Now RNA polymerase, of course,
plays an important role
an essential role in transcription; because,
without RNA polymerase cells can't make RNA.
Eukaryotic cells have a different setup for
RNA polymerases then prokaryotic cells.
In prokaryotic cells, again,
it's a simpler world.
All of the transcription in the prokaryotic
cells is done by one RNA polymerase.
It does all the work that's there.
Eukaryotes breakdown function
by different polymerases.
So many RNA polymerases that are
made have a hand structure
and that hand structure is like we saw with
the DNA polymerases in a previous lecture.
That hand is designed to hold onto a DNA.
You can see the hand structure here in green.
And RNA polymerases like DNA polymerases
only work in the 5 prime to 3 prime direction.
Now one difference with RNA polymerase
is they do not require a primer.
DNA polymerases always require a primer.
So RNA polymerases are simpler in that respect.
Now another difference with RNA polymerases are
at least 1 difference with the process of
of transcription compare to DNA replication
is that only one strand of
the DNA template is copied.
Now we can see on the
right a representation
of an RNA polymerase that is bound to a DNA and you
can see that the strands have been peeled apart.
You can also see on this figure that
the strand that's being copied
by the RNA polymerase and you can
see the RNA coming off in red
the strands that's being copied
is called the template strand.
Whereas the strand that is not being
copied is called the coding strand.
The transcript comes off of the DNA so
for usually during transcription only a small region
of the part being transcribed
will have base pairs between
the RNA and the DNA. The rest of it falls
off the end as you see here.
Now RNA polymerases in eukaryotic
cells, as I said, are specialized in function.
Eukaryotic cells or most eukaryotic cells
have three different RNA polymerases.
These have individual functions for making
specific RNAs, as we shall see.
Plants have as many as 5 and
I won't go into that there.
Prokaryotic cells, as I noted,
only have one RNA polymerase.
And some viruses actually encode
their own RNA polymerase.
Now if you think about that, that's
an indication of the importance
of the RNA and perhaps
in the case of the virus
the specific needs of the
virus, for a specific type of RNA.
The sigma subunit of the
prokaryotic RNA polymerase,
as I noted earlier, is the
functional part of the polymerase
it's really all this needed in most
cases to help the RNA polymerase
to bind to the DNA
and make the transcript.
In eukaryotic RNA polymerases
we have numerous subunits
but these subunits don't bind to
promoters and that's again unlike
the prokaryotic RNA polymerase.
And last but important certainly
from a medical perspective
is the fact that eukaryotic RNA polymerases are
susceptible to a poison called α-amanitin.
α-amanitin is produced by some
mushrooms and so you may have heard of
people having issues with death cap mushroom.
Death cap mushrooms create
this toxic substance called α-amanitin
and it is exquisitely a poison
for RNA polymerase too.
Very very tiny amounts of α-amanitin can
stop the functioning of RNA polymerase too.
If you have eaten death cap mushrooms
and you don't get a liver transplant very
shortly, you are probably not gonna be around.
The RNA polymerases that
I wanna discuss in eukaryotes
are the three that we find in almost all
eukaryotic cells and these are RNA polymerase I.
RNA polymerase I is a polymerase
that makes the large ribosomal RNAs.
These include the 28S,
the 18S and the 5.8S ribosomal RNAs.
Now you will notice it says
they are processed to that.
That means that these
ribosomal RNAs are made in larger
molecules and then chopping and cutting of pieces and
editing of that results in the final products here.
The second RNA polymerase in eukaryotic
cells is that of RNA polymerase II.
Now RNA polymerase II is doing a
bulk of the work in the cell it's
making the messenger RNAs which, of course, are
the RNAs that carry the genetic
information to the ribosome
for translation into proteins.
They make most of the small nuclear
RNAs that I will talk about later
and the micro RNAs that are involved
in helping to regulate gene expression.
RNA polymerase III performs functions of making
mostly the small RNAs in the eukaryotic cells.
This includes the 5S ribosomal RNA,
that's the smallest one,
the transfer RNAs and other small
RNAs that the cell may need.
Well, with this presentation I have gone through
the complexity of eukaryotic RNA synthesis
as well as the discussion of the RNA
polymerases that are involved in cells.
In other presentations, I will
go through in some more detail
the different functions of RNA and
the way that RNA is actually used
to control its own gene expression.