Which statement describes the Shine-Dalgarno sequence and its role?

• A. It is a ribosome binding site that aligns the ribosome with the start codon to initiate translation.
• B. It acts as a promoter element used for transcription initiation.
• C. It terminates translation.
• D. It binds repressors to inhibit transcription.

Answer: (D)

The Shine-Dalgarno sequence is a ribosome binding site in prokaryotic mRNA that helps align the ribosome with the start codon to initiate translation. It base-pairs with the 3' end of the 16S rRNA of the small ribosomal subunit, positioning the start codon in the correct place for initiation. It sits a short distance upstream of the start codon, coordinating the assembly of the translation apparatus rather than regulating transcription. This makes the description that it binds repressors to inhibit transcription incorrect, since the Shine-Dalgarno region functions at the level of translation initiation, not transcription.