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Translation is the synthesis of a polypeptide from the information encoded in mRNA, carried out by ribosomes with the help of tRNAs. The ribosome has three tRNA-binding sites — A (aminoacyl), P (peptidyl), and E (exit) — and moves along the mRNA in the 5'-to-3' direction. Each elongation cycle involves: codon recognition by a charged tRNA in the A site, peptide bond formation by peptidyl transferase (a ribozyme), and translocation shifting the ribosome one codon. Translation begins at AUG and terminates when a stop codon enters the A site, releasing the finished protein.
Use a codon table and manually translate a short mRNA sequence step by step, tracking tRNA movements through A, P, and E sites. Compare prokaryotic (70S, Shine-Dalgarno) and eukaryotic (80S, Kozak sequence) initiation.
You know the genetic code: each three-nucleotide codon specifies an amino acid. Translation is the molecular machinery that reads codons in sequence and assembles the corresponding amino acids into a polypeptide. The central player is the ribosome, which is far more than a passive scaffold — it is a molecular machine with moving parts and, crucially, catalytic activity residing in its RNA.
The ribosome has three named sites that track the state of the tRNAs involved in each round of elongation. The A (aminoacyl) site receives an incoming charged tRNA — a tRNA carrying its specific amino acid — whose anticodon must complement the mRNA codon currently positioned there. The P (peptidyl) site holds the tRNA attached to the growing polypeptide chain. The E (exit) site holds the now-uncharged tRNA just before it leaves the ribosome. Each elongation cycle proceeds in three coordinated steps: a charged tRNA enters the A site and base-pairs with the codon; peptidyl transferase catalyzes the transfer of the polypeptide from the P-site tRNA to the A-site amino acid, forming a new peptide bond; and the ribosome translocates one codon in the 5'-to-3' direction, shifting the tRNAs from A to P, P to E, and ejecting the E-site tRNA. The surprising fact revealed by structural and biochemical studies is that peptidyl transferase is not a protein enzyme — it is the rRNA of the large ribosomal subunit. The ribosome is a ribozyme, and this discovery supports the idea that protein synthesis evolved in an RNA-based world.
Translation does not begin at just any AUG in the mRNA. Initiation requires signals that position the ribosome at the correct start codon. In prokaryotes, a Shine-Dalgarno sequence a few nucleotides upstream of the AUG base-pairs with the 16S rRNA of the small ribosomal subunit, delivering the ribosome directly to the right position. In eukaryotes, the 40S subunit loads at the 5' cap and scans in the 3' direction until it encounters an AUG in a favorable Kozak context. This difference has profound implications: prokaryotic mRNAs are often polycistronic (multiple genes on one mRNA, each with its own Shine-Dalgarno) while eukaryotic mRNAs are usually monocistronic, translated from a single start site.
The reading frame set at AUG is non-negotiable. Once the ribosome commits to an AUG, it reads every subsequent codon as exactly three nucleotides, without gaps or shifts. This is why a frameshift mutation — a single nucleotide insertion or deletion in the coding sequence — is typically catastrophic: it changes the triplet grouping of every codon downstream, producing a completely garbled amino acid sequence that almost always includes a premature stop codon. A single nucleotide change therefore does not just alter one amino acid; it destroys the entire C-terminal portion of the protein.
Termination is triggered by stop codons (UAA, UAG, UGA), which have no corresponding tRNAs. Instead, protein release factors bind the A site when a stop codon arrives, hydrolyze the bond between the polypeptide and the final tRNA, and release the finished protein. The ribosomal subunits then dissociate and are recycled. In active cells, many ribosomes can translate the same mRNA simultaneously, forming polysomes — a strategy that greatly amplifies protein output from a single mRNA molecule.