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1、Thenormalcellcycleconsistsoffourphases.ThefirstthreeincludeGbtheperiodofnormalmetabolism;Sphase,duringwhichnormalsynthesisofbiologicalmoleculescontinues,DNAisreplicated,andhistonesaresynthesized;andG2,abriefperiodofmetabolismandadditionalgrowth.TogethertheG],S,andG2phasesarecalle
2、dinterphase・ThefourthphaseofthecellcycleisMphase,theperiodofmitosis,duringwhichthereplicatedchromosomescondenseandmoveandthecelldivides・Itisbelievedthatpropertiesofthecellcytoplasmcontrolthecellcycle,alongwithexternalstimulatorsandinhibitorssuchaschalones・正常细胞循环由4个时期组成。头三期包括正常新陈代
3、谢；S期,正常新陈代谢同时，DNA复制，组蛋白合成；G?期，短期的新陈代谢利少许生长。GhS,和G2称分裂间期。最后是M期，有丝分裂期，复制的染色体组浓缩，移动并细胞分裂。据称是染色质控制了细胞循环，伴随外部激活因子和抑制因子如抑索。Biologistsdividethemitoticcycleintofourphases・Althebeginningofprophase前期lhechromosomeseachconsistoftwohighlycondensedchromatidsattachedtoeachotheratacentromere・Asp
4、rophaseendsandmelaphase中期begins,lhecondensed]农穹汕勺chromosomesbecomeassociatedwiththespindle纺锤体.Eventuallylhechromosomesbecomearrangedinaplane(calledthemetaphaseplate)atarightangletothespindlefibers.Next,duringanaphase后期，thetwosisterchromatidsofeachchromosomesplit,andonefromeachpai
5、risdrawntowardeachpoleofthecell.During(elophase末期nuclearenvelopes包膜begintoformaroundeachseiofchromosomes,anddivisionofthecytoplasmtakesplace・生物学家将有丝分裂划分为4个阶段。分裂前期，高度浓缩的两个染色单体通过着丝粒连接在一起。在分裂前期后期和分裂中期前期，浓缩的染色体与纺锤体相连，最后以正确的角度排列在赤道板上。在分裂后期，两个姊妹单体分离，分别拽向细胞两极。在分裂末期，在每套染色体周围形成核膜，细胞质发生分裂。
6、Meiosis减数isaspecialformofcelldivisionthatlakesplaceinthereproduclive生殖organsthatproducesexcells.Likemitosis,ittakesplaceafterDNAreplicationhasoccurredandinvolvestwosequential连续的nucleardivisions(meiosisIandmeiosisII)・Thesedivisionsresultinfourdaughtercells,eachwithhalfthenumberofc
7、hromosomesoftheparentcell.Thephenomenonofcrossingoverduringmeiosisresultsinexchangesofgeneticinformationbetweenchromosomes・Hence因止匕,thehomologous同源的chromosomesdistributed分布todifferentprogeny后裔cellsarenotidentical同样的.减数分裂是性细胞分裂的特殊形式。如有丝分裂，它也是发生在DNA复制后并有连续的两个核分裂。产生4个子细胞，分别含冇亲本-半的染色
8、体数。这-减数分裂的交叉现象，结果导致染色体之间的遗传信息交流。因此，同源染色体
