PyrimidinDimere

PyrimidinDimere, or pyrimidine dimers, are covalent linkages that form between adjacent pyrimidine bases in nucleic acids when they are exposed to ultraviolet (UV) light. The formation of these photoproducts distorts the DNA or RNA helix and can impede replication and transcription if left unrepaired.

The two main classes of pyrimidine dimers are cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PP). CPDs arise when a cyclobutane ring forms between the C5 and C6 atoms of adjacent pyrimidines, commonly thymine-thymine (TT) sequences but also occurring with thymine-cytosine (TC) and cytosine-cytosine (CC) combinations. 6-4PPs involve a covalent bond between the C6 atom of one pyrimidine and the C4 atom of the neighboring pyrimidine, producing a distinct type of distortion. In DNA, thymine-containing dimers are the most frequently observed; in RNA, uracil-containing dimers can form under UV exposure as well.

Biological significance stems from their ability to block DNA replication and transcription, leading to mutations if not repaired. In humans and many organisms, pyrimidine dimers contribute to UV-induced skin damage and carcinogenesis, among other effects. The persistence of dimers increases mutational risk if the lesion is misread or bypassed during replication.

Repair of pyrimidine dimers is primarily accomplished by nucleotide excision repair (NER), which recognizes distorted DNA and excises a short damaged segment. In some organisms, photoreactivation enzymes (photolyases) use light energy to reverse CPDs directly. Detection methods for these lesions include antibody-based immunoassays and sequencing or gel-based approaches that identify DNA distortions.

Overall, pyrimidine dimers are key photoproducts formed by UV exposure with important implications for genomic stability and cellular health.