GENETIC CONTROL OF CELL INTEGRATION 315 



mitochondria, nuclei, and cytoplasmic cell sap are sharply delimited from 

 one another. The method lends itself to studies of incorporation pat- 

 terns after treatments as short as one minute. By feeding mycelia with 

 tritiated uridine for various times, followed by washing and further 

 growth in an excess of nonradioactive uridine, it was shown that, during 

 the first four minutes, radioactivity appears only in the nuclear fraction, 

 subsequently appearing in the cytoplasmic fractions. When the tritiated 

 compound was fed for only one minute, followed by an excess of un- 

 labeled uridine, the radioactivity appeared first in the nuclear fraction 

 and, after several minutes, moved to the cytoplasm. Within the limits of 

 sensitivity of this system, it appears that all the RNA is formed within 

 the nuclei and is subsequently transported to the cytoplasm. The 

 method is ingenious, providing definitive information about the major 

 constituents, but cannot be employed to study constituents present in 

 low amount. 



The uptake of radioactive precursors into RNA has also been followed 

 by radioautography of a number of different types of cells. An example 

 of the results are those obtained by Taylor with rapidly growing cells of 

 the Chinese hamster in tissue culture. Using tritiated cytidine as the 

 source of the radioactive label, it was seen that, after exposure of cells 

 for 5 to 10 minutes, only the nuclear RNA contained detectable label. If 

 these cells were washed after their short exposure to cytidine-H' , and 

 kept for varying lengths of time in fresh medium with an excess of un- 

 labeled cytidine and uridine, the radioactivity could be followed as it 

 moved from nucleus to cytoplasm. There was no increase in radio- 

 activity in the nucleus after washing out the labeled precursor, indicating 

 the absence of a precursor pool in these cells. Consequently, it was 

 possible to ascribe all changes occurring after washing to movement of 

 labeled material within the cell. As can be seen in Figure 11.5, the 

 disappearance of radioactivity from the nucleus coincided with its ap- 

 pearance in the cytoplasm. 



The increase in total radioactivity in the cytoplasm above that initially 

 present in the nucleus has been interpreted by Taylor as the result of 

 more efficient detection of disintegrations in the thin, spread out cytoplasm 

 than in the thick nuclear region. In the absence of an estimate of the 

 quantitative effect of this geometrical problem, the results do not exclude 

 the possibility of some cytoplasmic RNA synthesis, in addition to the 

 transfer of nuclear RNA to the cytoplasm. 



Another point of considerable interest in this work was the demonstra- 

 tion of complete cessation of RNA synthesis (as measured by incorpora- 

 tion of cytidine) during the period from prophase to telophase when the 

 chromosomes were highly condensed. There was no apparent decrease 



