Now splicing is an important process and I
should emphasize that splicing occurs
almost exclusively in eukaryotes.
There is a only a couple of genes known anywhere in
the prokaryotic kingdom where splicing actually occurs.
Splicing occurs on almost all types
of RNA found in eukaryotic cells.
But mostly commonly we find it in messenger RNA.
It is found in some tRNAs and in some ribosomal RNAs
but most commonly we find it in messenger RNAs.
Now splicing, as I said, involves
the removal of sequences internal
to a pre-RNA, in this case a pre-mRNA.
This removal of sequences takes out sequences
in the RNA that we call introns.
So thinking back to the original DNA, the original
DNA had a sequence that was transcribed
and that DNA sequence
contain intron sequences.
The introns in the RNA
that's produced get removed
and so that's what's being
depicted on the screen.
We see the removal of the introns by
a structure called the spliceosome.
And the spliceosome, as we will see,
is a complex of RNA and proteins
that performs this function.
The removal of the intron results in production
of a molecule we call a lariat structure.
And all that really means is that,
that intron had one end of itself
bound to another part of itself to make a
sort of loop structure, as you see on the image,
that has a form of like that of a lariat.
The spliceosome is a complex of both RNAs
and protein. The RNAs are called
snRNAs; because, they correspond to
what we refer to as small nuclear RNAs.
And when complex to proteins we called them
small nuclear ribonucleoproteins or S-N-R-N-Ps.
Some people call those snrnps although that's
probably not exactly an accurate pronunciation.
Now this image depicts the sort of chemical
way in which this process occurs.
We can see for example
the pre-messenger RNA at the top.
It has an exon on the left and an exon on the right.
Now I need to tell you what those terms are.
The exons are the parts that get
joined in the splicing process.
Intron is the part
that gets removed.
Now it's very important that
this process proceed precisely;
because, if the splicing is made one nucleotide
or another in the wrong direction
then that will also have significant effects
because it will affect the genetic code
that is coming in the mRNA
that's being translated.
So this process must occur accurately
if the protein that is going to be made from
this mRNA is itself going to be made properly.
So there are couples of things that we see in
introns sequences and you can see them
labelled above that are common
among almost all introns.
At the 5 prime end of the intron sequence,
we see a sequence of GU
that almost always occurs
and probably helps to make sure
that the orientation and the
removal of the intron happens
at the same place every time.
At the 3 prime end of the
intron, we see a sequence of AG
and that almost always
also occurs within introns.
Then about 30 nucleotides
or so ahead of that AG,
there is a sequence that is
an A, as you can see here,
and that A is usually adjacent to a sequence
of pyrimidines that are not shown on this figure.
Now in this splicing process
what happens is the A that
you see on the image
makes a nucleophilic attack on the G
at the 5 prime end of the intron.
That nucleophilic attack results
in the covalent bonding between
the G and the A, as you can see here and
we have started to form the intron.
The structure gets resolved when the
joining of the exon on the left
occurs with the exon on the right.
That results in this cutting of the bond
between the G on the right and the exon.
That results in the removal of the
lariat structure that you can see
and the production of the spliced messenger RNA here.
Now common intron sequences help to
orient and make this process possible
and without these common intron sequences the cell
would have no way of knowing where to make the cut.
Now one other thing I didn't note about the
lariat structure is that it has an unusual bond in it.
You recall from the structure of nucleic acid
that the nucleotides are joined together in
a 5 prime to 3 prime orientation such as the
3 prime end of one nucleotide
is covalently bonded to the 5
prime structure of the next one.
So that 5 prime 3 prime, 5 prime 3 prime
carries all the way through an RNA and a DNA.
But this bond right here is
unusual in that. Instead of having a
5 prime-3 prime bond, it has
a 5 prime-2 prime bond.
And that's because the 2 prime hydroxyl
is available and it's what making the
nucleophilic attack that I mentioned.
It's an unusual structure. But this is the
one place in biology where it occurs.
The excised inton comes
out as a lariat, as I said,
and that lariat can also
be used for other things.
In a few cases the lariat itself actually
contains another gene, that's rather unusual.
Or the lariat can simply be degraded and the
base is used inside of that structure.
Now this figure shows
the joining of the snRNPs
to the inton for making
this process happen.
You can see in this process that U1
is added first, U2 is added second
and then the complex of U4,
U5, and U6 are added third.
Now we are not gonna look up close in
personal at that individual structure but
if I set to say that's there is very precise and specific
structure that's created that is known as the spliceosome.
That spliceosome some people
has compared to ribosome
in the sense that they both contain
proteins and they both contain RNAs
and they both perform
functions on an RNA.
Well, this one isn't performing translation.
But this one is now excising introns.
The snRNPs and the snRNAs that they contain
appear to play a very important
role in aligning the A
that's going to make the nucleophilic
attack on the G in the right orientation.
So once that orientation is setup then
that excision of the intron and the
formation of a lariat structure can actually occur.
So there is actually a sequential series
of events that happen in this process.
After the structure has formed we see removal
of couple the snRNPs, the U4, the U1 for example
and then released of the others and finally
the spliced messenger RNA along with a lariat structure.