22-21 



THE YEAST CELL 



tations may have taken place during the development of the colony 

 on the plate, so that by the time the mutant was tested by transferrii 

 to liquid medium its growth rate was more rapid than when it was 

 originally isolated. 



Fig. 22-6 Growth of Mutant Colonies on Test Agar Plates. 

 A, Table 7, Exp'. II, Culture 7; B, Table 7, Exp. II, Culture 18 



Since the mutants grow more slowly than the original in the 

 presence of sufficient pantothenate there is selection against them 

 during growth in complete medium prior to plating on the test agar, 

 and because of this, it is not possible to calculate the variance from 

 the data in table 22-2. It is apparent that mutations do occur in the 

 presence of a high concentration of pantothenate and that there is 

 considerable variation in the numbers of mutants. It appears that 

 the mutations are random and because of the selection against them, 

 are not ordinarily encountered. The mutation rate, calculated from 

 the data from the experiment in which some of the cultures con- 

 tained no mutants is 2.82 x 10"9 per cell per generation. The mu- 

 tation rate, calculated from the data from the experiment in which 

 some of the cultures contained no mutants is 3.21 x 10"^ per cell 

 per generation. The mutation rate was calculated from the formula 

 devised by Luria and Delbruck (1943) as given by Demerec and 



Fano (1945) in which the mutation rate a, - -1-6 log 10 PQ, where 



N 

 Pq is the proportion of cultures in which there are no mutants and 

 N is the average number of cells per culture. Both large and smal) 

 r.nlonipc: Mjf^rr^ inrluded in the calculation of the mutation rate. 



