Translation

Core Concept

Translation is the ribosome-catalyzed synthesis of protein from an mRNA template. The mRNA's nucleotide sequence is read in triplets (codons), each specifying an amino acid or a stop signal. Transfer RNAs (tRNAs), each charged with its cognate amino acid by a specific aminoacyl-tRNA synthetase, decode the codons through base pairing. Translation proceeds in three phases — initiation at the AUG start codon, elongation via repeated rounds of tRNA delivery and peptide-bond formation, and termination at a stop codon — consuming GTP at several steps to ensure speed and accuracy.

Key Components

• Ribosome — a large (60S) + small (40S) subunit complex with three tRNA-binding sites: A (aminoacyl), P (peptidyl), E (exit).
• mRNA — carries the codon sequence; the start codon AUG (methionine) sets the reading frame.
• tRNA — adaptor molecules with an anticodon and an attached amino acid.
• Aminoacyl-tRNA synthetases — enzymes that attach each amino acid to its correct tRNA (aminoacylation).
• Initiation factors (eIFs) — assemble the ribosome at the start codon (eIF4E binds the 5′ cap; eIF2 delivers initiator Met-tRNA).
• Elongation factors (eEF1/eEF2) — deliver aminoacyl-tRNAs and translocate the ribosome.
• Release factors (eRF1/eRF3) — recognize stop codons and trigger termination.

Mechanism

The genetic code is read 5′→3′ in non-overlapping triplets. Each tRNA carries a specific amino acid and recognizes its codon via the anticodon. In the ribosome, the incoming aminoacyl-tRNA enters the A site, a peptide bond forms between the A-site amino acid and the P-site peptidyl chain (catalyzed by the rRNA-based peptidyl transferase center), and the ribosome translocates by one codon so the growing chain moves to the P site and the spent tRNA exits through the E site. This cycle repeats until a stop codon recruits release factors, which hydrolyze the completed polypeptide from the final tRNA.

How It Works

1. Initiation: eIF4E binds the cap, the 40S subunit scans to AUG, and the 60S subunit joins with initiator Met-tRNA in the P site.
2. Elongation — codon recognition: a cognate aminoacyl-tRNA•eEF1•GTP enters the A site.
3. Peptide bond formation: the peptidyl transferase center links the P-site chain to the A-site amino acid.
4. Translocation: eEF2•GTP moves the ribosome one codon, shifting tRNAs from A→P and P→E; the E-site tRNA exits.
5. Repeat: steps 2–4 occur for each codon.
6. Termination: a stop codon (UAA/UAG/UGA) is recognized by eRF1, and eRF3•GTP triggers hydrolysis and release of the polypeptide; subunits dissociate.

Energy and Directionality

Translation is the cell's most energetically expensive process, and its energy budget is dominated by GTP hydrolysis: charging each tRNA with an amino acid costs ~2 ATP-equivalents (ATP → AMP), and elongation consumes two GTPs per added amino acid (one for aminoacyl-tRNA delivery, one for translocation), plus additional GTP for initiation and termination. Directionality is absolute — the mRNA is read 5′→3′ and the polypeptide is built N-terminus to C-terminus — and is enforced by the AUG reading frame and the ordered A→P→E tRNA flow.

Experimental Evidence

• Nirenberg & Matthaei (1961) — synthetic poly(U) RNA directed synthesis of polyphenylalanine, cracking the first codon and proving mRNA is the template.
• Crick's frameshift experiments — triplet, non-overlapping reading of the code.
• Aminoacylation assays — purified synthetases attach amino acids to tRNAs in vitro, defining the adaptor hypothesis.
• Puromycin — an aminoacyl-tRNA mimic that terminates elongation, used to map the A/P site and the peptidyl transferase center.
• Ribosome structures — X-ray crystallography and cryo-EM captured the A/P/E sites and showed the peptidyl transferase center is composed of rRNA (ribozyme).

Technique

Methods include polysome profiling (measure translational activity), ribosome footprinting/Ribo-seq (map translated codons genome-wide), pulse-chase and ³⁵S-methionine labeling (measure protein synthesis), in vitro translation systems, and cryo-EM/X-ray crystallography of ribosomal states.

How it works

1. Initiation: eIF4E binds the cap, the 40S subunit scans to AUG, and the 60S subunit joins with initiator Met-tRNA in the P site.
2. Elongation — codon recognition: a cognate aminoacyl-tRNA•eEF1•GTP enters the A site.
3. Peptide bond formation: the peptidyl transferase center links the P-site chain to the A-site amino acid.
4. Translocation: eEF2•GTP moves the ribosome one codon, shifting tRNAs from A→P and P→E; the E-site tRNA exits.
5. Repeat: steps 2–4 occur for each codon.
6. Termination: a stop codon (UAA/UAG/UGA) is recognized by eRF1, and eRF3•GTP triggers hydrolysis and release of the polypeptide; subunits dissociate.

Common confusions

• "The ribosome reads DNA" — it reads mRNA; DNA is the template for transcription.
• "Amino acids directly read codons" — tRNAs (via anticodons) decode codons; amino acids are attached to tRNAs by synthetases.
• "The start codon is read anywhere" — the ribosome scans to AUG and the reading frame is fixed from there.
• "The ribosome's catalytic activity is protein-based" — the peptidyl transferase center is rRNA (a ribozyme).
• "Energy comes mainly from ATP" — elongation is driven largely by GTP hydrolysis (plus ATP for aminoacylation).

Quick review

• mRNA read 5′→3′ in triplets; AUG start, UAA/UAG/UGA stop.
• tRNA + aminoacyl-tRNA synthetases decode the genetic code.
• Initiation (eIFs, cap, Met-tRNA) → elongation (A/P/E sites, peptide bond, translocation) → termination (release factors).
• rRNA catalyzes peptide-bond formation; ~2 GTP/amino acid.
• N→C growth; ribosome is the target of many antibiotics.
• Studied by Ribo-seq, polysomes, in vitro translation, cryo-EM.