The origin recognition complex is a highly conserved heterohexameric protein complex that associates with DNA at or near initiation of DNA replication sites. All six ORC subunits are essential for initiation of DNA replication, and ORC may be involved in regulation of gene expression in response to stress. ORC binding to DNA permits the ordered binding of other proteins such as cdc6 and MCMs to form pre-replication complexes. Pre-RCs form between telophase and early G1 phase of the cell cycle and are inactivated at the onset of DNA synthesis, allowing coordinated regulation of DNA replication and cell division. Modification of one or more of the six ORC subunits may be responsible for its inactivation during S phase, but the chromatin binding behavior of the ORC subunits during the cell division cycle is still under investigation.
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