What are cyclins and cyclin-dependent kinases, and how do they interact?

Cyclins and Cyclin Dependent Kinases (CDKs) are the two types of regulators in the cell cycle. Cyclins regulate the cell cycle exclusively when they are bound to CDKs. Cyclins rise and fall in a cyclical manner, hence their name. Different types of cyclins are expressed in different stages of the cell cycle such as G1 cyclins (Cyclin D), S phase cyclins (cyclin E), and mitotic cyclins (Cyclin B). CDKs are catalytic proteins involved in the cell cycle that possess enzymatic function after being activated by cyclin. These proteins are involved in advancing the cell to the next phase of the cell cycle. In order to be activated, the cyclin-CDK complex must be phosphorylated in specific regions. The levels of CDK proteins remain stable throughout the cell cycle. However, concentrations of cyclin vary and determine when the cyclin-CDK complexes form. The different cyclins and CDKS bind at specific points during the cell cycle and therefore modulate different checkpoints.