tRNA in Polypeptide Synthesis: Translation, Step by Step

By What Peptides Editorial Team · Updated 2026-09-14 · Part of Peptide Structure, Classification & Scientific Terminology

Transfer RNA is the adapter that makes the genetic code readable as chemistry. One end carries an anticodon triplet that pairs with a codon on messenger RNA; the other end carries the matching amino acid, joined by a high-energy ester bond. Because the pairing rules are separate from the amino acid attached, one 76 to 90 nucleotide fold can serve any of the standard residues, and the ribosome never needs to know which amino acid it is joining. This page follows a round of elongation from charging through peptidyl transfer to translocation, names the machinery at each step, and gives the energy accounting. It is descriptive molecular biology written for study and reference.

Two features of transfer RNA explain most of what follows. The first is the CCA sequence at the 3' end, which is the attachment site: the amino acid is esterified to the ribose of the terminal adenosine. The second is the set of modified bases in and around the anticodon, which tune pairing and let one transfer RNA read more than one codon. Errors are controlled at two separate checkpoints, one in the synthetase and one on the ribosome, and the measured error rate of translation sits around one wrong residue in ten thousand. For the code itself see how codons map to residues; for the bond that results, the chemistry is set out in our peptide structure and classification reference.

Charging: aminoacyl-tRNA synthetases and the energy input

Charging happens in two steps inside one enzyme. First the amino acid attacks the alpha phosphate of ATP, forming an aminoacyl-adenylate and releasing pyrophosphate; hydrolysis of that pyrophosphate by pyrophosphatase makes the step effectively irreversible. Second, the hydroxyl on the terminal adenosine of the transfer RNA attacks the carbonyl of the aminoacyl-adenylate, releasing AMP and forming an ester between the amino acid carboxyl group and the ribose. Two phosphate bonds are therefore spent to make one ester, and that ester is the stored energy later used to drive peptide bond formation on the ribosome. This is why charging is described as activation rather than simply as attachment.

The enzymes come in two structural classes of ten each, differing in fold, in oligomeric state and in which ribose hydroxyl they esterify first. Recognition is not based on the anticodon alone; identity elements are distributed across the acceptor stem, the discriminator base and the anticodon, which is why a single base change in an acceptor stem can cause mischarging. The enzymes that handle isosteric residues, such as isoleucyl-tRNA synthetase with valine, carry a separate editing site that hydrolyses the wrong product, a proofreading arrangement known as the double sieve.

• ATP is converted to AMP and pyrophosphate during charging, so two phosphate bonds are spent per amino acid attached.
• The ester bond between the amino acid and the terminal adenosine stores the energy used later for peptide bond formation.
• Class I and class II synthetases differ in fold, in oligomeric state and in which ribose hydroxyl they use first.
• Identity elements sit in the acceptor stem and the discriminator base as well as in the anticodon.
• Editing sites hydrolyse mischarged amino acids before the wrong residue reaches the ribosome.

The ribosome: A, P and E sites and an RNA-catalysed reaction

The ribosome holds transfer RNAs in three positions. The A site accepts an incoming aminoacyl-tRNA, delivered as a ternary complex with elongation factor Tu and GTP in bacteria, or with eEF-1A in eukaryotes. The P site holds the transfer RNA carrying the growing chain. The E site is the exit position, occupied by a deacylated transfer RNA on its way out. Decoding is a two-stage selection: initial binding is reversible and rejects most near-cognate species, then correct codon-anticodon geometry in the decoding centre accelerates GTP hydrolysis on the factor, and a second step follows before the aminoacyl end is accommodated into the peptidyl transferase centre.

Peptide bond formation itself needs no soluble factor and no nucleotide. The amine of the A-site aminoacyl-tRNA attacks the ester carbonyl that links the P-site chain to its transfer RNA, and the growing chain is transferred wholesale to the A-site RNA, longer by one residue. The reaction is catalysed by the peptidyl transferase centre of the large subunit, and crystallographic work placed no protein side chain within about 18 angstroms of the reaction centre, so the catalytic activity belongs to ribosomal RNA and the ribosome is a ribozyme. Catalysis appears to be mostly positioning, with substrate-assisted proton transfer involving the ribose hydroxyl of the transfer RNA rather than a protein acid-base group.

Translocation, termination, and what limits fidelity

After the bond forms, the A-site transfer RNA carries the chain and the P-site transfer RNA is empty, so the ribosome must move one codon. Elongation factor G in bacteria, or eEF-2 in eukaryotes, binds with GTP and drives a ratchet-like rotation of the subunits that advances the messenger RNA and the transfer RNAs, leaving the peptidyl-tRNA in the P site and opening the A site for the next round. GTP hydrolysis releases the factor. Adding it up, bacteria spend about four high-energy phosphate bonds per residue: two from charging and two from the elongation factors, with additional expenditure at initiation and termination. Elongation in bacteria runs at roughly 15 to 20 residues per second, and mammalian systems are slower.

Termination uses no transfer RNA at all. A stop codon in the A site is recognised by a release factor protein whose conserved GGQ motif positions a water molecule to attack the ester linking the chain to the transfer RNA, releasing the finished polypeptide. Bacteria use two factors to cover the three stop codons; eukaryotes use one factor that recognises all three with a partner GTPase. Fidelity comes from several filters in series: synthetase selection, editing of mischarged products, and two-step decoding on the ribosome with kinetic proofreading after GTP hydrolysis. Measured substitution rates land near one wrong residue in ten thousand, and most such errors are tolerated because a single residue change rarely destroys a fold.

One structural detail explains why a cell needs fewer transfer RNAs than codons. Pairing at the third codon position tolerates non-standard geometry, and modified bases such as inosine at position 34 expand what one anticodon can read, so bacteria commonly carry 40 to 50 species against 61 sense codons. The cloverleaf secondary structure, with its acceptor stem, D arm, anticodon arm and T arm, folds into the L shape that fits the ribosome. A specialised transfer RNA also reads UGA as selenocysteine when the message carries the right recoding signal. See how a 191-residue protein is numbered for what a finished chain looks like once this machinery has run.

This page is part of the Peptide Structure, Classification & Scientific Terminology guide.