154 MUTATION AND PLANT BREEDING 



effect on subsequent DNA synthesis. Only with treatment of at least 

 20 minutes is maximum MFD obtained and this decline is not asso- 

 ciated with any change in the pattern of DNA synthesis. The same 

 holds true for RNA and protein synthesis in the culture. Net RNA 

 and protein syntheses begin only after 25 to 30 minutes postirradiation 

 incubation, and treatments during the first 20 minutes causing maxi- 

 mum MFD do not appear to appreciably change the subsequent 

 pattern of RNA and protein synthesis. It is apparent from these 

 experiments that delay in DNA synthesis cannot be held responsible 

 for MFD following UV irradiation. 



Mutation fixation 



The potential mutation eventually becomes established in the 

 cell to the extent that it is no longer susceptible to conditions pro- 

 moting MFD. This process has been termed "Mutation Fixation" 

 (MF) (4). It should not be confused with the final process of mutation 

 induction involving DNA synthesis (see below). CMP challenge (see 

 Figure 2 for outline of technique) is most frequently used for demon- 

 starting MF. The CMP challenge technique is based on the principal 

 that, when irradiated cells are incubated in complete medium, all 

 processes involved in mutation induction will proceed. However, 

 when CMP is added after a given incubation interval, all potential 

 mutations remaining "unfixed" at the time of CMP addition will be 

 eliminated by the MFD processes. In Figure 2 comparison of the 

 MS curve with that for MF (determined by CMP challenge) shows 

 that the potential mutations are stabilized in the cell for a consider- 

 able period prior to initiation of the MF process. As late as 30 minutes 

 after irradiation all potential mutations remain subject to processes 

 promoting MFD, but after this time decreasing numbers are affected 

 by CMP challenge and after 75 minutes none are susceptible. Neither 

 MS nor MF is correlated with gross protein synthesis since this process 

 is not initiated until 10 minutes after MF has begun. 



Mutation fixation and mutation expression in relation to 

 macromolecular syntheses 



When the progress of RNA, DNA, and protein synthesis are 

 simultaneously followed with MF in a UV-irradiated culture, an 

 interesting correlation is apparent (Figure 3). Progression of MF is 

 closely correlated with RNA synthesis. Both processes are initiated at 



