o o o 



II II 11 



Adenosine -O-P tOP-O-P-O 

 /> 



o" I o o 



o 



I 



Nucleoside - O - P - O' 



II 



O 



Fig. 4. Nucleophilic Attack of a Nucleoside Monophosphate on ATP. 



the simplest of the nucleotide condensation products, that it is ATP which 

 condenses with nicotinamide mononucleotide to form diphosphopyridine nu- 

 cleotide, with riboflavin phosphate to form FAD, with pantetheine phosphate 

 to form the precursor of coenzyme A and so forth. This pattern has been 

 amplified by the discovery of identical mechanisms for the activation of fatty 

 acids and amino acids and it has been demonstrated further that uridine, 

 cytidine and guanosine coenzymes are likewise formed from the respective 

 triphosphates of these nucleosides. 



This mechanism (Fig. 4), in which a nucleophilic attack (10) on the pyro- 

 phosphate-activated adenyl group by a nucleoside monophosphate leads to 

 the formation of a coenzyme, was adopted as a working hypothesis for studying 

 the synthesis of a DNA chain. As illustrated in Fig. 5, it was postulated that 

 the basic building block is a deoxynucleoside 5 '-triphosphate which is attacked 

 by the 3'-hydroxyl group at the growing end of a polydeoxynucleotide chain; 

 inorganic pyrophosphate is eliminated and the chain is lengthened by one unit. 

 The results of our studies on DNA synthesis, as will be mentioned presently, 

 are in keeping with this type of reaction. 



Properties of the DNA-synthesizing enzyme. 

 First let us consider the enzyme and comment on its discovery (8, 11, 12). 

 Mixing the triphosphates of the four deoxynucleosides which commonly occur 

 in DNA with an extract of thymus or bone-marrow or of Escherichia coli 



S-54 



