What regulates the transition from the G1 to S stage of the cell cycle?

The transition from the G1 to S stage of the cell cycle is primarily regulated by the Cyclin E/CDK2 complex. This complex plays a pivotal role in the progression through the cell cycle by phosphorylating target proteins that prepare the cell for DNA synthesis. Cyclin E is synthesized in response to growth signals and it binds to CDK2, activating its kinase activity, which is necessary for the cell to move from the G1 phase, where the cell grows and prepares for DNA replication, into the S phase, where DNA synthesis occurs.

This transition is crucial as it ensures that the cell has adequately prepared all necessary components and conditions before entering DNA synthesis, thereby maintaining genomic integrity. The activity of the Cyclin E/CDK2 complex is tightly regulated by various factors, including checkpoint proteins that monitor the cell’s readiness to replicate its DNA.

In contrast, the other choices involve different phases or functions within the cell cycle. Cyclin B/CDK1 mainly regulates the transition from G2 to M phase, while Cyclin A/CDK2 is involved in the S phase itself and the G2 phase transition. Cyclin D1 is important for the progression through early G1 but does not directly mediate the transition to S phase. Therefore