BARBARA E. HOLMES AND LORXA K. MEE 



7 hours before mitosis. In the very early stages of rat Hver regeneration, 

 just before the first mitoses take place, enlarged cells with nuclei carrying 

 their double loads of DNA are found. Since the total uptake of isotopes 

 into the DNA up to this point can be measured and the number of enlarged 

 cells counted, it should be possible to decide here also whether double the 

 amount of isotope needed for new synthesis has been taken up. 



It is possible that this apparent doubling is due to an occurrence at actual 

 mitosis. Various authors have been led to believe that there is a doubling 

 of DNA content at mitosis or directly after. It can also be pointed out, 

 though this is not meant as a serious suggestion, that a throwing out of old, 

 unlabelled DNA at mitosis might occur and would reduce the actual amount 

 of DNA present while causing a sudden rise in the specific activity of the 

 DNA now extractable from the cells. In our curves w^e find a sudden rise 

 in the specific activity of DNA phosphorus just before the first burst of 

 mitoses in the tissue, though no new ^^p is now entering the DNA. 



c 



< 



Q 



WOO 



VJ 



500 



100 

 



Mitotic rotes shown by isolated points 

 • Control 

 V Afier 150 T 

 G After li 50 r 



-30% 



50 



40 



20 



70 



t 



n 76 24 30 



Hours after liepatectomy 



Figure 1. Effect o/ 1 50r and 450r X-ray doses on rate of formation of DNA (^'P used) 



Another possible way, and quite a different one, of accounting for the 

 X-ray resistant 50 per cent of DNA synthesis w^as brought out by the work 

 of Pelc and Howard^. They showed that DNA synthesis could be inhibited 

 for some hours b)' very small doses of X-rays given shortly before the s\n- 

 thesis began. During the period of actual DNA synthesis even considerably 

 larger doses had no effect on it. It therefore seemed possible that, when a 

 growing tissue containing cells in every stage of the mitotic cycle was 

 irradiated, a proportion of them would be in an insensitive stage and a 

 proportion in a sensitive stage as regards DNA synthesis. Thus it might 

 happen that only part of the synthesis would be inhibited. 



The early stages of rat liver regeneration provide very easy and straight- 

 forward material for investigating this possibility. In fact Kelly^ has 

 already shown that the formation of DNA can be inhibited by total body 

 irradiation in the pre-synthesis period much more easily than by irradiation 

 in the synthesizing period in this material also. It can be seen from Figure 1 

 that no synthesis of DNA takes place in the remaining lobe during the first 

 12 or 15 hours after partial hepatectomy. Synthesis then begins and the 



221 



