1. Mismatched repair. Corrected to reflect the template strand. Dam methylase methylates the repair site where incorrect nucleotides were placed. Immediatley after passage of replication, template strand is methylated while the new strand is not, permitting the two strands from being distinguished from one another.

2. Abnormal bases, alkylated bases, pyrimidine dimers. Base excision repair, removal of one base by DNA glycosylases that recognize common DNA lesions and remove them. AP endonucleases cut the DNA strand containing the AP site, DNA polymerase I replaces the DNA and DNA seals the nick.

3. Lesions causing structural strain. Nucleotide-excision repair, exonuclease hydrolyzes 2 phosphodiester bonds, one on either side of the distortion.

4. Pyrimidine dimers, methylation. direct repair, pyrimidine dimers caused by UV-induced reaction.

Review: Chapter 25 Learning Objectives