Step by Step
Init
Initiation — ribosome assembles at AUG
The small ribosomal subunit binds the mRNA at the 5' cap (eukaryotes) or Shine-Dalgarno sequence (prokaryotes) and scans for AUG. The initiator tRNA (carrying Met) binds the P site. The large subunit joins — translation begins.
APE
The three ribosomal sites — A, P, E
The ribosome has three tRNA binding sites: A site (Aminoacyl) — where new charged tRNA arrives. P site (Peptidyl) — where the growing peptide chain is held. E site (Exit) — where uncharged tRNA leaves.
Memory trick: APE — A = Arrival, P = Peptide chain, E = Exit. The tRNA moves A → P → E and then leaves.
Elong
Elongation cycle — one amino acid added per cycle
(1) A new charged tRNA enters the A site (codon-anticodon matching). (2) Peptide bond forms between the amino acid in the A site and the chain in the P site (catalyzed by rRNA). (3) Translocation: ribosome moves one codon along mRNA — tRNA shifts A→P→E. Repeat.
Stop
Termination — stop codon releases the protein
When a stop codon (UAA, UAG, UGA) enters the A site, no tRNA recognizes it. Instead, a release factor protein binds, triggering hydrolysis of the bond between the polypeptide and the last tRNA. The protein is released and the ribosome disassembles.
Applied Walkthrough
1
Ribosome assembles at AUG. Met-tRNA occupies the P site. A site is open.
2
Second codon (e.g., UUU) — Phe-tRNA arrives at A site. Peptide bond forms: Met-Phe chain moves to A site tRNA.
3
Translocation: ribosome moves one codon. Met-tRNA exits at E site. Phe-tRNA (now holding the chain) moves to P site. New A site opens.
4
Cycle repeats until stop codon. Release factor enters A site — protein released, ribosome disassembles.
Exam Application
Exams test the three ribosomal sites (A, P, E) and what occupies each, the elongation cycle steps in order, and what happens at termination. Know that the initiator Met-tRNA starts in the P site (not the A site), and that release factors (not tRNA) recognize stop codons.
⚠ Common Trap
Students think the first tRNA enters the A site — it doesn't. The initiator Met-tRNA goes directly to the P site. Also: stop codons are NOT recognized by tRNA — they are recognized by protein release factors. This distinction is frequently tested.
✓ Quick Self-Check
1. What do A, P, and E stand for in the ribosomal sites?
A = Aminoacyl (arrival), P = Peptidyl (peptide chain held here), E = Exit (uncharged tRNA leaves).
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2. Where does the initiator Met-tRNA bind?
The P site — not the A site. This is an exception to the normal entry point.
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3. What catalyzes peptide bond formation during elongation?
rRNA — it acts as a ribozyme (peptidyl transferase activity) within the large ribosomal subunit.
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4. What recognizes stop codons during termination?
Release factor proteins — not tRNA. No tRNA has an anticodon for stop codons.
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5. In what direction does the ribosome move along mRNA during translocation?
5' to 3' — the ribosome moves toward the 3' end of the mRNA, one codon at a time.
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