316 



CELL HEREDITY 



100 



80 



60 



40-, 



20 



hr 



1 hr 



2 hr 



3 hr 



4 hr 



FIGURE 11.5. Incorporation and transfer of RNA in cells of the Ctiinese hiamster in 

 tissue culture. Cells were exposed to cytidine-H for 5 to 10 minutes, tfien trans- 

 ferred to fresh medium and fixed at the times shown on the graph. Curve A records 

 grain counts of radioautographs over the nucleus and curve 6, over the cytoplasm 

 in the same cells. Initial uptake was entirely into the nucleus, and later labeling of 

 cytoplasmic RNA could be ascribed entirely to transfer from the nucleus. (The high 

 final values in the cytoplasm have been attributed to increased efficiency of counting 

 grains in the thin cytoplasm over that of the compact nucleus.) Since the intracellular 

 concentration of RNA may b>e assumed to remain constant during the 4-hour period, the 

 plotted points are comparable to relative specific activities (after Taylor, 1960, i. 

 Biophys. & Biochem. Cytol., 7:455). 



in the rate of RNA synthesis during the prolonged (six hours out of a 14- 

 hour doubhng time) period of DNA synthesis. This observation provides 

 no direct information on the question whether DNA and RNA molecules 

 may be replicated simultaneously from the same template, because DNA 

 synthesis occurs in different parts of the chromosome complement at 

 different times during the six-hour replication period. 



These experiments, as well as others of a similar nature, have shown 

 that RNA synthesis occurs primarily, if not exclusively, in the nucleus 

 in a variety of cell types. It is possible that these findings apply only 

 to the particle-bound RNA of high molecular weight, since it has not 

 been fullv demonstrated whether the s-RNA or other fractions of low 

 molecular weight are retained by the fixation techniques emploved. 



Having focused our attention upon the nucleus as the site of RNA 

 synthesis, we may ask where in the nucleus does this process occur? Is it 

 associated with the chromosomes, the nucleolus, or bothr* Attempts by 



