Final answer:
Cyclins bind to Cdks to form a complex that is necessary for the progression through the phases of the cell cycle.
Step-by-step explanation:
The cell cycle requires a cyclin to bind to a Cdk because the Cdk/cyclin complex is necessary for the progression through the different phases of the cell cycle. Cdks are enzymes that phosphorylate other proteins, which activates them and allows the cell cycle to proceed. When cyclins bind to Cdks, it triggers the phosphorylation and activation of proteins that advance the cell to the next phase of the cycle.