L. H. GRAY 



usually visualized as having arisen in two steps, viz- by a breaking of indi- 

 vidual chromatid threads and by the illegitimate fusion of broken ends. In 

 attempts to interpret their observations, different authors have expressed 

 views ranging between the extremes in which oxygen is considered only to 

 increase the initial breakage or only to affect the probability of illegitimate 

 reunion^' •■^^■^^■^'•^^'^^. In a recent careful analysis of aberrations produced 

 by a single exposure of Vicia roots to '^'^Co y-rays, Neary and Evans ^ find 

 quantitatively the same influence of oxygen on chromatid breaks, isochro- 

 matid breaks, chromatid interchanges, and the formation of small achromatic 

 regions on the chromatid thread, formerly confused with chromatid breaks. 

 They conclude that while their results support the view that oxygen increases 

 the initial or primary effect of radiation on the chromosomes, certain details 

 indicate that the primary event may not be the actual breakage of chroma- 

 tids. The lo\v frequency of true chromatid breaks and the absence of iso- 

 chromatid aberrations showing no reunion are more in accord with the 

 exchange mechanism for aberration production, as proposed by RevelP". 



Wolff and Atwood^^ studied the influence of oxygen on chromatid aber- 

 rations produced in Vicia root tip cells by fractionated X-ray exposures. 

 Interpreting the influence of fractionation on the yield of those types of 

 aberrations which involve two chromosomes along the classical lines deve- 

 loped by Sax3-.33 and by Lea^^ and Catcheside^^ they have concluded that 

 the presence of oxygen at the time of irradiation increases damage to a 

 mechanism involved in the healing of broken chromosome ends as well as to 

 the primary breakage. From subsequent experiments in which soaked Vicia 

 seeds were exposed to fractionated doses 75 min apart, Wolff and Luippold^^ 

 concluded that the rejoining of ends of chromosomes broken during the 

 first part of the exposure was dependent not only on the presence of oxygen, 

 but on continued oxidative phosphorylation, during the interval between 

 the two irradiations, since oxygen was ineffective in seeds treated with 

 dinitrophenol and in seeds kept in an atmosphere of 95 per cent carbon 

 monoxide : 5 per cent oxygen in the dark, but not in the light. Whether the 

 primary event in the production of chromosome aberrations is the chromatid 

 break as postulated by Sax, or the chromatid interchange as postulated by 

 RevelF'^, the evidence presented by Wolff, Atwood,' and Luippold is most 

 naturally interpreted, as the authors suggest, to indicate a metabolic require- 

 ment for chromosome aberration production which involves cellular respir- 

 ation. Nevertheless, it would be very valuable if support could be obtained 

 for this view from experiments with cells, rather than tissues, since with 

 tissues there must always be some doubt concerning the effect of the 

 supplementary treatments on the oxygen tension at the target cells. Thus, 

 dinitrophenol is known to increase cellular respiration and is quoted^" as 

 increasing basal metabolic rate as much as tenfold. This agent will, there- 

 fore, give rise to a corresponding increase in tissue oxygen gradients, and a 

 fall in the oxygen tension at the target cells which might even become com- 

 pletely anoxic. Carbon monoxide has an opposite effect on cellular respir- 

 ation, but treatment with carbon monoxide (under pressure) before and 

 during a single aei'obic irradiation, is known to increase radiation damage to 

 Tradescantia microspores^*^ and Vicia meristem cells^". Beatty, Beatty and 

 Collins^" have observed that when Tradescantia microspores are irradiated in 



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