Question

So when cells are infected with SRC or in response to application of a mitogenic ligand, one of the first genes to be transcribed is cyclin E, which immediately complexes with CDK. However, cells do not immediately pass through the Restriction point and enter S phase. Why isn’t simply having the cyclin-CDK complex present “enough” to drive through the checkpoint?

Answer 1

Solution: The different cell cycle stages are regulated by Cyclin CDK complexes. But simple complex formation of Cyclin CDK is not enough for entering consecutive cell cycle stages. This complex undergoes further regulations like Phosphorylation, dephosphorylation, ubiquitin mediated degradation etc. Mainly CDKs are regulated by activating and inhibitory phosphorylations. CAKs induce activating phosphorylation whereas Wee 1 kinases induce inhibitory phosphorylation. For the activation of Cyclin CDK complexes we have to remove this inhibitory phosphorylation using specific phosphatases.