DNA Repair Fix the damaged base photoreactivation - in bacteria, thymine dimers formed by UV radiation are separated by DNA photolyase using visible light as the energy source O6-methylguanine methyl transferase fixes guanines that have been methylated by alkylating agents Excision repair - remove damaged bases and use a DNA polymerase to replace them with the correct bases AP repair - DNA glycosylase removes damaged bases from the sugar, the resulting apurinic or apyrimidinic site (AP site) is removed from the DNA UV damage repair - excise area around thymine dimer and resynthesize the strand Xeroderma pigmentosum - no UV repair, high skin cancer rates and death by age 30 Mismatch repair enzymes nick DNA on both sides of the mismatch, Pol I and ligase do the rest how to recognize the "right" strand? In E. coli the "old" DNA is methylated at G*ATC Recombination repair (post-replication repair) Pol III skips the damaged region, leaving a gap RecA promotes recombination with the sister duplex (the damaged region is still there but the DNA is intact) Error-prone repair (part of SOS response) RecA also has a protease activity that attacks a regulatory protein called LexA w/o LexA the SOS genes are induced the SOS gene products allow DNA synthesis opposite damaged DNA many mistakes are made, however, it's better than nothing Bell CSU Chico Library This document is copyright of Jeff Bell Last Update: Wednesday, August 12, 1998