What are cyclins? What role do they play in the cell cycle?

Cyclins are proteins that bind and activate cyclin dependent kinases (CDKs). Cyclins do this in order to help regulate the progression of the cell cycle. Cyclins activate and provide substrate specificity for the CDKs. Common classes of cyclins include G1-phase cyclins, S-phase cyclins, G1/S phase cyclins, and M-phase cyclins. M-phase cyclins form M-CDK complexes and stimulate the cell’s entry into mitosis. G1 cyclins form the G1-CDK complex and direct the cell’s progress through the G1 phase. Cyclin degradation is also crucial for progression through the cell cycle. Specific enzymes break down cyclins at specific times in the cell cycle. As cyclin levels decrease, the corresponding CDKs become inactive; the cell cycle may stop if cyclins fail to degrade.