VI FINAL REMARKS 295 



al. (1955) came to similar conclusions in the case of sulfur and nitrogen mustards. 

 The evidence which has just been reviewed shows conclusively ihdit protein synthesis 

 is possible in the absence of DMA synthesis; RNA synthesis is also independent of DNA 

 synthesis. But RNA and protein synthesis, on the other hand, are very closely 

 linked together. 



We now come to the second question : does DNA play a direct role in protein 

 synthesis? As compared to the wealth of information we have regarding the role 

 of RNA in protein synthesis (see section iv of this Chapter, p. 272), the present 

 evidence is still meagre. 



Definite indications in favour of a direct role of DNA in protein synthesis come 

 from the work of Gale and Folkes (1954, 1955a); they found that either RNA or 

 DNA are able to restore the ability to incorporate amino acids into the proteins of 

 disrupted Staphylococci whose nucleic acids had previously been removed. As a 

 matter of fact, DNA is even more effective than RNA in promoting amino-acid 

 incorporation into the proteins of the nucleic acid-depleted cells, regarding both the 

 rate and the final amount of incorporation attained. It is for this reason that Gale 

 and Folkes (1954) believe that DNA, while not capable of synthesizing proteins 

 itself, might bring about exchange reactions in the proteins; DNA would act as 

 an organizer for RNA synthesis, which in turn would catalyse protein synthesis. 



A different type of experiment, by Allfrey (1954) and by Mirsky (1956), leads 

 to similar results and conclusions : thymus nuclei isolated in sucrose are able to 

 incorporate labelled amino acids into their proteins, the process requiring the 

 presence of oxygen. The important fact is that the incorporation reaction is de- 

 pendent on the presence of DNA, since breakdown of DNA by the enzyme 

 deoxyribonuclease (DNase) strongly inhibits the amino acid uptake. Addition 

 of thymus DNA to the DNase-treated nuclei restores the activity to some extent. 

 These interesting observations lead Mirsky (1956) to the conclusion that protein 

 synthesis in the cytoplasm depends upon the presence of RNA, while it depends on 

 DNA in the nucleus. 



The validity of this conclusion will be discussed after reviewing what is known 

 about the role of the nucleus in growth and protein synthesis. 



(b) The role of the cell nucleus in growth and protein synthesis 



We shall chiefly deal with experiments performed on nucleated and non-nu- 

 cleated fragments of unicellular organisms. The most interesting case is perhaps 

 that of Acetabularia meditertanea, because of its unusual ability to regenerate in the 

 absence of the nucleus (Hammerling, 1934). Hammerling's (1934) experiments 

 clearly show that non-nucleated fragments of this giant unicelhdar alga are capa- 

 ble of prolonged survival and considerable regeneration: there is no question that, 

 in this rather exceptional case, growth and even morphogenesis are possible for 

 quite a long time in the absence of the nucleus. According to our own observations 

 (Brachet, Ghantrenne and Vanderhaeghe, 1955), it takes 3 weeks at least before 

 the regenerative power disappears from the non-nucleated pieces. 



Extensive biochemical observations performed in this laboratory have led to 

 the following main conclusions (Brachet, Ghantrenne and Vanderhaeghe, 1955) : 

 non-nucleated pieces of Acetabularia are capable of incorporating labelled precur- 



lAterature p. 2gr) 



