No abstract available

[en] Some current controversies concerning the mechanism of post- replication repair in mammalian cells are discussed. We have found that two xeroderma pigmentosum cell lines normal in excision-repair are abnormal in postreplication repair. The rate of postreplication repair is slower than that in normal cells and is caffeine sensitive. In normal human fibroblsasts, caffeine has little effect

No abstract available

[en] During replication of DNA after ultraviolet irradiation, gaps are left in the newly-synthesized DNA strands in both bacterial and animal cells and these gaps are subsequently sealed by a process known as postreplication repair. In order to test whether it is the ultraviolet-induced pyrimidine dimers which are responsible for the production of these daughter-strand gaps in animal cells, we have used chick embryo fibroblasts. In these cells the pyrimidine dimers are photoreactivable, i.e. they can be split by an enzymatic process dependent on visible or near ultraviolet light. Our results indicate that chick cells possess a postreplication repair system similar to that in mammalian cells; gaps are produced in the newly-synthesized strands and then filled in. If the ultraviolet-irradiated cells are first photoreactivated to remove most of the dimers, the number of daughter-strand gaps produced is much less than without photoreactivation. This suggests that the dimers are indeed responsible for the formation of many of the gaps in the newly-synthesized DNA. Ultraviolet light also inhibits the overall rate of DNA synthesis. This inhibition is, however, only partly overcome by photoreactivation

[en] The ability to repair damage in DNA is of paramount importance to man. A defect in this ability has drastic consequences, giving rise to disorders of an extremely serious nature. To elucidate the mechanisms of repair of DNA human mutants, above all cells of xeroderma pigmentosa strains, were investigated as to their response to UV and gamma radiation, respectively, as well as to their sensitivity to chemical mutagens

No abstract available

[en] After 16 Jm^-₂ of UV-irradiation non-dividing normal cells recover normal rates of RNA synthesis within 24 h, whereas in cells from donors with Cockayne syndrome (CS) the rate of RNA synthesis gradually declines. Cultures of a mixed population from 2 CS donors were fused with polyethylene glycol: subsequently they were UV-irradiated and RNA synthesis was measured autoradiographically in mono-, bi-, and multinuclear cells. Genetic complementation was indicated by high levels of RNA synthesis in bi- and multinuclear cells when compared with mononuclear cells. Using this assay, 11 CS strains have been assigned to three complementation groups: 2 into group A, 8 into group B and 1 into group C. The strain in group C is derived from an individual who also had xeroderma pigmentosum (XP), and was the sole known representative of XP-complementation group B. (orig.)

[en] Pyrimidine dimer sites associated with the newly-synthesized DNA were detected during post-replication repair of DNA in UV-irradiated human fibroblasts. These pyrimidine dimer sites were inferred from a decrease in the molecular weight of pulse-labelled DNA after treatment with an extract of Micrococcus luteus containing UV-specific endonuclease activity. In DNA synthesized immediately after irradiation the frequency of these daughter strand dimer sites was 7-20% of that in the parental DNA. Such sites were found in fibroblasts from normal donors and from xeroderma pigmentosum patients (with defects in excision-repair or post-replication repair). They were excised from the DNA of normal cells. As the time between UV-irradiation and pulse-labelling was increased, the frequency of dimer sites associated with the labelled DNA decreased. If the pulse-label was delivered 6 h after irradiation of normal cells or excision-defective xeroderma pigmentosum cells, no dimer sites were detected in the labelled DNA. It has usually been assumed that daughter-strand dimer sites were the result of recombinational exchanges. The assay procedure used in these experiments and in similar experiments of others did not distinguish between labelled DNA containing pyrimidine dimers within the labelled section, and labelled DNA which did not contain pyrimidine dimers but was attached to unlabelled DNA which did contain dimers. The latter structures would arise during normal replication immediately following UV-irradiation of mammalian cells. Calculations are presented which suggest that a significant proportion and conceivably all of the dimer sites associated with the daughter strands may have arisen in this way, rather than from recombinational exchanges as has been generally assumed. (author)

[en] The relationship between pyrimidine dimers (measured as endonuclease-sensitive sites) and newly-synthesized DNA was examined in UV-irradiated normal and XP variant human fibroblasts. Following irradiation of normal cells, the frequency of pyrimidine dimer sites in sections of DNA which had been synthesized immediately before the UV-irradiation was similar to that in the bulk DNA. The frequency of pyrimidine dimer sites in the parental strands of replicating DNA in irradiated normal cells was similar to that in the bulk DNA. In UV-irradiated XP variant cells, the size of DNA synthesized in the presence of caffeine immediately after irradiation accurately corresponded with the average interdimer distance in the parental DNA. This suggested that in this experimental situation each pyrimidine dimer gives rise to a discontinuity or a termination site in the daughter strand. (author)

No abstract available