Discussion 47 



material? Are you not just shifting the focus of observation from a 

 mult i- individual or a multicellular community to the multimolecular 

 community of the single cell, and does this really escape any of the 

 problems ? 



Danielli : This was one of the problems I was hoping not to have to 

 deal with extensively! Here, one has to move into a little more detail 

 about the relationships of genes and macromolecules. As far as I under- 

 stand the situation at the moment, the function of the nuclear genes is to 

 control the manufacture of particular types of macromolecules : I would 

 hesitate to say that the macromolecules which are made under the 

 control of one gene are always absolutely identical. The things which a 

 particular gene "makes", generally speaking, are presumably very 

 similar, but I should hesitate to say that there is any known way of 

 establishing that they are absolutely identical. Secondly, there is the pro- 

 cess organizing these different types of macromolecules into particular 

 functional units, which again is genetically controlled but which is a 

 totally different process and different level of activity of the cell com- 

 pared with the actual manufacture of the macromolecules. I can see 

 that in senescence some degree of breakdown may take place both in 

 the manufacture of the macromolecules and in competence to organize 

 them into specific structures, and I think each of these things must be 

 examined separately. 



Landowne: Certainly the macromolecules, once made, are relatively 

 .stable in their environment. Do you consider there is much physico- 

 chemical repair and reversibility in position and in bonding within a 

 constructive bit of architecture, so to speak? 



Danielli: That depends on which sort of molecule you look at. If 

 you look at deoxypentose nucleic acid in the non-dividing cell, the 

 evidence is that it does not change much. If you look at haemoglobin, 

 you will not find it greatly changed, with the exception, perhaps, of the 

 haem moiety. If you look at some adaptive enzymes you will find that 

 they don't seem to change very much. On the other hand, Gale's work 

 shows that amino acids can pop in and out of some sort of proteins quite 

 easily under some circumstances ; and if amino acids keep on popping in 

 and out of a molecule, how can you call it either an old molecule or a new 

 molecule ? 



Landowne: If they continue to pop in and out with the same rapidity 

 and in the same manner over a period of time, then the molecule is 

 behaving as a young molecule. When the exchange is altered, whether 

 it is being altered intrinsically or extrinsically, we have a change, and 

 this might give rise to something which is called an effect of age. 



Danielli: That then would be a failure in the control process which 

 determines what pops in and out. 



Landowne: Yes, and is this not exactly the sort of thing which we deal 

 with in cells that divide ? 



Bourne: I think the same problem arises in consideration of the 

 skeleton. There is a constant turnover of calcium and phosphorus in 

 the skeleton and after a time one has a completely different series of 

 molecules composing one's skeleton to what one had earlier. Are you 



