CSIR NET Life Sciences Unit 3: DNA Repair Pathways (BER, NER, MMR) & Part C Solved Problems (Part 2 of 5)

CSIR NET Life Sciences | Unit 3 Masterclass: Part 2 of 5

Mastering DNA Repair Pathways: BER, NER & Mismatch Repair for CSIR NET Part C

High-Yield Overview: DNA repair mechanisms safeguard genomic integrity against constant mutational pressure, making them a top-tier topic for analytical questions in the CSIR NET exam. Understanding enzyme specificity and pathway execution is vital for tackling high-value Part C experimental scenarios.

• Exam Weightage: Consistently tested with 1-2 questions (6 to 9 marks) in every CSIR NET session.
• Part C Focus: Experimental setup using mutant strains, damaged bases, or inhibitor assays.
• Core Mastery: Distinguishing substrate specificity among BER, NER, and MMR pathways.

1. Comparative Breakdown of Core Excision Pathways

Excision repair mechanisms remove modified or mispaired bases before replication turns them into permanent mutations. Each pathway operates on distinct lesions using specific catalytic enzymes.

Feature	Base Excision Repair (BER)	Nucleotide Excision Repair (NER)	Mismatch Repair (MMR)
Primary Substrate	Non-bulky modified bases (Uracil, 8-oxoG)	Bulky lesions, UV pyrimidine dimers	Replication misincorporations & insertion loops
Key Recognition	DNA Glycosylases	UvrA/UvrB (E. coli) or XPC/RAD23B (Eukaryotes)	MutS / MutL (Hemimethylation via MutH)
Excision Mode	AP Endonuclease cleaves abasic site	Dual incision (UvrC / XPG-XPF) removing ~12-30 nt oligomer	Exonuclease digests strand from nick to mismatch

2. Molecular Cascade & Enzymatic Logic

• BER Cascade: Glycosylase removes base → AP site generated → AP Endonuclease nicks backbone → Polymerase & Ligase seal gap.
• NER Cascade: Damage sensing → Double incision flushes out damaged oligomer → Repair synthesis by Pol δ/ε + RFC/PCNA.
• MMR Strand Discrimination: In E. coli, transient hemimethylation at 5'-GATC-3' sites by Dam methylase allows MutH to specifically nick the newly synthesized (unmethylated) strand.

⚠️ Common Trap / Examiner's Trick

Watch out for Glycosylase vs. AP Endonuclease traps! DNA Glycosylases do not cleave the phosphodiester backbone; they only cleave the N-glycosidic bond to generate an AP (apurinic/apyrimidinic) site. The phosphodiester backbone is cut subsequently by AP Endonuclease. Mixing up these two steps is a classic Part B/Part C distractor!

🧪 Experimental Thinking: Solving Part C Data Questions

Scenario: You are analyzing a mutant E. coli strain showing a 100-fold increase in spontaneous transition mutations post-replication. You test methylation status and find all 5'-GATC-3' sites fully methylated on both strands instantly.

Step-by-Step Analytical Logic:

1. If Dam methylase is hyperactive, both strands become methylated before MutS/MutL/MutH can process replication errors.
2. MutH cannot distinguish between template and daughter strands when both are methylated.
3. Mismatch repair is blocked, causing uncorrected errors to accumulate during subsequent rounds of replication.

⚡ Quick Self-Check: 10 High-Yield Part C Practice Questions

Q1. A cell line deficient in DNA Glycosylase is exposed to deaminating agents. Which type of DNA damage will accumulate uncontrollably?

A) Thymine dimers

B) Uracil bases in DNA

C) Double-strand breaks

D) Interstrand crosslinks

Reveal Solution & Explanation

Correct Answer: B
Cytosine deamination produces Uracil in DNA. Uracil-DNA Glycosylase specifically recognizes and removes Uracil via BER. Without it, Uracil accumulates in DNA.

Q2. In E. coli Mismatch Repair, which protein directly senses and binds to the mispaired nucleotide base?

A) MutH

B) MutL

C) MutS

D) UvrB

Reveal Solution & Explanation

Correct Answer: C
MutS recognizes and binds directly to mismatched base pairs or small insertions/deletions. MutL recruits MutH, which nicks the unmethylated strand.

Q3. Xeroderma Pigmentosum (XP) in humans is caused by inherited defects in which repair pathway?

A) Base Excision Repair

B) Nucleotide Excision Repair

C) Mismatch Repair

D) Non-Homologous End Joining

Reveal Solution & Explanation

Correct Answer: B
XP results from mutations in NER genes (e.g., XPA through XPG), leading to hypersensitivity to UV radiation and inability to repair pyrimidine dimers.

Q4. During bacterial NER, which protein complex executes the actual dual incision on both 5' and 3' sides of the DNA lesion?

A) UvrA2B

B) UvrC in complex with UvrB

C) UvrD helicase

D) DNA Polymerase I

Reveal Solution & Explanation

Correct Answer: B
UvrC binds to the UvrB-DNA complex and cuts the phosphodiester backbone on both sides of the damage (5' and 3' incisions).

Q5. An AP site in DNA lacks which of the following chemical moieties?

A) Nitrogenous base

B) Deoxyribose sugar

C) Phosphate group

D) 3'-OH group

Reveal Solution & Explanation

Correct Answer: A
An AP site (apurinic/apyrimidinic site) is a location in DNA that lacks a purine or pyrimidine base while keeping the sugar-phosphate backbone intact.

Q6. In eukaryotic mismatch repair, strand discrimination occurs predominantly through which mechanism in the absence of Dam methylase?

A) DNA glycosylase tagging

B) Nicks present in the newly synthesized daughter strand (e.g., Okazaki fragments)

C) Histone acetylation tags

D) RNA primer incorporation

Reveal Solution & Explanation

Correct Answer: B
Eukaryotes lack Dam methylase. Strand recognition relies on pre-existing single-strand nicks in the lagging (Okazaki fragments) and leading strands during replication.

Q7. What is the primary role of UvrD in the bacterial Nucleotide Excision Repair pathway?

A) Sensing structural distortion

B) Cleaving the 5' end of the lesion

C) Helicase activity to release the excised oligonucleotide

D) Resealing the backbone nick

Reveal Solution & Explanation

Correct Answer: C
UvrD is a 3'-to-5' DNA helicase that unwinds and releases the excised damaged single-stranded segment along with UvrC.

Q8. 8-oxoguanine (8-oxoG) is a dangerous oxidative DNA lesion because it mispairs with which base during replication?

A) Cytosine

B) Thymine

C) Adenine

D) Uracil

Reveal Solution & Explanation

Correct Answer: C
8-oxoG can adopt a Hoogsteen base-pairing conformation with Adenine, leading to G:C to T:A transversion mutations if unrepaired.

Q9. A researcher mutates the mutH gene in E. coli. What will be the cellular phenotype?

A) Inability to repair pyrimidine dimers

B) Loss of strand-specific cleavage during mismatch repair

C) Complete arrest of replication fork initiation

D) Immediate accumulation of double-strand break lethal mutations

Reveal Solution & Explanation

Correct Answer: B
MutH is the endonuclease responsible for cutting the unmethylated strand at GATC sites. Without MutH, MMR strand cleavage fails.

Q10. Photoreactivation via DNA photolyase repairs pyrimidine dimers using energy absorbed from which spectrum?

A) Infrared light (>800 nm)

B) Visible / Near-UV light (300–500 nm)

C) X-rays

D) Far-UV light (100–200 nm)

Reveal Solution & Explanation

Correct Answer: B
DNA Photolyase uses chromophores (FADH-) to absorb light in the visible/near-UV region (300–500 nm) to directly reverse pyrimidine dimers.

Mastered DNA Repair Pathways? Keep Your Preparation Momentum Going!

Did you miss Part 1? Read Part 1: DNA Replication in Prokaryotes vs. Eukaryotes to strengthen your fundamentals.

Check out our CSIR NET Examination 2026 Complete Guide for full syllabus strategies and exam patterns.

Continue to Part 3: Prokaryotic Gene Regulation (Lac & Trp Operons + Mutations )→

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