What is the primary function of cyclins in the cell cycle?

Cyclins play a crucial role in regulating the cell cycle by forming complexes with cyclin-dependent kinases (CDKs). The primary function of cyclins is to activate CDKs, which are enzymes that drive the cell through various phases of the cell cycle by phosphorylating target proteins. This phosphorylation is essential for the transition between phases, such as from G1 to S phase and from G2 to M phase. Cyclin levels fluctuate throughout the cell cycle, aligning with their regulatory roles, and their binding to CDKs is what activates these kinases.

The interaction between cyclins and CDKs is finely tuned; without cyclins, CDKs remain inactive. Thus, cyclins are not just regulatory proteins but are vital for ensuring that the cell cycle progresses smoothly and accurately, preventing errors in cell division that could lead to issues such as cancer.

In relation to the other options, they do not capture the primary role of cyclins as effectively. For instance, while DNA phosphorylation occurs during the cell cycle, it is not a direct function of cyclins. Cyclins do not block mitosis; instead, they facilitate the transition to mitosis by activating the necessary kinases. Regarding the inhibition of cyclin degradation, that is part of more complex regulatory