A diploid organism has two sets of chromosomes (2n): one set derived from its male parent, and the other from its female parent. As the organism grows and develops, its cells undergo mitotic divisions. In mitosis, each chromosome behaves independently of its homolog, and its two chromatids are sent to opposite poles during anaphase. Each daughter nucleus ends up with 2n chromosomes. In meiosis, things are very different. Figure 11.18 compares the two processes.

In meiosis I, chromosomes of maternal origin pair with their paternal homologs during synapsis. To clarify, you have two copies of chromosome 1 of the human genome in your cells. One copy came from your mother and the other from your father. In meiosis I, these two chromosome 1’s pair up. And so on with the other 22 pairs of chromosomes. Pairing of maternal and paternal homologs does not occur in mitosis. Segregation of the homologs during meiotic anaphase I ensures that each newly formed cell receives one member of each homologous pair (see steps 4–6 of Figure 11.15). Returning to the example of chromosome 1, at the end of meiosis I in your cells, each daughter nucleus contains one copy of chromosome 1, whereas the diploid cell that began meiosis had two copies. Taking the other 22 pairs of chromosomes into account, at the end of meiosis I in humans, each of the two daughter cells has 23 of the original 46 chromosomes.

Recall that before meiosis begins each chromosome is duplicated in S phase of interphase. So the beginning cell of meiosis had four copies of each chromosomal DNA molecule. After meiosis I each daughter cell therefore has two copies. In meiosis II, each product of meiosis I divides further to form two haploid gametes, with one copy of each chromosomal DNA molecule. So the end product of meiosis II is four haploid gametes.

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Crossing over is one reason for the genetic diversity among the products of meiosis; another is *independent assortment, whereby each haploid cell receives an entire set of genes, but only one of each pair from each parent, from the diploid cell in a random fashion. It is a matter of chance which member of a homologous pair goes to which daughter cell at anaphase I. For example, imagine there are two homologous pairs of chromosomes in the diploid parent nucleus. A particular daughter nucleus could receive the paternal chromosome 1 and the maternal chromosome 2. Or it could get paternal 2 and maternal 1, or both maternal, or both paternal. It all depends on the way in which the homologous pairs line up at metaphase I.

Note that of the four possible chromosome combinations just described, only two produce daughter nuclei with full complements of either maternal or paternal chromosome sets (apart from the material exchanged by crossing over). The greater the number of chromosomes, the less probable it is that the original parental combinations will be reestablished, and the greater the potential for genetic diversity. Most species of diploid organisms have more than two pairs of chromosomes. In humans, with 23 chromosome pairs, 2²³ (8,388,608) different combinations can be produced just by the mechanism of independent assortment. Taking the extra genetic shuffling afforded by crossing over into account, the number of possible combinations is very large indeed! Crossing over and independent assortment, along with the processes that result in mutations, provide the genetic diversity needed for the differential survival and reproduction of diverse individuals—the basis of evolution by natural selection.

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You have seen how meiosis I is fundamentally different from mitosis. In contrast, meiosis II is similar to mitosis in that it involves the separation of sister chromatids into daughter nuclei (see steps 7–11 in Figure 11.15). However, because of crossing over during meiosis I, the sister chromatids are not necessarily identical to one another as they would be in mitosis. Chance assortment of the chromatids during meiosis II contributes further to the genetic diversity of the meiotic products. The final products of meiosis I and meiosis II are four haploid daughter cells, each with one set (n) of chromosomes, each of which is genetically non-identical.