the discovery in 1880 of mitochondria, no Golgi apparatus, and no flagella. Furthermore,
Proterospongia haeckeli, a choanoflagellate that lives in colonies of they have a highly reduced metabolism and among the smallest
cells joined by adhesive proteins. genomes of any known eukaryote. The cellular simplicity of
Molecular sequence comparisons now confirm the close microsporidians does not mean that they are early-evolved
relationship between animals and choanoflagellates. For example, organisms, however. Molecular sequencing studies show that
the complete genome of the choanoflagellate Monosiga brevicollis microsporidians are the descendants of more complex organisms.
shows that a number of signaling molecules known to play a Their simplicity is an adaptation for life as an intracellular parasite.
role in animal development are present in our choanoflagellate It is now widely accepted that microsporidians are closely related
relatives, although their function in these single-celled organisms to the fungi.
remains largely unknown. More generally, many genes once Other protistan opisthokonts have been identified in recent
thought to be unique to animals have now been identified years. These are mostly bacteria-eating unicells or parasites of
in choanoflagellates and other unicellular opisthokonts, aquatic animals, but like the choanoflagellates described earlier,
underscoring the deep evolutionary roots of animal biology. their biology is beginning to illuminate the evolutionary path to
Microsporidia form another group of single-celled complex multicellularity in animals and fungi.
opisthokonts. Microsporidia are parasites that live inside animal
cells. Only their spores survive in the external environment, Amoebozoans include slime molds that produce
where they await the opportunity to infect a host and complete multicellular structures.
their life cycle. Microsporidia infect all animal phyla, so the Previously in this chapter, we learned one thing about amoebas:
approximately 1000 known species probably represent only the they move and feed by extending cytoplasmic fingers called
tip of the iceberg. More than a dozen species have been isolated pseudopodia. As its name implies, the superkingdom Amoebozoa
from human intestinal tissues. Microsporidia infections can be (Fig. 27.11a) is a group of eukaryotes with amoeba-like cells
particularly devastating in AIDS patients. (illustrated by Amoeba proteus in Fig. 27.11b). More than 1000
Beyond their importance in human health, microsporidians amoebozoan species have been described. In some amoebozoan
have attracted the attention of biologists because of features species, large numbers of cells aggregate to form multicellular
they lack: Microsporidian cells have no aerobically respiring structures as part of their life cycle.
 CHAPTER 27 E U K A RYOT I C C E L L S : O R I G I N S A N D D I V E R S I T Y 565

FIG. 27.11 Amoebozoans. This group includes protists with an amoeboid stage in their life cycle. Photo sources: b. M. I. Walker/Science Source; c. AFIP/
Science Source.

a. c. Entamoeba histolytica, the infectious agent in amoebic

b. Amoeba proteus, with clearly visible pseudopodia dysentery
Opisthokonts

Amoebozoans

Archaeplastids

Stramenopiles

Alveolates

Rhizarians

200 µm 10 µm
Excavates

Although there are amoeba-like cells that are not members one giant cell. The plasmodia can move and so seek out and feed
of the Amoebozoa superkingdom, all members of this group have on the bacteria and small fungi commonly found on bark or
cells similar to that of Amoeba, at least at some stage of their life plant litter on the forest floor. Triggered by poorly understood
cycle. Amoebozoans play an important role in soils as predators environmental signals, plasmodia eventually differentiate to form
on other microorganisms. Some amoebozoans cause disease in stalked structures called sporangia that can be 1 to 2 mm high (Fig.
humans. For example, Entamoeba histolytica (Fig. 27.11c), an 27.12b). Within the mature sporangium, cell walls form around the
anaerobic protist that causes amoebic dysentery, is responsible for many nuclei, producing discrete cells. These cells undergo meiosis,
50,000–100,000 deaths every year. generating haploid spores that disperse into the environment.
Beyond considerations of human health, the amoebozoans Germination of these spores begins the life cycle anew.
of greatest biological interest are the slime molds. In plasmodial Cellular slime molds (Fig. 27.13) are a second type of slime
slime molds (Fig. 27.12), haploid cells fuse to form zygotes that mold. These slime molds spend most of their life cycle as solitary
subsequently undergo repeated rounds of mitosis but not cell amoeboid cells feeding on bacteria in the soil. Starvation,
division to form colorful, often lacy structures visible to the however, causes these cells to produce the chemical signal cyclic
naked eye (Fig. 27.12a). These structures, called plasmodia, are AMP (Chapter 9), which induces as many as 100,000 cells to
coenocytic, which means they contain many nuclei within aggregate into a large multicellular slug-like form (Fig. 27.13a).

FIG. 27.12 Plasmodial slime molds. (a) Plasmodia, such as this pretzel slime mold, are coenocytic structures containing many nuclei.
(b) Plasmodia generate sporangia, stalked structures that produce spores for dispersal. Sources: a. Matt Meadows/Science Source;
b. Ray Simons/Getty Images.

a b

10 mm 1 mm
566 SECTION 27.3 E U K A RYOT I C D I V E R S I T Y

algae within this branch, but this remains

FIG. 27.13 Cellular slime molds. (a) Starvation causes amoeboid feeding cells of cellular a topic of continuing research.
slime molds to aggregate into a multicellular “slug” that can migrate along surfaces, The least conspicuous archaeplastids
leaving behind a trail of slime. (b) “Slugs” differentiate to form stalked sporangia are the glaucocystophytes (Fig. 27.14b),
that produce spores. Source: Carolina Biological Supply Company/Phototake. a small group of single-celled algae
found in freshwater ponds and lakes.
a b
We might skip over them entirely,
except for one illuminating observation:
Glaucocystophyte chloroplasts
retain more features of the ancestral
cyanobacterial endosymbiont than any
other algae. Their chloroplasts have walls
of peptidoglycan, the same molecule
found in cyanobacterial walls, and
their photosynthetic pigments include
biliproteins, also found in cyanobacteria.
More abundant and diverse is the
second major archaeplastid group, the
1 mm 1 mm red algae (Fig. 27.14c). About 5000
species are known, mostly from marine
environments. Most are multicellular
and some have differentiated tissues
This “slug” can migrate by a coordinated movement of its cells and complex morphology. Like other archaeplastids, most have
governed by actin and myosin, the same proteins that control walls made of cellulose. The principal photosynthetic pigments
muscle movement in animals. Migrating “slugs” can forage for in red algae are a form of chlorophyll called chlorophyll a and
food, leaving behind a trail of mucilage as they move—that’s the biliproteins also found in glaucocystophytes and most
why they’re called slime molds. Like plasmodial slime molds, the cyanobacteria.
“slugs” of cellular slime molds eventually differentiate to form Red algae can be conspicuous as seaweeds on the shallow
stalk-borne sporangia that produce the spores responsible for seafloor. One subgroup of red algae, known as the coralline algae,
dispersal of the organism and renewal of the life cycle (Fig. 27.13b). secretes skeletons of calcium carbonate (CaCO3) that deter
Slime molds are not true intermediates in the evolution of predators and help the algae withstand the energy of waves in
multicellularity—that is, they are not closely related to animals, shallow marine environments. If you examine the margin of coral
fungi, or other groups in which complex multicellularity evolved. reefs, where waves crash into the reef, you will often see that it is
Nonetheless, the cellular slime mold Dictyostelium has been coralline algae and not the corals themselves that resist breaking
extensively studied by biologists as a model organism because it waves and so enable reefs to expand upward into wave-swept
has much to teach us about fundamental eukaryotic processes of environments.
cell signaling and differentiation, cellular motility, and mitosis. Red algae are important as food in certain cultures, for
example, as laver in the British Isles (particularly in Wales, in cakes
Archaeplastids, which include land plants, are similar to oatcakes) and as nori in Japan. Without knowing it,
photosynthetic organisms. you often ingest carageenan, a polysaccharide synthesized by red
After the opisthokonts, the most conspicuous and diverse algae. Carageenan molecules stabilize mixtures of biomolecules
eukaryotes fall within the superkingdom Archaeplastida, and promote gel formation, and so they are used in products
another major branch of the eukaryotic tree (Fig. 27.14a). This is ranging from ice cream to toothpaste. Agar, another red algal
where we find the land plants, whose 400,000 described species polysaccharide, is found universally in microbiology laboratories
dominate eukaryotic biomass on this planet (Chapter 33). The because the gel it forms at room temperature provides a nearly
archaeplastids contain three major phylogenetic groups, and with ideal substrate for bacterial growth.
the exception of a few parasitic and saprophytic (living on dead The land plants and their algal relatives form Viridoplantae,
matter) species, all are photosynthetic. This indicates that the last the third branch of Archaeplastida. In this chapter, we are
common ancestor of living archaeplastids was photosynthetic. concerned with the protistan members of this group, the green
In fact, archaeplastids are the direct descendants of the protist algae (Fig. 27.14d). Green algae include a diverse assortment of
that first evolved chloroplasts from endosymbiotic cyanobacteria. species (about 10,000 have been described) that live in marine
Some molecular analyses also place cryptophyte and haptophyte and, especially, freshwater environments. Green algae range from
 CHAPTER 27 E U K A RYOT I C C E L L S : O R I G I N S A N D D I V E R S I T Y 567

FIG. 27.14 Archaeplastids. (a) Archaeplastids are a photosynthetic group descended from the protist that acquired photosynthesis from an
endosymbiotic cyanobacterium. They include the (b) glaucocystophytes, (c) red algae, and (d) green algae and their descendants,
the land plants. Photo sources: b. Jerome Pickett-Heaps/Science Source; c. Dr. D. P. Wilson/Science Source; d. D. J. Patterson.

a. b. Glaucocystophyte
Opisthokonts Most diverse groups

Amoebozoans Groups with complex

multicellularity
Archaeplastids Groups containing
photosynthetic
Stramenopiles organisms

Alveolates

Rhizarians 2 µm

c. Red algae
Excavates

Glaucocystophytes

Red algae

Chlorophytes

100 µm

Flagellated
unicells Green
algae d. Green algae

Viridoplantae
Streptophyte
algae

Land plants 10 µm

tiny single-celled flagellates to meter-scale seaweeds (Fig. 27.15). branch, the chlorophytes, radiated mostly in the sea and includes
They all are united by two features: the presence of chlorophyll a common seaweeds seen globally along seashores (Fig. 27.15e).
and chlorophyll b in chloroplasts that have two membranes, and a Chlorophyte green algae have also played a major role in laboratory
unique attachment for flagella. studies of photosynthesis (Chlamydomonas, Fig. 27.15a) and
It is thought that green algae originated as small flagellated multicellularity (Volvox, Fig. 27.15d).
cells, and today most species that branch early on the green algal For humans, however, the more important branch of the green
tree still live as photosynthetic cells in marine or fresh waters (Fig. algal tree is the one that diversified in freshwater and, eventually,
27.15a). The larger diversity of green algae, however, is found on on land. Called streptophytes, the species on this branch show
two branches that diverged from these flagellated ancestors. One a progression of form from unicells at the base through simple
568 SECTION 27.3 E U K A RYOT I C D I V E R S I T Y

FIG. 27.15 Diversity of green algae. (a) Chlamydomonas reinhardtii, a tiny flagellated unicell widely used in laboratory research on
photosynthesis; (b) Spirogyra, a freshwater green alga with distinctive helical chloroplasts; (c) a desmid, among the most diverse
and widespread of all green algae in fresh water; (d) Volvox, a simple multicellular organism; and (e) Acetabularia, a macroscopic
(but single-celled!) green alga found in coastal marine waters in tropical climates. Sources: a. DeAgostini/Getty Images; b. Eric Grave/Phototake;
c. Gerd Guenther/Science Source; d. Frank Fox/Science Source; e. Wolfgang Poelzer/age fotostock.

a c e

FPO
50 µm

5 µm d

50 µm 50 µm 1 mm

cell clusters and filaments on intermediate branches (Figs. and these protists also play important roles in the ecology of lakes,
27.15b and 21.15c), to complex multicellular algae that form rivers, and soils.
the closest relatives of land plants (Chapter 33). As was true of The superkingdom Stramenopila (see Fig. 27.9) includes
choanoflagellates and animals, green algae display features of cell unicellular organisms and giant kelps, algae and protozoa, free-
biology and genomics that tie them unequivocally to land plants. living cells and parasites. All have an unusual flagellum that bears
two rows of stiff hairs (Fig. 27.16). Most also have a second,
j Quick Check 2 Do plants and animals have a common
smooth flagellum.
multicellular ancestor?
Despite the presence of heterotrophic species, most
stramenopiles are photosynthetic. There are about a dozen
Stramenopiles, alveolates, and rhizarians dominate groups of stramenopile algae, of which the brown algae and
eukaryotic diversity in the oceans. diatoms deserve special mention. Brown seaweeds are common
The phylogenetically related superkingdoms called the along rocky shorelines across the world, and in the Sargasso Sea,
stramenopiles, alveolates, and rhizarians may not be household huge masses of ropy brown algae (aptly named Sargassum)
names, but perhaps they should be. The surface ocean hosts three float at the surface. Easily the most impressive brown algae are
species belonging to these groups for every marine animal species, the kelps, giant seaweeds that form forests above the seafloor
 CHAPTER 27 E U K A RYOT I C C E L L S : O R I G I N S A N D D I V E R S I T Y 569

FIG. 27.16 Stramenopiles. This electron micrograph of a FIG. 27.17 Kelps. These complex multicellular brown algae can
unicellular stramenopile shows a flagellum with two form “forests” tens of meters high in the ocean. Source:
rows of stiff hairs. Like this cell, most stramenopiles Mark Conlin/Alamy.
also have a second, smooth flagellum. Source: D. J.
Patterson,image used under license to MBL (micro*scope).

Hairy
flagellum

Smooth
5 µm
flagellum

(Fig. 27.17). Kelps not only develop functionally distinct tissues,

but also have evolved distinct organs specialized for attachment,
photosynthesis, flotation, and reproduction.
The most diverse stramenopiles, however, are the diatoms
(Fig. 27.18). Recognized by their distinctive skeletons of silica,
diatoms account for half of all photosynthesis in the sea (and, so,
nearly a quarter of all photosynthesis on Earth). The 10,000 known
diatom species thrive in environments that range from wet soil to
the open ocean.
Among heterotrophic stramenopiles, the oomycetes, or water
molds, are particularly interesting. Originally classified as fungi,
these multicellular pathogens cause a number of plant diseases,

a b
FIG. 27.18 Diatoms. Diatoms are the most
diverse stramenopiles, and
among the most diverse of all
protists. Their shapes, outlined
by their tiny skeletons made of
silica (SiO2), can be (a) like a
pill-box, (b) elongated, or
(c) twisted around the long
axis of the cell. Some diatom
10 µm 50 µm
species form colonies, like
those shown in (d). Sources:
c d
a. and b. Steve Gschmeissner/Science
Source; c. Andrew Syred/Science
Source; d. Biophoto Associates/Science
Source.

50 µm 100 µm
570 SECTION 27.3 E U K A RYOT I C D I V E R S I T Y

supplied to coastal waters in large

FIG. 27.19 Alveolates. (a) The arrows point to alveoli, flattened vesicles just beneath the amounts, sometimes for natural reasons
surface of the cell that give alveolates their name. (b) Dinoflagellates have cell walls but commonly because of human
made of interlocking plates of cellulose. (c) A ciliate has many short flagella, or cilia, activities such as sewage seeps or runoff
that line the cell. Sources: a. Gert Hansen–SCCAP, University of Copenhagen; b. Electron Microscopy from highly fertilized croplands.
Unit, Australian Antarctic Division © Commonwealth of Australia; c. Andrew Syred/Science Source. The alveolates also include a second
a diverse (8000 described species) group
of protists called ciliates (Fig. 27.19c).
Ciliates are also distinct at the cellular
level, with two nuclei in each cell and
1 µm numerous short flagella called cilia.
They are heterotrophs that feed on
b c bacteria or other protists. In ecological
terms, ciliates are among Earth’s most
important heterotrophic protists.
Dinoflagellates and ciliates are both
diverse and ecologically important, but
Flagella
it may be the third principal group of
alveolates, the Apicomplexans, that
attracts most scientific attention. The
reason is simple: The apicomplexan
Plasmodium falciparum and related
species cause malaria, a lethal disease
that sickened millions and killed
nearly 600,000 people in 2013 (Case 4:
Malaria). Although Plasmodium cells are
25 µm 50 µm
heterotrophic, they contain vestigial
chloroplasts that suggest evolutionary
loss of photosynthesis. Recently, this
hypothesis has been confirmed by the
including the potato blight that devastated Ireland during the discovery in Sydney Harbour, Australia, of photosynthetic cells
1840s (Chapter 33). Other species infect animals, including closely related to parasitic Plasmodium.
humans.
Like stramenopiles, the superkingdom Alveolata is a Photosynthesis spread through eukaryotes by
diverse group that includes both photosynthetic and repeated endosymbioses involving eukaryotic algae.
heterotrophic species. And, again, like stramenopiles and many A curious feature of the eukaryotic tree is that photosynthetic
other eukaryotic groups, alveolates are united by a feature eukaryotes are distributed widely but discontinuously among its
observed only with careful electron microscopy—small vesicles branches (see Fig. 27.9). How can we explain this pattern?
called cortical alveoli that are packed beneath the cell surface As explored in Fig. 27.20, two hypotheses have been
and, in some species at least, store calcium ions for use by the suggested: Either photosynthesis was established early in
cell (Fig. 27.19a). eukaryotic evolution and then subsequently lost in some lineages,
Most photosynthetic alveolates belong to a large subgroup or eukaryotes acquired photosynthesis multiple times by repeated
called dinoflagellates. Dinoflagellates are distinguished by two episodes of endosymbiosis. The second hypothesis, it turns out,
flagella that produce a rotary motion of the cell as it moves is correct. The twist is that most of the symbionts involved in
through its environment; many also have a distinctive covering the spread of photosynthesis across eukaryotic phylogeny were
of organic plates (Fig. 27.19b). About 4000 dinoflagellate species photosynthetic eukaryotes, not cyanobacteria.
have been described, but recent genomic surveys of the oceans Once more, DNA sequence comparisons provide the decisive
make it clear that they are much more diverse—by nearly an data. We can construct phylogenetic trees for photosynthetic
order of magnitude. About half of the known dinoflagellate eukaryotes using chloroplast genes and then repeat the exercise
species are photosynthetic, but most of these are also capable of using nuclear genes. Comparing the two trees, as shown in
heterotrophic growth. Dinoflagellates are important components Figure 27.20, reveals that two groups, chlorarachniophytes and
of the photosynthetic biota in many parts of the oceans, and euglenids, gained photosynthesis by establishing green algal cells
they affect humans when they form red tides, large and toxic as endosymbionts. (The algal cells contained chloroplasts derived
blooms in coastal waters. Red tides occur when nutrients are from an earlier symbiotic assimilation of cyanobacteria.) Other
HOW DO WE KNOW?

FIG. 27.20

How did photosynthesis spread b. Phylogenetic relationships based on nuclear genes

through the Eukarya? Opisthokonts

Amoebozoans
BACKGROUND Many different branches on the eukaryotic tree include Glaucocystophytes
photosynthetic species, commonly interspersed with non-photosynthetic Red algae Archaeplastids
lineages. Did eukaryotes gain photosynthesis once, by the endosymbiotic Green algae
incorporation of a cyanobacterium, and then lose this capacity multiple Diatoms
Brown algae Stramenopiles
times? Or, is the history of photosynthetic endosymbioses more complicated,
involving multiple events of symbiont capture and transformation?
Dinoflagellates
HYPOTHESIS 1 Photosynthesis was established early in eukaryotic Alveolates
evolution and was subsequently lost in some lineages.
Chlorarachniophyte algae
HYPOTHESIS 2 Eukaryotes acquired photosynthesis multiple times by Rhizarians
repeated episodes of endosymbiosis.
Cryptophyte algae
PREDICTIONS The two hypotheses make different predictions. If
photosynthesis were acquired in a single common ancestor of all living
eukaryotes and then lost in some lineages, we would expect chloroplast Euglenids
and nuclear gene phylogenies to show the same pattern of branching.
Excavates
If photosynthesis were acquired multiple independent times, we would
expect chloroplast and nuclear phylogenies to show different patterns.

EXPERIMENT Chloroplasts have DNA, so scientists developed a RESULTS Molecular sequence comparisons show that evolutionary
phylogenetic tree based on molecular sequence comparison of chloroplast relationships among chloroplasts do not mirror those based on
genes (Fig. 27.20a). This phylogeny was then compared with a second nuclear genes. The chlorarachniophyte algae and photosynthetic
phylogeny, for all eukaryotes, based on molecular sequence comparison of euglenids (shown in green) fall in a monophyletic group with the
nuclear genes (Fig. 27.20b). green algae when chloroplast DNA is analyzed (Fig. 27.20a).
However, these groups lie far from
green algae in the phylogeny based
a. Phylogenetic relationships based on chloroplast genes Nuclear genes place brown algae
and diatoms among the on nuclear genes (Fig. 27.20b).
Brown algae
Diatoms
stramenopiles, dinoflagellates Similarly, brown algae, diatoms, most
within the alveolates, and
Most photosynthetic photosynthetic dinoflagellates, and
cryptophytes on their own poorly
dinoflagellates resolved branch. Chloroplast genes, cryptophyte algae (shown in red) form
Cryptophyte algae however, relate them to red algae. a monophyletic group with the red
These groups gained
Red algae algae in the phylogeny based on
photosynthesis by incorporating red
Red algae algal cells as endosymbionts. chloroplast DNA (Fig. 27.20a), but
Green algae analysis of nuclear genes places them in
Euglenids Nuclear genes place different groups (Fig. 27.20b).
chlorarachniophytes and euglenids
Green algae
among the rhizarians and excavates, CONCLUSION The chloroplast and
Chlorarachniophyte algae respectively, but chloroplast genes
Green algae relate them to green algae. Both nuclear phylogenies show different
Green algae groups gained photosynthesis by patterns of branching, supporting the
incorporating green algal cells as
Green algae endosymbionts.
hypothesis that photosynthesis spread
Green algae through the Eukarya by means of
Land plants Chloroplasts form a monophyletic multiple eukaryotic endosymbionts.
Glaucocystophytes grouping nested within the
cyanobacteria—a principal line of
SOURCE Hackett, J. D., et al. 2007. “Plastid
Cyanobacteria evidence supporting the
Endosymbiosis: Sources and Timing of
endosymbiotic origin of chloroplasts.
the Major Events.” In Evolution of Primary
Producers in the Sea, edited by P. G. Falkowski
For branches in black, chloroplast relationships match the phylogeny and A. H. Knoll, 109–132. Boston: Elsevier
indicated by nuclear genes—these groups diverged from the ancestor Academic Press.
in which endosymbiosis first gave rise to chloroplasts.

571
572 SECTION 27.4 T H E F O S S I L R E CO R D O F P ROT I S T S

Chlorarachniophyte algae also retain

FIG. 27.21 Successive endosymbiotic events that established photosynthesis in another feature that supports the eukaryotic
chlorarachniophyte algae. origin of their chloroplasts: a small organelle
Mitochondrion Chloroplast derived called a nucleomorph (Fig. 27.21). The
Nucleus of primary from endosymbiotic nucleomorph in chlorarachniophyte algae
eukaryotic host cell Cyanobacterium cyanobacterium is, in fact, the remnant nucleus of a green
algal symbiont. It retains only a few genes,
but those show close phylogenetic affinity
with green algae. Nucleomorph genes in
cryptophyte algae similarly provide evidence
for a red algal symbiont.
Primary How many endosymbiotic events are
endosymbiosis
required to account for the diversity of
photosynthetic eukaryotes? There was the
primary endosymbiosis that gave rise to
Phagocytic eukaryote Photosynthetic eukaryote green and red algae, two further events that
(Green alga)
established chloroplasts derived from green
algae in euglenids and chlorarachniophytes,
Nucleus of second Nucleomorph (vestigial and at least one to three events that
(Rhizarian) eukaryotic nucleus of eukaryotic
host cell endosymbiont) established chloroplasts derived from red
algae in cryptophytes, haptophytes, and
stramenopiles.
The dinoflagellates add a remarkable coda
to this accounting. Like stramenopiles, most
dinoflagellates have chloroplasts derived from
Secondary red algae. Three dinoflagellate groups, however,
endosymbiosis
have chloroplasts that are clearly derived
from green, cryptophyte, and haptophyte
algae. Include Paulinella and at least 8 to 10
Second phagocytic eukaryote Chlorarachniophyte
(Rhizarian)
endosymbiotic events are required to explain
eukaryotic photosynthesis. Eukaryotes never
evolved oxygenic photosynthesis—only the
groups (including brown algae, diatoms, most photosynthetic cyanobacteria accomplished that. Instead, eukaryotes appropriated
dinoflagellates, and cryptophyte algae) gained photosynthesis as the biochemistry of cyanobacteria and spread it throughout the
well, but most likely by ingesting a photosynthetic endosymbiont domain by the horizontal transfer of entire cells.
that was a red algal cell.
j Quick Check 3 How do the molecular sequences of genes in
Several features of cell biology support the view that
chloroplasts show how photosynthesis spread throughout the
eukaryotes acquired photosynthesis multiple times by repeated
eukaryotic domain?
episodes of endosymbiosis. The chloroplasts of euglenids and
chloarachniophyte algae contain chlorophyll a and b, like green
algae, whereas cryptophytes contain chlorophyll c, a form of
chlorophyll not known from either red or green algae, along with 27.4 THE FOSSIL RECORD
chlorophyll a and biliproteins, both found in red algae. (Diatoms, OF PROTISTS
brown algae and their relatives contain chlorophyll a and c, but not
biliproteins.) Not all algae and protozoa fossilize easily, but those that do provide
We noted earlier that chloroplasts in green and red algae a striking record of evolution that both predates and parallels the
are bounded by two membranes, but most of the algae shown better-known fossil record of animals. Fossilized protists do not
in red and green in Fig. 27.20 have chloroplasts bounded by four contain DNA, nor do they preserve the features of cell biology that
membranes. The presence of four membranes is expected under define eukaryotes, such as nuclear membranes and organelles. Thus,
the hypothesis of eukaryotic endosymbionts: The inner two for a fossil to be identified as eukaryotic, it must preserve other
membranes are those that bounded the chloroplast in the green features of morphology found today in Eukarya, but not in Bacteria or
or red algal endosymbiont, the third membrane is derived from Archaea. For this reason, fossils that can be identified with certainty
the outer cell membrane of the symbiont, and the fourth, outer as eukaryotic are limited to those left by eukaryotes that synthesized
membrane is part of the cell membrane of the host (Fig. 27.21). distinctive and preservable cell walls at some stage of their life cycle.
 CHAPTER 27 E U K A RYOT I C C E L L S : O R I G I N S A N D D I V E R S I T Y 573

eukaryotic. Interlocking plates, long and branching arms, and

FIG. 27.22 Eukaryotic fossils in Precambrian sedimentary rocks. complex internal layering have been imaged by both scanning and
(a) Simple protist in 1500-million-year-old rocks transmission electron microscopes (Fig. 27.22a). Comparison
from Australia. (b) Simple multicellular red algae in with living organisms suggests that such fossils could only be
1100 –1200-million-year-old rocks from Canada. formed by organisms with a cytoskeleton and endomembrane
(c) Amoebozoan test in 750-million-year-old rocks system, the hallmarks of eukaryotic biology.
from the Grand Canyon, Arizona. Sources: a. Courtesy of At present, it appears that the eukaryotic domain first
Andrew H. Knoll, b. courtesy of N. Butterfield, University of Cambridge appeared no later than 1800 million years ago and possibly
(from Butterfield NJ. 2000. Bangiomorpha pubescens n. gen., n. sp.: earlier, although the record is difficult to trace in older rocks.
Implications for the evolution of sex, multicellularity, and the None of these earliest eukaryotic microfossils can be placed with
Mesoproterozoic/Neoproterozoic radiation of eukaryotes. confidence into one of the living superkingdoms. Conceivably, at
Paleobiology 26: 386–404, c. Porter et al. 2003. Vase-shaped least some of the oldest protistan microfossils could predate the
microfossils from the Neoproterozoic Chuar Group, Grand Canyon: last common ancestor of living eukaryotes.
a classification guided by modern testate amoebae. Journal of In sedimentary rocks thought to be 1100–1200 million years
Paleontology 77(3): 409– 429.) Photo courtesy of Susannah Porter, old, we find the oldest fossils that can be linked clearly to a living
University of California, Santa Barbara. group of eukaryotes (Fig. 27.22b). These fossils preserve features
found today only in the red algae. We can therefore conclude that
a b
by 1200 million years ago Archaeplastida had already diverged from
other eukaryotic superkingdoms, photosynthesis had become
established in eukaryotes, and simple multicellularity had already
evolved within the domain.
Much more eukaryotic diversity is recorded in rocks
800–700 million years old, including green algae and the remains
of test-forming amoebozoans (Fig. 27.22c). Molecular fossils
further indicate the presence of ciliates and dinoflagellates in
50 µm
the emerging eukaryotic world and show that algae were
expanding to become major photosynthesizers in the oceans.
c
How can we explain this diversification? One hypothesis is
that 800–700 million years ago, protists first evolved (or
expanded) the ability to eat other protists rather than preying
only on Bacteria and Archaea. This ability initiated an evolutionary
20 µm 25 µm arms race that favored divergent means of preying on other
organisms and avoiding predation. Both fossils and molecular
Time (billion years) clocks support this ecological hypothesis for eukaryotic
4 3 2 1 0 diversification, perhaps facilitated by a small increase in the
oxygen content of seawater.

Protists have continued to diversify during the age

al f (c)
e

(a)

on roup be

ils
f lif

Fir of pr (b)

of animals.
oss
nim tists
tes

n
eo

te t ca
yo

Modern protistan diversity took shape alongside evolving

o
c

ryo ha
en

kar

g
Div uka es t
vid

animals during the Phanerozoic Eon, 541 million years ago

eu

st a
t
st e

sil

g e yo

to the present. For example, foraminiferans and radiolarians,

ati
fos

vin kar
e
Old

ific

marine protozoans with shells of calcium carbonate and silica,

est

a li eu

ers
Old

to ssil

respectively, appear for the first time in rocks deposited near

ked st fo

the beginning of the Phanerozoic Eon and have continued to

e
Old

diversify since that time (Fig. 27.23). Some algal seaweeds also
lin

evolved skeletons of calcium carbonate at this time, and diversified

along with animals in Phanerozoic oceans. Why did algae and
Fossils show that eukaryotes existed at least 1800 protozoans evolve mineralized skeletons? Presumably to keep
million years ago. from being eaten.
Sedimentary rocks deposited in coastal marine environments Marine protists underwent a new round of radiation during
1800–1400 million years ago contain microscopic fossils (called the Mesozoic Era, 252–66 million years ago. The most significant
microfossils for short) up to about 300 µm in diameter. Many changes occurred among photosynthesizers. During the preceding
of these have complicated wall structures that identify them as Paleozoic Era, green algae and cyanobacteria were the primary
574 SECTION 27.4 T H E F O S S I L R E CO R D O F P ROT I S T S

photosynthesizers in the oceans, but as the Mesozoic Era began,

FIG. 27.23 (a) Foraminifera and (b) Radiolaria, showing new groups of photosynthetic dinoflagellates and coccolithophorid
skeletons of calcium carbonate and silica, algae appeared, followed by diatoms. Today, these three groups
respectively. Sources: a. Astrid & Hanns-Frieder Michler/Science dominate photosynthesis in many parts of the sea.
Source; b. M. I. Walker/Science Source. Within the oceans, diatom and coccolithophorid evolution
had another important consequence. The silica skeletons of
a
diatoms are now the principal means by which SiO2 is transferred
from seawater to sediments. Similarly, the CaCO3 scales made by
coccolithophorids and the skeletons of planktonic foraminifera
are a primary source of carbonate sediments on the seafloor
f
(Fig. 27.24). In this way, the ongoing evolution of protists in the
age of animals has continued to transform marine ecosystems.
Today, microscopic eukaryotes dominate photosynthetic
production in many parts of the oceans and play a major role
in biogeochemical cycles. And for thousands of protists, plants
and animals are simply habitats, ripe for colonization—we need
look no further than the many eukaryotic vectors of human
200 μm disease. The key conclusion is that a remarkable, and still largely
undocumented, diversity of eukaryotic organisms has evolved
through time to form an integral part of ecosystems across our
b
planet, shaping in ways both positive and negative the world in
which we live.•

FIG. 27.24 Coccolithophorids. (a) Coccolithophorid algae, like

Emiliania huxleyi, form tiny scales of calcium carbonate.
(b) A satellite image captures a coccolithophorid bloom
200 μm off the coast of Norway, visible as light blue streamers
of calcium carbonate in the otherwise dark blue ocean.
Sources: a. ©The Trustees of the Natural History Museum, London;
b. NASA image courtesy Jacques Descloitres, MODIS Rapid Response
Team at NASA GSFC.

a b

2 µm
 CHAPTER 27 E U K A RYOT I C C E L L S : O R I G I N S A N D D I V E R S I T Y 575

Core Concepts Summary 27.3 Eukaryotes were formerly divided into four
kingdoms, but are now divided into at least seven
27.1 Eukaryotic cells are defined by the presence superkingdoms.
of a nucleus, but features like a dynamic cytoskeleton The terms “protist,” “algae,” and “protozoa” are useful, but do
and membrane system explain their success in not describe monophyletic groups. page 562
diversifying.
Protists are eukaryotes that do not have features of animals,
Eukaryotic cells have a network of proteins inside the plants, and fungi; algae are photosynthetic protists; and protozoa
cell that allow them to change shape, move, and transfer are heterotrophic protists. page 562
substances in and out of the cell. page 554
The seven superkingdoms of eukaryotes are Opisthokonta,
Eukaryotic cells have dynamic membranes that facilitate
Amoebozoa, Archaeplastida, Stramenopila, Alveolata, Rhizaria,
movement and feeding. page 554
and Excavata. page 563
Eukaryotic cells compartmentalize their machinery for
energy metabolism into mitochondria and chloroplasts, Opisthokonts are the most diverse eukaryotic superkingdom.
freeing the cell to interact with the environment in novel They include animals and fungi, as well as choanoflagellates (our
ways not available to prokaryotes. page 555 closest protistan relatives). page 563

The eukaryotic genome is larger than that of prokaryotes, Amoebozoans produce multicellular structures by the
allowing new mechanisms of gene regulation. page 556 aggregation of amoeba-like cells, and include organisms that
cause human disease and those important in biological research.
Sexual reproduction promotes genetic diversity in eukaryotic
page 564
populations. page 556
The archaeplastids, which include land plants, are photo-
27.2 The endosymbiotic hypothesis proposes that the synthetic, and are divided into three major groups. page 566
chloroplasts and mitochondria of eukaryotic cells were
The related superkingdoms called the stramenopiles,
originally free-living bacteria.
alveolates, and rhizarians dominate eukaryotic diversity in the
The endosymbiotic hypothesis is based on physical, oceans. page 568
biochemical, and genetic similarities between chloroplasts
and cyanobacteria, and between mitochondria and 27.4 The fossil record extends our understanding of
proteobacteria. page 557 eukaryotic diversity by providing perspectives on the
Chloroplasts and mitochondria have their own genomes, timing and environmental context of eukaryotic evolution.
but their genomes are small relative to free-living bacteria,
Fossils in sedimentary rocks as old as 1800 million years have
to which they are closely related, mostly because of gene
unmistakable signs of eukaryotic cells, including complicated
migration to the nuclear genome. page 559
wall structures. page 573
Symbiosis between a heterotrophic host and photosynthetic
partner is common throughout the eukaryotic domain; reef- The earliest fossil eukaryotes that can be placed into one of the
forming corals are an example. page 559 present-day superkingdoms are 1100 –1200 million years old and
belong to the red algae. page 573
Photosynthesis spread through Eukarya by means of multiple
independent events involving a protozoan host and a Eukaryotic fossils diversified greatly about 800 million years ago,
eukaryotic endosymbiont. page 559 reflecting a new capacity for predation on other protists and,
perhaps, a modest increase in oxygen. page 573
Most eukaryotic cells have mitochondria, but a few do not.
Evidence suggests that cells lacking mitochondria once had Protists continue to diversify to the present. Green algae
them but lost them. page 559 and cyanobacteria dominated primary production in earlier
oceans, but, since about 200 million years ago, dinoflagellates,
The eukaryotic nuclear genome contains genes unique to
coccolithophorids, and diatoms have become the primary
Eukarya, but also genes related to Bacteria and Archaea,
photosynthetic organisms in the oceans. page 574
suggesting that the ancestor of the modern eukaryotic cell
was either a primitive eukaryote descended from archaeal Protists have evolved to take advantage of the environments
ancestors or an archaeon that engulfed a bacterium. provided by animals and plants; these protists include many that
page 560 cause disease in humans. page 574
576 SELF-ASSESSMENT

Self-Assessment 5. Name the seven superkingdoms of eukaryotes and an

organism in each one.
1. List key features that distinguish a eukaryotic cell from a
6. State when the eukaryotic cell is first thought to have
prokaryotic cell.
evolved and the evidence that supports this date.
2. Describe the forms of energy metabolism found in
eukaryotes.
Log in to to check your answers to the Self-
3. Describe the origin and evolution of the chloroplast and Assessment questions, and to access additional learning tools.
mitochondrion.

4. Present two hypotheses for the origin of the eukaryotic

cell.
 CHAPTER 28

Being
Multicellular
Core Concepts
28.1 Complex multicellularity arose
several times in evolution.
28.2 In complex multicellular
organisms, bulk flow
circumvents the limitations of
diffusion.
28.3 Complex multicellularity
depends on cell adhesion,
communication, and a genetic
program for development.
28.4 Plants and animals evolved
complex multicellularity
independently of each other,
and solved similar problems
with different sets of genes.
28.5 The evolution of large
and complex multicellular
organisms, which required
abundant oxygen, is recorded
by fossils.
irawansubingarphotography/Getty Images.

577
578 SECTION 28.1 T H E P H Y LO G E N E T I C D I S T R I B U T I O N O F M U LT I C E L LU L A R O RG A N I S M S

Chapters 26 and 27 introduced the extraordinarily diverse, but eukaryotes have evolved those. Simple multicellular organisms,
largely unseen, world of microorganisms. Among the Bacteria, composed of multiple similar cells, occur widely within the
Archaea, and protists, each cell usually functions as an individual, eukaryotic tree of life, and a few of these evolved into complex
growing and reproducing, moving from one place to another, multi-cellular organisms characterized by differentiated tissues and
taking in nutrients, and both sensing and transmitting molecular organs. How are these different types of organization distributed on
signals. In contrast, complex multicellular organisms contain as the eukaryotic tree of life?
many as a trillion or more cells that work in close coordination.
In your own body, for example, different cells are specialized for Simple multicellularity is widespread among
specific functions, so that while your body as a whole can perform eukaryotes.
the broad range of tasks accomplished by microorganisms, A recent survey of eukaryotic organisms recognized 119 major
individual cells for the most part cannot. Cells lining the intestine groups within the superkingdoms discussed in Chapter 27. Of
absorb food molecules, but nutrients must then be transported these, 83 contain only single-celled organisms, predominantly
to other parts of the body. Lungs take up oxygen from inhaled cells that engulf other microorganisms or ingest small organic
air, but this, too, must be distributed to other tissues and organs. particles, photosynthetic cells that live suspended in the water
And while cells at the body’s surface sense signals from the column, or parasitic cells that live within other organisms. Each
environment, the signals affecting interior cells come mostly of the 36 remaining branches exhibits some cases of simple
from surrounding cells. multicellularity, mostly in the form of filaments, hollow balls, or
The biological gulf between microbes and complex sheets of little-differentiated cells (Fig. 28.1).
multicellular organisms is enormous, but complex multicellularity Simple multicellular eukaryotes share several properties.
has evolved a half dozen times. In the chapters that follow, we In many, cell adhesion molecules cause adjacent cells to stick
explore the structure and diversity of plants, fungi, and animals. together, but there are few specialized cell types and relatively
Here, however, we address the broader question of how complex little communication or transfer of resources between cells. Most
multicellular organisms evolved in the first place. or all of the cells in simple multicellular organisms retain a full
range of functions, including reproduction, and so the organism
usually pays only a small penalty for individual cell death.
28.1 THE PHYLOGENETIC Importantly, in simple multicellular organisms, nearly every cell
DISTRIBUTION OF is in direct contact with the external environment, at least during
MULTICELLULAR ORGANISMS phases of the life cycle when the cells must acquire nutrients.
Simple multicellularity occurs most prominently among
Most prokaryotic organisms are composed of a single cell, algae, although stalklike colonies of particle feeders have evolved
although some form simple filaments or live in colonies. A few in at least three groups of heterotrophic protists, and simple
types of bacterium, notably some cyanobacteria, differentiate filamentous fungi absorb organic molecules as sources of carbon
to form several distinct cell types. No bacteria, however, develop and energy. In addition, four eukaryotic groups have achieved
macroscopic bodies with functionally differentiated tissues. Only simple multicellularity by a different route, aggregating during

FIG. 28.1 Simple multicellularity. Cells in simple multicellular organisms show little differentiation and remain in close contact with the external
environment. Many groups of eukaryotic organisms display simple multicellularity: (a) Uroglena, a stramenopile alga found commonly
in lakes; (b) Epistylis, a stalked ciliate protozoan that lives attached to the surfaces of fish and crabs; and (c) Prasiola, a bladelike green
alga only a single cell thick, found on tree trunks and rock surfaces. Sources: a. Jason Oyadomari; b. D. J. Patterson; c. Fabio Rindi.

a b c

25 µm 50 µm 1 mm
 CHAPTER 28 B E I N G M U LT I C E L L U L A R 579

FIG. 28.2 Coenocytic organization. Nuclei divide repeatedly but are not partitioned into individual cells. Two different groups of green algae
have evolved large size as coenocytic organisms: (a) Codium, found abundantly along temperate coastlines, and (b) Caulerpa, common
in shallow tropical seas and aquaria. Sources: a. Marevision/age fotostock; (b) Wolfgang Poelzer/WaterFrame/age fotostock.

a b

just one stage of the life cycle. Slime molds are the best-known do not reproduce, instead supporting the few that do. This requires
example (Chapter 27). cooperation among cells, but it creates opportunities for cells to
Six groups (two algal, three protozoan, and one fungal) include “cheat”—to use nutrients for their own proliferation rather than
species characterized by coenocytic organization. In coenocytic the growth and reproduction of the organism as a whole. Cancer
organisms, the nucleus divides multiple times, but the cell does is the most conspicuous and lethal example of non-cooperation,
not, so the nuclei are not partitioned into individual cells. The perhaps the defining disease of complex multicellular organisms.
result is a large cell—sometimes even visible to the naked eye— As we will see, complex multicellular organisms evolved
with many nuclei. The green algae Codium and Caulerpa, found from simple multicellular ancestors, but most taxonomic groups
along the shorelines of temperate and tropical seas, respectively, containing simple multicellular organisms never gave rise to
are common examples of coenocytic organisms (Fig. 28.2). complex descendants.
Coenocytic rhizarians on the deep seafloor can be 10 to 20 cm
long. There is no evidence that any coenocytic organisms evolved Complex multicellularity evolved several times.
from truly multicellular ancestors, nor have any given rise to Complex multicellular organisms are conspicuous parts of our
complex multicellular descendants. daily existence. Plants, animals, and, if you look a little more
What selection pressures favored the evolution of simple closely, mushrooms and complex seaweeds are what we notice
multicellular organisms from single-celled ancestors? One when we view a landscape or the coastal ocean (Fig. 28.3).
selective advantage is that multicellularity helps organisms avoid Complex multicellular organisms differ from one another in
getting eaten. In an illuminating experiment, single-celled green many ways, but they share three general features. They have
algae were grown in the presence of a protistan predator. Within highly developed molecular mechanisms for adhesion between
10 to 20 generations, most of the algae were living in eight-cell cells. They display specialized structures that allow cells to
colonies that were essentially invulnerable to predator attack. communicate with one another. And they display complex
Another advantage is that multicellular organisms may be able patterns of cellular and tissue differentiation, guided by
to maintain their position on a surface or in the water column networks of regulatory genes. Without these features, complex
better than their single-celled relatives. Seaweeds, for example, multicellularity would be impossible.
live anchored to the seafloor in places where light and nutrients For example, plants and animals both have differentiated
support growth. Feeding provides a third potential advantage: cells and tissues with specialized functions. Only some tissues
In colonial heterotrophs such as the stalked ciliate Epistylis (see photosynthesize or absorb organic molecules; other tissues
Fig. 28.1b), the coordinated beating of flagella assists feeding by transport food and oxygen through the body; and still others
directing currents of food-laden water toward the cells. generate the molecular signals that govern development. Only a
There are also costs associated with multicellularity, small subset of all cells contributes to reproduction. Because of
particularly for complex multicellular organisms with this functional differentiation, cell or tissue loss can be lethal for
differentiated reproductive tissues. In these organisms, most cells the entire organism.
580 SECTION 28.2 D I F F U S I O N A N D B U L K F LO W

FIG. 28.3 Complex multicellular organisms. Complex multicellularity evolved independently in (a) red algae, (b) brown algae, (c) land plants,
(d) animals, and (e) fungi (at least twice). Sources: a. Dr. Keith Wheeler/Science Source; b. Dave Fleetham/Getty Images; c. Filmfoto/Dreamstime.com;
d. Jack Milchanowski/age fotostock; e. AntiMartina/Getty Images.

a b c

d e

There is one more feature of complex multicellular organisms characterizes animals, but it also evolved at least twice in the
that is key to understanding their biology: They have a three- fungi, once in the green algal group that gave rise to land plants,
dimensional organization, so only some cells are in direct contact once in the red algae, and once in the brown algae, producing the
with the environment. Cells that are buried within tissues, giant kelps that form forests in the sea.
relatively far from the exterior of the organism, do not have direct
j Quick Check 1 How do simple multicellular organisms differ
access to nutrients or oxygen. Therefore, interior cells cannot
from complex multicellular organisms?
grow as fast as surface cells unless there is a way to transfer
resources from one part of the body to another. Similarly, interior
cells do not receive signals directly from the environment, even 28.2 DIFFUSION AND BULK FLOW
though all cells must be able to respond to environmental signals
if the organism is to grow, reproduce, and survive. Complex We already mentioned a key functional challenge of complex
multicellular organisms, therefore, require mechanisms for multicellularity: transporting food, oxygen, and molecular signals
transferring environmental signals received by cells at the body’s rapidly across large distances within the body. How does oxygen
surface to interior cells, where genes will be activated or repressed get from the air in your lungs into your bloodstream? How does
in response. Development in complex multicellular organisms can, atmospheric carbon dioxide get into leaves? How does ammonia
in fact, be defined as increasing or decreasing gene expression in get from seawater into the cells of seaweeds? The answer to all
response to molecular signals from surrounding cells (Chapter 20). three questions is the same: by diffusion. But oxygen absorbed
Complex multicellularity evolved at least six separate times in by your lungs doesn’t reach your toes by diffusion alone—it is
different eukaryotic groups (Fig. 28.4). Complex multicellularity transported actively, and in bulk, by blood pumped through your
 CHAPTER 28 B E I N G M U LT I C E L L U L A R 581

constraint on the size, shape, and function of cells. Diffusion is

FIG. 28.4 Complex multicellularity within the eukaryotic tree effective only over small distances and so it strictly limits the size
of life. Complex multicellular organisms (found on and shape of bacterial cells, as discussed in Chapter 26. Diffusion
branches marked by blue) evolved at least six times also constrains how eukaryotic organisms function.
within three superkingdoms of eukaryotes. Oxygen provides a good example. Most eukaryotes
require oxygen for respiration. If a cell or tissue must rely on
Complex multicellularity
diffusion for its oxygen supply, its thickness is limited by the
concentration difference in oxygen between the cell or tissue and
Animals
its environment. The concentration difference itself depends on
Choanoflagellates Opisthokonts the amount of oxygen in the environment and the rate at which
Fungi (2 times) oxygen is used for respiration inside the organism. In shallow water
Dictyostelid slime molds that is in direct contact with the atmosphere, diffusion permits
Plasmodial slime molds animals to reach thicknesses of about 1 mm to 1 cm, depending on
Entamoebae Amoebozoans
their metabolic rate. Of course, a quick swim along the seacoast will
reveal many animals much larger than this. And, obviously, you are
Flabellinea
larger than this. How do you and other large animals get enough
Tubulinea
oxygen to all of your cells to enable you to survive?
Glaucocystophytes
Red algae Archaeplastids Animals achieve large size by circumventing limits
Green algae (land plants) imposed by diffusion.
Diatoms
Sponges can reach overall dimensions of a meter or more, but
they actually consist of only a few types of cell that line a dense
Brown algae Stramenopiles
network of pores and canals and so remain in close contact with
Oomycetes
circulating seawater (Fig. 28.5a). The large size of a sponge is
Dinoflagellates therefore achieved without placing metabolically active cells
Apicomplexans Alveolates at any great distance from their environment. Similarly, in
Ciliates jellyfish, active metabolism is confined to thin tissues that line
Cercozoans
the inner and outer surfaces of the body. Essentially, a large flat
surface is folded up to produce a three-dimensional structure
Foraminifera Rhizarians
(Fig. 28.5b). The jellyfish’s bell-shaped body is often thicker
Radiolaria
than the metabolically active tissue, but its massive interior is
Cryptophytes filled by materials that are not metabolically active. This material
Haptophytes constitutes the mesoglea, the jellyfish’s “jelly.” The mesoglea
Centrohelid heliozoans provides structural support but does not require much oxygen.
But what about us? How does the human body circumvent
Euglenids
the constraints of diffusion? Our lungs gather the oxygen we
Trypanosomes
Excavates need for respiration, but the lung is a prime example of diffusion
Diplomonads
in action, not a means of avoiding it. Because lung tissues have
Parabasalids a very high ratio of surface area to volume, oxygen can diffuse
efficiently from the air you breathe into lung tissue (Chapter 39).
A great deal of oxygen can be taken in this way, but how does
circulatory system. Bulk flow of oxygen, nutrients, and signaling it get from the lungs to our brains or toes? The distances are far
molecules, at rates and across distances far larger than can be too large for diffusion to be effective. The answer is that oxygen
achieved by diffusion alone, lies at the functional heart of complex binds to molecules of hemoglobin in red blood cells and then is
multicellularity. carried through the bloodstream to distant sites of respiration.
We circumvent diffusion by actively pumping oxygen-rich blood
Diffusion is effective only over short distances. through our bodies.
Diffusion is the random motion of molecules, with net movement
from areas of higher to lower concentration (Chapter 5). The Complex multicellular organisms have structures
distance through which molecules can diffuse quickly depends specialized for bulk flow.
in part on how great the concentration difference is. Because We have seen how humans circumvent the limitations of oxygen
diffusion supplies key molecules for metabolism, it exerts a strong diffusion. Many other animals similarly rely on the active
582 SECTION 28.3 H O W TO B U I L D A M U LT I C E L LU L A R O RG A N I S M

FIG. 28.5 Circumventing limits imposed by diffusion. (a) Sponges can attain a large size because the many pores and canals in their bodies
ensure that all cells are in close proximity to the environment. (b) Jellyfish also have thin layers of metabolically active tissue, but their
familiar bell can be relatively thick because it is packed with metabolically inert molecules (the mesoglea, or “jelly”). Sources: a. Andrew J.
Martinez/SeaPics.com; b. D. R. Schrichte/SeaPics.com.

a b

circulation of fluids to transport oxygen and other essential nutrients and signaling molecules upward and downward through
molecules, including food and molecular signals, across distances roots, stems, and leaves.
far larger than those that could be traversed by diffusion alone. Still other complex multicellular organisms also have
Indeed, without a mechanism like bulk flow, animals could not specialized tissues to transport essential materials over long
have achieved the range of size, shape, and function familiar to us. distances at rates faster than could be accomplished by diffusion.
Bulk flow is any means by which molecules move through Fungi transport nutrients through networks of filaments that may
organisms at rates beyond those possible by diffusion across be meters long, relying on osmosis to pump materials from sites
a concentration gradient (Fig. 28.6). In humans and other of absorption to sites of metabolism (Chapter 34). The giant kelps
vertebrate animals, the active pumping of blood through blood have an internal network of tubular cells that transports molecules
vessels supplies oxygen to tissues that may be more than a meter through a body that can be tens of meters long. We will discuss
distant from the lungs (Fig. 28.6a). Most invertebrate animals lack the biology of plants, fungi, and animals in subsequent chapters.
well-defined blood vessels but circulate fluids freely throughout For now, it is important to understand the functional similarities
the body cavity. among these diverse multicellular organisms. In general, when
Bulk flow also distributes nutrients through the body, some cells within an organism are buried within tissues, far from
transporting the organic molecules required for respiration large the external environment, bulk flow is required to supply those
distances from the intestinal cells that absorb these molecules cells with molecules needed for metabolism.
from the digestive tract. (Again, the molecules are transported
j Quick Check 2 How do mechanisms for bulk flow enable
actively in the bloodstream.) Endocrine signaling molecules such
organisms to achieve large size?
as hormones (Chapters 9 and 38) also move rapidly through the
body by means of the blood and other fluids.
Complex organisms other than animals also rely on bulk
flow. A redwood tree must transport water upward from its roots 28.3 HOW TO BUILD A
to leaves that may be 100 m above the soil. If plants relied on MULTICELLULAR ORGANISM
diffusion to transport water, they would be only a few millimeters
tall. How, then, do they move water? Plants move water by bulk We have emphasized three general requirements for complex
flow through a system of specialized tissues powered by the multicellular life: Cells must stick together; they must
evaporation of water from leaf surfaces (Fig. 28.6b; Chapter 29). communicate with one another; and they must participate in a
Vascular plants also have specialized tissues for the transport of network of genetic interactions that regulates cell division and
 CHAPTER 28 B E I N G M U LT I C E L L U L A R 583

FIG. 28.6 Bulk flow. (a) The circulatory system in animals and (b) the vascular system in plants allow these organisms to get around the size limits
of diffusion.

a. Human circulatory system b. Plant vascular system

Vascular tissues
The transport water and Other vascular tissues
circulatory nutrients upward from transport sugars
system the soil to leaves, downward from
transports where photosynthesis leaves to other parts
oxygen and takes place. of the plant.
nutrients to
tissues and
carbon Water flows upwards through specialized
dioxide and tissues, while sugars are transported downward
other wastes from the leaves where they are synthesized.
away from
tissues.

differentiation. Once these are in place, the stage is set for the Cadherins and integrins do not occur in complex multicellular
evolution of specialized tissues and organs observed in plants, organisms other than animals, but all such organisms require some
animals, and other complex multicellular organisms. means of keeping their cells stuck together. How do they do it?
Plants also synthesize cell adhesion molecules that bind cells into
Complex multicellularity requires adhesion tissues, but, in this case, the molecules are polysaccharides called
between cells. pectins (pectins are what make fruit jelly jell). Molecules that
If a fertilized egg is to develop into a complex multicellular control adhesion between adjacent cells are less well known in
organism, it must divide many times, and the cells produced other complex multicellular organisms, but the general picture is
from those divisions must stick together. In addition, they must clear. Without adhesion there can be no complex multicellularity,
retain a specific spatial relationship with one another in order for and different groups of eukaryotes have evolved distinct types of
the developing organism to function. Chapter 10 introduced the molecular “glue” for this purpose.
molecular mechanisms that hold multicellular organisms together.
We begin by reviewing those mechanisms briefly before placing How did animal cell adhesion originate?
them in evolutionary context. The genome of the choanoflagellate Monosiga brevicollis
In animal development, proliferating cells commonly become provides fascinating insight into the evolution of cell adhesion
organized into sheets of cells called epithelia that line the inside molecules. Choanoflagellates, the closest protistan relatives of
and cover the outside of the body. Within epithelia, adjacent animals, are unicellular microorganisms. Therefore, it came as
cells adhere to each other by means of transmembrane proteins, a surprise that the genes of M. brevicollis code for many of the
especially cadherins, which form molecular attachments between same protein families that promote cell adhesion in animals.
cells. Epithelial cells also secrete an extracellular matrix made The choanoflagellate genome contains genes for both cadherin
of proteins and glycoproteins, and these cells attach themselves and integrin proteins. Clearly, these proteins are not supporting
to this matrix by means of other transmembrane proteins called epithelia in M. brevicollis, so what are they doing?
integrins. Cadherins, integrins, and additional transmembrane One approach to an answer came from the observation that
proteins thus provide the molecular mechanisms for adhesion in cells stick not only to one another but also to rock or sediment
animals. surfaces. Proteins that originally evolved to promote adhesion
584 SECTION 28.3 H O W TO B U I L D A M U LT I C E L LU L A R O RG A N I S M

of individual cells to sand grains or a rock surface may have been deeper into eukaryotic phylogeny—they are found in single-
modified during evolution for cell–cell adhesion. It is also possible celled protists that branch near the base of the opisthokont
that adhesion molecules were originally used in the capture of superkingdom (Chapter 27). Such a phylogenetic distribution
bacterial cells, to make prey adhere to the predator. provides strong support for the general hypothesis that cell
An intriguing clue to the role of these proteins in adhesion in animals resulted from the redeployment of protein
choanoflagellates comes from careful laboratory studies (Fig. 28.7). families that evolved to perform other functions before animals
Although choanoflagellates are unicellular, simple multicellular diverged from their closest protistan relatives.
structures can be induced in a number of species. Molecular
signals induce some choanoflagellates to form a novel multicellular Complex multicellularity requires communication
structure, and, unexpectedly, the source of these signals is a between cells.
bacterium, the preferred prey of the choanoflagellates. The function In complex multicellular organisms, it is not sufficient for cells to
of the multicellular structures in these choanoflagellates has not yet adhere to one another. They must also be able to communicate.
been determined, but these observations support the hypothesis Communication is important during development, guiding the
that they aid predation. patterns of gene expression that differentiate cells, tissues, and
To date, cadherins have been found only in choanoflagellates organs. The functional integration of cells within tissues and tissues
and animals, but proteins of the integrin complex extend even within organs also depends on the flow of information among cells.

HOW DO WE KNOW?

FIG. 28.7
a b c

forming choanoflagellate Salpingoeca rosetta, 73–82, Copyright 2011,

How do bacteria

Dayel and others, Cell differentiation and morphgenesis in the colony-

Photo source: Reprinted from Developmental Biology, 357, Mark
influence the life cycles of
choanoflagellates?
BACKGROUND Choanoflagellates are the closest protistan

with permission from Elsevier.

relatives of animals. Cell adhesion, cell differentiation, and
multicellularity in these organisms are being studied to increase
our understanding of early events in the evolution of animal
multicellularity. Nicole King and her colleagues cultured the 5 µm 5 µm 5 µm
choanoflagellate species Salpingoeca rosetta in the laboratory.
High-speed videos revealed that these choanoflagellates
differentiate into several distinct cell types in the course of their RESULTS The multicellular state formed only when
life cycle. Actively swimming cells can be fast swimmers or slow A. machipongonesis was present (Fig. 28.7c).
swimmers that differ in shape and ornamentation (Fig. 28.7a).
When swimming cells make contact with a solid surface, they CONCLUSION Choanoflagellates differentiate several cell types
can differentiate to form a stalk that tethers them to the substrate upon cues from the environment. In particular, molecular signals
(Fig. 28.7b). Simple multicellular colonies in the shape of from prey bacteria can induce a simple form of multicellularity. The
rosettes (Fig. 28.7c) were also seen when the choanoflagellates direct ancestors of animals may have gained simple multicellularity
were cultured in the presence of many bacteria, including by a similar route.
Algoriphagus machipongonesis. FOLLOW-UP WORK Research is continuing to investigate the
HYPOTHESIS The bacterium Algoriphagus machipongonesis molecular mechanisms of cell differentiation, cell adhesion, and
induces the choanoflagellate to form rosette-like colonies. signal reception in these distant relatives of ours.

EXPERIMENT The scientists grew the choanoflagellates in

SOURCE Dayel, M., et al. 2011. “Cell Differentiation and Morphogenesis in the
a medium containing only A. machipongonesis, as well as in Colony-Forming Choanoflagellate Salpingoeca rosetta.” Developmental Biology
media without this prey bacterium. 357:73–82.
 CHAPTER 28 B E I N G M U LT I C E L L U L A R 585

FIG. 28.8 Gap junctions. Gap junctions facilitate the transport of ions and molecules between adjacent cells in animals.

Gap junctions
are intercellular
connections
made up of a
Gap ring of proteins.
junction

Plasma membrane

Intercellular space

As discussed in Chapter 9, cells communicate by molecular than sponges have gap junctions, protein channels that allow
signals. A signaling molecule (generally a protein) synthesized by ions and signaling molecules to move from one cell into another
one cell binds with a receptor protein on the surface of a second (Fig. 28.8). Gap junctions not only help cells to communicate with
cell, essentially flipping a molecular switch that activates or their neighbors, they allow targeted communication between a cell
represses gene expression in the receptor cell’s nucleus. Plants and and specific cells adjacent to it (Chapter 10).
animals both have receptors of the receptor kinase type, and these Plants, in contrast, have an intrinsic barrier to intercellular
initiate similar signaling pathways within the cell. As in the case communication: the cell wall. Careful observation of plant cell
of adhesion molecules, these signaling pathways, or components walls with the electron microscope reveals that these walls
of the pathways, have also been identified in their close protistan have tiny holes through which thin strands of cytoplasm extend
relatives. Evidently, many of the signaling pathways used for from one cell to the next (Fig. 28.9). Called plasmodesmata,
communication between cells in complex multicellular organisms these intercellular channels are lined by extensions of the cell
first evolved in single-celled eukaryotes. Again we can ask, what membrane (Chapter 10). Tubules running through these channels
function did molecular signals and receptors have in the protistan connect the endomembrane systems of the two cells. Like gap
ancestors of complex organisms? junctions, plasmodesmata permit signaling molecules to pass
All cells have transmembrane receptors that respond to signals between cells in such a way that they can be targeted to only one
from the environment. In some cases, the signal is a molecule or a few adjacent cells.
released by a food organism (such as the bacteria that induce Complex red and brown algae also have plasmodesmata,
simple multicellularity in choanoflagellates), and in some cases and complex fungi have pores between cells that enable
the cells sense nutrients, temperature, or oxygen level. Single- communication by means of cytoplasmic flow. As similar channels
celled eukaryotes also communicate with other cells within the do not occur in most other eukaryotic organisms, they appear
same species, for example to ensure that two cells can find each to represent an important step in the evolution of complex
other to fuse in sexual reproduction. Signaling between two cells multicellularity.
within an animal’s body can be seen as a variation on this more
general theme of a cell responding to other cells and the physical Complex multicellularity requires a genetic program
environment. for coordinated growth and cell differentiation.
While all eukaryotic cells have molecular mechanisms for All of the cells in your body derive from a single fertilized egg, and
communication between cells, complex multicellular organisms most of those cells contain the same genes. Yet your body contains
have distinct cellular pathways for the movement of molecules about 200 distinct cell types precisely arranged in a variety of
from one cell to another. For example, animals more complex tissues and organs. How can two cells with the same genes become
586 SECTION 28.3 H O W TO B U I L D A M U LT I C E L LU L A R O RG A N I S M

FIG. 28.9 Plasmodesmata. Plasmodesmata facilitate the movement of ions and molecules between cells in land plants and in complex red and
brown algae. Photo source: Biophoto Associates/Science Source.

Plasmodesmata Vacuole

Cell
interior

Cell wall

Plasma
membrane
Plasmodesma
Cell
interior

Cell wall

Endoplasmic
reticulum

Plasmodesmata are intercellular

connections lined by extensions
of the cell membrane. They also
contain a tubule connecting the
endomembrane systems of
neighboring cells.

different cell types? The answer lies in development, the system conditions. For example, if we experimentally starve dinoflagellate
of gene regulation that guides growth from zygote to adult cells, two cells undergo sexual fusion to form morphologically
(Chapter 20). and physiologically distinct resting cells protected by thick walls.
Development is the result of molecular communication That is, a nutrient shortage induces a change in gene expression
between cells. Cells have different fates depending on which that leads to the formation of resting cells. When food becomes
genes are switched off or on, and genes are switched off and on by available again, the cells undergo meiotic cell division to form new
the molecular signals that cells receive. A signal commonly alters feeding cells. Many other single-celled eukaryotes form resting
the production of proteins—inducing the reorganization of the cells in response to environmental cues, especially deprivation of
cytoskeleton, for example. As a result, a stem cell may become an nutrients or oxygen.
epithelial cell, or a muscle cell, or a neuron. This observation leads The innovation of complex multicellularity was to
to another question: What causes the same gene to be turned on in differentiate cells in space instead of time. In a three-dimensional
one cell and off in another? The ultimate answer is that two cells multicellular organism, only surface cells are in direct contact
in the same developing organism can be exposed to very different with the outside environment. Interior cells are exposed to a
environments. different physical and chemical environment because nutrients,
When we think about development as a process of oxygen, and light become less abundant with increasing depth
programmed cell division and differentiation, the link to within tissues. In effect, there is a gradient of environmental
unicellular ancestors becomes clearer. Many biological innovations signals within multicellular organisms. We might, therefore,
accompanied the evolution of complex multicellularity, but, as hypothesize that in the earliest organisms with three-dimensional
noted in Chapter 27, the differentiation of distinct cell types is not multicellularity, a nutrient or oxygen gradient triggered oxygen- or
one of them. Many unicellular organisms have life cycles in which nutrient-starved interior cells to differentiate, much as happens to
different cell types alternate in time, depending on environmental trigger the formation of resting cells in their single-celled relatives.
 CHAPTER 28 B E I N G M U LT I C E L L U L A R 587

With increasing genetic control of cellular responses to signaling results of these separate evolutionary events by examining cell
gradients, the seeds of complex development were sown. adhesion, signaling, and development in plants and animals.
Green algae provide a fascinating example that links cell Both plants and animals have evolved sophisticated systems
differentiation in unicellular and multicellular organisms. that cause adjacent cells to adhere to each other and that promote
The simple multicellular organism Volvox (see Fig. 27.15) has the targeted movement of signaling molecules between cells.
two types of cells, vegetative cells that photosynthesize and Plant and animal mechanisms, however, must differ because plant
control movement of the organism, and reproductive cells. Cell cells have cell walls and animal cells do not. Likewise, plants and
differentiation in Volvox is regulated by a gene also involved in the animals have evolved similar genetic logic to govern development,
formation of distinct cell types in the life cycle of Volvox’s single- but use mostly distinct sets of genes. In both plants and animals,
celled relative Chlamydomonas, supporting the hypothesis that many proteins switch genes encoding other proteins on or off,
the spatial differentiation of cells in multicellular organisms began so that the spatial organization of multicellular organisms arises
with the redeployment of genes that regulate cell differentiation from networks of interacting genes and their protein products.
in single-celled ancestors. Ancestral plants and animals simply recruited distinct families of
Bulk flow, which transports nutrients, oxygen, and genes to populate regulatory networks.
water within complex multicellular organisms, also carries
developmental signals. Signals carried by bulk flow can travel far Cell walls shape patterns of growth and development
greater distances through the body than signals transmitted by in plants.
diffusion alone. For example, in animals the endocrine system The plant cell wall (Chapter 5), made of cellulose, provides
releases hormones directly into the bloodstream, enabling them to structural support to cells. In fact, the cell wall provides the
affect cells far from those within which they formed (Chapter 38). mechanical support that allows plants to stand erect (Chapter 29).
Thus, the sex hormones estrogen and testosterone are synthesized The presence of cell walls has largely determined the evolutionary
in reproductive organs but regulate development throughout the fate of plants. For example, because their cells cannot engulf
body, contributing to the differences between males and females. particles or absorb organic molecules, most plants gain carbon and
In this way, signals carried by bulk flow can induce the formation energy only through photosynthesis. In addition, because all plant
of distinct cell types and tissues along the path of signal transport. cells except eggs and sperm are completely surrounded by cell
The genome of the choanoflagellate Monosiga brevicollis, walls, they have no pseudopodia and no flagella (in conifers and
discussed earlier, has been a treasure trove of information on the flowering plants, even sperm have lost their flagella). This being
antiquity of signaling molecules deployed in animal development. the case, plant cells cannot move. Therefore, the plant as a whole
In addition to expressing proteins that govern cell adhesion and is constrained to obtain nutrients, evade predators, and cope with
epithelial cohesion in animals, M. brevicollis expresses a number environmental stress without moving parts.
of proteins active in animal cell differentiation. For example, The inability of plant cells to move has major consequences
signaling based on specific receptor kinases was long thought to for development. At the level of the cell, the entire program of
be restricted to animals, but it also occurs in M. brevicollis. In other growth and development involves cell division, cell expansion
cases, individual components of signaling proteins are present (commonly by developing large vacuoles in cell interiors), and cell
in the M. brevicollis genome, but not the complex multidomain differentiation. The mechanical consequence is that plant growth
proteins formed by animals. This is true, for example, of several is confined to meristems (Fig. 28.10), populations of actively
protein complexes that are important in development. Similarly, dividing cells at the tips of stems and roots (Chapter 31). The cells
molecules that play an important role in plant development are also in meristem regions are more or less permanently undifferentiated
being identified in the genomes of morphologically simple green cells that repeatedly undergo mitosis. A few millimeters from the
algae. The key point is that genome sequences interpreted in light of region of active cell division in a stem or root meristem, cells stop
eukaryotic phylogeny are now enabling biologists to piece together dividing and begin to expand. Within another few millimeters,
the patterns of gene evolution that accompanied the evolution of this activity is curtailed as well. There, individual cells respond
morphological complexity and diversity in plants and animals. to signaling molecules that induce differentiation, forming the
mature cells that will function in photosynthesis, storage, bulk
flow, or mechanical support. In general, then, plant development
28.4 VARIATIONS ON A THEME: involves site-specific cell division, followed by differentiation into
PLANTS VERSUS ANIMALS distinct cell types that govern the function of the plant as a whole.
Because cell walls render plants immobile, plants have evolved
Plants and animals are the best-known examples of complex mechanisms to transport water and nutrients from the soil to
multicellular organisms. The study of phylogenetic relationships leaves that may be tens of meters distant without the use of
makes it clear that complex multicellularity evolved any moving parts or even the expenditure of ATP. And because
independently in the two groups. In other words, they do not plants are anchored in place, they are unable to move in response
share a common ancestor that was multicellular. We can see the to unfavorable growth conditions. Instead, plants respond to
588 SECTION 28.4 VA R I AT I O N S O N A T H E M E : P L A N T S V E R S U S A N I M A L S

development: Unconstrained by cell walls, animal cells can

FIG. 28.10 A meristem. Because of their walls, plant cells can’t move relative to one another. Blastula cells migrate, becoming
move. For this reason, roots grow by cell division of the reorganized into a hollow ball that folds inward at one location to
undifferentiated cells in the meristem region at the root form a second layer under the first. This multilayered structure is
tip. Photo source: Walker Mi./Getty Images. called the gastrula.
Gastrula formation brings new populations of cells into
direct contact with one another, inducing patterns of molecular
signaling and gene regulation that begin the long process of
Zone of cell growth and tissue specification. As cells proliferate, molecular
differentiation signals are expressed in some cells and diffuse into others,
generating what amounts to a three-dimensional molecular map
of the organism that guides cell differentiation. Gradients in
signaling molecules define top, bottom, front, back, left, and right.
Plants do much the same thing, but because animal cells can move

Zone of cell
expansion
FIG. 28.11 Gastrulation. Gastrulation is the movement of cells
during embryogenesis that in animals transforms a
blastula to a gastrula.

Zone of cell
division
Fertilized egg

Meristem

Cells that cap Mitosis

and protect
the meristem Blastula
(hollow ball)

environmental signals by adjusting their meristem activity and,

hence, growth pattern. Heat, drought, floods, and fire can all
leave their mark in altered patterns of growth—comparing well
and poorly watered tomato plants shows this effect. In addition,
plants can’t flee predators, so they have evolved mechanical
structures (hairs and spines) and poisons to keep from being eaten.
Indeed, it has been suggested that, in terms of responding to the
environment, growth plays a role in plants similar to that played
by behavior in animals.
The details of plant growth, development, reproduction, and Cells migrate
inward to
function are presented in Chapters 29–33. Here, the point to bear form a fold in
in mind is that the properties of a pine tree or a rice plant depend the cell layer.
fundamentally on the basic features of cell adhesion, intercellular
communication, and a regulatory network to guide development,
all carried out under the constraints imposed by cell walls.

Animal cells can move relative to one another.

Having outlined the growth and development of plants, we
can appreciate anew the remarkable ballet that characterizes
development in animal embryos (Fig. 28.11; Chapter 42).
Fertilized eggs undergo several rounds of mitosis to form a hollow
ball called a blastula, formed of a single layer of undifferentiated
cells. Then something happens that has no parallel in plant Gastrula
 CHAPTER 28 B E I N G M U LT I C E L L U L A R 589

during development, animal embryos are not restricted to growth cell adhesion, modes of communication between cells, and
only from localized regions like meristems. Cell division and tissue a genetic program to guide growth and development. These
differentiation occur throughout the developing animal body. three requirements had to be acquired in a specific order. If the
Because they are not constrained by cell walls, animals can products of cell division don’t stick together, there can be no
form organs with moving parts—muscles that power active complex multicellularity. Adhesion, however, is not sufficient. It
transport of food and fluids and allow movement. Thus, animals must be followed by mechanisms for communication between
have possibilities for function that are far different from those cells. Moreover, cells must be able to send molecular messages to
of plants. If the environment offers a challenge, like drought or specific targets or regions, assisted in animals by gap junctions and
predators, animals can respond by changing their behavior—for in plants by plasmodesmata.
example, they can move to a new location (Chapter 37). With these requirements in place, natural selection would
Plants and animals, then, display contrasting patterns of favor the increase and diversification of genes that regulate growth
development and function that reflect both their independent and development, making possible more complex morphology
origins from different groups of protists and the constraints and anatomy. The biological stage was finally set for the functional
imposed by cell walls in plants. key to complex multicellularity: the differentiation of tissues and
organs that govern the bulk flow of fluids, nutrients, signaling
j Quick Check 3 How do plants and animals differ in the ways their
molecules, and oxygen through increasingly large and complex
cells adhere, communicate, and differentiate during development?
bodies. This differentiation freed organisms from the tight
constraints imposed by diffusion. When all these features are
placed onto phylogenies, they show both the predicted order of
28.5 THE EVOLUTION OF COMPLEX acquisition and the tremendous evolutionary consequences of
MULTICELLULARITY complex multicellularity (Fig. 28.12).

The differences between an amoeba and a lobster seem vast, Fossil evidence of complex multicellular organisms
but research over the past decade increasingly shows how this is first observed in rocks deposited 579–555 million
apparent gap was bridged through a series of evolutionary years ago.
innovations. Throughout this chapter, we have emphasized In Chapter 23, we noted that phylogenies based on living
that complex multicellular organisms require mechanisms for organisms make predictions about the fossil record. Characters

a.
Bulk flow
Gap junctions
Insects, mammals, and other animals
with bilateral symmetry (~10,000,000)
Animals
Jellyfish and their relatives (10,000)
Adhesion, cell
signaling
Sponges (10,000)

Single-celled Choanoflagellates (150)

b.
Bulk flow
Vascular plants (350–400,000)
Land plants
Plasmodesmata Mosses and their
† relatives (20,000) FIG. 28.11 The evolution of complex
multicellularity. Phylogenetic
Simple multicellular
relationships of complex (a) animals
Adhesion, cell
signaling and (b) plants, with their close
Green algae (6000*)
evolutionary relatives, show similar
Single-celled patterns of character accumulation
and its consequences for biological
* Only species on the branch leading to land plants diversity. Estimated species
† Limited development of transport tissues in some mosses numbers are shown.
590 SECTION 28.4 T H E E VO L U T I O N O F CO M P L E X M U LT I C E L LU L A R I T Y

and groups associated with lower branches in phylogenies should animals, with distinct head and tail, top and bottom, left and
appear as earlier fossils than those associated with later branches. right, enter the record (Fig. 28.13b). At the same time, we begin
Although fossils don’t preserve molecular features, the record to see tracks and trails made by animals whose muscles enabled
supports the phylogenetic pattern shown in Fig. 28.12. them to move across and through surface sediments (Fig. 28.13c),
Single-celled protists are found in sedimentary rocks as an indirect but complementary record of increasing animal
old as 1800 million years, and a number of simple multicellular complexity. These, then, are the first known occurrences of the
forms have been discovered that are nearly as old (see Fig. 27.23). types of animal that dominate ecosystems today, both in the sea
Nonetheless, the oldest fossils that record complex multicellularity and on land. These animals must have possessed a diverse array of
are found in rocks that were deposited only 579–555 million years genes to guide development, something akin to those seen today in
ago. Fig. 28.13a shows one of the oldest known animal fossils, insects or snails.
a frondlike form preserved in 575-million-year-old rocks from Clearly, by 579–555 million years ago, the animal tree was
Newfoundland, near the eastern tip of Canada. This fossil and many beginning to branch. This, however, raises a new question: Why
others found in rocks of this age are enigmatic. There is complex did complex multicellular animals appear so much later than
morphology to be sure, but it is difficult to understand these simple multicellular organisms?
forms by comparing them to the body plans of living animals. The
Newfoundland fossils show no evidence of head or tail, no limbs, Oxygen is necessary for complex multicellular life.
and no opening that might have functioned as a mouth. They Earlier in the chapter, we discussed the key biological
appear to be very simple organisms that gained both carbon and requirements for complex multicellularity. There is a critical
oxygen by diffusion. These fossils are long and wide, but they are environmental requirement as well: the presence of oxygen.
not thick. In fact, it is likely that active metabolism occurred only There are other potential electron acceptors for respiration, like
along the surfaces of these structures. sulfate and ferric iron. However, they not only occur in much
Such animals lie near the base of the animal tree (Chapter lower abundances than oxygen, but they are not gases and so do
44), and probably would have developed under the guidance of not accumulate in air. Only the oxidation of organic molecules by
regulatory genes similar to those present in modern sponges or, O2 provides sufficient energy to support biological communities
perhaps, jellyfish. By 560–555 million years ago, more complex that include large and active predators like wolves and lions. And

FIG. 28.13 The earliest fossils of animals. (a) Among the oldest known macroscopic animal fossils is a frond-like form made up of tubular
structures, preserved in 575-million-year-old rocks from Newfoundland. (b) The oldest known animal fossil showing bilateral
symmetry is a mollusk-like form preserved in 560–555-million-year-old rocks from northern Russia. (c) The oldest tracks and
trails of mobile animals are found in rocks 560 million years old and younger. Sources: a. Guy Narbonne; b. Mikhail Fedonkin; c. Andrew Knoll,
Harvard University.

a b c
 CHAPTER 28 B E I N G M U LT I C E L L U L A R 591

bulk flow in turn permitted still larger

FIG. 28.14 Oxygen and the evolution of complex multicellularity. size, setting up a positive feedback that
eventually resulted in the complex
Archean Proterozoic Phanerozoic
marine organisms we see today.
(% of present-day level)

Complex multicellularity
Atmospheric oxygen

spread through the With morphological complexity

100
oceans only as the came new functions, including predation
oxygen content of the on other animals. Protozoan predators
atmosphere and oceans
1-10 capture other microorganisms, but do so
increased.
one cell at a time. In contrast, animals
0
that obtain food by filtering seawater
4000 3000 2000 1000 Present gather cells by the thousands, and larger
Time (millions of years before present) animals can eat smaller ones, opening
up endless possibilities for evolutionary
Life specialization and, therefore,
Eukaryotes diversification. Among photosynthetic
Simple multicellularity organisms, multicellular red and green
Animals
Complex Complex multicellular algae were able to establish populations
Complex algae
multicellular organisms radiated on in wave-swept coastlines and other
organisms Land plants
land later, as one line of
Complex fungi green algae adapted
environments where simpler organisms
photosynthesis on land. lacked the mechanical strength to hold
their position along the shoreline.
The key point is that complex
multicellular organisms didn’t succeed
by doing the same things as simpler
only oxygen in concentrations approaching those of the present eukaryotes. Complex multicellularity spread through the oceans
day can diffuse into the interior cells of large, active organisms. On because it opened up new and unprecedented evolutionary
our planet—and, probably, on all planets—no other molecule is possibilities.
both sufficiently abundant and has the oxidizing power needed to
support the biology of large complex organisms. Land plants evolved from green algae that could carry
On the present-day Earth, lake or ocean bottoms with oxygen out photosynthesis on land.
levels below about 8% of surface levels support only a limited Complex multicellular land plants originated about 465 million
diversity of animals, and those that they do support tend to be years ago, well after marine animals and algae. In some ways,
tiny. Large animals with energetic modes of life—carnivores, however, the pattern of evolution of land plants parallels that of
for example—live only in oxygen-rich environments. The clear animals. Ancestral green algae evolved molecular means for cell–cell
restrictions on animal form and diversity observed at lower oxygen adhesion and communication between cells, and the algae that form
levels today have important implications for the distribution the sister group to land plants share with them the innovation of
of large, active, and diverse animals through time. We noted plasmodesmata. Then, as they developed larger and more complex
in Chapter 26 that oxygen first began to accumulate in the three-dimensional bodies, plant ancestors evolved a succession of
atmosphere and surface oceans about 2400 million years ago, genes and their protein products to guide cell division, expansion,
but the chemistry of sedimentary rocks deposited after that time and differentiation. Land plants, however, faced a different
tells us that oxygen levels remained low for a very long time. Our challenge from that of animals. The diversification of plants across
modern world of abundant oxygen came to exist only 580–560 the land surface requires that photosynthesis, evolved earlier by
million years ago, about the time when fossils first record large aquatic organisms, be carried out within tissues bathed by air. It
multicellular organisms (Fig. 28.14). also requires that nutrients and water be absorbed from the soil and
Scientists continue to debate the causes of this environmental transported throughout the plants rather than simply taken in by
transformation, but the correspondence in time between oxygen diffusion from the surrounding water.
enrichment and the first appearance of large, complex animals Fossils indicate that by 400 million years ago, plants with
(and algae) suggests that more oxygen permitted greater size specialized tissues for bulk flow of water and nutrients had begun
and more energetic modes of life. Greater size in turn created to spread across the continents. As plant biomass built up on
opportunities for tissue differentiation, leading to transport of land, it created opportunities for additional types of complex
nutrients and signaling molecules by bulk flow. The evolution of multicellular organism. In particular, the fungi, which decompose
592 SECTION 28.4 T H E E VO L U T I O N O F CO M P L E X M U LT I C E L LU L A R I T Y

FIG. 28.15 Evolution of complex fungi. (a) This trunklike fossil, discovered in France, is actually a giant fungus that towered over early land
plants 375 million years ago. (b) This image shows the interior of the fungus, which consists of tubes that transported nutrients
through the large body. Sources: Carol Hotton; b. Martha E. Cook, School of Biological Sciences, Illinois State University.

a b

100 µm

organic matter, radiated by taking advantage of this resource The answer has to do with the network of genes that guide
(Fig. 28.15), evolving complex multicellularity in two distinct development. Since the 1950s, biologists have learned a remarkable
lineages (Chapter 34). amount about this genetic network and its host of interacting genes
(Chapter 20). Many of the genes involved in development are what
Regulatory genes played an important role in the we might consider middle managers: A molecular signal induces
evolution of complex multicellular organisms. the expression of a gene, and the protein product, in turn, prompts
Complex multicellular organisms account for a large proportion the expression or repression of another gene. It is the complex
of all eukaryotic species. Thus, one evolutionary consequence of interplay of these genetic switches—turning specific genes on in
complex multicellularity was an increase in biological diversity. one cell and off in another, depending on where those cells occur
Let’s compare the diversity of complex multicellular groups with in the developing body—that results in crabs with large pincers or
their simpler phylogenetic sisters. Choanoflagellates, for example, butterflies with patterned wings (Fig. 28.16).
number about 150 species; simple animals such as sponges and It makes sense that if regulatory genes guide development of
jellyfish about 20,000; and complex animals with circulatory the body in each complex multicellular species, then mutations
systems perhaps as many as 10 million. Plants and their relatives in regulatory genes may account for many of the differences we
show a comparable pattern. There are about 6000 species of green observe among different species. Our growing understanding
algae on the branch that includes land plants, but about 400,000 of how developmental genes underpin evolutionary change has
species of anatomically complex land plants capable of bulk flow. given rise to a whole new field of biology called evolutionary-
Similarly, complex fungi and red algae include more species by an developmental biology, or evo-devo for short.
order of magnitude than their simpler relatives. In evo-devo research, scientists compare the genetic programs
How did this immense diversity arise? At one level, the answer for growth and development in species found on different branches
is functional and ecological. Complex multicellular organisms of phylogenetic trees. In addition to wanting to discover the
can perform a range of functions that simpler organisms cannot, molecular mechanisms that guide development in individual species,
and so they have evolved many specific types of interaction with these scientists aim to understand the genetic differences associated
other organisms and the physical environment. Carnivory, for with differences in form and function among species. Evo-devo
example, requires both sophisticated sensory systems and jaws or is an exciting and rapidly expanding field of research because it is
limbs to locate and capture prey, but once established, carnivorous helping to illuminate the long-suspected relationship between the
animals radiated throughout the oceans and, eventually, on land. development of individuals and patterns of evolutionary relatedness
This explanation, however, prompts another question: What is among species. Continuing research promises to shed new light on
the genetic basis for the bewildering range of sizes and shapes the similarities and differences among plants, animals, and other
displayed by complex multicellular organisms? complex multicellular organisms. •
HOW DO WE KNOW?

FIG. 28.16

What controls color pattern

in butterfly wings?
BACKGROUND Butterfly wings commonly show a striking pattern
of color, including circular features known as eyespots. Eyespots are
adaptive, for example in deterring predation by birds.

HYPOTHESIS The expression of regulatory genes during wing

development governs eyespot formation on wing surfaces.

EXPERIMENT Paul Brakefield and his colleagues mapped the

Photo source: Reprinted by permission
expression of a regulatory gene called Distalless in developing
from Macmillan Publishers Ltd:
butterfly wings. Distalless genes were fused to genes for green NATURE 384, 236–242, (21
fluorescent protein (GFP), a protein that glows vivid green when November 1996), P.M. Brakefield,
et al. Development, plasticity and
illuminated under blue light. GFP is then expressed along with evolution of butterfly eyespot patterns,
Distalless, allowing the spatial pattern of Distalless expression in copyright 1996.
developing wings to be visualized.

RESULTS The expression pattern of Distalless (on the left in CONCLUSION Regulatory genes play an important role in butterfly
each panel of the figure) in developing wings closely resembles wing coloration, and mutations in these genes can account for
the pattern of eyespots on the wing (on the right in each panel). differences in wing color patterns among species.
The two patterns match for both wild-type (top left) and mutant SOURCE Brakefield, P. M., et al. 1996. “Development, Plasticity and Evolution of
butterfly wings. Butterfly Eyespot Patterns.” Nature 384: 236–242.

Core Concepts Summary 28.2 In complex multicellular organisms, bulk flow

circumvents the limitations of diffusion.
28.1 Complex multicellularity arose several times in Diffusion is the random motion of molecules, with net
evolution. movement occurring from regions of higher to regions of
Bacteria are unicellular or form simple multicellular structures. lower concentration. It generally acts over small distances,
page 578 placing limits on the size of multicellular organisms. page 581

Eukaryotes are unicellular or multicellular and exhibit both Bulk flow is an active process that allows multicellular
simple and complex multicellularity. page 578 organisms to nourish cells located far from the external
environment, thereby circumventing the constraints
Simple multicellularity involves the adhesion of cells
imposed by diffusion. page 582
with little cell differentiation. Complex multicellularity
involves cell adhesion, cell signaling, and differentiation and
specialization among cells. page 578 28.3 Complex multicellularity depends on cell
adhesion, communication, and a genetic program for
Complex multicellular organisms have evolved
development.
independently at least six times: in animals, vascular plants,
red algae, brown algae (the kelps), and at least twice in the Animal and plant cells are organized into tissues characterized
fungi page 580 by specific molecular attachments between cells. page 583

55
9933
594 SELF-ASSESSMENT

Choanoflagellates express some of the same proteins that The evolution of complex multicellularity observed in the
permit cell adhesion in animals, even though choanoflagellates fossil record correlates with increases in atmospheric oxygen
are unicellular. Experiments suggest that choanoflagellates use that occurred 580–560 million years ago. page 591
these proteins to capture bacteria, not for cell adhesion. Complex multicellular organisms evolved later on land, as
page 583 ancestral plants evolved the capacity to photosynthesize
Gap junctions in animals and plasmodesmata in plants allow surrounded by air rather than water. page 591
cells to communicate with each other in a targeted fashion. Evolutionary-developmental biology, or evo-devo, is a field
page 585 of research that looks at both individual development and
The cells of complex multicellular organisms are genetically evolutionary patterns in an attempt to understand the
programmed to differentiate into multiple cell types in space. developmental changes that allowed organisms to diversify and
page 586 adapt to changing environments. page 592

A number of gene families known to play developmental roles

in multicellular organisms are also present in their unicellular Self-Assessment
relatives, where at least some of them play a role in life cycle
differentiation. page 587 1. Describe differences between simple and complex
multicellularity.
28.4 Plants and animals evolved complex multicellularity 2. Describe the phylogenetic distribution of complex
independently of each other and solved similar problems multicellularity.
with different sets of genes.
3. Explain how diffusion limits the size of organisms.
The cell wall characteristic of plant cells provides structural
and mechanical support, but does not allow plant cells to move. 4. Explain how multicellular organisms get around the size
page 587 limits imposed by diffusion.

The cell wall of plants has led to distinct solutions to 5. Describe a key difference between multicellular plants
the problems of cell adhesion, cell communication, and and animals.
development. For example, plants grow by the activity of 6. Describe environmental changes recorded by the
meristems, populations of actively dividing cells at the tips of sedimentary rocks that contain the oldest fossils of large
stems and roots. page 587 active animals.
Animal cells do not have cell walls, allowing cell movement
that is not possible in plants. For example, during animal Log in to to check your answers to the Self-
development, cells of the embryo migrate inward to form a Assessment questions, and to access additional learning tools.
layered structure called a gastrula. page 588

28.5 The evolution of large and complex multicellular

organisms, which required abundant oxygen, is recorded
by fossils.

Oxygen may be required for the evolution of complex

multicellularity because of its chemical properties and its
abundance. page 590
CASE 6

Agriculture
Feeding a Growing Population
You may have noticed that while your grocery store or Cultivated bananas are vulnerable to infection in a
supermarket has several types of apple for sale, there way that wild bananas are not. The bananas that we eat
is usually only one type of banana. In fact, it’s possible are sterile; they do not produce seeds. This means they must
that every banana you have ever eaten has been the same be propagated vegetatively by replanting cuttings taken
variety. Now the popular yellow fruit may be in trouble. from parent plants. As a result, banana plantations contain
Years ago, the Western world favored a banana variety almost no genetic diversity. As Panama disease and black
known as the Gros Michel. In the first part of the twentieth sigatoka are proving, that lack of diversity can have
century, a devastating fungal infection called Panama disastrous effects.
disease wiped out Gros Michel plantations around the As sterile clones, bananas are particularly vulnerable
world. In the 1950s, banana growers turned instead to the to disease outbreaks. Other agricultural crops face similar
Cavendish, a variety that displayed some natural resistance threats. For example, a new, virulent strain of yellow wheat
to Panama disease. Half a century later, the Cavendish rust fungus emerged in the Middle East in 2010. In its first
remains the top-selling banana in supermarkets in North year, the disease wiped out as much as half of Syria’s wheat
America and beyond. crop and has since spread to several other countries in the
Yet the reign of the Cavendish may be coming to an region. Meanwhile, crop scientists have warned that Ug99,
end. For years, banana growers have battled black sigatoka, a new, virulent strain of an even more damaging fungal
a fungal infection pathogen that first surfaced in eastern Africa, could devastate
Agriculture has that can cause global wheat crops as it spreads.
losses of 50% or Wheat was one of the first crops that our ancestors
transformed our planet more of the banana cultivated when agriculture emerged 10,000 years ago. The
and our species; without yield in infected early farmers learned to choose wheat plants that were
regions. Making easiest to harvest and whose seeds germinated without
it, modern civilization matters worse, delay. Over time, wheat evolved by this process of artificial
would not be possible. Panama disease is selection. Those earliest farmers set in motion a long chain
once again posing a of genetic modifications that ultimately led to the high-yield
threat. A strain of the disease has emerged against which the wheat varieties grown today.
Cavendish plants have no resistance. The disease has spread In the wild, natural selection favored wheat whose seeds
across Asia and Australia, and experts fear it’s only a matter were protected by tough barbs and separated easily from
of time before it reaches prime banana-growing regions in the plant, allowing efficient dispersal across the landscape.
Latin America. Farmers, on the other hand, preferred wheat plants whose
For those who enjoy bananas sliced into their breakfast seeds remained attached to the plant and so were easier
cereal, the loss of bananas would be disappointing. For to harvest. But the tendency for seeds to remain attached
the millions of people in tropical countries who depend would interfere with the plant’s ability to self-seed in the
on bananas and related plantains for daily sustenance, the wild. By selecting for traits that would be disadvantageous
destruction of those crops would be much more serious. in nature, humans created a plant dependent on humans for
Agriculture has transformed our planet and our species; its survival. The codependency between domestic plants and
without it, modern civilization would not be possible. people is a hallmark of agriculture.
But as the case of the banana shows, there are challenges In addition to selecting desirable traits in their crop
to overcome if we’re to continue feeding a growing plants, early agriculturalists altered the environment to
population. be favorable for those plants. They provided water, chose

595
 Domestication of wheat. Wild wheat (left) and modern domesticated wheat
(right). The seeds of wild wheat are protected by tough barbs (the long spikes), and
they fall easily from the plant, enhancing dispersal. The seeds of domesticated wheat
remain attached to the plant, making the grain easier to harvest. Sources: (left) Bob
Gibbons/FLPA/Science Source; (right) Nigel Cattlin/Science Source.

planting sites to ensure optimal sunlight, and removed contain a gene from the bacterium Bacillus thuringiensis (Bt)
nearby plants that would compete with the crops for vital that confers pest resistance.
resources such as water and nutrients. By changing the At a glance, it would seem that we’ve successfully taken
community structure and the availability of resources domesticated species under our control, and indeed, we’ve
to support growth and reproduction, humans allowed made great strides in increasing yields to feed an ever-growing
domesticated crops to thrive. population. But evolution never takes a break. Crop pests and
By the 1940s, scientists had developed industrially pathogens continue to evolve ways to evade our savviest plant
fixed nitrogenous fertilizers that significantly boosted plant breeders.
growth. They had also learned enough about genetics and The nature of modern agriculture only compounds the
DNA to begin applying that knowledge to plant breeding. problem. Most modern farms consist largely of monocultures,
These advances led to an amazingly productive period single types of crop each grown over a large area. Growing
during the 1960s and 1970s now known as the Green only a single crop at a time makes it much easier to mechanize
Revolution. During that time, plant yields ballooned. Food planting and harvesting. In a given field of corn or wheat,
production actually out-stripped population growth through therefore, the individual plants are often genetically quite
the second half of the twentieth century. similar to one another. With so little genetic variation, each
Today, we are entering a new phase of agriculture, plant is vulnerable to pathogens in exactly the same way. This
with genetic engineering being added to the traditional means that a pathogen that can overcome one plant’s natural
approaches of crossbreeding and artificial selection. defenses has the potential to wipe out the entire field.
Most of the corn and soy products now consumed in the Lately, many consumers are taking a hard look at these
United States come from plants that have been genetically problems and at the source of the food on their plates.
engineered to resist certain pests and herbicides. Some of Critics of modern agriculture argue that monocultures,
the genes employed come from different species altogether. with their heavy reliance on fungicides and pesticides,
For example, corn and cotton plants are often engineered to are unsustainable and unhealthy. Proponents of genetic

596
 modification and high-tech agriculture argue that we In many parts of the world, agricultural productivity
will need every tool to meet food-production demands is limited by the availability of water or nutrients
for the expanding human population. Sometime in 2011, or both. Global climate change is predicted to cause
the human population surpassed 7 billion people. The regional droughts and flooding that may further weaken
population continues to grow, expanding at its fastest rate the global food supply. There will be no easy solutions
at any time in human history. By 2100, experts predict, as we move forward into civilization’s next phase
our planet will be home to more than 9 billion people. One of agriculture. One thing that is certain, however, is
point that both sides agree on is that unless agricultural that understanding how plants grow, reproduce, and
productivity continues to increase, more and more land protect themselves from pests will help us survive in an
will be needed to produce food. increasingly crowded world.

? CASE 6 QUESTIONS
Special sections in Chapters 29 –34 discuss the following questions related to Case 6.

1. How has nitrogen availability influenced agricultural productivity? See page 616.
2. How did scientists increase crop yields during the Green Revolution? See page 637.
3. What is the developmental basis for the shorter stems of high-yielding rice and wheat?
See page 649.
4. Can modifying plants genetically protect crops from herbivores and pathogens?
See page 683.
5. What can be done to protect the genetic diversity of crop species? See page 708.
6. How do fungi threaten global wheat production? See page 734.

597
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 CHAPTER 29

Plant Structure
and Function
Moving Photosynthesis
onto Land

Core Concepts
29.1 The evolution of land plants
from aquatic ancestors
introduced a major challenge
for photosynthesis: acquiring
carbon dioxide without
drying out.
29.2 Leaves have a waxy cuticle and
stomata, both of which are
important in regulating carbon
dioxide gain and water loss.
29.3 Xylem allows vascular plants to
replace water evaporated from
leaves with water pulled from
the soil.
29.4 Phloem transports
carbohydrates to the non-
photosynthetic portions of the
plant for use in growth and
respiration.
29.5 Roots expend energy to obtain
nutrients from the soil.
Frank Krahmer/Getty Images.

599
600 SECTION 29.1 P L A N T S T RU C T U R E A N D F U N C T I O N : A N E VO LU T I O N A RY P E R S P E C T I V E

In the ocean, where life first evolved, organisms are bathed in

seawater that often matches the osmotic concentration of their FIG. 29.1 The phylogenetic tree of plants. Plants can be divided
cells. In contrast, organisms on land are surrounded by air and into two groups: the vascular plants, which actively
thus at risk of drying out. Excessive water loss, or desiccation, is control their hydration, and the bryophytes, which do not.
a constant challenge for life on land, especially for photosynthetic Sister group of
organisms, which expose large surface areas to the air in order to green algae
obtain sunlight and carbon dioxide (CO2). In Chapter 27, we saw
that land plants evolved from aquatic green algae. What structural
Liverworts
and functional innovations enabled plants to colonize the land so
successfully?
Mosses
The story begins about 470 million years ago, when a group of
green algae began the transition from water to land. There are few Hornworts
fossils to show us what the first land plants looked like. We know
they were small, at most only a few centimeters tall. These early Lycophytes Land
colonists resembled their algal ancestors; they had no means of plants
obtaining water from the soil and, at best, only a limited capacity Ferns and
Green algae horsetails
to restrict water loss from cells. Therefore, their water content
would have fluctuated wildly. Water is the medium for life, and Bryophytes Gymnosperms
metabolism is possible only when the water content of cells is
Vascular plants
high. Thus, these first land plants would have been able to carry Angiosperms
out photosynthesis only when the surface of the land was wet.
Today, descendants of those first land plants dominate
terrestrial habitats, having evolved the ability to draw water
More than 95% of all land plant species found today are
from the soil and limit water loss from their leaves. These are the
vascular plants, and they provide almost all the photosynthetic
vascular plants, and their evolution transformed the physical
output of terrestrial environments. The phylogenetic tree shows
and biological environment on land. Vascular plants can carry
that vascular plants form a monophyletic group, with four main
out photosynthesis even when the soil surface is dry and can
subgroups. Lycophytes and ferns and horsetails are mostly
sustain the water content of photosynthetic leaves elevated as
small plants, although these groups once included tall trees. The
much as 100 m into the air. In many ways, the capacity to control
gymnosperms include pine trees and other conifers, and the
the uptake and loss of water made possible the extraordinary
angiosperms, or flowering plants, include oak trees, grasses, and
evolutionary success of vascular plants.
sunflowers.
The evolution of vascular plants was a game-changing event
in the history of life. To appreciate why this is so, we have to
29.1 PLANT STRUCTURE AND understand the “game” played by the groups that preceded the
FUNCTION: AN EVOLUTIONARY vascular plants. In particular, how are bryophytes (the three
PERSPECTIVE groups of nonvascular plants) able to survive on land? Bryophytes
do not produce roots and are totally dependent upon surface water
We begin with a quick sketch of evolutionary relationships among to sustain the water content of their photosynthetic cells. As the
land plants that provides a road map to land plant evolution. We environment dries, so do they, and all metabolic activity ceases.
reserve detailed discussion of plant diversity until we have discussed All cells require a high degree of hydration to function.
how plants function. As bryophytes dry out, they lose the ability to carry out
photosynthesis. However, many bryophytes exhibit desiccation
Land plants are a monophyletic group that includes tolerance, a suite of biochemical traits that allows their cells to
vascular plants and bryophytes. survive extreme dehydration without damage to membranes
Fig. 29.1 shows that all land plants are descended from green or macromolecules (Fig. 29.2). Bryophytes can then resume
algae; thus, land plants form a monophyletic group because they photosynthesis when surface water is once again available. In
include all (and only) the descendants of a common ancestor. Land contrast to bryophytes, vascular plants keep their photosynthetic
plants can be divided into two types: vascular plants, which can cells hydrated with water drawn from the soil. As a result, vascular
pull water from the soil, and bryophytes, which cannot. The first plants can photosynthesize even when the surface of the soil is dry.
plant groups to diverge are bryophytes—namely, mosses, along Bryophytes provide insights into how the first plants may
with the less-familiar liverworts and hornworts. have met the challenge of carrying out photosynthesis on land.
 CHAPTER 29 P L A N T S T RU C T U R E A N D F U N C T I O N : M OV I N G P H OTO S Y N T H E S I S O N TO L A N D 601

Sheets of cells called the epidermis line the leaf’s upper

FIG. 29.2 Desiccation tolerance in bryophytes. Syntrichia princeps, and lower surfaces; loosely packed photosynthetic cells make
a moss, can withstand complete desiccation (right) but up the mesophyll (literally, “middle leaf”); and the system
regains photosynthetic capacity rapidly when rewetted of vascular conduits called veins connects the leaf to the rest
(left). Source: Brent Mishler and Michael Hamilton. of the plant (Fig. 29.4). We discuss how water and carbohydrates
are transported through the vascular system in sections 29.3 and
29.4. In this section, we explore how the structure of leaves allows
vascular plants to photosynthesize in air without drying out.

CO2 uptake results in water loss.

Water is the resource that most often limits a plant’s ability to
grow and function. We see this easily when we forget to water our
houseplants or garden. And when you drive across a continent, you
see that wet places look very different from dry ones. Water has
such a significant effect on the growth and functioning of plants
because plants use large amounts of water. Why do plants require
so much water?
The answer is not water’s role as an electron donor in
photosynthesis (Chapter 8): That process accounts for less than 1%
of the water required by vascular plants. Instead, most of a plant’s
need for water arises as a consequence of CO2 uptake from the air.
As we saw in Chapter 25, CO2 is a minor constituent of air, at
present about 400 parts per million. This low concentration limits
the rate at which CO2 can diffuse into the leaf. Therefore, how fast a
plant can take up CO2 depends in large part on the degree to which
leaves can expose their photosynthetic cells to the surrounding
The ability to survive desiccation is likely to have been critical.
air. If we look inside a leaf, we see that each mesophyll cell is
But we must avoid the temptation to think of living bryophytes as
surrounded by air spaces. The mesophyll cells obtain the CO2 that
representative of the ancestral state of all land plants. The diversity
they need for photosynthesis directly from these air spaces. If the air
of bryophytes that we encounter today has been shaped by their
spaces within the leaf were completely sealed off, photosynthesis
long coexistence with vascular plants. Indeed, bryophytes thrive
in the shady, moist understory of forests, but only in places where
roots would not provide a significant advantage—on rocks and the
trunks of trees, for example, and in regions too dry or too cold for FIG. 29.3 Major organs of a vascular plant.
roots to penetrate the soil.
We return to bryophytes when we examine plant reproduction
Reproductive
(Chapter 30) and diversity (Chapter 33). For now, however, let’s organs
focus on the vascular plants.

29.2 THE LEAF: ACQUIRING CO 2 Leaf

Shoot
WHILE AVOIDING DESICCATION
Fig. 29.3 shows the general features of a vascular plant.
Stem
Aboveground, we see three major types of organ—leaves, stems,
and reproductive organs—which collectively form the shoot.
Roots, which are generally hidden from view in the ground, make
up the fourth major organ system. We can answer the question of
how vascular plants photosynthesize on land by examining how Roots
leaves, stems, and roots work together.
The leaf is the principal site of photosynthesis in vascular
plants. A cross section shows the leaf’s three major tissues:
602 SECTION 29.2 T H E L E A F : AC Q U I R I N G CO 2 W H I L E AVO I D I N G D E S I CC AT I O N

FIG. 29.4 Leaf structure. The uptake of CO2 by diffusion is accompanied by a much
larger outward diffusion of water vapor.

Cuticle
Upper
epidermis Mesophyll cells near the
upper surface of the leaf have
a columnar arrangement that
maximizes light interception.

Mesophyll ~200 ppm ~40,000 ppm

cells Mesophyll cells close to the

Water vapor gradient

CO2 stomata have a honeycomb-
like arrangement to allow CO2

CO2 gradient
to spread easily throughout
Lower the leaf.
epidermis
Vein
Cuticle
Guard cell
Stoma H2O
~400
400 ppm ~20,000
~20
20 000 ppm

The concentration difference driving the diffusion of CO2 into

the leaf is typically 100 times smaller than the concentration
difference driving the diffusion of water vapor out of the leaf.

would quickly run out of CO2 and come to a halt. But they’re not plants can sustain such high rates of water loss because they can
sealed off—the leaf’s air spaces are connected to the air surrounding access the only consistently available source of water on land: the
the leaf by pores in the epidermis. As photosynthesis lowers the soil. Water moves from the soil, through the bodies of vascular
concentration of CO2 molecules within the leaf’s air spaces relative plants, and then, as water vapor, into the atmosphere. Therefore,
to the concentration of CO2 in the outside air, the difference in the challenge of keeping photosynthetic cells hydrated is met
concentration between the inside and the outside of the leaf causes partly by the continual supply of water from the soil. As we discuss
CO2 to diffuse into the leaf, replenishing the supply of CO2 for next, it is also met by limiting the rate at which water already in
photosynthesis. leaf tissues is lost to the atmosphere.
The ability of leaves to draw in CO2 comes at a price: If CO2
j Quick Check 1 Why do plants transpire?
can diffuse into the leaf, water vapor can diffuse out (Fig. 29.4).
Furthermore, water vapor diffuses out of a leaf at a much faster
rate than CO2 diffuses inward. Molecules diffuse from regions of The cuticle restricts water loss from leaves but inhibits
higher concentration to regions of lower concentration, and the the uptake of CO2.
rate of diffusion is proportional to the difference in concentration. Epidermal cells secrete a waxy cuticle on their outer surface
On a sunny summer day, the difference in water vapor that limits water loss. Without a cuticle, the humidity within
concentration between the air spaces within a leaf and the air the internal air spaces would drop and the photosynthetic
outside can be more than 100 times greater than the difference in mesophyll cells would dry out. However, the cuticle prevents CO2
concentration of CO2. Add the fact that water is lighter than CO2, from diffusing into the leaf even as it restricts water vapor from
and so diffuses 1.6 times faster for the same concentration gradient, diffusing out of it.
and it becomes clear why, on a sunny summer day, several hundred As noted above, small pores in the epidermis allow CO2 to
water molecules are lost for every molecule of CO2 acquired for diffuse into the leaf (Fig. 29.5a). These pores are called stomata
photosynthesis. (singular, stoma). Stomata can be numerous—there can be
The loss of water vapor from leaves is referred to as hundreds per square millimeter. Yet because each one is small, less
transpiration, and the rates at which plants transpire are often than 1% to 2% of the leaf surface is actually covered by these pores.
quite high. A sunflower leaf, for example, can transpire an amount Thus, even with stomata, the epidermis is a significant barrier to
equal to its total water content in as little as 20 minutes. To put the diffusion of CO2 and water vapor.
this rate of water loss in perspective, you would have to drink The epidermis with its stomatal pores represents a
about 2 liters per minute to survive a similar rate of loss. Vascular compromise between the challenges of providing food
 CHAPTER 29 P L A N T S T RU C T U R E A N D F U N C T I O N : M OV I N G P H OTO S Y N T H E S I S O N TO L A N D 603

Cellulose consists of long molecules that provide strength to

FIG. 29.5 Stomata. (a) A scanning electron microscope (SEM)
plant cell walls. In guard cells, the cellulose molecules are oriented
image of stomata on a leaf surface; (b) guard cells of a
radially—that is, wrapped around the cell. This makes it easier for
stoma in open and closed positions. Photo source: Dr. Jeremy
swelling guard cells to expand in length rather than in girth. Because
Burgess/Science Source.
the guard cells are firmly connected at their ends, this increase
a.
in length causes them to bow apart, opening the stomatal pore.
Conversely, a decrease in guard cell volume causes the pore to close.
Guard cells control their volume by altering the concentration
of solutes, such as potassium ions (K1) and chloride ions (Cl2), in
their cytoplasm. ATP drives the uptake of solutes across the plasma
membrane against their concentration gradient. An increase in
solute concentration causes water to flow into the cell by osmosis,
whereas a decrease in solute concentration causes water to flow
out of the cell. Recall from Chapter 5 that osmosis is the diffusion
of water molecules across a selectively permeable membrane,
such as the cell’s plasma membrane, from a region of higher water
concentration to a region of lower water concentration. An increase
in the concentration of solutes in guard cells is accompanied by a
corresponding decrease in the concentration of water molecules.
Thus, as guard cells add solutes, water diffuses into the cells, causing
b.
them to swell.
Solutes H2O Solutes H2O Stomata close by the same process, only in reverse. Solutes leave
the guard cells, and as the solute concentration decreases, water
diffuses out of the cells, causing them to shrink.
To function effectively, stomata must open and close in
Guard
cell Guard cells response to both CO2 and water loss. Stomata are thus key sites
shrink. for the processing of physiological information. Light stimulates
stomata to open, while high levels of CO2 inside the leaf (a signal
Radially
oriented Guard cells that CO2 is being supplied faster than photosynthesis can take it up)
cellulose swell. cause stomata to close. Guard cell volume also changes in response
molecules
to signaling molecules. For example, abscisic acid, a hormone
Open, high-volume state Closed, low-volume state produced during drought, causes stomata to close. Thus, stomata
open when the conditions for photosynthesis are favorable, and
Uptake of solutes by guard Release of solutes
cells causes water to be causes water to close when a water shortage endangers the hydration of the leaf.
drawn in by osmosis. As the flow out of the Our own experience leads us to think of evaporation (for
guard cells swell, they bow guard cells, closing
apart, opening the stoma. the stoma. example, of sweat) as a means of preventing overheating. Is there
any evidence that plants control the temperature of their leaves
by regulating transpiration? The answer, perhaps surprisingly,
is no. Although transpiration does cool leaves, plants transpire
whether their leaves are too hot or too cold, or at their optimal
(CO2 uptake) and preventing thirst (water loss). However, because temperature. When leaves are above their optimal temperature for
both the dryness of the air and the wetness of the soil are variable, photosynthesis, cooling through transpiration is beneficial. But
leaves must be able to alter the porosity of the leaf epidermis when temperatures are below the optimum, transpiration results
to maintain a balance between the rates of water loss to the in leaf temperatures that are even less favorable. In addition,
atmosphere and water delivery from the soil. They can do this by evaporative cooling is most beneficial in hot environments, yet
opening and closing their stomata. often in these environments little water is available in the soil. For
this reason, plants in arid regions tend to rely on mechanisms other
Stomata allow leaves to regulate water loss than evaporative cooling, such as reflective hairs and waxes and
and carbon gain. small leaf size, to prevent their leaves from overheating.
Stomata are more than just holes in the leaf epidermis; they are
hydromechanical valves that can open and close. Each stoma CAM plants use nocturnal CO2 storage to avoid water
consists of two guard cells surrounding a central pore. The guard loss during the day.
cells can shrink or swell, changing the size of the pore between the In most plants, water loss and photosynthesis peak at the same
guard cells (Fig. 29.5b). How does this valve system work? time, when the sun is high overhead. What if a plant could open
604 SECTION 29.2 T H E L E A F : AC Q U I R I N G CO 2 W H I L E AVO I D I N G D E S I CC AT I O N

its stomata to capture CO2 at night, when cool air limits rates of By opening stomata only at night, CAM plants greatly increase
evaporation? the amount of CO2 gain per unit of water loss. Indeed, the
In fact, a number of plants have evolved precisely this CO2 : H2O exchange ratio for CAM plants is in the range of 1 : 50,
mechanism, called crassulacean acid metabolism, or CAM, to nearly 10 times higher than that of plants that open their stomata
help balance CO2 gain and water loss (Fig. 29.6). CAM (named after during the day. However, CAM has a drawback. The carbohydrate
the Crassulaceae family of plants, which uses this pathway) provides production by CAM photosynthesis is slower because ATP is
a system for storing CO2 overnight by converting it into a form that needed to drive the uptake of organic acids into the vacuole and
will not diffuse away. The storage form of CO2 is produced by the because only so much organic acid can accumulate overnight in
activity of the enzyme PEP carboxylase, which combines a dissolved the vacuole. CAM is therefore most common in habitats such
form of CO2 (bicarbonate ion, HCO32) with a 3-carbon compound as deserts, where water conservation is crucial, and among
called phosphoenol pyruvate (PEP). The resulting product is a epiphytes, plants that grow on the branches of other plants,
4-carbon organic acid that is stored in the cell’s vacuole. without contact with the soil.
When the sun comes up the next morning, the stomata close, Because CAM uses enzymes (for example, PEP carboxylase)
conserving water. At the same time, the 4-carbon organic acids and compartments (for example, vacuoles) that exist in all plant
are retrieved from the vacuole and transferred to the chloroplasts. cells, it is not surprising that it has evolved multiple times. CAM
The 4-carbon organic acids are then decarboxylated—that is, their occurs in all four groups of vascular plants shown in Fig. 29.1. It
CO2 is released. Because the stomata are now closed, this newly is most widespread in the angiosperms, occurring in 5% to 10%
released CO2 does not escape from the leaf. Instead, it becomes of species. Vanilla orchids and Spanish moss, two well-known
incorporated into carbohydrates by the Calvin cycle. As we saw in epiphytes, as well as many cacti, are CAM plants.
Chapter 8, the Calvin cycle requires a continual supply of energy
in the form of ATP and NADPH produced by the light reactions C4 plants suppress photorespiration by concentrating
of photosynthesis and so can operate only in the light. The CO2 in bundle-sheath cells.
3-carbon PEP molecules are converted into starch and stored in the Photorespiration adds another wrinkle to the challenge of
chloroplast until the sun goes down and they are again needed to acquiring CO2 from the air. Recall from Chapter 8 that either
capture and store CO2 in the vacuole. CO2 or O2 can be a substrate for rubisco, the key enzyme in the

FIG. 29.6 CAM photosynthesis. CAM plants store CO2 at night in the vacuole; leaves can then photosynthesize during the day without opening
their stomata.

Night Day

4-carbon 4-carbon
organic acid Vacuole organic acid
ATP
ADP + Pi
HCO3–
4-carbon
4-carbon PEP
carboxylase organic acid
organic acid
PEP
Rubisco
Carbohydrates
CO2

Chloroplast Starch Calvin cycle Starch PEP

(inactive
because of
lack of light)

CO2 Mesophyll cell

Night, Day,
stomata stomata
open closed
H 2O Stoma
 CHAPTER 29 P L A N T S T RU C T U R E A N D F U N C T I O N : M OV I N G P H OTO S Y N T H E S I S O N TO L A N D 605

FIG. 29.7 C4 photosynthesis. C4 plants suppress photorespiration by concentrating CO2 in bundle-sheath cells. Photo source: Ken Wagner/Phototake.

Bundle-sheath cell
Mesophyll cell

PEP 4-C
carboxylase
Rubisco Leaf vein
CO2 -
HCO3
CO2 Calvin cycle
PEP C4 cycle
Carbohydrates

ADP
3-C
Chloroplast ATP

Bundle-
Bundle- sheath
sheath Chloroplasts cells
cells
Cuticle
Upper
epidermis

Mesophyll
CO2 cells

Lower
Vein epidermis
Stoma Cuticle Stoma Vein
Guard cell
H2O

Calvin cycle. When CO2 is the substrate, the Calvin cycle produces use PEP carboxylase to produce 4-carbon organic acids that
carbohydrates through photosynthesis. When O2 is the substrate, subsequently supply the Calvin cycle with CO2. The Calvin cycle
there is a net loss of energy and a release of CO2, the process produces 3-carbon compounds (Chapter 8), and plants that do not
called photorespiration. (Photorespiration is similar to aerobic use 4-carbon organic acids to supply the Calvin cycle with CO2 are
respiration only in the sense that it uses O2 and releases CO2. thus referred to as C3 plants.
However, plants do not gain energy; they lose it.) Both CAM and C4 plants produce 4-carbon organic acids as
Photorespiration presents a significant challenge for land the entry point for photosynthesis. However, in CAM plants, CO2
plants for two reasons. The first is that air contains approximately capture and the Calvin cycle take place at different times; in C4
21% O2 but only 0.04% CO2. Although rubisco reacts more readily plants, they take place in different cells.
with CO2 than with O2, the sheer abundance of O2 means that C4 plants initially capture CO2 in mesophyll cells by means
rubisco uses O2 as a substrate some of the time. The second reason of PEP carboxylase, which combines a dissolved form of CO2
is that air provides much less of a thermal buffer than does water, (bicarbonate ion, HCO3⫺) with the 3-carbon compound PEP.
such that organisms on land experience higher and more variable This produces 4-carbon organic acids that diffuse through
temperatures than do organisms that live in water. Temperature plasmodesmata into the bundle sheath (Fig. 29.7), a cylinder of
has a major effect on photorespiration because the selectivity of cells that surrounds each vein. Once inside bundle-sheath cells,
rubisco for CO2 over O2 is reduced as temperatures increase. At the 4-carbon compounds are decarboxylated, releasing CO2 that
moderate leaf temperatures, O2 is the substrate instead of CO2 as is then incorporated into carbohydrates through the Calvin cycle
often as 1 time out of 4. At higher temperatures, O2 is even more (Chapter 8). The C4 cycle is completed as the 3-carbon molecules
likely to be the substrate for rubisco. generated during decarboxylation diffuse back to the chloroplasts
Some plants have evolved a way to reduce the energy and in the mesophyll cells, where ATP is used to re-form PEP.
carbon losses associated with photorespiration. These are the The significance of the C4 cycle is that it operates much faster
C4 plants, which suppress photorespiration by increasing the than the Calvin cycle because of the very low catalytic rate of
concentration of CO2 in the immediate vicinity of rubisco. C4 rubisco (Chapter 8). As a result, the concentration of CO2 within
plants take their name from the fact that they, like CAM plants, bundle-sheath cells builds up, reaching levels as much as five times
 higher than in the air surrounding the leaf. The
HOW DO WE KNOW? high concentration of CO2 in bundle-sheath
cells makes it unlikely that rubisco will use O2
as a substrate. Thus, the C4 cycle functions like
FIG. 29.8
a “fuel-injection” system that increases the
efficiency of the Calvin cycle in bundle sheath
How do we know that C4 cells.
C4 plants have high rates of photosynthesis
photosynthesis suppresses because they do not suffer the losses in
energy and reduced carbon associated with
photorespiration? photorespiration (Fig. 29.8). At the same
time, C4 plants lose less water because they
BACKGROUND Studies using radioactively labeled CO2 showed can restrict diffusion through their stomata
that some species initially incorporate CO2 into 4-carbon to a greater extent than a C3 plant while still
compounds instead of the 3-carbon compounds that are the first maintaining high concentrations of CO2 in
products in the Calvin cycle. These C4 plants also have high rates bundle-sheath cells. However, C4 photosynthesis
of photosynthesis. Is this a new, more efficient photosynthetic has a greater energy requirement than
pathway? Or do C4 plants have high rates of photosynthesis because conventional (C3) photosynthesis, as ATP must
they are able to avoid the carbon and energy losses associated with be used to regenerate PEP in the C4 cycle.
photorespiration? Thus, C4 photosynthesis confers an advantage
in hot, sunny environments where rates of
HYPOTHESIS C4 plants do not exhibit photorespiration.
photorespiration and transpiration would
METHOD “Air tests,” as these experiments were first called, otherwise be high. C4 photosynthesis has evolved
compared rates of photosynthesis in normal air (21% O2) and in as many as 20 times, but is most common among
an experimental gas mixture in which the concentration of O2 is tropical grasses and plants of open habitats with
only 1%. When the concentration of O2 is low, rubisco has a low warm temperatures. C4 plants include a number
probability of using O2 (instead of CO2) as a substrate, and thus of important crops, including maize (corn),
photorespiration does not occur. sugarcane, and sorghum, as well as some of the
most noxious agricultural weeds.
RESULTS
j Quick Check 2 How does the formation of
21% O2 4-carbon organic acids increase the efficiency of
Photosynthetic rate

The rate of
1% O2 photosynthesis water use in both CAM and C4 plants?
21% 1% is different in
O2 O2 21% O2 and 1%
O2 in C3 plants,
but not in C4
plants. 29.3 THE STEM:TRANSPORT
C3 plant C4 plantt OF WATER THROUGH
XYLEM
CONCLUSION Photosynthesis in C4 plants is not affected by
differences in O4 concentration, indicating that significant On a summer day, a tree can transport many
photorespiration is not occurring in these plants. In contrast, hundreds of liters of water from the soil to its
the photosynthetic rate of the C3 plants increased in the low O4 leaves. Impressively, this feat is accomplished
environment, indicating that photorespiration depresses rates of without any moving parts. Even more remarkably,
photosynthesis in 21% O4. trees and other plants transport water without
any direct expenditure of energy. The structure of
FOLLOW-UP WORK The higher photosynthetic efficiency of
vascular plants allows them to use the evaporation
C4 photosynthesis has prompted efforts, so far unsuccessful, to
of water from leaves to pull water from the soil.
incorporate this pathway into C3 crops such as rice.
Fig. 29.9 shows the cross section of a
sunflower stem. Like a leaf, it has a surface layer
SOURCE Bjorkman, O., and J. Berry. 1973. “High-Efficiency Photosynthesis.” of epidermal cells. This layer encloses thin-
Scientific American 229:80–93. walled, undifferentiated parenchyma cells in
the interior. Notice that the stem also contains
differentiated tissues that lie in a ring near the

606
 CHAPTER 29 P L A N T S T RU C T U R E A N D F U N C T I O N : M OV I N G P H OTO S Y N T H E S I S O N TO L A N D 607

The walls of xylem conduits are thick and contain lignin, a

FIG. 29.9 Xylem and phloem. A cross section of the stem of a chemical compound that increases mechanical strength and also
sunflower shows the two vascular tissues, xylem and prevents water from passing through the cell wall. Water enters
phloem. Photo source: Garry DeLong/Science Source. and exits xylem conduits through pits, circular or ovoid regions
where the lignified cell wall layer was not produced. Instead, pits
contain only the thin, water-permeable cell wall that surrounded
each cell as it grew. As we will see, pits play an important role in
xylem because they allow the passage of water, but not air, from
one conduit to another.
A xylem conduit can be formed from a single cell or from
Xylem Phloem multiple cells stacked to form a hollow tube. Unicellular
Epidermis conduits are called tracheids (Fig. 29.10a), and multicellular
conduits are called vessels (Fig. 29.10b). Because tracheids are the
product of a single cell, they are typically 5–50 mm in diameter and
less than 1 cm long. Vessels, which are made up of many cells called
vessel elements, can be much wider and longer. Vessel diameters
range from 10–500 mm, and lengths can be up to several meters.
Water enters a tracheid through pits, travels upward through
the conduit interior, and then flows outward through other
Thin-walled
parenchyma cells

Thick-walled FIG. 29.10 Xylem structure. Xylem conduits are formed by

cells that add
hollow cells with stiff walls and no cell content or
mechanical
strength to membrane. (a) Single-celled tracheids and
the stem (b) multicellular vessels are interconnected by pits.
a. Tracheids

outside of the stem. These are the vascular tissues, which form a
continuous pathway that extends from near the tips of the roots,
through the stem, and into the network of veins within leaves.
The outer tissue, called phloem, transports carbohydrates from
leaves to the rest of the plant body. The inner tissue, called xylem,
transports water from the roots to the leaves. b. Multicellular vessels
In addition, both the xylem and the phloem transport dissolved
nutrients and signaling molecules, such as hormones, throughout
the plant. Importantly, the xylem is the pathway by which mineral
nutrients absorbed from the soil are transported to the aboveground Pits
portions of the plant. The total concentration of solutes in the
xylem, however, is very low (typically less than 0.01%). We thus
focus on the xylem as a means for transporting water from soil to Thick,
leaves and discuss the uptake of nutrients in section 29.5. lignified
walls

Xylem provides a low-resistance pathway for the

movement of water.
Vessel
Water travels with relative ease through xylem: It would take element
10 orders of magnitude greater force to drive water through living
parenchyma cells than it takes to drive the same amount of water
through the xylem. This ease of flow is explained by the structure of
the water-transporting cells within the xylem, which have cell walls
but lack any cytoplasm or membranes at maturity. Water thus flows
unimpeded through the hollow centers of these cells (Fig. 29.10).
 HOW DO WE KNOW?

FIG. 29.11

How large are the forces that allow leaves to pull water
from the soil?
BACKGROUND The idea that water is pulled through the plant by through the branch when attached to the transpiring plant with
forces generated in leaves was first suggested in 1895. However, the flow rate generated by the vacuum pump.
without a way to measure these forces, there was no way to know
RESULTS Renner found that the flow rates generated by
how large they were.
transpiring plants were two to nine times greater than the flow
HYPOTHESIS In 1912, German physiologist Otto Renner rates generated by the vacuum pump.
hypothesized that leaves are able to exert stronger suctions than
CONCLUSION A transpiring plant pulls water through a
could be generated with a vacuum pump.
branch faster than a vacuum pump. Therefore, the pulling force
EXPERIMENT Renner measured the rate at which water flowed generated by a transpiring plant must be greater than that of
from a reservoir into the cut tip of a branch of a transpiring plant. He a vacuum pump. Because vacuum pumps create suctions by
next cut off about 10 cm of the branch and attached a vacuum pump reducing the pressure in the air, the maximum suction that a
in place of the transpiring plant. He then compared the flow rate vacuum pump can generate is 1 atmosphere. Thus, the maximum
height that one can lift water using a vacuum pump is 10 m
(33 feet). The forces that pull water through plants have no
comparable limit because they are generated within the partially
To
vacuum dehydrated cell wall. This is why plants can pull water from dry
pump soils and why they can grow to more than 100 m in height.

FOLLOW-UP WORK In 1965, Per Fredrick Scholander

developed a pressurization technique that uses reverse osmosis to
measure the magnitude of the suctions exerted by leaves of intact
Transpiring
Water flow Water flow plants. He used this technique to show that the leaves at the top
plant
of a Douglas fir tree 100 m tall exert greater pulling forces than
do leaves closer to the ground.

SOURCES Renner, O. 1925. “Zum Nachweis negativer Drucke im Gefässwasser

Flow rate

bewurzeler Holzgewachse.” Flora 118/119:402–408.; Scholander, P. F., et al.

1965. “Sap Pressure in Vascular Plants.” Science 148:339–346.

Transpiring Vacuum
plant pump

pits into an adjacent, partially overlapping, tracheid. Water also through longer conduits because the water does not need to
enters and exits a vessel through pits. Once the water is inside the flow so often from conduit to conduit across pits, which exert a
vessel, little or nothing blocks the flow of water from one vessel significant resistance to flow. Like the flow through pipes, water
element to the next. That is because during the development of flow through xylem conduits is also greater when the conduits
a xylem vessel, the end walls of the vessel elements are digested are wider. The flow is proportional to the radius of the conduit
away, allowing water to flow along the entire length of the vessel raised to the fourth power, so doubling the radius increases flow
without having to cross any pits. At the end of a vessel, however, sixteenfold. Because vessels are both longer and wider than
the water must flow through pits if it is to enter an adjacent vessel tracheids, plants with vessels achieve greater rates of water
and thereby continue its journey from the soil to the leaves. transport. Tracheids are the type of xylem conduit found in
The rate at which water moves through xylem depends on lycophytes, ferns and horsetails, and most gymnosperms, whereas
both the number of conduits and their size. Water flows faster vessels are the principal conduit in angiosperms.

608
 CHAPTER 29 P L A N T S T RU C T U R E A N D F U N C T I O N : M OV I N G P H OTO S Y N T H E S I S O N TO L A N D 609

Water is pulled through xylem by an evaporative moving water through the plant itself. To replace water lost by
pump. transpiration with water pulled from the soil, the leaves must
If you cut a plant’s roots off under water, the leaves continue to exert forces that are many times greater than the suctions that
transpire for some time. The persistence of transpiration when we can generate with a vacuum pump (Fig. 29.11). How do leaves
roots are absent demonstrates that the driving force for water exert this force?
transport is not generated in the roots, but instead comes from When stomata are open, water evaporates from the walls
the leaves. In essence, water is pulled through xylem from above of cells lining the intercellular air spaces of leaves. The partial
rather than being pushed from below. dehydration of the cell walls creates a force that pulls water toward
The forces pulling water upward through the plant are large. the sites of evaporation, much as water is drawn into a sponge.
Not only must these forces be able to lift water against gravity, This force is transmitted through the xylem, beginning in the leaf
they must also be able to pull water from the soil, which becomes veins, then down through the stem, and out through the roots to
increasingly difficult as the soil dries. In addition, they must the soil (Fig. 29.12). Water can be pulled through xylem because of
be able to overcome the physical resistance associated with the strong hydrogen bonds that form between water molecules.

FIG. 29.12 Xylem transport from roots to leaves. Water is pulled through the plant by forces generated in the leaves, preventing them from
drying out.

Mesophyll
cells

1 The evaporation
of water from leaves
Stoma causes water to flow
from the soil.

2 Hydrogen
bonds that form
Cell wall between water
molecules allow
water to be pulled
through the xylem.
Water
molecule

Xylem 3
Root The forces
hair that develop in
leaves must be
Water large enough to
flow overcome the
Soil capillary forces
particle in the soil.
610 SECTION 29.4 T H E S T E M : T R A N S P O RT O F C A R B O H Y D R AT E S T H RO U G H P H LO E M

Xylem transport is at risk of conduit collapse

FIG. 29.13 Collapse and cavitation. The forces exerted by and cavitation.
transpiring leaves can cause xylem conduits to collapse Water can be pulled through the xylem only if there is a continuous
inward or to cavitate, in both cases blocking flow. column of water that extends from the roots to the leaves. Thus,
a. Collapse b. Cavitation due c. Cavitation due to xylem conduits must be structured in such a way as to minimize two
to an air leak freeze and thaw distinct risks (Fig. 29.13). The first is the danger of collapse. If you
suck too hard on a drinking straw, it will collapse inward, blocking
Xylem conduit flow. Much the same thing can happen in xylem (Fig. 29.13a). To
filled with water prevent this, xylem has thick walls that contain lignin. Although
in metabolic terms lignin is more costly to produce than cellulose,
lignin makes conduit walls rigid, reducing the risk of collapse.
The second danger associated with pulling water through the
xylem is cavitation, the abrupt replacement of water by water
The tensile force Air leaks into Bubbles form as
of water pulls xylem conduit water freezes. vapor. Because cavitation disrupts the continuity of the water
the walls inward. through a pit. column, cavitated conduits can no longer transport water from the
soil. Cavitation can occur in either of two ways. First, an air bubble
Pits may be pulled through a pit because of lower pressure in the water
compared to the air (Fig. 29.13b). The bubble can then expand under
the tensile, or pulling, force exerted by leaves. The likelihood of
cavitation forming in this way increases during drought, when the
tensions in the xylem are large.
The second way that cavitation occurs in xylem conduits is by
gases coming out of solution during freezing (Fig. 29.13c). Gases
are much less soluble in ice than in water, so as a conduit freezes,
dissolved gas molecules aggregate into tiny bubbles that can cause
cavitation when the conduit thaws. The wide vessels found in
Thaw angiosperms are especially vulnerable to cavitation at freezing
temperatures, partly explaining why boreal (that is, subarctic)
forests contain few angiosperms.
The xylem is organized in ways that minimize the impact
of cavitation. For example, xylem consists of many conduits in
parallel, so the loss of any one conduit to cavitation does not result
in a major loss of transport capacity. Similarly, as water flows from
the soil to the leaves, it passes from one conduit to another, each
one of finite length. The likelihood that cavitation will spread is
thereby reduced because air can be pulled through pits only when
the tensile forces in the xylem are large.

j Quick Check 3 How is the statement that water is transported

through the xylem without requiring any input in energy by the plant
Collapse Cavitation Cavitation both correct and incorrect?

29.4 THE STEM: TRANSPORT

OF CARBOHYDRATES
THROUGH PHLOEM
Central to the success of vascular plants is their ability to pull
water from the soil. But to pull water from the soil, these plants
need roots, and these roots need to be able to grow belowground.
Without sunlight to power photosynthesis, growing roots depend
on carbohydrates imported from aboveground. Thus, vascular plants
develop a second transport tissue, the phloem, capable of moving
Collapsed xylem Air-filled xylem Air-filled xylem
carbohydrates efficiently across the entire length of the plant.
 CHAPTER 29 P L A N T S T RU C T U R E A N D F U N C T I O N : M OV I N G P H OTO S Y N T H E S I S O N TO L A N D 611

The phloem supports not only the growth of roots, but also the
development of non-photosynthetic organs such as reproductive FIG. 29.14 Phloem structure. Phloem conduits are formed of
structures and stems. living cells. (a) Sieve tubes and associated companion
cells and (b) scanning electron microscope image
Phloem transports carbohydrates from sources showing sieve plates of squash (Cucurbita maxima).
to sinks. Photo source: Michael Knoblauch.

Whereas the direction of xylem transport is typically from the soil

a.
to the leaves, phloem moves carbohydrates from wherever they
are synthesized or stored to wherever they are needed to support
growth and respiration. In plants, regions that produce or store
carbohydrates are referred to as sources. For example, leaves are
sources because they produce carbohydrates by photosynthesis,
and potato tubers (the part we eat) are sources because they
store carbohydrates that can be supplied to the rest of the plant
body when needed. Sinks are any portion of the plant that needs
carbohydrates to fuel growth and respiration—examples are roots, Phloem
cells
young leaves, and developing fruits. Thus, the direction of phloem
transport can be either up or down, depending on where the
source and sink are relative to each other.
In contrast to the xylem, phloem transport takes place
through living cells whose cytoplasm is rich in sugars, 10% to
30% by weight. For comparison, a can of regular Coke has a sugar
Sieve
content of approximately 10%. During development, the cells plate
through which sugars will flow lose much of their intracellular
structure, including the nucleus and the vacuole. At maturity, Nucleus
Sieve Plasmodesmata
phloem transport cells retain an intact plasma membrane tube
that encloses a modified cytoplasm containing only smooth
endoplasmic reticulum and a small number of organelles, Companion cell
including mitochondria. Cellular functions such as protein
synthesis are carried out by neighboring cells (called companion
cells in angiosperms) to which the phloem transport cells are
connected by numerous plasmodesmata.
Phloem transport cells are connected by pores, which allow
Smooth
sugars and other solutes to flow from one cell to another. Phloem endoplasmic
sap refers to the soluble components of the cytoplasm that can reticulum
pass through these pores. In angiosperms, sugar transport occurs Mitochondrion
though sieve tubes composed of cells that are connected by sieve Plasma
plates, which are modified end walls with large pores (1–1.5 µm in membrane

diameter) (Fig. 29.14). The plasma membrane of adjacent cells is Cell wall
continuous through each of these pores, so each multicellular sieve
tube can be considered a single cytoplasm-filled compartment.
Phloem transports carbohydrates as sucrose or larger
sugars. Phloem also transports amino acids, inorganic forms b.
of nitrogen, and ions including K1, which are present in much
lower concentrations. Finally, phloem transports informational
molecules such as hormones, protein signals, and even RNA.
Thus, phloem forms a multicellular highway for the movement of
raw materials and signaling molecules through the entire length
of the plant.

Carbohydrates are pushed through phloem by an

osmotic pump.
How does phloem transport sugars from source to sink? The
overall mechanism results from the buildup of sugars in sources
612 SECTION 29.5 T H E RO OT : U P TA K E O F WAT E R A N D N U T R I E N T S F RO M T H E S O I L

relative to a lower concentration of sugars in sinks. In sources, Like the xylem, the phloem is subject to risks that arise from
a high concentration of sugars in sieve tubes causes water to the way the flow is generated. The contents of damaged sieve
be drawn into the phloem by osmosis. Because the cell walls tubes can leak out, pushed by high turgor pressures in the phloem.
of the sieve tube resist being stretched outward, the turgor Damage is an ever-present danger because the sugar-rich phloem
pressure (Chapter 5) at the source end increases. At sinks, sugars is an attractive target for insects. Cell damage activates sealing
are transported out of the phloem into surrounding cells. This mechanisms that block flow through sieve plates and thus prevent
withdrawal of sugars causes water to leave the sieve tube, again phloem sap from leaking out. In some respects, these mechanisms
by osmosis, reducing turgor pressure at the sink end. It is the are comparable to the formation of blood clots in humans, except
difference in turgor pressure that drives the movement of phloem that phloem can seal itself much more rapidly, typically in less
sap from source to sink (Fig. 29.15). than a second.
In some plants, sugars are actively transported into sieve
j Quick Check 4 How is phloem able to transport carbohydrates
tubes using energy from ATP. In others, sugars diffuse passively
from the shoot to the roots, as well as from the roots to the shoot
from the sites of photosynthesis into the sieve tubes as the
(although not at the same time)?
sugar concentration builds up in photosynthesizing cells. These
mechanisms are quite effective. The pressures that develop in
sieve tubes are approximately 100 times the pressures that occur Phloem feeds both the plant and the rhizosphere.
in our own circulatory system. All the cells in a plant’s body contain mitochondria that carry out
The water that exits the phloem can remain within the sink respiration to provide a constant supply of ATP. Typically, about
or it can be carried back to the leaves in the xylem (Fig. 29.15). 50% of the carbohydrates produced by photosynthesis in one day is
The volume of water that moves through the phloem, however, is converted back to CO2 by respiration within 24 hours.
tiny compared to the amount that must be transported through Carbohydrates that are not immediately consumed in
the xylem to replace water lost by transpiration. Therefore, the respiration can be used as raw materials for growth, or they can be
number and size of xylem conduits greatly exceeds the number stored for later use. Carbohydrates stored within roots and stems as
and size of sieve tubes. starch, or in tubers (specialized storage organs such as potatoes), can
support new growth in the spring or following a period of drought.
Stored reserves can also be used to repair mechanical damage or
replace leaves consumed by insects or grazing mammals.
FIG. 29.15 Phloem transport from source to sink. Sources
What determines how carbohydrates become distributed
produce carbohydrates or supply them from storage,
within the plant? Phloem transport to reproductive organs
and sinks require carbohydrates for growth and
appears to have priority over transport to stems and developing
respiration.
leaves, and these have priority over transport to roots. In Chapter
31, we discuss the role that hormones play in controlling the
Companion
growth and development of plants and how these hormones may
cell
influence the ability of different sinks to compete successfully for
resources transported in the phloem.
Source Phloem also supplies carbohydrates to organisms outside the
(leaf cell) plant. A fraction of the carbohydrates transported to the roots spills
Sucrose out into the rhizosphere, the soil layer that surrounds actively
growing roots. This supply of carbohydrates stimulates the growth
Sieve tube of soil microbes. As a result, the density of microbial organisms
near roots is much greater than in the rest of the soil. These soil
bacteria decompose soil organic matter rich in nutrients such as
nitrogen and phosphorus. Thus, by releasing carbohydrates into the
Xylem Phloem soil, roots are thought to acquire more nutrients from the soil.

29.5 THE ROOT: UPTAKE OF WATER

AND NUTRIENTS FROM THE SOIL
Sink
(root cell)
As anyone who has dug a hole in a forest or a field can tell you,
plants make a lot of roots (Fig. 29.16). Laid end to end, the roots
Starch of a single corn plant would extend over 600 km. Why do plants
Water Carbohydrate
flow flow make such an extensive investment belowground? A major reason
 CHAPTER 29 P L A N T S T RU C T U R E A N D F U N C T I O N : M OV I N G P H OTO S Y N T H E S I S O N TO L A N D 613

is that, with the important exception of CO2 , everything that a

FIG. 29.16 Roots and root hairs. Extensive branching of roots vascular plant needs to build and sustain its body enters through
(a) and the presence of root hairs (b) create a large its roots.
surface area in contact with the soil. Sources: a. Topic Photo So far, we have focused on the importance of roots for
Agency/age fotostock; b. Photoshot/SuperStock. obtaining water from the soil to replace water lost to the
a b atmosphere as stomata open to allow CO2 to diffuse into the leaf.
However, to carry out photosynthesis, plants must acquire the
raw materials needed to produce the enzymes of the Calvin cycle,
as well as the components of the photosynthetic electron chain.
Without the substantial uptake of mineral nutrients from the soil,
plants would lack the ability to photosynthesize at rapid rates.
Thus, as vascular plants evolved roots to obtain water from the
soil, they evolved mechanisms for acquiring nutrients as well.

Plants obtain essential mineral nutrients from the soil

On average, plants are 85% water and only 15% dry material. About
96% of the dry material, by weight, is made up of three elements:
carbon, oxygen, and hydrogen. The remaining 4% is made up of
minerals, which are elements that come from the soil. Minerals
play essential roles in all metabolic and structural processes
(Table 29.1), despite making up only a small fraction of the plant’s
body. A deficiency in any one of them results in a plant that grows
poorly and has a low rate of photosynthesis.

TABLE 29.1 The mineral composition of plants.

CONCENTRATION
ELEMENT IN DRY MATTER (%) FUNCTIONS

Nutrients covalently bonded with carbon compounds

Nitrogen 1.5 Component of amino acids, nucleic acids, nucleotides, coenzymes
Phosphorus 0.2 Component of nucleic acids, nucleotides, coenzymes, phospholipids;
key role in reactions involving ATP
Sulfur 0.1 Component of three amino acids, coenzyme A, and other essential
organic compounds

Nutrients that remain in ionic form

Potassium 1.0 Cofactor for many enzymes, major cation in cell osmotic balance
Calcium 0.5 Cofactor for enzymes involved in hydrolysis of ATP and phospholipids,
second messenger in metabolic regulation, important structural role in cell walls
Magnesium 0.2 Enzyme cofactor, component of chlorophyll
Chlorine, manganese, sodium each ≤0.01 % Cofactor for enzymes involved in photosynthesis and other reactions

Nutrients involved in redox reactions

Iron, zinc, copper, nickel, each ≤0.01 % Component of enzymes that catalyze electron transfer in the electron
molybdenum transport chains used in cellular respiration and photosynthesis

Nutrients present in cell walls

Silica, boron, calcium each ≤0.01 % Contribute to the structural integrity of cell walls and defense against herbivores

Data from Table 5.2 in Lincoln Taiz et al., Plant Physiology and Development, 6th ed., 2015, Sinauer Associates, Inc.; Table 8.1 in Emmanuel
Epstein and Arnold J. Bloom, Mineral Nutrition of Plants: Principles and Perspectives, 2nd ed., 2004, Sinauer Associates, Inc.
614 SECTION 29.5 T H E RO OT : U P TA K E O F WAT E R A N D N U T R I E N T S F RO M T H E S O I L

The essential mineral nutrients can be divided up into four

groups. Nitrogen, phosphorus, and sulfur are components of FIG. 29.17 Water movement through endodermal cells. As water
many organic molecules, including amino acids, nucleic acids, carrying nutrients flows from the soil to the xylem of
and phospholipids. Another group of nutrients remain in an ionic roots, it must pass through the plasma membranes of
(charged) form and play important roles as enzyme cofactors and endodermal cells, which act as a selective barrier.
second messengers (for example, calcium) and in maintaining the
Water
osmotic balance of cells (for example, potassium). Magnesium ions movement
are critical for photosynthesis because they occur in chlorophyll.
Metal elements such as iron and zinc are primarily involved in
reactions that transfer electrons from one molecule to another.
The final group includes elements that contribute to the structural
integrity of the cell wall, such as calcium, silica, and boron.

Nutrient uptake by roots is highly selective. Endodermal Casparian

Nutrients dissolved in water from the soil enter roots and are cell strip
transported from the roots to the rest of the plant in the xylem.
Xylem, however, is never in direct contact with the soil. The living Root
cells that surround the xylem allow roots to exert a high degree hairs
of selectivity over which nutrients move from the soil into the Epidermis
xylem. Thus, the mineral nutrient composition of the water in the
Root cortex
xylem differs markedly from that of the water that can be easily
Endodermal
extracted from the soil. Some elements, for example nitrogen and cells
phosphorus, are more concentrated within the xylem, whereas Phloem
others, for example sodium and calcium, are more abundant in
Xylem
the soil solution. Given that water travels in a continuous pathway
from the soil to the leaves, how are roots able to concentrate some
nutrients and exclude others?
Like leaves and stems, roots have an outer cell layer of
epidermis (Fig. 29.17). Epidermal cells near the root tip produce
slender outgrowths known as root hairs. Root hairs greatly
increase the surface area of the root available for absorbing
nutrients. Inside the epidermis is the cortex, composed of
parenchyma cells. Depending on species, the cortex can vary in
thickness from a single cell to many cells. Xylem and phloem
are located at the center of the root and are surrounded by the
endodermis, a layer of cells that acts as a gatekeeper controlling
the movement of nutrients into the xylem.
Water flowing across the epidermis and the root cortex can
travel through the cells, crossing plasma membranes, and can
also flow through the relatively porous walls that surround each
cell. At the endodermis, flow through the cell wall is blocked by High concentrations of salts in the soil make it difficult for
the Casparian strip, a thin band of hydrophobic material that plants to obtain water. The reason is that to prevent salt from
encircles each cell. Water can gain access to the xylem only by building up to toxic levels in leaves, plants must prevent salt from
passing through the cytoplasm of the endodermal cells. entering the xylem. As a result, salt concentrations build up in
The Casparian strip allows the root to control what solutes and around roots. Plants must then pull harder to overcome the
enter the xylem. How does it do that? In order for solutes to osmotic effects of these salts if they are to obtain water from the
pass through the plasma membrane into the cytoplasm, they soil. Poor agricultural practices can lead to salt buildup in the soil
must be transported by ATP-powered protein complexes in the and cause crop yields to decline significantly.
membranes of endodermal cells. These transporters move only
certain compounds through the endodermis and actively exclude Nutrient uptake requires energy.
others. As a result, roots can concentrate nutrients that are needed Roots expend a great deal of energy acquiring nutrients from
in large amounts (for example, nitrogen) in the xylem and exclude the soil. We have already seen that energy is required to move
others that can damage plant cells (for example, sodium). nutrients across the cell membranes of the endodermis on their
 CHAPTER 29 P L A N T S T RU C T U R E A N D F U N C T I O N : M OV I N G P H OTO S Y N T H E S I S O N TO L A N D 615

journey to the xylem. Energy is also required to make specific produced by photosynthesis, and the fungus provides nutrients
nutrients in the soil accessible to the root. The mitochondria of they have obtained from the soil.
root cells must, therefore, respire at high rates to provide energy Mycorrhizae are of two main types (Fig. 29.18).
needed for nutrient uptake. Their high respiration rates explain Ectomycorrhizae produce a thick sheath of fungal cells that
why plants are harmed by flooding: Waterlogged soils slow the surrounds the root tip. Some of the fungus extends into the
diffusion of oxygen, limiting respiration. interior of the root, producing a dense network of filaments that
To understand how roots acquire nutrients and why this surrounds individual root cells. Endomycorrhizae do not form
requires energy, let’s first look at how nutrients move through the structures that are visible on the outside of the root. Instead, their
soil. Nutrients move by diffusion through the films of water that networks form highly branched structures, called arbuscules,
surround individual soil particles. In addition, when transpiration which grow inside root cells.
rates are high, the flow of water through the soil helps to convey Why do fungi enhance the ability of roots to obtain nutrients
nutrients toward the root. The active uptake of nutrients by root from the soil? One reason is that fungal networks can extend more
cells, a process that requires an expenditure of energy in the form than 10 cm from the root surface, greatly increasing the absorptive
of ATP, decreases nutrient concentrations at the root surface, surface area in contact with the soil. And because fungal filaments
creating a concentration gradient that drives diffusion from are so thin, they can penetrate tiny spaces between soil particles
the bulk soil to the root. However, because diffusion transports and thus access nutrients that would otherwise be unavailable to
compounds efficiently only over short distances, each root is roots. Another reason is that fungi secrete enzymes that make soil
able to acquire nutrients only from a small volume of
soil. This limitation explains why plants produce so
many roots and so many root hairs: Both structures
increase the volume of soil from which a root can obtain FIG. 29.18 Mycorrhizae. These fungus–root associations allow plants to use
nutrients. It also explains why roots elongate more carbohydrates to help gain nutrients from the soil.
or less continuously, exploiting new regions of soil to
obtain nutrients required for growth. Ectomycorrhizae Epidermis
Nutrients vary dramatically in the ease and speed
with which they move through the soil. For example, Root cortex
nitrate (NO32) and sulfate (SO422) are extremely mobile. Endoderm
In contrast, interactions with clay minerals keep zinc Phloem
and inorganic phosphate attached to soil particles. One Xylem
way that roots gain access to highly immobile nutrients Fungal
is by releasing protons or compounds that make the soil sheath
environment close to the root more acidic. This requires
energy in the form of ATP, but the decrease in pH
weakens the attachment of immobile nutrients to soil
particles. As a result, these formerly immobile nutrients Fungal cells surround but do not
are able to move by diffusion to the root. Fungal strands
penetrate root cells. Carbon and
nutrients are exchanged through
Soil microorganisms—both fungi and bacteria— extending from
the plasma membrane.
are more adept than plants at obtaining nutrients. the root

Fungi (Chapter 34) are particularly good at mobilizing

phosphorus and at decomposing organic matter, Endomycorrhizae
releasing nitrogen, while some bacteria can convert
gaseous nitrogen (N2) into a chemical form that can be Fungal cells
incorporated into proteins and nucleic acids (Chapter 26).
What plants have at their disposal is a renewable source Arbuscule
of carbohydrates. Using these carbohydrates, plants
support the growth of symbiotic fungi and bacteria, in
effect exchanging carbon for phosphorus or nitrogen.

Mycorrhizae enhance nutrient uptake.

Plants can increase their access to soil nutrients by
forming partnerships with fungi. Mycorrhizae are Fungal cells penetrate inside
symbioses between roots and fungi that enhance root cells, enhancing carbon
and nutrient exchange.
nutrient uptake: The plant provides carbohydrates
616 SECTION 29.5 T H E RO OT : U P TA K E O F WAT E R A N D N U T R I E N T S F RO M T H E S O I L

nutrients more available. Endomycorhizae are important in

enhancing phosphorus uptake, while ecotomycorrhizal fungi FIG. 29.19 Symbiosis between roots and nitrogen-fixing bacteria.
provide host plants with access to nitrogen that would be (a) Symbiotic root nodules on alfalfa; (b) the formation of a
otherwise unavailable. Both kinds of mycorrhizae may also root nodule. Photo source: Inga Spence/Science Source.
offer other benefits to plants: For example, mycorrhizal a.
roots may be less susceptible to infection by pathogens.
Mycorrhizal associations are costly to the plant,
consuming between 4% and 20% of its total carbohydrate
production. The plant has some control over whether or
not the roots become infected: When nutrients are readily
available, the percentage of mycorrhizal roots decreases.
Nevertheless, mutualistic associations between roots
and fungi are a characteristic feature of most groups of
plants. Mycorrhizal roots are thus the norm rather than
an exception. Fossils from more than 400 million years
ago show that interactions between plants and fungi are
ancient. Perhaps fungi helped plants gain a foothold on land. b.
Xylem Phloem Root
hair
Symbiotic nitrogen-fixing bacteria supply
nitrogen to both plants and ecosystems.
Nitrogen is the nutrient that plants must acquire in
greatest abundance from the soil, and in many ecosystems CHO

nitrogen is the nutrient that most limits growth. Nitrogen N

itself is not rare; 78% of Earth’s atmosphere is nitrogen
gas (N2). However, as discussed in Chapter 26, plants and
other eukaryotic organisms cannot use N2 directly. Only Nodule
certain bacteria and archaeons have the metabolic ability to Rhizobium
bacteria
convert N2 into a biologically useful compound in nitrogen
fixation. Nitrogen-fixing bacteria transform N2 into forms
such as ammonia (NH3) that can be used to build proteins, 1 Nitrogen-fixing bacteria 2 The bacteria take
nucleic acids, and other compounds. Some of these usable multiply outside the root up residence in a
forms of nitrogen are released into the soil, where they are in response to chemical root nodule formed
signals, then enter the by dividing root cells.
available to plants and other organisms. root through a root hair or
Although many nitrogen-fixing bacteria live freely break in the epidermis.
in the soil, some live in plant roots. When the level of
available soil nitrogen is low, some plants form symbiotic
relationships with nitrogen-fixing bacteria (Fig. 29.19). In
response to chemical signals transmitted between the root and
bacteria, bacteria multiply within the rhizosphere. At the same
time, root cells begin dividing, leading to the formation of a pea- symbiotic associations with nitrogen-fixing bacteria, but those
sized protrusion called a root nodule. Nitrogen-fixing bacteria that have—especially members of the legume (bean) family—
enter the root either through root hairs or through breaks in the provide an important way in which biologically available nitrogen
epidermis. They then move into the center of the nodule and enters into ecosystems. In addition, nitrogen fixation in legume
begin to fix nitrogen. Even though the bacteria appear to have roots helps to sustain fertility in many agricultural systems.
taken up residence within the cytoplasm of nodule cells, in fact
each bacterial cell is contained within a membrane-bound vesicle ? CASE 6 AGRICULTURE: FEEDING A GROWING POPULATION
produced by the host plant cell. How has nitrogen availability influenced agricultural
The plant supplies its nitrogen-fixing partners with productivity?
carbohydrates transported by the phloem and carries away the Harvesting crops removes nutrients—including nitrogen—from
products of nitrogen fixation in the xylem. As in mycorrhizal an ecosystem. These nutrients must be returned if the land is to
symbioses, the carbon cost to the plant is high: It is estimated remain fertile. In the early days of agriculture, farmers left fields
that as much as 25% of the plant’s total carbohydrate goes to unplanted for long periods between crops. During these periods,
nitrogen-fixing bacteria. Relatively few plant species have evolved nitrogen-fixing bacteria replenished soil nitrogen, either in
 CHAPTER 29 P L A N T S T RU C T U R E A N D F U N C T I O N : M OV I N G P H OTO S Y N T H E S I S O N TO L A N D 617

symbiotic association with roots or as free-living bacteria in accomplished only by prokaryotes. Like nitrogen fixation by
the soil. bacteria, the Haber–Bosch process requires large inputs of
As populations grew, fields could not be left unplanted for long energy; unlike nitrogen fixation by bacteria, it occurs only at
periods. Organic fertilizers such as crop residues, manure, and high temperatures and pressures. The prospect of an essentially
even human feces recycled some of the lost nitrogen, but unless unlimited supply of nitrogen in a form that plants can make use of
they are brought in from off the farm, they cannot replace all the has proved irresistible. Today industrial fertilizers produced by the
nitrogen removed during harvest. Haber–Bosch process account for more than 99% of fertilizer use.
Crop rotation has been widely practiced throughout the history Industrial fixation of nitrogen is approximately equal to the entire
of agriculture as a means of sustaining soil fertility. Every few years, rate of natural fixation of nitrogen, which is largely due to bacteria,
a field is planted with legumes harboring nitrogen-fixing bacteria. although a small amount of nitrogen is fixed by lightning.
Typically, the legume plants are not harvested, but instead are As we discuss in Chapter 49, human use of fertilizer is
treated as “green manure” that is allowed to decay and replenish soil altering the nitrogen cycle on a massive scale, posing a threat to
nitrogen. During the eighteenth and nineteenth centuries, legume biodiversity and the stability of natural ecosystems. Yet abundant
rotation contributed to the higher crop yields needed to support nitrogen fertilizers are a cornerstone of modern agriculture. The
increasing populations. Therefore, along with the mechanization high-yielding varieties of corn, wheat, and rice introduced in the
of agriculture, nitrogen-fixing symbioses contributed to the social mid-twentieth century achieve their high levels of growth and
conditions that fueled the Industrial Revolution. grain production only when abundantly supplied with nitrogen.
By the second half of the nineteenth century, more abundant The fact that plants such as legumes (including beans, soybeans,
and more concentrated supplies of nitrogen fertilizers were alfalfa, and clover) form symbiotic relationships with nitrogen-
sought. Mining of guano and sodium nitrate mineral deposits fixing bacteria suggests a possible way around this problem. If
helped meet the demand for the nitrogen, but with time these it were possible to engineer nitrogen fixation into crops such
natural nitrogen sources were depleted. A mechanism was needed as wheat and rice, either by inducing them to form symbiotic
to obtain nitrogen fertilizer from the vast storehouse of nitrogen relationships with nitrogen-fixing bacteria or by transferring the
in the atmosphere. genes that would enable them to fix nitrogen on their own, would
German chemist Fritz Haber developed an industrial that eliminate the need for nitrogen fertilizers? The answer is
method of fixing nitrogen in the years 1908–1911, and Karl yes—but with a caveat. As we have seen, nitrogen fixation requires
Bosch subsequently adapted it for industrial production. Their a large amount of energy, diverting resources that could otherwise
method allows humans to achieve what, until then, had been be used to support growth and reproduction. •

Core Concepts Summary The waxy cuticle on the outside of the epidermis slows rates of
water loss from leaves but also slows the diffusion of CO2 into
29.1 The evolution of land plants from aquatic leaves. page 602
ancestors introduced a major challenge for Stomata are pores in the epidermis that open and close, allowing
photosynthesis: acquiring carbon dioxide without CO2 to enter into the leaf and also allowing water vapor to
drying out. diffuse out of the leaf. page 602
Early-branching groups of land plants, commonly grouped CAM plants capture CO2 at night when evaporative rates are
as bryophytes, balance CO2 gain and water loss passively, low. During the day, they close their stomata and use this stored
photosynthesizing in wet conditions and tolerating desiccation CO2 to supply the Calvin cycle, resulting in increases in the
in dry ones. page 600 exchange ratio of CO2 and H2O. page 604

Vascular plants maintain the hydration of their photo- C4 plants suppress photorespiration by concentrating CO2 in
synthetic cells with water pulled from the soil. page 600 bundle-sheath cells. The buildup of CO2 in bundle-sheath cells
results from the rapid production of C4 compounds in the
29.2 Leaves have a waxy cuticle and stomata, both of mesophyll cells, which then diffuse into the bundle-sheath cells
which are important in regulating carbon dioxide gain and release CO2 that can be used in the Calvin cycle. page 605
and water loss.
29.3 Xylem allows vascular plants to replace water
The low concentration of CO2 in the atmosphere forces plants evaporated from leaves with water pulled from the soil.
to expose their photosynthetic cells directly to the air. The
outward diffusion of water vapor leads to a massive loss of Water flows through xylem conduits from the soil to the leaves.
water. page 601 page 607
618 SELF-ASSESSMENT

Xylem is formed from cells that lose all their cell contents as Plants exchange carbohydrates with symbiotic fungi in return
they mature. page 607 for assistance obtaining nutrients from the soil. page 615

Xylem conduits have thick, lignified cell walls. Water flows Plants supply carbohydrates to symbiotic nitrogen-fixing
into xylem conduits across small thin-walled regions called bacteria in exchange for available forms of nitrogen. page 616
pits. page 607
Agricultural productivity is closely linked to nitrogen supply.
Xylem conduits are of two types: tracheids and vessels. page 616
Tracheids are unicellular xylem conduits; vessels are formed
from many cells. page 607

The evaporation of water from leaves generates the driving

Self-Assessment
force for water movement in xylem. Because of the strong 1. Explain why vascular plants are better able to sustain
hydrogen bonds between water molecules, water can be pulled photosynthesis than are bryophytes.
from the soil and transported through xylem. page 609
2. Explain why transpiration is best explained as a
Pulling water through xylem creates the risk of mechanical consequence of acquiring CO2 from the atmosphere, as
failure, either by the collapse inward of conduit walls or by opposed to temperature regulation, nutrient transport, or the
cavitation in which a gas bubble expands to fill the entire use of water as a substrate in photosynthesis.
conduit. page 610
3. Draw and label the features of a leaf that are necessary
to maintain a well-hydrated interior while still allowing CO2
29.4 Phloem transports carbohydrates to the non- uptake from the atmosphere.
photosynthetic portions of the plant for use in growth
and respiration. 4. Diagram how the movement of solutes results in the
opening and closing of stomata.
Phloem transport occurs through living cells, which lack a
nucleus or vacuole at maturity. In angiosperms, these cells are 5. Diagram how the enzyme PEP carboxylase allows CAM
connected end to end by sieve plates, forming multicellular plants to reduce transpiration rates and C4 plants to suppress
sieve tubes. Sieve plates contain large pores that allow photorespiration.
phloem sap to flow from one sieve tube cell to another. 6. Contrast how transport is generated in xylem and in
page 611 phloem.
The active loading of solutes brings water into sieve tubes by 7. Describe the structure of xylem conduits and sieve tubes
osmosis, increasing the turgor pressure. At sites of use, the in relation to their function.
removal of solutes leads to an outflow of water and a drop in
8. Explain why phloem is necessary to produce roots.
turgor pressure. page 612
9. List four adaptations of roots that enhance the uptake of
The difference in turgor pressure drives the movement of
nutrients from the soil.
phloem sap from source to sink. page 612

Carbohydrates exuded from roots provide carbon and energy

Log in to to check your answers to the Self-
sources for the rhizosphere microbial community, stimulating
Assessment questions, and to access additional learning tools.
decomposition of organic matter and increasing nutrient
availability. page 612

29.5 Roots expend energy to obtain nutrients from

the soil.

The endodermis allows roots to control which materials enter

the xylem. The uptake of solutes by roots is therefore highly
selective. page 614
 CHAPTER 30

Plant
Reproduction
Finding Mates and Dispersing
Young

Core Concepts
30.1 Alternation of generations
evolved in plants by the
addition of a diploid
sporophyte generation that
allows plants to disperse spores
through the air.
30.2 Pollen allows the male
gametophyte to be transported
through the air, while
resources stored in seeds
support the growth and
development of the embryo.
30.3 Angiosperms (flowering
plants) attract and reward
animal pollinators, and they
provide resources for seeds
only after fertilization.
30.4 Many plants also reproduce
asexually.
JAPACK/amanaimagesRF/amanaimages/Corbis.

619
620 SECTION 30.1 A LT E R N AT I O N O F G E N E R AT I O N S

As plants moved onto land, the challenges of carrying out with the parent plant. Early-diverging groups of plants evolved the
photosynthesis in air were matched by the difficulties of capacity to disperse their offspring through the air, but they still
completing their life cycle. The algal ancestors of land plants required water for fertilization. How could these plants exist in both
relied on water currents to carry sperm to egg and to disperse water and air? They couldn’t—at least not at the same time. Instead,
their offspring. On land, the first plants were confronted with early land plants evolved a life cycle in which one generation, or
the challenge of moving gametes and offspring through air. Air is phase of the life cycle, released sperm into a moist environment
less buoyant than water, provides a poor buffer against changes and the following generation dispersed offspring through the air.
in temperature and ultraviolet radiation, and increases the risk of Thus, a pattern of alternating haploid and diploid generations,
drying out. As plants diversified on land, structures evolved that which occurs in all plants, is thought to have evolved as a means
allow gametes and offspring to survive being transported through of allowing fertilization in water while enhancing the dispersal of
air. For these structures to be useful, plant life cycles had to offspring on land.
undergo radical change. The modification of land plant life cycles
is a major theme in plant evolution. The algal sister groups of land plants have one
The gametes and offspring of many land plants are carried multicellular generation in their life cycle.
passively by the movement of the air (or in a few cases, water— Molecular sequence comparisons identify three groups of green
think coconuts). However, some plants evolved the capacity to algae as being closely related to land plants (Fig. 30.1). These close
harness animals as transport agents. In particular, the flowers relatives include Coleochaetales and Charales, two groups of green
and fruits of angiosperms influence animal behavior through algae that share a number of features with plants, including cellular
their colors, scents, and food resources. Animals attracted by the structures like plasmodesmata and developmental features like
food provided by flowers (or in a few cases, by the false promise apical growth (i.e, at the tip). Another group of green algae, the
of a mate) transport male gametes from one flower to another. microscopic Zygnematales, may also be closely related to plants,
Animals that eat fruits or inadvertently carry them attached to but this group has gone down a distinct evolutionary path and so
their fur are key agents of seed dispersal. As plants diversified on exhibits fewer plant-like features. The life cycles of all three groups,
land, coevolution with animals emerged as a second major theme however, share many features. Here we use Coleochaete and Chara to
in plant evolution. gain insights into the evolution of land plant life cycles. Coleochaete
is an inconspicuous green pincushion that grows on aquatic plants
or rocks in freshwater environments, and Chara is an erect alga
30.1 ALTERNATION OF GENERATIONS found along the margins of lakes and estuaries (Fig. 30.2).
In animals, the multicellular body consists of diploid (2n) cells.
Land plants are rooted in one place, yet must be able to disperse Coleochaete and Chara have a multicellular body as well, shown in
their offspring to minimize competition for space and resources Fig. 30.2, but it consists entirely of haploid (1n) cells. In animals,

FIG. 30.1 The phylogeny of land plants. This phylogenetic tree shows the evolutionary
trajectory of the environmental setting for fertilization and dispersal.

Fertilization Dispersal
Other green algae Water Water

Sister group of
Water Water
green algae

Liverworts Water (sperm) Air (spores)

Mosses Water (sperm) Air (spores)

Hornworts Water (sperm) Air (spores)

Lycophytes Land Water (sperm) Air (spores)

plants
Ferns and
Water (sperm) Air (spores)
horsetails
Green algae
Bryophytes Gymnosperms Air (pollen) Air (seeds)

Vascular plants Angiosperms Air (pollen) Air (seeds)

 CHAPTER 30 P L A N T R E P RO D U C T I O N : F I N D I N G M AT E S A N D D I S P E R S I N G YO U N G 621

the water, propelled by flagella, before

FIG. 30.2 Coleochaete (left) and Chara (right), two green algae that are closely related to settling in a new location. In Chara, the
land plants. Sources: (left) Dr. Ralf Wagner. email&lt;mikroskopie@dr-ralf-wagner.de&gt;/http://www. zygotes are carried by water currents
dr-ralf-wagner.de; (right) A. Jagel/age fotostock. until they settle and undergo meiosis.
In Chara and Coleochaete, both
100 µm 1 cm
fertilization and dispersal take place
in water. Living along the margins of
lakes and streams, however, Chara and
Coleochaete are occasionally left high and
dry by drought. Their photosynthetic
bodies shrivel quickly, but these algae
survive because their zygotes form
a protective wall that allows the cell
inside to tolerate exposure to air. When
water once again covers the resting
zygotes, they undergo meiosis and the
resulting haploid cells escape from
haploid gametes are formed by meiosis. In Coleochaete and Chara, their protective coat. This is the condition from which the more
haploid eggs and sperm are formed instead by mitosis. The egg is complicated life cycles of land plants evolved.
retained within the female reproductive organ and the sperm are
released into the water (Fig. 30.3). Fertilization results when one Bryophytes illustrate how the alternation of
egg and one sperm fuse to form a diploid zygote. The zygote then generations allows the dispersal of spores in the air.
undergoes meiosis to produce four haploid cells. These haploid cells In Chapter 29, we saw that bryophytes—liverworts, mosses, and
can develop into new multicellular (haploid) individuals, and thus hornworts—make up the earliest branches on the phylogenetic tree
they represent the beginning of the next generation. In Coleochaete, of land plants (see Fig. 30.1). They also share many reproductive
the haploid products of meiosis disperse by swimming through traits. Here, we use Polytrichum commune, a moss common in
forests and the edges of fields, to
illustrate how movement onto land was
accompanied by the evolution of two
FIG. 30.3 The life cycle of the green alga Chara corallina. The only diploid cell is the zygote. multicellular generations: one specialized
for fertilization and the other for
dispersal.
HAPLOID
(1n) Like Chara and Coleochaete,
Sperm are released Polytrichum has a photosynthetic
Following
into the water. fertilization, diploid body—the familiar green tufts observed
zygotes are on the forest floor—that is made of
Fertilization released and
haploid cells and that forms gametes by
dispersed by water
currents. mitosis. And like Chara and Coleochaete,
Polytrichum and other bryophytes release
swimming sperm into the environment.
The egg is
retained. For fertilization to occur, the sperm must
DIPLOID swim through films of water that coat
(2n)
moist surfaces to reach eggs retained
Diploid within reproductive organs.
Haploid zygote
photosynthetic Their reliance on external water
alga for fertilization helps explain why
One of the Dispersal
haploid cells
bryophytes produce their gametes near
develops into Haploid
the ground, where they are most likely to
a multicellular cells encounter a continuous film of water. It
haploid alga.
Meiosis The diploid zygote also helps explain why bryophytes tend to
undergoes meiosis be small, as sperm are able to swim only a
as its first division. couple of centimeters. Many bryophytes
release their sperm only when agitated by
622 SECTION 30.1 A LT E R N AT I O N O F G E N E R AT I O N S

branch of the phylogenetic tree. How

FIG. 30.4 The life cycle of the moss Polytrichum commune. The multicellular sporophyte might this new generation have made it
generation is a major innovation of land plants. Photo source: Malcolm Storey, 2010, www. possible for plants to colonize land?
bioimages.org.uk. To answer this question, let’s look
at the sporophyte of Polytrichum. Late
Sperm release The diploid zygote is in the growing season, the green tufts
is stimulated by retained, and the
of this moss sprout an extension—a
raindrops. sporophyte develops
Fertilization in place, supported small brown capsule at the end of a
mechanically and cylindrical stalk a few centimeters high
nutritionally by the
The egg is gametophyte. (Fig. 30.5a). This is the sporophyte,
retained. which originated from a fertilized egg.
Because the fertilized egg was retained
within the female reproductive organ,
HAPLOID DIPLOID
(1n) (2n) the sporophyte grows directly out of
Sporangium
Male Female Sporophyte the gametophyte’s body. Thus, the
Gametophytes Polytrichum specimen pictured in Fig.
30.5a shows both a gametophyte (green)
and several sporophytes (brown). The
Haploid gametophyte is photosynthetically
Meiosis
Dispersal spores self-sufficient, but the sporophytes must
obtain water and nutrients needed for
their growth from the gametophyte.
The significance of the multicellular
sporophyte is that it enhances the ability
Haploid spores are released into Sporopollenin The multicellular
the air. Those that land in a suitable wall sporophyte produces of plants to disperse on land. The capsule
site will germinate to form a new thousands of haploid at the top of the Polytrichum sporophyte
gametophyte generation. spores by meiosis.
is a sporangium, a structure in which
many thousands of diploid cells undergo
meiosis, producing huge numbers of
haploid spores. In contrast, fertilization
in Chara and Coleochaete is followed by
raindrops. Raindrops signal the presence of surface water, and their meiosis in only a single cell (the zygote).
impact can splash sperm farther than they could swim on their own. Spores are small and can be carried for thousands of kilometers
It is after the fusion of egg and sperm gives rise to a diploid by the wind. But their tiny size puts spores at risk of being washed
zygote that the life cycles of plants and their green algae relatives from the air by raindrops. The sporangia of many bryophytes
diverge. In plants, the zygote does not undergo meiosis, and release their spores only when the air is dry. In Polytrichum, the
it does not disperse. Instead, the zygote is retained within the sporangium has small openings from which it releases spores the
female reproductive organ, where it divides repeatedly by mitosis way a salt shaker releases salt. When the air is wet, the spores
to produce a new multicellular plant—this one made of diploid form clumps that are too big to fit through these opening. More
cells (Fig. 30.4). Some cells within this diploid plant eventually common is the presence of a ring of teethlike projections that
undergo meiosis, giving rise to haploid spores, cells that disperse curl over the top of the sporangia. These bend backward when air
and give rise to new haploid individuals. Because the diploid humidity is low, flinging the spores into the air (Fig. 30.5b).
multicellular plant produces spores, it is called the sporophyte, What protects the spores from drying out or from exposure
and this generation is called the sporophyte generation; because to damaging ultraviolet radiation as they travel through the air?
the haploid multicellular plant produces gametes, it is called the Earlier, we noted that the zygotes of Chara and Coleochaete secrete
gametophyte, and that generation is called the gametophyte a wall that protects the zygote within from desiccation. The wall
generation. The resulting life cycle, in which a haploid contains sporopollenin, a complex mixture of polymers that is
gametophyte generation and a diploid sporophyte generation remarkably resistant to environmental stresses such as ultraviolet
follow one after the other, is called alternation of generations, radiation and desiccation. In mosses and other land plants, it is the
and it describes the basic life cycle of all land plants. spore, not the zygote, that secretes sporopollenin. This suggests
Because land plants are descended from green algae with that early land plants retained a capacity present in their algal
only haploid gamete-producing individuals, we can infer that the ancestors—the ability to synthesize sporopollenin—but the
diploid sporophyte of land plants is an evolutionary novelty on this timing of gene expression changed in these plants.
 CHAPTER 30 P L A N T R E P RO D U C T I O N : F I N D I N G M AT E S A N D D I S P E R S I N G YO U N G 623

FIG. 30.5 Moss sporangia. (a) When spores of Polytrichum commune are mature, a cap falls off, allowing the salt shaker–like sporangium to
release the spores. (b) The sporangia of Homalothecium sericeum have a ring of teethlike projections that curl back as they dry, flinging
spores into the air. Sources: a. Keith Burdett/age footstock; b. Power and Syred/Science Source.

a b

Following dispersal, most spores germinate within a few habitats. The evolution of new structures to enhance dispersal on
days, but some can survive within the spore wall for months or land is a major theme in plant evolution.
even years. When conditions for growth are favorable, the spore
j Quick Check 1 In what ways are spores similar to gametes, and in
wall ruptures, and the haploid cell inside can then develop into a
what ways do spores and gametes differ?
multicellular and free-living gametophyte.

Dispersal enhances reproductive fitness in several Spore-dispersing vascular plants have free-living
ways. gametophytes and sporophytes.
With the evolution of the multicellular sporophyte generation, In Chapter 29, we saw that the evolution of vascular tissues
land plants were able to make many spores and disperse them allowed plants to grow tall. Xylem and phloem are present only
across large distances on land. Dispersal is one of the most in the sporophyte generation because spore dispersal is enhanced
important stages in a plant’s life cycle. To understand why, by height whereas gametophytes must remain small and
imagine what would happen if a plant’s spores (or seeds) simply close to the ground to increase the chances of fertilization. A
dropped straight down onto the ground below the parent plant. All major change in the plant life cycle is that in vascular plants
the plant’s offspring would then attempt to grow in a very small the sporophyte is the dominant generation both because the
space that might contain enough nutrients and light for only a few sporophyte is physically larger than the gametophyte and because
to establish themselves. In contrast, offspring dispersed by wind or its photosynthetic production is much higher than that of the
other means are more likely to settle in a spot where they will not gametophyte.
be forced to compete with closely related individuals for resources. The first two groups of vascular plants—the lycophytes
In addition, dispersal allows offspring to avoid pathogens and the ferns and horsetails—depend on swimming sperm for
and parasites. Viruses and bacteria travel easily from individual fertilization and disperse by spores that are released into the air
to individual in a densely packed population. An individual that (see Fig. 30.1). In these ways, their life cycle is similar to that of
settles away from the parent plant is less likely to encounter the bryophytes. However, the spore-dispersing vascular plants are
a pathogen. Finally, dispersal allows offspring to colonize new distinct from bryophytes in that both the gametophyte and the
624 SECTION 30.1 A LT E R N AT I O N O F G E N E R AT I O N S

FIG. 30.6 The life cycle of the bracken fern Pteridium aquilinum. This life cycle illustrates the alternation between a free-living gametophyte
generation that remains small and a free-living vascularized sporophyte generation that grows tall. Photo sources: (left) Biophoto Associates/
Science Source; (right) Jess Merrill/Alamy.

HAPLOID DIPLOID
(1n) (2n)

The sporophyte is
Sporophyte initially supplied
Sperm are released with nutrients by
when moisture is the gametophyte.
present. Fertilization

Vascular tissues (xylem and phloem)

Diploid sporophyte allow the sporophyte generation to
grow tall and become physiologically
independent of the gametophyte.

The egg is
retained.
Leaf
Gametophytes

Dispersal
Haploid gametophyte Haploid
spores
Sporangia are produced on
Spores are released into air. the lower surfaces of leaves.
Those that land in a suitable Meiosis
site will germinate and grow
into new gametophytes. Sporopollenin Underground stem
walls
Roots
Sporophyte

sporophyte generation are free-living, meaning that each is able to a weak link in the fern life cycle. In fact, because fern gametophytes
supply its own nourishment. tolerate drying out, they often live longer and withstand stressful
To illustrate this, let’s examine the life cycle of the bracken environmental conditions better than the much larger sporophytes.
fern Pteridium aquilinum. We begin with the largest component of As discussed in Chapter 29, desiccation tolerance is important for
the life cycle—the leafy diploid sporophyte generation (Fig. 30.6). plants that lack roots, which include both bryophytes and free-
If you examine a bracken leaf closely, you may find tiny brown living gametophytes.
packets along its lower margin. These are sporangia, and each Fern gametophytes typically produce either male or female
contains diploid cells that undergo meiosis to generate haploid gametes. The male gametes swim to the egg on a nearby
spores. The spores become covered by a thick wall containing gametophyte through a film of water. The union of a male and a
sporopollenin, making them well suited for dispersal through female gamete forms a diploid zygote, which develops in place and
the air. is supported by the gametophyte as it begins to grow. Eventually,
As the sporangia dry out, they break open and the spores are the developing sporophyte forms leaves and roots that allow it to
released. Most spores travel only short distances, but a few may become a physiologically independent, diploid plant.
end up far from their parent. Bracken ferns can be found on every Ferns release swimming sperm and thus are able to reproduce
continent except Antarctica, as well as on isolated oceanic islands, only when conditions are wet. That is true of the other spore-
testimony to the ability of spores to travel long distances. dispersing plants (lycophytes, horsetails, and bryophytes) as well.
Spores that land in a favorable location can germinate and grow What if a plant could eliminate the requirement for swimming
to form the haploid gametophyte generation. You have probably sperm? Seed plants did just that.
never seen a fern gametophyte—or never noticed one. Typically
of most ferns, the bracken gametophyte is less than 2 cm long and j Quick Check 2 In what ways is fern reproduction similar to moss
only one to a few cells thick (Fig. 30.6). The small size of the fern reproduction? In what ways is fern reproduction different from
gametophyte has led many to assume that the haploid generation is moss reproduction?
 CHAPTER 30 P L A N T R E P RO D U C T I O N : F I N D I N G M AT E S A N D D I S P E R S I N G YO U N G 625

protected by one or more layers of sporophyte tissue that surround

30.2 SEED PLANTS the sporangium wall. This entire structure, consisting of female
gametophyte, sporangium and protective outer layers, is the
Seed plants have the remarkable ability to bring male and female ovule. Ovules, if fertilized, develop into seeds.
gametes together when conditions are dry. How is this possible? The male gametophyte also develops while still attached to the
Standing beneath a pine tree on a spring day, you will see the sporophyte. Many cells within each sporangium undergo meiosis,
answer to this question all around you: tiny yellow grains that forming large numbers of haploid spores. Within each of these
cover the ground and everything else close to the tree. These spores, a male gametophyte develops, producing a handful of cells
are pollen grains, and they are produced by all seed plants. In by mitosis that all fit within the wall of the original spore. Pollen
fact, in seed plants we encounter two new structures: pollen consists of the multicellular male gametophyte and a surrounding
and seeds. Pollen is what liberates seed plants from the need to outer wall containing sporopollenin.
release swimming sperm into the environment, and seeds are The third step begins when pollen is shed. Pollination is
multicellular structures adapted for dispersing offspring. The the transport of pollen, either in the air or by an animal, from
evolution of pollen and seeds required far-reaching changes to the the sporangium where it was produced to a location near an
seed plant life cycle. ovule. Because the male gametophyte produces male gametes,
There are two groups of seed plants living today: pollination eliminates the need to release swimming sperm
gymnosperms, which include conifers such as pine trees, and into the environment. However, pollination alone is not
angiosperms, which are the flowering plants (Fig. 30.1). In this sufficient to ensure fertilization. The male gametophyte must
section, we examine aspects of the life cycle that are common germinate and grow through the tissues of the ovule to deliver
to all seed plants and use pine, a gymnosperm, to illustrate how the male gametes to the egg. For this reason, pollen should not be
seed plant reproduction occurs without releasing swimming thought of as flying sperm. The growth of the male gametophyte
sperm into the environment. In section 30.3, we examine aspects plays an essential role in bringing the male and female gametes
of angiosperm reproduction, such as flowers and fruits, which together.
distinguish their life cycle from other seed plants. The fourth and final step is the maturation of a fertilized
ovule into a seed. A seed consists of an embryo, stored resources,
The seed plant life cycle is distinguished by four and an outer, protective coat. Seeds increase the probability that
major steps. offspring will survive because their larger size allows the transport
Pollen and seeds are produced only by seed plants, taking on some of more resources that can be used to fuel the early growth of the
of the roles previously played by sperm and spores. To understand next generation.
these new structures and how they relate to the life cycles we have
already discussed, we consider four major steps that distinguish Pine trees illustrate how the transport of pollen in air
the life cycle of seed plants. allows fertilization to occur in the absence of external
The first step is the formation of two types of spore, each sources of water.
produced in separate sporangia. One type develops into a To illustrate how seed plants have freed themselves from a
male gametophyte and the other into a female gametophyte. dependence on surface moisture for fertilization, let’s look at the
Male gametophytes produce only male gametes, and female life cycle of a pine tree (Fig. 30.7).
gametophytes produce only female gametes. Although this In pines, as in all vascular plants, what we think of as “the plant”
condition is not unique to seed plants (it also occurs in a small is the diploid sporophyte. However, a significant part of the pine’s
number of spore-dispersing vascular plants), it had to have evolved life cycle—from the production of haploid spores to the growth
first or the rest of the modifications that result in the life cycle of the gametophyte from a spore, to the production of gametes,
exhibited by seed plants could not have evolved. and ultimately to fertilization—takes place within reproductive
The second step is unique to seed plants. Seed plant spores structures called cones. Pines develop two types of cone. Ovule
are not dispersed. Instead, gametophytes develop within their cones typically are located on upper branches and produce spores
sporangium and thus remain attached to the sporophyte. The that develop into female gametophytes, which produce female
significance of this is most apparent in the development of gametes. Pollen cones are commonly found in clusters near the tips
the female gametophyte. By developing in place, the female of branches lower in the tree; they produce spores that develop into
gametophyte can draw resources from the sporophyte to produce male gametophytes, which produce male gametes.
female gametes (eggs) and to store energy and raw materials Ovule cones contain sporangia, each one surrounded by a
that the embryo can make use of following dispersal. Within jacket of protective tissue. Within each sporangium, a single
each sporangium, only a single haploid spore develops into a cell undergoes meiosis, producing four haploid spores. Three of
female gametophyte, and this gametophyte continues growing these spores degrade, leaving a single functional spore inside the
until it fills the sporangium. The haploid female gametophyte is sporangium. This spore is not dispersed, but instead undergoes
626 SECTION 30.2 SEED PL ANTS

FIG. 30.7 The life cycle of the loblolly pine tree, Pinus taeda. Photo source: Charles O. Cecil/Alamy.

HAPLOID DIPLOID
Egg
(1n) Pollen is released into the air Multicellular female (2n)
and transported by wind; some gametophyte
land on ovule cones.
Before fertilization can occur, the
pollen must germinate to form a
pollen tube that grows to the
female gametophyte.
Pollination
Fertilization
Ovule
Haploid spores
Four Sporangium
cells (2n)
Pollen Seed
grain coat
Multicellular (2n)
A fertilized
male gametophyte Embryo ovule develops
Protective into a seed.
(2n)
Sporangium layers
(2n) Meiosis Female
gametophytes Female Seed dispersal
develop from gametophyte
Haploid (1n)
spores spores within
ovule cones.

Meiosis Winged
seeds

Germination

Male gametophytes
develop from spores
within pollen cones. Seeds that land in
a suitable site can
grow into new
sporophytes. Sporophyte

repeated mitoses to form a multicellular female gametophyte must germinate and the male gametophyte produce a pollen
consisting of a few thousand haploid cells, one or more of which tube that grows outward through an opening in the sporopollenin
differentiate as eggs. The female gametophyte remains attached to coat. In pines, pollination occurs as much as 15 months before
the sporophyte as part of the ovule fertilization. In fact, it is the arrival of the pollen that
Pollen cones are compact shoots with modified leaves that stimulates the development of the female gametophyte. When the
produce sporangia on their surface. Within the sporangia, many female gametes are ready to be fertilized, the male gametophyte’s
cells divide by meiosis to form haploid spores. The haploid spores pollen tube grows to the female gametophyte, attracted by chemical
then divide mitotically to form a multicellular male gametophyte signals. Two sperm, which lack flagella, travel down the pollen tube
inside the spore wall. In pine, the male gametophyte consists and one of them fuses with the egg to form a zygote. Fertilization
of only four cells at the time the pollen is shed from the parent in seed plants thus takes place without the male gametes ever being
plant. exposed to the external environment.
In pine, pollen is carried to the ovule by the wind. To reach the In seed plants, the relationship between sporophyte and
egg, however, and penetrate the tissues of the ovule, the pollen gametophyte has been reversed completely from that found in
 CHAPTER 30 P L A N T R E P RO D U C T I O N : F I N D I N G M AT E S A N D D I S P E R S I N G YO U N G 627

FIG. 30.8 Life-cycle evolution in land plants. A major trend in the evolution of land plants is a decrease in the size and independence of the
gametophyte generation and a corresponding increase in the prominence of the sporophyte generation.

Gametophyte Sporophyte
Sister group Photosynthetic, persistent Physically and physiologically
of green algae dependent on gametophyte
Liverworts
50 mm
Mosses 10 cm
Multicellular
sporophyte
Hornworts

Lycophytes
50 cm
Spore-dispersing 2 cm
Ferns and vascular plants
Vascularized sporophyte horsetails
generation
Gymnosperms 50 m
Green algae Seed plants < 0.5 cm
Pollen
Bryophytes Angiosperms Physically and physiologically
and seeds Photosynthetic, persistent
dependent on sporophyte
Vascular plants

bryophytes. The gametophyte, so prominent in mosses, is important food resource for animals and are the cornerstone of
reduced to a small number of cells that depend entirely on their agricultural production. Because seeds are larger than spores,
parent sporophyte for nutrition (Fig. 30.8). however, they are not as easily dispersed by wind. As a result,
seed plants have evolved structures to enhance seed dispersal.
j Quick Check 3 How do the male and female gametophyte
For example, pine seeds have flattened extensions, referred to
generations differ in seed plants?
as “wings,” that increase the distances over which they can be
carried by a breeze. Other seed plants produce brightly colored
Seeds enhance the establishment of the next and/or fleshy structures to attract and reward animals that
sporophyte generation. may inadvertently carry seeds farther away from the parent
The union of gametes triggers the ovule to develop into a seed plant than they could disperse on their own. Many plants
that can carry the new embryo away from the parent plant. dispersed by animals develop seed coats that allow their seeds
Because seeds develop from fertilized ovules, they contain to pass unharmed through an animal’s digestive system.
tissues produced by three generations (Fig. 30.9). On the
outside is the protective seed coat, which is formed from
tissues that surround the sporangium and thus is a product
FIG. 30.9 Structure of a pine seed. A pine seed contains
of the diploid sporophyte. At the center is the embryo, which
tissues from three generations: the seed coat formed
develops from the zygote and represents the next sporophyte
from diploid sporophyte layers, the haploid female
generation. In seed plants, the embryo develops an embryonic
gametophyte, and the diploid embryo, which will grow
root at one end and an embryonic shoot with one to several
into the next sporophyte generation. Source: Jim French.
embryonic leaves at the other. In gymnosperms such as
pine trees, the embryo is surrounded by the haploid female Female
Seed coat (2n) gametophyte (1n)
gametophyte, which provides the raw materials that support
the growth of the embryo. In angiosperms, the origin of this
storage tissue differs, but it serves the same role of nourishing
the embryo.
Compared with a unicellular spore, a seed is able to store
more resources to support the growth and establishment of a Embryo (2n)
new sporophyte generation. Many seeds contain high levels of
protein and energy-rich oils (Chapter 2). As a result, seeds are an
628 SECTION 30.3 F LO W E R I N G P L A N T S

During seed maturation, most seeds lose water. And as water Seeds span over 11 orders of magnitude in size (Fig. 30.10).
content falls, the seeds’ metabolic activity drops to extremely The 100-nanogram (ng) seeds of orchids are so small (100 ng 5
low levels and the embryo ceases to grow. The combination of 0.0001 mg) that they cannot germinate successfully without
low metabolic activity and ample stored resources allows seeds to obtaining nutrients from a symbiotic fungus. At the other end of
survive for long periods, in many cases one to several years, and the spectrum are the 30-kilogram (kg) seeds of the coco-de-mer,
sometimes much longer. Some seeds exhibit dormancy, meaning a palm tree found on islands in the Indian Ocean. Large seeds,
that they can delay germination even when conditions for growth, being better provisioned, are well adapted for the deep shade
notably temperature and moisture, are favorable. Dormancy of a forest understory. However, large seeds typically disperse
prevents seeds from germinating at the wrong time, for example shorter distances and they often have little or no dormancy. In
on an uncharacteristically warm day at a time of year when the contrast, for the same investment in resources, a plant can make
seedlings would be unable to survive. It also prevents seeds from many small seeds, which can persist in the soil until the right
germinating all at once, thus spreading the risk of making the combination of conditions triggers germination. As a result, most
transition from embryo to seedling over a larger time span and weeds produce small seeds.
range of conditions.
j Quick Check 4 Name three advantages of seeds over spores in
terms of the probability that the next sporophyte generation will
become successfully established.

FIG. 30.10 Seed diversity. Seeds vary tremendously in size, from

(a) the dustlike orchid seeds to (b) the large seeds of
the coco-de-mer. Sources: (top) Charles Marden Fitch/age
30.3 FLOWERING PLANTS
fotostock; (bottom) Kevin O’Hara/age fotostock. Let’s say that you want to send a postcard to a friend, but your only
means of doing so is to throw the card into the air and hope the
a wind carries it to your friend’s front door. How many cards would
you have to loft in order to achieve a high probability of success? The
answer, of course, is many, and this is precisely the reason that pines
generate huge numbers of pollen grains each year. Suppose instead
that you can give your card to the letter carrier whose route goes by
your friend’s house—your odds of success are now much greater.
This analogy gives some idea of a key evolutionary innovation that
appeared in the angiosperms. The “letter carriers” in angiosperm
fertilization are animals, and the evolution of flowers is what
allowed angiosperms to use animals to transport pollen.
Animal pollination is far more efficient than pollination by
wind—that is, a higher proportion of pollen reaches an egg. Thus,
a plant that relies on animal pollination can afford to produce less
pollen. Flowers initially evolved to make animal pollination possible
in dense tropical forests. However, as angiosperms diversified into
many habitats, some species reverted to wind pollination. Today,
angiosperms make use of a wide range of pollination strategies.
Here, we examine how angiosperms reproduce, beginning with
b
flowers, which when fertilized develop into fruits.

Flowers are reproductive shoots specialized for the

transfer and receipt of pollen.
Angiosperms are seed plants and their life cycle is similar to that
of pine trees (see Fig. 30.7), with a few important differences. One
of these differences is that in angiosperms pollen and ovules are
produced in a single structure—the flower (Fig. 30.11)— rather
than in separate cones. The significance of this arrangement is
that, because pollen and ovules are so close together, an animal
pollinator can deliver pollen to one plant and take up pollen to
carry to another in a single visit. Approximately 12% of flowering
plant species produce pollen and ovules in separate flowers,
but all of these species descend from ancestors whose flowers
 CHAPTER 30 P L A N T R E P RO D U C T I O N : F I N D I N G M AT E S A N D D I S P E R S I N G YO U N G 629

FIG. 30.11 Flower diversity. Shown here are (a) a lady’s-slipper orchid (Cypripedium reginae); (b) a magnolia (Magnolia grandiflora); (c) French
lavender (Lavandula stoechas); and (d) a tropical tree (Brownea grandiceps). Sources: a. Barrett & MacKay/age fotostock; b. Florida Images/Alamy;
c.. Jonathan Buckley/age fotostock; d. Tim Lamen/Getty Images.

a b

d c

produced both. Such “unisexual” flowers are often found in the whorls of floral organs (Fig. 30.12; Chapter 20). The outer whorls
approximately 20% of angiosperm species that have reverted to consist of sepals and petals, while the inner whorls are made up
wind pollination. of pollen-producing stamens and ovule-producing carpels. Let’s
Flowers are spectacularly diverse in size, color, scent, and look at the floral organs in more detail, starting with the carpels at
form, but they all have the same basic organization: concentric the center of the flower.

FIG. 30.12 Flower organization. The four whorls of organs in a flower are carpels, stamens, petals, and sepals. Photo source: Cora Niele/Getty Images.

The outer two whorls, petals and

sepals, serve to attract pollinators
and to protect the flower as it
develops.

Sepals
p

Petals

Stamens
Carpels

The central two whorls, carpels

and stamens, produce ovules
and pollen, respectively.
630 SECTION 30.3 F LO W E R I N G P L A N T S

FIG. 30.13 Spore formation and gametophyte development in (a) carpels and (b) stamens.

a. Ovule development
Spore formation Gametophyte development
Stigma
Within each sporangium, One of the haploid
one cell undergoes spores develops into a
meiosis, forming four female gametophyte
haploid spores. that fills the sporangium.
Style

Protective Protective
layers (2n) layers (2n)

Meiosis Mitosis
Ovary

Sporangia Sporangium Four haploid spores Female gametophyte (1n)

Carpels Ovule

b. Pollen development
Spore formation Gametophyte development

Within each sporangium Each haploid spore develops

many haploid cells into a male gametophyte
Anther undergo meiosis, surrounded by a sporopollenin
forming haploid spores. containing wall.
Each anther
contains 2-4
Meiosis Mitosis
sporangia.

Sporangium Sporopollenin
Filament wall
Many haploid spores Male gametophyte (1n)
Stamen Pollen

Carpels are modified leaves that have become folded over and grains at once, the fastest growing pollen tubes are the most likely
sealed along the edges to form a hollow chamber. Sporangia are to deliver sperm to an egg. In some plants, the style can be more
produced on the inner surface of this cavity (Fig. 30.13a). Within than 10 cm long—corn silks, for instance, are styles—creating the
each sporangium a single cell undergoes meiosis, forming four opportunity for competition, and thus natural selection, between
haploid spores, one of which develops into a female gametophyte. genetically distinct male gametophytes.
Recall that an ovule consists of a female gametophyte surrounded Immediately surrounding the carpels are the pollen-producing
by protective tissues and that ovules, when fertilized, develop into stamens. A stamen can have a leaflike structure bearing sporangia on
seeds. The fact that the ovules develop within the carpel is what its surface, but more commonly it consists of a filament that supports
gives rise to the name “angiosperm,” which is from Greek words a structure known as the anther, which contains several sporangia
meaning “vessel” and “seed.” In gymnosperms, the ovules are not (Fig. 30.13b). Within each sporangium, many cells undergo meiosis
enclosed, and their name, again from the Greek, literally means to form haploid spores. The male gametophyte then develops within
“naked seeds.” the spore wall, forming pollen. In most flowers, the anther splits
Each flower produces one or more carpels (typically 3 to 5, but open, exposing the pollen grains. Once exposed, the pollen can come
sometimes more than 20), but because the carpels are often fused, into contact with the body of a visiting pollinator or, in the case of a
there may be only a single structure at the center of the flower. At wind-pollinated species, be carried off by the wind. However, in some
the base is the ovary in which one to many ovules develop. The plants, for example tomato, small holes open at the top of anthers. To
ovary protects the ovules from being eaten or damaged by animals, extract the pollen, bees land on the flower and vibrate at just the right
but also makes it impossible for pollen to land directly on the frequency to shake loose the pollen inside. Orchids and milkweeds
surface of the ovule. To reach the ovules, pollen must land on the do not release individual pollen grains. Instead, they disperse their
stigma, a sticky or feathery surface at the top of the carpel(s), and pollen all together in a package with a sticky tag that attaches to a
grow down through the stalklike style. The female gametophytes visiting pollinator.
produce chemicals that guide the pollen tubes toward unfertilized The outer whorls of the flower produce neither pollen nor ovules
ovules. Because animal pollinators can deposit many pollen but instead contribute to reproductive success in other ways. Most
 CHAPTER 30 P L A N T R E P RO D U C T I O N : F I N D I N G M AT E S A N D D I S P E R S I N G YO U N G 631

FIG. 30.14 Flowers providing food or other rewards for their pollinators. (a) A butterfly collecting pollen from flowers; (b) a hummingbird
visiting a flower for nectar; (c) a bat visiting Agave palmeri for nectar; and (d) flies attracted to Rhizanthes lowii with its smell of rotting
flesh. Sources: a. Sue Kennedy/Flowerphotos/ardea.com; b. Glenn Bartley/age fotostock; c. Rolf Nussbaumer/age fotostock; d. David M. Dennis/age footstock.

a b

d c

flowers have two outer whorls, of which the outermost is made the same time as the diversification of bees (Hymenoptera) and
up of sepals. Sepals, which are often green, encase and protect the butterflies (Lepidoptera). The evolution of flowers allowed animals
flower during its development. In contrast, petals are frequently to specialize on new food resources. At the same time, animal
brightly colored and distinctively shaped. Their role is to attract pollinators greatly facilitated the movement of pollen between
and orient animal pollinators. In addition to serving as visual cues, plants within the same population.
petals in many flowers produce volatile oils. These are the source of Flowers communicate their presence through both scent and
the distinctive odors, some pleasant, some decidedly not, that many color. For pollination to be reliable, however, it must be in the
flowers use to advertise their presence to pollinators. interest of the animal pollinator to move repeatedly between
flowers of the same species. Flowers earn fidelity from their
j Quick Check 5 What is the name and function of the structures in pollinators by providing rewards, frequently in the form of food
each whorl of a flower? (Fig. 30.14). Many insects consume some of the pollen itself,
while many flowers also produce nectar, a sugar-rich solution
The diversity of floral morphology is related to modes attractive to bats and birds as well as to insects.
of pollination. Some species provide rewards other than food. For example,
The evolutionary histories of the angiosperms and their animal many orchids secrete chemicals that male bees need to make
pollinators are closely intertwined. A rapid increase in angiosperm their own sexual pheromones. Other flowers provide an enclosed
diversity occurred between 100 and 65 million years ago, about and sometimes heated chamber in which insects can aggregate. But
632 SECTION 30.3 F LO W E R I N G P L A N T S

by echolocation. In contrast, bird-pollinated flowers are often

FIG. 30.15 Ophrys speculum, a Mediterranean orchid, which red, a color most insects cannot see, and have no scent. Because
mimics the shape and color of female bees. These interactions between plants and pollinators affect gene flow, they
flowers induce males to attempt mating. Through can affect rates of speciation. A recent radiation in columbine
repeated attempts on different flowers, the males occurred as the plants shifted from bumblebees to hummingbirds
transfer pollen. Source: José Antonio Jiménez/age fotostock. to hawkmoths as their primary pollinator (Fig. 30.16).
Angiosperms are thought to have evolved in tropical forests.
Pollen cannot move freely through the air in the high density
of foliage that is present year round in these forests, favoring
the evolution of animal pollination. However, as angiosperms
diversified and spread around the globe, some evolved wind
pollination once again. The reversal to wind pollination is
associated with habitats where pollinators are not reliably
abundant, such as dry regions.
Wind pollination is most effective in species that are locally
abundant. For example, many grasses rely on wind for pollination.
Wind pollination is also common in temperate-zone trees, which
produce flowers in the early spring, before leaves expand and
before pollinators are active. Wind-pollinated species produce
large amounts of pollen, and their petals tend to be tiny. Hay fever
is an allergic reaction to pollen, virtually all of which is from wind-
pollinated species.

Angiosperms have mechanisms to increase outcrossing.

When pollen is deposited onto a stigma, the pollen must
germinate and form a pollen tube that grows down through the
style before fertilization can occur (Fig. 30.17). Typically, only
pollen from the same species will germinate and grow successfully,

some flowers do not provide rewards at all: Instead, they “trick” FIG. 30.17 Germination and growth of pollen tubes. The pollen
their pollinators into visiting. For example, flowers that look and tubes of Arabidopsis thaliana can be seen here because
smell like rotting flesh attract flies, and orchids that look like a they have been stained with a fluorescent dye. Source:
female bee even emit similar pheromones to attract male bees Courtesy Keun Chae.
(Fig. 30.15). Male bees find these orchids irresistible and, as they
attempt to copulate with the flower, deliver and receive pollen.
Producing rewards and attractants such as nectar and petals
requires resources. Thus, while animals can provide more efficient
pollination than can the wind, the resources spent may be wasted
if nonpollinating visitors “steal” pollen or nectar. For this reason,
many flowers have mechanisms to protect their investments. For
example, tubular cardinal flowers provide rewards to hummingbirds
able to probe their depths, and snapdragons provide rewards only to
insects that are strong enough to push apart their petals.
Many plants have evolved flowers that match rewards and
attractants to the metabolic needs and sensory capabilities
of their pollinators. For example, flowers pollinated by larger
pollinators such as bats and birds produce copious amounts of
nectar. Bat-pollinated flowers tend to be large, white or cream
colored, and strongly scented, all traits appropriate to the size and
nocturnal habits of their pollinators. The flowers are positioned
among branches and leaves in such a way that they can be found
 CHAPTER 30 P L A N T R E P RO D U C T I O N : F I N D I N G M AT E S A N D D I S P E R S I N G YO U N G 633
HOW DO WE KNOW?

FIG. 30.16

Are pollinator shifts associated with the formation

of new species?
BACKGROUND Columbines (Aquilegia species) are diverse and have visually striking flowers. Columbine flowers have nectar spurs, which are
modified petals that form tubular outgrowths. Blue flowers with short nectar spurs are pollinated by bumblebees, red flowers with medium
nectar spurs are pollinated by hummingbirds, and white or yellow flowers with long nectar spurs are pollinated by hawkmoths. The size of the
nectar spur corresponds to the length of the tongue of the pollinator.

HYPOTHESIS The recent radiation of columbines corresponds with shifts from short-tongued pollinators (bumblebees) to ones with
increasingly longer tongues (hummingbirds, hawkmoths).

METHOD Researchers mapped changes in flower color and nectar spur length onto a phylogenetic tree of columbines. To do this, they
assigned each of 25 species examined to one of three groups based on length of nectar spur, flower color, and most common pollinator
(bumblebees, hummingbirds, or hawkmoths). The group is indicated by the color circle at the tip of each branch in the phylogenetic tree in the
figure. The researchers then did a statistical analysis that suggested the most likely pollinator type at each node representing an ancestor, also
indicated by color circles in the figure. They used this information to infer the history of pollinator shifts during the radiation of this group.

Phylogeny of 25 North American columbine species

JO Bumblebee RESULTS Within the recent radiation of
SA columbines, there are multiple instances of
LA
shifts from a shorter-tongued to a longer-
BR
CA tongued pollinator, and no reversals.
* DE
FL CONCLUSION The evolution of floral
EX diversity and increasing nectar-spur length
FO.W in columbines is associated with pollinator
FO.E Hummingbird
shifts. Large changes in nectar-spur length
SH
TR occur disproportionately at speciation
* PU events, indicating a key role in adaptive
EL radiation.
Sp. nov.
COOC.CO FOLLOW-UP WORK Developmental
* COCO studies show that the diversity of nectar
COAL
spur length evolved as a result of changes
COOC.UT
BA
Hawkmoth in cell shape.
* MI
SC SOURCES Whittall, J. B., and S. A. Hodges. 2007.

CHAP “Pollinator Shifts Drive Increasingly Long Nectar Spurs

in Columbine Flowers.” Nature 447:706–709; Puzey,
CH.CHI
J. R., et al. 2011. “Evolution of Spur Length Diversity
HI in Aquilegia Petals Is Achieved Solely Through Cell
* CH.NM Shape Anisotropy.” Proceedings of the Royal Society,
PI London, Series B, 279:1640–1645.
LO.AZ
SK Photo sources: (top) Paul Oomen/Getty Images;
LO.TX (middle) Nancy Nehring/Getty Images; (bottom)
* Daniela Duncan/Getty Images

Short nectar spurs, blue petals, visited by bumble bees * indicates a likely evolutionary
shift in pollinator
Medium-length nectar spurs, red petals, visited by hummingbirds
Long nectar spurs, white or yellow petals, visited by hawkmoths

633
634 SECTION 30.3 F LO W E R I N G P L A N T S

thus preventing gametes from two different species from coming of angiosperm species are self-incompatible, meaning that
into contact. pollination by the same or a closely related individual does not
Plants do not have the elaborate courtship rituals that many lead to fertilization. Self-incompatible plants must be able to
animal species use to select an appropriate mate. Nevertheless, recognize that a gamete comes from a closely related individual.
angiosperms have a wide range of mating systems. At one Self-recognition is based on the proteins produced from self-
extreme are self-compatible species in which gametes from incompatibility genes, or S-genes. If the pollen’s proteins match
male (pollen) and female (ovules) gametophytes produced those of the carpel, the pollen either fails to germinate or
by flowers on the same plant can form viable offspring. Self- germinates but the pollen tube grows slowly and eventually stops
compatible plants can reproduce even when they are physically elongating. S-genes may have originated as a defense against
isolated from other individuals of the same species or when fungal invaders but subsequently evolved as a means of promoting
pollinators are rare. Many weedy species and the majority of crops outcrossing. Because there can be dozens of S-gene alleles in a
are self-compatible. population, only pollen transfers between closely related individuals
At the other extreme are species in which pollen must be are blocked. Apples are examples of self-incompatible plants, so
transferred between different plants. Approximately half of all apple orchards always contain a mixture of different varieties.

FIG. 30.18 The life cycle of an angiosperm.

When the pollen is mature, Before fertilization can

the anthers rupture, allowing occur, the pollen must
the pollen to be dispersed. germinate and grow
through the style.

Pollination Pollen Double

Following meiosis, tubes fertilization
each haploid spore
develops into a male
gametophyte Pollen tube
consisting of three
cells.

Fusion with sperm

Pollen
Triploid creates a diploid
Outer protective cell zygote and the first
Sperm
layers (2n) surrounding Diploid triploid endosperm
female gametophyte zygote cell.
Pollen tube Ovule
nucleus
Following meiosis, one Pollen tube
Sporopollenin of the haploid spores nucleus
wall develops into a female
gametophyte consisting Seed
of only eight nuclei and
seven cells.

Egg
Each carpel Endosperm (3n)
produces one
Meiosis or more ovules.
Embryo (2n)
Carpels
Seed coat (2n)
Stamen Dispersal and
germination

Fertilized ovules
develop into seeds.
The ovary develops
into a fruit.

Sporophyte
 CHAPTER 30 P L A N T R E P RO D U C T I O N : F I N D I N G M AT E S A N D D I S P E R S I N G YO U N G 635

FIG. 30.19 Double fertilization. In double fertilization one sperm fuses with the egg to form a zygote, while a second sperm fuses with the
diploid cell of the female gametophyte, leading to the formation of triploid endosperm.

Mature pollen grain

One sperm from the male gametophyte fuses
Sperm with the egg, forming the zygote. The other
unites with the two haploid nuclei in the
Pollen tube central cell of the female gametophyte to form
nucleus a triploid cell that gives rise to endosperm.

Sporopollenin Seed
Female
gametophyte wall
Endosperm
Triploid (3n)
cell

Central Embryo
cell with (2n)
Diploid
2 haploid zygote Seed
nuclei Egg Sperm Nucleus controlling coat (2n)
pollen tube growth

Angiosperms delay provisioning their ovules until endosperm develops only when fertilization has occurred. In many
after fertilization. flowering plants, including corn, wheat, and rice, the mature seed
As we have seen, a major trend in the evolution of plants is the consists largely of endosperm (Fig. 30.20). The embryo itself
decreasing size and independence of the gametophyte generation is small. As the seeds germinate, the embryo draws resources
and the increasing prominence of the sporophyte generation. In from the endosperm to help it become established. It is no
angiosperms, this trend is taken even further (Fig. 30.18). The overstatement to suggest that the human population is sustained
male gametophyte has only three cells: One of these cells controls mostly by endosperm produced in the seeds of cereal crops.
the growth of the pollen tube, while the other two are male In other angiosperms, the growing embryo consumes all the
gametes or sperm. In most angiosperms, the female gametophyte endosperm before the seed leaves the parent plant. For example, a
contains only eight nuclei, arranged as six haploid cells (one of
which gives rise to the egg) plus a central cell containing two
nuclei. The female gametophyte of angiosperms is thus too small
to support the growth of the embryo, as it does in gymnosperms. FIG. 30.20 Endosperm. Endosperm develops from the triploid
How, then, do flowering plants provide nutrition for the embryo cell produced by double fertilization. Photo source:
to grow? Greg Lawler/Alamy.
This is where the central cell of the female gametophyte,
which contains two haploid nuclei, comes into play. During In rice, the endosperm In a mature peanut seed, all
pollination, the two sperm travel down the pollen tube and fills most of the seed and of the endosperm’s stored
the embryo is small. resources have already been
enter the ovule (Fig. 30.19). One of the sperm fuses with the transferred to the embryo.
egg to form a zygote, just as occurs in all plants. The other sperm,
however, unites with the two haploid nuclei in the central cell to
form a triploid (3n) cell. This triploid cell undergoes many mitotic Seed coat Cotyledon
divisions, forming a new tissue called endosperm. In angiosperm (2n, previous (part of the
diploid embryo)
seeds, it is the endosperm that supplies nutrition to the embryo.
generation)
The process in which two sperm from a single pollen tube fuse
with the egg and the two haploid nuclei in the central cell is called Endosperm Shoot of
double fertilization. (3n) embryo
Double fertilization and endosperm formation offer a
distinct advantage. In pines and other gymnosperms, the female Embryo (2n) Root of
gametophyte provides food for the embryo. This provisioning embryo
Female gametophyte
occurs whether or not the egg is successfully fertilized, and so is tissue (1n) is too small to
see, even at this scale.
potentially a significant waste of resources. In angiosperms, the
636 SECTION 30.3 F LO W E R I N G P L A N T S

FIG. 30.21 Developmental transition from flower to fruit. Fruits, in this case a plum, develop from ovaries and provide a means of dispersing
seed. Sources: (left to right) Gert Tabak/Getty Images; Trevor Sims/age fotostock; McPHOTO/age footstock.

peanut has virtually no endosperm left at the time it is dispersed. ripening of the banana. Tomatoes and bananas are usually picked
Almost all the resources that were previously held within the in an immature state and are subsequently ripened by exposure to
endosperm have been transferred into two large embryonic ethylene. The fruits can therefore be transported while they are
leaves, called cotyledons, which form the two halves of the part still hard and less prone to damage. Because ethylene synthesis
of the peanut we eat. Thus, the embryo itself contains all of the requires oxygen, the unripe fruits are typically stored in a low-
stored resources that will nourish the growth of the seedling at oxygen environment. Without this precaution, the ethylene
germination. produced by the ripening of even a single fruit could set off a chain
reaction in which all the stored fruit would ripen at once.
Fruits enhance the dispersal of seeds. The tremendous diversity of form seen in fruits reflects the
The transformation of a flower into a fruit is as remarkable as the many ways in which angiosperm seeds are dispersed (Fig. 30.22).
metamorphosis of a caterpillar into a butterfly (Fig. 30.21). As The winged fruit of an ash tree is carried away on the wind, and a
the fertilized egg develops to form an embryo and the endosperm coconut can float on ocean currents for many months, protected
proliferates around it, the ovary wall develops into a fruit, from exposure to salt water by its thick outer husk. The burdock
stimulated by signaling molecules produced by the endosperm. fruit has small hooks that attach to fur (or clothing)—this small
In a tomato, for example, the fleshy fruit is a mature ovary that hitchhiker was the inspiration for Velcro. Other fruits are not
encloses the seeds. In some plants, other parts of the flower dispersed, but instead open to expose their seeds. For example,
may also become incorporated into the fruit. For instance, the as milkweed fruits mature and dry out, they split apart to release
fleshy part of an apple is formed from the outer part of the ovary hundreds of seeds, each with a tuft of silky hairs that floats
combined with the base of the petals and sepals. Other fruits, like them away on even a light breeze. Still others, such as Impatiens
pineapple, develop from multiple flowers. (jewelweed), produce fruits that forcibly eject their seeds.
Fruits serve two functions: They protect immature seeds from Animals are probably the most important agents of seed
being preyed on by animals, and they enhance dispersal once the transport, in many cases attracted by the nutritious flesh of the
seeds are mature. In the case of fruits dispersed by animals, it fruit. In fleshy fruits, the seeds are protected by a hard seed coat
is essential that the fruit not be consumed before the seeds are and pass unharmed through the animals’ digestive tract. In some
able to withstand a trip through their disperser’s digestive tract. species, this passage actually enhances germination rates. In many
Thus, immature fleshy fruits are physically tough and their tissues cases, however, the seed is consumed, and successful dispersal
highly astringent. As they ripen, the fruits are rapidly transformed occurs only when the animals gather more seeds than they eat.
in texture, palatability, and color. Ripening converts starches to Squirrels are a good example, storing many acorns that never
sugars and loosens the connections between cell walls so that the get eaten. Humans are an important dispersal agent—we can
fruit becomes softer. transport seeds long distances, sometimes far outside their usual
In a number of species, including apples, bananas, and range. In some cases, accidentally introduced plant species can
tomatoes, a gaseous hormone called ethylene triggers fruit disrupt native plant communities.
ripening. It is the production of ethylene that explains why
placing a ripe apple and an unripe banana together hastens the j Quick Check 6 From what flower structure(s) is a fruit derived?
 CHAPTER 30 P L A N T R E P RO D U C T I O N : F I N D I N G M AT E S A N D D I S P E R S I N G YO U N G 637

traits into a single variety. Because many of

FIG. 30.22 Fruits and seed dispersal. (a) The seeds of the milkweed Asclepias syriaca the traits that affect crop productivity are
are dispersed by wind; (b) a robin is eating berries of mountain ash (Sorbus the product of multiple genes, crop breeding
americana); (c) cocklebur (Xanthium strumarium) fruits are hitching a ride on remains a powerful tool for increasing
the fur of a cow; (d) a squirrel carries a fruit it will bury for later use. Sources: crop yields. The spectacular increases in
a. Don Johnston/age fotostock; b. Don Johnston/age fotostock; c. blickwinkel/Alamy; productivity that are often referred to as
d. David Norton/Alamy. the Green Revolution resulted, in part,
a b from controlling the movement of pollen
between plants.
Norman Borlaug, an American
agronomist, has been called “the father of the
Green Revolution” and “the man who saved
a billion lives.” In the mid-twentieth century,
Borlaug led a project to make Mexico self-
sufficient in bread wheat. Borlaug’s aim was
to breed wheat that could resist infection by a
devastating class of fungal pathogens known
as rusts. To do this, he and his group hand-
pollinated plants that exhibited resistance
to rusts with pollen from plants that grew
well under field conditions or had seeds that
produced good flour.
Under natural conditions, wheat usually
self-pollinates. To introduce new sources of
c pollen, Borlaug and his coworkers removed
the anthers of each flower with tweezers
before the flowers opened. They had to
cover each flower to prevent unwanted
d pollen from coming into contact with
the stigma. Finally, when the stigma was
receptive, they dusted it with
pollen collected from a plant with the
desired characteristics. Under the hot sun,
this is backbreaking work, but over a series
of years the pollinations carried out by
Borlaug and his coworkers achieved their
goal of breeding wheat varieties that were
disease resistant.
Yet another problem remained to be
solved. To obtain high yields requires large
amounts of fertilizer (Chapter 29), but under

? CASE 6 AGRICULTURE: FEEDING A GROWING POPULATION these conditions, the new wheat varieties produced so many seeds
that stalks became top heavy, falling over in the wind. Borlaug
How did scientists increase crop yields during the knew that an answer to this problem was to cross his plants with
Green Revolution? wheat varieties that had shorter, sturdier stems. But given the
Human civilization and plant reproduction are closely selective advantage of height in natural populations, where was he
intertwined. Not only do seeds and fruits make up a significant to find such a plant? Japanese wheat breeders had identified and
portion of our diet, the direct manipulation of plant reproduction preserved a mutant dwarf plant that had arisen spontaneously, and
plays a critical role in agriculture. Plant breeding began as the Borlaug obtained seeds of this dwarf variety.
artificial selection for plants with seeds that were easy to harvest Much hard work and many thousands of hand pollinations lay
and has today become a highly quantitative field in which ahead. But by 1963, more than 95% of Mexican wheat cultivation
controlled crosses between plants are used to combine favorable made use of Borlaug’s high-yielding semidwarf wheat varieties,
638 SECTION 30.4 A S E X UA L R E P RO D U C T I O N

resulting in wheat yields six times greater than in 1944, the year without sex” —for example, an embryo can form directly from
he began work in Mexico. In 1965, the seeds were exported in large a diploid sporophyte cell. Dandelions are an example of plant
numbers, first to India and Pakistan and soon to the rest of the species that produce seeds in the absence of sex. Apomixis would
world. Used in combination with greater investments in fertilizer be of tremendous utility in agriculture if it could be turned on
and irrigation, these varieties prevented the famines that had been or off at will. The reason for this is that, once a desirable variety
predicted by many as a result of rising human populations. For his was developed, additional seeds could be produced without
work in alleviating world hunger, Borlaug was awarded the Nobel introducing new genetic variation. This ability to breed true would
Peace Prize in 1970. be useful in self-incompatible species such as apple and blueberry.
Most plants that reproduce asexually do so without producing
seeds; instead, a new plant grows out of part of the parent plant.
30.4 ASEXUAL REPRODUCTION In these cases, dispersal of offspring occurs in different ways. In
mosses and liverworts, for example, tiny plantlets form by mitosis at
Many plants can also reproduce asexually, producing new the base of shallow cups. Raindrops landing in this cup can dislodge
individuals without the formation and fusion of gametes. While the tiny plantlets, splashing some away from the parent plant.
asexual reproduction avoids the challenges associated with However, most plants that reproduce asexually do so by
fertilization, asexually formed plants must still disperse if they growing to a new location and only then producing a new plant.
are to avoid competition for resources with the parent plant. You need look no further than a front lawn to find evidence of
Sexual reproduction relies on spores and seeds; how do asexually such vegetative reproduction. Grass forms horizontal stems
produced individuals move across the landscape? underground from which new upright grass shoots are produced
at a distance from the site where the parent plant originally
Asexually produced plants disperse with and without germinated. Strawberries, bamboo, and spider plants (a common
seeds. houseplant) also spread vegetatively by forming horizontal stems.
Some species of flowering plants are able to use seeds to disperse In many cases, the connections that were needed initially to
asexually formed individuals. In these species, seeds can develop produce a new plant become severed. When this happens, genetic
even in the absence of fertilization, a process called apomixis. A evidence is needed to determine if a plant is the result of sexual or
variety of underlying mechanisms exists that can result in “seeds asexual reproduction.

FIG. 30.23 Vegetative growth of aspen trees. Each distinct group of trees, distinguished by
leaf color, is a group of genetically identical individuals produced by vegetative
reproduction. Source: Scott Smith/Corbis.
 CHAPTER 30 P L A N T R E P RO D U C T I O N : F I N D I N G M AT E S A N D D I S P E R S I N G YO U N G 639

Vegetative reproduction also occurs in woody plants. For of thousands of years by continuing to produce new
example, when a redwood tree falls over, new upright stems stems. The current record holder is an aspen clone in Utah,
can form along the now-horizontal trunk. Perhaps the most consisting of nearly 50,000 stems, that is estimated to be
impressive example of the ability to spread vegetatively is the 80,000 years old.
quaking aspen (Populus tremuloides), a common tree of the Plants can reproduce vegetatively because of the way they
Rocky Mountains that produces brilliant yellow foliage in build their bodies. As we will see in the next chapter, growth
autumn (Fig. 30.23). Aspens reproduce vegetatively by producing and development occur throughout a plant’s life, and this is the
upright stems from a spreading root system. Although each shoot plant characteristic that allows plants to produce new upright
lives for less than 200 years, a single individual can persist for tens “individuals” from roots or horizontal stems. •

Core Concepts Summary is called an ovule. Ovules, when fertilized, develop into
seeds. page 625
30.1 Alternation of generations evolved in plants by Pollination is the transport of pollen to the ovule, where
the addition of a diploid sporophyte generation that fertilization takes place. page 625
allows plants to disperse spores through the air.
Seeds contain the diploid embryo, a seed coat, and, in
The life cycle of the algal sister groups of land plants has one gymnosperms, nourishment in the form of the female
multicellular haploid generation. page 620 gametophyte. page 625
The life cycle of land plants is characterized by an alternation The pollen and ovules of gymnosperms are produced in separate
of haploid and diploid generations, one generation specialized structures, whereas the pollen and ovules of angiosperms often
for fertilization and the other for dispersal. page 621 form in one structure, the flower. page 625
Gametes are haploid cells produced by mitosis from cells in A trend in the evolution of plants is the progressive reduction in
the multicellular haploid generation, which is referred to as size and independence of the gametophyte and the increasing
the gametophyte generation. page 622 role of the sporophyte. page 627
Spores are haploid cells produced by meiosis from the
multicellular diploid generation, which is referred to as the 30.3 Angiosperms (flowering plants) attract and reward
sporophyte generation. page 622 animal pollinators, and they provide resources for seeds
only after fertilization.
Bryophytes have a free-living haploid gametophyte that
makes gametes and a dependent diploid sporophyte that Flowers consist of four whorls of organs: ovule-bearing carpels,
makes spores that are released into the air. page 622 pollen-producing stamens, petals, and sepals. page 629
In spore-dispersing vascular plants, both the gametophyte Many flowers attract animals because they provide a reward, such
generation and the sporophyte generation can survive on as food, shelter, or chemicals. Animals, in turn, transfer pollen.
their own. page 623 page 631

Bryophytes and spore-dispersing vascular plants rely Many flowering plants have genetic and structural mechanisms
for fertilization on swimming sperm released into the to prevent self-fertilization. page 632
environment. page 623 Double fertilization, which is unique to angiosperms, is the
formation of a diploid zygote and triploid endosperm. The
30.2 Pollen allows the male gametophyte to be endosperm nourishes the embryo that develops from the zygote.
transported through the air, while resources stored page 635
in seeds support the growth and development of the
The fertilization of an ovule triggers the development of the
embryo.
ovary wall into a fruit, a structure that enhances seed dispersal.
Pollen is the male gametophyte that develops within the page 636
spore wall, and so can be transported through the air.
page 625 30.4 Many plants also reproduce asexually.
The female gametophyte of seed plants is retained and Asexual reproduction produces new individuals without the need
nourished by the sporophyte. The female gametophyte, for uniting gametes. Dispersal of offspring, however, remains
together with the surrounding layers of protective tissues, important. page 638
 PASS 2

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Apomixis is the formation of seeds from diploid cells, in the 3. Explain how the organs in the different whorls of
absence of fertilization. Seeds formed by apomixis are thus angiosperm flowers interact to promote fertilization.
genetically identical to the parent plant. page 638
4. Contrast the investments that angiosperms and
Vegetative reproduction can occur when a plant spreads gymnosperms make and the structures that they produce to
horizontally and develops new upright shoots. page 638 enhance pollination.
5. Diagram the structure of a mature angiosperm and a
mature gymnosperm seed, indicating the ploidy (1n, 2n, 3n) of
Self-Assessment each tissue and its role in seed development and germination.
1. Explain how the evolution of alternation of generations is 6. Describe two ways that plants can reproduce asexually
an adaptation for reproduction on land. and explain how each of these help to disperse offspring.
2. Diagram the relationship between the sporophyte
and gametophyte generations in bryophytes, ferns, Log in to to check your answers to the Self-
gymnosperms, and angiosperms. Show the relative sizes Assessment questions, and to access additional learning tools.
and physical interactions (if any) of the two generations.
 CHAPTER 31

Plant
Growth and
Development
Core Concepts
31.1 In plants, upward growth by
stems occurs at shoot apical
meristems, populations of
totipotent cells that produce
new cells for the lifetime of
the plant.
31.2 Hormones are chemical signals
that influence the growth and
differentiation of plant cells.
31.3 Lateral meristems allow plants
to grow in diameter, increasing
their mechanical stability and
the transport capacity of their
vascular system.
31.4 The root apical meristem
produces new cells that allow
roots to grow downward into
the soil, enabling plants to
obtain water and nutrients.
31.5 Plants respond to light, gravity,
and wind through changes in
internode elongation and the
development of leaves, roots,
and branches.
31.6 Plants have sensory systems
that control the timing of
developmental events.
Dietrich Rose/Getty Images.

641
642 SECTION 31.1 S H O OT G RO W T H A N D D E V E LO P M E N T

Plants build their bodies in ways that are fundamentally different

from what occurs in animals. To appreciate how different, let 31.1 SHOOT GROWTH AND
us remind ourselves briefly how our own bodies develop from DEVELOPMENT
fertilized eggs (Chapter 20). Humans and other mammals
grow by the repeated division of cells throughout the body. In tropical forests, vines weave their way upward through the
Many cells migrate from one place to another, especially early branches of trees. Many of these vines have long, thin stems with
in development. The first divisions of the fertilized egg form widely spaced leaves (Fig 31.1a). In contrast, a barrel cactus, living
stem cells that have the potential to give rise to many different in the desert, has thick stems and leaves modified to form sharp
cell types. Cells with the potential to become one of many spines (Fig. 31.1b). Although they look nothing alike, these two
different cell types are termed “totipotent.” As development plants are constructed in the same way. Plant shoots are modular,
proceeds, however, cells lose this totipotency and proceed down a meaning that they are formed of repeating units. Each unit consists
genetically determined developmental pathway. Once the body is of a node, the point where one or more leaves are attached, and an
mature, growth essentially stops. internode, the segment between two nodes (Fig. 31.1c). In vines
In contrast to animals, cell division in plants persists in the internodes are long and the leaves large, whereas in cacti the
populations of totipotent cells called meristems, and growth internodes are short and the leaves thin and sharp. The modular
commonly lasts throughout the life of the plant. A bristlecone structure of plants helps us understand how the capacity for
pine in western North America may be several thousand years continued growth gives rise to the tremendous variation in plant
old, but it continues to sprout new branches, new leaves, and form that we see around us.
new cones every year. Plants respond to the world around them
not by moving about, but by modifying their size and shape. Stems grow by adding new cells at their tips.
Plants acquire resources by growing to them, and they compete At the tip of each stem lies a tiny dome of cells called the shoot
with neighboring plants by growing beyond them. Furthermore, apical meristem (Fig. 31.2). The shoot apical meristem is a group of
because development in plants can be modified in response to totipotent cells that, like embryonic stem cells in animals, gives rise
information about local environmental conditions, growth fulfills to new tissues. As we will see, plants have meristems in several parts
many of the same roles in plants that behavior does in animals of their bodies, but upward growth occurs exclusively at the shoot
(Chapter 45). And although plants can’t move out of the way of tip. This is where cell division occurs, generating all the new cells
danger, plants can rebuild their bodies if damaged by fire or frost, that serve to elongate stems. The shoot apical meristem also initiates
or after being struck by a falling tree. Even reproduction requires leaves and produces new meristems, which allow plants to branch.
the continuing formation of new organs. An important characteristic of the shoot meristem is that it
This chapter focuses on how vascular plants build their maintains a constant size even though it is a region of active cell
bodies. We begin by asking how the shoot system, consisting of division. As new cells are added near the shoot tip, cells that are
leaves, stems, and reproductive structures, is formed. We then farther from the shoot tip cease to divide. Cells near the shoot tip
turn to the development of root systems, asking how plants maintain their ability to divide by the expression of meristem
grow belowground. Finally, we consider how plants sense the identity genes. The expression of meristem identity genes is
environment and how they respond to what they sense by controlled by a network of chemical signals produced by cells at the
modifying their growth and development. very tip of the stem. Because these signals can be transmitted only

FIG. 31.1 Shoot organization. (a) A tropical vine and (b) barrel cacti look very different, but their shoot systems are built from (c) the same
repeating units of nodes and internodes. Sources: a. Le Do/Shutterstock; b. LianeM/Alamy.

a. b. c. Leaf

Shoots are made up

Nodes
of repeating units of
nodes and internodes;
one or more leaves
are attached at each
node.
Internode
 CHAPTER 31 P L A N T G RO W T H A N D D E V E LO P M E N T 643

FIG. 31.2 The shoot apical meristem. The shoot apical meristem consists of undifferentiated cells that divide rapidly, giving rise to all the cells in
stems and leaves. (a) The seedling is rowan (Sorbus acuparia). (b) The photo shows a longitudinal section of the shoot apical meristem
of coleus (Solenostemon scutellariodes). (c) This scanning electron micrograph shows the shoot apical meristem of tomato (Solanum
lycopersicum). Sources: a. Ian Gowland/Science Source; b. M. I. Walker/Science Source; c. Siobhan Braybrook.

a. c.
Developing leaves (leaf primordia)

200 µm

Shoot apical
meristem

b.
Shoot apical meristem

in the internodes located immediately beneath the shoot apical

meristem (Fig. 31.3). The increase in cell size that occurs in this
Leaf primordium
region is impressive. If all of the cells in the stem of a 100-m
Zone of cell division
redwood tree remained the size they were when they left the
Meristem cells appear shoot apical meristem, the tree would be less than 5 m tall.
dark in this section Within an elongating internode, each cell grows many times
because the nuclei
(stained pink) fill more in length than in width. The reason is the orientation of
much of these the cellulose molecules, which are primarily wrapped around
actively dividing cells. the cell perpendicular to the axis of the stem rather than parallel
with it. Because cellulose is very strong, having a tensile strength
similar to that of steel, cells expand more easily in length than in
girth. The large central vacuole that characterizes mature plant
cells forms during cell elongation. In fact, most of the increase
in cell volume is due to the uptake of water and solutes that fill
over a limited distance, only the cells close to the shoot tip express the vacuole. This explains in part why plant growth is markedly
meristem identity genes. reduced during periods of drought.
As we move even farther from the shoot tip, cells reach their
j Quick Check 1 What are three functions of the shoot apical
final size and complete their differentiation into mature cell
meristem?
types. This organization into successive zones of cell division, cell
elongation, and cell maturation allows stems to grow without any
Stem elongation occurs just below the apical meristem. predetermined limit to their length.
In animals, cell division and cell enlargement typically go hand Once cells have matured, they no longer expand. Thus,
in hand. In plants, most of the increase in cell size occurs after during the upward growth of the shoot, it is the production and
cell division is complete. Cells that are too far from the shoot elongation of cells at the shoot tip that lift the meristem ever
tip to express meristem identity genes cease to divide, yet they higher into the air. Imagine that, as a 10-year-old, you carved
continue to grow. Most of the elongation of stems takes place your initials into a tree. Twenty years later, your initials will be
644 SECTION 31.1 S H O OT G RO W T H A N D D E V E LO P M E N T

FIG. 31.3 Stem elongation. Internodal elongation results from cells increasing many times in length, but only a small amount in width.

Shoot apical meristem

Elongating Growth
internodes

Within an elongating internode,

cells increase many times in
length, and the vacuole comes
to occupy over 90% of the
interior volume of the cell.

Fully expanded
internodes
Cell wall

Cytoplasm

Time Vacuole

Nucleus

exactly the same distance from the ground as they were on the The shoot apical meristem controls the production
day you inscribed them. The letters, however, will be wider—a and arrangement of leaves.
phenomenon we discuss later in this chapter. The earliest vascular plants were simple branching stems, and
photosynthesis took place along the length of the stem. As evolution
The development of new apical meristems allows
stems to branch.
Vascular plants evolved the ability to branch even before they FIG. 31.4 Axillary buds. In seed plants, branching results from
evolved roots or leaves. Branching was important to these first axillary buds that are produced at nodes. Photo source:
plants because it allowed them to produce more sporangia. Jacqueline S. Wong/ViewFinder Exis, LLC.
Branching allows present-day plants to support greater numbers of
both reproductive structures and leaves.
Shoot apical
In spore-dispersing vascular plants, branches form when the meristem
shoot apical meristem divides in two, giving rise to two stems,
each with its own shoot apical meristem. In seed plants, branches
grow out from axillary buds, which are meristems that form Axillary bud
at the base of each leaf (Fig. 31.4). Axillary buds have the same
structure and developmental potential as the apical meristem
Leaf
and express the same meristem identity genes. When a branch
develops, the axillary bud becomes the shoot apical meristem
of the new branch. Thus, most plants have many shoot apical
meristems: one at the tip of each stem and branch. Axillary buds
are formed at the same time as leaves, but they remain dormant
until triggered to grow, and they persist even after leaves are shed.
Thus, axillary buds provide seed plants with many points along
their stem where new branches can form.
 CHAPTER 31 P L A N T G RO W T H A N D D E V E LO P M E N T 645

proceeded, some branch systems became flattened, increasing the

FIG. 31.5 The evolution of leaves. Fossils show that leaves capture of light. These planar branches lost the capacity for continued
evolved from branches that (1) lost the ability for growth. By about 380 million years ago, the planar branches became
continued growth; (2) developed in a plane; and modified into structures recognizable as leaves (Fig. 31.5).
(3) expanded the photosynthetic surface to fill the area The evolution of leaves required several developmental
between veins. The photo shows Archaeopteris, one changes. Apical meristem identity genes were down-regulated and
of the earliest plants with flattened leaves. Photo source: leaf identity genes up-regulated, resulting in an organ specialized
Topham/The Image Works. for photosynthesis. New meristems evolved, enabling leaves to
expand into flattened structures for capturing sunlight. In fern
Early vascular In some of these early The development leaves, these regions of cell division are located along the leaf
plants lacked vascular plants, side of new meristems margin. In pine needles, they occur at the base of each needle. In
leaves. Their branches lost their allowed tissues to
shoots consisted apical meristems and form between these flowering plants, meristem cells can be distributed throughout
of photosynthetic became flattened side branches, the developing leaf, making possible a diversity of leaf shapes. In
stems. into a single plane. leading to the
development of contrast to cells in the shoot apical meristems, which can divide
leaves. continuously throughout the lifetime of a plant, leaf meristem cells
divide only for a relatively short period of time. This explains why,
at a certain point, leaves stop growing.
The arrangement of the leaves along a stem has a major
impact on how much sunlight each leaf receives and therefore
on how much photosynthesis is carried out. Leaves begin as small
bumps, called primordia (singular, primordium), which form on
the sides of the shoot apical meristem in a highly regular pattern (see
Fig. 31.2). Each species has a characteristic number of leaves attached
at each node along the stem. Some species have only a single leaf at
each node, whereas others have two or more (Fig. 31.6). The regular
placement of leaves reduces the shading of one leaf by another and
thus enhances the ability of plants to obtain sunlight.
When we think of leaves, it is the green photosynthetic ones
that first come to mind. However, many plants produce leaves that

FIG. 31.6 Diversity of leaf arrangements. Photo sources: (left to right)

Biosphoto/Jonathan Buckley/GAP; Anneke Doorenbosch/Alamy;
Christian Hütter/imagebroker/age fotostock.

Alternate Opposite Whorled

646 SECTION 31.1 S H O OT G RO W T H A N D D E V E LO P M E N T

quickly exceeds what diffusion can supply. If they are to grow to

FIG. 31.7 Non-photosynthetic functions of leaves. (a) Protection even a fraction of their full size, leaves must establish vascular
(bud scales); (b) climbing (tendrils); (c) trapping insects connections with the xylem and phloem in the stem. As we have
(spines); and (d) attracting pollinators (color). Sources: seen in Chapter 29, for leaves to carry out photosynthesis requires
a. Dr. Keith Wheeler/Science Source; b. Marga Werner/age fotostock; both a supply of water through the xylem and the ability to export
c. J Garden/Alamy; d. Adisa/Dreamstime.com. carbohydrates to the rest of the plant through the phloem.
When a developing leaf is still very small, files of cells within
a b
the leaf begin to elongate, forming discrete strands of procambial
Bud cells that extend from near the tip of the leaf to the mature
scales vascular tissues within the stem. Procambial cells ultimately
give rise to both xylem and phloem within the leaf veins and
the vascular bundles in the stalk attaching the leaf blade to the
stem. The common origin of xylem and phloem from procambial
cells explains why these two tissues are always found in close
association throughout the length of the plant.
The vascular system of an elongating stem is built from
the successive addition of vascular bundles that connect each
developing leaf to the vascular tissues already present within the
stem. Spore-dispersing vascular plants typically have one or several
vascular bundles in the center of their stems. In seed plants, the
most common configuration is a ring of vascular bundles near
the outside of the stem. The bundles have xylem conduits toward
c d Colored the center of the stem and phloem nearer the outside of the stem
leaves (Fig. 31.8). Typically, each leaf develops vascular connections with
several of the stem’s vascular bundles. The region between the
Flower epidermis and the vascular bundles is the cortex, and the region
inside the ring of vascular bundles is the pith. The placement of
the vascular bundles, which contain the stiff xylem conduits, near
the outside of the stem increases the stem’s ability to support its
leaves without bending over.

Flower development terminates the growth of shoot

meristems.
In vascular plants, sporangia are produced on the surfaces of
leaves. In some plants, such as ferns, sporangia are located on
leaves that also carry out photosynthesis. However, in conifers
and some other plants, sporangia form on the surfaces of highly
modified leaves and branches that form compact cones. In both
cases, the arrangement of the sporangia-bearing leaves is the same
as leaves that do not bear sporangia.
are specialized for functions other than photosynthesis, including Flowers develop from floral meristems formed by the
climbing, trapping insects, and attracting pollinators (Fig. 31.7). conversion of shoot meristems. This means that flowers can be
Plants that overwinter produce small, hard bud scales that protect produced at the tip of a shoot or at the base of leaves as the products
shoot apical meristems from water loss and damage due to cold. of axillary buds. Floral meristems differ from shoot meristems
Bud scales may not look like leaves, but they form from leaf in several ways. First, all of the cells within a floral meristem
primordia and are arranged in the same way around the stem as differentiate, and so floral meristems lose the capacity for continued
the green leaves produced in spring. growth. Second, although floral organs (sepals, petals, stamens, and
carpels) develop from primordia and are thought to be modified
Young leaves develop vascular connections to the leaves, their arrangement differs from the placement of leaves along
stem. the stem. Floral organs occur in whorls, with each whorl consisting
During the initial growth of leaf primordia, cells rely on diffusion of only a single organ type (see Fig. 30.12).
to obtain the resources needed to divide and expand. As the young Flowering is triggered by florigen, a protein produced in leaves
leaves increase in size, their demand for water and carbohydrates and transported through the phloem to apical meristems and
 CHAPTER 31 P L A N T G RO W T H A N D D E V E LO P M E N T 647

are restricted to the region close to the shoot tip. Other signaling
FIG. 31.8 Arrangement of vascular bundles. In most seed plants, molecules, or their precursors, enter the vascular system and are
the vascular bundles are arranged in a ring near the outside transported the entire length of the plant. Because plants grow
of the stem. Photo source: Steve Gschmeissner/Science Source. and develop in many places at once, signaling molecules that can
move from one part of the plant to another play an important role
in coordinating growth and development.

Hormones affect the growth and differentiation

of plant cells.
Hormones are chemical signals that influence both growth and
development. The term “growth” refers to irreversible increases
in size, while “development” encompasses the formation of new
tissues and organs, as well as the differentiation of specific cell
types. Hormones play two roles in plant growth and development.
First, they help establish and maintain basic patterns of the plant
body plan: How are leaves arranged? Where do branches form?
Epidermis
How much do internodes elongate? Second, they coordinate
Phloem
growth in different parts of the plant in response to both internal
Xylem and external environmental factors. For example, hormones guide
the growth of a stem toward sunlight and control which axillary
buds will grow into branches. Hormones affect plant growth and
Vascular development both by influencing physiological processes, such
bundle
as the activity of membrane transport proteins, and by altering
patterns of gene expression.
Traditionally, plant biologists have recognized five major
plant hormones: auxin, gibberellic acid, cytokinins, ethylene, and
abscisic acid (Table 31.1). How is a small number of hormones able
to shape the continuing growth and development of plants? One
Pith Cortex characteristic of these hormones is that each triggers a wide range
of developmental effects. For example, ethylene can lead to fruit
axillary buds. Florigen triggers the transition to floral meristems ripening (Chapter 30) and also slows the elongation of roots and
by initiating the down-regulation of meristem identity genes and stems. How a cell responds to a particular hormone depends both
the up-regulation of genes that govern floral identity. Once the upon its developmental stage and its position within the plant. In
meristem is launched along the trajectory for flower development, addition, the effect of any one hormone is often influenced by the
the identity of each whorl is controlled by the expression of presence of one or more other hormones.
homeotic genes (Chapter 20). Several chemicals with hormone-like properties have recently
been discovered. Some of these have broad effects on plant
development (for example, strigolactone), whereas others appear
31.2 PLANT HORMONES to be more specific (for example, florigen, whose only role is to
stimulate flower development).
A key question in the development of multicellular organisms To illustrate how hormones influence plant growth and
is what controls the identity, or “fate,” of individual cells. In development, we consider the role of three hormones, auxin,
plants, each cell is bound to its neighbor by the cell wall and thus gibberellic acid, and cytokinins, in establishing the basic patterns
fixed in place. This means that cell identity must be determined of shoot development. Later in the chapter, we consider the role of
by where a cell is located within the plant. For cells at the surface, hormones in root development (section 31.4) and plant growth in
the absence of neighboring cells on one side can be the signal response to light and gravity (section 31.5).
to become an epidermal cell. For cells in the interior, chemical
signals determine what sort of cell is formed. These signals provide Polar transport of auxin guides the placement of
information on both the identity of neighboring cells and the cell’s leaf primordia and the development of vascular
position in the plant. connections with the stem.
Some of the chemical signals that guide plant development The plant hormone auxin was first identified when it was
affect only a small area. For example, in section 31.1 we mentioned observed to cause shoots to elongate. As we will see in section
that the signals that cause cells to express meristem identity genes 31.5, auxin’s ability to trigger cell elongation underlies how shoots
648 SECTION 31.2 PL ANT HORMONES

TABLE 31.1 The Five Major Hormones

HORMONE SYNTHESIS AND TRANSPORT EFFECTS

Auxin O Synthesized primarily in shoot apical Induces cell wall extensibility.

meristems and young leaves. Inhibits outgrowth of axillary meristems.
OH
Transported by polar transport and through Stimulates formation of root meristems.
phloem.
Stimulates differentiation of procambial
N cells.
H Involved in phototropism and gravitropism.
Indole-3-acetic acid Stimulates ethylene synthesis.

Gibberellic acid Synthesized in the growing regions of both Stimulates stem elongation and cell division.
roots and shoots.
O H
Mobilizes seed resources for developing
embryo.
O
OH Stimulates cytokinin synthesis.
HO
H
HO O

Cytokinins Synthesized in both root and shoot Stimulates cell division.

Cl O meristems. Delays leaf senescence.
O Transported in both xylem and phloem.
OH Acts synergistically with auxin.

Cl
Zeatin

Ethylene Synthesized in elongating regions of roots Reduces cell elongation.

H H and stems, as well as in developing fruits. Triggers fruit ripening.
C C Gaseous; precursors can be transported in Stimulates formation of air spaces in roots.
H H xylem.

Abscisic acid Synthesized in root cap, developing seeds, Maintains seed dormancy.
and leaves. Stimulates root elongation.
OH COOH
O Transported from roots to leaves in xylem. Triggers closing of stomata.
Antagonistic with ethylene.

bend and grow toward light. Auxin also plays an important role in the outermost cell layer toward the shoot tip. High levels of auxin
establishing spatial patterns of shoot development. In particular, in the outermost layer of cells trigger the growth of new leaf
auxin influences the spacing and placement of leaf primordia and primordia. The movement of auxin into leaf primordia depletes
the formation of the vascular bundles connecting young leaves to auxin from surrounding regions and thus prevents new leaf
the mature vascular system of the stem. primordia from forming nearby. As the shoot continues to grow,
Auxin’s role in patterning results from the fact that its already formed leaf primordia become displaced from the shoot
direction of movement across a group of cells can be controlled. tip and spread farther and farther apart. This allows auxin moving
The net movement of auxin in a single direction creates regions upward in the outermost cell layer to accumulate to levels needed
of high and low concentrations of auxin. These gradients in to trigger new leaf primordia close to the shoot tip.
auxin concentration can guide the growth and development of Let’s now consider how auxin transport guides the formation
individual cells. of vascular bundles that connect the leaf to the stem. As auxin is
Let’s consider first how auxin movement influences the transported into leaf primordia, it promotes cell expansion and
placement of new leaf primordia. Auxin is synthesized in rapidly the young leaves grow. However, auxin does not remain in the
dividing cells within the shoot meristem and is transported in developing leaf. Instead, auxin that has accumulated at the tip of
 CHAPTER 31 P L A N T G RO W T H A N D D E V E LO P M E N T 649

FIG. 31.9 Polar transport of auxin. The polar transport of auxin results in the movement of this hormone from the tip of each developing leaf to
the base of the plant.
Direction of
auxin transport
Developing
leaves

1 In the cell wall (pH 5),

Shoot apical auxin is uncharged, so it
meristem diffuses easily across cell
membranes and into the cell.
Procambial
cells
2 In the cytoplasm (pH 7),
auxin is negatively charged
and can no longer diffuse
easily across membranes.
Auxin can exit the cell only
As auxin moves by polar transport from through auxin transport
the tips of developing leaves toward the proteins called PIN proteins.
stem, it triggers the development of
procambial strands. In the diagram above,
the procambial cells in the younger 3 PIN proteins are located
(inner) leaves are still differentiating, and only on the basal side of each
Direction of auxin transport

the connection to vascular tissues in the cell (the side farthest away
stem is not yet complete. from the shoot tip). Thus,
auxin can exit only through
the basal end of the cell.

Uncharged auxin
Negatively charged auxin
PIN transport proteins

the young leaf is transported back toward the stem, but this time the plasma membrane. However, because the pH of the cytoplasm
through cells in the interior of the leaf. As auxin moves through is higher than the pH of the cell wall, auxin loses a proton. As a
these cells, it causes the cells both to elongate and to produce result, auxin has a net negative charge when inside the cell and
more of the membrane transport proteins needed for auxin to cannot move easily across the plasma membrane. For auxin to
exit the cell. The cells that initially, by chance, have the highest leave the cell, an auxin-specific plasma membrane transport
auxin levels become highly efficient at transporting auxin toward protein known as PIN is required. By restricting the placement of
the stem. This, in turn, creates distinct “channels” through which PIN proteins to only one face of each cell, plants can control the
auxin drains from the leaf. These continuous strands of elongate direction of auxin movement across an otherwise undifferentiated
procambial cells eventually develop into veins and vascular group of cells. For example, if PIN proteins are inserted only on
bundles containing mature xylem and phloem. the side of cells farthest away from the shoot tip, auxin movement
These two examples illustrate how the movement of auxin will be toward the roots. In fact, the prevailing direction of auxin
in a single direction establishes spatial patterns that can guide transport in plants is from shoot apical meristems to the root tips.
further differentiation and development. But they leave open the
question of how undifferentiated cells control the movement of
auxin in the first place.
? CASE 6 AGRICULTURE: FEEDING A GROWING POPULATION

The coordinated movement of auxin across many cells in a What is the developmental basis for the shorter stems
single direction is referred to as polar transport (Fig. 31.9). Polar of high-yielding rice and wheat?
transport allows auxin to move independent of gravity at rates In natural ecosystems, competition for sunlight creates a strong
faster than the hormone could move by diffusion alone. However, selective advantage for growing tall. Compare the slender vine in
it requires an input of energy. Fig. 31.1, which grows in a dense forest, with the compact barrel
Polar transport depends on the difference in pH between the cactus that grows where there is little danger of being shaded
cell wall and the cytoplasm. In the cell wall, auxin carries no net by a neighboring plant. Whereas the ancestors of today’s crops
charge and thus can enter surrounding cells by diffusing across competed for sunlight by growing taller, a key element of the
650 SECTION 31.3 S E CO N DA RY G RO W T H

high-yielding rice and wheat varieties developed during the Green

Revolution is their shorter stems. These shorter-stemmed plants FIG. 31.10 Apical dominance. Source: Nigel Cattlin/Alamy.
can support the much-larger seed mass induced by high rates of
fertilizer application (Chapter 29). Moreover, by investing less
in stems, these plants can allocate a greater percentage of their
resources to the production of seeds. What happens inside plants
of these high-yielding crop varieties to make the stems shorter?
Gibberellic acid was first identified as a chemical produced
by fungal pathogens that caused the stems of young rice plants
to grow to many times their normal length. We now know that
it is produced naturally in plants, particularly in growing regions
such as developing leaves and elongating stems. Gibberellic acid
increases internode elongation by reducing the force needed
to cause cell walls to expand. This raises the possibility that the
high-yielding crop varieties with shorter stems have reduced
production of or sensitivity to gibberellic acid.
At the time that the semidwarf varieties were developed, the
underlying basis for their reduced stem growth was not known.
The genes responsible for reduced stem growth have now been
identified in both wheat and rice. The semidwarf wheats in fact
do have reduced sensitivity to gibberellic acid, and the shorter
rice varieties are deficient in the biosynthetic enzymes needed
to produce this hormone. In both cases, the result is shorter but
sturdier plants.
chemical signals is auxin, which moves by polar transport (and also
Cytokinins, in combination with other hormones, in the phloem) from the shoot tip toward the root. Auxin’s role in
control the outgrowth of axillary buds. suppressing branching can be demonstrated: If the shoot tip is cut
Branching allows plants to support more leaves, but branches off and a paste containing auxin is applied to the cut end of the
that are too close together result in leaves that overlap and shade stem, the axillary buds remain dormant. If instead the shoot tip is
one another. Because of the formation of axillary buds, seed treated with a substance that inhibits auxin transport, branching
plants have the potential to produce a new branch at each point is induced.
where a leaf was attached. To avoid overlap, only a few axillary Auxin suppresses the growth of axillary buds by inhibiting
buds actually develop into branches. Which become branches is the synthesis of cytokinins, the hormones that stimulate the
determined by the interaction of several hormones, including both outgrowth of axillary buds (Table 31.1). When the apical meristem
cytokinins and auxin. is removed, cytokinin synthesis increases in the stem nodes,
Cytokinins are plant hormones that were discovered when triggering the growth of axillary buds.
they were seen to stimulate cell division in tissues grown in the Recent studies indicate that this top-down control is
laboratory. Not surprisingly, cytokinins are produced in plant complemented by the action of another hormone, strigolactone.
meristems. Although cytokinins affect a variety of physiological and This hormone is made in roots and transported upward in the
developmental processes, they promote shoot growth by increasing xylem. Plants that are unable to synthesize strigolactone produce
the number of dividing cells in the apical meristem and, when many more branches than do wild-type plants. This indicates
applied directly to axillary buds, can stimulate them to develop into that strigolactone inhibits the outgrowth of axillary buds. One
branches. This suggests that cytokinins play a role in branching in hypothesis is that strigolactones are transported to the shoot
nature, but leaves open the question of what determines whether when either water or nutrients are limiting. Thus, the plant would
an axillary bud remains dormant or begins to grow. stop growing new branches whenever the root system is unable to
Removing the shoot tip stimulates the outgrowth of axillary supply the water and nutrients they require.
buds along that branch or stem (Fig. 31.10), suggesting that the
shoot apical meristem somehow suppresses the growth of axillary
buds. This suppression is referred to as apical dominance. Apical 31.3 SECONDARY GROWTH
dominance prevents branches from being formed too close to
the shoot tip, yet new branches can still be formed if the shoot Shoot apical meristems enable plants to grow in length and to
meristem is damaged. branch. But a plant unable to grow in diameter would not become
Apical dominance results because chemical signals from very tall before buckling under its own weight. And a plant
the shoot tip control the growth of axillary buds. One of these unable to add vascular tissue could not supply its ever-increasing
 CHAPTER 31 P L A N T G RO W T H A N D D E V E LO P M E N T 651

number of leaves with water. Growth in diameter thus serves two

functions: It strengthens the stem, and it increases the capacity of FIG. 31.11 Secondary growth. The vascular cambium is the
the vascular system. source of new xylem and phloem, while the cork
Primary growth is the increase in length made possible by cambium maintains an outer layer of bark.
apical meristems. Secondary growth is an increase in diameter.
It results from a new type of meristem, one that forms only after Pith Vascular cambium Cortex
elongation is complete. The fossil record shows that the ability to Primary xylem Secondary phloem Cork cambium
grow in diameter evolved millions of years after primary growth, Secondary xylem Primary phloem Cork
and it did so independently in several groups, testimony to the
benefits of being able to compete for sunlight by growing tall.
Today, secondary growth occurs almost exclusively in seed plants,
although this was not always the case in Earth’s history.

Shoots produce two types of lateral meristem. Gro

New cells must be added as a plant grows in diameter. The sources wth

of these new cells are lateral meristems, which form along the Gro
wth
length of a stem. Lateral meristems are similar to shoot apical
meristems in their ability to produce new cells that grow and
differentiate. However, lateral meristems differ from their apical
counterparts in several ways. First, lateral meristems surround the
stem, rather than occur at its tip. Second, because lateral meristems
form only after elongation is complete, the new cells they produce
grow in diameter but not in length. Finally, as a stem becomes
thicker, the number of meristem cells needed to encircle the stem
also increases. Thus, lateral meristems become larger over time.
Plants produce two distinct lateral meristems that together
Wood Bark
result in secondary growth. One of these, the vascular cambium
(plural, cambia), is the source of new xylem and phloem. The
xylem produced by secondary growth is familiar to you: It’s wood.
Plants with secondary growth are often described as woody,
whereas plants that lack secondary growth are described as
herbaceous. Most of the mass of a large tree consists of wood,
generated from the vascular cambium.
As the plant’s diameter increases, the epidermis formed during
primary growth eventually ruptures. Thus, plants with secondary
growth have a second lateral meristem, the cork cambium,
which renews and maintains a protective outer layer. This outer Secondary growth involves the formation of a
layer protects the stem against herbivores, mechanical damage, continuous vascular cambium that produces
secondary xylem toward the center of the
desiccation, and, in some species, fire.
stem and secondary phloem toward the
outside. The cork cambium maintains a
The vascular cambium produces secondary xylem protective outer layer.

and phloem.
The vascular cambium forms at the interface between xylem and
phloem. In stems, the vascular cambium derives from procambial This is because the undifferentiated and actively dividing cells
cells within each vascular bundle and parenchyma cells in between of the vascular cambium are weak and easily pulled apart. All of
the bundles. In forming vascular cambium, both types of cell take the material produced to the outside of the vascular cambium,
on a new identity, becoming meristem cells. including the secondary phloem, makes up the bark.
The vascular cambium produces new cells that differentiate Plants must produce more secondary xylem than secondary
on both its sides. Those to the inside of the vascular cambium phloem to replenish the water lost by leaves at high rates during
become secondary xylem, and those on the outside become the uptake of CO2 for photosynthesis (Chapter 29). As the vascular
secondary phloem (Fig. 31.11). The vascular cambium forms a cambium forms new secondary xylem cells along its inner face,
continuous layer, one cell thick, which surrounds the xylem and the newly formed cells push the vascular cambium outward, much
runs nearly the entire length of the plant. When you peel the bark the way the elongation of newly formed cells pushes the shoot
off a stem, it separates from the wood at the vascular cambium. apical meristem upward. The vascular cambium accommodates
652 SECTION 31.3 S E CO N DA RY G RO W T H

dark edge of a growth ring. Counting growth rings makes it

FIG. 31.12 Annual growth. In seasonal climates, tree rings possible to determine a tree’s age. Growth rings also provide a
provide a record of the annual accumulation of window into the past because by measuring the width of each ring,
wood. The tree rings shown are from douglas-fir it is possible to determine how much wood was laid down in a given
(Pseudotsuga menziesii). Source: H.D. Grissino-Mayer. year (Fig. 31.12). A tree produces more wood when moisture and
temperature levels are favorable for photosynthesis. Thus, annual
Wide growth rings growth rings provide a record of past climatic conditions, as well as
indicate higher events such as fire.
growth rates and thus
suggest that the tree
experienced more The cork cambium produces an outer protective layer.
favorable conditions
in these years. The cork cambium maintains a protective layer around a stem that
is actively increasing in diameter (see Fig. 31.11). The cork cambium
Narrow growth rings
forms initially from cortex cells that de-differentiate—that is, they
indicate slower regress to their earlier state—to become meristem cells. However,
growth rates, perhaps with time, this layer of actively dividing cells becomes increasingly
because of low
rainfall. distant from its source of carbohydrates—the phloem. As the cork
cambium becomes cut off from its carbohydrate supply, a new cork
cambium forms within the secondary phloem. Thus, each cork
cambium is relatively short- lived, and in many cases, new cork cambia
form in patches rather than as continuous layers. As a result, the bark
this outward movement by adding new cells that increase its of many trees has a patchy and fissured appearance (Fig. 31.13).
circumference. The secondary phloem, however, is unable to The cells produced by the cork cambium are called cork. As
produce any additional cells. As the production of secondary xylem cork cells mature, they become coated with suberin, a waxy
pushes the vascular cambium outward, the phloem becomes compound that protects against mechanical damage and the entry
stretched and, eventually, damaged. Consequently, the vascular of pathogens, while also forming a barrier to water loss. However,
cambium must continually produce new secondary phloem. this layer of waxy, nonliving cells impedes the inward diffusion of
Over time, xylem conduits lose their ability to transport oxygen needed to meet the respiratory demands of living cells. The
water. In long-lived trees, the xylem that is still active is located outer bark cells are less tightly packed in regions called lenticels,
in a sapwood layer adjacent to the vascular cambium, while the allowing oxygen to diffuse into the stem (Fig. 31.14).
heartwood, in the center of the stem, does not conduct water.
Heartwood is often darker in color than sapwood because it j Quick Check 2 Why do plants have two types of lateral meristem?
contains large amounts of chemicals such as resins. For this reason,
heartwood is typically more resistant to decay than sapwood. Wood has both mechanical and transport functions.
In many plants growing in seasonal climates, secondary xylem We have seen that as a tree grows, its secondary xylem fulfills the
cells produced at the end of the growing season are smaller than two roles of secondary growth: to provide trees with the strength
ones produced earlier. If you cut down a tree and inspect the top and stability to stand upright and support large crowns, and to
surface of the stump, the layer of smaller cells is visible as the transport water and nutrients from the roots to the leaves. In

FIG. 31.13 Outer bark. (a) Outside and (b and c) inside, showing how successive cork cambia are produced. The growth of many,
discontinuous cork cambia results in the patchy bark of pine and walnut trees. Sources: a. Alan Majchrowicz/age fotostock; b. Biosphoto/Dr. Keith
Wheeler/Science Photo Library.

a. Outside of tree, ponderosa pine (Pinus ponderosa) b. Cross section of bark, walnut (Juglans regia) c.

Cork cambium

Cork

Secondary phloem
 CHAPTER 31 P L A N T G RO W T H A N D D E V E LO P M E N T 653

In contrast, the wood of angiosperms is diverse in structure

FIG. 31.14 Lenticels in (a) water birch (Betula occidentalis) and because xylem cells mature into several distinct types that
(b) basswood (Tilia americana). Lenticels allow oxygen separate the functions of water conduction and mechanical
to diffuse through the outer bark to supply the respiratory support (Fig. 31.15b). Fibers are narrow cells with extremely
needs of the cells within the stem. Sources: a. Robert and Jean thick walls and almost no lumen; their primary function is
Pollock/Science Source; b. Garry DeLong/Science Source. support. In contrast, vessel elements can be extremely wide cells;
Lenticel their primary function is water transport (Chapter 29). The largest
vessel elements are close to 0.5 mm in diameter and thus visible to
a b the naked eye. Vessel elements can be much wider than tracheids
Cork because the fibers provide support. Because vessel elements
develop open connections that link them end to end with other
vessel elements, creating multicellular xylem vessels, angiosperms
can produce wood that transports water more efficiently than the
Cortex
wood of gymnosperms.
Secondary The ability to form xylem vessels has a number of important
phloem
consequences. One is that angiosperms can have higher rates of
Vascular
cambium
water flow through their stems than is possible with wood that
contains only tracheids. As a result, angiosperms sustain higher rates
Secondary of CO2 uptake and thus higher growth rates. In addition, because
xylem
vessels can be much more efficient than tracheids at transporting
water, angiosperms can devote a smaller fraction of their wood to
cells for water conduction, and a larger fraction to other cell types.
The wood of some angiosperms contains many fibers, in extreme
most gymnosperms, a single cell type, the tracheid, fulfills both cases resulting in wood that is so dense that it sinks. These dense
functions (Fig. 31.15a). As discussed in Chapter 29, tracheids are woods are extremely strong, allowing trees to create spreading
single-celled xylem conduits that have thick lignified cell walls. crowns with outstretched branches. Such “hardwoods” are useful
They are nonliving and empty at maturity. The wood of a pine tree, as flooring materials and furniture. The wood of other angiosperms
for example, is relatively uniform in structure, consisting almost contains a high percentage of living cells instead of fibers, and this
entirely of tracheids. This type of wood does not vary dramatically wood has a very low density. Balsa trees, whose wood is used in
from species to species. building model airplanes, are a good example.

FIG. 31.15 Wood structure. These scanning electron micrographs show (a) a gymnosperm, Douglas fir (Pseudotsuga menziesii), and (b) an
angiosperm, English elm (Ulmus procera). Sources: a. Scanning electron micrograph courtesy of the N.C. Brown Center for Ultrastructure Studies, State
University of New York College of Environmental Science and Forestry, Syracuse, NY; b. Andrew Syred/Science Source.

a. Gymnosperms b. Angiosperms

In gymnosperms, In angiosperms,
tracheids have fibers provide
both a mechanical mechanical
and a transport support, allowing
function. vessels to be
specialized for
water transport.

Tracheid

Vessels
Fiber
654 SECTION 31.4 RO OT G RO W T H A N D D E V E LO P M E N T

difference is that the root apical meristem is covered by a root cap

31.4 ROOT GROWTH AND that protects the meristem as it grows through the soil. As the root
DEVELOPMENT elongates through the soil, root cap cells are rubbed off or damaged.
Thus, the root cap is continually supplied with new cells produced
The roots that evolved in vascular plants enabled them to obtain by the root apical meristem. A third difference between roots and
water and nutrients from the soil and provided a mechanical stems, and one that can be seen without a microscope, is that the
anchor for plants capable of growing tall. As in the case of root apical meristem does not produce any lateral organs, whereas
leaves and wood, the fossil record shows that the first vascular the shoot apical meristem makes leaves. Root hairs, the single-celled
plants lacked roots. These plants survived only in extremely outgrowths of root epidermal cells that greatly increase the surface
wet environments in which they could absorb water through area (Chapter 29), are formed only after elongation has ceased.
horizontal, and possibly submerged, stems. The evolution of roots
allowed vascular plants to expand into drier habitats and ensured Root elongation and vascular development are
the stability of taller stems. coordinated.
The fossil record also indicates that roots evolved at least twice. A cross section taken through the zone of cell maturation shows
Nevertheless, the roots of all vascular plants share many features. a series of concentric tissue layers, starting with the epidermis on
This suggests that the physical challenges of growing through dense the outside, followed by the root cortex, and then the endodermis
soil and the demands of obtaining water and nutrients from that soil (Fig. 31.16). Recall that the endodermis plays a key role in the
place strong constraints on root structure. selective uptake of nutrients (Chapter 29). The Casparian strip, a
waxy band surrounding each endodermal cell, forces all materials
Roots grow by producing new cells at their tips. that will enter the xylem to cross the plasma membranes of these
In Chapter 29, we saw that plants produce large numbers of thin cells. At the center of the root is a single vascular bundle in which
roots to obtain sufficient nutrients from the soil. And to obtain the xylem is arranged as a fluted cylinder, with phloem nestled
water during periods without rain, plants produce roots that in each groove. In cross section, the xylem is arranged in a star
penetrate deep into the soil. Growing roots into the soil presents shape; the number of points of the star differs in different
challenges different from those associated with elongating stems plant species.
into the air. Although the dangers
of drying out are much reduced
belowground, deeper soil layers can have FIG. 31.16 The structure of a typical root, including the root apical meristem and root cap.
densities approaching that of concrete. Photo source: Walker Mi./Getty Images.
How are the thin and permeable roots
needed for water and nutrient uptake able
to penetrate and grow through the soil?
In one respect, roots grow in the
same way as shoots in that new cells are
produced at their tip. The root apical Phloem Xylem
Zone of cell
meristem is the source of new cells. After maturation
cell division, these cells elongate and Root hairs
become differentiated. They complete Epidermis
their differentiation only after elongation Cortex
has ceased (Fig. 31.16). Thus, as we move
Pericycle
back from the tip of the elongating root,
Endodermis
we encounter successive regions of cell
division, elongation, and maturation, just
as we saw in stems. Root apical Zone of cell
Roots, however, differ from stems in meristem elongation
several important ways. One difference is
that roots are much thinner than stems.
Plants with thin roots can more efficiently
produce the large surface area needed
Zone of cell
for uptake of water and nutrients from
division
the soil. Thin roots can also more easily
bend around soil particles and make use
Root cap
of channels made by invertebrate animals
that burrow through the soil. A second
 CHAPTER 31 P L A N T G RO W T H A N D D E V E LO P M E N T 655

As in stems, the hormone auxin plays a central role in specifying single layer of cells just inside the endodermis (Fig. 31.17). The
which cells differentiate into vascular tissues. Auxin produced in pericycle is immediately adjacent to the vascular bundle, so the
shoot meristems is transported downward through the mature stem new root is connected to the vascular system from the beginning.
and into the roots by the phloem. At the end of the fully formed Because the new root meristem develops internally, the new root
phloem, auxin moves by polar transport from cell to cell until it must grow through the endodermis, cortex, and epidermis before
reaches the root apical meristem and the root cap. As auxin moves reaching the soil.
through the elongation zone, it triggers the formation of procambial One consequence of initiating new root meristems from the
cells that subsequently become phloem and xylem. pericycle is that new roots can form anywhere along the root. As
In this way, the production of auxin by developing leaves a result, plants have tremendous flexibility in the development
and the polar transport of auxin from cell to cell ensures that the of their root systems and roots can proliferate locally in response
vascular system is continuous between roots and leaves. Phloem to nutrient abundance. For example, the presence of nitrate, the
extends closer to the root tip than do fully lignified xylem conduits. most common form of nitrogen in soil, triggers the formation
The root apical meristem does not need direct access to the xylem of lateral roots. Only a relatively small number of a plant’s roots,
because it can obtain water and nutrients directly from the soil however, continue to elongate and branch indefinitely. Most new
but depends entirely on the phloem for carbohydrates needed for roots are active for only a short time, after which they die and are
growth. shed. This turnover of fine roots allows the root system to respond
So far, we have described only the primary growth of roots. to changes in the availability of water and nutrients but represents
However, roots can undergo secondary growth as well. Through a major expenditure of carbon and energy.
secondary growth, roots increase in diameter and add new xylem Up to this point, we have described how new roots form from
and phloem. As in stems, the vascular cambium forms at the existing roots. New roots can also be produced by stems. Many
interface between the xylem and the phloem. Indeed, the vascular plants that lack secondary growth produce horizontal stems that
cambium must be continuous from the shoot to the roots to grow on or beneath the soil surface. In these plants, new roots
ensure that the secondary xylem and phloem provide an unbroken form at each successive node, so the uptake capacity of the root
connection between the roots and the leaves. In older roots with system is directly coupled to the elongation of the stem.
extensive secondary growth, a cork cambium is also formed. As When the stem of a plant is severed, new roots often form at
in stems, the cork cambium renews the outer layers of cells that the cut end. The formation of new root meristems is stimulated by
protect the living tissues from damage and predation. auxin, which is produced by the young leaves and accumulates by
polar transport at the cut end. Because their cut stems can form
The formation of new root apical meristems allows new roots, plants are able to survive damage and in some cases
roots to branch. even proliferate afterward. Many commercially important plants
Compared to stems, roots branch extensively. Even a plant are propagated through cut stems.
that has only a single stem aboveground will have many tens
j Quick Check 3 How is root development similar to and how is it
of thousands of elongating root tips. A major reason that roots
different from stem development?
branch so much is that most of the water and nutrients are taken
up in a zone near the root tip where root hairs are most abundant.
Thus, branching is necessary to create enough root surface area to The structures and functions of root systems are
supply the water and nutrient needs of the shoot. diverse.
For a root to branch, a new root apical meristem must be How a plant’s roots are distributed in the soil has a tremendous
formed. In roots, new meristems develop from the pericycle, a impact on the ability of the root system to supply the shoot with

FIG. 31.17 Branching in roots. New root meristems develop from the pericycle, and the developing lateral root grows through the root cortex
before reaching the soil. The light micrographs show willow (Salix) roots. Source: Science Source.

Emerging Epidermis
lateral root

Endodermis New root

apical
Pericycle meristem

Cortex
Lateral
Vascular root 100 μm
cylinder
656 SECTION 31.5 T H E E N V I RO N M E N TA L CO N T E X T O F G RO W T H A N D D E V E LO P M E N T

water. Some plants produce very deep roots that provide access to produce new leaves and shoots quickly following either damage or
groundwater that is independent of the variability of rainfall. For a period of drought or cold that has killed the shoots. Cassava and
example, roots have been observed in mine shafts and caves more sweet potato are examples of economically important species with
than 50 m below the soil surface. Producing and maintaining these storage roots.
non-photosynthetic structures is costly for the plant. Perhaps the most unusual rooting structures are the
At the other extreme are plants that produce shallow and “breathing” roots produced by trees that grow in water. For
spreading root systems. For example, barrel cacti produce roots that example, black mangroves produce pencil-sized roots that extend
penetrate less than 10 cm into the soil but extend out several meters vertically upward out of the sandy soil (Fig. 31.18d). Breathing roots
from the base of the plant. These roots cast a wide but shallow net to do not actually breathe; instead, they contain internal air spaces
capture as much water as possible from intermittent rain showers. that provide an easier pathway for oxygen to diffuse into the roots
Some plants produce distinctive roots whose principal than through the waterlogged soil. For this reason, breathing roots
role is not the absorption of water and nutrients from the soil must extend above the surface of the water.
(Fig. 31.18). For example, climbing plants such as poison ivy produce
roots along their stems that allow them to adhere to the sides of
trees (Fig. 31.18a). The conspicuous prop roots produced by some 31.5 THE ENVIRONMENTAL CONTEXT
tropical trees provide mechanical stability that allows these plants to OF GROWTH AND DEVELOPMENT
grow tall despite their slender stems (Fig. 31.18b).
Many plants produce swollen roots that store resources such The capacity for continued growth allows plants to respond to
as starch (Fig. 31.18c). Plants with belowground storage can changes in their environment by growing more in one direction
than another. To respond in a manner that
enhances fitness, however, plants must first
gain information about the world around them.
FIG. 31.18 Root diversity. (a) Climbing roots of poison ivy; (b) prop roots of a Pandanus
Plants rely on three types of sensory receptors:
tree; (c) storage roots of yams; and (d) breathing roots of black mangrove
Photoreceptors sense the availability of light
(shown at low tide). Sources: a. Doug Wechsler/age fotostock; b. A Jagel/age fotostock;
needed to drive photosynthesis; mechanical
c. PhilipYb/Shutterstock; d. Patrick Lynch/Alamy.
receptors sense physical influences such as
a b gravity and wind; and chemical receptors detect
the presence of specific chemicals, as well as
chemical gradients.
We saw in Chapter 9 how a signal can be
conveyed to a cell by a change in the shape of a
receptor molecule on the cell surface or within
the cell. Sensory receptors in plants work in a
similar manner. For example, the absorption of
light by a photoreceptor changes the chemical
properties of the photoreceptor, similar to the
photoreceptors in your eye. When a plant’s
sensory receptor is activated, it produces a signal
that triggers changes in the cell’s metabolism or
alters patterns of gene expression. Hormones
d c play a key role in translating information
gained by the plant’s sensory receptors into an
appropriate developmental response.

Plants orient the growth of their stems

and roots by light and gravity.
If you lay a plant on its side, its stems will turn
upward and its roots will bend downward.
Similarly, if you place a houseplant near a
window, the shoot will grow toward the light.
Tropism is the bending or turning of an
organism in response to an external signal such
as light or gravity. Plant stems are positively
phototropic, meaning they bend toward the
HOW DO WE KNOW?

FIG. 31.19

How do plants grow

toward light? Experiment 1

BACKGROUND In 1926, Frits Went, a Dutch graduate student,

followed up on the experiments of Charles and Francis Darwin.
In particular, Went sought to understand how the perception of
light by the tip of the plant led to a growth response farther
down the stem. He reasoned that if the signal from the shoot
tip was a chemical, he could isolate it by cutting the tip off of a Place gelatin Growth
block on top resumes.
growing plant and putting it on a block of gelatin. The gelatin of the cut
would act like a trap, capturing any chemical that moved out of plant.
the shoot tip.
Remove tip Place tip on a Experiment 2
HYPOTHESIS A chemical signal links the perception of light by of plant. gelatin block
for 1–4 hours.
the shoot tip with the growth of the stem toward light.

EXPERIMENT Went cut off the tips of young oat plants

(Avena sativa) and placed them on gelatin blocks for periods of
1 to 4 hours. Removing the tip of the plant caused the plants to
stop elongating. When the gelatin block was placed on top of
the cut plant (Experiment 1), growth resumed. This experiment
Place gelatin Growth resumes
demonstrated that a growth-inducing chemical was exported block to one such that the plant
from the tip of the young oat plants. side of the top bends away from
of the cut the gelatin side.
Interestingly, Went found that if the block of gelatin was
plant.
not centered on the tip but instead was a bit off center, the plant
bent away from the side of the gelatin block (Experiment 2).
In this case, the side of the plant exposed to the gelatin block
FOLLOW-UP WORK Went followed up his experiments by
grew faster, causing bending, because it received more of the
determining how the level of auxin relates to the degree of
chemical signal.
curvature. This assay became known as the Avena curvature test,
CONCLUSION A signal is produced in the tip of the plant and after the experimental species. Other experiments showed that
travels downward through the stem, toward the roots. This auxin moves in a directed fashion by polar transport. Later, auxin
signal stimulates growth. Light causes the signal to move to the was shown to be indole-3-acetic acid (IAA).
opposite side, stimulating growth on that side, which in turn
causes the plant to bend toward the light. This signal is now SOURCES Darwin, C., and F. Darwin. 1880. The Power of Movement in Plants.
London: John Murray; Went, F. W. 1926. “On Growth-Accelerating Substances
known to be a plant hormone, called auxin from the Latin word in the Coleoptile of Avena sativa.” Proceedings of the Royal Academy of Amsterdam
augere, “to increase.” 30:10–19.

light. Plant stems are also negatively gravitropic, meaning they were exposed, they did. This experiment established the shoot tip
grow upward against the force of gravity. In contrast, plant roots as the site where the light is perceived but left open the question of
grow down and away from the light. Thus, roots are positively what kind of signal caused the plants to bend (Fig. 31.19).
gravitropic and negatively phototropic. Photoreceptors allow plants to detect and respond to
Observing that grass seedlings bend toward light, Charles light. If a plant is exposed to light from only one side, only
Darwin and his son Francis conducted experiments to determine the photoreceptors on the illuminated side are activated.
what part of the plant detects light. When they covered just the Phototropism in plants involves a photoreceptor that absorbs
tip of the young plants, the plants no longer grew toward the light, blue wavelengths of light. The absorption of blue light by the
but when they buried them in fine black sand so that only their tips photoreceptor is thought to trigger a change in the placement of

657
658 SECTION 31.5 T H E E N V I RO N M E N TA L CO N T E X T O F G RO W T H A N D D E V E LO P M E N T

decrease the rate of cell elongation in roots, causing roots to bend

FIG. 31.20 Phototropism in shoots, controlled by the lateral away from light. The reason that auxin redistribution leads to such
redistribution of the hormone auxin. a different response in roots compared with shoots is that roots
a. and stems differ in their sensitivity to auxin and that in roots high
concentrations of auxin trigger the production of the hormone
Light ethylene, which reduces cell elongation.
from A similar mechanism orients plants with respect to gravity.
above
Moving a plant to a horizontal position results in the transport of
auxin to the lower side. The accumulation of auxin on the lower
surface causes stems to bend up and roots to turn down. How is the
When the plant able to detect which way is up and which way is down? Recall
light source is that plants store carbohydrates as starch. Starch is a large molecule
directly that has a density greater than water. Starch-filled organelles
Plant growth
overhead, the
auxin moves
evenly down
the stem and
Auxin

the plant
grows straight
upward. FIG. 31.21 Root orientation with respect to gravity. Starch-
filled statoliths settle to the bottom of cells, triggering
a redistribution of auxin to the lower side of the root,
where it slows elongation, causing the root to bend
b. downward. Photo source: Courtesy Dr. Christophe Jourdan.

Light
from
side
Auxin R

oo
tg
row
th
ow

Auxin

th
Auxin
Root growth

Vascular tissue
Shoot gr

Apical meristem
Root cap

When light reaches the Higher auxin concentration

plant on an angle, auxin is causes the shaded side to
redistributed to the shaded grow more rapidly, causing the

Gravity
side of the shoot. stem to bend toward the light.

Statoliths

PIN proteins so that auxin is transported to the shaded side of the

stem. As a result, concentrations of auxin are higher on the shaded
side and lower on the illuminated side. Because auxin stimulates
cell expansion in stems, the shoot grows faster on the shaded side
than it does on the illuminated side, causing the plant to bend
toward the light (Fig. 31.20).
Phototropism in roots is similar, in that illuminating a root from
the side causes auxin to be redistributed toward the shaded side. In
this case, auxin is transported to the shaded side within the root
cap, and then moves upward in the outermost layer of cells into
the zone of cell elongation. However, higher auxin concentrations
HOW DO WE KNOW?

FIG. 31.22

How do seeds detect the Lettuce Seeds Germinating After Exposure to Red and Far-Red
Light in Sequence
presence of plants growing SEQUENCE OF SUBSTRATE-LEVEL

overhead? LIGHT EXPOSURE

Dark (control)
GERMINATION (%)

8.5
Red 98
BACKGROUND To study the effect of light on seed germination,
scientists exposed lettuce seeds that had been kept continually in Red, far-red 54
the dark to different wavelengths of light and then counted what Red, far-red, red 100
fraction of the seeds germinated. Red light had the greatest ability Red, far-red, red, far-red 43
to stimulate germination, but surprisingly, far-red light inhibited Red, far-red, red, far-red, red 99
germination such that fewer seeds germinated than in the control
Red, far-red, red, far-red, red, far-red 54
seeds, which were kept in darkness. In the rush to conduct more
experiments, petri dishes with the light-treated seeds piled up Red, far-red, red, far-red, red, far-red, red 98
by the sink until someone noticed that the seeds that had been Time →
experimentally treated with far-red light were now germinating.

HYPOTHESIS This observation suggested that the inhibitory effect CONCLUSION Seed germination in lettuce is triggered by exposure
of far-red light can be overcome by a subsequent exposure to red to red light and is inhibited by exposure to far-red light in a reversible
light. fashion. As a result, plants are able to track changes in the relative
amount of red and far-red light, which provides information on the
EXPERIMENT The scientists exposed lettuce seeds to red and presence or absence of plants overhead (see Fig. 30.23).
far-red light in an alternating pattern, ending with either red light
or far-red light. They then placed the seeds in the dark for two days FOLLOW-UP WORK Additional studies showed that this result

and afterward counted the number of seeds that had germinated. was due to a single pigment in a photoreceptor, now known as
phytochrome, that is converted into an active form by red light and
RESULTS When the lettuce seeds were exposed to red light last, reversibly converted into an inactive form by far-red light.
nearly 100% of the seeds germinated. By contrast, when the last
exposure of the seeds was to far-red light, the percentage of seeds SOURCE Borthwick, H. A., et al. 1952. “A Reversible Photoreaction Controlling
that germinated was dramatically reduced. Seed Germination.” Proceedings of the National Academy of Sciences 38: 662–666.

thus sink to the bottom of the cell. The weight of these organelles than it is in the shade of another plant. When scientists first began
pressing on the cytoskeleton or membranes is thought to be the to study the effects of light on seed germination, they found that
signal that causes auxin to move toward the lower side of the red light is particularly good at stimulating germination, while far-
plant. In roots, gravity is a critical source of information regarding red light inhibits germination. A simple “flip-flop” experiment by
orientation. Specialized gravity-sensing cells in the root cap contain H. A. Borthwick and colleagues showed that the inhibitory effect
large starch-filled organelles known as statoliths (Fig. 31.21). of far-red light could be overcome by a subsequent exposure to
red light (Fig. 31.22). Ultimately, it was shown that only a single
Seeds can delay germination if they detect the photoreceptor was needed to detect both far-red and red light.
presence of plants overhead. Phytochrome is a photoreceptor that switches back and
For a small seed that has few stored reserves, germinating in the forth between two stable forms depending on its exposure to
shade of another plant could be fatal. But how can a seed detect light (Fig. 31.23a). Red light causes phytochrome to change into a
the presence of plants overhead? form that absorbs primarily far-red light (Pfr); far-red light causes
As sunlight passes through leaves, the red wavelengths are phytochrome to change back into the form that absorbs red light
absorbed by chlorophyll but the far-red wavelengths are not. The (Pr). Because red light stimulates seed germination, we know that
ratio of red to far-red light is thus much higher under an open sky Pfr is the active form of phytochrome. That is, red light causes

659
660 SECTION 31.5 T H E E N V I RO N M E N TA L CO N T E X T O F G RO W T H A N D D E V E LO P M E N T

phytochrome to change into its Pfr form

and Pfr sends a signal that triggers seed FIG. 31.23 Detection of shading. Phytochrome detects the depletion of red light under
germination. the canopy.
Phytochrome allows dormant seeds a.
to detect the presence of plants overhead
Red light Absorption of red or far-red light Far-red light
(Fig. 31.23b). Because it responds to the causes phytochrome to switch back
ratio of red to far-red light, the proportion and forth between two forms,
of phytochrome that is in the active Pfr Pr and Pfr.

form is much greater in open environments Inactive Active

form form
than in the forest understory. Furthermore,
because the ratio of red to far-red light is Pr Pfr Seeds germinate.
independent of light intensity, it provides
a reliable signal that leaves are overhead
even for seeds covered by a thin layer of soil.
Independent of whether light is present
or not, Pfr slowly converts back into the
Slow conversion of
inactive Pr form. The light-independent
Pfr to Pr in the dark
conversion of Pfr to Pr prevents seeds that
are too deep in the soil, yet still receive
some light, from germinating.
b.
Plants grow taller and branch less
when growing in the shade of other
plants.
A newly germinated seedling must reach
the soil surface before its stored resources
are used up or it will die. Seeds that
germinate in the dark produce completely Open environments
In open environments,
Pr 60% of phytochrome is Pfr
white seedlings with thin, highly elongated
in the active, Pfr, form.
internodes and small leaves (Fig. 31.24). By
putting all their resources into elongation,
seedlings grown in the dark maximize
their chances of reaching the light. If
and when they do, internodal elongation
slows markedly and green leaves expand.
These dramatic changes are triggered by
photoreceptors that signal the presence Forest understory
In the forest understory,
of light. In its transformation from a The absorption of red light by
Pr as little as 10% of phytochome Pfr
is in the active, Pfr, form.
dark-grown to a light-exposed seedling, chlorophyll means that the
the seedling receives signals from several light in the understory has a
greater amount of far-red
types of photoreceptor, including one light than red light.
that absorbs blue wavelengths and one,
phytochrome, that absorbs both red and far-
red wavelengths.
Even after a plant emerges above the soil, its ability to sense and that the taller, thinner stems are the result of a plant’s sensing
respond to light can enhance its chances for survival. Some species the presence of its neighbors. The reason a plant is able to sense
are adapted for shade, producing thin leaves that efficiently use low neighboring plants is that chlorophyll absorbs red, but not far-
levels of light; others grow best in direct sunlight. When such sun- red, wavelengths of light. Thus, as light passes through leaves, it
adapted species are grown in the shade of an into growing tall. These becomes depleted in red wavelengths. As a result, phytochrome
plant species produce stems that have long, thin internodes and is converted from the Pfr to the Pr form. Furthermore, light
fewer branches, and they invest less in root growth. reflected off the leaves of a plant’s neighbors is also depleted in
Interestingly, the effect is much less if the shade is produced red wavelengths, so phytochrome allows a plant to detect not only
by anything other than another plant. Experiments have shown when it is underneath another plant, but also when it is surrounded
 CHAPTER 31 P L A N T G RO W T H A N D D E V E LO P M E N T 661

FIG. 31.24 Seedlings grown in the light and the dark (left) or in full sun and shade (right). Photo source: Nigel Cattlin/Science Source.

Plants adapted to full sun develop

long, thin internodes, and fewer
leaves, when grown in shade.

Node

Node

Node

Node
Node

Node

Dark Light Full sun Canopy shade

by other plants that could grow to shade it in the future. Thus, production of ethylene, the production of ABA in drought-exposed
phytochrome provides an early-warning system that competitors roots leads to roots that penetrate deeper into the soil.
are nearby.
Exposure to wind results in shorter and stronger stems.
Roots elongate more and branch less when water In the 1980s, scientists studied the model plant Arabidopsis
is scarce. thaliana to learn which genes are expressed in response to different
The developmental sensitivity of plant shoots to their hormones. The experimental treatment involved spraying plants
environment is mirrored underground. When a plant experiences with different solutions containing different hormones; control
drought, it produces more roots, and these roots penetrate farther plants were sprayed with water. The hormone treatments were
into the soil, in part because they produce fewer lateral branches. successful in that several genes were strongly up-regulated.
By producing deeper roots, a plant increases its chances of However, the same genes were also turned on in the plants sprayed
reaching moister soil. with water. Further experiments showed that what the plants were
The root cap appears to play an important role in allowing responding to was neither the hormones nor the water, but the fact
roots to sense and respond to the amount of water in the soil. that their stems were bent back and forth when the plants were
Because the root cap is not well connected to the plant’s vascular sprayed. This led to the discovery of touch-sensitive genes in plants,
system, the water status of its cells provides a good indication of which are activated by mechanical perturbation.
the moisture content of the soil. As soils dry, root cap cells produce This discovery confirmed what foresters and horticulturists
a hormone called abscisic acid, abbreviated as ABA (Table 31.1). had long known. Plants exposed to wind produce stems that are
Abscisic acid stimulates root elongation, leading to roots that shorter and wider than ones grown in more protected sites. In fact,
penetrate deeper into the soil. It also triggers stomata to close, commercial greenhouses often install fans, in part so that their
reducing the demand for water. plants will produce stems robust enough that the plants can thrive
Abscisic acid stimulates root elongation by suppressing ethylene when moved outdoors. Flexing a stem back and forth triggers an
synthesis. Ethylene slows root elongation by influencing the increase in the synthesis of ethylene. As noted in section 31.4, cells
orientation of cellulose in the cell wall. Cells treated with ethylene treated with ethylene expand more in diameter and less in length,
expand in diameter rather than in length. Thus, by inhibiting the resulting in shorter and thicker stems.
662 SECTION 31.6 T I M I N G O F D E V E LO P M E N TA L E V E N T S

Maryland Mammoth flowers. Because the heated greenhouse was

31.6 TIMING OF DEVELOPMENTAL set to the same temperatures as those found during the summer,
EVENTS temperature could also be eliminated as a factor. Garner and Allard
were left with only one environmental variable that was different
As plants moved onto land, they encountered environments with between the summer and the winter: the length of the day.
distinct seasons. In temperate regions, temperatures vary from In a series of experiments in the early part of the last century,
summer to winter; in most tropical habitats, wet seasons alternate Garner and Allen artificially shortened the day length during
with dry ones. Thus, plants evolved mechanisms to control the summer and extended it during winter. Their results showed
timing of developmental transitions such as the production of that in Maryland Mammoth, as well as in many other plants,
flowers and other reproductive organs, the germination of spores flowering is controlled by day length. They coined the term
or seeds, or the formation of overwintering buds. To accomplish photoperiodism to describe the effect of the “photo period,”
this, plants need reliable information that tells them what season or day length, on flowering (Fig. 31.25). Short-day plants, like
it is. While temperature tends to be unreliable—think of unusually Maryland Mammoth, flower only when the day length is less than
warm or cold summers—day length is a dependable source of a critical value. When the light period exceeds this threshold, a
information about the seasons that works everywhere except close short-day plant continues to produce new leaves, but no flowers
to the equator. are formed. In contrast, long-day plants, which include radish,
lettuce, and some varieties of wheat, flower only when the light
Flowering time is affected by day length. period exceeds a certain length. Flowering of day-neutral plants
In the 1910s, Wightman Garner and Henry Allard, scientists at is independent of any change in day length.
the U.S. Department of Agriculture, set out to understand why Sensitivity to day length ensures that plants flower only
Maryland Mammoth, a variety of tobacco, does not produce flowers when they are large enough to support the development of a
during the summer like other tobacco plants. Instead, these plants large number of seeds and there is enough time for those seeds
continue to grow throughout the summer, reaching heights several to mature before the onset of winter. A short-day plant that
times that of a typical tobacco plant and producing many more germinates in the spring will not flower until late summer, when
leaves. Only in autumn does the Maryland Mammoth tobacco plant the decreasing day length falls below the critical value for that
finally produce flowers, too late in the year for seeds to mature. species (Fig. 31.26a). This flowering strategy allows short-day
One possible explanation for the observed delay in flowering plants to grow as large as possible and switch from producing
time is that Maryland Mammoth must achieve a greater size leaves to producing flowers in time to complete seed development
before it produces flowers. However, when Garner and Allard before winter. In contrast, many long-day plants produce flowers
grew Maryland Mammoth plants in a heated greenhouse during during midsummer. These plants germinate in spring and are
the winter, the plants flowered at the same time and size as triggered to flower when the day length exceeds a critical value
other tobacco plants. Therefore, size does not determine when (Fig. 31.26b).

FIG. 31.25 Control of flowering by daylength. Short-day plants flower only when hours of daylight are below a certain number. Long-day plants
flower only when hours of daylight are above a certain number. Photo sources: (top left) Darlyne A. Murawski/Getty Images; (bottom left) Liz Van
Steenburgh/Shutterstock; (top right) Dr. Jeremy Burgess/Science Source; (bottom right) Biosphoto/Dr. Jeremy Burgess/Science Photo Library.

Short-day plants Long-day plants

Midnight 6 am Noon 6 pm Midnight

Midnight 6 am Noon 6 pm Midnight

 CHAPTER 31 P L A N T G RO W T H A N D D E V E LO P M E N T 663

FIG. 31.26 Flowering strategies for short-day and long-day plants. (a) A short-day plant that germinates in the spring flowers in the late
summer when day length falls below a critical value. (b) A long-day plant flowers in the summer after a critical day length is reached.
(c) A long-day plant that requires vernalization does not flower until the critical day length is reached in the second summer.

a. Short-day plant b. Long -day plant

16
Critical daylength Plant flowers.
to trigger flowering

14 Plant flowers.
Hours of daylight

Plant germinates.
Plant germinates.
12

Although days are

short, the plant is
too small to respond
10 to daylength.

8
Dec June Dec Dec June Dec

c. Long-day plant that requires vernalization

16
Although the days are long Critical daylength
Plant flowers.
during the first summer, to trigger flowering
this species does not
flower without prior
14 exposure to cold
temperatures.
Hours of daylight

Plant germinates.

12

10

Vernalization

8
Dec June Dec June Dec

Plants use their internal circadian clock and phytochrome tells the plant whether it is light or dark during a
photoreceptors to determine day length. specific phase of the circadian cycle.
To determine day length, one first needs to know what time The actual mechanism used by plants to determine day length
it is, specifically how long it has been since the sun came up, relies on the fact that the plant’s circadian clock affects the
and then whether it is light or dark outside. Plants, like all transcription of many genes. For example, in Arabidopsis thaliana, a
organisms, have an internal circadian clock. Circadian clocks are long-day plant, a gene whose protein product is critical for triggering
biochemical mechanisms that oscillate with a 24-hour period flowering is transcribed at increasingly higher rates throughout
and are coordinated with the day–night cycle. Phytochrome is the day, peaking ~16 hours after dawn. After that, the rate of
the photoreceptor responsible for synchronizing the plant’s transcription declines. The protein product of this gene is targeted
internal oscillator with the first light of dawn. The circadian clock for rapid degradation unless light is present. Thus, whether the
tells the plant how long it has been since the sun came up, while protein builds up to a level sufficient to trigger flowering depends on
664 CO R E CO N C E P T S S U M M A RY

day length. During long days, the protein reaches levels needed to flower, when the chances for successful pollination and seed
trigger flowering because peak production occurs during daylight, development are greater (Fig. 31.26c). Even parts of the plant
when the protein is not targeted for destruction. During short that form long after the cold stimulus is past seem to “remember”
days, the protein product does not reach levels needed to trigger that they received the required cold treatment. This “memory”
flowering because the gene is transcribed at high rates only after the of winter is important because many plants that overwinter grow
sun has gone down, so the protein is rapidly degraded. rapidly in the spring. What is the basis of this “memory”?
This mechanism explains why interrupting the dark period One common mechanism acts through chromatin remodeling.
with a brief exposure to light alters the flowering response. In The DNA of all eukaryotic organisms is bound with proteins
an early set of experiments, both short-day plants and long-day to form chromatin. We saw in Chapter 19 that modifications
plants were grown under a photoperiod that would normally to chromatin, such as DNA methylation, can change gene
cause the short-day plants to flower and the long-day plants expression. In Arabidopsis, for example, vernalization results in
to produce only leaves. A brief exposure to light during the chromatin remodeling that turns off a gene whose protein product
dark period reversed this result. The short-day plants no longer represses flowering. Chromatin remodeling is stable through
produced flowers, but the long-day plants did. Furthermore, the mitotic divisions, explaining why newly formed parts of a plant
night interruption was effective only when the light contained “remember” winter. However, the slate is wiped clean during
red wavelengths, and the effects of red light could be reversed meiosis, and the requirement for vernalization is reinstated with
if exposure to far-red light immediately followed. These results each generation.
indicate that the light is detected by phytochrome. We now
understand that the night interruption acts like a switch Plants use day length as a cue to prepare for winter.
that converts phytochrome into its active form. In that form, We have seen how plants use day length to control when flowers
phytochrome can then affect the stability of proteins whose are produced. Day length also triggers other developmental
synthesis follows a circadian rhythm. events, particularly ones that allow plants that persist for more
Other experiments demonstrated that day length is sensed by than a year to prepare for winter. One such developmental
the leaves and not by the buds where the flowers will form. When change is the formation of storage organs, for example in roots.
conditions are right for flowering, the protein florigen is synthesized Carbohydrates that accumulate in these structures can support the
in leaves and transported through the phloem to the growing points growth of new leaves and stems the following spring.
of a plant. In this way, florigen is a chemical signal or hormone that A second developmental change in response to day length is
converts shoot meristems into floral meristems. the formation of overwintering buds. As the days begin to shorten,
plants stop producing photosynthetic leaves and begin forming
Vernalization prevents plants from flowering until bud scales. Bud scales surround and protect the meristem from ice
winter has passed. and water loss. The formation of bud scales accompanies a series of
In some species, flowering is induced only if the plant has metabolic changes that allow meristems to remain in a dormant
experienced a prolonged period of cold temperatures, a process state throughout the winter. For example, plugs are produced that
known as vernalization. A requirement for vernalization block the plasmodesmata between the meristem and the rest of
prevents long-day plants from flowering during their first summer, the plant. These plugs prevent any growth-stimulating compounds
and instead forces them to wait until the following spring to from reaching the meristem. •

Core Concepts Summary Stems contain zones of cell division, elongation, and maturation.
Most of the increase in size occurs in the zone of cell elongation.
31.1 In plants, upward growth by stems occurs at shoot page 643
apical meristems, populations of totipotent cells that In seed plants, branching occurs at axillary buds at the base of
produce new cells for the lifetime of the plant. each leaf. page 644

Stems are built from repeating modules consisting of nodes, Leaves differ from stems in that they differentiate fully and thus
where one or more leaves are attached, and internodes, which have no capacity for continued growth. page 645
are the regions of stem between the nodes. page 642 Leaves develop from leaf primordia produced by the shoot apical
meristem. page 645
The shoot apical meristem, located at the tip of each stem, is
the source of new cells from which the stem and the leaves are In most seed plants, the stem’s vascular bundles are organized as
formed. page 642 a ring. page 646
 CHAPTER 31 P L A N T G RO W T H A N D D E V E LO P M E N T 665

Shoot apical meristems and axillary buds can be transformed Roots alter their development in response to conditions in the
into floral meristems, at which point they lose their capacity for soil. page 655
continued growth. page 646
Some roots are involved in adhesion, storage, and gas diffusion.
31.2 Hormones are chemical signals that influence the page 656
growth and differentiation of plant cells.
31.5 Plants respond to light, gravity, and wind through
Traditionally, five plant hormones have been identified: changes in internode elongation and the development of
auxin, gibberellic acid, cytokinins, ethylene, and abscisic acid. leaves, roots, and branches.
page 647
Plant stems grow toward the light and away from gravity as a
Xylem and phloem differentiate from procambial cells. result of redistribution of auxin. page 656
page 649
Roots grow away from light and toward gravity as a result of
The polar transport of auxin through PIN proteins guides redistribution of auxin. page 656
the formation of leaf primordia and the development of
vascular channels (xylem and phloem) that connect leaves Starch-filled statoliths allow roots to sense and respond to
and stems. page 649 gravity. page 659

Gibberellic acid causes internodal elongation of shoots. Phytochrome is a photoreceptor that allows plants to
page 650 determine the amount of red versus far-red light. Because light
filtered through leaves has lower levels of red wavelengths,
Apical dominance is the suppression of the outgrowth of axillary
phytochrome allows plants to detect the presence of plants
buds caused by hormones produced by roots (strigolactone) and
overhead. page 659
shoots (auxin). page 650
Seedlings germinating in the dark produce elongated internodes
31.3 Lateral meristems allow plants to grow in diameter, and suppress leaf expansion to increase their ability to reach the
increasing their mechanical stability and the transport soil surface before their stored resources are depleted. page 660
capacity of their vascular system.
Some plants respond to the presence of neighboring plants by
The formation of two lateral meristems—the vascular cambium elongating more rapidly and suppressing branching, which allow
and the cork cambium—allows plants to grow in diameter. them to maximize height growth. page 660
page 651
Mechanical stress, such as the bending of stems in the wind,
The vascular cambium produces secondary xylem toward the results in stems that are short and wide. page 661
center and phloem toward the outside. page 651
31.6 Plants have sensory systems that control the timing
The cork cambium produces a protective outer layer. page 652
of developmental events.
Gymnosperm tracheids function in both mechanical support
Many plants are photoperiodic, meaning that their flowering is
and water transport. Angiosperm xylem produces fibers that
controlled by the day length. page 662
influence the strength of wood and vessel elements that
function in water transport. page 653 Plants measure day length through an interaction between
photoreceptors and their internal circadian clock. page 663
31.4 The root apical meristem produces new cells that
allow roots to grow downward into the soil, enabling Vernalization is a prolonged period of exposure to cold
plants to obtain water and nutrients. temperatures necessary in some temperate-zone species before
flowering can be induced. page 664
The root apical meristem is the source of new cells that allow
roots to elongate. page 654 Decreases in photoperiod trigger plants to prepare their
meristems for winter by producing bud scales and reducing
The root apical meristem has a cap that protects it from damage
internode elongation. page 664
as the root grows through the soil. page 654

The root’s vascular tissues are located in the center of the root,
surrounded by the pericycle, the endodermis, the cortex, and the Self-Assessment
epidermis. page 654
1. Diagram the zones of cell division, elongation, and
Lateral roots develop from new root apical meristems that form maturation, and explain why this organization allows stems to
in the pericycle. page 655 grow without a predetermined limit to their length.
 PASS 1

666666 SSEELCFT-IAOSN
S E3S0S.M
1 EN
AT

2. Name one role of the plant hormone auxin and 8. Give an example of how a plant’s ability to sense its
describe how auxin is transported within a plant. environment improves the plant’s chances for survival and
reproduction.
3. Explain why a plant that has a vascular cambium
also has a cork cambium. 9. Explain why a short-day plant that germinates in the
spring will not flower until late summer and why a long-day
4. Explain why the vascular cambium forms a
plant that germinates at the end of summer will not flower
continuous sheath that runs from near the tips of the
until late the following spring.
branches to near the tips of the roots, whereas the cork
cambium is discontinuous in both space and time. 10. Describe how vernalization can have an effect in cells that
were not formed at the time of the cold treatment.
5. List three structural differences between roots and
shoots that allow roots to grow through the soil. Explain
how you would tell whether an isolated piece of a plant Log in to to check your answers to the Self-
came from the shoot or from the root. Assessment questions, and to access additional learning tools.

6. Discuss the similarities and differences in the ways

stems and roots respond to gravity and light.
7. Diagram how the amount of red versus far-red light
allows plants to determine if they are in the open or in
the forest understory.
 CHAPTER 32

Plant
Defense
Keeping the World Green

Core Concepts
32.1 Plants have evolved
mechanisms to protect
themselves from infection
by pathogens, which include
viruses, bacteria, fungi, worms,
and even parasitic plants.
32.2 Plants use chemical,
mechanical, and ecological
defenses to protect themselves
from being eaten by
herbivores.
32.3 The production of defenses
is costly, resulting in trade-
offs between protection and
growth.
32.4 Interactions among plants,
pathogens, and herbivores
contribute to the origin and
maintenance of plant diversity.
Don Johnston/Getty Images.

667
668 SECTION 32.1 P ROT E C T I O N AG A I N S T PAT H O G E N S

Plants have complicated interactions with animals. As we saw

in Chapter 30, many plants rely on animals for pollination or FIG. 32.1 A potato plant infected with Phytophthora infestans,
seed dispersal. In return, plant seeds provide a rich source of an oomycete that causes blight. Source: Nigel Cattlin/
food for animals, and leaves furnish a vast, but lower-quality, Science Source.
food resource. The plant body also provides habitats for an array
of viruses, bacteria, and protists, not to mention other plants,
which may grow on or within their tissues. How do plants defend
themselves against the animals that would eat them? How do
they defend against pathogens? The answers to these questions
help us understand how plant populations are distributed in
nature, and they help us solve practical problems as well. In the
United States alone, crop damage by herbivores and pathogens
costs farmers more than $2 billion every year.
Plants can’t run away and hide as animals can. Nonetheless,
they can sense danger and they are able to deploy a diverse and
powerful defensive arsenal that includes physical and chemical
deterrents as well as aerial surveillance, armed guards, and sticky
traps.
Plants produce a wide array of chemicals that alter the
metabolism and influence the behavior of animals that seek
to consume them. These molecules also interact with our own
biochemistry, and so compounds derived from plants have been
used since antiquity as sources of medicine. Our early ancestors
turned to the leaves and bark of willow trees in much the same
way that we rely on aspirin—and for good reason. Willows produce
a compound that deters herbivores but in humans relieves aches
and pain; this compound in fact provides the chemical basis
of aspirin. Quinine, derived from the tropical cinchona tree, is immune system that allows them to recognize and respond
used in the treatment of malaria, while belladonna, a relative to pathogens that would otherwise exploit them. In the Irish
of the tomato, produces toxins that the ancient Romans used potato famine, which lasted from 1845 to 1852, these defenses
to eliminate rivals but in very low doses relieve gastrointestinal were breached, allowing P. infestans to spread and grow with
disorders. In fact, nearly 30% of all medicines are based on devastating effects on the potato crop and, in consequence, the
chemicals that plants produce to defend themselves. people of Ireland.

Plant pathogens infect and exploit host plants by a

32.1 PROTECTION AGAINST variety of mechanisms.
PATHOGENS The epidermis, with its thick walls and waxy cuticle, is a plant’s
first line of defense against pathogens. Viruses and bacteria cannot
In September 1845, potato plants across Ireland began to die. Their penetrate the cuticle and so commonly enter through wounds or
leaves turned black and the entire plant began to rot. When the on the piercing mouthparts of insects and nematodes. Stomata
potatoes were dug up, those that were not already rotten soon form a natural entry point for bacteria, oomycetes, and fungi
shriveled and decayed into a foul-smelling and inedible mush. The (Fig. 32.2). Some bacteria and fungi even secrete chemicals that
biological agent that had destroyed the potato crop looked like a prevent stomata from closing. Other pathogens, notably parasitic
fungus, but in fact was an oomycete called Phytophthora infestans, plants and some species of fungi, secrete enzymes that weaken
a type of protist (Chapter 27). epidermal cell walls. These invaders then force their way through
Phytophthora infestans is one example of the diverse the weakened epidermis. Mechanical cultivation can damage crop
pathogens that infect plants. Plant pathogens include viruses, plants, thus also providing entry points for pathogens.
bacteria, fungi, nematode worms, and even other plants, but what Once inside the plant, fungi can spread by growing through
they all have in common is the ability to grow on and in the tissues plant cell walls. In contrast, viruses move from one plant cell to
of plants, extracting resources to support their own growth and another through plasmodesmata. Because the genomes of most
causing an array of diseases. The outcome for the plant is often plant viruses are single strands of RNA, they can travel by the
death (Fig. 32.1). The fact that most plants appear disease free is plant’s natural mechanism for moving messenger RNA between
testimony to their ability to defend themselves. Plants have an cells. The plant vascular system provides a highway that allows
 CHAPTER 32 PL ANT DEFENSE: KEEPING THE WORLD GREEN 669

a fungal pathogen whose spores are carried in the xylem. More

FIG. 32.2 Entry points for pathogens. A scanning electron recently, the California wine industry has been threatened by
micrograph shows Phytophthora infestans gaining access Pierce’s disease, which results from infection by Xylella fastidiosa, a
to the leaf interior by growing through stomata. Source: bacterium that also moves through the xylem.
Clouds Hill Imaging Ltd/Science Source. Biotrophic pathogens obtain resources from living cells,
Stomata whereas necrotrophic pathogens kill cells before exploiting
them. Most biotrophic pathogens do not enter into the host
plant’s cells. Instead, they produce chemicals that stimulate the
host cells to secrete compounds that the pathogen can use as
food. Viruses, however, are biotrophic pathogens that do enter
the cell: They must hijack a living cell’s machinery to reproduce.
Necrotrophic pathogens produce toxins that kill cells; the
pathogens then feed on compounds that leak from the dead and
dying cells. Bacteria and fungal pathogens can be either biotrophic
or necrotrophic. For example, wheat leaf rust (Puccinia species) is a
biotrophic fungus that significantly reduces wheat yields (Chapter
34). Rice blast (Magnaporthe grisea) is a necrotrophic fungus that
each year reduces rice production by an amount that would feed
60 million people.
Parasitic plants obtain resources by infecting other plants
(Fig. 32.3). Parasitic plants produce structures that penetrate
into the stems or roots of other plants, eventually tapping into
the host plant’s vascular system. Some, like some mistletoes
(Fig. 32.3a), are xylem parasites: They obtain water and nutrients
Phytophthora
infestans from the xylem of the host plant without expending the costs
of producing roots themselves. Other plant pathogens target
both the xylem and the phloem. In the most extreme cases, the
pathogens to move over long distances. Viruses are transported parasite receives all its carbohydrates from its host and carries
within the phloem, whereas bacteria and fungi move through out no photosynthesis of its own. Rafflesia (Fig. 32.3b), famous
the xylem. During the 1950s, commercial banana production for producing the largest flower in the world, grows entirely
in Mesoamerica was nearly wiped out by Fusarium oxysporum, within the tissues of its host, emerging only when it produces its

FIG. 32.3 Parasitic plants. (a) Mistletoe carries out photosynthesis, but relies on its host plant for water and nutrients. (b) Rafflesia, which
produces the world’s largest flower, also obtains everything it needs from its host. (c) Cuscuta, or dodder, parasitizes the stems of many
plants. It does not photosynthesize and instead receives everything it needs from its host. Sources: a. © Mark Boulton/NHPA/Photoshot;
b. Paul Kennedy/Gettty Images; c. Clive Varlack/clivevbugs/AGPix.

a b c

50 cm
670 SECTION 32.1 P ROT E C T I O N AG A I N S T PAT H O G E N S

FIG. 32.4 The plant immune system. (a) In basal resistance, plasma membrane receptors recognize molecules produced by broad classes of
pathogens. (b) Specific resistance depends on R genes that allow plant cells to identify and deactivate AVR proteins produced by
specific pathogens.

a. Basal resistance b. Specific resistance

Pathogens
Pathogen secrete AVR
proteins that
AVR can enter
Receptor protein plant cells.
protein

Plant cell

Cell wall R proteins

Defense response No defense response Defense response

Plasma
membrane No infection Virulent infection Avirulent infection

A receptor protein in the plant In the absence of a When an R protein binds with an AVR
cell’s plasma membrane binds matching R protein, the protein, it prevents the AVR protein from
with a pathogen-derived molecule AVR protein blocks the blocking the plant’s basal resistance and
and triggers a defense response. plant’s basal resistance. directly activates defensive genes.

giant flowers. The ability to parasitize other plants has evolved mount an appropriate response. A key feature of the immune
independently multiple times. Currently, more than 4000 species system is that plants, like animals, can recognize a cell or virus as
of parasitic plants have been identified. foreign. How do they accomplish this?
How do parasitic plants find an appropriate host? Mistletoe Plant cells have protein receptors (Chapter 9) that bind to
fruits, which are eaten and dispersed by birds, have a sticky layer molecules produced by pathogens and recognize them as foreign
that adheres to the stems and branches of trees that the plant will to the plant body. Binding changes the conformation of the
parasitize. Dodder, a parasitic vine (Fig. 32.3c), orients its growth receptor, triggering a cascade of reactions that enhance the plant’s
toward a potential host plant, guided by volatile organic compounds ability to resist infection.
produced by the species it infects. One of the most destructive plant The plant immune system has two components (Fig 32.4).
pathogens is witchweed (Striga species), which infects the roots of The first consists of receptors located on the plasma membrane
a number of crop species, including corn, sorghum, and sugarcane. (Fig 32.4a). These receptors recognize highly conserved molecules
Striga seeds remain dormant in the soil until their germination is generated by broad classes of pathogens. Examples of these
triggered by chemicals released from the roots of host species. Striga molecules are flagellins, present in the flagella of bacteria, and
infects more than 40% of all agricultural areas in Africa, driving chitin, a component of the fungal cell wall. When one of these
down crop yields, in some cases to zero. molecules binds to the plant’s receptor, an array of defense
mechanisms is triggered that help protects the plant from
Plants are able to detect and respond to pathogens. infection. Protection from pathogens conferred by this component
Most plants are at risk of infection by only a small subset of of the immune system is called basal resistance.
the many known plant pathogens. Host plants of a particular The second component of the plant immune system allows
pathogen are the species that can be infected by that pathogen. plants to resist pathogens that have evolved means of overcoming
Not all infections significantly damage the host plant, however. basal resistance. Pathogens produce proteins called AVR proteins
Virulent pathogens can overcome the host plant’s defenses and that enter into plant cells and facilitate infection (“AVR” stands
lead to disease. In contrast, avirulent pathogens damage only a for “avirulence”). Some AVR proteins block defense responses
small part of the plant because the host plant is able to contain the triggered by the basal resistance of plants, while others cause the
infection. Whether an interaction is virulent or avirulent depends cell to secrete molecules that the pathogen needs to support its
on the genotypes of both the pathogen and the host plant. own growth. The second component of the plant immune system
In your own body, the innate immune system provides a first targets these AVR proteins.
line of defense against pathogens (Chapter 43). Plants, too, have Like basal resistance, this component of the immune system
an immune system that allows them to detect pathogens and also consists of receptors, but in this case they are located inside
 CHAPTER 32 PL ANT DEFENSE: KEEPING THE WORLD GREEN 671

the cell rather than on the plasma membrane (Fig. 32.4b). Plants
harbor hundreds of these receptors, called R proteins, each FIG. 32.5 Hypersensitive response. A tobacco plant infected with
expressed by a different gene—these genes are collectively called tobacco mosaic virus responds by producing a region
R genes (the “R” in both cases stands for “resistance”). Each of dead tissue that surrounds the site of infection and
R protein recognizes a specific AVR protein. When an R protein prevents the virus from spreading to other parts of the
binds with a pathogen-derived AVR protein, it both prevents the plant. Source: Norm Thomas/Science Source.
AVR protein from blocking the plant’s defenses and activates
additional defenses. Because this component of the immune
system depends on interactions between specific plant and
pathogen genes, it is commonly called the gene-for-gene model
of plant immunity. Protection from pathogens conferred by this
component of the immune system is called specific resistance.
The large number of R genes is the result of an evolutionary
arms race that occurs as pathogens evolve ways to evade their
host’s detection systems. Pathogens improve their success rate
when mutation results in an AVR protein that does not bind with
any of the R proteins produced by its host plant. For example,
P. infestans, the pathogen responsible for the Irish potato famine,
produces many more AVR proteins than related Phytophthora
species. The large number of AVR proteins found in P. infestans
increases the probability that at least one of these proteins is not
detected by the infected plant. Conversely, when pathogens are
successful, natural selection favors plant populations that produce
new R proteins capable of binding to the AVR proteins of the
invader.

j Quick Check 1 Can the specific component of the plant immune

system (the one using R genes) defend against both biotrophic and Site of initial infection
necrotrophic pathogens?

(Fig. 32.6). The widespread death of native elm trees in both

Plants respond to infections by isolating infected Europe and North America is the result of a vascular wilt disease
regions. caused by a fungus introduced from Asia but first identified in
Once a plant has detected a pathogen, how does it protect the Netherlands and therefore referred to as Dutch elm disease.
itself? One way is by reinforcing its natural barriers against
pathogens. For example, plants commonly defend themselves by Mobile signals trigger defenses in uninfected tissues.
strengthening their cell walls, closing their stomata, and plugging In 1961, American plant pathologist A. F. Ross reported
their xylem. Or the plant can counterattack. Plants produce a that when individual tobacco leaves were infected with the
variety of antimicrobial compounds, including ones that attack tobacco mosaic virus (TMV), they developed necrotic patches
bacterial or fungal cell walls. A third type of defense is known characteristic of a hypersensitive response. However, when
as the hypersensitive response. In this case, uninfected cells uninfected leaves of the same plant were subsequently exposed
surrounding the site of infection rapidly produce large numbers to the virus, they suffered little or no damage (Fig. 32.7). Ross
of reactive oxygen species, which trigger cell wall reinforcement called this ability to resist future infections systemic acquired
and cause the cells to die (Fig. 32.5). The death of surrounding resistance (SAR). Initially reported for viral infections, SAR
cells creates a barrier of dead tissue that prevents the spread of occurs in response to a wide range of pathogens, including
biotrophic pathogens. bacteria and fungi.
Plants can defeat pathogens by isolating infected regions to a How do uninfected leaves acquire resistance to pathogens?
far greater degree than is possible in most animals. For example, One hypothesis is that a chemical signal is transported from the
plants respond to the presence of xylem-transported pathogens infected region through the phloem. This chemical signal then
by plugging xylem conduits. If the infection is caught in time, triggers the expression of genes encoding many of the same
the pathogen is prevented from spreading throughout the plant. proteins that defend against the pathogen in infected cells. The
But if containment fails and the pathogen spreads, the plant identity of the mobile signal has proved elusive, but experiments
may contribute to its own demise by blocking its entire vascular show that salicylic acid—the chemical basis of aspirin—is required
system. Virulent pathogens that move through the xylem cause a for SAR. The methylated form of salicylic acid, known familiarly as
variety of disorders collectively referred to as vascular wilt diseases oil of wintergreen, is also a potent inducer of defense responses.
672 SECTION 32.1
HOW DO WE KNOW?

FIG. 32.7
FIG. 32.6 Vascular wilt diseases. Pathogens that
are transmitted through the xylem can kill
their host if plant defenses also block water Can plants develop immunity to
transport to the leaves. (a) Panama disease,
which affects bananas, is caused by a fungal specific pathogens?
pathogen. (b) Pierce’s disease, which affects
BACKGROUND Tobacco is susceptible to an infectious disease that causes its
grapes, is caused by a pathogenic bacterium.
leaves to turn a mottled yellow, hence the name of the disease—tobacco mosaic
Sources: a. Guy Blomme/Bioversity International;
disease. In the 1890s, experiments with filters showed that the infectious agent was
b. Bruce Fleming/Cephas Picture Library.
smaller than a bacterium, a finding that led to the discovery of viruses. Some plants
a succumb to the disease, but others do not. Are the plants that survive the initial
infection immune from further attacks?

HYPOTHESIS Plants that survive infection with tobacco mosaic virus (TMV) have
acquired immunity and therefore resist further attack.

EXPERIMENT American plant pathologist A. F. Ross infected one leaf on a tobacco

plant with TMV. One week later, he exposed another leaf on the same plant to the
virus.

RESULTS Plants that had been previously exposed to TMV showed no signs of
infection when exposed to the virus a second time.

7 days

TMV

Tobacco
plant

b Infect one leaf Infect a second

with TMV. leaf with TMV.

The second leaf

The first leaf exposed to TMV
exposed to TMV shows no visible
shows necrotic sign of infection
regions resulting or disease.
from a hypersen-
sitive response.

CONCLUSION Tobacco leaves that have never been exposed to TMV can acquire
resistance to the pathogen if another leaf on the same plant has been previously exposed.
This result indicates that a signal has been transmitted from the originally infected leaf to
the undamaged parts of the plant, and that the transmitted signal subsequently triggers
the development of an immune response that protects the plant from further infection.

FOLLOW-UP WORK Plants are now known to acquire immunity to a wide range of
pathogens in addition to viruses.

SOURCE Ross, A. F. 1961. “Systemic Acquired Resistance Induced by Localized Virus Infections in Plants.”
Virology 14 : 340–358.
 CHAPTER 32 PL ANT DEFENSE: KEEPING THE WORLD GREEN 673

are formed. Because plant cells do

FIG. 32.8 Plant defense by small interfering RNAs (siRNAs). siRNAs target complementary not normally make double-stranded
single-stranded RNA for destruction. RNA, the replicating viral genomes
are identified as foreign. Enzymes
1 A virus infects the produced by the plant cell cleave the
host plant cell by
injecting its double-stranded RNA molecules into
single-stranded
Plant cell 2 During the small pieces of 20 to 25 nucleotides,
RNA (ssRNA) replication of the
genome. viral genome, forming fragments called small
double-stranded interfering RNA, or siRNA
RNA molecules (Chapter 3). These fragments bind
ssRNA
(dsRNA) are formed.
with specific protein complexes in the
5 ssRNA molecules cell. The RNA–protein complexes then
that bind to the play a role in targeting and destroying
protein-bound
single-stranded RNA molecules that
siRNA strand are
destroyed. dsRNA 3 The plant cell have a complementary sequence—that
recognizes dsRNA is, the viral genome (Chapter 19). The
as foreign and virus is unable to replicate and thus
cleaves it into
cannot spread. In this way, the plant
fragments of 20–25
nucleotides called cell uses RNA chemistry to eliminate
siRNA small interfering viral infections.
RNA (siRNA).
When a cell is attacked by a virus,
the siRNA molecules that are produced
4 in response to the initial infection
siRNA binds to a plant protein
complex, which retains only one can also spread, allowing the plant
of the RNA strands. to acquire immunity against specific
viruses. The siRNA molecules move
through plasmodesmata, enter the
Because methyl salicylic acid vaporizes readily, the transport of this phloem, and from there spread throughout the plant. Thus, the
compound through the air may play a role in signaling the presence systemic response to viral infection consists of both general
of pathogens. Airborne signals may be particularly important for defense responses triggered by salicylic acid and the transport
leaves: Because leaves export carbohydrates rather than receive of highly specific molecules that can target and destroy viral
them, they are not able to receive signaling molecules transported genomes throughout the plant.
in the phloem.
Up-regulation of salicylic acid occurs when infection by a A pathogenic bacterium provides a way to modify
biotrophic pathogen triggers a hypersensitive response. Plants plant genomes.
respond to necrotrophic pathogens by producing different Some pathogenic bacteria alter their host plant’s biology
signaling molecules, notably jasmonic acid and the plant hormone by inserting their own genes into the host’s genome. One
ethylene. A general feature of these responses is that they activate such pathogen, Rhizobium radiobacter (formerly known as
defenses effective against broad classes of pathogens. Only in the Agrobacterium tumifaciens), is useful to humans, despite the fact
case of viral infections, discussed next, is the response targeted to that it can cause widespread damage to crops such as apples and
an individual pathogen. grapes. The reason? This bacterium’s naturally evolved ability to
incorporate its DNA into the genome of its host plant provides a
j Quick Check 2 How does the plant hypersensitive response differ
surprisingly easy way to genetically modify plants.
from systemic acquired resistance (SAR)?
Rhizobium radiobacter is a soil bacterium with virulent strains
that infect the roots and stems of many plants. Virulent bacteria
Plants defend against viral infections by producing enter a plant through wounds and move through the plant’s cell
siRNA. walls, propelled by flagella. A tumor, or gall, forms at the point of
Viruses can be potent infectious agents, but plants have evolved infection, often where the stem emerges from the soil, and for
responses to viral infection—including a hypersensitive response— this reason R. radiobacter infection is called crown gall disease.
that are much like those mounted against other pathogens. In R. radiobacter alters the growth and metabolism of infected cells by
addition, plants have a form of defense that targets the virus itself inserting some of its own genes into the chromosomal DNA of the
(Fig. 32.8). Most plant viruses have genomes made of single- host plant (Fig. 32.9).
stranded RNA (ssRNA). During the replication of the viral genome Virulent strains of R. radiobacter contain a plasmid, called the
inside the plant cell, double-stranded RNA molecules (dsRNA) Ti plasmid, which is a small (~200 kb), circular DNA molecule
674 SECTION 32.2 D E F E N S E AG A I N S T H E R B I VO R E S

FIG. 32.9 The formation of a crown gall tumor. Rhizobium radiobacter, a soil bacterium, can insert genes into the plant genome, altering the
growth and metabolism of the plant in ways that enhance the growth of R. radiobacter bacteria.

Bacterial Ti genes cause the host cells

chromosome to divide to form a tumor and
Ti plasmid to produce compounds that
the bacteria can metabolize.

Bacteria
enter the
plant
through
a wound.
Wound Plant Tumor
chromosome
Plant nucleus

A section of the bacterial Ti

plasmid is inserted into the
plant’s genome.
Specialized compounds
Rhizobium
metabolized by bacteria
radiobacter

containing the genes that are integrated into the host cell’s genome, themselves against organisms from caterpillars to cows only
as well as all the genes needed to make this transfer. Some of the increased. Given that they are unable to run away, plants would
transferred genes—including ones that result in the synthesis of seem to be the ideal food. But plants have evolved a diversity
the two plant hormones auxin and cytokinin (Chapter 31)—cause of mechanisms that deter would-be consumers. Because these
infected cells to proliferate and form a gall. Other transferred genes mechanisms deplete resources that could otherwise be directed
induce cells in the gall to produce specialized compounds that the toward growth and reproduction, plants have also evolved means
bacteria use as sources of carbon and nitrogen for growth. Note that of deploying their defenses in a cost-effective manner. Here,
because plant cells do not move (in contrast to animal cells), there is we explore the mechanical, chemical, developmental, and even
no possibility that the tumor-forming cells will spread to other parts ecological means by which plants protect themselves in a world
of the plant. teeming with hungry herbivores.
Biologists interested in genetically modifying crops and other
plants make use of R. radiobacter’s ability to insert genes into Plants use mechanical and chemical defenses to avoid
the host genome. The first step is to replace the genes in the Ti being eaten.
plasmid that are usually inserted into the plant genome. Genes Milkweeds (Asclepias species) are commonly found in open fields
for gall formation are removed, and genes of interest are inserted. and roadsides across North America. They illustrate well how
Examples of genes that have been transferred into plants in this plants protect themselves from herbivorous animals. In fact, most
way include genes whose products increase the plant’s nutritional generalist herbivores, which eat a diversity of plants, do not use
value or confer resistance against disease. In addition, researchers milkweeds as a food source. Only specialists, such as caterpillars of
need a way to identify which cells have taken up the genes, and so the monarch butterfly (Danaus plexippus), can feed on these well-
a marker gene, for example one conferring antibiotic resistance, is defended plants.
typically included. In this way, cells that survive an application of In part, milkweed’s defenses are mechanical. The leaves of many
the antibiotic are known to have successfully incorporated the Ti milkweed species are covered with dense hairs. An insect crawling
plasmid into their genome. on a leaf must make its way through as many as 3000 hairs per
square centimeter, a journey likely more daunting to an insect than
wading into a blackberry thicket is to a human. Monarch caterpillars
32.2 DEFENSE AGAINST HERBIVORES may spend as much as an hour mowing off these hairs before they
begin to feed. This long handling time represents a significant cost.
The first land plants were followed closely in time by the ancestors To understand why monarch caterpillars make this investment, let’s
of spiders and scorpions. Fossils show that these earliest land look at milkweed’s other defenses.
animals soon began to feed on plant fluids and tissues. As plants A network of extracellular canals filled with a white, sticky
diversified, so did herbivores, and the pressure on plants to defend liquid called latex runs through the milkweed leaf, both within
 CHAPTER 32 PL ANT DEFENSE: KEEPING THE WORLD GREEN 675

FIG. 32.10 How monarch caterpillars feed on milkweed. Young monarch caterpillars disarm milkweed defenses by digging trenches, while
older, larger caterpillars sever major veins. Both methods prevent the sticky latex from flowing into the feeding area.

a. Damaged milkweed b. Trench c. Vein cut

Exuded latex Small latex

accumulates. canals
Trench Severed vein

Large latex
canals

Flow of latex

Latex, which is sticky, exudes

from damaged areas and Small monarch caterpillars dig Large monarch
protects milkweeds from trenches that prevent latex from caterpillars sever
many herbivores. flowing into the feeding area. the midvein.

the major veins and extending into the regions between veins Most plant species have mechanical or chemical defenses
(Fig. 32.10). Latex canals are under pressure in the intact leaf. against herbivores, or even both. Mechanical defenses are varied.
Thus, when the leaf is damaged, the latex flows out, sticking to Hairs are common on leaves, and, as anyone who has ever rubbed
everything it comes into contact with and rapidly congealing on against a stinging nettle knows, these hairs are sometimes armed
exposure to air. A feeding insect is in danger of having its tiny with chemical irritants. Grasses and some other plants have a hard,
mouthparts glued together, with potentially fatal consequences. mineral defense consisting of silica (SiO2) plates formed within
Small monarch caterpillars cut trenches into the leaf that prevent epidermal cells. Silica wears down insect mouthparts, so the insects
latex from flowing onto the region where they are feeding. Larger feed less efficiently and grow more slowly. On a larger scale, some
caterpillars sever the midvein, relieving the pressure within the tropical trees have prickles, spines, or thorns (Fig. 32.11). Even
latex canals downstream of the cut. The caterpillars are then able something as simple as how tough a leaf is can have a large impact
to feed without risk on the part of the leaf that lies beyond the on the probability of the leaf’s being eaten. In fact, leaves are most
severed midvein. likely to be eaten while they are still growing and their tissues not
Milkweed latex is not only sticky but also toxic. In particular, fully hardened. Nonetheless, the most spectacular diversity in plant
milkweed latex contains high concentrations of cardenolides, defenses lies in the realm of chemistry.
steroid compounds that cause heart arrest in animals. (In small
doses, these compounds are used to treat irregular heart rhythms in Diverse chemical compounds deter herbivores.
humans.) Monarch caterpillars do not metabolize the cardenolides The cardenolides produced by milkweeds are only one example
that they consume, but instead sequester them within specific from the vast chemical arsenal available to plants for protection.
regions of their bodies as a chemical defense against their own Some plants produce alkaloids, nitrogen-bearing compounds that
predators. Because of the presence of these cardenolides, monarch damage the nervous system of animals. Commonly bitter tasting,
caterpillars and even the adult butterflies are highly toxic to alkaloids include such well-known compounds as nicotine, caffeine,
most birds and other animals that would otherwise eat them. morphine, theobromine (found in chocolate), quinine (a treatment
The protection conferred by these chemicals helps explain why for malaria), strychnine, and atropine. Alkaloids are a costly defense
monarch caterpillars have evolved to feed on leaves that are both because they are rich in nitrogen, an essential and often limiting
metabolically difficult and time-consuming to eat. element that plants need to build proteins for photosynthesis.
676 SECTION 32.2 D E F E N S E AG A I N S T H E R B I VO R E S

first extracted from the bark of the Pacific

FIG. 32.11 Mechanical defenses. (a) Spines on the beach palm make it hard for animals to yew tree.
climb or penetrate its stem, and hairs on the leaves of (b) nettle and (c) Arabidopsis Phenols form the third main class of
deter much smaller herbivores. Sources: a. Fletcher & Baylis/Science Source; b. Dr. Keith defensive compounds, illustrated by the
Wheeler/Science Source; c. Dr. John Runions/Science Source. tannins found widely in plant tissues.
a b Tannins bind with proteins, reducing
their digestibility. Plants store tannins in
cell vacuoles, and so these compounds
come into contact with the protein-rich
cytoplasm only when cells are damaged.
Herbivores attempting to feed on tannin-
producing plants obtain a poor reward
for their efforts. For this reason, natural
selection favors individuals that avoid
tannin-rich plants. Many unripe fruits
are high in tannins, and the unpleasant
experience of biting into an unripe banana
illustrates how tannins deter consumers.
c
For thousands of years, humans have
taken advantage of the protein-binding
properties of tannins, using extracts
from tree bark to process animal skins by
“tanning” to produce leather.
Some of the chemical defenses
used by plants are stored in a separate
compartment from the enzymes that
activate them. When you bite into a plant
belonging to the cabbage family, the
chemicals stored in the vacuole comingle
with enzymes in the cytosol. The enzymes
then catalyze the production of mustard
However, because alkaloids affect specific aspects of metabolism, oils from these compounds. Mustard oils give cabbage and its
even very small concentrations can deter herbivores. One of the relatives their distinctive smell. They serve as feeding deterrents
first chemotherapy drugs (Table 32.1), vincristine, is an alkaloid because they interfere with insect growth.
extracted from the Madagascar periwinkle. It inhibits microtubule Cassava roots, an important food crop in Africa and South
polymerization and thus prevents cell division (Chapter 11). America, release the toxin hydrogen cyanide when their cells are
Vincristine does not harm the plant’s own cells because it is stored damaged. Before cassava roots can be safely consumed, the roots
within an extracellular system of latex-containing canals. must be ground and the cyanide-producing chemicals removed in
A second group of defensive compounds, the terpenes, do not running water. Although some cassava varieties, so-called sweet
contain nitrogen. As a result, plants can produce them for defense cassava, lack these chemicals and can be eaten without any special
without having to make use of nitrogen that could otherwise be preparation, the bitter varieties are preferred crops in many cases
used in protein synthesis. Small terpenes are volatile, vaporizing because they often have higher yields.
easily, and so make up many of the essential oils associated with Some chemical defenses found in plants are protein based. Plants
plants. The distinctive smells of lemon peel, mint, sage, menthol, and animals use the same 20 amino acids to construct proteins,
pine resins, and geranium leaves are all due to terpenes. While we but some plants produce additional amino acids as well. Plants do
find these odors pleasant, they are feeding deterrents to mammals not incorporate these additional amino acids into their proteins,
such as squirrels and moose. In addition, terpenes obstruct the but herbivores that ingest them do. The resulting proteins can no
growth and metabolism of both fungi and insects. As a result, longer fulfill their function. Thus, insect herbivores that consume
pyrethrin, a terpene derivative extracted from chrysanthemums, is nonprotein amino acids grow slowly and often die early. A second
marketed commercially as an insecticide. Other compounds derived protein-based defense is the production of antidigestive proteins
from terpenes interfere with insect development. For example, called protease inhibitors. These proteins bind to the active site
exposure to compounds that are replicas of insect hormones cause of enzymes that break down proteins in the herbivore’s digestive
insects to molt prematurely and thereby keep their populations in system. This prevents proteins from being broken down into their
check. Taxol, which is used in chemotherapy, is a terpene that was individual amino acids and therefore reduces the nutritional value of
 CHAPTER 32 PL ANT DEFENSE: KEEPING THE WORLD GREEN 677

TABLE 32.1 A SAMPLING OF PLANT COMPOUNDS USED IN MEDICINE

CLASS EXAMPLE PLANT SOURCE MEDICAL USE

Alkaloids Atropine Deadly nightshade Dilate pupils; treat extremely

low heart rate
HO
Vincristine Madagascar periwinkle Treat cancer
Codeine Opium poppy Cough suppressant
N
O Morphine Opium poppy Painkiller

H Quinine Cinchona tree Treat malaria

Ephedrine Ephedra (Mormon tea) Asthma relief
HO
Morphine Turocurare (curare) Strychnos toxifera Muscle relaxant
Digitoxin Foxglove Treat congestive heart failure
and arrhythmia
Berberine Berberis (barberry) Antibiotic, anti-inflammatory

Terpenoids Menthol Peppermint Topical analgesic; mouthwash

Camphor Camphor laurel Anti-itch creams
Eucalyptol Eucalyptus species Mouthwash; cough suppressant
Taxol Pacific yew Treat cancer
OH
Artemisinin Artemisia annua Treat malaria

Menthol

Phenols O Salicylic acid Willow tree Treat aches and pains

OH

OH
Salicylic acid

the plant tissue. Insects that feed on plants that produce protease
inhibitors have reduced growth rates. FIG. 32.12 Ants attracted by an extrafloral nectary. These ants
receive a food reward and also consume any insect eggs
Some plants provide food and shelter for ants, which or larvae they encounter on the plant. Source: Alex Wild/
actively defend them. alexanderwild.com.
While many plants have mechanical defenses against herbivores
and nearly all have at least one form of chemical defense, a
smaller number of species have evolved ecological defenses
against herbivory. These plants “employ” animals as bodyguards
in exchange for shelter or nourishment, much the way that many
plants provide food or other rewards to animals that transfer
pollen and disperse seeds (Chapter 30).
Nectar is typically associated with flowers. However, many
plants produce nectar in glands located on their leaves (Fig. 32.12).
These extrafloral nectaries attract ants, which move actively
throughout the plant in search of nectar. When the ants encounter
the eggs or larvae of other insects, they consume these as well.
The value of this relationship to the plant is easily demonstrated:
When ants are prevented experimentally from patrolling certain
branches, those branches suffer higher rates of herbivore damage.
678 SECTION 32.2 D E F E N S E AG A I N S T H E R B I VO R E S

host plants. And while ant-plants can be grown without ants,

FIG. 32.13 The symbiosis between ants and the bullhorn acacia. herbivores would rapidly consume an undefended plant in nature
(a) Ants live in the plant’s thorns, and (b) the plant because ant-plants lack the chemical defenses present in related
provides food (the yellow food bodies). The ants species that do not host ants.
vigorously defend their home against animals and plants
alike. Source: Dan L. Perlman/EcoLibrary.org. Grasses can regrow quickly following grazing by
a mammals.
By far, the most important vertebrate herbivores are the bison,
zebra, antelope, and their relatives that populate the great plains of
East Africa and North America. Studies of herbivore damage in an
East African savanna indicate that vertebrates annually consume
approximately the same amount of leaf biomass as do insects.
Grasses are well adapted to cope with grazing mammals
(Fig. 32.14). The vertically oriented leaves grow from stems
with limited internodal elongation. This allows the shoot apical
meristem to remain close to the ground, where it is less likely to
be damaged. Grasses have persistent zones of cell division and
elongation at the base of each leaf. Because grass leaves elongate
from their base, grasses can quickly replace leaf tissues removed
b by a grazing mammal, fire, or a lawn mower. Only when grasses
produce their flowering stalks does the apical meristem emerge
above the leaves.
Grasses are thought to have evolved in forest environments.
In fact, grass-dominated ecosystems have become widespread

FIG. 32.14 Adaptation for grazing. The ability of grass leaves

to elongate from the base allows grasses to regrow
following grazing.

A small number of plants, several hundred in total, have

evolved a much closer relationship with ants. These so-called
ant-plants provide both food and shelter for an entire colony of
ants, which then defend their host. The bullhorn acacia (Acacia
cornigera), which grows in dry areas of Mexico and Central America,
produces hollow spines at the base of each leaf, as well as protein-
and lipid-rich food bodies on the tips of its expanding leaves.
Symbiotic ants (Pseudomyrmex ferruginea) live in the spines and Leaf
feed on the food bodies (Fig. 32.13). To protect this source of food blade
and shelter, the ants actively defend the plant. When the ants
detect an invader, they produce an alarm pheromone that causes
the entire colony to swarm over the plant, attacking any insects or Zone of cell
vertebrates (including scientists) that they encounter. The ants will division and
attack the growing tip of a vine that might climb up the host and elongation
even destroy plants growing in the soil beneath the host Acacia.
Apical
This system works well until it comes time to reproduce. meristem
Without some mechanism to control the hypervigilant ants, any
visiting pollinator would be attacked as though it were a truly
unwanted guest. So that pollinators may visit flowers unharmed, Prior to flowering, the apical Therefore, ...the apical
meristem of grasses is close if the top meristem is not
flowers of the bullhorn acacia release a chemical that repels the
to the ground, and there is is cut off... damaged and
resident ants. a persistent zone of cell the leaf blades
The interactions between ant-plants and their ants are highly division and elongation at can grow back.
the base of each leaf blade.
coevolved. The ant species are found only in association with their
 CHAPTER 32 PL ANT DEFENSE: KEEPING THE WORLD GREEN 679

only during the past 25 million years. The fossil record shows that Plants can sense and respond to herbivores
as grasslands expanded, grazing mammals evolved along with For more than 15 years, biologists have studied how coyote tobacco
them. Horses, for example, originated as small browsers that fed (Nicotiana attenuata), a relative of tobacco native to dry open
on the leaves of small trees and shrubs. As grasslands expanded, habitats of the American West (Fig. 32.15), defends itself against
larger horse species with teeth adapted to eating grass evolved. herbivores. These studies provide a fascinating window into how
Why would specialized teeth be advantageous? The small plates of plants allocate resources toward defense under natural conditions.
silica in grass cell walls increases their strength and may provide
mechanical defense against herbivores. Horses and other grazers
that feed on these grasses wear down their tooth enamel grinding
against the silica, so teeth with a thick enamel layer are favored FIG. 32.15 Nicotiana attenuata, the coyote tobacco. This species
by natural selection. It is not clear whether grasses evolved in has been developed as a model organism for studying
response to grazing or to disturbances such as fire, but without how plants protect themselves against herbivores.
question, their distinctive pattern of meristem activity minimizes (a) Nicotiana attenuata in a Utah desert; (b) Manduca
the damage caused by grazing mammals. sexta, a specialist herbivore, feeding on N. attenuata.
Sources: a. Danny Kessler, Max Planck Institute for Chemical Ecology,
Jena, Germany; b. Celia Diezel, Max Planck Institute for Chemical

32.3 ALLOCATING RESOURCES Ecology, Jena, Germany.

TO DEFENSE a

By now, it should be clear that plants invest substantial resources

in defense, resources that might otherwise be used for growth
and reproduction. Given the considerable cost of plant defenses,
their benefits must be correspondingly large. If a commonly
encountered pathogen or herbivore is lethal, the resources
invested in defense provide a clear reproductive advantage. But
what if the pathogen or herbivore usually damages only half of a
plant’s leaves? Or 10%? Or encounters the plant only infrequently?

Some defenses are always present, whereas others are

turned on in response to a threat.
In nature, a balance between cost and benefit is achieved in
several different ways. When a threat is common, plants produce
constitutive defenses—defenses that are always present. When a
threat is uncommon, however, there is an advantage to mounting
a defense only when the threat is encountered. A defense that is
activated only when the plant senses the threat is an inducible
defense. An inducible defense can be either an increase in the level
of an already present (constitutive) defense or the production
of a new type of defense. The ability to sense and respond to the
presence of herbivores allows plants to use resources efficiently.
When a plant is attacked by herbivores, it begins to express
a new set of genes. Many of these genes increase the production
of chemical defenses, while others stimulate the mechanical b
reinforcement of cell walls. In addition, herbivore damage triggers
the synthesis of jasmonic acid, a signal that is transmitted through
the phloem. Exposure to jasmonic acid induces the transcription
of defensive genes even in parts of the plant untouched by
herbivores. A version of the compound called methyl-jasmonic
acid is highly volatile and can move through the air to reach
portions of the plant that are not actively importing carbohydrates
through the phloem.

j Quick Check3 What would happen to a plant in which jasmonic

acid synthesis is blocked?
 HOW DO WE KNOW?

FIG. 32.16

Can plants communicate?

BACKGROUND Plants respond to herbivore damage by producing RESULTS When measured 52 hours after the initial damage,
chemicals that make their tissues less palatable. Field experiments both the damaged plants and the undamaged experimental plants
in which caterpillars were added to target plants suggested that exhibited elevated levels of defensive compounds in their leaves
undamaged neighboring plants increased their production of compared to control plants.
defensive chemicals, whereas plants farther away did not.
5

Leaf tannins (% tannic acid

equivalent/g dry weight)
HYPOTHESIS Volatile chemicals released from plants that 4
have been attacked by herbivores elicit a defensive response in
undamaged plants. 3

2
EXPERIMENT One set of plants was placed downwind of other
1
plants in which two leaves were torn, simulating herbivore
damage, and a set of control plants was placed downwind of an 0
Control Damaged Experimental
empty chamber. The concentrations of defensive compounds in all plants plants plants
sets of plants were measured and compared.
Air flow CONCLUSION Volatile chemicals released from damaged plant
tissues can trigger the production of defensive chemicals in
undamaged plants.

FOLLOW-UP WORK The volatile chemicals released from

damaged plants have been identified and shown to attract
predatory animals that feed on the herbivores or parasitize their
Damaged plants eggs or both. Thus, when plants respond to damaged neighbors,
(two leaves torn)
they are “eavesdropping” on signals that likely evolved to attract
predatory insects, rather than to warn neighboring plants.

SOURCE Baldwin, I. T., and J. C. Schulze. 1983. “Rapid Changes in Tree Leaf
Chemistry Induced by Damage: Evidence for Communication Between Plants.”
Experimental plants Control plants Science 221: 277–279.
(undamaged) (undamaged)

Nicotiana attenuata is attacked by a wide variety of herbivores, to leaves in the xylem, the activation of this inducible defense
both generalists and specialists. A major component of requires the transport of signaling molecules in the phloem from
N. attenuata’s defense against being eaten is the production of the the sites of insect damage to the roots.
alkaloid nicotine. Nicotine is an effective defense against many Nicotiana attenuata is also attacked by a specialist herbivore,
generalist herbivores. However, nicotine is also an expensive form the tobacco hornworm caterpillar, Manduca sexta. These
of chemical defense because it contains nitrogen, a nutrient whose caterpillars, which can reach 7 cm in length, have evolved the
availability can limit growth. Thus, N. attenuata plants that have ability to sequester and secrete nicotine. By this means, these
not been exposed to herbivores produce only low levels of nicotine caterpillars are able to consume leaves with high levels of nicotine.
as a constitutive defense. When N. attenuata is attacked by a If nicotine is not an effective defense against this voracious
generalist herbivore, nicotine concentrations in leaves increase herbivore, what can N. attenuata do to protect itself?
dramatically—typically fourfold, but in some cases as much as It turns out that N. attenuata can recognize specific chemicals
tenfold. Because nicotine is synthesized in roots and transported in the saliva of M. sexta. When damaged by most herbivores—or a

680
 CHAPTER 32 PL ANT DEFENSE: KEEPING THE WORLD GREEN 681

pair of scissors—N. attenuata responds by producing nicotine. But related species found on nutrient-poor sandy soils. They found
when attacked by M. sexta, N. attenuata does not waste time and that species from the nutrient-rich clay soils invested less heavily
resources synthesizing nicotine. Instead, it protects itself in an in defenses than did species from the sandy sites. Because the clay-
entirely different way, by attracting other insects that will attack soil species divert fewer resources to defense, they grow faster in
its herbivores, as we discuss next. either soil type compared to sandy-soil species—as long as they are
grown under a net that excludes herbivores (Fig. 32.17). However,
Plants produce volatile signals that attract insects that when grown out in the open, each species grows best in the soil
prey upon herbivores. type on which it is naturally found.
How does N. attenuata attract its insect allies? A hint comes from The clay-soil species grow poorly on sandy soils when not
the smell of a newly mown lawn. This distinctive smell originates protected by a net because, without ample soil nutrients available,
from chemicals released from the cut blades of grass. Some of the they are unable to replace tissues consumed by herbivores.
chemicals released when plants are damaged represent nothing Conversely, when grown on the nutrient-rich soils, species from
more than the exposure of cell interiors to the air. Others, however,
are produced specifically in response to damage. When N. attenuata
is attacked by M. sexta caterpillars, it produces volatile signals that FIG. 32.17 A trade-off between growth and defense. Plants from
attract insects that prey on the eggs and larvae of M. sexta. The nutrient-rich clay habitats can overcome herbivory by
adage “The enemy of my enemy is my friend” seems to hold true growing fast, whereas plants from low-nutrient sandy
when it comes to protecting plants from herbivore damage. habitats grow slowly and invest more in defense. The
Studies first conducted in the 1980s showed that undamaged photos show that, without defenses, clay-soil plants are
plants increase their synthesis of chemical defenses when prone to being eaten by herbivores. Source: Paul Fine
neighboring plants are attacked by herbivores (Fig. 32.16).
Clay soil plants growing in sandy soils
Although these plants were described in the popular press as
“talking trees,” the plants under attack are not altruistically
warning their neighbors. Instead, subsequent studies suggest that
the undamaged plants are detecting the signals released by their
neighbors that are directed at other insects, then ramping up their
own defenses. Recent work demonstrates that even the sound
of a neighboring plant being chewed on by insects can trigger
defensive responses in a nearby plant.

Nutrient-rich environments select for plants that

allocate more resources to growth than to defense.
Unprotected from herbivores Protected from herbivores
Some plant species allocate far more resources to defense than
others. Because defenses are costly to produce, you might think
that plants growing in nutrient-rich habitats should be the
In the presence of The species from clay soil
best defended. However, consider the counterargument: Plants herbivores, plants sites grew faster in both
growing on nutrient-poor soils might invest heavily in defense grew best in their habitats when protected
because they cannot afford to replace tissues lost to herbivores. own soil type. from herbivores.

Such plants would grow relatively slowly because their resources 0.6
6
are being used to build defenses rather than to produce new leaves
and roots. By contrast, plants from nutrient-rich habitats would
favor growth over defense because they can more readily replace 0.4
Increase in leaf area

lost or damaged tissues.

(cm2/day)

This situation is an example of a trade-off: Something is

gained and, at the same time, something is lost. You can’t have 0.2
your cake and eat it, too—you have to make a choice. With plants,
there can be a trade-off between growth and defense in which
allocating resources for one means that those resources cannot be 0.0
allocated for the other. Clay origin
A recent study in the Amazon rain forest has focused on a Sand origin
possible trade-off between growth and defense. Researchers -0.2
Unprotected Unprotected Protected Protected
compared species found on nutrient-rich clay soils with closely clay habitat sandy habitat clay habitat sandy habitat
682 SECTION 32.4 DEFENSE AND PL ANT DIVERSIT Y

the sandy habitats are poor competitors because they allocate consistent with an important role for plant–insect coevolution in
resources that could have been used for growth to defensive generating diversity. Further evidence that defense has influenced
compounds and structures. What we see is a classic example of a plant diversification comes from plant genes and secondary
trade-off. compounds. For example, the R genes that are instrumental in
pathogen recognition form one of the largest gene families in
j Quick Check 4 Does the trade-off between growth and defense
plants, while chemical defenses against herbivores include about
favor a single species that dominates in all soil types or different
6000 alkaloids and more than 10,000 terpenoid compounds. Such
species specialized for each habitat?
observations suggest that plants are locked in an evolutionary arms
race with herbivores and pathogens. This arms race is very much in
Exposure to multiple threats can lead to trade-offs. evidence in the efforts by farmers to limit crop losses by reducing
In the real world, plants are routinely confronted with more than the numbers of the pathogens and herbivores in fields and orchards.
one threat. A plant may be attacked simultaneously by both a
pathogen and an herbivore, or it might be in danger of being shaded The evolution of new defenses may allow plants
by a fast-growing neighbor at the same time that it must defend its to diversify.
leaves from being eaten. Given that the resources a plant can draw American scientists Paul Ehrlich and Peter Raven observed that
on are limited, how do plants respond to multiple threats? closely related species of plants were fed upon by closely related
Often, the response to multiple threats suggests a trade- species of butterflies. In 1964, they cited this observation as
off. For example, a plant exposed to a pathogen may be less evidence that plants and their herbivores are caught up in an
able to respond to a later herbivore attack. If tobacco plants are evolutionary arms race: Plants evolve new forms of defense, and
exposed to tobacco mosaic virus and systemic acquired resistance herbivores evolve mechanisms to overcome these defenses. In
is activated, they become more susceptible to the hornworm particular, Ehrlich and Raven hypothesized that the evolution of
caterpillar, Manduca sexta. Studies indicate that plants do not novel defenses has been an important force in the diversification
defend themselves as vigorously against subsequent herbivore of both plants and herbivorous insects.
attacks because of crosstalk between the signaling pathways for A novel form of defense may allow a plant population to expand
responding to biotrophic pathogens and those for responding to into new areas. If the population in a new area remains separated
herbivores. from the original population, it may evolve into a new species.
There can also be trade-offs between the threat of competition Similarly, a novel means of overcoming plant defenses may allow
with neighboring plants and the threat of attack by herbivores. For an insect or pathogen population access to new plant resources. If
example, when the phytochrome receptor in plants detects the these insects or pathogens became separated from their original
presence of neighboring plants (Chapter 31), plants allocate more population, then they might also evolve into new species. Ehrlich
resources to growing tall, but they also produce fewer defensive and Raven’s “escape and radiate” hypothesis thus predicts a burst of
chemicals in response to herbivore damage. Interactions among diversification following the evolution of novel defenses.
the signaling molecules associated with each of these processes This hypothesis makes intuitive sense, but it could only
underlie this trade-off. be tested with the advent of phylogenetic trees based on DNA
sequence comparisons (Chapter 23). Biologists knew that latex
or resin canals evolved independently in more than 40 different
32.4 DEFENSE AND PLANT DIVERSITY groups of plants. Reasoning that these features represent a novel
defense, they asked whether the groups that evolved latex or resin
Many plants spend a significant fraction of their resource and canals were more diverse than closely related groups that lacked
energy budgets on defensive chemicals and structures. Despite these forms of defense. In 14 of the 16 groups examined, the groups
this commitment, insects consume approximately 20% of all new with the protective canals were significantly more diverse than
plant growth each year, while pathogens destroy a substantial their closest relatives that lacked latex or resin canals. This pattern
additional amount. The magnitude of this loss means that supports the hypothesis that the evolution of latex and resin canals
pathogens and herbivores must be a potent force in both ecology provided plants with the freedom to expand into new habitats.
and evolution, limiting the success of some species while allowing Escalation of defenses, however, is not the only possible
some of their competitors to prosper. In nature, plant fitness may evolutionary outcome. Phylogenetic research shows that, as
be strongly influenced by the capacity to deter herbivores and milkweeds diversified, many of the newly evolved species
resist pathogens. produced fewer chemical defenses such as latex canals and
Interactions between plants and their consumers are thought cardenolides. Instead, these species are found in resource-
to have influenced patterns of plant diversification. Just like rich habitats and replace tissues by growing quickly following
pollinating insects, plant-eating insects diversified along with damage. The presence of specialized herbivores such as monarch
angiosperms (the flowering plants). The coincidence of timing is caterpillars that can disarm milkweeds’ defenses may help explain
 CHAPTER 32 PL ANT DEFENSE: KEEPING THE WORLD GREEN 683

an adult tree, so seedlings that grow farther

FIG. 32.18 Density-dependent mortality promotes tree diversity. Seedlings of rare away have a better chance of escaping these
species have a greater probability of surviving because they are more likely to pathogens. In both cases, high-density
escape infection by species-specific pathogens. populations of seeds and seedlings are more
at risk.
Seedlings of rare species are more Seedlings of common Such density-dependent mortality
likely to survive because they have a species are less likely to
greater chance of dispersing away survive because they have provides rare species with an advantage over
from the build up of pathogens a high probability of being more common species. In other words, the
under adult trees of the same species. infected by pathogens.
offspring of a rare species is more likely to
survive because its seeds are more likely
to be dispersed into a location where there
are no neighboring adult plants of the
same species. By favoring the survival of
less common species, density-dependent
mortality prevents one species from
outcompeting all others.
In the tropical rain forests of Ecuador
Common and elsewhere, only a small number of
Seed of species A
Seed of rare Rare common tree species are common; the majority of
species B species B species A tree species are rare. These rare species
are able to escape herbivore and pathogen
damage, but they need reliable ways of
getting pollen from one plant to another.
Thus, animal pollination, herbivory, and
infection by pathogens are thought to
Seedling of Region where species B’s Region where species A’s play complementary roles in promoting
rare species B pathogen is abundant pathogen is abundant angiosperm diversity.

? CASE 6 AGRICULTURE: FEEDING A

GROWING POPULATION
why these more recent plant species reduce investments in Can modifying plants genetically protect crops from
defense in favor of more rapid growth. herbivores and pathogens?
The density-dependence of herbivore and pathogen damage is a
Pathogens, herbivores, and seed predators can particular challenge for agriculture as practiced in most countries.
increase plant diversity. When acre after acre are covered by a single crop species, successful
The lowland rain forests of eastern Ecuador have the highest invaders are not slowed by low target density. On the contrary,
diversity of tree species recorded anywhere: More than pest populations build up to high levels, and the tendency to plant
1000 species of trees were identified in a 500 m  500 m plot. genetically uniform varieties increases the losses in yield. Not
Because trees compete for sunlight, water, and nutrients, the surprisingly, then, herbivores and pathogens exact a large toll on
species best able to acquire resources would be expected, over time, agricultural crops, and protecting crops from damage is a high
to outcompete all others. What allows so many tree species to priority for farmers.
coexist is one of the great questions of modern biology. In the 1970s, The production of pesticides and herbicides increased dramatically
two ecologists, Daniel Janzen and Joseph Connell, independently in the second half of the twentieth century. Together with irrigation
proposed that interactions with host-specific pathogens, herbivores, and fertilizers, these compounds are important contributors to
or seed predators play an important role. the increased yields of the Green Revolution. However, large-scale
The Janzen–Connell hypothesis, as it is now known, is based application of pesticides is not without risk. In addition to the
on the observation that seeds or seedlings often suffer high rates potential for toxic effects, widespread chemical treatments provide a
of mortality when growing directly beneath an adult tree of the strong selective force for the evolution of resistance. As pests become
same species (Fig. 32.18). Seed predators often gather in higher resistant to pesticides, one response is to apply more or stronger
densities near trees with mature fruit, so seeds that disperse far chemicals. The consequences of this approach, however, can spiral out
from their maternal plant may escape notice by seed predators. By of control as chemical applications indiscriminately kill off the natural
a similar argument, pathogens are common in the soil underneath predators of the pests themselves.
684 SECTION 32.4 DEFENSE AND PL ANT DIVERSIT Y

During the 1980s, farmers applied increasing amounts

of pesticides to rice paddies in South Asia, yet crop losses FIG. 32.19 Bt-crops co-planted with non-Bt varieties to prevent
continued to increase. This dangerous course was reversed by the the development of resistance. Source: John L. Obermeyer,
introduction of a program first developed in the 1950s. Farmers in IPM Specialist Department of Entomology, Purdue University.

this program carefully determine what pests are actually present,

whether or not they are at damaging levels, and whether or not Refuge,
Bt corn non-Bt corn
there are natural predators of the pests. Only after understanding
the ecology of the farm do farmers remove pests, starting with
mechanical means, then biological control using natural predators,
and, only as a last resort, industrial pesticides. Farmers using this
approach, called integrated pest management, can minimize
routine pesticide use and therefore prevent pest species from
evolving resistance.
Another approach is to improve the ability of crops to defend
themselves against herbivores and pathogens through breeding
and genetic modification. Crop breeders must emulate—on a
faster timescale—the role played by evolution in selecting for
resistance to newly evolving pests. For example, Norman Borlaug’s
original task was to develop wheat varieties for Mexico that were
resistant to fungal pathogens. To do this, he obtained seeds of wheat resistant to Bt toxins survive by feeding on the non–Bt-expressing
varieties from around the world that showed evidence of pathogen plants. These non-resistant pests interbreed with individuals
resistance. He then crossed these disease-resistant varieties with feeding on the Bt-expressing plants, thus slowing the evolution
varieties adapted to the local conditions. Although it took thousands of Bt-resistance. The efficacy of this practice, however, is called
of hand-pollinations, Borlaug was successful at introducing into question by reports of Bt-resistant pests and the need to apply
resistance genes into wheat varieties that grew well in Mexico. increasing amounts of industrial pesticides to Bt crops.
Crop breeding is a powerful way to alter the genetic makeup If there is one practical lesson to be learned from our
of cultivated species. However, because it relies on sexual experience with Bt, it is that pathogens and herbivores will
reproduction, crop breeding is limited to the genetic diversity that continue to evolve new ways of circumventing the defenses
exists within varieties that can interbreed. In contrast, Rhizobium of crop plants. Thus, agriculture will continue to require new
radiobacter allows scientists to introduce specific genes found in forms of crop protection. While the use of chemical pesticides
distantly related organisms into cultivated species. One of the first will probably remain widespread, crop protection will
commercial uses of this technology was to introduce chemical increasingly be based on the application of genetics, through
defenses from a bacterium into crop plants. both crop breeding and biotechnology. In either case, success
Bacillus thuringiensis (Bt) is a soil-dwelling bacterium that will depend upon the availability of existing genes honed by
produces proteins toxic to insects. It has been used to control natural selection. Without access to genes involved in pathogen
insect outbreaks in agriculture since the 1920s. At first, farmers recognition (R genes) or in the production of chemical and
applied spores containing the toxins or the toxins themselves mechanical defenses, we will have the tools, but not the raw
directly to plants. In 1985, the genes that encode the toxins from materials, to protect our crops.
B. thuringiensis were inserted into tobacco, where their expression In some cases, crop protection will rely on genes from
conferred substantial protection against insects. Today, Bt- distantly related organisms, as illustrated by Bt. Commonly,
modified crops are some of the most widely planted genetically however, defense strategies for crops will rely on genes from
modified plants. other plants, often ones that are closely related. For example, the
The introduction of Bt crops markedly reduced the application specific R genes used for pathogen recognition typically function
of pesticides, to the benefit of both farmworkers and the only within a single species or closely related species. Thus, to
environment. However, the danger of planting Bt-expressing develop potatoes that are resistant to the protist Phytophthora
crops widely is that constant exposure to Bt toxins increases the infestans that causes potato blight, scientists are transferring R
probability that pest populations will evolve resistance. Organic genes from a wild relative of potato. The continuation of such
farmers are particularly concerned because they continue to use efforts requires a commitment to safeguarding the genetic
traditional application of B. thuringiensis spores to control insect diversity found in natural populations of crop species and their
outbreaks. To prevent the evolution of resistance, U.S. farmers are close relatives. Only by tapping into these natural genetic
required by law to plant a fraction of their fields, known as a refuge, resources can breeders produce crops that stay one step ahead of
with non–Bt-expressing plants (Fig. 32.19). Pests that are not the constantly evolving threats of pathogens and herbivores. •
 CHAPTER 32 PL ANT DEFENSE: KEEPING THE WORLD GREEN 685

Core Concepts Summary 32.3 The production of defenses is costly, resulting in

trade-offs between protection and growth.
32.1 Plants have evolved mechanisms to protect Plants produce both constitutive defenses, which are produced
themselves from infection by pathogens, which include whether or not a threat is present, as well as inducible defenses,
viruses, bacteria, fungi, worms, and even parasitic plants. which are triggered when a plant detects that it is being
Pathogens enter plants through damaged tissue or stomata. attacked. page 679
page 668 Plants produce volatile signals that attract insects that prey on
Plant pathogens spread by growing or moving through the herbivores. page 681
plant vascular system. Viruses move in the phloem, whereas A trade-off is sometimes observed between plant growth and
bacteria and fungi move in the xylem. page 669 defense, in which allocation to defense results in slower growth
Biotrophic pathogens gain resources from living cells; rates. page 681
necrotrophic pathogens kill cells before exploiting them.
In nutrient-rich environments, species that allocate more
page 669
resources to growth and less to defense can outcompete plants
Plants have an immune system that allows them to detect and that invest more resources to defense and less to growth; the
respond to pathogens. page 670 opposite is true on nutrient-poor soils. page 681

One part of the plant immune system acts generally and

recognizes highly conserved molecules produced by broad 32.4 Interactions among plants, pathogens, and
classes of pathogens. page 670 herbivores contribute to the origin and maintenance of
plant diversity.
A second part of the plant immune system acts specifically
and consists of “resistance” or R genes that encode R proteins, The “escape and radiate” pattern of plant evolution suggests that
which recognize pathogen-derived AVR proteins page 670 plants undergo a burst of diversification following the evolution
of a new form of defense against a pathogen or an herbivore.
Plants respond to pathogens by actively killing the cells page 682
surrounding the infection (the hypersensitive response) and by
sending a signal to uninfected tissues so that they can mount a The Janzen–Connell hypothesis proposes that interactions with
defense (systemic acquired resistance). page 671 pathogens and herbivores increase plant diversity. page 683

The bacterium Rhizobium radiobacter infects plants by inserting Herbivores and pathogens are a major concern for agriculture.
some of its genes into the plant’s genome, resulting in the page 683
formation of a tumor and providing a way to genetically Crop protection includes the use of chemical pesticides,
engineer plants. page 673 integrated pest management, application of spores or toxins
from Bacillus thuringiensis to plants, and inserting genes that
32.2 Plants use chemical, mechanical, and ecological encode for toxins from B. thuringiensis to make Bt-modified
defenses to protect themselves from being eaten by plants. page 684
herbivores.
Plant defenses against herbivory include dense hairs, latex,
and chemicals. page 675
Self-Assessment
Alkaloids are nitrogen-bearing compounds that interact with
1. Describe how pathogens enter and move within the plant
the nervous system of animals. page 675
body.
Terpenes, volatile compounds that give rise to many of the
essential oils we associate with plants, deter herbivores. 2. Distinguish between biotrophic and necrotrophic plant
page 676 pathogens.

Tannins bind with proteins, reducing their digestibility. 3. Name and describe the two components of the plant
page 676 immune system.
Ant-plants, such as the bullhorn acacia, provide food and 4. Describe and contrast the hypersensitive response and
shelter for ants, which defend their host plant. page 677 systemic acquired resistance.
686 SELF-ASSESSMENT

5. Describe a feature of Rhizobium radiobacter that 9. Draw a phylogenetic tree that illustrates an “escape and
makes it a useful tool in biotechnology. radiate” pattern of diversification for plants that evolve novel
defenses.
6. Describe three ways that plants protect themselves
from being eaten by herbivores. 10. Describe one benefit and one disadvantage of herbicide and
pesticide use in agriculture.
7. Explain why there are often trade-offs between plant
growth and plant defense.
Log in to to check your answers to the Self-
8. Explain how pathogens and herbivores can increase
Assessment questions, and to access additional learning tools.
plant diversity.
 CHAPTER 33

Plant Diversity
Core Concepts
33.1 Angiosperms make up
approximately 90% of all plant
species found today.
33.2 Bryophytes form persistent,
photosynthetic gametophytes
and small, unbranched
sporophytes; today they
grow in environments where
the ability to pull water from
the soil does not provide an
advantage.
33.3 Spore-dispersing vascular
plants today are primarily
small plants that grow in moist
environments, but in the past
included tall trees.
33.4 Gymnosperms produce seeds
and woody stems and are most
common in seasonally cool or
dry regions.
33.5 Angiosperms are distinguished
by flowers, fruits, double
fertilization, and xylem
vessels; their diversity is the
result of traits that increase the
efficiency of completing their
life cycle and building their
bodies.
Gaby Wojciech/age fotostock.

687
688 SECTION 33.1 P L A N T D I V E R S I T Y : A N E VO LU T I O N A RY OV E RV I E W

Five hundred million years ago, the land surface supported ultraviolet radiation and temperature fluctuations, and they can use
little more than a green crust made up of a mixture of water currents to transport sperm and disperse offspring. Surviving
photosynthetic and non-photosynthetic microbes. Then, through on land required the evolution of new ways of growing and
the process of natural selection, a group of freshwater algae reproducing. Here, we highlight key adaptations that allowed plants
began the transition to survival on land. Fast-forward 465 million to photosynthesize and reproduce on land, explored in Chapters
years—to the present time—and plants are everywhere. Much 29–31, and discuss how plant diversity changed as new ways of
of what makes up a plant, including roots, leaves, tree trunks, growing and reproducing evolved.
and seeds, evolved quickly. Within the first quarter of the time
since these algal ancestors began to colonize the land, all of Four major transformations in life cycle and structure
these features were present. But one thing was missing: flowers. characterize the evolutionary history of plants.
Flowering plants, also known as angiosperms, do not appear The first major transformation was the evolution of alternation
on the scene until the last quarter of this history, and yet today of generations (Fig. 33.1; Chapter 30). To understand this
they make up a remarkable 90% of all plant species. Many of the change, let’s first consider the presumed ancestral life cycle. The
plant groups that diverged before angiosperms appeared are still algal relatives of plants produce multicellular bodies made up
present, but where they live and grow has been shaped by the entirely of haploid cells. These algae form gametes by mitosis,
evolution of flowering plants. and fertilization results in a zygote, which is the only diploid
cell. The algal ancestor of land plants is thought to have released
male gametes (sperm) into the surrounding water and relied on
33.1 PLANT DIVERSITY: AN water currents to carry the zygote away from the parent plant.
EVOLUTIONARY OVERVIEW The zygote then underwent meiosis, producing haploid cells that
developed into new multicellular algae.
The closest living relatives of plants are green algae that grow in Like their green algal relatives, the first land plants would
streams and ponds (Fig. 33.1). Some features found in plants are have produced a multicellular body composed entirely of haploid
also present in the close algal relatives, including cellular structures cells and that produces gametes. During periods of rain, the male
such as plasmodesmata and the enzymes used to reduce CO2 loss gametes could swim to female gametes in surface water layers.
during photorespiration. But for the most part, plants are quite But zygotes dispersed this way would not be able to travel very far
distinct from their aquatic progenitors. The reason is that, when from the parent plant. To enhance dispersal, land plants evolved
plants moved onto land, they had to survive in a new medium, air. alternation of generations: The multicellular haploid generation
Algae can rely on water for hydration, support, and protection from alternates with a multicellular generation composed of diploid

FIG. 33.1 Phylogenetic tree of land plants. The number of species is shown in parentheses.
Note that 90% of all land plant species are angiosperms.

Other green algae

(10,000
Sister group of green algae species)

Liverworts (8000 species)

Mosses (15,000 species)

Multicellular sporophyte
Protected embryos
Hornworts (100 species)

Lycophytes (1200 species)

Vascular tissues Ferns and horsetails (11,500 species)

Green algae
Bryophytes Gymnosperms (1020 species)

Vascular plants Seeds and pollen Angiosperms (>380,000 species)

Flowers
 CHAPTER 33 PL ANT DIVERSIT Y 689

cells. Specifically, the diploid zygote develops by mitosis into a large. These plants could only reproduce where and when surfaces
multicellular spore-producing plant, called a sporophyte, while were sufficiently wet. Eliminating this dependence—through the
still attached to and supported by the haploid gamete-producing evolution of pollen and seeds—is the third major evolutionary
plant, called a gametophyte. In the first diverging groups of event in the history of plant diversification.
plants, the multicellular diploid generation produces many The life cycle of seed plants differs from that of spore-
haploid spores by meiosis and, because it is typically erect, can dispersing vascular plants in a number of important ways
release these spores into the air where they can be carried off by (Chapter 30): (1) Spores are not dispersed; instead, they germinate
a breeze. Furthermore, because the walls of the spores contain and develop into morphologically distinct male and female
sporopollenin, a polymer that is highly resistant to decay, the gametophytes while still attached to the sporophyte. (2) Male and
spores can survive prolonged exposure to air. Thus, the sporophyte female gametes are brought together by the transport of the male
generation and the formation of spores represent evolutionary gametophyte, which is so small that is fits within the spore wall,
innovations that enhance dispersal on land. forming a pollen grain. (3) Following fertilization, the embryo and
The first plants lacked roots and thus would have depended surrounding tissues develop into a seed, a multicellular structure
entirely on surface water both for fertilization and to maintain that replaces unicellular spores as the dispersal unit. In seed
the hydration of their cells. As a result, these plants would have plants, alternation of generations persists, but the dominant phase
been small, with their photosynthetic tissues and gamete- of the life cycle is the sporophyte, and pollen and seeds carry out
producing structures remaining in close contact with the ground. tasks previously accomplished by swimming sperm and spores.
Photosynthesis would have taken place only when conditions The effect of this transformation is a life cycle that is freed from
were wet enough to keep cells well supplied with water. Only the dependence on surface moisture for fertilization and in which
sporophyte generation is likely to have extended above the surface, embryos are dispersed along with resources that they can draw
to increase the chances of spores being carried off by a breeze. upon during germination.
This situation radically changed with the second major event The fourth evolutionary event is the evolution of the
in the evolutionary history of plants: the evolution of vascular flowering plants, also referred to as the angiosperms. Angiosperms
plants. These plants produce elongate cells for the internal are seed plants and thus their life cycle contains all the traits just
transport of water and other materials. Xylem cells transport water described. In addition, four new reproductive features are thought
and dissolved nutrients, and phloem cells transport carbohydrates to have contributed to angiosperm diversity and success. The
produced by photosynthesis (Chapter 29). The cell walls of the first is the flower, a reproductive structure that attracts animal
xylem conduits contain lignin, a chemical that greatly strengthens pollinators, increasing the efficiency of pollen transfer compared
the cellulose wall. Conduits made rigid by lignin allow vascular to wind pollination. The second is the carpel—the closed “vessel”
plants to pull water from the soil and to transport it efficiently in which seeds develop and that gives angiosperms their name.
through their stems. As a result, vascular plants are taller and able Because pollen must grow through the carpel to reach the female
to photosynthesize over a much wider range of conditions than gametophyte, interactions between pollen and carpel genotypes
plants dependent solely on surface moisture. The phloem conduits can affect the probability of fertilization. A third feature is
that transport sugars allow roots to grow into the soil, where there double fertilization, in which one sperm nucleus from the male
is no sunlight to power photosynthesis. gametophyte fuses with the egg, and a second sperm nucleus
In many vascular plants, xylem and phloem are formed only fuses with two haploid nuclei from the female gametophyte. The
as stems and roots elongate; they cannot be added to an already first union results in the diploid zygote, and the second gives rise
formed stem. However, some vascular plants evolved the ability to endosperm, which forms the nutritive tissue within the seeds
to produce additional or “secondary” xylem and phloem through of angiosperms. A consequence of double fertilization is that
the formation of a vascular cambium, a layer of actively dividing angiosperms do not expend resources for the next generation until
and differentiating cells that surrounds stems and allows them to the egg is fertilized. A fourth feature is the development of fruits,
increase in diameter (Chapter 31). A second layer of dividing cells, structures that surround seeds and attract animals to enhance
the cork cambium, maintains an intact layer of protective outer seed dispersal.
bark. Vascular and cork cambia provide the support and water Angiosperms are also distinguished by the presence of
transport capacity needed for plants to grow tall and to support wood containing xylem vessels (Chapter 29). Most other seed
increasing numbers of leaves. Thus, the evolution of secondary plants produce only tracheids, which both support the stem
growth opened the way to the development of trees and forests. and transport water. In contrast, angiosperms have thick-
As plants moved onto land, they retained their ancestral pattern walled, elongate fibers that support the stem and xylem vessels
of releasing swimming sperm into the environment. As a result, that transport water. Because xylem vessels are not a stem’s
the gamete-producing generation remained small, constrained by sole source of support, they can be wide and long, allowing
the requirement for surface moisture for fertilization, even as the angiosperms to transport water through their stems at higher
evolution of vascular tissues allowed the sporophyte to become rates than plants with tracheids. Their uptake of CO2 for
690 SECTION 33.2 B RYO P H Y T E S

photosynthesis increases as well because CO2 uptake is directly plant species increased dramatically. Thus, angiosperms’ hold on
linked to water lost to the atmosphere. As a result, angiosperms plant diversity is the result of both squeezing out other groups and
compete well for light and space and are the dominant plants in increasing the number of species that can coexist.
most terrestrial ecosystems. One way that angiosperm evolution may have increased the
overall number of plant species on Earth was by transforming the
Plant diversity has changed over time. environments into which they diversified. Climate models suggest
If we tabulate the numbers of species within each major branch on that angiosperms may have been necessary for the formation of
the phylogenetic tree shown in Fig. 33.1, one fact stands out: Of tropical rain forests as we know them today. Without the higher
the nearly 400,000 species of plants present today, approximately rates of transpiration exhibited by angiosperms, many tropical
90% are angiosperms. Furthermore, angiosperms are relative regions would have higher temperatures and lower rainfall. The
newcomers (Fig. 33.2). They appeared in the fossil record about warmer, drier conditions would have been less conducive to the
140 million years ago. The oldest known evidence of land plants luxuriant plant growth found in tropical rain forests. The new
is found in rocks approximately 465 million years old. Thus, for tropical forests, with their dense shade and humid understories,
more than 300 million years, terrestrial vegetation was made up provided new habitats into which both angiosperms and non-
of plants other than angiosperms. During this period, lycophytes, angiosperms could evolve. Thus, although early angiosperms
ferns and horsetails, and gymnosperms, as well as many groups outcompeted many species for space and light, their evolution also
of now-extinct plants, dominated the land, forming forests and stimulated radiations of new species in at least some of the groups
landscapes unlike those that surround us today. that preceded them.
Once the flowering plants gained an ecological foothold,
however, the number of angiosperm species increased at an
unprecedented rate. As the number of angiosperm species rose, 33.2 BRYOPHYTES
the number of species in other groups fell. Yet the total number of
Liverworts, mosses, and hornworts diverged before the evolution
of xylem and phloem (Fig. 33.1). The hypothesis shown in
Fig. 33.1 implies that liverworts, mosses, and hornworts evolved
FIG. 33.2 The diversity of vascular plant groups through time. independently for hundreds of millions of years and that they
First appearing about 140 million years ago, angiosperms do not form a monophyletic group. Recall from Chapter 23 that
have come to dominate the fossil record. Source: Adapted from a monophyletic group includes a common ancestor and all of its
A. H. Knoll and K. J. Niklas, 1987, “Adaptation, Plant Evolution, and the descendants, whereas a paraphyletic group includes a common
Fossil Record,” Review of Palaeobotany and Palynology, 50:127–149. ancestor and some of its descendants. However, it is important to
recognize that the phylogenetic relationship between liverworts,
80
mosses, and hornworts remains uncertain. Because these three
groups share many features, we refer to them collectively as
70 Angiosperms bryophytes.
Conifers
As the living representatives of the first plant groups to
Mean number of species per fossil assemblage

Cycads
60 Gingkos diverge after plants moved onto land, bryophytes provide us
Ferns and horsetails with insights into how plants gained a foothold in the terrestrial
Lycophytes
environment. The insights from bryophytes are particularly
Extinct seed plant groups
50 Extinct spore-dispersing plant groups welcome because the first plants are poorly represented in the
fossil record. Only tiny spores and fragments of a cuticle-like
covering record these early events. Of course, we must not lose
40
sight of the fact that bryophytes have continued to evolve as the
Angiosperms conditions for life on land have changed over the past 400+ million
30 years. Where bryophytes grow today and what they look like have
been shaped by their long coexistence with vascular plants.
20 Lycophytes Conifers j Quick Check 1 Look at Fig. 33.1 and determine whether green
algae, bryophytes, and vascular plants are monophyletic or
Ferns and horsetails paraphyletic.
10

Cycads
Bryophytes are small, simple, and tough.
400 350 300 250 200 150 100 50 2 Mosses are the most widely distributed of the bryophyte groups
Millions of years ago and the most diverse, with about 15,000 species. You may have
 CHAPTER 33 PL ANT DIVERSIT Y 691

FIG. 33.3 Bryophyte diversity. Bryophytes include (a) mosses (Polytricum commune is shown); (b) liverworts (Pellia ephiphylla, a thalloid liverwort,
is shown); and (c) hornworts (Anthoceros species). Sources: a. NHPA/Photoshot; b. ARCO/Reinhard H/age fotostock; c. Daniel Vega/age fotostock.

a b c

Sporophyte Gametophyte Gametophyte Sporophyte Gametophyte Sporophyte

seen moss growing on a shady log or on top of rocks by a stream. do not provide an advantage—for example, on the surfaces of rocks.
However, mosses grow in all terrestrial environments, from Many bryophytes live on the branches and trunks of trees rather
deserts to tropical rain forests. Liverworts (about 8000 species) than on the ground. Plants that grow on other plants are called
and hornworts (about 100 species) are less widespread and also less epiphytes (from the Greek words epi, “on,” and phyton, “plant”).
diverse. In considering the diversity of these three groups, let’s Bryophytes are well suited to growing on other plants because they
start by reviewing some of their features that allow them to carry do not depend on the soil as a source of water.
out photosynthesis on land.
Bryophytes have small, simple bodies (Fig. 33.3). Some The small gametophytes and unbranched sporophytes
produce only a flattened photosynthetic structure called a thallus. of bryophytes are adaptations for reproducing on land.
Others consist of slender stalks and have a leafy appearance. The easily visible and persistently photosynthetic bryophyte
However, these leaflike structures are quite different from the bodies described above are those of the haploid, gamete-producing
leaves of vascular plants in that they are only one to several cells generation. Because bryophytes release swimming sperm into
thick and lack internal air spaces. Both thalloid and leafy species the environment, their mode of fertilization also limits their
are found in the liverworts, whereas all mosses are of the leafy size. Bryophyte sperm must swim through films of surface
type and all hornworts are of the thalloid type. water or be transported by the splash of a raindrop. Sperm can
Because bryophytes do not produce lignified xylem conduits, travel in this manner only a relatively limited distance and only
they cannot pull water from the soil. Instead, they absorb water, if water is present. A few mosses grow to be more than half a
nutrients, and CO2 through their surfaces. For this reason, meter tall, but these giants of the bryophyte world are found only
bryophytes have little or no waxy cuticle to protect their in the understory of very wet forests. In most bryophytes, the
photosynthetic tissues. Bryophytes can absorb enough water to gametophyte grows to only a couple of centimeters in height.
remain metabolically active when the environment is wet, but Following fertilization, the diploid sporophyte develops
they must be able to tolerate desiccation when the environment is while remaining physically attached to and, in varying degrees,
dry. Thus, while the small size and lack of differentiated structures nutritionally dependent upon the gametophyte. Recall that the
might suggest that bryophytes are delicate, nothing could be diploid, spore-producing generation represents a new component
further from the truth. Bryophytes can be found from the equator of the life cycle that evolved as plants moved onto land. This
to both the latitudinal and altitudinal limits of vegetation, and sporophyte generation addresses the challenges plants face in
from swamps to deserts. dispersing offspring through the air. In most bryophytes, the
Before the evolution of vascular plants, bryophytes likely unbranched sporophyte extends several centimeters above the
covered much of the land surface. However, because bryophytes gametophyte, greatly increasing the chances of a gust of wind
are small and they can only photosynthesize when surface water is dispersing spores through the air.
present, they are poor competitors for light and space with vascular In mosses and liverworts, the sporophyte is short-lived, drying
plants. Today, bryophytes thrive in local environments where roots out after the spores are dispersed. However, in hornworts, the
692 SECTION 33.2 B RYO P H Y T E S

FIG. 33.4 Moss specializations. (a) The yellow moose-dung moss, Splachnum luteum, attracts insects that then
transport its spores. (b) Dawsonia superba has internal transport tissues that allow it to grow more than
40 cm tall. Sources: a. Biopix: A Neumann; b. blickwinkel/Alamy.

a b

sporophyte can live nearly as long as the gametophyte because it Another example of convergent evolution is the presence of
can produce new cells at its base, similar to the way grass blades cells specialized for the transport of water and carbohydrates.in
elongate (Chapter 32). Thus, bryophytes illustrate an important some of the largest bryophytes (Fig 33.4b). The structure of these
trend in the evolution of plants, an increase in the persistence of cells indicates that they evolved independently from the xylem
the spore-producing generation. and phloem of vascular plants. In particular, the water-conducting
cells in bryophytes do not have lignified cell walls and thus are not
Bryophytes exhibit several cases of convergent sufficiently rigid to pull water from the soil. Nevertheless, because
evolution with the vascular plants. of these elongated cells, water and carbohydrates can move from
Bryophytes are interesting partly because they are so different one part of the plant to the other efficiently.
from the more familiar vascular plants. However, given that Stomata are present on the sporophytes of some mosses
they have long evolved in parallel with vascular plants, it is not and hornworts, particularly in regions where a waxy cuticle also
surprising that bryophytes have evolved similar solutions to develops. As discussed in Chapter 29, stomata are pores in the
environmental challenges, a process referred to as convergent epidermis of vascular plants that open and close, providing an
evolution (Chapter 23). For example, some mosses depend on active means of controlling water loss. Bryophyte stomata may
insects to transport their spores (Fig. 33.4a). The brightly colored play an additional role: A sporophyte with open stomata will dry
sporangia of these mosses emit volatile chemicals that mimic out quickly, so its spores are more easily released into the air.
compounds released by rotting flesh or herbivore dung. Attracted Further study is needed to determine if these stomata are the
by these chemicals, insects land on the sporangia, and their legs result of convergent evolution.
become covered with spores. When the insect subsequently
lands on a real pile of dung, some of the spores fall off into what j Quick Check 2 If the stomata of bryophytes and of vascular plants
is, for them, an ideal site for supporting the growth of a new are not an example of convergent evolution, what is an alternative
gametophyte. explanation for their appearance in these groups?
 CHAPTER 33 PL ANT DIVERSIT Y 693

Sphagnum moss plays an important role in the global fossil fuels. Thus, any increase in the decomposition rate of these
carbon cycle. naturally formed peats has the potential to increase significantly
In most ecosystems, bryophytes make only a small contribution the CO2 content of Earth’s atmosphere.
to the total biomass. The one exception is peat bogs, wetlands in What might cause an increase in the decomposition rate? Peat
which dead organic matter accumulates rather than decomposes. bogs are vulnerable to changes in the water table. As water levels
A major component of peat bogs is sphagnum moss, any of the fall, the peat becomes exposed to the air, allowing it to be broken
several hundred species of the genus Sphagnum. These mosses down by microbes. Most peat bogs are located in northern latitudes,
play a key role in creating wet and acidic conditions that slow where the effects of climate change are predicted to be greatest. If
rates of decomposition. They have specialized cells that hold onto rising summer temperatures result in higher rates of transpiration
water, much like a sponge, and they secrete protons that acidify from surrounding forests, the result could be a lowering of the water
the surrounding water (Fig. 33.5). Unlike many plants with roots, table and a relatively rapid release of CO2 from this moss-dominated
sphagnum moss thrives under wet, acidic conditions. In addition, ecosystem. Thus, peat bogs have the potential to accelerate rates of
sphagnum mosses produce phenols, organic compounds that slow climate change over the next century.
decomposition under waterlogged conditions.
Peat bogs occupy only 2% to 3% of the total land surface, but
they store large amounts of organic carbon—on the order of 65 33.3 SPORE-DISPERSING
times the amount released each year from the combustion of VASCULAR PLANTS
Vascular plants first appear in the fossil record approximately
425 million years ago. The evolution of xylem and phloem gave
FIG. 33.5 Sphagnum moss. Peat bogs have low rates of organic vascular plants advantages in size and hydration. They were able
decomposition due to the wet and acidic conditions to outcompete bryophytes for light and other resources and to
created by sphagnum moss. Sources: (top) Duncan Shaw/ become the dominant plants on land.
Science Source; (middle) All Canada Photos/Alamy; (bottom) Vascular plants can be divided into two groups according
blickwinkel/Alamy. to how they complete their life cycle. Lycophytes, as well as
ferns and horsetails, disperse by spores and rely on swimming
sperm for fertilization, just like bryophytes. Gymnosperms and
angiosperms are seed plants: They disperse seeds and rely on the
aerial transport of pollen for fertilization. In this section, we focus
on the spore-dispersing vascular plants: lycophytes, and ferns and
horsetails. In both groups, the sporophyte dominates in physical
Peat bog size; the gametophyte is only a few centimeters across, thalloid
in structure, and typically tolerant of desiccation. Thus, our
discussion emphasizes the sporophytes of these plants.

Rhynie cherts provide a window into the early

evolution of vascular plants.
Fossils recovered from a single site in Scotland document key
stages in the evolution of vascular plants. Four hundred million
years ago, near the present-day village of Rhynie, a wetland
Water-holding cell
community thrived in a volcanic landscape. Mineral-laden springs
Sphagnum
moss similar to those found today in Yellowstone Park encased the
newly deposited remains of local populations in chert, a mineral
made of silica (SiO2). These fossils preserve plants, fungi, protists,
and small animals in remarkable detail.
Rhynie cherts provide our best view of early vascular plants.
These plants were small (up to about 15 cm tall) photosynthetic
structures that branched repeatedly, forming sporangia at the tips
Photosynthetic
of short side branches (Fig. 33.6). They had no leaves, and their
cell only root structures were small hairlike extensions on the lower
parts of stems that ran along the ground. The stems had a cuticle
with stomata, and a tiny cylinder of vascular tissue ran through the
 694 SECTION 33.3 S P O R E - D I S P E R S I N G VA S C U L A R P L A N T S

lycophytes are the earliest branching group of vascular plants, they

FIG. 33.6 Rhynia, a fossil vascular plant from the 400-million- are the sister group to all other vascular plants (see Fig. 33.1).
year-old Rhynie chert. All vascular plants are Fossils indicate that the leaves and roots of lycophytes evolved
descended from simple plants such as this. Photo independently from those of ferns, horsetails, and seed plants.
sources: (top) Courtesy of Hans Kerp © Palaeobotanical Research A distinctive feature of lycophyte leaves is that they have only a
Group, University of Münster; (middle & bottom) Hans Steur, The single vein, in contrast to the more complex leaf venation found
Netherlands. in other vascular plants. About 1200 species of lycophytes can be
found today. Some grow low to the ground and produce branching
stems covered by small leaves (Fig. 33.8a) or leaves in rows held
in a single plane (Fig. 33.8b), while others grow as epiphytes in
tropical forests. The most distinctive living lycophytes are the
Stoma quillworts (genus Isoetes), which are small plants that live along
the margins of lakes and slow moving streams (Fig. 33.8c). A few
species are desiccation tolerant and can live in ponds that dry up in
summer. Uniquely among living lycophytes, they have a vascular
Photosynthetic cambium. Seeing a quillwort today, you would never guess that its
Sporangium tissue in stem
relatives once included enormous trees.

Small
cylinder
FIG. 33.7 An early lycophyte preserved in the 400-million-
of xylem year-old Rhynie chert. The fossil has helically arranged
leaflike structures, adventitious roots (that is, roots that
arise from the shoot and not from the root system), and
Epidermis a central strand of xylem, features much like those of
with cuticle living lycophytes. Sources: (top) Photo by William Chaloner, Crown
Copyright; (bottom) Photo by John Hall © Botanical Society of America.

Hairlike
rooting
structures

center. Many of the gametophytes also had erect cylindrical stems,

not at all like the reduced gametophytes of living vascular plants.
Rhynie cherts provide another important glimpse of early plant
evolution: Some fossils have a branched sporophyte that includes
cell walls that are reinforced with lignin, but there is no evidence Helical arrangement
of leaflike structures,
of vascular tissue. Evidently, plants evolved the upright stature seen in cross section
we associate with living vascular plants before they evolved the near the shoot tip.
differentiated tissues of the vascular system (xylem and phloem).
Once vascular tissues had evolved, however, they played a key role
in shaping subsequent evolution of the plant sporophyte.

Lycophytes are the sister group of all other

vascular plants.
One of the fossil populations in Rhynie cherts is notably larger
and more complex than the others. These fossils show leaflike
structures arranged on the stem in a spiral pattern. Through the
stem runs a thick-lobed cylinder of xylem (Fig. 33.7). These are Lobed cylinder of xylem,
seen in cross section of
among the earliest fossils of lycophytes, a group of vascular plants the stem.
that can be found from tropical rain forests to Arctic tundra. Because
 CHAPTER 33 PL ANT DIVERSIT Y 695

FIG. 33.8 Lycophyte diversity. Living lycophytes include about 1200 species, in three major groups: (a) Lycopodium annotinum, showing
yellow sporangia-bearing leaves at the shoot tips of this 5-to-30-cm tall plant; (b) Selaginella wildenowii, in a close-up showing the
flattened arrangement of its 3-mm-long leaves; and (c) Isoetes lacustris, an aquatic lycophyte that grows from 8 to 20 cm tall.
Photo sources: a. Philippe Clement/Nature Picture Library; b. blickwinkel/Alamy; c. Biopix: JC Schou.

a b c

Lycopodium group (350–400 species)

Lycophytes Selaginella group (700 species)

Ferns and horsetails
Vascular
Gymnosperms plants Isoetes (~50 species)
Angiosperms
Tree-sized lycophytes (extinct)
Vascular cambium

Ancient lycophytes included giant trees that

dominated coal swamps about 320 million years ago.
Fossils show that ancient lycophytes evolved additional features
convergently with seed plants, including a vascular cambium and
cork cambium that enabled them to form trees up to 40 m tall
(Fig. 33.9). Swamps that formed widely about 320 million years
ago were dominated by tree-sized lycophytes. Unlike the vascular
cambium of seed plants, the vascular cambium of these lycophytes
(Fig. 33.10) produced relatively little secondary xylem and no
secondary phloem. Instead, the giant lycophytes relied on thick
bark for mechanical support. Thus, tree-sized lycophytes were
markedly different from the trees familiar to us today.
These forest giants were not outcompeted by seed plants, but
persisted for millions of years in swampy environments alongside
early seed plants. It was environmental change that spelled their
doom. When changing climate dried out the swamps, the large
lycophytes disappeared, along with their habitat. Little Isoetes was
left as the only living reminder of a once-dominant group of plants.
These ancient forests left one other legacy: the coal deposits
b we mine today. As trees died and fell over into the swamp, their
bodies decomposed slowly. Over time, as material accumulated

FIG. 33.9 Giant lycophytes. (a) Artist’s depiction of giant lycophytes.

(b) A fossil grove of lycophyte trees showing rooting
structures and thick stems. Sources: a. John Gurche; b. Biophoto
Associates/Science Source.
 CHAPTER 30 P L A N T R E P RO D U C T I O N : F I N D I N G M AT E S A N D D I S P E R S I N G YO U N G 696
HOW DO WE KNOW?

FIG. 33.10

Did woody plants evolve more than once?

OBSERVATIONS Today, the vascular cambium occurs almost seed plants and progymnosperms did not share a common ancestor
exclusively among seed plants, with only limited development that had a vascular cambium. Moreover, anatomical research
of secondary xylem in some adder’s tongue ferns and quillwort shows that the vascular cambium of extinct woody lycophytes
lycophytes (Isoetes species). The fossil record, however, shows that and the giant horsetails Archaecalamites and Calamites generated
other plants, now extinct, also had a vascular cambium. secondary xylem but not secondary phloem, unlike the vascular
cambium of seed plants. Living horsetails do not have a vascular
HYPOTHESIS The vascular cambium evolved convergently in
cambium, but fossils show that they are descended from ancestors
several different groups of vascular plants.
that did make secondary xylem and have lost this trait through
EXPERIMENT AND RESULTS The vascular cambium is recorded evolution.
in fossils by xylem cells in rows oriented radially in the stem
CONCLUSION Fossils and phylogeny support the hypothesis that
or root. Thus, the giant tree lycophytes of Carboniferous coal
the vascular cambium and, hence, wood evolved more than once,
swamps (the top photo) had a vascular cambium, as did extinct
reflecting a strong and persistent selection for tall sporophytes
tree-sized relatives of the horsetails (the bottom photo) and other
among vascular plants.
extinct horsetail relatives. Phylogenetic trees generated from
morphological features preserved in fossils unambiguously show SOURCE Taylor, T. N., E. L. Taylor, and M. Krings. 2009. Paleobotany. Amsterdam:
that woody lycophytes, woody horsetail relatives, and the group of Elsevier.

X Rhynia A small branch of Lepidodendron, an ancient giant lycophyte

Lycopodium Secondary Vascular
Selaginella Cortex xylem Pith cambium

Isoetes
X Lepidodendron
X Psilophyton
Adder’s tongue ferns
Whisk ferns
Extant horsetails
X Calamites
X Archaecalamites
2.5 mm
X Sphenophyllum
Ferns
A small stem of Calamites, an ancient giant horsetail
X Progymnosperms
Seed plants
Secondary xylem
Vascular cambium present
Vascular cambium lost Hollow center
X Extinct
Vascular cambium

5 mm

Photos source: Andrew Knoll, Harvard University.

696
 CHAPTER 33 PL ANT DIVERSIT Y 697

FIG. 33.11 Diversity of ferns and horsetails. (a) The adder’s tongue fern Ophioglossum, (b) the whisk fern Psilotum, (c) the horsetail Equisetum,
(d) a marattioid fern, (e) an aquatic fern (Salvinia), and (f) a polypod fern (in the middle of the photo is Athyrium felix-femina, the
common lady-fern). Photo sources: a. Jonathan Buckley/age fotostock; b. Biophoto Associates/Science Source; c. CuboImages srl/Alamy; d. Courtesy Gary
Higgins; e. FLPA/Chris Mattison/age fotostock; f. Ron Watts/First Light/Corbis.

Lycophytes a b c
Ferns and horsetails
Gymnosperms
Angiosperms

Adder’s tongue
ferns (114 species)
Whisk ferns (15 species)

Horsetails (15 species)

Distinct shoot
organization Marattioid ferns (240 species)
d e f

Earlier branching
Leptosporangium leptosporangiate
ferns (1030 species)

The branching order Aquatic ferns (85 species)

of horsetails,
marattioid ferns, and Tree ferns (710 species)
leptosporangiate
ferns is uncertain. Polypod ferns (9300 species)

and became buried, the combined action of high temperature and and their cells accumulate high levels of silica, earning them the
pressure converted the dead organic matter first into peat and then name “scouring rush.” Today, horsetails are small plants, most less
into the carbon-rich material we call coal. Thus, a major source of than a meter tall. However, tree-sized versions grew side by side
energy we use today derives originally from photosynthesis carried with the ancient lycophyte trees (see Fig. 33.9).
out by lycophyte trees.

Ferns and horsetails are morphologically and

FIG. 33.12 A fern leaf uncoiling from a “fiddlehead.” Source: Andre
ecologically diverse.
Nantel/Shutterstock.
Ferns and horsetails form a monophyletic group that is the sister
group to the seed plants (Fig. 33.11). The majority of species in this
group are ferns. You can recognize ferns by their distinctive leaves
that uncoil during development from tightly wound “fiddleheads”
(Fig. 33.12). This group also includes horsetails and whisk ferns,
which traditionally were considered distinct groups because of
their unique body organizations. Whisk ferns have photosynthetic
stems without leaves and roots (see Fig. 33.11b). Thus, they look
similar to fossils of early vascular plants. Molecular sequence
comparisons, however, support the hypothesis that whisk ferns
are the simplified descendants of plants that produced both leaves
and roots.
The horsetails, represented by 15 living species of the genus
Equisetum, produce tiny leaves arranged in whorls, giving the stem
a jointed appearance (see Fig. 33.11c). Horsetail stems are hollow,
698 SECTION 33.3 S P O R E - D I S P E R S I N G VA S C U L A R P L A N T S

FIG. 33.13 A tree fern and an aquatic fern. (a) Silver tree ferns (Cyathea meullaris) grow 10 m tall or more. (b) Azolla ferns floating on the
surface of still freshwater. Each fern is about 10 mm in diameter. Sources: a. AA World Travel Library/Alamy; b. Huetter, C/age fotostock.

a b

they were the most diverse group of plants (Fig. 33.2). Then, as
angiosperms evolved, their diversity markedly decreased. It is not
clear why angiosperm evolution should have so affected ferns and
horsetails, which had coexisted with a variety of seed plant groups
for more than 200 million years.
However, the most diverse group of ferns today evolved after
the rise of the angiosperms. These are the polypod ferns (about
9300 species; see Fig. 33.11f). Thus, many of the fern species
In contrast, many ferns produce large leaves—in some cases present today are likely to have evolved to occupy habitats newly
up to 5 m long, although typically the photosynthetic surfaces are created by the development of angiosperm forests. Approximately
divided into smaller units called pinnae. Yet, because ferns are 40% of living ferns are epiphytes, many of them growing in
not capable of secondary growth, their stems remain slender and tropical rain forests dominated by angiosperm trees.
frequently grow underground, with only the leaves emerging into Polypod ferns may owe some of their evolutionary success to
the air. Many ferns exhibit little stem elongation and produce their an enhanced capacity for spore dispersal. Horsetails, whisk ferns,
leaves in tight clumps. However, some species, such as bracken and a few other fern groups make large sporangia similar to those
fern, produce spreading stems with widely spaced leaves. Because produced by early vascular plants. Most ferns, however, have a
bracken is toxic to cattle and other livestock, its tendency to distinctive sporangium (called a leptosporangium) whose wall is
invade pastures is a serious problem. only a single cell thick. Polypod ferns have leptosporangia with a
Despite the absence of secondary growth, some ferns can grow line of thick-walled cells that run along the sporangium surface.
quite tall. Tree ferns, which can grow more than 10 m tall, produce When the spores mature, the sporangium dries out and these cells
thick roots near the base of each leaf that descend parallel to the contract, forcibly ejecting spores into the air.
stem to the ground. These roots increase the mechanical stability
of the plant and also allow it to transport more water (Fig. 31.13a). An aquatic fern contributes to rice production.
Other ferns grow tall by producing leaves that twine around the Azolla is a free-floating aquatic fern that has been used for
stems of other plants, using the other plant for support. At the other centuries in Asia as a biofertilizer of rice paddies (see Fig. 33.13b).
extreme are tiny aquatic ferns such as Salvinia (see Fig. 33.11e) and Its success as a fertilizer is due to its symbiotic association with
Azolla (Fig. 33.13b) that float on the surface of the water. nitrogen-fixing cyanobacteria. Azolla is common worldwide on
still water such as ponds and quiet streams. Its leaves contain an
Fern diversity has been strongly affected by the internal cavity in which colonies of nitrogen-fixing cyanobacteria
evolution of angiosperms. become established. The cyanobacteria provide Azolla with
The fossil record shows that ferns originated more than a source of nitrogen, while the fern provides shelter and
360 million years ago, and between 300 and 100 million years ago carbohydrates. The cyanobacteria are reported to have higher rates
 CHAPTER 33 PL ANT DIVERSIT Y 699

of nitrogen fixation when they are housed within an Azolla leaf fertilization and the formation of a seed. Seeds in turn enhance
than when they live freely in the environment. dispersal. The nutrient-rich tissue in the seed makes it more
When added to newly planted rice fields, this fast-growing fern likely that the embryo will grow into a photosynthetically self-
suppresses weed growth by forming a dense, floating layer. At the sufficient plant. Before these innovations, the ancestors of seed
same time, it provides organic nitrogen that eventually finds its plants evolved the ability to form large woody stems through
way into the developing rice plants. The floating fertilizer factories the formation of both a vascular and a cork cambium. Therefore,
provided by Azolla may have made possible the long history of rice woody stems and roots are the ancestral condition in seed plants.
cultivation in Asia. Although the fossil record shows that there were once
many different groups of seed plants, today there are only two:
gymnosperms and angiosperms. Gymnosperms total only a bit
33.4 GYMNOSPERMS more than 1000 species, whereas angiosperms total 380,000
species or more. Despite their much lower diversity, in cool or dry
Seed plants have come to dominate terrestrial environments since parts of the world, gymnosperms such as pine trees can make up
their first appearance about 365 million years ago. While the coal- more of the forest canopy than do angiosperms. In this section,
swamps of 300 million years ago were populated by spore-producing we examine the evolutionary history of the four groups of living
giants, the understory of these strange forests included plants that gymnosperms (Fig. 33.14) and explore how their diversity and
produced pollen and seeds. By the time of the dinosaurs, seed plants distribution has been affected by the rise of the angiosperms.
had become the largest and most abundant plants on Earth.
What factors have contributed to the success of seed plants? Cycads and ginkgos are the earliest diverging groups
One is that they do not require external water for fertilization. of living gymnosperms.
Instead, the male gametophyte is transported through the air in It’s easy to confuse cycads with palm trees because both have
pollen. As a result, seed plants are able to reproduce successfully unbranched stems and large leaves (Fig. 33.15). However, the
in both rain forest and desert. Pollination, if successful, leads to presence of cones rather than flowers clearly marks cycads as
gymnosperms. Cones are modified branch tips inside
which reproductive structures develop, with pollen and
ovules produced in separate cones. In cycads, the cones
FIG. 33.14 Seed plant diversity. The phylogenetic tree shows a hypothesis of
are large and woody, and thus provide protection for the
evolutionary relationships among gymnosperms. Photo sources: (top
developing pollen and seeds.
to bottom) Robert Davis/age fotostock; Michael P. Gadomski/Science Source; Pete
Cycads have low photosynthetic rates and they grow
Ryan/Getty Images; Patti Murray/age fotostock.
slowly. Although they have large stems, their vascular
cambium produces little additional xylem, and most
Lycophytes
of their bulk is made up of unlignified (nonwoody)
Ferns and horsetails
Gymnosperms
cells in the center of the stem (pith) and surrounding
Angiosperms the vascular tissues (cortex). Cycads are able to grow
in nutrient-poor environments by forming symbiotic
associations with nitrogen-fixing cyanobacteria. Cycads
Cycads (~300 species) are also able to survive wildfires because their apical
meristem is protected by a dense layer of bud scales.
The approximately 300 species of cycads occur
most commonly in tropical and subtropical regions.
Ginkgo (1 species) Most species have a limited and often fragmented
distribution, suggesting that they were much more
widespread in the past. Most cycad populations are small
and often vulnerable to extinction. Cycads may owe
Conifers (~630 species) their persistence in part to the fact that they rely on
insects for pollination, as well as to their ability to live
in nutrient-poor and fire-prone environments. Insects
transfer pollen more efficiently than wind, and therefore
insect-pollinated species with small populations are able
Gnetophytes (~90 species)
to reproduce successfully. Many cycads are pollinated by
beetles, which are attracted by chemical signals produced
by the cones. The large seed cones provide shelter for the
Angiosperms (>380,000 species) insects, and the pollen cones provide them with food.
700 SECTION 33.4 GYMNOSPERMS

FIG. 33.15 Cycads. (a) Most living cycads have thick, unbranched stems that support large leaves and cones. Shown here is Encephalartos
friderici-guilielmi. (b) Many cycads, such as Encephalartos transvenosus, have large cones. (c) Mature seeds are large (here are shown
those of Cycas circinalis) and contain a relatively large female gametophyte that provides nutrition for the embryo and germinating
sporophyte. Sources: a. A. Laule/age fotostock; b. & c. Andrew Knoll, Harvard University.

a b

Following fertilization, brightly colored, fleshy seed coats attract fleshy seeds, perhaps a feature inherited from their common
a variety of birds and mammals that serve as dispersal agents. seed plant ancestor. Ginkgos date back about 270 million years,
Because of their small numbers and fragmented distribution, and during the time of the dinosaurs, they were common trees
two-thirds of all cycads are on the International Union for the in temperate forests. Like cycads, ginkgos declined in abundance,
Conservation of Nature’s “red list” of threatened species, the diversity, and geographic distribution over the past 100 million
highest percentage of any group of plants. years, as a result of both changing climates and angiosperm
Cycads are an ancient group, with fossils at least 280 million diversification.
years old. During the time of the dinosaurs, they were among the Ginkgos have long been cultivated in China, especially near
most common plants. Cycad diversity and abundance declined temples, and it is unclear whether any truly natural populations
markedly during the Cretaceous Period, at the same time the exist in the wild. Today, however, Ginkgo once again is found all
angiosperms rose to ecological prominence. A recent study shows over the world, as its ability to tolerate the pollution and other
that present-day cycad species date from a burst of speciation that stresses of urban environments has made it a tree of choice for
took place approximately 12 million years ago. At that time, the street plantings.
positions of the continents and ocean circulation patterns were
changing in such a way that climates became increasingly cooler Conifers are woody plants that thrive in dry
and more seasonal. This climate change provided new habitats for and cold climates.
emerging species while dooming older species to extinction. The approximately 630 species of conifers are, for the most part,
A second group of gymnosperms has only a single living trees. Conifers include the tallest (over 100 m) and oldest (more
representative: Ginkgo biloba. This species forms tall, branched than 5000 years) trees on Earth (Fig. 33.17). Well-known conifers
trees, with fan-shaped leaves that turn brilliant yellow in autumn include pines, junipers, and redwoods. Most conifers have strong
(Fig. 33.16). Ginkgo is wind pollinated, but, like cycads, develops apical dominance, and thus develop a single straight stem from
 CHAPTER 33 PL ANT DIVERSIT Y 701

FIG. 33.16 Ginkgo biloba. (a) The fan-shaped leaves of Ginkgo turn yellow in autumn. (b) The fleshy seeds enclose a large female gametophyte,
as in cycads. Sources: a. JTB/Photoshot; b. Kathy Merrifield/Science Source.

a b

which extend much smaller horizontal branches. Conifers have of the great questions in the evolutionary history of plants is why
traditionally served as masts for sailing ships, and today are used conifers were displaced from tropical latitudes by the angiosperms
for telephone poles. Conifer xylem consists almost entirely of but continued to thrive in cold and dry environments.
tracheids, and so its wood is strong for its weight, making it well An important difference between angiosperms and conifers
suited as a building material. Conifers supply much of the world’s is that angiosperms transport water in the xylem vessels,
timber, as well as the raw material for producing paper. whereas conifers transport water in tracheids. In wet and warm
Most conifers are evergreen, meaning that they retain their environments, the xylem vessels of angiosperms can be both wide
often needle-like leaves throughout the year. In some species, and long, increasing the efficiency of water transport through
the leaves are remarkably long lived, remaining green and their stems. Thus, one hypothesis is that the evolution of xylem
photosynthetic for decades. Many conifers produce resin canals in vessels gave angiosperms a competitive advantage in moist
their wood, bark, and leaves that deter insects and fungi. Conifer tropical environments. With more water flowing through their
resins are harvested and distilled to produce turpentine and other stems, angiosperms could have higher rates of photosynthesis and
solvents. In addition, conifer resins are the source of a number of
important chemicals, including the drug taxol used in the treatment
of cancer. Amber, prized for its beauty, is fossilized resin. FIG. 33.17 Conifers. Giant sequoia (Sequoiadendron giganteum)
Conifer reproduction tends to be slow: After pollination, it can can reach 85 m in height and have stems that are 8 m in
take more than a year before the seeds are ready to be dispersed. diameter. Source: Charles J. Smith.
Pollen is produced in small cones, and seeds in larger cones. The
seed cones become woody, providing protection, as the fertilized
ovules develop slowly into seeds. Conifers are wind pollinated, and
most species also rely on wind for seed dispersal. However, some
conifers, such as junipers and yews, produce seeds surrounded
by fleshy and often brightly colored tissues that attract birds and
other animals.
Conifers dominate the vast boreal forests of Canada, Alaska,
Siberia, and northern Europe. Conifers tend to become more
abundant as elevation increases, and they are common in dry
areas, such as the western parts of North America and much of
Australia. Only a small number of conifer species are found in
tropical latitudes, most commonly at higher elevations. Before
the rise of the angiosperms, however, conifers were more evenly
distributed across the globe, including in the lowland tropics. One
702 SECTION 33.5 ANGIOSPERMS

typical of angiosperms, notably the

FIG. 33.18 Gnetophytes. The three genera in this group include some of the most formation of xylem vessels and double
distinctive seed plants: (a) Welwitschia mirabilis; (b) Gnetum species; and fertilization. However, double fertilization
(c) Ephedra species. Sources: a. M. Philip Kahl/Science Source; b. Ahmad Fuad Morad in gnetophytes does not result in the
[Flickr page] 072112; c. Bob Gibbons/Science Source. formation of a triploid tissue, as it does in
a angiosperms. Moreover, the water transport
capacity of gnetophyte xylem, while higher
than other gymnosperms, is less than that
of the angiosperms sharing their habitats.
For years, many botanists viewed
gnetophytes as the sister group of
angiosperms. DNA analysis, however,
indicates that gnetophytes are not closely
related to the angiosperms but are, instead,
more closely related to conifers. Thus,
both xylem vessels and double fertilization
evolved independently in this group.
The fossil record shows that
gnetophytes originated as much as 200
b c
million years ago, reaching their greatest
diversity at approximately the same time
as the angiosperms began to diversify.
The subsequent decline in gnetophyte
diversity may result from competition with
angiosperms.

j Quick Check 3 Double fertilization

evolved independently in gnetophytes and in
flowering plants. Name two other features
usually associated with angiosperms that
evolved independently in gymnosperms.

grow larger leaves. Once conifer populations began to dwindle,

they could have disappeared completely because wind pollination 33.5 ANGIOSPERMS
requires large populations. In colder or drier regions, the dangers
of cavitation due to drought or freeze followed by thaw (Chapter The evolution of angiosperms transformed life on land. The total
29) constrain the size of angiosperm vessels. This helps level the number of plant species increased significantly as angiosperms
playing field and explains the greater coexistence of conifers and diversified. Some non-angiosperm plant groups thrived as they
angiosperms in cold or dry climates. expanded into new niches created by angiosperms, while others
declined as many of their species went extinct. Terrestrial
Gnetophytes are gymnosperms that have ecosystems became more productive as the more efficient
independently evolved xylem vessels and double angiosperm xylem allowed higher rates of photosynthesis.
fertilization. As a result, more food became available for insects, birds, and
Gnetophytes are a small group of gymnosperms (about 90 species) mammals. All of these animal groups diversified along with the
made up of three genera that look completely different from one flowering plants.
another (Fig. 33.18). Welwitschia mirabilis is found only in the
deserts of southwestern Africa. It produces only two straplike leaves Angiosperms may have originated in the shady
that elongate continuously from their base. Gnetum grow in tropical understory of tropical forests.
rain forests, where they form woody vines and small trees with The last common ancestor of angiosperms and living gymnosperms
large broad leaves. Ephedra, which is native to arid regions, produces is thought to have lived more than 300 million years ago. Yet
shrubby plants with photosynthetic stems and tiny leaves. the first unambiguous fossils of angiosperms appear in rocks
Gnetophytes have long attracted the attention of plant about 140 million years ago. This 160-million-year gap means
biologists because they exhibit a number of traits that are that there is no closely related sister group to help us understand
 CHAPTER 33 PL ANT DIVERSIT Y 703

pollen and ovules close to each other, the movement of animals

FIG. 33.19 Angiosperm phylogeny. Most angiosperm species from one flower to another provides an efficient mode of pollen
belong to the monocots and eudicots. Photo sources: transfer.
(top to bottom) Kenneth Setzer; Tim Gainey/Alamy; Liz Cole/ Insect pollination may have allowed the first angiosperms to
age fotostock; Jo Whitworth/age fotostock;Tropicals JR Mau/ exploit understory habitats of tropical rain forests. Wind provides
PhotoResourceHawaii.com; Image Source/Getty Images. an effective means of pollen transfer in open habitats or in plants
Lycophytes
that can produce reproductive structures at the top of the canopy.
Ferns and horsetails In the dense understory of a tropical forest, the likelihood that air
Gymnosperms movements alone would bring pollen to an ovule is close to zero. All
Angiosperms of the living members of the earliest angiosperm groups produce
flowers that are visited by insects. Fig. 33.20 shows the flower of
Japanese star anise, which is visited by beetles. The fruit of a close
Amborella relative (Illicium verum) is the source of a popular seasoning used in
Flowers; double
fertilization; fruits (1 species) Asian cooking.
Let’s turn now to a second important feature of angiosperms,
the formation of wood containing xylem vessels. In the humid
Gymnosperm-like understory of a tropical forest, where transpiration rates are
Water lilies
ancestor (<1%) extremely low, there would seem to be little advantage in
producing wood with a greater capacity for water transport. In
fact, the sole existing species of the earliest-diverging angiosperm
group—Amborella trichopoda—produces only tracheids.
Xylem Star anise
Furthermore, early-diverging angiosperm groups that do produce
vessels and
relatives xylem vessels do not conduct water at higher rates than those that
(<1%) do not. Thus, xylem vessels in angiosperms may not have evolved,
at least initially, as a means of enhancing water transport.
Magnoliids
(2%)

FIG. 33.20 Japanese star anise, an early-branching angiosperm.

Monocots The flowers of Illicium anisatum are pollinated by
(23%) beetles. Source: Biosphoto/Richard Bloom/GAP.
One cotyledon

Eudicots
Pollen with (75%)
three apertures

what type of environment the early angiosperms evolved in. But

we can look at where living plants from the earliest-branching
groups of angiosperms are found today (Fig. 33.19). Most grow
in the understory of tropical rain forests; water lilies, which grow
in shallow ponds, are one notable exception. This observation
suggests that angiosperms may have evolved in wet and shady
habitats. How could life in the understory of a tropical forest
dominated by conifers have contributed to the evolution of flowers
and xylem vessels?
Recall from Chapter 30 that flowers are a plant’s way of
enlisting animals to help them reproduce. Brightly colored petals
and volatile scents advertise the presence of flowers, while
rewards such as nectar encourage pollinators to return repeatedly
to individuals of the same species. Because many flowers produce
704 SECTION 33.5 ANGIOSPERMS

Instead, the formation of separate cell types for water different species and from closely related individuals (including
transport and mechanical support may have given the early self) suggests that these genetic recognition systems contribute to
angiosperms greater flexibility of form. Vines and sprawling shrubs the reproductive isolation required for speciation (Chapter 22).
are two examples of growth forms exhibited by early diverging It is important to recognize that diversity results from both
angiosperms. Both growth forms allow plants to grow toward high rates of species formation and low rates of species loss. The
small patches of sunlight that filter through the forest canopy. In fossil record suggests that the persistence of species may play
the heavily shaded tropical rain forest understory, this ability may a particularly important role in angiosperm diversity. Wind-
have provided a distinct advantage. pollinated plants can reproduce only when their populations have
a relatively high density. Since no wind-carried pollen is likely to
Angiosperm diversity results from flowers and xylem fall on isolated individuals, species with low population density are
vessels, among other traits, as well as coevolutionary more likely to go extinct. In contrast, animal pollinators actively
interactions with animals and other organisms. searching for rare species are much more likely to find them, and
The fossil record provides evidence that for the first 30 to 40 so animal-pollinated angiosperm populations can persist at low
million years of their evolution, angiosperms were neither population densities.
diverse nor ecologically dominant. In fact, fossil pollen indicates Another reason rare species may be more persistent is that
that, between about 140 and 100 million years ago, gnetophytes they are less likely to encounter soil pathogens and seed predators.
diversified just as much as angiosperms. Later in the Cretaceous These threats to survival tend to gather near an adult of the same
Period, beginning about 100 million years ago, angiosperm diversity species. The seeds of rare species are more likely to land away
began to increase at a much higher rate. Trees belonging to the from an individual of the same species and thus suffer less from
magnoliids, the branch of the angiosperm tree that today includes this density-dependent mortality than do more common species.
magnolias, black pepper, and avocado, emerged as ecologically However, this ecological strategy works only if rare isolated
important members of forest canopies. More important, however, individuals can remain in contact with a large enough population
was the divergence of the two groups that would come to dominate of potential mates, underlining again the key role of flowers and
both angiosperm diversity and the ecology of many terrestrial animal pollinators in persistence.
environments: the monocots and the eudicots. If animal pollination has so many advantages, why did
Before looking at the diversity of these two groups, let’s approximately 20% of angiosperm species subsequently evolve
consider how characteristics of angiosperms may have promoted a dependence on wind for pollination? Wind-pollinated species
their diversification. While many factors likely have contributed, grow primarily in seasonal environments, where fewer animal
we’ll focus on two themes. The first is that angiosperms are more pollinators are available than in tropical forests, and the pressures
efficient at building their bodies and completing their life cycle than resulting from soil pathogens and seed predators may be much less.
are other plants. Recall that the xylem of angiosperms has vessels That angiosperm diversity is the result of multiple factors is
supplying water required for photosynthesis and fibers providing reinforced by the observation that some of these “angiosperm”
mechanical support. Because water transport and mechanical features are also present in other groups of plants. For example,
support are separated, angiosperms are able to grow efficiently into cycads rely on animal pollination, whereas gnetophytes produce
a wide variety of forms, ranging from short-lived herbaceous plants xylem vessels, and yet neither group is particularly diverse.
to trees with large spreading crowns. This diversity of shapes and The initially slow but increasingly rapid buildup of angiosperm
sizes reduces competition for light and space. diversity may reflect the compounding effect of coevolutionary
Angiosperms complete their life cycle as efficiently as they interactions with pollinators and dispersers, as well as adaptive
build their bodies. Not only are animal-pollinated plants able to radiations triggered by chemical arms races with pathogens
reduce their production of pollen, but also double fertilization and herbivores (Chapter 32). This is what is meant by the adage
reduces the costs of reproduction by allowing angiosperms to delay “Diversity begets diversity,” a phenomenon further explored in
provisioning their offspring until after fertilization. Angiosperms Chapter 47.
can thus reproduce quickly and with a minimum of resources. One
possible consequence is that angiosperms can make use of habitats Monocots are diverse in shape and size despite not
and resources that are only fleetingly available, such as ephemeral forming a vascular cambium.
pools or open areas that appear following a fire. Monocots or monocotyledons make up nearly one-quarter
A second theme focuses on the ways in which angiosperms of all angiosperms. Monocots come in all shapes and sizes and are
interact with other types of organisms. The interaction of flowers found in virtually every terrestrial habitat on Earth (Fig. 33.21).
and animal pollinators contributes to angiosperm diversity. Plants Coconut palms, hanging Spanish moss (a relative of pineapple), and
whose flowers attract different pollinators tend to more readily tiny floating duckweeds, less than 2 mm across, are all monocots,
diverge to form new species than wind-pollinated species. In as are the sea grasses of tropical lagoons and the lilies and daffodils
addition, the ability of carpels to block the growth of pollen from in the garden. Some monocots grow in arid regions—including
 CHAPTER 33 PL ANT DIVERSIT Y 705

a b
FIG. 33.21 Monocot diversity and phylogeny. This diverse group
of angiosperms includes (a) Agave shawii, a desert
succulent; (b) Leucojum vernum, a spring wildflower;
(c) bamboo, a forest grass; and (d) Costus species,
which grow in the rain forest understory. (e) Monocot
phylogeny indicates that orchids (~22,000 species)
and grasses (~11,500) make up much of monocot
diversity. Photo sources: a. Biosphoto/François Gohier; b. Kerstin
Hinze/naturepl.com/NaturePL; c. Axle71/Dreamstime.com;
d. Tropicals JR Mau/PhotoResourceHawaii.com.

of plants in terms of what we eat. Grasses, for example corn, rice,

d c wheat, and sugarcane, are staples in the diets of most people. Other
monocots that make their way to our dinner table include banana,
yams, ginger, asparagus, pineapple, and vanilla.
Monocots take their name from the fact that they have one
embryonic seed leaf, or cotyledon, whereas all other angiosperms
have two. However, monocots are distinct in form in so many
other ways that one rarely has to count the number of cotyledons
to identify a member of this large, monophyletic group. Monocots
represent a major evolutionary departure in the way seed plants
build their bodies, in that a vascular cambium is never formed.
How then has this group been so successful?
In monocot leaves, the major veins are typically parallel and
the base of the leaf surrounds the stem, forming a continuous
e. sheath. This type of leaf base means that only one leaf can be
attached at any node, consistent with the formation of a single
cotyledon. Although monocots do not form a vascular cambium,
Many aquatic or marshy plants they can still produce stems that are quite large. For example, both
(4500 species)
corn and coconut palms are monocots. In monocots, all of the
increase in stem diameter occurs in a narrow zone immediately
Yams (2400 species) below the apical meristem. As a result of this lateral expansion, the
vascular bundles become distributed throughout the stem instead
Lilies (1500 species) of being arranged in a ring as in all other seed plants. Monocots
that have slender stems—for example, some species of climbing
Orchids, daffodils, agave, apraragus
(26,000 species)
yams—have their vascular bundles in a ring.
The lack of a vascular cambium has a profound impact on
Palms (2300 species)
the way monocots form roots. Because individual roots cannot
Grasses, pineapples (18,000 species) increase their vascular capacity, monocots continuously initiate
new roots from their stems. Thus, the root systems of monocots are
Gingers, bananas (3000 species) more similar to those found in ferns and lycophytes than in other
seed plants. Finally, monocot flowers typically produce organs in
multiples of 3 (for example, having 3, 6, or 9 stamens), whereas
eudicot flowers typically produce organs in multiples of 4 or 5.
Monocots have a relatively poor fossil record, in part because
agave, which is used to make tequila. Others, such as orchids, grow they do not produce wood. What factors might have led to such
primarily as epiphytes in tropical forests. A few monocots form a radical revision in how they build their bodies? This question
vines—for example, the rattan palms, whose stems are used to requires that we look at where and how living monocots grow.
produce rattan furniture. Monocots are the most important group One hypothesis is that monocots evolved from ancestors that
 CHAPTER 30 P L A N T R E P RO D U C T I O N : F I N D I N G M AT E S A N D D I S P E R S I N G YO U N G 706
HOW DO WE KNOW?

FIG. 33.22

When did grasslands expand over the land surface?

BACKGROUND Today, prairies, steppes, and other grasslands occur widely in the interiors of continents. Grasses, however, do not fossilize
readily. How, then, can we understand how grasslands developed through time?

Map is from Edwards, E. J., C. O. Osborne, C. A. E. Stromberg, S. A. Smith,

Integrating Evolutionary and Ecosystem Science.” Science 328:587–591

and the C4 Grasses Consortium. 2010. “The Origins of C4 Grasslands:
Forests
C3 grasslands
C4 grasslands
Crops

HYPOTHESIS Grasslands expanded as climate changed over the past 50 million years.

OBSERVATIONS AND EXPERIMENTS Grasses commonly make phytoliths, small structures of silica (SiO2) in their cells. These preserve well,
providing a direct record of grass expansion. Moreover, mammals that feed on grasses evolved high-crowned teeth, which also preserve well,
giving us an indirect record of grassland history. Finally, C4 grasses, which are adapted to hot sunny environments with limited rainfall, have a
distinctive carbon isotopic composition imparted by the initial fixation of CO2 by PEP carboxylase (Chapter 29). Measurements of 13C and 12C
in mammal teeth, soil carbonate minerals, and more recently, tiny amounts of organic matter incorporated into phytoliths, allow scientists to
track C4 grasslands through time.

produced creeping, horizontal stems as they grew along the within an environment that makes only modest demands for
shores of lakes and other wetlands. Many monocots today, water transport.
including the earliest diverging groups of monocots, grow in such We may never know for sure in what environment the
habitats, and many of the features of the monocot body plan are monocots first evolved. We thus turn our attention to one of the
well adapted to environments with loose substrates, flowing most diverse groups within the monocots: the grasses. Many
water, and fluctuating water levels. For example, their leaf base grasses produce stems that grow horizontally and branch, allowing
provides a firm attachment that prevents leaves from being pulled them to cover large areas. Grasses have linear leaves that elongate
off by flowing water. Furthermore, many monocots produce from the base, allowing them to survive grazing as well as fire
strap-shaped leaves that elongate from a persistent zone of cell and drought. Many grasses have evolved the ability to tolerate dry
division and expansion located at the base of the leaf blade. By environments by producing roots that extend deep into the soil.
continually elongating from the base, monocot leaves can extend In addition, C4 photosynthesis has evolved within the grasses
above fluctuating water levels. Finally, it is easier to imagine multiple times. As described in Chapter 29, C4 photosynthesis
evolutionary changes that affected the vascular system occurring allows plants to avoid photorespiration and thus photosynthesize

706
 RESULTS Studies of phytoliths, mammal tooth structure, and carbon isotopic composition of teeth enamel from the North American
midcontinent clearly show that grasslands expanded 20 to 15 million years ago, and C4 grasslands expanded later, about 8 to 6 million
years ago.

Eocene Oligocene Miocene Pliocene

Middle Late Early Late Early Middle Late
of high-crowned teeth

3.0
Relative importance

2.5
2.0

1.5
1.0

100
Percent of grass
species that live
in open habitat

80
60
40
Based on analysis of
20
phytolith assemblages
0
100 The shading shows the maximum
that is C4 plants

(top of shaded area) and minimum

Percent of diet

80
value (bottom of shaded area)
60 recorded at each point in time.
40
Based on carbon isotope composition
20 of horse tooth enamel
50 45 40 35 30 25 20 15 10 5 0
Millions of years ago

CONCLUSION In North America, grasslands expanded as atmospheric CO2 levels declined and climates became drier. Other continents
show evidence of a similar linkage of grassland expansion to climate change.

FOLLOW-UP WORK At present, atmospheric CO2 levels are increasing rapidly, which may affect the competitive abilities of C3 and C4
grasses. Scientists are working to understand how global change will influence grasslands and other vegetation.

SOURCE Stömberg, C. A. E. 2011. “Evolution of Grasses and Grassland Ecosystems.” Annual Review of Earth and Planetary Sciences 39: 517–544.

with greater efficiency. Grasses are among the most successful of all angiosperm species (Fig. 33.23). Eudicots are well
group of plants, becoming widespread within the past 20 million represented in the fossil record, in part because their pollen is
years as climates changed (Fig. 33.22). Today, nearly 30% of easily distinguished. Each eudicot pollen grain has three openings
terrestrial environments are grasslands. from which the pollen tube can grow, whereas pollen from all
other seed plants has only a single opening. Eudicots take their
j Quick Check 4 What are some of the distinctive features of
name from the fact that they produce two cotyledons, whereas
monocots?
monocots produce one. But, because the magnoliids and early
angiosperm groups also have two cotyledons, this largest of all
Eudicots are the most diverse group of angiosperms. angiosperm groups is referred to as the “eu-” or “true” dicots.
Eudicots first appear in the fossil record about 125 million years Many eudicots produce highly conductive xylem. High rates
ago and, by 90 to 80 million years ago, most of the major groups of water transport, and thus high rates of photosynthesis, may
we see today were present. Today, there are estimated to be explain why eudicot trees were able to replace magnoliid trees as
approximately 160,000 species of eudicots, nearly three-quarters the most ecologically important members of forest canopies. In

707
708 SECTION 33.5 ANGIOSPERMS

eudicots. Important eudicot trees in temperate regions include

FIG. 33.23 Eudicot diversity and phylogeny. Eudicots include oaks, willows, and eucalyptus.
(a) red oak trees (Quercus rubra), (b) tropical At the other end of the size spectrum are the herbaceous
passionflower vines (Passiflora caerulea), (c) Banksia eudicots, which make up a substantial fraction of eudicot diversity.
shrubs, native to Australia, and (d) beavertail cactus Herbaceous plants do not form woody stems. Instead, the
(Opuntia basilaris). (e) The tree shows a phylogeny of aboveground shoot dies back each year rather than withstand a
eudicots, with well-known species indicated. Groups period of drought or cold. At the extreme are annuals, herbaceous
with <3000 species are not shown. Photo sources: a. Michael plants that complete their life cycle in less than a year, persisting
P Gadomski/Getty Images; b. Sergio Hayashi/Dreamstime.com; during the unfavorable period as seeds. Annuals are unique to
c. Joanne Harris/Dreamstime.com; d. Danita Delimont/Getty Images. angiosperms and the majority of these annuals are eudicots.
a b
Why might eudicots be successful as both trees and herbs?
One possibility is that the ability to produce highly conductive
xylem may allow herbaceous eudicots to grow quickly and to
produce inexpensive and thus easily replaced stems. Examples of
herbaceous eudicots include violets, buttercups, and sunflowers.
The herbaceous growth form appears to have evolved many
times within eudicots as tropical groups represented by woody
plants expanded into temperate regions. For example, the
d c common pea is an herbaceous plant that resides in many vegetable
gardens; its relatives include many tropical rain forest trees.
Peas are members of the legume, or bean, family, some of which
can form symbiotic interactions with nitrogen-fixing bacteria
(Chapter 29). The ability to trade carbon for nitrogen may explain
why legumes are important in many habitats. Many of the trees
of the African savannas, as well as trees in seasonally dry regions
throughout the world, are legumes.
Eudicots are diverse in other ways as well. Most parasitic
plants and virtually all carnivorous plants are eudicots, as are
water-storing cacti that grow in deserts. Some, such as roses
and blueberry, are woody shrubs. Others grow as epiphytes in
the canopy of rain forests, and still others, such as grapes and
e. honeysuckle, are vines. Perhaps the most unusual are the strangler
Buttercups, poppies (4500 species) fig trees. These begin as epiphytes and then produce roots that
descend to the forest floor and fuse to form a solid cylinder that
Grapes (850 species) “strangles” their host tree. Finally, eudicots make a significant
Apples, roses, violets, legumes,
contribution to the diversity of the dinner table. Apples, carrots,
figs, oaks, walnuts, pumpkins, pumpkins, and potatoes are all eudicots, as are coffee, cacao (the
willows, passionflower, birches source of chocolate), and tea. Many plants with oil-rich seeds,
(50,000 species)
such as olives, walnuts, soybean, and canola, are eudicots, as are
Cotton, chocolate, eucalyptus, buckwheat and quinoa.
papaya, cabbage, Arabidopsis
(20,000 species)
The reproduction, growth, and physiology of angiosperms are
summarized in Fig. 33.25 on pages 710 and 711.
Cacti, carnations (11,500 species)

Blueberry, coffee, tea, tomato,

? CASE 6 AGRICULTURE: FEEDING A GROWING POPULATION

milkweeds, potato, tobacco, What can be done to protect the genetic diversity
mints (60,000 species) of crop species?
Of the more than 400,000 species of plants on Earth today, we eat
Sunflower, carrots, honeysuckle
(10,000 species) surprisingly few. Of those we do eat, nearly all are angiosperms.
The only exceptions are the female gametophyte of a small
number of gymnosperms (pine nuts and ginkgos), as well as
addition, many eudicot trees lack strong apical dominance and the young leaves of a few ferns. Most of the plants we eat come
produce crowns with many spreading branches. Today, tropical from species that are grown in cultivation, and many of these
rain forest trees are an important component of the diversity of cultivated species have, as a consequence of artificial selection,
 CHAPTER 33 PL ANT DIVERSIT Y 709

FIG. 33.24 “Centers of origin” where different crop species were domesticated.

Citrus, sugarcane, rice

Sugarcane, banana,
yam, coconut
Barley, soybean, rice

Corn, tomato,
bean, cotton
Cabbage, lettuce, Cotton, potato,
olives, oats, wheat peanut
Wheat, barley Potato
Grapes Rubber, chocolate
Sorghum, wheat,
coffee

lost the ability to survive and reproduce on their own. Of the to collect seeds from around the globe. Vavilov’s observations of
approximately 200 such domesticated species, 12 account for over cultivated plants and their relatives led him to hypothesize that
80% of human caloric intake. Just three—wheat, rice, and corn— plants had been domesticated in specific locations from which
make up more than two-thirds. they had subsequently expanded as a result of human migration
Crop breeders select for varieties that grow well under and commerce (Fig. 33.24). Vavilov called these regions “centers
cultivation and are easy to harvest, but a consequence of selective of origin,” and he believed that they coincided with the centers of
breeding is a decrease in genetic diversity. Thus, not only do we diversity for both crop species and their wild relatives. In World
depend upon a small number of species for food, agriculture itself War II, during the siege of Leningrad in which more than 700,000
contributes to a narrowing of plant diversity. Because pathogens and people perished, scientists at the Vavilov Institute sought to
pests continue to evolve, any loss in genetic diversity of crop species protect what was then the world’s largest collection of seeds. Their
creates substantial risks. For example, in 1970, the fungal pathogen dedication—at least one of the self-appointed caretakers died of
Bipolaris maydis, a disease-causing organism that had previously starvation, despite being surrounded by vast quantities of edible
destroyed less than 1% of the U.S. corn crop annually, destroyed seeds—illustrates the priceless nature of the genetic diversity on
more than 15% of the corn crop in a single year. The severity of the which our food supply rests.
epidemic was due to the genetic uniformity of the corn varieties Today, seed banks help preserve the genetic diversity of crop
planted at that time. In natural plant populations, genetic variation species and their wild relatives. However, seed banks can store
for pathogen resistance makes it highly unlikely that a newly only a fraction of the genetic diversity present in nature. To
evolved pathogen strain will be able to infect every plant. help make up the difference, some have suggested establishing
Nicolai Vavilov, a Russian botanist and geneticist, was one protected areas that coincide with Vavilov’s centers of origin. As
of the first to recognize the importance of safeguarding the we discuss in Chapter 49, new threats lie just over the horizon.
genetic diversity of both crop species and their wild relatives. In Meeting these threats will require that every genetic resource be
the early twentieth century, he mounted a series of expeditions brought to bear. •
V I S UA L S Y N T H E S I S Angiosperms:
FIG. 33.25 Structure and Function
Reproduction Integrating concepts from Chapters 29 –31, and 33
Many angiosperms rely on animals
for pollination and dispersal.

1 Flowers: Animals in
search of nectar and
other rewards carry 2 Pollination: Animal pollination is more efficient
pollen from one flower than releasing pollen into the wind. Animal-pollinated
to another. plants can reproduce even when they are rare,
increasing the number of species that can coexist.

Seed
Apple 3 Double fertilization:
blossom Angiosperms supply resources for
seed development only after the
Triploid Ovary egg has been fertilized, during the
endosperm formation of triploid endosperm.
Embryo

Ovule

Triploid cell
Diploid
4 Fruits: Following zygote Pollen tube
fertilization, flowers nucleus
develop into
structures that
enhance seed
dispersal, often by
attracting animals that
carry the fruits away. Growth
Fruit Meristems allow angiosperms to grow and develop throughout their life.

5 Dispersal: 6 Primary growth: Apical 7 Secondary growth: The vascular

Transport of seeds meristems are persistent centers cambium produces new xylem and
away from the parent of cell division and growth that phloem, allowing the plant to
plant reduces allow plants to produce new support more leaves and providing
competition with stems, leaves, and roots the mechanical strength to grow tall.
relatives and provides throughout their entire life.
a means of escaping
pathogens.

Shoot apical
meristem

Seed

Root apical
meristem

710
Photosynthesis
Efficient water transport through stems allows angiosperms
to support high rates of photosynthesis.

Carbohydrates

H2O CO2

Leaves: To absorb CO2

needed for photosynthesis, Bark: The cork
leaves open pores called cambium maintains an
stomata, resulting in a outer layer that
massive loss of water vapor. protects the phloem
and vascular cambium
from damage.
Carbohydrates

H2O

Phloem: Plants transport

carbohydrates from sources to
sinks through living sieve tubes,
Xylem: Water lost from allowing them to produce organs
leaves is replaced by water such as roots that are not
pulled from the soil and photosynthetic.
transported through
stiff-walled xylem vessels.

Nutrients

Nutrients
Carbohydrates

Endomycorrhizae: Plants
H 2O exchange carbohydrates for
nutrients with mycorrhizal
fungi, an example of a
symbiotic relationship.

Roots: Extensive branching

of roots creates the surface
area needed to obtain water
and nutrients from the soil.

711
712 CO R E CO N C E P T S S U M M A RY

Core Concepts Summary Three hundred million years ago, lycophytes included large
trees that dominated swamp forests. Today, lycophytes are small
33.1 Angiosperms make up approximately 90% of all plants that either grow in the forest understory as epiphytes or
plant species found today. occur in shallow ponds. page 694

There are thought to be nearly 400,000 species of plants living Ferns and horsetails are morphologically diverse. Ferns produce
today. Of these, approximately 90% are angiosperms. page 690 large leaves that uncoil as they grow; horsetails have tiny leaves;
and whisk ferns with no leaves at all. page 697
The other 10% of plant species is distributed among the other
six major groups of plants. page 690 Although ferns have a long history, most present-day species
Angiosperms first appear in the fossil record about 140 million are the result of a radiation that occurred after the rise of the
years ago, more than 300 million years after plants first moved angiosperms. page 698
onto land. page 690
33.4 Gymnosperms produce seeds and woody stems
The evolution of angiosperms resulted in a rapid and dramatic
and are most common in seasonally cool or dry regions.
increase in total plant diversity. page 690
As angiosperm diversity increased, other plant groups, such as Of the many groups of seed plants that have evolved, only two
gymnosperms and ferns, declined in diversity. page 690 can be found today: the gymnosperms, with fewer than 1000
species, and the angiosperms, with more than 380,000 species.
Moist tropical rain forests dominated by angiosperms provided page 699
new types of habitat into which other plants could evolve.
page 690 Gymnosperms are composed of four groups of woody plants:
cycads, ginkgos, conifers, and gnetophytes. page 699
33.2 Bryophytes form persistent, photosynthetic
Cycads produce large leaves on stout, unbranched stems.
gametophytes and small, unbranched sporophytes;
Although they once were widely distributed, they now
today, they grow in environments where the ability to
occur in small, fragmented populations, primarily in the
pull water from the soil does not provide an advantage.
tropics and subtropics. Many cycads are insect pollinated, and
Bryophytes constitute a paraphyletic group consisting of three all form symbiotic associations with nitrogen-fixing bacteria.
groups of plants: mosses, liverworts, and hornworts. page 690 page 699
Bryophytes are small plants that produce one of two Ginkgo is the single living species of a group that was distributed
morphological types: a flattened thallus or an upright leafy globally before the evolution of the angiosperms. Ginkgo is wind
type. They do not form roots, but instead absorb water through pollinated and produces tall, branched trees. page 700
their surfaces. page 691
Conifers include the tallest and longest-lived trees on Earth.
The persistent component of the life cycle is the gametophyte.
Wind-pollinated and largely evergreen, they are found primarily
Sporophytes range from tiny and non-photosynthetic in
in cool to cold environments. page 700
some liverworts to relatively long-lived and photosynthetic in
hornworts. page 691 Before the angiosperms appeared, conifers were widespread.
Their almost complete absence in the tropics and persistence
There are several examples of convergent evolution between
in temperate regions may be due to their dependence upon
bryophytes and vascular plants, including insect dispersal of
wind pollination and having xylem formed entirely of
spores in some mosses and the evolution of internal transport
tracheids. page 701
cells in some mosses and liverworts. page 692
Sphagnum moss is the dominant plant of peat bogs. page 692 The gnetophytes are a small group, containing only three
genera and few species, that has independently evolved xylem
Sphagnum moss produces water-holding cells that allow
vessels. page 702
it to soak up water, and it acidifies the environment. Both
characteristics help slow decomposition, so large amounts of
organic carbon build up year after year. page 692 33.5 Angiosperms are distinguished by flowers,
fruits, double fertilization, and xylem vessels;
33.3 Spore-dispersing vascular plants today are their diversity is the result of traits that increase the
primarily small plants that grow in moist environments, efficiency of completing their life cycle and building
but in the past included tall trees. their bodies.
Only two groups of seedless vascular plants have living relatives Xylem vessels make it possible for angiosperms to have a
today: lycophytes, and ferns and horsetails. page 693 diversity of form and to grow toward light. page 704
PASS1

CHAPTER 33 PL ANT DIVERSIT Y 713

Angiosperms reproduce quickly and with less use of

resources as a result of pollination and double fertilization,
Self-Assessment
allowing them to diversify into ephemeral habitats. 1. Using information in Fig. 33.2, draw a pie chart that depicts
page 704 proportional plant diversity today.

Angiosperm diversity may result as much from low rates of 2. Imagine a world in which mosses, liverworts, and
extinction as from high rates of speciation. page 704 hornworts form a monophyletic group. How would your ability
to infer what the first land plants looked like be affected?
Plants having flowers can reproduce even if they are far
apart, allowing rare species to persist and reproduce. 3. Describe three environments that allow bryophytes to
page 704
coexist with vascular plants.
4. Describe the habitats in which lycophytes are found
The angiosperm phylogeny indicates a major split between
today.
two diverse groups, monocots and eudicots. page 704
5. List three ways that ferns, which lack secondary growth,
Monocots include grasses, coconut palms, bananas, ginger,
are able to elevate their leaves and thus access more sunlight.
and orchids. page 704
6. Describe how fern diversity has been affected by the
Monocots have a single cotyledon, or embryonic seed leaf, evolution of the angiosperms.
and they do not form a vascular cambium. page 705
7. Contrast the ways in which the evolution of angiosperms has
Eudicots are characterized by pollen grains with three affected the distribution of cycads and of conifers.
openings through which the pollen tube can grow.
8. Explain how xylem produced by conifers differs from that
page 707
of angiosperms and how that difference may have influenced
Eudicots are diverse, and include many familiar plants such the present-day distribution of conifers.
as legumes, roses, cabbage, pumpkin, coffee, tea, and cacao.
9. Compare the movement of pollen from an animal-
page 708
pollinated angiosperm and a wind-pollinated conifer, noting
Modern agriculture is based on just a few plant species with what features of angiosperm reproduction increase the
low genetic diversity, making crops vulnerable to pests and efficiency (or lower the costs) of pollen transfer.
pathogens. page 709 10. Name two features that have contributed to the diversity
and success of angiosperms and discuss possible advantages of
these features.

Log in to to check your answers to the Self-

Assessment questions, and to access additional learning tools.
this page left intentionally blank
 CHAPTER 34

Fungi
Structure, Function,
and Diversity

Core Concepts
34.1 Fungi are heterotrophic
eukaryotes that feed by
absorption.
34.2 Fungi reproduce both sexually
and asexually, and disperse by
spores.
34.3 Other than animals, fungi are
the most diverse group of
eukaryotic organisms.

Chris Ted/Getty Images.

715
716 SECTION 34.1 G RO W T H A N D N U T R I T I O N

In tropical rain forests, vascular plants make up most of the Hyphae permit fungi to explore their environment
biomass. Animals eat the plants, obtaining food for growth and for food resources.
reproduction, but they feed selectively: They consume leaves, Most fungi consist of highly branched, multicellular filaments
fruits, and seeds, but largely avoid a much more abundant called hyphae (Fig. 34.1). The hyphae are slender, typically
tissue—wood. In fact, most leaves also escape grazing and 10 to 50 times thinner than a human hair. The numerous long,
eventually fall, dead, onto the forest floor. Animals, too, may be thin hyphae provide fungi with a large surface area for absorbing
eaten by predators, but some die of other causes and contribute nutrients.
their remains to the soil. Given this constant rain of biological Hyphae maintain their slender form by growing only at their
materials, we might expect wood, leaves, and animal carcasses tips. Elongating hyphae thus penetrate ever farther into their
to accumulate in great piles, but a walk through the rain forest environment, encountering new food resources as they grow.
reveals that the forest floor is remarkably clean. What happened Where resources are low, growth is slow or may stop entirely.
to all the dead plant and animal tissues? On the other hand, when fungi encounter a rich food resource,
On land, the organisms principally responsible for the they grow rapidly and branch repeatedly, forming a network of
decomposition of plant and animal tissues are fungi, one of branching hyphae called a mycelium (plural, mycelia). Mycelia
the most abundant and diverse groups of eukaryotic organisms. can grow to be quite large—the largest known individual of the
Many of us have seen mushrooms and toadstools in a meadow or fungus Armillaria ostoyae covers over 2000 acres in the Blue
woodland, but most fungal biomass and most of the metabolic Mountains of Oregon and weighs many hundreds of tons. We may
work that fungi do occurs within the soil, out of sight. Fungi play be impressed by blue whales and redwoods, but in fact some fungi
a critical role in cycling carbon because of their ability to locate are the largest living organisms on Earth.
and break down the complex molecules and bulky tissues in A strong but flexible cell wall is key to hyphal growth and
plant and animal bodies. Moreover, because they form intimate nutrient transport. In fungi, cell walls are made of chitin, the
relationships with living plants and animals, fungi affect the same compound found in the exoskeletons of insects. Chitin is a
growth and reproduction of many other organisms. For example, modified polysaccharide that contains nitrogen. Because of their
fungi associated with plant roots dramatically increase plant chemical makeup, fungal cell walls are thinner than plant cell
productivity by enhancing the uptake of mineral nutrients walls. Fungal cell walls prevent hyphae from swelling as water
from the soil (Chapter 29). Other fungi are major agricultural flows into the cytoplasm by osmosis. Thus, the wall prevents cells
pests (Chapter 32). Fungi enable us to turn wheat into bread, from rupturing when exposed to dilute solutions such as those in
barley into beer, and milk into cheese, but still other fungi cause
athlete’s foot, yeast infections, and, especially in patients with
compromised immune systems, overwhelming infections that can
lead to death. FIG. 34.1 Fungal hyphae. These thin filaments have enormous
surface area that increases absorption of nutrients. Shown
here are hyphae of Trichophyton, a fungus that infects
34.1 GROWTH AND NUTRITION the skin of humans, as seen under a scanning electron
microscope. Source: Susumu Nishinaga/Science Source.
Fungi are heterotrophs, meaning that they depend on preformed
organic molecules for both carbon and energy. Unlike animals,
fungi do not have organs that enable them to ingest food and
break it down in a digestive cavity (Chapter 40). Instead, fungi
absorb organic molecules directly through their cell walls. This
mode of feeding presents two major problems. First, although
simple molecules like amino acids and sugars pass readily through
the cell wall, more complicated molecules do not. Fungi secrete a
diversity of enzymes that break down complex organic molecules
like starch or cellulose into simpler compounds that can be
absorbed. Fungi, then, digest their food first and take it into the
body afterward.
A second challenge is to find food in the environment. Other
heterotrophic organisms such as bacteria, protists, and animals
commonly move through their habitat, actively searching for
food. Fungi, however, have no means of locomotion, and so these
organisms use the process of growth itself to find nourishment.
 CHAPTER 34 F U N G I : S T RU C T U R E , F U N C T I O N , A N D D I V E R S I T Y 717

mycelia. In early-diverging groups, the hyphae have many nuclei

FIG. 34.2 Septa separating individual cells in fungi. Septa, but no cell walls to separate them. In later-diverging groups,
partitions between cells, have pores that allow transport nuclear divisions are accompanied by the formation of septa
of solutes along the hypha. (singular, septum), walls that partially divide the cytoplasm into
separate cells (Fig. 34.2). Each septum contains one or more pores
that allow water and solutes to move freely between cells. Septa
play an important role when hyphae are damaged. Injury activates
sealing mechanisms that plug pores in the septa, preventing the
loss of pressurized cytoplasm.

j Quick Check 1 Name two roles of hyphae.

Plasma
Hypha
membrane Not all fungi produce hyphae.
Pore
Yeasts are single-celled fungi found in moist, nutrient-rich
environments. In these circumstances, the search for food does
Cell wall
not require hyphae. All yeasts are descended from ancestors
that developed hyphae, but hyphal development has been lost
independently several times. Thus, the lack of hyphae is an
example of convergent evolution, in which the same trait develops
Septum independently in different lineages in response to similar selective
pressures (Chapter 23).
Most yeasts divide by budding. A small outgrowth increases
in size and eventually breaks off to form a new cell (Fig. 34.3).
The localized outgrowth that results in budding is similar to
Pore growth by elongation at hyphal tips. In fact, some yeasts can form
hypha-like structures under certain conditions. For example,
in Candida albicans, a yeast that infects humans, hyphae are
freshwater environments and in the soil. Moreover, the inflow of produced during the transition of the fungus from a harmless to a
water by osmosis provides the force that enables fungi to explore pathogenic form.
their surroundings: the inflow of water generates positive turgor
pressure (Chapters 5 and 29), pushing the hyphal tip ever deeper
into the local environment.
FIG. 34.3 The yeast Saccharomyces cerevisiae. Yeasts are
Fungi transport materials within their hyphae. unicellular fungi that often divide by budding smaller cells
Fungi can transport food and signaling molecules across long off larger ones. Source: Andrew Syred/Science Source.
distances in mycelia. As a result, they are able to grow between
resource patches and to produce reproductive structures such as
mushrooms that rise up above the ground. Molecules taken up
actively from the environment drive water into the cell by osmosis
and thus increase turgor pressure. At the same time, growth and
respiration consume these molecules, resulting in a decrease in
turgor pressure. Such differences in turgor pressure along hyphae
drives bulk flow in a manner similar to phloem transport in plants
(Chapter 29). Bulk flow carries materials obtained in a nutrient-
rich location so that they can fuel hyphal elongation across
nutrient-poor locations. Transport by bulk flow also allows fungi to
build relatively large reproductive structures aboveground. All the
raw materials used to build a mushroom must be transported from
hyphae in contact with a source of nutrients, such as the soil or a
rotting log.
A continuous stream of cytoplasm, not divided by barriers,
is essential for the long-distance movement of materials within
718 SECTION 34.1 G RO W T H A N D N U T R I T I O N

Yeasts are common on the surfaces of plants, and to a lesser

extent on the surfaces and in the guts of animals. Humans have FIG. 34.4 Decomposition of wood by fungi. (a) A dead tree being
long used the yeast Saccharomyces cerevisiae to ferment plant decomposed by brown rot fungi. (b) These scanning
carbohydrates and thereby to produce leavened bread and alcoholic electron microscope images show (left) normal cedar
beverages. Saccharomyces cerevisiae is also an important model wood and (right) rotted cedar wood from the coffin
organism used in laboratory studies of genetics. in the tomb of King Midas in Gordion, Turkey. Sources:
a. Science Photo Library/Alamy; b. Timothy R. Filley, Robert A.

Fungi are principal decomposers of plant tissues. Blanchette, Elizabeth Simpson, and Marilyn L. Fogel. Nitrogen cycling

Most fungi use dead organic matter as their source of energy and by wood decomposing soft-rot fungi in the “King Midas tomb,” Gordion,

raw materials. As noted in Chapter 25, on land the organic matter Turkey PNAS 2001 98 (23) 13346-13350. Copyright 2001 National

in dead tissues on and within soils far exceeds the amount in Academy of Sciences, U.S.A. Photos courtesy of Robert A. Blanchette,

the living biomass. Thus, the ground beneath our feet provides University of Minnesota.

tremendous resources for heterotrophic growth. Bacteria and

a
some protists can use this resource, but for the most part it is the
fungi that convert dead organic matter back to carbon dioxide and
water. This helps to keep the biological carbon cycle in balance
(Chapter 25) and returns nutrients to the soil, where they will be
available for new plant growth.
A brief consideration shows why fungi have the advantage on
a forest floor or in soil. Like fungi, bacteria secrete enzymes that
break down organic molecules in their immediate environment,
but in the soil bacteria have only a limited capacity to move from
place to place. Flagella can propel them a few micrometers through
a thin film of water, but when local resources become depleted,
bacteria must wait for new food to appear. In contrast, the growing
hyphae of fungi can search actively through the soil for new food b
resources. Furthermore, because fungi can penetrate their food,
they are particularly well suited for breaking down the bodies of
multicellular organisms, such as tree stems.
Fungi can gain nutrition from dead animal, protozoan, or even
bacterial cells, but the most abundant biomolecules on and within
soils are cellulose and lignin, the principal components of plant
cell walls (Chapter 31). Cellulose is a polymer of the sugar glucose,
so it is a particularly rich source of carbon and energy. Cellulose
is difficult to degrade because individual cellulose polymers bind
tightly to one another. In addition, in wood, cellulose and lignin
are closely associated, which makes the cellulose even harder
to get at. Lignin is difficult for enzymes to break apart, in part
because it lacks a regular chemical structure. A few bacteria are
known to degrade lignin, but in nature, fungi account for most of example, are vulnerable to a diverse array of fungal pathogens—
the decomposition of wood (Fig. 34.4). rusts, smuts, and molds—that cause huge losses in agricultural
To break down lignified plant cell walls, fungi require oxygen. production, as we discuss in section 34.3.
However, oxygen concentrations are typically low in water- Invasion is the key to success for plant pathogens (Chapter 32).
logged environments, and in that case decay proceeds slowly if Successful pathogens must be able to get past a plant’s physical
at all. For this reason, woody biomass, once submerged in water, or chemical defenses. This explains why fungal pathogens exhibit
tends to accumulate, leading to the buildup of partially decayed host specificity, meaning that they are able to infect only one or
plant material as peat. Over geologically long time intervals, peat a small number of host species. In many cases, fungi infect plants
becomes coal, an energy staple of the industrial world that owes its through wounds, which provide a route through a plant’s outer
existence to the inhibition of fungal decay. defenses. Some fungi enter through stomata, while others penetrate
epidermal cells directly, degrading the wall with enzymes and then
Fungi are important plant and animal pathogens. using turgor pressure to push their hyphae into the plant interior.
Fungi not only feed on dead plants and animals, but they are The extent of infection usually depends on the ability of hyphae to
also well adapted to infect living tissues (Fig. 34.5). Plants, for expand into new tissues. Vascular wilt fungi (Chapter 32) send their
 CHAPTER 34 F U N G I : S T RU C T U R E , F U N C T I O N , A N D D I V E R S I T Y 719

sugar-rich droplets that attract insects, which then transport their

FIG. 34.5 Fungal infection of living tissue. (a) This stained section spores to other sites.
of heart tissue (pink) is infected with the fungus Candida Belowground, infection is typically transmitted by hyphae
albicans (the purple threadlike structures). (b) A scanning that penetrate the root. Hyphae can feed on organic matter in
electron micrograph shows hyphae of Ceratocystis ulmi, a the soil until they encounter the root of a suitable host plant. The
vascular wilt fungus, growing in xylem vessels of an elm persistence of fungal hyphae in soils means that populations of
tree. Sources: a. Science Source; b. Biophoto Associates/Science Source. pathogenic fungi can build up in the soil near a host plant. This
is why seedlings germinating close to an adult tree of the same
a
species are at risk of infection, whereas seedlings of distantly
related species may be spared. By favoring the establishment of
seedlings of less common species, fungal pathogens are thought
to play an important role in maintaining high levels of species
diversity in tropical rain forests (Chapter 32).
Insects and other invertebrates are also susceptible to fungal
pathogens. Certain fungi exploit living animals in a unique way: by
trapping them. Some fungal predators form sticky traps with their
hyphae, whereas others actually lasso their prey (Fig. 34.6). The
hyphae of these fungi form rings that can inflate within seconds,
trapping anything within the ring. Nematodes, tiny worms
common in soils, inadvertently pass through the rings, setting off
hyphal inflation. The nematodes are then trapped, providing food
for the fungus.
Fungal infection is relatively rare in vertebrates. Severe
infections are more frequent among fish and amphibians than in
b mammals, perhaps because fungi grow poorly at mammalian body
temperatures. An apparent exception is the fungal infection that
has caused dramatic declines in North American bat populations.
But these fungi infect the bats during their winter hibernation,
when the bats have lowered body temperatures that conserve
energy. In humans, most fungal infections are annoying rather

FIG. 34.6 A fungal lasso. Hyphae of Drechslerella doedycoides

form inflated rings that capture nematode worms moving
through the soil. Source: Reprinted from Current Opinion in
Neurobiology, 22, Jennifer K Pirri, Mark J Alkema, The neuroethology of
C. elegansescape Fig. 3, Copyright 2011, with permission from Elsevier.
Reproduced with permission from Mark Alkema.

hyphae through xylem vessels, enabling them to spread throughout

the entire plant. Old photographs of towns in the eastern United
States show streets shaded by American elm trees, but today nearly
all those trees are dead, victims of a vascular wilt fungus introduced
into the United States early in the twentieth century.
Aboveground, plant infections are usually transmitted by
fungal spores, carried either by the wind or on the bodies of
insects. For example, many fungi that attack fruits are transmitted
by insects also seeking those fruits; yeasts that feed on nectar
within flowers hitch rides on pollinators. Some fungi secrete
720 SECTION 34.1 G RO W T H A N D N U T R I T I O N

than life threatening—athlete’s foot and yeast infections are

prime examples. However, in individuals with immune systems FIG. 34.7 Mutualistic relationship between fungi and plants.
compromised by HIV or illness, fungal infection can be fatal. Mycorrhizal fungi (white) surround roots (brown) and
produce extensively branched hyphae that extend into soil.
Many fungi form symbiotic associations with plants Source: Dr. Jeremy Burgess/Science Source.

and animals.
While some interactions between species benefit the fungus at the
expense of its host, others benefit both partners. In Chapter 29,
we saw that mycorrhizal fungi supply plant roots with nutrients
such as phosphorus and nitrogen from the soil and in return
receive carbohydrates from their host (Fig. 34.7). There are two
main types of mycorrhizae. The hyphae of ectomycorrhizal fungi
surround, but do not penetrate, root cells. In contrast, the hyphae
of endomycorrhizal fungi penetrate into root cells, where they
produce highly branched structures that provide a large surface
area for nutrient exchange. As we discuss in section 34.3, the
endomycorrhizal fungi constitute a monophyletic group dating back
over 400 million years. In contrast, ectomycorrhizae are present in
multiple groups and appear to have evolved more than once.
Other fungi, called endophytes, live within leaves. While
endophytes are much less studied than mycorrhizae, they may
be as widespread. Endophytic hyphae grow within cell walls
and in the spaces between cells. Long thought to be harmless,
endophytes are now recognized as beneficial. They may help beetles. These beetles maintain fungal gardens within tunnels that
the host plant by producing chemicals that deter pathogens and they excavate in damaged or dead trees.
herbivorous insects.
Mutually beneficial associations between fungi and animals Lichens are symbioses between a fungus and a green
are much less common, but a few examples are known, none more alga or a cyanobacterium.
striking than insects that actually grow fungi for food. The fungi Lichens are familiar sights in many environments, often forming
benefit because the insects provide shelter, food, and protection colorful growths on rocks or tree trunks (Fig. 34.8). Lichens
from predators and pathogens. Insect–fungal agriculture has look, function, and even reproduce as single organisms, but
evolved at least three times: in leaf-cutter ants that live in tropical they are actually stable associations between a fungus and
forests, in a group of African termites, and in some wood-boring a photosynthetic microorganism, usually a green alga but

FIG. 34.8 Lichens. A lichen is a mutualistic association between a fungus and a photosynthetic microorganism. Sources: (left to right) Wallace Garrison/
Getty Images; Stephen Sharnoff; Stephen Sharnoff.
 CHAPTER 34 F U N G I : S T RU C T U R E , F U N C T I O N , A N D D I V E R S I T Y 721

FIG. 34.9 Lichen anatomy. The fungus provides structure, and photosynthetic algae form a thin layer under the surface. The scanning electron
micrograph shows a section through Xanthoria flammea. Photo source: Eye of Science/Science Source.

Lichens consist mostly of fungal

hyphae that take up water and
Fungal nutrients from the soil.
hyphae

Photosynthetic algal cells form a

Algal thin layer just under the surface
cells of the lichen.

sometimes a cyanobacterium. Nearly 15% of all known fungal Surprisingly, these associations are not species specific. There
species grow as lichens. are approximately 13,500 known lichens, but only about 100
The dual nature of lichens was first proposed by a Swiss participating photosynthetic species. This means that different
botanist, Simon Schwendener, in 1867, but his hypothesis lichens can have the same algae or cyanobacteria. Conversely,
found little favor at the time. The existence of a composite a given fungal partner may associate with more than one
“organism” challenged the idea that life could be divided photosynthetic species without affecting the lichen’s outward
into discrete categories such as animals and plants and raised form. Because of the intimate association between lichen
the question of how an association of distinct species could morphology and fungal species, the lichen and fungus are assigned
function as an integrated whole. Given what we now understand the same scientific name.
about the widespread nature of mutualisms, the opposition to Lichens spread asexually by fragmentation or through the
Schwendener’s proposal may seem surprising. Yet it was only in formation of dispersal units consisting of a single photosynthetic
1939, when Eugen Thomas showed that lichens could be separated cell surrounded by hyphae. Sexual reproduction, at least by
into their individual parts and then reassembled, that their dual the fungi, is common, and the photosynthetic cells reproduce
nature became widely accepted. asexually by mitotic cell division. Whether sexual or asexual
Lichens consist mostly of fungal hyphae, with the reproduction is more important in allowing lichens to establish in
photosynthetic algae or cyanobacteria forming a thin layer just new habitats is not known.
under the surface (Fig. 34.9). The hyphae anchor the lichen Lichens are remarkable for their ability to grow on the surfaces
to a rock or tree, aid in the uptake and retention of water and of rocks and tree trunks. They are among the first colonizers of
nutrients, and produce chemicals that protect against excess light lava flows and the barren land left after glacial retreat. In these
and herbivorous animals. In turn, the photosynthetic partners habitats, lichens obtain nutrients from rainfall or by secreting
provide a source of reduced carbon, such as carbohydrates. organic acids that help release some nutrients from even rocky
Cyanobacteria living in lichens are capable of nitrogen fixation surfaces. Not surprisingly, lichens in these harsh environments
(Chapter 26) and thus can use atmospheric N2 as a source of grow slowly. Some lichens grow on soil, for example reindeer
nitrogen. The two partners exchange nutrients through fungal “moss,” which grows in the Arctic tundra and is eaten by caribou.
hyphae that tightly encircle or even penetrate the walls of the Reindeer moss is thought to compete successfully for space by
photosynthetic cells. As a result, lichens are able to thrive where secreting chemicals that hinder the growth of plants.
neither partner could exist on its own. Given their small size and exposure to the environment,
It remains an open question whether either partner can exist lichens must be able to tolerate drying out from time to time.
independently in nature. In cases where the fungal partner can All lichens have a high tolerance for desiccation, and can tolerate
be grown in the laboratory in culture, it produces a relatively wide fluctuations in temperature and light. In contrast, lichens are
undifferentiated hyphal mass. Thus, while the bulk of the lichen quite sensitive to air pollution, particularly sulfur dioxide (SO2 ).
structure comes from the fungus, chemical signals from the For this reason, lichen growth is sometimes used as an indicator of
photosynthetic partner influence the form and shape of the fungus. industrial pollution.
722 SECTION 34.2 R E P RO D U C T I O N

within soils or host organisms can spread locally but cannot disperse
34.2 REPRODUCTION over great distances, so fungi produce spores that can be carried by
the wind (Fig. 34.10a), in water, or attached to (or within) animals.
Like plants, fungi face two challenges in completing their life In early-diverging fungi that live in aquatic environments, spores
cycles. First, to maintain genetic diversity within populations, have flagella that allow them to swim. The great majority of fungi,
they must find other individuals to mate with. Second, they must however, live on land, and their spores lack flagella. Instead, the
be able to disperse from one place to another. The majority of spores are encased in a thick wall that protects them as they are
fungi reproduce both sexually and asexually. Sexual reproduction dispersed over habitats unsuitable for growth.
in many fungi is characterized by a feature of the life cycle that The probability that any given spore will come to rest in a
distinguishes it from all other eukaryotes, and some asexual favorable habitat is low, so fungi produce large numbers of spores.
fungi have a unique way to generate genetic diversity. Fungal Fungal spores remain viable, able to grow if provided with an
adaptations for dispersal broadly resemble those of plants. Fungi appropriate environment, for periods ranging from only a few
rely on wind, water, or animals to carry spores through the hours in some species to many years in others. Thus, spores allow
environment. Recall from Chapter 30 that spores are specialized fungi to use resources that are patchy in time as well as in space. In
cells well adapted for dispersal and long-term survival. fact, a shortage of resources is one of the cues that triggers spore
We most often come into contact with fungi through formation.
their reproductive structures. Fungal spores commonly cause Spores can form by meiotic cell division as part of sexual
respiratory illness, and mushrooms attract our attention reproduction, and they can also form asexually. Asexual spores
because they are delicious or poisonous. Yet many aspects of are formed by mitotic cell division and therefore are genetically
fungal reproduction are not easily observed. In this section, we identical to their parent. In many species, asexual spores are
emphasize the most general principles and patterns. produced within sporangia that form at the ends of erect hyphae,
allowing the release of the spores into the air. A close look at
Fungi proliferate and disperse using spores. a moldy piece of bread reveals that the surface is covered with
The tissues, living or dead, that support fungal growth often have a hyphae carrying sporangia containing asexual spores (Fig. 34.10b).
patchy distribution. For this reason, fungi must be able to disperse Fungal species vary in the extent to which they rely on
from one food source to another. The extensive networks of hyphae asexual as opposed to sexual spore production. In some groups,

FIG. 34.10 Fungal spores dispersed by wind. Sources: a. Biosphoto/Yves Lanceau; b. (top) Gregory G. Dimijian/Science Source; b. (bottom) Garry DeLong/Getty Images.

Sporangium

Spores
 CHAPTER 34 F U N G I : S T RU C T U R E , F U N C T I O N , A N D D I V E R S I T Y 723

FIG. 34.11 Fruiting bodies, complex multicellular structures built from hyphae. Fruiting bodies of Hygrophorus miniatus, known commonly as
the vermillion waxcap. Photo source: Matt Meadow/Getty Images.

Fruiting body

Hyphae Spores

Mycelium

such as the group that includes the most common mushrooms, eject their spores, achieving velocities of more than 1 m/s that
sexual reproduction is the dominant means of spore production. allow the tiny spores to penetrate and travel beyond the
On the other hand, in a small number of fungi that includes the layer of unstirred air that surrounds the fruiting body. Once
endomycorrhizal species, spores produced by meiotic cell division airborne, the spores are transported efficiently through air
have never been observed. Although sexual reproduction appears (Fig. 34.12). Other fungi rely on external agents such as
to be absent in some fungal species, asexual fungi have other raindrops or animals to move their spores around. In section
mechanisms for producing genetic diversity, as discussed later in 34.3, we will encounter examples of the diverse dispersal
this section. mechanisms found in the fungi.

Multicellular fruiting bodies facilitate the dispersal The fungal life cycle often includes a stage in which
of sexually produced spores. haploid cells fuse, but nuclei do not.
Fungi employ an astonishing array of mechanisms to enhance Like other sexually reproducing eukaryotes, fungi have life
spore dispersal. Particularly conspicuous are the multicellular cycles that include haploid (1n) and diploid (2n) stages. The
fruiting bodies produced by some fungi, which are structures nuclei in fungal hyphae are haploid, and the fungal life cycle is
that allow the dispersal of sexually produced spores. (Fungal therefore similar to the life cycles of eukaryotic organisms that
fruiting bodies should not be confused with the fruits of exist normally in the haploid stage (see Fig. 27.3a). In these
flowering plants, which are unrelated and very different haploid-dominant organisms, asexual reproduction takes place
structures.) Mushrooms, stinkhorns, puffballs, bracket fungi, through the production of haploid spores by mitosis, while sexual
truffles, and many other well-known structures are fungal reproduction takes place as two haploid cells (often male and
fruiting bodies. female gametes) fuse to form a diploid zygote, which undergoes
Fruiting bodies are highly ordered and compact structures meiosis as its first division. However, sexual reproduction in
compared with the mycelia from which they grow, yet they are fungi differs from all other haploid-dominant organisms in one
constructed entirely of hyphae (Fig. 34.11). In fact, in many important respect: In fungi, the fusion of haploid cells is not
cases their mechanisms of spore dispersal demand a high degree immediately followed by the fusion of their nuclei.
of structural precision. Yet we know little of the developmental In most fungi, the sexual phase of the life cycle takes
processes that allow tip-growing hyphae to produce such complex place through the fusion of hyphal tips rather than specialized
and regular structures. reproductive cells, or gametes. For mating to occur, two hyphae
The fruiting bodies of many fungi rise above the ground or grow together and release enzymes that digest their cell walls at
grow from the trunks of dead trees, so the sexually produced the point of contact. Once the dividing wall is gone, the contents
spores are released high above the ground. But elevation by of the two hyphal cells merge, forming a single cell with two
itself is not enough to ensure dispersal. Thus, many fungi forcibly haploid nuclei. Eventually, the two haploid nuclei merge, forming
HOW DO WE KNOW?

FIG. 34.12

What determines the shape

Species of Pandanus, 1998, with permission

10 µm 10 µm 10 µm

R. Dulymamode,P. F. Cannon, A. Peerally,

Fungi from Mauritius Anthostomella
of fungal spores that are

Photo source: (left) Reprinted from

Mycological Research, 102/11,
ejected into the air?

from Elsevier.
BACKGROUND Many fungi disperse their spores by ejecting
them forcibly into the air. For spores to be picked up by the wind,
however, they must escape a layer of still air called the boundary Fungal spores with outlines (blue) of computer-modeled shapes
layer that lies close to the ground. Escape from the boundary layer that minimize drag.

is easier if spores have a size and shape that minimize drag, the
RESULTS Nearly three-quarters of the examined spores had
resistance of air to the movement of an object.
shapes that came within 1% of the shape calculated to minimize
HYPOTHESIS In fungi that eject their spores into the atmosphere, drag (shown in the figure as blue outlines). Related species that do
natural selection favors spore shapes that minimize drag. not eject spores forcibly were less likely to have drag-minimizing
shapes.
EXPERIMENT Working from computer models for minimizing drag
CONCLUSION Natural selection has acted on fungi to facilitate
on airplane wings, scientists modeled spore shapes that minimize
spore dispersal by wind.
drag. These models took into account spore size as well as the
physical characteristics of the air through which spores travel. The
SOURCE Roper, M., et al. 2008. “Explosively Launched Spores of Ascomycete
scientists then measured spore shape for more than 100 species of Fungi Have Drag-Minimizing Shapes.” Proceedings of the National Academy of
spore-ejecting fungi. Sciences, USA 105:20583–20588.

a diploid zygote. The zygote divides by meiotic cell division, giving edible mushrooms found on market shelves consist entirely of
rise to sexually produced haploid spores. dikaryotic hyphae.
In most sexually reproducing organisms, when two gametes Dikaryotic fungi account for more than 98% of all known
merge, their nuclei fuse almost instantly to form a diploid zygote. In fungal species, suggesting that the separation of plasmogamy
fungi, however, the cytoplasmic union of two cells (plasmogamy) and karyogamy in time and space may confer an evolutionary
is not always followed immediately by the fusion of their nuclei advantage. One such advantage can be seen when we think about
(karyogamy). Instead, the haploid nuclei retain their independent the environments where fungi live. Mating takes place principally
identities, resulting in what is referred to as a heterokaryotic within the soil or inside the trunks of rotting trees, sites where
(“different nuclei”) stage (Fig. 34.13). In the heterokaryotic stage, hyphae are most likely to come into contact. Dispersal, however,
a cell contains nuclei from two parental hyphae, but the nuclei is most effective when spores can be released into the air. The
remain distinct. The heterokaryotic stage ends with pairs of nuclei separation of plasmogamy and karyogamy allows mating and
fusing, which leads to the formation of a diploid zygote. spore production to occur where each is most effective. In some
In some groups, the heterokaryotic stage consists of a single dikaryotic fungi, however, the separation of plasmogamy and
cell with many haploid nuclei. In other groups, plasmogamy is karyogamy is neither distant in space nor long in time. In these
followed by mitosis, which produces hyphae in which each cell fungi, the highly branched dikaryotic hyphae may serve primarily
contains two haploid nuclei, one from each parent. Heterokaryotic as a means of increasing the number of cells in which karyogamy
cells with two genetically distinct haploid nuclei are referred to will eventually take place, therefore leading to the production of
as dikaryotic, or n + n, cells. In fungi with dikaryotic cells, called more sexually produced spores.
dikaryotic fungi, there may be a small number of dikaryotic cells At present, we really don’t know why fungi proliferate n + n
or extensively developed hyphae made up of dikaryotic cells. Some cells rather than forming a multicellular diploid phase, and there

724
 CHAPTER 34 F U N G I : S T RU C T U R E , F U N C T I O N , A N D D I V E R S I T Y 725

FIG. 34.13 Generalized fungal life cycle. In many fungi, plasmogamy and karyogamy are
separated by a heterokaryotic stage.

HAPLOID
(1n)

Dispersal and
germination Spores
Asexual cycle

Mycelium

Dispersal and
germination Plasmogamy
(fusion of cytoplasm)

Spores Sexual cycle

Heterokaryotic cell
Meiosis

DIPLOID Zygote Karyogamy HETEROKARYOTIC

(2n) (fusion of nuclei) (unfused nuclei from
different parents)

may be no single answer to the question of why the dikaryotic individual is determined by a mating-type gene. Fertilization can
fungi have this unique cell type. take place only between individuals that have different alleles at
the mating-type gene. In some species, there are only two mating-
j Quick Check 2 What is the difference between a diploid cell
type alleles. In this case, mating patterns are identical to those for
and a dikaryotic cell?
species with male and female sexes. If the two mating-type alleles
are spread evenly throughout the population, an individual should
Genetically distinct mating types promote be able to mate with 50% of the general population.
outcrossing. Some fungi have more than two mating-type alleles. These
Under most conditions found in nature, genetically diverse fungi have a greater likelihood of encountering a compatible
populations persist better than those lacking diversity. Indeed, genotype in the general population. That likelihood is even higher
sexual reproduction is widely viewed as a means of promoting for fungi in which mating type is determined by two different
genetic diversity for long-term ecological success in variable mating-type genes that each have multiple alleles. Fungi in the
environments (Chapters 11 and 42). With the exception of early- group that includes the common mushrooms can have as many
diverging aquatic groups, fungi do not produce male and female as 20,000 different mating-type alleles. In this case, the odds of
gametes. What prevents an individual from mating with itself? finding a compatible mate are close to 100%.
Fungi have different mating types that are genetically Genetic compatibility is a prerequisite for mating. How do
determined and prevent self-fertilization. The mating type of an fungi go about finding a suitable mate? The answer is that they
726 SECTION 34.3 DIVERSIT Y

reproduction is that parasexual species do not

FIG. 34.14 Parasexuality. Crossing over during mitosis is thought to provide genetic undergo meiosis.
diversity in asexual fungi. Entirely asexual species of fungi posed
Heterokaryotic cell a problem for early taxonomists because
these species lack many of the morphological
characters such as fruiting bodies or
sporangium-bearing stalks previously
used to define different groups. With the
Nuclear fusion advent of DNA sequence data, it has become
(karyogamy) increasingly clear that many asexual species
are closely related to sexual forms, indicating
1 Haploid nuclei within that their evolutionary history as asexual
a heterokaryotic cell species is not long. A major exception are the
undergo karyogamy to
form a diploid nucleus. Glomeromycota, for which fossils as old as
Mitosis and 450 million years have been reported.
crossing over
2 Rare crossing over
during mitosis results 34.3 DIVERSITY
in novel genotypes,
and cells lose excess
chromosomes When Aristotle classified the living world
through time. into animals and plants, he grouped fungi
Chromosome with plants because of their lack of motility.
loss
As discussed in Chapter 27, molecular
3 The result is nuclei
sequence comparisons now make it clear
with the restored
haploid chromosome that fungi are actually related more closely to
count but novel gene animals. In fact, it would not be amiss to refer
combinations.
to mushrooms and molds as our “cousins.”
Fungi share a number of features with
animals: Their motile cells, when present,
have a single flagellum attached to their
secrete chemical signals that attract fungi with mating types posterior ends; fungi and many animals synthesize chitin; and
different from their own. The chemical nature of these signals fungi store energy as glycogen, as do animals. Nevertheless, the
differs among groups. last common ancestor of fungi and animals was a single-celled
About 20% of fungi appear to lack sexual reproduction microorganism that lived in aquatic environments about 1 billion
altogether. Among these are such well-known groups as Penicillium years ago. Multicellularity evolved independently in fungi and
(the source of the antibiotic penicillin), Aspergillus (the major animals (Chapter 28), and so the fungi have many features that are
industrial source of vitamin C), and all the endomycorrhizal fungi. unique in the biological world.
Spores formed by mitosis (asexual reproduction) allow these fungi
to proliferate and disperse. Fungi are highly diverse.
How are asexual species able to persist without a mechanism By any measure, fungi are diverse, although just how diverse is not
for generating genetic diversity? The short answer is, they known. About 75,000 species have been formally described, but
don’t. Fungal species that lack an observable sexual cycle are estimates of true fungal diversity run as high as 5 million species.
hypothesized to have another mechanism of generating genetic Among eukaryotes, the only organisms more diverse than fungi
diversity: the crossing over of DNA during mitosis (Fig. 34.14). are the animals. The availability of DNA sequence data has greatly
Such species are described as parasexual. In parasexual fungi, advanced our understanding of phylogenetic relationships within
haploid nuclei within a heterokaryotic cell undergo karyogamy to the fungi, which are shown in Fig. 34.15. The tree clearly shows how
form a diploid nucleus—so far, a familiar pattern. Next, however, the characters present in familiar mushrooms accumulated through
during mitosis, a rare crossing over may occur between the two sets the course of evolution: first cell walls of chitin, then hyphae,
of chromosomes, resulting in novel genotypes. Cells lose excess then regularly placed septa and, finally, the complex multicellular
chromosomes over time, such that the haploid state is restored. reproductive bodies we call mushrooms. Motile single cells are
Like sexual reproduction, these processes produce novel gene found only in the most ancient groups, while dikaryotic cells and
combinations. The key difference between parasexuality and sexual complexity of form are found only on the most recent branches.
 CHAPTER 34 F U N G I : S T RU C T U R E , F U N C T I O N , A N D D I V E R S I T Y 727

j Quick Check 3 From the phylogenetic tree shown in Fig. 34.15,

FIG. 34.15 A phylogeny of the fungi. Approximate numbers of would you say that complex multicellular fruiting bodies evolved
described species are given in parentheses. once or twice (independently) in fungi?
Animals
Fungi evolved from aquatic, unicellular, and flagellated
ancestors.
Chytrids Fungi are opisthokonts, members of the eukaryotic superkingdom
Chitin (1000 species)
in cell
that includes animals (Chapter 27). Among fungi, the
walls Chytridiomycota (commonly referred to as chytrids) were the
first groups to diverge. Molecular clock analysis estimates that
chytrids began to diversify 800 to 700 million years ago. Today,
Hyphae Zygomycetes
(1000 species) there are about 1000 species of chytrids found in aquatic or moist
environments (Fig. 34.16). Many chytrids are single cells with
walls of chitin. They may also form short multinucleate structures,
Glomeromycetes but they lack the well-defined hyphae characteristic of other
(200 species) fungi. Chytrids, therefore, do not form a true mycelium, although

Fungi
in some species elongated cellular outgrowths called rhizoids
penetrate into organic substrates. Rhizoids anchor the organism in
Complex Basidiomycetes place and absorb food molecules.
multicellular (25,000 species) Chytrids generally disperse by flagellated spores. Sexual
fruiting bodies
reproduction appears to be rare, but there is substantial life-cycle
diversity within this group. Chytrids lack a heterokaryotic stage
Dikarya

Regular septa but form flagellated gametes that swim through their aqueous
environment.
Most chytrids are decomposers, and a few of them are
Ascomycetes
pathogenic. Notably, infection by the chytrid Batrachochytrium
(48,000 species)
dendrobatidis may be a cause of a widespread decline in amphibian
populations (Chapter 49).
Complex
multicellular
fruiting bodies

FIG. 34.16 Chytrids. Chytrids are aquatic fungi that attach to

decomposing organic matter by rhizoids. Source: John
Hardy, University of Wisconsin–Stevens Point Department of Biology.
The evolutionary history of fungi can be viewed as the
transition from a flagellated ancestral form that lived in aquatic
environments to soil-dwelling forms in which hyphae divided
by septa develop complex morphologies visible to the naked eye.
Moreover, in the earliest diverging fungal group, karyogamy
follows quickly on the heels of plasmogamy and there is no
heterokaryotic stage. Thus, another evolutionary trend is
the progressive separation in time between plasmogamy and
karyogamy in fungal life cycles. Many of the distinguishing
features of fungi evolved as adaptations to the challenges faced
by nonmotile heterotrophs on land that absorb nutrients directly
from their substrate.
We also see that the numbers of species are not spread evenly
across the phylogeny. Instead, more ancient groups include less
than 2% of known species, while the two dikaryotic groups include
more than 98% of known fungi. Clearly, dikaryotic fungi are
well adapted to many different habitats, and these include other
organisms, both living and dead.
728 SECTION 34.3 DIVERSIT Y

Zygomycetes produce hyphae undivided by septa.

Moving through the phylogenetic tree, we next encounter several FIG. 34.17 Spore dispersal in a zygomycete. In Pilobilus, light-
groups traditionally united as the Zygomycota, or zygomycetes sensing pigments orient the turgor-propelled ejection
(see Fig. 34.15). These groups share a number of traits. of the sporangia. Source: Carolina Biological Supply Company/
Phototake.
Collectively, zygomycetes make up less than 1% of the known
fungal species. Some are decomposers, specializing on dead leaves,
animal feces, and food. There are probably zygomycetes in your
kitchen. Others live on and in plants, animals, and even other
fungi. Zygomycetes have many typical fungal traits, including
the growth of a mycelium and the production of aerial spores.
These traits may have evolved as adaptations for finding food
and dispersing on land. The loss of flagellated spores may also
reflect adaptation for life on land. Unlike more complex fungi,
zygomycetes do not form regular septa along their hyphae, nor do
they produce multicellular fruiting bodies.
Earlier, we introduced the black bread mold Rhizopus, which
is a zygomycete (see Fig. 34.10b). Rhizopus and its relatives are
specialists on substrates containing abundant, easy-to-digest
carbon compounds, such as bread, ripe fruits, and the dung of
herbivorous animals. These fungi consume their substrates rapidly,
and once their meal is finished, they release large numbers of
aerial spores to locate another food source.
Sexual reproduction occurs when two compatible hyphal tips
fuse to form a thick-walled structure containing many nuclei
of each mating type. Karyogamy and meiosis are followed by
germination of the haploid cell to form an elevated stalk. Each
stalk develops a sporangium that contains spores produced famous are those that form endomycorrhizae. Molecular clock
asexually by mitotic cell division. If you look closely at a bread estimates suggest that glomeromycetes diverged from other fungi
mold, you can see the white filamentous hyphae and small black 600 to 500 million years ago, before the evolution of vascular
sporangia on slender stalks. Each sporangium can produce as many plants. Establishment of glomerophyte–root symbiosis changed
as 100,000 spores that are dispersed by the wind, explaining how the course of evolution for both participants. Today, most vascular
these organisms seem to get everywhere. plant species harbor endomycorrhizae, dramatically increasing
Although zygomycete fungi do not form multicellular fruiting nutrient uptake from the soil.
bodies, some have evolved truly spectacular means of dispersal. Glomeromycetes are hard to study because they cannot
Pilobilus, which consumes the dung of herbivorous animals, has be grown independently of their plant partners, and so many
a life cycle similar to Rhizopus except that, instead of releasing unanswered questions remain. For example, no evidence of
individual spores, it forcibly ejects the entire sporangium sexual reproduction has ever been found in this group. Yet
(Fig. 34.17). Turgor pressure generated in the supporting stalk genetic studies of glomeromycete populations show some genetic
propels the sporangia as far as 2 m. Light-sensitive pigments in diversity, which is thought to result from parasexual processes.
the stalk’s hyphae control the orientation of this water cannon, Glomerocytes produce extremely large (0.1 to 0.5 mm diameter)
ensuring that the spores have the best chance of escaping the multinucleate spores asexually. These large spores can persist in
dung pile and landing on vegetation that is attractive to grazing soils until they come into contact with an uninfected root.
herbivores. Feeding herbivores disperse the spores further,
while supplying them with fresh dung for food. The remarkable The Dikarya produce regular septa during mitosis.
dispersal of Pilobilus sporangia benefits other species, as well: The final major node, or branching point, in the fungal phylogeny
Parasitic nematode worms hitch a ride on the sporangia to find a marks the origin of the Dikarya, a group that includes about 98%
new host. of all described fungal species. A key feature of the dikaryotic fungi
is that every mitotic division is accompanied by the formation of
Glomeromycetes form endomycorrhizae. a new septum. This innovation allows these fungi to control the
The Glomeromycota, or glomeromycetes, are a monophyletic number of nuclei within each cell and thus to proliferate dikaryotic
group of apparently low diversity (there are about 200 described cells. The Dikarya include all the edible mushrooms, the yeast
species) but tremendous ecological importance. All known species used in the production of beer, bread, and cheese, the major
glomeromycetes occur in association with plant roots. Most wood-rotting fungi, and pathogens of both crops and humans.
 CHAPTER 34 F U N G I : S T RU C T U R E , F U N C T I O N , A N D D I V E R S I T Y 729

FIG. 34.18 Two groups of dikaryotic fungi. (a) The morel (Morchella esculenta) is an ascomycete, whereas
(b) the shiitake mushroom (Lentinula edodes) is a basidiomycete. Sources: a. Matt Meadows/Getty
Images; b. Topic Photo Agency/age fotostock.

a b

The Dikarya contain two monophyletic subgroups (see to absorb nutrients from the substrate for a while before nuclear
Fig. 34.15), each named for the shape of the cell in which the key fusion occurs.
reproductive processes of nuclear fusion (karyogamy) and meiosis
take place. The Ascomycota, or ascomycetes, are sometimes Ascomycetes are the most diverse group of fungi.
called sac fungi because nuclear fusion and meiosis take place in Ascomycetes make up 64% of all known fungal species. They
an elongated saclike cell called an ascus (askos is Greek for “leather include important wood-rotting fungi, many ectomychorrizal
bag”). The Basidiomycota, or basidiomycetes, are popularly species, and significant pathogens of both animals and plants.
known as club fungi because the nuclear fusion and meiosis take Ascomycetes form the fungal partner in most lichens—
place in a club-shaped cell called a basidium. (“Basidium” is derived approximately 40% of all ascomycete species occur in lichens.
from the Greek basis, which means “base” or “pedestal”; the Ascomycetes loom large in human history, contributing the
reference is to the position of sexually produced spores at the tips baker’s and brewer’s yeasts used to make bread and beer,
of this club-shaped body.) the antibiotic penicillin, and model systems employed in
Although ascomycetes and basidiomycetes have the same laboratory investigations of eukaryotic cell biology and genetics.
basic life cycle, they diverged from each other as much as Ascomycetes are used to produce soy sauce, sake, rice vinegar,
600 to 500 million years ago and evolved complex multicellular and miso, and to transform milk into Brie, Camembert, and
structures independently of each other. The morel in the Roquefort cheeses. Athlete’s foot and other skin infections are
supermarket is an ascomycete, whereas the shiitake mushroom caused by ascomycetes, as are many more serious fungal diseases.
next to it is a basidiomycete (Fig. 34.18). In addition to asci Ascomycetes include both species that form multicellular
and basidia, the two groups differ in several other features. For fruiting bodies and ones that form unicellular yeasts. A
example, the septal pores that connect adjacent cells within remarkable feature of ascomycete fruiting bodies is that they
hyphae are distinct in the two groups, as are the ways in which contain both dikaryotic and haploid cells. The proliferation of
cells can plug the pores to minimize damage to a mycelium. Also, dikaryotic cells leads to the production of many asci, while the
when ascomycetes reproduce, mating cells form and their nuclei growth of haploid cells contributes to the bulk of the fruiting
fuse in relatively close succession. Thus, the heterokaryotic stage body. Two ascomycete lineages consist largely of yeasts. The
is brief. In contrast, in the basidiomycetes, the nuclei of two fused earliest branching members in these groups, however, produce
cells may remain separate for a long time. In this case, hyphae hyphae. Thus, the unicellular nature of yeasts is a derived feature
containing both types of nuclei (dikaryotic hyphae) can continue evolved through loss of hyphae and is not an ancestral condition.
730 SECTION 34.3 DIVERSIT Y

FIG. 34.19 The sexual cycle of a common ascomycete, the brown cup fungus (Peziza species). Photo source: Ed Reschke/Getty Images.

HAPLOID 7 After dispersal 1 Ascomycetes

(1n) each spore can have haploid
germinate to produce mycelia that
a new mycelium. absorb nutrients.

Spores
Dispersal and
germination
Eight HETEROKARYOTIC
spores (1n)

Plasmogamy 2 Specialized cells

produced by hyphae
having different mating
types can undergo
Mitosis plasmogamy, resulting
in a heterokaryotic cell
containing many
6 This cell – Mating haploid nuclei.
subsequently Sexual cycle type
Four haploid
undergoes nuclei (1n)
meiosis and + Mating
each haploid type
Mycelia
product divides
once by mitosis
to produce
3 Dikaryotic (n + n)
hyphae that branch
eight spores.
repeatedly grow from
the heterokaryotic cell.
Meiosis
Ascus
(dikaryotic)
Diploid nucleus
(zygote) (2n)
Karyogamy
5 Within each
ascus, the two Dikaryotic
haploid nuclei hyphae
fuse (karyogamy) Dikaryotic
to form a diploid hyphae
nucleus.
4 Some
dikaryotic cells
DIPLOID differentiate to Haploid
(2n) form asci. hyphae

Fig. 34.19 illustrates the life cycle of a common ascomycete, of an ascomycete that grows as an ectomycorrhizal fungus on tree
the brown cup fungus. In ascomycetes, meiosis is followed by roots. Not only do truffles encase their spores in protective tissues,
a single round of mitosis, resulting in asci that contain eight but they also develop underground. How, then, can their spores
haploid spores. When mature, the spores are ejected from the be dispersed? Developing truffles release androstenol, a hormone
top of the asci, expelled by turgor pressure. In many ascomycetes, also produced by boars before mating. The hormone attracts female
fruiting bodies elevate the asci on one or more cup-shaped pigs, which unearth and consume the fruiting body. The spores pass
surfaces, from where the spores are easily caught and carried away through the pig’s digestive tract without damage and are released
by wind. into the environment in feces, thereby dispersing the spores.
In the fruiting bodies of some ascomycetes, the asci are Another ascomycete is thought to have played a role in the
completely enclosed by a layer of tissue and thus must be dispersed events that unfolded in Salem, Massachusetts, in 1692. That year,
by other organisms rather than by the wind. The truffles prized in several girls came down with an unknown illness manifested by
cooking provide an example. The edible truffle is the fruiting body delirium, hallucinations, convulsions, and a crawling sensation on
 0

731 SECTION 34.1 A

HOW DO WE KNOW?

FIG. 34.20

Can a fungus influence the RESULTS Infected ants have repeated convulsions (indicated
by vertical bars in the graph), but uninfected ants show no such
behavior of an ant? behavior (not shown). Transitions from erratic wandering to a
“death grip,” in which the ant bites into a leaf (indicated by red
triangles in the graph and shown in the photograph), occurred at
BACKGROUND A curious death ritual unfolds in the rain forest of about the same time of day. Dissections showed that the ants’ death
Thailand. Spores of the ascomycete fungus Ophiocordyceps infect grip results from jaw-muscle wasting caused by the fungus.
Camponotus leonardi ants, growing hyphae inside their bodies. The
fungus eventually kills the ant, and fruiting bodies emerge from the 12
dead ant’s head to disperse spores that begin the life cycle anew. 11
Camponotus leonardi nests and forages for food high in the forest 10
canopy. Infected ants, however, undergo convulsions that cause 9
them to fall to the forest floor. The infected ants wander erratically 8

Individual ant
and are unable to climb more than a few meters above the ground 7
before convulsions make them fall again. In their final act, the ants 6
bite into the undersides of leaves and die. The fungus is then able 5
to complete its life cycle within the humid forest understory. Is 4 Each horizontal
3 entry records
the ants’ behavior, including both the convulsions and leaf biting,
2 behavioral events
induced by the fungi? for a single ant.
1

09:00 10:00 11:00 12:00 13:00 14:00 15:00

Hour
Observations of numerous infected ants (vertical bars indicate convulsions; red
triangles indicate the time of ant biting).
Photo source: David Hughes.

CONCLUSION The fungi change the behavior of infected ants,

inducing a stereotypical behavior that facilitates completion of the
fungus’s life cycle. The molecular basis of this manipulation is not yet
known.

FOLLOW-UP WORK The zombie-like behavior induced by

An infected ant attached to a leaf by having bitten into it in a “death grip.”
the fungus is an example of what has been called the extended
phenotype: the idea that a phenotype should include the effects that
HYPOTHESIS Parasitic fungi manipulate ant behavior to complete
an organism has on its environment—in this case, its effects on host
their own reproductive cycle.
behavior.
OBSERVATIONS Infected and uninfected ants were observed for SOURCE Hughes, D. P., et al. 2011. “Behavioral Mechanisms and Morphological
many hours and behavioral events were recorded. Symptoms of Zombie Ants Dying from Fungal Infections.” BMC Ecology 11:13–22.

the skin. The girls were thought to be bewitched, and subsequent Basidiomycetes include smuts, rusts, and mushrooms.
accusations led to the infamous Salem witch trials. They were The basidiomycetes, which make up 34% of all described fungal
executed for witchcraft, but today many scholars prefer a simpler species, include three major groups (Fig. 34.21). Two of these—
diagnosis for the illness: poisoning by the ascomycete fungus the smuts and rusts—consist primarily of plant pathogens.
ergot. Ergot is a common pathogen of rye and related grasses, Smut fungi, which infect the reproductive tissues of grasses and
and ingestion of contaminated plants can lead to the symptoms related plants, take their name from the black sooty spores that
reported in Salem. Like many defensive compounds produced by they produce. Ustilago maydis, the corn smut, turns developing
plants, alkaloid molecules produced by ergot and their derivatives corn kernels into soft gray masses that are a culinary delicacy in
are used in medicine in low doses, such as in the treatment of Mexico. Because smuts infect seeds, their spread is magnified by
migraines. the harvest, storage, and eventual sowing of crop seeds. Before the
Ascomycetes are even known for producing so-called zombie introduction of chemical seed treatments in the 1930s, infection
ants, a topic explored in Fig. 34.20. by Tilletia (the stinking smut) commonly caused the loss of up to

731
732 SECTION 34.3 DIVERSIT Y

50% of the wheat harvest. Smuts remain a problem in parts of the

FIG. 34.21 Basidiomycetes. (a) Corn smut (Ustilago maydis); world where untreated seeds are still planted.
(b) pseudoflowers that form when plants are infected by Rusts can also have profound impacts on plant function and
the rust Puccinia monoica; (c) a toadstool fungus—the development. For example, the rust Puccinia monoica alters leaf
highly toxic Amanita. Sources: a. Inga Spence/Science Source; development in its host plant Arabis, resulting in a pseudoflower
b. Bitty Roy, University of Oregon; c. David Clapp/Getty Images. that attracts insects by visual and olfactory cues as well as nectar
rewards. These pseudopollinators transport spores, thus enhancing
a
outcrossing as well as dispersal of the fungi, but provide no service
to the plant. We discuss the biology of rusts in greater detail in the
next section when we examine Ug99, a highly virulent wheat rust
first reported in Uganda in 1999.
The third basidiomycete group is characterized by the
formation of multicellular fruiting bodies, including the iconic
toadstools with their central stalk and umbrella-like cap. However,
this group also includes the diverse shapes seen in stinkhorns,
puffballs, and bracket fungi (Fig. 34.22).
Fig. 34.23 shows the life cycle of a typical basidiomycete
mushroom. The life cycle is similar to that of ascomycetes (see
Fig. 34.19), except that the fruiting body is made up entirely of
b c
dikaryotic hyphae instead of a combination of dikaryotic and
haploid hyphae as in ascomycetes. In addition, nuclear fusion
(karyogamy) takes place in a specialized cell called a basidium
rather than in an ascus. Finally, the haploid products of meiosis
(the spores) do not undergo mitosis, so four spores are produced
from each basidium rather than eight spores from each ascus.

j Quick Check 4 When you eat a mushroom, what stage of its life
cycle are you consuming?

Most species in this group live by forming ectomycorrhizal

associations or by decomposing wood and other substrates. The
fruiting bodies that we see elevated above a rotting log or the soil
are connected to extensive mycelia that provide resources for
fruiting-body development.

FIG. 34.22 Diverse fruiting bodies of basidiomycetes. (a) Stinkhorn (Dictyophora indusiata); (b) puffball (Calvatia gigantea); (c) bracket fungi
(Ganoderma australe). Sources: a. Bill Gozansky/age fotostock; b. Wally Eberhart/Getty Images; c. Bob Gibbons/Science Source.
a b c
 CHAPTER 34 F U N G I : S T RU C T U R E , F U N C T I O N , A N D D I V E R S I T Y 733

FIG. 34.23 The sexual cycle of a basidiomycete that forms multicellular fruiting bodies.

1 Basidiomycetes 2 Two hyphae of different

have haploid
mycelia that absorb
mating types can undergo
plasmogamy, forming a
3 Dikaryotic hyphae
nutrients. heterokaryotic cell that produce multicellular
Dikaryotic fruiting bodies
produces a dikaryotic mycelium. mycelium (mushrooms), within which
some dikaryotic cells
6 After Plasmogamy differentiate into basidia.
dispersal, each
spore can
germinate to Fruiting body
produce a (dikaryotic)
new + Mating
mycelium. type
– Mating
type

Haploid
mycelia

Sexual cycle

Dispersal and Basidium

germination containing four
haploid nuclei
Basidia
(dikaryotic)

Spores (haploid) Gills lined with

basidia

Meiosis Karyogamy

5 This cell subsequently

undergoes meiosis to 4 Within each basidium, the two
produce four haploid haploid nuclei fuse (karyogamy)
spores. Diploid to form a diploid nucleus.
HAPLOID DIPLOID nuclei HETEROKARYOTIC
(1n) (2n)

Elevation alone does not ensure dispersal because the spores raindrops displace groups of spores (the “eggs” in the nest)
must penetrate a boundary layer of air that surrounds the fruiting onto nearby vegetation. Herbivores consume the spores when
body. Many basidiomycetes depend on surface tension to catapult they feed on the vegetation and transport them to a new dung
their spores through the air. The placement of their four spores pile. Stinkhorns produce spores in a stinky, sticky mass that
on the ends of short stalks is the key to this mechanism. Both the attracts insects, which then carry off spores that stick to their legs
spores and the supporting basidial cell actively secrete solutes that and body.
cause water to condense on their surfaces. At first, the droplets are
independent, but as they grow they eventually come into contact ? CASE 6 AGRICULTURE: FEEDING A GROWING POPULATION
with one another, and at this point the high surface tension of How do fungi threaten global wheat production?
water causes them to merge into a single, smooth droplet. This Throughout much of human history, crops have been vulnerable
change in shape shifts the center of mass of the water with such to infection by Puccinia graminis, the black stem rust of wheat.
force that it catapults the spore into the air. Puccinia graminis infects wheat leaves and stems through their
Basidiomycetes that form conspicuous fruiting bodies have stomata and then extends within the plant’s living tissues to fuel
diverse mechanisms for dispersing their spores. For example, its own growth. Rust-colored pustules erupt along the stem and
puffballs function like bellows: The impact of raindrops forces the then release a vast number of spores, leaving the host plant to
loose, dry spores out of a small hole at the tip of the ball (see Fig. wither and die. Crop losses from P. graminis can be devastating.
34.10a). Bird’s nest fungi function like splash cups: In this case, In the past, both the United States and the Soviet Union tried to
734 CO R E CO N C E P T S S U M M A RY

develop black stem rust as a biological weapon. The U.S. version

was cluster bombs (since destroyed) containing turkey feathers FIG. 34.24 The spread of Ug99. Ug99 is a highly resistant
smeared with rust spores, produced with the aim of disrupting stem rust of wheat and a major threat to global food
crop yields in enemy countries. production. Sources: After “The Spread of Wheat StemRust UG99
Many rusts have a life history in which they alternate between Lineage,” Food and Agriculture Organization of the United Nations,

different host species. For example, to complete its life cycle, JPEG image, 2010, http://www.fao.org/agriculture/crops/rust/

P. graminis must infect wheat first and then barberry bushes stem/rust-report/stem-ug99racettksk/en/; (photos) Yue Jin,

(Berberis species). After a particularly severe outbreak of wheat USDA-ARS, Cereal Disease Laboratory.

rust during World War I, in which crop losses equaled one-third

of total U.S. wheat consumption, a major program of barberry
eradication was started to bring stem rusts under control. This ? ?
program reduced the threat, but did not totally eliminate it. 2007 ?
The Green Revolution seemed to give growers a decisive ?
2009
advantage over stem rust. During World War II, Norman Borlaug,
whose original training was in plant pathology, was sent to
Mexico by the Rockefeller Foundation to help combat crop losses 2006
due to stem rust. Borlaug’s breeding efforts led to resistant 2003
wheat varieties. Much of this resistance could be attributed to
1998/9 2001
variation in a small number of genes. Stem rusts were apparently
vanquished—that is, until the appearance of Ug99, a wheat stem
rust first identified in Uganda in 1999 (hence the name). Ug99
is capable of defeating all major resistance genes. Although new
genes that confer resistance to Ug99 have recently been identified, ? ?
2009
it will take time before these can be incorporated into cultivated
varieties. Given that wheat accounts for 20% of daily global calorie
consumption by humans and that 90% of cultivated wheat has Ug99 sites
little or no resistance to Ug99, the appearance of this resistant Movement
fungus is cause for major concern. Possible spread
At present, Ug99 shows every sign of spreading (Fig. 34.24)— High wheat
production
not surprising given that a single hectare of infected wheat
produces upward of a billion spores. It currently devastates wheat Low wheat
production in Kenya and has been found in Yemen (2006), Iran production

(2007), and South Africa (2009). As you read this, Ug99 is poised to
reach the major wheat growing regions of Turkey and South Asia.
Even more rapid spread is possible if spores accidentally lodge on how plants defend themselves against rusts and what makes
cargo or airline passengers. Ug99 so virulent. The resurgence of P. graminis as a serious
Just before his death in 2009, Norman Borlaug urged the threat to world food production underscores the importance of
world to take this threat seriously. Today, plant breeders are safeguarding the genetic diversity of agricultural species so that
actively searching ancestral wheat varieties for genetic sources of the inevitable evolutionary battles with pathogenic species may be
resistance to Ug99, while other scientists are trying to understand waged successfully. •

Core Concepts Summary Most fungi have numerous branched filaments called hyphae,
which absorb nutrients. page 716
34.1 Fungi are heterotrophic eukaryotes that feed by
When fungi encounter a rich source of food, their hyphae form
absorption.
a network called a mycelium. page 716
Fungi are heterotrophs that depend on preformed organic
Fungal cell walls are made of chitin, the same compound found
molecules for both carbon and energy. page 716
in the exoskeletons of insects. page 716
Fungi break down their food and then absorb it. page 716
In early fungal groups, the hyphae form a continuous
Fungi use the process of growth to find and obtain food in their compartment, with many nuclei but with no cell walls, but in
environment. page 716 later evolving groups, nuclear divisions are accompanied by the
0

CHAPTER 34 F U N G I : S T RU C T U R E , F U N C T I O N , A N D D I V E R S I T Y 735

formation of septa that divide the cytoplasm into separate cells. Zygomycetes produce hyphae without septa and make up less
page 717 than 1% of known fungal species. page 728

Some fungi, such as yeasts, do not produce hyphae. page 717 The glomeromycetes are a group of fungi of low diversity but
tremendous ecological importance because of their association
Most fungi feed on dead organic matter. page 718
with plant roots. page 728
Fungi are critical elements of the carbon cycle, converting dead
Most fungi belong to the Dikarya (ascomycetes and
organic matter back into carbon dioxide and water. page 718
basidiomycetes), which form septa along hyphae. During the
Many fungi feed on living animals and plants, causing diseases. heterokaryotic stage, these fungi produce cells that contain two
page 718 haploid nuclei, one from each parent. page 728
Fungi have repeatedly evolved mutually beneficial relationships Ascomycetes include wood-rotting fungi, many ectomychorrizal
with plants and animals. page 720 species, plant and animal pathogens, the fungal partner in most
Lichens are stable associations between a fungus and a lichens, and baker’s and brewer’s yeasts. page 729
photosynthetic microorganism that look, function, and even Basidiomycetes include familiar toadstools and puffballs, as well
reproduce as single organisms page 720 as plant pathogens page 732

34.2 Fungi reproduce both sexually and asexually, and

disperse by spores. Self-Assessment
Fungi produce haploid spores that are dispersed by wind, water,
1. Describe two ways in which fungi differ from other
or animals. page 722
heterotrophic organisms in how they obtain and digest their
Spores can be produced asexually (by mitosis) or sexually (by cell food.
fusion and meiosis). page 722 2. Explain how hyphae and cell walls made of chitin allow
During sexual reproduction, many fungi produce spores within fungi to obtain nutrients from their environment.
multicellular fruiting bodies that enhance spore dispersal. 3. Describe how fungi contribute to the terrestrial carbon
page 723 cycle.
The fungal life cycle is similar to haploid-dominant life cycles 4. Name the two organisms that make up a lichen and
found in many eukaryotic organisms, except that plasmogamy describe how each partner benefits from the association.
(cell fusion) and karyogamy (nuclear fusion) are often separated,
5. Describe how fungi disperse.
resulting in a stage (called a heterokaryotic stage) in which
there are genetically distinct haploid nuclei in 6. Draw the life cycle of an ascomycete, and indicate the
one cell. page 724 heterokaryotic stage.

Genetically determined mating types prevent self-fertilization 7. Draw the life cycle of a basidiomycete, and indicate the
in many fungi. page 725 heterokaryotic stage.

Asexual fungal species have mechanisms for generating genetic 8. Name and describe several key innovations in the
diversity; these are thought to involve fusion of haploid nuclei, evolutionary history of fungi that allowed them to move from
followed by crossing over during mitosis and chromosome loss. water to land.
page 726 9. Explain how the evolution of vascular plants has
provided opportunities for fungal diversification.
34.3 Other than animals, fungi are the most diverse
group of eukaryotic organisms.
Log in to to check your answers to the Self-
About 75,000 fungal species have been formally described, but Assessment questions, and to access additional learning tools.
total diversity may be as high as 5 million species. page 726

Chytrids are the first diverging groups of fungi. They occur in

aquatic or moist habitats and only about 1000 species have been
identified. page 727
CASE 7

Predator–Prey
A Game of Life and Death
In his 1850 poem In Memoriam A.H.H., Alfred, Lord Northwest. When sea star numbers fall, mussel numbers
Tennyson wrote of “Nature, red in tooth and claw.” increase. The mussels crowd out other species, such as
The phrase has endured as a powerful description barnacles and seaweed, which compete for space on the
of the brutality of wild nature. Thanks to nature rocks. The result is a drop in the overall number of species in
documentaries and our human fascination with wild that habitat.
animals, most of us have witnessed plenty of images of The same patterns have been found among birds of prey
this red-toothed nature: a lion chasing down a zebra, a in the Italian Alps. These raptors—including the goshawk
hawk scooping a mouse into its talons, a shark sinking and four types of owl—are the top predators of their food
its teeth into a sea lion. chains. Spanish researchers found that locations where the
Predator–prey interactions are important in every birds were present had a greater diversity of trees, birds,
ecosystem on Earth. After all, every living animal can be and butterflies than did
classified as either “predator” or “prey”—and often, both comparable control sites.
labels apply. To a fly, a toad is a fearsome predator. To a One reason that Predator–prey
snake, that same toad may be choice prey. the predator and prey dynamics have
Very few species enjoy the luxury of having no populations eventually
natural predators; these are called top predators. For achieve a delicate balance is influenced both
the vast majority of animals, the threat of predation is that both are well adapted the evolution of
simply a fact of life. Not surprisingly, that threat has been to their roles. Predation
a powerful evolutionary force. Predator–prey dynamics has exerted powerful individual organisms
have influenced both the evolution of individual evolutionary pressure that and the shape of the
organisms and the shape of the ecosystems in which has influenced anatomy and
they live. physiology over the long ecosystems in which
On Isle Royale, an island in Lake Superior, ecologists term. That pressure has they live.
have been studying moose and wolves since 1958 in the shaped predators by giving
longest-running study of a single predator–prey system them claws, teeth, venom,
in the world. The moose arrived on the island about 100 and powerful muscles for hunting prey.
years ago, presumably by swimming about 15 miles from Of course, predation pressure has also shaped the form
the nearest shoreline. They arrived to find a predator- and function of prey. Some animals, including some insects
free paradise, and the moose population quickly and frogs, have evolved toxins to deter would-be predators.
exploded. Then, about 1950, a pair of wolves crossed an These toxic species often exhibit warning colors that tell the
ice bridge to Isle Royale. As the wolf population grew, predators to steer clear. Other prey animals have evolved
the moose population declined, until a delicate balance camouflage colors and forms to help them hide from hungry
was established. carnivores. Still others have developed protective behaviors,
As a predator population grows, the prey population such as living together in herds for security against
shrinks—a seemingly logical relationship. But predator– predation.
prey interactions are complex and sometimes even Often, predator and prey evolve in lockstep, driving
counter-intuitive. Removing a top predator can actually each other’s adaptations. Clams may have evolved thicker
reduce biodiversity. Sea stars, for instance, prey on shells to protect themselves from hungry crabs. In turn, the
mussels in the rocky intertidal zone of the Pacific crabs evolved larger, stronger claws for cracking clamshells.

736
 Wolves and moose on Isle Royale. The
populations of this predator and prey have
Canada been studied for more than 50 years, and the
Minnesota Isle Royale two have achieved a delicate balance over
time. Sources: (wolf) Samuel R. Maglione/Science
Source; (moose) Ron Erwin/age fotostock; (graph)
Data from J. A. Vucetich and R. O. Peterson, 2012, “The
Wisconsin
population biology of Isle Royale wolves and moose: an
Michigan
overview,” www.isleroyalewolf.org.

50 2500
Wolves
40 Moose 2000
Number of wolves

Number of moose
30 1500

20 1000

10 500

0
1959 1969 1979 1989 1999 2009
Year

This pattern of back-and-forth change is often described as When predators are nearby, prey species experience
an evolutionary arms race. And it has happened time and fear and anxiety. Stress hormones produced by prey animals
time again. can influence an animal’s physiology in a number of ways.
The interactions between eaters and eaten have also In snowshoe hares, levels of the stress hormone cortisol
influenced the evolution of physiological systems. The skills increase when predators such as lynx and coyote are
that a predator needs to hunt its prey and that its prey needs plentiful. The hormones trigger behaviors, such as alertness
to escape depend in large part on adaptations of their sensory and fearfulness, which help the hares avoid becoming a
systems, musculoskeletal systems, nervous systems, and lynx’s lunch. But the behavioral benefit comes at a cost:
even their circulatory and respiratory systems. Research has shown that stressed hares give birth to fewer
Consider sensory systems. Animals rely on visual, and smaller offspring.
auditory, tactile, and chemical stimuli to warn them of In short, predation has left a physical imprint on both
approaching predators—or to guide them to suitable prey. A predator and prey, from nose to tail—their body shapes, their
bat relies on sonar to locate insects. A spider responds to the behaviors, their physiology, even their muscle fibers have
flutter of silk when an insect becomes ensnared in its web. been influenced over time by predator–prey interactions.
Gazelles are always attentive, watching and listening for In the modern world, our own species, Homo sapiens,
signs of a cheetah or lion in the grass. occupies a unique position at the top of the food web. While

737
 humans are occasionally killed by animals such as bears or that it was our hunting of other animals that led us to
sharks, we are, for the most part, no longer constrained by evolve big brains and the ability to work together. Perhaps
the fear of predation by other species. But it wasn’t always both factors played a role. Either way, the importance of the
so. Some scientists have proposed that fear of predation predator–prey system can’t be ignored. To understand our
among early humans led to the evolution of cooperative roots, it seems, we must take a good look at nature, red in
social behavior and large brains. Others have theorized tooth and claw.

? CASE 7 QUESTIONS
Special sections in Chapters 35– 41 discuss the following questions related to Case 7.

1. What body features arose as adaptations for successful predation? See page 742.
2. How have sensory systems evolved in predators and prey? See page 771.
3. How do different types of muscle fiber affect the speed of predators and prey?
See page 797.
4. How does the endocrine system influence predators and prey? See page 821.
5. How do hormones and nerves provide homeostatic regulation of blood flow as well as
allow an animal to respond to stress? See page 843.
6. Does body temperature limit activity level in predators and prey? See page 854.
7. Can the loss of water and electrolytes in exercise be exploited as a strategy to hunt
prey? See page 880.

738
 CHAPTER 35

Animal Nervous
Systems
Core Concepts
35.1 Animal nervous systems
allow organisms to sense and
respond to the environment,
coordinate movement, and
regulate internal functions of
the body.
35.2 The basic functional unit of the
nervous system is the neuron,
which has dendrites that
receive information and axons
that transmit information.
35.3 The electrical properties
of neurons allow them to
communicate rapidly with one
another.
35.4 Animal nervous systems can
be organized into central and
peripheral components.
Thomas Deernick, MCMIR/Getty Images.

739
740 SECTION 35.1 N E RVO U S S YS T E M F U N C T I O N A N D E VO LU T I O N

Gazelle grazing on an African plain are alert

to the threat of predators nearby. Members of FIG. 35.1 Predator and prey. Interactions of predator and prey, such as this cheetah and
the herd scan the distant landscape for visual gazelle, are mediated by the nervous system. Source: Winfried Wisniewski/Corbis.
cues that might reveal the stealthy approach
of a cheetah (Fig. 35.1). Olfactory (smell) and
auditory (sound) cues from the environment
can also warn of the cheetah’s approach. Herd
members communicate the presence of the
cheetah to one another by the directed attention
of a dominant male or the pawing of a hoof. At
what point is the threat real? When does each
animal decide to stop grazing and turn to run?
What direction should the escape take to keep
the herd together as a group?
The cheetah scans the horizon, its keen
vision spotting the grazing gazelle. The
cheetah’s ability to recognize wind direction
enables it to move downwind of the gazelle
to minimize auditory and olfactory cues that
might otherwise be carried by the air toward
suspecting prey. At what point does the cheetah decide to sprint in more complex bodies with increasingly specialized organ systems
pursuit of its quick-running prey? for respiration, circulation, digestion, and reproduction, their
The perceptions and reactions of these two animals depend nervous systems also became more complex. These more complex
on the activity of their nervous systems. In this and the nervous systems could regulate internal body functions in
following chapter, we explore the organization of the nervous response to sensory cues received from the environment. They
system, including the brain and the nerve cells that compose also made possible more sophisticated behaviors that relied on
it. The nervous system has the remarkable ability to process improved decision making, learning, and memory. These abilities,
all kinds of information. It takes in information from the possessed by many invertebrate and vertebrate animals, increased
environment through the animal’s sensory organs and processes their fitness by improving their ability to survive, find and select a
that information through the activity of networks of nerve mate, and reproduce.
cells. The results of that processing guide an animal’s motor
responses. Nervous system processes underlie the perceptions and Animal nervous systems have three types
interaction of cheetah and gazelle as predator and prey, as well of nerve cell.
as the complex emotional and cognitive responses triggered, for All animal nervous systems are made up of three types of nerve
example, by seeing a friend or relative after a long separation. cell: sensory neurons, interneurons, and motor neurons.
Each type of neuron has a different function. Sensory neurons
receive and transmit information about an animal’s environment
35.1 NERVOUS SYSTEM FUNCTION or its internal physiological state. These neurons respond to
AND EVOLUTION physical features such as temperature, light, and touch or to
chemical signals such as odor and taste. Interneurons process
Most multicellular animals possess a nervous system, a network the information received by sensory neurons and transmit it to
of many interconnected nerve cells. This network allows an different body regions, communicating with motor neurons at the
animal to sense and respond to the environment, coordinate the end of the pathway to produce suitable responses. For example,
action of muscles, and control the internal function of its body. a motor neuron may stimulate a muscle to contract to produce
Nerve cells, or neurons, are the basic functional units of nervous movement. Other motor neurons may adjust an animal’s internal
systems. physiology, constricting blood vessels to adjust blood flow or
Nervous systems first evolved in simple animals. These early causing wavelike contractions of the gut to aid digestion. As a
nervous systems could sense basic features of the environment, result, nervous system function is fundamental to homeostasis,
such as light, temperature, chemical odors, and physical forces the ability of animals, organs, and cells to actively regulate and
such as touch or pressure. Guided by these cues, early nervous maintain a stable internal state (Chapter 5).
systems could trigger movements useful for obtaining food, Most nerve cells have fiberlike extensions, some of which
finding a mate, or choosing a suitable habitat. As animals evolved receive information and others that transit information. The result
 CHAPTER 35 A N I M A L N E RVO U S S YS T E M S 741

is a network of interconnected nerve cells that form a circuit.

These circuits allow information to be received, processed, and FIG. 35.2 A simplified phylogenetic tree of multicellular animals.
delivered. In most animals, interneurons form specialized circuits All multicellular organisms, except sponges, have a
that may consist of a diverse array of nerve cells having different nervous system. Photo sources: (top) Andrew J. Martinez/Science
shapes, sizes, and chemical properties. Source; (middle) UMI NO KAZE/a.collectionRF/Getty Images; (bottom)
The ability to process information first evolved with the Christof Wermter/age fotostock.
formation of ganglia (singular, ganglion). Ganglia are groups
of nerve cell bodies that process sensory information received
from a local, nearby region, resulting in a signal to motor Sponges
neurons that control some physiological function of the animal.
They can be thought of as relay stations or processing points in
nerve cell circuits. Eventually, the centralized concentration of
neurons that we call a brain evolved, and this organ was able to Multicellularity
Cnidarians
process increasingly complex sensory stimuli received from the (jellyfish, corals,
environment or anywhere in the body. Neurons in the brain sea anemones)
form complex circuits, allowing the brain to regulate a broad
Nervous system
array of behaviors, as well as to learn and retain memories of
past experiences. Bilaterians
(flatworms, insects,
vertebrates, and
Nervous systems range from simple to complex. other animals)
Even single-celled animals have cell-surface receptors that enable
them to sense and respond to their environment. By contrast,
most multicellular animals have a nervous system. In general,
the organization and complexity of an animal’s nervous system system of these animals has relatively few neurons arranged like a
reflects its lifestyle. Sessile, or immobile, animals, like sea net, but no ganglia or central brain (Fig. 35.3a). The nerve net of a
anemones and clams, have relatively simple sense organs and sea anemone is organized around its body cavity. Sensory neurons
nervous systems. In contrast, active animals such as arthropods sensitive to touch signal when one of the anemone’s tentacles has
(which include insects, spiders, and crustaceans), squid, and captured food prey, and then motor neurons react to coordinate
vertebrates have more sophisticated sense organs linked to a the tentacle’s movement toward the mouth for feeding. Sensory
brain and more complex nervous systems. The nervous systems neurons responding to a noxious stimulus signal the sea anemone
of these animals guide homeostatic regulation of their bodies and to retract or detach from its substrate.
coordinate more complex behaviors that require decision making, More complex nerve connections develop in the head end of
forming memories, and learning. flatworms, segmented annelid worms, insects, and cephalopod
Sponges are the only multicellular animals that lack a nervous mollusks, such as squid and octopus. These animals are all
system (Fig. 35.2). Sponges are marine animals that you may bilaterians, animals like us with symmetrical right and left sides
recognize from aquariums or photographs of coral reefs. They and a distinct front and back end (see Fig. 35.2; Chapter 44).
have tissues and organs, but lack any form of body symmetry. Flatworms are representatives of one of the earliest branching
Many small pores along the side and a larger opening at the top groups of bilaterians, and their bodies are well adapted for forward
draw nutrient-laden water into a central cavity. The cells lining locomotion. Flatworms are elongated, flattened organisms that
the cavity capture food particles and dissolved organic matter by glide along a streambed, trapping small animals under their
endocytosis, then metabolize these nutrients in the cell interior. bodies and sucking them in through a muscular opening in their
The functions of different body regions of sponges are coordinated underside. Sensory neurons are most numerous in the animal’s
in the absence of nerve cells or a nervous system. Instead, groups front end, so the flatworm can perceive the environment ahead of
of cells specialized for functions such as contracting or taking in it as it moves forward. The flatworm’s nervous system can compare
water respond to local chemical and physical cues. For example, inputs from different sensory neurons and then send signals to the
a pore-like cell that allows water to enter the sponge closes appropriate muscles to move toward a target. Paired ganglia (one
when touched. on each side of the animal) in the head end of flatworms handle
Among multicellular organisms, the simplest nervous systems this processing task (Fig. 35.3b).
are found in cnidarians, which are radially symmetric animals In animals with an organized nervous system, information is
such as jellyfish and sea anemones (Fig. 35.2). At one end of a transmitted to different regions along the length of the animal’s
cnidarian’s body is a mouth, surrounded by tentacles, that opens body by distinct nerve cords and nerves. These are bundles of the
into a central body cavity where food is digested. The nervous long fiberlike extensions from multiple nerve cells.
742 SECTION 35.1 N E RVO U S S YS T E M F U N C T I O N A N D E VO LU T I O N

FIG. 35.3 Animal nervous systems. (a) Sea anemones have a nerve net; (b) flatworms have paired ganglia; (c) earthworms, (d) insects,
(e) squid, and (f) frogs have a brain.

a. Sea anemone b. Flatworm c. Earthworm d. Insect e. Squid f. Frog

Eye
Eyespots Brain
Nerve Brain Brain
net Paired Eye
Ventral
ganglia nerve
cord Ventral Dorsal
Ventral nerve Eye spinal
nerve Segmental cord Brain cord
cord ganglion Giant
axon
Nerves

In earthworms, paired ganglia, each made up of many been retained over the course of evolution to code and reliably
interneurons, are located in each of the body’s segments transmit information. The ability to receive sensory information
(Fig. 35.3c). These segmental ganglia help to regulate the muscles from the environment, and transmit and process this information
within each body segment that move the earthworm’s body. Paired within a nervous system, is therefore shared across a wide diversity
ganglia near the head regulate the activity of motor neurons that of animal life, contributing to the success of multicellular animals.
control movements of the mouth for feeding. Ganglia serve to
regulate key processes in local regions and organs of the animal’s ? CASE 7 PREDATOR–PREY: A GAME OF LIFE AND DEATH
body. For example, local ganglia assist in processing information What body features arose as adaptations for successful
from the eyes (Chapter 36) or control the digestive state of an predation?
animal’s gut (Chapter 40). The homeostatic regulation of internal A notable feature of most animal nervous systems is that nervous
physiological processes is a fundamental role of nervous systems. system tissue, including specialized sense organs such as eyes,
Centralized collections of neurons forming a brain are becomes concentrated at one end of the body. For example, the
present in earthworms and other annelid worms (Fig. 35.3c), paired ganglia and eyespot of the flatworm are located at one end
insects (Fig. 35.3d), and mollusks (Fig. 35.3e). Sense organs also of its body, as are the brain and sense organs of the earthworm,
developed in the head region, from eyespots in flatworms that squid, and insect, as shown in Fig. 35.3. The concentration of
respond to light to more sophisticated image-forming eyes in nervous system components at one end of the body, defined as
octopus and squid. The evolution of a brain in these invertebrate the “front,” is referred to as cephalization. Cephalization is a key
animals enabled them to learn and perform complex behaviors. feature of the body plan of most multicellular animals, including
Many of these nervous system capabilities are shared with all vertebrates.
vertebrate animals. Cephalization evolved independently multiple times in
In common with certain invertebrates, such as arthropods different animal groups and is therefore thought to confer
and cephalopod mollusks, the evolution of a brain is a key feature certain advantages. Cephalization is thought be an adaptation for
of vertebrate nervous systems (Fig. 35.3f). It enabled these forward locomotion because it allows animals to take in sensory
organisms to evolve complex behaviors that rely on learning and information from the environment ahead of them as they move
memory and, in certain vertebrates, the ability to reason. At the forward. In addition, the nearness of the sensory organs to central
same time, animal brains became increasingly complex in the ganglia or the brain makes it possible to process this information
number of nerve cells they contain (fruit fly, about 0.25 million; quickly to enable a suitable behavioral response. As the quality
cockroach, about 1 million; mouse, about 75 million; humans, and amount of sensory information taken in increased, brain size
about 100 billion), in the number of connections between and complexity increased. Cephalization is also considered to be
neurons, and in the variety of shapes and sizes of nerve cells and an adaptation for predation, allowing animals to better detect and
specialized connections between them. capture prey.
Although animal nervous systems differ in organization and Although cephalization is a feature of many animals, it has
complexity, their nerve cells have fundamentally similar molecular been particularly well studied in vertebrates. In vertebrates, the
and cellular features and use fundamentally the same mechanisms brain, many sense organs, and mouth are all located in the head.
to communicate with neighboring cells. These mechanisms have Vertebrates also evolved several novel features, including a jaw,
 CHAPTER 35 A N I M A L N E RVO U S S YS T E M S 743

teeth, and tongue. These are all thought to be adaptations for signals from other nerve cells or, in the case of sensory nerves,
predation, or more generally the acquisition and processing of food. from specialized sensory endings. These signals travel along the
As a result of evolutionary selection for enhanced sensory dendrites to the neuron’s cell body. At the junction of the cell
perception and the ability to respond to important cues in the body and its axon, the axon hillock, the signals are summed. If
environment, the brain, sensory organs, and nervous system the sum of the signals is high enough, the neuron fires an action
of many animals are complex in their organization. These are potential, or nerve impulse, that travels down the axon. An action
linked to more sophisticated abilities that allow for a broad range potential is a brief electrical signal transmitted from the cell body
of behaviors. These abilities are critical to the success of both along one or more axon branches.
predators and prey and underlie the complex interactions that Axons generally transmit signals away from the nerve’s cell
occur among members of a species when they mate, reproduce and body. The end of each axon forms a swelling called the axon
disperse, and care for their young. terminal. An axon terminal communicates with a neighboring
cell through a junction called a synapse. A space, the synaptic
cleft, separates the end of the axon of the presynaptic cell and the
35.2 NEURON STRUCTURE neighboring postsynaptic cell. The synaptic cleft is commonly only
about 10 to 20 nm wide.
We have seen that nerve cells can be classified as sensory neurons, How does a signal cross the synaptic cleft? Molecules called
interneurons, or motor neurons. Yet all three receive and transmit neurotransmitters convey the signal from the end of the axon
information, and all three share the same basic organization. to the postsynaptic target cell. The arrival of a nerve signal at the
Neurons have extensions that receive information and relay it axon terminal triggers the release of neurotransmitter molecules
to the cell body. They also have other extensions that transmit from vesicles located in the terminal. The vesicles fuse with the
information away from the cell body. Although all neurons share axon’s membrane, releasing neurotransmitter molecules into
this basic plan, they differ remarkably in size and number of the synaptic cleft. The molecules diffuse across the synapse
extensions, according to their specific function. and bind to receptors on the plasma membrane of the target
cell. The binding of neurotransmitters to these receptors causes
Neurons share a common organization. a change in the electrical charge across the membrane of the
Neurons all share some basic features. These include a cell body receiving postsynaptic cell, continuing the signal in the second
from which emerge two kinds of fiberlike extension, dendrites cell. Most neurons communicate by the hundreds to thousands
and axons (Fig. 35.4). These extensions are the input and output of synapses that they form with other cells. The massive number
ends of the nerve cell. Both types of cellular extension can be of interconnections enables the formation of complex nerve cell
highly branched, enabling neurons to communicate over large circuits and the transmission of information from one circuit to
distances with many other cells. A neuron’s dendrites receive others in the nervous system.

FIG. 35.4 Neuron organization. Nerve cells typically have dendrites that receive signals and axons that transmit signals.

1 Stimuli are
received by 2 Synaptic stimuli are Synapse
the dendrites
summed at the axon
and cell body.
hillock, where an action Axon Postsynaptic Synaptic cleft
potential is triggered if terminal cell
the sum of the arriving
signals is high enough.
Neuro-
transmitter
Axon
hillock
tial
Action poten Receptor
Presynaptic
Nucleus cell
Axon Postsynaptic
Cell body membrane
3 Action potentials are conducted to the axon terminal,
where they cause the release of neurotransmitters. These
bind to receptors on the postsynaptic cell membrane,
creating a new signal in the postsynaptic neuron.
Dendrites
744 SECTION 35.2 N E U RO N S T RU C T U R E

The degree of branching of a neuron’s extensions can vary

FIG. 35.5 Variation in neuron shape. Neurons display a diversity from one neuron to the next. A neuron’s dendrites form a finely
of shapes, reflecting their different functions. branched field that receives inputs from many other nerve cells.
a. Sensory b. Interneuron c. Motor A neuron with more dendritic branches receives inputs from
neuron neuron more sources than one with fewer branches. Some neurons
receive information only from nearby regions through a few
dendrites that branch close to the cell body. Other neurons receive
information from many branching dendrites. Axons typically have
Dendrites fewer branches than dendrites.
Signals from all the dendrites, converging on the neuron cell
Cell body, determine the strength, or frequency, and timing of signals
body
carried by the neuron’s axon. The process of bringing together
Axon information gathered from different sources is referred to as
the integration of information. The degree of branching and
the number of synapses that a neuron makes with neighboring
nerve cells, therefore, reflect how information is processed and
integrated by the cell as part of a larger circuit.
Some neurons do not synapse with other neurons, but instead
with other types of cell that produce some physiological response Neurons are supported by other types of cell.
in the animal. Examples are muscle cells, which contract in Neurons in many body regions, and in particular within the brain,
response to nerve signals, and secretory cells in glands, which are supported by other types of cell that do not themselves transmit
release hormones in response to nerve signals. electrical signals. Glial cells are a major class of supporting cell.
Indeed, the human brain has more glial cells than neurons.
j Quick Check 1 How are signals transmitted from one end of a
Glial cells have many functions. They surround neurons and
nerve cell to the other, and from one neuron to another?
provide them with nutrition and physical support. Some glial cells,
called astrocytes, support endothelial cells that make up blood
Neurons differ in size and shape. vessels in the brain. These endothelial cells are linked by tight
In spite of their common organization, neurons show a remarkable junctions (Chapter 10) to form a selective barrier, called the blood–
diversity in size and shape. Some neurons, such as interneurons, are brain barrier, which limits the types of compounds that can diffuse
very short, whereas others, like the
motor neuron that extends from your
spinal cord to your big toe, are several FIG. 35.6 Pyramidal cell. (a) These neurons, drawn by Santiago Ramón y Cajal (1852 –1934), show
feet long. The number of extensions the triangular cell body, highly branched dendrites, and single axon. (b) The triangular
coming off the cell body can also shape of the cell body is clear in the photomicrograph at right. Sources: a. Science Source; b. Bob
vary in different neurons (Fig. 35.5). Jacobs, Ph.D., Laboratory of Quantitative Neuromorphology, Department of Psychology, Colorado College.
Some sensory neurons involved in
smell, sight, and taste have just two a
extensions coming off the cell body
(Fig. 35.5a), whereas other neurons
have many more (Fig. 35.5b and 35.5c).
The cell bodies of some neurons
have distinctive shapes. For example,
pyramidal neurons, as their name
suggests, have triangular-shaped b
cell bodies (Fig. 35.6). These cells,
located in the mammalian brain,
were elegantly drawn by the Spanish
neuroanatomist Santiago Ramón y
Cajal, who shared the 1906 Nobel
Prize in Medicine or Physiology with
the Italian scientist Camillo Golgi
“in recognition of their work on the
structure of the nervous system.”
 CHAPTER 35 A N I M A L N E RVO U S S YS T E M S 745

from the blood into the brain. This barrier prevents pathogens and by a wire. In this section, we discuss the electrical properties
toxic compounds in the blood from entering the brain. Because of neurons and how signals are propagated along neurons. We
nerve cells have limited capacity to regenerate after damage, also describe special features of some neurons that enable even
protection of the brain and spinal cord is critically important. faster signal transmission and more rapid responses by animals to
Nevertheless, lipid-soluble compounds such as alcohol and certain environmental cues.
anesthetics readily diffuse across the blood–brain barrier, and
thus they can affect the functioning of the brain and an animal’s The resting membrane potential is negative and results
mental state. They can also damage and even destroy neurons. in part from the movement of potassium ions.
During development, glial cells help orient neurons as they The inside surface of a neuron carries an electrical charge that is
develop their connections. They also provide electrical insulation due to the presence of charged ions, and the same is true of the
to vertebrate neurons that allows nerve signals to be transmitted outside surface. However, the electrical charge on the inside and
rapidly, a topic we discuss next. the electrical charge on the outside are not the same because
unequal numbers of charged ions are located inside and outside
the cell. When no signal is present, there are more negative ions
35.3 NEURON FUNCTION inside the cell, and the inside is thus negatively charged relative to
the outside.
We have seen that neurons send signals from one neuron to The charge difference between the inside and the outside of a
the next by releasing neurotransmitters that diffuse across the neuron due to differences in charged ions is the cell’s membrane
synaptic cleft and bind to receptors. In Chapter 9, we discussed potential, measured in volts. Other cells also have membrane
that this is a general mechanism of cell communication: A potentials, but only nerve cells and muscle cells respond to
signaling cell releases a chemical signal that binds to a receptor changes in their membrane potential. Therefore, these cells are
on a receiving cell. However, once the receptor has bound the considered electrically excitable.
neurotransmitter and is activated, in neurons the signal is not The key to producing an electrical signal in a nerve cell is the
propagated by a chemical signal diffusing through the cell, which movement of positively and negatively charged ions across the cell
would be much too slow. Instead, as we have seen, neurons send membrane. This movement creates a change in electrical charge
signals electrically from one end of the cell to the other. across the membrane that constitutes the electrical signal.
All neurons are electrically excitable cells that transmit Let’s first consider the membrane potential when the neuron
information in the form of electrical signals. Nerve signals is at rest and no signal is being received or sent. Under these
travel at high speeds, up to 200 m/s (450 mph). Although the conditions, the cell’s membrane voltage is negative on its inside
speed of nerve signals is fast by biological standards, it is slow relative to its outside (Fig. 35.7). The resting membrane potential
by comparison with the speed of electrical signals transmitted of the cell is said to be polarized. This means that there is a

FIG. 35.7 Neuron resting membrane potential. The resting potential of a neuron is negative and results primarily from the movement of K+ ions
out of the cell.

Extracellular
fluid

0
+
K+ movement
Voltage (mV)

0
-40 +40
+ + + + + + + + + + + + + + + + + + + + + -80
V +80

Resting potential
–70
– – – +– – – – – – – – – – – – – – – – – –
K channel Na+–K+ pump
–
K+ Na+
Cytoplasm Time

The Na+–K+ pump moves Na+ K+ channels allow K+ ions to Membrane potential
ions out of the cell and K+ “leak” out of the cell, resulting can be measured with
ions into the cell. in a negative resting potential small glass electrodes.
on the inside relative to the
outside of the cell.
746 SECTION 35.3 N E U RO N F U N C T I O N

buildup of negatively charged ions on the inside surface of the How is the action potential generated? At the axon hillock,
cell’s plasma membrane and of positively charged ions on its the summed membrane depolarization of the cell’s dendrites
outer surface. The negative voltage across the membrane at rest causes voltage-gated sodium channels to open, allowing
is referred to as the cell’s resting membrane potential. The Na1 ions to enter the cell. Voltage-gated channels open and
resting membrane potential ranges from –40 to –85 millivolts close in response to changes in membrane potential (Fig. 35.8;
(mV) depending on the type of nerve cell and most commonly Chapter 9). As Fig. 35.8 shows, they play a key role in how
is about –65 to –70 mV. The voltage of the cell’s interior can neurons fire action potentials.
be measured with respect to its outside voltage by small glass The influx of Na1 causes a flow of positive charge, like a
electrodes on the inside and outside of the cell. current, along the inside of the cell, toward more negatively
At rest, nerve (and muscle) cells have a greater concentration charged nearby regions. This flow of charge is reduced at longer
of sodium (Na1) ions outside the cell than inside, and a greater distances and quickly dissipates so that the membrane potential
concentration of potassium (K1) ions inside the cell than outside. returns to a resting state unless additional excitatory stimuli
This distribution of ions results in part from the action of the further depolarize the membrane.
sodium-potassium pump, discussed in Chapter 5. The sodium- If the excitatory signal is strong enough to depolarize the
potassium pump uses the energy of ATP to move three Na1 ions membrane of the nerve cell body to a voltage of approximately
outside the cell for every two K1 ions moved in (Fig. 35.7). The 15 mV above the resting membrane potential (about –55 mV),
action of the pump makes the inside of the cell less positive, and the nerve fires an action potential at the axon hillock. An action
therefore more negative, than the outside of the cell. potential is a rapid, short-lasting rise and fall in membrane
The exact value of the resting membrane potential, however, potential, as shown in Fig. 35.8. The critical depolarization
depends on the movement of K1 ions back out of the cell by voltage of –55 mV required for an action potential is the cell’s
passive diffusion through potassium ion channels (Fig. 35.7). As threshold potential. Nerve cells in different animals have
discussed in Chapter 5, ion channels are protein pores embedded slightly different resting and threshold potentials, but all operate
in the cell membrane. The most important ions that move across similarly with respect to firing an action potential. When
the cell membrane are sodium (Na1), potassium (K1), chloride the threshold potential is exceeded, the nerve fires an action
(Cl2), and calcium (Ca21). When a nerve cell is at rest, more K1 potential in an all-or-nothing fashion. This means that once the
ion channels are open, giving K1 greater permeability compared threshold potential has been exceeded, the magnitude of the
with all other ions. As a result, K1 ions move out of the cell. action potential is always the same and is independent of the
The movement of K1 ions from the inside to the outside causes strength of the stimulating input. At this point, a rapid spike in
positive ions to build up on the outside of the cell, whereas voltage to approximately 140 mV occurs over a very brief time
negatively charged ions (largely proteins) remain on the inside of period of 1 to 2 msec.
the cell, making the inside of the nerve cell more negative than An action potential results from dramatic changes in the
its outside. state of voltage-gated Na1 and voltage-gated K1 channels in the
The relative proportion of ions does not by itself determine axon membrane. As the cell membrane crosses its threshold
the resting membrane potential. This is because the number of potential, a large number of voltage-gated Na1 channels
charged ions that build up at the cell’s surface is a tiny fraction of suddenly open, allowing Na1 ions to rush inside the cell. The
the total number of charged ions and proteins located inside and rise in voltage that results causes additional voltage-gated Na1
outside the cell. It is the movement of K1 ions relative to other channels to open. This is an example of positive feedback, in
ions, particularly Na1 ions, that largely determines the resting which a signal (depolarization) causes a response (open voltage-
membrane potential. gated Na1 channels) that leads to an enhancement of the signal
(more depolarization) that leads to an even larger response
Neurons are excitable cells that transmit information (more open voltage-gated Na1 channels).
by action potentials. During the rising phase of the action potential, voltage-gated
When a nerve cell is excited, its membrane potential becomes K1 channels also begin to open, but these channels are slow
less negative—that is, the inside becomes less negative, or more to activate relative to the rapidly opening voltage-gated Na1
positive, than the outside of the cell. The increase in membrane channels. As a result, relatively few K1 ions leave the cell and
potential is therefore referred to as a depolarization of the the cell membrane potential continues to rise due to the large
membrane. This depolarization does not occur over the entire number of Na1 ions entering the cell.
cell at once. Rather, it starts at the terminal end of the dendrite, As you can see from Fig. 35.8, the rising phase of the action
in response to neurotransmitter binding to membrane receptors, potential (rapid depolarization) is followed by the falling phase
and travels to the cell body, losing strength along the way. If the (rapid repolarization). What causes this sudden reversal of the
depolarization is still strong enough at the axon hillock of the membrane potential? There are two important factors. First,
cell body, the cell fires an action potential that carries the signal voltage-gated Na1 channels begin to close once the membrane
from the cell body to the terminal ends of the axon. potential becomes positive. Second, voltage-gated K1 channels
 CHAPTER 35 A N I M A L N E RVO U S S YS T E M S 747

FIG. 35.8 An action potential. An action potential results from the opening and closing of voltage-gated ion channels and allows the membrane
potential briefly to be positively charged.

3 As the voltage rises to

+40mV, Na+ channels close and
voltage-gated K+ channels
remain open, allowing K+ ions to
leave the cell and causing the
+40 membrane potential to become
more negative.

2 Voltage-gated Na+ channels – – –

open and Na+ rapidly enters the
cell, causing a positive spike in the
membrane potential (positive inside
+ + +
relative to outside). K+ channels
open more slowly, allowing fewer
K+ ions to leave the cell.

0 Voltage-gated
Na+ channel
Voltage-gated
K+ channel
– – –

5
Membrane potential (mV)

+ Gradually the membrane

+ +
returns to resting, as excess K+
K+
ions are returned to the cell,
Na+ assisted by Na+-K+ pumps.

+ + +
1 Summed input
depolarizes the cell
Threshold of membrane at the axon – – –
excitation hillock above the
threshold potential.

–70

Resting potential

4 An overshoot in the amount of K+

ions that leave the cell causes the cell
membrane to be hyperpolarized. This
results in a refractory period during
which the nerve cannot fire another
action potential.

1 2 3
Time (msec)
748 SECTION 35.3 N E U RO N F U N C T I O N

continue to open in response to the change in voltage. Because The duration of the refractory period varies for different kinds of
they are slower to respond than the voltage-gated Na1 channels, nerve cell but limits a nerve cell’s fastest firing frequency to less
the membrane voltage peaks and then falls as K1 ions diffuse than 200 times per second. Most nerve cells fire at lower rates.
out of the axon through the open voltage-gated K1 channels. It might seem that many Na1 and K1 ions need to diffuse
The voltage inside the axon does not return immediately to across the axon membrane to produce an action potential.
the resting membrane potential. Instead, it briefly falls below However, only about 1 in 10 million Na1 and K1 ions need to cross
the resting potential (in what is known as hyperpolarization the membrane. Thus, enough ions are always available to generate
or undershoot), then returns to the resting potential after repeated action potentials.
another few milliseconds as K1 channels close to restore the The nature of the action potential, which is both all-or-
resting concentration of Na1 and K1 ions on either side of the nothing and uniform in its electrical features, means that the
cell membrane. The continuous action of the sodium-potassium action potential itself contains little information. Most neurons
pumps also helps to reestablish the resting membrane code information by changing the rate and timing of action
potential. potentials. Generally, a higher firing frequency codes for a more
The period during which the inner membrane voltage intense stimulus, such as a brighter light or louder noise, or
falls below and then returns to the resting potential is the a stronger signal transmitted by the nerve cell to other cells
refractory period (Fig. 35.8). During the refractory period, it contacts.
a neuron cannot fire a second action potential. The refractory
period results in part from the fact that when voltage-gated Neurons propagate action potentials along their axons
Na1 channels close, they require a certain amount of time by sequentially opening and closing adjacent Na+ and
before they will open again in response to a new wave of K+ ion channels.
depolarization. In addition, open voltage-gated K1 channels Once initiated, an action potential propagates along the axon (Fig.
make it difficult for the cell to reach the threshold potential. 35.9). The local membrane depolarization initiated at the axon

FIG. 35.9 Propagation of action potentials. Local depolarization of the membrane triggers the opening of nearby voltage-gated Na+ channels,
producing an action potential that spreads along the membrane. The action potential moves in only one direction (shown here as left to
right) because of the refractory period.

Extracellular Voltage-gated Voltage-gated Membrane

fluid K+ channel Na+ channel Na+
+ + + – – – – – – + + + + + + +

– – – + + + + + + – – – – – – –
Cytoplasm
– – – + + + + + + – – – – – – –

+ + + – – – – – – + + + + + + +

Depolarizing Resting

K+
+ + + + + + + + – – – – + + + +

– – – – – – – – + + + + – – – –

– – – – – – – – + + + + – – – –

+ + ++ + + + ++ ++ – – – – + + + +

Repolarizing Depolarizing Resting

 CHAPTER 35 A N I M A L N E RVO U S S YS T E M S 749

hillock triggers the opening of neighboring voltage-

gated Na1 channels farther along the axon. The inward FIG. 35.10 Saltatory conduction. Myelination insulates axons, enabling faster
sodium current depolarizes the membrane above conduction of action potentials. Photo source: Don W. Fawcett/Science Source.
threshold, triggering the opening of nearby voltage-
gated Na1 channels still farther along the axon. Layers of myelin insulate the axon.
By this means, the depolarization—the location As a result, action potentials “jump”
of the action potential spike—spreads down the from node to node, increasing the
speed of conduction.
axon. Neighboring voltage-gated K1 channels
subsequently also open and close to reestablish
a resting membrane potential after an action
potential has fired. Action potentials are thus self-
propagating. Action potentials propagate only in Nucleus – –
+ +––
++
one direction, normally from the cell body at the Cell body Node of
– +
Ranvier –––– –– – –
axon hillock to the axon terminal. The refractory ––
+ + –
period following an action potential prevents the Glial cell + ++
membrane from reaching threshold and firing an Nucleus +++
Axon
action potential in the reverse direction. of glial cell ––+ +
– –– –
The conduction speed of action potentials Myelin sheath
is limited by the neuron’s membrane properties
and the width of its axon. Yet fast transmission At nodes of Ranvier, there
is a buildup of + charges
speeds are essential for animals to respond quickly inside and – charges
to stimuli. For example, when you mistakenly outside the axon.
touch something very hot, your fingers sense
and relay this information rapidly to ensure that
Myelin sheath
you withdraw your hand quickly and avoid worse
injury. Axon

The neurons of vertebrate animals have an

adaptation that lets them respond rapidly to
stimuli in their environment. Glial cells form
lipid-rich layers or sheaths called myelin that wrap
around the axons of these neurons (Fig. 35.10).
Myelin gives many nerves their glistening white
appearance. The myelin sheath insulates the axon’s membrane,
spreading the charge from a local action potential over a much
greater distance along the axon’s length than would be possible in plasma membrane by studying the giant axon in squid that
the absence of myelin. stimulates contractions of their body muscles for swimming
At regular intervals, the axon membrane is exposed at sites (Fig. 35.11). For this work, they shared the 1963 Nobel Prize in
called nodes of Ranvier that lie between adjacent segments Physiology or Medicine along with John Eccles, who worked out
wrapped with myelin (Fig. 35.10). Voltage-gated Na1 and K1 the membrane properties of synapses, which we explore next.
channels are concentrated at these nodes. As a result, rather
than being conducted in a continuous fashion, as is the case for Neurons communicate at synapses.
unmyelinated axons, the action potentials in myelinated axons We have seen that nerve cells communicate at specialized
“jump” from node to node. This saltatory propagation (from junctions called synapses. There are two types of synapse,
the Latin saltus, “jump”) of action potentials by myelinated axons electrical and chemical. Electrical synapses provide direct
greatly increases the speed of signal transmission. electrical communication through gap junctions that form
between neighboring cells (Chapters 10 and 28). They enable
j Quick Check 2 Multiple sclerosis is a disease in which the
rapid communication but limit the ability to process and
immune system attacks and destroys the myelin sheath surrounding
integrate information. Electrical synapses are found in the giant
nerve cells. What effect would you expect the loss of myelin to
axons of squid, as well as in large motor nerves in fish used for
have on the speed of nerve impulses?
escape swimming. Electrical synapses can also be found in the
The British neurophysiologists Alan Hodgkin and Andrew mammalian brain. They likely help to speed up information
Huxley first worked out the electrical properties of the axon processing, but their role in the brain has not been well studied.
 HOW DO WE KNOW?

FIG. 35.11

What is the resting

membrane potential? How a. Squid, Loligo forbesi

does electrical activity

Arm

Eye

change during an action Brain

Nerve

potential? Tentacle
Giant
axon
BACKGROUND In order to record the voltage of the inside of
a nerve cell relative to the outside, a small electrical recording
device called a microelectrode can be inserted into a neuron. The
technique is more easily performed on large cells, such as the
squid giant axon. This axon, as its name suggests, is quite large, b.
measuring 0.5 mm in diameter (Fig. 35.11a). The large diameter A reference
electrode is
of the axon allows electrical signals to be propagated to the placed
0
-40 +40
muscles very quickly. Vertebrate neurons are much smaller; they -80
V +80 outside the
rely on myelin sheaths rather than large size for rapid conduction cell in
A microelectrode contact with
of electrical signals. is inserted into an the solution
axon. bathing the
EXPERIMENT In 1939, two British neurophysiologists, axon.
Alan Hodgkin and Andrew Huxley, inserted a microelectrode
into a squid giant axon and placed a reference electrode on
the outside (Fig. 35.11b). They then used a separate set of
Axon
electrodes (not shown) to depolarize the cell to threshold,
triggering an action potential. Axon
membrane
RESULTS Fig. 35.11c is a trace from Hodgkin and Huxley’s 1939
paper showing the resting membrane potential and the course of
an action potential recorded by the electrode inside a giant squid c.
axon. Note that the resting potential (–45 mV in squid) is negative +40
on the inside of the axon relative to the outside, and that the
action potential is a rapid spike in potential, with the inside of the
Voltage (mV)

cell quickly becoming positive, then negative again. The large size 0
of the spike was a surprise.

FOLLOW-UP WORK This work was performed before anyone

had identified the ion channels responsible for the changes in
current across the membrane. Subsequent work focused on
identifying these channels and their roles in generating the action –70
1 2 3 4
potential. Time (msec)

SOURCES Hodgkin, A. L., and A. F. Huxley. 1939. “Action Potentials Recorded

from Inside a Nerve Fibre.” Nature 144:710–711; R. D. Keynes. 1989. “The Role
of Giant Axons in Studies of the Nerve Impulse.” Bioessays 10:90–94.

750
 CHAPTER 35 A N I M A L N E RVO U S S YS T E M S 751

FIG. 35.12 A chemical synapse. Neurons communicate with other neurons or with muscle cells by releasing neurotransmitters at a synapse.

1 Synaptic
transmission begins
with action potential
conduction to the
axon terminal.

Axon
terminal
Ca2+ channel
2 Depolarization
of the axon Ca2+
terminal opens
voltage-gated
5 After inactivation,
Ca2+ channels. neurotransmitters are
re-absorbed into the
presynaptic terminal and
stored in vesicles until the
Acetylcholine next action potential arrives.
(neurotransmitter)
Presynaptic
3 Vesicles respond membrane
by fusing with the
presynaptic Synaptic
membrane, releasing cleft
neurotransmitters
into the synaptic
cleft.

Postsynaptic
membrane
Na+

Receptor

4 Neurotransmitters bind with receptors

on the postsynaptic cell that are
ligand-gated ions, causing a change in
membrane potential.

Chemical synapses are by far the more common type of with the presynaptic membrane and release neurotransmitter
synapse in animal nervous systems. The signals conveyed molecules into the synaptic cleft by exocytosis. The
at chemical synapses are neurotransmitters contained neurotransmitters diffuse rapidly across the cleft and bind to
within small vesicles in the axon terminal. When an action postsynaptic membrane receptors of the neighboring cell. The
potential reaches the end of an axon (Fig. 35.12), the resulting binding of neurotransmitters opens or closes ion channels,
depolarization induces voltage-gated Ca21 ion channels to open. causing a change in the postsynaptic cell membrane potential
These channels are found only in the axon terminal membrane. that allows the signal to propagate along the next neuron.
Because of their higher concentration outside the cell, Ca21 These postsynaptic membrane receptors are called ligand-
ions diffuse through these channels into the axon terminal. gated ion channels because when the neurotransmitter binds
In response to the rise in Ca21 concentration, the vesicles fuse as a ligand it causes the ion channel to open. This allows specific
752 SECTION 35.3 N E U RO N F U N C T I O N

ions to enter the postsynaptic cell, changing its membrane

potential. Ligand-gated ion channels underlie the signal FIG. 35.13 Integration of information. Multiple synapses—often
transmission that occurs at synapses, regulating the activity of hundreds of them—from communicating neurons
nerve and muscle cells. enable a neuron to integrate diverse sources of
The neurotransmitter’s effect on the postsynaptic information. Source: Prof. Mary B. Kennedy, Division of Biology
membrane ends shortly after binding. Neurotransmitters and Biological Engineering, Caltech.

become unbound from the receptor, causing the ligand-gated

ion channels to close, and neurotransmitters in the synaptic
cleft are either broken down by deactivating enzymes or taken
up again by the presynaptic cell and reassembled in newly
formed vesicles. Each yellow-green
dot represents an
j Quick Check 3 How do the structural and chemical properties axon terminal from
of the synapse ensure that communication proceeds in a single another neuron
synapsing on the
direction between nerve cells? dendrites of this
(green) postsynaptic
cell.
Signals between neurons can be excitatory
or inhibitory.
Neurotransmitters bound to postsynaptic membrane
receptors can either stimulate or inhibit the firing of action
potentials in the postsynaptic neuron. The binding of some
neurotransmitters depolarizes the postsynaptic membrane by
opening ligand-gated Na1 channels. As a result, Na1 ions diffuse
into the cell, making the membrane potential more positive. The
positive change in membrane potential is termed an excitatory
postsynaptic potential, or EPSP.
In contrast, the binding of other neurotransmitters can neurotransmitters. Thus, postsynaptic cells can receive both
hyperpolarize the postsynaptic membrane, making the excitatory and inhibitory signals. Commonly, the dendrites or
membrane potential more negative through the opening cell body of a single postsynaptic nerve cell make connections to
of ligand-gated Cl2 or K1 channels. Cl2 ions diffuse into the hundreds or even thousands of axons from other nerve cells (Fig.
cell or K1 ions diffuse out of the cell, causing the membrane 35.13).
potential to become more negative, or hyperpolarized. The The postsynaptic nerve cell sums the excitatory and
negative change in membrane potential is called an inhibitory inhibitory synaptic inputs (the summed EPSPs and IPSPs) that
postsynaptic potential, or IPSP. Membrane depolarization it receives through its dendrites and cell body (Fig. 35.14). If the
makes a neuron more likely to respond and transmit an action sum of synaptic stimulation results in a membrane potential
potential, whereas membrane hyperpolarization inhibits the that exceeds threshold at the axon hillock, the neuron fires an
neuron from sending an action potential. action potential, communicating an impulse to cells that it in
Excitatory synapses tend to transmit relevant information turn contacts. If the sum of inputs does not exceed threshold, no
between nerve cells, and inhibitory synapses often serve to filter action potential fires (Fig. 35.14a).
out unimportant information. For example, when a grazing EPSPs and IPSPs can be summed over time and space. When
gazelle senses a predator, its nervous system must transmit summed over time, the frequency of synaptic stimuli determines
the key sensory information about the presence of a predator whether the postsynaptic cell fires an action potential (Fig.
but filter out irrelevant information such as the flies buzzing 35.14b). This is referred to as temporal summation. When
over its back and ears. Inhibitory synapses are also important summed over space, the number of synaptic stimuli received
for controlling the timing of muscle activity required for from different regions of the postsynaptic cell’s dendrites
coordinated movement, discussed in Chapter 37. determines if the cell fires an action potential (Fig. 35.14c). This
There are a number of different neurotransmitters, but a is referred to as spatial summation. Sometimes, excitatory
single nerve cell releases only one type of neurotransmitter. and inhibitory signals may cancel each other out (Fig. 35.14d).
Consequently, a particular nerve cell exerts either an excitatory Temporal and spatial summation of EPSPs and IPSPs are the
or an inhibitory effect on neighboring cells, but not both. fundamental forms of information processing carried out by
In contrast, postsynaptic membranes typically contain the nervous system. They provide mechanisms for determining
multiple types of membrane receptor that bind different whether a particular sensory stimulus is responded to or ignored.
 CHAPTER 35 A N I M A L N E RVO U S S YS T E M S 753

FIG. 35.14 Summation of excitatory and inhibitory postsynaptic potentials. (a) Widely spaced EPSPs do not sum. Other EPSPs and IPSPs can
be summed (b) in time or (c) space, or even (d) cancel each other out.
a. b. c. d.

No summation: Multiple EPSPs Temporal summation: Spatial summation: Single Cancellation: An EPSP and an
widely spaced in time do not Multiple EPSPs arrive quickly EPSPs at two or more different IPSP may cancel each other so
set off an action potential. at a single synapse and set synapses set off an action no action potential is set off.
off an action potential. potential.
Presynaptic axon Postsynaptic cell
EPSP

EPSP EPSP EPSP EPSP

EPSP IPSP

3 EPSPs, No Summation 3 EPSPs, Temporal Summation 3 EPSPs, Spatial Summation EPSP and IPSP
+40
Membrane potential (mV)

Resting Threshold
potential potential

–55
–70

0
EPSP EPSP EPSP EPSP EPSP EPSP EPSP IPSP
Time EPSP
EPSP EPSP IPSP
EPSP EPSP
EPSP
EPSP

The role of temporal and spatial summation is explored further in Acetylcholine is one of the key neurotransmitters produced
Chapter 36. in a variety of nerve cells. It is the excitatory neurotransmitter
More than 25 neurotransmitters are now recognized, and released by motor neurons to stimulate muscle fibers
more are likely to be discovered. The amino acids glutamate (Fig. 35.15). All vertebrate motor synapses rely on acetylcholine
(excitatory), glycine (inhibitory), and GABA (inhibitory) are key and are therefore excitatory only. The release of acetylcholine
neurotransmitters that operate at synapses in the brain. The produces a depolarization of the muscle cell membrane by
related amino acid derivatives dopamine, norepinephrine, and opening ligand-gated Na1 channels, causing the muscle to
serotonin are also neurotransmitters acting in the brain. Simple contract (Chapter 37).
peptides serve as neurotransmitters for sensory neurons. More
recently, two gases, nitrous oxide and carbon monoxide, have been j Quick Check 4 Acetylcholinesterase is the enzyme that breaks
discovered to function as messengers between some nerve cells down and inactivates acetylcholine. Some nerve gases used
even though they do not behave as standard neurotransmitters as chemical weapons block acetylcholinesterase. What effect
that bind to membrane receptors. would such nerve gases have on muscle contraction?
754 SECTION 35.4 N E RVO U S S YS T E M O RG A N I Z AT I O N

that extends from the brain. Nerves are composed mainly of axons
FIG. 35.15 Vertebrate motor endplate. The motor endplate is from many different nerve cells. For example, the optic nerve
an excitatory synapse between a motor nerve and contains axons that travel from nerve cells in the eye to the brain.
a muscle cell; acetylcholine is the neurotransmitter. Sensory and motor nerves make up the peripheral nervous
Source: David B. Fankhauser, Ph.D., Professor of Biology and system (PNS). These nerves communicate with the central
Chemistry, University of Cincinnati, Clermont College. nervous system (CNS), made up of the brain, a main nerve cord,
or—in the case of animals such as flatworms that lack a brain—
Motor neuron axon Muscle cell
centralized information-processing ganglia. Sensory organs at or
near the body surface transmit information by afferent neurons,
which send information toward the CNS. After processing the
information, the ganglia and brain send signals through efferent
neurons, which send information away from the CNS. The signals
delivered by efferent neurons in peripheral nerves coordinate
the activity of muscles and glands in different regions of the
animal’s body. This organization ensures that an animal achieves
a coordinated behavioral and physiological response to the stimuli
received from its environment, which is fundamental to its ability
to maintain homeostasis. Among multicellular animals with
nervous systems, only the diffuse nerve nets of cnidarians lack
Acetylcholine binds to muscle membrane
receptors, causing a depolarization of the defined central and peripheral components.
muscle cell and contraction. The peripheral nervous system also includes interneurons and
ganglia that integrate and process information in local regions of
the animal’s body. For example, vertebrates and invertebrates have
ganglia that lie outside segments of their primary nerve cord that
coordinate localized function within a region of their body, such as
35.4 NERVOUS SYSTEM the muscles that control the limb of a running insect, the muscles
ORGANIZATION that allow an earthworm to dig, or the secretory activity of a
particular region of the digestive system of a fish or a mammal.
The brain is the body’s command center: It receives sensory However, the bulk of information processing, particularly in
information from the eyes, ears, nose, tongue, skin, and internal animals that exhibit a greater capacity for learning and memory,
organs, and sends instructions to the rest of the body. Up to this occurs in the CNS.
point, we have examined how individual neurons send signals to Let’s look more closely at the organization of the vertebrate
other nerve and muscle cells. How are these neurons organized in nervous system. The central nervous system of vertebrates
the body to sense stimuli, process them, and issue an appropriate includes both the brain and spinal cord (Fig. 35.16). The spinal
response? How does the brain coordinate the movement of limbs cord is a central tract of neurons that passes through the vertebrae
necessary to walk or run, and adjust heart rate and breathing to transmit information between the brain and the periphery
in response to exercise? In this section, we look at how the of the body. The vertebrate spinal cord is divided into segments,
nervous system is organized to allow the brain and the body to each controlling body movement in a particular region along
communicate with each other. the animal’s length. Each spinal cord segment contains axons
from peripheral sensory neurons, a set of interneurons, and a set
Nervous systems are organized into peripheral of motor neuron cell bodies. These are distinct from, but often
and central components. associated with, the ganglia mentioned above that lie outside the
As animals evolved the ability to sense and coordinate responses spinal cord.
to increasingly complex stimuli, their nervous systems became In humans and other vertebrate animals, the peripheral
organized into peripheral and central components (Fig. 35.16). Your nervous system is organized into left and right sets of cranial
eyes, receptors for the sense of touch, and other sensory organs are nerves located within the head and spinal nerves running from
located on the surface of the body, where they can receive signals the spinal cord to the periphery. Most of the cranial nerves and
from the environment. In contrast, the brain, centrally located all of the spinal nerves contain axons of both sensory and motor
ganglia, and a main nerve cord are located in the interior. neurons. Cranial nerves link specialized sensory organs (eyes, ears,
Nerves form the lines of communication between these tongue) to the brain. Cranial nerves also control eye movement,
nervous system structures. In general, nerve cell bodies are facial expression, speech, and feeding. Some cranial nerves, such
grouped together in sensory organs, ganglia, and a main nerve cord as the olfactory and optic nerves, contain only sensory axons.
 CHAPTER 35 A N I M A L N E RVO U S S YS T E M S 755

FIG. 35.16 Human central (yellow) and peripheral (blue) nervous systems.

Peripheral Nervous Central Nervous

System (PNS) System (CNS)

Cranial nerves Brain

Spinal cord

Spinal nerves

Axons

Blood
vessels

Connective
tissue

Structure of a nerve

Spinal nerves exit from the spinal cord through openings located sensing and responding to external stimuli, whereas involuntary
between adjacent vertebrae to thread through the trunk and limbs components typically regulate internal bodily functions. Both
of an animal’s body. These nerves receive sensory information nervous system components are found in invertebrate and
from receptors in nearby body regions along the length of the body vertebrate animals.
and send motor signals from the spinal cord back to those regions In insects and crustaceans, for example, nerve circuits from
(Fig. 35.16). the involuntary component regulate the animal’s digestive
system. Nerve circuits connecting sensory structures such as the
Peripheral nervous systems have voluntary antennae and eyes to the animal’s brain and muscles are part of
and involuntary components. the voluntary system.
As bodies with distinct internal organ systems evolved, two In vertebrates, the peripheral nervous system is divided into
separate components of the nervous system emerged. When somatic (voluntary) and autonomic (involuntary or visceral)
a gazelle senses a predator, some nerve circuits send a signal to components. The somatic nervous system is made up of sensory
run, an action that is under conscious control, while others signal neurons that respond to external stimuli and motor neurons
the heart to beat faster and blood vessels supplying muscles to that synapse with voluntary muscles. This system is considered
dilate, actions that occur unconsciously. Conscious reactions are voluntary because it is under conscious control. However, many
under the control of the voluntary component of the nervous reflexes are controlled lower in the spinal cord (discussed in the
system, and unconscious ones are under the control of the following section) or by the brainstem, independent of conscious
involuntary component. Voluntary components mainly handle control by the central nervous system.
756 SECTION 35.4 N E RVO U S S YS T E M O RG A N I Z AT I O N

sympathetic nervous system increases

FIG. 35.17 The human involuntary (autonomic) nervous system. The autonomic heart rate and breathing rate, stimulates
system, which controls involuntary functions, is divided into sympathetic and glucose release by the liver, and inhibits
parasympathetic systems. digestion. The parasympathetic nervous
system, in contrast, slows the heart and
Parasympathetic Division Sympathetic Division
stimulates digestion as well as metabolic
Dilates pupil of eye. processes that store energy—in other
words, it enables the body to “rest and
digest.”
Inihibits salivary In addition to having different
Constricts gland secretion. functions, sympathetic and
pupil of eye.
parasympathetic nerves also have
Accelerates heart. different anatomical distributions, as
Stimulates Sympathetic
salivary gland. ganglia shown in Fig. 35.17. Sympathetic nerves
leave the CNS from the middle (thoracic
Relaxes bronchi
in lungs. and lumbar) region of the spinal cord,
Slows heart.
forming ganglia along much of the length
Inhibits activity of the spinal cord. Parasympathetic
Constricts
bronchi in lungs. of stomach and nerves leave the CNS from the brain by
intestines. cranial nerves and from lower (sacral)
Stimulates activity levels of the spinal cord.
of stomach and Inhibits activity
intestines. of pancreas.
The nervous system helps
to maintain homeostasis.
Stimulates activity Stimulates glucose
of pancreas. release from liver; The autonomic nervous system plays
inhibits gallbladder. a key role in the ability to maintain
Stimulates homeostasis, or a steady physiological
gallbladder. Stimulates state, in the face of changing
adrenal medulla. environmental conditions. In
Promotes voiding Chapter 5, we discussed how the
from bladder. Inhibits voiding selectively permeable cell membrane
from bladder.
helps maintain homeostasis within
Promotes erection
individual cells. For example, it keeps ion
of genitals. Promotes
ejaculation concentrations within narrow ranges for
and vaginal normal cell function. Similarly, the firing
contractions. of action potentials by neurons requires
particular ion concentrations on either
side of the membrane. Temperature and
The autonomic nervous system (Fig. 35.17) controls internal pH are other parameters that do not vary much because enzymes
functions of the body such as heart rate, blood flow, digestion, often work effectively only in narrow temperature and pH ranges.
excretion, and temperature. It includes both sensory and motor Recent evidence suggests that nerve cell firing rates are maintained
components, which usually act without our conscious awareness. at a steady level in the brain of rats, regardless of sensory stimulus
The autonomic nervous system, in turn, is divided into two major or deprivation, or whether the animals are awake or asleep. A shift
subdivisions, a sympathetic division and a parasympathetic in the homeostatic set point of nerve cell firing rates may therefore
division. Both divisions continuously monitor and regulate underlie disorders of low brain activity, such as autism.
internal functions of the body. Generally, sympathetic and Homeostasis is also maintained for the whole body. Many
parasympathetic nerves have opposite effects, but not always. For physiological parameters are maintained in a narrow range of
example, sympathetic neurons stimulate the heart to beat faster, conditions throughout the body, including pH, temperature, and
whereas parasympathetic neurons cause the heart to beat slower. ion concentrations. Similarly, the water content of the body as
The sympathetic nervous system generally results in arousal a whole is kept stable through the careful regulation of ions and
and increased activity. It is the pathway activated when animals other solutes, as discussed in Chapter 41.
are exposed to threatening conditions, resulting in what is referred The concept of homeostasis was first described as regulation
to as the “fight or flight” response. As part of this response, the of the body’s “interior milieu” in the late 1800s by the French
 CHAPTER 35 A N I M A L N E RVO U S S YS T E M S 757

FIG. 35.18 Temperature regulation by negative feedback in (a) a house and (b) a mammal. In negative feedback, a response (such as heat)
opposes the stimulus (cold), leading to a stable state (a steady temperature).
a. b.

Stimulus Cold Cold

+ +

Sensor Thermostat Hypothalamus

+ +

Muscles
Effector Heater
(shivering)

– –

Response Heat Heat

physiologist Claude Bernard, who is often credited with fluctuates. Nerve cells in the hypothalamus (located in the base
bringing the scientific method to the field of medicine. The term of the brain) act as the body’s thermostat (Fig. 35.18b). When a
“homeostasis” was coined by the American physiologist Walter decrease in the temperature in the environment causes a drop
Cannon, whose book The Wisdom of the Body (first published in in body temperature, the lowered body temperature signals the
1932) popularized the concept. hypothalamus to activate the somatic nervous system to induce
Maintaining steady and stable conditions takes work in shivering and the production of metabolic heat, as discussed in
the face of changing environmental conditions. That is, a cell Chapter 40. At the same time, the hypothalamus activates the
(or organism) actively maintains homeostasis. For example, autonomic nervous system, causing peripheral blood vessels to
many animals are often faced with long periods of drought that constrict. The reduction in blood flow near the body’s surface
challenge their ability to remain hydrated and maintain a stable reduces heat loss to the surrounding air. By contrast, an increase
water and ion balance. These animals must rapidly respond by in temperature signals sweat glands to secrete moisture and
changing the permeability of their skin and respiratory organs so peripheral blood vessels to dilate to aid heat loss from the skin.
that they can retain as much water as possible. Homeostatic regulation, therefore, relies on negative feedback
How does the body, and in particular the nervous system, to maintain a set point, which in this case represents an animal’s
maintain homeostasis? Homeostatic regulation often depends on preferred body temperature. The ability to maintain a constant
negative feedback (Fig. 35.18). In negative feedback, a stimulus body temperature is known as thermoregulation, and it is just one
acts on a sensor that communicates with an effector, which of many physiological set points that the body actively maintains,
produces a response that opposes the initial stimulus. For example, as we discuss in subsequent chapters.
negative feedback is used to maintain a constant temperature
in a house. Cool temperature (the stimulus) is detected by a Simple reflex circuits provide rapid responses
thermostat (the sensor). The thermostat sends a signal to the to stimuli.
heater (the effector), producing heat (the response). The response An animal that has perceived a predator has an advantage if it can
(heat) opposes the initial stimulus (cool temperature), and no move quickly. Fast responses are made possible by simple reflex
further heat will be produced until the temperature drops below circuits that bypass the brain. These circuits connect sensory
the temperature setting of the thermostat. In this way, a stable neurons (detecting the presence of the predator) directly with
temperature is maintained (Fig. 35.18a). motor neurons (providing the quick movement necessary to
In a similar way, humans and other mammals maintain escape the predator). Reflex circuits are common in both the
a steady body temperature even as the temperature outside somatic and autonomic components of the nervous system.
758 SECTION 35.4 N E RVO U S S YS T E M O RG A N I Z AT I O N

FIG. 35.19 The knee-extension reflex. The knee-extension reflex involves a sensory neuron, a single synapse, and a motor neuron.

Spinal cord

3
2 A stretch receptor in an
The sensory neuron synapses
with a motor neuron in the
extensor muscle responds by Stretch Extensor
spinal cord.
sending a signal along the receptor muscle
sensory nerve. 3
Neuron from
stretch receptor
5
Patella
2 4
Interneuron for
Motor neuron to reciprocal inhibition
1 extensor muscle
etched
originally stretched
of flexor muscle
Patellar 4 The motor neuron sends
tendon
an excitatory signal to the
Motor neuron to
flexor muscle
5 An inhibitory interneuron
Flexor muscle same extensor muscle, which inhibits contraction of the
responds by contracting. opposing flexor muscle.
1 A physician strikes the
patellar tendon with a reflex
hammer.

Simple spinal reflex circuits connect sensory and motor dendrite and cell body to the axon. In the spinal cord, the axon
neurons directly in the spinal cord of vertebrates. The large of the sensory neuron forms synapses with motor neurons that
Mauthner neurons of fish are an example of such a circuit. Sensory travel from the spinal cord back to the muscle where the stretch
neurons respond to threatening visual, tactile, or vibratory cues originated. The signal from the muscle stretch receptor stimulates
from the environment and transmit signals through the spinal the motor neurons to increase the activation of the muscle: the
cord to the hindbrain of a fish, activating a large Mauthner neuron muscle contracts, and the leg extends at the knee.
on the side opposite the stimulus. The Mauthner cell in turn This reflex arc does not include an interneuron: It is composed
rapidly activates motor neurons along the body, causing the fish of just two neurons and one synapse. Transmission is delayed
to bend away from the threatening stimulus to initiate a rapid by communication at synapses, so the muscle contracts quickly
escape. At the same time, the Mauthner cell and motor nerves on because only one synapse is required to relay the sensory
the other side of the fish are inhibited. The giant axons of squid information back to the muscle.
are part of a similar reflex circuit that allows squid to swim quickly As well as being a useful medical test, the knee-extension
away from a predator. reflex has a normal physiological role. In running or landing from a
The knee-extension reflex in humans is an example of a jump, the knee joint first flexes (bends), stretching the quadriceps
simple nerve circuit that includes only a single synapse between muscles. This action sends the same sensory signal to the spinal
two neurons—a sensory neuron and a motor neuron (Fig. 35.19). cord as when a physician’s hammer taps the muscle’s tendon. As
Physicians commonly use this reflex to evaluate the performance a result, after flexing, the knee reflexively extends (straightens).
of the peripheral nervous and muscular systems. The strike of the Knee flexion followed by extension allows rapid adjustments in
physician’s hammer below the kneecap activates the extensor muscle force and knee position that help stabilize the body during
muscles on the front of the thigh, including the quadriceps running and jumping.
muscle, causing the bent leg to straighten. Because muscles can only contract, opposing movements at
Let’s follow this reflex pathway. It starts with specialized a joint such as flexion and extension require flexor and extensor
“stretch” receptors located on the dendrites of a sensory neuron muscles on either side of the joint (Chapter 37). These are often
in the extensor muscles of the leg. These dendrites extend to activated out of phase (at different times) so that, when one set of
cell bodies in ganglia alongside the spinal cord. These cell bodies muscles is activated (contracting), the other is inhibited (relaxed).
have axons that extend into the spinal cord. The stretch receptors The alternating extension and flexion of a leg during walking
sense the stretch of the muscle that occurs during movement or and running provide an example. This pattern of joint and limb
in response to a physician’s strike of a reflex hammer. In response movement is achieved by reciprocal inhibition: When stretch
to a stretch, a signal is sent from the stretch receptor, through receptors of the knee extensor muscles are activated to stimulate
 CHAPTER 35 A N I M A L N E RVO U S S YS T E M S 759

these muscles to extend the knee, they also inhibit the activity of control the movement of the right and left limbs. In this case,
opposing muscles that flex the knee (Fig. 35.19). interneurons cross the spinal cord to control the timing of activity
Reciprocal inhibition of opposing sets of muscles occurs by flexor and extensor muscles of the opposite limb. Reciprocal
in the spinal cord. Axons of the stretch receptor neurons not inhibition is also involved in the Mauthner cell circuit of fish to
only synapse with motor neurons of extensor muscles, but also ensure that only one side of the fish’s body bends away from
synapse with inhibitory interneurons that inhibit motor neuron the stimulus to escape. The local spinal circuits that provide
stimulation of the opposing flexor muscles. This inhibitory reflex reciprocal inhibition are therefore fundamental to the alternating
pathway contains two synapses, one between the sensory neuron motion of the body and limbs that characterizes the movements
and interneuron and the second between the interneuron and the of most animals.
motor neuron to the flexor muscle. In other cases, more complex circuits in the brain may act
to coordinate sensory input and motor output. These circuits
j Quick Check 5 Why is an interneuron needed to provide reciprocal
integrate other sources of sensory information, such as vision and
inhibition of the flexor muscle when an extensor muscle is activated?
balance, with conscious commands from the brain to voluntarily
Reciprocal inhibition also operates between the right and control motor behavior. In the next chapter, we look more closely
left sides of the body. For example, reciprocal inhibition helps to at these sensory circuits.•

Core Concepts Summary Glial cells provide nutritional and physical support for
neurons. page 744
35.1 Animal nervous systems allow organisms to
35.3 The electrical properties of neurons allow them
sense and respond to the environment, coordinate
to communicate rapidly with one another.
movement, and regulate internal functions of the body.
Neurons have electrically excitable membranes that code
Nerve cells, or neurons, receive and send signals and are the
information by changes in membrane voltage and transmit
functional unit of the nervous system. page 740
information in the form of electrical signals called action
Animal nervous systems include three types of neuron: potentials. page 745
sensory neurons that respond to signals, interneurons
Ion channels open and close in response to changes in
that integrate and process sensory information, and motor
membrane voltage, underlying the production of action
neurons that produce a response from muscle. page 740
potentials in nerve cells. page 746
Ganglia are localized collections of nerve cell bodies that
Action potentials fire in an all-or-nothing fashion; a brief
integrate and process information. page 741
refractory period follows. page 746
Simply organized animals, such as cnidarians, have a nerve
In vertebrates, glial cells also produce the myelin sheath that
net to coordinate sensory and motor function. page 741
insulates axons, increasing the speed of nerve impulses.
Forward locomotion led to the evolution of specialized sense page 749
organs in the head, along with concentrated groupings of
Action potentials are conducted in a saltatory fashion
nerve cells to form ganglia and a brain. page 741
in myelinated axons, firing at nodes of Ranvier, where the
axon membrane is exposed and not insulated by myelin.
35.2 The basic functional unit of the nervous
page 749
system is the neuron, which has dendrites that receive
information and axons that transmit information. Communication across the synapse occurs when an arriving
action potential triggers the release of neurotransmitters
Neurons share a common organization: They have dendrites
from vesicles within the axon terminal. page 751
that receive inputs, a cell body that receives and sums the
inputs, and axons that transmit signals to other nerve cells. Neurotransmitters released from presynaptic vesicles bind to
page 743 receptors in the postsynaptic membrane, causing either an
excitatory or an inhibitory stimulus. page 752
Most neurons communicate by chemical synapses formed
between an axon terminal and a neighboring nerve or Excitatory stimuli depolarize the membrane, producing an
muscle cell. page 743 excitatory postsynaptic potential (EPSP), whereas inhibitory
760 SELF-ASSESSMENT

stimuli hyperpolarize the membrane, producing an inhibitory 3. Diagram and label the basic features of a neuron,
postsynaptic potential (IPSP). page 752 indicating where information is received and where it is sent.
4. Graph an action potential, showing the change in
35.4 Animal nervous systems can be organized into electrical potential on the y-axis and time on the x-axis.
central and peripheral components. Indicate on the graph the phases when voltage-gated Na1 and
Animal nervous systems are organized into central and K1 ion channels are opened and when they are closed.
peripheral components called the central nervous system (CNS) 5. Explain what is meant by saying action potentials are
and peripheral nervous system (PNS). page 754 “all-or-nothing.”
The central nervous system includes the brain and one or 6. Explain why action potentials propagate along an axon
more main trunks of nerve cells, such as the spinal cord. The only in a single direction.
peripheral nervous system is distributed throughout the animal’s
7. Create a table that lists the sequence of events that
body and is composed of sensory and motor nerve cells.
create an action potential, identifying the membrane potential
page 754
at each stage, which channels are opening or closing, and in
In many invertebrates and vertebrates, the peripheral nervous which direction ions are moving across the cell membrane.
system is divided into voluntary and involuntary components.
8. Briefly describe how myelinated axons increase the speed
page 755
of signal transmission.
In vertebrates, the voluntary component is referred to as the
9. Diagram a chemical synapse, labeling the vesicles that
somatic nervous system, and the involuntary component is
contain neurotransmitter molecules and the receptors that
referred to as the autonomic nervous system. page 755
bind the neurotransmitter to produce either an inhibitory or
The autonomic system regulates body functions through an excitatory stimulus in the postsynaptic cell.
opposing actions of the sympathetic and parasympathetic 10. Describe how neurotransmitter binding to receptors on a
divisions. page 756 postsynaptic cell causes inhibition or excitation.
The nervous system helps to regulate physiological functions to 11. Describe how temporal summation of EPSPs from two
actively maintain stable conditions inside a cell or an organism, a presynaptic neurons results in a larger depolarization of
process known as homeostasis. page 756 the postsynaptic cell compared with how an inhibitory
Homeostasis is often achieved by negative feedback, in which presynaptic neuron causes an IPSP that negates the EPSP of
the response inhibits the stimulus. page 757 an excitatory neuron, and how spatial summation of three
excitatory presynaptic neurons produces an even larger
Simple reflex circuits can involve as few as two neurons: a depolarization of the postsynaptic neuron.
sensory neuron from the periphery that synapses with a motor
12. List which functions of an animal are controlled by voluntary
neuron in the spinal cord that sends a signal to a muscle.
and by involuntary components of the nervous system.
page 757
13. Diagram a simple circuit that includes a sensory neuron that
synapses with a motor neuron to produce a reflex. Indicate
Self-Assessment where in the nervous system this synapse is found.

1. Diagram a simple nervous system of an animal that lacks

cephalization and compare that system with the general Log in to to check your answers to the Self-
organizational features of a nervous system that exhibits Assessment questions, and to access additional learning tools.
cephalization.
2. Name the three basic types of neuron and describe their
functions.
 CHAPTER 36

Animal Sensory
Systems and
Brain Function

Core Concepts
36.1 Animal sensory receptors
detect physical and chemical
stimuli by changes in
membrane potential.
36.2 Specialized chemoreceptors
relay information about smell
and taste.
36.3 Hair cells convey information
about gravity, movement, and
sound.
36.4 The ability to sense light and
form images depends on
photosensitive cells with light-
absorbing proteins.
36.5 The brain processes and
integrates information from
multiple sensory systems, with
tactile, visual, and auditory
stimuli mapped topographically
in the cerebral cortex.
36.6 Cognition is the ability of
the brain to process and
integrate complex information,
remember and interpret past
events, solve problems, reason,
and form ideas.
Custom Medical Stock Photo—All rights reserved.

761
762 SECTION 36.1 A N I M A L S E N S O RY S YS T E M S

To see, your eyes detect light, which is a form of electromagnetic These cells either communicate with neurons, as for taste and
radiation. To hear, your ears detect sound waves. To smell, your sight, or are themselves neurons, as for smell. Note that the short-
nose detects odor molecules present in the air. In all these cases, hand term “sensory receptor” refers to the entire sensory cell or
your sense organs are detecting a physical or chemical stimulus sensory neuron, not just the membrane protein.
in the environment. The first half of this chapter explores how Sensory receptors represent a key cellular unit of the sensory
specialized sensory cells detect these signals and code them as component of animal nervous systems. In most multicellular
information that can be transmitted and processed by the nervous animals, the sensory receptors are organized into specialized
system. How is a dog able to distinguish thousands of different sensory organs that convert particular physical and chemical
odors? How is a hawk able to see a small rodent far below? How are stimuli into signals that are communicated to the brain. Cnidarians
humans able to detect small differences in the pitch of a sound? (including jellyfish, corals, and anemones) and roundworms
The second half of the chapter follows nervous system (including the laboratory organism Caenorhabditis elegans) evolved
pathways from these sensory cells to the brain, on the way sensory receptors that sensed physical contact, and cnidarians and
exploring basic principles of brain function. Processing of sensory flatworms were among the first multicellular animals to evolve
information in the brain takes place in regions specialized for simple light-sensing organs.
each sense, and within many of these regions information is The conversion of physical or chemical stimuli into nerve
represented in the form of topographic maps. impulses is called sensory transduction. For example, receptors
located in the ear convert the energy of sound waves into nerve
impulses that allow an animal to distinguish loud versus soft
36.1 ANIMAL SENSORY SYSTEMS sounds and high-pitched versus low-pitched sounds. Although the
sense organs of different animals share many similar properties,
Animals can sense the physical properties of their environment, differences have also evolved. Consequently, different animals
including light, chemicals, temperature, pressure, and sound, that perceive the world differently. Many insects, for instance, are
are useful in finding mates and food and avoiding predators and sensitive to ultraviolet light; nocturnal snakes can see at night
noxious environments. Early in evolutionary history, organisms by sensing infrared radiation; and dogs can distinguish odor
evolved specialized protein receptors located in the cell membrane compounds as much as 100 million times lower in concentration
that were able to detect these critical features in their environment. than humans can detect (Fig. 36.1).
For example, before the evolution of nerve cells and a nervous
system, bacteria evolved membrane receptors that sensed osmotic Specialized sensory receptors detect diverse stimuli.
pressures that might otherwise rupture their cell membrane. Animals have evolved a diverse array of sensory receptors that
Bacteria and sponges also evolved membrane receptors that detect respond to different stimuli, among them touch, light, and the
chemicals—including nutrients—in their environment. oscillations in air pressure we call sound waves. How is a physical
The senses of smell, taste, and sight in multicellular organisms phenomenon in the world outside an organism transformed into
with a nervous system also rely on membrane receptors. These a nerve impulse? The initial transformation takes place inside
receptors are embedded in specialized sensory receptor cells. the sensory receptor (Fig. 36.2). A physical or chemical stimulus

FIG. 36.1 Diverse sensory receptors. (a) Many insects can detect ultraviolet light. (b) Rattlesnakes can see at night by detecting infrared radiation.
(c) Dogs have a keen sense of smell. Sources: a. Russell Burden/Getty Images; b. Andy Hunger/age fotostock; c. Monika Ondrušová/Dreamstime.com.

a b c
 CHAPTER 36 A N I M A L S E N S O RY S YS T E M S A N D B R A I N F U N C T I O N 763

FIG. 36.2 Sensory transduction, which converts an external stimulus into a change in membrane potential. (a) Chemoreceptors are located
in the antennae of the luna moth; (b) mechanoreceptors below the cuticle of a roundworm; and (c) photoreceptors in the eyes of a
vertebrate. Photo sources: a. Rolf Nussbaumer Photography/Alamy; b. Sinclair Stammers/Science Source; c. Eyal Nahmias/Alamy.

a. Chemoreceptors b. Mechanoreceptors c. Vertebrate photoreceptors

Pressure

A molecule in the
environment Light
Na+
Cuticle

Signal to Signal to
open ion close ion
channel channel

Protein Protein
receptor receptor
Cell is Cell is Cell is
depolarized. depolarized. hyperpolarized.

is converted into a change in the sensory receptor’s membrane receptors in their cell membranes that respond to molecules in the
potential. Recall from Chapter 35 that a cell’s membrane potential environment.
is the electrical charge difference between the inside and the In animals, chemoreceptors are sensory receptor cells that
outside of the cell membrane, and that a depolarization in respond to molecules that bind to specific protein receptors on
membrane potential is the first step in firing an action potential. the cell membrane (Fig. 36.2a). Many animals detect food in
Sensory receptors either fire action potentials themselves or their environment by sensing key molecules such as oxygen
synapse with neurons that fire action potentials, which are then (O2), carbon dioxide (CO2), glucose, and amino acids. Female
transmitted to the central nervous system. mosquitoes track CO2 levels to detect prey for blood meals,
In many cases, the sound wave, touch, or other stimulus and coral polyps respond to simple amino acids in the water,
causes ion channels in the cell’s plasma membrane to open. The extending their bodies and tentacles toward areas of greater
influx of ions changes the membrane potential by altering the concentration to feed. Other arthropods, such as flies and crabs,
distribution of charged ions on either side of the membrane. Let’s have chemosensory hairs on their feet. These animals taste
look at examples of how three different types of stimuli from the potential food sources by walking on them. Salmon rely on
environment—molecules, touch, and light—can be transformed chemoreception to detect chemical traces of the home waters of
into a nerve signal. the river where they hatched, and where they will return to mate
and spawn.
Chemoreceptors are universally present in animals. Chemoreception underlies the sense of smell and taste.
The most ancient type of sensory detection is chemoreception. All Consider taste as an example. In most cases, the binding of
organisms respond to chemical cues in their environment. Even molecules to a protein receptor on a taste receptor causes
the earliest branching groups of Bacteria and Archaea have protein the protein receptor to change conformation. That change in
764 SECTION 36.1 A N I M A L S E N S O RY S YS T E M S

conformation in turn triggers the opening of Na1 channels receptors for balance, gravity sensing, and hearing, discussed in
through G protein signal transduction pathways (Chapter 9). The section 36.3.
influx of Na1 ions depolarizes the receptor cell. In the case of taste
receptors, no action potential fires, but the depolarization travels Electromagnetic receptors sense light,
far enough down the receptor’s short axon to trigger the release of thermoreceptors sense temperature, and nociceptors
neurotransmitters. Other chemoreceptors, such as those involved sense pain.
in the sense of smell or located on antennae to detect chemicals in Electromagnetic receptors are sensory cells that respond to
the air, are sensory neurons that do fire action potentials. electrical, magnetic, and light stimuli. Of these, light-detecting
photoreceptors are the most common and diverse. Most animals
Mechanoreceptors are a second general class sense light in their environment. Photoreceptors have been
of ancient sensory receptors. common since the first ones evolved in cnidarians (jellyfish) and
Mechanoreceptors respond to physical deformations of their ctenophores (comb jellies) more than 500 million years ago.
membrane produced by touch, stretch, pressure, motion, and Light-detecting photoreceptors are the sensory receptors in
sound. Mechanoreceptors are found in all multicellular animals, eyes (Fig. 36.2c). Photoreceptors respond to individual photons of
and even bacteria have pressure-sensitive protein receptors in light energy. In most invertebrates, light causes Na1 channels to
their cell membrane. In both bacteria and multicellular organisms, open, depolarizing the cell. By contrast, vertebrate photoreceptors
evidence suggests that the protein receptor is also a sodium ion close Na1 channels in response to light, causing the cell to become
channel. Thus, deformation of the receptor membrane opens hyperpolarized. Most receptors excite neurons that they synapse
sodium channels directly, causing a depolarization of the endings with, but vertebrate photoreceptors are unusual in that they can
of the cell’s dendrites (Fig. 36.2b). Early pressure-sensitive protein either excite or inhibit neurons in the eye. Photoreception in the
receptors in bacteria sensed internal cell pressure: When water vertebrate eye is discussed in section 36.4.
moving into the cell by osmosis increases the risk of bursting Some fish, such as catfish, contain specialized
(Chapter 5), the stretching of the membrane opens channels that electroreceptors arranged in a lateral line along their bodies.
let water leave the cell. Other mechanoreceptors in roundworms Electroreceptors enable these fish to detect weak electrical signals
and anemones are linked to externally projecting cilia that sense emitted by all organisms. They likely evolved as an adaptation for
forces at the animal’s body surface. locating prey or potential predators in poorly lit habitats where
One well-studied example of a mechanoreceptor is a vision was less useful. Some specialized “weakly electric” fish
touch receptor in the roundworm Caenorhabditis elegans. actually generate an electromagnetic field by emitting pulses
Roundworms, also called nematodes, are one of the most diverse from an electric organ located in the tail. Disturbances in the
and widespread groups of animals on Earth. Deformation of the field detected by electroreceptors of the lateral line system signal
surface of the worm—its cuticle—exerts pressure on proteins the location of nearby prey. These fish also inhabit rivers with
connected to an ion channel. The mechanical force changes the poor visibility. The bill of the duckbilled platypus also contains
shape of the ion channel, causing it to open and produce a change electroreceptors that locate prey in dimly lit water.
in membrane potential. Finally, specialized receptors with dendritic branches in the
A mechanoreceptor in humans and other mammals is the skin can respond to heat and cold (thermoreceptors) or to pain
sensory receptor found in the skin that senses touch and pressure. (nociceptors). Thermoreceptors help to control an animal’s
The cell bodies of these neurons are located in ganglia near the metabolism, and they also regulate body temperature by controlling
spinal cord, with extensions going in two directions: to the skin patterns of blood flow that in turn alter rates of heat gain and loss
and to the spinal cord. In the skin, the neuron has branched (Chapter 40). As a result, they help to maintain homeostasis. Pain
tips containing ion channels sensitive to deformations of the receptors send action potentials to the brain or spinal cord when
membrane. These branched tips are the initial sensors of touch exposed to excessive heat, force, or chemical damage. A quick
and pressure. If the stimulus is strong enough, local depolarization withdrawal from the painful stimulus is the typical response.
leads to the firing of an action potential that travels all the way
to the spinal cord. Thus, in contrast to taste chemoreceptors, this Stimuli are transmitted by changes in the firing rate
type of mechanoreceptor transmits an action potential. The of action potentials.
axon is much too long for simple depolarization to spread to its Sensory reception depends on converting the energy of a physical
other end. or chemical stimulus into an action potential, either in the sensory
Mechanoreceptors are also found at the base of whiskers receptor itself or in the neuron it synapses with. How do these
that rodents, cats, dogs, and other mammals use to sense touch nerve impulses convey information to the brain? In effect, action
with their snouts. Stretch receptors found in muscles are also potentials can be considered a code that the brain deciphers.
mechanoreceptors that influence a muscle’s motor activation, To convert information from the environment into a code,
helping to control its length and force. A very different group of the nerve impulses conveyed by sensory organs carry out the
specialized mechanoreceptors called hair cells are the sensory following functions:
 CHAPTER 36 A N I M A L S E N S O RY S YS T E M S A N D B R A I N F U N C T I O N 765

are received repeatedly over time by

FIG. 36.3 Sensory receptor signals. The firing rate of action potentials increases with (a) the the same sensory cell. When a sensory
intensity of the stimulus and (b) a novel compared to a continuous stimulus. cell is stimulated more frequently, the
excitatory postsynaptic potentials (EPSPs)
a.
sum, and the cell is more likely to become
depolarized above threshold and fire an
action potential.
Sensory receptors also distinguish
Action potential between discrete (intermittent) and
Membrane

firing rates continuous stimuli, providing a way to

potential

correlate with
the intensity of ignore background noise. Novel stimuli
the stimulus. are generally the most important for an
Low firing rate High firing rate
animal. Consequently, sensory receptors
initially respond most strongly when a
b.
stimulus is first received (Fig. 36.3b). If
the stimulus continues over a longer time
period, sensory receptors typically reduce
their firing rate through a process called
Adaptation to adaptation (this is not the adaptation that
Membrane

continuous
potential

results from natural selection). Adaptation

stimuli reduces
the firing rate of firing rate enables an animal’s sensory
over time. receptors to focus on novel stimuli, which
High firing rate Initially high firing rate
are those likely to be most important. You
can experience adaptation by touching
your skin lightly at a particular location.
1. Convey the strength of the incoming signal—for example, Initially, skin mechanoreceptors fire in response to the touch, but
the brightness of light, the loudness of a sound, or the after a while their firing rate diminishes and the sensation of a touch
intensity of an odor. becomes less noticeable. Adaptation also ensures that the clothes
2. Carry information about even weak signals when necessary. touching your skin do not cause an irritating and distracting long-
lasting sensation.
3. Convey the location of a signal’s source—for example,
Sensory receptors locate a signal’s source by lateral inhibition
where the body is touched.
(Fig. 36.4). Lateral inhibition enhances the strength of a sensory
4. Filter out unimportant background signals. signal locally but diminishes it peripherally. Sensory receptors
have multiple extensions. Some make contact with nearby
Because nerve cells transmit action potentials in an all-or- interneurons, while others make contact with interneurons
nothing fashion, information about the strength of the signal is farther away, in adjacent regions. In lateral inhibition, receptors
coded by the number of nerve cells involved and the nerve cell’s send inhibitory signals to adjacent regions of the sensory organ,
firing rate, which is the number of action potentials fired over a and send excitatory signals to interneurons within their local
given period of time. Most commonly, weak signals produce low or region. This increases the contrast in signal strength between
background firing rates and strong signals produce high firing rates adjacent regions. For example, lateral inhibition in the retina of
(Fig. 36.3a). For example, the firing rate of mechanoreceptors the eye enhances edge detection and sharpens image contrast.
increases as the tactile pressure they sense increases. Lateral inhibition in the skin enables the sense of touch to be
Several mechanisms increase a nerve cell’s sensitivity to even localized to a small region of the body surface. The inhibitory and
weak signals. Following sensory transduction, the signal typically excitatory signals are transmitted to the interneurons by synapses
undergoes initial processing. As discussed in Chapter 35, sensory that have different neurotransmitters.
stimuli may be summed over space or time. Signals from multiple We have not yet described all the information in the code. How
sensory receptors often converge onto a neighboring neuron, which do action potentials convey pitch or color or a particular odor? This
increases its firing rate proportionally to the number of signals information is conveyed by the identity of the particular sensory
received. This is an example of spatial summation. Combining the receptors that are activated. Sensory organs are constructed
synaptic input increases the receptor organ’s sensitivity to weak so that, for example, tone A activates “tone A” sensory neurons
external signals. in the ear, red light activates “red” sensory neurons in the eye,
Another way to increase sensitivity to external signals is and the fragrance of vanilla activates “vanilla” sensory neurons
through temporal summation, summing sensory stimuli that in the nose. The way in which the “correct” sensory receptor is
766 SECTION 36.2 S M E L L A N D TA S T E

Smell and taste depend on chemoreception of

FIG. 36.4 Lateral inhibition. The firing rate of action potentials is
molecules carried in the environment and in food.
higher in the center of the sensory receptive field than in
The sense of smell in mammals depends on specialized sensory
surrounding regions.
neurons that line the nasal cavity (Fig. 36.5). These neurons
have long, hairlike extensions containing membrane receptors
that project into the nasal mucus. Odor molecules are captured
by the mucus during inhalation and bind to membrane receptors.
Each sensory receptor has one type of membrane receptor, but
Mechanoreceptors there are typically many distinct sensory receptors, each detecting
a different molecule. Humans have nearly 1000 genes (about 5%
of our genome) that express particular odorant receptor proteins.
Mice have about 1500 genes that express odorant receptor proteins
Interneurons (about 7% of their genome). The olfactory system can detect specific
odor molecules in quite low concentrations.
When bound to odor molecules, the sensory receptors
produce excitatory postsynaptic potentials (EPSPs). If enough
Background Background odor molecules bind to the receptor cell, the EPSPs are summed
Lateral inhibition of and transmit an action potential to an olfactory bulb interneuron
sensory receptor cells that receives information only from sensory receptors sensitive
Membrane

enhances edge and

potential

border detection by to a particular odor molecule. The interneurons in the olfactory

reducing excitation bulb, in turn, transmit the combined information from different
of adjacent
High firing rate interneurons.
chemoreceptors to the brain. These neurons communicate directly
with the brain by the olfactory nerve, one of the cranial nerves
(Chapter 35).
Excitatory Inhibitory Interneuron Taste is achieved by clusters of chemosensory receptor cells
synapse synapse output located in specialized taste buds, the sensory organs for taste
(Fig. 36.5). Some fish have taste buds in their skin that sense
amino acids in the water, helping them to locate food. The taste
buds of terrestrial vertebrates are in the mouth; most of the taste
excited varies for each of the senses. In the next three sections, buds in mammals are on the tongue. In humans, the tongue
we discuss how this happens for smell and taste, hearing, and has about 10,000 taste buds, which are contained within raised
vision. structures called papillae, giving the tongue its rough surface. Each
taste bud has a pore through which extend fingerlike projections
j Quick Check 1 Aspirin and ibuprofen reduce pain by inhibiting
called microvilli that contact food items. The microvilli provide
the synthesis of chemicals called prostaglandins. What effect
a large surface area that is rich in membrane receptors. These
would you predict prostaglandins to have on the firing rate of
receptors bind to specific chemical compounds that give the food
nociceptors?
its taste. Taste bud sensory cells synapse with afferent neurons,
which fire action potentials when a sufficient number of sensory
cells are depolarized by a particular food item. Compared with
36.2 SMELL AND TASTE olfaction, taste generally requires higher concentrations of
food chemicals coming into contact with taste bud receptors to
In many animals, specialized sensory organs for smell (olfaction)
stimulate receptor depolarization.
and taste (gustation) have evolved that detect a variety of
Human taste can be divided into five categories—sweet,
chemical compounds. These organs contain distinct types of
sour, bitter, salty, and savory. The tongue contains only five types
protein membrane receptor, and each type binds to a specific class
of taste receptor, each specialized for one of these categories.
of molecule. A large number of genes code for different chemical
Variation in taste is achieved by combining the signals received by
odor receptors, and the presence of so many receptors enables the
receptors in the five categories. Your sense of taste is substantially
detection of broad categories of smell and taste.
enhanced by airborne odors that emanate from a food or beverage
In contrast to the senses of hearing and sight, the sensory
and are sensed by chemoreceptors in the nose. Signals from taste
receptors of smell and taste do the “work” of detecting an odor or
bud and olfactory receptors are combined in the brain, eliciting the
taste by themselves, without other structures being necessary. All
subtle sensation of a particular flavor.
that is needed is exposure to the environment and protection from
damage. Both are achieved by the placement of chemoreceptors in j Quick Check 2 Why is your sense of smell and taste diminished
the nasal and mouth cavities. when you have a cold?
 CHAPTER 36 A N I M A L S E N S O RY S YS T E M S A N D B R A I N F U N C T I O N 767

FIG. 36.5 The senses of smell and taste. Olfactory receptors respond to chemical odors in the nasal passages and communicate directly with the
brain. Taste cells are chemosensory receptors located in taste buds.

Olfactory
3 Interneurons integrate
nerves the odorant information
received by olfactory
receptors before sending
Interneuron it to the brain.

Interneuron
dendrites 2 Action potentials
produced in response to
Nasal the binding of odorants
bone to membrane receptors
are sent to the olfactory
Axon interneurons.

Olfactory 1 Olfactory sensory

sensory neurons sense
Microvilli Olfactory neuron odorants that bind
epithelium to specific receptors
on chemosensitive
Supporting hairs that project
cell into the mucus.
Taste Mucus Chemo-
sensory layer sensitive
cell hair

Sensory
neurons
A taste bud

of sound or body orientation. Thus, for hair cells to provide

36.3 GRAVITY, MOVEMENT, information about mechanical stimuli, they must interact with
AND SOUND other structures within the sensory organ in which they reside.
These more elaborate structures within the sensory organ
The ability to orient with respect to gravity, to detect motion, influence how individual hair cells respond to physical motion,
and to hear all depend on specialized mechanoreceptors called influencing the information they provide.
hair cells that sense movement and vibration. They are found
in fishes and amphibians, in which they detect movement of the Hair cells sense gravity and motion.
surrounding water, and in many invertebrates, in which they sense Hair cells detect gravity and motion in many animals. Hair cells
gravity and other forces acting on the animal. They are also found contained in the lateral line system of fishes and sharks sense
in the ears of terrestrial vertebrates, in which they sense sound, water vibrations that indicate nearby threats or prey, as well
body orientation, and motion. as their own motion (Fig. 36.6a). The lateral line is a sensory
In all these cases, hair cells sense mechanical vibrations. These organ along both sides of the body that uses hair cells to detect
vibrations move small, nonmotile, hairlike projections from the movement of the surrounding water.
surface of the hair cell called stereocilia. In turn, the motion A sense of gravity helps animals to orient their body within
of stereocilia causes a depolarization of the cell’s membrane by the environment, providing a sense of “up” and “down.” Even
opening or closing ion channels. Despite their name, stereocilia buoyant aquatic animals depend on a sense of gravity to keep
are more similar to microvilli than to cilia since cilia can move oriented to the water’s surface or within the water column. Gravity-
on their own but stereocilia cannot. Hair cells themselves do sensing organs called statocysts are found in most invertebrates
not fire action potentials but instead, when depolarized, release (Fig. 36.6b). The statocyst is an internal chamber lined by hair cells
neurotransmitters that alter the firing rate of adjacent neurons. with stereocilia that project into the chamber. Small granules of
Unlike chemoreceptors, hair cells are generally similar in sand or other material form a dense particle called a statolith that
structure; there is not a specific hair cell for a particular pitch is free to move within the statocyst organ. By pressing down on hair
768 SECTION 36.3 G R AV I T Y , M OV E M E N T , A N D S O U N D

the hair cells to activate sensory neurons. The brain interprets

FIG. 36.6 Motion and gravity sensation. (a) Hair cells within the differences in the motion of the hair cells among the three
lateral line system of fish sense water vibrations for motion semicircular canals to resolve angular motions of the head. As a
and prey detection. (b) Gravity-sensing organs (statocysts) result, mammals and birds react much faster to head motion than
in lobsters provide cues about body orientation. to visual cues, and this ability helps them to stabilize their gaze.
a. Lateral line
Lateral line j Quick Check 3 Why are you unstable when you try to walk after
you spin in place or take a ride on a merry-go-round?

Hair cells detect the physical vibrations of sound.

What our ears interpret as sound are alternating waves of pressure
traveling through the air. The frequency of the waves—the
number of waves per second—determines the pitch of a sound,
and the amplitude, or height, of the waves determines its
loudness. Sound waves cause the stereocilia of hair cells to bend,
b. Statocyst and the hair cells become excited, depolarizing and releasing

Stereocilia FIG. 36.7 The vestibular system. The vestibular system senses
angular rotations of the head from movements of fluid
within three semicircular canals that deflect hair cells. It
Hair detects gravity and posture by means of statoliths in two
cells statocyst chambers.

Resting Rotating

The statolith moves

and depresses hair
cell stereocilia,
providing gravity
detection.

Gelatinous Pressure exerted

mass by fluid
Sensory
neurons Semi-
circular
canal

Stereocilia
cells at the “bottom” of the chamber, the statolith activates those
cells. Statoliths help anemones and jellyfish orient themselves Hair
cell
in the water and direct their tentacles. Statoliths of lobster and
crayfish can be replaced by magnetic particles and subjected to an
Sensory nerve Support cell
experimental magnetic field that repositions the statolith to the
top or sides of the chamber. In this case, the lobsters and crayfish
respond by swimming upside down or on their sides.
Semicircular
The mammalian inner ear contains organs that sense motions canals
of the head and its orientation with respect to gravity. These
organs make up the vestibular system (Fig. 36.7), which consists
of two statocyst chambers and three semicircular canals. These
statocyst chambers are similar to those of invertebrates, providing
a sense of gravity and body orientation with respect to motion.
Hair cells located within the semicircular canals sense angular
motions of the head in three perpendicular planes, providing a
sense of balance. When the head rotates, gelatinous fluid in the
Cochlea
semicircular canals is accelerated, deflecting the stereocilia of Statocyst chambers
 CHAPTER 36 A N I M A L S E N S O RY S YS T E M S A N D B R A I N F U N C T I O N 769

FIG. 36.8 The human ear. (a) The tympanic membrane and bones of the middle ear transmit external sound vibrations to the oval window of the
inner ear. (b) Hair cells in the cochlea respond to the vibrations, sending signals to the brain that distinguish sound amplitude and pitch.
(c) A scanning electron micrograph shows the V-shaped arrangement of stereocilia on hair cells. Photo source: SPL/Science Source.

a. Human ear b. Cochlea

Semi- Vestibulocochlear nerve: Upper canal
Temporal bone circular Vestibular branch Cochlear duct
Malleus Incus Stapes canal Cochlear branch Organ of Corti

Cochlear branch of
vestibulocochlear
nerve

Basilar
Cochlea membrane Lower canal
External
auditory
canal
Tectorial
Pinna Oval membrane
window Stereocilia
Tympanic Middle Hair cell
membrane ear cavity
(eardrum)

Outer ear
Middle ear Basilar membrane Sensory
neurons
Inner ear
c. Hair cells

neurotransmitters. The process seems simple, but how can the

bending of stereocilia produce all the variations in pitch and
loudness you hear in the world around you? Hair cell
Insects hear by sensing airborne vibrations with small hairs
on their antennae or other regions of their body. Hairs of different
Stereocilia
lengths detect different frequencies because the shorter hairs
are stiffer than the longer hairs and vibrate at higher frequencies.
Frequency is measured in hertz (Hz), or cycles per second. Male
mosquitoes have small antennal hairs that vibrate in response
to the 500-Hz hum of a female mosquito’s flight, which attracts
them to the female to mate. up to 80,000 Hz. Birds generally hear in the same frequency
Many insects also have specialized ears that sense sound by range as mammals, which is one reason bird songs are pleasing
means of a tympanic membrane. The tympanic membrane to our ear. Frogs and reptiles generally hear at lower frequencies
is a thin sheet of tissue at the surface of the ear that vibrates in than mammals and birds. Vertebrate ears can also make fine
response to sound waves, amplifying airborne vibrations. The distinctions between close frequencies and can detect softer
vibration excites hair cells attached to the inside of the tympanic sounds because external structures amplify the sounds before they
membrane, and these hair cells in turn excite other neurons to fire reach the hair cells.
action potentials. Crickets and other singing insects use their ears The ears of mammals have an external structure, the
to sense the chirping or buzzing of potential mates. pinna, which enhances the reception of sound waves contacting
Hearing is most widespread and developed in terrestrial the ear. Notice the heightened sensitivity that this structure
vertebrates. The ears of amphibians, reptiles, and birds have an provides for hearing when you cup your hands behind your ears.
external tympanic membrane that vibrates when sound waves It is no coincidence that dogs and rabbits perk up their ears when
strike its surface. This membrane is similar to the tympanic alerted by a sound: This action orients them to the direction of the
membrane of insects but evolved independently and therefore is sound’s source.
an example of convergent evolution. Terrestrial vertebrate ears Fig. 36.8 shows the structure of the human ear. The pinna
can detect a broad range of sound frequencies. For humans, the is part of the outer ear, which also includes the ear canal and
range of audible frequencies is from 20 to 20,000 Hz. For dogs, tympanic membrane, or mammalian eardrum, which transmits
the audible range is 70 to 44,000 Hz, and some rodents can hear airborne sounds into the ear. The middle ear contains three small
770 SECTION 36.3 G R AV I T Y , M OV E M E N T , A N D S O U N D

bones, or ossicles, called the malleus,

incus, and stapes, which amplify the FIG. 36.9 Activation of hair cells. Upward movement of the basilar membrane pushes hair cells
waves that strike the tympanic membrane. into the tectorial membrane, causing bending of the stereocilia.
The stapes connects to a thin membrane
Tectorial membrane
called the oval window of the cochlea
1 No sound
in the inner ear. The cochlea (which vibrations are present.
means “snail” in Latin) is a coiled chamber Hair cell Pivot point for The basilar membrane
within the skull that contains hair cells that basilar membrane is immobile.
convert pressure waves into an electrical
impulse that is sent to the brain. Basilar membrane resting position
The mammalian eardrum evolved
from the tympanic membrane of reptiles 2 Sound vibrations
and earlier amphibians. The three middle push the basilar
membrane upward.
ear bones are also evolutionarily related Shear force The hair cell stereocilia
to three bones in reptiles, but only the bend against the
tectorial membrane,
stapes functions in reptiles to transmit
triggering the release
vibrations of the tympanic membrane to Upward movement of neurotransmitters.
the inner ear. In reptiles, the malleus and of basilar membrane
incus help support jaw movements during
feeding. As mammals evolved from their 3 The downward
reptilian ancestors, these two bones were motion of the basilar
membrane relative to
incorporated with the stapes to form the the tectorial
Shear force
middle ear bones that transmit sounds membrane bends
stereocilia in the
from the eardrum to the inner ear. opposite direction,
The process of hearing can be separated causing the hair cells
into three stages: to repolarize.
Downward movement
1. Amplification. Sound vibrations of basilar membrane
received by the outer ear are
transmitted by the eardrum and
amplified by the three bones in the middle ear (Fig. 36.8a). into a nerve signal. The fluid vibrations within the cochlear
Through piston-like actions, the stapes transmits the energy canals induce motions of the basilar membrane relative to the
of its movements to vibrations of the oval window of the tectorial membrane, bending the stereocilia of the hair cells
cochlea in the inner ear. Vibrations transmitted from the back and forth in localized regions of the cochlea (Fig. 36.9).
eardrum through the middle ear to the oval window are Bending of the stereocilia stimulates them to release excitatory
amplified more than 30 times because of the larger size of neurotransmitters that cause postsynaptic neurons to fire action
the eardrum compared to the oval window and the lever-like potentials. These are sensed as sound by neuronal networks in the
action of the three bones. auditory cortex, the area of the brain that processes sound.
Sound is characterized by amplitude (loudness) and frequency
2. Transfer of sound vibration to fluid pressure waves. The
(pitch). Louder sounds produce larger fluid vibrations that cause
cochlea contains fluid and an upper and a lower canal
the stereocilia to bend more, increasing their release of excitatory
separated by a basilar membrane and cochlear duct
neurotransmitters. The release of more neurotransmitters
(Fig. 36.8b). Vibrations of the oval window cause fluid
increases the firing rate of the postsynaptic sensory neuron. The
pressure waves in both canals at nearly the same time.
firing rate indicates intensity, or, in the case of hearing, loudness.
3. Mechanoreception by hair cells within the cochlea. The Many people can distinguish pure tones that differ by only
cochlear duct houses the organ of Corti, which contains a fraction of a percentage point in frequency. The ability to
specialized hair cells with stereocilia. These hair cells are discriminate different sound frequencies is largely the result of
supported by the basilar membrane. The stereocilia of differences in the mechanical properties of the basilar membrane
mammalian hair cells form a “V” and project into a rigid along its length. At the apex, the basilar membrane is widest but
tectorial membrane that does not move (Fig. 36.8c). thinnest and most flexible. At the base, the basilar membrane
is narrow but thick and stiff. Thus, the basilar membrane is
So far, we have seen pressure waves in the air transformed mechanically tuned to respond in different regions to different
into vibrations of solid structures in the ear and then into frequencies. Low-frequency vibrations excite hair cells in the apex
waves of fluid. In the cochlea, motion is finally transduced of the basilar membrane, mid-frequency vibrations excite hair cells
 CHAPTER 36 A N I M A L S E N S O RY S YS T E M S A N D B R A I N F U N C T I O N 771

in the middle region of the basilar membrane, and high-frequency

vibrations excite hair cells close to the base. 36.4 VISION
Sound amplitude and frequency, together with vestibular
Most multicellular animals have photoreceptor cells that
sensing of gravity and head motion, are transmitted by the
respond to light. Animals with simple forms of photosensitivity
vestibulocochlear nerve (another cranial nerve) to the brain.
can move toward or away from light sources. The photoreceptors
of other animals are arranged to form eyes that produce an
? CASE 7 PREDATOR–PREY: A GAME OF LIFE AND DEATH
image. The eyes of some of these animals, such as those of annelid
How have sensory systems evolved in predators worms and arthropods, are adapted to detecting the motion
and prey? of shapes and simple patterns of light. Other animals, such as
Many insect-eating bats rely on echolocation to find and vertebrates and cephalopod mollusks like squid and octopus,
apprehend their flying prey (Fig. 36.10). As it flies, a bat emits perceive sharper images.
short bursts of high-frequency sound. These sound pulses bounce Although animals have evolved a variety of light-sensing
off surrounding objects and are reflected back to the bat. The organs, all rely on the same light-sensitive protein, called opsin,
echoes are detected by the ears and processed by the brain to locate to convert light energy into electrical signals in the receptor cell.
the prey. Bats typically increase their call rate as they approach the Opsins are ancient molecules. They evolved early in the tree of
prey to locate it more accurately and judge its flight trajectory and animal life as receptor proteins sensitive to many different stimuli,
speed. The sharpness of an image is measured by its resolution, including light. They are G protein-coupled receptors. As we saw
the smallest distance between two features or objects that can be in Chapter 9, G protein-coupled receptors activate G proteins,
perceived. Bats can resolve prey and other objects at less than 1 leading to a cellular response. In this case, the cellular response is
mm, a resolution that exceeds that of the most sophisticated sonar a change in membrane potential. The observation that different
developed by human engineers. types of animal eye use a common light-sensitive protein suggests
Together with mosquitoes and other flying insects, a common evolutionary origin for animal eyes about 500 million
nocturnal moths are one of the bat’s primary food sources. In years ago in the Cambrian Period.
response to the evolution of echolocation by bats, several moths
have evolved the ability to emit sounds that interfere with the Animals see the world through different types
bat’s sonar signal. These moths are more likely to escape capture. of eyes.
The evolution of sophisticated sensory systems in both groups Although opsins are the universal photoreceptor protein among
of animals, and the brain organization that goes with it, is the animals, the way the visual world is perceived—what it looks like
result of a kind of evolutionary arms race. It illustrates how animal to the organism—depends on the structure of the eye in which
sensory systems and brains can achieve impressive detection and the receptors are embedded. Here, we look at the three main types
information-processing performance that improves an animal’s of eye structure shown in Fig. 36.11: eyecups, compound eyes,
chance of survival. and single-lens eyes. Each has very different capabilities.
In flatworms, two simple eyecups on the back surface of
the head detect the direction and intensity of light sources
FIG. 36.10 A bat about to catch a moth by high-frequency (Fig. 36.11a). Each eyecup contains photoreceptors that point up
auditory echolocation. Source: Daniel Tuitert/age fotostock. and to the left or right. A pigmented epithelium blocks light from
behind, so the photoreceptors receive light only from above and in
front of the animal. In response to light, flatworms turn to move
directly away from the light source, seeking a dark region that
hides them from potential predators. To find a dark region, the
flatworm nervous system compares the light intensity received
by photoreceptors of the two eyecups, and then moves in the
direction of lower light intensity. Many invertebrates use simple
light-sensitive photoreceptors in a similar way.
A key innovation for detecting images was the evolution of
a lens. Two major types of image-forming eye evolved in other
invertebrates: compound eyes and single-lens eyes. Insects and
crustaceans have compound eyes that consist of individual light-
focusing elements, each having a lens, called ommatidia (singular,
ommatidium) (Fig. 36.11b). The number of ommatidia in the
eye determines the resolution, or sharpness, of the image. The
compound eyes of ants have only a few ommatidia, and those of
fruit flies have about 800. In contrast, the eyes of dragonflies have
772 SECTION 36.4 VISION

FIG. 36.11 Three types of eye. Photo sources: a. David M. Dennis/age fotostock; b. Eye of Science/Science Source; c. Danté Fenolio/Science Source.

a. Eyecup b. Compound eye c. Single-lens eye

The flatworm The compound eye of Lens The

Photoreceptors Planaria uses insects, such as the single-lens
simple Light common housefly, are eye of a squid
photoreceptors composed of Light focuses light
and a pigmented hundreds of on a retina
Light epithelium to ommatidia, each with and allows for
sense the direction Lenses a lens, that individually a high degree
Pigment
of light. Photoreceptors sense light. Photoreceptors of acuity.
layer

10,000 or more. Dragonflies are predators, and their high number of perceive ultraviolet light. Many pollinating insects sense UV light
ommatidia is an adaptation for visually tracking their prey. to locate the flowers they prefer to visit for nectar (Fig. 36.12).
Within a single ommatidium, light is focused through a The single-lens eyes of vertebrates and cephalopod mollusks
lens onto a central region formed from multiple overlapping like squid and octopus work like a camera to produce a sharply
photoreceptors. Each ommatidium is sensitive to a narrow angle of defined image of the animal’s visual field (see Fig. 36.11c). In spite
light (about 1° to 2°) in the animal’s visual field. Compound eyes of of their similar outward appearance, there is clear evidence that
insects and crustaceans provide a mosaic image because individual single-lens eyes evolved independently in vertebrates and in some
light regions are sensed by separate ommatidia. It is likely that the mollusks (Fig. 36.13). Other invertebrates, such as some spiders
image is sharpened within the brain, but the resolution of images and annelid worms, also evolved single-lens eyes independently.
produced by compound eyes is not nearly as good as that produced An advantage of single-lens eyes compared to compound eyes
by single-lens eyes. Nevertheless, compound eyes are extremely is that the single lens can focus light rays on a particular region
good at detecting motion and rapid flashes of light, more than of photoreceptors, improving both image quality and light
300 per second compared with 50 per second for a human eye. sensitivity. As a result, animals with single-lens eyes can detect
Insect eyes also have good color vision, and many insects can prey effectively by their motion and shape.

FIG. 36.12 The same flower seen with visual and UV light. FIG. 36.13 Convergent evolution of single-lens eyes. Single-lens
Many pollinating insects see ultraviolet light, which eyes evolved independently in vertebrates (fish and
can dramatically alter the image that attracts them to a mammals) and in some mollusks (octopus and squid).
particular flower. Source: Bjorn Rorslett/Science Source.
Fish Mammals Clams Snails Octopus, squid
Seen with visual light Seen with UV light Single-lens
eyes

Vertebrates Mollusks

Single-lens eyes Compound eyes

 CHAPTER 36 A N I M A L S E N S O RY S YS T E M S A N D B R A I N F U N C T I O N 773

vitreous humor, which makes up most of

FIG. 36.14 Anatomy of the human eye. The eyes of other mammals and of birds have a the eye’s volume.
similar organization. To form an image, the cornea and lens
bend incoming light rays, focusing them
on the retina (Fig. 36.14). In doing so, the
light rays cross, inverting the image on the
retina and changing up to down and left
Sclera to right. The retina is a thin tissue located
Choroid in the posterior of the eye. It contains
Cornea the photoreceptors and nerve cells that
Retina
Aqueous sense and initially process light stimuli.
humor Fovea Humans, other mammals, and birds focus
Iris (region of
sharpest light on the retina by changing the shape
Pupil
acuity) of the lens (Fig. 36.15). When we focus on
Lens
objects at close range, as when we read a
Optic book, the ciliary muscles contract, making
nerve the lens more rounded to bend the light
rays more. When we focus on distant
Ciliary Retinal
muscle objects, the ciliary muscles relax, allowing
blood
vessels the lens to flatten, reducing the bending
of light. In contrast, the eyes of octopus
and squid focus by moving the lens back
and forth within the eye, as a camera does,
Vitreous humor rather than by changing its shape.

We focus much of our subsequent discussion on the human

eye, but many of its organizational features and properties apply to
vertebrates more generally. FIG. 36.15 Focusing an image. The eyes of humans and other
mammals and of birds focus the light of an image on the
The structure and function of the vertebrate eye retina by changing the shape of the lens.
underlie image processing.
Vertebrate eyes are protected within the eye socket of the When focusing on close objects, the ciliary
skull. The anatomy of the eye is shown in Fig. 36.14. The eye is muscles contract, making the lens more round
and increasing the bending of light rays.
surrounded by the sclera, a tough, white outer layer. Below the Retina
sclera, a thinner layer, called the choroid, carries blood vessels to Lens
nourish the eye. Mucus secretions produced over the sclera keep
Fovea
the eye moist. The portion of the sclera in the front of the eye is
the cornea, which is transparent. Light passes through the cornea,
enters the eye through an opening called the pupil, and then
passes through a convex lens. Small muscles called ciliary muscles
Ciliary
attach between the sclera and lens. Contraction or relaxation of muscle Optic
these muscles adjusts the shape of the lens to focus light images. nerve
At the front of the eye, surrounding the pupil, the choroid
forms a donut-shaped iris that opens and closes to adjust the
amount of light that enters through the pupil, in the manner of
the aperture of a camera. In bright light, the iris constricts to make
the pupil small. In dim light, the iris dilates to allow more light to
enter the pupil. The iris also gives the eye its color.
The focusing muscles and lens separate the eye into two
regions. The interior region in front of the lens is filled with a
clear watery liquid, the aqueous humor. This liquid is produced When focusing on distant objects, the ciliary
continuously and drained by small ducts at the base of the eye. The muscles relax, allowing the lens to flatten
and reducing the bending of light rays.
large cavity behind the lens is filled with a gel-like substance, the
774 SECTION 36.4 VISION

FIG. 36.16 Opsin bound to the light-absorbing pigment retinal. Retinal changes conformation from cis to trans when it absorbs a photon of
light. Rendered by Dale Muzzey.
Extracellular Opsin
fluid

Light wave

cis alkene
bond
cis-retinal
Retinal

trans-retinal
trans alkene
bond
Cytoplasm

Why do so many animals have two eyes? One reason is that two action potentials, but when stimulated by light they increase or
eyes provide a wider field of vision. Close one eye and you will see decrease the firing rate of neurons in the retina of the eye. That is,
the visual field of the other eye. With both eyes open, the visual the reduction in neurotransmitter triggers ESPNs in the dendrites
field is wider than the visual field of either eye alone. You’ll also of some connecting neurons, and ISPNs in others. Differences in
notice considerable overlap in the visual field of both eyes. This the firing rates of these neurons provide information about the
overlap exists in humans and other primates, as well as in other intensity and location of light.
mammals and birds of prey, and makes possible binocular vision, the
ability to combine images from both eyes to produce a single visual Color vision detects different wavelengths of light.
image. Binocular vision allows for depth and distance perception. Color vision is crucial for many invertebrate and vertebrate
Prey animals, such as some birds, antelope, and rabbits, have animals. It is achieved by photoreceptor cells called cone cells
eyes positioned more to the side, expanding their visual field up that contain opsins sensitive to different wavelengths of light
to 360°, but limiting or preventing binocular vision. Animals that (Fig. 36.17). The more numerous rod cells are also sensitive
require greater visual sharpness, such as predatory hawks, cats, to light, and most sensitive to blue-green light. Because all rod
and snakes, have eyes pointing forward, increasing their binocular cells have the same opsin (called rhodopsin), the brain interprets
field of view.

Vertebrate photoreceptors are unusual because they

hyperpolarize in response to light. FIG. 36.17 Vertebrate photoreceptors: rods and cones. Photo
In photoreceptors of the retina, opsin molecules are arranged in source: Science Source.
cylindrical groups in the plasma membrane of most photosensitive Discs (containing
cells. Each opsin protein contains a light-absorbing pigment rhodopsin)
molecule, called retinal. Retinal is a derivative of vitamin A that Discs Cytoplasm
(containing
changes conformation when it absorbs a photon (Fig. 36.16). cone opsin)
Vertebrate photoreceptor cells have leaky Na1 channels that
Cytoplasm
let some Na1 ions into the cell even at rest. As a result, their
resting membrane potential (in the dark) is less negative (about
Mitochondria Mito-
–35 mV) than that of nerve cells. When a photoreceptor is in the chondria
dark and its resting potential is –35 mV, it continuously releases Nucleus
the neurotransmitter glutamate. However, when retinal absorbs
a photon of light, it undergoes a conformational change from a cis
Nucleus
to a trans configuration, causing Na1 channels to close completely.
Without Na1 ions entering the cell, the cell membrane becomes Synaptic
hyperpolarized (rather than depolarized) and the cell’s release ending Cone Rod

of glutamate is reduced. Photoreceptors themselves do not fire Synaptic ending

 CHAPTER 36 A N I M A L S E N S O RY S YS T E M S A N D B R A I N F U N C T I O N 775

In contrast, rod cells are absent in the fovea but predominate

FIG. 36.18 Absorption spectra of the three opsins in human in the periphery, making it easier to detect motion in the
cone cells. Each cone cell expresses just one of the periphery, particularly at night. The eyes of many nocturnal
three opsins. predatory mammals, such as foxes and cats, contain mainly rod
cells, enhancing nighttime vision. These animals also have a
420 534 564 pigment located behind the retina that reflects light past the
100 photoreceptors, enhancing the ability to sense light under low-
Absorbance (%)

light conditions. The reflection of light by this pigment creates

the “eyeshine” that you see when a flashlight or a car’s headlights
shines on a nocturnal predator’s eyes at night. Great white sharks
50 are able to hunt at night because they also have a reflective
pigment within the retina.

j Quick Check 4 What is the primary role of rod cells, and what are
0 the two roles of cone cells in the retina?
400 500 600 700
Violet Blue Cyan Green Yellow Red
Wavelength (nm) Local sensory processing of light determines basic
features of shape and movement.
The rods and cones detect the intensity, color, and pattern of light
the light detected by rod cells as shades of gray, not as blue- entering the eye through the lens. However, it is their interaction
green. The human retina contains about 6 million cone cells and with a highly ordered array of neurons in the retina that begins
about 125 million rod cells. Because of their greater number and the first steps of visual sensory processing (Fig. 36.19). The
sensitivity to light, rod cells enable animals to see in low light. retina consists of five layers of cells that form a signal-processing
Together, rod and cone cells constitute approximately 70% of network just in front of a pigmented epithelium. The rods and
all sensory receptor cells in the human body, highlighting the
importance of vision in perceiving our environment.
Vertebrate cone cells likely evolved from a rod cell precursor.
FIG. 36.19 Cellular organization of the human retina. The retina
Most non-mammalian vertebrates have four types of cone cell,
is made up of five layers of cells: rods and cones,
each with a different opsin, whereas most mammals have only
bipolar cells, ganglion cells, horizontal cells, and
two types of cone cell, and two opsins. It is likely that two opsins
amacrine cells.
were lost early in mammal evolution, when mammals were small,
nocturnal, and burrowing. Old World primates, apes, and humans,
as well as some New World primates, regained trichromatic (three-
color) vision by means of gene duplication, likely in response to
selection for better ability to locate fruit, a key part of their diet. Retina
Each human cone cell therefore has one of three opsins, which
absorb light at blue, green, or red wavelengths (Fig. 36.18).
Stimulation of cone cells in varying combinations of these three
opsins allows humans and other primates to see a full range of
color (violet to red). Fish, amphibians, reptiles, and birds also have Light
good color vision, including for many the ability to see ultraviolet. Ganglion cells Bipolar cells Rod and cone cells
rays
Cone cells require higher levels of light than rod cells to
become stimulated. The low sensitivity of cone cells to light makes
it hard to detect color at night. Cone cells are most concentrated
within the fovea of the retina, the center of the visual field of
most vertebrates (see Fig. 36.15). Cone cells provide the sharpest
vision. Animals with particularly sharp vision, such as hawks and
other birds of prey, have an extremely concentrated number of
cone cells in the fovea, approaching 1 million/mm2 (compared
with about 150,000/mm2 in the human fovea). Birds of prey can
see small prey on the ground from high in the air. Many birds also
have eyes with two foveae, one projecting forward for binocular
vision and one projecting to the side to enhance the sharpness of
Optic Amacrine cell Horizontal cell Cone Rod Pigmented
their side vision. nerve epithelium
 HOW DO WE KNOW?

FIG. 36.20

How does the retina process visual information?

BACKGROUND In the 1950s, the American neurophysiologist Stephen Kuffler was interested in understanding how the retina helps
to process light information before it is sent to the brain. He focused on the activity of ganglion cells in the retina because they receive
input from the photoreceptors and bipolar cells.

EXPERIMENT Kuffler stimulated different regions of a cat’s retina with localized points of light while recording the action potentials
produced by ganglion cells.

RESULTS Kuffler found that there are two types of ganglion cell: on-center and off-center cells. On-center ganglion cells fire more
action potentials when light shines on the center of the cell’s receptive field compared to the surrounding region, and off-center cells
fire more when light is shown in the periphery and less on the center. These patterns are explained by lateral inhibition of input by the
photoreceptors and varying excitation or inhibition of bipolar cells to the ganglion cells in the retina.

On-center Off-center
ganglion cell ganglion cell

OFF region ON region

ON region Excitation OFF region Inhibition

Light in Light in
center Periphery center Periphery
Center Center

Action
Light Light
potentials

Inhibition Excitation

Periphery Periphery
Light off Light off
center Center center Center

Light Light

FOLLOW-UP WORK In the 1960s, Hubel and Wiesel found similar center–surround neural receptive fields, though with enhanced
opposition, in part of the thalamus and in the visual cortex of the brain. Cells with these fields enable cats and other mammals to detect
shapes of a given orientation moving through their visual field. Similar center–surround receptive fields have also been found in the
somatosensory and auditory cortex, highlighting the use of lateral inhibition to enhance sensory acuity and edge detection. Other studies
have found similar center–surround sensory processing in invertebrates and other vertebrates.

SOURCES Kuffler, S. W. 1953. “Discharge Patterns and Functional Organization of Mammalian Retina.” Journal of Neurophysiology 16:37–68; Hubel, D. H. 1963. “The Visual
Cortex of the Brain.” Scientific American 209:54–62.

cones are at the back of the retina, so that light must pass through on to bipolar cells, which also do not fire action potentials, but
several layers before reaching these light-sensitive cells. Because rather adjust their release of neurotransmitter in response to
they are photoreceptors, rod and cone cells hyperpolarize in the input from multiple rod and cone cells. Depending on their
response to light but do not fire action potentials. They synapse receptors, some bipolar cells are inhibited when rod and cone cells

776
 CHAPTER 36 A N I M A L S E N S O RY S YS T E M S A N D B R A I N F U N C T I O N 777

reduce their release of glutamate, whereas other bipolar cells are

excited by the reduction in glutamate release. FIG. 36.21 The skull of Phineas Gage. Phineas Gage survived an
The bipolar cells, in turn, synapse on to ganglion cells located accident in which a tamping iron passed through his
on the front of the retina. If activated, ganglion cells transmit skull, but his personality changed and his intellectual
action potentials by the optic nerve (a cranial nerve) to the capacity diminished. Source: The National Library of Medicine.
visual cortex in the brain, the part of the brain that processes
images. The optic nerve begins at the front of the retina and exits
at the back, creating an area without light-sensitive cells: This is
the blind spot. In octopus and squid, photoreceptors are in the
front of the retina, not in the back as they are in vertebrates. As a
result, octopus and squid don’t have a blind spot.
In addition to these three primary layers of neurons, two
additional sets of nerve cells combine information laterally across
the retina. Horizontal cells communicate between neighboring
groups of photoreceptors and bipolar cells, enhancing contrast and
adjusting photoreceptor sensitivity to light levels. Although their
function is less well understood, we know that amacrine cells
communicate between neighboring bipolar cells and ganglion
cells, enhancing motion detection and likely reinforcing the
adjustment of photoreceptor light sensitivity by horizontal cells.
Photoreceptor cells, bipolar cells, ganglion cells, horizontal cells,
and amacrine cells form a neural network that processes visual
information before it is sent to the brain for further processing
and interpretation. Visual signals from approximately 131 million
photoreceptors converge to approximately 1 million ganglion
cell neurons that communicate with the brain. Properties of The remainder of this chapter explores the functional
this network were first studied in the mammalian retina by the organization of the vertebrate brain. We discuss examples that
American neurophysiologists Stephen Kuffler, David Hubel, and show how the brain organizes sensory information, and how
Torsten Wiesel in the 1950s and 1960s (Fig. 36.20). Hubel and memories of sights and sounds, for example, are formed.
Wiesel shared the 1981 Nobel Prize in Physiology or Medicine for
their later work on how visual information is processed in the brain. The brain processes and integrates information
received from different sensory systems.
With the evolution of greater cognitive ability, animal
36.5 BRAIN ORGANIZATION brains became larger and more complex. Different brain regions
AND FUNCTION became specialized to carry out different functions. The vertebrate
brain is organized into a hindbrain, midbrain, and forebrain,
Up to this point, we have looked at how sensory information, with a cerebral cortex formed from a portion of the forebrain
such as smell, taste, hearing, and vision, is detected by sensory (Fig. 36.22a). The hindbrain and midbrain control basic body
receptors. In each case, the sensory information is sent to the functions and behaviors, and the forebrain, particularly the
brain, where it is processed and integrated. Much of what we know cerebral cortex, governs more advanced cognitive functions. The
about how the brain functions has been accumulated over many cerebral cortex of mammals—and particularly of primates and
years by studies of patients who suffered brain injuries. humans—is greatly expanded relative to that of other vertebrates.
A well-known example is Phineas Gage, a railroad worker In adult vertebrates, the hindbrain develops into the
who in 1848 survived severe damage to his brain’s frontal lobe. cerebellum and a portion of the brainstem (Fig. 36.22a). The
A tamping iron he was using to set a dynamite charge caused the remainder of the brainstem develops from the midbrain. The
dynamite accidentally to explode, sending the iron through his cerebellum coordinates complex motor tasks, such as catching a
skull and the frontal cortex of his brain (Fig. 36.21). Gage had been ball or writing. It integrates both motor and sensory information.
hard working, responsible, and well liked; after the accident, he The brainstem, consisting of the medulla, pons, and midbrain,
was given to fitful arguments, lacked self-control, and seemed less initiates and regulates motor functions such as walking and
intelligent. These changes provided some of the first evidence that controlling posture, and coordinates breathing and swallowing.
particular brain regions control particular aspects of a person’s The brainstem activates the forebrain by relaying information
behavior and cognitive function. from lower spinal levels. High levels of activity within the
778 SECTION 36.5 B R A I N O RG A N I Z AT I O N A N D F U N C T I O N

FIG. 36.22 Human brain development. (a) The brain develops from the forebrain, midbrain, and hindbrain. (b) Part of the forebrain develops
into the limbic system, which controls drives and emotions.
a. Three major brain regions b. Limbic system

Cerebral
cortex
Thalamus
Midbrain Hindbrain Hypothalamus
Cerebellum

Forebrain Midbrain
Pons Brainstem
Medulla
Spinal Spinal
cord cord Hippocampus
The brain develops
as three regions: a 25-day brain Adult brain
hindbrain, midbrain
and forebrain, with Forebrain Cerebral cortex, thalamus, hypothalamus
the forebrain
becoming elaborated Midbrain Midbrain (part of brainstem)
as the cerebral cortex
in humans and most
Hindbrain Pons and medulla (part of brainstem), cerebellum
other vertebrates.

brainstem maintain a wakeful state; low levels enable sleep. If the The brain is divided into lobes with specialized
midbrain is damaged, loss of consciousness and then coma result. functions.
The forebrain consists of an inner brain region that forms The cerebral hemispheres are the largest structures of the
the thalamus and the underlying hypothalamus, and a more mammalian brain (Fig. 36.23). They consist of a highly folded
anterior region that develops into the cerebrum, the outer left outer layer of gray matter about 4 mm thick that forms the
and right hemispheres of the cerebral cortex (Fig. 36.22a). The cerebral cortex, which is made up of densely packed neuron cell
thalamus is a central relay station for sensory information sent to bodies and their dendrites (Fig. 36.23a). The folds greatly increase
higher brain centers of the cerebrum. The hypothalamus interacts the surface area of the cortex and are the result of selection for an
closely with the autonomic and endocrine systems to regulate increased number of cortical neurons within the limited volume
the general physiological state of the body. In humans and most of the skull. Deep inside the cerebral cortex is the white matter,
other primates, the cerebral cortex is the largest part of the brain, which contains the axons of cortical neurons. The fatty myelin
overseeing sensory perception, memory, and learning. Whereas produced by glial cells surrounding the axon makes this region of
the forebrain is essential to normal behavior in mammals, it the brain white. These axons are the means by which neurons in
appears less important for other vertebrates. different regions of the gray matter communicate with neurons in
Other inner components of the forebrain constitute the other regions of the cortex and in deeper regions of the forebrain,
limbic system, which controls physiological drives, instincts, midbrain, and hindbrain.
emotions, and motivation and, through interactions with Major regions of the cerebral hemispheres are defined by
midbrain regions, the sense of reward (Fig. 36.22b). Stimulation clearly visible anatomical lobes separated in most cases by deep
of the limbic system can induce strong sensations of pleasure, crevices called sulci (singular, sulcus) (Fig. 36.23b). The frontal
pain, or rage. A posterior region of the limbic system, the lobe is located in the anterior region of the cerebral cortex (behind
hippocampus, is involved in long-term memory formation, your forehead). It is important in decision making and planning.
discussed in the next section. The parietal lobe is located posterior to the frontal lobe and is
Sensory information reaches the cerebral cortex from the separated from the frontal lobe by the central sulcus. The parietal
cranial nerves and nerves passing through the spinal cord. This lobe controls body awareness and the ability to perform complex
information passes through the brainstem, and then through tasks, such as dressing. The temporal lobe lies below the parietal
the thalamus, the central relay station for sensory information. lobe. It is involved in processing sound, as well as performing other
From the thalamus, information for each of the senses goes to a functions discussed below. Located behind the parietal lobe, at the
different region of the brain specialized to further process that back of the brain, is the occipital lobe, which processes visual
information, in a manner discussed next. information.
 CHAPTER 36 A N I M A L S E N S O RY S YS T E M S A N D B R A I N F U N C T I O N 779

FIG. 36.23 Human brain organization. (a) The folding of the brain and (b) the lobes of the cerebrum.

a. Cross section b. Lateral view Primary

motor Central
cortex sulcus Primary
somatosensory
cortex

Gray matter
(neuron cell
bodies)
Frontal lobe
Sulcus
Parietal lobe
White matter Occipital lobe
(neuron
axons with Temporal lobe
myelin and Cerebellum
glial cells) Auditory
cortex Brainstem
Visual
cortex

The surface of the brain is highly folded, The cerebrum is organized into lobes.
increasing the surface area of the cortex,
where neuron cell bodies are located.

The central sulcus separates the primary motor cortex somatosensory cortex integrates tactile information from
of the frontal lobe from the primary somatosensory cortex specific body regions and relays this information to the motor
of the parietal lobe (Fig. 36.24). “Command” neurons in the cortex. Both cortices make connections to the opposite side
primary motor cortex produce complex coordinated behaviors of the body by sending axons that cross over in the brainstem
by controlling skeletal muscle movements. The primary and spinal cord. As a result, the right cortex controls the left

FIG. 36.24 Somatosensory topographic map.

Forearm

k
Arm ow

He er

Trun
Nec ad

Hip
Wr

H
uld

Lit and
Elb
ist

t
Mi Ring le
Sho

dd Leg
Front I n d le
Th ex Foo
t
um
b
Left Right No Eye To e s
se
hemisphere hemisphere Fac Genitalia
Up
Primary motor per e
lip
Central cortex Lips
sulcus Primary Lowe
r lip
somatosensory
cortex Gum and
jaw

Tongue

nx
Phary
inal
- a b dom
a
Intr
Back
780 SECTION 36.6 M E M O RY A N D CO G N I T I O N

side of the body and the left cortex controls the right side of include object identification and naming. An individual with
the body. damage to the temporal lobe may lose the ability to recognize
faces, even though he or she may still recognize others by voice or
Information is topographically mapped into the other body features.
vertebrate cerebral cortex. The occipital lobe topographically maps visual information
The primary motor cortex and somatosensory cortex are received from the optic nerves. The topographic mapping of
organized as maps that represent different body regions, as information from retinal ganglion cells enables the brain to detect
shown in Fig. 36.24. The neurons that control movements of the patterns and motion that are precisely related to different regions
foot and lower limb are located near the mid-axis of the motor of the visual field. As a result, highly visual animals, such as
cortex. Those that control motions of the face, jaws, and lips humans, primates, and birds, are able to detect complex patterns
map to the side and farther down in the primary motor cortex. and movements in their environment.
There is a similar topographic map in the somatosensory cortex
for pressure and touch sensation. Regions such as the fingers,
hands, and face that are involved in fine motor movements and 36.6 MEMORY AND COGNITION
distinguishing fine sensations are represented by larger areas of
the cortex. The ability of the brain to process and integrate complex sources of
The auditory cortex in the temporal lobe is similarly organized information, interpret and remember past events, solve problems,
as a map. This region processes sound information transmitted reason, and form ideas is broadly referred to as cognition. In
from the cochlea of the ear. Neurons in the auditory cortex are this section, we consider memory and learning, as examples of
organized by pitch: Neurons sensitive to low frequencies are cognitive abilities.
located at one end and neurons sensitive to high frequencies at the
other. With the evolution of language in humans, regions farther The brain serves an important role in memory
back in the temporal lobe developed into language and reading and learning.
centers. These centers are linked by pathways to a specialized Memory is the basis for learning. An animal that can remember its
motor center controlling speech, located at the base of the primary encounter with a predator or noxious plant will avoid that danger
motor cortex of the frontal lobe. Functions of the temporal lobe in the future. A memory is formed by changes to neural circuits

FIG. 36.25 Synaptic plasticity. Memory and learning depend on long-term potentiation (LTP) of synapses within neural circuits.

Repeated release of the

neurotransmitter
glutamate stimulates the
production of new
Glutamate glutamate receptors,
Glutamate increasing the strength of
receptor the signals received by
the cell.
Enlarged
dendrite

New
dendrite
forming

Repeated stimulation also induces

new dendritic synapses to grow,
further strengthening the signal
connection between the two cells
and thereby reinforcing a particular
memory.
 CHAPTER 36 A N I M A L S E N S O RY S YS T E M S A N D B R A I N F U N C T I O N 781

within specific regions of the brain, specifically, by changes in Cognition involves brain information processing
the synaptic connections between neurons in a neural circuit. and decision making.
These changes enable an animal, for example, to recognize other Cognitive brain function ultimately gives rise to a state of
individuals in its social group or to recognize the threat of a consciousness—an awareness of oneself and one’s actions
predator. In humans and other primates, the hippocampus, a brain in relation to others. Consciousness allows judgments to be
structure in the limbic region, plays a special role in memory made about events and experiences in one’s environment. When
formation. We can remember much of the information we receive you suddenly see someone you recognize from long ago, rapid
for several minutes. However, unless reinforced by repetition neural associations within the cortex identify that person by
or by paying particular attention, the information is typically name, retrieve multiple memories of past events you shared
forgotten. The hippocampus transforms reinforced short-term with that person, and elicit conscious thoughts, including your
memories into long-term memories. Long-term memory is opinion of that person. At the same time, extraneous sensory
essential for learning. information is filtered and other thoughts recede, allowing you
The hippocampus forms long-term memories by repetitively to focus your attention on the person you have just seen and
relaying information to regions of the cerebral cortex. Although recognized.
the details remain uncertain, memory and learning depend on Mental processes are studied by relating changes in brain
establishing particular neural circuits in the hippocampus and activity to changes in an animal’s behavioral state. The study
cerebral cortex. These circuits can be activated to recall a memory of the brain is challenging because of the complexity of the
that is triggered by a particular sound, or sight, or other relevant circuits involved. Nevertheless, considerable progress is being
stimulus. Establishing a neural circuit requires establishing made through non-invasive neuroimaging techniques
a particular set of connections between neurons. Thus, the (Fig. 36.26). These techniques allow the interactions of different
formation of a memory circuit requires changes in synapses. Some brain regions within human and other animal subjects to be
synapses are weakened or removed altogether, and others are investigated while the subjects perform a particular mental or
formed or strengthened. motor task. The results of neuroimaging studies can be linked to
The ability to adjust synaptic connections between neurons is studies of the molecular and cellular properties of nerve cells, to
called synaptic plasticity (Fig. 36.25). Long-term potentiation
(LTP) is an example of synaptic plasticity that is believed to
underlie memory and learning. In response to repeated excitation,
FIG. 36.26 Non-invasive imaging of brain function. PET scan
neurons in excited circuits within the hippocampus release the
imaging reveals regions of metabolic brain cell activity.
neurotransmitter glutamate, opening Na1 and Ca21 channels. The
The reddish areas show the locations of (upper left) the
increased Ca21 stimulates signaling pathways leading to protein
visual area activated by sight, (upper right) the auditory
synthesis. New receptors are placed in the postsynaptic cell
area activated by hearing, (lower left) the tactile area
membrane and new dendrites are formed, strengthening synaptic
activated by touching braille, and (lower right) areas of
signaling between the two cells. These changes make the cells
the frontal cortex activated by generating words. Source:
more responsive to subsequent stimulation.
WDCN/University College London/Science Source.
Through repeated activation of neurons within the
hippocampus and associated regions of the cerebral cortex, neural
circuits are established that may be reinforced by LTP to create a
memory that can be retrieved over long periods of time. Although
memories may be associated with certain regions of the cortex,
they also rely on the interaction of nerve cells and networks
within multiple brain regions.
Although the brain regions involved in memory and learning
vary among different animals, the basic features of how memories
are constructed by means of synaptic plasticity within neural
circuits are likely broadly shared across animals capable of forming
memories. For example, the ability of an octopus to learn and
remember how to perform motor tasks rivals that of certain
vertebrates. Even the roundworm C. elegans has been found to be
capable of memory and learning: Individual roundworms learn to
avoid pathogenic bacteria in favor of bacteria that are nutritious.

j Quick Check 5 Which three areas of the human brain are

important for remembering and recognizing a face?
782 CO R E CO N C E P T S S U M M A RY

advance our understanding of how nerve cells form and summarized her review of studies on animal cognition and
maintain neural circuits among different brain regions, and consciousness as follows:
of how these circuits are linked to particular behaviors and
cognitive processes. Our near-certainty about [human] shared experiences is
Do animals other than humans have a conscious state of based, amongst other things, on a mixture of the complexity
awareness? Although the conscious awareness of other animals of their behavior, their ability to “think” intelligently, and
is difficult to study, it seems clear that many animals exhibit on their being able to demonstrate to us that they have
conscious states of awareness that influence their experience a point of view in which what happens to them matters
and behavior. Donald Griffin, known for his codiscovery as an to them. We now know that these three attributes—
undergraduate of bat echolocation, advocated the view not only complexity, thinking, and minding about the world—are
that animals are conscious, but also that animal consciousness is also present in other species. The conclusion that they, too,
amenable to scientific study. Behavioral biologist Marion Dawkins are consciously aware is therefore compelling. •

Core Concepts Summary The statocysts of invertebrates are sensitive to gravity, orienting
an organism to “up” and “down.” page 767
36.1 Animal sensory receptors detect physical and The vestibular system of the vertebrate ear provides a sense of
chemical stimuli by changes in membrane potential. balance and gravity. page 768
Sensory receptors include chemoreceptors, The tympanic membrane in the outer ear transmits
mechanoreceptors, thermoreceptors, pain receptors, and airborne sound waves to the middle ear, where the signal is
electromagnetic receptors. page 764 amplified. The amplified sound waves are transmitted to
Action potential firing rate correlates with the strength of a the cochlea in the inner ear, where sound waves are converted
stimulus. Generally, strong stimuli induce high firing rates to fluid pressure waves. These waves are then sensed by hair
and weak stimuli induce low firing rates. page 764 cells that convert the signal to an electrical impulse. page 769

Animal sensory receptors increase their sensitivity to stimuli 36.4 The ability to sense light and form images depends
by temporal and spatial summation, enhance their acuity by on photosensitive cells with light-absorbing proteins.
lateral inhibition, and adapt to continuous stimuli. page 765
Opsin is a G protein-coupled receptor and the universal
36.2 Specialized chemoreceptors relay information photoreceptor protein in all animal eyes. page 771
about smell and taste. The eyecups of flatworms detect light and dark. page 771
Odorant molecules bind to membrane receptors on the The compound eyes of arthropods sense light using individual
surface of olfactory chemoreceptors. page 766 light-focusing elements called ommatidia, providing low acuity
Olfactory sensory cells are neurons that fire action potentials but rapid motion detection. page 771
when sending odor information to the brain. page 766 The single-lens eyes of vertebrates and cephalopod mollusks
In humans, five taste receptors—for sweet, bitter, sour, salty, focus images on a retina, which contains photoreceptor cells
and savory—are activated in combination to determine a and interneurons that process the light stimuli. page 773
specific taste. page 766 The protein opsin present in photoreceptor cells converts
Taste receptors are sensory cells that do not fire action light energy into chemical signals, altering the firing rate of
potentials but communicate by synapses with afferent neurons. page 774
neurons. page 766 Photoreceptor cells of the retina include rod cells, which detect
light intensity, and three types of cone cells, which detect
36.3 Hair cells convey information about gravity, different wavelengths of light and allow color vision. page 774
movement, and sound.
Photoreceptor cells, bipolar cells, ganglion cells, horizontal
Hair cells are specialized mechanoreceptors that detect cells, and amacrine cells form a network that processes visual
motion and vibration. page 767 information in the retina. page 775
 CHAPTER 36 A N I M A L S E N S O RY S YS T E M S A N D B R A I N F U N C T I O N 783

36.5 The brain processes and integrates information Self-Assessment

from multiple sensory systems, with tactile, visual, and
auditory stimuli mapped topographically in the cerebral 1. Give an example of a chemosensory, mechanosensory, and
cortex. electromagnetic sensory cell.

The vertebrate brain is organized into a hindbrain, midbrain, and 2. Diagram a generalized sensory receptor cell and show
forebrain, with the forebrain elaborated into the cerebral cortex how it changes its firing rate in response to detected stimuli.
in birds and mammals. page 777 3. State a hypothesis that explains why all animals have
chemoreceptors.
The cerebrum is divided into frontal, parietal, temporal, and
occipital lobes, which are specialized for different functions. 4. Describe the three main stages by which the
page 778 mammalian ear detects and codes sound.

Somatosensory, motor, auditory, and visual information is 5. Compare and contrast the roles of rod cells and cone
topographically mapped to specific areas in the cerebral cortex. cells in the retina.
page 780 6. Describe the role of the cornea and lens in vertebrate
eyes.
36.6 Cognition is the ability of the brain to process and
7. Describe three different types of eye in animals.
integrate complex information, remember and interpret
past events, solve problems, reason, and form ideas. 8. Draw the brain, label the lobes, and describe their
primary functions.
The cerebral cortex integrates and processes information from
diverse sources, giving rise to problem solving, reasoning, and 9. Describe the importance of topographic mapping
decision making. page 780 of sensory input to the cortex, using the primary
somatosensory cortex as an example.
The brain stores memories of past experiences and enables
learning through the creation of more long-lasting neural 10. Explain how brain function can be understood by studying
circuits. page 781 patients with brain injuries.

Consciousness is an awareness of one’s own identity in relation

to others. page 781 Log in to to check your answers to the Self-
Assessment questions, and to access additional learning tools.
this page left intentionally blank
 CHAPTER 37

Animal
Movement
Muscles and Skeletons

Core Concepts
37.1 Muscles are biological
motors composed of actin
and myosin that generate
force and produce movement
for support, locomotion,
and control of internal
physiological functionsl.
37.2 The force generated by
muscle depends on muscle
size, degree of actin–myosin
overlap, shortening velocity,
and stimulation rate.
37.3 Hydrostatic skeletons,
exoskeletons, and
endoskeletons provide animals
with mechanical support and
protection
37.4 Vertebrate endoskeletons
allow for growth and repair
and transmit muscle forces
across joints.
Gustoimages/Science Source.

785
786 SECTION 37.1 M U S C L E S : B I O LO G I C A L M OTO R S T H AT G E N E R AT E F O RC E A N D P RO D U C E M OV E M E N T

The ability to move is a defining feature of animal life, allowing which evolved at least 600 million years ago. Thus, basic features
animals to explore new environments and avoid inhospitable of muscle organization and function are conserved across the
ones, escape predators, mate, feed, and play. An animal’s motor vast diversity of eukaryotes. As we will discuss, the geometry and
and nervous systems (Chapters 35 and 36) work together to organization of these proteins largely determines how muscles
enable it to sense and respond to its environment. How are contract to produce force and movement.
movements produced, and how are they controlled? What
determines an athlete’s performance? How do muscles and the Muscles use chemical energy to produce force
skeleton work together to provide movement and support? And and movement.
how do muscles convert the chemical energy of ATP into force Muscles are composed of elongated cells called muscle fibers.
and movement during a contraction? This chapter explores the Muscle fibers use ATP generated through cellular respiration
organization and function of the muscles and skeletons that (Chapter 6) to generate force and change length during a
power the movements of larger multicellular animals and provide contraction. This conversion of chemical energy into muscle work
mechanical support of their bodies (Fig. 37.1). underlies much of the energy cost of animal movement discussed
in Chapter 40.
A force is a push or pull by one object interacting with another
37.1 MUSCLES: BIOLOGICAL MOTORS object. The forces generated by skeletal muscles, for example,
THAT GENERATE FORCE AND act as pulling forces at specific sites of the skeleton. The work
performed by a muscle is equal to force times length change. For
PRODUCE MOVEMENT
example, work is increased when either the force produced or
We rely on our muscles for all kinds of movement. Muscles the distance the skeleton is moved increases. Similarly, a muscle
function as biological motors within the body because they that produces a pulling force twice as much as another muscle but
generate force and produce movement. Some of these movements moves the skeletal segment only half the distance does the same
are obvious—running, climbing stairs, raising your hand, playing work as that other muscle.
the piano. We are less aware of others—moving food through Because muscles only exert pulling forces, pairs of muscles
the digestive tract, breathing in and out, pumping blood through are arranged to produce movements in two opposing directions at
the body. A muscle’s ability to produce movement depends on specific joints of the skeleton. For example, muscles pull on the
electrically excitable muscle cells containing proteins that can be bones of your leg and foot to swing them forward when you kick a
activated by the nervous system. ball. Opposing sets of muscles contract to swing your leg back and
The contractile machinery of muscles is ancient. The proteins flex your knee before and after the kick.
underlying muscle contraction are found in eukaryotes that lived Muscles can shorten very quickly, and they can produce large
more than 1 billion years ago, and the first muscle fibers common forces for their weight. The forces they produce can be many times
to all animals are found in cnidarians (jellyfish and sea anemones), an animal’s body weight, allowing animals to move quickly. Some
small muscles contract extremely quickly:
The muscles of many flying insects
contract 200 to 500 times per second.
FIG. 37.1 Muscles and bones. Muscles and bones work together to produce movement.
Even in vertebrate animals, muscles can
Photo source: Tom Brakefield/Corbis.
contract as many as 50 to 100 times per
second. Other muscles, such as the closing
muscle of a clam, contract slowly, but
can do so for prolonged periods without
tiring. To see how muscles produce forces
and contract to change length, we now
consider how muscle fibers are organized
at the molecular level.

Muscles can be striated or smooth.

All muscles contain the same contractile
proteins that enable them to shorten and
produce force—actin and myosin (Chapter
10). These proteins are organized into
bundles called filaments that interact
with one another to cause muscles to
shorten.
 CHAPTER 37 A N I M A L M OV E M E N T : M U S C L E S A N D S K E L E TO N S 787

FIG. 37.2 Light micrographs of (a) skeletal muscle, (b) cardiac muscle, and (c) smooth muscle. Skeletal and cardiac muscles have a striated, or
striped, appearance; smooth muscle does not. Sources: a. Innerspace Imaging/Science Source; b. Manfred Kage/Science Source; c. SPL/Science Source.

a b c

Although all muscle fibers have filaments of actin and myosin,

the filaments are arranged differently in different types of muscle. FIG. 37.3 Skeletal muscle organization. Muscles are made up of
Muscles are divided into two broad groups on the basis of their bundles of muscle fibers (cells), each of which contains
function and appearance under a light microscope (Fig. 37.2). myofibrils.
As their name suggests, striated muscles appear striped under
a light microscope because the actin and myosin filaments are
arranged in a regularly repeating pattern. Striated muscles include
skeletal muscles (Fig. 37.2a), which connect to the body skeleton
to move the animal’s limbs and torso, and cardiac muscle (Fig. Muscle
37.2b), which contracts to pump blood. Skeletal muscle cells are
elongated and have many nuclei in each cell, whereas cardiac
muscle cells are typically less elongated than skeletal muscle cells, Blood
often branched, and contain one or more nuclei per cell. vesse
vessels

In contrast to skeletal and cardiac muscle, smooth muscles Muscles, like the
appear unstriated under the light microscope because the biceps that flexes the Nerve
elbow joint, connect
organization of actin and myosin filaments in smooth muscles is to a rigid skeleton. Muscle Connective
irregular (Fig. 37.2c). Smooth muscles are found in the walls of bundle tissue
vertebrate and certain invertebrate (squid and octopus) arteries to
regulate blood flow, in the respiratory system to control airflow, Connective
and in the digestive and excretory systems to help transport food tissue
and waste products. Smooth muscles contract slowly compared
with cardiac and skeletal muscles. Consequently, bivalve mollusks Muscles are composed
rely on smooth muscle fibers to keep their shells closed for long Muscle of elongate cells called
fiber muscle fibers that are
periods without having to expend much energy. (cell) embedded in
surrounding
connective tissue.
Skeletal and cardiac muscle fibers are organized into
Nucleus
repeating contractile units called sarcomeres.
The visible bands in skeletal and cardiac muscles are an important Mitochondrion
clue to the mechanism that produces muscle contractions. Whole
muscles are made up of parallel bundles of individual muscle fibers
Individual fibers have many
(Fig. 37.3). Recall that a muscle fiber is a muscle cell. Each muscle myofibrils with a striated
fiber in turn contains hundreds of long, rodlike structures called appearance due to their regular
Myofibril molecular organization.
myofibrils. Myofibrils contain parallel arrays of actin and myosin
filaments that cause a muscle to contract.
788 SECTION 37.1 M U S C L E S : B I O LO G I C A L M OTO R S T H AT G E N E R AT E F O RC E A N D P RO D U C E M OV E M E N T

Let’s consider the structural organization of myosin and actin

FIG. 37.4 Thin and thick filaments. A thin filament is made up of actin
filaments in skeletal muscles in more detail (Fig. 37.4). Each
subunits arranged in a double helix. A thick filament is made
myosin molecule consists of two long polypeptide chains coiled
up of numerous myosin molecules arranged in parallel.
together, each ending with a globular head. Consequently, the
Thin (actin) myosin molecule resembles a double-headed golf club. The myosin
filament molecules are arranged in parallel to form a thick filament, with
Actin Tropomyosin the numerous myosin heads extending out from their flexible necks
molecule along the myosin filament. Actin subunits are arranged as a double
Myosin helix to form a thin filament. The protein tropomyosin runs in
molecule the grooves formed by the actin helices.
The thin filaments are attached to protein backbones called
Z discs that are regularly spaced along the length of the myofibril
(Fig. 37.5a). The region from one Z disc to the next is called the
Thick (myosin)
filament sarcomere. Sarcomeres are the functional units of muscles. In

FIG. 37.5 Muscle sarcomere. (a) Longitudinal and (b) cross-sectional views of a sarcomere,
including two transmission electron micrographs. The sarcomere is the region
Muscle
between Z discs. It is the contractile unit of a muscle. Photo sources: a. Don W. Fawcett/
fiber
Science Source; b. Biophoto Associates/Science Source.

Myofibril

Sarcomere

b.
Z discs
Actin (thin) Myosin (thick)
a.

Titin Z disc Thin filament Thick filament Z disc

A longitudinal section shows the organization A cross section shows the regular hexagonal
of thick (myosin) and thin (actin) filaments. arrangement of thick and thin filaments.

Z disc Z disc

Sarcomere
 CHAPTER 37 A N I M A L M OV E M E N T : M U S C L E S A N D S K E L E TO N S 789

when rabbit or frog myofibrils were

FIG. 37.6 Sliding filament model. Muscle contraction, or shortening, results from the sliding stimulated to contract to different
of actin thin filaments relative to myosin thick filaments. Filament sliding changes the lengths. Their results led them to
sarcomere banding pattern between Z discs. Photo source: Reprinted by permission from Macmillan hypothesize that muscles produce
Publishers Ltd: NATURE 173, H. Huxley and J. Hanson, Changes in the Cross-Striations of Muscle during force and change length by the sliding
Contraction and Stretch and their Structural Interpretation, Copyright 1954. of actin filaments relative to myosin
filaments. Their theory is known
as the sliding filament model of
muscle contraction.
The banding patterns of
sarcomeres revealed the extent
Z disc Thin (actin) Thick (myosin) Z disc
filament filament of overlap between actin and
Z discs myosin filaments. When myofibrils
Contraction Relaxation contracted to short lengths, the
sarcomeres were observed to have
increased actin–myosin overlap.
When myofibrils were stretched to
longer lengths, actin–myosin overlap
decreased (Fig. 37.6). However, the
lengths of the myosin and actin
filaments never varied. Consequently,
other words, the shortening (contraction) of a muscle is ultimately nearly all of the length change during a muscle contraction
the result of the shortening of thousands of sarcomeres along a results from the sliding of actin filaments with respect to myosin
myofibril. filaments within individual sarcomeres. The length change of the
Sarcomeres are arranged in series along the length of the whole muscle fiber is therefore a sum of the fractions by which
myofibril. Sets of actin thin filaments extend from both sides of each sarcomere shortens along the fiber’s length.
the Z discs toward the midline of the sarcomere (Fig. 37.5a). In A muscle’s ability to generate force and change length is
the middle of the sarcomere, not directly contacting the Z discs, largely determined by the properties of its sarcomeres. Whereas
are myosin thick filaments. The thin filaments overlap with the sarcomere length is quite uniform in vertebrate animals (about
myosin thick filaments, forming two regions of overlap within a 2.3 µm at rest), it is variable in invertebrate animals (ranging
sarcomere. Toward either end of the sarcomere, as well as in the from as short as 1.3 µm to as long as 43 µm). Longer sarcomeres
middle, are regions where the actin and myosin filaments do not allow a greater degree of shortening. Thus, length changes are
overlap. The regions that overlap appear dark under an electron more limited in vertebrate muscles compared to those in some
microscope, while the regions of non-overlap appear lighter in invertebrate muscles with long sarcomeres. For example, when
color. A third large protein, called titin, links the myosin filaments the sarcomeres in an octopus tentacle contract, they can produce
to the Z discs at the ends of the sarcomere. Titin is believed large motions of the tentacle. However, because vertebrates have
to help with assembly and protect the sarcomeres from being shorter sarcomeres than an octopus, vertebrate muscles can
overstretched, thus contributing to muscle elasticity. shorten to produce movements more quickly.
As we have seen, the regular pattern of actin and myosin Interactions between the myosin and actin filaments are
filaments within sarcomeres along the length of the fiber gives what cause a muscle fiber to shorten and produce force. Along the
skeletal muscles their striated appearance (Fig. 37.5a). In cross myosin filament, one of the two heads of a myosin molecule at a
section, thick and thin filaments are arranged in a hexagonal given point in time binds to actin at a specific site to form cross-
lattice. This arrangement allows each thick filament to interact bridges between the myosin and actin filaments. The myosin
with six adjacent thin filaments (Fig. 37.5b). The precise filaments pull the actin filaments toward each other by means
geometry of thin and thick filaments is critical to their interaction, of these cross-bridges. The key to making the filaments slide
as we discuss next. relative to each other is the ability of the myosin head to undergo
a conformational change and therefore pivot back and forth. The
Muscles contract by the sliding of myosin and actin myosin head attaches to the actin filament and pivots forward,
filaments. sliding the actin filament toward the middle of the sarcomere. The
Using high-resolution light microscopy, two independent teams myosin head then detaches from the actin filament, pivots back,
(physiologists Hugh Huxley and Jean Hanson in the United reattaches, and the cycle repeats. The movement of the myosin
States and Andrew Huxley and Rolf Niedergerke in England) were head is powered by ATP. Together, these events describe the cross-
able to quantify the changing banding pattern of sarcomeres bridge cycle.
790 SECTION 37.1 M U S C L E S : B I O LO G I C A L M OTO R S T H AT G E N E R AT E F O RC E A N D P RO D U C E M OV E M E N T

FIG. 37.7 Cross-bridge cycle. The myosin head binds to actin and uses the energy of ATP to pull on the actin filament, causing muscle
shortening.

1 The myosin head ATP

binds ATP, leading to
detachment from actin.

Thick
filament

Myosin ATP
head
Thin
filament

4 ADP and Pi are released,

2 The myosin head catalyzes
producing a power stroke that the hydrolysis of ATP, forming
ADP causes the thin filament to slide ADP and Pi, and cocking the
+ myosin head back.
relative to the thick filament,
causing the sarcomere to shorten.

ADP

ADP Pi
Cross-bridge

3 The myosin head

binds actin, forming
a cross-bridge.

The cross-bridge cycle is just that—a cycle (Fig. 37.7). Let’s The myosin head then binds a new ATP molecule (step 1
look at it in more detail: again), allowing it to detach from actin, and the cycle repeats. After
detachment, the myosin head again hydrolyzes ATP, allowing it to
1. The myosin head binds ATP. Binding of ATP allows the return to its original conformation and bind to a new site farther
myosin head to detach from actin and readies it for along the actin filament. With the release of ADP and Pi, the myosin
attachment to actin. head undergoes another cycle of force generation and movement.
2. The myosin head hydrolyzes ATP to ADP and inorganic Individual muscle contractions are the result of many
phosphate (Pi). Hydrolysis of ATP results in a conformational successive cycles of cross-bridge formation and detachment.
change in which the myosin head is cocked back. ADP and Pi During these cycles, muscle cells convert the chemical energy
remain bound to the myosin head. Because ADP and Pi are released by ATP into force and the kinetic energy of movement.
bound rather than released, the myosin head is in a high- Muscle fibers that contract especially quickly, such as those that
energy state. power insect wing flapping or produce the sound of cicadas and
rattlesnake tails, express myosin molecules that have high rates of
3. The myosin head then binds actin, forming a cross-bridge.
ATP hydrolysis, allowing faster rates of cross-bridge cycling, force
4. When the myosin head binds actin, the myosin head development, and shortening. Thus, myosin functions as both a
releases ADP and Pi. The result is another conformational structural protein and an enzyme.
change in the myosin head, called the power stroke. Because each thick filament can interact with as many as six
During the power stroke, the myosin head pivots forward actin filaments, at any one time numerous cross-bridges anchor
and generates a force, causing the myosin and actin the lattice of myosin filaments, while other myosin heads are
filaments to slide relative to each other over a distance detached to find new binding sites. When summed over millions
of approximately 7 nm. The power stroke pulls the actin of cross-bridges, these molecular events generate the force that
filaments toward the sarcomere midline. shortens the whole muscle.
 CHAPTER 37 A N I M A L M OV E M E N T : M U S C L E S A N D S K E L E TO N S 791

j Quick Check 1 When an animal dies, its limbs and body become
stiff because its muscles go into rigor mortis (literally, rigor mortis FIG. 37.8 Excitation–contraction coupling. Depolarization
means “stiffness of death”). Why would the loss of ATP following (excitation) leads to shortening (contraction) of the
death cause this to happen? muscle. Photo source: Don W. Fawcett/Science Source.

a. Scanning electron micrograph of a motor nerve endplate

Calcium regulates actin–myosin interaction through
excitation–contraction coupling.
We have discussed what makes muscles contract. We next
consider when they contract. Skeletal and smooth muscle fibers Motor
are both activated by the nervous system. Whereas vertebrate nerve
axon
skeletal muscles are innervated by the somatic nervous system,
smooth muscles are innervated by the autonomic nervous system,
as are cardiac muscles (Chapter 35).
Like nerve cells, muscle fibers are electrically excitable. Skeletal
Muscle
muscle fibers are activated by impulses transmitted by motor fiber
nerves to synaptic junctions (Fig. 37.8). Motor neuron axons have
branches that allow them to synapse with multiple muscle fibers.
When action potentials traveling down a motor neuron arrive at b. Muscle excitation–contraction coupling
the neuromuscular junction, the neurotransmitter acetylcholine
is released into the synaptic cleft. The neurotransmitter binds 1 An action potential from a
with postsynaptic receptors on the muscle cell at a region called motor neuron leads to release of
acetylcholine and depolarization Plasma Infolding
the motor endplate, triggering the opening of Na⫹ channels. The of the muscle cell. membrane of plasma
resulting influx of Na⫹ ions in turn initiates a wave of depolarization membrane
that passes from the neuromuscular junction toward both ends of Nerve Nucleus

Action
the muscle fiber (Chapter 35). Motor Muscle cell

p
How does membrane depolarization lead to the cross-bridge endplate

otentia
cycle? Actin and myosin filaments can form cross-bridges only Myofibrils
when the myosin-binding sites on actin are exposed. At rest, these
2
l
The depolarization
binding sites are blocked by the protein tropomyosin. The wave is conducted into the
of depolarization in the muscle cell initiates a chain of events interior of the fiber by
that moves tropomyosin away from these binding sites, allowing the infolding of
plasma membrane.
cross-bridges between actin and myosin to form and the muscle to
contract (Fig. 37.8). Sarcoplasmic
Let’s look at the chain of events that concludes with the reticulum
exposure of the myosin-binding sites in more detail. You saw in
Chapter 5 that eukaryotic cells contain several types of membrane- 3 Depolarization
bound internal organelle. The myofibrils of muscle cells are leads to the release
of Ca2+ from the
surrounded by a highly branched membrane-bound organelle sarcoplasmic
called the sarcoplasmic reticulum (SR), a modified form of the reticulum.
endoplasmic reticulum (Fig. 37.8). Depolarization initiated in the
plasma membrane is conducted to the SR through infoldings of Ca2+
Tropomyosin
the plasma membrane (Fig. 37.8). When the muscle is at rest, the
SR contains a large internal concentration of calcium (Ca2⫹) ions 4 Ca2+ binds to
transported in by calcium pumps in its membranes. troponin, which
causes movement
A muscular contraction is initiated when depolarization of the Actin Troponin of tropomyosin,
Myosin-binding sites
muscle fiber causes the SR to release Ca2⫹. The Ca2⫹ diffuses into on actin blocked exposure of
myosin-binding
the myofibrils and binds to a protein called troponin, causing the Myosin–binding sites sites on actin, and
troponin molecule to change shape. This conformational change on actin exposed formation of
of troponin, in turn, causes tropomyosin to move, exposing cross-bridges to
produce shortening
myosin-binding sites along the actin filament. Now myosin cross- of the muscle.
bridges can form with actin, producing a contraction. Note that at
this stage in the cross-bridge cycle, the myosin head has already
792 SECTION 37.2 M U S C L E CO N T R AC T I L E P RO P E RT I E S

hydrolyzed ATP, is bound to ADP and Pi, and is in the cocked-back blood flow to particular body regions, and the gut moves digestive
“ready” position. Binding to actin then allows the power stroke to contents through the gastrointestinal tract.
occur. In this way, contraction is initiated immediately following
depolarization and release of Ca21.
The process by which membrane depolarization leads to Ca21 37.2 MUSCLE CONTRACTILE
release from the SR and the formation of myosin-actin cross- PROPERTIES
bridges is called excitation–contraction coupling because
excitation of the muscle cell is coupled to contraction of the Actin and myosin are evolutionarily conserved across all animal
muscle, producing force and movement. Together, these events life, so all muscles have similar force-generating properties. The
are the molecular “switch” that causes a muscle to contract. The huge muscles of a weight lifter are evidence that larger muscles
muscle relaxes when neural stimulation ends. Acetylcholine can exert more force. Why is that? The summed force produced
is broken down or reabsorbed, and Ca21 is actively transported by individual myofibrils of activated muscle fibers determines the
back into the sarcoplasmic reticulum, allowing tropomyosin force the whole muscle can exert. This means that larger muscles
molecules to once again block myosin-binding sites along the with more fibers produce greater forces than smaller muscles
actin filaments. with fewer fibers. However, the force that a muscle produces also
depends on its contraction length and the speed of contraction, as
j Quick Check 2 Curare is a paralyzing compound that blocks the we discuss next.
action of the neurotransmitter acetylcholine at the muscle fiber’s
motor endplate. What effect do you think curare has on the release Muscle length affects actin–myosin overlap and
of calcium ions from the sarcoplasmic reticulum of the muscle cell? generation of force.
Why is it difficult to jump while standing on your toes? A muscle’s
ability to generate force depends in part on how much it is
Calmodulin regulates Ca2+ activation and relaxation
stretched before contraction begins. The sliding filament model
of smooth muscle.
for muscle contraction, like any good model, makes specific
How is the contraction of smooth muscle regulated? The smooth
predictions. In this case, it predicts that the amount of overlap
muscle that controls many internal organs can be activated by
between actin and myosin filaments within the sarcomere
the autonomic nervous system and also responds to stretch of
determines the number of cross-bridges that can form. The more
the muscle, local hormones, and other local factors such as pH,
overlap, the more force that can be produced. Conversely, when
oxygen, carbon dioxide, and nitric oxide by intracellular signaling.
a muscle fiber is pulled to long lengths and then contracts, it
For example, smooth muscle in the walls of arteries contracts
generates less force because the overlap between myosin and
when stimulated by the release of epinephrine to reduce blood
actin filaments is reduced. When a muscle fiber contracts at short
flow to a body region (Chapter 39), and smooth muscle within
lengths, it also produces less force because myosin filaments begin
the mammalian uterus contracts when oxytocin is released
to run into the Z disc at each end of the sarcomere, disrupting the
to stimulate contractions (Chapter 42). As in skeletal muscle,
geometry of the myosin filament lattice and hindering cross-bridge
these initiators trigger the release of Ca21 from the SR. However,
formation. At intermediate lengths, the muscle fiber generates
in smooth muscle, Ca21 also enters through voltage-gated and
maximum force because actin–myosin filament overlap is greatest.
stretch-receptor calcium channels in the cell’s plasma membrane.
These predictions were tested by experiments (Fig. 37.9).
Smooth muscle lacks the troponin–tropomyosin mechanism
This influence of muscle fiber length on force generated is
for regulating contraction. Instead, activation of smooth muscle
the reason that you can’t jump well if you are on your toes and
cells results when the protein calmodulin binds with Ca21
your ankles are fully extended: This position shortens your calf
released from the SR or entering through the cell’s membrane.
muscle fibers. Conversely, if your ankles are overly flexed, your
The calmodulin–Ca21 complex activates a myosin kinase that
calf muscles are lengthened, and again jumping force is limited.
phosphorylates the smooth muscle myosin heads, causing them
Through training, athletes and dancers learn techniques, such
to bind actin and begin the cross-bridge cycle. A second enzyme
as bending the knees before jumping, so that their leg muscles
dephosphorylates the myosin heads, disrupting their ability to
contract at intermediate lengths to maximize muscle-force output.
bind to actin, and the muscle relaxes.
The relationship between force and length of striated muscle
Smooth muscle contracts slowly compared with skeletal
also explains Starling’s Law for the function of the heart as a pump
muscle, using less ATP per unit time. The SR of smooth muscle
(discussed in Chapter 39), which ensures that the heart contracts
cells is less extensive and has many fewer calcium pumps than the
more strongly when it is filled by larger amounts of blood
SR of skeletal muscle cells. As a result, calcium is returned more
returning from the veins.
gradually from the myofibrils back to the SR, or pumped back out
through the cell’s membrane, slowing the relaxation of smooth j Quick Check 3 Why can you lift a larger load when your elbow is
muscle. Using the slow contractions of smooth muscle, clams hold slightly flexed and your biceps muscle is at an intermediate length
their shells closed for long periods, arteries contract and restrict than you can when your elbow is fully extended?
HOW DO WE KNOW?

FIG. 37.9

How does filament overlap

affect force generation in 100

muscles?

Force (% of maximum)
80

60

BACKGROUND From the 1930s through the 1960s, British muscle 40

physiologists studied the properties of striated frog muscle fibers.
20
In the first set of experiments, A. V. Hill examined how a muscle’s
1.27 1.67 2.0 2.25 3.65
shortening velocity affects its force production. In the second set 0
1.0 1.5 2.0 2.5 3.0 3.5 4.0
of studies, American physiologists Albert Gordon and Fred Julian,
Sarcomere length (␮m)
together with British physiologist Andrew Huxley, studied how
a muscle’s length when it contracts affects the muscle’s force-
generating ability.

HYPOTHESIS Gordon, Huxley, and Julian hypothesized that

changes in overlap between myosin and actin affect the number of
cross-bridges and thus the force the muscle can produce.
100
EXPERIMENT The three scientists isolated a single muscle fiber Force (% of maximum)
and used optical microscopy to measure sarcomere length. They 80
kept this length constant while the fiber was stimulated to produce
force. Measurements of force were then obtained at different 60
sarcomere lengths. The length at which the muscle fiber generated
40
maximum force is indicated on the graphs as 100%. Other lengths
are expressed relative to this length. 20

RESULTS The top graph shows the data obtained by Gordon,

0
Huxley, and Julian, and the bottom graph reinterprets their data 40 60 80 100 120 140 160
in terms of force and percent of muscle length. Their experiments Percent of muscle length

showed that the muscle fiber produced maximal force at an

intermediate sarcomere length (approximately 2.3 μm). At this FOLLOW-UP WORK Other studies confirmed that changes in
length, the greatest number of cross-bridges were hypothesized myosin cross-bridge formation are linked to changes in actin and
to form, generating maximal force. As the sarcomere length was myosin filament overlap at different sarcomere lengths, supporting
decreased or stretched, and the sarcomere then stimulated to the sliding filament model of muscle contraction.
contract, force declined. These results were consistent with a
SOURCE Gordon, A. M., A. F. Huxley, and F. J. Julian. 1966. “The Variation in
decrease in the number of cross-bridges that could form as a result Isometric Tension with Sarcomere Length in Vertebrate Muscle Fibres.” Journal of
of decreased actin–myosin overlap. Physiology 184:170 –192.

Muscle force and shortening velocity are inversely example, your arm moves faster when you throw a light ball than
related. when you throw a heavy one. The reason is that the faster a muscle
In experiments carried out in the 1930s, A. V. Hill (who won fiber shortens, the fewer cross-bridges can form within it, reducing
the 1922 Nobel Prize in Physiology or Medicine for his work the force that each fiber and the muscle as a whole can produce.
on muscle energy use) observed that a muscle shortens fastest The term “muscle contraction” suggests that muscles shorten
when producing low forces (Fig. 37.10). To produce larger forces, when stimulated to generate force, but in fact muscles may also
the muscle must shorten at progressively slower velocities. For exert force and remain a uniform length or even be lengthened.

793
 794 SECTION 37.2 M U S C L E CO N T R AC T I L E P RO P E RT I E S

calf and quadriceps muscles are stretched to support your body in

FIG. 37.10 Force–velocity relationship of muscle. Skeletal landing from a jump. It also happens during every stride you take
muscles exert greater force when they shorten more during a run as your leg joints flex when a leg contacts the ground,
slowly and produce even greater force when they are stretching contracting leg muscles. Active muscle lengthening
stretched relative to being isometric. reduces the muscle’s energy consumption because fewer fibers are
recruited to generate a given level of force.
When a muscle Injury can result if a muscle is stretched too rapidly or
is actively repetitively. Rapid and repetitive stretching often occurs during
lengthened, it
generates prolonged downhill hiking, causing muscle damage and soreness,
greater force A muscle’s force-generating particularly in the quadriceps muscles in the front of the thigh.
(lengthening ability varies inversely with
contraction). its shortening velocity. Suitable training and warm-up exercises help muscles to stretch
without damage.
175%
Antagonist pairs of muscles produce reciprocal
motions at a joint.
Isometric force Muscles can generate force only by pulling, not pushing, on
(velocity = 0)
Force

100% the skeleton. Therefore, to produce reciprocal joint and limb

movements, they are arranged as pairs of antagonist muscles
that pull in opposite directions. Flexion is the joint motion
that moves bone segments closer together, and extension is
the joint motion that moves them farther apart. For example,
muscles at the elbow joint are organized as flexors (the biceps
0% muscle) that contract to bring the arm toward the shoulder, and
–50% 0 50% 100%
extensors (the triceps muscle) that contract to extend the arm
Shortening
velocity away from the shoulder (Fig. 37.11). Additional pairs of antagonist
Active lengthening muscles are present for movements in other directions. Muscles
in the body wall of cnidarians (jellyfish and sea anemones) and
segmented annelid worms, as well as the smooth muscles in the
walls of intestines, are organized in longitudinal (lengthwise) and

FIG. 37.11 Antagonist muscles. Skeletal muscles can produce

force only by pulling on a skeletal attachment, so
Consequently, the term “contraction” needs to be broadened they are arranged as antagonists to produce opposing
beyond its familiar reference to muscle shortening to include motions at a joint.
those instances when a muscle exerts force while remaining the
same length or being actively stretched. The biceps shortens The triceps shortens to
to flex the elbow joint. extend the elbow joint.
When a muscle contracts but cannot shorten (for example,
when you carry a heavy bucket of water), the muscle’s shortening
velocity is zero (Fig. 37.10), and it generates an isometric force (iso-
means “same” and metric refers to “measure”). That is, the muscle
still generates force, even though it stays the same length and does
not shorten. Isometric exercises use this type of muscle contraction:
Biceps Triceps
Holding a weight in a fixed position or exerting force against an
immovable object helps maintain muscle strength and may be Joint
motion
beneficial in rehabilitation after injury by avoiding joint motion.
Muscles can also be stretched when the external load against
which they contract exceeds the force the muscle produces. This
action is known as an active lengthening contraction. During a Joint
Flexion Extension
lengthening contraction, the muscle generates more force (up to motion
75% more, Fig. 37.10) than when it remains isometric or shortens.
This kind of contraction occurs when you lower a box from a shelf
and your biceps muscle lengthens as it exerts force, or when your
 CHAPTER 37 A N I M A L M OV E M E N T : M U S C L E S A N D S K E L E TO N S 795

FIG. 37.12 (a) Twitch and (b) fused tetanus muscle contractions. Source: Adapted from E. P. Widmaier, H. Raff, and K. T. Strang, 2011, Vander’s Human
Physiology, New York: McGraw-Hill.

a. b. Muscle force sums to higher

An action potential from a
motor nerve causes a muscle levels when action potentials
fiber to produce a twitch stimulate the muscle at higher
contraction with a slight rates, reaching a tetanus.
delay in force.
Nerve
+30 stimulus (S) S S S S S SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS
potential (mV)
Muscle fiber
membrane

0 Muscle fiber
action potential
3 Tetanus
–90

Relative force
2 Summation
force (Newtons)

0.3
Muscle fiber

Muscle
0.2 contraction Twitch
1
0.1

0 0
0 20 40 60 80 100 120 140 100 200 300 400 500 600 700 800 900 1000
Time (ms) Time (ms)
Latent
period

circumferential (circular) layers, allowing them to function as Measurements in animals indicate that skeletal muscles
muscle antagonists that control movement and shape. commonly produce tetanic contractions, which generate steady
In contrast, when muscles combine to produce similar and large forces. Tetanus is a normal physiological response of a
motions, they are termed muscle agonists. Muscles arranged as muscle, but it can also be induced by a toxin from the bacterium
agonists increase the strength and improve the control of joint Clostridium tetani, so named because it causes severe muscle
motion. For example, the three heads of the triceps are agonists of spasms, particularly of the jaw, chest, back, and abdomen.
each other (Fig. 37.11). A motor neuron and the population of muscle fibers that it
innervates are collectively termed a motor unit (Fig. 37.13).
Muscle force is summed by an increase in stimulation
frequency and the recruitment of motor units.
How do animals vary the amount of force exerted by a muscle? As
we have seen, skeletal muscles are stimulated by motor nerves, FIG. 37.13 Motor units. A motor unit consists of the motor neuron
which conduct action potentials to the neuromuscular junction. and the population of cells (fibers) it innervates.
The force exerted by a muscle depends on the frequency of
stimulation by the motor nerve. As action potentials become more
Motor
frequent, the amount of calcium that is released to activate cross-
neurons
bridge formation in the muscle fibers also increases.
A single action potential results in a twitch contraction of
a certain force (Fig. 37.12a). If sufficient time is allowed for the
muscle to pump Ca21 back into the sarcoplasmic reticulum, the Motor
contraction ends, force falls to zero, and a second stimulus can endplate
elicit a twitch contraction of the same force. However, if a second Myofibril Muscle
action potential arrives before the muscle has relaxed, a greater cell

force is produced because Ca21 remains within the myofibrils

to activate more cross-bridges. With increasing frequency of
stimulation, muscle force increases (Fig. 37.12b). This property of
muscle contraction is called force summation.
At a sufficiently high stimulation frequency, muscle force
Motor unit 1
reaches a plateau and remains steady as stimulation continues.
Motor unit 2
This muscle contraction of sustained force is called a tetanus.
796 SECTION 37.2 M U S C L E CO N T R AC T I L E P RO P E RT I E S

FIG. 37.14 Slow-twitch (darkly stained) and fast-twitch (lightly stained) skeletal muscle fibers. These two types of fiber provide differences
in endurance and the speed of contraction. (a) Mammalian limb muscle; (b) muscles of two different fish species. Photo source: Biophoto
Associates/Science Source.

a. b.
Fast-twitch glycolytic fibers (large diameter)

Fast-twitch
glycolytic
fibers are
light yellow.

Mackerel Tuna

Fast-twitch
fibers

Slow-twitch Slow-twitch
oxidative fibers
fibers are
dark red.

Slow-twitch oxidative fibers (small diameter)

Motor units can include relatively few to several hundred muscle depends on fast-twitch fibers to sprint or lunge at its prey. A key
fibers. The number of muscle fibers innervated by a given motor difference between the two types of fiber is that slow-twitch fibers
nerve, and hence the size of the motor unit, affects how finely a obtain their energy through oxidative phosphorylation (aerobic
vertebrate muscle’s force can be controlled. For example, finger respiration), and fast-twitch fibers obtain their energy mainly
muscles have small motor units allowing fine adjustments through glycolysis (Chapter 7).
in muscle force, whereas leg muscles have large motor units Although slow-twitch fibers develop force more slowly,
providing larger adjustments in force. Together with stimulation they have greater resistance to fatigue in response to repetitive
frequency, a vertebrate muscle’s force output depends on the stimulation—that is, their loss of force over time is less. Their
number of motor units (and therefore the number of muscle greater endurance is explained by the ability of their mitochondria
fibers) that are activated. Within each motor unit, muscle fibers to supply ATP to muscle fibers by aerobic respiration. Slow-twitch
generally share similar contractile and metabolic properties. fibers have many mitochondria and are well supplied by capillaries.
Differences in these properties can give muscles very different They contain an abundance of myoglobin, an oxygen-binding
capabilities, as discussed in the next section. protein related to hemoglobin that facilitates oxygen delivery
to the mitochondria (Chapter 39). The iron in myoglobin gives
j Quick Check 4 What are the two mechanisms by which the force
muscles with these fibers their red appearance. Slow-twitch
of contraction can be increased in vertebrate muscles?
muscle fibers express a chemically “slow” form of myosin with a
relatively low rate of ATP hydrolysis, limiting their speed of cross-
Skeletal muscles have slow-twitch and fast-twitch bridge cycling and force development.
fibers. White fast-twitch muscle fibers have fewer mitochondria and
The difference between a sprinter and a marathoner is not just capillaries, contain little myoglobin, and rely heavily on glycolysis
training, but also the properties of their muscles. There are to produce ATP. Fast-twitch fibers express a chemically “fast” form
different types of muscle fiber (Fig. 37.14), allowing variation in of myosin with a high rate of ATP hydrolysis, favoring rapid force
the way muscles contract. In vertebrates, these are most simply development and movement. However, fast-twitch fibers also
classified as red slow-twitch fibers and white fast-twitch fibers fatigue quickly. Because white fibers are larger than red fibers,
(Fig. 37.14a). Slow-twitch fibers are found in muscles that contract they generate more force. In general, most skeletal muscles are
slowly and consume ATP more slowly to produce force. An animal composed of mixed populations of red and white fiber types,
uses slow-twitch fibers to control its posture or move slowly and providing a broad range of contractile function.
economically. Fast-twitch fibers generate force quickly, producing Athletes who have more oxidative slow-twitch fibers excel at
rapid movements, but they consume ATP more quickly. An animal endurance and long-distance competitions. In contrast, athletes
 CHAPTER 37 A N I M A L M OV E M E N T : M U S C L E S A N D S K E L E TO N S 797

who have larger numbers of fast-twitch fibers perform better red muscle that they can keep warm (Fig. 37.14b), enabling their
in sprint races and weight-lifting competitions. Because the muscles to contract longer and faster to enhance their swimming.
distribution and properties of muscle fiber types are strongly When escaping from a predator or attacking prey, fish recruit their
inherited, it is largely true that champions are born and not made. larger fast-twitch white trunk musculature for rapid swimming.
Nevertheless, aerobic training can significantly increase the energy The evolutionary arms race between predators and prey
output through oxidative phosphorylation of an individual’s muscle has resulted in selection for particular muscle fiber types and
fibers, just as weight lifting and speed training can enhance an other musculoskeletal specializations for rapid movement and
individual’s strength and sprinting ability. Contrary to popular maneuvering. Features of the skeleton that contribute to these
belief that weight lifting builds muscle by straining the muscle to specializations are discussed next.
build new fibers, it actually builds muscle by increasing the size of
existing muscle fibers through the synthesis of additional myosin
and actin filaments. This synthesis increases the cross-sectional area 37.3 ANIMAL SKELETONS
of the fiber and thus its force-generating capacity.
Most animal skeletons provide a rigid set of elements that meet
at joints (articulate) to transmit muscle forces for movement
? CASE 7 PREDATOR–PREY: A GAME OF LIFE AND DEATH
and body support. They also enable many animals to manipulate
How do different types of muscle fiber affect the their environment by digging burrows, building nests, or
speed of predators and prey? constructing webs to catch food. Animals have evolved three types
We have just seen that slow-twitch muscle fibers provide of skeletal system: hydrostatic skeletons, exoskeletons, and
endurance for sustained activity, whereas larger fast-twitch endoskeletons. All three types of skeleton have rigid elements
fibers produce greater speed and strength but fatigue quickly. that resist the pull of antagonist sets of muscles.
The relative distribution of these fiber types in the muscles of
different animals is influenced mainly by genetic heritage. This Hydrostatic skeletons support animals by muscles that
affects the locomotive behavior and capacities of both predator act on a fluid-filled cavity.
and prey animals. Whereas cheetahs have exceptional speed, their Hydrostatic skeletons evolved early in multicellular animals
muscles tire quickly. As their prey, antelope have muscles that with the first cnidarians (jellyfish and anemones) approximately
enable endurance long-distance running and contract rapidly for 600 million years ago. They are found in nearly all multicellular
maneuvering. animals as well as in many vascular plants. In animals that depend
Highly aerobic animals such as dogs and antelope have large on a hydrostatic skeleton, fluid contained within a body cavity
fractions of slow-twitch fibers and specialized fast-twitch muscle serves as the supportive component of the skeleton. Muscles exert
fibers that have a high oxidative capacity and use glycolysis for pressure against the fluid to produce movement.
rapid ATP synthesis. These muscle fibers allow the animals to move Two sets of opposing muscles surround the fluid, controlling
for extended periods of time at relatively fast speeds. In contrast, the width and length of the body cavity and, in many cases, of the
cats have mainly fast-twitch fibers, enabling them to sprint whole animal (Fig. 37.15a). Circular muscles reduce the diameter
and pounce quickly. Nevertheless, cats and other animals use of the body cavity, and longitudinal muscles reduce its length. A
particular muscles with high concentrations of slow-twitch fibers sea anemone can bend its body in various directions to feed or to
(these are the dark-staining fibers shown in Fig. 37.14a) for slower resist water currents by closing its mouth to keep its fluid volume
postural movements and stealth. Animals such as sloths and lorises constant and contracting its circular and longitudinal muscles.
that move slowly have mostly oxidative slow-twitch fibers and few Sea anemones extend themselves into the water column by
glycolytic fast-twitch fibers. contracting their circular muscles and relaxing their longitudinal
More generally, animals with high body temperatures and muscles. Controlling the amount of fluid in their body cavity also
high metabolic rates (birds and mammals) have larger numbers allows sea anemones to adjust their shape. When threatened, a sea
of oxidative fibers compared with animals that have lower body anemone rapidly retracts by contracting its longitudinal muscles
temperatures and lower metabolic rates (reptiles). As a result, and allowing water to escape from its body cavity.
lizards use glycolytic fast-twitch fibers to sprint for brief periods Similarly, earthworms burrow through the soil by moving a
but must then recover from the acid buildup produced by their series of hydrostatic segments along their body (Fig. 37.15b). By
muscles’ anaerobic activity in longer periods of rest (Chapter 40). contracting longitudinal muscles in a few segments, they shorten
In contrast, birds and mammals can sustain activity over longer those segments and also widen them to anchor those segments
time periods because they have oxidative muscle fibers supported against the soil. They then extend intervening segments between
by aerobic ATP supply. anchor points by contracting circular muscles, causing the
Most fish have a narrow band of slow-twitch red muscle fibers segments to lengthen and move the animal’s body through the soil.
that runs beneath their skin, which they use for slow steady Because of their simple but effective organization, hydrostatic
swimming. Tuna and some sharks have evolved deeper regions of skeletons are well adapted for many uses. In animals such as squid
798 SECTION 37.3 A N I M A L S K E L E TO N S

FIG. 37.15 Diverse examples of hydrostatic skeletal support. (a) In sea anemone; (b) in earthworm; (c) intervertebral discs; (d) articular
cartilage. Photo sources: a. Carole Valkenier/Getty Images; b. Nigel Cattlin/Alamy.
a. b.

Sea anemones contract Earthworms use the

longitudinal and circular muscles hydrostatic support of
in the body wall to compress the each segment to change
fluid in the body cavity for shape and move by
support and to change shape. contracting longitudinal
and circular muscles.

Partition
between
Fluid-filled Blood
segments
body cavity vessel
Epidermis Gut
Longitudinal
muscles Longitudinal
Circular muscles muscle
Fluid-filled
body cavity Circular muscle

Body wall

c. d.
Spinal cord
Nerve root

Ligament
Vertebra

Disc Joint cavity

Articular
cartilage
Spongy
bone

Humans and other vertebrates

rely on hydrostatic support in the
discs of the vertebral column.
Compact bone

Articular cartilage is a hydrostatic

tissue that cushions joints and
allows them to rotate smoothly.

and jellyfish, the hydrostatic skeleton has become specialized for bony vertebrae of the backbone (Fig. 37.15c). Each disc has a wall
jet-propelled locomotion. Large circular muscles contract to eject reinforced with connective tissue that surrounds a jelly-like fluid.
fluid from the body cavity, propelling the animal in the opposite Intervertebral discs enable the backbone to twist and bend. Disc
direction. The muscles of other mollusks also exert pressure walls damaged by repeated stress can rupture, causing significant
against a hydrostatic skeleton. By this means, for example, a clam lower back pain. Articular cartilage, another fluid-filled tissue,
uses its muscular “foot” to burrow into the sediment, and an forms the joint surfaces between adjacent bones (Fig. 37.15d). The
octopus adeptly manipulates its flexible tentacles. (Clams also fluid within the cartilage resists the pressure between the ends of
have shells that form an exoskeleton to protect the body.) These articulating bones, cushioning forces transmitted across the joint.
same principles allow an elephant to control its trunk, and our
tongue to manipulate food and assist in speech. Exoskeletons provide hard external support
Even vertebrate animals with rigid endoskeletons have and protection.
hydrostatic elements that provide flexibility and cushion loads An exoskeleton is a rigid skeleton that lies outside the animal’s
transmitted by the skeleton. These include intervertebral discs soft tissues. The first mineralized skeletons arose with sponges
and cartilage. Intervertebral discs are sandwiched between the about 650 million years ago, but the more common exoskeletons
 CHAPTER 37 A N I M A L M OV E M E N T : M U S C L E S A N D S K E L E TO N S 799

FIG. 37.16 Examples of exoskeletons. The shells of (a) a bivalve mollusk (b) and nautilus, made of calcium carbonate, surround and protect internal
soft body parts. The exoskeleton of arthropods, such as (c) a grasshopper, is made of stiff cuticle that surrounds internal muscles and
tendons and other internal body parts. Sources: a. Rita van den Broek/age fotostock; b. Reinhard Dirscherl/Getty Images; c. Jim Simmen/Getty Images.

a b c

subsequently evolved in other invertebrate groups adapted for cuticle of arthropods is soft, so the animal can grow before the
life in aquatic and terrestrial environments (Fig. 37.16). Because cuticle hardens. Mature cuticle consists of two layers. A thin outer,
they are on the exterior of the animal, exoskeletons provide hard waxy layer minimizes water loss. Water loss is especially dangerous
external support and protection, allowing muscles to attach from for terrestrial arthropods, given their small size. The much
the inside. However, a main disadvantage is that exoskeletons thicker inner layer, whether flexible or stiff, is tough and hard
limit growth. to break. The cuticle remains flexible at joints, allowing motion
Different invertebrates have different kinds of exoskeleton, between body segments. Because a rigid exoskeleton restricts
but all types have relatively few cells for their volume of tissue. growth, arthropods shed their cuticle at intervals, a process called
The shells of bivalve mollusks, such as clams and mussels, or the molting. Molting allows arthropods to expand and grow before
cephalopod mollusk Nautilus that has existed largely unchanged forming a new rigid exoskeleton.
for nearly 500 million years, form a hard, mineralized calcium Marine crustaceans, such as crabs and lobsters, incorporate
carbonate exoskeleton (Figs. 37.16a and 37.16b). The calcium calcium carbonate in their cuticle. Calcium carbonate makes
carbonate is reinforced by proteins and is therefore an example the exoskeleton hard and stiff, much like the hardened cuticle
of a composite material, one that combines substances with of terrestrial insects, helping to protect these crustaceans from
different properties. Because of its composite nature, the shell predators.
is less brittle and more difficult to break than if it were made While offering several protective benefits, exoskeletons pose
of just one substance. The shell expands as the organism grows risks as well. Animals are vulnerable when the newly formed
and epidermal cells deposit new layers of mineralized protein in exoskeleton has not yet hardened. Exoskeletons are also hard to
regular geometric patterns. You can see the growth rings if you repair. If a skeleton is damaged while growing, the animal must
examine a mollusk shell. Although some marine bivalves achieve produce an entirely new one. Because the exoskeleton is a thin-
large size over many years, growth is limited by the rate at which walled structure, it is prone to breaking if its surface area is very
new skeletal material can be deposited on the outside of the shell. large. Consequently, the imperviousness of the monstrous insects
Arthropods have more complex exoskeletons. More than half depicted in science-fiction films is improbable—they would easily
of all known animal species, including insects and crustaceans, break from a blow to their exoskeleton.
are arthropods. Arthropods are defined in part by their jointed
legs, and another important feature is their exoskeleton formed The rigid bones of vertebrate endoskeletons are
of a cuticle that covers their entire body (Fig. 37.16c). Arthropod jointed for motion and can be repaired if damaged.
exoskeletons, which first evolved in aquatic crustaceans, protect Endoskeletons first evolved about 350–500 million years ago
animals from desiccation and physical insults. Consequently, when the first vertebrates evolved cartilaginous endoskeletons.
exoskeletons were key to the success and diversification of insects Lampreys are descendants of these early vertebrates.
in terrestrial environments about 450 million years ago. Subsequently, bony fishes evolved a more rigid mineralized
The cuticle of insects is composed mainly of chitin, a internal skeleton, a defining feature of this diverse group of
nitrogen-containing polysaccharide. When initially formed, the vertebrates. Sharks and other elasmobranchs (the cartilaginous
800 SECTION 37.4 V E RT E B R AT E S K E L E TO N S

fishes) evolved unusual skeletons

of calcified cartilage, losing the FIG. 37.17 The vertebrate skeleton, showing axial and appendicular regions.
earlier mineralized bony skeletons
a. Human b. Cat
of their bony fish ancestors.
A bony or cartilaginous
endoskeleton lies internal to most of
an animal’s soft tissues. In contrast
to exoskeletons, the endoskeletons
of vertebrate animals can grow
extensively and, when broken, can
be repaired. Endoskeletons also
provide protection for certain key
internal organs, such as the brain,
lungs, and heart.
The bones of vertebrate
skeletons consist of a variety of
tubular, rodlike, and platelike Axial skeleton
elements that form a scaffold to Appendicular skeleton
which the muscles attach. Because
bones are rigid and hard, there are
joints between adjacent bones to
enable movement. Muscles attach
to the skeleton by connective
tissue and specialized tendons in close association with the protein collagen. Generally, bone
made of collagen. Tendons transmit consists of two-thirds hydroxyapatite mineral and one-third
muscle forces, allowing the forces type I collagen protein. The combination of mineral and protein
to be redirected and transmitted makes bone a composite material, like exoskeletons. Bone is
over a wide range of joint motion. rigid (because of the properties of the mineral), but also hard to
Tendons, like a spring, also store break because it absorbs much energy before fracture (because
and recover elastic energy. of the properties of the collagen). Bone gradually becomes more
The vertebrate skeleton can be divided into axial and mineralized and brittle as a normal process of aging. The genetic
appendicular regions (Fig. 37.17). The axial skeleton consists bone disorder osteogenesis imperfecta leads to bones that are
of the skull and jaws of the head, the vertebrae of the spinal brittle and fragile because the osteoblasts produce insufficient
column, and the ribs. Bones of the skull protect the brain and collagen in the extracellular matrix.
sense organs of the head. The appendicular skeleton consists of Articular cartilage located at the joint surfaces of a bone
the bones of the limbs, including the shoulder and pelvis. The forms a cushion between bones meeting at a joint. The cartilage
axial and appendicular regions reflect the evolutionary ancestry is a gel-like matrix that resists fluid being squeezed out when
of vertebrates. The axial skeleton formed first, to protect the forces press on the cartilage during movement. Articular cartilage
head and provide support and movement of the animal’s body by consists of 70% water, 15% large, branched molecules called
bending its body axis. As a result, the axial skeleton is the main proteoglycans, and the remaining 15% type II collagen. The
skeletal component of fishes. The appendicular skeleton includes collagen fibers reinforce the cartilage, allowing the cartilage to
the pectoral and pelvic fins of fish, which evolved into limbs when cushion and distribute loads as the joint moves. Cartilage may
vertebrate animals first moved onto land. In a special group of degenerate for a variety of reasons, including rheumatoid arthritis
lobed-fin fish, the fin bones became adapted for an amphibious and repetitive physical injury, but in general it lasts over an
lifestyle, evolving into the limb bones that terrestrial vertebrates individual’s lifetime.
use for support against gravity and for movement on land. j Quick Check 5 Glass is strong and rigid but can be easily broken.
Skeletal tissues have relatively few cells for their volume In contrast, a bone is strong and rigid but hard to break. How can
because they consist mostly of an extensive extracellular matrix this be?
that is secreted by specialized cells, forming connective tissue
external to the cells (Chapter 10). In addition to tendon, three
main extracellular tissues are produced in the formation of the 37.4 VERTEBRATE SKELETONS
vertebrate skeleton: bone, tooth enamel, and cartilage.
Bone tissue–forming cells called osteoblasts synthesize and Although both vertebrate skeletons and invertebrate exoskeletons
secrete calcium phosphate as hydroxyapatite mineral crystals are rigid and have flexible joints, certain features distinguish them.
 CHAPTER 37 A N I M A L M OV E M E N T : M U S C L E S A N D S K E L E TO N S 801

presence of blood vessels triggers the

FIG. 37.18 Bone formation from a cartilage model. (a) Most bones other than the skull are transformation of cartilage into hard,
formed as a cartilage first, followed by vascularization and the transformation into mineralized bone.
bone tissue by osteoblasts. (b) Cartilage model of a human fetal hand, showing initial
sites of bone tissue formation at about 9 weeks of embryonic development. Photo source: Two main types of bone are
Science Photo Library/Science Source. compact and spongy bone.
a. Epiphysis b. The cartilage template of a bone is
replaced by one of two main types
of bone (Fig. 37.19). Both types
Growth
Proliferating plate contribute to the overall form of
cartilage cells many bones in the body. Compact
bone forms the walls of the bone’s
Blood vessel
shaft. It consists mainly of dense,
mineralized bone tissue containing
bone cells derived from osteoblasts
Diaphysis and the network of blood vessels
Bone supplying them with oxygen and
Cartilage cells marrow
and extracellular nutrients.
matrix Bone A second type of bone, called
3 As cartilage is further spongy bone, consists of small plates
transformed into bone, and rods known as trabeculae with
1 Initially the new cartilage continues
spaces between them. Spongy bone is
future bone is 2 Bone starts to form where to be added at the growth
formed of blood vessels penetrate, causing plate, causing the bone to found in the ends of limb bones and
cartilage. cartilage to degenerate locally. grow in length. within the vertebrae. It reduces the
weight of bone in these regions, but it

Here we explore the biology of vertebrate skeletons, focusing

on how bones develop and grow and how they are repaired and FIG. 37.19 Compact and spongy bone. Source: Dave King/Getty Images.
remodeled.

Vertebrate bones form directly or by forming a Epiphysis

cartilage model first. Spongy
Vertebrate bones form and develop by two distinct processes. bone
Bones of the skull and the ribs form when precursor cells
differentiate into osteoblasts that immediately begin producing
bone as a skeletal sheet or membrane. These bones are often Compact
referred to as membranous bones. bone
This method of forming bone is slow, however, and, if most
bones in the body formed this way, the fetus would take a very long Marrow
time to develop. Most other bones of the body—the vertebrae, cavity
Diaphysis
shoulder, pelvis, and limb bones—are formed as cartilage first
(Fig. 37.18). The precursor cells initially become cartilage-producing
cells, known as chondroblasts, which in the embryo grow into
cartilage models of most bones of the developing body (Fig. 37.18a).
For example, at approximately 9 weeks of development, a human
fetal hand consists mainly of this cartilage model (Fig. 37.18b).
This embryonic cartilage is the same as the articular cartilage that
remains at the ends of bones to form the joint surfaces. Because
cartilage is a pliable, fluid-filled matrix, it grows by expansion within
Epiphysis
the structure as well as at the surface, enabling the fetal skeleton
to grow rapidly. As the fetus grows and its skeleton matures,
the cartilage model of the bone is invaded by blood vessels. The
802 SECTION 37.4 V E RT E B R AT E S K E L E TO N S

is also stiff so that forces are transmitted effectively from the joint
to the bone shaft. Bone marrow, a fatty tissue found between FIG. 37.20 Hinge and ball-and-socket joints. The elbow and knee
trabeculae and also within the bone’s central cavity, contains are examples of hinge joints. The shoulder and hip are
many important cells, including blood-forming cells, other stem examples of ball-and-socket joints.
cells (Chapter 20), immune system cells (Chapter 43), and fat cells.
Ball-and-socket

Bones grow in length and width, and can be repaired. Highly mobile
ball-and-socket
Bones can grow in length, at least until an animal reaches maturity. joints allow
Growth in length occurs at a growth plate (see Fig. 37.18), a region motion along
of cartilage between the middle region, called the diaphysis, and all three axes.

the end, called the epiphysis. The growth plate remains in place
when the rest of the cartilage is transformed into bone. The growth Hinge
plate adds new cartilage toward the bone’s diaphysis, enabling
bone length to continue to increase after birth (see Fig. 37.18). Hinge joints
At maturity, each growth plate fuses as the remaining cartilage limit motion
to one
is replaced with bone, preventing further growth in length. The primary
fusion of growth plates is typical of most mammals and birds. In axis.
amphibians and reptiles, the growth plates often do not fuse and
growth may continue at a slower pace over much of their lifetime.
Limb bones grow in diameter when osteoblasts deposit new
bone on the bone’s external surface. At the same time, bone is
removed from the inner surface, expanding the marrow cavity.
Bone removal is slower than bone growth, thickening the walls
during growth. Bone is removed from the marrow cavity by a
group of cells called osteoclasts that secrete digestive enzymes
and acid to dissolve the calcium mineral and collagen. These
dissolved compounds are reabsorbed and recycled for bone
formation in other regions of the skeleton. Through this process,
the vertebrate skeleton serves as an important store of calcium
and phosphate ions. For example, in female birds, calcium
removed from the skeleton is regularly used to form the eggshell.
A great advantage of endoskeletons compared with
exoskeletons is that a damaged endoskeleton can be repaired by
osteoblasts and osteoclasts forming and removing mineralized They allow you to flex and extend your fingers as well as spread
tissue in particular regions of the bone. Generally, physically them laterally or move them together when making a fist or
active younger adults have thicker bones than less active younger grasping objects.
adults because physical loading stimulates osteoblasts to produce Joints with a broader range of motion are generally less stable.
more bone during growth. However, in older adults, bone tissue is The shoulder is the most mobile joint in the human body, but it
gradually lost as osteoclasts remove more bone than is produced is also the most often dislocated or injured. In contrast, the ankle
by osteoblasts. Bone loss is particularly severe in women after joint of dogs, horses, and other animals is a stable hinge joint that
menopause, in part because of hormone shifts as well as reduced is unlikely to be dislocated, but its range of motion is limited to
physical activity, and can lead to osteoporosis, a condition that flexion and extension. Because muscles are arranged as paired sets
significantly increases the risk of bone fracture. of antagonists to move a joint in opposing directions, hinge joints
are controlled by as few as two antagonist muscles (generally
Joint shape determines range of motion and skeletal referred to as a flexor and an extensor). In contrast, ball-and-socket
muscle organization. joints have at least three sets of muscle antagonists to control
The shapes of the bone surfaces that meet at a joint determine motion in three different planes. As a result, a more complex
the range of motion at that joint. Joints range from simple hinge organization of muscles is needed to control the movements of the
joints that allow one axis of rotation to ball-and-socket joints arm at the shoulder joint or the leg at the hip joint.
that allow rotation in three axes (Fig. 37.20). The human elbow
joint and the ankle joint of a dog are examples of hinge joints. The Muscles exert forces by skeletal levers to produce
shoulder and hip joints are examples of ball-and-socket joints, joint motion.
which allow the widest range of motion, as when you throw or By serving as a rigid set of levers, the skeleton enables muscles
kick a ball. The joints at the base of each finger are intermediate: to transmit forces that cause joint rotation. Muscles that attach
 CHAPTER 37 A N I M A L M OV E M E N T : M U S C L E S A N D S K E L E TO N S 803

farther from a joint’s axis of rotation produce stronger but slower

FIG. 37.21 Torque. Torque or rotational strength (F ⫻ r) depends movements, whereas muscles that attach closer to a joint produce
on the muscle’s force (F ) and the distance the force weaker but faster movements. This is why you open a door by
acts from the joint’s axis of rotation (r). pulling on a doorknob located opposite from its hinges. The
doorknob’s location increases the lever distance, making it easier
F 35 kg
to pull the door open than if it were placed next to the hinge.
The strength of rotational movement, or torque, is determined
by the product of a muscle’s force ( F) and lever distance ( r) from
the axis of joint rotation: F ⫻ r. Therefore, muscles produce
The rotational strength of the stronger rotational movements either by exerting a larger force
elbow is determined by the force
(F) exerted by the biceps muscle or by having a longer lever distance between the joint and muscle
and the lever distance (r). attachment site. Let’s consider a simple example: a weight carried
by your hand (Fig. 37.21). In this case, the axis of rotation is the
elbow, and contraction of the biceps muscle causes the arm to flex
Site of muscle
attachment
at the elbow. The strength of elbow rotation is determined by the
biceps muscle’s force (F) and the lever distance (r) between the
Joint
axis of elbow joint and the site of muscle attachment, where the muscle
rotation pulls on the arm bone.
In addition to the muscle’s force ( F) and lever distance (r), the
5 cm
Muscle distance from the joint to the site where the force is exerted ( L) also
r 35 cm
lever distance 5 kg affects movement (Fig. 37.21, Fig. 37.22). Animal joints located a
L long distance from the muscle attachment site (large r) and a short

FIG. 37.22 Functional anatomy. The armadillo forelimb skeleton is adapted for digging, whereas the horse forelimb is adapted for speed and
weight support. Sources: Based on M. J. Smith and R. J. G. Savage, 1956, “Some Locomotory Adaptations in Mammals,” Zoological Journal of the Linnean Society
42:603–622. Photos ( left to right): Morales/age footstock; J. L. Klein & M. L. Hubert / Science Source.

Armadillo Horse

Line of muscle force

Line of muscle
Armadillo forelimbs are force Horse limbs are long
relatively short for the r for the animal’s size,
animal’s size, have a lot have reduced weight
of weight in the foot, and L toward the hoof, and
have large r compared to have small r compared
L, which produce large to L, which produce
digging forces. L rapid limb movements.

Force exerted on ground Force exerted on ground

804 CO R E CO N C E P T S S U M M A RY

distance from where the output force is exerted (small L) produce the muscle attachment site (small r) and a long distance from where
strong but slow movements. Moles, armadillos, and spade-foot the output force is exerted (large L) produce faster movements but
toads are examples of digging animals that have this type of muscle– with less force. These are found in animals adapted for high-speed
joint arrangement. In contrast, joints located a short distance from running, such as antelope, cheetahs, and horses. •

Core Concepts Summary Muscles exert more force when they contract at slow
velocities compared with when they contract at high
37.1 Muscles are biological motors composed of actin velocities. page 793
and myosin that generate force and produce movement Because muscles can transmit force only by pulling on the
for support, locomotion, and control of internal skeleton, they are arranged as antagonist pairs to produce
physiological functions. reciprocal motions of a joint or limb. page 794
There are two main types of muscle: striated (skeletal and In vertebrates, a motor unit consists of a single motor neuron
cardiac) and smooth. page 787 and the muscle fibers (cells) it innervates. page 795
Skeletal and cardiac muscle fibers appear striated when Muscle force is increased by increasing the motor neuron firing
viewed under the microscope because of the regular spacing of rate and, in vertebrates, by increasing the number of motor units
sarcomeres along their length. page 787 that are activated. page 796
Smooth muscle fibers, which regulate airflow for breathing, Vertebrate muscles have two types of fiber: red slow-twitch
blood flow through arteries, and the passage of food through fibers that contract slowly over longer time periods and white
the gut, lack regular sarcomere organization and appear smooth fast-twitch fibers that contract rapidly but fatigue quickly.
when viewed under the microscope. page 787 page 796
Skeletal muscle fibers are long thin cells composed of parallel
sets of myofibrils built up from smaller parallel arrays of actin
37.3 Hydrostatic skeletons, exoskeletons, and
and myosin filaments. page 787
endoskeletons provide animals with mechanical support
The sarcomere is the basic contractile unit of a skeletal and protection.
muscle. Sarcomeres are arranged in series along the length of a
myofibril. page 788 The rigid element of a hydrostatic skeleton is an incompressible
fluid within a body cavity, used to support the body and change
Muscles change length and produce force by the formation of shape. page 797
actin–myosin cross-bridges, causing myosin and actin filaments
to slide relative to each other. page 789 Invertebrate exoskeletons form an external rigid support system,
which protects the animal and limits water loss, but also limits
Muscles are stimulated by motor neurons at the fiber’s growth and repair. page 798
motor endplate, leading to depolarization of the muscle cell
that triggers the release of Ca21 ions from the sarcoplasmic Vertebrates have a bony endoskeleton that provides rigid
reticulum. Ca21 binds troponin, which moves tropomyosin off support and protection of body organs, and that can grow and be
the myosin-binding sites on actin, allowing myosin heads to repaired. page 799
form cross-bridges with actin. page 791 The vertebrate endoskeleton is organized into axial (central)
Excitation–contraction coupling is the process by which and appendicular (limb) components. page 800
depolarization of the muscle cell leads to its shortening.
Vertebrate endoskeletons consist of bone and cartilage.
page 792
page 800

37.2 The force generated by muscle depends on Bone is a composite tissue that consists of calcium phosphate
muscle size, degree of actin–myosin overlap, shortening mineral and type I collagen and that is rigid and hard to break.
velocity, and stimulation rate. page 800

Muscles exert their greatest force when actin–myosin Cartilage is a fluid-based gel reinforced by type II collagen and
filaments have maximal overlap, allowing the most cross- proteoglycans, providing cushioning support at joint surfaces
bridges to form. page 792 page 800
 CCHHAAPPTTEERR 3377 AANNI IM
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MUUSSCCLLEESS AANNDD SSKKEELLEETO
TONNSS 880055

37.4 Vertebrate endoskeletons allow for growth and events to how muscles generate force and shorten when they
repair and transmit muscle forces across joints. are stimulated to contract, identifying the role that ATP plays.

Vertebrate bones develop by one of two processes: either 4. Draw a graph of the isometric force–length relationship
directly or by way of a cartilage model. page 801 of striated muscle, indicating where maximal overlap between
actin and myosin filaments occurs.
Cartilage forms much of the embryonic skeleton and
remains as growth plates within the bone to provide rapid 5. Draw a graph of the force–shortening velocity
growth after birth, as well as forming the bone’s joint relationship of striated muscle.
(articular) surfaces. page 801 6. Compare the force produced by a muscle over time
when it is stimulated by a single twitch stimulus with the
Bone has two basic structures: Compact bone forms the
force produced by multiple stimuli at low frequency, and then
solid walls of a bone’s shaft, and spongy bone forms a mesh
with the force produced when the stimulation frequency is
that supports the cartilage at the bone’s ends. page 801
increased.
Bone formation by osteoblasts and removal by osteoclasts is
7. Name the two basic structural elements common to all
a continuing process of growth, shape change, and repair.
animal musculoskeletal systems.
page 802
8. Compare and contrast three features of an exoskeleton
The shape of a bone’s joint surfaces largely determines the
and an endoskeleton.
range of motion and stability of a joint. page 802
9. Identify the two primary components of bone tissue and
Muscles produce joint movements by transmitting forces by
explain how each of these contributes to a bone’s strength,
means of rigid skeletal levers. page 802
stiffness, and resistance to fracture.
10. Diagram a limb bone, such as the tibia, showing the regions
Self-Assessment of articular cartilage, spongy trabecular bone, compact bone
tissue, and the marrow cavity.
1. Diagram a sarcomere, showing the basic organization of
thin (actin) and thick (myosin) filaments. Indicate on your
diagram the regions where myosin cross-bridges form Log in to to check your answers to the Self-
Assessment questions, and to access additional learning tools.
with actin.
2. Describe the sequence of events that occurs when an
action potential arrives at a muscle fiber’s motor endplate,
causing the muscle to be depolarized.
3. Describe the sequence of events involved in myosin
cross-bridge attachment and detachment. Relate these
this page left intentionally blank
 CHAPTER 38

Animal
Endocrine
Systems

Core Concepts
38.1 Animal endocrine systems
release chemical signals called
hormones into the bloodstream
that respond to environmental
cues, regulate growth and
development, and maintain
homeostasis.
38.2 Hormones bind to receptors on
or inside target cells, and their
signals are amplified to exert
strong effects on target cells.
38.3 In vertebrate animals, the
hypothalamus and the pituitary
gland control and integrate
diverse bodily functions and
behaviors.
38.4 Chemical communication
can also occur locally
between neighboring cells
or, through the release
of pheromones, between
individuals, coordinating social
interactions.
FLPA/Chris Mattison/age fotostock.

807
808 SECTION 38.1 A N OV E RV I E W O F E N D O C R I N E F U N C T I O N

We have seen how the nervous system allows an animal to physiological functions of the organism. In particular, we examine
sense and respond to its environment, coordinating the animal’s the mechanisms by which hormones trigger cellular responses in
movement and behavior (Chapters 35–37). The nervous system also their target organs. Because most hormones are evolutionarily
works closely with the endocrine system to regulate an animal’s conserved, their chemical structure is similar across a diverse array
internal physiological functions. This coordination is accomplished of animals. However, their functions often evolve rapidly, enabling
by signals sent from the nervous system to the endocrine system, hormones to serve new and broader roles.
which distributes chemical signals throughout the body. The
endocrine system is made up of an interacting set of secretory
glands and organs, located in different regions of the body, which 38.1 AN OVERVIEW OF
respond to nervous system signals as well as to blood-borne signals ENDOCRINE FUNCTION
from other organs to regulate internal bodily functions.
The signals communicated by the nervous and endocrine The word “hormone” often popularly connotes teenagers and
systems differ greatly in their modes of transmission and the times the changes that occur to their bodies as they grow and mature.
over which they act. The nervous system sends signals rapidly by Hormones, as we will see, do play a key role in animal growth and
action potentials running along nerve axons, and communication development—but that is only one of their roles. Hormones play
occurs between adjacent nerve cells by means of neurotransmitters. diverse physiological roles in the body, in particular regulating an
The endocrine system, by contrast, relies on cells and glands that organism’s response to the environment and helping to maintain
secrete chemical signals called hormones, which influence the stable physiological conditions within cells or within the animal
actions of other cells in the body. Whereas some hormones act as a whole. We start by highlighting these functions before
locally, many others are released into the bloodstream, allowing addressing the molecular mechanisms of how hormones work in
them to circulate and influence distant target cells throughout the section 38.2.
body. As a result, endocrine communication is generally slower and
more prolonged than the rapid and brief signals transmitted by The endocrine system helps to regulate an organism’s
nerve cells. Hormones exert specific effects on particular target cells response to its environment.
within the body by binding to receptors on or in target cells. The Organisms face constant changes in their environment. These
downstream influence of hormones is amplified in a series of steps changes often present physiological challenges or stresses that
so that a small amount of hormone can have dramatic effects in the the organism must respond to by altering the functional state
body (Chapter 9). of its body. Changes in light or temperature, the threat of a
In this chapter, we explore the properties and actions of predator, or the presence of a potential mate stimulate responses
hormones that help to coordinate and maintain a broad set of of an animal’s nervous system and endocrine system. Endocrine

FIG. 38.1 Growth and development in (a) moths and caterpillars and in (b) grasshoppers.

a. b.
Metamorphosis Molting

Pupa Adult 5th nymph

Adult
Molting Molting
Eggs

Hatching
4th nymph Eggs
5th instar

1st instar Hatching

Molting
Molting Molting

1st nymph
4th instar

2nd instar 3rd nymph

Molting
Molting 2nd nymph
3rd instar Molting
Molting
 CHAPTER 38 A N I M A L E N D O C R I N E S YS T E M S 809

responses are commonly triggered by sensory signals received by exoskeleton is referred to as molting. Each molting produces
the nervous system that are relayed to the endocrine system. The a new larval stage. In some insects, such as moths, the animal’s
endocrine system, with its slower and more prolonged signaling, body also changes dramatically, undergoing metamorphosis at
reinforces physiological changes in the animal’s body that better a key stage in development (Fig. 38.1a). In other insects, such
suit the environmental cues received by its nervous system. as grasshoppers and crickets, the animal grows larger after each
When a gazelle sees or smells a predator, its endocrine system molting, but with little change in body form (Fig. 38.1b).
helps to ready its body for rapid escape. When a female cardinal Molting and metamorphosis are regulated by hormones
sees a bright red, singing male early in spring, endocrine signals released from tissues in the insect’s head. British physiologist
released from within the female cardinal’s brain initiate changes in Vincent Wigglesworth studied their effects on the blood-sucking
its reproductive organs and increase its behavioral responsiveness, insect Rhodnius. Normally, Rhodnius goes through five juvenile
encouraging it to select the male as its mate. And when particular stages before developing into its final adult body form. Like
smells stimulate a sense of hunger and digestive function in grasshoppers and crickets, it does not undergo metamorphosis.
animals such as dogs and humans, the animals are attracted Each of the developmental stages is triggered by a blood meal.
to food. In each of these cases, the endocrine system helps an Wigglesworth showed that he could prevent molting if the head
organism respond appropriately to environmental cues. of the insect were removed shortly after a blood meal (Fig. 38.2).
Rhodnius can survive for long periods despite having its head
The endocrine system regulates growth and removed. If the insect takes a blood meal and the head is removed
development. a week later, the animal’s body undergoes a molt and grows into its
Growth and development require broad changes in many different adult form. Wigglesworth hypothesized that a diffusing substance
organ systems. The release of circulating hormones from the from the animal’s head controls its molt. In the first case, this
endocrine system accomplishes these changes, regulating how substance does not have time to reach the body to trigger a molt,
animals develop and grow. For example, we discuss in Chapter 42 but in the second it does.
the importance of the sex hormones estrogen and testosterone The German endocrinologist Alfred Kuhn found a similar
in determining female or male sexual characteristics both in the result in experiments performed on moth caterpillars. Tying the
embryo and during puberty. caterpillar’s head off from the rest of the body prevented the body
As another example, let’s consider how the well-studied from molting. Wigglesworth and Kuhn considered the diffusing
endocrine system of insects regulates their growth and substance to be a brain hormone. Subsequent work showed that
development. Insects produce a rigid exoskeleton (Chapter 37), cells in a specialized region of the insect brain secrete the peptide
which must be periodically shed and replaced by a larger new now known as prothoracicotropic hormone (PTTH) (Table 38.1).
one to enable the insect to grow (Fig. 38.1). Shedding of the In Rhodnius, PTTH is released after a blood meal and acts to trigger

TABLE 38.1 Major Invertebrate Hormones

SECRETING TISSUE TARGET GLAND

OR GLAND HORMONE OR ORGAN ACTION
Brain Brain hormone Prothoracic gland Stimulates release of ecdysone
(PTTH) (peptide) to trigger molt and metamorphosis

Corpora allata Juvenile hormone All body tissues Inhibits metamorphosis to adult stages; stimulates
(peptide) retention of juvenile characteristics

Prothoracic gland Ecdysone All body tissues Stimulates molt and, in absence or low levels of juvenile
(steroid) hormone, stimulates metamorphosis

Nervous system Ecdysone All body tissues Stimulates growth and regeneration in sea anemones,
(steroid) flatworms, nematodes, annelids, snails, and sea stars

Brain Melanocyte- Chromatophores Stimulates pigmentation changes in cephalopods

stimulating hormone (octopus, squid, and cuttlefish)
(peptide)

Brain and eye stalks Chromatotropins Chromatophores Stimulates pigmentation changes in crustaceans
(peptides)

Reproductive gland Androgen Reproductive tract Regulates development of testes and male secondary
(peptide) sexual characteristics of crustaceans
HOW DO WE KNOW?

FIG. 38.2

How are growth and development controlled in insects?

BACKGROUND During the 1930s, British physiologist Vincent RESULTS Wigglesworth showed that if a bug is decapitated
Wigglesworth studied how a blood meal taken by the bug less than an hour after a blood meal, it fails to molt. If a bug is
Rhodnius triggered its molting and growth. This work pioneered decapitated 1 week after the blood meal, it molts (Fig. 38.2b,
the discovery of hormonal substances that stimulated its growth. Experiment 1). He also found that if he used a fluid tube to join
Rhodnius goes through five successive larval stages (called nymphs) the body of a bug decapitated immediately after a blood meal with
before becoming a winged adult, as shown in Figure 38.2a. Each the body of one that wasn’t decapitated until a week after feeding,
developmental step is triggered by a blood meal. the bodies of both bugs molted (Fig. 38.2b, Experiment 2). This
experiment demonstrated that the diffusing hormone could trigger
HYPOTHESIS Wigglesworth hypothesized that a substance
molting in the bug that lacked the hormone because of immediate
(specifically, a hormone) that diffuses from the head triggers molt
decapitations.
in Rhodnius.
CONCLUSION A substance diffuses from the head in Rhodnius and
EXPERIMENT Wigglesworth decapitated juvenile bugs at different
triggers the molting process.
intervals of time after a blood meal and observed whether or not
molting occurred. (Decapitation does not kill the insect.)
a.
Development of Rhodnius

Blood Blood Blood Blood Blood

meal meal meal meal meal

Egg 1st nymph 2nd nymph 3rd nymph 4th nymph 5th nymph Adult

b.

Experiment 1 Experiment 2

3rd nymph Fluid Adult

decapitated tube formed
1 hr after
blood meal

3rd nymph
Adult
decapitated
formed
1 week after
0 1 hr 1 week blood meal

Time of decapitation after blood meal

FOLLOW-UP WORK Subsequent studies by the German SOURCES Wigglesworth, V. B. 1934. “The Physiology of Ecdysis in Rhodnius
prolixus (Hemiptera). II. Factors Controlling Moulting and ‘Metamorphosis.’ ”
endocrinologist Alfred Kuhn confirmed the result in experiments Quarterly Journal of Microscopical Sciences 77:191–223; Nagasawa, H., et al.
performed on moth caterpillars. More recent molecular analyses 1984. “Amino-Terminal Amino Acid Sequence of the Silkworm Prothoracicotropic
Hormone: Homology with Insulin.” Science 226:1344 –1345; Kawakami, A.,
performed by Japanese molecular endocrinologists Nagasawa and
et al. 1990. “Molecular Cloning of the Bombyx mori Prothoracicotropic Hormone.”
colleagues (1984) and Kawakami and colleagues (1990) isolated Science 247:1333 –1335.
and sequenced the structure of the hormone from silkworm moths.
It is now known as prothoracicotropic hormone, or PTTH, because
it acts on the prothoracic gland of the insect to release the molting
hormone, ecdysone.

810
 CHAPTER 38 A N I M A L E N D O C R I N E S YS T E M S 811

the animal’s molt by stimulating the release of a second hormone other neurosecretory brain cells secrete the peptide hormone PTTH,
from the prothoracic gland. which stimulates the release of ecdysone from the prothoracic
Two decades later, the German endocrinologist Peter Karlson gland, triggering the transition from larval to pupal and adult forms.
isolated and purified this second hormone, termed ecdysone. PTTH These studies reveal how small amounts of hormones released
triggers molting by stimulating the release of ecdysone, a steroid from key glands within the animal’s body regulate major stages of
hormone that coordinates the growth and reorganization of body growth and changes in body form during metamorphosis. When
tissues during a molt. The action of ecdysone is a good example of an animal’s body requires coordinated changes in multiple organ
how a hormone can precisely coordinate broad changes in body systems, hormonal regulation by the endocrine system plays a
organization and function, in contrast to the specific regulation that critical role.
nerves provide. Karlson was able to purify a small amount of ecdysone Hormones also regulate growth in humans and other
from large numbers of silk moth Bombyx caterpillars. He ultimately vertebrate animals. Growth hormone controls the growth
isolated a mere 25-mg sample from 500 kg of moth larvae! This tiny of the skeleton and many other tissues in the human body.
amount highlights the general principle that relatively few hormone Growth hormone is produced by the pituitary gland, which is
molecules can have a large effect on an organism. located beneath the brain. Tumors of the pituitary that cause an
In his studies of Rhodnius, Wigglesworth also noted that, overproduction of growth hormone lead to gigantism, and tumors
whatever the larval stage, the decapitated bug always molted into that result in too little growth hormone cause pituitary dwarfism.
an adult after its blood meal. By removing just the front brain The role of the pituitary gland in regulating body function is
region of the head, Wigglesworth was able to show that a region described later in the chapter.
in the head close to the brain releases a hormone that normally
prevents the earlier larval stages from molting into the adult form The endocrine system underlies homeostasis.
(Table 38.1). This hormone, called juvenile hormone, is released in Endocrine control of internal body functions is also central to
decreasing amounts during each successive larval molt. After the homeostasis, the maintenance of a steady physiological state
fifth and final stage, the level of juvenile hormone is so low that within a cell or an organism (Chapters 5 and 35). Without
it no longer blocks maturation, and at the final molt the insect some way to maintain a stable internal environment, changing
undergoes its final growth into the adult form (Fig. 38.3). environmental conditions would lead to dangerous shifts in an
The brains of insects, similar to the nervous systems of other animal’s physiological function. For example, an animal’s body
invertebrate and vertebrate animals, contain neurosecretory weight depends on the regulation of its energy intake relative to
cells. These cells are neurons that release hormones, which act energy expenditure. Disruption of hormones that regulate appetite
on endocrine glands or other targets within the insect instead of and food intake can lead to obesity on the one hand or to weakness
secreting neurotransmitters that bind to another neuron or to muscle. and lethargy on the other. Similarly, hormones that regulate the
In the case of insects, peptide hormones released from neurosecretory concentration of key ions in the body, such as Na1 and K1, are
brain cells act on the corpora allata (paired endocrine glands in fundamental to healthy nerve and muscle function (Chapters 35
insects) and the prothoracic gland (Fig. 38.3).Neurosecretory cells and 36) and fluid balance within the body (Chapter 41).
stimulate the corpora allata to produce juvenile hormone so that How does the body, and in particular the endocrine system,
molting larvae retain juvenile characteristics. At appropriate times, maintain homeostasis? Maintaining homeostasis depends on

FIG. 38.3 Hormonal control of growth and development in Rhodnius.

Prothoracic
Brain gland
Juvenile hormone produced by the
+ corpora allata prevents molting into an
Ecdysone Ecdysone triggers adult form. When juvenile hormone drops
PTTH hormone molting. to low levels, the insect molts into an adult.

Juvenile +
Corpora allata – + – + – + – + – +
hormone

The brain produces PTTH

in response to a blood
meal, which acts on the Egg 1st nymph 2nd nymph 3rd nymph 4th nymph 5th nymph Adult
prothoracic gland to
produce ecdysone.
812 SECTION 38.1 A N OV E RV I E W O F E N D O C R I N E F U N C T I O N

gland modifies its own subsequent production of hormone, either

FIG. 38.4 Negative feedback. Negative feedback results in steady increasing or decreasing it. There are two general types of feedback,
conditions, or homeostasis. negative feedback and positive feedback.
Homeostasis typically depends on negative feedback. In
Stimulus
Chapter 35, we saw how both the temperature in a house and the
core body temperature of an animal are maintained at a constant
+ level by this type of control.
Many physiological parameters—such as blood-glucose and
calcium levels—are maintained at relatively steady levels by negative
Sensor
The sensor and feedback. For example, a change in glucose or calcium level causes a
effector may be a response that brings the level back to the starting point, called
+ single cell, an
endocrine organ,
the set point. In the case of the endocrine system, a change in level
or a series of (the stimulus) is detected by a sensor in the endocrine organ
endocrine organs. (Fig. 38.4). The endocrine organ (the effector) releases a hormone,
– Effector
and the hormone causes a response in the body that opposes the
initial stimulus such that the glucose or calcium level moves back
toward the set point. The reversal of these levels causes secretion of
the hormone to be decreased, and there is a limit to further change.
Constant feedback between the response and sensor maintains a set
Response
point. That is, the maintenance of homeostasis is an active process.
Let’s examine an example in more detail. Like core body
temperature, the amount of glucose in the blood of animals is
feedback from the target organ to the endocrine gland that maintained at a steady level (Fig. 38.5). If glucose levels are too low,
secretes the hormone. Because hormones are transmitted through cells of the body do not have a ready source of energy. If glucose
the bloodstream, this feedback can occur over varying distances levels are too high for too long, they can damage organs.
within the body, coordinating the functions of several organs Maintaining steady blood-glucose levels is challenging because
at any one time. In response to this feedback, the endocrine glucose levels rise immediately after a meal when glucose is
absorbed into the bloodstream
from the intestine, then fall
FIG. 38.5 Control of glucose levels in the blood by negative feedback. as glucose is taken up by cells
to meet their energy needs.
a. b. How then are constant levels
maintained in the body? After a
Stimulus High blood glucose Low blood glucose
meal, when blood glucose rises, b
(right after a meal) (several hours after a meal)
(beta) cells of the pancreas secrete
+ + the hormone insulin, which
circulates in the blood (Fig. 38.5a).
In response to insulin, muscle
Sensor Pancreas Pancreas
and liver cells take up glucose
from the blood and either use it or
Muscle
convert it to a storage form called
Insulin Liver Glucagon glycogen (Chapter 7). In this way,
Cells
insulin guards against high levels
of glucose in the blood.
+ +
What happens if blood-
Effector – Pancreas – glucose levels fall too low several
Body cells take Muscle and liver take up Muscle and liver break down
hours after a meal? In this case,
up glucose. glucose and store it as glycogen. glycogen and release glucose. a different population of cells in
the pancreas, called a (alpha) cells,
secretes the hormone glucagon,
which has effects roughly
opposite to those of insulin (Fig.
Response Decrease in Increase in
38.5b). Glucagon stimulates
blood glucose blood glucose
the breakdown of glycogen into
 CHAPTER 38 A N I M A L E N D O C R I N E S YS T E M S 813

glucose and its release from muscle and liver cells. The result is oxytocin (stimulus) by the pituitary gland causes the uterine
that blood-glucose levels rise. In both cases, the stimulus (either muscles (effector) to contract more forcefully. The uterine
high or low blood-glucose levels) is sensed by cells of the pancreas contractions in turn stimulate (positive feedback) the pituitary
( b cells or a cells) and triggers a response (secretion of insulin or of gland to secrete more oxytocin, causing the uterine muscles to
glucagon, bringing blood-glucose levels back to the set point). Note contract more forcefully and more frequently.
that in each case the response feeds back to the secreting cells to
reduce further hormone secretion.
When the control of blood-glucose levels by insulin fails, 38.2 PROPERTIES OF HORMONES
a disease called diabetes mellitus results. When untreated,
diabetic individuals excrete excess glucose in their urine because Because hormones are released into the bloodstream, they have
blood-glucose levels are too high. Diabetes causes cardiovascular the potential to affect many organ systems. How do hormones
and neurological damage, including loss of sensation in the circulating in the blood target specific tissues? How can one
extremities, particularly the feet. hormone exert its effect on one cell type and another hormone
exert its effect on a different cell type? The answer lies in the
j Quick Check 1 Diabetes mellitus is a disease characterized
presence of receptors on cells within target organs that bind
by high blood-glucose levels. What two different physiological
specific hormones.
conditions can produce diabetes?

In some instances, it is necessary to accelerate the response Hormones act specifically on cells that bind
of target cells for a period of time. Positive feedback provides this the hormone.
enhancement (Fig. 38.6). In positive feedback, a stimulus causes Hormones circulating in the bloodstream bind to receptors
a response in the same direction as the initial stimulus, which located either on the surface of or inside target cells. Therefore,
leads to a further response, and so on. In positive feedback in the it is the presence or absence of a receptor for a given hormone
endocrine system, a stimulus leads to secretion of a hormone that that determines which cells respond and which ones do not.
causes a response, and the response causes the release of more For example, when the hormone oxytocin is released into the
hormone. The result is an escalation of the response. A positive bloodstream of mammals, it affects only cells that express a
feedback loop reinforces itself until it is interrupted or broken by receptor on the cell surface capable of binding oxytocin as it
some sort of external signal outside the feedback loop but from flows by in the bloodstream. These cells are uterine muscle cells
within the body. and secretory cells in breast tissue. When oxytocin binds these
Positive feedback occurs in mammals during birth cell-surface receptors, uterine muscle cells are stimulated to
(Chapter 42). In response to uterine contractions, the hormone contract, and secretory cells in breast tissue release milk during
oxytocin is released from the pituitary gland. The release of breastfeeding. The cells of other organs do not express the
receptor, so the hormone can exert its effect only on these specific
tissues and no others.
The binding of a hormone to its receptor triggers changes in
FIG. 38.6 Positive feedback. In positive feedback, a stimulus
the target cell, resulting in a cellular response (Chapter 9). The
causes a response, and that response causes a further
specific action of a hormone depends on the kind of response
response in the same direction.
that it triggers in the target cell. For example, the binding of a
hormone can alter ion flow across the cell membrane, activate
Stimulus
an intracellular signal transduction cascade that leads to changes
in the biochemical activity of a target cell, or initiate more
+ substantial changes in gene and protein expression.

j Quick Check 2 What general features make a chemical

Sensor compound a hormone, and how do hormones achieve specificity
The sensor and for certain kinds of target cell?
effector may be a
+ single cell, an
endocrine organ, Two main classes of hormone are peptide and amines,
or a series of
and steroid hormones.
+ Effector
endocrine organs.
Some hormone receptors are located on the cell surface, others
are located inside the cell. Where a receptor is located depends
on characteristics of the hormone molecules themselves.
Hormones can be grouped into two general classes: hydrophilic
molecules, which cannot diffuse across the plasma membrane, and
Response
hydrophobic molecules, which can. The hydrophilic hormones are
814 SECTION 38.2 P RO P E RT I E S O F H O R M O N E S

peptide hormones and amine hormones, and the hydrophobic Because steroid hormones are hydrophobic, they diffuse
hormones are steroid hormones, examples of which are freely across the cell membrane to bind with receptors in the
illustrated in Fig. 38.7. cytoplasm or nucleus, forming a steroid hormone–receptor
Peptide hormones and amine hormones are both derived from complex (Fig. 38.8b). Hormone–receptor complexes that form in
amino acids (Figs. 38.7a and 38.7b). Peptide hormones are short the cytoplasm are transported into the nucleus of the cell. These
chains of amino acids, whereas amine hormones are derived from complexes most commonly act as transcription factors: They
a single aromatic amino acid, such as tyrosine or phenylalanine stimulate or repress gene expression and thereby alter the proteins
(Chapter 4). All steroid hormones are derived from cholesterol produced by the target cell. Consequently, steroid hormones
(Fig. 38.7c). Whereas peptide hormones (which are sometimes typically have profound and long-lasting effects on the cells and
large enough to be considered proteins) can evolve through tissues they target, on timescales of days to months.
changes in their amino acid sequence, steroid hormones cannot. The sex hormones estrogen, progesterone, and testosterone
Evolutionary changes in steroid hormone function depend instead (Table 38.2) are steroid hormones that regulate the development
on changes in their synthetic enzymes or changes in the receptors of the vertebrate reproductive organs. Cortisol is a steroid
they bind and the cellular responses they trigger. hormone that regulates a vertebrate animal’s response to stress
Peptide and amine hormones are more abundant than steroid and inhibits inflammation. Steroid hormones similar to cortisol are
hormones and are more diverse in their actions. Because most used medically to reduce the symptoms of certain inflammatory
peptide and amine hormones are hydrophilic and cannot diffuse
across the plasma membrane (exceptions are thyroid hormones),
nearly all these hormones bind to membrane receptors on the
surface of the cell (Fig. 38.8a). Peptide and amine hormones FIG. 38.7 (a) Peptide, (b) amine, and (c) steroid hormones.
alter the biochemical activity of the target by initiating signaling
a. Peptide hormones
cascades within the target cell, as discussed in Chapter 9: The
Oxytocin and ADH
bound receptor activates an enzyme or other molecule within Oxytocin are peptide
the cell, and it in turn activates another and so on. Typically, the hormones that are
Cys Tyr Ile Gln Asn Cys Pro Leu Gly 9 amino acids long.
activated enzymes are protein kinases, which phosphorylate Both are released
other proteins. These signaling cascades can lead to changes in by the posterior
Antidiuretic hormone (ADH) pituitary gland and
gene expression or they can alter metabolism by turning on or off
Cys Tyr Phe Gln Asn Cys Pro Arg Gly share a similar
metabolic enzymes. For example, a peptide or amine hormone structure (only 2
could trigger a cell to grow, divide, change shape, or to release amino acids differ).
another hormone. Peptide and amine hormones act on timescales
b. Amine hormones
of minutes to hours.
L-dopa Dopamine
There are many examples of peptide hormones. In vertebrates, HO HO
for instance, growth hormone stimulates protein synthesis and
HO O HO
the growth of many body tissues, particularly the musculoskeletal
NH2
system (Table 38.2, on pages 816–817). As we saw, insulin and NH2 OH
glucagon are peptide hormones released by the pancreas that
regulate glucose metabolism by the liver, muscles, and other Norepinephrine Epinephrine
tissues. Gastrin and cholecystokinin are peptide hormones that HO NH2 HO
regulate mammalian digestive function (Chapter 40). In addition NH
to serving as a neurotransmitter released by the sympathetic HO HO
OH OH
nervous system (Chapter 35), epinephrine and norepinephrine
(also known as adrenaline and noradrenaline) are amine
hormones, supporting an animal’s fight-or-flight response. Amine hormones are derived
from aromatic amino acids.
All four shown here are
The steroid hormones all
derived from phenylalanine.
share a similar structure and
are derived from cholesterol.
c. Steroid hormones
OH
Progesterone Testosterone Cortisol Cholesterol
O O
OH OH
HO HO

O O O
HO
 CHAPTER 38 A N I M A L E N D O C R I N E S YS T E M S 815

of a signal from one endocrine gland to the next in a hormonal

FIG. 38.8 Mechanism of action of (a) peptide and (b) steroid pathway, and the second set is signal transduction in the target
hormones. cell. Signals are amplified at each step of the pathway, resulting in
a. Cell-surface receptors a large effect on the target cell or organ.
Peptide and Hormonal signaling pathways between endocrine glands and
amine hormones tissues are often referred to as endocrine axes. In vertebrates,
Receptor hormonal signals are amplified along a pathway called the
hypothalamic–pituitary axis that goes from the hypothalamus to
Plasma membrane
the anterior pituitary gland, and then from the anterior pituitary
of target cell
gland to target glands or tissues in the body. The hypothalamus
initially releases trace amounts of peptide hormones called
releasing factors (Fig. 38.9). The releasing factors bind to receptors
Peptide and amine on cells in the anterior pituitary gland, leading that organ to release
hormones are hydrophilic
and bind to cell-membrane a much larger amount of associated hormones. For example, the
receptors activating second hypothalamus secretes a corticotropin releasing factor, which
messenger pathways, which
stimulates the anterior pituitary gland to release a larger amount of
change the metabolic state
or can affect gene expression adrenocorticotropic hormone (ACTH).
of the target cell.

b. Intracellular receptors
Receptor FIG. 38.9 Amplification of a hormonal signal.
Steroid hormone protein
Stress
New protein
mRNA
+
Steroid hormones are
hydrophobic and diffuse into
the target cell, where they Hypothalamus
bind a cytoplasmic or
nuclear receptor that allows
them to act as transcription Corticotropin
factors to alter the gene releasing factor
expression of the cell. 0.1 μg

+
Nucleus
Anterior pituitary
DNA
Hormone– At each step,
receptor the hormonal
complex signal is Adrenocorticotropic
amplified, so hormone (ACTH)
that a small 1 μg
amount of
+
disorders. However, they carry risks because they also suppress corticotropin
releasing
the immune system, increasing the chance of infection and factor leads to
Adrenal cortex
diminishing wound healing. a large effect
(in this case,
j Quick Check 3 Why do peptide and steroid hormones bind the production
of glucose by Cortisol
different kinds of receptor, and how does this difference affect the the liver). 40 μg
resulting signaling pathways in the target cell?
+

Hormonal signals are amplified to strengthen

their effect. Liver

Hormones are typically released in small amounts. So how is it

possible for them to have large effects on the overall physiology
of an organism? The answer is that hormone signals are amplified
(Chapter 9). Amplification occurs during a series of signaling
Glucose
steps, both before the hormone binds to a cell receptor and inside 5600 μg
the cell afterward. The first set of signaling steps is the passing
816 SECTION 38.2 P RO P E RT I E S O F H O R M O N E S

TABLE 38.2 Major Vertebrate Hormones

SECRETING TISSUE TARGET GLAND

OR GLAND HORMONE OR ORGAN ACTION

Hypothalamus Releasing factors (peptides) Anterior pituitary Stimulate secretion of anterior pituitary hormones
gland

Anterior pituitary gland Thyroid-stimulating hormone Thyroid gland Stimulates synthesis and secretion of thyroid
(TSH) (glycoprotein) hormones by the thyroid gland

Follicle-stimulating hormone Gonads Stimulates maturation of eggs in females; stimulates

(FSH) (glycoprotein) sperm production in males

Luteinizing hormone (LH) Gonads Stimulates production and secretion of sex hormones
(glycoprotein) in ovaries (estrogen and progesterone) and testes
(testosterone)

Adrenocorticotropic hormone Adrenal glands Stimulates production and release of cortisol

(ACTH) (peptide)
Growth hormone (GH) Bones, muscles, liver Stimulates protein synthesis and body growth
(protein)

Prolactin (protein) Mammary glands Stimulates milk production

Melanocyte-stimulating Melanocytes Regulates skin (and scale) pigmentation

hormone (peptide)

Posterior pituitary gland Oxytocin (peptide) Uterus, breast, brain Stimulates uterine contraction and milk let-down;
influences social behavior

Antidiuretic hormone (ADH; Kidneys, brain Stimulates uptake of water from the kidneys;
vasopressin) (peptide) involved in pair bonding

Thyroid gland Thyroid hormones (peptides) Many tissues Stimulate and maintain metabolism for development
and growth

Calcitonin (peptide) Bone Stimulates bone formation by osteoblasts

Ovaries Estrogen (steroid) Uterus, breast, other Stimulates development of female secondary sexual
tissues characteristics and regulates reproductive behavior

Progesterone (steroid) Uterus Maintains female secondary sexual characteristics

and sustains pregnancy

Hormones released by the anterior pituitary gland in turn bind its effects, it causes the liver to convert glycogen and amino acids
cell receptors in the target organ. In this case, ACTH acts on cells to glucose. This action yields 56,000 times more glucose than the
of the adrenal cortex, stimulating their secretion of the hormone initial weight of releasing factors secreted by the hypothalamus.
cortisol. Cortisol acts on many different cells and tissues in the We can also see an example of amplification in invertebrates in
body, causing what is known as an acute stress response. Among the regulation of insect molting and metamorphosis (see Fig. 38.3).
 CHAPTER 38 A N I M A L E N D O C R I N E S YS T E M S 817

TABLE 38.2 Major Vertebrate Hormones (continued)

SECRETING TISSUE TARGET GLAND

OR GLAND HORMONE OR ORGAN ACTION

Testes Testosterone (steroid) Various tissues Stimulates development of male secondary sexual
characteristics; regulates male reproductive behavior
and stimulates sperm production

Adrenal cortex Cortisol (steroid) Liver, muscles, Regulates response to stress by increasing blood
immune system glucose levels and reduces inflammation

Adrenal medulla (Nor-) Epinephrine Heart, blood vessels, Stimulates heart rate, blood flow to muscles, and
(peptide) liver elevation of blood glucose level as part of
fight-or-flight response

Parathyroid glands Parathyroid hormone Bone Stimulates bone resorption by

(PTH) (protein) osteoclasts to increase blood Ca2+ levels

Pancreas Insulin (protein) Liver, muscles, fat, Stimulates uptake of blood glucose and storage
other tissues as glycogen

Glucagon (protein) Liver Stimulates breakdown of glycogen and glucose

release into blood

Somatostatin (peptide) Digestive tract Inhibits insulin and glucagon release; decreases
digestive activity (secretion, absorption, and motility)

Stomach Gastrin (peptide) Stomach Stimulates protein digestion by secretion of digestive

enzymes and acid; stimulates gastric motility

Small intestine Cholecystokinin (peptide) Pancreas, liver, Stimulates secretion of digestive enzymes and
gallbladder products from liver and gallbladder

Secretin (peptide) Pancreas Stimulates bicarbonate secretion from pancreas

Pineal gland Melatonin (peptide) Brain, various organs Regulates circadian rhythms

This pathway has two signaling steps in which an endocrine gland binds to cell receptors in the target organ. Signaling cascades within
releases a hormone. First, the brain releases the hormone PTTH. these cells further amplify the hormone signal. Thus, amplification
That hormone signals the prothoracic gland, leading to the release applies both to signal transduction cascades within a cell and to
of a much larger amount of ecdysone, the hormone that regulates the chemical signals that are transmitted as endocrine hormones
growth and metamorphosis of the insect’s body. Ecdysone in turn between glands and tissues.
818 SECTION 38.3 T H E V E RT E B R AT E E N D O C R I N E S YS T E M

Hormones are evolutionarily conserved molecules (Fig. 38.10). The hypothalamus, in turn, relays these signals to the
with diverse functions. pituitary gland, which lies just below it. The pituitary gland is a
Most hormones have an ancient evolutionary history. The central regulating gland of the vertebrate endocrine system:
structures of many hormones are evolutionarily conserved. For It releases several hormones that coordinate the action of many
example, some vertebrate hormones can also be found in many other endocrine glands and tissues. These glands, illustrated in
invertebrates. Since the first vertebrate animals diverged from Fig. 38.10, control the growth and maturation of the body, regulate
invertebrates over 500 million years ago, some animal hormones
are even older. In many cases, their roles in many invertebrate
animals have yet to be discovered.
Typically, the same hormone serves different functions FIG. 38.10 The hypothalamus–pituitary axis. The hypothalamus
in vertebrates and in invertebrates, and may even serve sends signals to the pituitary gland, which in turn sends
different functions within distinct groups of vertebrates. signals to diverse cells and organs throughout the body.
Thyroid-stimulating hormone (TSH), a hormone released by
Hypothalamus
the anterior pituitary gland that targets the thyroid gland, Releasing factors
regulates metabolism in mammals but triggers metamorphosis
in amphibians and feather molt in birds. It has even been found Anterior pituitary Posterior pituitary
in snails and other invertebrates that lack a thyroid gland. Its gland gland
TSH, FSH, LH, ACTH, Oxytocin, ADH
function in snails appears to be stimulation of the number of
GH, prolactin
sperm or eggs produced.
Recent genomic analysis has shown that the receptors for
many hormones evolved well before the hormones with which
they now interact. Even though the structure of a hormone or
its receptor is often largely unchanged across diverse groups of
organisms, hormones and their receptors can readily be selected to
take on new roles as organisms evolve new behaviors and exploit
new environments. An example is the shift in function of TSH to
regulate metabolism in mammals and feather molt in birds from
its role stimulating sperm or egg production in snails.
Another intriguing finding is that many peptides originally
identified as hormones in various tissues have also been found TSH
to function as neurotransmitters in the nervous system. For Thyroid
example, oxytocin, which stimulates uterine contraction and the Thyroid hormones
release of milk, also serves as a neurotransmitter in the brain and
is believed to influence social behavior, as well as stimulate sexual Prolactin Oxytocin
arousal in mammals. Similarly, antidiuretic hormone, a peptide Breast
hormone that regulates water uptake in the kidneys (Chapter 41),
also functions as a neurotransmitter in the brain influencing ACTH
mammalian mating and pair-bonding behavior. Hence, the same
compound, expressed in two different cell types, may have ADH
entirely different functions. The roles of hormones as chemical
messengers are varied within an organism and easily changed over
GH
the course of evolution. Adrenal gland Kidney
Bones
Cortisol,
and soft
aldosterone,
tissue epinephrine Uterus
38.3 THE VERTEBRATE
Oxytocin
ENDOCRINE SYSTEM
FSH/LH
The vertebrate endocrine system regulates changes in the animal’s
physiological and behavioral states in response to sensory cues
received by its nervous system both from the environment and from
internal organs. Whereas certain sensory signals may communicate
directly to endocrine glands or tissues in the body, many are
processed within the brain and transmitted to the endocrine Testis Ovary
Testosterone Estrogens,
system by the hypothalamus, which is located in the forebrain progesterone
 CHAPTER 38 A N I M A L E N D O C R I N E S YS T E M S 819

reproductive development and metabolism, and control of The pituitary gland is divided into anterior and posterior
water balance. These effects reflect the broad range of roles that regions (Fig. 38.11). This division is not arbitrary—the two regions
the endocrine and nervous systems perform in regulating body have distinct functions, organizations, and embryonic origins.
maturation and maintaining homeostasis. The anterior pituitary gland forms from epithelial cells that
The pituitary gland communicates with many cells, tissues, develop and push up from the roof of the mouth, whereas the
and organs of the body. Some of these, like the thyroid and adrenal posterior pituitary gland develops from neural tissue at the
glands, secrete hormones in response to signals from the pituitary base of the brain. Both sets of cells form pouches that develop into
gland and therefore have exclusively endocrine functions. Others, glands, which come to lie adjacent to each other. The anterior and
like the lungs, kidneys, and digestive tract, harbor endocrine cells posterior pituitary glands should therefore be thought of as two
that secrete hormones and also have other physiological roles in distinct glands, not one.
the body. Consequently, the vertebrate endocrine system is not Because of these developmental differences, the anterior and
localized in one part of the body, but is present throughout. posterior pituitary glands receive input from the hypothalamus
in different ways. As does the brain of arthropods, the vertebrate
The pituitary gland integrates diverse bodily functions hypothalamus contains neurosecretory cells, which are part
by secreting hormones in response to signals from the of the brain and therefore are neurons. As with invertebrate
hypothalamus. neurosecretory cells, these cells release hormones into the
The hypothalamus is the main route by which nervous system bloodstream. Some of these neurosecretory cells communicate
signals are transmitted to the vertebrate endocrine system. The with the anterior pituitary gland. In this case, they secrete
function of the hypothalamus is to transmit these signals to the hormones called releasing factors into small blood vessels that
pituitary gland, the endocrine gland that acts as a control center travel to and supply the anterior pituitary gland (Fig. 38.11). In
for many other endocrine glands in the body. response, cells of the anterior pituitary gland release hormones
into the bloodstream.
These hormones circulate
throughout the body and bind
FIG. 38.11 The anterior and posterior pituitary glands. The hypothalamus communicates with the anterior
to receptors on target cells,
pituitary gland by secreting releasing factors into the blood. The hypothalamus communicates
tissues, and organs.
with the posterior pituitary gland by extending axons of neurosecretory cells into it.
In contrast,
communication between
the hypothalamus and the
The hypothalamus of the
midbrain communicates posterior pituitary gland does
with the pituitary gland not involve hypothalamic
located below.
releasing factors. Instead,
the posterior pituitary
Neurosecretory gland contains the axons of
cells Hypothalamus neurosecretory cells whose
cell bodies are located in the
hypothalamus. These axons
release hormones directly into
the bloodstream, and these
hormones are transported in
the blood to distant sites (Fig.
38.11). Consequently, the
posterior pituitary is part of
Some neurosecretory Other neurosecretory
cells in the hypothalamus cells of the hypo-
the nervous system itself.
Releasing
secrete releasing factors factors thalamus extend their In response to signals from
into the bloodstream that axons all the way to the hypothalamus, distinct
cause cells in the anterior Anterior the posterior pituitary
pituitary gland to release pituitary gland, where they hormones are secreted by
hormones. gland release their hormones the anterior and posterior
into the bloodstream.
Blood pituitary glands (Table 38.2).
vessels The hormones released by the
Posterior
anterior pituitary hormone act
Hormones pituitary
gland on an endocrine gland to cause
release of other hormones.
Endocrine cells Hormones Hormones that control
820 SECTION 38.3 T H E V E RT E B R AT E E N D O C R I N E S YS T E M

the release of other hormones are called tropic hormones. In of two peptide hormones, thyroxine (T4) and triiodothyronine
response to thyroid-stimulating hormone (TSH), the thyroid (T3). These peptide hormones are sufficiently hydrophobic that
gland releases thyroid hormones that regulate the metabolic state they enter cells, binding to intracellular receptors to increase the
of the body. In response to follicle-stimulating hormone (FSH) resting metabolic rate of cells throughout the body. As levels of T3
and luteinizing hormone (LH), the ovaries release estrogen and and T4 rise, these hormones act as a brake on the release of TSH
progesterone and the testes release testosterone. In response to by the anterior pituitary gland. This regulation of TSH by negative
adrenocorticotropic hormone (ACTH), the adrenal glands release feedback is critical to maintaining stable TSH levels and a stable
cortisol, which has diverse effects that include stimulating glucose metabolic state of the body as a whole. Overproduction of thyroid
release into the bloodstream, maintaining blood pressure, and hormones (hyperthyroidism) creates symptoms that reflect an
suppressing the immune system. overly active metabolic state (increased appetite and weight loss),
The anterior pituitary gland also secretes the hormones whereas thyroid hormone deficiency (hypothyroidism) creates
growth hormone (GH) and prolactin. Growth hormone acts symptoms that reflect a metabolic state that is too low (fatigue
generally on the muscles, bones, and other body tissues to and sluggishness). Both conditions are diagnosed by blood tests
stimulate their growth, and prolactin stimulates milk production that monitor circulating levels of thyroid hormones, and both can
in the breasts of female mammals in response to an infant’s be treated by medication.
suckling at the mother’s nipple. Because the thyroid hormones T3 and T4 require iodine for
The hormones released by the posterior pituitary gland their formation, individuals who do not acquire enough iodine
include oxytocin and antidiuretic hormone (ADH; also called from their diets produce too little thyroid hormone. Additionally, in
vasopressin). These are two evolutionarily related peptide cases of iodine insufficiency, the anterior pituitary gland increases
hormones of similar structure. As we discussed earlier, oxytocin its production of TSH because it is no longer receiving negative
plays several roles related to female reproduction: It causes uterine feedback. Over time, in response to TSH the thyroid gland enlarges
contraction during labor and stimulates the release of milk during to form a goiter, which is observed as an enlargement of the throat.
breastfeeding. Antidiuretic hormone acts on the kidneys (Chapter The introduction of iodized salt has eliminated goiter formation and
41), regulating the concentration of urine that an animal excretes, hypothyroidism in many countries, but in many underdeveloped
which is critical to maintaining water and solute balance in the body. areas these remain significant public health problems.
In addition to their roles in reproduction and kidney function, The gonadotropic hormones are FSH and LH. They target the
oxytocin and antidiuretic hormone are released by cells in the female and male gonads—the ovaries and testes (see Fig. 38.10;
brain and may play roles in social behaviors. For instance, recent Table 38.2). In response to FSH and LH, the ovaries and testes each
evidence indicates that oxytocin may be important in regulating secrete sex hormones that regulate their own development as well
maternal behavior toward infants. Recent research suggests as the development of secondary sexual characteristics. Female
that oxytocin may also have a role in behaviors such as trust as a sex hormones include the steroids estrogen and progesterone. The
prelude to mating in vertebrate animals. These functions reflect principal male sex hormone is the androgen testosterone. These sex
the long evolutionary history of oxytocin, as it plays a role in the hormones are common to a vast majority of vertebrates, regulating
social behavior of insects. sexual differentiation, gonadal maturation, and reproductive
Recent evidence also suggests that antidiuretic hormone behavior. The role of these sex hormones in regulating reproductive
plays a parallel role in regulating male social behavior and parental function is discussed in greater detail in Chapter 42.
behavior in mammals. This is not surprising because oxytocin and Testosterone is a naturally occurring anabolic steroid that
antidiuretic hormone have very similar chemical structures (see stimulates the synthesis in the testes of proteins needed for sperm
Fig. 38.7). The integration of reproductive function with parental production and the development of male sexual features and body
and social behavior for the care of young is likely favored strongly tissue growth, particularly in muscles. Anabolic steroids promote
by natural selection. Receptors for these hormones are expressed protein synthesis to build body tissues and anabolic metabolism to
within the brain. Thus, in addition to acting on reproductive and store energy within cells. A variety of synthetic anabolic steroids
renal organs of the body, oxytocin and antidiuretic hormone affect that mimic the action of testosterone have been developed to treat
mammalian behavior by acting directly on the brain. muscle wasting due to loss of appetite and diseases such as cancer
and AIDS. Anabolic steroids are used illegally by some male and
Many targets of pituitary hormones are endocrine female athletes to promote muscle strengthening, undermining
tissues that also secrete hormones. fair competition. The risks to health of continued use of anabolic
As we have seen, some of the hormones released by the anterior steroids are considerable: liver damage, heart disease, and high
pituitary gland act on endocrine organs that themselves then blood pressure. Tissues become damaged because elevated
release hormones. These tropic hormones are TSH, FSH and LH, circulating levels of testosterone produced by anabolic steroids
and ACTH (see Fig. 38.10; Table 38.2). Here, we look at their target inhibits the normal synthesis of tropic hormones by the anterior
organs in more detail. pituitary gland.
TSH acts on the thyroid gland, which is located in the front ACTH released by the anterior pituitary gland is also a tropic
of the neck (see Fig. 38.10; Table 38.2) and leads to the release hormone. As noted in section 38.1 in explaining how amplification
 CHAPTER 38 A N I M A L E N D O C R I N E S YS T E M S 821

strengthens the action of downstream hormones in a signaling gland responds to light sensed by a “third eye,” photosensitive cells
pathway, ACTH acts on the cortex (the outer portion) of the paired located at the skull’s surface between the animal’s actual eyes. In
adrenal glands (see Fig. 38.10; Table 38.2). The adrenal glands are both cases, melatonin levels rise at night. In diurnal animals (those
located adjacent to the kidneys (ad- meaning “near” and “renal” that are active during the day), the rise in melatonin levels at night
referring to the kidneys). In humans, each small adrenal gland lies causes the animals to sleep. In nocturnal animals (those that are
just above each kidney. During times of stress, such as starvation, active during the night), the rise in melatonin levels causes them
fear, or intense physical exertion, ACTH stimulates adrenal cortex to become active. The release of melatonin in response to daily
cells to secrete cortisol. The release of cortisol in response to stress (and seasonal) light cycles helps to maintain circadian rhythms
affects a broad range of bodily functions, including blood-glucose of the body. Circadian rhythms are cycles of about 24 hours in
levels (Chapter 41), immune function (Chapter 43), and blood which an animal’s biochemical, physiological, and behavioral state
pressure (Chapter 39). Whereas acute (short-term) responses that shifts in response to changes in daily and seasonal environmental
lead to the release of ACTH and elevated cortisol by the adrenal conditions. To overcome jet lag, travelers are advised to seek
cortex are healthy physiological responses that enable animals to sunlight and often to take melatonin before sleeping to help reset
respond to stressful situations, chronic (long-term) stress often their 24-hour circadian rhythm.
leads to a broad range of unhealthy conditions that include impaired In several animals, including ground squirrels, bats, bears, and
cognitive and immune function, sleep disruption, and fatigue. rattlesnakes, the pineal gland regulates hibernation, controlling the
animal’s metabolic state over longer time periods. In many animals,
Other endocrine organs have diverse functions. release of melatonin also regulates seasonal breeding cycles.
Although many endocrine organs receive signals from the
hypothalamic–pituitary axis, some respond instead to internal
physiological states of the body or to external environmental ? CASE 7 PREDATOR–PREY: A GAME OF LIFE AND DEATH
cues. One of these is the parathyroid gland, which is located How does the endocrine system influence predators
on the thyroid gland. The parathyroid gland secretes parathyroid and prey?
hormone (PTH). Working together with calcitonin, secreted by The endocrine system works closely with the nervous system to
the thyroid gland, PTH controls the levels of calcium in the blood enable animals to respond to external cues in the environment.
(Table 38.2). The two hormones act by regulating the actions of In Chapters 35 and 36, we discussed how prey animals, such as
bone cells (osteoblasts and osteoclasts; Chapter 37) that control gazelles, use visual and olfactory cues processed by the nervous
bone formation and bone removal. system to detect the distant movement of a predator, such as a
When circulating levels of calcium fall too low, the parathyroid lion (Fig. 38.12). We saw how the sympathetic nervous system,
gland releases PTH, which stimulates osteoclasts to reabsorb
bone mineral, halting bone formation and releasing
calcium into the bloodstream. When calcium levels
are too high, the production of PTH is inhibited and FIG. 38.12 Predator and prey. Animals rely on rapid integration of sensory
calcitonin is released to shift bone metabolism toward information with coordination of bodily functions to escape a predator
net bone formation, building bone that stores calcium or to catch their prey. Source: Federico Veronesi/Getty Images.
in the skeleton. Note that, as with blood-glucose
levels, calcium levels are maintained in a narrow range
by negative feedback: The response to the hormone
(increasing or decreasing levels of calcium in the
blood) is the opposite of the stimulus (low or high
levels of calcium in the blood) so that a stable set
point is maintained.
Several organs of the digestive system, including
the pancreas, stomach, and duodenum, produce
hormones that regulate appetite and digestion. Three of
these hormones are discussed in Chapter 40.
The pineal gland is located in the thalamic region
of the brain. In response to darkness, the pineal gland
secretes melatonin, a hormone that helps control an
animal’s state of wakefulness (Table 38.2). Melatonin
secretion is inhibited when environmental light cues
sensed by the retina are conveyed to the gland by
the autonomic nervous system. In other vertebrates,
such as lampreys, some fishes, and reptiles, the pineal
822 SECTION 38.4 OT H E R F O R M S O F C H E M I C A L CO M M U N I C AT I O N

in the fight-or-flight response, can trigger broad physiological

changes, such as increased heart and breathing rates and changes in FIG. 38.13 Modes of chemical signaling. Chemical signaling can
metabolism that ready the predator to attack or the prey to respond. occur (a) over long distances (endocrine), or (b) locally
These actions are coordinated by the endocrine system. (paracrine and synaptic).
The sympathetic nervous system sends axons to the adrenal
a. Long-distance signaling
medulla (see Figure 38.10), the inner part of the adrenal gland. In
response to stimulation by the sympathetic nervous system, cells Endocrine
Blood vessel
of the adrenal medulla secrete two hormones, epinephrine and signaling
involves
norepinephrine (also known as adrenaline and noradrenaline). release of a
These are hormones released by the adrenal gland with targets hormone Target cell
throughout the body, leading to the physiological changes of into the
bloodstream
the fight-or-flight response. Interestingly, epinephrine and that affects
norepinephrine also act as neurotransmitters in the brain. In fact, distant cells.
cells of the adrenal medulla are modified nerve cells that have lost Hormone
their axons and dendrites. travels in
bloodstream
The same set of responses occurs when, walking along Receptor
a deserted sidewalk, we note a shadowy movement or hear
a footstep behind us. The perceived threat sets in motion a
coordinated physiological reaction, mediated by the combined b. Local signaling
action of the nervous and endocrine systems. These changes make
us alert and ready for action to avoid or resist a perceived threat.
Secretory
They also inhibit digestion and eliminate a sense of appetite. vesicle
Earlier, we considered the importance of homeostasis in In paracrine
signaling, a
maintaining a variety of parameters, such as temperature and
cell signals
glucose levels, at a steady level. In this case, there is an adjustment neighboring
of the set points of different organs, which is critical in enabling cells by
releasing a
an animal to respond to external cues and adjust its physiological chemical,
response appropriately. When the real or perceived threat is gone, such as a
the body is able to return to its prior state by reestablishing earlier growth
factor, that
set points. acts locally.
Set points are also changed when an animal acclimatizes to Secreting cell
a new environment, such as when it moves from fresh water
Local regulator
to a marine environment or to a new altitude. An adaptive diffuses through Target
cell
change in body function to a new environment is referred to as extracellular fluid.
acclimatization. Examples of acclimatization are the increase
in the number of red blood cells and their capacity to carry more
oxygen at higher altitude (Chapter 39) and seasonal changes in the
fur of mammals that achieve the insulation required to maintain
Synaptic
a stable body temperature (Chapter 40). These physiological cleft
changes result from establishing new set points in a process Ner
ve s
coordinated by the endocrine system. ignals

38.4 OTHER FORMS OF CHEMICAL Synaptic Neuro-

transmitter
signaling
COMMUNICATION involves the
release of a Receptor
As we have seen, hormones are chemical compounds secreted neurotransmitter
between two
by glands that enter the circulation and act on distant cells, adjacent nerve
Receiving
those having receptors that bind the hormones. However, other cells, or between
a nerve cell and nerve cell
chemical signals act over short distances, and still others are a muscle cell.
released into the environment to signal other individuals of the
same species. Here, we explore additional modes of chemical
communication and the compounds that produce them.
 CHAPTER 38 A N I M A L E N D O C R I N E S YS T E M S 823

FIG. 38.14 Pheromone action. (a) Pheromones are means of attracting mates, as in these ladybugs; (b) wolves secrete pheromones in their
urine that mark their territory; (c) honeybees swarm to defend their nest after a bee that stings a threatening animal releases an alarm
pheromone. Sources: a. Krzysztof Bisztyga/age footstock; b. Jorg & Petra Werner/Animals Animals/Earth Scenes; c. Steven Smith/Alamy.

a b c

Local chemical signals regulate neighboring be extremely rapid and brief compared to the other modes of
target cells. chemical communication.
Whereas hormones enter the bloodstream to be transmitted to
more distant target cells (Fig. 38.13a), other chemical compounds Pheromones are chemical compounds released into
act locally on neighboring cells (Fig. 38.13b). In order to take the environment to signal physiological and behavioral
up the chemical signal, these cells must have receptors for the changes in other species members.
compound to bind to before it degrades or diffuses into the Many animals release small chemical compounds into the
bloodstream. environment to signal and influence the physiology and behavior
Chemical compounds that act locally are said to have paracrine of other members of their species. These water- or airborne
function (Fig. 38.13b). If the compounds act on the secreting compounds are called pheromones. For example, bees and wasps
cell itself, they are said to have autocrine function—that is, the release pheromones to find a nest, locate food, and regulate the
chemical signals may stimulate or inhibit their own secretion. development and activities of other colony members. Even plants,
Growth factors (also referred to as cytokines and distinct from such as orchids, release pheromones. Some plant pheromones
the growth hormone released by the anterior pituitary gland) and attract pollinating bees or wasps by mimicking the chemical
histamine are examples of paracrine chemical agents released in signals released by the females of these species. There are many
small amounts that act locally on neighboring cells. different pheromones, which serve very different functions, as
Growth factors (Chapter 9) enhance the differentiation shown in Fig. 38.14.
and growth of particular kinds of tissue. For example, bone Female silk moths use pheromones to attract males over
morphogenetic proteins (BMPs) promote the formation of bone long distances. The sex pheromone bombykol, released from
in skeletal growth, and fibroblast growth factors (FGFs) stimulate a gland in the female’s abdomen, signals her location to males
the formation of connective tissue. Similarly, nerve growth factors looking for a mate. Sex pheromones play a key role in the mating
(NGFs) promote the survival and growth of nerve cells. When behavior of many invertebrate and vertebrate species. In most
tissues are damaged, specialized cells nearby release histamine. mammals, amphibians, and reptiles, pheromones are detected
This chemical signal triggers the dilation of blood vessels, allowing by a vomeronasal organ with chemosensory neurons in the nasal
blood proteins and white blood cells to move into the region region of the skull. The release of sex pheromones to attract a
to fight infection and induce repair (Chapter 43). The release mate and trigger ritualistic social and mating behaviors is one of
of histamine causes the swelling, redness, and warmth that the most common types of pheromone signaling (Chapter 45).
commonly surround a wound. Humans and nonhuman primates lack a vomeronasal organ, and
Another form of short-range chemical signaling is the most studies indicate that they do not produce pheromones.
release of neurotransmitters into the synapse between directly Other pheromones signal territorial boundaries or predatory
communicating nerve cells or between nerve cells and muscles threats. Dogs and other canids (this family includes wolves and
at neuromuscular junctions (Fig. 38.13b). Synaptic signaling can coyotes), as well as cats, secrete pheromones in their urine that
824 SECTION 38.4 OT H E R F O R M S O F C H E M I C A L CO M M U N I C AT I O N

FIG. 38.15 Trail pheromones in ants. Here, a chemical trail has FIG. 38.16 A brightly colored male cichlid fish. The behavior and
been laid down by worker ants signaling a food source markings of this fish are under hormonal control and
for the colony. Source: LatitudeStock–Patrick Ford/Getty Images. triggered by social context. Source: blickwinkel/Alamy.

a trail, or into and out of your house, you have seen worker ants
using pheromone cues that signpost the way to food.
Aquatic species also depend on pheromone signaling. Fish,
salamanders, and tadpoles release alarm pheromones into the
water to warn of a threatening predator. Prey species that share
the same habitat may even sense and respond to the release of
another species’ alarm pheromone.
Fish also release pheromones to coordinate mating and to
regulate social interactions. In dense populations of cichlid fish
living in Lake Tanganyika in central Africa, the ratio of brightly
colored breeding males (Fig. 38.16) to females is highly regulated.
Only about 10% of males are brightly colored and defend their
territory to attract and mate with females. This restriction on the
mark their territories. Social seabirds, such as cormorants and
number of dominant breeding males is regulated by behavioral
boobies, mark their nesting sites by pheromones.
cues and chemical pheromones. When a dominant male dies,
Ants and other social insects deploy alarm pheromones to
subordinate males fight for the vacated territory. When a new
warn of an attack by an advancing army from another colony. The
male assumes dominance, it becomes brightly colored to attract
release of alarm pheromones from a stinging bee causes other bees
females and defend its newly won territory. The dominant
to swarm and join the attack to defend their nest. In many other
breeding male reinforces its dominance by releasing pheromones
species, an animal under attack releases an alarm pheromone,
in its urine that signal females and subordinate males in its
warning its neighbors. Similarly, social mammals such as deer
breeding area. The color change is mediated by hormones released
warn of an approaching predator by means of alarm pheromones.
from t