82 THE BIOSYNTHESIS OF PROTEINS 



Hybrid helices can also form in which one strand with a DNA backbone 

 wraps around another strand with a RNA backbone in such a manner that 

 the strands are held together by complementary hydrogen bonds formed 

 between purine and pyrimidine residues (Rich, 1960). Should RNA be 

 copied from DNA by such a process, a complete correlation between 

 RNA and DNA composition would be expected only if both DNA chains 

 were copied and if each DNA unit would produce about the same number 

 of RNA replicas. 



Belozersky and Spirin (1958) determined the base composition of DNA 

 and total RNA of nineteen bacterial species belonging to various groups. 

 As shown in the tables, the composition of DNA varies widely, from 

 species in which the Adenine Thymine pair accounts for 70 per cent of the 

 bases, to types in which it accounts for about 25 per cent only. In the same 

 organisms, the composition of total RNA differs much less from species to 

 species. However, a certain correlation is observed between the composi- 



/Q_i_p\ / 



tion of RNA and DNA: the ratio ,—-, — — -' seems to vary as a linear func- 



/(A+U) ^ 



tion ofrj^—r^ in DNA (Belozersky, 1959). 



Although the composition of global RNA does not correspond to that 

 of global DNA of the same organism, the observed correlation together 

 with the fact that DNA composition in a given species is rather homogen- 

 ous down to the level of smaller molecules (Rolfe and Meselson, 1959) 

 suggests that a fraction of the RNA has indeed a composition which corre- 

 sponds to that of DNA. Belozersky's results are therefore compatible with 

 the idea that a fraction of bacterial RNA is organized under the control of 

 DNA, and might possibly carry specific genetic information. 



Volkin and Astrachan (1957) made very interesting observations on RNA 

 synthesis at the beginning of phage infection. The distribution of ^sp 

 incorporated into RNA indicates that the small RNA fraction which rapidly 

 forms after infection has the same base composition as phage DNA (with 

 uracil substituted for thymine). Using the same method. Yeas and Vincent 

 (1960) reached the conclusion that yeast contains a relatively small fraction 

 of RNA with a high turnover rate, whose composition is very similar to 

 that of yeast DNA, with uridylic acid replacing thymidylic acid. The 

 authors, for this reason, suspect that this fraction may be a primary gene 

 product, a specific RNA copied from DNA and which carries information 

 to centres of protein synthesis. A new RNA fraction which may deserve 

 special attention has recently been isolated by Sibatani et al. (1960) from 

 animal tissues. It seems to be made at the same time as DNA and to be 

 metabolically as inert as DNA. 



None of these observations can be taken as definite evidence that specific 



