n TPPP TP 



n dGPPP dGP 



+ DNA^=^DNA - 

 n dAPPP dAP 



n dCPPP dCP 



+ 

 4(n)PP 



Fig. 6. Equation for Enzymatic Synthesis of DNA. 



and diesterase present in the coli cell. The occurrence of what appears to be 

 a similar DNA synthesizing system in animal cells as well as in other bacterial 

 species has been observed (14). We must wait for purification of the enzymes 

 from these sources in order to make valid comparisons with the coli system. 



The requirements for net synthesis of DNA with the purified coli enzyme 

 (15) are shown in the equation in Fig. 6. All four of the deoxynucleotides which 

 form the adenine-thymine and guanine-cytosine couples must be present. The 

 substrates must be the tri- and not the diphosphates and only the deoxy 

 sugar compounds are active. DNA which must be present may be obtained 

 from animal, plant, bacterial or viral sources and the best indications are 

 that all these DNA samples serve equally well in DNA synthesis provided 

 their molecular weight is high. The product, which we will discuss in further 

 detail, accumulates until one of the substrates is exhausted and may be 20 

 or more times greater in amount than the DNA added and thus is composed 

 to the extent of 95 % or more of the substrates added to the reaction mixture. 

 Inorganic pyrophosphate is released in quantities equimolar to the deoxy- 

 nucleotides converted to DNA. 



Should one of these substrates be omitted, the extent of reaction is di- 

 minished by a factor of greater than io 4 and special methods are now required 

 for its detection. It turns out that when one of the deoxynucleotide substrates 

 is lacking, an extremely small but yet significant quantity of nucleotide is 

 linked to the DNA primer. We have described this so-called "limited reac- 

 tion" (16), and have shown that under these circumstances a few deoxy- 

 nucleotides are added to the nucleoside ends of some of the DNA chains but 



s-66 



