What is the mechanism of cyclin action?

The cyclin A/CDK2 complex ends the S phase by phosphorylating CDC6 and E2F1. The complex facilitates the cell-cycle transition from S phase to G2 phase and also activates CDK1 by cycling A. This leads the cells to then enter the M phase. During the initiation of mitosis, CDK1 activity is regulated by the cyclin B/CDK1 complex. As the quantity of cyclin A/CDK2 complex increases, the complex initiates DNA replication. Cyclins on their own do not have any enzymatic function. They need to bind to CDKs in order to activate them. Once they are bound, they form maturation promoting factors that phosphorylate target proteins and lead to the different stages of the cell cycle.