128 



S. S. COHEN 



CI 



C3 



base 

 C 



o 



II 



C-O-P 



boses or protein 



I 

 base 



C 



bases or protein 



base 

 C 



base 



base 

 C 



base 



XPO. 



TOT 



C_0-P C-O-P partial cleavage^ „ C — OH C— O-p, 



\ I l\ I A\ I \ 



'"'5 O-C OHO-C H O-C 



\ I 



O-C 



\ 



c 



I 



C-O-P. 



i\ I 



C O O-C 



I " 



O O 



= P O-P-OH 



O O 



H H NH, 



I ^ 

 aminoacid RiC — COOH 

 or anhydride 



boses or protein 



base 



I 



C 



\. 



base 



I 



C 



\ 



C-O-R C-O-R 



I \ I 



C O-C 



I 



o 



1 



0=P— OH 



1 



° R 

 I I 

 o=c— C NH2 



Fig, 25. A possible meclianism for the participation of DNA in metabolic activity 

 without replacement of carbon, nitrogen, and phosphorus atoms. 



V. CONTEOLLING MECHANISMS OF EnZYME AND PrOTEIN SYNTHESIS 



A. Introduction 



Until now this survey has been concerned for the most part with the 

 control of polymer synthesis imposed by existing cell structure. The develop- 

 ment of cell structure is in turn controlled by genetic and environmental 

 factors; mdeed, such factors are determinative not only in the formation of 

 structure but also in their operation. Determining genetic factors are built 

 into cell structure in nucleus and cytoplasm and confer the potentialities for 

 metabolic and synthetic reactivities; the reahzation of these potentialities, 

 i.e., phenotypic expression, is dependent upon the existence of a suitable 

 internal and external milieu, i.e., pH, ionic strength, temperature, pressure, 

 nutrients, substrates, and other substances. 



