GENERAL DISCUSSION 351 



There must be added to this the particular sensitivity of high-i)oly- 

 mer DNA, and its synthesis, to ionizing radiations. 



Therefore the key point in radiation injuries to the cell and in the 

 search for the means of protecting it from such injuries, in my opinion, 

 is the study of the biophysics and biochemistry of the processes which 

 lead to the occurrence of genetic injuries to the chromosomes and the 

 quest for means of liquidating these changes. 



This latter problem appeared until quite recently to be hopeless and 

 without any method of approach. Now, as we have heard, in particular 

 in the contributions of HoUaender and of Shapiro, possibilities have 

 emerged whereby these genetic processes may be affected at least at 

 the very initial stages of their development. 



ALEXANDER: The hypothesis which Bacq and I have advanced that an 

 imjDortant type of radiation damage that leads to cell death consists 

 of the disorganization of intracellular structures does not mean that we 

 are only concerned with the cytoplasm. The electron microscope has 

 revealed the existence of an extensive cyto-skeleton in the cytoplasm, 

 though the prediction that such a structure must exist was made more 

 than 30 years ago by R. A. Peters. He argued that the presence of 

 enzymes and their substrates required that they be separated by fine 

 lipid films. We now know that in the nucleus, there are also enzymes 

 which must be kept away from other nuclear material which they can 

 attack. Fine structures of the type seen in the cytoplasm must there- 

 fore also exist in the nucleus. The fact that these have not been re- 

 vealed by the electron microscope can, I believe, be attributed to techni- 

 cal factors. Our theory that the mechanism of action for radiation in 

 relation to cell death is therefore not confined to the breakdown of 

 structures in the cytoplasm, but includes the breakdowai of structures 

 in the nucleus. 



PASSYNSKY : Any general theory of radiobiological action must at the 

 present time postulate the destruction of the cell through intermediate 

 biochemical processes from the primary destruction of several mole- 

 cules or of a certain volume of molecular dimensions. Damage to the 

 chromosomes as the unique structures of the cell satisfies this require- 

 ment. The role of the damaged chromosomes in the genetic conse- 

 quences of the radiation remains completely inviolate, but many papers 

 at the present symposium and, indeed the last discussion, show that 

 the concept of the primary nature of this damage is evidently in need 

 of some modification. Another view has been expressed namely that at 

 the instant of irradiation, some intracellular membranes are damaged. 



