Each time a cell divides, the whole genome must be duplicated and the two copies faithfully separated between the mother and the daughter cells. This is achieved by a complex mechanism that involves maintaining the newly replicated and the old DNA molecules (the two sister chromatids) attached while building the separating machinery. Sister chromatid cohesion (SCC) is being actively investigated in yeast.' The cohesin complex, composed of the Smci, Smc3, Scc1/Mcd1 and Scc3 subunits, forms a ring that is essential for cohesion. Two additional protein hetero-duplexes are required: the Scc2-Scc4 and the Pds5-Rad61/Wpl1 complexes. Finally, the Ctf7/ Eco1 acetyltransferase plays an important regulatory role by acetylating Smc3 during S phase; this acetylation is essential to establish cohesion, by a mechanism that is being actively studied.
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