Final answer:
p53 acts to preserve genomic integrity by inducing cell cycle arrest in the presence of DNA damage through the production of p21 and inhibition of Cdc25 phosphatase. Consequently, p21 and phosphorylated Cdc25 work together to halt cell cycle progression, facilitating DNA repair or triggering apoptosis if repair is not possible.
Step-by-step explanation:
p53 is a pivotal protein involved in cell cycle regulation. It plays a crucial role in preventing the cell cycle from progressing in the presence of DNA damage. When p53 detects damaged DNA during the G1 phase, it triggers the production of p21, which serves as an inhibitor of Cdk/cyclin complexes, thus enforcing the cell cycle arrest. As a result, p21 prevents these complexes from phosporylating the retinoblastoma protein (Rb), which blocks progression from G1 to S phase. Additionally, in response to cellular stressors, p53 phosphorylation also leads to cell cycle arrest by inhibiting Cdc25 phosphatase, which would otherwise activate cyclin-dependent kinases required for entry into mitosis. If DNA repair is unachievable, p53 can induce apoptosis to prevent the proliferation of cells with damaged DNA.
Moreover, the activity of Cdc25 phosphatase is also regulated by proteins such as Chk1 and Chk2, which upon activation in response to DNA damage, phosphorylate Cdc25, thus sequestering it and preventing the activation of cyclin-dependent kinases, leading to cell cycle arrest.