G. W. BEADLE 



that postulated by Krebs and Henseleit for the mammalian liver. In 

 the bread mold it is known that mutation in any one of seven different 

 genes will interi'upt the synthesis of ornithine or its conversion to 

 arginine. So far as the data go, they are consistent with the assump- 

 tion that each of the seven genes is normally concerned with a different 

 chemical reaction in the system. It is an interesting point that it was 

 possible to establish the presence of the ornithine cycle in Neurospora 

 because of the existence of the mutant strains indicated. 



Tatum and Bonner (50) have shown that tryptophan is normally 

 synthesized in Neurospora through the condensation of indole and 

 serine. Evidently the indole is somehow derived from anthranilic 

 acid, for there exist two mutant strains, one of which accumulates 

 anthranilic acid when it is grown under suitable conditions, while 

 the other is able to grow normally when supplied with anthranilic 

 acid in place of indole or tryptophan (51). The gene by which the 

 first strain differs from wild type is evidently concerned with the reac- 

 tion by which anthranilic acid is converted into indole, whereas the 

 mutant gene of the second strain appears to be concerned with failure 

 of some reaction essential to the synthesis of anthranilic acid. It is 

 obvious, in this case, that genetics has provided a tool of great useful- 

 ness in investigating the biosynthesis of the important amino acid, 

 tryptophan. 



Relations similar to those mentioned above are known for other 

 biosyntheses in the bread mold and in other organisms. By following 

 methods developed by Beadle and Tatum (8), it has been possible to 

 obtain a series of mutant strains of Neurospora in each of which some 

 particular reaction has been blocked. These are concerned with the 

 synthesis of amino acids, vitamins, purines, pyrimidines, and other 

 compounds of biological importance (6,21,48,49). 



We can be sure from such cases as those just cited that genes 

 function in directing biochemical reactions. We know, further, that 

 this direction may involve enzymes as intermediates between gene and 

 reaction. All our information is consistent with the hypothesis that 

 in all cases in which genes control specific reactions they do so indirectly 

 through enzymes. In other words, genes direct enzyme specificities, 

 and enzymes control reactions. This is not a new idea. Bateson (4), 

 Moore (29), Troland (52), Goldschmidt (16), Muller (30), Alexander 

 and Bridges (1), Haldane (18,19), Wright (55), and others have sug- 



