Final answer:
During G1 phase, cells grow, produce essential proteins, and can repair DNA damage. The G1 checkpoint, managed by proteins like p53 and p21, regulates this process and prevents progression to the S phase if DNA damage is detected or unrepaired.
Step-by-step explanation:
During the G1 phase of the cell cycle, cells primarily perform three key activities. First, cells grow in size and synthesize enzymes and proteins necessary for DNA replication. Secondly, cells have the potential to repair DNA damage if it exists.
This DNA repair is largely governed by the proteins Rb, p53, and p21, which act at the G1 checkpoint. The presence of damaged DNA triggers p53 to halt the cell cycle to facilitate DNA repair. If repair is feasible, enzymes are recruited to mend the damaged DNA.
However, if the damage is irreparable, p53 may initiate apoptosis to prevent the propagation of defective chromosomes. Concurrently, p21 production is induced, which inhibits the activity of Cdk/cyclin complexes, thus ensuring that the cell does not progress to the S phase under stressful conditions or with damaged DNA.