118 Nature of the Genetic Material 



be a clustering of potential breaking points away from loci which are 

 easily caused to mutate. Such an assumption or one involving similar 

 chromosomal conditions appears to be a more plausible explanation. 



To continue with Muller's quotation: "If now, instead of com- 

 paring the two different kinds of material, we compare the two 

 diflFerent agents, ultraviolet and X- or gamma rays, we find an 

 analogous situation. For, with both materials, ultraviolet produces far 

 more gene-mutations, in comparison with gross rearrangements, then 

 do X- or gamma radiation. In the plant material [Stadler, 1933, 1941] 

 it is also clear that ultraviolet produces far more gene mutations in 

 comparison with minute rearrangements (small deficiencies) and in 

 comparison with single breaks, than do X- or gamma radiation." Again 

 the argument does not seem convincing to me. Ultraviolet specifically 

 acts, as proved by Stadler as well as Knapp, by absorption at 2537 A 

 in the purine and pyramidine groups (see fig. 4) of the DNA mole- 

 cule. How this absorption produces a mutation we do not know. If 

 these rings are not themselves the genie material, which is most 

 improbable, the action must be indirect. This means the same type of 

 action through formation of a peroxide, which we know is hable to 

 produce breaks. Why these breaks tend to be more frequently on the 

 submicroscopic level, that is, two very nearby breaks as opposed to 

 more distant breaks which may reunite into visible rearrangements, is 

 diflBcult to say. It seems to have something to do with the specific 

 absorption in the DNA moiety, while X rays seem to strike all mole- 

 cules alike. However this may be, I cannot see a cogent reason for 

 separating point mutation and breakage. 



Another argument was recently proposed by Lefevre, Jr., Ratty, 

 and Hanks (1953). They found that X rays used to produce Notch 

 deficiencies in Drosophila never produced point mutations at the 

 split locus, which is one of the deficient loci in the Notch chromosome. 

 They think that this shows that X rays can produce breaks only, and 

 not point mutation, which therefore are two different things. I should 

 conclude that the X ray is such a powerful agent that there is an over- 

 whelming probability that the two distant breaks are produced simul- 

 taneously, which leads to visible rearrangements of all kinds; while 

 two nearby (submicroscopic) breaks, in the absence of a distant one 

 with which to form a visible arrangement, are rather rare. 



Muller finishes his discussion by saying: "All this clearly shows a 

 difference in the processes whereby these changes originate. It thereby 

 becomes legitimate to draw a distinction between gene mutation, 

 on the one hand, and structural changes, even of minute size, on the 



