Sexual Reproduction: Meiosis, Germ Cells, and ... - U-Cursos
Sexual Reproduction: Meiosis, Germ Cells, and ... - U-Cursos
Sexual Reproduction: Meiosis, Germ Cells, and ... - U-Cursos
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MEIOSIS 1281In mammalian females, egg precursor cells (oocytes) begin meiosis in thefetal ovary but arrest after diplotene, after the synaptonemal complex has disassembledin meiosis I. They complete meiosis I only after the female has becomesexually mature <strong>and</strong> the oocyte is released from the ovary during ovulation;moreover, the released oocyte completes meiosis II only if it is fertilized. Thus,there are special stop <strong>and</strong> start mechanisms during meiosis in female mammals.In humans, some oocytes remain arrested in meiosis I for 40 years or more,which is presumably at least part of the reason why nondisjunction increasesdramatically in older women. In mammalian males, by contrast, meiosis onlybegins in sperm precursor cells (spermatocytes) in the testes at puberty <strong>and</strong> thengoes on continuously, without the stop <strong>and</strong> start mechanisms that operate infemale meiosis. It takes about 24 days for a human spermatocyte to completemeiosis.There is also a big difference in the error rates of meiosis in mammalianfemales <strong>and</strong> males, <strong>and</strong> this is especially striking in humans. About 20% ofhuman eggs are aneuploid, compared with 3–4% of human sperm, <strong>and</strong>, largelyas a result of this, up to 25% of all human fetuses are aneuploid, <strong>and</strong> most ofthese result from nondisjunction in oocytes at meiosis I. Mammalian fertilizationtypically involves the ovulation of a small number of eggs at one end of thefemale reproductive tract <strong>and</strong> the entry of millions of sperm at the other. Giventhe relative scarcity of eggs, one might have expected that egg developmentwould be subject to more stringent quality control than sperm development, butthe opposite is the case. If meiosis goes wrong in male cells, a cell-cycle checkpointmechanism (discussed in Chapter 17) is activated, which arrests meiosis<strong>and</strong> leads to cell death by apoptosis. Such checkpoint mechanisms apparentlydo not operate in female meiotic cells: if homolog segregation fails to occur normally,the cells continue through meiosis <strong>and</strong> produce aneuploid eggs. The malegerm line, on the other h<strong>and</strong>, is thought to be the main source of another type ofgenetic error. Because many more mitotic cell divisions occur on the way to theproduction of a sperm, <strong>and</strong> each round of DNA replication is liable to error, theaverage number of new mutations contributed by fathers is larger than thenumber contributed by mothers.The production of gametes involves more than just meiosis, <strong>and</strong> the otherprocesses also differ for eggs <strong>and</strong> sperm. As we will see, by the end of meiosis, amammalian egg is fully mature, whereas a sperm that has completed meiosis hasonly just begun its differentiation. Before discussing these gametes, however, wefirst consider how certain cells in the mammalian embryo initially become specifiedto develop into germ cells <strong>and</strong> how these cells then become committed todeveloping into either sperm or eggs, depending on the sex of the individual.SummaryHaploid gametes (eggs, sperm, pollen, <strong>and</strong> spores) are produced by meiosis, in whichtwo successive cell divisions follow one round of DNA replication to give rise to fourhaploid cells from a single diploid cell. <strong>Meiosis</strong> is dominated by a prolonged prophaseI, which can occupy 90% or more of the total meiotic period. At the start of prophase I,the chromosomes have replicated <strong>and</strong> consist of two tightly joined sister chromatids.Homologous chromosomes (homologs) then pair up side-by-side <strong>and</strong> become progressivelymore closely juxtaposed as prophase I proceeds. The tightly aligned homologs(bivalents) undergo genetic recombination, forming crossovers that can later be seenas chiasmata, which help hold each pair of homologs together during metaphase I.Both crossing-over <strong>and</strong> the independent segregation of the maternal <strong>and</strong> paternalcopies of each chromosome during meiosis I have important roles in producinggametes that are genetically different from one another <strong>and</strong> from both parents. <strong>Meiosis</strong>-specific,kinetochore-associated proteins help ensure that both sister chromatids ina homolog attach to the same spindle pole; other kinetochore-associated proteinsensure that the homologs remain connected at their centromeres during anaphase I, sothat homologs rather than sister chromatids are segregated in meiosis I. After the longmeiosis I, meiosis II follows rapidly, without DNA replication, in a process that resemblesmitosis, in that sister chromatids are pulled apart at anaphase.(A)time (days)(B)time (days)03691201234567MOUSELEPTOTENEZYGOTENEPACHYTENEDIPLOTENE+DIAKINESISLILYLEPTOTENEZYGOTENEPACHYTENEDIPLOTENE+DIAKINESISprophase Icompletionof meiosis I <strong>and</strong>entire meiosis IIprophase Icompletionof meiosis I <strong>and</strong>entire meiosis IIFigure 21–15 Comparison of timesrequired for each of the stages ofmeiosis. (A) Approximate times for amale mammal (mouse). (B) Approximatetimes for the male tissue of a plant (lily).Times differ for male <strong>and</strong> female gametes(sperm <strong>and</strong> eggs, respectively) of thesame species, as well as for the samegametes of different species. <strong>Meiosis</strong> in ahuman male, for example, lasts for24 days, compared with 12 days in themouse. In human females, it can last40 years or more, because meiosis Iarrests after diplotene. In all species,however, prophase I is always muchlonger than all the other meiotic stagescombined.