So now let’s take a look at
how the whole thing ends.
We’ve gone along around the cycle,
then we’ve switched from P to A and
translocate it and so on and so forth.
We’re growing the
It’s getting longer and longer.
And now, we have reached the end of the
coding sequence on the messenger RNA.
Now, just as there is a
5’ untranslated region,
that helps the ribosome associate and
helps the messenger RNA find its way,
there’s also a 3’ untranslated region,
so we will get to the stop codon.
There will still be a little tail of RNA.
We’re not necessarily completely
sure how those come into play
at this point in time, but we
know they have some sort of role.
But anyway, the point here
is we come to a stop codon
and the stop codon does
not code for a tRNA.
It codes for a release factor.
I think of release factors
as if they are bombs.
Once that the release factor
binds into that site,
we’ll see that the whole
thing falls apart.
So it’s not a tRNA, binds into the site, and
it causes the whole thing to blow apart.
We release the polypeptide chain
and then the large subunit
dissociates from the small subunit
and we end up having both large
and small subunit dissociated.
So let’s say
we are dealing with proteins that are
going to be used inside the cell
versus proteins, polypeptides, that are
going to be used outside the cell.
You recall from cell structure, there
was rough endoplasmic reticulum,
which had ribosomes
studded all over it.
And there is smooth endoplasmic
reticulum, which does not.
But there are also
What’s the difference?
So some proteins are to go and
some proteins are to stay.
You’ll recall that we use the endoplasmic
reticulum for proteins to go.
We package them, send them to the Golgi,
who then puts them in the
right transport vehicles.
That’s another lecture.
So what happens if we are to go?
We need to get associated with
the endoplasmic reticulum.
First of all though, will
be a signal sequence on the
beginning of that
All sorts of things happen to these
polypeptide chains once they’ve been formed.
But the signal sequence will
trigger a certain set of reactions
and it involves a signal
which is going to help dock this
ribosome on the endoplasmic reticulum.
So first of all, they’re not associated
and once it’s recognized that
there’s this signal sequence,
the particle will bind on and help the
ribosome find the endoplasmic reticulum.
So the signal recognition particle
binds to the signal peptide
and that’s going to arrest
the elongation cycle.
So the elongation stops,
then we go all the way over
to the endoplasmic reticulum
and we’re going to dock the ribosome
on the endoplasmic reticulum
and associate it with a pore
so that it can then
restart protein synthesis
and filter its polypeptide into
the endoplasmic reticulum.
So it’s going to –
The polypeptide enters through a pore
and synthesis is going to resume
so the ribosome hanging out on the
outside of the endoplasmic reticulum
continues to grow the polypeptide chain
using that whole elongation cycle business.
And now the polypeptide is inside the
endoplasmic reticulum and it’s ready to go.
It’s ready for export.
So now, let’s take a quick look at a
summary of all of this gene expression