To investigate the mechanism by which UV irradiation causes S-phase-dependent chromosome aberrations and thereby genomic instability, we have developed an assay to study the DNA structure of replication forks (RFs) in UV-irradiated mammalian cells, using pulse-field gel electrophoresis for the DNA analysis. We demonstrate that replication stalling at UV-induced pyrimidine dimers results in the formation of single-strand DNA (ssDNA) regions and incomplete RF structures. In normal and in nucleotide-excision-repair (NER)-defective xeroderma pimentosum (XP) cells, stalling at dimers is rapid and prolonged and recovery depends on dimer repair or bypass.
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