Translation turns mRNA into protein. A ribosome, loaded with mRNA and tRNAs carrying amino acids, reads codons in sequence and links amino acids into a polypeptide chain. The process has three stages: initiation, elongation, and termination.
The ribosome: three sites, one movement
Translation
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The growing polypeptide Charged tRNA arriving Stop codon: no tRNA exists for it GTP spent
The catalyst is RNA, not proteinPeptide bond formation is done by peptidyl transferase, which is part of the large subunit's ribosomal RNA. The ribosome is a ribozyme, and that is a favorite exam fact because it undercuts the assumption that catalysis means protein.
Reading frame is everythingThe code is read three bases at a time with no punctuation, so an insertion or deletion that is not a multiple of three shifts the frame and garbles everything downstream. A substitution changes at most one amino acid; a frameshift changes the whole tail of the protein.
Where the ribosome sitsTranslation starts free in the cytosol. If the emerging chain begins with a signal sequence, the whole ribosome is towed to the rough ER and finishes there, which sends the protein into the secretory pathway. No signal sequence means the protein stays in the cytosol.
One tRNA moves right to left through three sites, and the chain it carries grows by one residue each time. Everything else, the factors, the GTP, the start and stop signals, is arrangement around that single movement.
Initiation
The small ribosomal subunit binds the mRNA near the start codon. In bacteria, the Shine-Dalgarno sequence (AGGAGG, upstream of AUG) base-pairs with 16S rRNA. In eukaryotes, the small subunit binds the 5’ cap and scans along the mRNA until it finds the first AUG in a favorable context (the Kozak sequence, GCCACCAUGG).
The initiator tRNA carrying methionine (or formyl-methionine in bacteria) base-pairs with the start AUG in the P site. The large ribosomal subunit joins, completing the ribosome.
Elongation
Elongation repeats three steps for every codon:
A site entry: an aminoacyl-tRNA with an anticodon matching the current codon binds in the A site (with GTP hydrolysis by EF-Tu / eEF1A).
Peptide bond formation: the peptidyl transferase activity of the large subunit (catalyzed by 23S/28S rRNA - a ribozyme) transfers the growing chain from the P-site tRNA to the amino acid on the A-site tRNA. The growing chain now hangs from the A-site tRNA.
Translocation: the ribosome shifts one codon (with GTP hydrolysis by EF-G / eEF2). The A-site tRNA moves to the P site; the P-site tRNA moves to the E site and leaves. A new codon is exposed in the A site.
The ribosome moves along the mRNA 5’ to 3’, elongating the polypeptide from N-terminus to C-terminus.
Termination
When the A site encounters a stop codon (UAA, UAG, or UGA), a release factor enters the A site instead of a tRNA. Release factors trigger hydrolysis of the bond linking the polypeptide to the P-site tRNA. The finished polypeptide is released. The ribosome dissociates.
Energy Cost
Translation is expensive. Per amino acid added:
1 ATP to charge the tRNA with its amino acid (aminoacyl-tRNA synthetase).
1 GTP during A-site entry (EF-Tu / eEF1A).
1 GTP during translocation (EF-G / eEF2).
Plus initiation and termination costs. Building a 300-residue protein burns about 1200 high-energy phosphate bonds.
What are the three steps of each elongation cycle on the ribosome?
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(1) A-site entry: aminoacyl-tRNA matches the codon in the A site. (2) Peptide bond formation: the peptidyl transferase (rRNA catalytic activity) joins the growing chain to the new amino acid. (3) Translocation: the ribosome moves one codon, shifting tRNAs from A → P → E.
Why is the ribosome called a ribozyme?
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Because its catalytic activity (peptidyl transferase - forming peptide bonds) is performed by rRNA, not protein. The 23S rRNA in bacteria (28S in eukaryotes) catalyzes peptide bond formation. This makes the ribosome an RNA-based enzyme, supporting the idea that RNA-catalyzed reactions predated protein enzymes.
What happens when the ribosome reaches a stop codon?
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A release factor enters the A site (no tRNA anticodon matches a stop codon). The release factor triggers hydrolysis of the bond between the P-site tRNA and the polypeptide, releasing the finished protein. The ribosome then dissociates into large and small subunits, which recycle to initiate new translation.
