Replisome passage through the cohesin ring

Following eukaryotic genome replication, the ring-shaped cohesin complex embraces the two newly synthesized sister chromatids, enabling their faithful segregation during cell divisions. Replisome passage through cohesin rings has been envisioned as a fail-safe mechanism that ensures co-entrapment of replication products—whether replisomes can indeed pass through cohesin rings remains unknown. Here, we use biochemical reconstitution and single-molecule fluorescence microscopy to directly visualize replisome-cohesin encounters. We find that the translocating eukaryotic replicative Cdc45-Mcm2-7-GINS (CMG) helicase, unlike other obstacles of similar size, readily passes through cohesin rings. Fully reconstituted replisomes also pass cohesin rings to leave both replication products trapped inside. Replisome passage is primarily aided by DNA polymerases α and ε, a finding that necessitates re-evaluation of canonical cohesion establishment factor roles. Our findings demonstrate the existence of a simple mechanismthat links genome replication with chromosome segregation: replisome passage through cohesin rings.