[Lecturer] Hello. All right.
So I wanted to take a few minutes
to review translation in a little bit more detail
and try to draw it out.
I've actually tried this, tried recording me,
actually drawing on my whiteboard in my office
and realized you couldn't really see very much.
So I'm gonna try it this way,
which is just drawing it on PowerPoint
and recording my screen for you
and hope that becomes helpful.
You know, I really am a big proponent
of online courses, online learning.
However, there are some limitations.
Sometimes it's just easier if you could just be
in the room together and draw on the board.
But barring that, we will try to use technology
to make the best of the situation.
So translation, remember translation is the second step,
it follows transcription.
Transcription being the stage of copying
a complimentary mRNA strand from the sequence
of a gene in the nucleus.
And then that mRNA leaves the nucleus, enters the cytoplasm,
and is translated from its mRNA code,
which is read, and is used to instruct ribosome
on which sequence of amino acids to construct.
Remember that mRNA does not become protein,
does not become amino acids.
It simply provides the instructions for which amino acids
to assemble together to make a protein.
Remember, one protein, if you stretch it out,
is composed of a linear sequence of amino acids
in a particular order.
And you have 20, in humans,
we have 20 normally found amino acids
that could be ordered in any particular order,
a number of them, to form the, you know,
over a million, 1 million different types of proteins
that we end up having in our cells.
So let's take a look at what's actually going on here.
If we start out by,
if we start out by drawing an mRNA, let's draw a stretch.
Remember, mRNA is, it's usually depicted as a straight line.
When it's depicted as a straight line,
remember that it's made up of bases similar to DNA,
but with the exception of, do you remember what?
That uracil is replacing thymine.
So wherever there would've been a thymine,
it's replaced by uracil.
And that is done so that the mRNA is able
to leave the nucleus and be translated in the cytoplasm.
And we want to keep the DNA in the nucleus
just to keep it nice and protected and safe.
And mRNA, remember, is just a temporary copy
of the information in the DNA
so it can leave the nucleus.
It's not such a big deal if it ends up getting damaged
or lost, because it's not the original, it's just a copy.
All right, so I'm gonna label this,
so that it's clear, this is our mRNA.
All right, gonna put that down here.
And the mRNA, of course,
has a sequence to it of bases,
and we'll draw that out here.
Let's say, let's give us an example
of A-U-G-C-G-A-U-A-U-G-C-C-U-A-A.
Okay, let's say that's our example.
Okay, so that's our mRNA sequence.
Okay?
So now how does it get read?
Well, you remember from the lectures,
that we have these blob, these,
they're always drawn as blobs, these ribosomes.
Let's say that's our ribosome
and it is bound to a triplet of bases.
Remember what a triplet of bases is called?
It's called a codon.
Okay, so it's down there.
And let's say that it has already started working
on this mRNA so that it's already read this codon,
this first codon, this second codon,
and this third codon, oops, and this third codon.
So that would have resulted in the amino acids
being assembled that correspond to those,
which would be methionine,
arginine,
tyrosine.
Okay?
That would be the first three.
Let's make that a different color.
So it's clear that we'll make.
We'll make the amino acids red so that way it's clear
that they're different from bases.
All right.
And what's going to read this particular codon
and say, okay, the next amino acid is whatever it is,
and then actually attach it?
The missing link here is tRNA.
All right, I'm going to try to draw a tRNA,
make it, it's gonna look like a kindergartner did it.
Okay.
I apologize.
I'm not the best drawer in the world.
All right, let's make it a little bit bigger
so we can actually see it.
Okay, so this is tRNA.
tRNA is a stretch of nucleotides similar to mRNA
except it forms this funny extra shape,
and it's made up, so it's made up of bases.
So we would, if we were to zoom in here,
we'd see this is C-G-A-U-U-C-G-U-U-U-A-G, whatever.
It's gonna be a sequence of bases.
Only part that we really care about is right here
in this bottom loop, which is called the anticodon loop.
Anticodon loop.
So I'm actually gonna make this
a little bit bigger so it'll fit the size
of what I've drawn for everything else here.
That should do it.
Okay.
Now, all right.
And so let's say that this tRNA has the following sequence
in its anticodon loop, C, G, G.
All right, well, there, so I said that,
as I said that the tRNA is just RNA.
That has been folded up in a particular structure,
that has this anti-codon loop at the bottom.
And the magic actually happens with this particular kind
of protein called a tRNA synthetase, tRNA synthetase.
And what it does is it recognizes the particular tRNA,
it recognizes its anticodon loop,
and it says, "Okay, you're a CGG.
All right, that's really nice.
That's what you are.
And that means you get a glycine amino acid."
Because if we look on our codon table,
what will be complimentary to CGG is GCC,
and we know that GCC codes for glycine,
as mRNA codes for glycine.
So we're gonna get a glycine here,
and let's, we're gonna get a glycine here,
and it's gonna get stuck onto the end of this tRNA.
So we have nucleotides, nucleotides, nucleotides,
nucleotides, one amino acid.
Okay?
And that's the glue that holds it all together.
It's weird, but it is true.
Okay, so now what happens?
Well, we have our ribosome sitting here.
It's like, okay, is there a tRNA around
that might want to bind to this GCC?
And this tRNA says, "Well, I do."
And it comes in.
La da da da da, boom.
Oh, nice.
So we have really nice complimentary binding,
C to G, G to C, G to C.
So it's stuck on there for a second, which is great.
And it has this glycine on the end.
And it says, "You know what?
I think glycine deserves to be on this amino acid chain
more than it deserves to be on me,"
is what the tRNA says
if we wanna personify them a little bit.
So it kindly hands off its glycine onto this chain
of forming amino acids attached to the ribosome.
And then the tRNA says, "All right, our work here is done."
And then it leaves.
Let's take a step back here.
So when the tRNA actually has the glycine attached to it
before it transfers it, this is in a state called charged.
So this tRNA is charged, it's charged up, it's ready to go.
It has its amino acid ready, it's ready to get in there,
get to that ribosome, attach to the mRNA,
and deliver its glycine.
So let's say it's done that, let's draw that one more time.
Okay, it comes in, boop, it binds. Lovely.
It delivers its glycine to the chain of amino acids there.
And let's draw this together.
And then the tRNA dislodges and leaves.
So now this tRNA here doesn't have an amino acid attached
to it, and it's called uncharged.
Well, those tRNA synthetase proteins, remember those?
They'll recognize it and say,
"Hey, you look like you could use a glycine."
And the tRNA synthetase will recharge it, charge it back up,
and now this tRNA is ready to go if there's another mRNA
that has the codon that's complimentary to it,
it will come in and bind and deliver that glycine.
All right, let's take a quick peek at how this all wraps up.
Let's say now what's gonna happen is that our mRNA
is going to shift downward
to the next codon, the next triplet.
Okay, so it's moved down now to this codon, UAA.
Hmm.
And then the ribosome sits there, and it's like,
"Okay, bring on the tRNA that matches UAA,"
and it's waiting and it's waiting
and it's waiting, and it never shows up.
'Cause there is no tRNA that is complimentary to UAA
because it is, UAA is a stop codon.
So, because it never gets a tRNA here,
the ribosome basically says, all right,
well I guess our work here is done.
We've reached the end, and the complex disassembles.
What you get is your chain of amino acids
now is free to go on.
I mean, obviously it would be longer than four.
Proteins are made up of hundreds
and hundreds of amino acids at least.
And so it would go on,
and it would form its tertiary structure,
which basically means it would fold up on itself
and go off to perform its function.
So this is now, this is now the protein, here in red.
The ribosome would fall off,
and the mRNA would be degraded pretty soon after that.
Okay, so then those are all the parts and pieces
to translation, and I hope that makes sense.
And yeah, basically, just keep in mind
that there are different, there's a different tRNA
for every combination of bases
that can be found in a codon,
with the exception of the three stop codons,
because those don't have tRNAs.
So that way, all of translation stops, everything falls off,
and your protein's all done and ready to go.
All right, hope that helps.
Let me know if it was useful to you.
If so, I can add some of these in where they're helpful.
Let me know if there's something else you want me
to talk through in this way, if that would be useful to you.
But I think that will do it for now.
Thanks so much. Bye.