Which cyclin helps in the transition from the G1 phase to the S phase?

The transition from the G1 phase to the S phase of the cell cycle is a critical point where cells prepare to replicate their DNA. Cyclin D is primarily responsible for driving this transition. It activates Cyclin-dependent kinases (CDKs), specifically CDK4 and CDK6, which together promote the progression into the S phase by phosphorylating target proteins that enable the cell to move from a state of growth to one of DNA synthesis.

During the G1 phase, Cyclin D levels rise in response to external growth signals, and as it binds to its respective CDK, it initiates a cascade of events that facilitate cell cycle progression. This preparation includes the transcription of genes required for DNA replication and the formation of necessary molecular machinery for the S phase.

In contrast, Cyclin E, which is also involved in the transition to the S phase, acts later and works in conjunction with CDK2 to further push the cell toward DNA replication. While Cyclin A and Cyclin B are crucial for other phases of the cell cycle, particularly for the S phase and mitosis respectively, they do not play the primary role in the G1 to S phase transition like Cyclin D does. Hence, the focus on Cyclin D as