322 Discussion 



doing it, and you are just moving the problem one stage further. If it 

 comes apart in the cell, there must be a very subtle mechanism. 



Lederberg: It is not quite so bad as that. If you build on additional 

 nucleotides, that will do the unwinding and provide the energy for the 

 turning as well. You have a directed mechanism. 



Walker: Is the Kuhn model satisfactory from the stereochemical point 

 of view ? Can urea get in to direct its hydrogen-bonding power to break 

 the hydrogen bond? 



Alexander: The structure is quite open, and urea can get in quite easily. 

 The problem is never one of the urea splitting the hydrogen bonds 

 between the bases, but of getting the strands apart. 



Walker: There are directional stereochemical effects involved in the 

 hydrogen bonding. 



Alexander: The dissociation only occurs with 4 m urea. The hydrogen 

 bonds are opened up and remade with the urea, and there is no directional 

 effect ; the only directional effect occurs when they are originally formed. 

 We cannot in the test tube reconstitute the two isolated strands to 

 reform DNA. 



Lederberg: How much re-aggregation do you get if you dialyse the 

 urea away ? 



Alexander: Very much. Once the urea is dialysed away the single 

 strand aggregates and, in the electron microscope, DNA after treatment 

 with urea has the appearance of a matted mass and no long thin strands 

 which are typical of native DNA can be seen. 



Hayes: I understand that when DNA transforming principle is treated 

 with urea in order to break the molecular weight, especially to pre- 

 vent the aggregation of large lumps, it still retains its transforming 

 potentiality. 



Alexander: Herring sperm head DNA and the DNA obtained from 

 Esch, coll are the only specimens that we found to be split by urea. DNA 

 which we have obtained from thymus is not split by urea, until it has 

 been treated with relatively high concentrations of versene. We first 

 thought this was because metals formed additional bonds which held 

 the strands of DNA together ; but we now believe that it is due to protein 

 contamination. Only from herring sperm, because this sperm is such a 

 particularly clean material, does one get DNA which is really free from 

 protein— there is less than 0-01 per cent protein in it. As soon as the 

 protein contamination is of the order of • 5 per cent then the material 

 does not split any more. 



