It has been proposed but never proven that cohesion between sister chromatids distal to chiasmata is responsible for holding homologous chromosomes together while spindles attempt to pull them toward opposite poles during metaphase of meiosis I. Meanwhile, the mechanism by which disjunction of homologs is triggered at the onset of anaphase I has remained a complete mystery. In yeast, cohesion between sister chromatid arms during meiosis depends on a meiosis-specific cohesin subunit called Rec8, whose mitotic equivalent, Sccl, is cleaved at the metaphase to anaphase transition by an endopeptidase called separin. We show here that cleavage of Rec8 by separin at one of two different sites is necessary for the resolution of chiasmata and the disjunction of homologous chromosomes during meiosis.

During meiosis, two rounds of chromosome segregation occur after a single round of DNA replication, producing haploid progeny from diploid progenitors. Three innovations in chromosome behaviour during meiosis I accomplish this unique division. First, crossovers between maternal and paternal sister chromatids (detected cytologically as chiasmata) bind replicated maternal and paternal chromosomes together. Second, sister kinetochores attach to microtubules from the same pole (mono-polar orientation), causing maternal and paternal centromere pairs (and not sister chromatids) to be separated. Third, sister chromatid cohesion near centromeres is preserved at anaphase I when cohesion along chromosome arms is destroyed. The finding that destruction of mitotic cohesion is regulated by Polo-like kinases prompted us to investigate the meiotic role of the yeast Polo-like kinase Cdc5. We show here that cells lacking Cdc5 synapse homologues and initiate recombination normally, but fail to efficiently resolve recombination intermediates as crossovers. They also fail to properly localize the Lrs4 (ref. 3) and Mam1 (ref. 4) monopolin proteins, resulting in bipolar orientation of sister kinetochores. Cdc5 is thus required both for the formation of chiasmata and for cosegregation of sister centromeres at meiosis I.

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؉ gene is essential for viability and is expressed specifically at S phase of the cell cycle. Genetic analysis revealed that rpa1؉ is the locus of the S. pombe radiation-sensitive mutation rad11. The rad11 allele exhibits pleiotropic effects consistent with an in vivo role for RPA in both DNA repair and DNA synthesis. The mutant is sensitive to both UV and ionizing radiation but is not defective in the DNA damage-dependent checkpoint, consistent with the hypothesis that RPA is part of the enzymatic machinery of DNA repair. When incubated in hydroxyurea, rad11 cells initially arrest with a 1C DNA content but then lose viability coincident with reentry into S phase, suggesting that DNA synthesis is aberrant under these conditions. A significant fraction of the mutant cells subsequently undergo inappropriate mitosis in the presence of hydroxyurea, indicating that RPA also plays a role in the checkpoint mechanism that monitors the completion of S phase. We propose that RPA is required to maintain the integrity of replication complexes when DNA replication is blocked. We further suggest that the rad11 mutation leads to the premature breakdown of such complexes, thereby preventing recovery from the hydroxyurea arrest and eliminating a signal recognized by the S-phase checkpoint mechanism.The integrity of the eukaryotic genome is maintained in part by mechanisms that ensure the fidelity of DNA replication and mediate the repair of DNA damage. Two classes of genes involved in these mechanisms have been identified by genetic experiments. One class encodes proteins that carry out the enzymatic steps in DNA replication and repair. The second class is involved in coordinating these processes with other cell cycle events. The products of the latter genes comprise surveillance mechanisms (or checkpoints) that act to delay cell cycle progression until DNA replication and/or repair are completed (40, 49). The checkpoint mechanism that monitors DNA replication normally prevents the cell from entering mitosis when DNA synthesis is blocked by inhibitors such as hydroxyurea. Similarly, the DNA damage checkpoint(s) causes mitotic delay following UV or ionizing radiation.A large number of radiation-sensitive mutants of the fission yeast Schizosaccharomyces pombe have been isolated and characterized (37). Most are defective in the enzymatic pathways responsible for repairing lesions in the DNA. However, mutants in seven linkage groups (rad1, rad3, rad9, rad17, rad26, hus1, and chk1) are competent to repair DNA damage but fail to arrest in G 2 following irradiation, indicating that they are defective in the DNA damage checkpoint (2, 44). Interestingly, mutants in six of the groups are also unable to arrest in S phase in response to inhibitors of DNA replication, suggesting that there is considerable overlap of the DNA damage and S-phase checkpoints (2,3,16,21,44). The overlap is not complete, however, since one mutant (chk1 strain) is defective exclusively in the DNA damage checkpoint (26,48).The signals recognized by the DNA damage an...