Cyclins

Cyclins are key regulators of the cell cycle, different cyclins are expressed at different stages of the cell cycle.

Cell cycle and cyclins. Cyclin D, D1-3, and E are synthesized and are active in the G₁ phase. They regulate the transition from the G₁ to the S phase. To do so, they must be phosphorylated (P). Cyclins A and B1-2 control the transition from G₂ to M. To exercise this action, cyclin B must bind to cdc2, a constitutively produced inactive kinase. The complex cyclin B/cdc2 is activated by kinases and phosphatases (cdc25) inducing mitosis (M). Following mitosis, the cells can reenter the cell cycle or pass to a quiescent state (G₀).