Translation is the synthesis of a polypeptide whose amino-acid sequence is specified by codons in messenger RNA.

The molecular machine that performs translation is the ribosome, a complex of ribosomal RNA (rRNA) and proteins. A ribosome has a small subunit that helps decode the mRNA and a large subunit whose rRNA-rich catalytic center promotes peptide-bond formation.

Translation uses three main inputs:

• an mRNA template;
• charged tRNAs carrying amino acids;
• the ribosome and accessory factors.

Initiation

The ribosome assembles on an mRNA and establishes the correct reading frame at a start codon, commonly AUG. An initiator tRNA pairs with that codon, and the complete ribosome forms around the mRNA-tRNA complex.

Elongation

The ribosome contains three functional tRNA positions commonly called the A, P and E sites.

1. A charged tRNA enters the A site and its anticodon is tested against the next mRNA codon.
2. The growing polypeptide is transferred from the tRNA in the P site to the amino acid carried in the A site, forming a new peptide bond.
3. The ribosome moves one codon along the mRNA. The tRNAs shift positions, and the empty tRNA eventually exits through the E site.

Repeated cycles extend the polypeptide from its N-terminus toward its C-terminus while the ribosome moves along mRNA $5'\rightarrow3'$.

Termination

When a stop codon enters the decoding site, no ordinary tRNA matches it. Instead, a release factor promotes release of the completed polypeptide and disassembly or recycling of the translation machinery.

For an mRNA segment

5'-AUG GCU UAC UGA-3'

the ribosome reads

AUG | GCU | UAC | UGA
Met | Ala | Tyr | Stop

and produces a short polypeptide whose sequence begins Met-Ala-Tyr.

The ribosome therefore does not invent the genetic code. Charged tRNAs embody the codon-to-amino-acid correspondence; the ribosome repeatedly selects matching tRNAs and catalyzes their assembly into a directional polypeptide chain.