220 



CELL HEREDITY 



8.5 



34 68 135 270 540 IC 

 X-ray Dose (r) 



2160 4320 



FIGURE 8.10. Number of mef-2^ mutants induced in E. coli met-2~ by increasingly 

 large doses of X-rays (after Demerec and Sams, in: Buzzati-Traverso, 1960, Immediafe 

 and Low Level Effects of Ionizing Radiafions, p. 283, London, Taylor and Francis, Ltd.). 



tor mutation because for every aliquot of dose the same number of muta- 

 tions are induced, according to the equation: 



M = kD 



(8.3] 



where M is the number of induced mutants, k is some dose-independent 

 constant describing the sensitivity of the target, and D is the dose of 

 radiation. Were two hits in one target or one hit in two targets re- 

 quired, the curve would be exponential, the first dose yielding prepara- 

 tory hits and later doses proportionately more hits within targets already 

 hit once. 



That the whole cell is not the target particle is indicated by a calcula- 

 tion of the target size. This is done on the assumption that a larger 

 target would be hit more often than a smaller one by radiation which, 

 like X-rays, is of known density and randomly distributed in space. It 



