162 A. N. BELOZERSKY AND A. S. SPIRIN 



(e.g., of closely related species) may widely differ as to the composition of 

 only some nucleic acid molecules, the main mass of the molecules being 

 the same or very similar in their composition. In this case, even drastic 

 differences in the composition of a few of the molecules of two species will 

 not be observed since they will be veiled or become completely imperceptible 

 because of the similarity of the composition of the main mass of DXA or 

 RNA. 



Yet, in spite of the above mentioned restrictions, the determination of 

 the nucleic acid composition is the only method which, to the present time, 

 has been systematically and successfully applied to the study of various 

 aspects of the chemical specificity of total nucleic acids. Proceeding from 

 this assumption, when considering further different aspects of nucleic acid 

 specificity, we shall refer almost solely to the specificity of their composi- 

 tion. 



1. Age Specificity of Nucleic Acids and Related Problems 



This section deals with a group of problems concerning the nucleic acid 

 composition of microorganisms as it depends on various physiological and 

 functional states of the cells. Besides the age aspect, we shall be concerned 

 with the effect of environmental conditions on nucleic acid composition, as 

 well as the effect of one or another change in cell metabolism on the quality 

 of nucleic acids. 



The problems to be discussed in this section are of key importance for 

 the elucidation of all other aspects of the nucleic acid specificity of bacteria. 

 If nucleic acids appreciably altered their composition, either in the course 

 of the development of cells or under the effect of certain conditions, then 

 the nucleic acids of various microorganisms would have to be compared in 

 cells of the same physiological and functional states only. This would con- 

 siderably complicate the study of nucleic acid specificity. But fortunately, 

 according to the data available, nucleic acids seem, by the criteria available 

 at present, to possess a very stable compositional specificity which does 

 not exhibit appreciable alterations either in the course of the growth and 

 development of the cells or as a result of the conditions of cultivation. 



a. Nucleic Acid Corn-position at Different Ages of the Culture 



(1) DNA. The first data proving the invariability of the DXA composi- 

 tion in the course of growth of a bacterial culture were obtained on Pseudo- 

 monas hydrophila 126 and, somewhat earlier, on Haemophilus influenzae. 127 

 Abrams' claim of alterations in the DNA in the course of growth of the 



126 K. K. Reddi, Biochim. et Biophys. Acta 15, 585 (1954). 



127 S. Zamenhof, in "Phosphorus Metabolism" (W. D. McElroy and B. Glass, eds.), 

 Vol. 2, p. 336. Johns Hopkins, Baltimore, 1952. 



