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a nuclear envelope re-forms

Cytokinesis

The cytoplasm divides

*Only in cells in which the spindle has broken down, chromosomes have relaxed, and the nuclear envelope has re-formed in telophase I. Other types of cells skip directly to metaphase II after cytokinesis.

*Only in cells in which the spindle has broken down, chromosomes have relaxed, and the nuclear envelope has re-formed in telophase I. Other types of cells skip directly to metaphase II after cytokinesis.

Consequences of Meiosis

What are the overall consequences of meiosis? First, meiosis comprises two divisions; so each original cell produces four cells (there are exceptions to this generalization, as, for example, in many female animals; see Figure 2.22b). Second, chromosome number is reduced by half; so cells produced by meiosis are haploid. Third, cells produced by meiosis are genetically different from one another and from the parental cell.

Genetic differences among cells result from two processes that are unique to meiosis. The first is crossing over, which takes place in prophase I. Crossing over refers to the exchange of genes between nonsister chromatids (chromatids from different homologous chromosomes). At one time, this process was thought to take place in pachytene (Figure 2.15b), and the synaptonemal complex was believed to be a requirement for crossing over. However, recent evidence from yeast suggests that the situation is more complex, as shown in Figure 2.16. Crossing over is initiated in zygotene, before the synaptonemal complex develops, and is not completed until near the end of prophase I.

After crossing over has taken place, the sister chromatids may no longer be identical. Crossing over is the basis for intrachromosomal recombination, creating new combinations of alleles on a chromatid. To see how crossing over produces genetic variation, consider two pairs of alleles, which we will abbreviate Aa and Bb. Assume that one chromosome possesses the A and B alleles and its homolog possesses the a and b alleles ( FIGURE 2.17a). When DNA is replicated in the S stage, each chromosome duplicates, and so the resulting sister chromatids are identical ( FIGURE 2.17b).

In the process of crossing over, breaks occur in the DNA strands and the breaks are repaired in such a way that segments of nonsister chromatids are exchanged ( FIGURE 2.17c). The molecular basis of this process will be described in more detail in Chapter 12; the important thing here is that, after crossing over has taken place, the two sister chro-matids are no longer identical — one chromatid has alleles A and B, whereas its sister chromatid (the chromatid that underwent crossing over) has alleles a and B. Likewise, one chromatid of the other chromosome has alleles a and b, and the other has alleles A and b. Each of the four chromatids now carries a unique combination of alleles: A B, a B, A b, and a b. Eventually, the two homologous chromosomes separate, each going into a different cell. In meiosis II, the two chromatids of each chromosome separate, and thus each of the four cells resulting from meiosis carries a different combination of alleles ( FIGURE 2.17d).

The second process of meiosis that contributes to genetic variation is the random distribution of chromosomes in anaphase I of meiosis following their random alignment during metaphase I. To illustrate this process, consider a cell with three pairs of chromosomes I, II, and III (< Figure 2.18a). One chromosome of each pair is maternal in origin (Im, IIm, and III^; the other is paternal in origin (Ip, IIp, and IIIp). The chromosome pairs line up in the center of the cell in metaphase I and, in anaphase I, the chromosomes of each homologous pair separate.

How each pair of homologs aligns and separates is random and independent of how other pairs of chromosomes align and separate ( FIGURE 2.18b). By chance, all the maternal chromosomes might migrate to one side, with all the paternal chromosomes migrating to the other. After division, one cell would contain chromosomes Im, IIm, and IIIm, and the other, Ip, IIp, and IIIp. Alternatively, the Im, IIm, and IIIp chromosomes might move to one side, and the Ip, IIp, and IIIm chromosomes to the other. The different migrations would produce different combinations of chromosomes in the resulting cells ( FIGURE 2.18c). There are four ways in which a diploid cell with three pairs of chromosomes can divide, producing a total of eight different ffiOne chromosome |2| ...and the homologous chromosome possesses the a and b genes.

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