276 



CHROMOSOME ABERRATION PRODUCTION 



at normal atmospheric pressure. As mentioned previously, preliminary 

 experiments have also been completed which strongly suggest that the 

 effect of temperature on aberration frequency is, at least in large part, 

 actually an oxygen effect (Giles and Beatty, unpubhshed). Similar 

 evidence concerning the temperature effect on sex-linked lethals in 

 Drosophila has already been obtained by Baker and Sgourakis (4). 



1.1 



-55 0.9 - 



2P0.7 



•E 0.5 - 



0.3 - 



0.1 - 



20 40 60 80 100 



Percentage of oxygen in exposure chamber 



Fig. 2. Reproduced by permission from Science 112 : 643, 1950. 

 The Biochemical Mechanism of the Oxygen Effect 



It is clear from the results which have just been presented that oxygen 

 has a marked effect in increasing the radiosensitivity of Tradescantia 

 chromosomes, as measured by the frequency of x-ray-induced inter- 

 changes and deletions. We shall assume, as most of the evidence seems 

 to indicate, that this effect arises only if oxygen is present in cells during 

 the actual period of irradiation, and results from the production by x- 

 rays of more chromosome breaks under these circumstances. The sim- 

 plest explanation for this situation would appear to be that, in the 

 presence of oxygen, irradiation results in the production within the 

 nucleus of some substance (or substances) which causes an increase in 

 chromosome breakage and that the amount of this substance produced 

 is positively correlated with the amount of oxygen present. It thus be- 

 comes of interest to determine, if possible, what this substance is. 



Since these cells are composed largely of water, it seems very probable 

 that the substance is a product of irradiated water, more particularly, a 

 product characteristically formed when oxygen is present in irradiated 



