RADIOACTIVE TRACERS 



855 



The ratio of the rate of incorporation of ^^p or ^^C into DNA and RNA 

 in the exposed organism shows appreciable variations. These depend on 

 the nature of the precursor, the time left for the labelled compound to 

 take part in the metabolic steps involved and the dilution of the newly- 

 formed labelled DNA or RNA by "old" non-labelled ones present. As the 



InTest. mucosa 

 ONA Protein 



Liver 

 ONA Protein 



o 



Muscles Kidneys 



DNA Protein- DNA Protein 



c 

 o 



O 



Fig. 1. Effect of irradiation, 700 r X-rays, on the incorporation 

 of "C of injected CHg^COONa into tissue fractions of the 



growing mouse. 



300 



olOO 



days 



Fig. 2. Effect of irradiation on the incorporation of ^^P into the 

 DNA of the spleen of the rat (Kelly et al.) 



total RNA content shows greater variations with time than that of the 

 DNA content, the dilution effect will show a more intricate pattern in 

 the case of RNA than in that of DNA. The great difference in the synthetic 

 rate of nuclear and cytoplasmic PNA will also contribute to make the 

 effect of irradiation on the incorporation of ^^p or i^C into PNA much 

 more dependent from the experimental conditions than that into DNA. 

 While the effect of irradiation on PNA formation is mostly less pro- 

 nounced than on DNA formation, this interference may have very far 

 reaching consequences as well. 



