Mechanism of Translation

• Translation = process of polymerisation of amino acids into a polypeptide according to the sequence of codons on mRNA.
• It is the second step of gene expression in the central dogma: DNA → mRNA → protein. 
• Takes place on ribosomes present in the cytoplasm (and on rough ER in eukaryotic cells).

1. mRNA – carries genetic code in the form of codons (triplets of bases), each specifying an amino acid or stop signal.
2. tRNA – adaptor molecule with an anticodon that pairs with a codon and brings the specific amino acid to the ribosome.
3. Ribosome – site for decoding and peptide bond formation; consists of small and large subunits.
4. Amino acids – building blocks that join to form polypeptide chains.
5. Aminoacyl‑tRNA synthetase – enzyme that “charges” tRNA by attaching the correct amino acid; one synthetase per amino acid.
6. Energy sources – ATP (amino acid activation) and GTP (several steps of translation).
7. Protein factors – initiation, elongation and release factors assist various stages.

• Codon – three‑nucleotide sequence on mRNA coding for one amino acid / stop signal.
• Start codon – AUG; usually codes for methionine and marks the beginning of translation.
• Stop codons – UAA, UAG, and UGA – do not code for any amino acid and signal termination.

The P site and A site of a ribosome.

A-C. Steps in the protein synthesis: A-B. Consecutive stages in the attachment of tRNA/amino acid complexes by their anticodons to the codons on mRNA and the formation of a peptide bond between adjacent amino acids; C. Relative movements of mRNA and ribosome exposing a new triplet (frame) for the attachment of tRNA/amino acid complex.

Simplified summary diagram of the major steps involved in protein synthesis in a cell

A. Amino Acid Activation and tRNA Charging

• Each amino acid is first activated by ATP.
• Aminoacyl‑tRNA synthetase attaches the activated amino acid to its specific tRNA → forms aminoacyl‑tRNA (charged tRNA).

B. Initiation

• Small ribosomal subunit binds mRNA near the start codon.
• Initiator tRNA carrying methionine recognises AUG and binds at P site by codon‑anticodon pairing.
• Large ribosomal subunit joins → complete initiation complex is formed.
• Initiation needs: mRNA, small and large ribosomal subunits, initiator tRNA, initiation factors, GTP.
• Start codon is always placed at P site during initiation.

C. Elongation

• Charged aminoacyl‑tRNA enters the A site according to next codon.
• Peptidyl transferase (rRNA‑based activity of ribosome) forms peptide bond between new amino acid at A site and chain at P site.
• Ribosome translocates: mRNA shifts by one codon; tRNA from A moves to P; empty tRNA from P moves to E and exits.
• Steps 1–3 repeat; polypeptide length increases one amino acid at a time.

D. Termination

• When a stop codon (UAA/UAG/UGA) enters A site, no tRNA can bind.
• Specific release factors recognise stop codon and trigger release of the completed polypeptide from P site.
• Ribosomal subunits dissociate; mRNA and tRNAs are released.

A newly formed polypeptide is often not fully functional immediately after translation. It may undergo folding, cleavage, or chemical modification to become an active protein.

Examples of modifications

• folding into a specific three-dimensional structure.
• removal of extra terminal segments.
• addition of chemical groups required for function.

Many proteins remain in the cytoplasm, but some are transported to membranes, organelles, or secretory pathways after synthesis. Ribosomes attached to membranes help synthesise proteins meant for export or membrane insertion.

• Free ribosomes generally synthesise proteins used within the cytoplasm.
• Membrane-bound ribosomes often synthesise proteins that move into the endomembrane system or outside the cell.

Synthesis of proteins on free and membrane-bound polyribosomes