272 Discussion 



to a very high temperature. If you wanted to work this out you would 

 have to introduce the equivalent temperature, in other words the 

 temperature which would produce that particle, so that your tempera- 

 ture would be very high and you would not expect any volume change. 

 The same applies to the effect of temperature. There should be no effect 

 of temperature on the initiating step, but if you do get an effect of 

 temperature it is an effect on the subsequent steps. 



Lajtha : Does one get these ploidy effects only if one irradiates under 

 dry conditions, or do they occur if the organism is suspended in dilute 

 medium ? 



Gray : They are ordinary living cells, they are not dry. 



Lajtha: The opposite happens in mammalian cells and in living bean 

 roots where the actual synthetic stage during which the 2 n DNA in- 

 creases to 4 n seems to be less sensitive to small doses of radiation than 

 the interphase stage with its stationary 2 n amount of DNA. 



Gray: If you score the amount of chromosome damage per cell, this 

 amount of damage increases with ploidy, but of course it may be less 

 lethal to the cell because damage to any one chromosome is more 

 likely to be covered by the other chromosome sets. Diploid yeast is less 

 sensitive than the haploid; the same applies to Neurospora, the higher 

 ploidy is less sensitive despite the fact that more actual chromosomal 

 damage is produced. 



Koller: Miss Lamy, working with MuUer in Edinburgh, found no 

 difference in radiation sensitivity between the diploid and the triploid 

 Drosophila. 



Gray: This was concluded in yeast in the higher ploidies also. 



Alexander: I think an important point arises both out of Dr. Gray's 

 and Dr. Swanson's papers and that is : is the breakage of the chromo- 

 somes already a metabolic event or is this the primary chemical change 

 following directly on the absorption of the energy from the radiation ? 

 I believe that the amount of energy which Lee and others have calcu- 

 lated as necessary for giving a break is not sufficient to turn the nucleo- 

 protein from the chromosome from a gel into a sol. This would have to 

 be the case if the chromosome-break was produced by chemical action. 

 I think even for the break it may be necessary to postulate some 

 metabolic process. This view, I believe, is supported by the experiments 

 on the effect of radiation on the properties of the nucleoprotein gel 

 which I mentioned earlier (this symposium, p. 57). Irradiation of whole 

 cells produces much greater changes than irradiation of nuclei. A 

 possible interpretation is that the nucleoprotein is not damaged by a 

 few hundred r at the concentration at which it is present in the cell, but 

 that an enzyme acting on the nucleoprotein is released by radiation. 

 Dounce showed that there are enzymes in the cell which can liquify this 

 nucleoprotein gel extremely efficiently, and that only the most careful 

 preparations of nuclei which are absolutely clean from all adhering 

 matter can give nucleoproteins which remain as stable gels. 



Swanson: If you are referring to chromosome breaks, wouldn't this 

 presuppose that there should be a posttreatment effect? 



Alexander: Yes, but it might have to be extremely rapid. In this 



