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In this module:

The Genetic Code
Transfer RNA


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.


The normal sequence of the ß chain of hemoglobin has a Glu in the sixth position. This amino acid is coded for by a GAG codon in the DNA sequence.


In a mutant form of ß hemoglobin, the second nucleotide, A, is a T instead.


A Val occupies the sixth position instead of Glu. The hemoglobin molecule now has the characteristics of the mutant sickle hemoglobin. More information on sickle cell anemia is available in the Protein 1 module (click to open a new window explaining how the symptoms of sickle cell anemia are caused by the amino acid substitution, then close that window when you are done to return to this module).

Occasionally a mutation produces benefits for an organism. Natural selection acts on the variation produced by mutations to bring about gradual changes in species. Thus mutations are the engine that drives evolution.