RNA AND CONTROL OF CELLULAR PROCESSES 89 



infective and produced progeny with the protein characteristics of 

 the RNA donor, not of tlie protein donor. 



All these facts totietlier, some of which will be discussed more 

 extensively, contributed to the growing acceptance of the hypothesis 

 DNA -^ RNA -^ protein. I want to present in this paper some facts 

 in favor of this hypothesis and speculations about how this hypoth- 

 esis could explain the processes controlling cell function, 



RNA as Gene Product 



A Nuclear Origin of RNA. Modern research in biochemical 

 genetics showed first that specific biochemical reactions are under 

 genetic control, and later, tliat genes must control the production 

 of enzymes responsible for these reactions. Since the genes are lo- 

 cated in chromosomes, inside the nucleus, they cannot directly form 

 proteins, which are made in the cytoplasm. The information con- 

 tained in the genes must be carried to the cytoplasm by a "messen- 

 ger. " The substance which serves as a "messenger" must conform to 

 certain a priori conditions. It has to be synthesized in the nucleus, 

 where the genes are located, and it has to be transferred into the 

 cvtoplasm. There should be no other place in the cell where its 

 synthesis is possible. In the cytoplasm the substance should direct 

 protein synthesis. It appears now that RNA is most likely to be the 

 substance. 



Recentlv, evidence has accumulated for the nuclear origin of 

 RNA. It has been possible, by the use of radioactive tracers, to 

 detect the site of formation of new RNA and to follow the fate of 

 RNA in the cell. Studies of incorporation of P'- into different cell 

 fractions showed that the nucleus is particularly active in the incor- 

 poration of phosphorus into RNA ( Marshak and Calvet, 1949; Jeener 

 and Szarfarz, 1950; Smellie, 1955). Many such experiments have 

 indicated that the nuclei may be the main sites of RNA synthesis, 

 although opposite opinions have also been expressed (Barnum et al., 

 1953 ) . Clearer proof of nuclear origin of RNA has been obtained 

 since by the use of autoradiography. 



Goldstein and Plant (1955) fed amoebas P''- which was incor- 

 porated into the nucleic acids. Extraction of the fixed material with 

 the appropriate solvents and treatment with crvstalline ribonuclease 

 allowed the study of the incorporation into RNA, separatelv from 

 the incorporation into DNA and other phosphorylated compounds^ 



