Abstract 9316168 Orr-Weaver The Drosophila gene mei-S332 has been shown to be critical for maintaining sister-chromatid cohesion in meiosis because mutations in the gene cause the sister chromatids to prematurely disjoin late in the first meiotic division. The defect in mei-S332 mutants is manifest at a time when the sister chromatids are held together only at their centromere regions, suggesting that the product of this gene acts at the centromere or kinetochore to promote cohesion in mitosis. The molecular mechanisms by which mei-S332 controls sister-chromatid cohesion will be elucidated by isolating the gene, identifying the protein it encodes, and analyzing where that protein is localized on the chromosomes. The nature of unusual sex-specific mutations will be determined. Experiments will investigate whether the mei-S332 protein is modified at the metaphase-anaphase transition, the time at which sister-chromatid segregation will be identified by their interaction with the mei- S332 protein, using biochemical and genetic approaches. Two maternal effect mutants have been identified in Drosophila, grauzone and cortex, that have the opposite phenotype as mei-S332 mutants. In these mutants meiosis is arrested at metaphase II, prior to the separation of sister chromatids. These genes may act in a regulatory manner to signal the metaphase-anaphase transition, or they may promote the separation of sister-chromatids, possibly by causing the loss of cohesion. Genetic and cell biology experiments will define the role of these genes in meiosis and mitosis and their relationship to mei-S332. The grauzone gene will be isolated in order to understand the molecular basis of its action in sister-chromatid segregation. *** The proper segregation of chromosomes is essential to ensure that a complete complement of genetic information is transmitted to progeny or daughter cells. Prior to chromosome segregation a copy of each chromosome is replicated. During mitosis th ese sister chromatids segregate to daughter cells. In the first meiotic division the sister chromatids migrate as a unit to the same pole as the homologous copies of each chromosome segregate; the sister chromatids do not segregate until the second meiotic division. One aspect of chromosome segregation that is poorly understood is how the sister chromatids remain attached until their separation in mitosis or meiosis. Sister-chromatid cohesion is likely to be mediated through structural functions that hold the sister chromatids together and regulatory functions that time their dissociation. The control of sister-chromatid segregation will be investigated using Drosophila as a model system because it affords the combined approaches of genetics, cell biology, and molecular biology. %%%
