694 



RADIATION BIOLOGY 



identified by using purified, crystalline ribonuclease. It is suggested that 

 the differences in the reports of various workers concerning the accumula- 

 tion or loss of ribonucleic acid after irradiation may be due in part to 

 differences in dosage and time between treatment and examination, and in 

 part to differences in the response of the tissues being studied. 



Hevesy (1945) has noted that cells are highly radiosensitive if the 

 synthesis of nucleic acids and associated materials is proceeding rapidly, 

 but are more resistant if synthetic activity is low. Differences in sensi- 

 tivity between actively dividing and differentiated tissues are explained 

 on the assumption that cells in actively growing tissues do not have time 

 to recover from the deleterious effects of radiation before initiating divi- 

 sion, whereas there is ample time for recovery in differentiated cells. 

 The high content of desoxyribonucleic acid in tumor cells may be a 

 factor in their radiosensitivity (Sparrow, 1944). 



Another problem that has been attacked by chemical and cytochemical 

 methods concerns the nature of the changes induced by ionizing radia- 

 tions that lead to the adhesion and deformation of chromosomes (the 

 "primary" or "physiological" effect). Adhesion of chromosomes sug- 

 gests that changes have occurred in the viscosity of their constituent 

 materials. It has frequently been assumed that such changes in viscosity 

 are due to depolymerization of desoxyribonucleic acid (see, for example, 

 Darlington, 1942). There is little question that X rays can depolymerize 

 salts of desoxyribonucleic acid in vitro, as has been shown, for example, 

 by Sparrow and Rosenfeld (1946) ; Taylor, Greenstein, and Hollaender 

 (1948); G. C. Butler (1949); Scholes, Stein, and Weiss (1949); Limperos 

 and Mosher (1950) ; and Smith and Butler (1951). The data of Sparrow 

 and Rosenfeld for sodium thymonucleate are given in the last column of 

 Table 9-16. 



Table 9-16. Relation of X-ray Dosage to Relative Viscosities of Solutions of 



Thymonucleohistone and Sodium Thymonucleate 



(Sparrow and Rosenfeld, 1946) 



