SYNCHRONIZED GROWTH IN TETRAHYMENA CELLS 



153 



this laboratory by Dr. G. G. Holz Jr. showed that of the five substances 

 mentioned the three latter are strong inhibitors of logarithmic growth 

 in a chemically defined medium. 



On the level of the co-factors the system is strongly influenced by 

 the cycling heat, as first demonstrated by Plesner ( 1956, 1958a, 1958b ) . 

 At the end of the last temperature shock the contents by dry weight of 

 ATP and GTP in Tetrahymena are much increased relative to what is 

 observed in the logarithmic cell ( see Table II ) . This is partly reversed 

 at the time of the first synchronous division. Immediately before the 

 division there is a transitory and impressive rise in the concentration of 

 nucleoside triphosphate ( Plesner, 1958, a, b ) . 



Some drugs, namely 6-methylpurine and 8-azaguanine, do delay 

 division if they are added before 50 time units (see Figure 14). We 

 shall discuss the results on the basis of the suggestion that in our system 

 these anti-metabolites interfere with the co-factors containing adenine 

 and guanine. The bulky literature on 8-azaguanine ( Chantrenne, 1958; 

 Mandel and Markham, 1958; Way and Parks, 1958 ) is rather in support 

 of this view. If the cells are incubated with 6-metliylpurine before 50 

 time units, then the first synchronous division is blocked or delayed. If 

 incubation is later, then the drug delays or blocks only the later divi- 

 sions. In the course of 24 hours, stronger concentrations of 6-methyl- 



100 

 time units 



Figure 14. Delay of the first synchronous division in washed cells by 



( 1 ) 8-azaguanine 



(2) 8-azaguanine 

 (3a) 8-azaguanine 

 (3b) 6-methylpurine 



(4) 5-fluoro-2'-desozyuridine 



( 5 ) a-denopterin 



(6) 5-bromouracil 



3.5 X 10-4 M 

 0.7 X 10-* M 

 4.0 X 10-4 M 



1.6 X 10-3 M 

 4.0 X 10-4 M 

 1.3xlO^M 

 1.6 X 10-3 M 



The exposure to a drug is continuous from the time indicated by a point. 



