130 



S. S. COHEN 



We may picture the position of genetic units in a metabolic sequence in 

 the simplest form, as sho^\ai in formula (VII). 



B. 



C. 



->D. 



t t t 



Enzyme^ Enzymeg Enzymec 



t t t 



Gene A Gene B Gene C 



(VII) 



At the level of a specific reaction, this apparently simple genetic control 

 may be complicated by a network of other possible reactions (see Fig. 26) 

 for all of which known examples exist. 



'Primary 

 gene 



Recctants 



Factors affecting rate of 

 production and molecular 

 ,, species of enzymes 



Produces 



Degradation of enzyme 



Rate of production 

 of inhibitors 



Fig. 26. A summary illustrating some factors of genetic origin that influence reaction 

 rates (Wagner and Mitchell, 1955). 



As this scheme demonstrates, the normal or distorted actions of a single 

 gene can and do impinge on a wide variety of more or less directly related 

 reactions and thereby can and do influence an ever-widening circle of meta- 

 bolic patterns, cellular structures, and morphological units. As a result of 

 these complicating factors, the task of determining the primary site of gene 

 action has much in common with determming the primary effects of viral 

 infection. We may note in addition that many genes appear to be able to 



