DEOXYPENTOSE NUCLEIC ACIDS 65 



complexes of bacterial origin can be studied. The most widely 

 investigated nucleoproteins, of both the deoxy and pentose types, 

 namely, the viruses and phages, probably represent more com- 

 plex types of organization than are encountered in the conjugated 

 proteins that I have mentioned above. 



Two principal methods are used for the isolation of deoxy- 

 ribonucleohistone and of similar complexes with protamine, 

 namely, (a) extraction with strong salt solution; (b) extraction 

 with solutions of very low ionic strength. The procedures using 

 strong salt solutions are most widely used, though they would 

 appear more objectionable, if the dissociation of the nucleo- 

 protein and the formation of an artifact is to be avoided. These 

 nucleoproteins are soluble at a very low electrolyte concentration. 

 As the latter increases, the solubility first drops rapidly, reaching 

 a minimum in isotonic solutions, and then rises gradually. Though 

 variations occur, freshly prepared nucleohistone is, in general, 

 soluble at NaCl concentrations higher than 0.7 or 0.8 M. 



The course of dissociation of two nucleoproteins (calf thymus 

 and wheat germ) at rising NaCl concentrations is illustrated in 

 Fig. 2 (p. 45, Ref. 8) and is based on experiments of Crampton 

 et aiy and of Lipshitz and Chargaff^^. The observation that a 

 progressive separation between protein and nucleic acid takes 

 place has made possible the fractionation of the latter, as will be 

 mentioned below. 



3. DEOXYPENTOSE NUCLEIC ACIDS 



The position of the phosphate bridges holding together the 

 mononucleotides of a polynucleotide chain has been studied in 

 detail only in calf thymus deoxyribonucleic acid; the available 

 evidence points to this link going from the 5' position to the 3' 

 position on the neighboring nucleosides^. Whether this generali- 

 zation holds for many, or all, deoxypentose nucleic acids cannot 

 yet be stated. Wherever the sugar moiety could be identified, it 

 proved to be 2-deoxyribose^; but here, again, no general state- 

 ment can be made. As to the nitrogenous constituents, two 



References p. 75 



