66 NITROGEN METABOLISM 



to synthesize a compound essential for life, the mutant is 

 unable to grow unless it can obtain from the environment 

 at least one of the products of the reaction which it is unable 

 to accomplish. Such a reaction is often referred to colloqui- 

 ally as a 'genetically blocked reaction'. Beadle and Tatum 

 predicted that it should be possible to deduce the sequence 

 of reactions in biosyntheses from the range of compounds 

 which replace the substances required by nutritionally- 

 exacting mutants. The organisms used in these studies 

 include Penicillium notatum, Aspergillus niger. Asp. nidulans, 

 Ophiostoma, Esch. coli and B. subtilis, but the ascomycetes 

 Neurospora crassa and N. sitophila still remain the most 

 suitable if precise genetic data is also required. Most natural 

 (i.e. 'wild type') strains of Neurospora grow on a simple 

 medium containing mineral salts, biotin, an inorganic 

 source of N and an organic source of C and energy (e.g. 

 sucrose, sorbitol) and must therefore possess the wide 

 variety of enzymes required for the synthesis of all the 

 normal constituents of cytoplasm. The vegetative phase 

 reproduces asexually by conidia and micro-conidia; sexual 

 reproduction is only possible between gametes from parents 

 of opposite mating types. Much is known concerning the 

 genetics of Neurospora [see 5] and since the vegetative phase 

 is haploid, there are in contrast with diploid organisms, no 

 problems concerning the dominance of one character over 

 another. Strains whose nutritional requirements are different 

 from the parent type may arise naturally by spontaneous 

 mutation, but such mutations are often few in number and 

 natural selection does not favour their survival. In order to 

 increase the chance of isolating such mutants, the mutation 

 rate is artificially increased by exposing the conidia to ultra- 

 violet light. X-rays or chemical mutagens, e.g. mustard gas. 

 The conidia are then transferred to the protoperithecia of 

 wild type Neurospora of opposite mating type, and in conse- 

 quence asci develop, each ascus containing eight spores. One 

 spore from each ascus is transferred to a solid medium con- 

 taining the minimal requirements for growth (minimal 

 medium), and after incubation the colonies that have 

 developed will be of the wild type. Their position is noted 



