Evolution of the Genetic Code

Abstract

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.

Rare-event sampling analysis uncovers the fitness landscape of the genetic code

“…Such studies have examined, for instance, evolutionary simulation of the genetic code [8], and methods to improve the cost function to incorporate protein amino-acid frequency [4,15]. However, this choice of random genetic code ensemble is inconsistent with examples of the nonstandard genetic code [16][17][18], whereby the code differs in only a single or a few codons from the SGC, altering the block structure of the SGC. A justifiable choice of the random genetic code ensemble is such that each codon can take 20 possible amino acids, with the constraint that the whole code contains 20 different amino acids (Fig 1C ); thus, the number of possible codes in the ensemble is approximately 20 64 � 10 83 , which is much larger than the number of possible codes in the ensemble (20!…”