130 ROLE OF THE CELL NUCLEUS 



But the main role of the cell nucleus is different. It is the main, 

 if not the exclusive, site of nucleotide synthesis and not only of 

 simple mono- and dinucleotides such as DPN, but also of the much 

 more complex and important polynucleotides, DNA and RNA. 

 There is growing evidence in the case of DNA in favour of the 

 structure and mechanism of replication proposed by Watson and 

 Crick (1953): DNA would be made of two complementary helices, 

 which would separate from each other and reform their counterpart 

 when DNA is synthesized. Thus DNA synthesis would be a func- 

 tion of DNA itself or, at any rate, of the chromosome. The mecha- 

 nism of RNA synthesis remains more obscure but, as we have seen 

 repeatedly in this chapter, synthesis of RNA is always much more 

 active in the nucleus than in the cytoplasm and there is considerable 

 evidence for the view that part at least of cytoplasmic RNA is of 

 nuclear origin. It is very gratifying that the very different methods 

 of investigation described in this chapter (autoradiography, work 

 on enucleate organisms and work on homogenates) have yielded 

 similar results and led to the same general conclusion : the nucleus 

 is the more important site of RNA synthesis, 



A logical consequence of such a conclusion is that the nucleus 

 must exert a stronger control on cytoplasmic RNA synthesis (or 

 maintenance) than on cytoplasmic protein synthesis. We have 

 pointed out earlier that the nucleus exerts only a remote, indirect 

 control on protein synthesis. This is exactly what would be expected 

 if the nucleus produced an intermediary substance involved in pro- 

 tein synthesis; there is little doubt that this intermediary is RNA, 

 from all the facts that have been presented and discussed in this 

 book. 



The results obtained from experiments on unicellular organisms 

 are in good agreement with views which have been expressed re- 

 peatedly since Caspersson (1941, 1950) presented them first. DNA, 

 which is the primary genetic substance, would synthesize RNA; 

 proteins would, in turn, be synthesized under the influence of RNA. 

 The template hypothesis provides an easy explanation for specific- 

 ity. Specific DNA molecules (or parts of molecules) corresponding 

 to each gene would act as a template for RNA; there would thus 



