Where is the Shine-Dalgarno sequence located and what is its role?

Where is the Shine-Dalgarno sequence located and what is its role?

• A. It is the ribosome-binding site on mRNA in prokaryotes.
• B. It is the cap structure at the 5' end in eukaryotes.
• C. It is the start codon region.
• D. It is a promoter sequence on DNA.

Answer: (A)

The Shine-Dalgarno sequence is the ribosome-binding site on bacterial mRNA that helps start translation. It sits near the 5' end of the mRNA, a short distance upstream of the start codon (typically about 6–8 nucleotides before AUG). This purine-rich motif often looks like AGGAGG and serves as a contact point for the 16S rRNA component of the small ribosomal subunit. By base-pairing with this region of the rRNA, the Shine-Dalgarno sequence correctly positions the start codon in the ribosome’s P site, guiding the initiation machinery to start translation at the right place and in the proper reading frame. This mechanism is specific to prokaryotes; in eukaryotes, translation initiation relies on a 5' cap and scanning rather than a Shine-Dalgarno interaction. The promoter sequence on DNA is involved in transcription, not translation, and the start codon region refers to the codon where translation begins—not the binding site that helps recruit the ribosome.

The Shine-Dalgarno sequence is the ribosome-binding site on bacterial mRNA that helps start translation. It sits near the 5' end of the mRNA, a short distance upstream of the start codon (typically about 6–8 nucleotides before AUG). This purine-rich motif often looks like AGGAGG and serves as a contact point for the 16S rRNA component of the small ribosomal subunit. By base-pairing with this region of the rRNA, the Shine-Dalgarno sequence correctly positions the start codon in the ribosome’s P site, guiding the initiation machinery to start translation at the right place and in the proper reading frame. This mechanism is specific to prokaryotes; in eukaryotes, translation initiation relies on a 5' cap and scanning rather than a Shine-Dalgarno interaction. The promoter sequence on DNA is involved in transcription, not translation, and the start codon region refers to the codon where translation begins—not the binding site that helps recruit the ribosome.