Chapter *32 



BIOCHEMICAL GENETICS (II) 



I 



"t has been hypothesized that the 

 cistron has a single, primary effect 

 .on the phenotype. This hypothe- 

 sis is of value insofar as it provides motiva- 

 tion for studies aimed at determining whether 

 a given segment of genetic substance has, in 

 fact, a single, primary action. We found, in 

 the last Chapter, that investigations of specific 

 genes, causing inborn errors of metabolism, 

 gave support to this view. There is an addi- 

 tional implication of the one cistron-one 

 primary effect hypothesis. If we knew the 

 nature of such a primary effect it should 

 always prove to be the product of one cistron. 

 It may be possible to subject this prediction 

 to test also. In order to do so it will be neces- 

 sary to decide which particular aspects of the 

 phenotype are primary products of cistronic 

 action. 



We have already discussed, in the previous 

 Chapter, cases where the primary effect of a 

 mutant cistron is upon the capacity of an 

 enzyme to catalyze a particular reaction. 

 Since different enzymes catalyze different 

 reactions, each is said to show specificity of 

 action. The mutants referred to resulted in 

 a change in enzyme specificity. Let us pre- 

 sume that the specificities of all enzymes 

 found in protoplasm are the result of the pri- 

 mary action of cistrons. If this is so, then it 

 ought to be possible to study any particular 

 enzyme and find that its specificity can be 

 changed for genetic reasons. Let us proceed 

 to study this hypothesis experimentally as a 

 specific, if limited, test of the more general 

 concept, one primary effect-one cistron. 

 281 



We have already described the use of 

 Neurosponi as material for studies of genetic 

 recombination (beginning p. 121). Certain 

 characteristics of Neurospora also make the 

 organism very favorable material for bio- 

 chemical studies. Neurospora can manufac- 

 ture all of the components it needs for exist- 

 ence and reproduction from a very simple, 

 basic, food medium. This basic medium may 

 consist solely of water, an array of inorganic 

 salts, including sources of nitrogen, phos- 

 phorus, sulfur, and various essential trace 

 elements, a carbon and energy source such 

 as cane sugar, and a single vitamin, biotin. 

 From these raw materials it can synthesize 

 some 20 different amino acids, all essential 

 vitamins (but biotin), compounds like purines 

 and pyrimidines, and everything else it needs 

 for its total activity. According to the hy- 

 pothesis under consideration, it should be 

 possible to induce mutations in cistrons, 

 which should then fail to correctly specify 

 enzymes, so that various chemical syntheses 

 should become blocked. 



Previous work has established that the last 

 step in the synthesis of vitamin By, or thiamin, 

 is normally accomplished by the enzymatic 

 combination of a particular thiazole with a 

 particular pyrimidine. If enzymes owe their 

 specificities to cistrons, it should be possible 

 to induce a mutation in the cistron that 

 normally specifies this Bi-forming enzyme. 

 If the mutant no longer specifies active Bi- 

 forming enzyme, no Bi will be made. Then, 

 since Bi is required for growth, the mutant 

 mold will require Bi in its diet in order to 

 grow. 



What is done ^ is to obtain haploid spores 

 (produced asexually by the haploid organism 

 grown from an ascospore) and treat them 

 with a mutagenic agent (like X rays or ultra- 

 violet light). The treated spores are then 

 grown on the basic medium which has been 

 supplemented with vitamin Bi. Under these 

 circumstances the spores that grow will in- 

 ^ Based upon work of G. W. Beadle and E. L. Tatum. 



