Assembly of eukaryotic replication forks. The generalized structure of an origin in S. cerevisiae is shown at the top. The prereplication complex (preRC) assembles in G₁ phase (middle). The initiator, ORC, binds to the conserved A element and the B1 element. MCM helicases are loaded onto the DNA by Cdc6 and Cdt1. After the cell progresses to S phase (bottom), the origin forms replication forks, as cyclin-dependent protein kinases (CDK and DDK) facilitate assembly of other proteins to form the replication complex (RC) from which replication fork movement commences.