In bacteria, the Shine-Dalgarno sequence aligns the ribosome with the start codon by base pairing to which RNA component?

• A. Aligns the ribosome with the start codon via base pairing to 16S rRNA.
• B. Initiates transcription by promoter binding.
• C. Terminates translation at stop codon.
• D. Is a ribozyme involved in splicing.

Answer: (A)

The important idea is how the bacterial ribosome is positioned to start translation. The Shine-Dalgarno sequence on the mRNA base pairs with a complementary region at the 3' end of the 16S rRNA in the small ribosomal subunit. This base pairing pulls the start codon into the correct place in the ribosome (the P site) so translation begins at the right AUG and in the right frame. It’s a translation-initiation interaction, not a transcription event, not a termination event, and not related to splicing. So the correct pairing is with the 16S rRNA.