Repair of DNA strand breaks after gamma-irradiation of protoplasts isolated from cultured wild carrot cells

We have isolated protoplasts of cultured wild carrot (Daucus carota L.) cells, lysed them directly on top of alkaline sucrose gradients, and measured single-stranded DNA of molecular weight 1·10⁸ by velocity sedimentation. DNA sedimentation studies on γ-irradiated protoplasts indicate that the energy absorbed in DNA per strand break is 85 eV in air and 260 eV in nitrogen. Isolated wild carrot protoplasts can repair 50% of the DNA strand breaks within 5 min after a dose of 20 krad, and by 1 h none can be detected.