540 RADIATION BIOLOGY 



conclusion previously arrived at from a comparison between the effects of 

 neutrons and those from X and 7 rays. 



That all or by far the greater part of the mutagenic effect of ultraviolet 

 in most of the algal, fungal, and higher plant material studied is caused by 

 activations arising in the chromatin itself is shown by the rather close 

 agreement between the curve showing the relative amounts of absorption 

 of different wave lengths of ultraviolet by nucleic acid, i.e., the absorption 

 spectrum for this substance, and that showing the relative amounts of 

 effect of different, nearly monochromatic wave lengths in producing muta- 

 tions, i.e., the action spectrum. Indications of the mutational action 

 spectrum had first been obtained by Noethling and Stubbe (1934, 1936; 

 Stubbe and Noethling, 1936) for Antirrhinum pollen and by Stadler and 

 Sprague (1936) for maize pollen. Detailed action spectra were later 

 obtained for Sphaerocarpus spermatozoids by Knapp and co-Avorkers 

 (Knapp, 1938; Knapp et al., 1939; Knapp and Schreiber, 1939), for 

 Trichophyton spores by Hollaender and co-workers (Hollaender, 1939; 

 Hollaender and Emmons, 1941; Hollaender, Raper, and Coghill, 1945), 

 and for maize pollen by Stadler and co-workers (Stadler, 1939, 1941; 

 Stadler and Uber, 1942). In all these studies a fairly close agreement 

 was found between the mutagenic action spectrum, on the one hand, and 

 the nucleic acid absorption spectrum, on the other. This nucleic acid 

 absorption spectrum, in its turn, depends mainly on absorption by its 

 contained purines and pyrimidines. For Drosophila spermatozoa, where 

 absorption by the body wall obscures the picture, it has been found that, 

 although ultraviolet of 3100 A is still mutagenic, wave lengths of 3300 A 

 or more have no appreciable effect (Reuss, 1938; Mackenzie and Muller, 

 1940; Mackenzie, 1941 ;Demerec, Hollaender, e^aZ., 1941). For Drosophila 

 pole cells, 2537 A appears to be about ten times as effective as radiation 

 between 2900 and 3200 A, as would be expected on the basis of the nucleic 

 acid absorption spectrum (data of Meyer and Muller; see Muller, 1952c). 



The fairly good fit of the action spectrum to the absorption spectrum 

 in the material referred to does not, however, justify the inference that 

 activations produced by ultraviolet in the chromosomal nucleic acid are 

 more efficient mutagenically than those produced elsewhere ; for, with the 

 wave lengths used, in the given material, much the greater part of the 

 absorption in the nucleus (and, in some of the material used, in the whole 

 cell) is in fact by this very component. Hence, if activations at this 

 site were of only average efficiency, they might, nevertheless, play the 

 predominant role in mutagenesis. For this reason alone the absorption 

 spectrum representing them would tend to correspond with the action 

 spectrum of mutagenesis. That activations in other substances also can 

 cause mutations is indicated by more detailed consideration of the action 

 spectra obtained l)y Hollaender and co-workers (Hollaender, 1939; Hol- 

 laender and Emmons, 1941; Hollaender, Raper, and Coghill, 1945). 



