Abstract
WE have used cells that were derived from individuals with xeroderma pigmentosum (XP) and are defective in excision repair1 to demonstrate the appearance of ultraviolet lesions in the DNA made after ultraviolet irradiation. Normal human and XP cells were irradiated and labelled with either 3H-thymidine or 32PO4 for 6–8 h, the label was removed, and the cells were further incubated for 18 h to allow for DNA chain elongation and joining of the short DNA segments synthesised after irradiation. The cells were then osmotically shocked, treated with a repair endonuclease contained in Micrococcus luteus extract for 5 min at 37° C and pH 8.0, and sedimented in 5–20% alkaline sucrose gradients. The enzymatic assay2 and the sedimentation procedure3 have been described elsewhere. Figure 1 shows that daughter DNA from irradiated and non-irradiated normal human cells contains no repair endonuclease-sensitive sites. This is expected because normal human cells excise and repair most of the pyrimidine dinners formed after ultraviolet irradiation4. Figure 2 shows that DNA synthesised by irradiated XP cells does contain repair endonuclease-sensitive sites, although the number appears to be small. Under our experimental conditions there is approximately one sensitive site for every fifteen to thirty-five dimers in the parental DNA5.
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BUHL, S., REGAN, J. Repair Endonuclease-sensitive Sites in Daughter DNA of Ultraviolet-irradiated Human Cells. Nature 246, 484 (1973). https://doi.org/10.1038/246484a0
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DOI: https://doi.org/10.1038/246484a0
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