What signals the start codon in translation?

• A. The initiator tRNA recognizing AUG, aided by sequence context (Kozak consensus in eukaryotes).
• B. A Shine-Dalgarno sequence base-pairs with rRNA to locate the start codon.
• C. Release factors facilitate start codon recognition.
• D. The poly-A tail directs initiation.

Answer: (B)

In bacteria, the start of translation is signaled by the Shine-Dalgarno sequence in the mRNA. This short, purine-rich region sits upstream of the start codon and base-pairs with a complementary sequence at the 3' end of the 16S rRNA in the small ribosomal subunit. This interaction correctly positions AUG in the ribosome’s P site, so the initiator tRNA (carrying formylmethionine) can pair with AUG and begin translation.

The initiator tRNA recognizing AUG is part of the initiation process, but it doesn’t by itself locate the start codon—the Shine-Dalgarno–rRNA base pairing does that recruitment and positioning. Release factors are involved in termination, not initiation, and the poly-A tail influences mRNA stability and translation efficiency rather than signaling the start of translation. In eukaryotes, a different mechanism (the Kozak context and scanning from the 5' cap) signals initiation, not the Shine-Dalgarno sequence.