The small subunit finds the start codon AUG, the initiator tRNA carrying methionine pairs with it in the P site, and the large subunit joins to complete the ribosome. Press Play or step forward to begin elongation.
Imagine the cell's DNA is a giant cookbook locked in the library (the nucleus). You cannot take the cookbook out, so you photocopy one recipe onto a card and carry it to the kitchen. That photocopy is the mRNA, and the kitchen is the ribosome. Little delivery carts, the tRNAs, each bring exactly one ingredient (an amino acid) whenever the recipe calls for it. Following the card in order, the kitchen assembles the ingredients into a finished dish — a protein.
The recipe is read in three-letter words. A codon is a group of $3$ mRNA bases, and each codon names one amino acid. With four bases in three slots there are $4^3 = 64$ codons, which between them specify the $20$ standard amino acids plus a "stop" — so the code is redundant (several codons can mean the same amino acid). Each tRNA carries a matching three-base anticodon that base-pairs with a codon (A–U, G–C), guaranteeing the right ingredient arrives. The ribosome holds three working slots in a row: the A site receives each new tRNA, the P site holds the chain, and the E site lets the spent tRNA exit.
Translation runs in three phases. Initiation: the small subunit finds the start codon AUG and an initiator tRNA carrying methionine sets the reading frame, then the large subunit joins. Elongation repeats a three-beat cycle — codon recognition (a new tRNA tests its anticodon at the A site) → peptide-bond formation (the large subunit's rRNA, a ribozyme, joins the amino acid to the chain) → translocation (the ribosome shifts one codon toward the 3′ end, tRNAs move A→P→E, and the empty tRNA leaves). Termination: at a stop codon a release factor takes the A site and the finished polypeptide is freed.
Press Play and pause on the peptide-bond step to see the chain hand off to the new amino acid. Toggle Anticodon pairing on and off to compare the codon below with the anticodon above each docked tRNA. Switch the Initiator from eukaryote to prokaryote to see the first residue change from Met to fMet, and turn on Cutaway to look inside the large subunit at the exit tunnel where the polypeptide threads out.
| Stage | What happens | Key players |
|---|---|---|
| 1 · Initiation | Small subunit binds mRNA and scans to the start codon AUG; initiator tRNA (Met) enters the P site; large subunit joins | mRNA, small + large subunits, initiator tRNA |
| 2 · Codon recognition | A new aminoacyl-tRNA enters the A site; its anticodon is tested against the next codon | A-site tRNA, mRNA codon |
| 3 · Peptide bond | The large-subunit rRNA (a ribozyme) joins the P-site chain to the A-site amino acid; the chain transfers to the A-site tRNA | Peptidyl transferase center (rRNA) |
| 4 · Translocation | The ribosome moves one codon toward 3′; tRNAs shift A→P and P→E; the empty tRNA exits the E site | Ribosome, GTP, elongation factors |
| 5 · Elongation (repeat) | Codon recognition, peptide bond, and translocation repeat, adding one amino acid per codon as the chain lengthens | Successive tRNAs, growing polypeptide |
| 6 · Termination | A stop codon (UAA/UAG/UGA) enters the A site; a release factor binds; the finished polypeptide is freed and the subunits separate | Stop codon, release factor |
Because a codon is three bases drawn from four letters, there are $64$ possible codons but only about $20$ amino acids to specify, so most amino acids have more than one codon — the code is degenerate (redundant). The codon AUG is special: it both starts translation and codes for methionine, so almost every freshly made polypeptide begins with Met (it is often trimmed off later). The three stop codons — UAA, UAG, UGA — code for no amino acid and instead signal the end.
The chemistry that links amino acids together, forming the peptide bond, happens at the peptidyl transferase center in the large subunit. Remarkably, the catalyst there is not protein but ribosomal RNA (rRNA). An enzyme made of RNA is called a ribozyme, so the ribosome itself is a ribozyme — a key clue supporting the idea that RNA-based catalysis came early in the history of life.
In eukaryotes, transcription occurs in the nucleus and the finished mRNA is exported to the cytoplasm, where ribosomes (free in the cytosol or studding the rough endoplasmic reticulum) translate it. In prokaryotes, which have no nucleus, ribosomes can begin translating an mRNA while it is still being transcribed, and the first amino acid is a modified methionine called formyl-methionine (fMet).
Protein synthesis is how a cell builds a protein from a gene, and its final stage is translation. A messenger RNA (mRNA) copy of the gene is read by a ribosome in groups of three bases called codons. Each codon specifies one amino acid, and transfer RNAs (tRNAs) bring the matching amino acids one at a time. The ribosome links them with peptide bonds into a chain (a polypeptide) that folds into a working protein. "Translation" captures the switch from the four-letter language of nucleic acids to the twenty-letter language of amino acids.
Key takeaway: translation reads mRNA codons three bases at a time and strings the matching amino acids into a polypeptide.They are two stages of gene expression in order. Transcription comes first: an enzyme copies one strand of a DNA gene into messenger RNA, so it is DNA rewritten into RNA. Translation comes second: a ribosome reads that mRNA and builds a protein, so it is RNA decoded into protein. Transcription stays in the same nucleic-acid language (like copying a sentence); translation switches languages (RNA to protein). In eukaryotes transcription is in the nucleus and translation is later in the cytoplasm.
Key takeaway: transcription makes the mRNA from DNA; translation reads that mRNA to build the protein.A codon is three consecutive mRNA bases that code for one amino acid (or a stop signal). With four possible bases in three positions there are 4 × 4 × 4 = 64 codons, which specify only 20 standard amino acids plus stop — so the code is redundant, with most amino acids having more than one codon. AUG does double duty as the start codon and codes for methionine; UAA, UAG, and UGA are stop codons that code for no amino acid.
Key takeaway: a codon is three bases; 64 codons cover 20 amino acids plus stop, so the code is redundant.A ribosome has three tRNA binding sites that a tRNA passes through in order: A, then P, then E. The A site (aminoacyl) accepts each newly arriving tRNA, where its anticodon is checked against the codon. The P site (peptidyl) holds the tRNA attached to the growing chain. The E site (exit) holds the now-empty tRNA just before it leaves. Each cycle forms a peptide bond and then translocates, shifting tRNAs from A to P and P to E.
Key takeaway: tRNAs move A to P to E — A receives, P holds the chain, E releases the spent tRNA.Transfer RNA (tRNA) is the adapter linking the code to real amino acids. Each tRNA has two ends: one carries a specific amino acid, the other has a three-base anticodon. The anticodon base-pairs antiparallel with a matching mRNA codon (A with U, G with C), so the right amino acid is delivered for that codon. Enzymes called aminoacyl-tRNA synthetases attach the correct amino acid to each tRNA beforehand.
Key takeaway: tRNA is an adapter whose anticodon reads the codon while its other end carries the matching amino acid.The peptide bond linking each new amino acid to the chain forms in the large subunit, at the peptidyl transferase center. The catalyst there is ribosomal RNA (rRNA), not protein. An enzyme made of RNA is a ribozyme, so the ribosome is a ribozyme: its RNA does the chemistry. This was strong evidence for the RNA world hypothesis, that early life used RNA both to store information and to catalyze reactions.
Key takeaway: peptide bonds form in the large subunit, and because rRNA catalyzes them, the ribosome is a ribozyme.It starts at the start codon AUG. The small subunit attaches near the 5-prime end of the mRNA and moves until it finds AUG; an initiator tRNA carrying methionine pairs with it, and the large subunit joins to complete the ribosome. Elongation repeats until the ribosome reaches a stop codon (UAA, UAG, UGA). No tRNA recognizes a stop codon; instead a release factor binds the A site, the finished polypeptide is freed, and the subunits come apart to be reused.
Key takeaway: translation begins at AUG with an initiator methionine tRNA and ends at a stop codon, where a release factor frees the protein.