ROLE OF THE CELL NUCLEUS 129 



4. CONCLUSIONS 



There is no doubt that protein synthesis, including synthesis of 

 specific proteins such as hemoglobin or enzymes, is possible in the 

 absence of the nucleus and, therefore, in the absence of DN A. One 

 of the limiting factors in protein synthesis by enucleate cell frag- 

 ments is the energy production. This explains why Acetabularia, 

 which contains chloroplasts and is capable of perfect photosyn- 

 thesis in the absence of the nucleus, is so much superior to Amoeba 

 proteus in this respect. Enucleate fragments of amoebae are unable 

 to feed and it is no wonder that no net protein synthesis can occur 

 in them. Reticulocytes and enucleate egg fragments occupy an inter- 

 mediary position between these two extremes. They are capable of 

 appreciable protein synthesis or turnover but the reticulocytes are 

 living in a nutrient medium and the egg fragments are supplied with 

 large reserves of yolk. From this viewpoint, the variety of results 

 obtained in enucleation experiments becomes logical and we arrive 

 at the conclusion that the differences between enucleate fragments 

 of Amoeba or Acetabularia are of a quantitative, rather than a 

 qualitative, nature. 



With regard to the relationships existing between the nucleus and 

 the cytoplasm in the mechanisms of energy production, it is pos- 

 sible that the nucleus might produce co-enzymes required for 

 glycolytic and oxidative reactions. In fact, there is some evidence 

 for the view that diphosphopyridine nucleotide (DPN), as well as 

 several other nucleotides, are synthesized in the nucleus (Hogeboom 

 and Schneider, 1952; Baltus, 1955). On the other hand, we know 

 that the main sites of energy production are located in the cyto- 

 plasm (mitochondria, chloroplasts) and we have seen that suppres- 

 sion of these cytoplasmic mechanisms of energy production have 

 far-reaching consequences on the morphology and RNA content 

 of the nucleolus in Acetabularia. Similar observations can be made 

 on other cells, for example, on amphibian eggs treated with dini- 

 trophenol. In conclusion, the nucleus depends on the cytoplasm for 

 ATP production, but it might be a source of coenzymes for the 

 cytoplasm. 



References p. 133/135 



