NUCLEAR SYNTHESIS OF RNA 



531 



Tetrahxmena were centrifuged out of the radioactive medium and re- 

 suspended in medium containing unlabelied cytidine at the same concen- 

 tration. This first washing resulted in a twenty-five-fold dilution of the 

 [^H]-cvtidine and was completed at 12 min. At 12 min. all incorporated 

 cytidine is still localized in the nucleus (Fig. 2(1^)). The washing procedure 

 was repeated three more times to give a 150000-told dilution of the 

 isotope by 30 min. 



After the first washing the incorporation of radioactivity into nuclear 

 RNA continues for another 40 min., indicating a large pool of PH]- 

 cytidine or its derivatives that could not be washed out of the living 

 Tetrahymena with medium containing unlabelied cytidine. Cytoplasmic 

 label begins to appear at about 15 min. Forty minutes after the first washing 



160-1 



(40 



120- 



(r(00 



FOURTH WASHING 

 COMPLETE 



FIRST WASHING 

 COMPLETE 



':^l 



NUCLEUS 



=5^ 



-H 



40 50 60 



TlME(min) 



70 



100 



Fig. 3. The two curves describe the total amount of incorporation of [^H]- 

 cytidine into RNA of the nucleus and cytoplasm. .A.t 12 min. the cells were washed 

 free of the isotope with non-labelled medium. The range for each point indicates 

 95 "^'o confidence limits. 



the density of label in the nucleus begins to decrease and by 100 min. has 

 fallen to about io",j of the peak level of 50 min. Only a small fraction of 

 this decrease can be ascribed to dilution through cell division. During this 

 decrease the cytoplasmic label per cell increases until the rate of cyto- 

 plasmic labelling per cell equals the rate of dilution by cell division. This 

 transient balance occurs at about 80 min. In the last 20 min. of the experi- 

 ment, cytoplasmic label per cell is decreased more rapidly by cell division 

 than it is built up by newly labelled RNA. At 100 min. approximately 

 one-third of the nuclei contain no radioactivity (Fig. 2{d)). The remaining 

 two-thirds of nuclei contain small amounts of activity, about half of which 

 is RNase removable. The remaining trace of acti\ity is presumed to be 

 incorporated into DNA. 



