CONTROL OF POTENTIAL ENZYMIC CONSTITUTION 59 



THE POTENTIAL ENZYMIC CONSTITUTION 

 It is obvious that all organisms cannot form all enzymes. 

 Otherwise any attempts at classification would fail. Although 

 the enzymic constitution of an organism can and does undergo 

 vast changes with alterations in the growth environment, theie 

 is still a limit to the changes that can occur and the enzymes 

 that can be produced by any one organism. In higher 

 organisms the enzymic constitution is controlled by the genetic 

 composition of the cell. Genes are hypothetical units which 

 determine the carry-over of characteristics from mother- to 

 daughter- cell. In nucleate cells it is possible to observe 

 changes in the form of the nucleus during division of the cell ; 

 the nucleus forms a skein instead of a solid body, the skein 

 breaks up into short rod-like structures known as chromosomes 

 and, in normal division, each chromosome divides into two 

 before division, one of each pair passing into each daughter- 

 cell; after the daughter cell has spUt off, the chromosomes join 

 up again into a skein which then collapses to form the new 

 nucleus. The genes occupy definite positions on the chromo- 

 somes and damage to a chromosome in a certain place will be 

 accompanied by loss of the property associated with possession 

 of the gene lying at that place. Chromosomes have not yet 

 been clearly demonstrated in 'bacteria, although structures 

 allied to chromosomes have been described in some of the 

 filamentous organisms. However, the general behaviour of 

 bacteria, the inheritance of enzymic properties from one cell to 

 another, and the occurrence of "mutants" suggest that some 

 form of genetic control of enzyme constitution occurs in these 

 organisms. 



Neurospora crassa, a mould which commonly occurs on 

 bread, has certain characteristics concerning the arrangement 

 of its spores which make it easy to study from a genetical point 

 of view. It has been found that it is possible to alter the 

 genetic constitution of the mould by irradiation with X-rays. 

 When this is done an occasional " hit " is made by the radiation 

 on the chromosome and the absorption of a quantum of energy 



