Fork dynamics during replication elongation

During an unperturbed S phase, DNA synthesis is catalyzed by the assembly of the multisubunit replisome at the replication fork (RF) to promote leading- and lagging-strand synthesis. Replisome components include the MCM helicase complex, which unwinds DNA to allow access of the Pol-alpha primase; PCNA and the PCNA loader, RFC; and the replicative polymerases Pol-delta and Pole. When a replication block, such as one resulting from hydroxyurea-mediated depletion of nucleotide pools, is encountered, fork progression stalls. The replisome is stabilized by factors that associate with the RF, allowing stalled forks to restart replication once the block has been removed. These factors include DNA helicases (Rrm3, Sgs1), checkpoint mediators (Mrc1, Tof1, Csm3) and a histone chaperone-nucleosome assembly factor (Asf1). If RFs are not stabilized, the fork collapses, leading to ssDNA gaps and double-strand breaks, which activate the intra-S phase checkpoint. Alternative, error-prone pathways are used to restart replication and can result in genome instability. The Ino80 ATPase subunit of the INO80 nucleosome remodeling complex is recruited to stalled RFs to stabilize the replisome. INO80 could act on nucleosomes either in front of or behind the fork to preserve RF integrity.