<Li> It can initiate apoptosis (i.e., programmed cell death) if DNA damage proves to be irreparable . </Li> <Li> It is essential for the senescence response to short telomeres . </Li> <P> Activated p53 binds DNA and activates expression of several genes including microRNA miR - 34a, WAF1 / CIP1 encoding for p21 and hundreds of other down - stream genes . p21 (WAF1) binds to the G1 - S / CDK (CDK4 / CDK6, CDK2, and CDK1) complexes (molecules important for the G1 / S transition in the cell cycle) inhibiting their activity . </P> <P> When p21 (WAF1) is complexed with CDK2, the cell cannot continue to the next stage of cell division . A mutant p53 will no longer bind DNA in an effective way, and, as a consequence, the p21 protein will not be available to act as the "stop signal" for cell division . Studies of human embryonic stem cells (hESCs) commonly describe the nonfunctional p53 - p21 axis of the G1 / S checkpoint pathway with subsequent relevance for cell cycle regulation and the DNA damage response (DDR). Importantly, p21 mRNA is clearly present and upregulated after the DDR in hESCs, but p21 protein is not detectable . In this cell type, p53 activates numerous microRNAs (like miR - 302a, miR - 302b, miR - 302c, and miR - 302d) that directly inhibit the p21 expression in hESCs . </P>

What stage of the cell cycle does p53 work