Voiceover: Let's talk about post-transcriptional regulation
which is regulation basically once DNA
has been transcribed into mRNA
and I've drawn out this little schematic for you here
and it kind of just shows you how a DNA strand
has a corresponding RNA strand
and then the mRNA strand afterwards
and I'll sort of explain what all the different colors
and words mean in just a little bit.
So once DNA is transcribed by RNA polymerase
into the corresponding RNA strand,
this RNA strand needs to get what I call
a tidy little haircut and then don some
protective outerwear before it can leave
the comfort of the nucleus for
its big debut into the cytoplasm in the form of
a fully processed messenger RNA or mRNA strand.
Now keep in mind that this form of regulation
occurs in eukaryotes only and this modification
also helps to stabilize the mRNA to protect it
from premature degradation before it
gets translated into a protein.
So as you can see here, DNA gets transcribed
one to one, base for base, into RNA
and you can see here that there are sections
of the RNA that ultimately make it
into the finished mRNA,
these short segments, which are turned exons,
and they are the sequences that code
for the ultimate protein product.
And then there are short non-coding segments of RNA
that get cut or spliced out and this would
be the haircut that I alluded to earlier
and so these are called introns,
and this is accomplished by a large
molecular entity called the spliceosome.
So the spliceosome binds on either side of an intron,
loops the intron into a circle,
and then cleaves it off, and then ligates
the two cut ends of the exposing exons together,
kind of cinches them together.
And an easy way to remember which sequences
are exons, which ones are introns,
is that exons exits the nucleus
and introns stay in the nucleus
so exons kind of stands for exit.
Now even though the mRNA has gotten
its nice little haircut, it's not quite ready
to leave the nucleus just yet.
It has to grab what is called
a 5' prime cap and a 3' prime poly-A tail.
Now what are those things that I just mentioned?
So a 5' prime cap refers to changes at
the 5' prime end of the mRNA
and remember that this is the phosphate end
of the nucleotide bases in the mRNA
and some people like to remember this as
F for five prime and for fosphate
so that's how you can kind of
keep the two ends straight.
And capping at the 5' prime end converts
this end of the mRNA to a 3' prime end by
a 5' prime to 5' prime linkage which
basically just protects the mRNA from
exonucleases which degrade foreign RNA.
The cap also promotes ribosomal binding
for translation and also helps the regulation
of nuclear export of the mRNA.
Now the poly-A tail, that goes on the other end,
the 3' prime end of the mRNA
which has the terminal hydroxyl group
and so what do I mean when I say poly-A tail?
Well, poly-A tail refers to polyadenylation
in which multiple adenosine monophosphates
or basically adeonine bases are added
to act as a buffer for exonucleases
in order to increase the half life of mRNA
and again, protect it from degradation.
And so the purpose of the poly-A tail
is really very similar to the 5' prime cap
which is basically to protect from degradation,
help with promoting translation,
and regulating nuclear export.
The poly-A tail also does one more thing
and it kind of just helps with transcription
termination for the RNA polymerase that's
transcribing the messenger RNA.
Polyadenylation is catalyzed by
an enzyme called polyadenylate polymerase
which as the adenosine monophosphates using
adenosine triphosphate as the substrate
and the poly-A tail is built until
it's about 250 or so nucleotides long.
So overall the 5' prime cap and the poly-A tail
help to stabilize the mRNA for translation.
That's the key point to take home from here.
So once the mRNA has donned its cap and tail
and had its introns spliced out,
its now ready to exit the nucleus
to be translated into a protein.
Now additionally, there's one more type
of RNA regulation called RNA editing,
which is a process that results in seqeuence
variation in the RNA molecule
and is catalyzed by various enzymes.
RNA editing is relatively rare
and these events may include insertion,
deletion, and base substitution of nucleotides
within the edited RNA molecule.
Now one of these enzymes is called
adenosine deaminase acting on RNA
or ADAR enzymes which convert specific
adenosine residues to inosine in an mRNA molecule
by hydrolytic deamination.
Another type of editing is called
cytosine deaminase acting on RNA, or CDAR,
which involves deamination of cytosine
to uridine by cytidine deaminase.
RNA editing is currently being extensively studied
in relation to infectious diseases because
the editing process alters viral enzymes
and their function so kind of an exciting,
new emerging concept in post-transcriptional regulation.