Regulation of DNA replication during meiosis in fission yeast

The interval between meiotic nuclear divisions can be regarded as a modified mitotic cell cycle where DNA replication is blocked. Mechanisms regulating this critical aspect of meiosis that allows haploid cells to be generated from a diploid progenitor were investigated in this project. Licensing is restricted after meiosis I due to down-regulation of Cdc18 and Cdt1. Late meiotic expression of Cdc18 and Cdt1, which load the MCM helicase onto replication origins, can lead to partial DNA replication after meiosis I. This implies that block to initiation via licensing forms an important component of this regulation. As detecting any minor DNA re-replication after meiosis I requires a technique more sensitive than flow cytometry for detection of total cell DNA contents, I also investigated a procedure to allow incorporation and detection of 5-ethynyl-2'-deoxyuridine (EdU) in fission yeast.Additional inactivation of Spd1 or stabilization of Dfp1 after MI when Cdc18 and Cdt1 are also expressed does not enhance re-replication, but cyclin-dependent kinase Cdc2 plays a role in preventing re-replication during the MI-MII interval. Unexpectedly, when the licensing block is subverted, replication forks only move a short distance in the interval between meiosis I and II, implying that the elongation step of DNA replication is also inefficient. In addition, I show that the regulation of entry into meiosis II is not delayed by a partial round of DNA replication or DNA damage, indicating that replication and DNA damage checkpoints do not operate in late meiosis.