150 



CELLS, TISSUES, AND ORGANISMS 



they develop and again disappear in the controls ( + ) and in the 

 samples with the drug. The first samples ( A ) are incubated at the time 

 when the last temperature shock ends. The last samples (F) are incu- 

 bated at 67 minutes. These F-cells, in other words, are exposed during 

 the last 33 minutes before maximum division takes place ( at 100 min- 

 utes ) in the last control. The later the samples are removed, the earlier 

 the controls divide. This is simply because the bottle is incubated at 

 28° C, whereas the samples are incubated and inspected at room tem- 

 perature. Samples E and F are both incubated after a critical point. 

 Even the strongest concentrations of the drug are without effect. This is 

 more clearly seen when the delays relative to the controls are plotted as 

 in Figure 11. The time from E.H. to maximum division varies some 

 from one experiment to another. Following Flesner ( 1961, in press), we 

 assign to it the value 100, so that we can refer events in the cells' time 

 to a scale which goes from ( E.H. ) to 100 ( division ) . 



The effect of the drug depends on its concentration and on how 

 early it is added. When it is added after 50 to 60 time units, the cells 

 go through the first division without delay, but they do not divide 

 again. The blocked cells are viable for at least 24 hours in the drug. 

 These results strongly suggest that the synchronized cells, in order later 

 to divide, must perform a certain amount of protein synthesis. This 

 takes them beyond a critical point, after which they divide in standard 

 time. The washed cells depend either on the presence of free intra- 

 cellular amino acids or on simultaneous breakdown of their own pro- 

 teins for new synthesis. In cells which remain in the proteose-peptone 

 during the synchronous division, higher concentrations of p-fluoro- 

 phenylalanine are required to delay division. However, with such con- 

 centrations (8 X lO"'^ M), the cycle of sensitivity from E.H. to the first 



100 

 time units 



Figure 11. Another plot of 

 the same experiments as in 

 Figure 10. The successive 

 concentrations of the drug 

 from 1 to 5 are: 5.5 x IQ-* M, 

 5.5 x 10-5 M, 5.5 x 10-6 m, 5.5 

 X 10-7 M, and 5.5 x 10-^ M. 

 The delay in division (ordi- 

 nate) is plotted against the 

 time of immersion in the 

 drug. The drug is without 

 effect after a critical point 

 around 50 time units. 



