EFFECT OF OXYGEN TENSION 



ON THE PRODUCTION OF CHROMOSOME 



BREAKAGE BY IONIZING RADIATIONS : 



AN INTERPRETATION* 



C. p. SWANSON 



The Johns Hopkins University, BaUimore, Maryland, and Biology 

 Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 



It was first demonstrated by Thoday and Read^ (1947) that anoxia 

 reduces the eflfectiveness of any given dose of X-rays. This is strikingly 

 illustrated when the frequency of chromosome aberrations is used as the 

 criterion of effect, but it should be recognized that the protective action of 

 anoxia extends to other phenomena as well : survival of organisms, delay 

 in cell division, and the stickiness of chromosomes. The time is not yet ripe, 

 however, for an integrated interpretation of these multiple effects even if it 

 is presumed that they are related to a common mechanism. It has been 

 recognized that the sensitivity of cells to X-rays, as regards breakage, 

 stickiness, and delay in cell division, is greatest at late prophase and very 

 early metaphase, which suggests possible interrelations. This discussion 

 will, however, be confined to chromosome breakage in the microspores of 

 Tradescantia, and a more extensive interpretation will be attempted in the 

 light of new evidence. Firstly, it will be well to recount those circumstances 

 which have been established with some certainty, and then to reinforce and 

 extend them with additional observations made recently. 



All other things being equal, it is the oxygen tension of the cell which 

 determines, in part, the frequency of detectable chromosome aberrations. 

 The oxygen-dependent portion of the total frequency will vary in magnitude 

 with different ionizing radiations, but that oxygen is indeed part of a reactive 

 system which influences breakage can be considered established by the 

 studies of Giles and his co-workers. They have irradiated cells in hydrogen, 

 helium, and in vacuum, and the results are essentially similar. Clearly, 

 then, the oxygen tension of the cell is an important factor in the breakage 

 of chromosomes, a finding reinforced by the observation that it is the oxygen 

 tension during irradiation that is important, post-treatment changes in 

 oxygen tension being without detectable effect. Gray- ( 1 953) has adequately 

 discussed these aspects of chromosome breakage in terms of radiochemistry, 

 and it is apparent that a more meaningful picture is beginning to emerge. 



Secondly, it is now quite clear that the degree of reduction resulting from 

 anoxic irradiation is not the same for all types of aberrations even when the 

 quality of radiation is the same. This was first demonstrated by Riley, 

 Giles and Beatty^^ (1952), who showed that chromatid deletions were less 



* Work performed under Contracts No. W-7405-eng-26 and No. AT-fSO-D-BSl, U.S. 

 Atomic Energy Commission. 



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