Translation

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Translation is the process of copying information from a string of mRNA to a protein. This is done by ribosomes, which translate the anticodon of a tRNA molecule according to the genetic code.

Overview

Translation may be divided into three stages:

Initiation

Translation is initiated when an mRNA molecule and the small subunit of a ribosome bind together, along with other proteins called 'initiation factors' and 'elongation factors', forming an 'initiation complex'. The small ribosomal subunit recognizes and binds to the initiation factors, which themselves have bound to a specific nucleotide sequence at the beginning (the 5' end) of the mRNA transcript. In prokaryotes this sequence is called the "Shine-Dalgarno sequence" after the scientists who discovered it, whereas in eukaryotes a different sequence called the "5' cap" is responsible.

In eukaryotes, once the initiation complex has been formed, it moves along the RNA (towards the 3' end) until reaching the codon 'AUG' , which codes for Methionine (see: Genetic Code). In prokaryotes, the Shine-Dalgarno sequences (there is usually more than one place where translation can begin in prokaryotes) are near a start/methionine codon (also AUG, but sometimes UUG or GUG can act as a start codon) and so the small subunit and initiation factors will already be in place. Because the large subunit is not yet bound, the tRNA with methionine attached (tRNA^Met) is escorted to the small unit by an initiation factor, called IF2, in prokaryotes or a special initiator tRNA (called tRNA_i^Met) in eukaryotes. Once it has bound, the initiation and elongation factors dissociate and large ribosomal subunit now binds to the small subunit around the tRNA, forming the complete ribosome to begin elongation.

Elongation

Assisted by elongation factors (EF-Tu in prokaryotes and EF-1α in eukaryotes), the aminoacyl-tRNAs enter the ribosome and bind to the codon in the A (acceptor) site. Now, the methionine and the next amino acid (whatever it happened to be) are next to each other, and the enzymatic (peptidyl transferase) activity of the large subunit will form a peptide bond between the carboxy end of the methionine and the amino end of the neighboring amino acid, forming a polypeptide, and the methionine will dissociate from tRNA^Met.

Now, the ribosome moves along the mRNA by three nucleotides (in a process called translocation). The now uncharged tRNA of methionine will be ejected via the E-site of the large subunit, the next amino-acyl tRNA will enter as before, and a second peptide bond will be formed. This cycle will continue until reaching a stop codon (see: Genetic Code), when termination will occur.

Termination/Release

When a stop codon is encountered, a 'release factor' protein (one type in eukaryotes, three types in prokaryotes) will enter instead of a tRNA. In the presence of this protein, the neighboring amino acid will be hydrolysed (bond forms with water instead of another amino acid) and dissociate from it's tRNA as per usual. Now, without a tRNA bound to it, the polypeptide is free to dissociate. The subunits of the ribosome also dissociate, free to rebind again on another mRNA transcript.

References

Hartl, Daniel L. & Jones, Elizabeth W. (2005), Genetics: Analysis of genes and genomes Jones and Bartlett Publishers. 6th Edition, p417-23
Garrett, R.H. & Grisham, C.M. (2005), Biochemistry Thomson Brooks/Cole Third Edition, 996-1011