Base excision repair in E. coli.

1. A base pair mismatch is recognized by DNA glycosylase, which removes the base from the sugar, creating an apurinic/apyrimidinic (AP) site.
   
   
   
2. AP endonuclease makes a cut in the phosphodiester backbone at the AP site.
   
   
   
3. The gap is filled by DNA polymerase I and DNA ligase
   
   
   
4. The error is corrected.