Volume 1

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

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    the triplet and if the organism coded only a few, rather crude proteins. As the number and sophistication of proteins increased, a time would be reached when the substitution of new amino acids would disrupt too many pre-existing proteins and would therefore be lethal. At this point, the code would be frozen; this theory has accordingly been termed the frozen accident model. To explain the universality of the code, it is again necessary to postulate that this event must have happened at an early stage of evolution, with all cells descended from the single population involved. There is therefore no entirely satisfactory explanation for the universal nature of the code; and, indeed, no way to elucidate its evolution since this must have occured at a very early stage and even the simplest organisms living today possess a highly sophisticated system for translating messengers into proteins. Even the mechanistic theories of stereochemical fit do not provide entirely satisfactory explanation for universality; we may suppose that, even if the first stage of evolution of the code was a direct interaction between polypeptide and polynucleotide, this direct relationship must have been lost as the polypeptides gained the catalytic activities which were eventually to develop into the translation apparatus. It is reasonable to suppose that there would still be opportunity for some evolution of codon meanings during this process, which again drives us back to the idea that the first cell to develop an efficient code must be the progenitor of all organisms. Stereochemical fit alone is therefore inadequate as an explanation of univer sality, because it seems probable that some, at least, of the twenty amino acids in present use may have been added to the repertoire of the cell during the stage of expansion after the original stereochemical fit had begun to be superceded by a translation apparatus. Whilst this idea overcomes some of the difficulties in allowing for specificity of stereochemical fit—for it means that only a few amino acids need be distinguished, not necessarily by the sixty-four codons in use today but perhaps by a much smaller selection of triplets— at the same time it reopens the question of the universal origin of the code by alllowing selection to operate before the code is finalised. We do not know, of course, which, if any, of the selective criteria so far considered may have been used during this evolution, but we may be sure that the organised degeneracy of the code is not fortuitous. In summary, then, we may imagine the initial stages of evolution of the code to have arisen from some interaction between polypeptide and polynucleotide; it is otherwise difficult to see how a code which depends on its own translation products could have evolved. An early priority in evolution must have been the development of some system for holding together the successful replicating system—that is, an early cell membrane. Selective processes must have acted to develop a more accurate translation apparatus from this starting point; we can only speculate on the nature of these selective forces and on to what extent they may have enlarged and altered the codon dictionary.