Induction of Chromosomal Aberrations 245 



is apparent that their actions differ among themselves, and 

 that all differ in their action from the ionizing radiations. 

 Chemically, MH is more similar to EOC than it is to DEPE. 

 Both MH and EOC are relatively unreactive compounds 

 whereas DEPE has a high chemical reactivity. The biological 

 effect of MH or EOC may or may not be related to the fact 

 that both possess a structural resemblance to the bases in 

 nucleic acids: EOC is a purine derivative, and MH, as first 

 realized by Loveless, is a structural isomer of uracil. Of the 

 three mutagens discussed, MH is the only one having acidic 

 properties. This is reflected in the strong pH dependence of 

 this compound for effectiveness; it is a powerful mutagen at 

 a pH of 5, but at the concentrations used its effect dimin- 

 ishes rapidly at higher pH levels, and practically vanishes 

 at a pH of 7 or above. These facts suggest that the 

 unionized compound penetrates much faster than the ionized 

 form. 



In spite of their structural similarity, the biological effect 

 of MH bears no closer resemblance to that of EOC than it does 

 to that of DEPE. Both MH and DEPE act in early inter- 

 phase with the first aberrations appearing between 8 and 12 

 hours after treatment (Revell, 1953; McLeish, 1953). Aber- 

 rations induced by EOC appear within 2 hours after treatment, 

 indicating that it, like X-rays, acts in late interphase or early 

 prophase (Kihlman, 1955). MH and DEPE also show the 

 same relationship to temperatures ; in both cases the effective- 

 ness of the compound increases rapidly with increasing 

 temperature. The effectiveness of EOC increases in the range 

 of temperatures from 0° to 10° C, but then decreases at higher 

 temperatures. Few aberrations are obtained with EOC at 

 temperatures above 25° C. 



On the other hand, the biological effectiveness of EOC and 

 MH, but not that of DEPE, is greatly reduced when oxidative 

 phosphorylation is inhibited during the period of treatment. 

 Their differences are also emphasized when one considers 

 their specificity of action which is reflected in a non-random 

 distribution of aberrations. In Vicia root-tip cells, DEPE 



