Which initiation event represents the binding of the small subunit with tRNA to mRNA at the 5' end?

• A. Binding of small subunit w/ tRNA to mRNA (5' end)
• B. Binding of initiator tRNA to small subunit
• C. Detection of start codon
• D. Attachment of large subunit

Answer: (A)

The key idea is how translation starts: first, the small ribosomal subunit binds to the messenger RNA at its 5' end together with the initiator tRNA, forming the initial initiation complex. This binding sets up the reading frame and positions the initiator tRNA so that the start codon will be recognized next. Once the start codon is detected and properly aligned in the P site, the large ribosomal subunit joins to form the complete ribosome and elongation can begin. The other steps—the initiator tRNA binding to the subunit, start codon detection, and the large subunit attachment—occur in this sequence, with the initial small-subunit–mRNA–tRNA assembly being the foundational event.