380 



Discussion 



the constancy may break down. On the other hand, the RNA content 

 tends to be proportional to the growth rate, but here again the tendency 

 may break down in extreme cases. 



We have been interested in following the change in the RNA/DNA 

 ratio in cells of Bad. lactis aerogenes (Aerobacter aerogenes) during the 



< 



Z 



< 



z 



100 200 300 400 500 

 Cell mass 



600 700 



Fig. 1 . The variation in the RNA/DNA ratio during a growth 

 cvclc. 

 Ordinate : RNA/DNA 

 Abscissa : cell mass. 

 The cell mass was determined turbidimetrically and is ex- 

 pressed in terms of the number of cells of standard size in 

 millions/ml. 



growth cycle in a simple synthetic medium containing glucose, phos- 

 phate buffer (pH 7 1) and magnesium and ammonium sulphates. 

 The general pattern which emerges is shown in Fig. 1. 



In these experiments the inoculum was taken from a fully grown 

 culture of an earlier growi:h cycle and would consist of cells whose RNA/ 

 DNA ratio was about 2. Reference to Fig. 1 will show that there is a 

 considerable rise in the RNA/DNA ratio in the early part of the cycle 

 followed by a slower fall as growth proceeds, the ratio eventually 

 settling down to a value of about 2. The curve was not greatly dif- 

 ferent when the air supply to the culture was replaced by nitrogen. In 

 the normal course of one of these growth cycles the pH of the culture 

 medium gradually changes from 71 to about 4. Keeping the pH 



