22-31 



THE YEAST CELL 



_Q 



Mutation rate = 8.25 x 10 ' per cell per generation. 



Numbers alone designate large colonies, ranging approximately from 0.4 to 

 0.6 mm. in diameter. The letters s and vs designate from small and very 

 small colonies. 3,3s means there are 3 large and 3 small colonies on the 

 plate; "nu" means not uniform. The film of yeast cells on some of the 

 plates appeared uneven although there were no colonies visible macroscopi- 

 cally. Microscopic examination revealed that a considerable number, around 

 a fifth, of the colonies had reached a size of several hundred cells; these 

 plates were classified "nu". The proportion of cultures showing mutants 

 at the end of the time given in the table was used in calculation of the 

 mutation rates. 



COMPETITION BETWEEN ADAPTED AND UNADAPTED 



CELLS IN LIMITING CONCENTRATIONS 



OF PANTOTHENATE 



Gene mutations to pantothenate sjmthesis occur at random both 

 in the presence and absence of pantothenate, but adaptations to growth 

 in the absence of pantothenate only occur in media in which the con- 

 centration of pantothenate is less than two micrograms per liter 

 and do not occur in concentrations above this point. An attempt was 

 made to discover why adaptation occurs at the 27 of pantothenate 

 per liter .level or below, but not above this point. A series of dupli- 

 cate tubes were set up containing decreasing amounts of pantothen- 



