Induction of Chromosomal Aberrations 247 



Discussion 



An attempt can be made to formalize our notions of inter- 

 actions at the chromosomal and physiochemical levels within 

 the cell during and after exposure to ionizing radiation or 

 chemical mutagens. At the chromosomal level the two major 

 categories of events are breakage and rejoining, but each is 

 subdivisible. As to the breakage category, it is unlikely that 

 all breaks consist of fully broken chromatids or chromosomes. 

 Although difficult to assess, the concept of potential as dis- 

 tinguished from primary breakage (Thoday, 1953) gains some 

 credence from the infrared studies done in these laboratories. 

 Probably the ion density of the radiation would be the 

 principal factor involved in determining the spectrum of 

 chromosomal damage (Swanson, 1955a), i.e., the greater the 

 ion density the greater the portion of primary breaks as 

 opposed to potential breaks, and it seem likely, although no 

 proof is at hand, that the oxygen level of the cell would also be 

 a contributing factor (Swanson, 19556). The rejoining system 

 can be operationally divided into restitution and recombina- 

 tion, these being competitive actions for the disposal of broken 

 ends of chromosomes in the nucleus. The great majority of 

 broken ends do not, of course, contribute to observable 

 aberrations, but if we adhere to the idea of the existence of 

 potential breaks then a reduction in the number of damaged 

 sites could come about through the repair of potential breaks 

 or the restitution of primary breaks. There is no possibility of 

 distinguishing between them at present. 



The experimental facts permit us to modify the Latar jet- 

 Gray scheme in the following way: 



I II III IV V 



Primary Secondary 



Physical radio- radio- Metabolic Observable 



events chemical chemical events effects 



events events 



The distinction between stages II and III is made on the basis 

 of time, with II covering the short-lived radicals and III 



