80 THE BIOSYNTHESIS OF PROTEINS 



considerations. The intermediate depositary of genetic information must 

 be able to receive the information from DNA, to record it in some way and 

 to convey it to the sites of protein synthesis. As we shall see, RNA seems 

 to fulfil these a priori requirements. 



(a) Do RNA molecules carry genetic information? Virus RNA can indeed 

 transfer to a living cell the specific information required for making at least 

 the protein of the virus (see Chapter I). It is therefore quite reasonable to 

 suspect that some fraction of cellular RNA might play a similar role, with- 

 out prejudice as to what fraction this may be. 



There is every reason to believe that the genetic information contained 

 in DNA or in virus RNA consists in a specific arrangement of the purines 

 and pyrimidines on the phosphate-carbohydrate backbone. The two 

 families of nucleic acids are so closely related, that they must speak closely 

 similar languages. The translation of DNA information into RNA language 

 must be a rather simple matter. The easiest way to visualize how RNA 

 could receive information from DNA and store it in its own structure, is to 

 assume that specific RNA is made under direct control of DNA. Interesting 

 attempts have been made at formulating in terms of structural chemistry 

 possible template processes by which a unique sequence of bases might be 

 imposed upon a nascent RNA chain which would take shape in close 

 contact with DNA. Thus Lockingen and De Busk (1956) developed this 

 idea in rather general terms, and suggested that the strands of the DNA 

 double helix could separate and a RNA chain would organize under the 

 influence of the bases of single DNA strands. Although the DNA structure 

 which was then assumed by the authors is probably outdated by now, the 

 same idea might soon be considered again in a revised form since single 

 strand DNA have been shown to exist (Sinsheimer, 1959). 



Stent (1958) and Zubay (1958) developed models in which an RNA chain 

 would be organized in the deep grove of a DNA double helix. The se- 

 quence in RNA is supposed to be imposed by that of the pairs of bases in 

 DNA (Fig. 23). 



Experimental results obtained by Rich and associates with synthetic 

 polynucleotides make processes of this type rather plausible. When equiva- 

 lent amounts of polyadenylic and polyuridylic acids are mixed together in 

 dilute salt solution, a double helix forms in which each adenine in the 

 polyadenylic is hydrogen bonded to a uracil in the polyuridylic chain (Rich, 

 1957; Felsenfeld, 1958; Steiner and Beers, 1959). The structure of this 

 double helix is very similar to that of DNA. If enough magnesium ions 

 or polyamines are added, the double helix is able specifically to bind 

 another polyuridylic acid chain. A three-stranded molecule is thus formed, 

 one component of which is loosely bound to a very stable double helix 

 (Felsenfeld and Rich, 1957; Zubay, 1958; Rich, 1959a, 1959b). 



