Translation is the process of converting mRNA codon sequences into an amino acid polypeptide chain. mRNA synthesised in the nucleus of an eukaryote cell migrates through the nuclear pore into the cytoplasm. Later, tRNA, amino acids and ribosomes in the cytoplasm complete the process of translation.
The amino acids and tRNAs in cytoplasm become activated by aminoacylation, during which, the amino acids binds with ATP in the presence of aminoacyl RNA synthetase and is activated into aminoacyl–AMP–enzyme complex. This complex reacts with tRNA and transfers the amino acids to it. As a result, the tRNAs becomes activated as aminoacyl-tRNA and the enzyme and AMP are liberated. Each tRNA has an anticodon loop which determines the amino acid that will link to it. This anticodon of tRNA is complementary to the codon sequence on the mRNA and only binds to that codon.
In the inactive state, a ribosome consists of two subunits – a large subunit and a small subunit. The large subunit consists of two sites – the P or Peptidyl site, also called the donor site and A or Aminoacyl site, also called the acceptor site. At these sites, the two subsequent aminoacyl-tRNAs bind and facilitate the formation of a peptide bond between the two amino acids. For efficient translation, mRNA consists of the start codon, AUG that codes for Methionine which is a region that codes for the polypeptides, a stop codon which is either UAA, UAG or UGA and untranslated regions or UTR present at both the 5 prime-end before the start codon and the 3 prime-end after the stop codon.
Translation occurs in three phases: initiation, elongation and termination. In the initiation phase, the small subunit of the ribosome moves along the mRNA strand to bind to the start codon, AUG. The first aminoacyl-tRNA carrying the methionine with an anticodon UAC binds to the start codon at the P site, after which the large ribosomal subunit binds to the small ribosomal subunit. Another aminoacyl-tRNA now binds to the A site of the large ribosomal subunit and the first peptide bond is formed between these amino acids.
The peptide bond formation is catalysed by the peptidyl transferase enzyme. In the elongation phase, the ribosome runs along the mRNA in the 5 prime to 3 prime direction, moving from codon to codon and adding subsequent amino acids to the growing polypeptide chain. This is caused by the elongation factor, translocase, and this process is called translocation.
In the termination phase, once the ribosomes reaches the stop codon-UAG, a release factor or RF binds to it and terminates the translation and releases the polypeptide chain. Even the small and large subunits of the ribosomes dissociate and the mRNA degenerates.