Discussion 381 



constant at 7 • 1 by the graded addition of alkali had little effect on the 

 result. 



WTiat is of particular interest to us at present is the stimulus for the 

 sudden increase in the RNA/DNA ratio in the early part of the growth 

 cycle but so far we have no information. The addition of amino acids 

 such as asparagine and glutamic acid did not prevent the early rise. 

 During the early part of the growth cycle there is a considerable increase 

 in cell size. For instance, by the time the RNA/DNA ratio has reached 

 its maximum value the cell mass has increased fivefold but the actual 

 number of cells is usually twice the number inoculated. Thereafter, the 

 cell size diminishes throughout the growth cycle. 



The falling part of the curve, on the other hand, can be explained by 

 the progressive dilution out of material having an RNA/DNA ratio of 

 6 by material with the ratio of 2 reached at the end of the growth cycle. 



Lipmann : Dr. Dean, would you comment on the absolute amounts of 

 DNA and RNA during this cycle? 



Dean : The DNA per unit of mass tends to keep constant, whereas the 

 RNA per unit of mass tends to fall ; so the change is taking place in the 

 RNA more than in the DNA of each cell. 



Lipmann : Then there is an initial formation of RNA and dilution of 

 the RNA as it is formed, and it is not apparently formed after that? 



Dean: We are diluting out the material with the high RNA content 

 with material having a low standard amount of RNA. 



Magasanik : Regarding the observation by Dr. Dean and Sir Cyril that 

 the phosphatase seems to increase in the early part, I think there is 

 some evidence that, at the beginning of glucose fermentation, there is a 

 drop in the intracellular phosphate. 



Hess: Only in the ascites cells, to my knowledge. 



Magasanik: Dr. Torriani studied the formation of phosphatase in 

 Esch. coli, and found that the alkaline phosphatase was subject to 

 repression, i.e. in cells growing on a phosphate-containing medium this 

 enzyme was present in small amounts. It was shown by Levinthal and 

 Garen that cells grown without inorganic phosphate formed the enzyme 

 in fabulous amounts, up to 5 per cent of the total weight of the cell. It 

 seems that inorganic phosphate, as Dr Pardee has described previously, 

 repressed the formation of phosphatase, which is an enzyme responsible 

 for the production of phosphate. I wonder whether the observation by 

 Dr. Dean and Sir Cyril that the phosphatase first increases and then 

 decreases again, might not reflect the level of inorganic phosphate in the 

 cell at the beginning of growth ; when the level of phosphate inside the 

 cell drops, one would expect a rapid synthesis of phosphatase which 

 would be followed, after a high level of phosphate has been reached, by 

 the inhibition of further synthesis of the enzyme. 



Hinshelwood: The cells are all the time in a medium containing 

 plenty of phosphate. 



Magasanik: In the ascites tumour cells in a phosphate-containing 

 medium, we have seen that the intracellular level drops when glucose 

 metabolism begins. 



Hinshelwood: We wondered whether phosphatase perhaps plays a role 



