Translation is the process by which ribosomes read the sequence of messenger RNA (mRNA) and assemble amino acids into a polypeptide, the chain that forms a protein. It is a central stage of gene expression, distinct from transcription, which produces RNA from a DNA template. Translation converts information written in nucleotides into an amino-acid sequence through the genetic code, using transfer RNAs as molecular adaptors. Its principal stages are initiation, elongation, and termination, followed by recycling of the translation machinery. (ncbi.nlm.nih.gov)
Information and molecular machinery
The ribosome reads mRNA in the 5′-to-3′ direction, interpreting successive groups of three nucleotides called codons. In the standard genetic code, 61 codons specify amino acids and three—UAA, UAG, and UGA—normally signal termination. Several codons can specify the same amino acid. The selected initiation site establishes the reading frame, determining how subsequent nucleotides are grouped. Insertions or deletions that disrupt this frame can change the entire downstream protein sequence. (ncbi.nlm.nih.gov)
Transfer RNA (tRNA) connects codon recognition with amino-acid delivery. Each tRNA has an anticodon that pairs with an appropriate mRNA codon and an attachment site for an amino acid. Aminoacyl-tRNA synthetases are enzymes that attach the appropriate amino acids to their corresponding tRNAs, using ATP. Correct charging is essential because decoding normally checks the codon–anticodon interaction rather than directly identifying the attached amino acid. Flexible “wobble” pairing allows some tRNAs to recognize more than one codon. (ncbi.nlm.nih.gov)
Ribosomes contain a small and a large subunit, each composed of proteins and ribosomal RNA (rRNA). The small subunit participates in decoding; the large subunit contains the peptidyl-transferase center that forms peptide bonds. This catalytic activity depends on rRNA, making the ribosome a ribozyme. Bacterial ribosomes are designated 70S, whereas eukaryotic cytosolic ribosomes are 80S; these labels describe sedimentation behavior rather than additive masses. (ncbi.nlm.nih.gov)
Initiation
Initiation positions an initiator tRNA and the start codon in the ribosome’s P, or peptidyl, site. Initiation factors coordinate assembly of the small subunit, mRNA, and initiator tRNA; joining of the large subunit produces a complex ready for elongation. AUG is the usual start codon, although alternative initiation codons occur. (ncbi.nlm.nih.gov)
In many bacteria, a Shine–Dalgarno sequence upstream of the start codon pairs with small-subunit rRNA, helping position the ribosome. Bacterial initiator tRNA carries N-formylmethionine. In eukaryotes, the common pathway instead recruits initiation machinery near the mRNA’s 5′ cap. The small-subunit complex scans toward the 3′ end until it recognizes a suitable start codon, where initiation uses methionine. Not every eukaryotic transcript follows this pathway: some support internal recruitment without conventional cap-dependent scanning. (ncbi.nlm.nih.gov)
Elongation and termination
During elongation, an aminoacyl-tRNA enters the ribosome’s A, or aminoacyl, site with an elongation factor and guanosine triphosphate (GTP). Appropriate codon recognition promotes GTP hydrolysis and accommodation of the tRNA. The growing chain is transferred from the P-site tRNA to the amino acid on the A-site tRNA, forming a new peptide bond. The polypeptide therefore grows from its amino terminus toward its carboxyl terminus. (ncbi.nlm.nih.gov)
Translocation then advances the ribosome by one codon. The chain-bearing tRNA moves toward the P site, while the deacylated tRNA passes through the E, or exit, site and leaves. Elongation factors and GTP help coordinate these movements. Repeated cycles couple decoding to chain extension, while kinetic selection steps reduce incorporation of incorrectly matched tRNAs. (ncbi.nlm.nih.gov)
When a normal stop codon enters the A site, a release factor replaces the incoming aminoacyl-tRNA. It promotes hydrolysis of the bond linking the completed polypeptide to its tRNA. Recycling subsequently separates the ribosomal components and releases the remaining RNA molecules for further use. Multiple ribosomes can translate one mRNA simultaneously, forming a polysome. (ncbi.nlm.nih.gov)
Cellular location and regulation
In bacteria, translation can begin before transcription of the same message has finished. In eukaryotes, nuclear transcription and cytoplasmic translation are spatially separated. Cytosolic ribosomes may remain free or become associated with the endoplasmic reticulum while producing many secreted and membrane proteins. Mitochondria and chloroplasts possess translation systems for proteins encoded by their own genomes. Some organellar genetic codes differ from the standard code. (ncbi.nlm.nih.gov)
Translation is regulated both globally and transcript by transcript. Changes in initiation-factor activity can alter overall protein synthesis, while proteins binding particular mRNA sequences can selectively inhibit initiation. For example, iron-responsive proteins regulate ferritin translation through elements in its mRNA leader. Initiation control permits cells to change protein production without first changing transcript abundance. Protein folding can also begin while a chain is still emerging from the ribosome. (ncbi.nlm.nih.gov)
Experimental investigation
Ribosome profiling examines translation by sequencing mRNA fragments protected from nuclease digestion by ribosomes. Mapping these “footprints” identifies translated regions and measures ribosome occupancy along transcripts. Combined with measurements of mRNA abundance, it reveals differences in translational activity that transcript measurements alone cannot capture. A 2009 study demonstrated genome-wide, nucleotide-resolution profiling in budding yeast and documented extensive translational regulation under nutrient-rich and starvation conditions. Ribosome occupancy nevertheless requires interpretation: the density of footprints reflects both ribosome recruitment and how rapidly ribosomes progress through a coding region. (pmc.ncbi.nlm.nih.gov)