STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 



99 



been proved to occur in the absence of RNA or witliin cells in the absence of 

 RNA synthesis. On the other hand, the phenomenon of protein synthesis in 

 enucleate cells and in ceU-free preparations devoid of nuclei indicates 

 unequivocally that protem synthesis in the cytoplasm is not tied to the 

 presence of DNA or its synthesis. 



3. RNA Synthesis 



a. Incorporation Data. RNA is found among all the subcellular fractions 

 which have been so far discussed. Using the colorless flagellate, Polytomella 

 coeca, grown successfully in P^^ and P^^, Jeener (1952a,b) isolated five 

 different fractions of sedimentable particles containing RNA and analyzed 

 their relative rates of RNA synthesis. He observed that the rates of RNA 

 synthesis in a given fraction were proportional to the amounts of RNA 

 present. He concluded, in addition, that the smallest particles did not behave 

 as precursors to the larger particles. 



TABLE XI 

 Base Compositions of RNA's of Rat Liver"- ^ 



Adenine Guanine Cytosine Uracil 



purine A -f- U 



pyrimidine G + C 



"ElsoneiaZ. (1955). 



^ Moles per 100 moles of nucleotide. 



Although most of the RNA is in the cytoplasm, a significant amount is 

 found in the nucleus. As in the case of DNA, the base composition of the 

 cytoplasmic RNA is the same in all cytoplasmic fractions in various tissues 

 of the same animal (Elson et al., 1955); indeed, the base compositions of the 

 RNA of the various cytoplasmic fractions remain the same whether one is 

 dealing with resting or regenerating rat fiver (Cox, 1957). However, these 

 and other workers are agreed that the base composition of nuclear RNA is 

 substantially different from that of cytoplasmic RNA (Crosbie et al., 1953), 

 as indicated in Table XI. It may be supposed that the nuclear and cyto- 

 plasmic fractions have independent origins, or that RNA is made first in one 

 site and then disassembled for reassembly at another site. 



