ROLE OF NUCLEIC ACIDS 89 



Perry and Errera (1960) showed that in Hela cells irradiation of the 

 nucleolus with a microbeam of ultraviolet light (257 mfx) inhibits the 

 incorporation of tritiated cytidine into both cytoplasmic and nuclear 

 (extranucleolar) RNA. Kinetic studies are interpreted as indicating that 

 RNA synthesis takes place independently both in the nucleoli and in the 

 nucleus (outside the nucleoli), and that both fractions pass into the cyto- 

 plasm. According to the same studies, less than 10 per cent of the cyto- 

 plasmic RNA can be formed in the cytoplasm of Hela cells (Perry, 1960). 

 Working with a similar material Feinendegen et al. (1960) came to the con- 

 clusion that RNA synthesis begins in the chromatin portion of the nucleus, 

 and that RNA later passes into the cytoplasm. Amano and Leblond (1960) 

 also concluded that, in liver, nucleolar and other nuclear RNAs are made 

 independently and that they behave as precursors of cytoplasmic RNA. 



Further support to the hypothesis of a nuclear origin of cytoplasmic 

 RNA will be found in studies on the labelling of RNA of Drosophila larvae 

 by radioactive phosphate (Herbert, 1954) or tritiated uridine (Zalokar, 

 1960). 



Clear evidence in the same direction was obtained by Zalokar (1959, 

 1960b) in experiments with hyphae of Neurospora crassa the content of 

 which had been stratified by high speed centrifugation. In short exposures 

 to tritiated uridine followed by dilution of the isotopic precursors with 

 non-labelled uridine, the nuclear RNA is labelled first, tritium-marked 

 compounds later appear in the cytoplasm and the labelling of ergastoplasm 

 increases with time at the expense of nuclear label. Again there remains 

 the possibility that the labelled molecules which pass from the nucleus to 

 the cytoplasm are not intact RNA molecules, but products from which 

 cytoplasmic RNA is built up. But the possibility that RNA does pass is very 

 good. 



The works reviewed above dealt with several types of animal and plant 

 cells and made use of various experimental approaches. Their results are 

 not always in entire agreement with one another; however, a general con- 

 clusion can be safely drawn, namely that the nucleus is a very important 

 centre of ribosenucleic acid synthesis ; moreover, it is likely that a large part 

 of the cytoplasmic RNA is made within the nucleus in the vicinity of 

 chromatin. Clearly, these are very good reasons to retain the hypothesis that 

 the genetic messenger which carries genetic information from DNA to the 

 cytoplasmic centres of protein formation is probably made of RNA. 



(c) Where is the genetic messenger? It should not be forgotten, however, 

 that the very existence of a messenger carrying the genetic information has 

 not been established by direct experiments, it is postulated only because 

 most proteins arise at a distance from DNA or can be made in the absence 

 of DNA. The experiments which come closest to establish directly the exist- 

 ence of genetic information outside DNA, and to provide some indications 



