Final answer:
Cyclins specifically direct their associated Cdks to phosphorylate substrates for advancing the cell to the next phase of the cell cycle.
Step-by-step explanation:
Cyclins bind to cyclin-dependent kinases (Cdks) to regulate the cell cycle. They specifically direct their associated Cdks to the substrates that need to be phosphorylated for advancing the cell to the next phase of the cell cycle.
Cyclins and Cdks bind at specific points in the cell cycle to regulate different checkpoints. The levels of cyclin fluctuate throughout the cell cycle, while the levels of Cdk proteins remain relatively stable.