Which statement correctly contrasts bacterial and eukaryotic translation initiation?

• A. Eukaryotes rely on a 5' cap and scanning by the ribosome with initiation factors.
• B. Initiation in bacteria requires a cap-binding complex.
• C. Bacteria use a Shine-Dalgarno sequence to align the ribosome with the start codon; eukaryotes rely on a 5' cap and scanning by the ribosome with initiation factors.
• D. Initiation in bacteria uses scanning by the ribosome; eukaryotes use Shine-Dalgarno.

Answer: (C)

Initiation in prokaryotes uses a Shine-Dalgarno sequence in the mRNA that base-pairs with the 16S rRNA of the small ribosomal subunit to align the start codon with the P site. In contrast, eukaryotes rely on recognition of the 5' cap by initiation factors and then the ribosomal subunit scans along the 5' untranslated region to locate the start codon, often in a favorable Kozak context. This contrast—bacteria positioning the start with a Shine-Dalgarno sequence, versus eukaryotes starting via cap-dependent scanning with initiation factors—captures the key difference.

The other options are misleading because bacteria do not require a cap-binding mechanism, and the roles of scanning vs. Shine-Dalgarno are reversed or incomplete in those statements.