The extractability of DNA from u.v.-irradiated HeLa cells following lysis is an inverse function of the u.v.-dose. The effect was examined immediately after exposures ranging from 500 ergs/mm 2 to 3000 ergs/mm 2 , and the cells were irradiated either at room temperature (22c) or in cold (4°c). The unextracted DNA was recovered from the protein fraction with 0.5 per cent trypsin solution. Dose-response curves show that the magnitude of the effect increased linearly with dose, and the amount of DNA recovered from irradiated cells decreased for about 1 per cent for every 100 ergs/mm 2 of u.v. light for cells exposed at room temperature. The effect is twice as large, when the cells are irradiated in cold. The amount of DNA cross-linked to proteins remained unchanged from 0 up to 10 hours after exposure, i.e. for as long as the experiments were carried out.

No abstract