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1280 Chapter 21: Sexual Reproduction: Meiosis, Germ Cells, and FertilizationFigure 21–14 Crossovers betweenhomologs in the human testis. In theseimmunofluorescence micrographs,antibodies have been used to stain thesynaptonemal complexes (red), thecentromeres (blue), and the sites ofcrossing-over (green). Note that all ofthe bivalents have at least 1 crossoverand none have more than 3. (Modifiedfrom A. Lynn et al., Science296:2222–2225, 2002. With permissionfrom AAAS.)10 mmThe molecular details of crossing-over are discussed in Chapter 5 (see Figure5–64). Briefly, a conserved meiosis-specific protein called Spo11 initiates crossing-overby creating a double-strand break in the DNA of either a maternal or apaternal chromatid. A very large multienzyme recombination complex, containingdouble-strand DNA repair enzymes, assembles on the break and catalyzeshomologous recombination. In most cases, these events do not result in acrossover. In some cases, however, homologous recombination leads to acrossover, where DNA segments are exchanged between two nonsister chromatidsin a reciprocal fashion. As discussed earlier, after desynapsis, eachcrossover can be seen in the microscope as a chiasma (see Figure 21–10A). Asillustrated in Figure 21–10B, each of the two sister chromatids of a homolog canform one or more crossovers with either of the two sister chromatids of its partnerhomolog.Crossing-Over Is Highly RegulatedCrossing-over has two distinct functions in meiosis: it helps hold homologstogether so that they are properly segregated to the two daughter cells producedby meiosis I, and it contributes to the genetic diversification of the gametes thatare eventually produced. As might be expected, therefore, crossing-over is highlyregulated: the number and location of double-strand breaks along each chromosomeis controlled, as is the likelihood that a break will be converted into acrossover. Although the double-strand breaks that occur in meiosis I can belocated almost anywhere along the chromosome (see Figure 21–14), they are notdistributed uniformly: they cluster at “hot spots”, where the chromatin is accessible,and occur only rarely in “cold spots”, such as the heterochromatin regionsaround centromeres and telomeres.At least two kinds of regulation influence the location and number ofcrossovers that form, neither of which is well understood. Both operate beforethe synaptonemal complex assembles. One ensures that at least one crossoverforms between the members of each homolog pair, as is necessary for normalhomolog segregation in meiosis I. In the other, called crossover interference, thepresence of one crossover event inhibits another from forming close by, perhapsby locally depleting proteins required for converting a double-strand DNA breakinto a stable crossover.Meiosis Is Regulated Differently in Male and Female MammalsThe basic mechanisms of meiosis have been conserved in evolution in all sexuallyreproducing eucaryotes. In all of these organisms, for example, most of meiosis isspent in prophase I, although the details of the timing of different stages varyamong organisms (Figure 21–15). There are, however, some remarkable differencesin the regulation of meiosis in different species and in different sexes of thesame species. The difference between the two sexes is very striking in mammals.
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 and 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 and 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 and thengoes on continuously, without the stop and 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 and males, and this is especially striking in humans. About 20% ofhuman eggs are aneuploid, compared with 3–4% of human sperm, and, largelyas a result of this, up to 25% of all human fetuses are aneuploid, and 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 and 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 meiosisand 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 and produce aneuploid eggs. The malegerm line, on the other hand, 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, and 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, and the otherprocesses also differ for eggs and 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 and how these cells then become committed todeveloping into either sperm or eggs, depending on the sex of the individual.SummaryHaploid gametes (eggs, sperm, pollen, and 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. Meiosis 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 and consist of two tightly joined sister chromatids.Homologous chromosomes (homologs) then pair up side-by-side and 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 and the independent segregation of the maternal and paternalcopies of each chromosome during meiosis I have important roles in producinggametes that are genetically different from one another and from both parents. Meiosis-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 andentire meiosis IIprophase Icompletionof meiosis I andentire 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 and female gametes(sperm and eggs, respectively) of thesame species, as well as for the samegametes of different species. Meiosis 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.
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