In eukaryotic translation initiation, the ribosome begins scanning from which feature toward the start codon?

• A. The 3' end of the mRNA
• B. The 5' cap
• C. The start codon
• D. The poly-A tail

Answer: (B)

Scanning in eukaryotic translation initiation starts at the 5' cap. The small ribosomal subunit, guided by initiation factors that recognize the cap (the 5' end of the mRNA), binds there and then moves along the 5' untranslated region toward the start codon. This 5'→3' scanning continues until the start codon is found, where translation begins. The poly-A tail at the 3' end and the start codon itself are not the starting point for scanning; they relate to later steps—the start codon marks where synthesis starts, and the poly-A tail helps with mRNA stability and circularization but does not initiate scanning.