ABSTRACT
The spectrum of damage that cellular DNA can suffer is extensive. Although various types of environmental factor are hazardous because they can bring about specific damage to DNA, considerable DNA damage appears to arise from the activities of reactants generated naturally within cells themselves. In the face of this range of problems, organisms have evolved an impressive array of enzymatic repair systems to restore the normal nucleotide sequence and structure of damaged DNA. Moreover, these systems are capable of repairing damage caused by both synthetic and naturally produced reactants. This chapter aims to review very broadly the various types of enzyme-based repair mechanisms that exist. These include the photolyases and alkyl transferases, which are enzymes that can reverse DNA damage directly, as well as the more complex nucleotide and base excision systems that require the participation of considerable numbers of enzymes. In addition, post-replication mismatch repair systems that can eliminate gaps arising in DNA strands at positions directly opposite damaged
nucleotides that block DNA replication are described, together with mechanisms available to facilitate the repair of double-strand DNA breaks.
