Here's why:
* Regulation of the cell cycle: CDKs are crucial for regulating the progression through the different phases of the cell cycle (G1, S, G2, M).
* Activation by cyclins: CDKs are inactive on their own. They need to be activated by binding to regulatory proteins called cyclins. Different cyclins are expressed at different stages of the cell cycle, leading to the activation of specific CDKs.
* Phosphorylation of target proteins: Once activated, CDKs phosphorylate (add a phosphate group to) specific target proteins, which are involved in various cell cycle processes like DNA replication, chromosome condensation, and nuclear envelope breakdown.
In summary, CDKs act like "molecular switches" that trigger the progression of the cell cycle by activating specific proteins at appropriate times.
While CDKs are the primary players, other enzyme classes also play roles in the cell cycle:
* Phosphatases: Remove phosphate groups from proteins, counteracting the effect of CDKs and providing a mechanism for control.
* Ubiquitin ligases: Mark proteins for degradation, helping to regulate the levels of key cell cycle proteins.
However, CDKs are the core players in the complex and tightly regulated machinery of the cell cycle.
