286 ORIGIN OF STRUCTURES AND FUNCTIONS 



is contradicted by numerous observations which show that, 

 during the process of development of cells, their DNA con- 

 tent may diminish markedly until it disappears completely, 

 as occurs in the unfertilised sea-urchin's egg.^"^ On the basis 

 of a study of the development of the mycelium of Actino- 

 myces globisporus streptomycini N. S. Demyanovskaya and 

 A. N. Belozerskii^"* have shown that at a definite develop- 

 mental stage DNA apparently disappears. In its place there 

 is found in the mycelium another nucleic acid containing not 

 thymine but another base, X. This acid is apparently a 

 precursor of DNA, which is later synthesised from it. 



We still know very little about the biosynthesis of nucleic 

 acids but all the facts at our disposal suggest that it is no less 

 complicated a process than the biosynthesis of proteins and is 

 by no means a simple autocatalytic process of self-reproduc- 

 tion. 



The studies on the enzymic synthesis of nucleic acid which 

 have recently been started by M. Grunberg-Manago and 

 others^°^ in S. Ochoa's laboratory are of very great interest 

 from this point of view. These workers used an enzyme isol- 

 ated from Azotobacter which catalyses the synthesis of poly- 

 nucleotides from nucleoside-5'-diphosphate with the libera- 

 tion of inorganic orthophosphate according to the equation: 



n(A— R— P— P)^^(A— R— P)„ -f r?P 



A = the base, R = ribose, P = pliosphoryl radical. 



One may thus suppose that the reaction whereby poly- 

 nucleotides are formed, like that whereby polysaccharides are 

 formed, is a process of reversed phosphorolysis. The authors 

 therefore called their enzyme ' polynucleotide phosphoryl- 

 ase '. They showed that it was possible, by using this enzyme, 

 to synthesise a substance similar to ribonucleic acid from 

 separate mononucleotides. From this it follows that the bio- 

 synthesis of nucleic acids, like that of the other compounds 

 found in protoplasm, is brought about by means of a compli- 

 cated enzymic apparatus. Thus, on the one hand, the synthesis 

 of proteins requires the presence of nucleic acids while, on 

 the other, the synthesis of nucleic acids requires the presence 

 of proteins (enzymes). 



