8 



THE BIOSYNTHESIS OF PROTEINS 



biochemical studies. Let us consider one classical case, that of some trypto- 

 phan-less mutants oi Neurospora. 



The wild type mould will grow readily on a very simple medium con- 

 taining salts, sucrose as the carbon source, and biotin. The mycelium forms 

 spores which, when transferred to fresh medium, will give a new 

 mycelium identical to the original one, and thus indefinitely. However, 

 from time to time a spore will not be able to grow on the basic medium, 

 unless tryptophan is added. Genetic analysis shows that this tryptophan 

 requirement is inherited as a single mendelian character ; it is the result of a 

 single mutation. 



Conidia 



Conidia 



Mycelium (JV) of 

 mating type A 



Trichogyne 



Protoperithecimn 

 of mating type A 

 (maternal parent) 





i. 



Rsaalta of 



first roeiotic 



division 



Ascoepore 

 germination 



Zygote C2iVJ 

 nuclem 



Results of 



second meiotic 



division 



Mycelium (N) of 

 mating type a 



Protoperithecium 

 of mating type a 

 (maternal parent) 



Aflcoepore 

 germination 



Ascoepores 

 from mitosis 



Fig. 8. The life cycle of Neurospora crassa (from Wagner and Mitchell, 



1955). 



Tryptophan is a constituent of the proteins of the mould, and it is 

 normally produced in the wild type. If the mutant cannot grow in the 

 absence of the exogeneous tryptophan, this means that the mutant cannot 

 make tryptophan for itself as the wild type used to do, and that it must 

 depend on the medium for securing the amino acid. The mutation has 

 probably blocked biochemical processes involved in tryptophan bio- 

 synthesis. 



Certain tryptophan-less mutants were found to excrete indol into the 



