Meiosis: A Special Type of Cell Division
■ Meiosis involves one cycle of chromosome replication followed by two cycles of cell division to produce haploid germ cells from a diploid premeiotic cell (see Figure 21-34).
■ During meiosis I, replicated homologous chromosomes pair along their lengths in a process called synapsis. At least one recombination between chromatids of homologous chromosomes almost invariably occurs.
■ Most of the cell-cycle proteins that function in mitoti-cally dividing cells also function in cells undergoing meio-sis, but some proteins are unique to meiosis.
■ In sS. cerevisiae, expression of G1 cyclins (Clns) is repressed throughout meiosis. Meiosis-specific Ime2 per-
pair toward one spindle pole and the other homologous pair toward the other) whether attached to the meiosis I spindle (left) or meiosis II spindle (right). Similarly, the meiosis II chromosomes (red) separated normally (i.e., one chromatid toward one spindle pole and the other chromatid toward the opposite pole) independent of which spindle they were attached to. These results indicate that the association of kinetochores with spindle microtubules and the stability of cohesins linking chromosomes are determined by factors that are associated with the chromosomes and not by the spindles or soluble components of cells in meiosis I and II. [See L. V. Paliulis and R. B. Nicklas, 2000, J. Cell Biol. 150:1223.]
forms the function of G1 cyclin-CDK complexes in promoting initiation of DNA replication during meiosis I. DNA replication does not occur during meiosis II because neither Ime2 nor G1 cyclins are expressed.
■ In sS. cerevisiae, recombination (crossing over) between chromatids of homologous parental chromatids and cohesin cross-links between chromatids distal to the crossover are responsible for synapsis of homologous chromosomes during prophase and metaphase of meiosis I. A specialized cohesin subunit, Rec8, replaces the Sccl cohesin subunit during meiosis.
■ During early anaphase of meiosis I, Rec8 in the chromosome arms is cleaved, but a meiosis-specific protein associated with the kinetochore protects Rec8 in the region of the centromere from cleavage. As a result, the chro-matids of homologous chromosomes remain associated during segregation in meiosis I. Cleavage of centromeric Rec8 during anaphase of meiosis II allows individual chromatids to segregate into germ cells (see Figure 21-35b).
■ Monopolin, another meiosis-specific protein, is required for both chromatids of homologous chromosomes to associate with microtubules emanating from the same spindle poles during meiosis I.
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