Which feature best distinguishes nucleotide excision repair (NER) from base excision repair (BER) in terms of DNA damage recognition and removal?

Explanation

## Comparison: NER vs BER ### Key Structural and Functional Differences **Key Point:** The fundamental distinction between NER and BER lies in the **type of lesion recognized**, the **extent of DNA removal**, and the **enzymatic machinery** employed. | Feature | Base Excision Repair (BER) | Nucleotide Excision Repair (NER) | |---------|---------------------------|----------------------------------| | **Lesion type** | Small, non-distorting (oxidative damage, alkylation, deamination) | Bulky, helix-distorting (UV-induced thymine dimers, benzo[a]pyrene adducts) | | **Lesion recognition** | Glycosylases (e.g., uracil-DNA glycosylase, 8-oxoguanine glycosylase) | XPA, XPC proteins; TFIIH helicase complex | | **Nucleotides removed** | 1–2 nucleotides (short-patch) or up to 10 (long-patch) | 25–30 nucleotides on 3' side, 5–10 on 5' side | | **Incision pattern** | Endonuclease VIII or APE1 cleaves at apurinic site | Dual incisions flanking the lesion | | **Cofactors** | DNA polymerase β, DNA ligase I | DNA polymerase δ/ε, PCNA, RPA, DNA ligase I | | **Clinical association** | Spontaneous mutations, aging | Xeroderma pigmentosum (XP), skin cancer predisposition | ### High-Yield Distinction **High-Yield:** NER is the **only repair pathway** that can remove bulky, distorting lesions. This is why **xeroderma pigmentosum** (defective NER) presents with extreme UV sensitivity and early-onset skin cancers, whereas BER defects cause different phenotypes (e.g., OGG1 mutations in familial adenomatous polyposis). **Mnemonic: BULKY = NER** - **B**ulky lesions → NER - **U**V-induced damage → NER - **L**arge excision (25–30 nt) → NER - **K**inky DNA (helix distortion) → NER - **Y**es to thymine dimers → NER ### Why Option 1 is Correct Option 1 directly captures the **lesion specificity** and **excision extent** — the two most clinically and mechanistically important discriminators. NER's removal of 25–30 nucleotides (vs. BER's 1–2) is a hallmark feature tested in NEET PG. [cite:Lehninger Principles of Biochemistry 8e Ch 25] [cite:Alberts Molecular Biology of the Cell 6e Ch 5]