Which event completes ribosome assembly by attaching the large subunit?

• A. Attachment: of large subunit
• B. Detection: of start codon
• C. Binding: of initiator tRNA to small subunit
• D. Binding: of small subunit w/ tRNA to mRNA

Answer: (A)

Ribosome assembly is completed when the large subunit binds to the small subunit that already carries the mRNA and initiator tRNA, forming the functional ribosome ready for elongation. In bacteria, the 30S initiation complex with mRNA and initiator tRNA is created first, then the 50S subunit joins to make the 70S ribosome. In eukaryotes, the same idea holds: after the small subunit has positioned the mRNA and initiator tRNA, the large subunit joins to form the 80S ribosome.

This joining is the final step that makes the ribosome a complete machine with the A, P, and E sites active for translation. The other steps—detecting the start codon, binding the initiator tRNA to the small subunit, or forming the small-subunit–mRNA–tRNA initiation complex—are all earlier parts of the initiation process and do not by themselves complete ribosome assembly.