Biochemical reconstitution of sister chromatid cohesion establishment during DNA replication

Concomitant with DNA replication, the ring-shaped cohesin complex encircles both newly synthesized sister chromatids, enabling their faithful segregation during cell divisions. Our molecular understanding of how cohesin co-entraps both replication products remains incomplete. Here, we reconstitute sister chromatid cohesion establishment using purified budding yeast proteins. Cohesin rings, initially loaded onto template DNA, remain DNA bound during complete DNA synthesis. Some of these cohesin rings encircle both sister chromatids, consistent with the idea that replisomes traverse through cohesin rings. Often, however, cohesin ends up embracing only one of the two replication products, suggesting that a two-step capture mechanism operates during cohesion establishment. Additionally, DNA replication initiates new cohesin recruitment as a further means to generate sister chromatid cohesion. Our results illustrate that more than one pathway leads to sister chromatid cohesion, and they make cohesion establishment amenable to direct biochemical exploration.