540 L. G. LAJTHA 



occurring in bone marrow, 35 her observations are not surprising. In any 

 case in short term experiments in vitro, unless large enough doses are given 

 to depolymerize significant amounts of DNA in situ (requiring doses of the 

 order of 10 5 -10 6 r.), even cell death would not decrease the amount of DNA 

 present. 



Investigating formate-C 14 incorporation into DNA, Totter found a 50% 

 depression in rabbit bone marrow cells in vitro, 61 3 hours after 3000 r. and 

 even greater depressions were reported in cultures of human bone marrow 

 cells after 5000 r., using P 32 or adenine-C 14 incorporation as the index of 

 DNA synthesis. 7 These latter investigations, based on autoradiography of 

 cells, indicated a "complete inhibition" of uptake. In view of later experi- 

 ments it should be emphasized that this was an overstatement. In autora- 

 diographic studies cells showing significant grain counts above background 

 are scored; if the grain counts are not very high then a 50-70% depression 

 of uptake per cell may depress the grain count to near background level 

 and the cells may be scored "negative." Later investigations with better 

 labels (P 32 is a poor label for any autoradiographs and adenine-C 14 is not 

 ideal for DNA work as cells have to be treated with ribonuclease prior to 

 autoradiography) indicated that the depression of uptake is in fact of the 

 order of 50% after 2000-5000 r. 62 



5. The Question of Differential Effects of 

 Radiation on DNA Synthesis 



Harrington and Lavik suggested that there may be a differential effect 

 of radiation on the incorporation of different precursors into DNA. 63 They 

 have injected P 32 , orotic acid-C 14 or formate-C 14 into rats 30 minutes after 

 100 r. TBR and isolated DNA from thymus 24 hours later. The incorpora- 

 tion of both P 32 and orotic acid-C 14 was depressed by 50-60%, and so was 

 formate-C 14 incorporation into thymine and guanine. However, adenine-C 14 

 incorporation was unaffected, and formate-C 14 incorporation into adenine 

 showed inconsistent depression. 



These experiments were repeated in the Donner laboratory 64 ' 65 but the 

 findings of Harrington and Lavik were not confirmed. Incorporation of 

 both P 32 and adenine-C 14 was depressed by about 40 %, under experimental 

 conditions similar to those of Harrington and Lavik. 



In chick embryos suspended in saline in vitro, Passonneau and Totter 



61 J. R. Totter, Radiation Research 1, 232 (1954). 



62 L. G. Lajtha, R. Oliver, T. Kumatori, and F. Ellis, Radiation Research 8, 1 (195S). 



63 H. Harrington and P. S. Lavik, Arch. Biochem. Biophys. 54, 6 (1955). 



64 E. L. Bennett and B. Krueckel, Univ. Calif. Radiation Lab. Rept. No. 2827/28 

 (1955). 



65 L. S. Kelly and E. L. Bennett, Radiation Research 5, 485 (1956). 



