- [Voiceover] So let's take a piece of mRNA
that we're going to translate,
and I want to make it very clear that we're
dealing with prokaryotic mRNA,
and I'm saying that because eukaryotic mRNA
is actually going to be a little bit different,
and we'll see later in what ways.
So let's start with our 5' side,
so we have this non-coding region that's in yellow,
and that's just a part of the mRNA
that the ribosome is not going to translate,
so whatever is written there is not going to end up
in our polypeptide chain,
and the non-coding regions do have certain functions.
We're just not gonna get into it right now.
And then, after the non-coding region, we have the
Shine-Dalgarno sequence that you see here in blue,
and that's actually the part on the mRNA
that the ribosome recognizes and will bind to,
so I'm just gonna draw a little ribosome
over here to help us remember that,
and then we have another non-coding region,
which can be a little bit
larger or smaller than how I drew it,
and then finally, we hit the start codon, AUG,
that tells our ribosome to begin translating,
and so the ribosome will read all of these codons,
and put together the corresponding polypeptide,
and I'm actually going to circle all the codons,
so that we get a picture of how many amino acids
are going to be in this
particular polypeptide chain that we're making,
and so we actually have 10 codons here,
and so, because we have 10 codons,
this polypeptide chain will have 10 amino acids.
I'm just gonna abbreviate amino acids 'aa.'
And just to give you a little bit of perspective,
so, this is a very small polypeptide chain.
Most of the polypeptide chains in our body
have between 200 and 300 amino acids.
Okay, so, at some point, the ribosome will
hit the stop codon right here in orange,
and the stop codon can be one of three codons.
It can be UAA, UAG, or UGA,
and then we have another non-coding region,
and, sorry if I didn't make it clear,
but the stop codon basically
tells the ribosome to stop translating.
And then again, the non-coding region here in yellow,
which I'm just gonna write 'NCR.'
So, let's begin translating.
The first step is initiation,
and so, what happens during initiation is,
the ribosome recognizes this
Shine-Dalgarno sequence,
and binds to the mRNA,
and then it moves to the right,
kind of scanning the mRNA,
until it hits this start codon, AUG,
and you can see there are, these boxes represent
three basic sites on the ribosome.
There is the E site, the P site, and the A site,
and in just a few minutes, I'll explain why
they are given those letters as names,
and so, the next thing that is going to happen is,
so we're looking at our AUG code on here on the P site,
and the corresponding tRNA molecule
is going to line up with our AUG,
so you can see the anti-codon on this tRNA molecule is UAC,
and that's complimentary to AUG,
and it's holding on to the amino acid.
It's a little bit difficult to see,
so I'm gonna draw it.
It's holding on to the amino acid
formyl-methionine,
and the first, this is going to be the first amino acid
in our polypeptide chain,
and I want you to just keep in mind that
this is in prokaryotes.
In eukaryotes, it's just going to be plain methionine,
and so, what is formyl-methionine?
Well, it's just the amino acid methionine,
but with a formyl group attached to it
somewhere on the molecule,
and so it's a modified version of methionine,
and so, this actually takes care of initiation,
the first step in translation,
and let's move on to the next step.
So, initiation was our first step,
and now we're gonna move on to our second step,
which is elongation,
and in this step, we're actually gonna
start putting together our polypeptide,
and so let's just pretend
like this codon right over here
is AAA,
and so the corresponding tRNA molecule,
with the anti-codon UUU,
is going to line up with that codon,
and just in case you're wondering
how does it know to line up,
or how exactly does it happen,
so, really, there are a bunch of tRNA molecules
floating around the ribosome,
which of course, I didn't draw,
because it would just make the picture very cluttered,
and it's actually like trial and error.
The tRNAs will kind of all try to bind
to this particular AAA site,
and they're all just gonna kind of not stick unless
the anti-codon is UUU.
Okay, so let's go back to our process of elongation.
This bond right over here between the tRNA
and the formyl-methionine is going to break,
and if you recall, it's a very high-energy,
or unstable bond, so when it's broken,
it releases a lot of energy,
and that energy is then used
to drive a reaction
that will create a bond between these two amino acids,
and that bond is, I'm gonna write it over here,
a peptide bond.
So, a peptide bond is a bond between two amino acids.
So, this is a peptide bond,
and now what's going to happen,
is the ribosome's going to shift to the right,
by a distance of approximately one codon,
and so the tRNA that's in the P site
will move to the E site,
and the tRNA that's in the A site
will move to the P site,
and so, let's see what that actually looks like.
So here's our next step.
So before, our tRNA
with the UAC was in the P site,
but now it moved to the E site,
and it's actually going to exit.
It's going to leave, so it exits,
and in our P site, we have this tRNA molecule
with the anti-codon UUU,
and it's holding on to these two amino acids,
and our A site is empty,
and let's just say that the codon here in our A site
is CCC,
and, I didn't draw these compartments in a very exact way,
because in a ribosome, they're also not in an exact way,
but it's just to give you a picture.
So the tRNA molecule with the anti-codon
that's complimentary to CCC
is going to come and line up with the
codon, just like that.
And again, we're going to break the bond
between this tRNA and the amino acid that it's attached to,
and the energy from that bond being broken
will drive a different reaction
that will make this peptide bond
between those two amino acids.
And again, the ribosome will now shift to the right
by a distance of about one codon,
and let's see what this looks like.
So, we're still in the middle of elongation,
and so, the tRNA that was in the P site moved to the E site,
the tRNA that was in the A site moved to the P site,
and now our A site is empty.
I want to just pause for a second,
and describe why
these sites are given these names.
And so, the E site stands for exit site,
because the tRNA molecule exits from that site,
so it's the exit site.
The P site is named the P site
because it stands for the
peptidyl tRNA site,
and so this tRNA that sits in the P site
is the peptidyl tRNA because it's the tRNA
that's holding on to the growing polypeptide chain,
and the A site is called the A site because it stands for
the aminoacyl tRNA site,
and actually, maybe I should have drawn it
closer to the actual site, so
that's our A site,
it stands for aminoacyl tRNA site,
P stands for peptidyl tRNA site,
and going back to our aminoacyl tRNA site,
so, an aminoacyl tRNA is simply a tRNA molecule
that's holding on to the amino acid
that it's supposed to hold on to,
and so we call this the aminoacyl tRNA site
because that's where the aminoacyl tRNAs first come.
So let's take our aminoacyl tRNA,
and let's put it into the site that it belongs,
and if the anti-codon is AAA,
then that tells us
that the mRNA codon just beneath it
is going to be UUU,
and again, the bond between this tRNA molecule
and the amino acid is going to break,
and a new bond will form between
this amino acid and the phenylalanine,
and so hopefully you're getting the picture,
and, once this happens,
the ribosome again moves to the right,
and we're not gonna go through all the codons,
but it's basically going to read all of this,
and let's fast-forward until it hits
the stop codon,
which leads us to the last step in translation.
And so, the last step in translation is termination,
and this happens when the
stop codon enters the A site.
You can see it again right over here,
and that tells the ribosome to stop translating,
and so the two sub-units will separate,
the mRNA will be degraded,
or broken down, and parts will be recycled,
and we've actually achieved our goal.
We made a polypeptide, a chain of amino acids.