Sexual Reproduction: Meiosis, Germ Cells, and ... - U-Cursos

1274 Chapter 21: Sexual Reproduction: Meiosis, Germ Cells, and Fertilizationrecombination. They then line up at the equator of the meiotic spindle, afterwhich the duplicated homologs rather than the sister chromatids are pulledapart and segregated into the two daughter cells. Only in division II of meiosis(meiosis II), which occurs without further DNA replication, are the sister chromatidspulled apart and segregated to produce haploid daughter cells. In thisway, each diploid cell that enters meiosis produces four haploid cells, each ofwhich inherits either the maternal or paternal copy of each chromosome, butnot both (see Figure 21–5A).replicatedpaternalchromosome 1replicatedmaternalchromosome 1centromereDuplicated Homologs (and Sex Chromosomes) Pair During EarlyProphase IDuring mitosis in most organisms, homologous chromosomes behave independentlyof each other. During meiosis I, however, it is crucial that homologs recognizeeach other and associate physically in order for the maternal and paternalhomologs to undergo genetic recombination and to segregate to differentdaughter cells at anaphase I. Special mechanisms mediate these intimate interactionsbetween homologs.The progressive juxtaposition of homologs occurs during a very prolongedmeiotic prophase (prophase I), which can take hours in yeasts, days in mice, andweeks in higher plants. Like their mitotic counterparts, duplicated meioticprophase chromosomes initially appear as long threadlike structures, in whichthe sister chromatids are so tightly glued together that they appear as one. It isduring early prophase I that the homologs begin to associate along their lengthin a process called pairing, which, in some organisms at least, occurs initiallythrough interactions between complementary DNA sequences (called pairingsites) in the two homologs; in most organisms, stable pairing requires geneticrecombination between the homologs. As prophase I progresses, the homologsbecome more closely juxtaposed, forming a four-chromatid structure called abivalent (Figure 21–6A). As we discuss later, genetic recombination begins duringpairing in early prophase I, with the production of programmed doublestrandbreaks in chromatid DNA; some of these recombination events will laterresolve into crossovers, where a fragment of a maternal chromatid is exchangedfor a corresponding fragment of a homologous paternal chromatid (Figure21–6B; also see Figure 5–64).The pairing of homologs requires chromosome movements, but it is notknown what drives these movements. The replicated chromosomes undergomajor rearrangements within the nucleus during prophase I. The ends of thechromosomes (the telomeres) are tightly bound to the inner surface of the nuclearenvelope. They are initially distributed diffusely there, but they then cluster transientlyat one spot on the envelope and, later still, disperse again (Figure 21–7).Neither the mechanism nor the roles of these rearrangements are known,although they are thought to make prophase I faster and more efficient. One possibilityis that they help prevent chromosome entanglements during prophase I.In fission yeast, telomere clustering is required for homolog pairing and crossingover,but in some organisms it occurs after pairing is well underway.(A)bivalentsisterchromatids(B)chiasmaFigure 21–6 Homolog alignment andcrossing-over. (A) The structure formedby two closely aligned duplicatedhomologs is called a bivalent. As inmitosis, the sister chromatids in eachhomolog are tightly connected alongtheir entire lengths, as well as at theircentromeres. At this stage, the homologsare usually joined together by a proteincomplex called the synaptonemalcomplex (not shown; see Figure 21–9).(B) A later-stage bivalent in which a singlecrossover event has occurred betweennon-sister chromatids. It is only when thesynaptonemal complex disassembles andthe paired homologs separate a little atthe end of prophase I, as shown, that thecrossover is seen microscopically as a thinconnection between the homologs calleda chiasma.(A)(B)5 mmFigure 21–7 Rearrangements oftelomeres during prophase indeveloping bovine eggs. The nucleus isstained blue and the telomeres arestained red. During prophase I, thetelomeres are bound to the inner surfaceof the nuclear envelope. At first, they aredispersed around the nuclear envelope(not shown). Then they become clusteredat one region of the envelope (A); finally,toward the end of prophase I, theydisperse again (B). (From C. Pfeifer et al.,Dev. Biol. 255:206–215, 2003. Withpermission from Elsevier.)