IV. CELLULAR CONTROL OP DNA BIOSYNTHESIS 165 



inducer for the synthesis of nuclear deoxyribotides. No correlative 

 studies were made between the utilization of these compounds and 

 nuclear DNA synthesis. 



It has also been shown that sea urchin embryos contain large amounts 

 of acid-soluble deoxyribosidic compounds (Sugino et al., 1960; Sugino, 

 1960) amounting to more than 5 or 6 times the intracellular DNA 

 content. In these studies it was shown that the DNA content of the cell 

 doubled during the cell division cycle. However, no studies of the time- 

 course of DNA synthesis as related to the disappearance of acid-soluble 

 deoxyribosidic compounds were made. The acid-soluble material appears 

 to be derived from cytidine, which would indicate that it cannot alone 

 serve as an adequate reservoir of DNA precursor material. Nevertheless, 

 it is possible that further, more detailed studies on this system could 

 reveal a relationship similar to that found in amphibian embryos. That 

 this may be the case is indicated by the immunity of echinoderm embryo 

 development to agents inhibiting purine or pyrimidine deoxynucleoside 

 biosynthesis (Karnofsky and Bevelander, 1958; Karnofsky and Basch, 

 1960). This immunity persists until the blastula stage and indicates that 

 until this time the developing embryo is not dependent on de novo purine 

 or pyrimidine biosynthesis. It is possible that further studies in these 

 systems of the time relationship between DNA synthesis and deoxyribo- 

 tide utilization may substantiate a product-precursor relationship be- 

 tween the two. 



In these systems, then, the possibility exists that changes in the 

 quantity of precursor material may control the synthesis of DNA. 



Other situations in which deoxyribosides, or deoxyribotides, accumu- 

 late quantitatively prior to DNA synthesis are not known. IMaterial 

 from a number of different sources have been analyzed for their content 

 of deoxyribosides or deoxyriboside derivatives (Schneider, 1955, 1957; 

 Schneider and Brownell, 1957; Rotherham and Schneider, 1958, 1960; 

 Potter and Schlesinger, 1955; Potter et al., 1957; Potter and Buettner- 

 Janusch, 1958; LePage, 1957; Ord and Stocken, 1958; Parizek et al, 

 1958; Davidson and Bishop, 1957; Okazaki and Okazaki, 1958; Okazaki 

 et al, 1959; Okazaki, 1959; O'Donnell et al, 1958; Okazaki et al, 1960; 

 Lark, 1961). With one exception (Kanazir, 1954), the amount of material 

 was only a small fraction (from 0.2-5%, depending upon the system) 

 of the DNA content of the cell. Moreover, with the exception of two 

 observations only pyrimidine deoxynucleosides or deoxynucleotides have 

 been reported. In one of these exceptions where deoxyadenosine triphos- 

 phate was observed in a rat carcinoma (LePage, 1957), this amounted to 

 less than 10% of the quantity of pyrimidine deoxynucleosides or deoxy- 

 nucleotides (Schneider, 1957). 



