Radiation, nucleotide excision-repair

DNA base damage is also frequent after radiation (Ward, 1986). As compared to ssb, this DNA lesion is regarded as a possible source of mutation and is repaired through specific DNA repair pathways, such as base excision repair and nucleotide excision repair. 

The product yields in y-irradiated DNA are given in Tables 12.5-12.7. FAPY-G has always been observed, but 8-oxo-G yields were extremely low, when y-ir-radiations were carried out under N2. This may serve as a caveat for the common practice to use 8-oxo-G as a kind of marker for free-radical DNA damage (for assays see Chap. 13.2). Besides the other products reported in Table 12.5, 8 cG lesions per 106 bases are formed per Gy in y-irradiated N20-saturated DNA (50 pg ml-1) solution (Dizdaroglu et al. 2001a corresponding to 1.6 x 10 9 mol J-1), and this lesion has also been observed in y-irradiated cultured human cells (Dizdaroglu 1986 for its elimination by nucleotide excision repair see Kuraoka et al. 2000). 

Ionizing radiation can be a cause of DNA breaks (Hutchinson, 1985 Ward, 1998). However, DNA breaks are also induced through processes of DNA repair (Friedberg et al., 1995). In mammalian cells, there are two major repair pathways nucleotide excision repair (NER) and base excision repair (BER) (Friedberg et al., 1995 Lindahl and Wood, 1999). However, DNA breaks produced during the two repair pathways are characteristically different. 

Figure 2 DNA damage induced by ionizing radiation. A) DNA damage and repair. All the constitutive elements of DNA (sugar-phosphate backbone and bases) are possibly modified by ionizing radiation. Single strand breaks (SSB), oxidized bases and abasic site are processed by base excision repair (BER), double strand breaks (DSB) by homologous recombination and non homologous end joining (HR and NHEJ) and DNA-protein crosslinks by nucleotide excision repair (NER). B) Quantitative measurement of radiation-induced and spontaneous DNA damage.

In addition, and while UVR-mediated DNA damage occurs in aquatic autotrophic organisms [168,193-196], repair mechanisms of the DNA molecule (see Chapter 9) are also present [193]. However, the presence of one or other mechanism (i.e., photoreactivation, nucleotide excision repair or recombination repair) is clearly dependant on the species under study and the radiation conditions at which the cells are exposed (see Chapter 9). 

Nucieotide excision- repair Spontaneous, chemical, or radiation damage to a DNA segment Removal of an approximately 30-nucleotide oligomer and replacement... 

Sensitivities to X, y and UVC-radiations have been related to impaired DNA repair processes LY-S cells have been foimd to repair DNA dsb (double strand breaks) more slowly than the more radiation-resistant LY-R cells (3) LY-R cells are nucleotide excision-deficient, in contrast with the more UVC-resistant LY-S cells (4,5). In this work we review experimental data (5, 6-8) concerning LY cell response to combined UVC + Bz (benzamide) and Bz in combination with X or y rays. These results are best explained by assuming an impaired DNA ligase function in LY-S cells. 

Response to UVC Radiation and Bz. Similar features of the cellular response were examined after UVC + Bz treatm t as in the case of X or y rays + Bz (5). The results are summarized in Table 2. In contrast with LY-R cells and numerous other cell lines, LY-S cells were sensitized to UVC radiation by Bz. As in the case of X or y + Bz treatment, die distinct features of LY-S cell response to UVC + Bz treatment can be explained by low ligase I activity and hence, the necessity to activate ligase n. In LY-R cells, as in xeroderma pigmentosum cells (15), the rate of DNA incisions seems too low to activate poly(ADP-ribose) polymerase. On the other hand, LY-R cells may synthesize NAD+ more efficiently than LY-S cells (Table 1), and be able to maintain a stable NAD+ level in spite of poly(ADP-ribose) polymerase activation. In fact, when a double-labelling modification of the DNA unwinding method was used to examine the difference in sb frequency between Bz-treated and untreated UVC irradiated cells, the results were identical for LY-R and LY-S cells. We interpreted this as an indication of a Bz-sensitive base excision repair system (16,17) operating in both cell strains (in contrast with a nucleotide excision functional only in LY-S strain). Lack of nucleotide excision presumably makes LY-R cells sufficiently sensitive to UVC radiation that the base excision repair is not limiting for survival. 

The 5 -3 exonucelase activity plays two known roles. The first of these is the excision of RNA primers in lagging strand replication (Figures 24.4 and 24.5) via a nick translation mechanism in which the 5 -3 exonuclease excises ribonucleotides just as the polymerase is replacing them with deoxyribonucleotides. The second known function of the 5 -3 exonuclease is to cleave nucleotides from DNA. This may be an important function for repair of DNA that has been damaged by radiation or chemicals. Again, the mechanism involves nick translation. 

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