Explain the start of translation in bacteria and in eukaryotes.

• A. Bacteria begin translation at the start codon guided by the Shine-Dalgarno sequence; formylmethionine is used on the initiator tRNA; eukaryotes use a Kozak sequence and the initiator Met-tRNAi bound at the 5' cap.
• B. Bacteria use a Kozak sequence to start translation; eukaryotes use Shine-Dalgarno; both use formylmethionine.
• C. Bacteria initiate translation at any AUG codon without signals; eukaryotes require scanning.
• D. Bacteria and eukaryotes initiate translation exclusively in the nucleus.

Answer: (A)

Start of translation is guided by different cues in bacteria and eukaryotes, reflecting how each system recognizes where to begin reading the mRNA. In bacteria, the small ribosomal subunit binds the mRNA at a Shine-Dalgarno sequence, a short purine-rich region that base-pairs with the 16S rRNA. This interaction correctly positions the AUG start codon in the ribosome’s P site so the initiator tRNA can pair with AUG and form the initiation complex. The initiator tRNA in bacteria carries formylmethionine, so the first amino acid added is fMet.

In eukaryotes, initiation relies on a 5' cap structure and a suite of initiation factors. The cap is recognized by these factors, and the ribosome assembles into a preinitiation complex that binds near the cap and then scans the 5' untranslated region until it encounters a start codon in a favorable Kozak context. The initiator tRNA in eukaryotes is methionyl-tRNAi carrying methionine (not formylated). Translation then proceeds in the cytoplasm.

This combination of a bacterial Shine-Dalgarno-guided start with formylated methionine, versus a cap-dependent, scanning eukaryotic start with unmodified methionine, is why the statement is the best description of how translation begins in the two domains. The other options misstate the ribosome recruitment signals, the initiator amino acid, or the cellular location of initiation.