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Thymocytes were irradiated with fast electrons up to 6 Mrad in the presence and absence of oxygen. The cells were treated before irradiation with a cold shock to prevent any repair rejection during irradiation. The DNA isolated subsequently was analysed for double-strand breaks (dsb), actual single-strand breaks (ssb) and alkali-induced strand breaks (alisb). We observed a linear increase of all types of lesions with dose and an o.e.r. for dsb of 3-6, for ssb of 4-9 and for alisb of 2-1. The data do not deviate significantly from those, measured on thymocytes irradiated without cold shock. In DNA of irradiated thymocytes, the frequency of 3' and 5' hydroxyl and 5' phosphate end-groups was analysed enzymatically. In both the ssb and alisb, about 11 per cent of the terminals carry 5'OH end-groups and 20-40 per cent 5' phosphate groups. On the 3' terminals, 60-80 per cent of the ssb are identified as 3'OH end-groups, whereas on the alisb only a small amount of 3'OH end-groups if found. The frequency of characterized end-groups shows the same oxygen effect as the corresponding strand breaks. Therefore, in the presence and absence of oxygen, the same mechanism may be responsible for formation of DNA strand breaks in vivo.

Purified, dry DNA and dry nucleoprotein were irradiated with 60Co-γ-rays under a nitrogen or oxygen atmosphere. The DNA was isolated from the irradiated nucleoprotein. In the DNA the following radiation induced changes were investigated: Double strand breaks, single strand breaks and crosslinks between the DNA molecules. An oxygen effect (OER > 1) was found for all of these events except for crosslinks in irradiated DNA. In the nucleoprotein, the oxygen effect is more marked than in pure DNA.

Irradiation of aqueous solutions of DNA causes liberation of free nucleobases and of compounds which behave chromatographically like nucleosides. These components can be separated from irradiated DNA by DEAE-Sephadex chromatography. Their amount increases linearly with dose, resulting in a G-value of 0'16 to 0·18. Further separation of the liberated products by ion-exchange chromatography on Dowex 50 x4 and by thin-layer chromatography on cellulose plates shows that about 60 per cent of the liberated material consists of nucleosides, part of which carries a damaged sugar molecule. Different methods used to determine the formation of malonic dialdehyde in irradiated DNA lead to the conclusion that this type of sugar damage does not contribute very much to the breakage of the DNA backbone. Several mechanisms leading to single strand breaks in DNAare discussed.