Lesions, double-stranded

Although the reduction potentials of DNA bases and UV induced DNA lesions inside a DNA double strand or inside the active site of a DNA photolyase, together with the reduction potential of the photoexcited FADH- in the photolyases, are not known, currently available redox potentials indicate that the single electron reduction of a nucleobase or a UV induced dimer lesion by a reduced and deprotonated flavin coenzyme is a weakly exothermic process. The reduced and deprotonated FADH- in its photoexcited state is... 

Lohse J, Dahl O, Neilsen PE (1999) Double duplex invasion by peptide nucleic acid a general principle for sequence-specific targeting of double stranded DNA. Proc Natl Acad Sci USA 96 11804-11808 Lonn U, Lonn S, Nylen U, Windblad G (1990) Bleomycin-induced DNA lesions are dependent on nucleosome repeat length. Biochem Pharmacol 39(1) 101-107 Lopez-Larraza DM, Bianchi NO (1993) DNA response to bleomycin in mammalian cells with variable degrees of chromatin condensation. Environ Mol Mutagen 21(3) 258—264 Lopez-Larraza DM, Padron J, Rond NE, Vidal Rioja LA (2006) Chromatin condensation and differential sensitivity of mammalian and insect cells to DNA strand breaks induced by bleomycin. Mutat Res 16 April [Epub ahead of print]... 

Steighner RJ, Povirk LF (1990) Bleomycin-induced DNA lesions at mutational hot spots imphcations for the mechanism of double-strand cleavage. Proc Natl Acad Sci USA 87(21) 8350-8354 Straney DC, Crothers DM (1987) Effect of drug-DNA interactions upon transcription initiation at the lac promoter. Biochemistry 26(7) 1987-1995... 

PCR amplification of the single-stranded (ss) DNApool will result in multiple copies of a double-stranded DNA pool. Chemical lesions occurring during the chemical synthesis of the ss DNA pool, and the possibility that some sequences are more amplified than others during PCR procedures, may result in a limited pool size and predominance of sequence motifs in the random sequence (see below). Therefore it is necessary to sequence and analyze a small number of individual sequences of the random pool (2). 

Simple experimental approaches to this problem recently started postulate that the repair of clustered DNA damage leads to conversion of nonlethal lesions, e.g., dihydrothymine, or mutagenic lesions, such as 8-OxoGuanine, into lethal double strand breaks. These early experiments have studied kinetics and influence of excision of base lesion within clustered DNA damage by E. coli and nuclear extracts [27,123-129]. 

Steenken et al. have concluded that in double-stranded DNA direct hydrogen atom abstraction from 2 -deoxyribose by G(-H) radical is very unlikely due to steric hindrance effects and a small thermodynamic driving force [94]. The EPR studies performed in neutral aqueous solutions at room temperature have shown that, in the absence of specific reactive molecules, the lifetime of the G(-H) radical in double-stranded DNA is as long as -5 s [80]. Therefore, the fates of G(-H) radicals are mostly determined by the presence of other reactive species and radicals. Thus, the G(-H) radical can be a key precursor of diverse guanine lesions in DNA. In the next section we begin from a discussion of the site-selective generation of the G(-H) radical in DNA, and then continue with a discussion of the reaction pathways of this guanine radical. 

FIGURE 25-37 Models for recombinational DNA repair of stalled replication forks. The replication fork collapses on encountering a DNA lesion (left) or strand break (right). Recombination enzymes promote the DNA strand transfers needed to repair the branched DNA structure at the replication fork. A lesion in a single-strand gap is repaired in a reaction requiring the RecF, RecO, and RecR proteins. Double-strand breaks are repaired in a pathway requiring the RecBCD enzyme. Both pathways require RecA. Recombination intermediates... 

It was surmised that X-ray-induced base damage would lead mostly to recessive lesions and that the significant damage in cells other than haploids was, therefore, of a different nature. In due course, it was shown that the principal agent of lethality was the formation of double-strand breaks, since in strains defective in doublestrand break repair, inactivation and the induction of double-strand breaks were in a 1 1 ratio (Ho, 1975). 

Whatever happens to the lesions induced by ionizing radiation other than doublestrand breaks, from the late 1960s onward, almost all research on repair of ionizing radiation damage has focussed on the repair of double-strand breaks or perhaps one should state rather that ionizing radiation has been the principal means of... 

The main types of damage that can be formed in DNA (base damage, apy-rimidnic/apurinic (AP) site, single-strand break (SSB), double-strand break (DSB), tandem lesions and various clustered lesions) are shown schematically in Fig. 12.1. There are, however, further lesions such as DNA/DNA and DNA/pro-tein cross-links. 

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