Pyrimidine dimers

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We are exposed to UV light whenever we walk into the sun. This UV light makes the chemical changes in the DNA. Within the UV spectrum UV-A (320-400 nm) reaches the earth surface, UV-A is often used in tanning booths. UV-B (295-320 nm) and UV-C (100-295nm) with wavelength of 300 nm and below are blocked by ozone layer. However in the laboratory UV-C is typically used to cause UV damage ^[1].

Ultraviolet light is absorbed by a double bond thymine and cytosine in DNA, creating covalent bonds between them ^[2]. The two common UV products are cyclobutane pyrimidine dimer (CPD) and 6, 4 photoproduct. The hallmark of UV mutations are tandem C-C→T-T, T-T→A-A mutations.

Cyclobutane Pyrimidine Dimer is a four membered ring structure between 5, 6 double bonds of pyrimidines^[3,4]. T-T dimers thymine dimers formed in between two thymine’s are most abundant of CPDs. Cyclobutane Pyrimidine Dimers (also known as cyclobutyl dimer) are repaired with in seconds by Nucleotide excision repair machinery, however if not repaired then results in GC- AT transition mutations (Purine →Purine, Pyrimidine→Pyrimidine) and could cause skin cancer.

6, 4 Photoproduct or 6, 4 pyrimidine-pyrimidone (6-4) adducts are formed with C-6 of the 5’ pyrimidine and C-4 of the 3’ pyrimidine. The frequency of occurrence of 6, 4 Photoproduct is one third of CPD. 6, 4 Photoproduct is responsible for maximum mutagenicity induced by UV ^[5].

[edit] References

David S. Goodsell, The Molecular Perspective: Ultraviolet Light and Pyrimidine Dimers, The Oncologist, Vol. 6, No. 3, 298-299, June 2001
S. E. Whitmore, C. S. Potten, C. A. Chadwick, P. T. Strickland, W. L. Morison,Effect of photoreactivating light on UV radiation-induced alterations in human skin, Photodermatol Photoimmunol Photomed. 2001 Oct;17(5):213-17
(02)00533-1a.pdf (short code: fig001hah); 2 December 2002, Expert reviews in molecular medicine
Mathews & Vanholde, Biochemistry, 2nd Edition. Benjamin Cummings Publication
Mitchell DL, Nairn RS, The biology of the (6-4) photoproduct. Photochem Photobiol. 1989 Jun;49(6):805-19