Now, at the same time as this is happening,
we have another series of things
happening, which is called RNA splicing.
The primary RNA transcript
from eukaryotic cells
that is not coding.
So we have it split up
into introns and exons.
And they’re not as neat
and symmetrical as these,
but these will exhibit
the whole concept.
Introns are inactive
and exons are expressed.
At least, that’s the
way I remember them
because it’s not truly true
that introns are inactive.
But exons are expressed.
They’re the DNA that’s going
to end up in the mature mRNA.
So we cut out the introns
and produce a mature mRNA
and you have a 5’ cap
and a poly-A tail.
So the way that the cell manages
cutting out the introns
is through the use of a
tool called a spliceosomes.
The spliceosome is like a pair
of giant molecular scissors
and it is composed of
which are small nuclear
Try saying that one.
That’s why we call them snRNPs
because it’s much easier.
So these small nuclear RNA
portion of the snRNP
is going to base pair with
the 5’ end of the intron
as well as the branch
site of the intron.
You’ll see how that comes
into play shortly.
And we’ll also see some
bind with the 3’ end.
They come together to
form a whole spliceosome
and then they cut the DNA and
stick one piece, the Lariat loop,
back onto the branch site and then cut
the 3’ end and excise the intron.
So the intron is removed.
Now the two exons can be stuck together
and we are good to go with
introns being removed.
But let’s go a little bit further
and look at the possibilities
because we’ve now learned that we
can have alternative splicing.
So, yes, we have introns and exons
and exons are the things that
are going to be expressed.
But the pieces that you
choose to be expressed
or the cell chooses to be expressed
could be different depending
on how the spliceosomes cut.
So we could out introns in this fashion and
produce a protein that looks like protein A
or we could cut it in this fashion and
stick those pieces together as exons
and have them express
a different protein.
And then we could have a
different variety even still,
so alternative splicing
ends up with the production
of a variety of different proteins
from the same messenger RNA.
Now, it sort of seems like this
might be violating that one gene,
one polypeptide thing.
I mean, you could argue that
it’s still one polypeptide.
But as definitely seeing that the protein
comes out in a little different way,
but this is one of the ways
that we have with so few genes,
we have so much possibility
for a variation.
It’s these mechanisms of
So we can one coding sequence, actually
results in multiple possible proteins.
So you may have heard that
we got done early sequencing
the human genome back in the year 2000.
And part of the reason was it turned out
that we had a lot less genes
than we had anticipated.
For example, we have much less than
corn, which was quite confusing to us.
Because apparently, we are
more complex than corn
and this alternative splicing is
perhaps one of the explanations
for why we could be more complex
because we have the possibility
of multiple protein products
from one coding sequence of DNA.
So let’s recap what’s happened
in this RNA processing phase.
We had to take our primary RNA transcript
and make it into mature messenger RNA.
We added a 5’ cap to protect him from the
environment out there in the cytosol
as well as help him
bind the ribosomes.
In addition to that, we added a poly-A
tail to protect him from enzymes
that might be out there trying
to degrade the messenger RNA.
And then we cut out introns in order
to make a mature messenger RNA
that’s ready to go
out into the cytosol
and be translated into a
polypeptide by ribosomes out there.
That is precisely the topic
of our next lecture.
So I look forward to
seeing you for that one.
At this point though, you should
be able to describe the process
of transcription in prokaryotes
as well as differentiate between
the initiation phase, the elongation
phase, and termination phase,
and explain differences in prokaryotic
and eukaryotic transcription
as well diagram some of the
aspects of RNA processing.
Thank you so much for your attention.
I will see you shortly.