The most lethal region of the ultraviolet light spectral region is 260 nm, for it is this wavelength that is maximally absorbed by nucleic acids. Even when exposure to ultraviolet rays is insufficient to cause death, mutations are generated. These thymine dimers are recognized and bound by the enzyme called photolyase. Upon exposure to visible light, photolyase is activated and the enzyme cuts the covalent bonds between the adjacent thymine residues and restores the structure to the DNA molecule. Hence, bacteria exposed to ultraviolet radiation followed by exposure to bright light have more cells capable of survival and growth than a similar culture exposed to ultraviolet light alone.
Humans do have a repair system to remove UV induced thymine dimers.
Rather than having a single enzyme just cut covalent bonds of the dimers to restore DNA structure, our repair system utilizes several enzymes to actually excise a small piece of DNA containing the dimer followed by new DNA synthesis to replace the piece that was removed. We know that this is very important in preventing DNA damage. Individuals that lack this repair system, suffer from a genetic defect called xeroderma pigmentosum. Such individuals are very sensitive to sunlight and usually are afflicted with cancer by their teenage years. Ultraviolet light is very injurious to the eyes and skin.