- So let's say I have a piece of mRNA.
And this mRNA was transcribed or copied from a piece of DNA.
And it's holding information.
And let's say that each one of these rectangles
on our mRNA is one nucleotide.
Every group of three nucleotides
is going to code for one amino acid.
So three nucleotides, or one codon.
A codon is simply three nucleotides.
Code for one amino acid.
And what we want to do is we want
to read this piece of mRNA.
And put together all the amino acids that it codes for
in the precise order that they're coded for
to make a polypeptide.
So a polypeptide is a chain of amino acids strung together.
And now I want to take this polypeptide
and fold it into all sorts
of interesting shapes to make a protein.
And some proteins actually have more
than one polypeptide chain,
like the protein hemoglobin for example.
So hemoglobin is a protein in our blood
that helps us carry oxygen.
And it's made up of four polypeptide chains
that are folded in a very precise shape or a conformation.
So the question is, how do we get from this piece
of mRNA to the polypeptide that we want?
In other words, how do we read this piece of mRNA?
And the answer to that question is,
through a process known as protein translation.
And protein translation happens in an organelle
in a cell called the ribosome.
Which is the picture that you're seeing right in middle.
And so let's recap.
So protein translation is a process by which we're
going to take a piece of mRNA and we're going to
run it through the ribosome.
And the ribosome is going to put together
the corresponding chain of amino acids
known as a polypeptide.
And where are these ribosomes found?
Ribosomes are found in the cytoplasm of the cell.
And they're also found in a different organelle
called the endoplasmic reticulum.
And there are two types of endoplasmic reticulum.
There's the smooth endoplasmic reticulum
and the rough endoplasmic reticulum.
And the ribosomes are found
in the rough endoplasmic reticulum.
And so by looking at this picture
you can see that the ribosome is made up of two units.
A larger one and a smaller one.
And here we get into some differences
between eukaryotic cells and prokaryotic cells.
So let's make a quick chart.
We have eukaryotic cells and prokaryotic cells.
And in eukaryotic cells, the larger sub unit is 60 S.
It's kind of way to measure it.
And in a minute we'll get to what that S stands for.
And the smaller unit is 40 S.
But if the two units are together combined they are 80 S.
And in prokaryotic cells the larger unit is 50 S.
The smaller unit is 30 S.
But if we combine them together, they are 70 S.
And you're probably thinking that I'm really bad at math
because 60 plus 40 is not 80 and 50 plus 30 is not 70.
So to clarify that let's talk about what this S is.
So the S stands for a unit of measurement called, Svedberg.
And the Svedberg measures the rate
of sedimentation of the ribosomes.
So let's explain what that is.
So if I were to take a test tube
and if I put some ribosomes into this test tube,
some big ones, some little ones,
some larger sub units, some smaller ones,
some of them are combined large and small units.
And of course the ribosomes are suspended
in some sort of liquid.
So if I were to take this test tube
and spin it down in the centrifuge
these ribosomes would all fall down to the bottom.
Settle at the bottom of the tube at different rates.
So the Svedberg measure is the rate at which
the ribosomes will fall to the bottom of the test tube.
And because the Svedberg does not necessarily measure size
the numbers are not additive.
They don't necessarily add up.
So the next thing I wanna ask you is,
so the ribosome is putting together it's polypeptide,
he's stringing together the amino acids,
but where are the amino acids coming from?
Like, where are they?
And the answer to that question is
that floating around in the cytoplasm
and in the endoplasmic reticulum we have
these molecules called tRNA.
And they hold onto amino acids
and bring the amino acids to the ribosome.
So let's see what a tRNA molecule looks like.
And I'm just gonna get rid of some of this
RNA that's coming out of the ribosome.
And here's our tRNA molecule.
And I did not draw this to scale.
So a tRNA molecule is really much smaller than the ribosome.
So it has this clover like shape
and in real life it's actually a 3D structure.
So we know it's made up of RNA.
So what does that T stand for?
That T stands for the word transfer.
So this is transfer RNA and the reason it's called
transfer RNA is because it's job is literally
to transfer amino acids to the ribosome.
So it's kind of like a mail man.
It holds on to this package, the amino acid.
And it's job is to bring the package, or amino acid,
to the proper address, which is the ribosome.
Or more specifically, a particular place
on the mRNA that's in the ribosome.
But we're not gonna get into that
in such great detail right now.
So let's focus on this tRNA molecule.
I want to look at one particular place
on the tRNA molecule.
This part right over here.
And Im just gonna make some room.
And I picked that part because if you can see between
the two yellow lines there are three nucleotides there.
And I want to blow up those three nucleotides.
And let's say that these three nucleotides are GG and G.
And so the codon that's complementary to GGG is CCC.
And the codon CCC happens to code
for the amino acid proline.
And so this particular tRNA molecule
is going to carry the amino acid proline right over here.
And it happens to be that the bond
between the amino acid and this tRNA molecule
is a very high energy bond.
It's relatively unstable and just keep that in mind
because we'll talk about that a little bit more later.
Let's do this again so that we get
a clear idea of what's going on.
Let's say that the codon
that we're looking at did not read GGG.
Let's say it read UUU.
And again, remember we're focusing
on this bit right over here.
And so the codon that's complementary to UUU is AAA.
And AAA codes for the amino acid lysine.
And so this particular tRNA molecule
is going to carry the amino acid lysine right over here.
And so the part on the tRNA molecule,
the part over here that determines
which amino acid the tRNA molecule is going to hold
is called the anticodon.
And then it's complementary codon
right over here is just a codon.
And just bear in mind that the anticondon
and codon are complementary to each other.
And try not to get confused.
The tRNA molecule is not going to hold onto
the amino acid that's coded for by the anticodon.
So UUU happens to code for the amino acid phenylalanine.
But this tRNA molecule does not carry phenylalanine.
The tRNA molecule carries the amino acid
that's coded for by the codon
that's complementary to the anticodon.
So in our case, the anticodon is UUU.
The complementary codon is AAA.
AAA codes for lysine.
And so this particular tRNA molecule
is going to carry the amino acid, lysine.