In base excision repair, which sequence correctly describes the enzymes acting to repair a damaged base?

• A. DNA glycosylase; AP endonuclease; DNA polymerase; DNA ligase
• B. AP endonuclease; DNA glycosylase; DNA polymerase; DNA ligase
• C. DNA polymerase; DNA ligase; DNA glycosylase; AP endonuclease
• D. DNA ligase; DNA polymerase; DNA glycosylase; AP endonuclease

Answer: (A)

Base excision repair starts by a DNA glycosylase recognizing and removing the damaged base, which creates an apurinic/apyrimidinic (AP) site. Then an AP endonuclease cuts the DNA backbone at that AP site, producing a nick that provides a 3′-OH end for synthesis. A DNA polymerase then fills in the missing nucleotides (short-patch or long-patch BER, depending on the situation), and finally DNA ligase seals the nick to restore an intact backbone. If the backbone is cut before the base is removed, or ligation occurs before the gap is created, the repair can’t proceed correctly. This is why the sequence starts with glycosylase, then AP endonuclease, then polymerase, then ligase.