224 



CELL HEREDITY 



2380 2480 2537 2650 2804 



Wavelength, A 



2967 



FIGURE 8.13. The absorption spectrum of DNA compared with the action spectrum of 

 mutants induced by ultraviolet light. The points are for mutation in E. coli from str-d 

 to str-s, and the curve is drawn for calf thymus DNA (from Hollaender and Zelle, 1954, 

 First Interm. Photobiological Congress, Wageningen, Netherlands, H. Veenman and 

 Zonen, p. 128). 



acids absorb the different wave lengths to different extents and, upon do- 

 ing so, enter an excited state. The absorption spectrum of DNA is 

 shown in Figure 8.13. Superimposed on it is a curve for the number of 

 mutations induced by the same wave lengths. This is called the action 

 spectrum of mutation. The similarity suggests that absorption of ultra- 

 violet light by DNA is the cause of mutation. This finding also fits the 

 target theory well, but, again, there is reason to suspend judgment re- 

 garding its validity. First of all, a linear relationship between ultra- 

 violet dose and number of induced mutants is not found (Figure 8.14). 

 Furthermore, there is some biochemical evidence involving pre- and post- 

 treatments of the irradiated cells which suggest that the effect of ultra- 

 violet light is not direct. Other substances than DNA, for example 

 RNA and the purines and pyrimidines themselves, have essentially the 

 same absorption spectra. It could be that the initial absorption of the 

 ultraviolet quanta occurred in these substances which exist in the cell 

 outside of the gene. In a chemically modified form they might be in- 

 corporated in the replicating DNA and give rise to the mutation ob- 

 served. If this were the case, the target would be one of these sub- 

 stances rather than the gene itself. In support of this idea is the fact 

 that pretreatment of bacteria with purines and pyrimidines or their 

 ribosides increases the number of mutations induced by subsequent treat- 

 ment with ultraviolet light. On the other hand, the indirectness men- 



