Size and frequency of gaps in newly synthesized DNA of xeroderma pigmentosum human cells irradiated with ultraviolet light.

Native newly synthesized DNA from human cells (xeroderma pigmentosum type) irradiated with ultraviolet light releases short pieces of DNA (L-DNA) when incubated with the single-strand specific S1 nuclease. This is not observed in the case of unirradiated cells. Previous experiments had shown that the L-DNA resulted from the action of S1 nuclease upon gaps, i.e., single-stranded DNA discontinuities in larger pieces of double-stranded DNA. We verified that the duplex L-DNA, that arises from the inter-gap regions upon S1 nuclease treatment, has a size which approximates the distance between two pyrimidine dimers on the same strand; this has been observed at different fluences of ultraviolet-light and indicates that the gap is related to or opposite the dimer. A method was devised to measure the size of the gaps. A Poisson distribution analysis of the percentage of the L-DNA produced as a function of S1 nuclease concentration made this possible. 65% of the gaps corresponded to stretches of 1,250 nucleotides and 35% to stretches of 150 nucleotides. These parameters have been considered in the proposition of a model for DNA synthesis on a template containing pyrimidine dimers.