Translation is the biological process by which genetic information encoded in a messenger RNA (mRNA) molecule is decoded to synthesize a specific, functional protein. This conversion represents the second half of gene expression, following the transcription of DNA into mRNA. It is a core tenet of the central dogma of molecular biology, describing the flow of genetic information from nucleic acids to protein. By translating the sequence of nucleotides in mRNA into a sequence of amino acids, the cell constructs the complex machinery and structural components necessary for life.
Required Components for Protein Synthesis
Protein synthesis requires a coordinated collection of molecular machinery. The messenger RNA (mRNA) serves as the template, carrying genetic instructions in the form of codons—three-nucleotide sequences that specify individual amino acids. The cellular machine responsible for reading this template is the ribosome, a large complex of ribosomal RNA (rRNA) and proteins, which separates into a large and a small subunit until translation begins.
Transfer RNA (tRNA) molecules function as adaptor molecules, each carrying a specific amino acid at one end. The other end contains an anticodon, a three-nucleotide sequence complementary to an mRNA codon, ensuring the correct amino acid delivery. This reaction is energetically demanding, relying on the hydrolysis of Guanosine Triphosphate (GTP) and Adenosine Triphosphate (ATP) to power the assembly and movement of these components.
The Initiation Phase
The initiation phase marks the start of protein synthesis by assembling all necessary components into a functional complex. Translation begins when the small ribosomal subunit binds to the mRNA and scans for the start signal. This signal is the start codon, typically the three-nucleotide sequence AUG, which specifies the amino acid methionine.
Once the AUG start codon is located, an initiator transfer RNA (tRNA) carrying methionine pairs its anticodon to the codon. This resulting complex, consisting of the small ribosomal subunit, the mRNA, and the initiator tRNA, is known as the initiation complex. The large ribosomal subunit then joins this complex, positioning the initiator tRNA in the central P (peptidyl) site and leaving the adjacent A (aminoacyl) site open for the next charged tRNA.
The Elongation Phase
Elongation is the cyclical stage where the polypeptide chain grows one amino acid at a time. The cycle begins with codon recognition, as a new transfer RNA carrying its amino acid enters the A site of the ribosome. For this tRNA to be accepted, its anticodon must correctly match and base-pair with the codon exposed on the mRNA in the A site.
After pairing, the ribosome catalyzes the formation of a peptide bond between the amino acid in the P site and the amino acid in the A site. This reaction is mediated by the enzyme peptidyl transferase, located in the large ribosomal subunit. It transfers the growing polypeptide chain from the tRNA in the P site to the amino acid on the tRNA in the A site, leaving the tRNA in the P site “uncharged.”
The final step in the cycle is translocation, powered by the hydrolysis of GTP. The ribosome moves exactly one codon (three nucleotides) down the mRNA strand in the 3′ direction. This movement shifts the tRNAs: the uncharged tRNA moves from the P site to the E (exit) site, and the tRNA carrying the polypeptide chain moves from the A site to the P site. The E site tRNA is then released from the ribosome. This leaves the A site vacant, exposing the next codon and allowing the cycle to repeat until a stop signal is encountered.
The Termination Phase
Termination brings protein synthesis to an end when the ribosome encounters a stop codon on the messenger RNA. The three stop codons—UAA, UAG, and UGA—do not code for any amino acid and lack complementary transfer RNA molecules. When one of these stop codons enters the A site, it signals the binding of a protein known as a release factor. The release factor triggers the hydrolysis of the bond connecting the completed polypeptide chain to the final tRNA in the P site.
This reaction frees the newly synthesized polypeptide chain. Following the release of the protein, the entire complex disassembles. The large and small ribosomal subunits detach from the mRNA and from each other, making all components available for a new round of translation.
Protein Folding and Modification
Once the completed polypeptide chain is released, it must achieve a specific three-dimensional structure to become a functional protein. This process is known as protein folding, where the chain spontaneously contorts into its unique, active shape. Achieving the correct form is often assisted by specialized proteins called chaperones. Chaperones bind to the newly formed polypeptide and prevent improper folding or aggregation with other proteins.
The protein may also undergo post-translational modifications (PTMs) before it is fully active or directed to its final destination. Common modifications include the addition of chemical groups like phosphorylation, which acts as a molecular switch to turn the protein’s activity on or off, or cleavage, where parts of the polypeptide chain are cut away. These modifications and proper folding ensure the protein is biologically active and correctly targeted.