166 MUTATION AND PLANT BREEDING 



started immediately and was correlated with expression of the second 

 mutant fraction. 



Summary and Conclusions 



Several years ago Witkin (26) expressed the opinion that "the 

 time interval between absorption of radiant energy and produc- 

 tion of stable genetic changes can no longer be regarded as infini- 

 tesimal." She pointed out that many postirradiation treatments 

 altered the ultimate fate of a potential genetic change. Our experi- 

 ments have indicated that the basis for the delay between appli- 

 cation of inducing agents and production of stable genetic changes 

 is due to the necessity for replication of the genetic DNA. 



Our experiments with X-ray-induced mutations indicate that 

 certainly with this inducing agent mutations are produced through 

 at least two mechanisms, or at two different levels of genetic structure 

 development. Recent studies by Witkin and Thiel (28) and by 

 Kada, Brim, and Marxovich (11) indicate that the same is probably 

 true for UV-induced mutations. In one type of mutation induc- 

 tion with either agent the initial damage is unstable and capable 

 of modification by postirradiation treatments. The second type is 

 stable, and is not influenced significantly by postirradiation condi- 

 tions. Both types are analyzed in our X-ray experiments, but Ave 

 have dealt only with the former type in the UV experiments. 



In regard to the unstable type of UV-induced mutation, while 

 our studies suggest that the potential mutation is initially in the 

 form of a mutagenic nucleic acid precursor formed by action of the 

 radiation on a purine- or pyrimidine-containing monomer (4), we 

 have no conclusive evidence for this; and the possibility remains 

 that the initial irradiation damage is to the cellular DNA. Sins- 

 heimer (22, 23) has shown that exposure of uridylic or cytidylic 

 acids to UV results in stable and unstable byproducts, and our 

 experiments indicate that cytidine and uridine are effective com- 

 pounds in increasing UV-induced mutation frequency. 



Studies on the postirradiation events involved in mutation 

 induction by UV have led to delineation of several biochemical 

 interrelations influencing mutation induction and probably involved 

 in DNA replication. Those biochemical interactions occurring prior 

 to any detectable macromolecular syntheses and tending to increase 



