Several Systems Exist for DNA Repair

Adjacent pyrimidine bases in a DNA strand form dimers with high efficiency after absorbing ultraviolet light (fig. 26.16). By contrast, purines are quite resistant to damage by ultraviolet. Pyrimidine dimers formed within an otherwise intact DNA duplex have provided a useful substrate to assay for DNA repair. These dimers can be repaired directly by enzymatic photoreactivation (fig. 26.17). The 

Structure of a thymine dimer formed in DNA by exposure to short-wavelength ultraviolet light. 

Thymine dimers may be monomerized from DNA by enzymatic photoreactivation. In this case no nucleotides are removed in the repair reaction. 

Pyrimidine dimers and other forms of DNA damage can be removed by a general excision repair mechanism. The first reaction in this form of repair involves forming nicks about the damaged region of the DNA. In (a) we see the mode of incision of UV-irradiated DNA by the pyrimidine-dimer-specific glycosylase and AP endonuclease 

Systems that function with the help of visible light are quite common. This relates to the fact that sunlight is a mixture of ultraviolet and visible light. The potentially harmful effects of moderate doses of ultraviolet are overcome by the repair processes triggered by the visible light. Ultraviolet tanning lamps usually eliminate most of the visible light and consequently could be quite harmful if used excessively or even in moderation. 

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