692 RADIATION BIOLOGY 



the chemical composition of cellular materials. The experimental pro- 

 cedures have involved cytochemical study of irradiated cells and chemical 

 analysis of materials extracted therefrom. Supplementary studies have 

 been made on nucleic acids and proteins that were irradiated after extrac- 

 tion from the cells. Attention has been focused, in all these studies, on 

 nucleic acids and proteins because they are considered to be the major 

 structural components essential to the synthetic and reparative processes 

 of the cell. It has been established that ionizing radiations effect 

 changes in the physical and chemical properties of these materials, and 

 in their quantities and proportions. The more important question of 

 the alterations effected in their patterns of organization remains largely 

 unexplored. 



Cytochemical methods have shown that X rays and 7 rays, within the 

 dosage range of 40-4000 r, produce metabolic disturbances in prolifer- 

 ating and undifferentiated cells that are characterized by inhibition of the 

 synthesis of desoxyribonucleic acid in the nucleus, and accumulation of 

 ribonucleic acid, mainly in the cytoplasm (Mitchell, 1940, 1942, 1943, 

 1944). The two types of nucleic acid were identified in these studies by 

 digestion of the cells with the enzymes ribonuclease and desoxyribo- 

 nuclease. Determinations of the amounts of nucleic acid were made by 

 a method of ultraviolet photomicrography. Comparable areas of non- 

 irradiated and irradiated tissues were photographed with wave length 

 2537 A, which is absorbed by the purine and pyrimidine bases of the 

 nucleic acids. The sections were then digested with one of the nucleases 

 and rephotographed. Differences in the amount of blackening of the 

 photographic plate afforded an estimate of changes effected by irradiation 

 in the amounts of the nucleic acids. 



Inhibition of the synthesis of desoxyribonucleic acid by ionizing radia- 

 tions has been confirmed in other experiments. Cytochemical studies 

 have used spectrophotometric methods to determine the amount of 

 nucleic acid in irradiated and control sections of the same tissue. Absorp- 

 tion of ultraviolet rays (wave length 2537 A) by purines and pyrimidines 

 was determined on unstained preparations (Ely and Ross, 1948a, b), and 

 absorption of visible light (around wave length 5460 A) was determined 

 on preparations stained by the Feulgen reagent (Stowell, 1945; Petrakis, 

 Ashler, and Ferkel, 1949). Radioactive phosphorus has also been used 

 to determine the amount of newly formed desoxyribonucleic acid in 

 X-ray-treated and nonirradiated tissue (e.g., studies by von Euler and 

 von Hevesy, 1942, 1944; Hevesy, 1945 — see Table 9-15; and Holmes, 

 1947). Hevesy (1945) reported that doses of X rays at or above the 

 therapeutic level inhibited the formation of desoxyribonucleic acid about 

 equally in growing and differentiated tissue. Approximately 75 per cent 

 of the inhibiting action disappeared within 2 hours after irradiation. 



Inhibition of synthesis of desoxyribonucleic acid after irradiation has 



