RADIOACTIVE TRACERS 



857 



organisms, though their vital staining revealed that they were living all 

 through the 48 hours period of the experiment. These and other obser- 

 vations prove that interference with DNA synthesis does not necessarily 

 involve visible cell damage. 



FoRSSBERG and Klein observed, as can be seen in Fig. 3, an increase 

 in the total nitrogen and PNA content of the exposed ascites tumour 

 cells with time in contrast to their DNA content, and the same result 

 is demonstrated by Fig. 4, which is taken from a paper of Kellner 



0,25 



0,20 



CD 



cn 



c 



i 0,15 

 o 



<u 



>■ 



5 0,10 



O) 



o 



0,05 - 



0,0 



o 



/ )E( RNA ana 



/_ ^/ Growth 



DNA 

 _i 



20 30 



Time -minutes 



40 



50 



Fig. 4. Effect of irradiation with ultraviolet light on growth and 

 formation of constituents of E. coli. 



(1953) who irradiated E. coli with ultra-violet light. Following the change 

 in the protein, PNA and DNA content of these bacteria, he found the 

 formation of the last mentioned compound to be blocked after exposure, 

 but not the formation of the first mentioned ones. Irradiation by ultra- 

 violet radiation thus affects DNA formation immediately. Thirty minutes 

 after irradiation with Roentgen rays was the shortest time interval after 

 which 32p incorporation into the DNA of rat sarcoma was investigated 

 (cf. Table 1), a very pronounced effect being observed. 



As mentioned on p. 855 in the experiments described, irradiation did 

 not depress the incorporation of "C into cellular proteins. Nor was it 

 found to depress the incorporation of 35g into nuclear proteins of the 

 root of Vicia faba (Howard and Pelc, 1951). 



Accumulation of cellular constituents 



The marked interference with one of the main processes, the formation 

 of DNA going on in the course of the mitotic cycle, is one of the most 

 conspicuous radiation effects. Other cell constituents accumulate and 



