What is the role of the activated G1 cyclin/cdk complex in the cell cycle?
The correct answer and explanation is:
The activated G1 cyclin/CDK complex plays a critical role in regulating the cell cycle, specifically in the transition from the G1 phase to the S phase, which is essential for DNA replication and cell division. The G1 phase is the first phase of the cell cycle where the cell grows, synthesizes proteins, and prepares for DNA replication. The cyclin-dependent kinase (CDK) complex is activated when G1 cyclins bind to CDKs, which are enzymes that phosphorylate target proteins to trigger specific cellular events.
Once the G1 cyclin/CDK complex is activated, it initiates a series of molecular events that push the cell through the G1 checkpoint, also known as the “restriction point.” The restriction point is crucial because it determines whether the cell will proceed through the rest of the cell cycle or enter a resting state (G0 phase). If the cell passes the restriction point, it is committed to completing the cell cycle and dividing.
The G1 cyclin/CDK complex functions by phosphorylating key proteins that regulate DNA replication and cell cycle progression. For example, it phosphorylates the retinoblastoma (Rb) protein, leading to the release of E2F transcription factors. E2F then activates the transcription of genes necessary for DNA replication and progression into the S phase. Additionally, the activated G1 cyclin/CDK complex helps regulate other proteins that promote cell growth and inhibit inhibitors of the cell cycle.
In summary, the G1 cyclin/CDK complex is vital for controlling the transition from G1 to S phase by phosphorylating proteins that initiate DNA replication and cell division. Its proper function ensures that cells divide at the right time, preventing uncontrolled cell division that could lead to cancer or other diseases.