Mutations

The replication process of DNA is extremely accurate. In bacteria, the DNA polymerase proofreads the pairing process by checking the newly attached nucleotide to confirm that it is correct. If it is not, the polymerase removes the incorrect nucleotide, backs up, and attaches a new nucleotide. If a mismatch should escape the proofreading ability of the DNA polymerase, other, mismatch repair, enzymes will correct the error. Repair mechanisms occur in eukaryotic cells as well but are not well understood.

Radiation (such as ultraviolet and x-ray) and various reactive chemicals can cause errors in DNA molecules. One kind of DNA error occurs when the bases of two adjacent nucleotides in one DNA strand bond to each other rather than make proper pairs with nucleotides in the complementary DNA strand. A thymine dimer, for example, originates when two adjacent thymine nucleotides in the same strand base-pair with each other instead of with the adenine bases in the complementary strand. Such errors can be fixed by excision repair enzymes that splice out the error and use the complementary strand as a pattern, or template, for replacing the excised nucleotides.

If a DNA error is not repaired, it becomes a mutation. A mutation is any sequence of nucleotides in a DNA molecule that does not exactly match the original DNA molecule from which it was copied. Mutations include an incorrect nucleotide (substitution), a missing nucleotide (deletion), or an additional nucleotide not present in the original DNA molecule (insertion). When an insertion mutation occurs, it causes all the subsequent nucleotides to be displaced one position, producing a frameshift mutation. Radiation or chemicals that cause mutations are called mutagens. Carcinogens are mutagens that activate uncontrolled cell growth (cancer).