MUTATION AS A CHEMICAL PROCESS 



209 



TABLE 8.1 

 The Relation of Bacterial Mutation to Division Rate 



;From Novick and Szilard, 1950, Proc. Natl. Acad. Sci., 36:708) 



Generation Time, 

 Hours 



Mutation Rate 

 per Bacterium 

 per Generation 



Mutation Rate 



per Bacterium 



per Hour 



and t is time. At any time, then, the proportion of mutants is: 



M 



N 



= at + C 



(8.2) 



where C is the initial proportion of mutants at time zero (fg). This is the 

 equation for the curve in Figure 8.2. 



Novick and Szilard changed the rate of growth of bacteria in the 

 chemostat by changing the flow rate, and observed that the mutation 

 rate, a, was an inverse function of the rate of growth (Table 8.1). The 

 relation held for a broad range of growth rates. It seems, then, the 

 mutations occur at a constant rate in time, irrespective of the number of 

 generations that take place in any interval. At first glance this might 

 seem to suggest that mutation is not a function of gene replication; it 

 might even occur while the gene is not replicating. But it is likely that, 

 when bacteria grow at different rates, the period during which the gene 

 is replicating may be prolonged or shortened accordingly. Perhaps mu- 

 tations have a measurable probability of taking place only while gene 

 replication is going on. If this were so, then mutation could be constant 

 with time and yet could occur as a copy-error. 



FIGURE 8.2. The rate of increase of 

 mutants in the chemostat (from Novick 

 and Szilard, 1950, Proc. Nafl. Acad. 

 Sci. Wash., 36:708). 



O CD 

 C O 



(0 I-H 



ro;=;400 



5 „ 200 - 



00 



10 20 



Generations 



