Photoreactivation or Light repair of DNA

Ultraviolet light is a physical mutagen and can induce mutation. Ultra violet radiation (254 nm) causes formation of pyrimidine dimers (cyclobutane ring), when two pyrimidine bases occurs together in single strand of DNA.
Thymine dimer is most common one but cytosine dimer as well as thymine-cytosine may also occurs. Thymine dimer is a state in which two adjacent thymine molecules are chemically joined distorting the structure of DNA, so that impeding transcription and replication process.
This pyrimidine dimer formation is lethal to the cell unless it is corrected. A repair mechanism known as photo reactivation can repair this mutation.
When UV radiated population of bacteria is subsequently exposed to visible light of wave length of 300-450nm, the survival rate increases and frequency of mutation decreases. This is due to activation of photo reactivating enzyme photolyase, which splits thymine dimer. In the dark, the enzyme bind with thymine dimer and in presence of visible light the enzyme split the thymine dimers.
Upto 80% of thymine dimers existing in genome can be photoreactivated.