ORC or Origin Recognition Complex is a multisubunit complex (6 subunits) existing in the replication procedure of DNA. It marks the replication origin by binding to and marking ori sequences in all eukaryotes in an ATP-dependent manner. During the G1 phase of the cell cycle, when CDK levels are low, ORC recruits cdc6 and cdt1 proteins. Cdc6 and cdt1 block DNA helicase. During the transition to S phase CDK levels increase, CDK phosphorylates cdc6 and cdt1, and they are removed. Now DNA helicase can open the double stranded DNA and replication begins. DDK also participates in ORC activation and triggers the transition into DNA replication.

it:Origin recognition complex