22-23 THE YEAST CELL 



thenate-free medium were each inoculated with about 1000 cells of 

 culture #2154 and then followed closely to determine the number of 

 cells present when the adaptation occurred and what proportion of 

 the cells adapted. Samples were taken and the cells counted with a 

 haemocytometer at frequent intervals during the growth period to 

 determine if a change in growth rate occurred; at the same time 

 samples were plated on plus and minus pantothenate agar plates to 

 detect the presence of mutants. The data are shown in Table 22-3. 



Cell counts from the beginning and the end of the logarithmic 

 phase of growth were used in the calculation of generation time from 

 the formula given by Buchanan and Fulmer (1928), in which genera- 

 t X logjQ2 



tion time = where t is the length of time over 



logiob - logio ^ 

 which the generation time is to be calculated, b is the amount of 

 growth at the end of the time, and B, the amount of growth at the be- 

 ginning. The generation time thus calculated is the average time 

 required for cell division. 



One of the duplicate tubes did not begin to show appreciable visi- 

 ble growth until about 1000 hours (46 days) and then exhibited a gen- 

 eration time of 42 hours. The other tube began to show appreciable 

 growth at around 1900 hours (80 days), and had a generation time of 

 87 hours during the rapid phase of growth. The generation time of a 

 fully adapted culture after several transfers on minus pantothenate 

 medium is about 12 hours. Cell counts were used for the zero level 

 of pantothenate as a measure of growth rather than turbidity read- 

 ings. Evaporation of the water from the tube is a relatively unim- 

 portant factor during the first month of inoculation, but may become 

 serious later, although more than one -half of the water is never 

 lost and this only occurs in the tubes after 3 months or so. No ef- 

 fort was made to correct for this error. 



Figure 22-7 shows the number of cells in one of these cultures 

 grown about three months at SO'-'C, plotted on semilogarithmic 

 graph paper. The inoculum does not fall on the line, but there is 

 fairly close fit of all the points plotted during the logarithmic phase 

 of the growth of the culture. There is an obvious change in the rate 

 of growth from that of the original inoculum, after which, the rate 

 remains constant with a generation time of 82 hours. 



The adaptation to the medium which establishes this generation 

 time may be called the "primary" adaption. This adaptation is ob- 

 viously different from the "secondary" adaptation which produces 

 cells with a generation time of around 12 hours in pantothenate -free 

 medium and which presumably originates by the mutation of a single 

 cell subsequently demonstrated to be a segregating gene mutation. 



Extrapolation of the curve shows that to achieve the primary 

 adaptation at least one cell of the original inoculum must have adapted 

 and does not exclude the possibility that the entire population may 

 have adapted. This is an extraordinarily high mutation rate (if it be 



