What sequence in bacteria helps align mRNA with the ribosome to start translation?

• A. Kozak sequence
• B. Shine-Dalgarno sequence
• C. Polyadenylation signal
• D. TATA box

Answer: (B)

Shine-Dalgarno sequence acts as the bacterial ribosome binding site that guides the small ribosomal subunit to the correct start codon. This short mRNA segment is located just upstream of the start codon and base-pairs with the anti-Shine-Dalgarno region at the 3' end of the 16S rRNA in the small subunit. This pairing correctly positions AUG in the P site, allowing the initiator tRNA to deliver formyl-methionine and start translation. The typical distance between this site and the start codon helps ensure the reading frame is set properly.

The other options come from different contexts: the Kozak sequence helps initiate translation in eukaryotes, the polyadenylation signal relates to mRNA processing and stability, and the TATA box is a promoter element involved in transcription initiation.