Figure 1.

APC/C oscillation regulates pre-RC formation. In G1, APC/C activity is high and prevents accumulation of cyclin A and geminin, thereby allowing pre-RC formation. At the G1/S transition, Emi1 levels increase, inhibiting the APC/C. This allows the accumulation of cyclin A and geminin. Geminin associates with Cdt1 and prevents recruitment of MCM to the origins. Cyclin A/cdk phosphorylates Cdc6 (which translocates it out of the nucleus) and Cdt1 (which targets it for degradation by the proteasome). As cells enter G2, Emi1 is degraded, activating the APC/C. The APC/C then degrades cyclin A and geminin as cells enter the next cell cycle.