ISOLATION AND COMPOSITION OF DEOXYPENTOSE NUCLEIC ACIDS 361 



fluctuation in composition in different nucleic acid preparations of the same 

 origin has been stressed before in connection with the results presented in 

 Tables V and VI. But, as has been pointed out (Section V.l), the number of 

 possible permutations within a nucleic acid chain, even without a change in 

 composition, is truly enormous; and many fine points cannot be considered 

 at the present state of our knowledge, as, for instance, whether two nucleic 

 acids of the same composition, but differing in some details of sequence or 

 in their terminal nucleotides, are to be regarded as different entities. One 

 must, for the time being, rely on relatively massive changes in composition. 



Although the possibility that a deoxypentose nucleic acid preparation 

 from a given species represents a mixture of many different individuals has 

 been discussed occasionally, it is only in recent times that successful frac- 

 tionation experiments have been recorded. Attempts to separate highly 

 polymerized preparations by fractional centrifugation^^'* or adsorption on 

 charcoaP^" were of no avail. Indications of heterogeneity, with respect to 

 their metabolic origin, of deoxypentose nucleate preparations from rat 

 tissues have been presented by Bendich et al}^'' The evidence rests on dif- 

 ferences in the incorporation of isotope observed with two nucleic acid 

 preparations differing in their solubility in physiol. saline. In the absence 

 of analytical information it is not possible to say whether the fractions 

 differed in composition, nor, in fact, whether they were pure nucleic acids. 

 In the light of the experiments of Crampton el al}° on the dissociation and 

 reassociation of nucleohistone, the material insoluble in physiol. saline prob- 

 ably represented a, perhaps partially degraded, protein nucleate. 



A process best described as the fractional dissociation of a nucleoprotein 

 has permitted the separation of many deoxypentose nucleate preparations 

 into a series of fractions of divergent purine and pyrimidine contents. The 

 procedure was first applied to calf thymus nucleohistone"^ and later ex- 

 tended to the fractionation of other nucleic acids through their complexes 

 with histone,"^ globin,"^ or polylysine.-^" Subsequently, a related procedure 

 was described in a preliminary form in which fractionation was achieved by 

 the gradual elution of the sodium nucleate from a histone-kieselguhr col- 

 umn. ^^^ 



2. Fraction.\l Dissociation of Nucleohistone or Protein Nucleates 



The experiments are based on the observation^" that when nucleohistone, 

 prepared as described in Section II.3.a.(l), is treated with chloroform in 

 the absence of electrolytes the entire nucleic acid P is found in the resulting 



2" A. Bendich, P. J. Russell, Jr., and G. B. Brown, J. Biol. Cheni. 203, 305 (1953). 



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



"9 C. F. Crampton, R. Lipshitz, and E. Chargaff, J. Biol. Chem., 211, 125 (1954). 



"° P. Spitnik, R. Lipshitz, and E. Chargaff, in preparation. 



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



