The cell cycle is the process by which cells divide and multiply. It is regulated by a number of proteins, including cyclins and cdks.
Cyclins are proteins that bind to and activate cdks, which are enzymes that phosphorylate other proteins. This phosphorylation activates or inhibits the proteins, depending on the particular protein target. In this way, cyclins and cdks control the timing of events in the cell cycle.
Cyclin levels rise and fall throughout the cell cycle, with different cyclins being active at different stages. For example, cyclin D is active in early G1 phase, while cyclin E is active during late G1 phase. This ensures that each stage of the cell cycle proceeds in an orderly fashion.
Cdks are also regulated by inhibitors, which prevent them from binding to and activating cyclins. These inhibitors help to ensure that the cell cycle proceeds correctly and prevents cells from dividing too frequently.