5,6-Dihydro-5(α-thyminyl)thymine

5,6-Dihydro-5(α-thyminyl)thymine, also known as T<>T or Thy<>Thy, is a cyclobutane pyrimidine dimer (CPD) formed by the photochemical reaction of two adjacent thymine bases in DNA. It's a specific type of DNA damage induced primarily by ultraviolet (UV) radiation.

CPDs like 5,6-Dihydro-5(α-thyminyl)thymine disrupt the normal structure of DNA and can interfere with processes such as DNA replication and transcription, potentially leading to mutations and cell death. The formation of this dimer involves the saturation of the 5,6-double bond of the thymine base and the formation of a cyclobutane ring between the two thymine molecules. The "α" notation refers to the specific stereochemistry and connection between the two thymine moieties within the dimer structure.

Repair mechanisms exist within cells to correct this type of DNA damage. One primary repair pathway is nucleotide excision repair (NER), which recognizes and removes the damaged segment of DNA containing the dimer, followed by DNA synthesis to fill in the gap using the undamaged strand as a template. Another repair mechanism, photoreactivation (not present in all organisms, including humans), utilizes the enzyme photolyase and visible light to directly reverse the dimerization process.

The level of 5,6-Dihydro-5(α-thyminyl)thymine in DNA can be used as a biomarker of UV radiation exposure and DNA damage. Its detection and quantification are important in fields like photobiology, dermatology, and cancer research.