INTRODUCTION TO LIVING WORLD

Biology is the natural science that studies life and living organisms, including their physical
structure, chemical processes, molecular interactions, physiological mechanisms, development
and evolution. Its name is derived from the Greek words "bios" (life) and "logos" (study).
Biologists study the structure, function, growth, origin, evolution and distribution of living
organisms. There are generally considered to be at least nine "umbrella" fields of biology, each
of which consists of multiple subfields.
o Botany: the study of plants, including agriculture
o Zoology: the study of animals, including animal behavior
o Biochemistry: the study of the material substances that make up living things
o Cellular biology: the study of the basic cellular units of living things
o Molecular biology: the study of biological molecules
o Genetics: the study of heredity
o Ecology: the study of how organisms interact with their environment
o Physiology: the study of the functions of organisms and their parts
o Evolutionary biology: the study of the origins and changes in the diversity of life over time
Adding to the complexity of this enormous idea is the fact that these fields overlap. It is
impossible to study zoology without knowing a great deal about evolution, physiology and
ecology. You can't study cellular biology without knowing biochemistry and molecular biology as
well.

 Framework of understanding
All the branches of biology can be unified within a framework of five basic understandings about
living things. Studying the details of these five ideas provides the endless fascination of biological
research:
o Cell Theory: There are three parts to cell theory;
i. The cell is the basic unit of life,
ii. All living things are composed of cells, and
iii. All cells arise from pre-existing cells.
o Energy: All living things require energy, and energy flows between organisms and
between organisms and the environment.
o Heredity: All living things have DNA and genetic information codes the structure and
function of all cells.
o Equilibrium: All living things must maintain homeostasis, a state of balanced equilibrium
between the organism and its environment.
o Evolution: This is the overall unifying concept of biology. Evolution is the change over
time that is the engine of biological diversity.
 (A hierarchy of biological organization: - Biological organization extends from the microscopic
scale of the molecules and cells that make up organisms to the global scale of the entire living
planet)

 Biology and its sub-branches

Biology is often studied in conjunction with other branches, here are some examples:
o Anatomy – the study of organism’s structures
 Comparative anatomy – the study of evolution of species through similarities and
differences in their anatomy
 Histology – the study of tissues, a microscopic branch of anatomy
o Astrobiology (also known as exobiology, exopaleontology, and bioastronomy) – the
study of evolution, distribution, and future of life in the universe
o Biochemistry – the study of the chemical reactions required for life to exist and function,
usually a focus on the cellular level
o Biological engineering – the attempt to create products inspired by biological systems or
to modify and interact with the biological systems
o Biogeography – the study of the distribution of species spatially and temporally
o Bioinformatics – the use of information technology for the study, collection, and storage
of genomic and other biological data
o Biolinguistics – the study of the biology and evolution of language
o Biomechanics – the study of the mechanics of living beings
o Biomedical research – the study of health and disease
o Biophysics – the study of biological processes by applying the theories and methods
traditionally employed in the physical sciences
o Biotechnology – the study of the manipulation of living matter, including genetic
modification and synthetic biology
 Synthetic biology – research integrating biology and engineering; construction of
biological functions not found in nature
o Botany – the study of plants
 Phycology – scientific study of algae
 Plant physiology – concerned with the functioning, or physiology, of plants
 Astrobotany - the study of plants in space
o Zoology – the study of animals, including classification, physiology, development,
evolution and behavior, including:
 Ethology – the study of animal behavior
 Entomology – the study of insects
 Herpetology – the study of reptiles and amphibians
 Ichthyology – the study of fish
 Mammalogy – the study of mammals
 Ornithology – the study of birds
o Cell biology – the study of the cell as a complete unit, and the molecular and chemical
interactions that occur within a living cell
o Chronobiology – the study of periodic events in living systems
o Cognitive biology – the study of cognition
o Conservation biology – the study of the preservation, protection, or restoration of the
natural environment, natural ecosystems, vegetation, and wildlife
o Cryobiology – the study of the effects of lower than normally preferred temperatures on
living beings
o Developmental biology – the study of the processes through which an organism forms,
from zygote to full structure
 Embryology – the study of the development of embryo (from fecundation to birth)
 Gerontology – study of ageing processes
o Ecology – the study of the interactions of living organisms with one another and with the
non-living elements of their environment
o Evolutionary biology – the study of the origin and descent of species over time
o Genetics – the study of genes and heredity
 Genomics – the study of genomes
 Epigenetics – the study of heritable changes in gene expression or cellular
phenotype caused by mechanisms other than changes in the underlying DNA
sequence
o Immunology – the study of the immune system
o Marine biology (or biological oceanography) – the study of ocean ecosystems, plants,
animals, and other living beings
o Microbiology – the study of microscopic organisms (microorganisms) and their
interactions with other living things
 Bacteriology – the study of bacteria
 Mycology – the study of fungi
 Parasitology – the study of parasites and parasitism
 Virology – the study of viruses and some other virus-like agents
o Molecular biology – the study of biology and biological functions at the molecular level,
some cross over with biochemistry
o Nanobiology – the application of nanotechnology in biological research, and the study of
living organisms and parts on the nanoscale level of organization
o Neuroscience – the study of the nervous system
o Paleontology – the study of fossils and sometimes geographic evidence of prehistoric life
o Pathobiology or pathology – the study of diseases, and the causes, processes, nature,
and development of disease
o Pharmacology – the study of the interactions between drugs and organisms
o Physiology – the study of the functions and mechanisms occurring in living organisms
o Phytopathology – the study of plant diseases (also called Plant Pathology)
o Psychobiology – the application of methods traditionally used in biology to study human
and non-human animal’s behavior
o Quantum biology – the study of the role of quantum phenomena in biological processes
o Systems biology – the study complex interactions within biological systems through a
holistic approach
o Structural biology – a branch of molecular biology, biochemistry, and biophysics
concerned with the molecular structure of biological macromolecules
o Theoretical biology – the branch of biology that employs abstractions and mathematical
models to explain biological phenomena
 History of biology

o Our fascination with biology has a long history. Even early humans had to study the
animals they hunted and know where to find the plants they gathered for food. The
invention of agriculture was the first great advance of human civilization. Medicine has
been important to us from earliest history as well. The earliest known medical texts are
from China (2500 B.C.), Mesopotamia (2112 B.C.), and Egypt (1800 B.C.).
o In classical times, Aristotle is often considered to be the first to practice scientific zoology.
He is known to have performed extensive studies of marine life and plants. His student,
Theophrastus, wrote one of the West's earliest known botanical texts in 300 B.C. on the
structure, life cycle and uses of plants. The Roman physician Galen used his experience
in patching up gladiators for the arena to write texts on surgical procedures in A.D. 158.
o During the Renaissance, Leonardo da Vinci risked censure by participating in human
dissection and making detailed anatomical drawings that are still considered among the
most beautiful ever made. Invention of the printing press and the ability to reproduce
woodcut illustrations meant that information was much easier to record and disseminate.
One of the first illustrated biology books is a botanical text written by German botanist
Leonhard Fuchs in 1542. Binomial classification was inaugurated by Carolus Linnaeus in
1735, using Latin names to group species according to their characteristics.
o Microscopes opened up new worlds for scientists. In 1665, Robert Hooke, used a simple
compound microscope to examine a thin sliver of cork. He observed that the plant tissue
consisted of rectangular units that reminded him of the tiny rooms used by monks. He
called these units "cells." In 1676, Anton von Leeuwenhoek published the first drawings
of living single celled organisms. Theodore Schwann added the information that animal
tissue is also composed of cells in 1839.
o During the Victorian era, and throughout the 19th century, "Natural Science" became
something of a mania. Thousands of new species were discovered and described by
intrepid adventurers and by backyard botanists and entomologists alike. In 1812, Georges
Cuvier described fossils and hypothesized that Earth had undergone "successive bouts of
Creation and destruction" over long periods of time. On Nov. 24, 1859, Charles Darwin
published "On the Origin of Species," the text that forever changed the world by showing
that all living things are interrelated and that species were not separately created but
arise from ancestral forms that are changed and shaped by adaptation to their
environment.
o While much of the world's attention was captured by biology questions at the
macroscopic organism level, a quiet monk was investigating how living things pass traits
from one generation to the next. Gregor Mendel is now known as the “father of genetics”
although is papers on inheritance, published in 1866, went largely unnoticed at the time.
His work was rediscovered in 1900 and further understanding of inheritance rapidly
followed.
 o The 20th and 21st centuries may be known to future generations as the beginning of the
"Biological Revolution." Beginning with Watson and Crick explaining the structure and
function of DNA in 1953, all fields of biology have expanded exponentially and touch
every aspect of our lives. Medicine will be changed by development of therapies tailored
to a patient's genetic blueprint or by combining biology and technology with brain-
controlled prosthetics. Economies hinge on the proper management of ecological
resources, balancing human needs with conservation. We may discover ways to save our
oceans while using them to produce enough food to feed the nations. We may "grow"
batteries from bacteria or light buildings with bioluminescent fungi. The possibilities are
endless; biology is just coming into its own.

 Introduction to Biological Classification

Do you understand the meaning of classification? Classification basically is the arrangement of
things in taxonomic groups in accordance with the observed similarities. It helps in understanding
the group as a whole with simple ease. Now, here we’ll discuss with you the biological
classification and how it has simplified things for us.

o What is Biological Classification?

Biological classification is the scientific procedure that involves the arrangement of the organisms
in a hierarchical series of groups and sub-groups on the basis of their similarities and
dissimilarities.
Need for Classification;
Right from the archaic times, several attempts have been made to classify the living organisms.
The first man to attempt a scientific basis of classification was Aristotle. He used simple
morphological characters to classify plants as trees, shrubs, and herbs. He classified the animals
into two groups:
i. Enaima (with red blood)
ii. Anaima (without red blood)
However, a need for a proper system of biological classification was always felt.

Need for classification of living organisms;

 The study of one or two organisms is not sufficient to know the essential features of the
group.
 All kinds of organisms do not occur in one locality.
 Classification helps in knowing the relationship between the different groups of
organisms.
 It helps in knowing the evolutionary relationship between organisms.

o Types of Classification System/History of classification system

Based on the types of system of classification, organisms are classified into the following
kingdoms.
A. Two Kingdom Classification System
In the year 1758m Linnaeus (the father of taxonomy system) divided all the living organisms into
two kingdoms. These are;
1. Plantae and
2. Animalia.
Features of Kingdom Plantae: The significant features of the kingdom Plantae are listed here;
 They have a cell wall.
 Autotrophic mode of nutrition is followed. The reserve food is starch.
 A big central vacuole is present.
 There aren’t any excretory organs, nervous system, sense organs and muscular system.
 No locomotion is seen except in some lower algae.
 Plantae absorbs inorganic nutrients from outside.
 They experience unlimited growth but have well-defined growing points.
 The response to external stimuli is slow.
Features of Kingdom Animalia: The significant features of the kingdom Animalia are listed here;
 The cell wall is absent.
 There are no inorganic crystals present in their cells.
 Central vacuole is absent.
 Growth is limited and well-defined growing points are not present.
 Heterotrophic mode of nutrition is used.
 Show quick response to external stimuli.
 The muscular system is present.
 Locomotion is present.
 Excretory organs, nervous system and sense organs are present.
 Reserve food as glycogen.
Limitations of two kingdom classification system;
 Fungi are included in kingdom plantae in spite of the fact they lack chlorophyll, cellulosic
cell wall and are either saprophyte or parasite unlike typical plants.
 Some of the organisms like viruses and lichens can’t be placed in either of these two
kingdoms because of peculiar characteristics.
—Viruses lack protoplasm and exhibit characters of living organisms only inside a living
cell.
—Lichens are peculiar in being association of an alga and a fungus having neither distinct
plant n o r animal characters.
 It puts together eukaryotes with prokaryotes.
 This system does not indicate the gradual evolution of early organisms into plants and
animals.
B. Three Kingdom Classification System
In the year 1866, Ernst Haeckel, classified living organisms into three kingdoms i.e.

1. Plantae,
2. Protista,
3. Animalia.

The new kingdom Protista included all those organisms, which lack the capability of tissue
differentiation. This group included algae, fungi, and Protozoa. Later, kingdom Protista was
reserved only for the unicellular organism.

Limitations of Three Kingdom Classification System;

 No separation of Prokaryotes and eukaryotes.
 Both unicellular and multicellular organisms are classified under Protista.
C. Four Kingdom Classification System
In addition to Protista, Plantae and Animalia, the four-kingdom classification system included
Monera;
1. Plantae,
2. Protista,
3. Animalia,
4. Monera.
The studies with electron microscope made it clear that bacteria and related organisms have a
different nuclear structure as compared to others. These are the prokaryotes. As a result of this,
Copeland in the year 1956, introduced the kingdom-Monera. Fungi continued to remain with
Plantae in this system.
The main drawback of this system is that fungi are not properly placed.
D. Five Kingdom Classification System
In the year 1969, this classification came into existence. RH Whittaker proposed this system. He
created a separate group for fungi.
1. Plantae,
2. Protista,
3. Animalia,
4. Monera,
5. Fungi.
The primary criterion for classification here were:
 Cell structure
 Modes of nutrition
 Reproduction
 Thallus organization
 Phylogenetic relationships

E. Six Kingdom Classification System

Carl Woese a Professor in the Department of Microbiology, University of Illinois, came up with
the Six Kingdom Classification System in the year 1990. It was also known as the three-domain
system as in its organism classification was done in three domains, i.e., Archaea, Bacteria and
Eukarya.

It majorly used the basic principles of the five-kingdom system but divides the Monera into two
domains Archaebacteria, Eubacteria and other eukaryotes in the third kingdom.
1. Archaea
Archaea domain includes prokaryotic organisms. These have a monolayer core of lipids in the cell
membrane and distinct nucleotides in their 16S RNA. It contains a single kingdom called
Archaebacteria. This kingdom includes early prokaryotes. These are methanogens, halophiles
and thermoacidophiles.
2. Bacteria
The bacteria domain consists of typical prokaryotes that lack membrane covered cell organelles.
These do not have microchambers for separating various metabolic activities. It also has a single
kingdom-Eubacteria.
Kingdom-Eubacteria
The members of this kingdom have peptidoglycan cell wall, naked DNA in coiled form, glycogen
food reserves. There is no sap vacuole and 70S ribosomes are present. The members of this
kingdom are bacteria, mycoplasma, Actinomycetes, rickettsiae, spirochaetes, cyanobacteria,
Firmicutes.
3. Eukarya
The domain eukarya contain all the eukaryotes. The four kingdoms of this domain are:

 Protista
 Animalia
 Plantae
 Fungi
“NOTE: Traditionally, some textbooks from the United States used a system of six kingdoms
(Animalia, Plantae, Fungi, Protista, Archaea/Archaebacteria, and Bacteria/Eubacteria) while
textbooks in countries like Great Britain, India, Greece, Australia, Latin America and other
countries used five kingdoms (Animalia, Plantae, Fungi, Protista and Monera).”

 Taxonomy
Taxonomy is the branch of biology that classifies all living things. It was developed by the Swedish
botanist Carolus Linnaeus, who lived during the 18th Century, and his system of classification is
still used today. Linnaeus invented binomial nomenclature, the system of giving each type of
organism a genus and species name. He also developed a classification system called the
taxonomic hierarchy, which today has eight ranks from general to specific: domain >> kingdom
>> phylum >> class >> order >> family >> genus >> species.
 o The Taxonomic Hierarchy
A taxon (plural: taxa) is a group of organisms that are classified as a unit. This can be specific or
general. For example, we could say that all humans are a taxon at the species level since they are
all the same species, but we could also say that humans along with all other primates are a taxon
at the order level, since they all belong to the order Primates. Species and orders are both
examples of taxonomic ranks, which are relative levels of grouping organisms in a taxonomic
hierarchy. The following is a brief description of the taxonomic ranks that make up the taxonomic
hierarchy.
  Domain
A domain is the highest (most general) rank of organisms. Linnaeus did invent some of the
taxonomic ranks, but he did not invent the domain rank, which is relatively new. The term domain
wasn’t used until 1990, over 250 years after Linnaeus developed his classification system in 1735.
The three domains of life are Bacteria, Archaea, and Eukaryota. Archaea are single-celled
organisms similar to bacteria; some archaea live in extreme environments, but others live in mild
ones. Eukaryota, or every living thing on earth that is not a bacterium or archaeon, is more closely
related to the domain Archaea than to Bacteria.

Taxonomic ranks are always capitalized, except for species. This allows people to differentiate
between bacteria (the organisms; could refer to all bacteria or just two specific bacteria) and
Bacteria (the domain, which includes all bacteria).

 Kingdom
Before domains were introduced, kingdom was the highest taxonomic rank. In the past, the
different kingdoms were Animalia, Plantae, Fungi, Protista, Archaea, and Bacteria (Archaea and
Bacteria were sometimes grouped into one kingdom, Monera). However, some of these
groupings, such as Protista, are not very accurate. Protista includes all eukaryotic organisms that
are not animals, plants, or fungi, but some of these organisms are not very closely related to one
another. There is no set agreement on the kingdom classification, and some researchers have
abandoned it altogether. Currently, it continues to be revised; in 2015 researchers suggested
splitting Protista into two new kingdoms, Protozoa and Chromista.
 Phylum
Phylum (plural: phyla) is the next rank after kingdom; it is more specific than kingdom, but less
specific than class. There are 35 phyla in the kingdom Animalia, including Chordata (all organisms
with a dorsal nerve cord), Porifera (sponges), and Arthropoda (arthropods).
 Class
Class was the most general rank proposed by Linnaeus; phyla were not introduced until the 19th
Century. There are 108 different classes in the kingdom Animalia, including Mammalia
(mammals), Aves (birds), and Reptilia (reptiles), among many others. The classes of Animalia that
Linnaeus proposed are similar to the ones used today, but Linnaeus’ classes of plants were based
on attributes like the arrangement of flowers rather than relatedness. Today’s classes of plants
are different than the ones Linnaeus used, and classes are not frequently used in botany.
 Order
Order is more specific than class. Some of Linnaeus’ orders are still used today, such as
Lepidoptera (the order of butterflies and moths). There are between 19-26 orders of Mammalia,
depending on how organisms are classified—sources differ. Some orders of Mammalia are
Primates, Cetaceans (whales, dolphins, and porpoises), Carnivora (large carnivores/omnivores),
and Chiroptera (bats).
 Family
Family is, in turn, more specific. Some families in the order Carnivora, for example, are Canidae
(dogs, wolves, foxes), Felidae (cats), Mephitidae (skunks), and Ursidae (bears). There are 12 total
families in the order Carnivora.
 Genus
Genus (plural: genera) is even more specific than family. It is the first part of an organism’s
scientific name using binomial nomenclature; the second part is the species name. An organism’s
scientific name is always italicized, and the genus name is capitalized while the species name is
not. Genus and species are the only taxonomic ranks that are italicized. The scientific name for
humans is Homo sapiens. Homo is the genus name, while sapiens is the species name. All other
species in the genus Homo are extinct. Some were ancestral to humans, such as Homo erectus.
Others lived at the same time, were closely related, and interbred with Homo sapiens, such as
Homo neanderthalensis, the Neanderthals.

 Species
Species is the most specific major taxonomic rank; species are sometimes divided into
subspecies, but not all species have multiple forms that are different enough to be called
subspecies. There are an estimated 8.7 million different species of organisms on Earth, but the
vast majority have yet to be discovered and categorized. While each genus name is unique, the
same species names can be used for different organisms. For example, Ursus americanus is the
American black bear, while Bufo americanus is the American toad. The species name is always
italicized, but never capitalized. It is the only taxonomic rank that is not capitalized. In scientific
articles where the species name is used many times, it is abbreviated after the first full use by
using just the first letter of the genus name along with the full species name. Homo sapiens is
abbreviated to H. sapiens.
Examples of Taxonomy; The scientific classification of humans is as follows:

Another example of taxonomy is the diagram below, which shows the classification of the red
fox, Vulpes vulpes (sometimes the genus and species names are the same, even though these
are two different ranks).
 o Viruses, Viroids and Lichens
After going through all the classification groups of the plant and animal kingdom, don’t you feel
we missed something out? What about the viruses and lichens? We hear so much about viruses
but where are they on the classification list? Whittaker, in his classification of the five kingdoms,
did not mention any acellular organisms like viruses and viroids, and lichens.

Viruses
 Almost all of us have suffered the ill effects of common cold or ‘flu’. Therefore, we know
what effects viruses can have on us, even if we do not associate it with our condition.
Viruses did not find a place in classification since they are not truly ‘living’. This is based
on the understanding that living organisms that have a cell structure.
 The viruses are non-cellular organisms. They, in fact, have an inert crystalline structure
outside the living cell. Once they infect a cell, they take over the machinery of the host
cell to replicate themselves, killing the host. Would you call viruses living or non-living?
 Pasteur. D.J. Ivanowsky (1892) gave the name virus. It means venom or poisonous fluid.
According to his research, certain microbes caused the mosaic disease of tobacco.
 These organisms were smaller than bacteria because they passed through bacteria-proof
filters. M.W. Beijerinek (1898) demonstrated that the extract of the infected plants of
tobacco could cause infection in healthy plants. He named the fluid as Contagium vivum
fluidum (infectious living fluid).
 W.M. Stanley (1935) discovered that viruses could be crystallised. These virus crystals are
composed largely of proteins. They are inert outside their specific host cell. Viruses are
nothing but obligate parasites.
Genetic Material of Viruses
 In addition to proteins, viruses also contain genetic material, that could be either RNA or
DNA. No virus contains both RNA and DNA. A virus is a nucleoprotein and the genetic
 material is infectious. Speaking in strictly general terms, viruses infecting plants have
single-stranded RNA. On the other hand, viruses that infect animals have either single or
double-stranded RNA or they might have double-stranded DNA.
 Bacterial viruses or bacteriophages usually have a double-stranded DNA structure. By
bacteriophages, we mean viruses that infect the bacteria. The protein coat, capsid made
of small subunits (capsomeres) protects the nucleic acid. They have these capsomeres
arranged in various geometric forms like helical or polyhedral forms.
Viroids

 In 1971 T.O. Diener discovered a new infectious agent. This agent was more minute than
the viruses. it was responsible for causing the potato spindle tuber disease. He found a
free RNA. It lacked the protein coat that is found in viruses. Hence, the name viroid. The
RNA of the viroid was of low molecular weight.
Lichens

 Lichens are symbiotic associations i.e. mutually useful associations, between algae and
fungi. The algal component called phycobiont and the fungal component is what we call
as mycobiont. These components are autotrophic and heterotrophic, respectively.
 Algae prepare food for fungi. On the other hand, fungi provide shelter and absorb mineral
nutrients and water for its partner. Their association is so close that if one saw a lichen in
nature one would never imagine that they had two different organisms within them.
Lichens are very good pollution indicators – they do not grow in polluted areas.

 Cell: Prokaryotic and Eukaryotic cell

o The cell was discovered by Robert Hook in 1665.

o Some cells have membrane bound organelles and some do not have.
o Two types of cells are found in an organism depending upon the internal structure of cell
that is Eukaryotic and Prokaryotic.
o Cells are also known as 'a small room'. It is the functional and structural unit of life. It is a
small united area where all kinds of actions and reactions collectively take place.
o Organisms that are made up of single cell are known as single-celled organisms or
unicellular and from many cells are known as multi-cellular organisms.
o When scientists studied cells of various living organisms from an electron microscope, it
was found that many organisms have no distinct nucleus surrounded by cell membrane.
o Thus, two types of cells are found in the organisms: eukaryotic and prokaryotic depending
on whether cells contain membrane-bound organelles or not. Their genetic materials are
enclosed by a nuclear envelope or not.
o Prokaryotes are simple, small cells, whereas eukaryotic cells are complex, large structured
and are present in trillions which can be single celled or multicellular.
o Prokaryotic cells do not have a well-defined nucleus but DNA molecule is located in the
cell, termed as nucleoid, whereas eukaryotic cells have a well-defined nucleus, where
genetic material is stored.
o Based on the structure and functions, cells are broadly classified (Eukaryotic and
Prokaryotic).
o Prokaryotic Cells are the most primitive kind of cells and lack few features as compared
to the eukaryotic cell.
o Eukaryotic cells have evolved from prokaryotic cells only but contain different types of
organelles like Endoplasmic reticulum, Golgi body, Mitochondria etc., which are specific
in their functions. But features like growth, response, and most importantly giving birth
to the young ones are the commonly shared by all living organisms.

o Prokaryotic cell:
 Prokaryotes are single-celled organisms that are the earliest and most primitive forms
of life on earth.
 As organized in the Three Domain System, prokaryotes include bacteria and
archaeans.
 Some prokaryotes, such as cyanobacteria, are photosynthetic organisms and are
capable of photosynthesis.
 Many prokaryotes are extremophiles and are able to live and thrive in various types
of extreme environments including hydrothermal vents, hot springs, swamps,
wetlands, and the guts of humans and animals (Helicobacter pylori).
 Prokaryotic bacteria can be found almost anywhere and are part of the human
microbiota. They live on your skin, in your body, and on everyday objects in your
environment.
 Prokaryotic cells are not as complex as eukaryotic cells. They have no true nucleus as
the DNA is not contained within a membrane or separated from the rest of the cell,
but is coiled up in a region of the cytoplasm called the nucleoid.
 Prokaryotic organisms have varying cell shapes. The most common bacteria shapes
are spherical, rod-shaped, and spiral.
 Figure: - Prokaryotic Cell: Bacterial cell

 Using bacteria as our sample prokaryote, the following structures and organelles can be
found in bacterial cells:
 Capsule - Found in some bacterial cells, this additional outer covering protects the
cell when it is engulfed by other organisms, assists in retaining moisture, and helps
the cell adhere to surfaces and nutrients.
 Cell Wall - The cell wall is an outer covering that protects the bacterial cell and
gives it shape.
 Cytoplasm - Cytoplasm is a gel-like substance composed mainly of water that also
contains enzymes, salts, cell components, and various organic molecules.
 Cell Membrane or Plasma Membrane - The cell membrane surrounds the cell's
cytoplasm and regulates the flow of substances in and out of the cell.
 Pili (Pilus singular)- Hair-like structures on the surface of the cell that attach to
other bacterial cells. Shorter pili called fimbriae help bacteria attach to surfaces.
 Flagella - Flagella are long, whip-like protrusion that aids in cellular locomotion.
 Ribosomes - Ribosomes are cell structures responsible for protein production.
 Plasmids - Plasmids are gene carrying, circular DNA structures that are not
involved in reproduction.
 Nucleoid Region - Area of the cytoplasm that contains the single bacterial DNA
molecule.
 Most prokaryotes reproduce asexually through a process called binary fission. During
binary fission, the single DNA molecule replicates and the original cell is divided into two
identical cells.
 Steps of Binary Fission:
 Binary fission begins with DNA replication of the single DNA molecule. Both copies
of DNA attach to the cell membrane.
 Next, the cell membrane begins to grow between the two DNA molecules. Once
the bacterium just about doubles its original size, the cell membrane begins to
pinch inward.
 A cell wall then forms between the two DNA molecules dividing the original cell
into two identical daughter cells.
 Although E. coli and other bacteria most commonly reproduce by binary fission, this mode
of reproduction does not produce genetic variation within the organism.
 Prokaryotic cells lack organelles found in eukaryotic cells such as mitochondria,
endoplasmic reticula, and Golgi complexes.
 According to the Endosymbiotic Theory, eukaryotic organelles are thought to have
evolved from prokaryotic cells living in endosymbiotic relationships with one another.
 Like plant cells, bacteria have a cell wall. Some bacteria also have a polysaccharide capsule
layer surrounding the cell wall. It is in this layer where bacteria produce biofilm, a slimy
substance that helps bacterial colonies adhere to surfaces and to each other for
protection against antibiotics, chemicals, and other hazardous substances.
 Similar to plants and algae, some prokaryotes also have photosynthetic pigments. These
light-absorbing pigments enable photosynthetic bacteria to obtain nutrition from light.
o Eukaryotic cell:
 Eukaryotic cells are larger than prokaryotic cells and have a “true” nucleus,
membrane-bound organelles, and rod-shaped chromosomes.
 The nucleus houses the cell’s DNA and directs the synthesis of proteins and
ribosomes.
 Mitochondria are responsible for ATP production; the endoplasmic reticulum
modifies proteins and synthesizes lipids; and the Golgi apparatus is where the sorting
of lipids and proteins takes place.
 Peroxisomes carry out oxidation reactions that break down fatty acids and amino
acids and detoxify poisons; vesicles and vacuoles function in storage and transport.
 Animal cells have a centrosome and lysosomes while plant cells do not.
 Plant cells have a cell wall, a large central vacuole, chloroplasts, and other specialized
plastids, whereas animal cells do not.
 The nucleus is an organelle that is surrounded by a double membrane called the
nuclear envelope. The nuclear envelope separates the contents of the nucleus from
the rest of the cell.
 Eukaryotic cells also have a cell membrane (plasma membrane), cytoplasm,
cytoskeleton, and various cellular organelles.
 Animals, plants, fungi, and protists are examples of eukaryotic organisms.
 Animal and plant cells contain many of the same kinds or organelles.
 There are also certain organelles found in plant cells that are not found in animal cells
and vice versa.
 Examples of organelles found in plant cells and animal cells include:
 Nucleus - a membrane bound structure that contains the cell's hereditary
(DNA) information and controls the cell's growth and reproduction. It is
commonly the most prominent organelle in the cell.
 Mitochondria - as the cell's power producers, mitochondria convert energy
into forms that are usable by the cell. They are the sites of cellular respiration
which ultimately generates fuel for the cell's activities. Mitochondria are also
involved in other cell processes such as cell division and growth, as well as cell
death.
 Endoplasmic Reticulum - extensive network of membranes composed of both
regions with ribosomes (rough ER) and regions without ribosomes (smooth
ER). This organelle manufactures membranes, secretory proteins,
carbohydrates, lipids, and hormones.
 Golgi complex - also called the Golgi apparatus, this structure is responsible
for manufacturing, warehousing, and shipping certain cellular products,
particularly those from the endoplasmic reticulum (ER).
 Ribosomes - these organelles consist of RNA and proteins and are responsible
for protein production. Ribosomes are found suspended in the cytosol or
bound to the endoplasmic reticulum.
 Lysosomes - these membranous sacs of enzymes recycle the cell's organic
material by digesting cellular macromolecules, such as nucleic acids,
polysaccharides, fats, and proteins.
 Peroxisomes - Like lysosomes, peroxisomes are bound by a membrane and
contain enzymes. Peroxisomes help to detoxify alcohol, form bile acid, and
break down fats.
 Vacuole - these fluid-filled, enclosed structures are found most commonly in
plant cells and fungi. Vacuoles are responsible for a wide variety of important
functions in a cell including nutrient storage, detoxification, and waste
exportation.
 Chloroplast - this chlorophyll containing plastid is found in plant cells, but not
animal cells. Chloroplasts absorb the sun's light energy for photosynthesis.
 Cell Wall - this rigid outer wall is positioned next to the cell membrane in most
plant cells. Not found in animal cells, the cell wall helps to provide support and
protection for the cell.
 Centrioles - these cylindrical structures are found in animal cells, but not plant
cells. Centrioles help to organize the assembly of microtubules during cell
division.
 Cilia and Flagella - cilia and flagella are protrusions from some cells that aid in
cellular locomotion. They are formed from specialized groupings of
microtubules called basal bodies.
 Single-celled organisms: Types, Ecology and roles of microorganisms

o A unicellular organism, also known as a single-celled organism, is an organism that

consists of only one cell, unlike a multicellular organism that consists of more than one
cell.
o Unicellular organisms fall into two general categories: prokaryotic organisms and
eukaryotic organisms.
o Prokaryotes include bacteria and archaea.
o Many eukaryotes are multicellular, but we are discussing about single-celled organisms,
the group includes the protozoa, unicellular algae, and unicellular fungi.
o Unicellular organisms are thought to be the oldest form of life, with early protocells
possibly emerging 3.8–4 billion years ago.
o Microorganisms, by their omnipresence, impact the entire biosphere. Microbial life
plays a primary role in regulating biogeochemical systems in virtually all of our planet's
environments, including some of the most extreme, from frozen environments and
acidic lakes, to hydrothermal vents at the bottom of deepest oceans, and some of the
most familiar, such as the human small intestine.
o Microbial ecology (or environmental microbiology) and Types of Microorganisms:
 Ecology of microorganisms: their relationship with one another and with their
environment. It concerns the three major domains of life—Eukaryota, Archaea,
and Bacteria—as well as viruses.
 Microorganisms make up a large part of the planet’s living material and play a
major role in maintaining the Earth’s ecosystem.
 Microorganisms or microbes are microscopic organisms that exist as unicellular,
multicellular, or cell clusters.
 Microorganisms are widespread in nature and are beneficial to life, but some can
cause serious harm.
 They can be divided into six major types: bacteria, archaea, fungi, protozoa,
algae, and viruses.
 Bacteria
 They are unicellular organisms. The cells are described as prokaryotic
because they lack a nucleus. They exist in four major shapes:
i. Bacillus (rod shape),
ii. Coccus (spherical shape),
iii. Spirilla (spiral shape), and
iv. Vibrio (curved shape).
 Most bacteria have a peptidoglycan cell wall; they divide by binary fission;
and they may possess flagella for motility. The difference in their cell wall
structure is a major feature used in classifying these organisms.
  According to the way their cell wall structure stains, bacteria can be
classified on the basis of Gram staining;
i. Gram-positive, and
ii. Gram-negative.
 Bacteria can be further divided based on their response to gaseous oxygen
into the following groups:
i. Aerobic (living in the presence of oxygen),
ii. Anaerobic (living without oxygen), and
iii. Facultative anaerobes (can live in both environments).
 According to the way they obtain energy, bacteria are classified as;
i. Autotrophs (make their own food by using the energy of sunlight or
chemical reactions; in which case they are called chemoautotrophs), and
ii. Heterotrophs (obtain their energy by consuming other organisms.
Bacteria that use decaying life forms as a source of energy are called
saprophytes).
 Archaea or Archaebacteria
 They differ from true bacteria in their cell wall structure and lack
peptidoglycans. They are prokaryotic cells with avidity to extreme
environmental conditions. Based on their habitat, all Archaeans can be
divided into the following groups:
i. Methanogens (methane-producing organisms),
ii. Halophiles (archaeans that live in salty environments),
iii. Thermophiles (archaeans that live at extremely hot temperatures,
optimum growth temperature of 50 °C-110 °C),
iv. Psychrophiles (cold-temperature Archaeans, optimum growth
temperature of less than 15 °C ),
v. Alkaliphiles (optimum growth pH of greater than 8),
vi. Acidophiles (optimum growth pH of less than 3), and
vii. Piezophiles or barophiles (prefer high pressure up to 130 MPa, such as
deep ocean environments).
 Archaeans use different energy sources like hydrogen gas, carbon dioxide,
and sulphur. Some of them use sunlight to make energy, but not the same
way plants do. They absorb sunlight using their membrane pigment,
bacteriorhodopsin. This reacts with light, leading to the formation of the
energy molecule adenosine triphosphate (ATP).
 Fungi (mushroom, molds, and yeasts)
 They are eukaryotic cells (with a true nucleus).
 Most fungi are multicellular and their cell wall is composed of chitin.
  They obtain nutrients by absorbing organic material from their
environment (decomposers), through symbiotic relationships with plants
(symbionts), or harmful relationships with a host (parasites).
 They form characteristic filamentous tubes called hyphae that help absorb
material. The collection of hyphae is called mycelium. Fungi reproduce by
releasing spores.
 Protozoa
 They are unicellular aerobic eukaryotes.
 They have a nucleus, complex organelles, and obtain nourishment by
absorption or ingestion through specialized structures.
 They make up the largest group of organisms in the world in terms of
numbers, biomass, and diversity.
 Their cell walls are made up of cellulose.
 Protozoa have been traditionally divided based on their mode of
locomotion: flagellates produce their own food and use their whip-like
structure to propel forward, ciliates have tiny hair that beat to produce
movement, amoeboid have false feet or pseudopodia used for feeding
and locomotion, and sporozoans are non-motile.
 They also have different means of nutrition, which groups them as
autotrophs or heterotrophs.
 Algae
 They also called cyanobacteria or blue-green algae, are unicellular or
multicellular eukaryotes that obtain nourishment by photosynthesis.
 They live in water, damp soil, and rocks and produce oxygen and
carbohydrates used by other organisms.
 It is believed that cyanobacteria are the origins of green land plants.
 Viruses
 They are noncellular entities that consist of a nucleic acid core (DNA or
RNA) surrounded by a protein coat.
 Although viruses are classified as microorganisms, they are not considered
living organisms.
 Viruses cannot reproduce outside a host cell and cannot metabolize on
their own.
 Viruses often infest prokaryotic and eukaryotic cells causing diseases.
o Role of Microorganisms:
 They are the backbone of all ecosystems, but even more so in the zones where
photosynthesis is unable to take place because of the absence of light.
 In such zones, chemosynthetic microbes provide energy and carbon to the other
organisms.
  Other microbes are decomposers, with the ability to recycle nutrients from other
organisms' waste products.
 These microbes play a vital role in biogeochemical cycles.
 The nitrogen cycle, the phosphorus cycle, the Sulphur cycle and the carbon cycle
all depend on microorganisms in one way or another.
 For example, the nitrogen gas which makes up 78% of the earth's atmosphere is
unavailable to most organisms, until it is converted to a biologically available form
by the microbial process of nitrogen fixation.
 Due to the high level of horizontal gene transfer among microbial communities,
microbial ecology is also of importance to studies of evolution.
o Although some prokaryotes live in colonies, they are not specialized into cells with
differing functions. These organisms live together, and each cell must carry out all life
processes to survive.
o In contrast, even the simplest multicellular organisms have cells that depend on each
other to survive.
o Microbes, especially bacteria, often engage in symbiotic relationships (either positive or
negative) with other microorganisms or larger organisms.
o Although physically small, symbiotic relationships amongst microbes are significant in
eukaryotic processes and their evolution.
o Symbiosis is any type of a close and long-term biological interaction between two
different biological organisms.
o The types of symbiotic relationship that microbes participate in include mutualism,
commensalism, parasitism, amensalism and Mimicry these relationships affect the
ecosystem in many ways.
 Mutualism or interspecies reciprocal altruism is a long-term relationship between
individuals of different species where both individuals benefit.
 Commensalism describes a relationship between two living organisms where one
benefits and the other is not significantly harmed or helped.
 In a parasitism relationship, the parasite benefits while the host is harmed.
Parasitism takes many forms, from endoparasites that live within the host's body
to ectoparasites and parasitic castrators that live on its surface and
micropredators like mosquitoes that visit intermittently. Parasitism is an
extremely successful mode of life; about 40% of all animal species are parasites,
and the average mammal species is host to nematodes, cestodes, and trematodes.
 Amensalism is an asymmetric interaction where one species is harmed or killed
by the other, and one is unaffected by the other. There are two types of
amensalism, competition and antagonism (or antibiosis). Competition is where a
larger or stronger organism deprives a smaller or weaker one from a resource.
Antagonism occurs when one organism is damaged or killed by another through a
chemical secretion.
  Mimicry is a form of symbiosis in which a species adopts distinct characteristics of
another species to alter its relationship dynamic with the species being mimicked,
to its own advantage.
o Economic importance of microbes:
 It is important for us to know about microorganisms and its uses as they are both
beneficial as well as harmful to other life forms. They play a crucial role in the
ecosystem. Maintaining a balance between the ‘good’ and ‘bad’ microorganisms
is the key to coexisting with them. But here we will discuss about economic
importance of microbes as follows;
i. Production of dairy products:
 Bacteria are the key players here. Bacteria help in fermentation which
helps in making different forms of dairy products from milk like curd,
buttermilk, butter, cheese.
 Streptococcus is the most common genus of bacteria that are used in the
commercial production of this product.
ii. Bread Baking:
 A species of Streptococcus is added to the dough before making bread to
bring about the required fermentation.
iii. Alcoholic Drinks:
 Alcoholic drinks are prepared or manufactured by the process of
fermentation. Each drink is derived from a different starting product such
as potato and grapes. Then it is fermented, distilled and alcohol is
prepared.
 The commonly used microorganism here is different types of fungus like
yeast. Some even use bacteria and fungus. Alcoholic drinks include wine,
rum, vodka etc.
iv. Organic acids:
 Organic acids are commercially prepared using fungi.
 Acetobacter, Rhizopus, Penicillium are a few fungi that are used to ferment
substrates such as fruits and sugar-containing syrups.
 Examples of acids that are derived and manufactured on a large scale using
fungi are acetic acid, citric acid, gluconic acid, fumaric acid and lactic acid.
v. Enzymes:
 Many microbes are used in the derivation of enzymes such as lipase,
lactase, protease, peptidase to name a few.
vi. Steroid production:
 Some bacterial and fungal species are used in the preparation of steroids
that are then injected into the human body for different purposes.
vii. Help in sewage treatment:
  Not only are microorganisms helpful to our body, they are also helpful to
the environment. They help in the secondary treatment stage of sewage
treatment.
viii. Used as insecticides:
 Certain bacterial and fungal species are used to keep certain insects and
pests away from crops.
ix. Fertility of soil:
 Microorganisms play a very important role in maintaining the fertility of
the soil. They help in the composting process which forms manure.
 Also, microorganisms present in the soil help aerate it and enrich the soil
with nitrates and other nutrients. These nutrients are needed by the crops
for an abundant harvest.
x. Microbes as Bio-Control Agents:
 Bio-control is the use of biological methods for controlling plant diseases
and pests. These chemicals are also harmful for human beings and animals.
Thus, polluting the environment (soil, groundwater).
 (A) Chemical pesticides decrease the growth of weeds, reduce attack from
pathogens and drive away or kill insects, worms and birds, which happen
to feed on crop plants.
 (B) These undesirable species can range from agricultural pests to water
contaminants to virulent pathogens. They are undesirable because these
species are a detriment to human interests in an ecosystem.
 (C) Microbes used for bio-control reduce the target species population
through many ecological mechanisms, including pathogenism,
competition, production of allelochemicals and other interactions.
 (D) Bacteria, fungi and viruses can all act as bio-control agents due to the
large diversity of target species and the variety of methods of action. The
important examples of microbial bio-control agents include Bacillus
thuringiensis, Pseudomonas and Beauveria bassiana.
xi. Biological Control of Pests and Diseases:
 Bio-control is a holistic approach that seeks to develop an understanding
of the interactions between various organisms and use this knowledge to
control pests, weeds, etc.
 Bio-controlling requires familiarity with various life forms, their habitat,
predators, life style, etc., to use them in bio-control measures and reducing
the dependence on chemicals and pesticides.
 Bio-control microbes control their target species through a web of
biological interactions.
xii. Microbes in Production of Biogas:
  Biogas is a mixture of gases, but the major content is methane gas. It is
produced by the microbial activity in digestion of biomass with the help of
certain bacteria. Biogas is used as fuel.
 The type of gas produced depends upon the microbes and the organic
substrates they utilise. Certain bacteria, which grow anaerobically on
cellulosic material, produce large amount of methane along with CO2 and
H2.
 These bacteria are called methanogens. Methanogens produce large
amount of methane (50-70%), CO2 (30-40%) and H2. Methanogens, are
also present in anaerobic sludge during sewage treatment. They are also
present in rumen (a part of stomach) of cattle, where they help in
breakdown of cellulosic material in the food and thus, play important role
in nutrition of cattle.
xiii. Production of vitamins:
 An essential vitamin that people need for proper digestion is Vitamin B 12.
Fungi are responsible for manufacturing B12.
xiv. Production of antibiotics and antivirals:
 Bacteria and viruses are isolated and their antigens and enzymes are
extracted. These antigens help in the development of antibiotics and
antivirals.
xv. Biotechnology and research:
 So many labs use bacteria, fungi and especially viruses for research studies.
Non- virulent forms of these microorganisms are injected into subjects
going through clinical trials.
 This in future helps in the development of medicines, vaccinations and
cure for diseases. And DNA and RNA studies also make use of them.
 GENETICS
o Genetics is the study of genes, and tries to explain what they are and how they work.
Genes are how living organisms inherit features or traits from their ancestors; for
example, children usually look like their parents because they have inherited their
parents' genes. Genetics tries to identify which traits are inherited, and explain how these
traits are passed from generation to generation.
o Some traits are part of an organisms' physical appearance; such as a person's eye-color,
height or weight. Other sorts of traits are not easily seen and include blood types or
resistance to diseases. Some traits are inherited through our genes, so tall and thin people
tend to have tall and thin children. Other traits come from interactions between our genes
and the environment, so a child might inherit the tendency to be tall, but if they are poorly
nourished, they will still be short. The way our genes and environment interact to produce
a trait can be complicated. For example, the chances of somebody dying of cancer or heart
disease seems to depend on both their genes and their lifestyle.
o Genes are made from a long molecule called DNA, which is copied and inherited across
generations. DNA is made of simple units that line up in a particular order within this large
molecule. The order of these units carries genetic information, similar to how the order
of letters on a page carry information. The language used by DNA is called the genetic
code, which lets organisms read the information in the genes. This information is the
instructions for constructing and operating a living organism.
o The information within a particular gene is not always exactly the same between one
organism and another, so different copies of a gene do not always give exactly the same
instructions. Each unique form of a single gene is called an allele. As an example, one
allele for the gene for hair color could instruct the body to produce a lot of pigment,
producing black hair, while a different allele of the same gene might give garbled
instructions that fail to produce any pigment, giving white hair. Mutations are random
changes in genes, and can create new alleles. Mutations can also produce new traits, such
as when mutations to an allele for black hair produce a new allele for white hair. This
appearance of new traits is important in evolution.
o Every cell in the body with a nucleus (a compartment in most cells) has the same complete
set of genes. A gene is made of DNA (deoxyribonucleic acid) and is basically a type of
genetic instruction. Those instructions can be used for making molecules and controlling
the chemical reaction of life. Genes can also be passed from parent to offspring; this is
inheritance.
o Some genes are active ('on') in some tissues and organs but not in others. This is what
makes the difference between a liver cell and a lung cell. Genes are turned on and off
during development and in response to environmental changes, such as metabolism and
infection.
o The basic laws of inheritance are important because they can reveal how a genetic trait
of interest or a disorder can be passed from generation to generation.
o Each person has 22 pairs of chromosomes. For each pair of chromosomes, one comes
from the mother and one comes from the father. Thus, because genes are on
chromosomes, there are actually two copies of each gene, one paternal in origin and the
other maternal. We also get one sex chromosome from each parent, for a total of 46
chromosomes.

Karyotype: a picture of a person's chromosomes arranged into pairs by size from 1-22.
The X and Y chromosomes determined gender (female: XX; male: XY).
o Genes can change or mutate, although this happens only rarely. A mutation is a
permanent change in DNA.
o Given our trillions of cells, some mutation is occurring all the time. While certain
mutations are harmful, in many cases there is no effect on traits. Some mutations are
even beneficial. Only mutations in sperm or egg can be passed from parent to child.
o Our bodies can sometimes recognize and destroy cells with harmful mutations, but not
always. This is how cancer starts. In general, the genome (all the DNA in your body) is
quite stable, and the genetic makeup we are born with remains throughout our lives. It is
this stability that makes genetic testing a little different from other medical testing. For
example, your cholesterol level or your blood count may change with time, but your genes
do not change.
o Every person is born with genetic differences, called variation. Variation is why each
individual is unique at the level of genes and traits. Most variation is harmless, but some
causes disease. Genetic and trait variation allows populations to adapt more readily to
different environmental challenges. In fact, variation in populations is necessary for the
evolution of species. Because many traits and conditions are the result of combinations
of genes and environment, we see a wide range of variation for most traits in the
population.
o Although our genetic makeup is constant throughout life, our genes alone do not
determine our future. All genes work in the context of environment. Changes in the
environment, such as diet, exercise, exposure to toxic agents, or medications can all
influence our genes and traits.
o Mendel's Laws of Inheritance
 Inheritance can be defined as the process of how a child receives genetic
information from the parent.
 The whole process of heredity is dependent upon inheritance and it is the reason
that the offsprings are similar to the parents.
 This simply means that due to inheritance, the members of the same family
possess similar characteristics.
 It was only during the mid-19th century that people started to understand
inheritance in a proper way.
 This understanding of inheritance was made possible by a scientist named Gregor
Mendel who formulated certain laws to understand inheritance known as
Mendel’s laws of inheritance.

Mendel experimented on a pea plant and considered 7 main contrasting traits in the plants
 Between 1856-1863, Mendel conducted the hybridization experiments on the
garden peas.
 During that period, he chose some distinct characteristics of the peas and
conducted several cross-pollination/ artificial pollinations on the peas lines that
showed stable trait inheritance and underwent continuous self-pollination.
 Such pea lines are called true-breeding pea lines.
 Why was pea plant selected for Mendel’s Experiments?
 He selected a pea plant for his experiments:
a) The pea plant can be easily grown and maintained.
b) There are naturally self-pollinating but can also be cross-pollinated.
c) It is an annual plant; therefore, many generations can be studied
within a short period of time.
d) It has several contrasting characters.
 Mendel conducted 2 main experiments to determine the laws of
inheritance. These experiments were:
a) Monohybrid Cross Experiment
b) Dihybrid Cross Experiment
 While experimenting, Mendel found that certain factors were always being
transferred down to the offsprings in a stable way.
 Those factors are now called genes i.e. genes can be called as the units of
inheritance.
 Mendel’s Experiments:
 Mendel experimented on a pea plant and considered 7 main contrasting
traits in the plants. Then, he conducted both the experiments to determine
the aforementioned inheritance laws. A brief explanation of the two
experiments is given below.
a) Monohybrid Cross
 In this experiment, Mendel took two pea plants of opposite
traits (one short and one tall) and crossed them.
 He found the first generation offsprings were tall and called
it F1 progeny.
 Then he crossed F1 progeny and obtained both tall and
short plants in the ratio 3:1.
 Mendel even conducted this experiment with other
contrasting traits like green peas vs yellow peas, round vs
wrinkled, etc.
 In all the cases, he found that results were similar.
 From this, he formulated the laws of Segregation and
Dominance.
 Inheritance is the acquiring of genetic characteristics or
traits from parents by their offspring.
 In the year 1860, Gregor Mendel unlocked the mystery of
genetics. He cultivated pea plants and observed their
pattern of inheritance from one generation to the next
generation. His investigation led to the discovery of three
laws of inheritance, famously known as Mendel’s Laws of
Inheritance.
 The Law of Dominance, Law of Segregation and Law of
Independent Assortment are the three Mendel’s laws of
inheritance. These laws came into existence by the
experiments on pea plants in a variety of differing traits.
Mendel started his research with monohybrid cross.
 Mendel observed that traits which were absent in the F1
generation had reappeared in the F2 generation. These
observations led to the formulation of the Law of
Dominance and the Law of Segregation.

Monohybrid Cross

 In this section, let us take a detailed look at the two laws of

inheritance, namely, Law of dominance and Law of
segregation.
 Law of Dominance states that:
“When parents with pure contrasting traits are crossed
together, only one form of trait appears in the next
generation. The hybrid offsprings will exhibit only the
dominant trait in the phenotype.”
 Law of dominance is known as the first law of
inheritance. In this law, each character is controlled
by distinct units called factors, which occur in pairs.
If the pairs are heterozygous, one will always
dominate the other.
Law of dominance explains that in a monohybrid
cross between a pair of contrasting traits, only one
parental character will be expressed in the F1
generation and both parental characters will be
expressed in the F2 generation in the ratio 3:1.
The one which expressed in the F1 generation is
called the dominant trait and the one which is
suppressed is called recessive trait. In simple words,
the law of dominance states that recessive traits are
always dominated or masked by the dominant trait.
This law can be described by Mendel’s experiment.
A monohybrid cross is a cross between the two
monohybrid traits (TT and tt). Here plants which
have the same characters but differ in only one
character were crossed.
For monohybrid cross, Mendel began with a pair of
pea plants with two contrasting traits i.e., one tall
and another dwarf. The cross-pollination of tall and
dwarf plants resulted in tall plants and the offsprings
were called F1 progeny. The trait which is expressed
in the phenotype is called the dominant trait while
the one that is not is called the recessive trait.
He then continued his experiment with self-
pollination of F1 progeny plants. This resulted in
both tall and short plants in the ratio of 3:1 which
gave rise to the law of segregation.
 Law of Segregation states that:
“During the formation of gamete, each gene separates
from each other so that each gamete carries only one
allele for each gene.”
Law of segregation is the second law of inheritance.
This law explains that the pair of alleles segregate
from each other during meiosis cell division (gamete
formation) so that only one allele will be present in
each gamete.
 In a monohybrid cross, both the alleles are
expressed in the F2 generation without any
blending. Thus, the law of segregation is based on
the fact that each gamete contains only one allele.
This law is based on four basic concepts:
 A gene exists in more than one form of allele.
 When gametes are produced by meiosis, the
allelic pairs separate leaving each gamete
with a single allele.
 Every organism inherits two alleles for each
trait.
 The two alleles of a pair are different, i.e.,
one is dominant and one is recessive.
 Mendel chose to perform monohybrid cross of a pair of
contrasting traits. The observations of monohybrid cross
led to the formulation of Law of Segregation and Law of
Dominance.
 Followed by this, Mendel performed dihybrid cross taking
two contradicting traits together for crossing.

b) Dihybrid Cross
 In a dihybrid cross experiment, Mendel considered two
traits, each having two alleles.
 He crossed wrinkled-green seed and round-yellow seeds
and observed that all the first-generation progeny (F1
progeny) were round-yellow. This meant that dominant
traits were round shape and yellow color.
 He then self-pollinated the F1 progeny and obtained 4
different traits wrinkled-yellow, round-yellow, wrinkled-
green seeds and round-green in the ratio 9:3:3:1.
 Dihybrid Cross

 After conducting for other traits, the results were found to

be similar. From this experiment, Mendel formulated his
third law of inheritance i.e. law of Independent Assortment.
Here we will discuss the Law of Independent Assortment.
 Law of Independent Assortment states that
“the alleles of two more genes get sorted into gametes
independent of each other. The allele received for one
gene does not influence the allele received for another
gene”.
Mendel’s experiment always portrayed that the
combinations of traits of the progeny are always
different from their parental traits. Based on this, he
formulated the Law of Independent Assortment.
Reasons for Independent Assortment;
 Independent assortment takes place during
the process of meiosis. In this process, the
chromosomes are halved and are known as
haploid.
 To understand the law of independent
assortment, it is very important to
understand the law of segregation. In this,
two different genes are sorted into different
gamete cells. On the other hand, law of
 independent assortment occurs when the
maternal and paternal genes are divided
randomly.
Mendel’s Experiment on Law of Independent
Assortment;
 The Law of Independent Assortment states
that during a dihybrid cross (crossing of two
pairs of traits), an assortment of each pair of
traits is independent of the other. In other
words, during gamete formation, one pair of
trait segregates from another pair of traits
independently. This gives each pair of
characters a chance of expression.
 In the dihybrid cross, he chose round-yellow
seed and wrinkled green seed and crossed
them. He obtained only round yellow seeds
in the F1 generation. Later, self-pollination
of F1 progeny gave four different
combinations of seeds in the F2 generation.
He obtained round-yellow, wrinkled-yellow,
round green and wrinkled green seeds in the
phenotypic ratio 9:3:3:1.
 The phenotypic ratio 3:1 of yellow: green
colour and the ratio 3:1 of the round:
wrinkled seed shape during monohybrid
cross was retained in the dihybrid cross as
well. Thus, he concluded that characters are
distributed independently and inherited
independently. Based on this observation,
he developed his third law – Law of
Independent Assortment.
 The dihybrid crosses between the parental
genotype RRYY (round yellow seeds) and
rryy (green wrinkled seeds) explains the law.
Here the chances of formation of gametes
with the gene R and the gene r are 50:50.
Also, the chances of formation of gametes
with the gene Y and the gene y are 50:50.
Thus, each gamete should have either R or r
and Y or y.
  The Law of Independent Assortment states
that the segregation of R and r is
independent of the segregation of Y and y.
This results in four types of gametes RY, Ry,
rY, and ry. These combinations of alleles are
different from their parental combination
(RR, YY, rr and yy).
Example of Law of Independent Assortment;
Let us consider an example of rabbits with two
visible traits:
 fur color (black or white)
 eye color (green or red)
Two-hybrid rabbits are crossed. Both the rabbits
have a genotype BbGg. Before breeding each rabbit
produced gametes. During this, the alleles are
separated and the copy of each chromosome is
assigned to different gamete. That means,
regardless of the parental phenotype, the baby
rabbits inherit different combinations of the traits.
Alternatively, a baby rabbit can have a genotype
Bbgg.

 Conclusions from Mendel’s Experiments:

 The genetic makeup of the plant is known as the genotype. On the
contrary, the physical appearance of the plant is known as phenotype.
 The genes are transferred from parents to the offsprings in pairs known as
allele.
 During gametogenesis when the chromosomes are halved, there is a 50%
chance of one of the two alleles to fuse with the other parent.
 When the alleles are same, they are known as homozygous alleles when
the alleles are different, they are known as heterozygous alleles.
 In shorts, Formulation of Laws of Inheritance:
 The two experiments lead to the formulation of Mendel’s laws known as
laws of inheritance which are:
A. Law of Dominance
B. Law of Segregation
C. Law of Independent Assortment
 Law of Dominance
This is also called as Mendel’s first law of inheritance. According to the law
of dominance, hybrid offsprings will only inherit the dominant trait in the
 phenotype. The alleles that are suppressed are called as the recessive traits
while the alleles that determine the trait are known as the dormant traits.
 Law of Segregation
The law of segregation states that during the production of gametes, two
copies of each hereditary factor segregate so that offspring acquire one
factor from each parent. In other words, allele (alternative form of the
gene) pairs segregate during the formation of gamete and re-unite
randomly during fertilization. This is also known as Mendel’s third law of
inheritance.
 Law of Independent Assortment
Also known as Mendel’s third law of inheritance, the law of independent
assortment states that a pair of trait segregates independently from
another pair during gamete formation. As the individual heredity factors
assort independently, different traits get equal opportunity to occur
together.

o What are dominant and recessive alleles?

 Different versions of a gene are called alleles. Alleles are described as either
dominant or recessive depending on their associated traits.
 Since human cells carry two copies of each chromosome.
 They have two versions of each gene.
 These different versions of a gene are called alleles.
 Alleles can be either dominant or recessive.
 Dominant alleles show their effect even if the individual only has one copy
of the allele (also known as being heterozygous).
 For example, the allele for brown eyes is dominant, therefore you
only need one copy of the 'brown eye' allele to have brown eyes
(although, with two copies you will still have brown eyes).
 If both alleles are dominant, it is called codominance. The resulting
characteristic is due to both alleles being expressed equally.
 An example of this is the blood group AB which is the result of
codominance of the A and B dominant alleles.
 Recessive alleles only show their effect if the individual has two copies of
the allele (also known as being homozygous).
 For example, the allele for blue eyes is recessive, therefore to have
blue eyes you need to have two copies of the 'blue eye' allele.
 Illustration to show the inheritance of dominant and recessive alleles for
eye color.
o What are sex-linked genes?
 Some genes are found on the sex chromosome, X.
 These genes are inherited with the X chromosome (from the mother if it is a boy
or from either mother or father if it is a girl).
 Females have two X chromosomes (XX), while males have one X chromosome and
one Y chromosome (XY).
 This means females have two alleles for X-linked genes while males only have one.
 Some genetic diseases, are caused by sex linked genes, for example haemophilia.
 The allele for haemophilia is recessive so two copies is needed for a female to have
the disease.
 However, because males only have one X chromosome, they only need one copy
of the haemophilia allele to have the disease.
 This means haemophilia is much more common in males than in females.

o Gene interaction:
 When expression of one gene depends on the presence or absence of another
gene in an individual, it is known as gene interaction.
 The interaction of genes at different loci that affect the same character is called
epistasis.
 The term epistasis was first used by Bateson in 1909 to describe two different
genes which affect the same character, one of which masks the expression of
other gene.
  The gene that masks another gene is called epistatic gene, and the gene whose
expression is masked is termed as hypostatic gene.
 Epistasis is also referred to as inter-genic or inter-allelic gene interaction.
 The important features of gene interaction are briefly described below:
 Number of Genes:
 The epistatic gene interaction always involves two or more genes.
This is an essential feature of gene interaction.
 Affect same Character:
 The epistatic genes always affect the expression of one and the
same character of an individual.
 Expression:
 The phenotypic expression of one gene usually depends on the
presence or absence of epistatic gene. The gene which has masking
effect is called epistatic gene and the gene whose effect is masked
is known as hypostatic gene.
 Modification of Dihybrid Segregation Ratio:
 Epistasis leads to the modification of normal dihybrid or tri-hybrid
segregation ratio in F2 generation.
 Genetic Control:
 Epistasis is usually governed by dominant gene, but now cases of
recessive epistasis are also known.

o Epistasis:
 Due to the phenomenon of dominance a recessive allele remains obscure in the
hybrid. But when two different genes which are not alleles, both affect the same
character in such a way that the expression of one masks, inhibits or suppresses
the expression of the other gene, it is called epistasis. The gene that suppresses is
said to be epistatic, and the gene which remains obscure is hypostatic.
 “Spread is epistatic to color.” What does that mean, exactly? At the most basic
level, it means that when a bird has the spread phenotype, it has no visible
pattern. Spread hides pattern. One level deeper, it means that even though a
spread bird still has two copies (or alleles) of the pattern gene, it has no pattern
phenotype.
 The term epistasis describes a certain relationship between genes, where an allele
of one gene (e.g., ‘spread’) hides or masks the visible output, or phenotype, of
another gene (e.g., pattern). Epistasis is entirely different from dominant and
recessive, which are terms that apply to different alleles of the same gene (e.g.,
‘bar’ is dominant to ‘barless’ and recessive to ‘check’).
 An Analogy for Genes Working Together
 Epistasis involves not genes so much as the proteins they code for. (So do
dominant and recessive, for that matter.) Genes with epistatic
relationships tend to code for proteins that work together in the same
processes. An analogy might be easier to understand.
 Let’s say workers A, B, and C carry out the steps for painting a design on a
poster. Like genes, a, b, and c are the instructions.
 Worker A puts paint into the tray; a tells it how.
 Worker B adds dye to the paint; b tells it what color.
 Worker C paints a design on the poster; c tells it what design.
Like workers in an assembly line, proteins work together to carry out processes.

 If we put ourselves in a position like that of researchers trying to learn

about a genetic pathway, the instructions and the workers would be
invisible. The only part of the process that we would get to see is the
output: the poster, which is like the “phenotype.”
  Now imagine how different versions of the instructions would change the
output. For example, b could say to add red dye. From looking at the
output, we can tell that the instructions for B have changed. “Add blue”
and “add red” are like different alleles of b. And if a version of c said to
draw a square, we could start to figure out that color and shape are
controlled by different instructions.
 This is not epistasis (we’ll get to that soon). Its genes working together to
control an output.

 Adding Epistasis
 Epistasis typically applies to a certain allele, or version, of a gene. Epistasis
depends on how the protein that the allele codes for actually functions. In
our analogy, epistasis depends on what the workers do in our process.
 Now we’ll add epistasis to our example. Let’s say a version (or allele) of a
is broken so that it contains no instructions. Worker A wouldn’t be able to
put paint into the tray, and we would end up with a blank poster—even
though workers B and C are still doing their jobs.
 This broken version of a is epistatic to b and c: the final product (a blank
poster) shows no evidence of what B and C have been told to do. We can’t
tell if B’s instructions said to add red or blue, or if C’s said to draw a circle
or a square.

This version of a is epistatic to b and c: it hides their output.

 This version of c is epistatic to a and b: it hides their output.

 The important aspect of epistasis is that it doesn’t just influence the

phenotype, it hides the output of another gene or genes.
 Now let’s imagine that a is working, but c is broken. This version, or
“allele,” of c is epistatic to a and b: the output shows no evidence of what
A and B are doing. Even though the output is again a blank poster, the
cause is different than when a was broken.
 Because of its role in the process, no allele of b can be epistatic to a or c.
Changing the color of the dye, or even adding no dye at all, cannot hide
what workers A and C are doing.
 Types of Epistasis:
A. Dominant Epistasis
Epistasis due to dominant genes (AA or Aa) is called dominant epistasis.
B. Recessive Epistasis
Epistasis due to recessive genes (aa) is called recessive epistasis.

o The cell cycle or cell-division cycle:

 The cell cycle is the series of events that take place in a cell leading to duplication
of its DNA (DNA replication) and division of cytoplasm and organelles to produce
two daughter cells.
 In bacteria, which lack a cell nucleus, the cell cycle is divided into the B, C, and D
periods.
  The B period extends from the end of cell division to the beginning of DNA
replication.
 DNA replication occurs during the C period.
 The D period refers to the stage between the end of DNA replication and
the splitting of the bacterial cell into two daughter cells.
 In cells with a nucleus, as in eukaryotes, the cell cycle is also divided into two
main stages: interphase (I) and the mitotic (M) phase (including mitosis and
cytokinesis).
 During interphase, the cell grows, accumulating nutrients needed for
mitosis, and undergoes DNA replication preparing it for cell division.
 During the mitotic phase, the replicated chromosomes and cytoplasm
separate into two new daughter cells.
 To ensure the proper division of the cell, there are control mechanisms known as
cell cycle checkpoints.

Life cycle of the cell

 The cell-division cycle is a vital process by which a single-celled fertilized egg

develops into a mature organism, as well as the process by which hair, skin, blood
cells, and some internal organs are renewed.
 After cell division, each of the daughter cells begin the interphase of a new cycle.
Although the various stages of interphase are not usually morphologically
distinguishable, each phase of the cell cycle has a distinct set of specialized
biochemical processes that prepare the cell for initiation of cell division.
 Phases:
 The eukaryotic cell cycle consists of four distinct phases: G1 phase, S phase
(synthesis), G2 phase (collectively known as interphase) and M phase
(mitosis and cytokinesis). M phase is itself composed of two tightly coupled
processes: mitosis, in which the cell's nucleus divides, and cytokinesis, in
which the cell's cytoplasm divides forming two daughter cells. Activation
of each phase is dependent on the proper progression and completion of
the previous one. Cells that have temporarily or reversibly stopped dividing
are said to have entered a state of quiescence called G0 phase.

Schematic of the cell cycle

 After cell division, each of the daughter cells begin the interphase of a new
cycle. Although the various stages of interphase are not usually
morphologically distinguishable, each phase of the cell cycle has a distinct
set of specialized biochemical processes that prepare the cell for initiation
of cell division.
 G0 phase (quiescence)
 G0 is a resting phase where the cell has left the cycle and has
stopped dividing. The cell cycle starts with this phase. The word
"post-mitotic" is sometimes used to refer to both quiescent and
senescent cells. Non-proliferative (non-dividing) cells in
multicellular eukaryotes generally enter the quiescent G0 state
from G1 and may remain quiescent for long periods of time,
possibly indefinitely (as is often the case for neurons). This is very
common for cells that are fully differentiated. Cellular senescence
occurs in response to DNA damage and external stress and usually
constitutes an arrest in G1. Some cells enter the G0 phase semi-
permanently and are considered post-mitotic, e.g., some liver,
kidney, and stomach cells. Many cells do not enter G0 and continue
to divide throughout an organism's life, e.g., epithelial cells.
 Cellular senescence is also a state that occurs in response to DNA
damage or degradation that would make a cell's progeny
nonviable; it is often a biochemical alternative to the self-
destruction of such a damaged cell by apoptosis.
 Interphase (I)
 Interphase is a series of changes that takes place in a newly formed
cell and its nucleus before it becomes capable of division again. It
is also called preparatory phase or intermitosis. Typically
interphase lasts for at least 91% of the total time required for the
cell cycle.
 Interphase proceeds in three stages, G1, S, and G2, followed by the
cycle of mitosis and cytokinesis. The cell's nuclear DNA contents are
duplicated during S phase.
 G1 phase (First growth phase or Post mitotic gap phase)
The first phase within interphase, from the end of
the previous M phase until the beginning of DNA
synthesis, is called G1 (G indicating gap). It is also
called the growth phase. During this phase, the
biosynthetic activities of the cell, which are
considerably slowed down during M phase, resume
at a high rate. The duration of G1 is highly variable,
even among different cells of the same species.[3]
In this phase, the cell increases its supply of
proteins, increases the number of organelles (such
as mitochondria, ribosomes), and grows in size. In
G1 phase, a cell has three options.
 To continue cell cycle and enter S phase.
 Stop cell cycle and enter G0 phase for
undergoing differentiation.
 Become arrested in G1 phase hence it may
enter G0 phase or re-enter cell cycle.
The deciding point is called check point (Restriction
point). This check point is called the restriction point
or START and is regulated by G1/S cyclins, which
cause transition from G1 to S phase. Passage
through the G1 check point commits the cell to
division.
 S phase (DNA replication)
The ensuing S phase starts when DNA synthesis
commences; when it is complete, all of the
chromosomes have been replicated, i.e., each
chromosome consists of two sister chromatids.
Thus, during this phase, the amount of DNA in the
cell has doubled, though the ploidy and number of
chromosomes are unchanged. Rates of RNA
transcription and protein synthesis are very low
 during this phase. An exception to this is histone
production, most of which occurs during the S
phase.
 G2 phase (growth)
G2 phase occurs after DNA replication and is a
period of protein synthesis and rapid cell growth to
prepare the cell for mitosis. During this phase
microtubules begin to reorganize to form a spindle
(preprophase). Before proceeding to mitotic phase,
cells must be checked at the G2 checkpoint for any
DNA damage within the chromosomes. The G2
checkpoint is mainly regulated by the tumor protein
p53. If the DNA is damaged, p53 will either repair
the DNA or trigger the apoptosis of the cell. If p53 is
dysfunctional or mutated, cells with damaged DNA
may continue through the cell cycle, leading to the
development of cancer.
 Mitotic phase (chromosome separation)
 The relatively brief M phase consists of nuclear division
(karyokinesis). It is a relatively short period of the cell cycle. M
phase is complex and highly regulated. The sequence of events is
divided into phases, corresponding to the completion of one set of
activities and the start of the next. These phases are sequentially
known as:
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
 Stages of mitosis
 Mitosis is the process by which a eukaryotic cell separates the
chromosomes in its cell nucleus into two identical sets in two
nuclei. During the process of mitosis, the pairs of chromosomes
condense and attach to microtubules that pull the sister
chromatids to opposite sides of the cell.
 Mitosis occurs exclusively in eukaryotic cells, but occurs in different
ways in different species. For example, animal cells undergo an
"open" mitosis, where the nuclear envelope breaks down before
the chromosomes separate, while fungi such as Aspergillus
nidulans and Saccharomyces cerevisiae (yeast) undergo a "closed"
mitosis, where chromosomes divide within an intact cell nucleus.
 Cytokinesis phase (separation of all cell components)
 Mitosis is immediately followed by cytokinesis, which divides the
nuclei, cytoplasm, organelles and cell membrane into two cells
 containing roughly equal shares of these cellular components.
Mitosis and cytokinesis together define the division of the mother
cell into two daughter cells, genetically identical to each other and
to their parent cell. This accounts for approximately 10% of the cell
cycle.
 Because cytokinesis usually occurs in conjunction with mitosis,
"mitosis" is often used interchangeably with "M phase". However,
there are many cells where mitosis and cytokinesis occur
separately, forming single cells with multiple nuclei in a process
called endoreplication. This occurs most notably among the fungi
and slime molds, but is found in various groups. Even in animals,
cytokinesis and mitosis may occur independently, for instance
during certain stages of fruit fly embryonic development. Errors in
mitosis can result in cell death through apoptosis or cause
mutations that may lead to cancer.
 Cell cycle checkpoints:
 They are used by the cell to monitor and regulate the progress of the cell
cycle. Checkpoints prevent cell cycle progression at specific points,
allowing verification of necessary phase processes and repair of DNA
damage. The cell cannot proceed to the next phase until checkpoint
requirements have been met. Checkpoints typically consist of a network of
regulatory proteins that monitor and dictate the progression of the cell
through the different stages of the cell cycle.
 There are several checkpoints to ensure that damaged or incomplete DNA
is not passed on to daughter cells. Three main checkpoints exist: the G1/S
checkpoint, the G2/M checkpoint and the metaphase (mitotic) checkpoint.
 G1/S transition is a rate-limiting step in the cell cycle and is also known as
restriction point.[14] This is where the cell checks whether it has enough
raw materials to fully replicate its DNA (nucleotide bases, DNA synthase,
chromatin, etc.). An unhealthy or malnourished cell will get stuck at this
checkpoint.
 The G2/M checkpoint is where the cell ensures that it has enough
cytoplasm and phospholipids for two daughter cells. But sometimes more
importantly, it checks to see if it is the right time to replicate. There are
some situations where many cells need to all replicate simultaneously (for
example, a growing embryo should have a symmetric cell distribution until
it reaches the mid-blastula transition). This is done by controlling the G2/M
checkpoint.
 The metaphase checkpoint is a fairly minor checkpoint, in that once a cell
is in metaphase, it has committed to undergoing mitosis. However, that's
 not to say it isn't important. In this checkpoint, the cell checks to ensure
that the spindle has formed and that all of the chromosomes are aligned
at the spindle equator before anaphase begins.
 While these are the three "main" checkpoints, not all cells have to pass
through each of these checkpoints in this order to replicate. Many types of
cancer are caused by mutations that allow the cells to speed through the
various checkpoints or even skip them altogether. Going from S to M to S
phase almost consecutively. Because these cells have lost their
checkpoints, any DNA mutations that may have occurred are disregarded
and passed on to the daughter cells. This is one reason why cancer cells
have a tendency to exponentially accrue mutations. Aside from cancer
cells, many fully differentiated cells type no longer replicate so they leave
the cell cycle and stay in G0 until their death. Thus, removing the need for
cellular checkpoints. An alternative model of the cell cycle response to
DNA damage has also been proposed, known as the post replication
checkpoint.
 Checkpoint regulation plays an important role in an organism's
development. In sexual reproduction, when egg fertilization occurs, when
the sperm binds to the egg, it releases signaling factors that notify the egg
that it has been fertilized. Among other things, this induces the now
fertilized oocyte to return from its previously dormant, G0, state back into
the cell cycle and on to mitotic replication and division.
 p53 plays an important role in triggering the control mechanisms at both
G1/S and G2/M checkpoints. In addition to p53, checkpoint regulators are
being heavily researched for their roles in cancer growth and proliferation.

o Meiosis: A Type of Cell Division

 Meiosis is a type of cell division that reduces the number of chromosomes in the
parent cell by half and produces four gamete cells.
 This process is required to produce egg and sperm cells for sexual reproduction.
 During reproduction, when the sperm and egg unite to form a single cell, the
number of chromosomes is restored in the offspring.
 Meiosis begins with a parent cell that is diploid, meaning it has two copies of each
chromosome.
 The parent cell undergoes one round of DNA replication followed by two separate
cycles of nuclear division.
 The process results in four daughter cells that are haploid, which means they
contain half the number of chromosomes of the diploid parent cell.
 Meiosis has both similarities to and differences from mitosis, which is a cell
division process in which a parent cell produces two identical daughter cells.
 Meiosis begins following one round of DNA replication in cells in the male or
female sex organs.
 The process is split into meiosis-I and meiosis-II, and both meiotic divisions have
multiple phases.
 Meiosis-I is a type of cell division unique to germ cells, while meiosis-II is similar
to mitosis.

Stages of meiosis
 Meiosis-I:
 The meiotic division have many phases in meiosis-I, that is;
1. Prophase-I
2. Metaphase-I
3. Anaphase-I
4. Telophase-I
 During prophase-I, the complex of DNA and protein known as
chromatin condenses to form chromosomes. The pairs of
replicated chromosomes are known as sister chromatids, and they
remain joined at a central point called the centromere. A large
structure called the meiotic spindle also forms from long proteins
called microtubules on each side, or pole, of the cell. Between
prophase I and metaphase I, the pairs of homologous chromosome
form tetrads. Within the tetrad, any pair of chromatid arms can
overlap and fuse in a process called crossing-over or
recombination. Recombination is a process that breaks,
recombines and rejoins sections of DNA to produce new
combinations of genes.
 In metaphase-I, the homologous pairs of chromosomes align on
either side of the equatorial plate.
 Then, in anaphase-I, the spindle fibers contract and pull the
homologous pairs, each with two chromatids, away from each
other and toward each pole of the cell.
 During telophase-I, the chromosomes are enclosed in nuclei. The
cell now undergoes a process called cytokinesis that divides the
cytoplasm of the original cell into two daughter cells. Each
daughter cell is haploid and has only one set of chromosomes, or
half the total number of chromosomes of the original cell.
 Meiosis-II:
 This is a mitotic division of each of the haploid cells produced in meiosis I.
Meiosis-II also have different phases like miosis-I which are follows like
that;
1. Prophase-II
2. Metaphase-II
3. Anaphase-II
4. Telophase-II
 During prophase-II, the chromosomes condense, and a new set of
spindle fibers forms. The chromosomes begin moving toward the
equator of the cell.
  During metaphase-II, the centromeres of the paired chromatids
align along the equatorial plate in both cells.
 Then in anaphase-II, the chromosomes separate at the
centromeres. The spindle fibers pull the separated chromosomes
toward each pole of the cell.
 Finally, during telophase-II, the chromosomes are enclosed in
nuclear membranes. Cytokinesis follows, dividing the cytoplasm of
the two cells.
 At the conclusion of meiosis, there are four haploid daughter cells that go
on to develop into either sperm or egg cells.

o Evidence of Nucleic Acid as A Genetic Material

 Our modern understanding of DNA's role in heredity has led to a variety of
practical applications, including forensic analysis, paternity testing, and genetic
screening. Thanks to these wide-ranging uses, today many people have at least a
basic awareness of DNA.
 It may be surprising, then, to realize that less than a century ago, even the best-
educated members of the scientific community did not know that DNA was the
hereditary material!
 In this article, we'll look at some of the classic experiments that led to the
identification of DNA as the carrier of genetic information.
 Protein v/s DNA:
 The work of Gregor Mendel showed that traits (such as flower colors in
pea plants) were not inherited directly, but rather, were specified by genes
passed on from parents to offspring. The work of additional scientists
around the turn of the 20th century, including Theodor Boveri, Walter
Sutton, and Thomas Hunt Morgan, established that Mendel's heritable
factors were most likely carried on chromosomes.
 Scientists first thought that proteins, which are found in chromosomes
along with DNA, would turn out to be the sought-after genetic material.
Proteins were known to have diverse amino acid sequences, while DNA
was thought to be a boring, repetitive polymer, due in part to an incorrect
(but popular) model of its structure and composition.
 Today, we know that DNA is not actually repetitive and can carry large
amounts of information, as discussed further in the article on discovery of
DNA structure. But how did scientists first come to realize that "boring"
DNA might actually be the genetic material?
 Frederick Griffith: Bacterial transformation:
 In 1928, British bacteriologist Frederick Griffith conducted a series of
experiments using Streptococcus pneumoniae bacteria and mice. Griffith
 wasn't trying to identify the genetic material, but rather, trying to develop
a vaccine against pneumonia. In his experiments, Griffith used two related
strains of bacteria, known as R and S.
 R strain, when grown in a petri dish, the R bacteria formed colonies,
or clumps of related bacteria, that had well-defined edges and a
rough appearance (hence the abbreviation "R"). The R bacteria
were nonvirulent, meaning that they did not cause sickness when
injected into a mouse.
 S strain, S bacteria formed colonies that were rounded and smooth
(hence the abbreviation "S"). The smooth appearance was due to a
polysaccharide, or sugar-based, coat produced by the bacteria. This
coat protected the S bacteria from the mouse immune system,
making them virulent (capable of causing disease). Mice injected
with live S bacteria developed pneumonia and died.
 As part of his experiments, Griffith tried injecting mice with heat-killed S
bacteria (that is, S bacteria that had been heated to high temperatures,
causing the cells to die). Unsurprisingly, the heat-killed S bacteria did not
cause disease in mice.

Griffith Experiment - Transformation

 The experiments took an unexpected turn, however, when harmless R

bacteria were combined with harmless heat-killed S bacteria and injected
into a mouse. Not only did the mouse develop pnenumonia and die, but
when Griffith took a blood sample from the dead mouse, he found that it
contained living S bacteria!
  Griffith concluded that the R-strain bacteria must have taken up what he
called a "transforming principle" from the heat-killed S bacteria, which
allowed them to "transform" into smooth-coated bacteria and become
virulent.
 Avery, McCarty, and MacLeod: Identifying the transforming principle:
 In 1944, three Canadian and American researchers, Oswald Avery, Maclyn
McCarty, and Colin MacLeod, set out to identify Griffith's "transforming
principle."
 To do so, they began with large cultures of heat-killed S cells and, through
a long series of biochemical steps (determined by careful
experimentation), progressively purified the transforming principle by
washing away, separating out, or enzymatically destroying the other
cellular components. By this method, they were able to obtain small
amounts of highly purified transforming principle, which they could then
analyze through other tests to determine its identity.
 Several lines of evidence suggested to Avery and his colleagues that the
transforming principle might be DNA.
 The purified substance gave a negative result in chemical tests
known to detect proteins, but a strongly positive result in a
chemical test known to detect DNA.
 The elemental composition of the purified transforming principle
closely resembled DNA in its ratio of nitrogen and phosphorous.
 Protein- and RNA-degrading enzymes had little effect on the
transforming principle, but enzymes able to degrade DNA
eliminated the transforming activity.
 These results all pointed to DNA as the likely transforming principle.
However, Avery was cautious in interpreting his results. He realized that it
was still possible that some contaminating substance present in small
amounts, not DNA, was the actual transforming principle.
 Because of this possibility, debate over DNA's role continued until 1952,
when Alfred Hershey and Martha Chase used a different approach to
conclusively identify DNA as the genetic material.
 The Hershey-Chase experiments:
 In their now-legendary experiments, Hershey and Chase studied
bacteriophage, or viruses that attack bacteria. The phages they used were
simple particles composed of protein and DNA, with the outer structures
made of protein and the inner core consisting of DNA.
 Hershey and Chase knew that the phages attached to the surface of a host
bacterial cell and injected some substance (either DNA or protein) into the
host. This substance gave "instructions" that caused the host bacterium to
 start making lots and lots of phages—in other words, it was the phage's
genetic material. Before the experiment, Hershey thought that the genetic
material would prove to be protein.
 To establish whether the phage injected DNA or protein into host bacteria,
Hershey and Chase prepared two different batches of phage. In each
batch, the phage was produced in the presence of a specific radioactive
element, which was incorporated into the macromolecules (DNA and
protein) that made up the phage.
 One sample was produced in the presence of S^{35}, a radioactive
isotope of sulfur. Sulfur is found in many proteins and is absent
from DNA, so only phage proteins were radioactively labeled by
this treatment.
 The other sample was produced in the presence of P^{32}, a
radioactive isotope of phosphorous. Phosphorous is found in DNA
and not in proteins, so only phage DNA (and not phage proteins)
was radioactively labeled by this treatment.

The Hershey-Chase experiments

  Each batch of phage was used to infect a different culture of bacteria. After
infection had taken place, each culture was whirled in a blender, removing
any remaining phage and phage parts from the outside of the bacterial
cells. Finally, the cultures were centrifuged, or spun at high speeds, to
separate the bacteria from the phage debris.
 Centrifugation causes heavier material, such as bacteria, to move to the
bottom of the tube and form a lump called a pellet. Lighter material, such
as the medium (broth) used to grow the cultures, along with phage and
phage parts, remains near the top of the tube and forms a liquid layer
called the supernatant.
 When Hershey and Chase measured radioactivity in the pellet and
supernatant from both of their experiments, they found that a large
amount of P ^{32} appeared in the pellet, whereas almost all of the S^{35}
appeared in the supernatant. Based on this and similar experiments,
Hershey and Chase concluded that DNA, not protein, was injected into
host cells and made up the genetic material of the phage.

o Concept of Genetic Code

 Genetic code, the sequence of nucleotides in deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA) that determines the amino acid sequence of proteins.
 Though the linear sequence of nucleotides in DNA contains the information for
protein sequences, proteins are not made directly from DNA.
 Instead, a messenger RNA (mRNA) molecule is synthesized from the DNA and
directs the formation of the protein.
  RNA is composed of four nucleotides: adenine (A), guanine (G), cytosine (C), and
uracil (U).
 Three adjacent nucleotides constitute a unit known as the codon, which codes
for an amino acid.
 For example, the sequence AUG is a codon that specifies the amino acid
methionine.
 There are 64 possible codons, three of which do not code for amino acids but
indicate the end of a protein.
 The remaining 61 codons specify the 20 amino acids that make up proteins.
 The AUG codon, in addition to coding for methionine, is found at the beginning of
every mRNA and indicates the start of a protein. Because most of the 20 amino
acids are coded for by more than one codon, the code is called degenerate.
 The genetic code, once thought to be identical in all forms of life, has been found
to diverge slightly in certain organisms and in the mitochondria of some
eukaryotes.
 Nevertheless, these differences are rare, and the genetic code is identical in
almost all species, with the same codons specifying the same amino acids.
 The deciphering of the genetic code was accomplished by the American
biochemists Marshall W. Nirenberg, Robert W. Holley, and Har Gobind Khorana in
the early 1960s.

The genetic code: Nucleotide triplets (codons) specifying different amino acids in protein chains
o What is the 'Central Dogma?
 The ‘Central Dogma’ is the process by which the instructions in DNA are
converted into a functional product. It was first proposed in 1958 by Francis
Crick, discoverer of the structure of DNA.
 The central dogma of molecular biology explains the flow of genetic information,
from DNA to RNA, to make a functional product, a protein.
 The central dogma suggests that DNA contains the information needed to make
all of our proteins, and that RNA is a messenger that carries this information to
the ribosomes.
 The ribosomes serve as factories in the cell where the information is ‘translated’
from a code into the functional product.
 The process by which the DNA instructions are converted into the functional
product is called gene expression.
 Gene expression has two key stages - transcription and translation.
 In transcription, the information in the DNA of every cell is converted into small,
portable RNA messages.
 During translation, these messages travel from where the DNA is in the cell
nucleus to the ribosomes where they are ‘read’ to make specific proteins.
 The central dogma states that the pattern of information that occurs most
frequently in our cells is:
 From existing DNA to make new DNA (DNA replication)
 From DNA to make new RNA (transcription)
 From RNA to make new proteins (translation).

Central Dogma
o Some basic terminology
 Allele: one of two or more alternative forms of a gene that arise by mutation and
are found at the same place on a chromosome.
 Ancestry: the lineage through which an individual is descended.
 Aneuploidy: an abnormal deviation in the total chromosome number, typically
due to the addition or loss of a chromosome. In humans, this means any
chromosome number other than 46.
 Autosome: any chromosome that is not a sex chromosome (X or Y). A gene on an
autosome is called “autosomal”.
 Base: along with a sugar and phosphate group, one of the three components
that makeup a nucleotide. Bases are the “informational” part of the nucleotide
that are responsible for DNA’s ability to contain hereditary information.
 Bipedalism: the act of an organism moving using its two rear limbs; for humans,
one of the important traits associated with the ancestor of modern humans.
 Carrier: an individual that has one copy of a disease-causing allele of a gene.
Carriers do not express the disease because two copies of the “bad” allele are
required. However, they may pass the disease-causing allele to offspring.
 Chromosome: a strand of DNA tightly wrapped around proteins called histones.
Chromosomes are the way DNA is organized in a cell.
 Colonoscopy: a medical procedure done to look at the large bowel and part of
the small bowel and typically used as a screening method for cancer detection
 Common ancestor: the evolutionary hypothesis that all living organisms are
descended from a common ancestor; for humans, this is significant with respect
to common ancestry with chimpanzees and other primates.
 Cross-breed: to produce an organism from two genetically dissimilar parents.
 DNA (deoxyribonucleic acid) - a double-stranded molecule made up of four
building blocks called nucleotide bases (different chemicals that are abbreviated
A, T, C, and G) that are arranged in a certain order throughout a genome. The
human genome has 3 billion pairs of bases. The order of these nucleotides is
critical to the accuracy of the instructions of a gene. The most important
molecules encoded by genes are RNA and proteins.
 DNA testing: the characterization of an individual’s genetic makeup using various
methods.
 Dominant: an allele that is expressed in an organism’s phenotype, masking the
effect of the recessive allele when present.
 Down-regulation: decreasing the rate of gene expression.
 Enzyme: a large biological molecule, typically a protein, that catalyzes a chemical
reaction.
 Epigenetics: the study of chemical markers that are added to DNA and affect its
3-D structure related to how tightly it is packaged to form chromosomes. This
mechanism impacts whether or not a gene will be expressed.
 Eugenics: the desire to improve the overall genetic makeup of the human
population through selective breeding and elimination of “undesirable” traits.
Eugenics was a somewhat popular movement in the early 20th century with the
idea that certain races had qualities that were both superior to others and
genetically heritable.
 Gene - the functional and physical unit of heredity passed from parent to
offspring. Typically, a gene is a segment of DNA that occupies a particular
location on a chromosome, and encodes for a protein. If a gene is like a chapter
in a book, the chromosome is the book itself. Proteins and RNA influence how an
organism looks, how well its body metabolizes food and fights infection, and
even how it behaves. The number of human genes is about 20,000-25,000.
Different genes can vary in length and cover thousands of bases.
 Gene expression: conversion of the information encoded in a gene first into
messenger RNA and second into a protein.
 Gene therapy: a technique used to correct defective genes that are responsible
for disease development typically through the introduction of a copy of the
normal gene; a set of strategies that manipulate the expression of particular
genes or corrects abnormal genes.
 Genealogy: in terms of genetics, the study of the genetic lineages of humans and
other species.
 Genetic counselor: a healthcare professional who has received graduate
education and training in medical genetics and counseling.
 Genetic distance: the difference in frequencies of traits between populations;
used to compare genetic similarities between populations of the same species or
similarities between separate species. Genetic distance arises when breeding
stops, so it can be used as a measure of evolutionary relatedness.
 Genetic isolation: a population in which very little interbreeding with other
populations occurs, usually due to geographical or ecological separation; often
leads to speciation or differentiation within one species for certain traits.
 Genetic markers: a specific DNA sequence, that typically differs from normal,
that may increase the likelihood of developing a particular disease and can
therefore be used as an indicator of increased disease risk.
 Genome: the complete set of genes in an organism. The total genetic content in
one set of chromosomes.
 Genome sequencing: determination of the order in which the bases are
arranged within all the DNA of an organism.
 Genotype: the genetic makeup of an individual, often times in reference to a
particular gene.
 Heterozygous: having two different alleles for a given gene; both alleles at
corresponding loci are dissimilar.
 Histone: any of a group of small proteins that DNA wraps around to become
compacted within the nucleus of a cell.
 Hominid: a taxonomic group of primates that includes the early ancestors of
modern humans.
 Homo sapiens: the early primate species that originated in Africa and gave rise
to all modern humans.
 Homozygous: having identical alleles for a given gene; both alleles at
corresponding loci are identical.
 Human migration: the movement of humans from one geographical region to
another; over thousands of years, genetically distinct populations may develop.
 Karyotype: an array of all the chromosomes found in a cell of an individual.
Typically, the chromosomes are stained to reveal size, banding pattern, or other
distinguishing feature to enable the identification of any abnormalities.
 Law of Independent Assortment: each member of an allele pair on homologous
chromosomes separates independently of the members of other pairs on other
chromosomes so that the resulting allele combinations are random.
 Law of Segregation: the members of allele pairs on homologous chromosomes
separate during the formation of gametes and are distributed to different
gametes so that every gamete receives only one allele of the pair.
 Locus (plural=loci): The physical location of a gene on a chromosome.
 Mastectomy: a surgery to remove all breast tissue from a breast as a way to
treat or prevent breast cancer.
 Mendelian genetics: an approach to heredity that focuses on patterns of
inheritance from generation to generation. For example, if two heterozygotes
mate, they have a 25% chance of producing a homozygous dominant offspring, a
50% chance of producing a heterozygous offspring, and 25% chance of producing
a homozygous recessive offspring.
 Molecule: the fundamental unit of a substance composed of atoms bonded
together in a particular structure.
 Mutation: accidental, random changes in a DNA sequence caused by
environmental factors and replication errors.
 Nucleotide: a compound consisting of a sugar and base linked to a phosphate
group. Nucleotides form the basic structural unit of nucleic acids such as DNA.
 Nucleus: a membrane bound structure within a cell that contains the genetic
material (DNA).
 Pangenesis: an incorrect theory of heredity; each cell produces hereditary
particles that circulate in the blood and eventually collect and are incorporated
into reproductive cells to be passed onto offspring.
 Phenotypic/phenotype: the observable or physical characteristics of an
individual, as a result of genetic expression and environment.
 Polyps: an abnormal growth of tissue projecting from a membrane lining.
 Race: the system of classifying humans into particular populations based on
common ancestry, cultural background, language, geographical location, etc.
 Recessive: an allele that produces its characteristic phenotype only when the
paired allele is the same; will be masked if a dominant allele is present.
 Relatedness: in genetics, the degree to which one person is related to another;
or, the degree to which one species is related to another.
 Sex-linked genes: a gene located on a sex chromosome. In humans, the sex
chromosomes are the X chromosome and Y chromosome.
 Single-Nucleotide Polymorphism (SNP): genetic variation in a DNA sequence
that occurs when a single nucleotide in a genome is altered; SNPs are usually
considered to be point mutations that have been evolutionarily successful
enough to recur in a significant proportion of the population of a species.
 X-linked: a gene or DNA segment located on the X chromosome.
 Vector: a bacteriophage, plasmid, or other agent that carries and transfers
genetic material from one cell into another.
 Zygote: the cell formed when two gametes (sperm and egg) are fused via sexual
reproduction; earliest stage in embryonic development.