Mutations
Mistakes in base-pairing are corrected by DNA polymerase and other mismatch repair enzymes.
Radiation and various reactive chemicals can cause thymine dimers where adjacent nucleotides bind to each other instead of to the complimentary strand. Such errors are usually fixed by splicing out the affected nucleotides and replacing them (excision repair).
A mutation is a DNA error that is not repaired. Some mutations include using an incorrect nucleotide (substitution), deleting a nucleotide (deletion), and adding a nucleotide (insertion). When insertion occurs, all the subsequent nucleotides are misplaced causing a frameshift mutation.
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