Covalent linkage of adjacent Thymine-Thymine (T^T) pairs

Pyrimidine dimers (typically between adjacent T^T bases) form when DNA is exposed to Ultraviolet light. The covalent linkage of adjacent T residues as a four-carbon cyclo-butyl ring is reversible by photoreactivation.

Figure ©2012 TA Brown, Introduction to Genetics (1st ed.); additional text ©2014 by Steven M. Carr