Translation (biology)
Translation is the process by which a ribosome decodes the nucleotide sequence of a messenger RNA (mRNA) and synthesizes the corresponding sequence of amino acids to produce a polypeptide chain. It occurs in three phases — initiation, elongation, and termination — and takes place at ribosomes in the cytoplasm of both prokaryotes and eukaryotes. The genetic code, read in triplets called codons, determines which amino acid is incorporated at each step, with transfer RNA (tRNA) molecules acting as adaptors between the mRNA codons and the amino acids.
Phases of Translation and Their Key Events
| Phase | Key Event | Key Molecules | Result |
|---|---|---|---|
| Initiation | Ribosome assembles at start codon (AUG) | Small subunit, tRNA-Met, eIF factors | Ribosome ready to elongate |
| Elongation | Aminoacyl-tRNA enters A site; peptide bond formed | tRNA, EF-Tu, GTP, peptidyl transferase | Polypeptide chain grows |
| Translocation | Ribosome moves 3 nucleotides (one codon) along mRNA | EF-G, GTP | Next codon enters A site |
| Termination | Stop codon (UAA/UAG/UGA) reached | Release factors RF1/RF2 | Polypeptide released |
| Post-translation | Protein folds; may be modified | Chaperones, kinases, glycosyltransferases | Functional protein |
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Related Terms
Codon
A codon is a sequence of three consecutive nucleotides (a triplet) in mRNA that specifies a particular amino acid or signals the start or stop of protein synthesis. With four possible bases (A, U, G, C) at each of three positions, there are 4³ = 64 possible codons, which encode 20 standard amino acids plus three stop codons and one start codon (AUG, which codes for methionine). The redundancy of the genetic code — where multiple codons can specify the same amino acid — is called degeneracy and provides robustness against point mutations.
Transfer RNA (tRNA)
Transfer RNA (tRNA) is a small non-coding RNA molecule, typically 73–95 nucleotides long, that acts as a physical adaptor during translation by carrying a specific amino acid to the ribosome and recognising the corresponding mRNA codon via its anticodon loop. Each tRNA has a characteristic cloverleaf secondary structure that folds into an L-shaped three-dimensional conformation; the 3' CCA end accepts the amino acid (charged by aminoacyl-tRNA synthetase), while the anticodon loop base-pairs with the mRNA codon in the ribosomal A site. There are at least 61 functional tRNA species in cells (matching all sense codons), and their accurate aminoacylation is essential for translational fidelity.
Ribosomal RNA (rRNA)
Ribosomal RNA (rRNA) is the most abundant class of RNA in the cell, constituting approximately 80% of total cellular RNA, and forms the structural and catalytic core of the ribosome by associating with ribosomal proteins. In prokaryotes, the ribosome (70S) contains three rRNA species: 16S rRNA (small 30S subunit), 23S rRNA, and 5S rRNA (large 50S subunit); eukaryotic ribosomes (80S) contain 18S (small 40S subunit), 28S, 5.8S, and 5S rRNAs (large 60S subunit). The 23S/28S rRNA harbours the peptidyl transferase centre — the ribosome's catalytic site for peptide bond formation — making rRNA a ribozyme.
From Latin translatio, meaning "a carrying across" or "transfer." In molecular biology, the term metaphorically describes converting the language of nucleotides into the language of amino acids, a usage established in Francis Crick's 1958 central dogma paper.