EFFECT OF X-KAYS ON NUCLEIC ACID IN JENSEN-SARCOMA 693 



The investigation was performed by using the radioactive indicator 

 method. This method makes it possible to distinguish between tlie mole- 

 cules of nucleic acid which have been formed before and after the beginn- 

 ing of the experiment. The latter, which have been formed in an active 

 medium, viz. in cells containing active phosphate, will be active (i. e. 

 will contain radioactive phosphorus) as opposed to the inactive mole- 

 cules already present before commencement of the experiment. 



DESCRIPTION OF THE METHOD 



When a very small amount of sodium phosphate, labelled with an 

 admixture of radioactive phosphorus (i?P), is introduced by, for example, 

 injection into the experimental animal, the labelled phosphate ions soon 

 enter the sarcoma cells and take part in the synthetic processes occurring 

 in these cells with the same probability as the other phosphate ions 

 already present in the sarcoma cells. If molecules of nucleic acid are 

 synthesized in the sarcoma cells they will be radioactively labelled. 

 If all the nucleic acid molecules present in the sarcoma at the end of 

 the experiment have been formed in the course of the experiment, then 

 1 mgm of nucleic acid phosphorus will show the same content of ^^p^ 

 i. e. the same radioactivity, as 1 mgm of phosphate phosphorus. If, on 

 the other hand, at the conclusion of the experiment 1 mgm of nucleic 

 acid phosphorus shows an activity amounting to, say, only 1 per cent 

 of the activity of 1 mg of phosphate phosphorus, then the amount of 

 nucleic acid formed during the experiment will be 1 per cent of the total 

 amount of nucleic acid present in the sarcoma. By making this state- 

 ment we assume that the measured activity of 1 mgm of phosphate after 

 completion of the experiment is equal to the activity present at any 

 other point in time during the course of the experiment (cf. the discus- 

 sions on p. 705). 



Preparation and Measurement of the Activity of Radioactive Phosphorus 



The radioactive phosphorus used in the experiments which will be 

 described below has usually been obtained by the action of neutrons 

 on carbon disulphide^-^^ A mixture of radium-beryllium containing 600 

 mgm of elementary radium served as the source of neutrons. We are very 

 greatly indebted to Professor Niels Bohr for the loan of this source 

 and also for many other pieces of equipment. In addition to this, radio- 

 active phosphorus was available which had also been produced by the 



1 O. Chievitz and G. Hevesy, Kgl. Danske Vidensk. Selskab. Biol. Medd. 

 13, 9 (1937). 



