Evolution of anticodons.
Anticodons are trinucleotides in transfer RNA (tRNA) molecules.  The latter carry amino acids for insertion into the polypeptide sequences of proteins during the translation of messenger RNA (mRNA) molecules.  Messenger RNA molecules are transcribed from genes.  Evolution of tRNA molecules has resulted in a set of anticodons for the 20 amino acids that are used in protein synthesis.  This set of anticodons is slightly different in mitochondrial codes from the set that used in the nuclear "universal" code.  Theories for the evolution of the code include frozen accident, doublet expansion, repeating triplets and coevolutionary distribution.  The number of codons has always been fixed at 64 by mathematical rules, but because an anticodon may pair with more than one codon, the number of anticodons is only 54 in the universal code, is smaller in mitochondrial codes, and was probably even smaller in archetypal primitive codes.  Evidence of anticodon evolution can be seen by comparing mitochondrial codes with the universal code.  Codes used by very primitive organisms that are now extinct might have specified fewer amino acids than are now used.
Jukes
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University of California, Berkeley Space Sciences Laboratory, Oakland 94608, USA.
D000596:Amino Acids / Q000096:biosynthesis* / Q000737:chemistry; D000926:Anticodon; D003062:Codon; D019143:Evolution, Molecular; D005815:Genetic Code; D008928:Mitochondria / Q000235:genetics; D008969:Molecular Sequence Data; D000069497:Origin of Life; D012343:RNA, Transfer
D000596:Amino Acids; D000926:Anticodon; D003062:Codon; D012343:RNA, Transfer