落尘小说

Chapter 4 The life of cells（第1页）

Chapter4Thelifeofcells

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，havingwaysofasexualreprodutothem。Itisbeyondthescopeofthisbooktodelvetoodeeplyiisequencesofsexualreprodu，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。

（b）Inanimalcellsrepliisstartedsimultamanypoiop（1）isdoublestrandedDrand，blad）showingtheplacesiionstarts（X）。DNApintocopybothstraiohearrows（2，3）。AnumberofDNApolymeraseeeeadresultingihehesizedDNAstrand（3）。AsthereplibubblesgrowthesefragmeogetherandcheckedforaaketiesofthestartingDNA（4）