Heterogeneity of Deoxyribonucleic Acid (DNA) 17 



that differences among DNA fractions may be obscured when the total 

 DNA containing such fractions is analyzed. 



To compare the metabolic behavior of these two gross DNA frac- 

 tions, and the metabolism of the DNA of various organs, isotopic 

 formate was administered to a group of rats. Half the animals were 

 examined 1 day after administration of the formate and the remainder 

 23 days later. The DNA fractions isolated from a number of organs 

 were broken down to free bases and the percentage change in the 

 isotope contents during the 23-day period determined (Table 2) ; the 



Table 2. Per Cent Apparent Retention of C 14 during 23-Day Interval 

 Following Administration of Labeled Formate to Adult Rats * 



* Taken from A. Bendich, P. J. Russell, Jr., and G. B. Brown, J. Biol. Chem., 203, 305 (1953). 



change is referred to as "apparent retention." These results 9 show 

 that the two fractions are metabolically dissimilar in any one organ 

 and that the metabolic picture is different from organ to organ. With 

 the possible exception of the DNA 2 fraction of pancreas, the "apparent 

 retention" of isotope differs from base to base for the individual frac- 

 tions. (An analogous result has been obtained in mouse- and rat-liver 

 DNA in studies with P 32 . 12 ) These studies suggest that the DNA 

 fractions of various organs of the rat have a heterogeneous metabolic 

 origin. Thus, in addition to differences between the two fractions 

 insofar as solubility and chemical composition are concerned, they 

 could be distinguished on the basis of their anabolic behavior. 



Further evidence for chemical heterogeneity of DNA has recently 

 been obtained by three independent techniques. One may be termed 

 fractional dissociation of calf-thymus nucleoprotein; the second in- 

 volves chromatography on columns of histone, now itself found to be 

 heterogeneous; the last to be described employs an anion exchanger 

 that is ostensibly homogeneous. 



Calf-thymus nucleoprotein, in the form of a loosely packed gel with 

 chloroform-octanol, yields a few nucleoprotein fractions upon extrac- 



