22-17 



THE YEAST CELL 



I n 



A B A 



V-» W^X-Y-Z- pantothenate W->X-*Y^Z^ pantothenate 



B 

 W'-*X -►¥*-► Z'-» pantothenate 



If the genes were involved in two different steps in a series of 

 steps in the synthesis of pantothenate, as diagrammed in I, the lack 

 of either gene would slow up the synthesis. As it is, synthesis 

 occurs rapidly when eithei gene is present. Therefore, the situa- 

 tion must be as diagrammed in II, where A is involved in one pos- 

 sible mode of synthesis, and B in another alternate route. 



60 80 100 



HOURS 



Fig. 22-5 Growth of Adapted Culture 2154 in a Series of 

 Concentrations of Pantothenate. 



The mutation B involves an addition to the synthetic abilities of 

 the organism. One would assume that a number of steps were in- 



