lUC 



UADlATiO.N lUol.dC'* 



For /cro-poinl nuitatioiis tho mto of inciviiHc in tlif iiumhcr of iniitji- 

 tioiis with incrcasiiijj; dose excoods an exponential liinction of dose. The 

 frcMiiienc}' of mutations increases until doses eausinj; ah<»ut 10 ' sur\i\al 

 are exeeeded, at wliich tim(> a plateau followed l)y a slight decline is 

 oliserved. A similar (leclinc in mutations at high doses has been observed 

 by IloUaender and Emmons (l<)89a. 1941 ; Kmmonsand Ilollaender, 1939) 

 with the fungus Trichophyton mcntagrophi/tes, Ilollaender, Raper, and 

 Coghill (1945) with ,1. terreus, and Hollaeiider . . . Demerec (1945) for 

 Ncitrospom. Xo good explanation exists for the occurrence of this 

 maximum in the mutation-dose relation. 



1 000 



?00Q 



3000 4000 

 DOSAGE. Ergs « mm'' 



50O0 



6000 



Fig. 10-1 1. Zero-point and end-point mutants induced by ultraviolet 2537 A in E. roli 

 B/r, and ultraviolet svirvival curve of B/r (seniiiojrarithmic plot). Solid circles, zero- 

 point mutants; open circles, end-point mutants. (Adapted from Demerec and Latarjet, 

 1946.) 



For end-point mutations, Demerec and Latarjet (1946) observed a 

 rapid increase in mutations at low doses, but as the dose continued to 

 increase the rate of increase in mutations decreased and contirmed more or 

 less exponentially to the highest doses tested, no maximum being observed 

 for the end-point mutations. The frequency of end-point mutations was 

 higher at all doses than that of zero-point mutations. 



With X rays, essentially similar results were obtained for both zero- 

 point and end-point mutations, the latter being approximately 200 times 

 as freciuent at all doses. The mutation-dose curves were approximately 

 linear, which is compatible with a one-hit mechanism, whereas the much- 

 greater-than-linear increase in mutations with increasing dose observed 

 for ultraviolet irradiation indicates a multiple-hit relation. 



In comparative experiments with strains B and B/r, nearly identical 

 phage-resistance mutation rates were observed at a given dose of either 

 ultraviolet or X ravs even though the inactivation of strain B was much 



