86 THE BIOSYNTHESIS OF PROTEINS 



region faster than in nucleoli and much faster than in cytoplasm (Gold- 

 stein and Micou, 1959). 



A nuclear element in which RNA accumulates in considerable amount 

 is the nucleolus. Nucleoli are often observed to stay in close contact with 

 certain regions of the chromosomes and there are many indications that the 

 nucleoli are in some way produced by regions of the chromosome, often 

 called nucleolar organizers (for a discussion see Brachet, 1957). During 

 oogenesis in amphibians, the lampbrush chromosomes to which we have 

 referred before are especially developed at the time when synthesis of yolk 

 protein is most active, and nucleoli are also very numerous at this stage. 

 The nucleoli might be regarded as a transitory storage place for RNA 

 made in the chromosomes. It must be noted, however, that the nucleolar 

 organizers are heterochromatic regions of the chromosomes, i.e. regions 

 characterized by special staining properties, and which are believed to 

 contain genetic controlling element involved in the quantitative expression 

 of characters, whereas the genes of the sort we have considered so far in 

 relation with protein structure are located in the euchromatin. 



The histochemical observations on the whole support the idea that some 

 RNA may be built up within the chromosome or in close contact with 

 the genetic material. They are compatible with the hypothesis that RNA 

 receives genetic information from DNA, although they cannot be taken as 

 evidence for it. 



The next task that the genetic messenger should accomplish is getting 

 out of the nucleus and reaching the centres of protein synthesis in the cyto- 

 plasm. What is the evidence for the transfer of RNA from nucleus to 

 cytoplasm? 



Precursors of RNA are usually incorporated more rapidly into nuclear 

 RNA than into the average cytoplasmic RNA (Jeener and Szafarz, 1950; 

 Smellie et ah, 1953, 1955; Barnum et al., 1953). From the kinetics of the 

 incorporation, it was concluded that the bulk of RNA in liver cytoplasm 

 does not derive from the nucleus, but that the possibility exists that some 

 cytoplasmic RNA might be produced in the nucleus (Smellie et al, 1953; 

 Barnum et «/., 1953). 



This conclusion rests on several assumptions which are very difficult to 

 check and which now appear as oversimplifications, since it is known how 

 heterogeneous both nuclear and cytoplasmic RNA are. 



Enucleation experiments were designed to establish whether RNA can 

 be formed at all in the absence of the nucleus. In Acetahularia, or in sea 

 urchin eggs, enucleation has no effect on incorporation of labelled orotic 

 acid or glycine into RNA (Malkin, 1954; Brachet and Szafarz, 1956; 

 Brachet et ah, 1955). Precursors are also incorporated into the RNA of 

 rabbit reticulocytes (Kruh and Borsook, 1955). Incorporation of precursors 

 into total cytoplasmic RNA cannot, however, be equated to RNA synthesis, 



