p53 inhibits entry into mitosis when DNA synthesis is blocked.

Human and mouse fibroblasts with normal p53 fail to enter mitosis when DNA synthesis is blocked by aphidicolin or hydroxyurea. Isogenic p53-null fibroblasts do enter mitosis with incompletely replicated DNA, revealing that p53 contributes to a checkpoint that ensures that mitosis does not occur until DNA synthesis is complete. When treated with N-(phosphonacetyl)-L-aspartate (PALA), which inhibits pyrimidine nucleotide synthesis, leading to synthesis of damaged DNA from highly unbalanced dNTP pools, p53-null cells enter mitosis after they have completed DNA replication, but cells with wild-type p53 do not, revealing that p53 also mediates a checkpoint that monitors the quality of newly replicated DNA.