How does translation initiation differ between bacteria and eukaryotes?

• A. Bacteria rely on Shine-Dalgarno alignment; eukaryotes use 5' cap-driven scanning with Kozak context.
• B. Both bacteria and eukaryotes initiate translation mainly through scanning from the 5' cap.
• C. Bacteria use cap-driven scanning; eukaryotes rely on Shine-Dalgarno sequence.
• D. Bacteria use internal ribosome entry sites; eukaryotes never use cap structures.

Answer: (B)

Translation initiation strategies differ between bacteria and eukaryotes in how the ribosome finds the start codon. In bacteria, the mRNA typically lacks a 5' cap, and the small ribosomal subunit recognizes a Shine-Dalgarno sequence in the mRNA. This sequence base-pairs with the anti-Shine-Dalgarno region of the 16S rRNA, correctly positioning the start codon in the P site for initiation. In eukaryotes, the mRNA has a 5' cap, and the preinitiation complex binds the cap and scans along the 5' untranslated region until it encounters an AUG in a favorable Kozak context, after which the large subunit joins to start elongation. So the bacteria rely on Shine-Dalgarno alignment to set the reading frame, while eukaryotes depend on cap-driven scanning with Kozak recognition to choose the start site. The statement that both groups initiate primarily by scanning from the 5' cap oversimplifies and misstates the bacterial mechanism.