Fig. 4. Multiple mechanisms of cohesion. Cohesion between any sister-chromatid loci is the result of cooperation between at least two mechanisms of cohesion: DNA catenations (top) and cohesin (bottom). On one hand, the cohesin complex – formed by Smc1, Smc3, Mcd1/Rad21/Scc1 and Scc3/SA1/SA2 – is thought to tether the two sister chromatids together by physical entrapment. DNA catenations, on the other hand, provide sister-chromatid cohesion by the intertwinement of the two chromatids. The contribution of each mechanism at a specific locus might be influenced by factors such as the spacing of catenations, the location of cohesin-binding regions, chromatin structure and changes in chromatid cohesion that are induced after DNA replication (e.g. DNA damage or de novo cohesin loading) (Kim et al., 2002; Nagao et al., 2004; Potts et al., 2006; Sjogren and Nasmyth, 2001; Strom et al., 2004; Strom et al., 2007; Unal et al., 2007). The existence of different cohesion mechanisms is advantageous because it allows differential regulation of cohesion at specific chromosome regions. Both cohesin and catenations are subject to complex regulatory mechanisms and have to be concertedly removed during anaphase, possibly by post-translational modifications such as phosphorylation and sumoylation. Some of these regulatory mechanisms are depicted here.