Sources of Genetic Variation in Meiosis
What are the overall consequences of meiosis? First, meiosis comprises two divisions, so each original cell produces four cells (although there are exceptions to this generalization, as, for example, in many female animals; see Figure 2.17b). 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 parent cell. These genetic differences result from two processes that are unique to meiosis: crossing over and the random separation of homologous chromosomes.
CROSSING OVER Crossing over, which takes place in prophase I, refers to the exchange of genetic material between nonsister chromatids (chromatids from different homologous chromosomes). After crossing over has taken place, the sister chromatids are no longer 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 2.13a). When DNA is replicated in the S phase, each chromosome duplicates, and so the resulting sister chromatids are identical (Figure 2.13b).
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In the process of crossing over, there are breaks in the DNA strands, and the breaks are repaired in such a way that segments of nonsister chromatids are exchanged (Figure 2.13c). The molecular basis of this process will be described in more detail in Chapter 9; the important thing here is that after crossing over has taken place, the two sister chromatids 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, and each ends up in 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 2.13d). You can see how crossing over affects genetic variation by viewing Animation 2.3.
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RANDOM SEPARATION OF HOMOLOGOUS CHROMOSOMES The second process of meiosis that contributes to genetic variation is the random distribution of chromosomes in anaphase I after their random alignment in metaphase I. To illustrate this process, consider a cell with three pairs of chromosomes, I, II, and III (Figure 2.14a). One chromosome of each pair is maternal in origin (Im, IIm, and IIIm); 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 align and separate (Figure 2.14b). By chance, all the maternal chromosomes might migrate to one side and all the paternal chromosomes 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 2.14c). There are four ways in which the chromosomes in a diploid cell with three homologous pairs can migrate, producing a total of eight different combinations of chromosomes in the gametes. In general, the number of possible combinations is 2n, where n equals the number of homologous pairs. As the number of chromosome pairs increases, the number of combinations quickly becomes very large. In humans, who have 23 pairs of chromosomes, 8,388,608 different combinations of chromosomes are made possible by the random separation of homologous chromosomes. You can explore the random distribution of chromosomes by viewing Animation 2.3. The genetic consequences of this process, termed independent assortment, will be explored in more detail in Chapter 3.
In summary, crossing over shuffles alleles on the same chromosome into new combinations, whereas the random distribution of maternal and paternal chromosomes shuffles alleles on different chromosomes into new combinations. Together, these two processes are capable of producing tremendous amounts of genetic variation among the cells resulting from meiosis. 
TRY PROBLEMS 29 AND 30
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CONCEPTS
Meiosis consists of two distinct processes: meiosis I and meiosis II. Meiosis I includes the reduction division, in which homologous chromosomes separate and chromosome number is reduced by half. In meiosis II (the equational division) chromatids separate.
CONCEPT CHECK 5
Which of the following events takes place in metaphase I?
Crossing over occurs.
The chromosomes condense.
Homologous pairs of chromosomes line up on the metaphase plate.
Individual chromosomes line up on the metaphase plate.
c
CONNECTING CONCEPTS
Mitosis and Meiosis Compared
Now that we have examined the details of mitosis and meiosis, let’s compare the two processes (Figure 2.15 and Table 2.3). In both mitosis and meiosis, the chromosomes condense and become visible; both processes include the movement of chromosomes toward the spindle poles; and both are accompanied by cell division. Beyond these similarities, the processes are quite different.
Mitosis results in a single cell division and usually produces two daughter cells. Meiosis, in contrast, comprises two cell divisions and usually produces four cells. In diploid cells, homologous chromosomes are present before both meiosis and mitosis, but the pairing of homologs takes place only in meiosis.
Another difference is that in meiosis, chromosome number is reduced by half as a consequence of the separation of homologous pairs of chromosomes in anaphase I, but no chromosome reduction takes place in mitosis. Furthermore, meiosis is characterized by two processes that produce genetic variation: crossing over (in prophase I) and the random distribution of maternal and paternal chromosomes (in anaphase I). There are normally no equivalent processes in mitosis.
Mitosis and meiosis also differ in the behavior of chromosomes in metaphase and anaphase. In metaphase I of meiosis, homologous pairs of chromosomes line up on the metaphase plate, whereas individual chromosomes line up on the metaphase plate in metaphase of mitosis (and in metaphase II of meiosis). In anaphase I of meiosis, paired chromosomes (each possessing two chromatids attached at the centromere) separate and migrate toward opposite spindle poles. In contrast, in anaphase of mitosis (and in anaphase II of meiosis), sister chromatids separate, and each chromosome that moves toward a spindle pole is unreplicated. TRY PROBLEMS 25 AND 26
| Event | Mitosis | Meiosis I | Meiosis II |
|---|---|---|---|
| Cell division | Yes | Yes | Yes |
| Chromosome reduction | No | Yes | No |
| Genetic variation produced | No | Yes | No |
| Crossing over | No | Yes | No |
| Random distribution of maternal | No | Yes | No and paternal chromosomes |
| Metaphase | Individual chromosomes line up | Homologous pairs line up | Individual chromosomes line up |
| Anaphase | Chromatids separate | Homologous chromosomes separate | Chromatids separate |