Pyrimidine Dimer

**Semantic type:** Nucleic Acid, Nucleoside, or Nucleotide

**Definition:** A pair of pyrimidine bases (thymine or cytosine in DNA or uracil in double-stranded RNA) within a nucleic acid covalently linked by a cyclobutane ring, the formation of which was induced by exposure to ultraviolet radiation. The presence of pyrimidine dimers inhibits DNA replication; improper repair of these lesions promotes DNA mutagenesis.