96 CONTROL MECHANISMS IN CELLULAR PROCESSES 



Similarly, the laiiipbrush chromosomes of Tritiirus ovocytes were 

 found to incorporate C" adenine into RNA (Brachet and Ficq, 

 1956; Gall, 1958) sooner than did nucleoli. In electron microscope 

 pictures and in cytochemical preparations, it has even been pos- 

 sible to observe the organization of RNA rich nucleolar material on 

 the chromosomes and their final movement into the nucleoli (Gall, 

 1958 ) . Guyenot and Danon ( 1952 ) have indicated previously that 

 lampbrush chromosomes express a highly active state of genes. 



In tissue cultures, Goldstein and Micou (1959b) could also follow 

 a transfer of RNA from chromatin to nucleoli. Similarly, Feinende- 

 gen et al. (1960) found in HeLa cells that the nucleoplasm becomes 

 labeled earlier than the nucleoli. On the other hand, Harris (1959) 

 maintains that the RNA turnover is equal in the nucleolus and the 

 rest of the nucleus. Further experiments with reliable quantitative 

 methods will be needed to establish beyond criticism the facts indi- 

 cated by autoradiographic methods. 



It may be premature, then, to formulate a hypothesis of RNA 

 production, but it is difficult to resist the temptation. An inviting 

 hypothesis was formulated by Bonner ( 1959 ) : RNA is formed in the 

 cliromosomes, transferred to the nucleolus where the protein part 

 of the nucleoprotein is made. The finished nucleoprotein particle, 

 a ribosome, is transferred to the cytoplasm from the nucleolus. The 

 known facts do not contradict this hypothesis; nevertheless, it cannot 

 be fully accepted until more is known about the movements of RNA 

 in the cell. 



Chemistry of RNA. In the preceding discussion, RNA was con- 

 sidered as a class of substances, all with similar function and similar 

 origin. This was an oversimplification, since various types of RNA 

 could be distinguished by chemical analysis or by differing rates of 

 incorporation of radioactive precursors in the cell (Siekevitz and 

 Palade, 1959). Some of these types may correspond to differences 

 which must exist among gene-produced RNA molecules, each built 

 to carry the information of the corresponding gene. 



Nuclear and cytoplasmic RNA differ in their base composition 

 (Vincent, 1955; Elson et al, 1955). This has been used by many 

 as an argument against the nuclear origin of RNA. The argument 

 is not valid, since it could be quite possible that there is one RNA, 

 which is a constituent of the nucleus, and that the cytoplasmic RNA, 

 also synthesized in the nucleus, is continuously removed to the cyto- 



