Initiation of translation in prokaryotes: The small ribosomal subunit is separated from
the large subunit with the help of two initiation factors: IF1 and IF3. This complex then binds
a to purine-rich region -- the Shine-Dalgarno sequence -- upstream of the AUG start codon
on the mRNA. The Shine-Dalgarno sequence is base-paired to a complementary sequence on
the 16S rRNA - a component of the small subunit. This alignment ensures that the start codon
is in the right position within the ribosome. Another initiation factor - IF2 - brings in
the initiator tRNA charged with the initiator amino acid N-formyl-methionine. 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.