Figure 1.

Eukaryotic initiation of DNA replication. Relevant complexes that have been characterized by electron microscopy are shown in surface view. (A) ORC is chromatin bound throughout the cell cycle (Electron Microscopy Data Bank [EMDB]: 1156). (B) ORC/Cdc6 is the landing platform MCM2–7/Cdt1 (EMDB: 5381). (C) The association of MCM2–7/Cdt1 (EMDB: 6671) with ORC/Cdc6 results in the OCCM (EMDB: 8540) formation with the dsDNA inserted into MCM2–7 hexamer. (D) Cdt1 and Cdc6 are released from the OCCM in an ATP hydrolysis-dependent reaction, and, upon recruitment of another Cdc6, the OCM the complex is formed. The OCM is an essential intermediate in the loading reaction and is responsible for the recruitment of a second MCM2–7/Cdt1 heptamer, although the details of the reaction are currently not known. (E) The final product of the loading reaction is a MCM2–7 DH embracing dsDNA (EMDB: 6338). This inactive complex is a stable DNA replication intermediate, which becomes activated only in S phase. (F) Preinitiation of DNA replication in S phase relies on Dbf4-dependent kinase (DDK)-dependent phosphorylation of the DH and a plethora of factors that interact with the DH. One of the landmarks of preinitiation complex formation is the binding of Cdc45 and GINS (from the Japanese go-ichi-ni-san, meaning 5-1-2-3, after the four related subunits of the complex: Sld5, Psf1, Psf2, and Psf3) to MCM2–7, resulting in formation of the replicative helicase: the Cdc45/MCM2–7/GINS (CMG) complex. (G) During the DNA-unwinding process, the CMG (EMDB: 8518) associates with both polymerases ε and α into a replication fork (RFK) to synthesize the leading and lagging strands. The helicase is propelled by the C-terminal AAA⁺ motor domains, and the unwinding takes place on the N-terminal face.