Cdks are important in the regulation of the cell cycle. they carry out their function by:
1. degrading cyclin proteins.
2. preventing the progression of a cell from one stage of the cell cycle to the next.
3. adding phosphate groups to target proteins.
4. removing phosphate groups from target proteins.

3. adding phosphate groups to target proteins.

Explanation:

CDKs are the kinases that add the phosphate groups to their target proteins. These proteins are present in an inactive form and are activated only when cyclins bind to them. The complex formed by cyclins and the respective CDKs activates the later. The activated CDKs then phosphorylate the target proteins. Some proteins are activated while the others become inactive upon phosphorylation. This is how CDKs regulate the progress of cells from one stage of the cell cycle to the other.

the pic attached shows the stages of mitosis. hope it !