GENETIC AND MOLECULAR ANALYSIS OF DNA DAMAGE REPAIR AND TOLERANCE PATHWAYS - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 106

and other cellular metabolic processes that result in formation of single and double
strand breaks. Thus, in addition to the UV-induced damages discussed above, cells must
cope with oxidized purines, oxidized bases and abasic sites (sites of base loss), as well
as single and double strand breaks. To deal with these UV-induced and other DNA
alterations, cells have developed several distinct yet in some cases overlapping repair
pathways.
Figure 2. DNA damages induced by normal cellular metabolism in air.
3. Photorepair
Pyrimidine dimer photoreactivation is a one-enzyme repair path. The enzyme
photolyase binds specifically to CPD in double stranded DNA, forming an enzyme-
substrate complex. That complex absorbs a photon in the long UV or visible range,
Figure 3. Mechanism of photorepair of CPD in DNA by photolyase.