Final answer:
Cdks are inactivated in G1 due to the fluctuating levels of cyclins, which are necessary for forming active Cdk/cyclin complexes, and the presence of Cdk inhibitors, which ensure the cell cycle does not advance without meeting specific conditions.
Step-by-step explanation:
Cyclin-dependent kinases (Cdks) are stably inactivated in G1 because the concentrations of cyclins fluctuate, dictating when the Cdk/cyclin complexes can form. Without specific concentrations of fully activated cyclin/Cdk complexes, the cell cycle cannot proceed through the checkpoints. Furthermore, Cdk inhibitors play a significant role in blocking the full activation of Cdks until certain cell cycle events are properly monitored and completed. This is important for cell cycle control and preventing uncontrolled cell division, which can lead to cancer. Mechanisms that utilize proteins such as p21—which is regulated by the tumor suppressor protein p53—weave an intricate network of checks and balances ensuring the cell does not proceed with division until it is safe to do so.