A.
The cell-cycle control system uses several distinct mechanisms to halt progress through the cell cycle if DNA is damaged, and it can do so at various transition points. The mechanism that operates at the G 1 -to-S transition, which prevents the cell from replicating damaged DNA, is especially well understood.
B.
DNA damage in G 1 causes an increase in both the concentration and activity of a protein called p53, which is a transcription regulator that activates the gene encoding a Cdk inhibitor protein called p21. The p21 protein binds to G 1 /S-Cdk and S-Cdk, preventing them from driving the cell into S phase.
C.
The arrest of the cell cycle in G 1 gives the cell time to repair the damaged DNA before replicating it. If the DNA damage is too severe to be repaired, p53 can induce the cell to kill itself through apoptosis, a form of programmed cell death.
D.
If p53 is missing or defective, the unrestrained replication of damaged DNA leads to a high rate of mutation and the generation of cells that tend to become cancerous. In fact, mutations in the p53 gene are found in about half of all human cancers.