Below is a narrated animation of prokaryotic translation. Click here to license this video on Alila Medical Media website.
Translation is the process of making polypeptide (protein) from the messenger RNA (mRNA).
The translation process involves the following components:
– mRNA or messenger RNA containing the genetic information to be translated.
– tRNA or transfer RNA bringing in the amino acids – the building blocks of the protein.
– Ribosome – the machine that performs the translation. The ribosome has two subunits: small and large.
– Several initiation factors (IFs), elongation factors (EFs) and release factors (RFs). These factors assist with initiation, elongation and termination of the process, respectively.
Steps of the translation process
Initiation: The small ribosomal subunit binds to the initiator tRNA carrying the initiator amino acid methionine (fMet). In eukaryotes, this complex then attaches to the cap structure at the 5’ end of an mRNA and scans for the start codon AUG. The process is mediated by several initiation factors. This is cap-dependent initiation. In some cases, the initiation complex binds to an internal ribosome entry sites (IRES) on the mRNA – this is cap-independent initiation. The rest of the events remain the same. In prokaryotes, the initiation complex recognizes and binds to a a purine-rich region – the Shine Dalgarno sequence – upstream of the AUG initiation codon.
At the start codon, the large ribosomal subunit joins the complex and all initiation factors are released. The ribosome has three sites: the A-site is the entry site for new tRNA charged with amino-acid or aminoacyl-tRNA; the P-site is occupied by peptidyl-tRNA – the tRNA that carries the growing polypeptide chain; the E-site is the exit site for the tRNA after it’s done delivering the amino acid. The initiator tRNA is positioned in the P-site.
Fig. 1: Translation initiation (eukaryotic, cap-dependent). Click on image to see it on Alila Medical Media website where the image is also available for licensing (together with other related images and videos).
Elongation: A new tRNA carrying an amino acid enters the A-site of the ribosome. On the ribosome, the anticodon of the incoming tRNA is matched against the mRNA codon positioned in the A-site. During this proof-reading, tRNA with incorrect anticodons are rejected and replaced by new tRNA that are again checked. When the right aminoacyl-tRNA enters the A-site, a peptide bond is made between the two now-adjacent amino-acids. As the peptide bond is formed, the tRNA in the P-site releases the amino-acids onto the tRNA in the A-site and becomes empty. At the same time, the ribosome moves one triplet forward on the mRNA. As a result, the empty tRNA is now in the E-site and the peptidyl tRNA moves to the P-site. The A-site is now unoccupied and is ready to accept a new tRNA. The cycle is repeated until the ribosome reaches a stop codon.
Fig. 2: Translation elongation. Click on image to see it on Alila Medical Media website where the image is also available for licensing (together with other related images and videos).
Termination: Termination happens when one of the three stop codons is positioned in the A-site. No tRNA can fit in the A-site at that point as there are no tRNA that match that sequence. Instead, these codons are recognized by a protein, a release factor. Binding of the release factor catalyzes the cleavage of the bond between the polypeptide and the tRNA. The polypeptide is released from the ribosome. The ribosome is disassociated into subunits and is ready for a new round of translation. The newly made polypeptide usually requires additional modifications and folding before it can become an active protein.
Fig. 3: Translation termination. Click on image to see it on Alila Medical Media website where the image is also available for licensing (together with other related images and videos).
