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Control of the initiation of DNA replication. The replication origin is bound by the origin recognition complex (ORC) throughout the cell cycle, but ORC functions only in late mitosis and early G1 when it associates with Cdc6. ORC–Cdc6 binds the Mcm helicase, which contains six closely related subunits arranged in a barrel shape. The helicase also associates with a protein called Cdt1. Using energy provided by ATP hydrolysis, the ORC and Cdc6 proteins load two copies of the Mcm helicase around the DNA next to the origin. At the onset of S phase, S-Cdk stimulates the assembly of several accessory proteins, including Cdc45 and GINS, on each Mcm helicase. Another protein kinase, DDK, phosphorylates subunits of the Mcm helicase. The result is a large protein complex called the CMG helicase (for Cdc45–Mcm–GINS), which unwinds the DNA at the origin. DNA polymerase and other replication proteins arrive at the origin, and DNA replication begins. For clarity, this diagram does not show synthesis of the lagging strand (discussed in Chapter 5). The ORC is displaced by the replication machinery, but new ORCs bind to both replication origins after their replication. S-Cdk and other mechanisms also inactivate the loading factors ORC, Cdc6, and Cdt1, thereby preventing loading of new Mcm helicases at the origins until the end of mitosis.