radiation interferes with a number of biochemical processes, especially 

 those concerned with protein and nucleic acid synthesis. Presumably this 

 reflects inactivation of essential enzymes. Depolymerization of DNA oc- 

 curs in vitro and probably also in vivo, which may well account for 

 chromosome stickiness. Mitosis is generally inhibited and often by com- 

 paratively small doses. It seems probable that most of the effects which 

 we term physiological stem from inactivation of enzymes. Such effects 

 may be temporary and reversible, or permanent and usually lethal. 



Mutational 

 There is a very direct relationship between exposure to ionizing radia- 

 tion and yield of mutations. Despite the fact that the mode of operation 

 is still unknown, since both input in terms of energy and output in 

 terms of mutation can be measured, it has been possible to establish a 

 number of fairly definite relationships. Some of the more important are: 

 (1) there is a direct relationship between yield and dose, (2) the yield 

 is independent of wavelength over a fair range and also, within limits, of 

 dose rate, (3) as ion density increases yield of mutations decreases for 

 any given total amount of ionization, and (4) different genes differ in 

 sensitivity. 



Chromosomal 

 Chromosomal changes resulting from radiation have been of par- 

 ticular interest to cytologists. Differences in both quantity and kind of 

 chromosomal alterations provide excellent data for the study of differ- 

 ential sensitivity and variations in chromosome organization. While de- 

 tailed discussion is beyond the technical level of this book, certain 

 principles and results which are of major importance can be outlined. 

 In general, between absorption of the radiant energy and the scored 

 configuration there are three steps: (1) primary effect, (2) chromosome 

 breakage, and (3) healing or union of broken ends. On the target theory 

 the primary effect results from the chromosome being traversed by an 

 ionization track, and on the biochemical theory by production of highly 

 reactive radicals. Breakage presumably results from disruption of bonds 

 important to integrity, either directly or as the result of strain, while 

 healing or union of broken ends might be considered to represent some 

 sort of "satisfaction" of free bonds. In any event it is the resultant rear- 

 rangements which can be observed and from which deductions are made. 

 Many analyses of irradiated material have provided the following in- 

 formation. 



RADIATION CYTOLOGY / 195 



