RADIOACTIVE TRACERS 



861 



Desoxynucleoproteins are not necessarily reproduced in a template 

 fashion. Chargaff (1955) is inclined to consider the adjoining nucleotide 

 rather than the one opposite to direct incorporation. In this as well as 

 in the first mentioned case breaking of bonds may suffice to interfere 

 with the formation of new nucleotide molecules. 



Breakage of hydrogen bonds may produce extensive damage. Breakage 

 of one hydrogen bond requires about 10"-*' calorics. The average energy 

 of an ionization being 32 eV= 1.25 x 10"^^ gm calories, a single ionization 



JOO 



oiOO. 



10: 



Mammory carcinoma 



a 500 r 



• 800 r 

 Lymphosarcoma 



■ eoor 



3 

 Days 



Fig. 8. Effect of exposure on the incorporation of ^sp following 

 intraperitoneal injection of labelled sodium phosphate. 



!.•> capable of breaking 100 hydrogen bonds (Frank and Platzman, 

 1955; Pollard, Guild, Hutchinson and Setlov, 1955). Butler 

 (1956) states that an energy of 18 MeV suffices for the breaking of a 

 phosphate bond in dry DNA, which may lead to changes going deeper 

 than the breaking of a hydrogen bond. He considers the possibility that 

 the DNA-histone bond is affected and that the breakage of a compara- 

 tively few histone-protein bonds may prevent DNA synthesis. 



Numerous and very extended studies were carried out on the effect 

 of radiation on DNA or DNA nucleotide suspensions. The viscosity of 

 suspensions was measured prior to and after exposure to irradiation, 

 and various other methods were applied as well. Butler (1956), for 

 example, reports that while 15 minutes of heating at 100<* C had very 

 little effect on the sedimentation constant of DNA, after a dose of 

 9000 r it becomes much more sensitive to action of heat. 



In early investigations a dose of thousands of Roentgens had to be 

 applied to obtain a marked depolymerisation of the DNA (Sparrow 

 and RosENFELD, 1946; Taylor, Greenstein and Hollaender, 1947) 



