Clinical genetics / edited by Arnold Sorsby.

Date:
1953
Catalogue details

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

Credit: Clinical genetics / edited by Arnold Sorsby. Source: Wellcome Collection.

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    the gene: structure in relation to action butyric acid. The mutant accumulates both of the corresponding a, ß dihydroxy- analogues in the medium, according to Adelberg and Tatum in 1950. Finally, it is possible that an accumulated precursor might stimulate an unrelated reaction, though it is not possible to point to a Hkely example. Inhibition by growth factors Attention should also be drawn to the inhibitions of the utihzation of growth factors by other normal growth factors. The result of these is often that a mutant will not grow or else grows very poorly in the presence of a mixed supplement, such as malt or yeast extract or casein hydrolysate. For example, a mutant that requires lysine (/>^-3, Fig. 46) is inhibited by arginine, tquimolar proportions reducing the growth rate to half, while a molar concentration of arginine twice that of lysine gives zero growth rate. The lysineless mutant, which will grow if supplied with a aminoadipic acid, is inhibited by arginine if grown on lysine and by asparagine if grown on a aminoadipic acid. Similarly, an arginineless mutant {arg-\. Fig. 43) is inhibited by an excess of lysine. The mutant which will grow with homoserine will also grow on a suitably balanced mixture of threonine and methionine, the molar ratio 1 to 0-8 giving optimal growth; on excess methionine growth is depressed and ceases at a molar ratio of 1 threonine to 2 methionine. In some kinds of mutants it appears that a normal reaction is caused to take place at a higher rate, the increased amount of product proving to be inhibitory towards the organism. The mutant which requires sulphonamide for growth at temperatures above about 33° C. (Emerson, 1950) appears to be of this nature. It appears to have an enhancement of a reaction in which methionine appears to be the substrate. The reaction has a high temperature coefficient and the unknown product is inhibitory. The reaction requires pam-aminobenzoic acid, the use of which can be antagonized by sulphonamide at concentrations insufficient to disturb the participation of /7-aniinobenzoic acid in other essential reactions. The reaction, or the effects of its product, can also be antagonized by means of threonine. THE GENE: STRUCTURE IN RELATION TO ACTION The gene is confined within the nucleus which in general constitutes the minor portion of the bulk of the cell. One possibility to consider is that the reactions might take place upon the surface of the gene itself within the nucleus, but there is evidence that many of the important enzyme systems are located in the cytoplasm. A major outstanding problem is then how the genes influence the enzymes they control, and also to what extent the enzymes are capable of reproducing themselves without the direct intervention of the genes. If any reactions do take place upon the genes themselves it is possible that they might have to be arranged together in proximity upon a chromosome. Likewise, if a given reaction or series of reactions; involving highly unstable intermediates, required a special organization of the enzyme system in the cytoplasm, this organization might be reflected in the arrange¬ ment of the genes in the chromosomes. Information upon these subjects is still fragmentary, but nevertheless is highly interesting and significant. The genes controlHng different steps in the same reaction chain are usually scattered indiscriminately amongst the various chromosomes. No general order 131