Eukaryotic Translation (Protein Synthesis), Animation.

Eukaryotic Translation (Protein Synthesis), Animation.


Steps of the translation process:
Initiation : The small ribosomal subunit binds to the initiator tRNA carrying the initiator
amino acid methionine. 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. 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. 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 is in the P-site. The A-site is now unoccupied and is ready to accept a new tRNA. The cycle
is repeated for each codon on the mRNA. 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 the 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.