Cyclobutane pyrimidine dimer

Cyclobutane pyrimidine dimer (CPD) is a type of DNA damage that occurs when two adjacent pyrimidine bases, specifically thymine or cytosine, in the DNA strand form a covalent bond, creating a cyclobutane ring. This process is typically triggered by exposure to ultraviolet (UV) radiation, particularly UVB light.

Formation

The formation of a CPD is a direct result of the absorption of UVB light by the DNA molecule. The energy from the UVB light is absorbed by the pyrimidine bases, causing them to enter an excited state. In this state, the pyrimidines can react with each other to form a cyclobutane ring, creating the dimer. This reaction is a photochemical reaction, meaning it is driven by light.

Effects

The formation of a CPD distorts the DNA helix and interferes with normal DNA replication and transcription. This can lead to mutations, which can potentially lead to cancer, particularly skin cancer. CPDs are the primary cause of mutations induced by sunlight, and are therefore a major factor in the development of skin cancer.

Repair

Cells have several mechanisms for repairing CPDs. The primary mechanism is nucleotide excision repair (NER), which involves the removal of the dimer and the synthesis of a new DNA strand in its place. Another mechanism is photoreactivation, which involves the use of an enzyme called photolyase that can reverse the formation of the dimer in the presence of light.

See also

• DNA repair
• Ultraviolet light
• Skin cancer

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