o 



6 



I 



I 



I 



I 



• 



II 

 I' 



A 



1 / 



II 



20 



Q. 

 U 



30 10 30 



FRACTION NUMBER 



Fig. 11. 



Sedimentation profiles of RNA in a pulse-chase experi- 

 ment at 16 hr. 



dashed curve in this figure indicates the pattern 

 of papilla formation in the same culture. It was 

 followed in all cultures, but is not always plotted 

 in the figures. It demonstrates that we have a 

 good, synchronized culture and that RNA syn- 

 thesis essentially stops before the appearance 

 of the first morphological event. I should per- 

 haps say that gross RNA synthesis has stopped. 

 The same figure illustrates the pattern of leucine 

 incorporation. You will notice that this goes up 

 and continues to rise when RNA synthesis is 

 dropping. It continues longer and drops more 

 slowly, although it has reached a low level by 

 195^ hr. 



DEERING: Is this label in the dilute salt 

 solution that you put in after you wash these? 



LOVETT: Every experiment I'm talking 

 about now is in the dilute salt solution after 155^ 

 hr. 



a 

 d 



I 

 I 



a. 

 o 



FRACTION NUMBER 



Fig. 12. 



Sedimentation profiles of pulse- labeled RNA prepared 

 at different stages of development. 



DEERING: Then the label is added after 15^ 

 hr? 



LOVETT: Well, we often check points be- 

 fore. In this experiment a few points were taken 

 before 15/^ hr, which is the reason for the drop 

 and subsequent rise in the RNA incorporation 

 curve. Changing the medium does modify this 

 somewhat. We don't, however, attach much 

 significance to this at present. 



The fact that uracil incorporation seemed 

 to stop so drastically and sharply suggested that 

 the cells were obviously turning off RNA syn- 

 thesis and, perhaps, undergoing considerable 

 degradation and turnover. As I said earlier, we 

 felt it important to determine where the nuclear 



170 



