1997).ĭuring and after condensation, the sister-chromatid cohesion previously established must be maintained until the metaphase/anaphase transition. 1997) and demonstrated to be required for both establishing and maintaining the condensation state of mitotic chromosomes ( Hirano and Mitchison 1994 Hirano et al. Components of the condensation machinery, a 13S protein complex termed condensin, have also been identified in Xenopus ( Hirano et al. cerevisiae smc genes and in some of the Schizosaccharomyces pombe cut genes lead to a reduction in chromosome compaction during mitosis and phenotypes indicating defects in chromosome segregation ( Saka et al. The isolation, in multiple species, of mutants defective in mitotic chromosome condensation demonstrated that condensation is a necessary prerequisite for proper chromosome segregation. Upon entry into mitosis, in prophase, chromosomes compact 5- to 10-fold in mammalian cells or 2-fold in yeast cells (for review, see Koshland and Strunnikov 1996). Although Scc1p can asssemble onto chromosomes in G 2 phase, chromosome nondisjunction nevertheless occurs if cells undergo S phase in the absence of Scc1p ( Uhlmann and Nasmyth 1998). Recently it was shown that Scc1p must associate with sister chromatids in S phase to ensure cohesion.
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(1997) found that Mcd1p/Scc1p associates with chromatin in late G 1/early S phase and dissociates from it at the onset of anaphase. (1997) demonstrated that one of these proteins, Mcd1p/Scc1p, physically associates with a member of the SMC family, SMC1, and that its levels are cell-cycle regulated, peaking in S phase, declining by late S phase, and remaining constant through telophase. 1997) provides additional evidence that sister-chromatid cohesion is established during S phase. cerevisiae proteins necessary for sister-chromatid cohesion ( Guacci et al. Results from FISH experiments in Saccharmomyces cerevisiae indicate that sister-chromatid cohesion is established during S phase ( Guacci et al. To ensure proper chromosome segregation, cohesion must be established between sister chromatids, and the simplest way to ensure the attachments are indeed between sisters would be to make these connections during or immediately after DNA replication. Third, during mitotic and meiotic spindle formation and chromosome congression, sister-chromatid cohesion must be maintained stably to resist the poleward pulling forces as kinetochores engage in microtubule interaction. Second, the dispersed interphase chromosomes must condense to facilitate chromosomal movement during segregation. First, cohesion between duplicated sister chromatids must be established during or immediately after DNA replication. Several events must be linked and coordinated to occur in a timely manner to ensure accurate segregation of chromosomes. Errors in chromosome segregation during mitosis or meiosis result in aneuploidy, which is associated with tumorigenesis, miscarriages, and congenital disorders such as Down syndrome. Thus it appears that MEI-S332 assembles into a multimeric protein complex that localizes to centromeric regions during prometaphase and is required for the maintenance of sister-chromatid cohesion until anaphase, rather than its establishment in S phase.Īccurate segregation of the genetic material is one of the most fundamental and essential cellular processes. MEI-S332 interacts with itself in the yeast two-hybrid assay and in immunoprecipitates from Drosophila oocyte and embryo extracts.
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The amino-terminal coiled-coil domain may facilitate protein–protein interactions between MEI-S332 and male meiotic proteins. The carboxy-terminal basic region is required for chromosomal localization. MEI-S332 contains two separable functional domains, as mutations within these domains show intragenic complementation. We find that MEI-S332 is first detectable on chromosomes during prometaphase, and this localization is independent of microtubules. Here, we analyze the timing of MEI-S332 assembly onto centromeres and the functional domains of the MEI-S332 protein.
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The Drosophila melanogaster MEI-S332 protein is present on centromeres in mitosis and meiosis and is essential for cohesion at the centromeres in meiosis II. Recently biochemical analysis with Xenopus extracts suggests that cohesion is established during S phase by a cohesion complex but that other proteins must maintain it in mitosis. Sister-chromatid cohesion is essential for the faithful segregation of chromosomes during cell division.