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Chapter4Thelifeofcells
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Thecellcycle
&ualmeicsofcelldivisitoDitoshattheUyofDenver，requiresignifitlymoreinstrusthaobuildamooorsuperputer.Firstofall，theeedstodupliolecules，thatisDNA，RNA，proteins，lipids，etc.Attheanellelevel，severalhueareasolgibodies，cytoskeletalstrudribosomesbythemillioobeduplicatedsothatthedaughtercellshaveeogrowand，inturhemselves.Alltheseprocessesmakeupthe‘cellecellswilldivideonadailybasis，otherslivefordecadeswithoutdividing.Thecellcycleisdividediingwithiheperiodbetweencelldivisions（about23hours），andmitosis（Mphase），theactualprocesstheinalintotwodaughtercells（about1hour）.Ihersplitiperiods:gap1（G1，4–6hours），asynthesisphase（S，12hap2（G2，4–6henerally，tihoutiDioedtotheSphase.AttheendofG1thereisat.IfandenergylevelsareiforDhecellisdivertedintoaphasecalledG0.In2001TimHunt，PaulNurse，awellreceivedtheNobelPrizefortheirworkindisghowthecellcytrolled.
TimHuntfoueinscalleds，whichaccumulateduriagesofthecellcetherightlevelisreached，thecellis‘allowed’tresstothehesaredestroyed.sthenstarttobuildupagain，keepiheprogressateatofthedonlyallressioageifthecorrelevelhasbeenreached.
Mitosis
&hecellisreadytodivide，iteofthecellcyclecalledmitosis（Mphase）whichisbrokendowniherphases:prophase，prometaphase，metaphase，aelophase.Thefirstisprophase，duringwhichtheessetobeediscretestruprometaphasethenuclearmembranebreaksdownandthenueindistinct.
Theesherpacted，dsupergastheyseintoclearlyvisiblepairedsausagelikestructures（seeFigure8c，d，e），eachmadeoftwoatids，joiherbyastructurecalledtheere.Theereprovidesthesiteofattaitotidle（themicrotubularframeworkwhigsabouttheseparationofetothedaughtercells）atastructurecalledthekiotidleisformedfromicrotubules，anizedbyapairoftriolesreviouslyreplidmigratedtoeitherendofthecell（asdesChapter2）.Thisstagepletesprometaphase，whichisfollowedbymetaphase，wherethemitotidlemicrotubulesapplytensiontotheestoalighetreofthespiwhittheyformthe‘metaphaseplate’.
&ageisanaphase，inwhiatidfromeaeisseparatedaooppositeeidle（Figure 9）.Thisisbroughtaboutbyashofthemicrotubulesthatruochorestothespindlepoles，aheniubulesthatruhespihehtaboutbyadjattubulesslidingoverohearrivalofeaatidsatthepolesofthespiesthelaststageofmitosis，calledtelophase.Topletedivisionofthewholetotwodaughtercells，atragofafilamentsformsaroundthemiddleofthegtheintotwohalvesratherlikeabeltpulliighter，aprocesscalledesis（Figure 9b）.Oercellshaveseparatedtheireswillbegintorelax，degtotheirinterphasefigurationaspartofthenewlyformednueeisformedatthesurfaceofthedeges，whichalreadycarrythebuildingbloeoresontheirsurfaces.Mitosisishetersinterphaseagaiobeginaprocessoffurtherredoublidivision，ortoleavethecellbarkonajourneyofdifferentiationtofulfllaspecializedtissuefun，aswillbedesChapter5.
9.Celldivisiohroughgahasetofatidspulledtoolebythemitotidle，（b）twodaughtercellsbegintheendofdivision，‘pineachotherwithatralfurrow
Meiosis
Meiosisisaredudivisioistoproducespecialcells（gametes）whichhaveasingleA（haploid），inreadinessforfusionwithaetoproduceacell（zygote）withthenormaltwocopiesofDNA（diploid），fromwhichtheembryowilldevelopintametesbespermandeggsasinmostanimals，pollenandovulesinplants，aherlifeformssugi.Theofanewanismbyfusioesfromtwanismsisdefinedassexualreprodu.Plants，ofcourse，areonthisprocessasarethemajorityofanimals，havingwaysofasexualreprodutotheItisbeyondthescopeofthisbooktodelvetoodeeplyiisequencesofsexualreprodu，butsufficetosaythattheixiaboutinthismannerprovidesthevarietywhiwhichthepressuresofioion.
Themeeiosisarerelativelysimple:tworoundsofedivisioedwithoutaroundofDioween.Bydividingadiploidcelltwice，fourhaploidgametesareproduced.Meiosisbeginswiththetwomatologous）es（oerernal）inadiploidiwhiaybe‘sed’inaprocesscalledgover.Thefirstdivisioesoneofeaosomestotwoercells，whidividedirectlyprodueteswhiowhavehalftheinalD（haploid）.Themolecularmeosomeseparatioubulesofthespindleinmeiosisareprettymuchthesameastheyareinmitosis.Numerically，spermprodusiderablyprodu，asafertilehumanmaleproduiimeasasi，whereashumanfemalesarebornwitharouolastalifetime.
Dion
Beforeadivide，itmustproducetwocopiesofitsDNA，oneforeachdaughtercell.ThetwostrandsofDNAfromthemothercellareseparated，andcopiesaremadeusingtheinalstrandsofDe.RememberingthattheidebaseAalairswithTandCwithG，iforandhasasequeherandwillhaveasequeheoppositeoldstrandisTAGditsdaughterstrandisATthisway，twoidenticalcopiesofaDNAsequehisiscalledsemi-servativereplidnormallyprovidesaistakesinggeiooalteratioicmessageassedtothedaughterAismadeierruptedburstthattakesaroundathirdofthetimethataneweedsbeforeitdividesagain（cellbacteria，thecellcyclemaylastoes，andacoupleofhoursiessuchasyeast.Inostmammaliancellcyclesarearound24hours.Onlyasmallproportiohehumanbodydividedaily.Forexample，wemanufaewlayerofskincellseveryday，ahesurfaceofouriinuously，butsomenervecellswilllastalifetime.liisrequired，ittakesplaarouioributedthroughoutthenuAisfediionmaerylikefilmintoaprojeergesastwofilms.Exactlyensatthemolecularlevelcouldfillabooktwicethesizeofthisohereisabriefoutliheoverallprocessofthetransmissioiatioweencells.
&requirementistoseparatethetwostrandsoftheDoprovidetemplatesfortheformatiorands，oneoneachhalfinalDNA.Thisis（relatively）simpleinprokaryoteswheretheDNAliesheeukaryotes，itwouldtakefartooloheDNAstartingatoneendonly，sothattheDNAisopenedupbyanenzymecalledhelicaseataround1000differentsitesaloh.Ifyousiderthetopologyofthisprogpiecesoftwistedstring，itrequiresoraiobecutbeforeashortstretchofdoublestrandedDNAwouhemainenzymeofrepliApolymerase）lototheoperandsatthese‘repliforks’aidebases（iorder）totherandsatarateof100basespersed（Figure 10）.Badothisatevees—upto1000basespersed.Despitetherapidrate，repliisextremelyaccurate，witheproofreadaanymismatucleotide，usuallyleavingonlyoneerrorineverybillioides.
Trans
TransisthefirststepintheprocessbywhietiatioeseirandoftheDNA（knownasthegstraemplatetomakeanRNAsequengerRNAure 11）.ThismessengerRNAwilltheemplateforproteinprodu.
10.Dion.（a）Areplifork.HelidsthedoublestrandedDNAsothateadbeApolymeraseenzymes，oraheentarystrandswiions（arrows）.IheDNAstrainuousandcircular，soreplistartsatoanduesroundtopletetheentireDNAmolecule
InasimilarliAissyheDebutolymeraseratherthanDNApolymerase.MessengerRNAalsousesadiffereidebase，uracil（U），ihethymi）iheransApassesoutoftheheleadingendisdatailisfixedtesoftheAsequedoeinareremovedbyaprocessAsplig.ThenewmRaggedwithproteinsthatwilltargetittoanuclearporepriortopassageioplasm，whereitwillbiharibosometoformthemaakiein.
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