Translation is the process by which the genetic code carried by mRNA is decoded by the ribosome to synthesize a polypeptide chain with a specific amino acid sequence. It is the final step in the central dogma of molecular biology.
The Genetic Code
The genetic code maps each three-nucleotide codon to one of twenty amino acids or a stop signal. Of the 64 possible codons, 61 specify amino acids and three are stop codons. The code is degenerate, with most amino acids encoded by multiple codons that typically share the first two bases. The code is read without overlap or punctuation in the 5-prime to 3-prime direction. The nearly universal nature of the genetic code supports the common ancestry of all life, with only minor variations found in mitochondria and some protozoa.
Aminoacyl-tRNA Synthesis
Translation requires that each amino acid be attached to its corresponding tRNA. Aminoacyl-tRNA synthetases catalyze this two-step reaction. First, the amino acid reacts with ATP to form an aminoacyl-AMP intermediate. Second, the activated amino acid is transferred to the 3-prime end of the tRNA. There is at least one synthetase for each amino acid, and each synthetase must discriminate between its correct amino acid and tRNA substrates. Editing mechanisms hydrolyze mischarged tRNAs, maintaining a high fidelity of about one error in 10,000.
Ribosome Structure
The ribosome is a large ribonucleoprotein complex composed of two subunits. The small subunit binds mRNA and provides the decoding center where codon-anticodon pairing is monitored. The large subunit contains the peptidyl transferase center, where peptide bond formation occurs, and the exit tunnel through which the nascent polypeptide emerges. Ribosomal RNA constitutes about two-thirds of the ribosome mass and is responsible for both decoding and catalysis. The 23S rRNA of the large subunit catalyzes peptide bond formation, making the ribosome a ribozyme.
Translation Initiation
Initiation establishes the reading frame by positioning the ribosome at the start codon, usually AUG encoding methionine. In bacteria, the Shine-Dalgarno sequence upstream of the start codon base-pairs with 16S rRNA. Initiation factors IF1, IF2, and IF3 assemble the 30S subunit, mRNA, and initiator fMet-tRNA, and the 50S subunit joins upon GTP hydrolysis.
Eukaryotic initiation is more complex. The cap-binding complex eIF4F recognizes the 5-prime cap, and the 40S subunit with initiator Met-tRNA scans along the mRNA until it reaches the first AUG in a favorable Kozak context. Over a dozen initiation factors coordinate this process, with eIF2-GTP delivering the initiator tRNA. Phosphorylation of eIF2-alpha is a major regulatory mechanism that globally inhibits translation during stress.
Elongation
Elongation proceeds through repeated cycles of aminoacyl-tRNA binding, peptide bond formation, and translocation. Elongation factor EF-Tu in bacteria, or eEF1A in eukaryotes, delivers aminoacyl-tRNA to the A site in a GTP-dependent manner. Correct codon-anticodon pairing triggers GTP hydrolysis and release of the factor. The peptidyl transferase center catalyzes the transfer of the growing peptide chain from the P site tRNA to the aminoacyl-tRNA in the A site. EF-G/eEF2 promotes translocation, moving the ribosome three nucleotides along the mRNA.
Termination
Termination occurs when a stop codon enters the A site. Release factors recognize stop codons and trigger hydrolysis of the peptidyl-tRNA bond. In bacteria, RF1 recognizes UAA and UAG, while RF2 recognizes UAA and UGA. Eukaryotes use eRF1, which recognizes all three stop codons, in complex with eRF3. The ribosome then disassembles with the help of ribosome recycling factors.
Post-Translational Events
The newly synthesized polypeptide undergoes folding, often assisted by chaperones. Signal sequences target proteins to specific cellular locations, including the endoplasmic reticulum, mitochondria, or nucleus. Co-translational translocation delivers secreted and membrane proteins to the ER as they are synthesized. Numerous modifications may occur, including proteolytic cleavage, phosphorylation, glycosylation, and disulfide bond formation, before the protein reaches its final functional form.
