Meiosis is the specialized cell division that generates gametes. In contrast to mitosis, molecular mechanisms and regulation of meiosis are much less understood. Meiosis shares mechanisms and regulation with mitosis in many aspects, but also has critical differences from mitosis. This review highlights these differences between meiosis and mitosis. Recent studies using various model systems revealed differences in a surprisingly wide range of aspects, including cell-cycle regulation, recombination, postrecombination events, spindle assembly, chromosome–spindle interaction, and chromosome segregation. Although a great degree of diversity can be found among organisms, meiosis-specific processes, and regulation are generally conserved.
Meiosis is a special mode of cell division, which makes haploid cells from a diploid cell. It is essential for sexual reproduction in eukaryotes and diploid organisms and produces gametes, such as eggs and sperm. Sexual reproduction is thought to be essential for long-term survival of species, as it generates diversity and mixes the genetic materials within the species. This consists of two opposite processes: meiosis, which reduces chromosome numbers from diploid to haploid, and conjugation (fertilization), which restores the diploid state by fusion of two haploid cells. Meiosis generates diversity through two events: recombination and chromosome segregation. Missegregation during meiosis results in aneuploidy in progeny or fertilized eggs. In the case of humans, it is reported that 20% of all eggs are aneuploids, most of which are results of chromosome missegregation . This is a major cause of infertility, miscarriages, and birth defects, such as Down’s syndrome, in humans. Despite the medical importance, little is known about the molecular mechanisms of meiotic chromosome segregation in humans. Understanding meiosis is not only important for its own ends, but also provides unique insights into the fundamental regulation of mitosis. As many excellent reviews already cover specific aspects of meiosis, this review gives an overview by highlighting key meiotic events and molecular regulation distinct from mitosis.
CELL-CYCLE CONTROL
In eukaryotic mitotic cycles, chromosome replication and segregation alternate. This is essential for maintaining the genome stability. This is achieved by two-step regulation of replication by Cd. The first step, called licensing, allows Mcm2–7 to be recruited to form the prereplicative complex at replication origins only in G1 when Cdk activity is low. An increase in Cdk kinase activity, together with Cdc7 kinase activity in late G1, triggers initiation of DNA replication. As a high Cdk activity inhibits the formation of the prereplicative complex, the origin will not be licensed until the mitotic exit. This dual function of Cdk ensures only one firing from each replication fork in one mitotic cycle.
In contrast, meiosis consists of two divisions without an intervening S phase, which is essential for reducing the ploidy. Suppression of the intervening S phase is achieved by maintaining the Cdk activity sufficiently high between two meiotic divisions. In Xenopus oocytes, incomplete degradation of cyclin B and a low amount of the Wee1 kinase keeps the Cdk activity high. Artificial inactivation of Cdk1 after meiosis I results in DNA replication between the two divisions .High Cdk1 activity inhibits the formation of the prereplicative complex by preventing binding of Mcm2–7 to replication origins. In Saccharomyces cerevisiae, a meiosis-specific protein kinase, Ime2, also contributes to phosphorylation of some of Cdk1 substrates to suppress replication between the two meiotic divisions.
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