Q.68 Shine Dalgarno’s sequence present in mRNA binds to
(A) 3′ end of rRNA
(B) 5′ end of rRNA
(C) 5′ end of tRNA
(D) 3′ end of tRNA
Shine-Dalgarno sequence in prokaryotic mRNA binds to the 3′ end of 16S rRNA, enabling ribosome positioning for translation initiation.
The correct answer is (A).
Mechanism Overview
The Shine-Dalgarno (SD) sequence (consensus AGGAGG, ~8 nt upstream of AUG start codon) base-pairs with the anti-SD sequence (CCUCCU) at the 3′ terminus of 16S rRNA in the 30S ribosomal subunit. This interaction aligns the ribosome precisely on mRNA, distinct from eukaryotic Kozak sequences.
Option Breakdown
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(A) 3′ end of rRNA: Correct—SD complements 3′ end of 16S rRNA (E. coli: 3′-AUUCCUCCACUAG-5′), confirmed by hybridization studies and mutations disrupting binding.
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(B) 5′ end of rRNA: Wrong—5′ end of 16S rRNA (5′-UUAUAA…) lacks complementarity to purine-rich SD; no role in initiation.
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(C) 5′ end of tRNA: Wrong—tRNA 5′ end (acceptor stem) binds amino acids/elongation factors, not mRNA positioning sequences.
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(D) 3′ end of tRNA: Wrong—tRNA 3′ end (anticodon loop) reads codons during elongation; initiation uses fMet-tRNA without SD interaction.
Introduction to Shine Dalgarno Sequence Binds rRNA
Shine dalgarno sequence binds rRNA at 3′ end of 16S rRNA positions bacterial ribosomes for translation—core GATE Life Sciences molecular biology concept. Discovered 1974, AGGAGG mRNA pairs with CCUCC anti-SD, absent in eukaryotes.
Binding Specificity
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Location: SD ~8 nt upstream of AUG; 3′ 16S rRNA tail exposes anti-SD.
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Mechanism: Watson-Crick pairing aligns P-site over start codon.
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Evidence: Mutations in either sequence abolish initiation efficiency.
Why Other Options Fail
| Option | Incorrect Reason |
|---|---|
| (B) 5′ rRNA | No sequence complementarity. |
| (C) 5′ tRNA | tRNA handles aminoacylation. |
| (D) 3′ tRNA | Anticodon reads CDS post-positioning. |
| (A) 3′ rRNA | Direct base-pairing confirmed. |
GATE Exam Tip
Shine dalgarno sequence binds rRNA distinguishes prokaryotic initiation. Visualize: mRNA SD ↔ 16S 3′ tail = (A). Essential for XL biotech/biochem sections.
