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THE LIFE-SPAN OF MUTAGENS PRODUCED 

 IN CELLS BY IRRADLVFION 



W. D. Jackson 



BotcuiY Department, University of Tasmania, Hobart 



INTRODUCTION 



The target theory as put forward by Lea^ assumes direct action between the 

 incident radiation and the cellular components. In chromosome breakage 

 it is assumed that each l^reak is produced by a particular particle. A basic 

 assumption is that the initial chemical agents which are produced at 

 ionization centres have a very short half-life and a short mean free path. 



On the basis of an unqualified target theory we would expect irradiation 

 to produce effects quite different from those produced by chemical mutagens. 

 Instead, practically all investigations have shown a surprising similarity. 

 Such similarity might be expected if irradiation were indirect in action 

 through the agency of a chemical phase, with production of mutagenic 

 substances or radicals which have a relatively long mean free path and 

 half-life. 



There is altogether much evidence which suggests such an indirect mode 

 of action although there would seem to be certain results which are still 

 adequately explained by the target theory based on direct action. 



Evidence for an indirect action has come through study of the oxygen 

 effect, where high oxygen tension gives enhanced damage with low ion 

 density irradiation. A change in oxygen concentration from atmospheric 

 to near zero concentrations causes a reduction of damage In' a factor of 

 two to three in a wide variety of material and in a number of responses such 

 as {1) chromosome breakage, (2) mitotic rate, {3) growth, {4) survival or 

 mean lethal dose, (5) skin reactions, {6) lethal mutations. 



Fractionation of the dose into a number of smaller doses separated in time 

 reduces the effect of irradiation. Decreasing the dose rate or intensity also 

 has this effect. It seems reasonably certain that damage also increases pro- 

 portionately with the linear ion density of the irradiation although the 

 effects generally pass through a maximum with respect to types of irradiation 

 ranked in increasing order of ion density. This is presumably due to the 

 opposing effects of linear ion density and total path length per unit volume. 



Some experimentation with these combined variables has been made. 

 For example, experiments of Gray and Scholes- show that the dose-rate 

 effect is only observable with low linear ion densities. Similarly, experiments 

 by Thoday and Read^ Giles et al.\ Swanson* and Bender** show that the 

 oxygen enhancement is only observable at low ion density. Kirby-Smith and 

 Dolphin' have reported experiments on varying the dose rate at two levels 

 of o.xygen tension. Here no change was found in dose-rate effect at low 



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