Fig. 2. Early models of sister chromatid pairing reactions. Top: `replication-coupled cohesin pairing' positions Ctf7p (red triangle) at the replication fork (DNA polymerases as hexagons) to tether together cohesin complexes as nascent sister chromatids emerge. Cohesins are depicted as rings (pre-loaded cohesin rings are shown in green; cohesin loading coincident with replication is shown in yellow) that encircle chromatin (they could also associate laterally; not shown). Several possibilities for cohesion are shown, including various cohesin ring associations or tethering by an additional factor (purple star). Bottom: `replication through a ring' allows Ctf7p-independent establishment. Here, the replication fork passes through a pre-loaded cohesin ring that entraps both sister chromatids, passively establishing cohesion.