Describe the initiation of translation in prokaryotes.

• A. The large subunit binds first, then the small, with no initiation factors.
• B. The small subunit binds to the mRNA at the Shine-Dalgarno sequence, initiation factors and GTP recruit the initiator tRNA (fMet-tRNA) to the P site, then the large subunit joins to form the complete ribosome.
• C. The initiator tRNA binds at the A site before the small subunit binds.
• D. The ribosome assembles at the start codon after promoter binding.

Answer: (B)

In prokaryotes, translation starts by correctly positioning the mRNA on the small ribosomal subunit using the Shine-Dalgarno sequence. The small subunit binds to this purine-rich region, aligning the start codon so that the initiator tRNA carrying formylmethionine can pair at the P site. Initiation factors and GTP assist in bringing that fMet-tRNA to the P site and stabilizing the initiation complex. Only after this setup does the large ribosomal subunit join to form the complete ribosome, placing the initiator tRNA in the P site and readying the system for elongation. This sequence—small subunit binding to the Shine-Dalgarno site, recruitment of the initiator tRNA to the P site with initiation factors and GTP, then joining of the large subunit—best reflects how initiation occurs in bacteria.