The time between chromosome replication and segregation can be from hours to decades. Cohesion is thus crucial for identifying replication products as sister chromatids from S-phase until mitosis. Early models posited active sister chromatid tethering reactions in which cohesins deposited onto each sister chromatid are converted to a paired state by replication-fork-associated establishment factors. Subsequent, but now largely marginalized, models suggested instead that establishment occurs passively - requiring only cohesin preloading and passage of the replication fork through huge cohesin rings. More recent models return to active establishment reactions but remain predicated on preloaded ring structures. Here, new models are presented in which replication-coupled cohesin deposition is followed by conversion to a pairing-competent C-clamp structure that does not require DNA entrapment.
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