Allied Biology - Layout

8/8/2019 Allied Biology - Layout

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1. CELL STRUCTUREAll organisms are made of cells. They are the building blocks.Definition of a Cell

A cell is the fundamental and simplest integrated and continuously changing unit of thestructure and function of an organism delimited by a semi permeable plasma membraneand capable of self reproduction.

Types of Cells

Based on this definition, there are two types (1) the prokaryotic cells and (2) theeukaryotic cells.

Viruses

Viruses are exceptions. They are primitive. A virus is neither a cell nor an organism. Theyare not complete cells.

COMPARISON OF PROKARYOTIC AND EUKARYOTIC CELLSSNo Characters Prokaryotic Cell Eukaryotic Cell

1 Size Small

1 - 10m

Large

10 - 100m

2 Plasma membrane Present Present

3 Mesosome Present Absent

4 Cell wall Present made ofamino sugars &Muramic acid

Present in plant cells madeof cellulose

5 Cytoplasm Streaming movementabsent

Streaming movementpresent

6 Mitochondria Absent Present7 Golgi body Absent Present

8 Lysosomes Absent Present

9 Endoplasmic reticulum Absent Present

10 Vacuoles Absent Present in plant cells

11 Ribosomes Small 70 S Large 80 S

12 Photosynthetic Chlorophyll-a, etc. Chloroplasts with


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machinery chlorophyll- a & b in plants

13 Nucleus Absent Present14 Nucleoid Present Absent

15 Nuclear membrane Absent Present

16 Nucleoplasm Absent Present

17 Chromosome 1, circular,

with DNA

More, thread-like, with DNA

18 Genetic material DNA DNA

19 DNA Circular Linear

20 Flagella Present no 9+2pattern

Present with 9+2 pattern


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2. BACTERIA(Prokaryote Cell)

Introduction

Bacteria are prokaryotes. They are small, microscopic, simple, unicellular and mostprimitive. From them arose the eukaryotes.

General features of Prokaryotes

A prokaryotic cell is a one-envelope system. It has the central nuclear components. Theymay be DNA, RNA and nuclear proteins. The nucleoli are absent. They are not coveredby membrane. They are surrounded by cytoplasm. The cytoplasmic organelles likeendoplasmic reticulum, Golgi complex, mitochondria (the respiratory enzyme system),Centrioles, basal bodies, cytoskeleton, etc. are not well defined. The nuclearcomponents and cytoplasm are together covered by a plasma membrane.

Ultra StructureThe structural details of a bacterium can be seen only under the electron microscope.

I. Outer Covering

Bacteria have an outer covering of three layers. They are,

a. Plasma Membrane

The protoplast of bacteria is covered by the plasma membrane. It is living and dynamic.

It is ultra thin (6 to 8 nm).


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Chemical Structure

Plasma membrane is made of lipid and protein molecules. These macromolecules arearranged in a particular pattern. It is called the fluid mosaic pattern.

Fluid Mosaic Pattern

Lipids are united with phosphates. They are called phospholipids. They are present intwo layers. They are polar. They have a head-end and a tail end. The tails are fatty-acylchains. The polar heads are on the surface. The tails face the interior.

The protein molecules are embedded within this bi-layer. They may be on the outer,middle or inner sides.

Functions of Plasma Membrane

The membrane proteins act as carriers or permeases. They selectively transportsubstances in and out. Some proteins are involved in oxidative metabolism. They act asenzymes and carriers for electron flow. It provides attachment for the circularchromosome (DNA).

Intrusions

There are two main types of intrusions (infoldings) of the plasma membrane.


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1) Mesosome or Chondrioid

This is an inward extension. It is made of convoluted membranes. It forms complexwhorls. It increases the surface area and enzymatic contents. Mesosomes are presentin chemoautotrophic and photosynthetic bacteria.

2) Chromatophores

They are seen in photosynthetic bacteria. They are membranous. They bearphotosynthetic pigments. They have different forms vesicles, tubes, bundled tubes,

stacks or thylakoids.B. Cell Wall

The cell wall is outer to plasma membrane. It is strong and rigid. It is chemicallydifferent from the cell wall of plants. It has proteins, lipids, polysaccharides and chitin,and not cellulose.

Ultra structure of the Cell Wall of Gram-negative Bacteria

It is thicker, amorphous, homogeneous and single-layered. It contains the followingchemicals: peptidoglycons, proteins, neutral polysaccharides and polyphosphatepolymers. It is made of two layers. They are,

a) Gel, Proteoglycon or Peptidoglycon

This is situated around the plasma membrane. It contains periplasmatic space.

b) Outer Membrane

It consists of a lipid bilayer.The lipids are phospholipids and lipopolysaccharides. In it aremany channels of porin polypeptide. Solutes diffuse through these channels.

C. Capsule

This is the outermost covering. It is a layer of slime or gel. It is thick, gum-like andmucilaginous. Plasma membrane secretes it. It protects and regulates water and ion

concentration.2. Cytoplasm

The cytoplasm is covered by the plasma membrane. It has two parts 1) hyaloplasmand 2) matrix or cytosol.


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Cytosol

This is the ground substance. All metabolic activities take place here. It consists ofwater, proteins, lipids, carbohydrates, different types of RNAs and many smallermolecules.

The cytosol is differentiated into two regions. They are 1) the nuclear area andcytoplasmic area. The nuclear area is less electron dense (light). The cytoplasmic area iselectron thick (dark). The second area has two types of inclusions. They are Ribosomesand Reserve materials.

Ribosomes

There are 1000s of ribosomes. They are 25 nm in diameter. They are made of RNA andproteins. The type of these ribosomes is 7oS. It is made of two subunits. One is larger.It is the 50S subunit. The other is smaller. It is the 30S subunit. Some ribosomes arenon-functional. They are separated as subunits. Protein synthesis takes place in them.During protein synthesis, many ribosomes unite to form polyribosomes.

Reserve Materials

They are stored in the cytoplasm. They are finely distributed or distinct granules. Thesecond is also called inclusion bodies or storage granules. There are three types ofreserve materials.

i) Organic Polymers

They serve as carbon reserves and energy reserves.

ii) Volutin

They are more in quantity. They are the reserves of inorganic phosphate. They arehighly refractive.

iii) Sulpher

It is formed from the oxidation of hydrogen sulphide. They are spherical droplets. Theyare energy reserves. They are seen in some sulpher bacteria.

3. Nucleoids

The Nucleoid is a specific and clear region of the cytoplasm. The nuclear material ispresent here. It is not separated from the cytosol by membrane. It includes the nuclearmaterial. It is the single, circular and double-stranded DNA. This is called the bacterial

chromosome. It is permanently attached to the plasma membrane at one point.Histone proteins are absent from them.


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There is only one species of RNA polymerase. It forms all the three types of RNAs

mRNA, rRNA and tRNA.4. Plasmids

They are extra-chromosomal genetic materials. They are small, circular and closed DNAmolecules.

5. Flagella and other structures

Flagella

Many bacteria are mobile. One or more flagella do this function. The flagella aresmaller and simpler than the eukaryotic flagella. They are 15 to 20 nm in diameter.Their length is upto about 20 m.

Structure

The flagellum has a helical tube. It has a single type of protein subunit called flagellin.

The tube is attached to a base through a hook. This is short and flexible. It can berotated. A motor does this.

The flagellum has three parts. They are tube, hook and base.

a. The tube is helical. It is formed of a single type of protein subunit calledflagellin.

b. The hook joins the tube and the base. It has a rotatory motor. It rotates

like the propeller of a ship.c. The motor is formed of four parts rotor (M ring), stator, bearing (S

ring) and rod. The rotor is a protein disc. It is embedded in the plasmamembrane. Using energy, it rotates rapidly. The stator forms the basefor rotation. It is also a protein disc. The rotor is attached to the hookand flagellum by a rod. When the rotor rotates, the flagellum alsorotates. The rod passes through the outer membrane of the cell wall.At that point, there is a hole. The bearing seals this hole.

The number and arrangement of flagella in bacteria has four types.

Monotrichous - A single flagellum at one end.

Lophotrichous - Many flagella at one pole.

Amphitrichous - At lease one flagellum at each pole.

Peritrichous - Flagella all over the surface.


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Axial Filaments

This is seen in some spirochetes. They move like snakes. They do not project out. Theylie on the cell surface.

Fimbriae or Pili

Some bacteria like the Gram-negative bacteria have this. They are non-flagellar, veryfine appendages. They do not move. They help in attachment. It also helpsconjugation. Bacteriophages attach to the bacteria on the pili. The plasmid genes code

them.Spinae

They are tubular, pericellular and rigid appendages. They are formed of the proteinspinin. They are seen in some Gram-positive bacteria. They aid in tolerating pH,temperature salinity, etc.


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3. EUKARYOTIC CELLIntroduction

The eukaryotic cells are two-envelope systems. There are secondary membranes aroundthe nucleus and other cell organelles. In the cytoplasm, they form the endoplasmicreticulum.

The eukaryotic cells are true cells. They occur in plants and animals. They are typicallycomposed of plasma membrane, cytoplasm and its organelles and a true nucleus. The

nucleus is separated from the cytoplasm by a thin, perforated nuclear membrane.

General Structure of a Eukaryotic Cell

Shape

The basic shape is spherical. The function of the cell determines the shape. So, theshape may be fixed or variable or irregular. Plasma membrane, exoskeleton, surface

tension, viscosity of the protoplasm, cytoskeleton, mechanical action of the nearby cells,etc. also determine the shape.

The cell shape varies from animal to animal, organ-to-organ and even in the sameorgan.

The different shapes are: polyhedral (many-sided), flattened, cuboidal, columnar,discoidal, spherical, spindle-shaped, elongated, branched, etc.

SizeThe eukaryotic cells are typically larger. They are larger than the prokaryotic cells. Theyrange from 10 to 100 m. The biggest single cell is that of the unicellular organism,Amoeba proteus. It is 1000 m. The single-celled alga, Acetabularia, is unusually long 10 cm. In multicellular organisms, the ostrich egg is the biggest 18 cm.

Cell Volume

The cell volume of each cell type is mostly constant. It is not related to the size of theorganism. This is called the law of constant volume.

Ratio of Volume to surface

This should be within a limited range. Large cells have proportionately large volume andless surface area. Small cells have proportionately less volume and large surface.Metabolically active cells are smaller in size.


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Cell Number

This varies from organism to organism. Unicellular organisms have one cell. Inmulticellular organisms, the number of cells is related to the size of the animal. Largeanimals have more cells. Small animals have less number of cells. An 80 kg man mayhave about 60 thousand billion cells. Though the number of cells is indefinite, it may beconstant also (ex.: Rotifer). This phenomenon is called eutely.

Ultra Structure of a Typical Animal Cell

The typical parts of an animal cell are Plasma membrane,

Cytoplasm and

Nucleus.

1. Plasma Membrane or Cell Membrane or Plasmalemma

This forms the cell boundary. It is a living, thin and delicate membrane. It is tri-laminaror 3-layered. The outer layers are dark. The middle layer is translucent.


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Its molecular structure shows a bimolecular lipid layer. Protein molecules areembedded or attached to its surfaces. Some carbohydrate molecules may also beattached to lipids (glycolipids) or proteins (glycoproteins). This is called unit membrane

structure.Plasma membrane is selectively permeable. It controls and selects the entry or exit ofmaterials. Due to this, the cell environment is maintained constant (homoeostasis).

This transport of materials may be active (using energy rich ATP) or passive (osmosis,diffusion, etc). In the first type, the carrier molecules (membrane proteins) play animportant role. They are called transport proteins or pumps.

Bulk entry of large molecules is called endocytosis. Bulk exit of large molecules is calledexocytosis.

The cell organelles like mitochondria, Lysosomes, Golgi bodies, endoplasmic reticulumare membrane-bound. These membranes are similar to plasma membrane.

2. Cytoplasm

Cytoplasm is inner to the plasma membrane. It is differentiated into the following.

Cytosol or Matrix

It is a colloidal organic fluid. Outside the nucleus, it is called the cytoplasm. It fills all thespaces of the cell. It forms the internal milieu. It determines many fundamentalproperties of cells. Many small molecules for cellular metabolism are dissolved orsuspended in it. It contains the soluble proteins and enzymes. The cytosol isdifferentiated into an outer, non-granular, viscous, clear and rigid ectoplasm or cellcortex and an inner, granular and less viscous endoplasm.


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Cytoskeleton

Inside the cytosol, there are many fibers. They are collectively called the cytoskeleton.They give shape, mobility and attachment to other cell structures. The fibers are ofthree types microtubules, microfilaments and intermediate filaments.


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Microtrabecular Lattice

This is a three-dimensional network. The cytoskeletal fibers are interlinked. It is flexibleand can change its shape. The cell organelles are attached to it.

Cytoplasmic Structures

Cytoplasm includes two types of structures. One type is living and the second is non-living. They are suspended in the cytoplasm.

Non-living or Cytoplasmic Inclusions or Paraplasm

They are the stored food and secretory substances. They are in the form of refractilegranules. They include oil drops, triglycerols, yolk granules, secretory granules, glycogengranules, etc.

Living or Cytoplasmic Organelles

There are many types of organelles. They are membrane-bound. They do specializedtasks.

Endoplasmic Reticulum

This is a living cytoplasmic organelle. It is in the form of a wide reticulum (network). Ithas membrane-bound channels. Some portion of it is attached with the plasmamembrane and the nuclear membrane. At some surface areas, ribosomes are attached.

This gives a rough appearance. So, this is called the Rough Endoplasmic Reticulum

(RER). The other surface areas are called Smooth Endoplasmic Reticulum (SER). TheSERs functions are lipid metabolism, glycogenolysis and drug detoxification.

The RER participated in photosynthesis. The produced proteins enter into the lumen ofthe endoplasmic reticulum. There, they rearrange, mature and become secondary andtertiary proteins. It also produces cellular membranes.


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Golgi apparatus

This is a living cytoplasmic organelle. It is in the form of a cup. It is membrane-bound.It is near the nucleus. It is formed of many cisternae. They are smooth. They areclosely packed together in parallel rows. Spherical vesicles surround them. They are


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also membrane-bound. They are of three types. In active cells, they are well formedand many.

Functions

Golgi apparatus

a. stores secretory materials like enzymes, mucin, melanin pigment, etc.,

b. processes proteins,

c. synthesizes polysaccharides and glycolipids,

d. sorts and sends proteins to the various parts of the cell,

e. adds membranous elements and

f. forms the acrosome of the sperm.

Lysosomes

Lysosomes are living cytoplasmic organelles. They are tiny, spherical (polymorphic) and

membrane-bound. The Golgi apparatus forms them. They contain about 50 hydrolyticenzymes. These enzymes digest intracellular and extra-cellular materials. They have anacidic medium (pH 5).

Ribosomes

Ribosomes are living cytoplasmic organelles. They are tiny and spherical. Their

diameter is from 150 to 200 . They contain RNA and proteins. They are the work-benches of protein -synthesis. They may be free or attached to the endoplasmicreticulum. Their sedimentation quotient is 80S (40S + 60S).

Mitochondria

Mitochondria are living cytoplasmic organelles. They are ribbon-shaped. Twomembranes surround each mitochondrion. The inner one contains many enzymes,

coenzymes and electron. transport particles. It projects inward as finger-like cristae.


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The mitochondrial matrix is a colloid. It contains enzymes used in Krebs citric acidcycle.

Mitochondria are the seat of oxidative phosphorylation.

This produces energy as ATP molecules. So, mitochondria are called the powerhouses

of the cell. They produce part of their needed proteins. So, they are also called thesemi-autonomous organelles.

Cytoplasmic Vacuoles

They are many, small or large, hollow and liquid-filled. They are formed from the Golgiapparatus or endoplasmic reticulum. A lipoprotein membrane surrounds them. Theirfunction is storage, transmission of materials and internal cell pressure maintenance.

Peroxisomes

These are small and surrounded by membrane. Inside are some enzymes. Theyparticipate in detoxification activities, degradation of amino acids and -oxidation offatty acids.

Microtubules and Microtubular Organelles

This is seen in almost all eukaryotic cells. They are fibers 24 nm in diameter. Its wall isthick. It is made of -tubulin and -tubulin protein subunits. The center is hollow. The


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microtubules can be assembled or dissembled by adding or removing protein subunits.Cilia, flagella, basal bodies and centrioles are formed from the microtubules.

3.Nucleus

The nucleus is in the center of the cell. It is round. It controls all the vital activities ofthe cytoplasm. It carries the hereditary material, the DNA.

The nucleus can be differentiated into the following parts.

Nuclear Membrane

There are two nuclear membranes an inner and an outer. The outer membrane is acontinuation of the endoplasmic reticulum. The nuclear membranes have pores, the

nucleopores.Nucleoplasm

This fills the nucleus. It is surrounded by the nuclear envelope. It contains materialsneeded for DNA replication, transcription, etc.

Nucleolus

The nucleolus is a sub-organelle. It is round and prominent. It stains dark. It has no

limiting membrane. It is formed by the ribosomal DNA (rDNA). This, in turn, is formedby the nuclear organizer. Nucleolus forms the ribosomes.


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Chromatin

The chromatin carries the genetic or hereditary units, the genes. The chromosomesform a network, the chromatin network. The chromatin has two parts, the euchromatinand the heterochromatin. The first is genetically active. The second has no genetic role.The chromatin contains one DNA molecule.


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4. CELL DIVISIONIntroductionThe cells vary widely in structure and function. But they have some important propertiesin common. Plant cells differ from animal cells in shape etc. Both cells are essentiallyalike in terms of genes, chromosomes and other structures and functions.

Cell division is a process of reproducing itself. The function of the cell division is aninherent property of the cell to produce identical cells. This leads multicellular

organisms to grow. Cell division is really a process of duplication or multiplication.W.flemming(1843-1915) accounted the mechanism of cell reproduction. It involved twointer related process,viz;

a. Mitosis, the nuclear division.

b. Cytokinesis.

MITOSIS

Division of a cell begins after a more or less protracted period known as interkinesis orinterphase . In the interphase nucleus, the chromosomes are not visible but theyappear as long , thin ,intertwined threads. In the intetphase accounts for about 90% ofthe entire cell cycle . in the interphase stage , the cell grows and makes copies of thechromosomes. The interphase has three phases,viz;

1. G - 1 phase: This is the first part of the interphase. At this phase the cell grows.

2. S phase: It is the synthesis part of the interphase. Here the chromosome IScopied.

3. G - 2 phase: It is the second part of the interphase. The result of the interphaseis the duplication of DNA. At the end of the of the interphase, the chromatincontaining DNA becomes invisible.

[Chromatin which contains all DNA of nucleus in plant or animal cell - that later

condense to form the choromosome]. Nucleolus and centrioles are formed.

The remaining part of the cell cycle, has 4 phases Prophase, Metaphase, Anaphaseand Telophase.


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PROPHASE

The nuclear membrane breaks down, and eventually disappears.

Chromosomes threads (chromatin) begin to coil up. Chromatin is formed intochromosome. The chromosomes shorten and thicken.

Centrioles separate and start to move away

Spindle fibres begin to form

Metaphase:

It is the longest stage of the mitosis. The centrosomes move to the opposite sides of the cell.


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The mitotic spindle apparatus is formed out of the centrosomes.

The chromosome are lined up along the equatorial plate of the cell. They areattached to the mitotic spindle apparatus.

The chromosomes are composed of two chromatids each.

Anaphase:

It is the shortest stage of the mitosis.

The newly formed chromatids are pulled along the spindle fibres towards theopposite poles from the equator.

Centrosome splitting marks the beginning differentiation of chromatids intochromosome.

Chromosome move apart to the opposite sides of the cell, as the cell grows

It results in each side of the cell having same number of the chromosomes.


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Telophase:

Nuclear membrane formed around the chromosomes.

Chromosomes at the opposite poles begin to uncoil and become less dense.

The indentical nuclei are fully formed.

Cytokinesis:

Cytoplasm slits in two to form 2 daughter cells. In anaimal cytokinesis , a cleavagefurrow is caused at the surface of the cell membrane . The cleavage furrow is made outof the actin microfilaments. Their furrow forms into ring. It further deepens andsqueezes the cell into 2 identical cells.


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Mitotic Apparatus

The cytoplasmic organelle, the centrioles forms a part of a large and elaborate structurecalled the mitotic apparatus. The centrioles are not observed in-vivo in most cells. It ismore often surrounded by a clear zone microcentrum or centrosome and then by adenser zone centrosphere. From the centrosphere, the asters radiate.

The term mitotic apparatus is applied to the assembly of structures that forms theachromatic figure in the mitosis. This structure includes the asters [astropheree] whichsurround the centrioles and also the mitotic spindle. The aster appears in fixed

preparations as a group of radiating refringent fibrils that converge towards themicrocentrum and continue in the centrosphere. The aster is also evident in-vivobecause of its refringence, but in this case the fibrillar structure of the aster is not seen,and its make-up seems homogenous.


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The following classes of spindle fibres are observed through the light microscope;

Chomosomal fibres joining the poles to the kinetochores of the chromosomes Continuous fibres extending from pole to pole.

Astral fibres and

Interzonal fibres.

Role of the mitotic apparatus:

The role of the spindle in the movements of the chromosomes during anaphase is stillamazing. The contraction and shortening of the spindle fibres appear to take place atthe early anaphase. Other evidence accounts that at anaphase, the two sets ofdaughter chromosomes are pushed apart by an elongation of the spindle fibres inbetween the poles. This 2 types of anaphase movements are concerned with thechromosomal fibres and the continuous fibres. This two types of movements may occurin more or less in different proportions according to the cell type.

The equilibrium dynamic model of Inoue explain the process of spindle action.According to this model, the equilibrium between a large pool of monomers and theoriented polymers form the spindle fibres or microtubules. Upon polymerization, somewater molecules are believed to dissociate. It aids in contraction and elongation offibres. The contraction and elongation of fibres are responsible for the regularmovements of chromosomes during cell division. The contraction and elongation offibres is due to the subtraction or addition of monomers from the polar regions. Thisleads to consequent reduction of fibre length and pulling of chromosomes towards thepole / consequent elongation of fibre length and pushing of chromosomes towards thepole.


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5. MeiosisIntroductionThe reduction in chromosome number from diploid to haploid is done by the specializedcell divisions of meiosis. Cells prepare for meiosis by replication of their genetic materialas in mitosis. However, instead of a single division as in mitosis, meiosis consists of twoconsecutive divisions.

The first meiotic division

In the first meiotic division, the number of cells is doubled but the number ofchromosomes is not. This results in reduction in chromosome number into half per cell.

Prophase I

The prophase I is important stage in meiosis. In this phase the 5 sub stages are seen.They are Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis. During these stagesthe chromosomes behave characteristically.

The chromosomes first become visible as thin threads within the nucleus at thestage called leptotene.

In the next phase, zygotene, the homologous pairs of chromosomes becomeclosely associated along their lengths. This process is called synapsis [or] joining.This formed a structure comprised of two chromosomes, a bivalent.

The bivalents shorten and thicken throughout the next stage pachytene.Synapsis is complete and the bivalents are held together throughout theirlength by a structure known as thesynaptenemalcomplex.

In diplotene stage crossing over occurs. At this point, portions of one chromatidof one chromosome may become exchanged with the corresponding regionfrom a non-sister chromatid of the other chromosome in the pair.

The chromosomes continue to condense. The synaptonemal complex breaksdown. The homologous chromosomes appear to repel each other. They remainheld together only at chiasmata, the points where crossing over have occured,and at the centromere. This phase is named diplotene

There is always at least one chiasma per chromosome arm. Some of the longerarms will have two or even three. This chiasma HOLDS the chromosomestogether until they separate in anaphase.[ At this point the oocyte enters
http://www.ucl.ac.uk/~ucbhjow/bmsi/synapto.htmlhttp://www.ucl.ac.uk/~ucbhjow/bmsi/synapto.htmlhttp://www.ucl.ac.uk/~ucbhjow/bmsi/synapto.htmlhttp://www.ucl.ac.uk/~ucbhjow/bmsi/synapto.html


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meiotic arrest where it will remain for many years until it is reactivated atovulation.]

The final stage of prophase is diakinesis in which the nuclear membrane breaksdown. The chiasmata slip to the ends of the chromosome arms.

Centrosomes begin moving to the poles of cell.

Metaphase I

The bivalents align at the cetre of the cell.

They are attached to the spindle fibres.

Anaphase I

The first division begins.

Each bivalent divides so that one chromosome moves to one pole and thesecond chromosome moves to the other. [ Each chromosome is still comprisedof two chromatids.]

The homologous chromosomes separate

Telophase I

This stage may be absent in some species.

The second meiotic division

The second meiotic division is simply like mitosis. The number of chromosomes

does not get reduced.

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Between I and II meiotic divisions, there occurs interkinesis similar to theinterphase of mitosis. It differs in respect to the non-synthesis of DNA.

In males this follows immediately on the heels of the first division with nointervening round of DNA synthesis. Prophase II, Metaphase II and Anaphase IIresemble mitosis but with only a haploid chromosome number.

The second meiotic division in the egg is not completed until fertilization. It isagain very unequal giving the mature egg and a small second polar body

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6. Nucleic Acid DNA - STRUCTUREIntroductionJames Watson, Francis Crick, Maurice Wilkins and Rosalind Franklin played a role indiscovering the structure of DNA, deoxyribonucleic acid the molecule that encodesgenes in all living things.

The nucleic acid, DNA, is a double stranded poly nucleotide structure. The nucleotides

are made of a nitrogenous base, a pentose sugar - a ribose and a phosphate.Nitrogenous bases

The nitrogenous bases are five types. They are grouped as purines and pyrimidinesbased on their structure. The purines are adenine and guanine having two ringedstructures. The pyrimidines are cytosine, thymine and uracil. Thymine is present inDNA. Thymine is replaced by uracil in RNA.

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Pentose Sugar

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Both DNA and RNA contain pentose sugar. In RNA, the sugar component is ribose, asindicated by the name "ribonucleic acid". In DNA or deoxyribonucleic acid, the sugar

component is deoxyribose. The prefix deoxy means that an oxygen atom is missingfrom the second carbon atom of the ribose sugar.

Nucleoside

When a sugar bonds together with a nitrogenous base, it is known as a nucleoside. Thenitrogen base binds with the first carbon atom of the sugar. As such four nucleosidesare known for DNA viz. adenosine, guanosine, cytidine and thymidine. In RNA,

thymidine is replaced by uridine.

Nitrogenous Base + Pentose sugar = Nucleoside

Nucleotides

The binding of phosphate at the fifth carbon of the sugar in the nucleoside forms the

nucleotide. The chemical bond formed between the sugar and phosphate is an esterbond. The nucleotides are referred as adenylic acid, guanilic acid, cytidic acid andthymidilic acid. In RNA, thymidilic acid is replaced by the uridilic acid.

Nucleoside + Phosphate = Nucleotide

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Chargaffs Rule [Chemical equivalence rule]

Erwin Chargaff's work showed certain relationships between the amounts of variousbases. These relationships are called Chargaff's rules. They are

The amount of adenine equals the amount of thymine.

The amount of guanine equals the amount of cytidine.

The amount of adenine plus guanine equals 50% of the total implying that 50%

of the bases in DNA are purines. The amount of thymine plus cytosine equals 50% of the total implying that 50%

bases in DNA are pyrimidines.

Chargaff documented that base compositions actually varied among species, that is, inall species, [A] = [T] and [C] = [G].

The ratio [C+G] / [A+T] are typically less than unity (that is, [C+G] is less abundant).

Maurice Wilkins, and Rosalind Franklin Work

Work on x-ray diffraction patterns by Maurice Wilkins and Rosalind Franklin revealedthat the nucleic acid molecule has a "helical shape" with repeating distances of 0.34 nm,2nm diameter between the strands and 3.4 nm intervals between the base pairs.

James Watson and Francis Crick model of DNA Double Helix

Watson and Crick constructed a DNA model based on the findings of Wilkins andFranklin. In that, the nitrogenous bases are facing the interior of the doublehelix. They accounted that adenine can form two hydrogen bonds withthymine. Cytosine forms three hydrogen bonds with guanine. The Watson &Crick Model explained the basis of Chargaffs rules. It is a well-established factthat where an A exists along a DNA strand, there is a T on the opposite strandand, where there is a C on one strand, there is a G on the partner strand.

Watson and Crick then connected base pairs with phosphodiester bonds thatspaced the bases 3.4 apart and rotated each subsequent base pair by 36.This rotation forms a right-handed double helix with 10 bases per turn andrepeating elements with every 3.4.

The backbone of each strand consists of alternating phosphates anddeoxyriboses. To be more specific, the phosphate of a nucleotide bonds both to

the 5' carbon of one deoxyribose and the 3' carbon of the next deoxyribose.

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This is how the nucleotides create the strand. The 5' and 3' refers to the placeof the carbons in the deoxyribose molecule.

The heterocyclic nitrogenous bases, which are attached to the deoxyriboses,project in towards the axis of the helix. The two polynucleotide strands woundaround each other. The strands of a DNA double helix are anti-parallel whichmeans that one chain runs 5'-3' and the other runs 3'- 5'.

The joining of the two strands is by the hydrogen bonds between the nucleotidebases of the two strands. The chemical bind occurs between the adenine and

thymine (2 hydrogen bonds) and between cytosine and guanine(3 hydrogen bonds). The two strands are complementary to each other. Thismeans that the strand running in the 5' to 3' direction will have base Adeninethat will pair with the base Thymine on the opposite strand running in the 3' - 5'direction. Thus the strands are complementary and anti-parallel to each ofthem.

The DNA Double Helix makes a complete turn in over 10 nucleotide pairs, so

each turn takes 34.4 . About 25 hydrogen bonds are created in this completeturn. The power of these 25 bonds is equal to 1 covalent bond (bond between

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carbon and oxygen). The diameter of a DNA Double Helix is 20 . The hydrogenbonds and the bonds between deoxyribose and phosphates are the main (but

not the only) chemical forces that create a DNA double helix.

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7. CANCERGrowth for the sake of growth is the ideology of cancer cells.

Introduction

Cell division results in growth and repair. Sometimes, cells of a particular type dividerapidly, abnormally and uncontrolled at the cost of other cells. This leads to cancer.These cells are called the neo-plastic cells. The resulting growth is called the neo-plasticgrowth or tumour. The tumours may be benign or malignant.

Benign Tumour

When a tumour is formed as a result of abnormal and continuous cell division restrictedto the site of origin, it is termed benign. These tumour cells will be well-differentiated.

Malignant Tumours

The tumour cells may be carried by the blood or lymphatic circulation to other parts.

They may also directly penetrate other parts. There, they may induce secondary ormetastatic tumours. These tumours are termed malignant. They cause death. Theycontain undifferentiated cells. Their nuclei and nucleoli are large.

Types of Cancer

Cancer is actually a bundle of diseases. There are many types. They grouped into four carcinomas, sarcomas, lymphomas and leukaemia.

Carcinomas

The cells of these tumours are formed by the epithelial cells of the ectoderm orendoderm. The tumours will be solid.

Examples: Nerve tissues (brain), tissues of body surface (skin), glands of body surface(breast), and cervical, etc. carcinomas.

Sarcomas

These solid tumours grow on the mesodermal connective, cartilage, bone and muscletissues.

Lymphomas

These tumours are caused by the excessive production of lymphocytes by the lymphnodes and spleen.

Example: Hodgkins disease.

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Leukemias

Leucocytes or WBCs neo-plastically produce these tumours.

Mixed Malignant Tumours

These are tumours formed jointly by both the ectodermal and mesodermal tissues.

Cancer Cells Characteristics

All differentiated cells can become neo-plastic or cancerous. The inability to control itsrate of division and becoming a tumour is called transformation. They retain the basic

characteristics of their types. In addition, they have the following special characteristics.Immortalization

The transformed cells can survive permanently. They are immortal. But, normal cellscannot.

Loss of Contact Inhibition

The plasma membranes of normal cells may contact in a culture. They stop movementand growth. This is called contact inhibition. But, the transformed cells do not have thisproperty. They grow continuously. The membranes are less adhesive. Even on contact,they dissociate easily. This helps them to penetrate into other organs.

Reduced Attachment of Cells

Normal cells tend to attach with each other easily. There is specificity in this. That is,kidney cells attach to only other kidney cells. But, the transformed cells do not attachwith each other easily. They attach with other types of cells too. This helps them toinvade other organs.

Ability to Invade

Transformed cells can invade into other tissues. This may be due to changes in plasmamembrane or release of protease enzymes.

Anchorage IndependentFor growth, normal cells need attachment to some substance. The transformed cells donot need them. This character is used to differentiate a normal and a transformed cell.

Serum Requirement Low

Compared to normal cells, the transformed cells can grow even in media containing lessserum. External substances like serum are needed to lower the levels of intracellular

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cyclic AMP. This, in turn, induces mitosis. As the transformed cells divide abnormally,they do not need serum.

Lectins Agglutinate Selectively

Lectins are proteins. In some animals, it is present. It binds to branched, surface-membrane receptors like oligosaccharides. This results in agglutination/clumping ofcells. This process is less in normal cells and more in transformed cells.

Molecular Changes in Cell Membrane Components

Differences between the surface cell membranes ofNormal Cells Transformed Cells

Four types of Phospolipids Four types of Phospolipids

Form lipid bilayer Form lipid bilayer

Glycolipids and glycoproteins inserted Glycolipids and glycoproteins inserted

4 types of gangliosides GM1a, GM1,GM2 and GM3

Only GM3

Some major surface glycoproteinsdisappear

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Cytoskeleton Dismantling

The microtubules and microfilaments form the cytoskeleton in normal cells. They arearranged in a regular fashion. This regulates cell movement. In transformed cells, theyare less and thinner. This is due to polymerization. This results in the movement ofmembrane proteins. Blebs, micro-villi and ruffles are also formed.

Membrane Surface Negative Charge Increases

Evident from higher anodic mobility.

Increased Sugar TransportDue to abnormal growth and multiplication, the tumour cells take in more sugar. This isseen transported across the membrane.

Immunological Surveillance

Some antigens present in the plasma membranes of cancer cells are not seen in normalcells. Example: EB Nuclear Antigen. They immunize against themselves. This is called

immunological surveillance. Under favourable conditions, this happens. Only when thisfails, tumours are formed.

Loss of Electrical Communication

The electrical connections between normal cells is absent in cancer cells.

Increase in the secretion of Proteolytic Enzymes

All cancer cells secrete large amounts of proteolytic enzymes.

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Aldolases

There are three forms of aldolases Isozymes A, B and C in mammalian tissues. Incancer cells, B is replaced by A.

Increase in Glycolysis

In cancer cells, lactic acid is produced more. This is due to glycolysis or anaerobicrespiration. Glucose intake increases.

Hypotheses for the occurrence of Cancer

There are three.

1. The Somatic Mutation Hypothesis

Somatic mutations in normal cells transform them into cancer cells.

Some genes are inactivated through the production of repressor proteins. Mutationsmay block the production of these repressor proteins and activate the genes. The genesmay be directly mutated also. These lead to alteration in the cells control mechanismand unregulated division or cancer. Role of chromosomal changes in malignanttransformation is inconclusive.

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2. The Viral Genes Hypothesis

There are evidences that viruses cause cancers in animals. The tumour-producing

viruses are called the oncoviruses.

Virus-Cancer Relationship, some difficulties:

Risk of injecting cancer-causing viruses into man

Viruses may be non-transmissible

Viruses may not have a separate identity

Virus genetic material may be incorporated into human chromosomes Confusion by the presence of harmless viruses.

Evidences:

Through electron microscopy and immunology

Epstein-Barr (EB) Virus Burketts Lymphoma

EB Virus Nasopharyngeal carcinoma Herpes simplex Type 2 Virus Cervical cancer

Retoviruses

Virus-like RNA and DNA particles

The wart virus only benign growth.

Incorporation of Viral DNA into the Host GenomeAll viruses do not normally grow inside all cells. Each has specificity. Such cells arecalled permissive cells. Cells in which viruses do not grow are the non-permissive cells.

Behaviour of Virus on entering host cells

A) Lytic Infection or Productive Infection: They multiply inside the host cells. Kill them.This is called the lytic phase. Example: Adenovirus.

B) Oncogene Formation: The viral DNA gets inserted and integrated into the host DNA.The virus is now termed a provirus. This causes cancer. The genes of a virus causingtransformation are called Oncogenes.

Carcinogens

Cancer-causing agents are called carcinogens. The carcinogens affect the normalgenetic processes. Due to this, the cells control mechanism is altered.

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For example, the ears of rabbit were painted with coal tar. It produced carcinomas inthe rabbit. The 3, 4 benzapyrene, present in the coal tar, was found to be responsible

for this. This shows that the carcinogen induced the carcinoma. Even when the tar wasremoved, the carcinomas grew. Its continued presence is not necessary for the growthof abnormality.

There are many types of carcinogens like Chemicals and Radiations.

Chemical Carcinogens

Mode of Action

Many chemical carcinogens chemically bind with DNA, RNA and proteins and inducecancer. Mostly, they are chemically transformed into derivatives. These are morecarcinogenic. Example: Nitrates are not mutagenic. But, nitrosamines are powerfulmutagens and carcinogens.

Chemical carcinogens directly transform cells to malignancy.

Carcinogens are mutagenic. The DNA sequence is altered. Carcinogens can alter gene expression also. This will lead to abnormal

differentiation. Thus, gene expression is altered.

Carcinogenic Induction of Cancer

Many mutational defects combine to cause cancer.

Carcinogens or their derivatives bind with DNA. Mutation takes place. Activities

of the genes are affected. Cancer results.

In some genes, some parts of a gene may code for a harmful effect. They mightbe suppressed. The carcinogens remove the suppression. The DNA partsactively code and produce cancer.

Oncogene Theory: All cells carry information for malignancy and cancer-causingviruses (the oncogenic viruses). Carcinogens induce these viruses. They cause

cancer directly or indirectly.

3. The Defective Immunity Hypothesis

Spontaneous mutations occur more. Cancers occur less. This indicates that 1) cancer-producing mutations are suppressed or 2) the newly produced cancer cells aredestroyed somehow. The second is more possible. This is called immunologicalsurveillance. This is a case of defective immunity.

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4) Other Theories

Holley, 1969 The surface membrane of cells is modified during early transformation.

Due to this, growth and division regulating nutrients (de-oxyglucose) increase inside thecell.

Comings, 1973 Cells have structural genes. They code for transforming factors.Regulator genes suppress them. On removal of suppression, cancer occurs.

Oncogenic Viruses

Viruses causing tumours are called oncoviruses. The following viruses cause it.

1. Papovaviruses

2. Parvoviruses

3. Adenoviruses

4. Herpes viruses

5. Poxviruses

All viruses do not normally grow inside all cells. Each has specificity. Such cells arecalled permissive cells. Cells in which viruses do not grow are the non-permissive cells.

Behaviour of Virus on entering host cells

A) Lytic Infection or Productive Infection: They multiply inside the host cells. Kill them.This is called the lytic phase. Example: Adenovirus.

B) Oncogene Formation: The viral DNA gets inserted and integrated into the host DNA.

The virus is now termed a provirus. This causes cancer. The genes of a virus causingtransformation are called Oncogenes.

Proteins

Early Proteins

Soon after infection, T antigen or tumour-antigen is detected in the cell nucleus.

Interferon inhibits the synthesis of T antigen. Interferon specifically inhibits viral proteinsynthesis and not cell protein synthesis. This shows that T antigen is coded by the viralDNA. Only after this, DNA replicates.

Late Proteins

The capsid proteins are the late proteins. Two of them are identified.

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Retrovirus

The RNA Tumour Viruses or Oncornaviruses contain RNA and cause cancer. They are

classified as retroviruses. Their RNA forms a DNA copy. This is a reverse method offorming RNA from DNA. So, they have properties of both RNA and DNA viruses. TheDNA genes are combined with the hosts chromosomal DNA. The virus particles containthe RNA genes (infectious).

Many retroviruses are found to cause different types of cancers in fish, reptiles, birdsand mammals. Infection in humans is not clear.

The infection of retroviruses is classified as productive and non-productive. In theformer, progenies are produced. In the latter, the progenies are not produced.

In some viruses, a part of the genome may be missing. They are the defective viruses. Acell may contain one virus. It may not affect the cell. Another virus may also infect it.This is called super-infection. Due to this, the first virus causes cancer. This second virusis called the helper virus.

Transmission of Oncornaviruses

Horizontal Vertical

One host to another

through contagion.

Parent to offspring.

Congenital Genetic

From mother to offspringthroughovum/placenta/milk.

Through father

or mother.

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8. MENDELIAN TRAITS IN MANThe most striking attribute of a living cell is its ability to transmit hereditary propertiesfrom one cell generation to another. Hereditary transmission through the union ofsperm and egg was understood by 1860, and the chromosomes present in the nucleuswere considered as the active factors. The establishment of haploid (n) number ofchromosomes in sperm and egg, during meiosis is significant. This is necessary forkeeping the chromosome number constant as diploid (2n) in the daughter generations.These merely suggest that chromosomes carry the hereditary information.

GREGOR MENDEL (1822 l884)

J. Gregor Mendel did his experiments during 1856 -1864 in garden peas, Pisum sativum. Mendelsfather had a great love for plants and henceinfluenced his son, as they worked together ingarden and orchard. Later Mendel was intenselyinterested, in plant hybridization. Mendelsexperiments, with pea plants were elegant. Hisinferences constituted the foundation for themodern science of genetics. Mendel conducted hisexperiments systematically and drew the

conclusions for the data collected. Mendelpresented them in a paper entitled Experiments inPlant hybridization and read before the BrunnNatural History Society in 1865, and published in the

proceedings of BNH Society. His paper was considered as classic in biology for itselegance and simplicity.

MENDELS CHOICE OF MATERIALMendel was successful because he

used peas, Pisum sativum, an annual plant which were easily grown andproduced successive generations rapidly.

selected easily observable 7 characters.

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t i tl t ll d th f tili ti Th h f t fl i


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strictly controlled the fertilization process. They have perfect flowers possessingboth male and female parts performing self-pollination. So, cross-pollination

does not occur ordinarily without the investigators intervention. used mathematics rigorously to analyze his results and to derive good judgment.

used large numbers of plants

studied traits that had two easily identified factors.

Mendel examined the following seven characters found in peas:

Flower colour, purple or white Flower position, axial or terminal

Seed colour, yellow or green

Seed shape, round/spherical or wrinkled

Pod shape, inflated or constricted

Pod colour, green or yellow

Stem height, tall or short

MENDELS METHOD OF CROSSING

Mendel needed to control fertilization. Self-fertilization was ensured by placing a bagover the flowers to make sure pollen from the stamens lands on the carpel of the sameflower. Cutting off stamens from a flower before pollen was produced, then dusting thecarpel of the flower with pollen from another plant ensured cross-fertilization. To

ensure reliability, Mendel used thousands of plants in each experiment. Mendel worked

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with true breeding plants: self fertilized plants which produced all offspring identical to


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with true-breeding plants: self-fertilized plants, which produced all offspring identical tothe parents.

Mendel first cross-fertilized two true-breeding plants for one characteristic. For exampletall plants were crossed with short plants (Mendel called these plants a P1 parentgeneration). The offspring produced are called F1 (1st filial) generation. The F1generation were then self-fertilized or cross-fertilized to produce a second generation,F2

TERMINOLOGIES

Alleles:The paired factors, held for a single character, are termed as alleles or allelomorphs.Allele is an alternative form of a gene. Usually there are two alleles for every gene.Sometimes as many as 3 or 4 [Multiple alleles for a single character].

Example: For the character height in pea plant, the tall plants may have either TT or Tt.

Homozygous:

When the alleles for a particular type of character are same (or) alike, it is termed ashomozygous for that character.

Example: Tall - (TT) and dwarf - (tt) types for character height in pea plant.

Heterozygous:

Here the alleles for a particular type of character are not same or alike. In such cases the

dominant allele is expressed.Example: Tall-(Tt)

Homologous:

Chromosomes with same genetic loci (or) having similar gene sequences are calledhomologous.

Hybrid:An individual / offspring resulting from a cross between two different contrastinggenomes is termed as hybrid. The heterozygous character is also said to be hybrid.

Phenotypes:

Any measurable (or) visible character of an individual are termed as phenotype. This isalways expressed in words.

Example: Height- Tall/Dwarf & Cotyledon colour - Yellow/ Green

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Genotypes:


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Genotypes:

The allelic composition of an individual to distinguish from the physical appearance is

termed as genotype. It is always expressed in alphabets / letters.Example: Tall - TT/Tt, Yellow - YY/Yy, Dwarf tt, Green yy

Normally, the alphabet / letter used will be the first letter of the dominant type of thecharacter. The capital alphabet / letter for the dominant type and the correspondingsmall letter for the recessive type should be used.

Example: Tall (T), Dwarf(t).

Dominant

A term applied to the trait (allele) that is expressed regardless of the second allele.

Recessive

A term applied to a trait that is only expressed when the second allele is the same (e.g.short plants are homozygous for the recessive allele).

Pureline:

An individual homozygous for a particular character is termed as pure line or truebreeding variety.

CONCEPTS OF MENDEL

The interpretation of Mendels experiments gave the concept of segregation andindependent assortment of alleles. By segregation, the separation of alleles resulting inpure gametes is described. The independent assortment explains the independentcombinations of different pairs of alleles during gamete formation. These concepts arethe basic foundation of Mendelian heredity.

PRINCIPLE OF SEGREGATION & MONOHYBRID CROSS

Mendel did his experiments by planting two varieties of Peas, which have two definite

alternate / contrasting traits for a character. He considered the character, height withone tall type /trait and other with dwarf (short) type. When they were selfed to severalgenerations, tall never turned into short and short never turned into tall traits. Thus theconsistency of the traits was studied. Mendel called these two traits as true or pure line.

Mendel crossed the true tall and dwarf varieties of garden peas. In the first (F1)generation, [F Symbolized filial from the Latin, meaning progeny] all plants were tall.When these F1 tall plants were self pollinated, the missing trait, dwarf re-appeared in

the F2 generation. In this experiment, among the total of 1064 F2 plants, 787 were tall

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and 277 were dwarf varieties, giving the ratio as 3:1. This type of cross between parents


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and 277 were dwarf varieties, giving the ratio as 3:1. This type of cross between parentsdiffering in only one trait or in which only one trait is considered is termed as Mono -

hybrid cross.Mendel called the trait appeared in the Fl as dominant and its alternative, which wasmasked, as recessive.

LAW OF DOMINANCE

In a cross between organisms, which are pure for contrasting traits of a particularcharacter, only one of the traits appears in the Fl generation. The other trait is masked

or disappeared. The appeared trait is said to be dominant, over the other trait. Thedisappeared trait is known as recessive.

The monohybrid cross shall be symbolized as above to explain the segregation of alleles.Alleles control the expression of traits of a character. Mendel assumed that everycharacter is determined by some factor. It is transmitted from parents to the offspringthrough the gametes. Mendels results of monohybrid crosses accounted the separation

of pairs of alleles resulting in the purity of gametes. Thus the concept of segregation ispresented as Mendels first principle.

LAW OF SEGREGATION

Mendels first law or law of segregation, states that the longitudinal separation of themembers of the pair of homologous chromosomes leads to the complete separation (or)segregation of their allelic genes.

Since every individual is the result of the union of two (opposite) gametes, eachcharacter must necessarily be represented by two factors. In modern days, the factorsare called as genes. Thus, the pure line tall plant and dwarf plants must have two genesfor tallness (TT), and dwarf ness (tt) respectively. Segregation, the separation of thepairs of genes, occurs in the formation of gametes. Each gamete produced by tall plantcarries only one T gene, likewise t gene in the gamete of dwarf plant. The union of thesegametes resulted in Tt. Because of the presence of T gene in F1, all plants were tall.While the F1 plants were producing the gametes, half of the gametes carried T gene andthe other half the t gene. The result of selfing the F1 indicated to Mendel that the geneswere entirely separated from each other during gamete formation.

The principle of segregation was further demonstrated, when the F1 plants werecrossed back with the recessive trait. The results showed the separation of paired genes,by expressing both tall and dwarf in equal proportions among progeny.

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PRINCIPLES OF INDEPENDENT ASSORTMENT AND DIHYBRID CROSS

Mendel crossed a variety of pea plants having round seeds and yellow cotyledons withwrinkled seeds and green cotyledons plants. From the previous monogenic studies,genes for both round and yellow were known to be dominant. Genes for wrinkled andgreen seeds were found to be recessive. Hence, as expected, in the cross between pure

line round yellow with wrinkled green, all the Fl progenies were round yellow. The self-

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pollination of F1 seeds produced 4 phenotypes in a definite ratio. From a total of 556 F2


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seeds, 3l5 round yellow, 108 round green, 10l.wrinkled yellow and 32 wrinkled green

were observed. These were found to fit very nearly a ratio of 9:3:3:1.From the observed results, Mendel inferred the members of pairs of alleles segregate,and also one pair behaved independently with respect to another pair. Thus the conceptof independent assortment is explained as Mendels second principle.

LAW OF INDEPENDENT ASSORTMENT

Mendels second law or law of independent assortment states that when a dihybrid

parent produces reproductive cells, the two pairs of alleles (genes) controlling twodifferent characters must have separated or assorted independently of each other.

Mendels dihybrids cross between plants with round - yellow seeds and those withwrinkled- green seeds shall be represented diagrammatically using the generalizedMendelian pattern. The pure line Round Yellow parent shall produce only one type ofgamete (RY), carrying one member from each pair of alleles. Similarly, the wrinkled

green parent shall produce one type of gamete (ry), having one member from each pairof alleles. On fusion of these gametes, the Fl plant will have, RrYy, exhibitingheterozygosity for both pairs of alleles. The self-pollination of the Fl plant produces fourtypes of gametes, following the independent assortment of pairs of alleles controllingtwo different characters. The random fusion of gametes leads to 16 progenies having 4phenotypes and 9 genotypes, which was shown in the Punnet checkerboard.

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By using forked line method also, the F2 ratio can be drawn;


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BACK CROSS AND TEST CROSS

A cross between F1 organism with either of its parents is termed as back cross. It isrepresented as follows;

F1 X Homozygous dominant parent

[OR]

F1 X Homozygous recessive parent

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But a cross between FI hybrids (or) an organism of unknown genotype with thei i i d Th h l d i h


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recessive organism is termed as test cross. The tests cross helps to determine the

genotype of the unknown organism. If the F1 organism (or) unknown genotype iscrossed with the recessive parent, at least one recessive trait is produced in a series ofoffsprings. But in the cross between unknown genotype and the homozygous dominanttrait, it cannot be possible to know the genotype of the organism in question. Intestcrosses, the phenotypic & genotypic ratio will always be equal i.e., 1:1.

Example:

Diagram Showing Test Crosses Involving Monohybrid And Dihybrid Parents

The principle of segregation is also demonstrated well in testcrosses. The result showsthe separation of paired alleles, by expressing all the parental traits in equal proportionsamong the progeny. Test cross, is also one of the types of backcrosses, and hence alltestcrosses are backcrosses, but all backcrosses are not testcrosses.

MENDELIAN TRAITS IN MAN

S.No. CHARACTER DOMINANT TRAIT RECESSIVE TRAIT

1. Hair colour Dark Blended / Light

2. Hair type Curly Straight

3. Body hair Intensive Sparse

4. Skin colour Normal / Colour Albinism / White

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5. Eye colour Dark Light / Blue


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6. Eye lashes Long Short

7. Vision Near sighted Normal

8. Vision Long sighted Normal

9. Vision Night blindness Normal

10. Ear lobes Free Attached

11. Hearing & speech Normal Deaf - Mutism

12. Lips Thick Thin13. Nostrils Wide Narrow

14. Stature Short Tall

15. Fingers and toes Supernumerary Normal

16. Blood groups A,B,AB O

17. Rh factor Positive Negative18. Blood clotting Normal Abnormal

19. Handedness Right Left

20. Clasping of hands Right over left Left over right

21. Teeth enamel Normal Brown

22. Tongue rolling Able Unable

23. Tongue folding Able Unable

24. Chin Normal Receding

25. Dimple Present Absent

26. Blood clotting [Sex linked] Normal Hemophilic

27. Vision [Sex linked] Normal Colour blindness

28. Hair [Sex linked] Normal Baldness

Pedigree Analysis

A pedigree is a diagram of family relationships that uses symbols to represent peopleand lines to represent genetic relationships. These diagrams make it easier to visualizerelationships within families, particularly large extended families. Pedigrees are often

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used to determine the mode of inheritance (dominant, recessive, etc.) of geneticdiseases A sample pedigree is below


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diseases. A sample pedigree is below.

In a pedigree, squares represent males and circles represent females. Horizontal linesconnecting a male and female represent mating. Vertical lines extending downwardfrom a couple represent their children. Subsequent generations are therefore written

underneath the parental generations and the oldest individuals are found at the top ofthe pedigree.

If the purpose of a pedigree is to analyze the pattern of inheritance of a particular trait,it is customary to shade in the symbol of all individuals that possess this trait.

In the pedigree above, the grandparents had two children, a son and a daughter. Theson had the trait in question. One of his four children also had the trait.

Background

A pedigree is a family tree showing a line of descent. It can be used to trace theoccurrence of inherited traits in parents and offspring through a number of generations.

By convention, circles represent females and squares, males. A line between a squareand a circle represents a union and a line down indicates offspring from the union. Filledin symbols represent individuals displaying the phenotype being studied. For example:

In pattern 1, the son and father are both affected. This is a reasonable indication that

the characteristic is dominant. An affected offspring must have at least one affected

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parent if the phenotype is dominant. Other features of pedigrees of a dominant traitare:


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are:

Heterozygous individuals will be affectedTwo affected parents can produce an unaffected child (both parents would beheterozygous)

In pattern 2, the daughter is affected but neither parent is. This can only happen if thecharacteristic is recessive and the offspring are homozygous, e.g. bb. Both parent mustbe heterozygous, Bb. Other features of pedigrees of a recessive trait are:

Heterozygous parents will be unaffected.

Two affected parents will always have an affected child.

Pedigrees are valuable tools in genetic counseling. It allows a pattern of inheritance tobe traced throughout generations of a family. This can allow identification of the geneticdisease and advice can be made available on the probability of a couple having anaffected child. Cystic fibrosis is an example of a recessive genetic disease. Huntington'schorea is an example of a dominant genetic disease.

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9. SYNDROMES


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Aneuploidy

Aneuploidy is an abnormal chromosomal condition. This is occurred due to non-disjunction in autosomes or allosomes. This may either result in loss or gain ofchromosomes in the gamete. Due to this, instead of 2N condition in the zygote, either2n + 1 or 2n - 1 condition is formed. This addition (gain) 0r deletion (loss) ofchromosome is called aneuploidy. 2n + 1 condition is referred as trisomy orhyperaneuploidy, by addition of chromosome(s). 2n - 1 condition is referred as

monosomy or hypoaneuploidy, by loss of chromosome(s).

Trisomy [2n + 1]

At fertilization the egg (23 chromosomes) and the sperm (23 chromosomes) fuse tocreate a conception, or zygote, which has 46 chromosomes. If a sperm or egg carries anextra copy of one of the chromosomes, due to non-disjunction at meiosis I or meiosis II,there will be a total of 24 chromosomes instead of 23 in the reproductive cell. If thissperm or egg is fertilized by a normal sperm or egg the result will be a total of 47

chromosomes instead of 46.

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Monosomy [2n 1]

If one of the gametes has lost any one chromosome, and is fertilized by a normalgamete the result is monosomy, for the chromosome involved. Monosomy is adeficiency in number of chromosomes and is defined as only one copy of a chromosomethat is normally present in two copies. These eggs and sperm, which contain one lesschromosome, have 22 chromosomes. When fertilized, the outcome is 45 chromosomesin total. In general, monosomies are less likely to survive when compared to trisomies.

Syndromes

Syndromes are chromosomal disorders. This is formed due to non-dis-junction either inautosomes or allosomes. This is causing genetical disorders. If non-dis-junction occurs inthe autosomes, it brings autosomal syndromes. Otherwise if non-dis-junction occurs inthe allosomes, it brings allosomal or sex chromosomal syndromes. Hence the syndromesare classified into as (i) allosomal or sex chromosomal syndromes and (ii). autosomalchromosomal syndromes.

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Klinefelters syndrome [2n + 1] 47 XXY

Klinefelters syndrome has [2n + 1] condition with 47 XXY chromosomes. This has


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y

occurred due to ND in XX pair of chromosomes. So, one of the gametes is having 22A +XX chromosomes. On fertilization by a normal gamete having 22A+Y chromosomes, thezygote will be receiving 22AA+XXY condition. XXY occurs in approximately 1 out of 1,000live male births, but many men with it do not develop KS. When KS does develop, itusually goes undetected until puberty or sometimes much later.

Symptoms may include:

For babies:

Smaller birth weight and slower muscle and motor development

For children and adults:

Tallness with extra long arms and legs

Abnormal body proportions (long legs, short trunk)

Enlarged breasts (common) Lack of facial and body hair

Small firm testes, small penis

Lack of ability to produce sperm (common)

Diminished sex drive, sexual dysfunction

Social and learning disabilities (common) Personality impairment

Normal to borderline IQ

Speech and language problemsChildren with KS often learn to speak laterthan other children. They may have a difficult time reading and writing.

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Karyotypes of Klinefelters Syndrome and Turners Syndrome


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Turners Syndrome [2n - 1] 47 X

Turner Syndrome is a chromosomal condition first described by Dr. Henry Turner in the1930s. Turners Syndrome has [2n - 1] condition with 47 X chromosomes. This hasoccurred due to ND in XX pair of chromosomes. So, one of the gametes is having 22A + 0chromosomes. On fertilzation by a normal gamete having 22A+X chromosomes, thezygote will be receiving 22AA+X condition. It affects only females, and is characterizedby short stature and the lack of normal sexual development at puberty. Turner

Syndrome (TS) affects one in 2,500 live female births and may account for up to 10% ofall miscarriages.

Signs and Symptoms

A webbed neck (extra folds of skin around the neck)

Facial features such as a narrow, high-arched palate, receding lower jaw; slightlydrooping eyes, low-set ears, or low hairline

Scoliosis (curvature of the spine)

Strabismus (lazy eye)

Elbows that turn slightly out to the side

Short fourth metacarpals (the ends of the bones that form the knuckles),

Flat feet, or small, narrow fingernails and toenails that turn up

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Female infants with this condition are shorter than average at birth, and mayhave fluid around the neck (cystic hydromel) and swelling of the hands and


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feet. A broad chest with widely spaced nipples, or specific heart abnormalitiesmay indicate the presence of Turner Syndrome at birth.

The intelligence is usually not affected. Sometimes the girl child with TS mayhave difficulty with spatial-temporal processing (imaging objects in relation toone another), non- verbal memory and attention. This may cause problemswith their sense of direction, manual dexterity, and social skills. They maystruggle with maths; however, their verbal and reading skills are often quiteexcellent.

The girl may grow at a normal rate until about three years old, then growthslows. She will not have the expected growth spurt at puberty. The ovaries willnot produce estrogen and develop eggs. The girl will not begin her menstrualperiods or develop breasts.

Females with Turner Syndrome often exhibit other physical characteristicswhich are associated with TS, and which may vary in severity betweenindividuals.

Downs Syndrome [2n + 1] 47 [21st trisomy]

Downs syndrome has [2n + 1] condition with 47 [21st trisomy] chromosomes. This hasoccurred due to ND in 21st pair of chromosomes. So, one of the gametes is having 23A+ X chromosomes. On fertilzation by a normal gamete having 22A+Y (or)

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22A+X chromosomes, the zygote will be receiving 47 chromosomes, with an additionalchromosome for the 21st pair.

Downs syndrome Symptoms

Hypotonia(low muscle tone)

Palmer Crease

Nasal Congestion

Respiratory Infections-bronchitis, pneumonia

Gastroesophageal Reflux

Hearing Problems

Hearing Loss

Mental Retardation

Gum disease

Teeth Problems

Leukemia

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Weight Problems obesity

Vision Problems Crossed eyes, Astigmatism, Nearsightedness, Fairsigntedness


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Physical Feautres

Broader nose, Flattened nasal bridge

Upward slanting eyes, small folds of skin at inner corners

Small mouth, shallow roof of mouth (makes the tongue look larger whenactually its typical size)

Teeth Late eruption, misshapen, missing out of place

Small ears with the top folded over, small ear passages

Smaller than normal head (not usually noticeable unless measured)

Back of the head flattened Soft spots on head bigger-take longer to close

Hands smaller, fingers shorter

Feet Smaller

Sandal Gap between big toe and rest of toes

Hair that is soft and thin

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Sensitive skin, can be mottled and fair

Shape of chest can be funnel shaped or pigeon shaped


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Most kids have only a few Down syndrome symptoms. And symptoms will vary fromchild to child.

congenital heart defect

mild to moderate low muscle tone

sandal gap

one ear is "primitive" (doctor's words) sensitive skin, mottled at times (but she is fair-skinned anyway)

upward-slanting eyes

flattened bridge of nose and face (this makes her look like a doll. Adorable!)

some teeth are slightly pointed

slightly developmentally delayed.

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10. BLOOD GROUPS, ANTIGEN-ANTIBODY REACTIONBlood


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Blood is a specialized biological fluid. It is circulating around the body through the bloodvessels. It is considered as a connective tissue

Components of Human Blood

1. Plasma =55%

2. Blood Cells =45%

3. Blood Platelets = 0.1%

Blood group

The different blood types A, B, AB and O provides the best example for the existence ofseries of multiple alleles in Man. The observation of agglutination or clumping of the redblood corpuscles by K Landsteiner (1900-1901) lead to the findings of different bloodgroups. Landsteiner found A, B and O blood types. Later, his students Von Decastelo

and Sturli (1902) dicovered AB blood type.

ANTIGEN ANTIBODY REACTION

Antigen or agglutinogen [a specific protein] present in the red blood cells was found toreact with anti-body or agglutinin [another specific protein] leading to agglutination orclumping of red blood cells. Two antigens A and B with corresponding two antibodies,anti-A and anti-B were found. Agglutination observed when the same type of antigen &

antibody were allowed to react. It was observed no individuals blood had the sametype of antigen and anti body. Some people were found to have A antigen, some hadB antigen and some had both A and B antigens and some had neither A nor B antigens.Based on the type of antigen in the RBC, the blood has been typed as A, B, AB and Orespectively. It was also found that A type blood carried B antibody, B type carried Aantibody, AB type with no antibodies and O type carried both A and B antibodies. Forthis reason, the clumping of corpuscles and ill effects were reported in the non- relatedblood type transfusion.

Agglutination

Agglutination means clumping together of blood cells in the presence of an antibody. Itis an allergic reaction to prevent the foreign materials from the environment or fromothers passing inside the individuals

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Rh System

The Rh system was discovered by Landsteiner and A.S.Wiener (1940) from rabbitsimmunized with the blood of a monkey, rhesus. The symbol Rh is after the speciesname of the monkey. Initially the genetic mechanism of Rh System was seemed to besimple with a single pair of genes R and r, to designate Rh positive (RR/Rr) negative (rr).Further studies of Niener and R.A.Fisher e+ al revealed the existence of series ofmultiple allies, viz R1, R2,R0, Rz,r,r and ry.

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In general, Rh-positive gene R is dominant over the Rh-negative gene r. These allelescontrol the antigen - agglutinogen characteristics of RBCs. The main difference of ABO

system with Rh system is the existence natural antibodies A and B in the formerh h h l b d h l h h f


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system with Rh system is the existence natural antibodies A and B in the former,whereas there is no such natural antibody in the later. The Rh positive transfusion canimmunize the Rh negative to produce positive antibodies(Rho), but not seen in viceversa transfusion

Rh incompatibility

Blood transfusion generally occurs between mother and child during pregnancy or

during parturition. An Rh positive(R-) father and an Rh negative (rr) mother may have anRh positive child. Always blood transfusion is possible between this mother and child,and hence mother is possibly immunized to produce Rh posilive(Rho) antibodies. Thefirst child of this parent, with this genetic background, is nearly always normal.Ordinarily, atleast one pregnancy (in some cases several) is needed to immunize (or)sensitize the mother. Thus the subsequent child of this parent would be still born andshowed symptoms of Rh incompatibility. This is clinically termed as Hemolytic disease of

New born (or) Erythroblastosis foetalis. The symptoms with anaemia, janudice and fluidaccumulation in the tissues of the embryo, leading to still born babies (or) infant deathis termed as Rh incompatibility. The genetic combination of Rh positive father and Rhnegative mother only associate with this ilI effects of Rh incompatibility.

To prevent this, the mother is injected with anti-Rh antibodies around the seventhmonth of pregnancy and again just after delivering an Rh-positive baby.

Sometime the parents (Rh positive male & Rh negative female) with only one or twochildren may never know of their Rh incompatibility, because of the requirement ofhigher dose Rh positive blood to immunize the Rh negative mother.

In general before blood transfusion, the blood grouping under ABO system is normallydone. But it is also necessary to know the Rh type of the blood for transfusion. Rhnegative individuals must always be transfused with Rh negative blood to avoidimmunization and subsequent reactions. If the Rh negative individual is wronglytransfused with Rh positive blood, the Rh positive (Rho) antibodies will be produced inthe Rh negative individual. Again if that individual is transfused with Rh positive blood,then agglutination is occured by Rho antibodies resulting in the death of the recipient.Hence, the Rh negative individual must not be given blood transfusion with Rh positivetype, to avoid the Rh incompatibility at a later stage.

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11. Blood coagulationCoagulation is a complex process to form a blood clot. It is an important part of

hemostasis [Hemostasis means the stoppage of blood loss from a damaged vessel] The
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hemostasis.[Hemostasis means the stoppage of blood loss from a damaged vessel]. Thedamagedblood vesselwall is covered by aplateletand fibrin-containing clot to stop bleeding.This helps to begin repair of the damaged vessel. Any disorder in coagulation may leadto an increased risk of bleeding (hemorrhage) or clotting (thrombosis).

Coagulation begins almost instantly after an injury to the blood vessel. This injury willdamage theendothelium(lining of the blood vessel). This releases aphospholipidcomponent

called tissue factorand fibrinogenthat initiate a chain reaction.Platelets immediately form aplug at the site of injury; this is called primary hemostasis. Secondary hemostasis occurssimultaneously: Proteins in theblood plasma, called coagulation factors or clotting factors,respond through a complex process to form fibrinstrands. This strengthens the plateletplug the clot.

Mechanism of Blood coagulation

The blood clotting system or coagulation is a proteolytic process. Each enzyme of thepathway is present in the plasma as a zymogen [ an inactive form ]. On activation, itundergoes proteolytic cleavage to release an active factor from the precursor molecule.The coagulation pathway has positive and negative feedback action to control theactivation process. The ultimate goal of the pathway is to produce thrombin. Thrombinconverts soluble fibrinogen into fibrin. Fibrin forms a clot. The generation of thrombin

can be divided into three phases,1. intrinsic pathways [ Contact activation pathway ],

2. extrinsic pathways [ Tissue activation pathway ],Both provide alternativeways for the generation of factor X, and.

3. the final common pathway, results in thrombin formation.

Intrinsic pathway

The intrinsic pathway is activated when the blood comes into contact with thesurface that are exposed as a result of tissue damage.

This pathway is slower to form fibrin.

The Hageman factor (factor XII), factor XI, prekallikrein, and high molecularweight kininogen (HMWK) are involved in this pathway of activation.

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The first step is the binding of Hageman factor to the surface exposed by aninjury. Thus the factor XII is activated.

A complex of prekallikrein and HMWK also interacts with the exposed surface
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A complex of prekallikrein and HMWK also interacts with the exposed surfacealong with the factor XII. During activation, the single chain protein of the nativeHageman factor is cleaved into two chains.

The light chain contains the active site.

This molecule is referred to as activated Hageman factor (factor XIIa).

Activated Hageman factor in turn activates prekallikrein.

The kallikrein cleaves further the factor XII. This triggers the clottingmechanism.

The activated factor XII activates factor XI.

This next step requires Ca++ to proceed efficiently.

The HMWK [high molecular weight kininogen] also binds to factor XI to facilitatethe activation process.

Eventually the intrinsic pathway activates factor X.

Factor X is the first molecule of the common pathway and is activated by acomplex of molecules containing activated factor IX, factor VIII, calcium, andphospholipid. This is provided by the platelet surface.

The precise role of factor VIII in this reaction is not clearly understood. Anyhow,

its presence in the complex is obviously essential.Extrinsic pathway

The extrinsic pathway is an alternative way for the activation of the clottingprocess. The main role of this pathway is to generate a "thrombin burst".

It provides a very rapid response to tissue injury.

It generates the activated factor X almost instantaneously, compared to theintrinsic pathway to activate factor X.

The main function of the extrinsic pathway is to increaset the activity of theintrinsic pathway.

There are two components unique to the extrinsic pathway, tissue factor orfactor III, and factor VII.

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Tissue factor is present in most human cells bound to the cell membrane. Theactivation process for tissue factor is not entirely clear.

On activation, the tissue factor binds rapidly to factor VII. Then factor VII isi d


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On activation, the tissue factor binds rapidly to factor VII. Then factor VII isactivated.

This form a complex of tissue factor, activated factor VII, calcium, and aphospholipid.

This complex then rapidly activat

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