Key Concepts of Section 19.8

• Meiosis is a specialized type of cell division in which meiosis-specific gene products modulate the mitotic cell division program (see Figure 19-34).

• The meiotic division comprises one cycle of chromosome replication followed by two cycles of cell division to produce haploid germ cells from a diploid pre-meiotic cell. During meiosis I, homologous chromosomes are segregated; during meiosis II, sister chromatids separate.

• Specialized environmental conditions induce a developmental program that leads to the meiotic divisions.

• During prophase of meiosis I, homologous chromosomes undergo recombination. At least one recombination event occurs between the chromatids of each pair of homologous chromosomes.

• Chiasmata and cohesins distal to them are responsible for holding the homologous chromosomes together during prophase and metaphase of meiosis I.

• At the onset of anaphase of meiosis I, cohesins on chromosome arms are phosphorylated and, as a result, cleaved by separase, but cohesins in the region of the centromere are protected from phosphorylation and cleavage. This protection is brought about by a meiosis-specific cohesin subunit and a protein phosphatase that associates with centromeres. As a result, the sister chromatids remain linked to each other during segregation in meiosis I.

• Cleavage of centromeric cohesins during anaphase of meiosis II allows individual chromatids to segregate into germ cells.

• Meiotic cohesins facilitate the attachment of sister kinetochores to microtubules emanating from the same pole during meiosis I.

• Incomplete CDK inactivation between the two meiotic divisions inhibits DNA replication.