BARBARA E. HOLMES AND LORNA K. MEE 



(48-60 hours, ^-P injected at 57 hours) the effect of the low dose is obvious. 

 Several other examples of the lack of immediate effect and the marked 

 delayed effect of the low dose are shown in Table I. 



It seems that the immediate 50 per cent reduction in synthesis seen after 

 clinical doses of X-rays is not accounted for by the action on cells in the 

 sensitive state only. There is still a possibility that the new synthesis of 

 DNA is stopped but that some sort of renewal of old DNA is taking place 



Table I. — Immediate and delayed effects q/"450r and 2,200 r 



Rate of Dj\A Synthesis Expressed as Percentage of Controls 



^-P used and injected 3 hours before killing 



* Mitotic counts irregular from l';i-26 hours, but always high from 27-33 hours 



at the same time and is not inhibited. If the renewal of the old DNA is 

 taking place without much expenditure of energy, by exchanges of base and 

 phosphate with free base and phosphate in the medium for instance, this 

 could well be true. To reconcile this idea with the template theory (and 

 particularly if we find that the synthesis of the new and the renewal of the 

 old takes place at the same time) we have to suppose that the components 

 of the old DNA become more mobile at the moment when the new DNA 

 is being formed, or has just been formed, upon it. 



REFERENCES 



^ Hevesy, G. Radioactive Indicators. Interscience Publishers, New York. 1948. 

 2 Vermund, H., Barnum, C. P., Huseby, R. A. and Stenstrom, K. W. Cancer Res. 

 1953, 13 633. 



223 



