BASE SUBSTITUTIONS

=Base Substitutions:= When one base is substituted for another, usually only a single amino acid in the polypeptide chain is affected. [image:http://i.imgur.com/oiuZESG.png] When the sequence of amino acids is altered, the mutation is known as a mis-sense mutation. Depending on the number of amino acids affected and their positions within the polypeptide chain, the protein may or may not function properly. However, base substitutions do not always cause a change in the sequence of amino acids. This is due to the degeneracy in the genetic code; the fact that there are 64 different possible triplets but only 20 different amino acids, some must code for the same amino acid. If the third base in a triplet is substituted for another base, the resulting triplet may still code for the same amino acid. A mutation that does not alter the sequence of amino acids is known as a silent mutation. [image:http://i.imgur.com/BQi1lJ7.png] The altered triplet may potentially not code for an amino acid at all! These sequences normally signal the end of a gene and are known as STOP Codons (because they stop translation / protein synthesis). A mutation that results in a premature STOP Codon is known as a nonsense mutation.