Single strand break radicals

Thus, the AR directly protect DNA from the UV-damage. This is manifested in the saving of the total amount of this biopolymer, preventing its deep degradation due to formation of double-stranded breaks, as well as preventing the single -stranded breaks without transition of supercoiled into the relaxed circular form of DNA. Evidence of effects increased with length of the alkyl radical of the AR molecule and with AR concentration increase. 

DNA-DNA crosslinks develop with time following exposure to HgCl2, probably resulting from its ability to interact directly with the DNA bases [253], while single-strand breaks may result from the production of oxygen radicals by mercuric chloride and also by its interaction with DNA bases [254, 255], The single-strand breaks resemble those induced by X-rays [256]. 

Ward, J.F., Blakely, W.F., and Joner, E.I., 1985, Mammalian cells are not killed by DNA single-strand breaks caused by hydroxyl radicals from hydrogen peroxide, Radiat.Res. 

There seems, however, to be a problem with the above reasoning. If all the free radical damage ends up on the DNA bases, then one has problems explaining significant lesions such as single-strand breaks. How then does one explain strand breaks The question could be rephrased by asking if there could be damage to the ribose-phosphate which is difficult to detect. 

Figure 5 Individual examples of simulated sites of damage induced by 3.2 MeV alpha particles in DNA. In each example, the outer and inner rows represent the sugar-phosphate moieties and the pairs of bases, respectively, with single base pair resolution (dots). An x or H represent energy deposition or reaction of hydroxyl radical leading to induction of a single strand break or base damage. A indicates hit sites that did not lead to strand breaks (SB) or base damage (BD). Nomenclature no strand break (No SB) single strand break (SSB), (SSB ), (2SSB) double strand break (DSB), (DSB+), (DSB + ).

Millar BC, Sapora O, Fielden EM, Loverock PS (1981) The application of rapid-lysis techniques in radiobiology. IV. The effect of glycerol and DMSO on Chinese hamster cell survival and DNA single-strand break production. Radiat Res 86 506-514 Milligan JR, Ward JF (1994) Yield of single-strand breaks due to attack on DNA by scavenger-derived radicals. Radiat Res 137 295-299... 

Ward JF, Blakely WF, Joner El (1985) Mammalian cells are not killed by DNA single-strand breaks caused by hydroxyl radicals from hydrogen peroxide. Radiat Res 103 383-392 Ward JF, Webb CF, Limoli CL, Milligan JR (1990) DNA lesions produced by ionizing radiation Locally multiply damaged sites. In Wallace SS, Painter RB (eds) Ionizing radiation damage to DNA Molecular aspects. Wiley-Liss, New York, pp 43-50... 

Bernhard and co-workers have performed a series of experiments to determine the mechanisms of DNA strand breakage by direct ionization of plasmid DNA. A big surprise in this work was the discovery that the total yield of single strand breaks exceeds the yield of trapped sugar radicals. Even at very low hydration levels (2.5 waters per nucleotide residue) nearly 2/3 of the strand breaks are derived from precursors other than deoxyribose radicals [74], The authors conclude that a majority of the strand breaks observed do not result from dissociative electron capture, homolytic bond cleavage from excited states, or from hydroxyl radical attack. Rather, the authors conclude that doubly oxidized deoxyribose is responsible for the high yield of strand breaks. 

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