132 ROLE OF THE CELL NUCLEUS 



Step I (DNA makes RNA) is a logical one; but it must be admitted 

 that there is no experimental proof so far for its existence. A ques- 

 tion mark should thus remain for the time being. 



With regard to steps 2 and 3, the recent experiments of Allfrey 

 and Mirsky (1957) suggest, as already mentioned, that the role 

 played by DNA in the synthesis of chromosomal proteins is an in- 

 direct rather than a direct one, and that the synthesis of these pro- 

 teins is preceded by a synthesis of nuclear RNA. We should there- 

 fore consider step 3 as well established and step 2 as doubtful. 



There is little information about the possible intervention of 

 RNA in the synthesis of nucleolar proteins (step 4). The indirect 

 evidence that we have to hand (cytochemical and autoradiography 

 observations) is in favour of such a view. Experiments on the 

 localized U.V. irradiation of the nucleolus, followed by autoradio- 

 graphic studies of the incorporation of amino acids into nucleolar 

 proteins, might give a more direct answer. Such studies are cur- 

 rently being made in the author's laboratory by Drs. Errera and 

 Perry (1959). The first results indicate that destruction of nucleolar 

 RNA exerts inhibitory effects on cytoplasmic RNA synthesis, but 

 the effects on cytoplasmic protein synthesis are not yet known. 



The last steps, 5, 6, 7, of the scheme have formed the main subject 

 of this book and need not be discussed further. We have seen that 

 part of the cytoplasmic RNA is of nuclear origin (step 5), but that 

 autonomous synthesis of proteins and chloroplastic RNA (step 7) 

 has been demonstrated in Acetabularia. Step 6 (the relationship be- 

 tween RNA and protein synthesis) was discussed in Chapter 1 and 

 there is no need to repeat what was said there. 



The scheme which we have presented remains obscure in many 

 respects and should not be regarded in a dogmatic fashion; com- 

 plicated as it looks, it is probably a considerable over-simplification 

 of the reality. But it will serve a useful purpose if it can be used to 

 test experimentally the various hypothetical steps, for one of the 

 great mysteries of life will be solved when we understand the chem- 

 ical relationships existing between the gene and the specific pro- 

 tein which is synthesized under its control. 



