substitution
substitution (in genetics) A point mutation in which one base pair in the DNA sequence is replaced by another. There are two types of substitution mutation: transition mutations, in which a pyrimidine base (i.e. thymine or cytosine) is replaced by another pyrimidine base or a purine base (adenine or guanine) is replaced by another purine base; and transversion mutations, in which a pyrimidine base is replaced by a purine base, or vice versa. Most substitutions tend to alter one amino acid in a protein chain, which may or may not affect the functioning of that protein. Sickle-cell anaemia is an example of a substitution mutation in which thymine is replaced by adenine in the triplet coding for the sixth amino acid in the β-chain of haemoglobin.
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substitution
