Final answer:
The p53 tumor suppressor protein is activated and stabilized by cellular stress or DNA damage. It arrests the cell cycle to allow DNA repair and can induce apoptosis if the damage cannot be repaired.
Step-by-step explanation:
The p53 tumor suppressor protein is activated and stabilized by cellular stress or DNA damage. When cellular damage occurs, p53 arrests the cell cycle until the damage is repaired. If the damage cannot be repaired, p53 can induce apoptosis, or cell death, through the intrinsic pathway.