Effects of X-Rays and Atomic Radiations on Living Cells 65 



The original 'wild' neurospora is an organism with remarkable 

 synthetic ability. All it requires to live and to grow, besides water and 

 some inorganic elements, are glucose or some other source of car- 

 bon, ammonia and the vitamin biotin. Out of these simple materials 

 it makes everything it requires. It makes twenty amino acids, a variety 

 of purines or pyrimidines, nucleic acids and proteins, aneurin, ribo- 

 flavin, pyridoxin, pantothenic acid, folic acid, choline, inositol, and 

 no doubt numerous other substances. 



The mutants produced by X-rays often lack the ability to make 

 one or other of these compounds and growth will not take place un- 

 less the necessary compound is supplied. It is thus quite easy to iso- 

 late a mutant which lacks the ability to make one compound (see 



C9 



i 



Wild type 



I*' 



V5/ 



Minimal 

 medium 



X-rays or 

 ultraviolet 



////// 



Conidia 

 (asexual spores) 



O 



O 



oi 





Complete 



medium 



(with vitamins, 



amino acids, 



etc.) 



U 





Crossed with 

 wild type of 

 opposite sex 



Fruiting body 



I 



Sexual spore 



(9 O 



^ 



1^ 



V^ V5^ Vi/ Vi/ 



Vitamins Amino Minimal Complete 

 acids 



FIG. 14. How mutants of the fungus, Neurospora, are made and tested for 

 their nutritional requirements. Spores of the 'wild type' of the fungus are 

 exposed to X-rays or ultra-violet light. They are then 'crossed' with spores 

 of the original wild type and the progeny is examined (this ensures that the 

 changes observed are genuine effects on the genes or mutations). The spores 

 so obtained are tested for their ability to grow in various solutions. In the 

 first place those which cannot grow in a minimal medium which supports 

 the wild type, but can grow when a complete set of supplements (vitamins, 

 amino acids, etc.) are isolated. Separate tests are then made to discover which 

 of the supplementary substances they require 



