Gene mutations that affect one or a few bases are called point mutations. There are three main types.Substitution: one base is replaced by a different base. Only one codon changes, so at most one amino acid is affected. Because the genetic code is degenerate, the new codon may code for the same amino acid (a silent mutation). It may code for a different amino acid (a missense mutation), as in sickle cell anaemia, where a substitution changes glutamic acid to valine in the β-globin chain. It may also create a stop codon (a nonsense mutation), which gives a shortened, usually non-functional protein.Insertion: an extra base is added. Deletion: a base is removed. Because the code is read in non-overlapping triplets, an insertion or deletion causes a frameshift. Every codon after the mutation is read differently, so many amino acids change and the protein is usually non-functional. An insertion or deletion of three bases adds or removes one amino acid without a frameshift.Worked example: the DNA triplets TAC GGA CTT become TAC GAC TT... if the second G is deleted. Every triplet from the mutation onwards is altered.
Larger chromosome mutations include duplication (a section is repeated), inversion (a section is reversed) and translocation (a section moves to another chromosome).Any change to the amino acid sequence can alter the tertiary structure of the protein, for example the shape of an enzyme's active site, and so change its function.
Larger chromosome mutations include duplication (a section is repeated), inversion (a section is reversed) and translocation (a section moves to another chromosome).Any change to the amino acid sequence can alter the tertiary structure of the protein, for example the shape of an enzyme's active site, and so change its function.