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GE0102-BIOLOGY FOR ENGINEERS

UNIT I

Introduction

Cell structure and function

Genetic information, protein synthesis, and protein structure

Cell metabolism

Homoeostasis

Cell growth, reproduction, and differentiation

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Introduction

Science is an organized system for the systematic study of particular aspects of the natural world. The

scope of science is limited to those things that can be apprehended by the senses (sight, touch, hearing, etc.).

Generally, science stresses an objective approach to the phenomena that are studied. Questions about natureaddressed by scientists tend to emphasize how things occur rather than why they occur. It involves the

application of the scientific method to problems formulated by trained minds in particular disciplines.

In a more formal sense, the scientific method refers to the model for research developed by Francis

Bacon (15611626). This model involves the following sequence:

1. Identifying the problem

2. Collecting data within the problem area (by observations, measurements, etc.)

3. Sifting the data for correlations, meaningful connections, and regularities

4. Formulating a hypothesis (a generalization), which is an educated guess that explains the existing data and

suggests further avenues of investigation

5. Testing the hypothesis rigorously by gathering new data6. Confirming, modifying, or rejecting the hypothesis in light of the new findings

A living organism is primarily physicochemical material that demonstrates a high degree of

complexity, is capable of self-regulation, possesses a metabolism, and perpetuates itself through time. To many

biologists, life is an arbitrary stage in the growing complexity of matter, with no sharp dividing line between

the living and nonliving worlds.

Living substance is composed of a highly structured array of macromolecules, such as proteins, lipids,

nucleic acids, and polysaccharides, as well as smaller organic and inorganic molecules. A living organism has

built-in regulatory mechanisms and interacts with the environment to sustain its structural and functional

integrity. All reactions occurring within an individual living unit are called its metabolism. Specific moleculescontaining information in their structure are utilized both in the regulation of internal reactions and in the

production of new living units.

Living organisms generally demonstrate:

1. Movement: the motions within the organisms or movement of the organisms from one place to another

(locomotion)

2. Irritability: the capacity of organisms to respond in a characteristic manner to changesknown as

stimuli in the internal and external environments

3. Growth: the ability of organisms to increase their mass of living material by assimilating new materials

from the environment

4. Adaptation: the tendency of organisms to undergo or institute changes in their structure, function, or

behavior that improve their capacity to survive in a particular environment

5. Reproduction: the ability of organisms to produce new individuals like themselves

Organization of Life

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The study of evolution is particularly useful for classifying organisms into groups because it

reveals how

Organisms are chronologically and morphologically (by form and structure) related to each other.

The classification of organisms is known as taxonomy. Taxonomists utilize evolutionary

relationships in creating their groupings. Although classification schemes are, of necessity,

somewhat arbitrary, they probably do reflect the family tree of todays diverse living forms.

All organisms belong to one of five major kingdoms. A kingdom is the broadest taxonomic

category.

The five kingdoms are Monera, Protista, Fungi, Plantae, and Animalia. The Monera consists of

unicellular organisms that lack a nucleus and many of the specialized cell parts, called organelles.

Such organisms are said to be prokaryotic (pro before; karyotic kernel, nucleus) and

consist of bacteria. All of the other kingdoms consist of eukaryotic (eu true) organisms, which

have cells that contain a nucleus and a fuller repertory of organelles. Unicellular eukaryotes are

placed in kingdom Protista, which includes the protozoans. Multicellular organisms that

manufacture their own food are grouped into kingdom Plantae; flowers, mosses, and trees are

examples. Uni- and multicellular plantlike organisms that absorb food from their environment areplaced in kingdom Fungi, which includes the yeasts and molds. Multicellular organisms that must

capture their food and digest it internally are grouped into kingdom Animalia; Eg. snakes and

humans.

Cell structure

Robert Hooke was the first scientist to describe cellular structure. He studied thin sections of cork

(dead plant cell walls) and noted its boxlike structure in 1665. The honeycomb arrangement of these box

units reminded him of the tiny rooms of a monastery, which are called cellulae in Latin.

In 1673, Anton van Leeuwenhoek refined the grinding process to produce lenses that could be

used effectively in simple microscopes and was the first to view organism (living things).

In 1809, Lamarck recognized that all living things show cellular structure. In 1824, Dutrochet

stated unequivocally that all living tissues are made up of tiny globular cells. Further, he realized that

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growth involved both an increase in the size of existing cells and an increase in the number of cells. In

1831, Robert Brown described the nucleus, which is a feature of almost all eukaryotic cells.

In 1838, Matthias Schleiden published his studies of the cellular structure of plants and

concluded that all plants were made of cells.

In 1839, Theodore Schwann concluded that all animals were made of cells. When Rudolph

Virchow asserted in 1858 that all cells come from preexisting cells, the cell assumed the role of acontinuous living chain in time by which life was to be understood.

CELL THEORY

The cell is the unit of life .All living things are made up of cells. All are arise

from preexisting cells.

In the kingdoms Monera and Protista, the entire organism consists of a single

cell.

In most fungi and in the animal and plant kingdoms, the organism is a highly

complex arrangement of up to trillions of cells.

The human brain alone contains billions of cells. So vital are cells and their

activities to an understanding of life that the cell doctrine has become a central organizing

principle in the field of biology.

Cells tend to be very small, rarely exceeding microscopic dimension for cells with high

metabolic rates.

The property of membranes that permits movement across their surface is called

permeability.

The internal environment of the cell is carefully maintained by the selective permeability

of the cell membrane. Many materials pass across the membrane in accordance with their

concentration gradients.

A variety of nonlipid materials, such as Na+ and K+, probably pass across the membranethrough special channels or pores.

These channels may be transient or relatively permanent and are thought to facilitate the

passage of particular molecules or ions on the basis of their diameter, charge, or ability to

form weak bonds between the migrant species and some constituent of the channel.

Channels that tend to be a permanent part of the membrane may exist in an open or

shut condition depending on the state of a protein gate; these gates provide a means

for altering permeability with a change in environmental conditions.

The enzyme ATPase has been suggested as an enzymic carrier in the movement of Na+

and K+ across the membrane.

Prokaryotes

In the nineteenth century, the cell was described merely as having a limiting outer membrane, an

interior nucleus, and a large mass of cytoplasm surrounding the nucleus. Early microscopes used thinly

sliced specimens through which light could be shown to illuminate cellular features. Improved staining

methods enabled researchers to heighten the visibility of cellular structures selectively.

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Many cellular organelles appear to be derived from membranes, which are thin sheets of living

material within a surrounding amorphous medium. Some organelles do not have a membrane structure.

These include ribosomes, microtubules and microfilaments, flagella, cilia, and centrioles. The

prokaryotes have only a limited repertoire of organelles in their cytoplasm, and these are generally

nonmembranous, such as the ribosomes. They lack cilia, centrioles, microfilaments, and microtubules.

Eukaryotes are rich in the numbers and kinds of organelles that are present, and they include bothmembranous and non membranous types. In both prokaryotic and eukaryotic cells, a cell membrane

(plasma membrane) is always present.

The cell membrane is the outer layer of the living cell. It controls the passage of materials into

and out of the cell. An older view of the cell membrane, the unit membrane hypothesis, describes the

membrane as an inner and outer dense protein layer surrounding a thicker but less dense phospholipid

layer. This sandwich structure was indicated by electron microscope studies of many membranes.

Channels were also seen to run through the membrane to the exterior.

Prokaryotes

Eukaryotes

Eukaryotic cells occur in all animals and plants, but there are a number of significant differences

between the cells of organisms in these two kingdoms. Plant cells almost always contain an extracellular

cell wall, which is made up of cellulose. Animal cells do not generally possess a cell wall. Cell walls are

also found in fungi and bacteria, but they are not composed of cellulose in these organisms. Plastids are a

feature of most plant cells but are not found in the cells of animals. Vacuoles are quite prominent in plant

cells, but are far less significant in or absent from animal cells. While animal cells invariably demonstrate

a pair of centrioles lying just outside the nucleus, centrioles are not usually found in plants.

Eukaryotes

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CELL STRUCTURE AND FUNCTION

ORGANELLES

PLASMA MEMBRANE:

The cell membrane is the outer layer of the living cell. It controls the passage of materials into

and out of the cell. An older view of the cell membrane, the unit membrane hypothesis, describes the

membrane as an inner and outer dense protein layer surrounding a thicker but less dense phospholipid

layer. This sandwich structure was indicated by electron microscope studies of many membranes.

Channels were also seen to run through the membrane to the exterior.

More recently, S. J. Singer and G. L. Nicholson have introduced the fluid mosaic model. Like the

earlier model, it proposes a double layer of phospholipids, with their polar ends facing the inner and outer

surfaces and the hydrophobic, non-polar ends opposed at the center of the bilayer. However, the fluid

mosaic model better explains the dynamic nature of the membrane proteins. According to this model,

these proteins may reside on the exterior or interior face of the lipid bilayer (extrinsic proteins) or may be

located in the phospholipid matrix (intrinsic proteins); some may be embedded in the bilayer but project

through to the exterior, the interior, or both surfaces of the membrane. The primary and tertiary structures

of the proteins are compatible with their locations on or in the membrane. Intrinsic proteins tend to have

predominantly hydrophobic amino acids, and they assume conformations that segregate any hydrophilic

amino acids from the hydrophobic bilayer; extrinsic proteins, conversely, tend to have hydrophilic

residues, which can bond with the polar end of the phospholipids and interact with the surrounding

aqueous solution. According to this model, some lateral circulation of phospholipid and protein ispossible.

FLUID MOSAIC MODEL

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NUCLEUS

The nucleus is a round or oval body lying in the center of the cell. It is surrounded by a double

membrane, the nuclear membrane or envelope.

These membranes coalesce in certain portions of the nuclear envelope, and in these regions, pores

(openings) may be formed that provide a route for materials to leave the nucleus directly.

Since the outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum,

the pores may actually permit passage from the interior of the nucleus to the channels of the

endoplasmic reticulum.

Within the nucleus, one or more nucleoli may be seen. These are dense bodies containing the

subunits for the ribosomes, the cytoplasmic organelles involved in the synthesis of protein. The

nucleolus is involved in the assembly and synthesis of ribosomes.

It is usually attached to a specific chromosome in the nucleus. Each chromosome exists as a tiny

individual rod or string throughout the life of the cell, but in the resting (nondividing) cell, the

chromosomes look like a single network of thin threads. The gene material of the cell is found in

the chromosomes.

NUCLEUS

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NUCLEOLUS

Within the nucleus, one or more nucleoli may be seen. These are dense bodies containing the

subunits for the ribosomes, the cytoplasmic organelles involved in the synthesis of protein.

The nucleolus is involved in the assembly and synthesis of ribosomes. It is usually attached to a

specific chromosome in the nucleus.

Each chromosome exists as a tiny individual rod or string throughout the life of the cell, but in the

resting (nondividing) cell, the chromosomes look like a single network of thin threads. The gene

material of the cell is found in the chromosomes.

NUCLEOLUS

CYTOSKELETON

The cytoskeleton (also CSK) is cellular "scaffolding" or "skeleton" contained within the

cytoplasm and is made out of protein.

The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent

research has identified the prokaryotic cytoskeleton.

It has structures such as flagella, cilia and lamellipodia and plays important roles in both

intracellular transport (the movement of vesicles and organelles, for example) and cellular

division.

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The concept of a protein mosaic that dynamically coordinated cytoplasmic biochemistry was

proposed by Rudolph Peters in 1929 ,

The term (cytosquelette, in French) was first introduced by French embryologist Paul Wintrebert

in 1931.

CENTRIOLE

Centrioles are present as a pair of cylindrical rods in many eukaryotic cells.

They lie just above the nuclear envelope (membrane), and since their longitudinal axes are

perpendicular to one another, theyform a cross.

The microtubular structure of the centriole is the same as that of the basal body and may have

arisen from primitive basal bodies during cellular evolution.

Centrioles probably play a role in the formation of the spindle apparatus, which is an essential

feature of both mitosis and meiosis.

CENTRIOLES

MITOCHONDRIA

Mitochondria are rounded or cigar-shaped organelles that are particularly prominent in cells withhigh metabolic activity. Their name derives from their threadlike appearance (Greek mitos,

thread) under the light microscope.

Mitochondria have a double wall: an outer smooth membrane which forms the outer boundary

and an inner membrane which is extensively folded. The folds, or cristae, project into the interior

of the organelle and have a variety of enzymes embedded in them.

These enzymes are involved in the systematic degradation of organic molecules to yield energy

for the cell. Like the chloroplasts of plants, the mitochondria contain their own DNA and

ribosomes; they replicate independently of the rest of the cell and appear to control the synthesis

of their membranes.

MITOCHONDRIA

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ENDOPLASMIC RETICULUM

The endoplasmic reticulum (ER) is a series of membranous channels that traverse the cytoplasm of most

eukaryotic cells. It forms a continuous network extending from the cell membrane to the nuclear

membrane. In some regions of the cell, it may appear as a series of flattened disks or sacs.

The endoplasmic reticulum serves many general functions, including the facilitation of protein folding

and the transport of synthesized proteins in sacs called cisternae. Correct folding of newly-made proteins

is made possible by several endoplasmic reticulum chaperone proteins, including protein disulfide

isomerase (PDI), ERp29, the Hsp70 family member Grp78, calnexin, calreticulin, and the peptidylpropyl

isomerase family. Only properly-folded proteins are transported from the rough ER to the Golgi complex.

In many parts of the cell, the endoplasmic reticulum is associated with small dense granules lying along

the outer border of its membrane. These structures are known as ribosomes. They impart a rough

appearance to the endoplasmic reticulum, so that the ER is called the rough endoplasmic reticulum (RER)

in these regions, which are usually associated with active protein synthesis.

The prime rough endoplasmic reticulum function is the production and processing of specific

proteins at ribosomal sites that are later exported. The ribosomes in the rough endoplasmic

reticulum do their job and create proteins which are then sent in to the rough endoplasmic

reticulum for advanced processing.

Rough endoplasmic reticulum function involves creation of two types of proteins. One is the type

which fortifies and gets embedded into the reticulum membrane. The other types are water

soluble membranes which after creation at ribosomal sites, pass through the membrane and into

the lumen.

The proteins that enter are further folded inside made possible by chaperone proteins present in

the lumen.

The next rough endoplasmic reticulum function is to transport these ready proteins to the sites

where they are required. They may also be sent to the Golgi bodies for further advanced

processing, through vesicles.

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The smooth endoplasmic reticulum (SER) does not contain ribosomes and is associated with cellular

regions which are involved in the synthesis and transport of lipids or the detoxification of a variety of

poisons.Smooth endoplasmic reticulum is found in a variety of cell types and it serves different functions

in each. It consists of tubules and vesicles that branch forming a network.

In some cells there are dilated areas like the sacs of rough endoplasmic reticulum. The network ofsmooth endoplasmic reticulum allows increased surface area for the action or storage of key enzymes and

the products of these enzymes. In the case of smooth endoplasmic reticulum in muscle cells, the vesicles

and tubules serve as a store of calcium which is released as one step in the contraction process. Calcium

pumps serve to move the calcium.

In adrenal cortical cells (as well as steroid producing cells in the gonads), the smooth endoplasmic

reticulum serves to metabolize the steroids and produced the final steroid hormone. After the side chain of

cholesterol is cleaved in the mitochondria, the product is passed to the smooth endoplasmic reticulum and

further modified. Then, it is passed back to mitochondria for final modifications.

ENDOPLASMIC RETICULUM

RIBOSOMES

Ribosomes are the components of cells that make proteins from all amino acids. One of the

central tenets of biology, often referred to as the "central dogma," is that DNA is used to make RNA,

which, in turn, is used to make protein. The DNA sequence in genes is copied into a messenger RNA

(mRNA). Ribosomes then read the information in this RNA and use it to create proteins. This process is

known as translation; i.e., the ribosome "translates" the genetic information from RNA into proteins.

Ribosomes do this by binding to an mRNA and using it as a template for the correct sequence of amino

acids in a particular protein. The amino acids are attached to transfer RNA (tRNA) molecules, which

enter one part of the ribosome and bind to the messenger RNA sequence. The attached amino acids are

then joined together by another part of the ribosome. The ribosome moves along the mRNA, "reading" its

sequence and producing a chain of amino acids.

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Ribosomes are made from complexes of RNAs and proteins. Ribosomes are divided into two

subunits, one larger than the other. The smaller subunit binds to the mRNA, while the larger subunit binds

to the tRNA and the amino acids. When a ribosome finishes reading a mRNA, these two subunits split

apart. Ribosomes have been classified as ribozymes, since the ribosomal RNA seems to be most

important for the peptidyl transferase activity that links amino acids together.

Ribosomes from bacteria, archaea and eukaryotes (the three domains of life on Earth), has

significantly different structures and RNA sequences. These differences in structure allow some

antibiotics to kill bacteria by inhibiting their ribosomes, while leaving human ribosomes unaffected. The

ribosomes in the mitochondria of eukaryotic cells resemble those in bacteria, reflecting the likely

evolutionary origin of this organelle. The word ribosome comes from ribonucleic acid and the Greek:

soma (meaning body).

Ribosomes are the workhorses of protein biosynthesis, the process of translating mRNA into

protein. The mRNA comprises a series of codons that dictate to the ribosome the sequence of the amino

acids needed to make the protein. Using the mRNA as a template, the ribosome traverses each codon (3

nucleotides) of the mRNA, pairing it with the appropriate amino acid provided by a tRNA. Molecules of

transfer RNA (tRNA) contain a complementary anticodon on one end and the appropriate amino acid on

the other. The small ribosomal subunit, typically bound to a tRNA containing the amino acid methionine,

binds to an AUG codon on the mRNA and recruits the large ribosomal subunit. The ribosome then

contains three RNA binding sites, designated A, P and E. The A site binds an aminoacyl-tRNA (a tRNA

bound to an amino acid); the P site binds a peptidyl-tRNA (a tRNA bound to the peptide being

synthesized); and the E site binds a free tRNA before it exits the ribosome. Protein synthesis begins at a

start codon AUG near the 5' end of the mRNA. mRNA binds to the P site of the ribosome first. The

ribosome is able to identify the start codon by use of the Shine-Dalgarno sequence of the mRNA in

prokaryotes and Kozak box in eukaryotes.

GOLGI BODIES

They exist as stacks of flattened sacs, or vesicles that are continuous with the channels of the

SER. Their major function is the storage, modification, and packing of materials produced for secretory

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export, since these organelles are particularly prominent in secretory cells such as those of the pancreas.

The outer portion of the Golgi apparatus releases its secretory material within membrane-enclosed

globules (secretory vesicles) that migrate to the surface of the cell. It may also provide material for the

cell membrane. The Golgi apparatus may actually be part of a dynamic system of membranous channels

within the cell in which all elements such as the nuclear envelope, the ER, the Golgi apparatus, and the

cell membrane are connected to each other without sharp boundaries.

GOLGI BODIES

LYSOSOMES

Lysosomes are similar in shape to mitochondria but are smaller and consist of a single boundary

membrane. They contain powerful enzymes that would digest the cellular contents if they were not

contained within the impermeable lysosomal membrane. Rupture of this membrane releases these

enzymes. The lysosome plays a role in intracellular digestion and may also be important in the destruction

of certain structures during the process of development. In the metamorphosis of a frog, lysosomal

enzymes help destroy those structures of the tadpole that are no longer useful in later developmental

stages. The raw materials arising from degradation of such regions as the tail are then used in the

formation of more mature parts. Lysosomes are also involved in such autoimmune diseases as rheumatoid

arthritis.They are frequently nicknamed "suicide-bags" or "suicide-sacs" by cell biologists due to their

role in autolysis. Lysosomes were discovered by the Belgian cytologist Christian de Duve in the 1950s.

There are a number of lysosomal storage diseases that are caused by the malfunction of the lysosomes or

one of their digestive proteins; examples include Tay-Sachs disease and Pompe's disease. These diseases

are caused by a defective or missing digestive protein, which leads to the accumulation of substrates

within the cell, impairing metabolism.

LYSOSOSME

VACUOLES

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Vacuoles are discrete, clear regions within the cell that contain water and dissolved materials. The

vacuole may act as a reservoir for fluids and salts that might otherwise interfere with metabolic processes

occurring in the cytoplasm. The membrane surrounding the vacuole is called a tonoplast. Many

protozoans have a contractile vacuole, which periodically contracts and forces fluid and salts out of the

cell. The structure serves to prevent an accumulation of fluids in organisms that live in fresh water.Vacuoles containing digestive enzymes may also be formed around ingested food particles in a variety of

cells. In the cells of many plants, a large central vacuole is a prominent feature; this vacuole may swell,

press against the rigid cell wall, and give the cell a high degree of rigidity, or turgor.

VACUOLES

CHLOROPLASTS

Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct

photosynthesis. Chloroplasts capture light energy to conserve free energy in the form ofATPand reduce

NADP toNADPH through a complex set of processes called photosynthesis.

Chloroplasts are observable as flat discs usually 2 to 10 micrometers in diameter and 1micrometer thick. In land plants, they are, in general, 5 m in diameter and 2.3 m thick. The chloroplast

is contained by an envelope that consists of an inner and an outer phospholipid membrane. Between these

two layers is the intermembrane space. A typicalparenchyma cell contains about 10 to 100 chloroplasts.

The material within the chloroplast is called the stroma, corresponding to the cytosol of the

original bacterium, and contains one or more molecules of small circular DNA. It also contains

ribosomes; however most of its proteins are encoded by genes contained in the host cell nucleus, with the

protein products transported to the chloroplast.

Within the stroma are stacks of thylakoids, the sub-organelles, which are the site of

photosynthesis. The thylakoids are arranged in stacks called grana (singular: granum). A thylakoid has a

flattened disk shape. Inside it is an empty area called the thylakoid space or lumen. Photosynthesis takes

place on the thylakoid membrane; as in mitochondrial oxidative phosphorylation, it involves the coupling

of cross-membrane fluxes with biosynthesis via the dissipation of a proton electrochemical gradient.

In the electron microscope, thylakoid membranes appear as alternating light-and-dark bands, each

0.01 m thick. Embedded in the thylakoid membrane are antenna complexes, each of which consists of

the light-absorbing pigments, including chlorophyll and carotenoids, as well as proteins that bind the

pigments. This complex both increases the surface area for light capture, and allows capture of photons

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with a wider range of wavelengths. The energy of the incident photons is absorbed by the pigments and

funneled to the reaction centre of this complex through resonance energy transfer. Two chlorophyll

molecules are then ionised, producing an excited electron, which then passes onto the photochemical

reaction centre. Chloroplasts can be interconnected by tubular bridges called stromules, formed as

extensions of their outer membranes.Chloroplasts appear to be able to exchange proteins via stromulesand thus function as a network.

CHLOROPLAST

Genetic information, protein synthesis and protein structure

Proteins

Proteins are an important class of biological polymers. Proteins are used to build cells, act as

hormones & enzymes, and do much of the work in the cell and are largely responsible for expressing the

information present in the DNA molecule.They range in size from a few kilodaltons to hundreds of

kilodaltons.Proteins are polymers (macromolecules) made of monomers called amino acids. All proteins

are built up from 20 amino acids, which constitute the monomers.

Proteins are made up of: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N) and some proteins contain

Sulfur (S)

A protein is a natural polymer, made up of amino acid monomers joined together by peptide bonds

(peptide or amide linkages).

* A dipeptide is made up of 2 amino acids joined together by a peptide bond (peptide or amide linkage)

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* A tripeptide is made up of 3 amino acids joined together by peptide bonds (peptide or amide

linkages)

* A tetrapeptide is made up of 4 amino acids joined together by peptide bonds (peptide or amide

linkages)

* A polypeptide is made up of many amino acids joined together by peptide bonds (peptide or amide

linkages)

A peptide bond (peptide or amide linkage) is a covalent bond formed between the carbon of the carboxyl

group of one amino acid and the nitrogen of the amine group of another amino acid as shown below:

H

|

- C - N -

||

O

Water is eliminated when the amino acids react to form a protein. This is known as a condensation

reaction, or a condensation polymerisation reaction.

There are four levels of protein structure: The shape of the protein is very important because it determines

the proteins function.

* Primary Structure: specific sequence of amino acids in the polypeptide chain linked by peptide

linkages (-CO-NH). The sequence in which the amino acids are arranged differs from protein to protein

and is referred to as the primary structure of the protein.

* Secondary Structure: the shape of the protein molecule caused by hydrogen-bonding between -C=Oand -N-H groups within the chain.The two main shapes are helix and sheet.Secondary protein

structures occur when protein chains coil or fold

* Tertiary Structure: When protein chains called polypeptides join together, the tertiary structure forms

because R groups interact with each other that causes folding and bending.

* Quarternary Structure: interactions between protein subunits that result in the protein being classified

as fibrous, globular or conjugated. In the watery environment of a cell, proteins become globular in their

quaternary structure.

Thermodynamic experiments have shown that the functional three-dimensional structure of the

protein is encoded in its primary amino acid sequence. Thus a polypeptide chain in solution will

automatically fold into its three dimensional structure given only that the conditions of temperature, ionic

strength, pH, etc., are within a fairly wide range. Ultimately the structure and therefore the function of

proteins are directly derived from the chemical nature of the amino acids.

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PROTEIN FUNCTIONS:

Biochemical importance of proteins is stated below.

Proteins are the basic constituents of cell cytoplasm.

Fundamental constituents of structure & functional organization of the cell.

Enzymes & hormones are proteins.

Proteins play a major part in the transport of oxygen & CO2by haemoglobin.

Proteins like thrombin & fibrinogen participates in blood clotting as clotting factors.

Antibodies are protein in nature which acts as defense against infections.

Some proteins like actin & myosin carry out the mechanical work in the muscle.

Rhodopsin of retina carries out the function of sensing the light.

The plasma proteins functions in maintaining the homeostatic control of the volume of circulating

blood & interstitial fluids.

Protein synthesis

Protein synthesis is the transcription and translation of specific parts of DNA to form proteins.

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Central dogma

Transcription

Transcription starts with an enzyme called RNA polymerase copying the DNA sequence to a similar

molecule called messenger RNA (mRNA). It replaces T with U (Uracil), a helper base, making it clear

that the mRNA is a copy. The bases (A, T, G, C) on one strand of the DNA specify the order of bases on

the new strand of mRNA (A, U, G, C). The DNA stays inside the nucleus, but the mRNA travels out into

the cytoplasm.

Translation

Translation is the part of protein synthesis where the ribosomes in the cytoplasm use transfer RNA

(tRNA) to attach to the mRNA and translate the bases into amino acids. tRNA molecules bring the

specified amino acids that the ribosome links together to make a protein.

Various steps involved in protein synthesis are given below.

DNA unwinds

mRNA copy is made of one of the DNA strands.

mRNA copy moves out of nucleus into cytoplasm.

tRNA molecules are activated as their complementary amino acids are attached to them.

mRNA copy attaches to the small subunit of the ribosomes in cytoplasm. 6 of the bases in the

mRNA are exposed in the ribosome.

A tRNA bonds complementarily with the mRNA via its anticodon.

A second tRNA bonds with the next three bases of the mRNA, the amino acid joins onto the

amino acid of the first tRNA via a peptide bond.

The ribosome moves along. The first tRNA leaves the ribosome.

A third tRNA brings a third amino acid.

Eventually a stop codon is reached on the mRNA. The newly synthesised polypeptide leaves the

ribosome.

Cell metabolism

ATP: ENERGY FOR CELLS

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Metabolism, synthesis, and active transport

ATP is consumed in the cell by energy-requiring (endothermic) processes and can be generated

by energy-releasing (exothermic) processes. In this way ATP transfers energy between spatially-separate

metabolic reactions. ATP is the main energy source for the majority of cellular functions. This includes

the synthesis of macromolecules, including DNA and RNA, and proteins. ATP also plays a critical role inthe transport of macromolecules across cell membranes, e.g. exocytosis and endocytosis.

Roles in cell structure and locomotion

ATP is critically involved in maintaining cell structure by facilitating assembly and disassembly

of elements of the cytoskeleton. In a related process, ATP is required for the shortening of actin and

myosin filament crossbridges required for muscle contraction. This latter process is one of the main

energy requirements of animals and is essential for locomotion and respiration.

Cell signalling

Extracellular signalling

ATP is also a signalling molecule. ATP, ADP, or adenosine are recognised by purinergic

receptors. Purinoreceptors might be the most abundant receptors in mammalian tissues. In humans, this

signalling role is important in both the central and peripheral nervous system.

Intracellular signalling

ATP is critical in signal transduction processes. It is used by kinases as the source of phosphate

groups in their phosphate transfer reactions. Kinase activity on substrates such as proteins or membrane

lipids is a common form of signal transduction. Phosphorylation of a protein by a kinase can activate this

cascade such as the mitogen-activated protein kinase cascade.

ATP is also used by adenylate cyclase and is transformed to the second messenger molecule

cyclic AMP, which is involved in triggering calcium signals by the release of calcium from intracellular

stores. This form of signal transduction is particularly important in brain function, although it is involved

in the regulation of a multitude of other cellular processes

DNA and RNA synthesis

In all known organisms, the deoxyribonucleotides that make up DNA are synthesized by theaction of ribonucleotide reductase (RNR) enzymes on their corresponding ribonucleotides. These

enzymes reduce the sugar residue from ribose to deoxyribose by removing oxygen from the 2' hydroxyl

group; the substrates are ribonucleoside diphosphates and the products deoxyribonucleoside diphosphates

(the latter are denoted dADP, dCDP, dGDP, and dUDP respectively.) All ribonucleotide reductase

enzymes use a common sulfhydryl radical mechanism reliant on reactive cysteine residues that oxidize

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to form disulfide bonds in the course of the reaction.RNR enzymes are recycled by reaction with

thioredoxin or glutaredoxin.

The regulation of RNR and related enzymes maintains a balance of dNTPs relative to each other and

relative to NTPs in the cell. Very low dNTP concentration inhibits DNA synthesis and DNA repair and is

lethal to the cell, while an abnormal ratio of dNTPs is mutagenic due to the increased likelihood of theDNA polymerase incorporating the wrong dNTP during DNA synthesis. Regulation of or differential

specificity of RNR has been proposed as a mechanism for alterations in the relative sizes of intracellular

dNTP pools under cellular stress such as hypoxia.

In the synthesis of the nucleic acid RNA, ATP is one of the four nucleotides incorporated directly into

RNA molecules by RNA polymerases. The energy driving this polymerization comes from cleaving off a

pyrophosphate (two phosphate groups). The process is similar in DNA biosynthesis, except that ATP is

reduced to the deoxyribonucleotide dATP, before incorporation into DNA.

Binding to proteins

Some proteins that bind ATP do so in a characteristic protein fold known as the Rossmann fold,

which is a general nucleotide-binding structural domain that can also bind the cofactor NAD.The most

common ATP-binding proteins, known as kinases, share a small number of common folds; the protein

kinases, the largest kinase superfamily, all share common structural features specialized for ATP binding

and phosphate transfer.

ATP in complexes with proteins generally requires the presence of a divalent cation, almost

always magnesium, which binds to the ATP phosphate groups. The presence of magnesium greatly

decreases the dissociation constant of ATP from its protein binding partner without affecting the ability of

the enzyme to catalyze its reaction once the ATP has bound. The presence of magnesium ions can serveas a mechanism for kinase regulation.

Two types of metabolic reactions

Anabolism

Catabolism

Anabolism is the set of constructive metabolic processes where the energy released by

catabolism is used to synthesize complex molecules. In general, the complex molecules that make up

cellular structures are constructed step-by-step from small and simple precursors. Anabolism involves

three basic stages. Firstly, the production of precursors such as amino acids, monosaccharides,

isoprenoids and nucleotides, secondly, their activation into reactive forms using energy from ATP, and

thirdly, the assembly of these precursors into complex molecules such as proteins, polysaccharides, lipids

and nucleic acids.

Catabolism is the set of metabolic processes that break down large molecules. These include

breaking down and oxidizing food molecules. The purpose of the catabolic reactions is to provide the

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energy and components needed by anabolic reactions. The exact nature of these catabolic reactions differ

from organism to organism and organisms can be classified based on their sources of energy and carbon

(their primary nutritional groups), as shown in the table below. Organic molecules being used as a source

of energy in organotrophs, while lithotrophs use inorganic substrates and phototrophs capture sunlight as

chemical energy. However, all these different forms of metabolism depend on redox reactions that

involve the transfer of electrons from reduced donor molecules such as organic molecules, water,ammonia, hydrogen sulfide or ferrous ions to acceptor molecules such as oxygen, nitrate or sulfate.In

animals these reactions involve complex organic molecules being broken down to simpler molecules,

such as carbon dioxide and water. In photosynthetic organisms such as plants and cyanobacteria, these

electron-transfer reactions do not release energy, but are used as a way of storing energy absorbed from

sunlight.

HOMEOSTASIS

Living cells, as well as larger multicellular organisms, can function adequately only within a

relatively narrow range of conditions. If the temperature within a cell should exceed 60C, the cell will

cease its vital functions. At higher temperatures, the lipids and proteins of the cell break down and the cell

falls apart. At very low temperatures, freezing and ice crystal formation challenge the functional and even

the structural integrity of the cells.

Definition: Maintenance of the relative stability of the physical and chemical aspects of the

internal environment within a range compatible with cellular function. The maintenance of constancy is

called homeostasis.

Homeostasis has been studied most intensively in multicellular animals, particularly vertebrates.

However, it is operative at all levels of life. Those processes that maintain homeostasis are known as

homeostatic mechanisms.

Components: 1) sensor

2) afferent pathway

3) integration center or comparator

4) efferent pathway

5) effector organ(s)

Negative feedback : a control system that causes the value of a physiological measurement to

change in the direction opposite to the initial deviation from set point.

Positive feedback: a control system that causes the value of a physiological measurement to

change in the same direction as the initial deviation from set point.

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The body loses heat at its surface, so heat must be brought to the surface where it can be

dissipated. The blood carries a great deal of the body heat. In overheated conditions, receptors

from skin and some internal structures activate feedback circuits that dilate the blood vessels at

the skins surface, thus bringing an increased volume of blood to the surface.

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CELL GROWTH, REPRODUCTION, AND DIFFERENTIATION

Most eukaryotic cells reproduce at a regular rate to produce new daughter cells that contain the

distributed materials of the original cell. The distribution of nuclear materials, particularly the

chromosomes, is known as mitosis. The apportionment of cytoplasm is called cytokinesis. Theseprocesses are part of a larger sequence of events known as the cell cyclea rhythmic recurrence of

growth within the cell followed by reproduction of the cell.

After cell division, each of the daughter cells begins 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.

Resting (G0 phase)

The term "post-mitotic" is sometimes used to refer to both quiescent and senescent cells.

Nonproliferative cells in multicellular eukaryotes generally enter the quiescent G0 state from G1 and mayremain 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 is 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

Before a cell can enter cell division, it needs to take in nutrients. All of the preparations are done

during the interphase. Interphase proceeds in three stages, G1, S, and G2. Cell division operates in a

cycle. Therefore, interphase is preceded by the previous cycle of mitosis and cytokinesis.

G1 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 had been considerably slowed down during M phase, resume at a

high rate. This phase is marked by synthesis of various enzymes that are required in S phase, mainly those

needed for DNA replication. Duration of G1 is highly variable, even among different cells of the same

species.

S phase

The ensuing S phase starts when DNA synthesis commences; when it is complete, all of the

chromosomes have been replicated, i.e., each chromosome has two (sister) chromatids. Thus, during this

phase, the amount of DNA in the cell has effectively doubled, though the ploidy of the cell remains the

same. 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.

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G2 phase

The cell then enters the G2 phase, which lasts until the cell enters mitosis. Again, significant

biosynthesis occurs during this phase, mainly involving the production of microtubules, which are

required during the process of mitosis. Inhibition of protein synthesis during G 2 phase prevents the cell

from undergoing mitosis.

Mitosis (M Phase/Mitotic phase)

The relatively brief M phase consists of nuclear division (karyokinesis). The M phase has been broken

down into several distinct phases, sequentially known as:

prophase

metaphase

anaphase

telophase

cytokinesis (strictly speaking, cytokinesis is not part of mitosis but is an event that directly

follows mitosis in which cytoplasm is divided into two daughter cells)

Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus

into two identical sets in two nuclei. It is generally followed immediately 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 mitotic (M) phase of the cell cycle

- 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.

Mitosis occurs exclusively in eukaryotic cells, but occurs in different ways in different species.

For example, animals 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. Prokaryotic cells,

which lack a nucleus, divide by a process called binary fission.

The process of mitosis 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 stages are

prophase, prometaphase, metaphase, anaphase and telophase. During the process of mitosis the pairs of

chromosomes condense and attach to fibers that pull the sister chromatids to opposite sides of the cell.

The cell then divides in cytokinesis, to produce two identical daughter cells.

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. This occurs most notably among the fungi and slime

moulds, but is found in various different groups. Even in animals, cytokinesis and mitosis may occurindependently, for instance during certain stages of fruit fly embryonic development. Errors in mitosis

can either kill a cell through apoptosis or cause mutations that may lead to cancer.

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Cell cycle

REGULATION OF CELL CYCLE

Regulation of the cell cycle involves processes crucial to the survival of a cell, including the

detection and repair of genetic damage as well as the prevention of uncontrolled cell division. The

molecular events that control the cell cycle are ordered and directional; that is, each process occurs in a

sequential fashion and it is impossible to "reverse" the cycle.

Role of cyclins and CDKs

Two key classes of regulatory molecules, cyclins and cyclin-dependent kinases (CDKs),

determine a cell's progress through the cell cycle. Many of the genes encoding cyclins and CDKs are

conserved among all eukaryotes, but in general more complex organisms have more elaborate cell cycle

control systems that incorporate more individual components. Many of the relevant genes were first

identified by studying yeast, especially Saccharomyces cerevisiae; genetic nomenclature in yeast dubs

many these genes cdc (for "cell division cycle") followed by an identifying number, e.g., cdc25 orcdc20.

Cyclins form the regulatory subunits and CDKs the catalytic subunits of an activated heterodimer; cyclins

have no catalytic activity and CDKs are inactive in the absence of a partner cyclin. When activated by a

bound cyclin, CDKs perform a common biochemical reaction called phosphorylation that activates or

inactivates target proteins to orchestrate coordinated entry into the next phase of the cell cycle. Different

cyclin-CDK combinations determine the downstream proteins targeted. CDKs are constitutively

expressed in cells whereas cyclins are synthesised at specific stages of the cell cycle, in response to

various molecular signals.

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CHECKPOINTS

Cell cycle checkpoints 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.

Several checkpoints are designed to ensure that damaged or incomplete DNA is not passed on to

daughter cells. Two main checkpoints exist: the G1/S checkpoint and the G2/M checkpoint. G1/S transition

is a rate-limiting step in the cell cycle and is also known as restriction point. An alternative model of the

cell cycle response to DNA damage has also been proposed, known as the postreplication checkpoint.

p53 plays an important role in triggering the control mechanisms at both G1/S and G2/M

checkpoints.

A dysregulation of the cell cycle components may lead to tumor formation. As mentioned above,

some genes like the cell cycle inhibitors, RB, p53 etc., when they mutate, may cause the cell to multiply

uncontrollably, forming a tumor. Although the duration of cell cycle in tumor cells is equal to or longer

than that of normal cell cycle, the proportion of cells that are in active cell division (versus quiescent cells

in G0 phase) in tumors is much higher than that in normal tissue. Thus there is a net increase in cell

number as the number of cells that die by apoptosis or senescence remains the same.

Cell cycle checkpoints

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BACTERIAL REPRODUCTION

Bacteria are prokaryotic organisms that reproduce asexually. Bacterial reproduction most

commonly occurs by a kind of cell division called binary fission. Binary fission results in the formation of

two bacterial cells that are genetically identical.

Bacterial Cell Structure

Bacterial cells typically contain the following structures: a cell wall, cell membrane, cytoplasm,

ribosomes, plasmids, flagella, and a nucleiod region.

Cell Wall - Outer covering of the cell that protects the bacterial cell and gives it shape.

Cytoplasm - A gel-like substance composed mainly of water that also contains enzymes, salts,

cell components, and various organic molecules.

Cell Membrane or Plasma Membrane - Surrounds the cell's cytoplasm and regulates the flow

of substances in and out of the cell.

Flagella - Long, whip-like protrusion that aids in cellular locomotion.

Ribosomes - Cell structures responsible for protein production.

Plasmids - Gene carrying, circular DNA structures that are not involved in reproduction.

Nucleiod Region - Area of the cytoplasm that contains the single bacterial DNA molecule.

Bacterial Reproduction: Asexual

Most bacteria reproduce by binary fission. During binary fission, the single DNA molecule

replicates and both copies attach to the cell membrane.

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.

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Binary fission is a method of asexual reproduction by most prokaryotes. In binary fission, the

living cell divides into two equal, or nearly equal, parts. It begins when the DNA of the cell is replicated.

Each circular strand of DNA then attaches to the plasma membrane. The cell elongates, causing the two

chromosomes to separate. The plasma membrane then invaginates (grows inward) and splits the cell into

two daughter cells through a process called cytokinesis.

Binary fission theoretically results in two identical cells. However, the DNA of bacteria has arelatively high mutation rate. This rapid rate of genetic change is what makes bacteria capable of

developing resistance to antibiotics and helps them exploit invasion into a wide range of environments.

Similar to more complex organisms, bacteria also have mechanisms for exchanging genetic

material. Although not equivalent to sexual reproduction, the end result is that a bacterium contains a

combination of traits from two different parental cells. Three different modes of exchange have thus far

been identified in bacteria.

Conjunction involves the direct joining of two bacteria, which allows their circular DNAs to

undergo recombination. Bacteria can also undergo transformation by absorbing remnants of DNA from

dead bacteria and integrating these fragments into their own DNA. Lastly, bacteria can exchange genetic

material through a process called transduction, in which genes are transported into and out of the cell by

bacterial viruses, called bacteriophages, or by plasmids, an autonomous self-replicating extra

chromosomal circular DNA.

BINARY FISSION

Bacterial Recombination:

Binary fission is an effective way for bacteria to reproduce, however it does produce problems.

Since the cells produced through this type of reproduction are identical, they are all susceptible to

the same types of antibiotics. In order to incorporate some genetic variation, bacteria use a

process called recombination. Bacterial recombination can be accomplished through conjugation,

transformation or transduction.

Conjugation

Some bacteria are capable of transferring pieces of their genes to other bacteria that they come in

contact with. During conjugation, one bacterium connects itself to another through a protein tube

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structure called a pilus. Genes are transferred from one bacterium to the other through this tube.

Transformation

Some bacteria are capable of taking up DNA from their environment. These DNA remnants most

commonly come from dead bacterial cells. During transformation, the bacterium binds the DNA andtransports it across the bacterial cell membrane. The new DNA is then incorporated into the bacterial

cell's DNA.

Transduction

Transduction is a type of recombination that involves the exchanging of bacterial DNA through

bacteriophages. Bacteriophages are viruses that infect bacteria. There are two types of transduction:

generalized and specialized transduction.

The bacteriophage attaches to a bacterium; it inserts its genome into the bacterium. The viral

genome, enzymes, and viral components are then replicated and assembled within the host bacterium. The

newly formed bacteriophages then lyse or split open the bacterium, releasing the replicated viruses.

During the assembling process however, some of the host's bacterial DNA may become encased

in the viral capsid instead of the viral genome. When this bacteriophage infects another bacterium, it

injects the DNA fragment from the previous bacterium. This DNA fragment then becomes inserted into

the DNA of the new bacterium. This type of transduction is called generalized transduction.

In specialized transduction, fragments of the host bacterium's DNA become incorporated into the

viral genomes of the new bacteriophages. The DNA fragments can then be transferred to any new bacteria

that these bacteriophages infect.

Mitosis

Mitosis is the process during which the chromosomes are distributed evenly to two new cells that

arise from the parent cell undergoing division. During the S phase of interphase before mitosis

proper, each chromosome will have replicated. The two chromosomal strands (chromatids) are

identical in their genetic material and are joined at a constricted region called the centromere.

Within the centromere are one or more rings of protein known as kinetochores. The kinetochores

will play a significant role in the attachment of the spindle fibers to the chromosomes and in thesubsequent migration of the chromosomes.

Mitosis has four main stagesprophase, metaphase, anaphase, and telophase (see Fig. 8.2). In

prophase, the relatively long first stage of division, the nuclear membrane breaks down and the

spindle forms.The chromosomes condense and begin to move toward the equatorial (middle)

plane of the cell.

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Metaphase is characterized by the precise lineup of the chromosomes along the equatorial plane.

At the start of anaphase, the centromeres of each chromosome split so that each chromatid now

exists as a separate chromosome. Guided by the mooring spindle fiber, one chromatid of each

pair is moved to one pole, while the other chromatid is moved to the opposite pole. Once the

chromosomes reach opposite poles, the last phase of mitosis, telophase, begins. The nucleolus,

which tended to disappear during prophase, begins to re-form at specific nucleolar organizingregions of certain chromosomes. The spindle apparatus breaks down to its constituent

macromolecules, and a new nuclear membrane begins to form around each of the two clumps of

chromosomes aggregated at their respective poles. Telophase may be regarded as a prophase run

backwards.

With the completion of the nuclear division events, the cytoplasm usually begins its divisiona

process known as cytokinesis. Although accomplished differently in animals and plants, the

results are the same: the creation of two separate cells.

Mitosis

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MEIOSIS

A union of gametes in sexual reproduction always yields a doubling of the chromosome

number. To maintain homeostasis in terms of chromosome number, the uniting gametes may be

haploid rather than diploid, owing to a unique pair of cell divisions that segregate homologouschromosomes into separate cells. The process by which this is accomplished is called meiosis,

from a Greek word meaning to diminish. Meiosis probably evolved as a modification of

mitosis and incorporates many of its features.

The First Meiotic Division

Meiosis begins in similar fashion to mitosis each chromosome replicates in the S phase of

interphase, and prophase begins after G2 with an increasing coiling and condensation of each of

the doublet chromosomes. As in mitosis, the nuclear membrane begins to break down, centrioles

move to opposite poles of the cell, and the chromosomes begin to migrate toward the equatorial

plane. Spindle fibers start to aggregate from microtubules, and nucleoli disappear. Importantdifferences, however, soon become apparent. The prophase of meiosis I is a very much longer

and more extensive process than the prophase of mitosis and is actually divided into substages.

The most dramatic difference occurs early in the prophase when the homologous chromosomes

mysteriously start to come together in pairs (synapsis).

Homologues touch at one or several points; then the chromatids appear to zip together to

form an intimate four-stranded structure known as a tetrad. When the tetrad begins to loosen late

in prophase, the individual chromosomes from each tetrad starts separating. At this point, there

may still be a few physical links between chromatids of one homologous chromosome and those

of another. These clinging structures that seem to defy the tendency of the homologues to

separate are called chiasmata. Each of the chiasmata formed along the various homologues

represents a point at which a section from one chromatid has physically broken off and

exchanged with the corresponding chromatid section on the homologous chromosome. Such an

exchange of chromosome parts between the chromatids of two homologous chromosomes is

known as crossing over and results in the formation of hybrid chromosomes with mixed genetic

material. The metaphase of meiosis I consists of a lining up of pairs of homologous

chromosomes, now largely separated, at the equatorial plane. These structures continue to be

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identified as tetrads, since the homologues are still closely apposed to one another. However,

instead of a single line of centromeres at the equator, which is characteristic of mitotic metaphase,

there will be a double line of centromeres. The total number of tetrads at the equator will be equal

to the haploid (1n) number. During the anaphase of meiosis I, no splitting of centromeres occurs.

Instead, whole chromosomes separate, with one homologue moving to one pole and the other to

the opposite pole. This results in single sets of chromosomes (with two chromatids) aggregatingat each of the poles and effectively reduces the diploid (2n) condition to the haploid (1n)

condition. This first division of meiosis is consequently called the reduction division.

In the ensuing telophase, it is chromosomes with two chromatids that slowly lose their

density, a new nuclear membrane forms around each haploid set of doublet chromosomes, and

the usual events of telophase ensue. A short stage called interkinesis occurs between telophase I

and prophase II. However, no synthesis of genetic material occurs, and in some cases, the

chromosomes do not completely lose their condensed configuration before moving into the

second meiotic division.

The Second Meiotic Division

In the second meiotic division, called the equational division, a haploid set of replicate

chromosomes in each new cell migrates to the equatorial plane and lines up in a single line of

centromeres. The centromeres now split, and the former chromatids of each chromosome migrate

to opposite poles. Each of the two cell products of meiosis I will produce two new cells, a total of

four haploid cells during the full meiotic process. In some cases, only one functional cell arises

from the meiotic process, since in many species, each of the two meiotic divisions produces one

functional cell and one very tiny polar body, which quickly degenerates. The first polar body may

even undergo the second meiotic division before disintegrating. The production of gametes

(gametogenesis) in females (oogenesis) is similar to gamete production in males

(spermatogenesis) in terms of the behavior of chromosomes. However, in the apportionment ofcytoplasm to the resultant cells and their modification, differences often arise between the sexes.

In developmental biology, cellular differentiation is the process by which a less

specialized cell becomes a more specialized cell type. Differentiation occurs numerous times

during the development of a multicellular organism as the organism changes from a simple

zygote to a complex system of tissues and cell types. Differentiation is a common process in

adults as well: adult stem cells divide and create fully-differentiated daughter cells during tissue

repair and during normal cell turnover. Differentiation dramatically changes a cell's size, shape,

membrane potential, metabolic activity, and responsiveness to signals. These changes are largely

due to highly-controlled modifications in gene expression. With a few exceptions, cellular

differentiation almost never involves a change in the DNA sequence itself. Thus, different cells

can have very different physical characteristics despite having the same genome.

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A cell that is able to differentiate into all cell types of the adult organism is known as

pluripotent. Such cells are called embryonic stem cells in animals and meristematic cells in

higher plants. A cell that is able to differentiate into all cell types, including the placental tissue,

is known as totipotent. In mammals, only the zygote and subsequent blastomeres are totipotent,

while in plants many differentiated cells can become totipotent with simple laboratory techniques.

In cytopathology, the level of cellular differentiation is used as a measure of cancer progression."Grade" is a marker of how differentiated a cell in a tumor is.