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First published online August 17, 2004
doi: 10.1242/10.1242/jcs.01352

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Modifying sister chromatid cohesion for meiosis

Institute of Molecular and Cellular Biosciences, and SORST, Japan Science and Technology Agency, University of Tokyo, Yayoi 1-1-1, Tokyo 113-0032, Japan

View larger version (31K): [in a new window]   Fig. 1. Chromosome segregation in mitosis and meiosis. Sister chromatid cohesion during mitosis, carried out by Rad21/Scc1 complexes, is lost along the whole chromosome length at anaphase. During meiosis, Rec8 substitutes for the Rad21/Scc1 subunit of the cohesin complex. Consequently, sister chromatid cohesion is lost in a stepwise manner, first along chromosome arms during meiosis I and second in the vicinity of centromeres during meiosis II.

View larger version (18K): [in a new window]   Fig. 2. The cohesin complex and its dissociation by separase. A proposed model for the cohesin complex and its dissociation by separase. In mitosis, the onset of anaphase is triggered by the separase-dependent cleavage of Rad21/Scc1, a cohesin subunit that might bind the two ends of Smc1-Smc3 heterodimer and thereby hold sister chromatids. During meiosis, Rad21/Scc1 is largely replaced by Rec8 and Psc3 is partly replaced by Rec11.

View larger version (18K): [in a new window]   Fig. 3. Monopolar attachment at meiosis I. The centromeres of fission and budding yeast are different. The requirement of Rec8 for monopolar attachment is obvious in fission yeast but not in budding yeast, in which the ectopic expression of Scc1 can restore monopolar attachment even in rec8 cells. Instead, monopolin complexes are required for monopolar attachment in budding yeast.

View larger version (27K): [in a new window]   Fig. 4. Shugoshin protects centromeric Rec8 during meiosis I. At the onset of anaphase I, Sgo1 protects centromeric Rec8 from separase cleavage; centromeric cohesion is thereby preserved until meiosis II. At the onset of anaphase II, the residual Rec8 is cleaved, which results in the separation of sister chromatids. In sgo1 cells, all Rec8 is disrupted at meiosis I; therefore, sister chromatid cohesion is lost prior to meiosis II. This brings about random segregation in meiosis II.