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[Biochemistry](https://durgical.com/en/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)](https://durgical.com/en/biochemistry/enzymes/ribosome/)
- [Transfer RNA (tRNA)](https://durgical.com/en/biochemistry/topics/transfer-rna/)
- [RNA processing (cap, poly-A tail and splicing)](https://durgical.com/en/biochemistry/topics/rna-processing/)
- [Central dogma of molecular biology](https://durgical.com/en/biochemistry/topics/central-dogma-of-molecular-biology/)
- [Types of mutations](https://durgical.com/en/biochemistry/topics/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).

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Translation](https://reactome.org/PathwayBrowser/#/R-HSA-72766) (Reactome, 2026); [Eukaryotic Translation Initiation](https://reactome.org/PathwayBrowser/#/R-HSA-72613) (Reactome, 2026); [How do genes direct the production of proteins?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/howgeneswork/makingprotein/) (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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