Experiments were designed to determine whether or not cultured human cells recover from sublethal ultraviolet light injury during DNA-synthesis (S) phase as rodent cells do. Human liver (Chang) cells were synchronized by 2.5 mM thymidine or hydroxyurea blockage of DNA synthesis or by mitotic selection of cells previously blocked by excess thymidine. Cell survival was determined on the basis of colony formation. Following all three synchrony methods cell survival increased from the early S phase to the end of the division cycle. Cells irradiated in the G1 phase and allowed to progress through the ensuing prolonged (about 30 hr) S phase survived subsequent irradiation as if they had not been irradiated. It was concluded that Chang liver cells recover from sublethal injury inflicted by ultraviolet light.

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