STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 101 



as shown independently in isolated nuclei. It is, of course, evident that these 

 data are consistent with the concept of the nucleolus, as a particularly active 

 center in the synthesis of RNA and protein, as described in Fig. 12. The close 

 association of heterochromatin with the nucleolus and the relation of the 

 latter to RNA metaboHsm (Ficq, 1956) have also led to the studies of quan- 

 titative relations of heterochromatin to RNA metabolism. Although addi- 

 tional heterochromatin does not increase the RNA content of Drosophila 

 eggs, to which a Y chromosome was added, it is reported to alter the base 

 composition of the RNA that is made (Schultz, 1956). 



It must now be asked if this nuclear RNA or component nucleotides are 

 now made available for the synthesis of cytoplasmic RNA, a result consistent 

 with the kinetics presented in Fig. 15. 



b. Enucleate Cells. Brachet and Szafarz (1953) studied the incorporation 

 of orotic acid-C^^ into fragments of Acetabularia. It was found that both 

 nucleate and enucleate fragments of the alga were able to make radioactive 

 nucleic acid. 



Plaut and Rustad (1956) have examined the uptake of adenine-C^^ into 

 fragments of Amoeba jiroteus. In this organism, the enucleate fragments 

 also take up adenine, but at a rate of about 50 % that of the nucleate 

 half cell. Acid fractionation of the fragments was not undertaken to dis- 

 tinguish nucleotide and RNA-adenine. However, as shown most recently by 

 Prescott (1957), the uptake of uracil-C^* in enucleate fragments levels off 

 after 72 hours and apparently none of the radioactivity contained in the 

 fragments is bound in acid-insoluble RNA. 



c. Nuclear Transfer. A direct demonstration of the transfer of nuclear RNA 

 to cytoplasm has been made by Goldstein and Plaut (1955). These workers 

 labeled amoebae by feeding an organism labeled with P^^. After 2 to 3 days 

 of feeding, essentially all nuclear P^^ was present in the nuclear RNA and 

 was not in DNA. These P^^-labeled nuclei were transferred by micromani- 

 pulation to unlabeled enucleated amoebae or to normal amoebae. The isotope 

 remained in the transplanted nucleus for 5 hours, but appreciable radio- 

 activity appeared in the cytoplasm after 12 hours and continued to increase 

 in this fraction with time. All of the radioactivity in nucleus and cytoplasm 

 was removable by treatment w^th ribonuclease, suggesting that the radio- 

 active material leaving the nucleus was not morganic P^^ and was at least at 

 the ribonucleotide level of organization. In experiments of transfer to normal 

 cells, label derived from one nucleus did not pass significantly to the second 

 unlabeled normal nucleus. Thus, the transfer may be effected from nucleus 

 to cytoplasm but not in reverse. The nature of the P^^-containing fragments 

 involved in this transfer are as yet unknown. 



d. Dissociation of RNA Synthesis from Protein Synthesis. In all mstances 

 discussed until now, RNA synthesis has been associated with a concomitant 



