300 GENE MUTATIONS CAUSED BY RADIATION 



the plus "tail" of the statistically random energy distribution which re- 

 sults from the operation of ordinary thermodynamic principles. These 

 high levels are due to mechanisms, utilizing the energy-transferring prop- 

 erties of some nucleotides, which cause an accumulation of the potential 

 energy from multiple quanta, absorbed at different times, and which 

 can then suddenly release this energy at a high level, as occurs, for in- 

 stance, on a molar scale in electric organs. At present, however, there 

 seems no need to postulate an energy level above 1.5 ev for spontaneous 

 mutations merely because of its being needed for ultraviolet mutations. 

 It seems more plausible to refer the difference to the nature of the chemi- 

 cal pathways and/or to a multiplicity of key events being necessary to 

 effect the occurrence of mutation. 



The above diversity of possibilities should show what a high degree 

 of caution is necessary when the attempt is made to interpret biological 

 events on the basis of simple physical principles, without regard to the 

 chemical complexities that may be involved. Nevertheless, despite our 

 uncertainty regarding the energy level necessary for spontaneous muta- 

 tion and the nature of the chemical steps involved, we can be sure that 

 the random encounters of thermal agitation play an important and neces- 

 sary part in the process of spontaneous mutation. Moreover, this being 

 the case, it is probable that the increase in spontaneous mutation fre- 

 quency that accompanies a rise in temperature within the range normal 

 to the organism is, in part at least, caused by the increase in thermal 

 agitation which the higher temperature entails. 



That the spontaneous gene-mutation process must usually depend 

 upon thermally activated reactions follows, for one thing, from the cal- 

 culations and data presented by Muller and Mott-Smith (56), later con- 

 firmed by others, which showed that natural radiation is entirely inade- 

 quate in amount to be an important cause of spontaneous mutations 

 in Drosophila at the rate at which they occur in that material. That 

 these thermally activated reactions which lead to gene mutations are 

 random events beyond individual control by cellular regulative processes 

 and that they are therefore subject to the statistical principles of ordinary 

 thermodynamics is attested to by their sporadic, pointwise distribution 

 in space and time. For one thing, this randomness is of such scope that, 

 as yet, it has not been found possible (at any rate in experiments that 

 have been confirmed) to bias the occurrence of ordinary spontaneous 

 mutations by applying special conditions that would favor one type 

 against another. But, more telling still, the randomness can be shown 

 to exist even in the range of microscopic or submicroscopic dimensions. 

 For, as the present writer pointed out in an early discussion of this 

 question (38, p. 470): "Mutation is due to an event of such minute 



