332 GENERAL DISCUSSION 



bacq: II. Mitosis is more radiosensitive than DNA synthesis, but 

 these two processes are not necessarily connected. Would anyone who 

 does not agree with this care to comment? 



HOLLAENDER : It sccms to me that this question has not Ijeen correctly 

 formulated. Only certain stages of mitosis are very sensitive. The later 

 stages of mitosis are more resistant than DNA synthesis. Both these 

 processes are unavoidably connected, because mitosis depends upon 

 DNA synthesis. But it seems to me that this problem has been formu- 

 lated in a topsy-turvy manner. 



I would like to emphasize one point which Gray touched upon. One 

 has to work very rajjidl}^ in order to study the first stages of the radi- 

 ation effect. Within 60 sec mitosis can be so far advanced that we can 

 no longer do anything. 



SO ska: Kar])fel in the Institute of Biophysics at Brno has found new 

 criteria for determining the sensitivity of mitosis to irradiation. He 

 has discovered in the cells of mouse bone-marrow that after irradi- 

 ation the ratio of the number of metajDhases to the number of prophases 

 is altered. The normal ratio, which is 0-5, is raised after irradiation to 

 1 to 2. This variation proceeds even at very low doses, about 5 r, but 

 only if the irradiation has been carried out between 15 and 19 hr prior 

 to mitosis, i.e. the most decisive interval of time before the beginning 

 of mitosis. 



TOBIAS: I would also like to disagree with the formulation of the 

 problem as put forward by the President.! 



What is mitosis? Mitosis is the cellular activity by which the DNA 

 of the mother cell transmits its information to the daughter cell. The 

 time required for mitosis expresses the time necessary for the trans- 

 mission of this information. 



Thus we can examine the delay in mitosis caused by radiation either 

 as a defect in the information, the source of which is the DNA, or as a 

 defect in the channel capacity which is apparently a cytoplasmic 

 process. 



One of the interesting observations which has been known for the 

 last 20 years consists in the fact that if mitosis is delayed for a certain 

 prescribed peiiod of time, then the cell in general cannot divide. This 

 is seen from the work of Henshaw on sea urchin eggs, and has been 

 quite recently shown in Bird's dissertation in our university on yeast 

 cells after irradiation. 



f From the Praesidium: tlie President said tliat he ventured such a formulation on 

 purpose in order to provoke discussion. 



