AP Syllabus focus:
‘Translation proceeds through elongation as amino acids are added, and terminates when a stop codon is reached and the completed protein is released.’
Translation’s core chemistry happens during elongation and termination: tRNAs repeatedly add amino acids to a growing polypeptide, then stop-codon recognition triggers release. These steps convert nucleotide sequence information into a finished protein.
Elongation of translation (adding amino acids)
Key structures used repeatedly
Ribosomes coordinate tRNAs and mRNA using three tRNA-binding sites:
Schematic overview of translation initiation and elongation highlighting the ribosome’s A, P, and E sites. It visually connects codon–anticodon recognition in the A site, peptide-bond formation, and ribosome translocation along mRNA (5′→3′), reinforcing how the polypeptide is extended each cycle. Source
A site (aminoacyl site): holds the incoming charged tRNA
P site (peptidyl site): holds the tRNA carrying the growing polypeptide
E site (exit site): where the uncharged tRNA leaves
The ribosome advances along mRNA in the 5′ to 3′ direction, and the polypeptide is synthesized from N-terminus to C-terminus.
Elongation (translation): the stage of translation in which amino acids are repeatedly added to a polypeptide as the ribosome moves codon-by-codon along the mRNA.
Stepwise elongation cycle
Each elongation cycle adds one amino acid and typically uses GTP for energy and directionality.
Three snapshots of the 70S ribosome during elongation: delivery of an aminoacyl-tRNA to the A site (via EF-Tu), peptide-bond formation, and EF-G–driven translocation. This figure helps students map where energy (GTP) is used and how tRNAs progress through A → P → E as the mRNA advances by one codon. Source
1) Codon recognition at the A site
A charged tRNA (tRNA + attached amino acid) enters the A site.
Correct pairing occurs between the mRNA codon and the tRNA anticodon.
Incorrect tRNAs are rejected, improving fidelity.
2) Peptide bond formation
The ribosome catalyses a peptide bond between:
the growing polypeptide on the P-site tRNA, and
the new amino acid on the A-site tRNA.
The polypeptide is transferred to the A-site tRNA, so the chain now sits in the A site.
A key AP idea is that ribosomal function depends on RNA as well as protein; catalysis is performed by ribosomal components rather than by the tRNAs themselves.
3) Translocation (ribosome moves)
The ribosome shifts one codon along the mRNA (5′ → 3′).
tRNAs shift positions:
A-site tRNA (now carrying the polypeptide) moves into the P site
P-site tRNA (now uncharged) moves into the E site and exits
The A site becomes available for the next charged tRNA, repeating the cycle.
Termination of translation (releasing the protein)
Termination begins when the ribosome encounters a stop codon in the A site (commonly UAA, UAG, or UGA).
Diagram of translation termination showing how a stop codon in the A site is recognized by a protein release factor rather than a tRNA. It illustrates hydrolysis of the peptidyl–tRNA linkage to free the completed polypeptide, followed by dissociation of the ribosomal subunits for recycling. Source
Stop codons do not code for an amino acid and therefore do not have matching tRNAs.
Termination (translation): the stage of translation in which a stop codon triggers release of the completed polypeptide and dissociation of the translation machinery.
Molecular events in termination
Stop codon recognition
A release factor (a protein, not a tRNA) binds in the A site when a stop codon is present.
Polypeptide release
The bond linking the polypeptide to the tRNA in the P site is hydrolysed, freeing the completed protein.
Disassembly and recycling
The ribosomal subunits separate from the mRNA.
The mRNA, tRNAs, and ribosomal subunits can be reused in additional rounds of translation.
Termination ensures that translation stops at the correct position, so protein length and amino acid sequence match the information encoded by the mRNA.
FAQ
Initial codon–anticodon pairing is checked before the ribosome commits to peptide bond formation.
Energy from GTP-associated factors helps discriminate correct vs incorrect matches, reducing misincorporation.
There is no amino acid to add at a stop codon.
Protein release factors mimic aspects of tRNA shape to fit the A site while triggering chemistry that releases the polypeptide.
Amino acids are linked by adding the incoming amino acid’s amino group to the growing chain’s carboxyl end.
This chemistry enforces growth from N to C regardless of the specific protein.
It is uncharged and can be recharged by a specific enzyme in the cytosol.
That enzyme reattaches the correct amino acid, allowing the tRNA to participate in another elongation cycle.
If termination is disrupted, the ribosome may stall at the stop codon.
Stalled ribosomes can reduce translation efficiency because the mRNA and ribosomal subunits remain occupied longer than normal.
Practice Questions
Describe what happens during translocation in elongation. (2 marks)
Ribosome moves one codon along the mRNA in the direction. (1)
tRNAs shift A P and P E, with the uncharged tRNA exiting. (1)
Explain how a stop codon leads to release of a completed polypeptide during translation. (5 marks)
Stop codon enters the A site (e.g., UAA/UAG/UGA). (1)
No tRNA matches a stop codon. (1)
A release factor binds to the A site instead. (1)
Hydrolysis breaks the bond between polypeptide and the P-site tRNA, releasing the polypeptide. (1)
Ribosome dissociates and components (subunits/mRNA/tRNAs) are recycled. (1)
