210 



CELL HEREDITY 



4 6 8 10 12 



Generations After Irradiation 



14 



FIGURE 8.3. The relation of bacterial mutation induced by ultraviolet light to the 

 rate of division at various temperatures; the same curve describes the results at 37, 

 25, and 16°C (from Witkin, 1953, Proc. Nafl. Acad. Sci. Wash., 39:427). 



This concept agrees with other work indicating that mutation is fastest 

 when cell di\isions are taking place, and that the temperature coefficients 

 of the two processes are the same (Figure 8.3). Yet Stubbe, in Germany, 

 long ago showed that mutations could occur even in stored seeds. The 

 rate was, however, not constant but increased with time, suggesting that 

 as aging progresses the mutation rate changes in accordance with 

 changes in metabolism. Perhaps the genes slowly replicated in the 

 stored seeds at an increasing rate. It has also been shown that non- 

 dividing bacteria can mutate at a slow rate which does remain constant 

 with time. The mutation rate is a function of the ease with which vari- 

 ous sugars can be utilized, and metabolism may be slowed down to such 

 an extent by low temperature or starvation that new mutations cannot 

 be observed to take place. By the use of chloramphenicol, which in- 



