DNA Synthesis 147 



be borne in mind that "prophase" is a descriptive term relating 

 to a certain morphological condition of the chromosomes. 

 The time of synthesis of DNA may well be a more significant 

 point in the cell cycle with regard both to the metabolism 

 and the reproduction of the nucleus, and it is possible that in 

 different tissues this synthesis and the onset of visible prophase 

 may occur at relatively different times in the cycle. 



2. The Effect of X-Rays 



In 1944 and later papers Hevesy (1948) and his colleagues 

 reported that X-rays decrease the rate at which ^^P is incor- 

 porated into DNA. This is true for growing rats, for several 

 normal organs of adult rats, and for Jensen rat sarcoma: the 

 percentage formation of new DNA is similarly depressed in 

 all tissues studied. This effect of X-radiation on the synthesis 

 of DNA was also found by Holmes (1947) in Jensen rat 

 sarcoma and in rat gut. Our results (Fig. 9) show that X-rays 

 decrease both the amount of DNA synthesized in cell nuclei, 

 and the number of cells which synthesize it in observable 

 amounts, during a period of 24 hours after irradiation. Since 

 only cells preparing for division synthesize DNA in this tissue, 

 there can be little doubt that the depression of this synthesis 

 is one of the factors responsible for the known effect of 

 irradiation in delaying the entry of cells into mitosis. The 

 apparent absence of X-ray effect on nuclear uptake of ^^S 

 brings up the possibility that some of the mitotic and chromo- 

 somal abnormality to be seen after irradiation may be due to 

 a disturbance in the relative rates of synthesis of DNA and 

 protein. This possibility forms the basis of further experi- 

 mental work now in progress. 



REFERENCES 



Caspersson, T. (1939). Chromosoma, 1, 147. 



DoNiACH, I., and Pelc, S. R. (1950). Brit. J. Radiol, 23, 184. 



Gray, L. H., and Scholes, M. E. (1951). Brit. J. Radiol, 24, 82. 



Hevesy, G. (1948). Radioactive Indicators. New York: Interscience. 



Holmes, B. E. (1947). Bril J. Radiol, 20, 450. 



Howard, A., and Pelc, S. R. (1951a)- J- exp. Cell Res., 2, 178. 



