ROLE OF NUCLEIC ACIDS 63 



the transfer occurs. Lack of experimental evidence has encouraged specu- 

 lation. It seems obvious that a substance able to carry the information 

 required for controlling the primary structure of a protein must be a rather 

 large molecule, most probably with a linear structure. There are good 

 reasons to suspect that the information carrier might be made of ribose- 

 nucleic acid. 



B. RIBOSENUCLEIC ACIDS AND PROTEIN SYNTHESIS 



The first pieces of evidence for a participation of RNA in protein syn- 

 thesis are found in the works of Brachet and of Caspersson on the dis- 

 tribution of RNA in various animal tissues. A striking correlation was 

 shown to exist between the amount of RNA and the intensity of protein 

 synthesis (cf. p. 41). The fact that newly-formed proteins are first detected 

 in ribosomes (cf. p. 43) is further evidence for the participation of RNA in 

 protein synthesis. Before considering what is known or suspected at present 

 about the function of ribosenucleic acids in protein synthesis, it is essential 

 to be aware of their extreme diversity. 



1. Plurality and Metabolic Heterogeneity of Cellular RNAs 



It is known from cytological studies that RNA is present in several cell 

 constituents : ergastoplasm, nucleoli, chromatin, mitochondria and certain 

 membranes (Brachet, 1940, 1942, 1957; Caspersson and Schulz, 1939; 

 Caspersson, 1941, 1950). During mitosis, RNA is also found in chromo- 

 somes and in the spindle (Brachet, 1942, 1957). 



Autoradiographic studies show in the most striking manner that differ- 

 ences in metabolism exist between these differently located RNAs. Thus in 

 the salivary glands of Drosophila (Herbert, 1954; McMaster-Kaye and 

 Taylor, 1958) radioactive phosphate is incorporated more rapidly into 

 the RNA of the nuclei and of the chromosomes than into cytoplasmic 

 RNA. Tritiated cytidine is found in nucleolus RNA earlier and in greater 

 amount than in any other cell constituent (Ficq, 1959; Perry, 1960). The 

 loops of the lampbrush chromosomes of amphibian oocytes are the site of 

 a rapid incorporation of adenine into RNA (Ficq et al, 1959; Sirlin, 1960 a, 

 b). in the giant chromosomes of diptera, labelled cytidine is incorporated 

 into RNA at well-defined positions along the chromosomes, at certain times 

 of the life cycle of the larvae (Rudkin and Woods, 1959). Differences in 

 RNA metabolism are thus found between different cell regions, even 

 within one subcellular structure. 



Biochemical studies on isolated cell constituents had also revealed the 

 metabolic heterogeneity of cellular RNA. Radioactive phosphate was 

 injected into an animal, and the liver was isolated and homogenized. The 



