How do cells correct errors in DNA that would disrupt their function

P21 protein binds and then inactivates cell division and then cell cycle arrest.

Cells correct error in their DNA by three processes: proofreading (errors that occur during replication are rectified), mismatch repair (the mismatched base pairs are corrected just after the DNA replication), and damage repair mechanisms (including direct reversal by cell enzymes, excision repair by replacement or removal of the wrong base, and double stranded base repair by repairing the breaks in the DNA by homologous or non-homologous recombination methods).