\V. D. JACKSON 



by oxygen. In addition to actual breaks they considered that potential or 

 latent breaks were formed, whose fate was determined by the physiological 

 conditions of the cell. Liming^" and Swanson^ adopted and extended this 

 scheme to include a spectrum of chromosome damage. They postulated that 

 the important effects of oxygen were to convert potential breaks into actual 

 breaks and to alter the rate of fusion of broken ends. Although this concept 

 brought the opposed views into a common orbit it did not settle the question 

 of the relevant parts played by oxygen in breakage and reunion, a question 

 which underlies basic knowledge of radiation damage to chromosomes. 



Much recent research in this field has been directed at separating breakage 

 and reunion by fractionation in various conditions of oxygen availability. 

 As well as experiments in which the oxygen tension has been varied directly, 

 trials where the effect of oxygen has been suppressed indirectly by 'pro- 

 tective' agents have been made. Other experiments have been conducted 

 in which the oxygen effect was enhanced by physiological states of lowered 

 respiration. These include, conditions of rest (such as in the seeds used in 

 the present work), near zero temperatures (at which oxygen is more soluble 

 and oxidase systems inefficient), and treatments of cells with respiratory 

 inhibitors. 



With X and y irradiation, and to a lesser extent with neutrons, the partial 

 protection obtained by irradiation in a vacuum or in an inert gas can be 

 obtained by treatments with strong reducing agents such as hydrosulph- 

 j^ggi9, 51,46, 20, 52_ Xhis protcctivc effect is expected from reducing agents 

 capable of lowering the concentration of dissolved oxygen. However, re- 

 ducing agents of biochemical nature which are relatively ineffectual in 

 removing molecular oxygen are known to protect biological materials from 

 damage by irradiation. Of these, glutathione, cysteine, 2,3-dimercapto- 

 propanol (BAL), thiourea and ethanol have proved effective in lessening 

 chromosome breakage by X and y irradiation^^- ^^' ^^^ 2". 



Wolff and co-workers have used this protective effect to investigate the 

 breakage and reunion hypothesis in fractionation experiments. By changing 

 the separation interval between two or more dose fractions, an interval can 

 be found, above which the fractions act independently, and below which the 

 open breaks induced by the first fraction interact with those of the second 

 to increase the number of exchange, or two-event aberrations. Wolff^^, 

 working with Viciafaba, found that BAL shortened this interval in addition to 

 decreasing the number of aberrations. He assumed that BAL hastened the 

 reunion of breaks, thus decreasing the period for which the ends are open. 

 This effect could be obtained by varying the dose rate^®. Here the intensity 

 required to produce a significant alteration in the number of exchanges was 

 increased in the presence of BAL. From these experiments Wolff assumed 

 that much of the increased breakage in the presence of oxygen was caused 

 by an increase of reunion over restitution. However, detailed experiments 

 with Atwood^^ led to a change in this hypothesis for it was found that the 

 reduced breakage due to BAL, although correlated with the reduction of 

 the period where breaks remained open, was not affected by this pheno- 

 menon. It was found that the reduction of the open period was also de- 

 pendent on dose, so that with large doses the ends remained open for longer 

 periods. Doses given in the absence of BAL caused an extension of the open 



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