What are the steps of the translation elongation cycle?

• A. Initiation of ribosome assembly; release of energy by ATP
• B. Arrival of aa-tRNA at A site; peptide bond formation; translocation
• C. Arrival of aa-tRNA at A site; peptide bond formation; translocation; GTP hydrolysis by EF-G
• D. Stop codon recognition and release factor recruitment

Answer: (C)

In the translation elongation cycle, the ribosome adds amino acids one by one through a predictable sequence: first, an aminoacyl-tRNA is delivered to the A site in a GTP-dependent way (with EF-Tu in bacteria or eEF1A in eukaryotes) to ensure the codon and anticodon match. Once the correct tRNA is in place, GTP is hydrolyzed and the new amino acid is linked to the growing chain by the peptidyl transferase reaction, forming a peptide bond. Next comes translocation, where the ribosome moves one codon along the mRNA; this shift relocates the peptidyl-tRNA from the A site to the P site and the deacylated tRNA to the E site, and this movement depends on GTP hydrolysis by EF-G. The cycle then repeats with the next aminoacyl-tRNA entering the A site. This sequence, including the GTP-driven translocation step, defines elongation.

Initiation is separate: it concerns ribosome assembly at the start codon, not the elongation steps. Termination occurs when a stop codon is reached, recruiting release factors, which is outside elongation.