Question

Illustrate the biochemical mechanism and the effect of UV in DNA (in detail). Draw your own figure to explain your answer.

Answer 1

The main source of UV radiation is sunlight, others are UV lamps and welder's arcs. UV light penetrates the skin for a few millimetres only so that its effect is limited to the epidermis. The UV radiation spectrum can be divided into three wavelength ranges: UV-A (320-400nm), UV-B (280-320nm) and UV-C (200-280nm). Of these UV-B is believed to be responsible for induction of cutaneous cancer. UV-C is not considered significant mutagen because it is filtered out by the ozone layer surrounding the earth.

UV-B leads to the formation of pyrimidine dimers in DNA. If the energy in a photon of UV light is absorbed by DNA, the result is a chemical reaction that leads to covalent cross linking of pyrimidine bases, particularly adjacent thymidine residues in the same strand of DNA. This distorts the DNA helix and prevents proper pairing of the dimer with bases in the opposite DNA strand. Pyrimidine dimers are repaired by the nucleotide excision repair pathway. Defective nucleotide excision repair pathway leafs to an autosomal recessive disorder called Xeroderma pigmentosum.