Translation (biology)

Translation is how cells build proteins, using RNA as a blueprint. This process, a central part of gene expression, is carried out by ribosomes—large complexes made of RNA and proteins. The resulting protein is a chain of amino acids, and the order of those amino acids is dictated by the sequence of nucleotides in the RNA. The RNA is read in groups of three nucleotides, called codons, and each codon corresponds to a specific amino acid. This matching system is known as the genetic code. The whole operation takes place outside the nucleus, where messenger RNA (mRNA) is decoded by a ribosome to produce a polypeptide chain. That chain later folds into a working protein, though folding can also begin during synthesis.

The ribosome itself has two subunits—in eukaryotes, a small 40S subunit and a large 60S subunit—that join together on the mRNA to provide a platform for translation. Each new amino acid is added one at a time to the growing end of the polypeptide, specifically at the C-terminus, making translation amine-to-carboxyl directed. The mRNA carries genetic information from the chromosomes to the ribosome, where its codons are read. Transfer RNAs (tRNAs), small noncoding RNA chains (74–93 nucleotides), bring the correct amino acids to the ribosome. Each tRNA has an anticodon, a triplet complementary to the mRNA codon, and a site for attaching its specific amino acid. Enzymes called aminoacyl tRNA synthetases link each tRNA to its matching amino acid, creating a charged aminoacyl-tRNA. Mistakes in this pairing can lead to mischarged tRNAs and incorrect amino acids in the protein, a low-level error called mistranslation that can sometimes benefit the cell.

The ribosome has three binding sites for tRNA, arranged 5' to 3' along the mRNA: the exit site (E), the peptidyl site (P), and the aminoacyl site (A). An incoming aminoacyl-tRNA binds to the A site, matching its codon on the mRNA. A peptide bond then forms between the amino acid in the A site and the growing polypeptide chain held by the tRNA in the P site, transferring the chain to the A-site tRNA. The ribosome then translocates: the now-uncharged tRNA moves to the E site and leaves, while the tRNA carrying the polypeptide shifts to the P site. A new aminoacyl-tRNA enters the A site, and the cycle repeats. Energy for this movement comes from the hydrolysis of GTP bound to the translocase EEF2, which shifts the ribosome one codon toward the 3' end. The total energy cost for a protein of n amino acids is 4n-1 high-energy phosphate bonds. Translation rates differ: prokaryotic cells can add 17–21 amino acids per second, while eukaryotic cells manage 6–9 per second.

Translation proceeds in four stages: initiation, elongation, termination, and recycling. During initiation, the small ribosomal subunit binds to the 5' end of the mRNA with help from initiation factors. The ribosome and its associated factors assemble on the mRNA, and the first tRNA attaches at the start codon.