182 



KARL G. LARK 



20 40 60 80 

 Minutes after elution 

 Fig. 9. Pattern of DNA synthesis and cell division following removal of the 



inhibitor deoxyadenosine (DOA). % #, control; X X, DOA; O O, no 



DOA after 35 minutes; A A, no DOA after 50 minutes. DOA was added at a high 



concentration (10"" il/) immediately before the onset of DNA synthesis. Inhibition 

 was reversed at different times (as indicated) by the addition of adenosine. Resumii- 

 tion of DNA synthesis occurred at the same time as the second step of DNA 

 synthesis in the control culture despite reversal of inhibition at different times. In 

 contrast, inhibition of cell division continued in proportion to the length of time 

 DOA was present. The timing of DNA synthesis appears to be independent of the 

 DNA content of the cell, or of whether or not a previous cycle of DNA sj^nthesis 

 was completed. (From Lark, 1960.) 



DNA to act as primer was the controlling factor. (Recijirocal implant 

 experiments demonstrated that starved plasmodia did not contain an 

 inhibitor of DNA synthesis.) 



In all of these studies the hypothesis of jirimer control was suggested 

 because of the inadequacy of alternative hypotheses. In no case was a 

 lirimcr-c'ontrollcd mechanism directly demonstrated. 



