Although DNA is replicated with very high accuracy (less than 10-8 error rate), it is not always copied perfectly. A mutation is a change in the nucleotide sequence for a particular protein. Mutations can add, delete, or substitute nucleotides within the sequence of a gene. Because many codons specify the same amino acid, many mutations are completely harmless. For example, the codons CGU, CGC, CGA, and CGG all code for the amino acid Arg, so any mutation in the third position of these codons will have no effect on the organism.
Other mutations can have much more serious effects, however. Additions or deletions of nucleotides can affect the entire structure of a protein downstream of the mutation, usually resulting in a nonfunctional protein. Even a single nucleotide substitution can have a large effect, as in sickle cell anemia or cystic fibrosis.
Click on the step numbers below to see how a mutation causes sickle cell anemia. Click on the mouse at left to clear the images and text.