One secondary oocyte and the f i rst polar body each

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"One secondary oocyte and thefirst polar body,each haploid(1 N)tor chromosomes and diploid (2N)for DNA content. The secondary oocyte retainsthe cytoplasm,proteins,fats,andorganelles.One mature ovum forms with up tothree polar bodies.Each is haploid(1N)for chromosomes and DNA contentOust showing for chromosome number 3). At fertilization,the polar bodiesseparate from the ovum.Telraplold (4N) for chromosome number and DNA content
64UnitIBasic Concepts FromMolecular Geneticsthe fifth month,several million diploid oogonia arepresent ineach of the two ovaries. Many ofthesediploid cellsundergo degeneration without further maturation.Thosethat progressto mature ovabegin thisjourney by enteringmeiosisI during thefetal period.
Meiosis IFortheearly part of meiosisI,oogonia undergothe sameprocessesat the samerate asspermatogonia. They first start byenteringthe cellcycleand proceedingthrough5phasewith DNAreplication. Like spermato- gonia,they bypassG2andenter prophase of metaphaseI.Theleptotene,zygotene,and pachytene stagescontinue,and the eventsthatoccur in these stages are very similarto thosethat occurduringprophaseIforspermatogenesis.However,the eventSin thediplotene stageforoogenesisdifer from those occurringduringspermatogen- esis. Thefourchromatids perchromosomepair lengthen rather thanCOntract, and thenucleus becomesquitelarge. The chromatids become very loose,taking ona brushlike appearance.Thethreads of DNA unwind atmany points,and much more crossing overamonghomologouschromatids occurs. NotonlyismoreDNAinclose contact forcrossing over,butalsothis stagelasts foryears,atleastuntil puberty. Thus,prophaseIof meiosisI is arrested foraprolonged period duringoogenesis.Bybirth,mostfemaleinfantshave aboutamillionprimary oocytestrapped in meiosisI inbothovaries,and nofurther proliferationof these cells occurs.Themajority of primary oocytes willregress and degenerate so that bythe time a girl beginspuberty,only about40,000oocyresremain.Duringthediplotene stage, other nonnuclear but essentialgrowthof the oocyces occurs, especially oftheproteins,fats,developmental information, andcytoplasmic organelles.(This contentis critical forproper developmentafter fertilizationoccurs.) So,the extended diplotene stage isnot trulydormant,although theprocess of meiosisis on hold.After puberty,groups of primary oocyres continue meiosisI because of hormonal influences. In thesecells,diakinesis occurs,withevents similar tothoseinspermatogenesis.InanaphaseI,however,theresults aredifer- em.Complete separationof thechromosomepairs (notthe chromatids) occursduring thisphase,resultingin one secondary oocyte and thefirst polar body (see Fig.

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Term
Fall
Professor
NoProfessor
Tags
DNA, dsDNA, Basic Concepts From Molecular Genetics

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