Enzymes proofread DNA during its replication and repair damage in existing DNA. Mistakes during the initial pairing of template nucleotides and complementary nucleotides occur at a rate of one error per 100,000 base pairs. DNA polymerase proofreads each new nucleotide against the template nucleotide as soon as it is added. If there is an incorrect pairing, the enzyme removes the wrong nucleotide and then resumes synthesis. The final error rate is only one mistake per 10 billion nucleotides. Mismatched nucleotides that are missed by DNA polymerase or mutations that occur after DNA synthesis is completed can often be repaired. In mismatch repair, special enzymes fix incorrectly paired nucleotides. o A hereditary defect in one of these enzymes is associated with a form of colon cancer. Incorrectly paired or altered nucleotides can also arise after replication. DNA molecules are constantly subjected to potentially harmful chemical and physical agents. Reactive chemicals, radioactive emissions, X-rays, ultraviolet light, and molecules in cigarette smoke can change
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