The reactive oxygen species may impose numerous damages to cellular DNA. Among them, formation of stable products resulting from oxidation of nucleobases is observed. These species constitute class of heterocyclic compounds with great heterogeneities of their physico-chemical properties. The modified nucleobases significantly differ from their canonical protoplasts by tautomeric equilibriums, protolytic properties in the gas phase or water solution and consequently by pK_a or pK_b values. The modified nucleosides are also characterised by profoundly altered aromaticity, oxidative susceptibility and N-glycosidic bond stabilities. First of all, however, they have overwhelmingly altered pairing properties, which are directly responsible for observed cytotoxic properties of these lesions. Besides, since many analogues are structurally different with respect of canonical bases their presence in DNA must impose many energetic, structural and dynamic modifications. Above aspects are discussed across the manuscript.