Now another important process recently
discovered that all RNAs also function in,
is a very critical and interesting
process called RNA interference.
Now RNA interference is a process that's
stimulated by the presence of double
stranded RNA inside of cell.
Now as we are gonna see this can happen either as
a result of the cell making double stranded RNA
or by a virus that is invaded.
And the invasion of a virus bringing in
double stranded RNA is
sure cue for a signal for a problem.
Now the two different forms of double stranded
RNA that can exists in the cell
are known as micro RNAs or miRNAs
and these have cellular origins.
The siRNAs or silencing RNAs
come from an external source
like a virus for example.
Now I note that biotechnologist are also using
double stranded RNAs as a means of controlling
genes for biotechnology purposes.
So that's another way that foreign
siRNA can get into cells.
Now the process is quite wide
spread in eukaryotic cells.
It occurs in plants. It occurs in animals
and it plays a wide variety of roles.
The actions performed in this process
are called RNA interference or RNAi.
And what these double stranded RNA molecules result in
is it the interfering of the
translation of targeted genes.
Now the ability to use this technology to
target and specifically stop the
production of certain proteins
allows a researcher to do incredible
things. But more importantly
it allows the cell that has this to
both protected cells from invaders
and also to control its own gene expression.
RNA interference operates through the
silencing of gene expression
and that silencing, as I said, is
interference in the way that proteins are made.
Now this occurs as a result of, first of all,
the appearance in the cell of a double stranded RNA
and there is an enzyme called dicer.
And just like a dicer that you might have in a kitchen,
this dicer's job is to take that double stranded RNA
and chop it into bite size chunks
and those bite size chunks are about
20 base pairs long, as we shall see.
So these 20 base pairs chunks at
this point are called silencing RNAs
if they came from an external
source and micro RNAs if they
came from a cellular source.
These pieces of RNA can then be bound
by the RNA induced silencing complex
or what is knows as RISC. Now I am gonna show
you this whole process in just a minute.
Now this figure illustrates the process that I was
just describing in words on the previous slide.
We can see actually two things
occurring right here.
In the process occurring on the right, where
I am describing a cellular process
that starts with the production of
an RNA that makes a double stranded RNA used
to create the miRNAs that I described.
In the process on the left we have simply
the dsRNA that is appeared in the cell
by the foreign source whether it's a virus or
is by a researcher placing that within there.
I am gonna start with the process
on the right to describe yet.
The cell has encoded within its genome
certain sequences that when transcribed
produce structure like you can see on the right.
That double stranded structure
with tails hanging off of it
and the poly A that you see
on it is called a pri-miRNA.
The name doesn't really matter.
But that pri-miRNA gets
processed to make what will
ultimately become the miRNA.
So there is an enzyme called drosha that
cuts some of the ins off of the pri-miRNA
and creates the smaller structure
that you see on the image.
The pri-miRNA is then moved out of
the nucleus, as you can see,
where it is then attached to
the enzyme known as dicer.
Now at this point the two processes the
siRNA and the miRNA become the same.
What dicer does is it takes
that double stranded
pri-miRNA or the double stranded RNA
that you see on the left
from the foreign source and chops them into
the 20 base pairs sequences that I have described.
You can see for example a perfect
duplex on the siRNA side
and sort of a mismatched duplex
with a bulge on the miRNA side.
That's very common for miRNAs.
Dicer after chopping this
into 20 nucleotide blocks
then peels away one of the strands.
And the peeling away of that one
strand leaves a single stranded
siRNA or a single stranded miRNA
that is then grabbed by the RISC. Now the RISC
takes that individual sequence
and carries it to a messenger RNA.
Now the significance of the fact that
there is a single strand at this point
is due to the fact that this single
strand would be complimentary
to a target messenger RNA, as we shall see.
Now after the RISC has complexed
with that single stranded RNA
whether it was an miRNA or whether it was an
siRNA at this point doesn't matter.
That RISC RNA complex then goes and seeks
messenger RNA. Messenger RNAs, of course, are
coding for individual proteins.
If RISC finds a sequence that's complementary
to the RNA that it is carrying
it aligns that sequence with the specific
region in the messenger RNA as you can see here.
And then an enzymatic activity in the RISC
complex called argonaute
actually cleaves the target
messenger RNA, as you can see here.
Now that cleaving of the target messenger RNA means that
you have destroyed the coding for a protein.
In this way this protein that was coded
by this messenger RNA can no longer be made.
Well, this has a couple of implication as you could imagine.
This has very obvious protection effects
for the cell against an invading virus.
If the invading virus makes a double stranded RNA
in the process of its life cycle
then this siRNA system will stop
the production of targeted virus proteins.
Now also it may seem a little odd
but the miRNA also plays a role
in regulating gene expression;
because, the miRNA is actually stopping in this case
production of a cellular gene
that would otherwise make this protein. And
it might seem as very inefficient for this
cell to make a messenger RNA and
then degrade the messenger RNA.
But that probably makes more sense than continuing to
make a protein that the cell wouldn't otherwise need.
So this miRNA system allows for an
additional level of protection
or an additional level of control
of a cellular gene expression.
In any event, what happens here is that
the translation of the messenger RNA stopped.
Now I showed mention the fact that this processing
that happens doesn't have to cut the RNA.
It can also involve a simple binding
of that miRNA or siRNA sequence to the messenger RNA
and stop translation by the
formation of that duplex alone.