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

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