FREE DEOXYRIBONUCLEOTIDES IN RADIATION DAMAGE 



157 



Table II. The effect of dCMP ^^P -phosphate and ^"^C -formate incorpora- 

 tion into bone-marrow cells of irradiated and non-irradiated guinea-pigs 



studied by autoradiography. 



Mean numbers of cells showing significantly liigher grain counts than the background, 

 expressed per hundred cells of that cell type. 



by incorporation into DNA. Similarly, the incorporation of formate, 

 which is to a considerable degree a specific precursor of DNA-thymine 

 in bone marrow in vitro, indicated a threefold and two-fold increase of 

 DNA-synthesis in non-irradiated and irradiated reticular cells respec- 

 tively. The results thus show that the increased DNA-synthesis in the 

 j)resence of deoxycytidylic acid is caused for a great part by increased 

 incorporation into the reticular cells. At the same time, the presence 

 of free deoxycytidylic acid may be a limiting factor for DNA-synthesis 

 in the reticular cells of bone-marrow. 



Under similar conditions, the effect of thymidylate was also studied. 

 32P-phosphate incorporation into DNA, determined in the entire bone- 

 marrow suspension after fractionation according to Schmidt and 

 Thannhauser without autoradiography has indicated increased DNA- 

 synthesis in the presence of thymidylate. Formate incorporation was 

 reduced at the same time, which reflects the dilution of the newly syn- 

 thesized i4C-thymidylate by added thymidylate. The first results 

 obtained by autoradiography have also shown increased 32p incorpora- 

 tion into irradiated reticular and "blast" cells in the presence of 

 thymidylate, but paradoxically they have also revealed an increase of 

 formate incorporation into both irradiated and non-irradiated reticular 

 cells. In "blast" cells, the expected reduction of formate incorporation 



was found. 



Deoxycytidylic and thymidylic acid had also an effect on the purine/ 

 pyrimidine ratio in the spleen of irradiated rats (Palecek and Soska, 

 1960). This ratio is increased to more than 1 on the fifth day after irra- 



