Models for cohesion and cohesion-mediated chromosome segregation.

(A) SeqA-dependent precatenane removal. Positive supercoils migrate behind the replisome, entwining newly replicated sister regions. Resolution of precatenanes by Topo IV (green) is delayed by SeqA (red), which binds to hemimethylated DNA tracts behind the fork. Five to ten minutes after fork passage, DNA is remethylated by Dam (blue), releasing SeqA, and allowing Topo IV to resolve inter-sister links. SeqA may inhibit Topo IV by restraining supercoils, which transforms Topo IV-reactive hooked juxtaposition crossings (lower left brackets) to Topo IV-unreactive unhooked crossings (lower right brackets). (B) Simultaneous release of cohesion along right-arm snap regions (red) promotes abrupt sister separation and results in individualized daughter nucleoids (middle). Premature cohesion loss results in poor sister individualization (top). Deficient removal of cohesion results in late/unfinished sister separation (bottom).