Which component helps the ribosome locate the start codon in bacteria?

• A. A Shine-Dalgarno sequence base-pairs with the 16S rRNA to align the ribosome with the start codon.
• B. A Kozak consensus sequence in eukaryotes.
• C. A polyadenylation signal that marks end of transcription.
• D. A stop codon that marks termination.

Answer: (A)

In bacteria, starting translation at the correct codon relies on a specific mRNA feature that guides the ribosome to the start site. The Shine-Dalgarno sequence is a short, purine-rich region just upstream of the start codon that base-pairs with a complementary region at the 3' end of the 16S rRNA within the small ribosomal subunit. This base-pairing anchors the ribosome so the start codon sits at the correct position for initiation with the initiator tRNA, establishing the proper reading frame. The Kozak sequence is used in eukaryotes, not bacteria; a polyadenylation signal relates to mRNA processing, and a stop codon signals termination. Therefore, the Shine-Dalgarno interaction is what helps the ribosome locate the start codon in bacteria.