![]() However, the DNA replication program must remain inhibited between MI and MII to achieve the hallmark of meiosis, reductive cell division. This oscillation is required for multiple essential chromosome–segregation events, including duplication of the spindle pole body (SPB, the yeast centrosome) ( Buonomo et al., 2003 Fox et al., 2017 Marston et al., 2003). After MI has been completed, CDK activity decreases, and then increases again upon entry into MII ( Carlile and Amon, 2008). ![]() During meiosis, the CDK–oscillation dependence of both events presents a unique problem between MI and MII, a period known as the MI–MII transition ( Figure 1A). A low–CDK state during G1 phase allows both events to initiate, and a high–CDK state is required for their completion. In mitotic cells, both DNA replication and chromosome segregation require cyclin-dependent kinase (CDK) activity to oscillate during the cell cycle. The lack of DNA replication between MI and MII is essential for the reduction in ploidy inherent to meiosis, but it is unclear how the meiotic program differs from mitosis to allow for two sequential chromosome segregation events without an intervening S phase. ![]() In contrast, mitotically–dividing cells maintain their ploidy by strictly alternating rounds of DNA replication and chromosome segregation. These gametes are produced by meiosis, a specialized cell division program during which a single round of DNA replication is followed by two rounds of chromosome segregation (the meiotic divisions), Meiosis I (MI) and Meiosis II (MII). The production of haploid gametes is required for sexual reproduction. Together, our data demonstrate that multiple kinases inhibit both helicase loading and activation between the meiotic divisions, thereby ensuring reductive cell division. Finally, we show that CDK and the polo–like kinase Cdc5 trigger degradation of Sld2, an essential helicase–activation protein. Further analysis uncovered two previously unknown mechanisms by which Ime2 inhibits helicase loading. CDK and the meiosis–specific kinase Ime2 cooperatively inhibit helicase loading, and their simultaneous inhibition allows inappropriate helicase reloading. Using Saccharomyces cerevisiae, we show that meiotic cells inhibit both helicase loading and helicase activation to prevent DNA replication between the meiotic divisions. Yet how these two events are uncoupled between the meiotic divisions is unclear. Both DNA replication and chromosome segregation are similarly regulated by CDK oscillations in mitotic cells. Meiotic cells undergo a single round of DNA replication followed by two rounds of chromosome segregation (the meiotic divisions) to produce haploid gametes.
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