162 



LIFE: ITS NATURE AND ORIGIN 



Considering its simpler needs, Neurospora's synthesizing catalysts 

 enable it to make its needed amino acids and vitamins from simpler 

 chemical compounds. By the use of standard bacteriological tech- 

 nique the mold can be kept uncontaminated on a "minimal" medium 

 containing water, certain inorganic salts, sugar or some other source 

 of carbon and energy, and a minute amount of biotin, a vitamin of 

 the B complex. 



In 1927, H. J. Muller (Nobel prize, 1946) made an outstanding 

 advance by showing 2 that heritable changes may be induced in animals 

 and plants by treating their sex cells with x-rays of selected nature; 

 and since then a huge literature has developed. This was at first 

 called the "artificial transmutation of the gene," but it is now known 

 that chromosomal breakage is often involved. Assuming that Neuro- 

 spora has genes which govern the production of the chemical sub- 

 stances needed for its life, Beadle bombarded sexual spores with ultra- 

 violet or x-ray photons, and then placed the treated spores on media 

 to which various growth factors had been added. If a treated strain 

 could grow on the special medium but not on the "minimal" medium, 

 it was obviously deficient in catalysts capable of making the substance 

 added to the fortified medium. Thus several dozen mutant strains 

 produced in Neurospora by irradiation are unable to synthesize the 

 amino acid arginine from the minimal medium, though normal 

 Neurospora can do so. These mutants thrive only when arginine is 

 supplied. 3 



In mammals, arginine is formed in the so-called ornithine cycle of 

 Krebs and Henseleit. 4 Neurospora appears to make arginine in the 

 same way, for some mutant strains cannot convert citrulline into 

 arginine, and therefore require arginine; others cannot convert orni- 

 thine into citrulline, and therefore require citrulline or arginine; still 

 others cannot synthesize ornithine, and therefore fail to grow in the 

 absence of ornithine, citrulline or arginine. Beadle states: "The 

 interesting thing about this from the standpoint of gene action is that 

 we find just what would have been predicted from the hypothesis that 

 enzymes are dependent upon specific genes and that for each enzyme 

 there is a specific gene, and vice versa. In each instance of defective 



NH; 



OOH 



\ /NH 2 



^ 



H 2 N\/0 



N 

 / 00H 



\y 



NH Z 



H 2 Nv^NH 



N 

 / OOH 



V 



NH 2 



H 2 N\zNH 2 

 



NH 2 

 / OOH 



J 



NH 2 



Ornithine 



Citrulline 



Arginine Ornithine A- urea 



