STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 129 



Recent advances on the participation and interaction of these genetic and 

 environmental factors in polymer syntheses have been obtained on biological 

 materials of relatively little interest to virologists. Of the major experimental 

 materials, DrosopJiila, yeast, Neurospora, Escherichia coli, and most recently 

 Sahnonella typhimurium, only the bacteria have seriously attracted the 

 interest of virologists.^ Very little of this type of information is available for 

 the cells which are infectable by animal and plant viruses. However, the 

 development of tissue cultures of animal cells is beginning to provide suitable 

 biological materials for these types of analysis. Thus, papers have begun to 

 appear on inheritance and mutation in clones of animal cells (Puck and 

 Fisher, 1956), on the detailed nutritional requirements of such cells (Eagle, 

 1955), and on their metabolic patterns and their variabihty as a function of 

 environmental conditions. 



We shall briefly consider three types of controlling elements that have 

 begun to yield information on the mechanisms of polymer synthesis and shall 

 in the main confine our attention to these aspects of metaboHc activity. 



B. Genetic Controls 



This topic has been the subject of many books and reviews, of which a 

 recent volume can serve as one guide to the literature (Wagner and Mitchell, 

 1955). Since the original work of Garrod on inborn metabohc errors in man, 

 it was recognized that a close relationship existed between the genetic units 

 of Mendelian inheritance and the control of metabolic activity. Garrod's 

 studies of this problem appeared at the turn of the century (1902) and Cuenot 

 probably made the first statement of a one gene-one enzyme hypothesis, in 

 1903, before the word "gene" was coined by Johannsen, in 1911. Many forms 

 of such a hypothesis now exist and state m general that genes exert their 

 physiological power through enzymes whose synthesis or activities they 

 control. The variations on this theme extend from the early view of Haldane 

 that a gene may manufacture particular chemical species of enzyme or an 

 antigen to the more recent and cautious statements to the effect that there is 

 a large class of genes, each of which controls the activity or the specificity of a 

 single enzyme. We may note that, in the latter definition of the one gene-one 

 enzyme hypothesis, not all genes necessarily affect enzymes, that the 

 moderating control may be either direct or indirect, and that a single enzyme 

 need not be controlled by a single gene. However, if the activity of a single 

 enzyme is controlled by more than one gene, it is postulated that the con- 

 tribution of the various genes must be different. 



^ There is the outstanding exception in the studies of the virus-like CO2 sensitivity 

 factor in Drosophila (L'Heritier, 1951). 



