1.3-减数分裂-英文版课件.ppt

Meiosis(减数分裂)Insexualreproduction,eachnewindividualbeginswiththeunionoftwogametes.Ifthegameteshadthesamechromosomenumberastheparents,thegametewouldinherittwiceasmanychromosomesaseachparent.Aspecialdivisiontoreducethechromosomenumber(tohalftheparentalnumber)isrequiredpriortofertilization.MeiosisMeiosis,thereductionaldivisionprecedingfertilization,isactuallytwocelldivisionswithonlyasingleroundofDNAreplication.DNAReplicatesMeiosisistwodivisionstepslinkedtoproduce(usually)4haploiddaughtercells,eachofwhichcontainsonlyonecopyofeachchromosome.MeiosisTwodivisionsarereferredtoasmeiosisIandmeiosisII;MeiosisI:sisterchromatidsmovetogethertoonepoleofthecell.MeiosisII:sisterchromatidsseparate(asinmitosis)totwopoles.Eachdivisionisdividedinto4stagesseeninmitosis(prophase,metaphase,anaphase,andtelophase).ThestagenameisfollowedbyaRomanNumeraltoindicatewhichdivisionisbeingdescribed.MeiosisI;

5stagesofProphaseI1)Leptonema(leptotenestage,细线期)2)Zygonema(zygotenestage，偶线期)3)Pachynema(pachytenestage，粗线期)4)Diplonema(diplotenestage，双线期)5)Diakinesis（终变期）MeiosisI;5stagesofProphaseI1)Leptonema（细线期）ThinthreadsofchromatinTipsassociatedwithnuclearmembraneMeiosisI;5stagesofProphaseI2)Zygonema（偶线期）Beginswith“bouquet”(花束)arrangementpairingofhomologouschromosomesSynapsis:synaptonemalcomplex（联会复合体）attacheshomologouschromosomesalongtheirentirelength“bivalents（双价体）”aretheassociatedchromosomepairsRecombinationnodulesarevisibleMeiosisI;5stagesofProphaseI3)Pachynema（粗线期）:Chromosomes(bivalents)shortenandthickenRecombinationnodulesarevisibleMeiosisI;5stagesofProphaseI4)Diplonema（双线期）:ContinuedchromosomalcondensationChromosomesstarttoseparate,canbeseenaspairedchromo