One way that UV can damage DNA (and cause mutations, kill cells, and cause skin cancer) is by fusing together two adjacent pyrimidine rings to produce a molecule 6-4PP.
The figure to the right (taken from here) shows the associated rearrangement of atoms.
What is amazing is that nature has a way to repair this mutation involving a flavoenzyme called photolyase. This paper elucidates (using ultrafast laser spectroscopy) the repair mechanism which is summarised in the schematic below. It contains steps involving electron transfer and proton transfer.
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