Discussion 273 



connection I was encouraged by the report from Dr. Hollaender (this 

 symposium p. 206) that mitotic suppression could be reduced by treat- 

 ment with hypertonic salt immediately after irradiation. A posttreat- 

 ment would have to be given quickly because if it is in fact a question of 

 allowing enzyme to get as a gel particle, then this may happen within 

 seconds after irradiation. The suggestion that the attack on the chromo- 

 somes does not follow directly from the uptake of energy certainly 

 raises the possibility that mutations may be prevented by a suitable 

 aftertreatment. 



Swanson: In terms of breaks we have been able to use only two 

 devices to modify their frequency by posttreatment : one is mechanical, 

 i.e. centrifugation, the other is infrared, which is another irradiation. 

 There is no other means of which I am aware. 



Alexander : One really wants a physical method, because chemicals 

 would have to diffuse in and it is obviously difficult for one diffusion 

 process to catch up with another one. 



Gray: Dr. Alexander, I wonder whether, in the experiments to which 

 you refer, the number of nuclei w hich are affected is proportional to dose, 

 or whether all nuclei are affected but to a degree which varies with the 

 dose. 



Alexander: I suspect that radiation affects many nuclei to some 

 extent, the effect is not confined to a few. These experiments are rather 

 difficult to do accurately because of the difficulty of isolating clean nuclei. 

 Some of the apparent discrepancies in the literature concerning the 

 effect of radiation on cellular nucleoproteins could be explained if a meta- 

 bolic process intervened since time factors then become all-important. 

 Butler: I think that would be explained if the nuclear damage caused 

 by radiation made the nucleoprotein more open to attack by enzymes. 

 Nucleoprotein preparations all slowly break down, and it appears to us 

 that that is due to the action of deoxyribonuclease, because we found 

 that the breakdown is accompanied by a degradation of the DNA. 

 If you examine the DNA from an intact gel and then examine the DNA 

 from the nucleoprotein gel which has been kept for some time, and has 

 become liquified, you find that it is the DNA that has become degraded. 

 Therefore we conclude that this is an action of deoxyribonuclease which 

 may well have come from the cytoplasm, so that your observation could 

 be explained as the action of radiation on the nucleus causing some 

 degree of damage to the nucleoprotein material which in some way 

 renders the entry of the deoxyribonuclease easier. 



Alexander: That was the exact interpretation which I had placed on 

 it, that is, the irradiation is not sufficient to liquify the gel. All it did was 

 to enable a metabolic process to take place. Butler's suggestion that the 

 nucleoproteins from cells become more susceptible to attack by enzymes 

 after irradiation has already been established by Cole and Ellis. But 

 this effect by itself could not explain the formation of chromosome 

 breaks since any enzymes capable of attacking nucleoproteins must be 

 stored well away from the chromosomes if cells are to survive at all, 



Butler: There would have to be initial radiation damage. You are 

 merely showing it up. 



