Thymine base damage, distortions

When replicative DNA polymerase encounters a thymine dimer, it cannot replicate past the site. Deoxyadenylate is incorporated opposite the first thymine base in the template, but the double helix distortion induced by the thymine dimer causes the structure to be recognized as a mismatch, and the polymerase "idles" at the damage site, converting dATP to dAMP by a continual process of insertion and exonucleolytic cleavage (due to proofreading). 

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