THE INHIBITION OF DNA SYNTHESIS BY IRRADIATION 



3 Skipper, H. E. and Mitchell, J. H. Cancer, 1951, 4 363. 



* Holmes, B. E. and Mee, L. K. Brit. J. Radiol. 1952, 25 273. 



^ Von Euler, H. and Hevesy, G. Arkiv. Kemi, Mineral, geol. 1944, A17, No. 30. 



« Stevens, C. E., Dagust, R. and Leblond, C. P. J. Biol. Chem. 1953, 202 177. 



' Howard, A. and Pelc, S. R. Heredity, 1953, 6 261. 



® Pelc, S. R. and Howard, A. Aarhus Conference of Radiobiology, 1953. 



^ Kelly, L. S. Proc. Amer. Ass. Cancer Res. 1954, 1 24. 



DISCUSSION 



Z. M. Bacq : If I have clearly understood, Mrs. Holmes has observed several times 

 after irradiation that DNA synthesis may be normal when there is no mitotic activity. 



Is it possible that mitosis and DNA synthesis are unrelated events ? 



A. Howard : It is obvious that a cell must synthesize DNA if it is to divide, but 

 DNA synthesis does not always lead to division. The existence of many cells having 

 tetraploid or higher amounts of DNA in tissues with low rates of division indicates 

 this clearly, and is one of the grounds for criticizing Leblond's conclusion that the 

 high rate of incorporation of ^^P relative to mitotic rate means that existing DNA 

 is being broken down. In view of the clear separation in time of the two events, 

 it would, I think, be surprising if DNA synthesis were inevitably followed by mitosis. 



224 



