Ultraviolet light-induced DNA damage: TT-dimer (1N4I) and photolyase (1TEZ)

Photolyases are DNA repair enzymes that remove cyclobutane pyrimidine dimers (CPD) arising in DNA strains as a result of UV light exposure.
According to wavelength tree types of UV radiation are distinguished: UVA, UVB, and UVC. Only the first two reach the Earth surface. No matter these types of radiation are relatively weak, the energy they carry is sufficient for causing chemical changes in DNA molecules [1].
UV light destroys bonds in heterocyclic parts of pyrimidine bases. If there are two adjacent pyrimidine nucleotides in a DNA strain, formation of a covalent bond between their nitrogen bases becomes possible [2]. As a result there appears a cyclobutane pyrimidine dimer that disturbs the tertiary structure of the DNA molecule.

Thymine, cytosine and thymine-cytosine dimers may exist in four different conformations: syn-syn, cis-anti, trans-syn, and trans-anti. These stereoisomers are generated at different frequencies. Cys-syn thymine dimers prevail (the picture: TT-dimer is shown in blue).
Alongside with cyclobutane pyrimidine dimers exposure to UV light may induce generation of pyrimidine-(6—4)-pirimidone photoproducts. In this case a bond between atoms 6 and 4 of adjacent pyrimidine cycles is established. These photoproducts are less abundant than cyclobutane pirimidine dimmers, and are often found within 5'-TC and 5'-CC sequences [4].

Pyrimidine dimers prevent normal DNA-polymerase activity resulting in point mutations. There exists a number of ways to repair pyrimidine dimers [5]. The simplest one is photoreactivaton, a process mediated by an enzyme DNA-photolyase, which uses the energy of visible light, preferably from the violet/blue end of the spectrum [6].

Photolyases are monomeric FAD-dependent enzymes (flavoproteins) with two chromophore groups, accepting photons [1]. Usually there are several molecules of the enzyme per cell. Different types of these enzymes responsible for repairing of different photoproducts can be found in the majority of living organisms, though in most mammals photolyases either inactive, or are not synthesized at all [7].

The image represents a photolyase molecule of blue-green algae Anacystis nidulans with a TT-dimer in its active center. The model was constructed using the data of crystal structure of the enzyme reported in 2004 [8].

More:
A plastic model of a B-DNA turn.
Molecular visualization of DNA-polymerase IV.