Box 3.4The origin and evolution of the genetic code

The evolution of the genetic code has provoked argument ever since DNA was established as the genetic material back in the 1950s. Many geneticists support the ‘frozen accident’ theory, which suggests that codons were randomly allocated to amino acids during the earliest stages of evolution, the code subsequently becoming ‘frozen’ because any changes would result in widespread disruption of the amino acid sequences of proteins. Various lines of evidence suggest that the code might have evolved in a less random manner (Knight et al., 1999; Szathmáry, 1999). First, controversial experimental results suggest that at least some amino acids bind directly to RNAs containing the appropriate codons, this occurring in the absence of the tRNA that mediates the interaction in present-day cells. These results suggest that the code might have originated from natural interactions between RNA molecules and amino acids. If correct, the implication is that there is some kind of chemical relationship between an amino acid and its codon(s). Second, the deviations from the standard code listed in Table 3.2 indicate that the same codon reallocations have occurred more than once. For example, three unrelated groups of protozoa use 5′-UAA-3′ and 5′-UAG-3′ as glutamine rather than termination codons, suggesting that these codons were reallocated in the same way on at least three separate occasions. Similarly, the use of 5′-UGA-3′ as a tryptophan codon must have evolved twice, once in mitochondria and once in the ancestors of Mycoplasma. If the relationship between codon and amino acid is entirely random, as suggested by the ‘frozen accident’ theory, then we would not expect to see the same codon reallocations recurring on different occasions.

From: Chapter 3, Transcriptomes and Proteomes

Cover of Genomes
Genomes. 2nd edition.
Brown TA.
Oxford: Wiley-Liss; 2002.
Copyright © 2002, Garland Science.

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