154 CELLS, TISSUES, AND ORGANISMS 



TABLE II 



Relative Amounts of Nucleotides in the Cell Before and 

 After Heat Shock* 



la 2b 3*= 



ATP 8 31 25 



GTP 7 79 32 



* These amounts were .determined by chromatography of norite eluates of 

 acetone powder extracts (Plesner, 1956, 1958a, 1958b, and personal communica- 

 tion, 1958 ) ; all values are for identical dry weights. 



* Logarithmic-phase population. 



" At the time of completion of the last synchronizing heat shock. 



' Immediately after the first synchronous division. 



purine are killing. The killing, blocking, or delaying eflFects are antago- 

 nized fully by adenine, to a lesser extent by adenosine, and hardly at all 

 by adenylic acid. Guanine antagonizes 6-methylpurine, but less efiFec- 

 tively than adenine does. Guanosine and guanylic acid are poor releas- 

 ers. In Tetrahymena, guanine is readily transformed into adenine, but 

 the reverse is not possible ( Kidder and Dewey, 1948; Flavin and GraflF, 

 1951 ) , so we conclude that 6-methylpurine antagonizes adenine. Some- 

 times 8-azaguanine delays the first synchronous division, but only if it 

 is added before 50 time units, as we have seen; this drug is antagonized 

 by guanine, guanosine, guanylic acid, and by the corresponding ade- 

 nine compounds. The desoxyribosides also are efiFective. The cells can 

 be sensitized to 8-azaguanine by heat (6-methylpurine was not studied). 



Interpretations 



A population of Tetrahymena cells can be induced to undergo one 

 or more synchronous divisions in a constant temperature environment 

 if first it is exposed to a series of temperature shocks. A shock is defined 

 as a relatively short period of stay at a temperature diflFerent from the 

 growth temperature (28° C.). Cold and heat shocks have similar 

 effects, only the latter seems slightly more effective (Zeuthen & Scher- 

 baum, 1954). For that reason they are applied in the standard syn- 

 chronization procedure. Qualitatively, a single heat shock does the 

 same as a series of shocks. The series only induces sharper synchrony. 



Studies in which single cells from the logarithmic growth phase 

 are exposed to single temperature shocks (Thormar, 1959) led to the 

 view (cf. review by Zeuthen, 1958) that the temperature shocks dis- 

 turb equilibrium reactions which the cell pushes in one direction as it 



