Ultraviolet light-induced dimerization

Ultraviolet light induces the formation of dimers between adjacent thymines in DNA (also occasionally between other adjacent pyrimidines). The formation of thymine dimers interferes with DNA rephcation and normal gene expression. Thymine dimers are eliminated from DNA by a nucleotide excision-repair mechanism (Figure 1-2-4). 

Figure 6.46 Representation of two mechanisms of repair of DNA damage such as ultraviolet light-induced dimerization. The upper line represents cut and patch repair, the lower sister-strand exchange. Thick lines represent newly synthesized DNA. Source From Ref. 12.

Direct Repair Several types of damage are repaired without removing a base or nucleotide. The best-characterized example is direct photoreactivation of cyclobutane pyrimidine dimers, a reaction promoted by DNA photolyases. Pyrimidine dimers result from an ultraviolet light-induced reaction, and photolyases use energy derived from absorbed light to reverse the dam-... 

Boyce, R.P. Howard-Flanders, P. (1964). Release of ultraviolet light-induced thymine dimers from DNA in E. coli. Proc. Natl. Acad. Sci. USA 51,293-300. 

PHOTOREACTIVATIQN Direct reversal of ultraviolet-light-induced injury to cells, accomplished by postirradiation exposure to visible light involves the cleavage of pyrimidine dimers by photoreactivating enzymes. 

Exposure of DNA to ultraviolet light induces a [2 + 2] cycloaddition reaction between the double bonds of adjacent thymines. The resulting thymine dimer, containing a cyclobutane ring, prevents DNA reproduction and can lead to the development of skin cancer. 

Figure 27.46. Cross-Linked Dimer of Two Thymine Bases. Ultraviolet light induces cross-links between adjacent pyrimidines along one strand of DNA.

A thymine dimer is a covalent linked between adjacent two thymine bases within a strand of DNA. Thymine dimers arise from ultraviolet-light induced formation of covalent bonds, as shown in Figure... 

The most well-known form of ultraviolet light—induced base damage is the pyrimidine dimer. Although the hallmark repair system for this type of... 

KATIYAR s K, PEREZ A and MUKHTAR H (2000b) Green tea polyphenol treatment to hiunan skin prevents formation of ultraviolet light B-induced pyrimidine dimers in DNA , Clin Cancer Res, 6 (10), 3864-9. 

Mutagenesis can occur as the result of exposure to ionizing radiation and certain chemicals. Ionizing radiations include cosmic rays, x rays, and ultraviolet light. It is of interest to note that melanoma, caused almost exclusively by exposure to the ultraviolet radiation from the snn, is the most rapidly increasing lethal cancer in the United States. In melanoma, ultra violet radiation induces what is called thymine dimers. This means that two thymine nucleotides next to each other in a DNA sequence form an abnormal bond. Thymine dimers in a... 

Possible replication errors may arise from chemical modification of the existing bases - the action of ultraviolet light on thymine, for example, is known to induce the formation of dimers between adjacent bases along the same chain (11.131). Such dimers cannot fit satisfactorily into a double helix without distorting it, and replication errors are liable to be introduced. Chain-branching resulting from the possible presence of triesters is also a potential source of error... 

Taylor, J.-S. and Nadji, S., Unraveling the origin of the major mutation induced by ultraviolet light, the C — T transition at dTpdC sites. A DNA synthesis building block for the cis-syn cyclobutane dimer of dTpdU, Tetrahedron, 47, 2579,1991. 

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