Glaser, S.; Molodtsov, M. I.; Diffley, J. F.; Uhlmann, F.

Following eukaryotic genome replication, the two newly synthesised sister chromatids remain paired by the ring-shaped cohesin complex, enabling their faithful segregation to daughter cells during cell divisions. Cohesin topologically embraces DNA already before DNA replication, and replisome passage through the cohesin ring is thought of as a fail-safe mechanism ensuring that cohesin entraps both replication products. Whether replisomes indeed pass through cohesin rings remains unknown. Here, we use biochemical reconstitution4,8,9 and single molecule fluorescence microscopy to directly visualise replisome-cohesin encounters. We find that the translocating eukaryotic replicative Cdc45-Mcm2-7-GINS (CMG) helicase is frequently an obstacle to cohesin, but that the likelihood with which CMG passes cohesin increases in the presence of replisome components with known sister chromatid cohesion functions, or by preventing cohesin from freely sliding along the template DNA. Cohesin retains topological DNA entrapment during CMG passage, suggesting that the helicase passes through the ring. The passage frequency increases further when fully reconstituted replisomes encounter cohesin rings, resulting in successful establishment of cohesion between the two replication products. Our findings demonstrate the existence of a simple mechanism that links genome replication with chromosome segregation, replisome passage through cohesin rings.