Most eukaryotic organisms reproduce sexually: the genomes of two parents mix to generate offspring that are genetically distinct from either parent. The cells of these organisms are generally diploid: that is, they contain two slightly different copies, or homologs, of each chromosome, one from each parent. Sexual reproduction depends on a specialized nuclear division process called meiosis, which produces haploid cells carrying only a single copy of each chromosome. In many organisms, the haploid cells differentiate into specialized reproductive cells called gametes—eggs and sperm in most species. In these species, the reproductive cycle ends when a sperm and egg fuse to form a diploid zygote, which has the potential to form a new individual. In this section, we consider the basic mechanisms and regulation of meiosis, with an emphasis on how they compare with those of mitosis.
meiosis Includes Two Rounds of Chromosome segregation
Meiosis reduces the chromosome number by half using many of the same molecular machines and control systems that operate in mitosis. As in the mitotic cell cycle, the cell begins the meiotic program by duplicating its chromosomes in meiotic S phase, resulting in pairs of sister chromatids that are tightly linked along their entire lengths by cohesin complexes. Unlike mitosis, however, two successive rounds of chromosome segregation then occur (Figure 17–53). The first of these divisions (meiosis I) solves the problem, unique to meiosis, of segregating the homologs. The duplicated paternal and maternal homologs pair up alongside each other and become physically linked by the process of genetic recombination. These pairs of homologs, each containing a pair of sister chromatids, then line up on the first meiotic spindle. In the first meiotic anaphase, duplicated homo-logs rather than sister chromatids are pulled apart and segregated into the two daughter nuclei. Only in the second division (meiosis II), which occurs without further DNA replication, are the sister chromatids pulled apart and segregated (as in mitosis) to produce haploid daughter nuclei. In this way, each diploid nucleus that enters meiosis produces four haploid nuclei, each of which contains either the maternal or paternal copy of each chromosome, but not both (Movie 17.7).
Figure 17–53 Comparison of meiosis and mitosis. For clarity, only one pair of homologous chromosomes (homologs) is shown.
(a) meiosis is a form of nuclear division in which a single round of chromosome duplication (meiotic s phase) is followed by two rounds of chromosome segregation. The duplicated homologs, each consisting of tightly bound sister chromatids, pair up and are segregated into different daughter nuclei in meiosis I; the sister chromatids are segregated in meiosis II. as indicated by the formation of chromosomes that are partly red and partly gray, homolog pairing in meiosis leads to genetic recombination (crossing-over) during meiosis I. Each diploid cell that enters meiosis therefore produces four genetically different haploid nuclei, which are distributed by cytokinesis into haploid cells that differentiate into gametes. (B) In mitosis, by contrast, homologs do not pair up, and the sister chromatids are segregated during the single division. Thus, each diploid cell that divides by mitosis produces two genetically identical diploid daughter nuclei, which are distributed by cytokinesis into a pair of daughter cells.
During mitosis in most organisms, homologous chromosomes behave independently of each other. During meiosis I, however, it is crucial that homologs recognize each other and associate physically in order for the maternal and paternal homologs to be bi-oriented on the first meiotic spindle. Special mechanisms mediate these interactions.