Discussion 49 



as long as you are not doing these things to a molecule the possibilities 

 of change are diminished, and consequently the permanence of that 

 particular feature is more assured. 



Gillman: Therefore, if you are going to discuss any implication for 

 the survival of the cell in that particular case, the less the molecular 

 chains alter and, therefore, the less the chance of error, the greater the 

 survival rate. Is this the line of thinking? 



Danielli: That is a generalization I should prefer not to make until 

 I had some experiments which justified it. I should say this idea was 

 one of those stones that you lift up to see what kind of animals are lurking 

 underneath! 



Gillman: The line along which our group is thinking is that change 

 and maintained capacity for change is the essence of youth far more than 

 stability. This is characterized in many features of growing organisms 

 and there seems to me to have been too much of a tendency in much of 

 the modern literature to regard change as a bad thing in terms of main- 

 taining juvenility or youth. 



Best: The interchange of fatty acids which Schoenheimer first 

 showed between, let us say, liver and muscle tissues, might be of great 

 importance in this connection. There are factors which vary the rate 

 of this "flux". They have not been worked out carefully, but it would 

 be extremely interesting to explore the effect of ageing on the rate of 

 this interchange. 



Gillman: We have indications that the slowing up or even stoppage of 

 molecular change, in certain structures in connective tissues, is more 

 closely associated with ageing and degeneration of those tissues than is 

 the rapid turnover seemingly characteristic of the young. 



Danielli: I do not think anyone knows just why it is that certain of 

 the proteins are apparently in a very dynamic state so that the amino 

 acids are popping in and out, and other intracellular proteins are in an 

 extremely stable state. I don't think we know the answer to that. But 

 I think a fundamental point, so far as cells are concerned, is that the 

 feature which distinguishes a living system from a dead one is that you 

 have continuous change which is operating within fixed reference points, 

 so that the whole system is controlled through changing. What happens 

 to any particular molecule is unimportant unless that molecule is a key 

 one in the control processes, such as deoxypentose nucleic acid. If a 

 controlling molecule like DNA is exposed to rapid metabolic changes 

 then the possibility of errors creeping into its constitution are much 

 greater than if it is maintained stable. On the other hand, you cannot 

 have everything in a cell stable or it would cease to be living — it would 

 be a fixed cell. In fact, the whole character of a living cell depends upon 

 its occupying a "halfway house" between complete plasticity and com- 

 plete stability. Take for instance water, in which none of the atoms are 

 held in a specific position for any perceptible length of time; you could 

 not base a living organism on that sort of structure. Equally, if you take 

 for instance a metal, the atoms may occupy one position for a great 

 length of time. Again, you cannot base a living organism on that. The 

 characteristic property of living cells is that they are made from 



