​A mutant strain of E. coli has defects in one of the DNA repair pathways, identified as the Base Excision Repair (BER) pathway.

The researcher examines the following parameters to determine where the mutation occurred:

(A) Topoisomerase II enzyme activity
(B) AP endonuclease activity
(C) Expression of MutL and MutS
(D) DNA glycosylase activity
(E) DNA ligase activity

Question:
Based on the changes in which of the above parameters, can the conclusion of BER pathway defect be drawn?

Correct option is B

The Base Excision Repair (BER) pathway is responsible for repairing small, non-helix-distorting lesions in DNA, such as oxidative damage, deamination, and alkylation.

Key Enzymes Involved in BER:

1. DNA Glycosylase (D) → Recognizes and removes damaged bases.
2. AP Endonuclease (B) → Cleaves the abasic (AP) site left after base removal.
3. DNA Polymerase + DNA Ligase (E) → Fills and seals the repaired DNA strand.

Since BER involves DNA glycosylases, AP endonucleases, and DNA ligase, mutations affecting B, D, and E would indicate a defect in BER.

Information Booster:

• Base Excision Repair (BER) is critical for fixing oxidative DNA damage.
• DNA glycosylases (D) remove the damaged base, forming an AP site (apurinic/apyrimidinic site).
• AP endonuclease (B) cleaves the AP site, creating a gap.
• DNA polymerase fills the gap, and DNA ligase (E) seals it.

Additional Information:

(A) Topoisomerase II enzyme activity →

• Not related to BER, but involved in DNA supercoiling.

(C) Expression of MutL and MutS →

• Related to Mismatch Repair (MMR), not BER.

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