Final answer:
p53 is stabilized by phosphorylation upon DNA damage, which allows it to carry out its role in halting the cell cycle for repair or triggering apoptosis. p21 is produced as a result and helps to enforce the cell cycle halt.
Step-by-step explanation:
When DNA damage is sensed, p53 is stabilized primarily by phosphorylation. The p53 protein is normally bound to an active Mdm2 protein, but upon DNA damage, ATM kinase phosphorylates both Mdm2 and p53. This event leads to the dissociation of p53 from Mdm2, subsequently stabilizing p53 and allowing it to halt the cell cycle for DNA repair or trigger apoptosis if repair is not possible. As p53 levels increase, it activates the production of p21, which further enforces the cell cycle halt by inhibiting Cdk/cyclin complexes.