Biochemistry · Topic
Translation and the genetic code !
In translation the ribosome reads the mRNA three bases at a time (codons) and joins the amino acids brought by tRNAs into a protein.
Updated 2026-11-01
Key facts
- Genetic code: 64 codons; 61 code for amino acids and 3 are stops (UAA, UAG, UGA). AUG = methionine and start.
- It is degenerate (several codons per amino acid), unambiguous, nearly universal and comma-free; wobble lies at the 3rd base.
- Initiation: the 40S subunit with the initiator tRNA (Met) scans the mRNA from the cap to the first AUG (Kozak sequence).
- Elongation: A (aminoacyl), P (peptidyl) and E (exit) sites; the 28S rRNA of the 60S subunit forms the peptide bond (a ribozyme).
- Cost: about 4 high-energy bonds per amino acid (2 from ATP to charge the tRNA, 1 GTP to enter the A site, 1 GTP for translocation).
In detail
- Activation: each aminoacyl-tRNA synthetase attaches its amino acid to the CCA end of the right tRNA using ATP (→ AMP + PPi); this is the step that ensures fidelity.
- Eukaryotic initiation: eIF4E recognises the cap, eIF2-GTP carries Met-tRNA, and once AUG is found the 60S joins. In bacteria, the Shine-Dalgarno sequence aligns the mRNA and the first amino acid is formyl-methionine.
- Elongation: eEF1A-GTP places the aminoacyl-tRNA in the A site; peptidyl transferase joins the peptide to the new amino acid; eEF2-GTP moves the ribosome one codon.
- Termination: a stop codon in the A site is recognised by release factors (eRF1, eRF3), which free the chain by hydrolysis.
- Several ribosomes can read the same mRNA at once (polysomes). Secreted proteins are made on the rough endoplasmic reticulum, guided by a signal peptide and the SRP particle.
- Then come folding (chaperones) and post-translational modifications: phosphorylation, glycosylation, collagen hydroxylation, vitamin K-dependent carboxylation, ubiquitination.
- Also called
- Protein synthesis, Genetic code
Clinical notes
- Antibiotics that target the bacterial (70S) ribosome: aminoglycosides and tetracyclines on the 30S subunit; macrolides, chloramphenicol, clindamycin and linezolid on the 50S.
- Diphtheria toxin and Pseudomonas exotoxin A ADP-ribosylate eEF2 and stop human translation; Shiga toxin and ricin inactivate the 28S rRNA.
- Nonsense mutations (premature stop codon) give truncated proteins, as in many cases of Duchenne dystrophy.
Related
- Ribosome (and the antibiotics that target it)
- Transfer RNA (tRNA)
- RNA processing (cap, poly-A tail and splicing)
- Central dogma of molecular biology
- Types of mutations
Test yourself
Think of the answer, then tap the question to see it.
What does it mean that the genetic code is degenerate?
Most amino acids have more than one codon; the difference usually lies in the third base.
Which ribosome component catalyses the peptide bond?
The 28S rRNA of the large subunit (23S in bacteria): the ribosome is a ribozyme.
How does diphtheria toxin stop protein synthesis?
It ADP-ribosylates elongation factor eEF2, which can no longer translocate the ribosome.
How many high-energy bonds does adding one amino acid cost?
About four: two to charge the tRNA (ATP → AMP) and two GTP during elongation (A-site entry and translocation).
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Translation (Reactome, 2026); Eukaryotic Translation Initiation (Reactome, 2026); How do genes direct the production of proteins?: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.