The p53 enzyme system prevents progression to which stage of the cell cycle?

Question

choices are:
1. G1
2.Cytokineses
3.S phase
4. Mitosis
5. G2
* I think it's S phase, It's not G1 right?

Answer 1

This should explain it. It's not the S phase, from this explanation.

http://www.khanacademy.org/science/biology/cellular-molecular-biology/stem-cells-and-cancer/a/cell-cycle-regulators