Evolution of the Genetic Code


From the present situation of genetic code so far elucidated for various species of extant organisms, it is speculated that the genetic code system had started from a limited number of amino acids and evolved to the universal genetic code which is being used by most extant organisms. During evolution, the genetic code is changeable by some factors such as directional mutational pressure on genomes, genome economization and evolution of tRNA (and its cognate aminoacyl transfer ribonucleic acid synthetase). The metazoan (multicellular animal) mitochondrial genetic code is interesting issue as the model case of evolution of genetic code, since the metazoan mitochondrial genetic code is greatly deviated from the universal genetic code, and varied among lineages. The present situation on the genetic code is discussed.

Key Concepts:

  • Genetic code has evolved along (with) evolution of life.

  • Codon reassignment requires both elimination of tRNAs for sense codons or release factors for termination codons recognizing the concerned codon and creation or recruitment of ‘novel’ tRNAs or release factors that can recognize the concerned codon.

  • Wobble base pairing between anticodon first position and codon third position is an important constraint for codon reassignment.

  • Simple mitochondrial genetic system (small genome size, reduced number of tRNA species) allowed larger variation of genetic code than other genetic systems.

Keywords: deviations from the universal genetic code; evolutionary implications; origins of the genetic code; mechanisms for the evolution of the genetic code

Figure 1.

Distribution of nonuniversal codons in (a) mitochondria and (b) organisms. Modified from Osawa et al. .

Figure 2.

Possible evolutional consequences of AAA codon changing from Lys to Asn in animal mitochondria depending on the ‘codon capture’ hypothesis. tRNAAsn*GUU is tRNAAsnGUU that possibly received structural modifications so as to read not only the AAU and AAC codons but also the AAA codon (or in this case such modification may not be necessary; see text). A similar process has been presented by Castresana et al. .

Figure 3.

Evolution of translation apparatus and genetic codes showing periods of GC‐ and AT‐pressure, and the main changes in codon assignments and tRNA anticodons. Modified from Osawa .



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Further Reading

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Wolstenholme DR (1992) Animal mitochondrial DNA: structure and evolution. International Review of Cytology 141: 173–216.

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Yokobori, Shin‐ichi, Ueda, Takuya, and Watanabe, Kimitsuna(Apr 2010) Evolution of the Genetic Code. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000548.pub2]