BIOSYNTHESIS OF NUCLEIC ACIDS 365 



The data for other organs (many of which were contributed by R. M. S. 

 Smellie, and which are discussed in more detail in Chapter 26) have, with 

 the possible exception of kidney (lines 46 and 47), shown generally lower 

 ratios than those for liver. The recognition of the intracellular heterogeneity 

 of PNA decreases the significance of comparison of the total PNA with the 

 DNA. However, with precursors other than phosphorus, few separations 

 of PNA fractions have yet been made, and the bulk of the data now avail- 

 able for consideration deals with total PNA, or with but a few of the PNA 

 fractions. 



h. Precursors of the Carbon-Nitrogen Skeleton 



Ammonia (Table V, lines 88-90) is also incorporated into PNA to a 

 greater extent than into the DNA but the ratios observed are smaller. 

 Bergstrand et al}°^ and Eliasson et al}^^ showed that the nitrogen of glycine- 

 N^^ was incorporated into the DNA purines of rat livers to a smaller extent 

 than into the PNA purines, and that the nPNA fraction is renewed more 

 rapidly than is the cytoplasmic PNA (cPNA). Numerous subsequent ex- 

 periments with glycine, variously labeled in the nitrogen or in either of its 

 carbons, have always shown this significant incorporation iilto the DNA 

 of liver and have yielded PNA : DNA ratios ranging from about 3 to 8 (Table 

 V, Hues 65-67, 69-71, 74-76, 85, 103, 107, and 111), although recently some 

 much higher ratios have been obtained with glycine (lines 77 and 78). It 

 was also shown that, as with phosphate, the purines of the DNA fraction 

 from regenerating liver show a greater incorporation of the glycine nitrogen 

 and a resultant lowering of the PNA: DNA ratio (Table V, lines 68, 79, 80, 

 and 105). Investigations of the relative incorporations of formate into liver 

 nucleic acids have shown (Table V, lines 51-63, 109) ratios of the order 

 found with glycine, including an increased incorporation into DNA in 

 regenerating liver (lines 58 and 59). 



In contrast to the smaller precursors, the incorporation of administered 

 adenine into the purines of the PNA of rat liver has been found to exceed 



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