The amino acids and mRNA are translated using tRNA.
There ia a start signal that indicates where to
start
translation. The
translation
uses tRNA to bind between
the mRNA and the amino acids. A stop codon indicates
where to
stop
translation.
Translation Initiation
There is a special initiatior tRNA with methionine that
binds to the P site of the small ribosomal subunit.
Eucaryotic initiator factors bind the small ribosome
subunit and tRNA to the 5' end of the
mRNA and cap, and then start searching along the mRNA to
find the AUG start codon. The Shine-Dalgarno sequence
is used as the start sequence for bacteria, and can be
the start sequence for many genes.
ATP powered helicases help
these initial parts to move. Once the start codon is found,
the large ribosomal subunit binds. Then another
aminoacyl-tRNA binds in the A site of the ribosome.
Translation Elongation
An RNA synthetase protein will bind each tRNA
to an amino acid at one site,
and the other site will specify an anticodon, which
can bind to a codon on the mRNA.
The ribosome changes conformation, shifting
the tRNA's through from the A (aminoacyl-tRNA),
to P (peptidyl), to E (exit) sites.
The A site is where the tRNA is matched to the mRNA.
Incorrect tRNA molecules will dissassociate. When
the correct tRNA binds, then the ribosome shifts,
emptying the A site, and moving the tRNA from the A
site to the P site. Any tRNA in the P site is moved
to the E site. The large subunit binds the
amino acids attached to the tRNA's together with a
polypeptide bond in the P site.
The ribosome moves along the mRNA by these conformational
changes. Elongation factors increase the speed and
decrease errors.
Translation Termination
A release factor protein that has a similar conformation
to a tRNA, binds to the A site of the ribosome. The
ribosome shifts, and instead of binding an amino acid
to the polypeptide chain, a water molecule is bound
instead. Then the ribosome and polypeptide chain
dissassociates from the RNA strand. Sometimes the
polypeptide chain will directly shape into a protein,
and other times, there are molecular chaperone proteins
that help to shape the protein conform to the proper
shape.
