As we know the cell cycle consists of 4 phases, G0, which is a resting state, G1, a growing phase, S where duplicates its DNA, G2 which grows and produces the necessary protein for cell division and M where the cell actually divides. Now in some phases, there are crucial checkpoints if the cells do not meet certain requirements, apoptosis is induced. One is in the G1 phase, the second at the end of G2 and the third is the metaphase, one of the most important poroteins involved in this regulation is p53, sometime called the guardian of the genome.