VI FINAL REMARKS 297 



Prescott's (1955) experiments: the activity of the nucleated halves is only twice 

 that of the non-nucleated halves when the amoebae have been cut from 1-6 days. 

 It can be concluded that, even in amoebae, protein metabolism is not under a 

 direct control of the nucleus. 



Amoebae are different from the other cells studied in that the non-nucleated 

 fragments are unable to feed; perhaps as a consequence of this starvation, the RNA 

 content of the cytoplasm drops rapidly after removal of the nucleus (Brachet, 

 1955a; James, 1954). 



To summarize : in all the organisms and cells studied so far, the nucleus only exerts a 

 remote control on protein metabolism. RNA and protein, as should be expected, always 

 run parallel: the extreme cases are A. proteus, where RNA content and protein 

 metabolism quickly decrease, and Acetabularia, where net synthesis of RNA and 

 proteins occurs in the absence of the nucleus. 



The nucleus can thus no longer be considered as the only center of protein 

 metabolism; but this does not mean that protein anabolism is negligible in the 

 nucleus. On the contrary, the autoradiography observations of Ficq (1955), Ficq 

 and Errera (1955a), Moyson (1955) and Sirlin (1955) clearly show that in grow- 

 ing oocytes (starfish, frog), in developing embryos and in mammalian Hver, 

 incorporation of amino acids is faster in the nucleus than in the cytoplasm. We 

 do not know yet how general these findings are : but there is no doubt that in rapidly 

 dividing cells and in protein-synthesizing cells, protein anabolism in the nucleus 

 itself is far from negligible. 



Among the constituents of the nucleus, it is the nucleolus which shows the highest 

 activity: in young starfish oocytes, according to Ficq (1955), incorporation of 

 amino acids into the nucleolar proteins can be 100 times greater than in any other 

 part of the cell. In the nucleolus, as elsewhere, RNA is intimately concerned with 

 protein anabolism: if the living oocytes are treated with ribonuclease, inhibition 

 of amino-acid incorporation is strongest in the nucleolus (Ficq and Errera, 1955b). 



This very high activity of the nucleolus in protein metabolism probably ex- 

 plains why removal of the nucleus in Acetabularia accelerates protein and RNA 

 synthesis: it is likely that the very large and basophilic nucleolus, in this alga, 

 efficiently competes with the cytoplasm for the uptake of the precursors required 

 for the synthesis of ribonucleoproteins. 



A consequence of this important role of the nucleolus in protein synthesis is that 

 Mirsky's (1956) suggestion that protein synthesis depends on DNA in the nucleus 

 possibly is an oversimplification: Mirsky's (1956) statement might well be true 

 for the thymus nuclei he studied, because their nucleoli are inconspicuous. It is very 

 unlikely that in protein-synthesizing cells where, as emphasized by Caspersson 

 (1940, 1950), the nucleoli are always very large, RNA is not also of great impor- 

 tance in protein synthesis. It seems more probable that, if DNA is directlv con- 

 cerned with protein synthesis, its role is limited to the production of the proteins 

 which are closely associated to chromatin and the chromosomes. 



(c) Nucleic acids and regulation of growth 



It should now be clear that nucleic acids are deeply and directly involved in 

 growth processes: additional evidence, if necessary, can be found in various reports 



Literature p. 2gg 



