19.8 Meiosis: A Special Type of Cell Division

In nearly all diploid eukaryotes, meiosis generates haploid germ cells (eggs and sperm), which can then fuse with a germ cell from another individual to generate a diploid zygote that develops into a new individual. Meiosis is a fundamental aspect of the biology and evolution of all eukaryotes because it results in the reassortment of the chromosome sets received from an individual’s two parents. Chromosome reassortment and homologous recombination between parent DNA molecules during meiosis guarantee that each haploid germ cell generated will receive a unique combination of alleles that is distinct from each parent as well as from every other haploid germ cell formed.

The mechanisms of meiosis are analogous to those of mitosis. However, several key features of meiosis allow this process to generate genetically diverse haploid cells (see Figure 6-3). In the mitotic cell cycle, each S phase is followed by chromosome segregation and cell division. In contrast, during meiotic cell division, one round of DNA replication is followed by two consecutive chromosome segregation phases. This process leads to the formation of haploid, rather than diploid, daughter cells. During the two divisions, maternal and paternal chromosomes are shuffled and divided so that the daughter cells are different in genetic makeup from the parent cell. In this section, we discuss the similarities between mitosis and meiosis as well as the meiosis-specific mechanisms that transform the canonical mitotic cell cycle machinery so that it brings about the unusual cell division that leads to the formation of haploid daughter cells.

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