Translation in an important and complex feature of the process of protein synthesis. Genetic information codes for proteins via protein synthesis, this is an essential process as proteins are responsible for the vast majority of cell function and structure. Protein synthesis involves mRNA and tRNA along with other proteins and has three main steps:
Translation is the most complex, it consists of the nucleotide sequence of mRNA being translated into the amino acid secquence of the specific protein. The direction that translation is carried out is very significant; it occurs in the same direction as transcription (5'-3') this results in proteins being produced more efficiently as translation can occur during transcription .
The mRNA and tRNA play very important specific roles during translation; firstly mRNA acts as a template for the production of the polypeptide chain from the genetic code. The genetic code have three important features:
The genetic code is degenerate because it has 64 codons but only 20 amino acids, therefore most amino acids are coded for by more than one codon. 61 of these codons are used for amino acids and 3 are used as stop codons which will end translation. Only 1 codon is used for the amino acid Methionine and this is the start codon (AUG).
The tRNA acts as an adaptor molecule to decode the mRNA into the protein, it interacts with the mRNA through its anticodon.The tRNA is also responsible for proof-reading the amino acid chain, this ensures that mistakes are very rare (less than 1 per 10000). This is done by many tRNA having an editing site as well as an activation site. These change or reject amino acids if they are larger or smaller than they should be.
There are 3 main steps in Prokaryotic translation; Initiation, Elongation and Termination.
This involves the initiation factors IF1, IF2, IF3 and GTP is required for energy.
IF1 and IF3 bind to the free 30S subunit, releasing it from the 50S subunit. IF2 forms a complex with GTP and binds to the 30S subunit, which attaches to an mRNA molecule. mRNA has a ribosome binding site (RBS), which is adjacent to the start codon AUG. The start codon is approximately 7-10 nucleotides away from the RBS. It is important to note that the 30S subunit is complementary to the ribosome binding site, so base pairing can occur with the 16S rRNA. A charged initiator tRNA (fMet-tRNAfmet), then binds to this start codon. IF3 is released, allowing a 50S subunit to bind to the 30S complex to form the 70S initiation complex which has a P (peptidyl) and A (acceptor) site . During this formation, IF1 is released and both IF2 and GTP are hydrolysed. GTP--> GDP + Pi.
Elongation requires the elongation factors EF-Tu, EF-Ts and EF-G as well as GTP to supply the energy. Elongation describes the process of aminoacyl tRNA molecules binding to the codon. A peptide bond is formed between the amino acid of the tRNA in the P site and the amino acid in the tRNA molecule that has just arrived at the A site; the formation of this peptide bond is catalysed by the 23S subunit. The amino acid in the P site is released from its tRNA molecule and the ribosome moves along so as to transfer the tRNA currently in the A site into the P site. This step is known as translocation. The uncharged tRNA i.e. tRNA without an amino acid, moves into the E (empty) site. 
- EF-Tu: It will bind to aminoacyl-tRNA in GTP form and then release the aminoacyl-tRNA to the ribosome when GTP is hydrolysed into GDP.
- EF-T: It induces dissociation of GDP in EF-Tu and restore EF-Tu to GTP form enable it to bind to another aminoacyl-tRNA.
- EF-G: It enhances translocation by displacing peptidyl-transferase in A-site to P-site of ribosome.
Amongst other proof reading mechanisms in translation, elongation factors are involved mainly in proof reading of amino acid sequences, in the newly forming polypeptide chain. This takes part in 2 ways:
- While leading the aminoacyl- tRNA towards the ribosome, theGTP bound elongation factor EF-Tu checks whether the match between the tRNA and the amino acid is correct. The exact details of how this is accomplished isn't clear, however 1 hypothesis suggests that correct matches between the tRNA and an amino acid has a narrow affinity for EF-Tu. This allows the EF-Tu to selectively choose the correctly matched tRNAs before bringing them to the ribosome.
- EF-Tu monitors the initial match between the codon and the anti-codon. When the aminoacyl-tRNA arrives in the A site of the ribosome, the GTP bound EF-Tu allows formation of hydrogen bonding between the mRNA and the tRNA, but bends the aminoacyl-tRNA into a conformation which prevents the interaction between the amino acid and the growing polypeptide chain. This prevents peptide bond from forming. Only when the correct codon-anti codon match is made, the ribosome triggers hydrolysis of GTP on the EF-Tu which releases the tRNA, and dissociates from the ribosome. This allows the tRNA in the A site to donate its amino acid, thus peptide bond forms between the newly recruited amino acid and the growing polypeptide chann
A stop codon attaches to the A site and the newly synthesised polypeptide chain occupies the P site. Proetins called release factors binds to the stop codon, initiating the release of the polypeptide chain which is transferred to the cytoplasm . Several release factors are involved as they recognise different amino acid sequences. These are RF1, RF2 and RF3. RF1 recognises UAA or UAG. RF2 recognises UAA or UGA. RF3 mediates interation between the ribosome and RF1 or RF2. RRF (ribosome release/ rec-cycling factor), EF-G and GTP hydrolysis promotes the dissociation of the ribosome from mRNA so the mRNA can be released.
- ↑ Berg et al., 2007:869
- ↑ Stryer, Biochemistry, Seventh edition, 2007: 936
- ↑ http://rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/translate.htm
- ↑ http://staff.jccc.net/pdecell/proteinsynthesis/translation/steps.html
- ↑ Jeremy M. Berg; Biochemistry; 7th edition;
- ↑ Bruce Alberts. Molecular Biology Of The Cell. 5th ed. New York: Garland Science. Page 377