208 Discussion 



Howard: It has been proposed that this was due to the depolymeri- 

 zation of DNA on the chromosomes. I don't think there is any proof for 

 that or perhaps any disproof either. I don't know what physical 

 chemists think about this idea, but the stickiness can be produced by 

 very low doses compared with what is necessary to depolymerize DNA 

 in most experiments in the test-tube. There has been no clear histo- 

 chemical evidence that there is a change in polymerization of DNA in 

 cells. 



Swanson: However, there is a pronounced oxygen effect here. 



de Hevesy: You and Dr. Pelc have stated that 35 r has an effect. 



Howard: Yes, we got a reduction in the number of cells synthesizing 

 DNA in a 12-hour period, certainly after a dose of 50 r. One can see a 

 maximal effect there. But we know that this dose has a big effect also 

 on the division, and while we have not done the appropriate timing 

 experiment after 35 or 50 r, it seems quite possible that delays in 

 division would explain that effect also. 



Latarjet: I should like to add in answer to Swanson's question that 

 Dr. Ephrussi-Taylor and I have some data on a purified DNA, according 

 to which its inactivation by X-rays acting mainly through direct effect 

 is not influenced by the presence of oxygen. In these experiments, 

 protection against indirect effect was secured by 10 per cent yeast 

 extract. The protection within the cell cytoplasm is certainly higher. 

 Therefore, if we consider those lesions, such as chromosome breaks, 

 which are oxygen-sensitive, we may say either (a) that they do not 

 result from a primary effect of the radiation on DNA ; or (b) that oxygen 

 does not act at the level of a primary radiochemical change on DNA. 



Swanson: This would be a metabolic event of some sort, and fits in 

 with what we believe. 



Howard: It is certain that in some cells, at any rate, division of the 

 nucleus does not always determine division of the cell. These two things 

 are separable in many cells, and perhaps the synthesis of DNA is also 

 separable from the division of the nucleus. 



Lajtha: I think that DNA synthesis and division are clearly separable. 

 One can inhibit mitosis with colchicine and certain concentrations of 

 heparin, and neither of them will inhibit DNA synthesis. The result will 

 be polyploid cells and arrest of metaphase. 



Spiegelman: I would like to suggest that one of the most useful 

 systems that might be employed to study this phenomenon is a phased 

 thymine-less mutant where you can control DNA synthesis, nuclear 

 division, and examine for sensitivity. Here, many of the important 

 parameters would be more or less under fairly precise control. In point 

 of fact you can quite easily phase a thymine-less nucleus by controlling 

 the thymine. 



Alper: Dr. Howard, would you be prepared to apply the same 

 reasoning to u.v. effects? 



Howard: I understand that DNA synthesis can be immediately and 

 finally stopped by u.v. irradiation, and in this respect it seems to act 

 rather differently from ionizing radiation. Perhaps the reason is that the 

 nucleic acid itself has such a high absorption. 



