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CONTENTS:~

DEFINITION INTRODUCTION CELL MEMBRANE

LIPIDSPROTEINSCARBOHYDRATESINTERCELLULAR JUNCTIONS

CELL-MEMBRANE~CYTOPLASMIC INTERACTION ENDOPLASMIC RETICULUM

RIBOSOMES MITOCHONDRIA GOLGI COMPLEX LYSOSOMES

PEROXISOMES

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CYTOSKELETONMICROFILAMENTS

INTERMEDIATE FILAMENTS

MICROTUBULES CENTRIOLES

PROJECTION FROM THE CELL SURFACE

CILIA

FLAGELLAMICROVILLI AND BASOLATERAL FOLDS

NUCLEUS

MITOSIS

MEIOSIS

CONCLUSION

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Cell is the smallest structural and functionalunit which forms the building blocks oflife.

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INTRODUCTION

Robert Hook (1665) in his book named

MICROGRAPHIA mentioned about structureresembling honey comb.

He called it CELL derived from latin word

Cellula meaning small room.

German scientists M.Schleinden andT.Schwann (1838-39) proposed CELLTHEORY~

i) All organisms were made up of cell.ii) the cell was the basic unit of structure andfunction of all oranisms.

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HONEYCOMB SHAPED CORK CELL

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Cellular Organisation:~

i)Prokaryotes-without delimited nucleus. e.g:bacteria,blue-green algae.

ii)Eukaryotes- nucleus delimited with membrane.e.g:Humans, mammals,plants etc.

Humans are multicellular with 1014 cells.

A typical human cell size is 10microns, typical mass

1nanogram. The largest cell are about 135microns are in the

anterior horn in the spinal cord.

Smallest are the granular cell in the cerebellum.

Longest cell can reach from the toe to the lower brainstem (pseudounipolar cell).

The largest known cell are unfertilized ostrich egg.

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The components of the cell can bediscussed under 3 headings~i) Cell membrane

ii)Cytoplasmiii)nucleus

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CELL MEMBRANE / PLASMA MEMBRANE /UNIT MEMBRANE~

It is the outer boundary of the cell. Function~

i)Regulates the internal environment of the cell.ii)Determines the immunological makeup of thecell and tissues.iii)Cholestrol molecules add structure rigidity to thecell.

iv)The number,makeup,position and mobility of theprotein molecules account for specific individualproperties of cells and tissues by forming specificreceptor molecules.

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Under light microscope mammalian cellstructures cannot be seen.

Under high magnification electron microscope

cell membrane appear as trillaminar structuremeasuring 75Ao in width (25Ao each).

Davson-Danielli concept of semipermiablenuclear membrane has been replaced by

FLUID-MOSAIC MEMBRANE theory 1stproposed by Singer.

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Cell membrane is madeup oflipids,proteins snd carbohydrates.

LIPIDS~

Trilamminar structure of membrane is

produced by the arrangement of lipidmolecules(phospholipids).

Each phohospholipid molecule have~i)polar head end,hydrophilic and stains dark.ii)two thin nonpolar tails,hydrophobic and

stains light.

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Types of phospholipids arephosphotidylcholine,phosphotidylserine,phosphotidylethanolamine and

sphingomyelin.

Cholestrol provides stability to themembrane.

Glycolipid are present on the outersurface of the membrane.

Galactocerebroside which is an

important constituent in myelin sheath.

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PROTEIN

Forms 50%massof the membrane.

Functions~ i)

Forms cell adhesion molecules. ii)Aspumps actively transporting ions across themembrane. iii)As carriertransporting substance by facilitated diffusion.

iv)As receptors for neurotransmitters andharmones. v)Asenzymes

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The protein which are embedded and passthrough the membrane are called IntegralProtein.

Proteins attatched to the surface of themembrane are called peripheral proteins.

Hydrophobic portion of protein located in theinterior of the membrane.

Hydrophilic portion located on the surface. Commonly proteins are attatched by

glysylphosphatidylinisitol anchors.

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Other proteins are lipidated that is they have lipid attatchedto them.

CARBOHYDRATES

The carbohydrates layer on the external surface of plasma

membrane forming cell boundry reffered to as the CellCoat or Glycocalyx.

Carbohydrates attatched to~ proteins formglycoprotein. Lipids form glycolipid.

This layer contains antigen ,include Major

Histocompatibility antigen, In erythrocytes the glycocalyx contain Blood Group

Antigen.

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Special adhesion molecules present enables thecell to adhere to specific type of cells andextracellular molecules.

Glycocalyx has gotve charge which causes

repulsion with cells of same charge mainting adistance of 20nm,

Whereas cells wih opposite charge are heldtogether.

Some glycoprptein molecules present in the cellmembrane are called Cell AdhesionMolecules(CAM).

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The intermediate protein holds the CAM in thecytosolic end of the cell.

Some CAM are calcium dependent while someare calcium independent~

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Type of CAM Subtypes Present In

CALCIUM

DEPENDENT

Cadherin(of various types)

Selectins

Integrins

Most cellsincluding

epithelia.Migratingcells.e.g.leucocytes.

B/w cells and

intercellularsubstance(20types).

CALCIUMINDEPENDENT

Neural cell adhesionmolecule(NCAM)Intercellular adhesionmolecule(ICAM)

Nerve cells.

Lecocytes.

I di i

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Intermediate proteinare~catenins,vinculin,alpha actinin.

INTERCELLULAR JUNCTION~

Ciassified as-

1)Occluding(tight junction) or Zona

occludens.2)Adhesive junctionsa)cell-to-celli)zonula adherence(Adhesive belts)

ii)Macula adherens(Desmosomes oradhesive spots)

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2)i)Z l Adh ( dh i b lt )

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2)i)Zonula Adherence(adhesive belts)

Plasma membrane thickened because of thepresence of a dense layer of protein on its

surface. The thickened area of the two sides are

separated by a gap of 25nm and is rich inglycoprotein

This forms a continous bsnd around theepithelial cell.

The CAM present are Cadherins.

ii)MaculaAdherens

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ii)MaculaAdherens(desmosomes,adhesive spots)

Most common type of junction b/w

adjoining cells. Desmosomes are present where strong

anchorage b/w cells are

needed.e.g.b/w cells of epidermis. These are small circumscribed area of

attatchment.

The gap b/w the thickenings of themembranes are held together by fibrils.

CAM seen in desmosomes arecadherin.

The link proteins are desmoplakins.

DESMOSOME

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DESMOSOME.

B)i)FOCAL ADHESIONS PLAQUES/FOCAL

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B)i)FOCAL ADHESIONS PLAQUES/FOCALCONTACT.

In focal adhesions the transmembranecomponents is a member of the integrin

family of adhesion molecules. Such contacts may send signals to the cell

and initiate cytoskeleton formation . HEMIDESMOSOMES

Similar to desmosomes ,but the thickening ofthe membrane is seen only on one side.

The external ends of CAMs are attatched tothe extracellular structure.

Cytoskeleton attatched to intermediateprotein are keratin, CAMs are integrins. GAP JUNCTION Plasma membrane are not in actual contact,

the being reduced from 20nm to 3nm.

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In transmission electron microscope this gap is seen

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In transmission electron microscope this gap is seencontaining bead like structure.

A minute canaliculus passing through each beadconnects the cytoplasm of the two cells thus allowingthe free passage .

the gap junctions are therefore also called MaculaeCommunicants.

Changes in ph or in calcium ion concentration canclose the channels of gap junctions.

Channel are arranged in hexagonal groups. Wall of each channel is made up of 6 protein element

called Nexins or Connexons. Inner end projects into the gap between the two cell

membrane.

Here similar nexins come in contact with similarnexins projecting into the space from the cellmembrene of the opposite cell, to complete thechannel.

Electric synapsis between some neurons.

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CELL MEMBRANE CYTOPLASMIC

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CELL MEMBRANE CYTOPLASMICINTERACTION.

Cell function depends on signals and

nutrients received through the cellmembrane.

Molecular movement within the cell-

Capping. Larger molecules enter the cell by theprocess of Endocytosis.

Absorption of fluid into the cell byvesicles are called as Pinocytosis.

Process of endocytosis to engulfforeign matter (bacterias) is refferred to

as Phagocytosis.

Releasing the molecules to the exterior

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Releasing the molecules to the exteriorare called Exocytic Vesicle and theprocess is called as Rverse Pinocytosis.

Area marked by the presence ofFusogenic proteins aid the formation ofendocytic and exocytic vesicles.

Receptor mediated endocytosis areseen in depressed area called CoatedPits.

The membrane the floor of the pits isthickened due to the presence of aprotein called Clathrin.

Process of transferring materials right

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Process of transferring materials rightthrough the thickness of a cell is calledTranscytosis.

The transport takes place throughinvagination of cell membrane calledCaveole. Protein caveole is associated

with caveole.

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CYTOPLASM

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CYTOPLASM

Cytoplasm of a typical cell contains variousstructures that reffered to as Organelles.They are~

ENDOPLASMIC RETICULUM:~

The membrane form the boundries ofchannels that may be arranged in the form of

flattened sacs (or cisternae)or tubules. Cytoplasm within the channel is called as

Vaculoplasm and that outside the channel isthe Hyaloplasm or Cytosol.

Membrane forming the endoplasmic reticulumare studded with minute particles of RNAcalled Ribosomes giving rough appearance,so they are called Rough/Granular

Endoplasmic Reticulum.

Membrane devoid of these ribosomes are

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Membrane devoid of these ribosomes arecalled as Smooth/ Agrannular EndoplasmicReticulum.

Rough endoplasmic reticulum represents the

site of protein synthesis. Ribosomes play a very important role this

process. They are in continuation with nuclear

membrane. Smooth endoplasmic reticulum are involved

with the ~i)production of lipids.ii)carbohydrates metabolism.iii)Detoxification of drugs and poison.iv)Metabolizing calcium to mediate some cellactivities.v)Releases calcium to trigger muscle

contraction.

RIBOSOMES PROTEIN

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RIBOSOMES- PROTEINPRODUCTION MACHINE

All living organism contain ribosomescomposed of appox.60%(rRNA) and40%protein.

Eukaryotes ribosome are made of 4

strands of RNA.

Prokaryotes ribosomes consists of 3strands of RNA.

They may lie free in cytoplasm or inrelation to rough endoplasmicreticulum.

Present singly-Monosomes

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Present singly Monosomes.In groups-polyribosomes.

It is made up of 2 subunits, 60s and 40s.

Thy engage in protein synthesis~ Freeribosomes for cell`s own use. Attatchedribosomes-for extracellular use.

MITOCHONDRIA~THE POWER

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MITOCHONDRIA~THE POWERGENERATORS OF THE CELL.

The term mitos=granule;chondrium=rod.

They vary in size and no, most being0.5microns to 2microns in length.

These cells are high in high metabolicactivity.(e.g secretory cells).

It is bounded by outer and innermembrane.

The two are separated by anIntermembranous Space.

The inner membrane is highly folded on

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The inner membrane is highly folded onitself forming incomplete partition calledCristae.

Space bounded by inner membrene isfilled by granular material called Matrix.

It contains some RNA and DNA which

carry information enables mitochondriato duplicate themselves during celldivision.

All mitochodrias are maternal in origin

since they are from unfertilized ovum. Mitochondrial DNA can be abnormal

resulting in disorder reffered as

Mitochondrial Cytopathy Syndrome.

In this muscle weakness degenerative

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In this muscle weakness, degenerativelesions in the brain and high levels oflactic acid.

Mitochondria contein many enzymessome of which play an importsnt partKerbs cycle(TCA cycle).

Enzymes for conversion of ADP to ATPare located in the intermembranousspace.

Enzymes for lipid synthesis and fattyacids metabolism are located in theouter membrane.

ENDOSYMBIOTIC THEORY

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ENDOSYMBIOTIC THEORY

symbiosis-living together

Lynn Margulis 1971.

Mitochondria and chloroplast derivedfrom ancient colonization of large(become eukaryotic cell) by smaller

bacteria.Host cell acquires respiration from the

precursor of mitochondrion andchloroplast.

Also acquires genetic information.

Eventually organelle lost the ability toexist as independent organisms.

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MITOCHONDRIA

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MITOCHONDRIA.

GOLGI COMPLEX

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GOLGI COMPLEX

These organelle consists of a series ofparallel doughnut shaped flat spaces or

cisternae. They are of irregular shape usually near the

nucleus.

Towards the margin of the flattened sacs,small rounded vesicles are present.

Functional point if view golgi complex isdivisible into 3 regions~

i)A region nearest the nucleus is the cisface(cis golgi).ii)opposite face(near the cell membrane)is thetrans face.

iii)The intermediate part as the medial golgi.

Cis face-

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Cis faceProtein are phosphorylated.

Medial golgi-Here sugar residues are added toproteins to form protein-carbohydratecomplexes.

Trans face-i)Proteolysis of some proteins convertthem from inactive to active forms.

ii)adding sugar residues to protein.iii) substance are sorted out and packetin appropriate vesicles.

GOLGI COMPLEX

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GOLGI COMPLEX

As protein pass through successive

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s p ote pass t oug success esacs of golgi they undergo a process ofpurification.

LYSOSOMES:- (LYTIC BODIES) These are bags of hydrolysing enzymes

that break down a large variety of

substance which could serve asnutrients or raw material for biosyntheticactivity.

Sometimes it digests the whole of the

cell or a part of it thats its referred asSuicide Bags.

Lysosomes belong to the acid vesicles

system.

LYSOSOMES

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LYSOSOMES

Its membrane acts as the Hydrogen

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y gpump.The stages in the formation of alysosomes are as follows~ i)Acid

hydrolase enzyme which are formed inendoplasmic reticulum are inactive dueto lack of acid medium- Primarylysosomes.

These vesicles fuse with other vesicles(endosomes)producing an acid mediumforming- Endolysosome or Secondery

lysosomes. With this hydrogen ions are pumped

into the vesicles which creates acidenvironment and activates the

enzymes.-Mature lysosomes.

Lysosomes are present in all cells except

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y p pmature erythrocyte.

Genetic defects can lead to absence ofspecific acid hydrolse that are normallypresent in lysosomes.e.g:-lysosomal storagediseases.

PEROXISOMES-CELLULAR DIGESTIVESYSTEM.

These are membrane bound vesiclescontainig enymes.

Contains enzyme catalase which destroys

hydrogen peroxide thus preventing thehydrogen accumulating in the cell.

Most prominent in the liver and in renaltubules.

CYTOSKELETON~ The Cell Scaffold.

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CYTOSKELETON The Cell Scaffold.

It providesi)Structural framework.

ii)facilitates intracellular transport,iii)Supports cell junctions,iv)transmits signals about cell contact

and adhesion.v)Permits motility.

3 Structural elements of the

cytoskeleton are~i)Microfilaments.ii)Intermediate filaments

iii)Microtubules.

MICROFILAMENTS~

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They are made up of protein actin,these aremolecules of (G-actin) which polymerises to

form long chains called F-actin,actinfilaments and microfilaments.

Actin filaments form a meshwork justsubjacent to the cell membrane called Cell

Cortex.

This maintains the shape of the cell.

The filement forming meshwork are held

together by a protein called Filamin. The filaments can separate and reform in

different orientation that is how shape of acell is altered.

INTERMEDIATE FILAMENTS~

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They are so called as their diameter isintermediate between that of

microfilaments (5nm) and ofmicrotubules(25nm).

They include-

cytokeratin- In epithelial cell.Neurofilament protein-Neurons.Desmin-Muscles.

Glial fibrillary acidic protein-Inastrocytes.Laminin-In the nuclear lamina of cells.

Vimentin-In many types of cell.

These filaments facilitates cell

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attachment to extracellular elements athemidesmosomes.

In the epithelium of the skin thefilaments undergo modification to formKeratin, they also form the main

constituents of hair and nails. The neurofilaments of neurons are

intermediate filaments and these helps

in maintaining the cylindrical shape ofthe axons.

The nuclear lamina contains the

intermediate filaments.

MICROTUNULES:-25microns in

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diameter.

Basic constituents of microtubules is the

protein tubulin.

The chain of tubulin form Protofilamentswhich run longitudinally.

The tubulin protofilaments are stabilisedby microtubule associatedprotein(MAPs).

Microtubule are formed in centrioleswhich constitute a microtubuleorganising center.

The role of microtubule are:~i) They provide stability to the cell They prevent

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i) They provide stability to the cell. They preventtubules of endoplasmic reticulum from collapsing.ii)Facilitates transport within the cell.iii)In dividing cells microtubules from the mitoticspindle.

Cilia are made up of microtubules (held together byproteins).

CENTRIOLES~

These are paired organelles found together near thenucleus located at right angles to each other .

Each centriole is made up of nine bundles ofmicrotubules arranged (3 per bundle) in a ring.

They have role in building cilia and flagella duringwhich they are referred to as Basal Bodies.

These are seen as mitotic spindles in mitosisand meiosis

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and meiosis. These structures are self replicating . VACUOLES~

Membrane bound sacs for storage digestionand water removal.

PROJECTION FROM THE CELL SURFACE Many cells show projections from the cell

surface. The various types of projections are~ 1)CILIA~ These are minute hair like projections from the

surface of the epithelium.

The free part of the cilium is called the Shaft. The region of attachment of the shaft to the

cell surface is called the Base. The extension of cell membrane forms the

outer covering .

The microtubules arranged in a definitef h i

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manner forms the inner core.

There is a central pair of tubules ,the outertubules are connected to the inner pair byradial structure (which are like spokes of awheel).

FUNCTIONS~

Cilia lining the epithelial surface move in co-ordination with one another the total effectbeing that like a wave.

It lines the respiratory tract helps in

movement of secretion. Helps in movement of ova through the

uterine tube.

Spermatozoa through male genital tract.

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Cilia like structure that perform some

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psensory function. E.g: olfactory cilia innasal mucosa-receptor for smell.

Kinocilia in some parts of internal ear. FLAGELLA~

Larger processes having same basic

structure of cilia. e.g:flagellumtail ofspermatozoon.

MICROVILLI AND BASOLATERAL

FOLDS~ Microvilli are finger like projections from

the cell surface.

CILIA AND FLAGELLA

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CILIA AND FLAGELLA

Each microvillus consists of an outer

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covering of plasma membrane and acytoplasmic core in which numerous

microfilaments are continous with actinfilaments of cell cortex .

e.g straited border epithelial cell lining

the small intestine.

NUCLEUS~(4-10microns in dia)

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The nucleus is a highly specialized organellethat serves as the information and

administrative center of the cell. Two major functions:~

i)It stores the cells hereditary material or DNA.

ii)It coordinates the cells activities, whichinclude intermediary metabolism,growth,protein synthesis andreproduction(cell division).

Nuclei appears to be made up of a delicatenetwork of fibres the making up of fibres of thenetwork is called Chromatin

Some places the chromatin is seen in the

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form of irregular dark mass calledHeterochromatin.

At other places the network is loose andstains lightly, such areas of chromatin isrefferred to as Euchromatin.

Nuclei which are large and in which relatively

large areas of euchromatin can be refferredas Open-faced nuclei.

Nuclei which are made up mainly of

heterochromatin are refferred to as Closed-faced nuclei.

The masses of heterochromatin ,the nucleusshows one or more rounded, dark staining

bodies called Nucleoli.

CHROMOSOMES~

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Chromatin becomes tightly coiled and takesappearance of no of short, thick, rod like

structures.Made up of Deoxyribonucleic acid(DNA)

Double helix structure- two strands coil.

Each DNA strand consists of nucleiotides.Each neucleiotidesugar,phosphate molecule

and base.

Base-adenine,guanine,cytosine and thymine.

Sugar is made of 5C atom9 (pentose).

Two chain of neucleotide arrangedantiparallel(3-5).

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Two parallel rods-like called Chromatids.

Joined by centromere (kinetochore).Highly coiled partconstrictionPrimary

constriction.

It has long arm and short arm

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It has long arm and short arm.

Metacentric-two arms of equal length.

Submetacentric-slightly away from center.Acrocentric-Diff is marked.

Telocentric-centromere at one end.

End of chromatidsecondary constriction-nucleolar organizing centers.

Distal partsatellite.

RIBONUCLEIC ACID(RNA)

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Contains ribos sugar.

Base replaced by uracil.

Types of RNA are~

mRNA, tRNA , rRNA.

m RNA carries message for proteinsynthesis to the ribosomes calledTranscription.

Formation of amino acids by thesem RNA and t RNA is celled Translation.

NUCLEOLI

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1-3microns in diameter , round in shape,metabolically active.

Central filamentous zone-Pars filamentous.Outer granular layer-Pars granulosa.

BasePars amorphousa.

Chromosome located near the neucleoli-Parschromosoma.

RNA is synthesized.

r RNA (long filamentous,fibrous zone)smaller pieces(ribosomal sub units,granular zone)leaves nucleolus throughpores cytoplasm, protein synthesis.

The space between the various constituentsf th l fill d b b ll d th

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of the nucleus are filled by a base called theNucleoplasm.

The nucleus is seen to be surrounded bydouble layer membrane or Nuclear envelop.

The space between the outer and inner layeris the Perinuclear space.

Deep to the inner membrane there is a layercontaining protein and a network of filaments,this layer is called The nuclear lamina.

Protein lamins form a scaffolding thatmaintaining the spherical shape of thenucleus.

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Several points the inner and outer layer

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of the nuclear membrane fuse leavinggaps called Nuclear Pores.

Pore is surrounded by dense proteinarranged in the form of 8 complexes.

Chromatin is made up of substance

called Deoxyribonucleic acid. Protein in the chromatin are Histons.

The structure formed by a histone

complex and the DNA fibre coiledaround it is called Nucleosome.

Filaments of chromatin are again coiled onthemselves called Supercoiling and it is

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themselves called Supercoiling and it isrepeated several times.

5 types of histones are recognised H1,H2A,

H2B, H3 and H4. Two molecules each of H2A, H2B, H3 andH4 join to form a granular mass, thenucleosome core. DNA filament is woundtwice around this core, the whole complexforming a nucleosome.

One nucleosome is connected to the next bya short length of linker DNA.

During cell division the entire chromatin

within the nucleus becomes very highlycoiled and takes on the appearance of short,thick and rod like structure calledchromosomes.

The number of chromosomes in eachll i fi d f i i d i

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cell is fixed for a given species and inman it is 46.

44 are autosomes and 2 are sexchrosomes.

The females sex is described as

homogametic and male isheterogametic.

Virchow states ~~Every cell is derieved

from a cell. Multiplication of cell takes place by

division of pre existing cells.

The daughter cell having chromosomesid i l i b d i i

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identical in number and in geneticcontent to those in the mother cell type

of cell division is called Mitosis. The daughter cells having the reduced

number of chromosomes to half the

normal number and genetic informationin the various gametes produced is notidentical ,this type of cell division is

called Meiosis.

MITOSISTh i d d i hi h h ll i

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The period during which the cell isactively dividing is the Phase of Mitosis.

Equal division of nuclear DNA known asKaryokinesis followed by Cytokinesis.

The period between two successive

divisions is called Interphase.G1 Stage~The greater part of interphaseis this phase. It is the period duringwhich cell carries the normal function.

S Stage(synthesis)~12 hrs before theonset of mitosis the synthesis of DNAtakes place and is completed in about

7hrs. G2 Stage~The last 5hrs before mitosis

tili d f th i f t i

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are utilized for synthesis of proteinrequired for cell division. Cell at G2

stage have a double complement ofDNA.

Mitosis is divided into number of stages

called~1)Prophase2)Metaphase

3)Anaphase4)Telophase.

The later part of metaphase is called

Prometaphase. Telophase the chromatin of the

chromosomes uncoils and elongates and the

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chromosomes uncoils and elongates and thechromosomes can no longer be identified.

During S phase of interphase the DNAcontent of the chromosomes is duplicated.

PROPHASE~ The chromatin of thechromosome become gradually more coiled

so that the chromosome acquire rod likeappearance.

The two centrioles separate and more to

opposite poles of the cell and produce anumber of microtubules that pass fromcentriole to the other and form a spindle.

The tubules radiating from each centriole

create star like appearance or Aster.

Two aster collectively called as Diaster orAmphiaster or Achromatic spindle

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Amphiaster or Achromatic spindle.

Nuclear membrane breakdown and

disappear. METAPHASE~

With the formation of spindle thechromosomes move to a position midwaybetween the two centrioles(i.e, at the equatorof the cell) where each chromosomesbecome attached to microtubules of the

spindle by its centromere. The plane along which the chromosome lie

during metaphase is the Equatorial Plate orMetaphase Plate.

ANAPHASE

C t f h h lit

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Centromere of each chromosome splitslongitudinally into two so that chromatids

becomes independent chromosomes. One chromosome of each such pair now

move along the spindle to either pole of thecell.

This is followed by telophase in whichchromosomes gradually elongates andnucleoli reappears.

The division of the nucleus is accompaniedby the division of the cytoplasm, the cleavageinto two separate cells is referred to asCytokinesis.

Mitosis can be arrested by chemicalsl hi i it t it i t t h

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colchicin, it stops mitosis at metaphaseand allow us to study chromosomes at

this stage. Neurons, cardic muscles do not

undergo mitosis and are said to be in

G0 phase.

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MITOSIS

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MEIOSIS Meiosis consists of two successive divisions in

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Meiosis consists of two successive divisions inwhich during the 1st division duplication of theDNA content of the chromosomes.

2nd division takes place as in mitosis.

1ST MEIOTIC DIVISION

The prophase of the 1st meiotic division is

prolonged and usually divided into a numberof stages as follows~

a) Leptotene~The chromosomes graduallybecomes thicker and shorter.

Zygotene~The pairing of the chromosomesoccurs which are referred to as Synapsis orConjugation. The two chromosomes togetherconstitute a bivalent.

c)Pachytene~ The two chromatids of eachchromosomes become distinct The bivalent

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chromosomes become distinct. The bivalentnow has four chromatids in it and it is called aTetrad.

The two central chromatid becomes coiledover each other so that they cross at anumber of points called Crossing Over.

The site where the chromatids cross theybecome adherent. The point of adhesion arecalled Chiasmata.

d)Diplotene~ Crossing over of geneticmaterial each of four chromatids of the tetradnow has a distinctive genetic content.The twochromosome of the bivalent now move apart.

Metaphase is same as in mitosis. In anaphase there is no splitting of the

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a ap ase t e e s o sp tt g o t ecentromeres.

Telophase is similar to that in mitosis.

The first meiotic division is followed by a shortinterphase in which there is no duplication ofDNA.

SECOND MEIOTIC DIVISION

It is similar to mitosis . The DNA content of the daughter cells is

reduced to half of the crossing over during 1stdivision

The daughter is not identical in geneticcontent. In meiosis random shuffling of the genetic

material takes place thus no two persons arealike.

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CONCLUSION

Th d t di th lt t t f

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Thus understanding the ultrastructure ofthe cell organelles and its functions is

very essential for the diagnosis and thetreatment plan of various diseases.