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Cltokinesis thencleaves the cell in two, so that each daughter cell inherits one of the two nuclei(Figure l7-3). <TACT><TCAA>Most cells require much more time to grow and double their mass of proteins and organelles than they require to duplicate their chromosomes anddivide. Partly to allow more time for growth, most cell cycles have exrra gapphases-a G1 phase between M phase and S phase and a Gz phase between Sphase and mitosis.
Thus, the eucaryotic cell cycle is traditionally divided intofour sequential phases:Gr, S, G2,and M. Gr, S, and G2together are called interphase (Figure l7-4, and seeFigure 17-3). In a typical human cell proliferating inculture, interphase might occupy 23 hours of a 24-hour cycle, with I hour for Mphase.
Cell growth occurs throughout the cell cycle, except during mitosis.The two gap phases are more than simple time delays to allow cell growth.They also provide time for the cell to monitor the internal and external environment to ensure that conditions are suitable and preparations are completebefore the cell commits itself to the major upheavals of S phase and mitosis. TheG1 phase is especially important in this respect. Its length can vary greatlydepending on external conditions and extracellular signals from other cells. Ifextracellular conditions are unfavorable, for example, cells delay progressthrough G1 and may even enter a specializedresting state known as Go(G zero),in which they can remain for days, weeks, or even years before resuming proliferation.
Indeed, many cells remain permanently in Geuntil they or the organismdies. If extracellular conditions are favorable and signals to grow and divide arepresent, cells in early G1 or G0 progress through a commitment point near theend of G1 knorvn as Start (in yeasts) or the restriction point (in mammaliancells).We will use the term Start for both yeast and animal cells. After passingthis point, cells are committed to DNA replication, even if the extracellular signals that stimulate cell growth and division are removed.Figure 17-3 The eventsofeucaryoticcell divisionas seenunder amicroscope,The easilyvisibleprocessesand cellof nucleardivision(mitosis)calledcollectivelydivision(cytokinesis),M phase,typicallyoccupyonly a smallfractionof the cellcycle.The other,muchlonger,part of the cycleis known aswhich includesS phaseandinterphase,the gap phases(discussedin text).Thefive stagesof mitosisare shown:anstateabruptchangein the biochemicalof the cell occursat the transitionfromA cellcan pauseto anaphase.metaphasein metaphasebeforethis transitionpoint,this point,the cellbut onceit passescarrieson to the end of mitosisandthrough cytokinesisinto interphase.M PHASEmitosis(nucleardivision)G 2P H A S E5 PHASE( D N Ar e p l i c a t i o n )cytokinesis(cytoplasmicG 1P H A S EFigure 17-4 The four phasesof the cellcycle.In most cells,gap phasesseparatethe major eventsof 5 phaseand M phase'Gr is the gap betweenM Phaseand5 phase,while G2is the gaP betweenS phaseand M phase.1056Chapter17:TheCellCycleCell-CycleControlls Similarin All EucaryotesSome features of the cell cycle, including the time required to complete certainevents, vary greatly from one cell type to another, even in the same organism.The basic organization of the cycle, however, is essentially the same in alleucaryotic cells, and all eucaryotes appear to use similar machinery and controlmechanisms to drive and regulate cell-cycle events.
The proteins of the cellcycle control system, for example, first appeared over a billion years ago.Remarkably, they have been so well conserved over the course of evolution thatmany of them function perfectly when transferred from a human cell to a yeastcell. we can therefore study the cell cycle and its regulation in a variety of organisms and use the findings from all of them to assemble a unified picture of howeucaryotic cells divide.
In the rest of this section, we briefly review the threeeucaryotic systems most commonly used to study cell-cycle organization andcontrol: yeasts,animal embryos, and cultured mammalian cells.Cell-CycleControlCanBe DissectedGeneticallyby AnalysisofYeastMutantsYeasts are tiny, single-celled fungi, with a cell-cycle control system remarkablysimilar to our own.
TWospecies are generally used in studies of the cell cycle. Thefission yeast schizosaccharomyces pombe is named after the African beer it isused to produce. It is a rod-shaped cell that grows by elongation at its ends. Division occurs when a septum, or cell plate, forms midway along the rod (Figurer7-5A). The budding yeast Saccharomyces cereuisiaeis used by both brewersand bakers. It is an oval cell that divides by forming a bud, which first appearsduring G1 and grows steadily until it separatesfrom the mother cell aftei mitosis (Figure l7-58).gene, becausewe avoid the complication of having a second copy of the gene inthe cell.Many important discoveries about cell-cycle control have come from systematic searchesfor mutations in yeasts that inactivate genes encoding essential components of the cell-cycle control system.The genes affected by iome ofYEAST(Schhosaccharo mycespo m be)GrISTARTYEAST(5accharomyces cerevisiae)@GrFigure 17-5 A comparisonof the cellcyclesof fissionyeastsand buddingyeasts.(A)The fissionyeasthasa typicaleucaryoticcellcyclewith Gr,5, G2,andM phases.The nuclearenvelopeof theyeastcell,unlikethat of a highereucaryoticcell,does not breakdownduringM phase.The microtubulesof themitotic spindle(lightgreen)forminsidethe nucleusand areattachedto spindlepole bodies(darkgreen)at its periphery.The cell dividesby forming a partition(knownasthe cellplate)and splittingintwo.
(B)ThebuddingyeasthasnormalG1and S phasesbut doesnot havea normalG2phase.Instead,a microtubule-basedspindlebeginsto form late in S phase;asin fissionyeasts,the nuclearenveloperemainsintactduringmitosis,and thespindleformswithinthe nucleus.lncontrastwith a fissionyeastcell,the celldividesby budding.('sllrw^uof Jo ^sauno)) uorlernleu66a;o ssarordaql 6uunp uruopua)iorqseqaoola^uareal)nuaqt'alnlltoaqt ur uaasaq louue) slql q6noqtly'use;doy(r66a aqt ra{e;ate;rnsaq1ur1olueubrd u/Aotqaqt pareldsrpspqq)rqM'snal)nuaqlJo alrsaql smoqsdol aqlreau]odsa;edaq1 'uo11ezrp1.ra;.ro;Ipear'66asndouayarnleur g-11 arn611V( xesraqnga;;o,(sa1lnol)'aseqdW alelJo )llsllal)ereq)aJeq)lq^^'spnqulol} }lxaaq} alalduro)a6relqllm {;uro;tun lsa.ue{aq}'}lnsale sy'srsautloy(lpue slsolltLlaql le ua,rotba^rllrJlsarluPrnLu9t)pf e ul (B)alelduor s;;a>'ernleredua]]ouue)]nq aseqdeue'a6e1suorle;ndod;euloua;r(r-;14)aql ol 6urptotteaztsut Ilen spnq'slla)lseaI 6utleta;r1otd;oe ul (V)'uoltetnw )p) e Iq palsarres;1allseel 6ulppnqlo I6oloqdrou aq1 ;-1 1 atn6t1r.uri0Z(8)ururs 0'sepIAIpSurureluoc o^Jqua ue aleJaue8 ot 'Sut,r,roJ8lnoqluIIaJ luEIS elSuISaql qcrqm ur'suorstt'trpaSanaalJpallel 'suoISIAIpIIeJ Jo asuenbas prder ,r{1$urqsr-uolsE ue sraSSlrtBBasndouay aql Jo uoUPZIpraC '(8-ZI arnBIJ) ,r(poqueutnqaql ul IIaJ a8pJa^euE ueq] urspldoldJ eJour sarull 000'00I suleluoJ pue Jala-ruerp ur ruru I JaAosr 'aldruexa ro!'sndouax 3or; 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