G2/M checkpoint is the second checkpoint within the cell cycle. This checkpoint prevents cells with damaged DNA from entering the M phase, while also pausing so that DNA repair can occur. This regulation is important to maintain genomic stability and prevent cells from undergoing malignant transformation.Ataxia telangiectasia mutated (ATM) and ataxia telangiectasia mutated and rad3 related (ATR) are key kinases that respond to DNA damage. ATR responds to UV damage, while ATM responds to DNA double-strand breaks (DSB). ATM and ATR activate kinases Chk1 and Chk2 which in turn inhibit Cdc25, the phosphatase that normally activates Cdc2. Cdc2, a cyclin-dependent kinase, is a key molecule that is required for entry into M phase. It requires binding to cyclin B1 for its activity.
The tumor suppressor gene p53 is an important molecule in G2/M checkpoint regulation. ATM, ATR and Chk2 contribute to the activation of p53. Further, p19Arf functions mechanistically to prevent MDM2's neutralization of p53. Mdm4 is a transcriptional inhibitor of p53. DNA damage-induced phosphorylation of Mdm4 activates p53 by targeting Mdm4 for degradation. Well known p53 target genes like Gadd45 and p21 are involved in inhibiting Cdc2. Another p53 target gene, 14-3-3σ, binds to the Cdc2-cyclin B complex rendering it inactive. Repression of the cyclin B1 gene by p53 also contributes to blocking entry into mitosis. In this way, numerous checks are enforced before a cell is allowed to enter the M phase.