MOLECULAR BIOLOGY

Molecular Biology is the branch of biology that studies the molecular basis of biological activity.

Living things are made of chemicals just as non-living things are, so a molecular biologist studies how
molecules interact with one another in living organisms to perform the functions of life.

Molecular biologists conduct experiments to investigate the structure, function, processing, regulation
and evolution of biological molecules and their interactionswith one another — providing micro-level
insights into how life works.

Although there are many kinds of molecules in every living thing, most molecular biologists focus on
genes and proteins. Proteins perform a huge diversity of functions within living cells and genes contain
the information required to make more proteins.

The Complex Relationship of Genes & Proteins

Genes are segments of information stored on gigantic nucleic acid molecules and proteins are molecules
in their own right, making both of these substances (and the relationship between them), extraordinarily
important to study.

Molecular biologists work to identify and understand the parts of biological pathways. Proteins can:

• Regulate and impact each other

• Respond to signals from genes

• Respond to signals from outside a cell

The long chain of these interactions is what many molecular biologists seek to fully document. Every
step in a functional pathway is something that a disease can disrupt or a drug can target. Understanding
the role of each such molecule is important for understanding more complex aspects of how organisms
live and work.

Molecular biologists can also seek to understand ways the structure of a molecule, including details such
as the location and shape of active sites on a protein, affect how a molecule functions. Collecting this
information not only provides basic knowledge into how biology works, but helps inform the efforts of
other scientists who seek to manipulate that biology. Those scientists include drug designers and genetic
engineers.

The Differences in Molecular Biology, Biochemistry & Genetics

Molecular biology has much in common with two related sciences: biochemistry and genetics. The three
sciences all concern themselves with details of how organisms work at the molecular level. However,
each focuses on a different area and have different applications.
Biochemistry

Biochemistry often devotes more of its attention to molecules other than proteins. It also focuses on
nucleic acids and to chemical effects that happen when larger quantities of a substance are present,
such as the effects of venoms. Additionally, biochemistry uses many methods based in organic chemistry
research.

Genetics

Genetics is focused specifically on heritable traits and how changes in the genetic code affect an
organism. This focus on heritability means that genetics is often best studied at the population level,
making it a much larger-scale sort of field than molecular biology.

Each of these three fields overlap and influence the others. Genetics, in particular, has shared much with
molecular biology, particularly with regards to the role of RNA. RNA can both store information like DNA
and perform active functions like a protein.

Molecular Biology Laboratory Methods

Molecular biology is perhaps most famous for its distinctive suite of laboratory methods. Molecular
biologists make extensive use of:

• Molecular cloning

• Polymerase Chain reaction

• Gel electrophoresis

• Blotting

These techniques facilitate collecting, isolating and quantifying molecules of interest.

Studying protein and nucleic acid structure benefits from a different set of techniques, such as X-ray
crystallography. Many molecular biologists make extensive use of computer modeling in their work.
Modern techniques, particularly those developed for genetics, are also altering the course of molecular
biology research.

Molecular biology is a large and growing field, whose importance has yet to be fully realized. Many more
advances in medicine, ecology and other areas will come out of molecular biology research as this
science continues to grow and advance.

A TYPICAL CELL
A cell, in biology, is the basic membrane-bound unit that contains the fundamental molecules of life and
of which all living things are composed. A single cell is often a complete organism in itself, such as a
bacterium or yeast. Other cells acquire specialized functions as they mature. These cells cooperate with
other specialized cells and become the building blocks of large multicellular organisms, such as humans
and other animals. Although cells are much larger than atoms, they are still very small. The smallest
known cells are a group of tiny bacteria called mycoplasmas; some of these single-celled organisms are
spheres as small as 0.2 μm in diameter (1μm = about 0.000039 inch), with a total mass of 10−14 gram—
equal to that of 8,000,000,000 hydrogen atoms. Cells of humans typically have a mass 400,000 times
larger than the mass of a single mycoplasma bacterium, but even human cells are only about 20 μm
across. It would require a sheet of about 10,000 human cells to cover the head of a pin, and each human
organism is composed of more than 30,000,000,000,000 cells.

animal cell

As an individual unit, the cell is capable of metabolizing its own nutrients, synthesizing many types of
molecules, providing its own energy, and replicating itself in order to produce succeeding generations. It
can be viewed as an enclosed vessel, within which innumerable chemical reactions take place
simultaneously. These reactions are under very precise control so that they contribute to the life and
procreation of the cell. In a multicellular organism, cells become specialized to perform different
functions through the process of differentiation. In order to do this, each cell keeps in constant
communication with its neighbours. As it receives nutrients from and expels wastes into its
surroundings, it adheres to and cooperates with other cells. Cooperative assemblies of similar cells form
tissues, and a cooperation between tissues in turn forms organs, which carry out the functions necessary
to sustain the life of an organism.

Consider how a single-celled organism contains the necessary structures to eat, grow, and
reproduceCells are the basic units of life.See all videos for this article

Special emphasis is given in this article to animal cells, with some discussion of the energy-synthesizing
processes and extracellular components peculiar to plants.

The nature and function of cells


CELL ORGANELLS

Nucleus

nucleus, in biology, a specialized structure occurring in most cells (except bacteria and blue-green algae)
and separated from the rest of the cell by a double layer, the nuclear membrane. This membrane seems
to be continuous with the endoplasmic reticulum (a membranous network) of the cell and has pores,
which probably permit the entrance of large molecules. The nucleus controls and regulates the activities
of the cell (e.g., growth and metabolism) and carries the genes, structures that contain the hereditary
information. Nucleoli are small bodies often seen within the nucleus. The gel-like matrix in which the
nuclear components are suspended is the nucleoplasm.

Because the nucleus houses an organism’s genetic code, which determines the amino acid sequence of
proteins critical for day-to-day function, it primarily serves as the information centre of the cell.
Information in DNA is transcribed, or copied, into a range of messenger ribonucleic acid (mRNA)
molecules, each of which encodes the information for one protein (in some instances more than one
protein, such as in bacteria). The mRNA molecules are then transported through the nuclear envelope
into the cytoplasm, where they are translated, serving as templates for the synthesis of specific proteins.

A cell normally contains only one nucleus. Under some conditions, however, the nucleus divides but the
cytoplasm does not. This produces a multinucleate cell (syncytium) such as occurs in skeletal muscle
fibres. Some cells—e.g., the human red blood cell—lose their nuclei upon maturation.

The nucleus represents the cell’s headquarters. There is typically one nucleus per cell. However, this is
not always the case. Skeletal muscle cells, for instance, have two.

The nucleus contains the majority of the cell’s DNA, and the mitochondria house a small amount. The
nucleus sends out messages to tell the cell to grow, divide, or die.

A membrane called the nuclear envelope separates the nucleus from the rest of the cell. Nuclear pores
within the membrane allow small molecules and ions to cross back and forth, while larger molecules
need to transport proteins to help them through.

Plasma membrane

To ensure each cell remains separate from its neighbor, a special membrane, known as the plasma
membrane, envelops the cell. Phospholipids make most of this membrane and prevent water-based
substances from entering the cell. The plasma membrane contains a range of receptors, which carry out
a number of tasks, including being:

• Gatekeepers: Some receptors allow certain molecules through and stop others.

• Markers: These receptors act as name badges, informing the immune system that they
are part of the organism and not foreign invaders.

• Communicators: Some receptors help the cell communicate with other cells and the
environment.

• Fasteners: Some receptors help bind the cell to its neighbors.

Cytoplasm

The cytoplasm is the interior of the cell that surrounds the nucleus. It includes the organelles and a jelly-
like fluid called the cytosol. Many of the important reactions that take place in the cell occur in the
cytoplasm.

Cytoskeleton

The cytoskeleton forms the scaffolding within the cytoplasm of the human cell. It helps the cell maintain
the correct shape. However, unlike regular scaffolding, the cytoskeleton is flexible. It plays a role in cell
division and cell motility — the ability of some cells to move, such as sperm cells, for instance.

The cytoskeleton also helps with cell signaling through the uptake of material from the endocytosis, or
the area outside the cell, and moving materials within the cell.

Endoplasmic reticulum

endoplasmic reticulum (ER), in biology, a continuous membrane system that forms a series of flattened
sacs within the cytoplasm of eukaryotic cells and serves multiple functions, being important particularly
in the synthesis, folding, modification, and transport of proteins . All eukaryotic cells contain an
endoplasmic reticulum (ER). In animal cells, the ER usually constitutes more than half of the
membranous content of the cell. Differences in certain physical and functional characteristics distinguish
the two types of ER, known as rough ER and smooth ER.

endoplasmic reticulum

Rough ER is named for its rough appearance, which is due to the ribosomes attached to its outer
(cytoplasmic) surface. Rough ER lies immediately adjacent to the cell nucleus, and its membrane is
continuous with the outer membrane of the nuclear envelope. The ribosomes on rough ER specialize in
the synthesis of proteins that possess a signal sequence that directs them specifically to the ER for
processing. (A number of other proteins in a cell, including those destined for the nucleus and
mitochondria, are targeted for synthesis on free ribosomes, or those not attached to the ER membrane;
see the article ribosome.) Proteins synthesized by the rough ER have specific final destinations. Some
proteins, for example, remain within the ER, whereas others are sent to the Golgi apparatus, which lies
next to the ER. Proteins secreted from the Golgi apparatus are directed to lysosomes or to the cell
membrane; still others are destined for secretion to the cell exterior. Proteins targeted for transport to
the Golgi apparatus are transferred from ribosomes on rough ER into the rough ER lumen, which serves
as the site of protein folding, modification, and assembly.

If the adaptive response fails, cells are directed to undergo apoptosis (programmed cell death).

SMOOTH ENDOPLASMIC RETICULUM ( SER)

SER by contrast, is not associated with ribosomes, and its functions differ. The smooth ER is involved in
the synthesis of lipids, including cholesterol and phospholipids, which are used in the production of new
cellular membrane. In certain cell types, smooth ER plays an important role in the synthesis of steroid
hormones from cholesterol. In cells of the liver, it contributes to the detoxification of drugs and harmful
chemicals. The sarcoplasmic reticulum is a specialized type of smooth ER that regulates the calcium ion
concentration in the cytoplasm of striated muscle cells.

The highly convoluted and labyrinthine structure of the ER led to its description in 1945 as a “lace-like
reticulum” by cell biologists Keith Porter, Albert Claude, and Ernest Fullman, who produced the first
electron micrograph of a cell. In the late 1940s and early 1950s, Porter and colleagues Helen P.
Thompson and Frances Kallman introduced the term endoplasmic reticulum to describe the organelle.
Porter later worked with Romanian-born American cell biologist George E. Palade to elucidate key
characteristics of the ER

Golgi apparatus

Golgi apparatus, membrane-bound organelle of eukaryotic cells (cells with clearly defined nuclei) that is
made up of a series of flattened, stacked pouches called cisternae. The Golgi apparatus is responsible for
transporting, modifying, and packaging proteins and lipids into vesicles for delivery to targeted
destinations. It is located in the cytoplasm next to the endoplasmic reticulum and near the cell nucleus.
While many types of cells contain only one or several Golgi apparatus, plant cells can contain hundreds.

Golgi apparatus
Secretory proteins and glycoproteins, cell membrane proteins, lysosomal proteins, and some glycolipids
all pass through the Golgi apparatus at some point in their maturation. In plant cells, much of the cell
wall material passes through the Golgi as well.

Mitochondria

mitochondrion is a membrane-bound organelle found in the cytoplasm of almost all eukaryotic cells
(cells with clearly defined nuclei), the primary function of which is to generate large quantities of energy
in the form of adenosine triphosphate (ATP). Mitochondria are typically round to oval in shape and
range in size from 0.5 to 10 μm. In addition to producing energy, mitochondria store calcium for cell
signaling activities, generate heat, and mediate cell growth and death.

The number of mitochondria per cell varies widely—for example, in humans, erythrocytes (red blood
cells) do not contain any mitochondria, whereas liver cells and muscle cells may contain hundreds or
even thousands. The only eukaryotic organism known to lack mitochondria is the oxymonad
Monocercomonoides species. Mitochondria are unlike other cellular organelles in that they have two
distinct membranes and a unique genome and reproduce by binary fission; these features indicate that
mitochondria share an evolutionary past with prokaryotes (single-celled organisms).

Most of the proteins and other molecules that make up mitochondria originate in the cell nucleus.
However, 37 genes are contained in the human mitochondrial genome, 13 of which produce various
components of the electron transport chain (ETC). In many organisms, the mitochondrial genome is
inherited maternally. This is because the mother’s egg cell donates the majority of cytoplasm to the
embryo, and mitochondria inherited from the father’s sperm are usually destroyed. mtDNA) is highly
susceptible to mutations, largely because it does not possess the robust DNA repair mechanisms
common to nuclear DNA. In addition, the mitochondrion is a major site for the production of reactive
oxygen species (ROS; or free radicals) due to the high propensity for aberrant release of free electrons.
While several different antioxidant proteins within the mitochondria scavenge and neutralize these
molecules, some ROS may inflict damage to mtDNA. In addition, certain chemicals and infectious agents,
as well as alcohol abuse, can damage mtDNA. In the latter instance, excessive ethanol intake saturates
detoxification enzymes, causing highly reactive electrons to leak from the inner membrane into the
cytoplasm or into the mitochondrial matrix, where they combine with other molecules to form
numerous radicals.

Some diseases and disorders associated with mitochondrial dysfunction are caused by mutations in
mtDNA. Disorders resulting from mutations in mtDNA demonstrate an alternative form of non-
Mendelian inheritance, known as maternal inheritance, in which the mutation and disorder are passed
from mothers to all of their children. The mutations generally affect the function of the mitochondrion,
compromising, among other processes, the production of cellular ATP. Severity can vary widely for
disorders resulting from mutations in mtDNA, a phenomenon generally thought to reflect the combined
effects of heteroplasmy (i.e., mixed populations of both normal and mutant mitochondrial DNA in a
single cell) and other confounding genetic or environmental factors. Although mtDNA mutations play a
role in some mitochondrial diseases, the majority of the conditions actually are the result of mutations
in genes in the nuclear genome, which encodes a number of proteins that are exported and transported
to mitochondria in the cell.

There are numerous inherited and acquired mitochondrial diseases, many of which can emerge at any
age and are enormously diverse in their clinical and molecular features. They range in severity from
relatively mild disease that affects just a single organ to debilitating and sometimes fatal illness that
affects multiple organs. Both inherited and acquired mitochondrial dysfunction is implicated in several
diseases, including Alzheimer disease and Parkinson disease. The accumulation of mtDNA mutations
throughout an organism’s life span are suspected to play an important role in aging, as well as in the
development of certain cancers and other diseases. Because mitochondria also are a central component
of apoptosis (programmed cell death), which is routinely used to rid the body of cells that are no longer
useful or functioning properly, mitochondrial dysfunction that inhibits cell death can contribute to the
development of cancer.

Ribosomes

The nucleus transcribes segments of DNA into ribonucleic acid (RNA), a molecule similar to DNA, which
directs the translation of RNA into proteins. Ribosomes read the RNA and translate it into proteins by
sticking together amino acids in the order the RNA defines.

Some ribosomes float freely in the cytoplasm while others attach to the ER.

lysosome

subcellular organelle that is found in nearly all types of eukaryotic cells (cells with a clearly defined
nucleus) and that is responsible for the digestion of macromolecules, old cell parts, and microorganisms.
Each lysosome is surrounded by a membrane that maintains an acidic environment within the interior
via a proton pump. Lysosomes contain a wide variety of hydrolytic enzymes (acid hydrolases) that break
down macromolecules such as nucleic acids, proteins, and polysaccharides. These enzymes are active
only in the lysosome’s acidic interior; their acid-dependent activity protects the cell from self-
degradation in case of lysosomal leakage or rupture, since the pH of the cell is neutral to slightly
alkaline. Lysosomes were discovered by the Belgian cytologist Christian René de Duve in the 1950s. (De
Duve was awarded a share of the 1974 Nobel Prize for Physiology or Medicine for his discovery of
lysosomes and other organelles known as peroxisomes.)
Cell types

Different types of cells are present in the human body. Below is a small selection of human cell types:

Stem cells

Stem cells are cells that must choose what they are going to become. Some differentiate to become a
certain cell type, and others divide to produce more stem cells. The embryo and some adult tissues,
such as bone marrow, house them.

Bone cells

There are at least three

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main types of bone cells:

• osteoclasts, which dissolve bone

• osteoblasts, which form new bone

• osteocytes, which help communicate with other bone cells

Blood cells

There are many types of blood cells, including:

• red blood cells, which carry oxygen around the body

• white blood cells, which are part of the immune system

• platelets, which help blood clot to prevent blood loss after injury

• neutrophils and basophils, and other types of white blood cells

Muscle cells

Also called myocytes, muscle cells are long, tubular cells. Muscle cells are important for a range of
functions, including movement, support, and internal functions, such as peristalsis — the movement of
food along the gut.

Sperm cells

These tadpole-shaped cells are the smallest in the human body.

Sperm cells cannot divide. They only carry one haploid, unlike the majority of cells, which carry diploids.

Female egg cell

Compared with the sperm cell, the female egg cell is a giant. It is the largest human cell. The egg cell is
also haploid so that the chromosomes from the sperm and egg can combine to create a diploid cell
during the process of fertilization.

Fat cells

Fat cells are also called adipocytes, the main adipose tissue constituents. They contain stored fats called
triglycerides that the body can use as energy. Once the body uses the triglycerides, the fat cells shrink.
Adipocytes also produce some hormones.

Nerve cells

Nerves cells form the communication system of the body. Also called neurons, they consist of two major
parts — the cell body and nerve processes, known as axons and dendrites.

The central body contains the nucleus and other organelles, and the nerve processes run like long
fingers, carrying messages far and wide. Some of the axons are around 1 meter

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long.