Final answer:
The G1 checkpoint can prevent a cell from entering the S phase through initiating DNA repair mechanisms. Proteins like p53 and p21 play key roles in detecting DNA damage and either repairing it or triggering apoptosis to maintain genomic integrity.
Step-by-step explanation:
When the G1 checkpoint detects damaged DNA, it can prevent progression into the S phase by initiating DNA repair mechanisms. Proteins such as Rb, p53, and p21 function primarily at the G1 checkpoint to ensure the integrity of the cell's genetic material before it enters S phase and begins DNA replication. The p53 protein plays a critical role in this process. If damaged DNA is detected, p53 can halt the cell cycle and recruit specific enzymes to facilitate repair. If the damage is irreparable, p53 is capable of inducing apoptosis, a process of programmed cell death, to prevent the propagation of the flawed genetic material. The rising levels of p53 lead to increased production of p21, which assists in reinforcing the cell cycle arrest by binding to, and inhibiting, the activity of Cdk/cyclin complexes.