Evolution.

Society for Experimental Biology

Date:: 1953

Catalogue details

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

Credit: Evolution. Source: Wellcome Collection.

477/484 (page 445)

POSTSCRIPT 445 a sudden mutation. Perhaps some of the phenomena discussed by Demerec are to be explained in this way. Again, the ' instabilities ' in genes reported by Auerbach as a result of treatment with chemical mutagens may simply reflect the fact that the mutagen starts a gene on a new path of chemical evolution, which is only rendered evident by a change in gene product when the quantitative chemical changes have built up sufficiently to result in a qualitative change in function. Quite apart from changes in influences exerted by the external environ¬ ment, the immediate intracellular environment of a gene will influence both its behaviour and its chemical evolution, if these are envisaged as occurring at the physico-chemical level. We know that the activity of enzymes is profoundly affected by the nature of neighbouring colloids; the same may be true of genes. Changes in chemical composition of genes will also depend upon changes in their immediate physico-chemical environment. Consequently shifting a gene from one situation to another in a chromosome must be expected to change both the degree of activity of the gene and the course of its chemical evolution. If it is admitted that the chemical evolution of genes may be modified by both external agents and sometimes by changes in position in a chromosome, it is also true that some influence on the chemical evolution of a gene will probably be exerted by inevitable fluctuations in concentration of the minor constituents of cells. Recent studies on chemical mutagens have indicated that a very large number of substances are mutagenic. Major chemical constituents of cells can rarely be highly mutagenic. For example, glucose phosphate must normally be almost devoid of mutagenic action. But the cellular substances which are normally estimated by chemical analysis represent only a small proportion of those substances which can be synthesized if all permutations and combinations of cellular enzymic activities occur. Whilst the cell enzymes are organized in 'production lines' to permit optimal production of a restricted number of substances and to minimize the production of many others, traces of the later substances must be produced. It is probable that many of these trace substances are mutagenic. We must therefore consider the possibility that the mutations occurring in nature are often produced by the small amounts of mutagens synthesized by a cell as an inevitable product of its enzyme complement. Mutation produced by changes in the chemical environment of genes need not be a single catastrophic event. As indicated above, the environmental change may induce a series of chemical modifications of a gene, which will only appear as a mutation when the accumulation of quantitative changes is sufficient to produce a qualitative change in gene function. A mutation which is an expression of chemical evolution of a gene would appear to be