Final answer:
p53 halts the cell cycle and recruits enzymes for DNA repair. It can also trigger apoptosis if DNA cannot be repaired. p21 enforces the halt in the cell cycle by inhibiting Cdk/cyclin activity.
Step-by-step explanation:
p53 is a multi-functional protein that acts during the G₁-S transition in the cell cycle. If damaged DNA is detected, p53 halts the cell cycle and recruits specific enzymes to repair it. If repair is not possible, p53 can trigger apoptosis to prevent the duplication of damaged chromosomes. p53 also triggers the production of p21, which binds to and inhibits the activity of the Cdk/cyclin complexes, enforcing the halt in the cell cycle. As p53 and p21 levels increase, it becomes less likely for the cell to move into the S phase.