32. THE NUCLEIC ACIDS OF MICROORGANISMS 161 



from the medium. 125 The study of the effect of other 8-azapurines and substituted 

 triazoles on E. coli and B. cereus showed that these substances also suppress bacterial 

 growth but the}' [8-azaguanine, 8-azaadenine, 8-azahypoxanthine, and 5(4)-amino- 

 l-H-l,2,3-triazole-4(5)carboxamide] are incorporated into RNA only in the form of 

 8-azaguanine. 108 



The entire problem relating to the discovery of new purine and pyrimidine 

 nucleotides occurring naturally in DXA and RNA, as well as the incorpora- 

 tion of unnatural analogs of purines and pyrimidines into both these nucleic 

 acids, opens new vistas for the study of the specificity of nucleic acids. 



IV. Nucleic Acid Composition and Specificity 

 In speaking about the quality and qualitative variety of nucleic 

 acids, we have in mind, first of all, their chemical specificity. As it has re- 

 peatedly been pointed out in the literature, the principal features by which 

 the specificity of nucleic acids may display itself are as follows: (1) the pro- 

 portions of various purine and pyrimidine bases, i.e., the nucleic acid com- 

 position; (2) nucleotide sequence; (3) macromolecular configuration. These 

 features may, strictly speaking, be regarded merely as the possibilities of 

 specific differences between individual molecules, i.e., in the purely chemical 

 aspect. When dealing with biological units, however, that is cells, organoids, 

 etc., we may encounter, and probably do encounter, the fact that the 

 nucleic acids of any such unit represent not one type of molecule but a whole 

 set of molecules of various structure and specificity. 80 In other words, we 

 may run into the heterogeneity of nucleic acids in biological objects. The 

 study of the specificity of individual molecules of the nucleic acids is not 

 possible however, at present, because of the difficulties not yet overcome 

 in the isolation, and, in particular, in the complete fractionation of the entire 

 set of cellular nucleic acids in the native state. In this connection, when 

 studying the specificity of composition and structure of nucleic acids, the 

 main point refers to the specificity of the total cell DNA or RNA. From 

 the biological viewpoint, this approach often proves of no less importance 

 than the study of the specificity of individual molecules. 



At present, the only available and widely used approach to the solution 

 of the problem of nucleic acid specificity is the study of their composition. 

 There is, however, a very important restriction to this approach which 

 must be noted; in the case of an equal composition, one is unable to decide 

 whether there really is no difference between the nucleic acids under study, 

 or whether there do exist specific differences which have to do with differ- 

 ences in the nucleotide sequence or some other structural differences. 

 Unfortunately, hitherto only preliminary attempts have been made with 

 respect to such an important and maybe decisive approach as the deter- 

 mination of the nucleotide sequence. 



Another no less important restriction in the study of the nucleic acid 

 composition lies in the fact that the total DNA or RNA of any two objects 



