100 CONTROL MECHANISMS IN CELLULAR PROCESSES 



of all ages incorporate precursors into DNA and RNA (McFall and 

 Stent, 1959; Abbo and Pardee, 1960). In yeast, the RNA content 

 increased steadily, irrespective of the state of DNA reproduction 

 (Mitchison and Walker, 1959). We can still assume that there is 

 no RNA formation during DNA replication, but in some cases, only 

 part of the DNA rephcates at a time, so that the rest of it can still 

 be active in RNA production. During mitotic division, however, 

 DNA is condensed in chromosomes and the RNA synthesis is effec- 

 tively prevented. It would be desirable to follow this interdepend- 

 ence of DNA and RNA synthesis in cells of a greater variety of 

 organisms. 



When it comes to the chemistry of DNA action, only models exist. 

 If the Watson-Crick model is accepted, it is not easy to see how it 

 could be used as a template (Wilkins, 1956). RNA synthesis may 

 require a free, single-stranded DNA molecule. This possibility was 

 recently discussed by Rich (1959, 1961), who was able to show that 

 a single strand DNA could combine with a single strand RNA in 

 a double helix. Helixes could be formed also with three strands, 

 or with RNA strands only. It is then conceivable that RNA uses 

 DNA as a template and is replicated in a manner similar to the repli- 

 cation of DNA, as proposed by Watson and Crick ( 1953 ) . 



In this case the base composition of RNA should be complemen- 

 tary to the base composition of DNA. This does not seem to be so, 

 since it was found that the base composition of RNA does not vary 

 much in different bacteria, while the DNA composition varies widely 

 (Belozersky and Spirin, 1958). Yet, Belozersky could observe a 

 slight correlation between the base compositions of both nucleic 

 acids and assumed that only a small proportion of bacterial RNA has 

 a base composition corresponding to DNA, while the major part is 

 composed of approximatelv equal amounts of each base. Volkin and 

 Astrachan (1957) could show that RNA newly synthesized after a 

 virus infection has a base composition more closely resembling that 

 of DNA. Yeas and Vincent ( 1960 ) found that the more active frac- 

 tion of newly formed RNA in yeast has a base composition similar to 

 yeast DNA. 



These observations are intriguing: on the one hand, they give 

 strong support to the liypothesis that DNA serves as a template for 

 the synthesis of RNA, on the other hand, they indicate that the cell 

 contains a large proportion of RNA which did not originate on DNA. 

 Is it possible that this non-DNA-dependent RNA is the RNA which 



