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Codon degeneracy
Degeneracy of codons is the redundancy of the genetic code, exhibited as the multiplicity of three-codon combinations specifying an amino acid. The degeneracy of the genetic code is what accounts for the existence of synonymous mutations.〔
==Background==
Degeneracy of the genetic code was identified by Lagervist.〔Kagerkvist, U. (1978.) "Two out of three: An alternative method for codon reading", ''PNAS'', 75:1759-62.〕 For instance, codons GAA and GAG both specify glutamic acid and exhibit redundancy; but, neither specifies any other amino acid and thus are not ambiguous or demonstrate no ambiguity.
The codons encoding one amino acid may differ in any of their three positions; however, more often than not, this difference is in the second or third position. For instance, the amino acid glutamic acid is specified by GAA and GAG codons (difference in the third position); the amino acid leucine is specified by UUA, UUG, CUU, CUC, CUA, CUG codons (difference in the first or third position); and the amino acid serine is specified by UCA, UCG, UCC, UCU, AGU, AGC (difference in the first, second, or third position).〔
Degeneracy results because there are more codons than encodable amino acids. For example, if there were two bases per codon, then only 16 amino acids could be coded for (4²=16). Because at least 21 codes are required (20 amino acids plus stop) and the next largest number of bases is three, then 4³ gives 64 possible codons, meaning that some degeneracy must exist.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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