408 



HADIATION Mlol.OGY 



f^ivoM close ol radiation Ix'iiiK much fi;ri'al(.'r in the picscncc of oxygen tluiii 

 in its iihsence. I'lie same mutation data are plotted against survival 

 ratio in Fig. 10-18 to illustrate more elearly that, for the streptomycin- 

 dependent strain, the freiiuency of back mutation is greater for a given 

 survival lexcl when the cells were irradiated in the absence of oxygen and 



20 



40 



60 



80 



100 



120 



140 



1600- 



1400- 



1200- 



1000. 



800- 



600- 



400- 



200- 



nr 



OXYGEN 



1 1 — 



40 50 



EXPOSURE.!" 



Ix 10* 



9x10' 



SxlO-* 



-7x10* 



•6x10' 



-5x10^ 



-4x10' 



-3xlO' 



-2x10' 



IxlO^ 



90 



Fig. 10-12. X-ray induction of back mutations as a function of kiloroentgens of 

 exposure. Left and bottom scales, comparison of back-mutation rates to strepto- 

 mycin nondependence of the streptom.ycin-dependent strain when irradiated in 

 oxygen (curve I) or in nitrogen (curve II). Right and top scales, comparison of the 

 back-mutation rates of the purineless strain to purine nondependence when irradiated 

 in oxygen (curve III) or in nitrogen (curve IV). (Anderson, 1951b.) 



hence had received approximately two and one-half times as much inci- 

 dent energy as the cells irradiated in oxygen. For the purineless back 

 mutation, the mutation-survival curves are identical for cells irradiated in 

 the presence and in the absence of oxygen. These results are significant 

 in indicating that greatly different quantitative relations may exist 

 between the induced-mutation rates and radiation dosage for different 

 mutation systems. Furthermore, although the oxygen-sensitive indirect 

 mechanism which influences inactivation of both strains appears to have 



