# DBT JRF 2020 — Question 76

Exam: DBT JRF 2020 · Section: B · Topic: Molecular Biology · Difficulty: Medium

## Question

Sequence-specific recognition of DNA by proteins occurs primarily through the

## Options

- **A.** histones
- **B.** minor groove
- **C.** polyphosphate backbone
- **D.** major groove

## Answer

Option D

## Explanation

Sequence-specific recognition of DNA by proteins (transcription factors, restriction enzymes, activators, repressors) occurs predominantly through the MAJOR GROOVE. Reasons: (1) The major groove (width ~22 Å, depth ~8.5 Å) is wider and deeper than the minor groove (width ~12 Å, depth ~7.5 Å), allowing proteins to insert α-helices (recognition helix of HTH, zinc finger, leucine zipper motifs) directly into the groove. (2) The major groove exposes unique, sequence-specific hydrogen bond donor/acceptor patterns for each of the 4 base pairs (A:T, T:A, G:C, C:G) — proteins 'read' these patterns by forming specific H-bonds with bases without opening the double helix. (3) Minor groove is narrower and shows less sequence variability; minor groove recognition occurs in some cases (TATA-box binding protein, TBP; HMG proteins).

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