Molecular Genetics

Special mechanisms ensure that sister chromatids segregate together during meiosis I and segregate to opposite poles during meiosis II.

Facts

• Kinetochores, of sister chromatids are controlled so that they do not attach to opposite poles during meiosis I..
• In S. cerevisiae, interference with the functioning of three genes results in bipolar attachment of sister kinetochores and failure of homologs to segregate in meiosis I. The products of these genes Mam1p (ref), Csm1P, and Lrs4 (ref), have been designated monopolins..
• Monopolins disappear from chromosomes before meiosis II.
• The centromere regions to which kinetochores are attached remain in cohesion during meiosis I while the remainder of the sister chromatids (except for sections distal to chiasmata) separate at diakinesis of the meiosis I prophase.
• Interference with the functioning of five genes results in weakening of sister chromatid cohesion. The protein products (Smc3p, Rec8p, SMC1beta, SMC3 and STAG3) form a cohesin complex.
• A protein, designated separase, has been identified that destroys the cohesin complex of proteins.
• Rec8p is bound by the protein Spo13p. This binding interferes with the cleavage of Rec8p by separase.

Interpretations

• Monopolins prevent bipolar attachment of kinetochores of sister chromatids by as yet unknown mechanisms. Monopolins thus guarantee that sister chromatids cosegregate during meiosis I.
• Failure of meiosis in the absence of sufficient monopolins probably results from the continued cohesion of the sister centromeres due to Rec8p (ref).
• The interaction of Spo13p and Rec8p prevents the complete dissolution of the cohesion of sister chromatids, further aiding in their cosegregation during meiosis I.

Further information

• Another protein required for the preservation of Rec8p at centromeres is Slk19p (ref). tis protein is centromere-localized until anaphase I.
• The Smc3p and Rec8p proteins are required for the formation of axial elements in leptotene. These proteins are localized on the axial elements and remain there. The lateral elements of the synaptonemal complex also contain cohesin complex proteins (rev).

This is page 1367 of Molecular Genetics by Ulrich Melcher, © 2003