65. Which one of the following statements regarding base excision DNA repair system is FALSE?
(1) It can be triggered by damaged DNA.
(2) The pol pathway facilitates replacement of a long polynucleotide stretch of DNA.
(3) The enzymes that remove bases from DNA are glycosylases and lyases.
(4) Damaged DNA that has not been repaired causes stalling of DNA polymerase III.

🧪 Introduction: What is Base Excision Repair?

Base Excision Repair (BER) is a critical DNA repair mechanism that protects cells from small, non-helix-distorting base lesions caused by oxidation, deamination, and alkylation. It’s especially important for repairing damaged bases that could otherwise lead to mutations during DNA replication.

Let’s dive into the BER system, understand its key components and steps—and most importantly, pinpoint a false statement often misunderstood in the context of BER.

🧬 Core Steps of Base Excision Repair

The BER pathway includes the following steps:

1. Recognition and removal of the damaged base

   • Enzyme: DNA glycosylase

   • It cleaves the N-glycosidic bond between the damaged base and the sugar-phosphate backbone, creating an AP (apurinic/apyrimidinic) site.

2. Cleavage of the DNA backbone at the AP site

   • Enzyme: AP endonuclease or lyase

   • This produces a single-strand break.

3. Synthesis of new DNA

   • Two sub-pathways:

     • Short-patch repair: Involves replacement of a single nucleotide.

     • Long-patch repair: Replaces 2–10 nucleotides. This pathway does not involve polymerase beta (pol β); instead, it uses DNA polymerase δ/ε.

4. Ligation

   • Enzyme: DNA ligase, seals the DNA strand.

❌ Identifying the FALSE Statement

Let’s evaluate the statements:

(1) “It can be triggered by damaged DNA.” ✅

Correct – BER is initiated when small base lesions are detected.

(2) “The pol β pathway facilitates replacement of a long polynucleotide stretch of DNA.” ❌

False – The pol β pathway is primarily involved in short-patch repair, typically replacing a single nucleotide.
The long-patch repair is facilitated by polymerases δ/ε along with PCNA (proliferating cell nuclear antigen).

(3) “The enzymes that remove bases from DNA are glycosylases and lyases.” ✅

Correct – DNA glycosylases remove the base, and AP lyases or endonucleases help cleave the backbone.

(4) “Damaged DNA that has not been repaired causes stalling of DNA polymerase III.” ✅

Correct – Unrepaired DNA lesions can lead to stalling of DNA replication machinery, including polymerase III in prokaryotes.

✅ Final Answer:

(2) The pol β pathway facilitates replacement of a long polynucleotide stretch of DNA.

📌 Conclusion

The Base Excision Repair pathway is essential for genomic stability, but understanding the specific roles of enzymes is key. A common misconception is about the role of DNA polymerase β—while it is a major player in short-patch BER, it is not responsible for long-stretch DNA synthesis, which is carried out by other polymerases.

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• base excision repair false statement

• DNA glycosylase and lyase

• pol beta in DNA repair

• long-patch vs short-patch repair

• DNA damage repair mechanisms