Final answer:
The transcription of p21 results in cell cycle arrest by inhibiting the activity of Cdk/cyclin complexes, and is part of the cellular response to DNA damage mediated by the p53 protein.
Step-by-step explanation:
The transcription of p21 produces cell cycle arrest. p21, which is induced by p53, functions primarily at the G₁ checkpoint and is known as a negative regulatory molecule. When p53 is activated in response to DNA damage or stress, it can induce the expression of p21, which in turn binds to and inhibits the activity of Cdk/cyclin complexes, enforcing a halt in the cell cycle.
This ensures that cells do not progress to the S phase with damaged DNA, which would lead to further genome instability. When the damage is irreparable, p53 can also trigger apoptosis to prevent the proliferation of cells with damaged genetic material.