Using DNA fiber autoradiography, DNA replication in cultured fibroblasts, derived from normal donors, and from XPII patient with increased sensitivity to ionizing radiation was estimated. Here evidence is provided on the fact that the fork movement, significantly decreased in XPII cells before irradiation, remains the same after exposure to X-rays. The density of replicon clusters simultaneously operating in tandem groups, which is initially much less in XPII cells compared to normal cells, also remains unchanged after exposure to X-rays (5 Gy), since the inhibition of DNA replication occurs to individual replicons only. Our data suggest that the inhibition of DNA replication in normal cells throughout the whole cluster, that drastically reduces the rate of DNA-chain growth, may provide an additional time to restore damaged chromosomes, i. e. it is part of the cellular defence mechanism. It seems likely that XPII cells are deficient in such a defence mechanism.
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