50 DEOXYPENTOSE NUCLEOPROTEINS AND PROSTHETIC GROUPS 



/. Isolation of deoxypentose nucleic acids 



The first step in the preparation of a deoxypentose nucleic 

 acid is, irrespective of whether this is done consciously or not, 

 the isolation of a nucleoprotein. Once the nucleoprotein or a 

 mixture of nucleic acid and protein has been extracted, the 

 removal of the protein is carried out by treatment with chloro- 

 form, saturation with sodium chloride, or with the aid of an 

 anionic detergent. The several methods have been reviewed 

 critically in a recent survey^. 



3. DEOXYPENTOSE NUCLEIC ACIDS 



One of the surest signs that one has reached a stage of truth is 

 that its affirmation begins to bore the audience. I shall, therefore, 

 limit myself to a brief consideration of the principal conclusions 

 regarding the composition and structure of nucleic acids, many 

 of which already had been formulated six or seven years ago^^ 

 i.e., in what were the early days of modern nucleic acid research. 



a. Is there more than one deoxypentose nucleic acid in nature? 



All evidence points to the existence of a very large number 

 of differently constituted nucleic acids; a number probably ex- 

 ceeding by far what can be revealed by constituent analysis. For 

 it should be remembered that, while differences in nucleotide 

 composition signify divergences in nucleotide sequence, an iden- 

 tical composition does not prove an identical sequence. An early 

 estimate of the possible number of isomers having the com- 

 position of calf thymus deoxyribonucleic acid, which assumed 

 a chain of 2,500 nucleotides, gave the figure of lO^^^^ Although 

 the differentiation between nucleic acids on the basis of dif- 

 ferences in the contents of their nitrogenous constituents is a 

 relatively crude expedient, since it does not permit a distinction 

 between sequence variations that are not accompanied by a 

 change in total composition, it can be shown in many cases that 

 nucleic acids of different origin differ in composition (compare 



