330 A. N. BELOZERSKIÏ 



the appearance of the ribonucleotides, followed by RNA, was primary. The 

 DNA appeared much later, parallel with more complicated functions and an 

 ever-increasing differentiation of protoplasm. The following considerations may 

 be put forward in support of RNA priority: (i) RNA is found in all morpho- 

 logical elements of protoplasm without exception, whereas DNA is locaUzed in 

 the cell nucleus exclusively. (2) Growth and reproduction become possible only 

 when the RNA level reaches a certain value characteristic of that ty^pe of cells. 

 A vivid example of this is afforded by bacteria in whose cells, during the lag- 

 phase period, there take place substantial alterations preparatory to subsequent 

 reproduction and growth processes. Chemical analysis, specifically the analysis 

 carried out in our laboratory by G. N. Zaïtseva [15], reveals that one of the most 

 typical changes in this period is the intense biosynthesis of RNA. It is only 

 when the RNA reaches a certain fixed level that the bacterial cell enters the 

 logarithmic phase of growth. (3) The RNA appears to be deprived of genetic 

 functions. This is confirmed sufficiently weightily by the latest investigations of 

 the laboratories of Schramm and Fraenkel-Conrat, who have proved RNA 

 responsible for the reproduction of corresponding viruses. (4) Ribonucleotides 

 participate in the most diverse phases of metaboUsm. Moreover, as this has been 

 estabhshed in recent years by Leloir and others, all four nucleotides in RNA 

 are associated with metabolism. Hence, through its nucleotides, RNA is closely 

 associated with diverse aspects of metaboHsm. 



DNA is much more highly speciaHzed than RNA, this being witnessed by 

 narrow locaUzation in the cell, higher specificity than that of RNA, a higher 

 degree of polymerization, and, finally, the fact that we still do not know of its 

 nucleotides being as omnipotent in metabolic processes as RNA nucleotides. 

 The very fact of greater specialization and differentiation appears to lend 

 support to the behef that DNA had originated later than RNA. 



It seems to us that our experimental findings concerning the nucleotide com- 

 position of DNA and RNA in diverse bacteria may also be interpreted as proof 

 of this notion. Indeed, in this group of organisms, parallel with a relatively con- 

 stant RNA composition, there took place substantial changes in DNA compo- 

 sition, proceeding simultaneously with the differentiation and specialization of 

 individual groups of bacteria. 



One thus gets the impression that RNA, which is associated with the most 

 general manifestations of vital activity, arose at an earlier stage in the develop- 

 ment of hfe, whereas the appearance of DNA is associated with the formation 

 of narrower and phylogenetically later properties of organisms. 



It is to be hoped that new data will be obtained in the near future, shedding 

 Hght on the intimate mechanisms through which the nucleic acids play their part 

 in the basic processes of hfe. The earnest of this is the acute interest felt in this 

 problem and the large scale of research in this group of compounds. 



REFERENCES 



1. A. N. BelozerskiI & A. S. Spirin, Usp. sovr. Biol., 41, 144, 1956. 



2. A. S. Spirin & A. N. BelozerskiI, Biokhimiya, 21, 768, 1956. 



3. A. S. Spirin, A. N. BelozerskiI & A. Pretel-Martinez, Dokl. Akad. Nauk 



S.S.S.R., III, 1297, 1956. 



