Thymine dimers bend DNA.

A 32-base-pair DNA fragment containing a thymine photodimer was constructed and ligated head-to-tail to obtain multimers of this sequence in which thymine dimers were in phase with the helix screw axis (approximately equal to 3 turns apart). The ligation products were analyzed by one- and two-dimensional gel electrophoresis and quantitative electron microscopy. These analyses show that the thymine photodimer introduces a bend of approximately equal to 30 degrees in DNA, which causes anomalously slow migration of DNA fragments in polyacrylamide gels and facilitates the formation of small covalent circles. Repair of thymine dimers by DNA photolyase abolishes the anomalous migration.