Discussion 321 



Alexander: We believe that the two strands of the DNA molecule can 

 be separated in dilute solution by the addition of concentrated urea (4 m) ; 

 the molecular weight falls to half without a change in dimensions. All 

 our physicochemical data is comparatively consistent with this data. In 

 fact, it would be difficult to envisage a process other than this splitting 

 by which the molecular weight of a molecule is reduced to half while its 

 length remains unchanged. The problem remains, why do they come 

 apart ? An important point to remember in this connexion is that they 

 only come apart if the solutions are extremely dilute, i.e. less than 0-05 

 per cent of DNA. Clearly, the only forces which we can invoke are 

 Brownian motion. If we had to postulate that one of the strands un- 

 wound around the other by a rotating motion, then for a molecule of the 

 dimensions of DNA this would take something like 140 days before 

 10,000 turns all in the right direction had occurred. Prof. W. Kuhn 

 has, however, solved this problem in a most ingenious manner 

 (1957, Experientia, 13, p. 301) : if after breaking all the hydrogen bonds 

 holding the purine and pyrimidine bases together by adding urea, the 

 double spiral only increases in diameter by some 20 per cent, then the two 

 strands are sufficiently far apart to be able to move through one another 

 without any trouble. This increase in diameter is readily achieved if one 

 assumes that the centre of the molecule remains fixed and the two ends 

 rotate in opposite directions about 500 times. This process occurs quite 

 quickly by Brownian motion (average time required for a molecule of 

 the size of DNA is about 30 to 40 seconds). If this has occurred, the two 

 molecules may well have to move apart, and this again by Brownian 

 motion only requires about 20 to 30 seconds. There is, therefore, a very 

 high probability that both processes will occur (i.e., the two strands will 

 have become separated) within a minute or so. 



In our experiments, we could not measure the time required since it 

 takes a minute or so for the concentrated urea to be satisfactorily mixed 

 into the DNA solution. This time of mixing is quite sufficient for the 

 separation to occur. I would like to stress that the method of separation 

 is not a physiological process and can only occur in the artificial situation 

 in which we have studied DNA. 



Hayes: It is very hard to visualize this without a model, unless you 

 are accustomed to thinking in terms of double helices, but how DNA 

 replicates is an important biological problem from the genetic point of 

 view. Is Prof. Kuhn's theory applicable to separation of the two helices 

 during DNA replication? 



Alexander: This particular mechanism which Prof. Kuhn has worked 

 out could not apply to the cell. It applies to the artificial situation of 

 DNA in very dilute aqueous solution. We cannot get urea to split DNA 

 when this is at a concentration greater than • 05 per cent. We can only 

 do it when it is extremely dilute, because once the two chains begin to 

 touch one another they interfere with the uncoiling process. 



Pontecorvo: Would the Levinthal speedometer-cable system of unwind- 

 ing apply to your conditions ? 



Alexander: That does not get you very much further because you 

 then have to postulate a directive and energy-consuming mechanism for 



DRUG RES. — 11 



