Question 43:
A sequence in an mRNA that is required for binding bacterial ribosomes is called:
The correct answer is (C) Shine-Dalgarno. This sequence in bacterial mRNA is essential for ribosome binding during translation initiation.
Option Analysis
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(A) Short tandem repeat: These are DNA microsatellite repeats (1-6 bp units) used in genotyping or forensics, not mRNA ribosome binding.
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(B) Long tandem repeat: Similar to short tandem repeats but longer (>6 bp), these relate to genomic instability or diseases like Huntington’s, irrelevant to translation.
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(C) Shine-Dalgarno: Correct. This purine-rich sequence (e.g., AGGAGG), ~8 bases upstream of the AUG start codon in bacterial mRNA, base-pairs with 16S rRNA’s anti-Shine-Dalgarno region to recruit the 30S ribosomal subunit.
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(D) Ras: A proto-oncogene protein family involved in cell signaling (GTPase), not an mRNA sequence.
The Shine-Dalgarno sequence in mRNA is a critical ribosomal binding site required for bacterial ribosomes to initiate protein synthesis. Located about 8 nucleotides upstream of the start codon (AUG), this purine-rich motif (typically 5′-AGGAGG-3′) ensures precise ribosome positioning on prokaryotic mRNA.
Role in Translation Initiation
In bacteria, the Shine-Dalgarno sequence base-pairs with the anti-Shine-Dalgarno region at the 3′ end of 16S rRNA in the 30S ribosomal subunit. This interaction recruits the ribosome, aligns it with the start codon, and enables tRNA binding for elongation—essential for polycistronic mRNAs in operons. Without it, translation fails, as seen in mutants.
Why Not Other Options?
Short and long tandem repeats are genomic DNA features for variation analysis, not translation. Ras is a signaling protein, unrelated to mRNA sequences. Only Shine-Dalgarno fits bacterial ribosome binding.
Exam Relevance for GATE Life Sciences
This topic appears in molecular biology sections of GATE XL. Key fact: It’s prokaryote-specific (eukaryotes use Kozak sequence). Tip: Memorize consensus (AGGAGG) and position for PYQs.
