CHROMOSOME ABERRATIONS IN ANIMALS 



693 



been attributed to failure of conversion of ribose into desoxyribonucleic 

 acid (Mitchell, 1940, 1942, 1943, 1944), since ribonucleic acid accumu- 

 lates in the irradiated cell. The greatest accumulation seems to be in the 

 cytoplasm, but an increase has also been detected in the nucleus by 

 Mitchell (1944), and by Roller (1947), who noted that nucleoli, which 

 are known to contain ribonucleic acid, are formed precociously in tumor 

 cells after their irradiation. 



In contrast with the foregoing observations, there have been some 

 reports that the amount of ribonucleic acid in the cell decreases after 

 its irradiation (e.g., Holmes, 1947; Ely and Ross, 1948a, b). The prob- 

 lem was examined by Petrakis et al. (1949), who made a quantitative 



Table 9-15. Ratio of Newly Formed Desoxyribonucleic Acid Molecules 

 before and after irradiation of jensen's sarcoma of the rat 



(von Hevesy, 1945) 



histochemical study of rat-liver epithelium over a 6-day period after 

 irradiation of the animals with either 600 or 1500 r. The results of these 

 investigators showed that the detectable amounts of ribonucleic acid 

 varied with the dose and the interval of time elapsing between irradia- 

 tion and fixation of the tissue. After the 600-r dose there was an increase 

 during the first 24 hours in the concentration of ribonucleic acid in cyto- 

 plasmic granules and nucleoli, as determined by their stainability with 

 the basic dye methylene blue; but subsequent examination indicated that 

 a marked decrease occurred between the second and sixth days. After 

 the 1500-r dose, there was a decrease in cytoplasmic basophilia during the 

 first 3 hours, followed by an increase during the next 3 hours to about 40 

 per cent in excess of the control value, and again a marked decrease 

 detectable at 24 hours after irradiation. Similar results, indicating that 

 the concentration of ribonucleic acid depends on the dose and on the time 

 interval between treatment and fixation of the cell, have been obtained 

 by the author and his associates in cytochemical studies of sections of 

 irradiated plant and animal tissues stained with various basic and acidic 

 dyes (unpublished data). In these studies the ribonucleic acid was 



