THE INHIBITION OF DNA SYNTHESIS BY 



IRRADIATION WITH SPECIAL PREFERENCE 



TO IRRADIATION IN THE EARLY STAGES 



OF LIVER REGENERATION 



Barbara E. Holmes and Lorna K. Mee 



Department of Radiotherapy, University of Cambridge 



It has been noticed by Hevesy\ by Vermund et al", by Skipper and 

 Mitchell^ and by ourselves^ and various other authors that irradiation with 

 a considerable range of X-ray doses will reduce the rate of deoxyribo- 

 nucleic acid formation in growing tissues to about 50 per cent of normal. 

 Provided that the technique of irradiation is so arranged that the animal 

 suffers no undue shock from pain or fear or constricting strapping, the 

 dose may be as high^ as 9,000 r and still not reduce the DNA forma- 

 tion below this level. This fact may be of interest in a general con- 

 sideration of mitotic process, since it has been suggested by Von Euler 

 and Hevesy^ and later by other authors, that the reconstitution of the DNA 

 already present, as well as the formation of new DNA takes place in dividing- 

 cells and that the new formation is sensitive to irradiation whereas the 

 reconstitution is not. These views receive support from the work of Hevesy^ 

 and of Stevens, Dauost and Leblond^ who show that the rate of DNA 

 formation, as indicated by isotope uptake, is double the amount required 

 for the formation of new cells actually appearing. The number of new cells 

 appearing is judged sometimes by the increase in weight of an organ or 

 tumour and sometimes by mitotic counts after colchicine. 



Most of these estimations of rate of DNA synthesis were based on ^-P 

 uptake and it was therefore possible that only the phosphate of the resting 

 DNA was being replaced. We have, however, carried out some double 

 labelling experiments with regenerating liver (using glycine or adenine 

 containing ^^C and injecting it at the same time into the same animal as 

 the ^^P), which suggest that the new formation of the basic part of the 

 nucleic acid is proceeding at the same rate as the new formation of the 

 phosphate. These experiments are still incomplete, but as far as we can 

 see, if the DNA of the cell is indeed broken down and rebuilt during the 

 mitotic cycle, it is the whole molecule which must be broken down and 

 rebuilt. This may be difficult to reconcile with a template theory of DNA 

 synthesis. 



Since the work of Howard and Pelc^ it has been realized, as Lajtha 

 has pointed out in this symposium, that synthesis of DNA is complete some 

 hours before mitosis and we cannot, therefore, suppose that the breakdown 

 of the original DNA is some part of the chromosome division and general 

 rearrangement of mitosis. In our material, Howard has been able to show 

 that the synthesis, as measured by adenine incorporation, is complete about 



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