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DNA/RNA photohydrates represent a class of well-known biomolecular lesion formed by the absorption of near- to mid- UV light. They are formed via a photo-induced nucleophilic hydrolysis reaction in which water is split (via nucleobase sensitisation) into H + OH radicals. These nascent radicals can then add across C5=C6 - forming a saturation of the preexisting double bond. If un-repaired, such lesions can lead to mutagenic carcinogenesis - which is responsible for several forms of cancer. Using high-level electronic structure theory (CASPT2), we map the key excited state reaction paths associated with the reactivity of DNA (guanine and thymine) and RNA (uracil) nucleobases with water. At the outset, we consider the intrinsic reactivity in the isolated gas phase - in which the water (cluster) + chromophore complex is free from environmental perturbations. We then extrapolate the thymine nucleobase to the bulk DNA environment in aqueous solution in order to ascertain the relative importance for hydrate formation in a more complex biological environment. In this latter study we use high-level mixed quantum/classical (QM/MM: CASPT2/AMBER) methods.