RADIOACTIVE TRACERS 



859 



dose to 150 r does not increase the percentage of cells not DNA syn- 

 thetizing. The results are interpreted by them to be due to a greater 

 radiosensitivity, to delay or inhibition of DNA synthesis on the part 

 of cells which are in approximately one third of the cell cycle (the first 



part of interphase) at the time of irradiation. 

 The marked effect of exposure to a 140 r 

 dose of (190 kV) X-rays on the uptake of ^^p 

 by DNA of such cells is seen in Fig. 5. 



Results very similar to those of How^ard 

 and Pelc were recently obtained by Laj- 

 THA, Oliver and Ellis, (1954) in investiga- 

 tions of the incorporation of added ortho- 

 phosphate-^^P into human marrow cultures 

 or adenine-i^C into DNA in the course of 

 the intermitotic cycle. The total cycle time 

 was found to be of the order of 40 to 48 

 hours for the average dividing bone marrow 

 cells. DNA synthesis took place during 12 

 to 15 hours in the second half of the cycle 



100 



80 - 



CD 60 

 on 



40 



20- 





if 



i 





I 



I 



1 



AC 



MR 



Fig. 5. Percentage of 

 cells showing autoradio- 

 graph after 140 r X-rays 



as is seen in Fig. 6. 



followed by 24 hours of ^^^^ ^nd was divided from the mitosis by a 



32P. Hatched columns: ., ^ r ^ ii • • • i 



,. , , , , three to lour hour non-synthesizing period, 



unirradiated controls. "^ 



Empty columns: unirra- 

 diated. "aC=AU cells of Vi- 

 cia faba in the final 2 mm 

 of the root. MR = Meriste- 

 matic resting nuclei. D 

 = Dividing cells. 



The important results obtained by Ho- 

 ward and Pelc and Lajtha, Oliver, 

 Ellis and Suit help us also to understand 

 why exposure to radiation often depresses 

 DNA formation to one to two thirds of that 

 taking place in the non-irradiated tissue 

 only. In such cases where exposure to radiation interferes to a minor 

 extent only or does not interfere at all with DNA synthesis, as in the 

 case of refractory tumours, the above explanation clearly does not suf- 

 fice. We shall revert to this point. 



M- 



25-30 hours 



G, 



12-15hours 3-4hours 



M 



S 



Gfl 



Fig. 6. The cell cycle to DNA synthesis, measured by the 

 incorporation of ^^P or adenine-^*C into DNA. 



Interference with the DNA molecule 



In view of the complexity of the processes involved it cannot be 

 stated with certainty, but it is very probable, that irradiation does not 

 block or mainly block the formation of intermediates of DNA formation 



