71 



individual cells as they grow. These measurements were made by my student, 

 David Prescott, who used a sensitive Cartesian Diver technique. 



Let us consider the growth in weight. The initial point is that at which 

 the ameba comes out of a division - is -born". Twenty-four hours later, this 

 ameba divides. The initial rate of increase in mass is the highest; growth pro- 

 gresses at a steady but decreasing rate and levels off some hours before the next 

 division. It levels off at a weight just double the birth weight. 



The ameba knows what its mature weight is going to be but doesn't de- 

 termine it by simple arithmetic. It does not necessarily double. If we have a 

 case where 2 sister amebae are of unequal size, one abnormally small, the other 

 correspondingly large; the smaller one starts growing more rapidly than the 

 larger one, and both end up at the same weight - the weight of their mother cell. 



Water content keeps pace with dry mass, that is, the curve for growth 

 in volume runs parallel with that for growth in dry weight. 



We don't have data on growth with respect to DNA, but just to round out 

 the story, we may refer to the findings of Pelc and Howard and others, on other 

 kinds of cells. They find that the doubling of DNA goes on between divisions and 

 is completed some time before the next division.* 



The story of RNA - and I would not for a moment represent it as general 

 - is rather unexpected. This study was made by Dr. Thomas W. James. The 

 RNA per cell does not increase at all during the period of maximum protein syn- 

 thesis, but undergoes a doubling during the period between the leveling off of pro- 

 tein synthesis and the onset of division. 



In a discussion of radiation effects, we have to consider the relation 

 between cell growth and cell division. The cell is not going to divide until it has 

 reached what I might call "maturity." The growth in mass, the growth in volume, 

 and the doubling of DNA are completed some time before the cell is reading to 

 divide. It is waiting for something to happen before it can divide. 



I should like to raise, in a general way, the question, whether it is le- 

 gitimate to think about a trigger to cell division. Does the absolute quantity of 

 something in the cell have to reach a certain level? Does some new reaction 

 have to take place before division can go forward? There are some cute experi- 

 ments on the ameba that make me think that it is legitimate to invoke a trigger 

 mechanism to cell division. 



Some 30 years ago, it was shown that an ameba will not divide unless it 

 has achieved a full complement of some X that it must contain. The cell divides 

 once every 24 hours and it is growing during this period, as I have described. 

 All you do is this. Each day you amputate a big chunk of cytoplasm from the cell, 

 undoing the growth it has accomplished during that day. Its growth cannot catch 

 up with the amputations. It does not divide but remains alive indefinitely. This 

 experiment tells me that the ameba must pile up a certain amount of X before 

 division is triggered, and because of our frustrating operations, the level of X 

 neve gets to the triggering amount. 



* - Since the Highland Park meeting. Dr. Walter Plaut in our laboratory, 

 has obtained evidence that the synthesis of DNA in ameba is completed during the 

 first 18 hours of the 24 hour interphase period. 



