METABOLISM OF THE NUCLEIC ACIDS 407 



irradiation causes partial inhibition of uptake of P^^ and of labeled orotic 

 acid and formate into the DNA of rat thymus, the incorporation of labeled 

 adenine into the DNA adenine and guanine is not inhibited. 



In view of the discrepancy between the incorporation of small-molecule 

 precursors and preformed purines into the purines of DNA, Bendich et alP'^^ 

 have attempted to separate the tissue DNA into fractions with differing 

 metabolic activities. Some fractionation of DNA into two products with 

 slightly different metabolic activities has been obtained, but the difference 

 between the fractions appears to be small in comparison with the discrep- 

 ancies existing between the uptake of different precursors by DNA. 



A different approach to this problem has been made by Chargaff et al.,^^ 

 who have obtained several DNA fractions from calf thymus nucleohistone 

 by the successive extraction of nucleohistone/chloroform/octanol gels 

 with sodium chloride solutions of increasing concentration. When the 

 relative proportions of the bases in the DNA's derived from these extracts 

 by alcohol precipitation were determined, it was observed that the DNA 

 obtained mth increasing strengths of salt solution contained decreasing 

 proportions of guanine and cytosine while the corresponding values for 

 adenine and thymine increased. Similar results have been obtained inde- 

 pendently by Brown and Watson^^ using the somewhat different technique 

 of chromatography of calf thymus DNA on columns of histone-treated 

 kieselguhr. By a process of stepwise elution of the DNA with increasing 

 concentrations of sodium chloride solution. Brown and Watson obtained 

 several separate DNA fractions which on analysis were found to contain 

 less guanine and cytosine and more adenine and thymine as the concen- 

 tration of the eluting fluid was increased. These findings suggest that DNA 

 isolated from a tissue consists not of one or two different DNA's but of a 

 complete family of closely related but chemically distinguishable sub- 

 stances which could behave metabolically in quite different ways. 



4. The Metabolism of the Pyrimidine Bases 



From the studies discussed in Chapter 23, it may be concluded that 

 carbon dioxide furnishes the carbon 2 of uracil while the methyl group of 



" A. Bendich, Exptl. Cell Research. Suppl. 2, 181 (1952). 



^* A. Bendich and P. J. Russell, Jr., Federation Proc. 12, 176 (1953). 



86 A. Bendich, P. J. Russell, Jr., and G. B. Brown, /. Biol. Chem. 203, 305 (1953). 



«8 E. Chargaff, C. F. Crampton, and R. Lipshitz, Nature 172, 289 (1953). 



" G. L. Brown and M. Watson, Nature 172, 339 (1953). 



88 H. S. Loring and J. G. Pierce, J. Biol. Chem. 153, 61 (1944). 



89 H. Arvidson, N. A. Eliasson, E. Hammarsten, P. Reichard, H. von Ubisch, and 

 S. Bergstrom, J. Biol. Chem. 179, 169 (1949). 



'» R. S. Hurlbert and V. R. Potter, /. Biol. Chem. 195, 257 (1952). 



91 L. L. Weed and D. W. Wilson, J. Biol. Chem. 189, 435 (1951). 



92 M. Edmonds, A. M. Delluva, and D. W. Wilson, J. Biol. Chem. 197, 251 (1952). 

 " E. P. Anderson and S. E. G. Aqvist, J. Biol. Chem. 202. 513 (1953). 



