Apoptosis is cell death. Each cell has stored enzymes capable of cutting its proteins apart, thus killing the cell. The enzymes act like rotating blades that cut up proteins.

P53 stops the cell cycle in interphase if there is damage to cell's DNA. Then apoptosis destroys the cell. If p53 has mutated, it can no longer stop the cell cycle. A damaged cell that is not destroyed by apoptosis may become a cancer cell.

What are agents that can damage DNA? Click here for an example.