Well, the last place that I will talk about
here where the RNAs get involved is, of course,
in the carrying of the genetic information
to the ribosome via the genetic code.
The genetic, as I have discussed
in another presentation,
is a three base sequence that determines
ultimately how protein were made.
Each three base sequence
called a codon in a messenger RNA
specifies the incorporation of an amino
acid into a growing polypeptide chain.
Now you can see the actual genetic
code that's up on the screen here.
And it's beautiful in its simplicity and
also beautiful in what functions it performs.
As I have noted before, there is a sequence called a
start codon that is involved in starting the synthesis
of every protein that's made in the cell.
That start codon is known as AUG
and it codes from methionine as you can see
in the first column near the bottom.
There are also three codons called stop codons
that tell the ribosome this is the place to stop
incorporating amino acid into a growing peptide chain.
They are known as UAA, UAG and UGA,
as you can see on the screen.
Now the genetic code really is
what we all rely on to exist.
Without the genetic code and the
proper use of the genetic code,
life as we know would not be possible.
The genetic code is universal.
That's one thing that is
very interesting about
it and it says a lot
about our relationships to every cell on the face of
the earth, because, every cell on the face of the earth
is using the same genetic code.
That turns out to be not
only just an interesting fact
but also important when we think about
how we want to use the genetic code
to make products in biotechnology.
So for example if I were interested in
producing let's say human growth hormone
and I wanna to take a bacterial cell and have
that bacterial cell make that human growth hormone.
I could actually do it surprisingly easily.
I could take the coding sequence
for the human growth hormone
and remove all the introns and
all the things that would be
happened to in the eukaryotic cells. So that I
have all the spliced version of that gene
and put that gene into a bacterial
cell under the control of the promoter.
If I take that gene and do that the bacterial
cell will transcribe through that
coding region for the
human growth hormone.
And then ribosome will grab it
and start to translate it.
And since the genetic code is the same for
a bacterium as it is for my cell
the bacterium will actually start producing
the human growth hormone protein.
That's a remarkable thing that we can do as a
result of this common genetic code across organisms.
Now another consideration with the genetic code
and it's one that isn't commonly discussed is
the genetic code relies completely on accuracy.
We saw during the marginals
on DNA replication,
the extreme links that cells went
to in terms of proofreading
and correcting errors and
correcting damage in the DNA;
because, we knew that the integrity
of the DNA was critical.
And the reason it comes down to be critical is
because the proteins that are made from it have to be
accurately made and if they are not accurately
made there can be a severe consequences.
But it's not just DNA replication that was important.
The process of splicing was also important.
Remember that RNA polymerase copied the DNA
and in eukaryotic cells those little
intervening pieces had to be removed.
And the removal of those pieces has to be precise; because,
if their one nucleotide one direction the other wrong way
then the entire message is ruined and the
protein that would be made from it is ruined.
The last thing that has to happen relative to
the actual sequences is the
processing of these individual RNAs.
The processing to make them so
that they are translatable,
to modify the RNAs as
necessary to improve their
function or to facilitate their function is essential.
So without these things going on the
genetic code would have no meaning.
Now it's interesting that the translation process
is not as accurate as DNA replication.
For a cell, let's say a bacterial cell,
a bacterial cell makes an error in replication
about 1 time in every 10 million bases.
That's a pretty remarkable set of
accuracy, especially when you consider
it's working at a thousand nucleotides a second.
That's a lot of faster than any typers and I don't know
any typer who can type that accurately. That's remarkable.
But translation doesn't need to be
as accurate as DNA replication. Now
that's the sort of flip side this thing.
We need the accuracy to have
the translation proceed properly.
But the translation can
actually have a few errors.
If the translation has errors the proteins
that would be made will not function.
They will be destroyed and as long as
those errors are relatively infrequent,
most of the protein that's
made will be functional.
Now if we have an error in the DNA
that has very different consequences;
because, an error in the DNA
well that ruins everything
all the way down to the line.
And as DNA is being passed from one generation to the next,
once you have got an error then that error carries forward.
So errors in DNA replication have major implications
to the genetic code and the translation of proteins.
But errors during the process of translation
can be tolerated to some extent.
So the ultimate function of
the genetic code or the
ultimate value of the genetic code is then
the accuracy of all these processes
and the last part of it is putting
the right amino acid onto a transfer RNA.
Now the transfer RNA, of course, is the molecule that
carries the amino acid to the ribosome.
And in the ribosome the anti-code unloop
of the transfer RNA pairs with the codon.
That can be fairly accurately established.
However, if the transfer RNA has brought
the wrong amino acid in
even through the codon-anticodon pairing is good,
the wrong amino acids will be
incorporated into the protein.
So it's very important thing that the
amino acid at the 3 prime end of the transfer RNA
be the one that corresponds to the
anticodon at the anticodon loop.
This sequence that's there
is actually read by an enzyme
called the aminoacyl-tRNA synthetase and
it does what you see in the screen here.
It reads the anticodon and puts the right
amino acid onto the 3 prime end.
have to get it right.
If they don't get it right then
everything else is out the door.
So the ultimate integrity
of the genetic code
is residing in the catalytic
activity of these enzymes.
So how do they perform what they do?
They actually look at both ends of a transfer RNA.
That means they have to be able to span
from the bottom where the anticodon is
up to the top where
amino acid gets put on.
So I tell you a secret here.
Most aminoacyl-tRNA synthetases
don't span the entire distance from
the anticodon up to the place
where the amino acid is attached.
They are not physically big enough.
And this was realized when
people begin to analyze the
3D structure of a transfer RNA.
And when they did that they
discovered that the transfer RNAs that we
typically write in a 2D form, as
you see here as a flat structure,
really isn't a flat structure, but
in fact is bent about in the middle.
And that brings the end
that gets the amino acid onto it
much closer to the anticodon.
So that the span of aminoacyl-tRNA synthetase
can actually cover both of those areas at once
and have the proper amino
acid put onto the transfer RNA.
So when the proper amino acid is
linked, as we have seen here, then the
transfer RNA is ready to go to the
ribosome and be translated.
Now this is pretty important to get right.
So cells invest a fair amount of accuracy
and a fair amount of energy
into the synthetases that do this.
There is one synthetase made
for each amino acid. That is its
specific. There are 20 synthetases
that are used to put amino acids onto tRNAs.
One for each of the amino acids
that goes into a protein.
Now those synthetases have to be able to read
slightly different anitcodons at the end; because,
the code is redundant meaning that
some amino acids are specified
by more than one set of codons.
But those synthetases have
built that into them
and, as a result, are able to put
the proper amino acid onto the tRNAs.
Now there are two different types
of synthetase that cells have.
One is called a type 1
synthetase and these synthetases
put the amino acid not
actually on the 3 prime end
it's on the end that has the 3 prime end
But they scrolled over 1 and
put on to the 2 prime hydroxyl
They kind of cheat. The type 2 hydroxyls
actually do what I always said along
is that the put on the 3 prime end.
That is they put on the end
and on the 3 prime hydroxyl.
In ether case it's going on
to the same piece of DNA and it's
going on to the orientation you see.
It just depends which portion of the
ribo sugar that it's actually going onto.
Now interesting thing about the
aminoacyl-tRNA synthetases is
like the DNA polymerases that we saw,
aminoacyl-tRNA syntheses also do proofreading.
So, as I said, translation doesn't have to
be a 100% accurate but it's important enough
that the aminoacyl-tRNA synthetases are checking
after they have put the
amino acid onto the tRNA.
They are checking to see that they got the right
amino acid corresponding to the antiocodon.
This helps improve the accuracy of the
process I have just described to you.
Well, I have talked in this lecture a lot
about RNA. I have talked about how RNAs
are processed. I have talked about how RNAs
can perform functions including their own
regulation and catalysis upon other RNAs.
In the end very end we have
seen how the RNAs have to be
carefully processed in order to be translated.
And I hope what I have left
with you in this discussion
is the importance not only
the function of RNA
but the things that RNA
is doing in making proteins.