Introduction

DNA is a highly reactive molecule that is sensitive to damage from a wide range of both physical and chemical agents. Lesions alter the structure of DNA and consequently interfere with critical aspects of DNA metabolism such as transcription, replication and recombination. The maintenance of genetic integrity is essential for cellular survival, but the eukaryotic genome constantly incurs modifications which are potentially cytotoxic or mutagenic. Such changes can result from cellular exposure to genotoxic compounds present in the environment and from normal products of cellular metabolism. An additional source of genome modifications, particularly relevant to organisms exposed to chronically high levels of natural sunlight, is the ultra-violet radiation contained within this polychromatic light source. UV-B (280–320 nm) is considered to be the most biologically effective radiation within sunlight, although longer wavelengths (UV-A, 320–390 nm, and visible light radiation) do play their part in lesion formation but to a lesser degree (Tyrrell, 1993). (More damaging UV-C radiation is screened by the atmosphere and does not penetrate to the earth's surface.)

DNA is considered to be the primary absorbing chromophore in the cell in the UV-B region of the spectrum. The peak absorption of DNA is dictated by its component nucleotides and occurs at around 260 nm, and there is a sharp drop in photons absorbed by DNA through the UV-B range and any absorbance at wavelengths greater than 320 nm is minimal (Davies, 1995).