148 A. N. BELOZERSKY AND A. S. SPIRIN 



In recent years, however, along with a purely quantitative approach, the 

 study of the nucleic acid quality, i.e. of their specificity, acquired an ever 

 greater importance. It was, in fact, in the bacteria that there was revealed 

 the possibility of the existence of qualitative differences between the nucleic 

 acids of different origin, i.e., the possibility of their biological specificity. 13, 14 



Today, therefore, microorganisms attract ever increasing attention as 

 the organisms in which the role of nucleic acid activity is most clearly re- 

 vealed. Vast experimental material has been obtained with microorganisms, 

 which elucidates the role of nucleic acids in the processes of protein synthe- 

 sis, hereditary phenomena, etc. That is why this group of organisms seems 

 to be particularly attractive for the study of the general problems of the 

 chemistry and the biochemistry of the nucleic acids. 



The present review is restricted, mainly, to the chemical aspects of the 

 problem. The following will be considered: (a) various aspects of variations 

 of the nucleic acid content in microorganisms; (6) constituents of microbial 

 nucleic acids; and (c) composition and specificity of microbial nucleic 

 acids. 



II. Nucleic Acid Content and Dynamics 



It is well known that the nucleic acid content of microbial cells undergoes 

 great variations depending on the growth phase, the conditions of cultiva- 

 tion, and the physiological and functional state of the cells, as well as on 

 the strain itself or on certain physiological or morphological changes (e.g., 

 acquisition of resistance to an antibiotic or transition into the L-form). 



In view of this fact, earlier data on the nucleic acid content in different 

 species of microorganisms 1, 2i 4 ' 5 are of somewhat limited value since they 

 do not take into consideration these circumstances which are often of deci- 

 sive importance. 



Variations of nucleic acid content with the age of the culture in bacteria 

 were studied for the first time by Belozersky 7 on cultures of Spirillum. 

 Later, similar chemical studies were carried out on a number of other 

 strains. 3, " It was noted that young cultures were always characterized by 

 a higher nucleic acid content that the old ones. In the process of aging, 

 there always occurred a regular decrease of the nucleic acid content. 



A detailed study of the dynamics of the nucleic acid content during 

 growth was, for the first time, carried out by means of spectrography in the 

 ultraviolet range in the investigations of Malmgren and Heden. 8 ' 9 These 

 studies demonstrated that an accumulation of nucleic acids takes place in 

 the bacterial cell at the lag-phase preparatory to division. After the nucleic 

 acid content has attained a certain level, the division of cells, i.e. the 

 multiplication of bacteria, becomes possible. From this moment, the amount 



13 O. T. Avery, CM. MacLeod, and M. McCarty, J. Exptl. Med. 79, 137 (1944). 



14 E. Chargaff, 2nd Intern. Congr. Biochem., Paris "Symp. sur le metabolisme micro- 

 bien,"p. 41 (1952). 



