Repair of Thymine-Thymine dimers
T^T dimers may be repaired by two mechanisms. (a) In photoreactivation repair, the PRE enzyme activated by blue light breaks the dimer, restoring the normal base pairing. Note that blue light can affect DNA because it is at the same end of the spectrum as UV radiation. (b) In excision repair, the uvr system excises the dimer, and the gap is filled in by the proof-reading activity of DNAPol I.