Damaged Reversal OR Photo Reactivation, Ultraviolet Light, Monomerization, Cytosine, Thymine Dimers, Prokaryotes, Eukaryotes, Pyrimidine

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	Damaged Reversal OR Photo Reactivation -Exposure of a cell to ultraviolet light can result in the covalent joining of two adjacent pyrimidines producing a dimer. Although cytosine-cytosine, cytosine thymine dimers are also formed, the principal products of UV irradiation are thymine thymine dimers. These thymine dimers prevent DNA polymerase from replicating the DNA strand beyond the site of dimer formation. In E coli, an enzyme called DNA photolyase (deoxyribodipyrimidine photolyase or photo reactivating enzyme) detects and binds to the damaged DNA site.
Then the enzyme absorbs energy from visible light, which activates it so it can break the bonds holding the pyrimidine dimer together. The enzyme then falls free of the DNA. This enzyme thus reverses the UV induced dimerization. Photo reactivation or photo-restoration is a light dependent DNA repair mechanism in which certain types of pyrimidine dimers are cleaved. This repair pathway is found in many prokaryotes and lower eukaryotes but absent in higher eukaryotes. Photo reactivation should not be confused with other, non enzymatic mechanisms of monomerization.