Translation


Translation is the process of polymerizing a polypeptide chain according to the sequence of 'instructions' on an mRNA transcript. It is called translation because it involves sequential information being translated from the 'language' of nucleotides on the mRNA strand to the 'language' of amino acids in a protein (see the central dogma). In both eukaryotes and prokaryotes, translation occurs on ribosomes; however, while eukaryotic translation is compartmentalized from transcription in the cell, the two processes are coupled in prokaryotes.

Triplets of bases on an mRNA molecule form a structure called a codon. The codon codes for a particular amino acid, by way of having a complementary anticodon base triplet on an amino acid-carrying tRNA (aminoacyl-tRNA). The pairing of codon-anticodon between mRNA and aminoacyl-tRNA along the entire length of the mRNA for all of its codons is how the correct amino acids are polymerized in the correct order. mRNA is translated in the 5' to 3' direction, and the nascent polypeptide is synthesised from the N-terminus to the C-terminus. The first amino acid in a polypeptide is universally methionine (met) and so the 'initiator' aminoacyl-tRNA will be tRNAmet.

Translation, like transcription, has three key stages: initiation, elongation and termination. These stages differ between prokaryotes and eukaryotes.

Prokaryotic translation:

In prokaryotes, mRNA is polycistronic and may be translated by several ribosomes simultaneously (forming a structure called a polysome). Prokaryotic initiation requires tRNA (charged with formyl-methionine), initiation factors (IF-1 through IF-3), GTP and magnesium ions. The initiation complex of these molecules helps to position mRNA on the ribosome ready to scan for the AUG start codon. Prokaryotes have a 70S ribosome with a 50S larger subunit; for the next process, elongation, three tRNA-binding sites on this 50S subunit are required: the P (peptidyl) site, the A (aminoacyl) site and the E (exit) site. Peptide bonds are formed between amino acids in the P site, while new aminoacyl-tRNAs are bound in the A site, and discharged (used) tRNAs are liberated on the E site. Termination requires a stop codon on the mRNA (UAG, UAA, UGA) and various proteins called release factors.

Eukaryotic translation:

In eukaryotes, mRNA is monocistronic and carries a 5' methylguanosine cap. This cap is recognised by protein factors (eIF - eukaryotic initiation factors) which help to position the mRNA on the small (40S) subunit of the ribosome. Initiation is the most different process when comparing eukaryotes to prokaryotes, and over 13 more initiation factors are required in eukaryotes. The initiator codon (AUG) is recognised by a tRNA carrying methionine (met. rather than fmet. as in prokaryotes; it is not formylated in eukaryotes) and, following initiation, polypeptide polymerization continues (this is elongation). Elongation is very similar in eukaryotes to prokaryotes, although eukaryotes lack the E site while keeping the P and A sites on the ribosome. Termination still requires the same stop codons (UAG, UAA, UGA) but only one release factor is needed as it binds to all these stop codons.