Cell cycle mitosis

Cell cycle mitosis

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  Cell cycle : Itis the series of events that take place in a cell that cause it to divide into two daughter cells. It consistof three majorphasesnamely 1. Interphase where replicationof geneticmaterial occurs 2. Mitosis(M phase) orthe nucleardivision 3. Cytokinesisorthe divisionof the cell. The M phase lastsonlyfor an hourin a periodof 24 hour requiredforaeukaryoticcell todivide.The interphase canbe furtherdividedintoG1(gapphase 1), S (synthesis) andG2 (gapphase 2) phases.Cells that are nolongercapable of division,whethertemporarilyorpermanently,remaininG0 phase.A cell mustreceive agrowth-promotingsignaltoproceedfromthe quiescentstage orG0 intoG1 phase and thusreenterthe cell cycle. The order of cell cycle eventsisG1 → S → G2 → M and thenfollowedbycytokinesis. Interphase: The G1 phase,S phase and G2 phase togetherformthe interphase. The nuclearenvelope remains intact.The chromosomesoccurinthe form of diffused,long,coiledand indistinctlyvisiblechromatinfibres.The DNA amountbecomesdouble.Due toaccumulationof ribosomal RNA (rRNA) andribosomal proteinsinthe nucleolus,the size of the latterisgreatlyincreased. In animal cells,adaughterpairof centriolesoriginatesnearthe alreadyexistingcentrioleand,thus,an interphase cell hastwopairsof centrioles.Inanimal cells,netmembrane biosynthesisincreasesjust before cell division(mitosis).Thisextramembraneisstoredasblebsonthe surface of the cellsaboutto divide.Eventsininterphasetakesplace inthree distinctphases. G1 Phase: Afterthe Mphase of previouscell cycle,the daughtercellsbeginG1of interphase of newcell cycle.G1 isa restingphase.Itisalsocalledfirstgapphase,as noDNA synthesistakesplace duringthis stage.It isalsoknownas the firstgrowth phase,since itinvolvessynthesisof RNA,proteinsand membraneswhichleadstothe growthof nucleusand cytoplasmof eachdaughtercell towardstheir enhancingsize.DuringG1phase,chromatinisfullyextendedandnotdistinguishable asdiscrete chromosomeswiththe lightmicroscope.Thus,itinvolvestranscriptionof three typesof RNAs,namely rRNA,tRNA and mRNA;rRNA synthesisisindicatedbythe appearance of nucleolusinthe interphase(G1 phase) nucleus.ProteinssynthesizedduringG1phase (a) regulatoryproteinswhichcontrol various eventsof mitosis(b) enzymes(DNA polymerase) necessaryforDNA synthesisof the nextstage and(c) tubulinandothermitoticapparatusproteins.G1phase ismost variable asto durationiteitheroccupies 30 to 50 per centof the total time of the cell cycle.Terminallydifferentiatedsomaticcells(endcellssuch as neuronsandstriatedmuscle cells) thatnolongerdivide,are arrestedusuallyinthe G1 stage,sucha type of G1 phase is calledG0 phase. S phase:Duringthe S phase or syntheticphase of interphase,replicationof DNA andsynthesisof histone proteinsoccur.Newhistonesare requiredinmassive amountsimmediatelyatthe beginningof the S periodof DNA synthesistoprovide the new DNA withnucleosomes.Atthe endof Sphase,each
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  chromosome hastwoDNA moleculesandaduplicatesetof genes.Sphase occupiesroughly35 to 45 percent time of the cell cycle.G2 phase:Thisisa secondgap or growthphase or restingphase of interphase.DuringG2phase,synthesisof RNA andproteinscontinueswhichisrequiredforcell growth. It may occupy10 to 20 percenttime of cell cycle.Asthe G2 phase drawsto a close,the cell entersthe M phase. The mitosis(Gr.,mitos=thread) occursinthe somaticcellsandit ismeantfor the multiplicationof cell numberduringembryogenesisandblastogenesisof plantsandanimals.Fundamentally,itremains relatedwiththe growthof an individual fromzygote toadultstage.Mitosisstartsat the culmination pointof interphase (G2phase).Itisa shortperiodof chromosome condensation,segregationand cytoplasmicdivision.Mitosisisimportantforgrowthof organism, replacementof cellslosttonatural frictionorattrition,wearandtear and for woundhealing.Hence,mitosisisremarkablysimilarinall animalsandplants.Itis a smoothcontinuousprocessandisdividedinto differentstagesorphases. Mitosisisa processof cell divisioninwhicheachof twoidentical daughtercellsreceivesadiploid complementsof chromosomessame asthe diploidcomplementof the parentcell.Itisusuallyfollowed by cytokinesisinwhich the cell itself dividestoyieldtwoidenticaldaughtercells.The basicsinmitosis include:1.Each chromosome ispresentasa duplicatedstructure atthe beginningof nucleardivision (2n).2. Each chromosome divideslongitudinallyintoidentical halvesandbecome separatedfromeach other.3. The separatedchromosome halvesmove inopposite directions,andeachbecomesincludedin one of the twodaughternuclei thatare formed.Mitosisisdividedintofourstages:prophase, metaphase,anaphase andtelophase.The stageshave the followingcharacteristics: 1. Prophase:The chromosomesare inthe form of extendedfilamentsandcannotbe seenwithalight microscope asdiscrete bodiesexceptforthe presence of one ormore dark bodies(i.e.nucleoli) inthe interphase stage.The beginningof prophase ismarkedbythe condensationof chromosomestoform visiblydistinct,thinthreadswithinthe nucleus.Eachchromosome isalreadylongitudinallydouble, consistingof twocloselyassociatedsubunitscalledchromatidswhichare heldtogetherbycentromere. Each pair of chromatidsisthe productof the duplicationof one chromosome inthe Speriodof interphase.Asprophase progresses,the chromosomesbecome shorterandthickerasa resultof intricate coiling.Atthe endof prophase,the nucleoli disappearandthe nuclearenvelope,amembrane surroundingthe nucleus,abruptlydisintegrates.Fig.4:Prophase 2. Metaphase:At the beginningof metaphase,the mitoticspindle formswhichare abipolarstructure and consistof fiber-like bundlesof microtubulesthatextendthroughthe cell betweenthe polesof the spindle.Eachchromosome attachedtoseveral spindle fibersinthe regionof the centromere.The structure associatedwiththe centromere towhichthe spindlefibersattachisknownasthe kinetochore. Afterthe chromosomesare attachedto spindle fibers,theymove towardsthe centerof the cell until all the kinetochoreslieonanimaginaryplane equidistantfromthe spindle poles.Thisimaginaryplaneis calledthe metaphase plate.Hence the chromosomesreachtheirmaximumcontractionandare easiest to count andexamine fordifferencesinmorphology.The signal forchromosome alignmentcomesfrom the kinetochore,andthe chemical nature of the signal seemstobe the dephosphorylationof certain kinetochore-associatedproteins.The role of the kinetochore isdemonstratedbythe findingthat metaphase isnotdelayedbyanunattachedchromosome whose kinetochore hasbeendestroyedbya focusedlaserbeam.The role of dephosphorylationisdemonstratedthroughthe use of anantibodythat reacts specificallywithsome kinetochoreproteinsonlywhentheyare phosphorylated.Unattached
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  kinetochorescombine stronglywiththe antibody,butattachmenttothespindle weakens the reaction. In chromosomesthathave beensurgicallydetachedfromthe spindle,the antibodyreactionwiththe kinetochore reappears.Throughthe signalingmechanism, whenall of the kinetochoresare under tensionandalignedonthe metaphase plate,the metaphasecheckpointispassedandthe cell continues the processof division. 3. Anaphase:Inanaphase,the centromeresdivide longitudinally,andthe twosisterchromatidsof each chromosome move towardopposite polesof the spindle.Once the centromere divide,eachsister chromatidistreatedas a separate chromosome.Chromosomemovementresultsfromprogressive shorteningof the spindle fibersattachedtothe centromeres,whichpullsthe chromosomesinopposite directionstowardthe poles.Atthe completionof anaphase,the chromosomeslieintwogroupsnear opposite polesof the spindle.Eachgroupcontainsthe same numberof chromosomesthatwaspresent inthe original interphase nucleus. 4. Telophase:Intelophase,anuclearenvelope formsaroundeachgroupof chromosomes,nucleoliare formed,andthe spindle disappears.The chromosomesundergoareversal of condensationuntil and unlesstheyare nolongervisible asdiscrete entities.The twodaughternuclei slowlygoestointerphase stage the cytoplasmof the cell dividesintotwobymeansof a graduallydeepeningfurrow aroundthe periphery. 5. Cytokinesis:The chromosomesmovedclose tothe spindlepole regions,andthe spindle mid-zone beginstoclear.In thismiddle regionof the spindle,athin line of vesiclesbeginstoaccumulate.This vesicle aggregationisanindicationtothe formationof a new cell wall thatwill be situatedmidwayalong the lengthof the original cell andhence formboundarybetweenthe newlyseparatingdaughtercells.