5.5.7 Translation of mRNA
Messenger RNA molecules are read and translated into protein by complex RNA–
protein particles termedribosomes. The ribosomes are termed 70S or 80S depending
on their sedimentation coefficient. Prokaryotic cells have 70S ribosomes whilst those
of the eukaryotic cytoplasm are 80S. Ribosomes are composed of two subunits that
are held apart by ribosomal binding proteins until translation proceeds. There are sites
on the ribosome for the binding of one mRNA and two tRNA molecules and the
translation process is in three stages.- Initiation: involving the assembly of the ribosome subunits and the binding of the
mRNA. - Elongation: where specific amino acids are used to form polypeptides, this being
directed by the codon sequence in the mRNA. - Termination: which involves the disassembly of the components of translation
following the production of a polypeptide.
Transfer RNA molecules are also essential for translation. Each of these are covalently
linked to a specific amino acid, forming anaminoacyl tRNA, and each has a triplet of
bases exposed which is complementary to the codon for that amino acid. This exposed
triplet is known as theanticodon, and allows the tRNA to act as an ‘adapter’ molecule,
bringing together a codon and its corresponding amino acid. The process of linking an
amino acid to its specific tRNA is termedchargingand is carried out by the enzyme
aminoacyl tRNA synthetase.
In prokaryotic cells the ribosome binds to the 5^0 end of the mRNA at a sequence
known as a ribosome binding site or sometimes termed theShine–Dalgarno sequence
after the discoverers of the sequence. In eukaryotes the situation is similar but
involves a Kozak sequence located around the initiation codon. Following translation
initiation the ribosome moves towards the 3^0 end of the mRNA, allowing an aminoacyl
tRNA molecule to base-pair with each successive codon, thereby carrying in amino
acids in the correct order for protein synthesis. There are two sites for tRNA molecules
in the ribosome, the A site and the P site, and when these sites are occupied, directed
by the sequence of codons in the mRNA, the ribosome allows the formation of a
peptide bond between the amino acids. The process is also under the control of
an enzyme, peptidyl transferase. When the ribosome encounters atermination codon
(UAA, UGA or UAG) a release factor binds to the complex and translation stops,
the polypeptide and its corresponding mRNA are released and the ribosome divides
into its two subunits (Fig. 5.19). A myriad of accessory initiation and elongation
protein factors are involved in this process. In eukaryotic cells the polypeptide may
then be subjected topost-translational modificationssuch as glycosylation and by
virtue of specific amino acid signal sequences may be directed to specific cellular
compartments or exported from the cell.
Since the mRNA base sequence is read in triplets, an error of one or two nucleotides in
positioning of the ribosome will result in the synthesis of an incorrect polypeptide. Thus it
is essential for the correct reading frame to be used during translation. This is ensured160 Molecular biology, bioinformatics and basic techniques