Recent Evidence for Evolution of the Genetic Code
The genetic code is evolving, and is not “frozen.” This is shown by 8 departures from the universal code: 5 of them are in mitochondria and 3 are in nuclear codes. We propose that these changes are preceded by disappearance of a codon from coding sequences in mRNA of an organism or organelle. The function of the codon that disappears is taken by other, synonymous codons, so that there is no change in amino acid sequences of proteins. The deleted codon then reappears with a new function. This may be implemented in one of three ways: (1) by a mutation in an anticodon; (2) by change to a different amino acid in aminoacylation of a tRNA molecule, while retaining the same anticodon, or; (3) in mitochondria, by change in the codon-anticodon pairing.
The first procedure occurred in changing codon UGA from stop to tryptophan, codons UAR from stop to glutamine, and codon AAA from lysine to asparagine. The second procedure occurred in changing codon CUN from leucine to threonine CUG form leucine to serine. The third procedure was in AGR from arginine to serine, AGR from serine to stop and in CAU from only pairing with AUG to pairing with AUA and AUG. All these changes were non-disruptive.
KeywordsGenetic Code Candida Cylindracea Universal Code tRNA Meet Mitochondrial Genetic Code
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