 During the process of DNA replication , errors occasionally occur in the polymerization of the second strand . These errors , called mutations , can affect the phenotype of an organism , especially if they occur within the protein coding sequence of a gene . Error rates are usually very low — 1 error in every 10 – 100 million bases — due to the " proofreading " ability of DNA polymerases . Processes that increase the rate of changes in DNA are called mutagenic : mutagenic chemicals promote errors in DNA replication , often by interfering with the structure of base @-@ pairing , while UV radiation induces mutations by causing damage to the DNA structure . Chemical damage to DNA occurs naturally as well and cells use DNA repair mechanisms to repair mismatches and breaks . The repair does not , however , always restore the original sequence . 
