Which RNA component base pairs with the Shine-Dalgarno sequence to position the start codon in bacteria?

• A. 28S rRNA
• B. 16S rRNA
• C. 5S rRNA
• D. tRNA

Answer: (B)

During bacterial initiation, the Shine-Dalgarno sequence on the mRNA finds its match on the ribosome to align the start codon. This base pairing occurs with the anti-Shine-Dalgarno sequence at the 3' end of the 16S rRNA in the small ribosomal subunit. When these twoRNA regions pair, they position the start codon in the P site so translation begins at the correct frame. The 16S rRNA is the RNA component that carries this matching sequence, enabling the ribosome to properly assemble and start protein synthesis. The other RNA options don’t provide that specific pairing: 28S rRNA is part of the eukaryotic large subunit, 5S rRNA is on the large subunit but doesn’t pair with the SD sequence, and tRNA carries amino acids after initiation rather than guiding start-site positioning.