Holding Your Own: Establishing Sister Chromatid Cohesion

Chromosome Segregation and Genome Integrity At cell division, both daughter cells receive only one copy of the parental genome. Not surprisingly, chromatin plays a role at numerous stages to ensure that genome segregation occurs with high fidelity. First, each chromosome is duplicated to produce two identical chromosomes called sister chromatids. Second, sister chromatids become paired, or glued together, along the entire chromosome length (Sumner 1991; Selig et al. 1992; Guacci et al. 1994). This pairing, or cohesion, provides a mechanism by which the identity of sister chromatids can be maintained over time. Third, sister pairs associate with the mitotic spindle apparatus via kinetochores, specialized protein complexes that assemble on centromeres. This association produces a highly defined geometry: The kinetochore of one chromatid associates with microtubules from the spindle pole opposite that of her sister. Cohesion between sister chromatids facilitates this orientation, most probably by physically constraining sister kinetochores to face in opposite directions (Rieder 1982; Waters et al. 1996; Michaelis et al. 1997; Tanaka et al. 2000a). Fourth, kinetochores associated with spindle microtubules generate poleward-pulling forces that act to separate sister chromatids (McNeill and Berns 1981; Rieder et al. 1986; Skibbens et al. 1995). Cohesion opposes these kinetochore pulling forces, stretching the intervening chromatin and generating tension across the chromatid pair. This tension is critical for coordinating sister chromatid movement during mitosis (Skibbens et al. 1995), and for cell cycle progression. In support of the latter, kinetochores capture microtubules by chance, such that initial attachments are not always correct (Nicklas 1997). Fortunately, cells have evolved a mitotic checkpoint mechanism that halts cell cycle progression in response to inappropriate kinetochore–microtubule interactions or in the absence of tension (Hoyt et al. 1991; Li and Murray 1991; Nicklas et al. 1995; Li and Nicklas 1997; Waters et al. 1998, 1999; Skibbens et al. 1999). Only after all of the chromatid pairs are properly oriented to the mitotic spindle apparatus does dissolution of cohesion occur, allowing sister chromatids to segregate away from each other and move into the newly forming daughter cells (Fig. 1). Numerous advances have facilitated the molecular dissection of cis and trans factors required for establishing sister chromatid cohesion. A seminal finding that propelled forward our understanding of cohesion and cell cycle regulation was that anaphase onset (sister separation) and exit from mitosis (M-phase to interphase transition) are separable events: The dissolution of sister chromatid cohesion could now be studied in an isolated context (Holloway et al. 1993; Surana et al. 1993; Irniger et al. 1995). Other advancements, including the adaptations of fluorescent in situ hybridization (FISH) and GFP-tagged repressor/operator methodologies, allowed for the visualization of individual sister chromatid loci in yeast cells, an organism in which mutants are readily generated and the genome sequence is known (Guacci et al. 1994; Straight et al. 1996; Michaelis et al. 1997; Megee and Koshland 1999). Finally, analyses of DNA from coimmunoprecipitates, in combination with functional assays of cohesion in minichromosomes, have provided a vehicle to identify DNA sequences that participate in cohesion (Dedon et al. 1991; Meluh and Koshland 1997; Blat and Kleckner 1999; Megee and Koshland 1999; Megee et al. 1999; Tanaka et al. 1999). In this review, we take a cerevisiae-centric view of the recent advances that have brought insights, as well as intrigue, to the mechanism by which cis and trans factors participate in cohesion establishment.