HAZEL SIVE: We're well
along our sequence
of molecular
information transfer.
The thing that I want to
talk with you about now
is kind of the culmination
of information transfer.
And that is the
process of translation.
Translation refers to a
process that produces protein
from an RNA template.
If we think about DNA
replication and transcription,
they are, in a
way, quite similar.
DNA replication is just
copying one language
into the same language.
Transcription is kind of like
changing fonts in your Word
document.
You go from Calibri to Georgia.
The letters are the same.
You can recognize them.
There's a U, the UT, thing.
But otherwise, the
letters are the same.
You're in the same language
with RNA transcription from DNA.
Translation is different.
We're producing a
protein here that's
made of amino acids,
its own language,
from a different language,
the language of nucleic acids.
And that sense of how
does nucleic acid encode
protein, a different
language, was a huge question
for early molecular biologists.
This process is done
in the cytoplasm,
unlike DNA replication
and transcription,
which are done in the nucleus.
And it uses the genetic code.
We'll write down what
the genetic code is.
And then we'll do
some explanation.
The genetic code refers to
triplets, three sets of three,
of RNA bases, each of
which, each triplet,
encodes one amino acid.
These triplets
are called codons.
And each encodes an amino acid.
The process requires
lots of components.
The key ones are a kind of RNA
called tRNA, or transfer RNA,
and also the organelles
called ribosomes
that we mentioned when
we flew through the cell
and listed the organelles.
I will also mention
that the kind of RNA
that is used for translation
is a special RNA called
messenger RNA, or mRNA.
And that is the protein
coding type of RNA.
The RNA is read
5-prime to 3-prime.
And the protein is made from
the amino to the carboxyl end.
The RNA is read
5-prime to 3-prime.
And the protein is made from
amino to carboxyl end, OK?
Let's look at a slide here.
This is what the
genetic code looks like.
You always see it
as a kind of table.
And there are three
parts to every codon.
These are the triplets--
GUU, GUC, GUA, GUG.
Those all encode the
amino acid valine.
AUG, very important
one, encodes methionine.
All proteins start with that,
you'll see why in a moment.
And then there are some codons
that don't code for anything.
They're UAA, UAG, and UGA.
Those stop protein synthesis.
We'll talk about those
more in a moment.
So let's carry on
with some board work
and lay out a bit more
about how translation works.
In any kind of language change,
you need an interpreter.
You need someone who
understands both languages.
You need something in
the case of translation
that understands both the
language of the nucleic acid
and the language of the protein.
And as we started
alluding to, this
has got something to
do with the codons that
encode each amino acid.
But the interpreter
is this type of RNA
called tRNA, small RNAs that
are present throughout living
cells.
And those are really the
interpreters for translation.
So tRNA is the interpreter.
And it's the interpreter
for two reasons.
Firstly, it recognizes
codons on the mRNA.
A tRNA has got something called
an anticodon that base pairs
with a codon on the mRNA.
For example, if this
is an mRNA codon,
it goes 5-prime AUG 3-prime.
You know it's RNA
because of that U.
The tRNA would have the
complement of that 3-prime
TAC--
not T, ah, UAC and 5-prime
because it's also RNA.
This would be the
codon on the mRNA
and the anticodon on the tRNA.
They can base pair
with one another.
So tRNAs will base pair
three nucleotides at a time
to the messenger RNA.
That's part of
being interpreters.
The other part is
that tRNAs also
carry the corresponding
amino acid that is covalently
bonded to the tRNA, very cool.
For example, for this
particular codon,
for the mRNA 5-prime AUG,
this encodes the amino acid
methionine.
And a particular tRNA that has
the anticodon 5-prime CAU also
carries methionine.
Methionine is kind
of interesting
because it's the amino acid
that all proteins start with.
I'll make a note
of it down here.
Methionine is the
starting amino acid.
So let's think how this works.
Let's write a small
piece of nucleic acid,
and then turn it into protein.
Let's write 5-prime AUG
AAA and ACU 3-prime.
This is our messenger RNA.
When we look at our messenger
RNA, the first thing to do
is to break it into codons.
I like to put a slash.
You can underline
them, if you want.
But we can put a slash.
We'll break our messenger
RNA sequence into codons.
Then we can start thinking about
what amino acids correspond
to those codons.
So codon 1 will
correspond to methionine.
Codon 2 corresponds to lysine.
And codon 3 corresponds
to threonine.
These are the three letter
codes for the amino acids.
The way translation works is
that you start at methionine.
That is your start signal.
You read the mRNA
code without spaces.
And you keep translating until
you come to a stop codon.
The name there is
a bit misleading
because they're not
actually codons.
They don't code
for any amino acid.
That's why they
stop translation.
The whole thing falls apart.
The ribosome falls apart.
The protein comes
off the ribosome.
And you're done when
you get to a stop codon.
So stop codons do not code.
And they end translation.
That is their job.
Let's look again at the
genetic code bearing
what I've told you in mind.
The way it is written is
with the first letter--
it's actually the first
nucleotide, the first base--
the second letter, and the
third letter, as indicated.
So the first letter--
here, all of these start with
an A. The second letter, all
of them start with a U. And
the third letter is different.
You can see that,
for some amino acids,
there are a number
of possibilities.
This one here called isoleucine
has a number of possibilities.
It has three possibilities.
This one here, threonine
has, again, a number
of different possibilities.
Methionine has just
one possible codon, OK?
For each of these
codons, again, there's
a matching tRNA that
carries the right amino acid
and recognizes
the correct codon.
Now, let's look how we
go from DNA to protein.
Here is a DNA template.
I'm showing you just one
strand of the DNA template.
I've taken away the other one--
the 3-prime to 5-prime strand.
It is translated-- it is
transcribed, excuse me,
into RNA in the anti-parallel
direction, 5-prime to 3-prime.
And I'm telling you that
it is messenger RNA.
And it's coding RNA.
And I've broken it
up now into codons.
And each of those
codons then corresponds
to a particular amino acid.
So transcription is going in the
5-prime to 3-prime direction,
as all nucleic acid
synthesis does.
Protein synthesis goes in the
amino to carboxyl direction,
as we discussed.
And you can match this all up
to go from the DNA template
to the corresponding
complementary RNA
and the corresponding protein
that would be translated
from that particular RNA.
We're not going
to go into detail
of how ribosomes work
and the exquisitely
complicated, beautiful process
of translation in the same way
as we haven't for
replication or transcription.
The idea is that
you get the basics,
you get the general
idea of translation.
And when you take a
higher level course,
you'll be able to understand
a lot more of the details.
But the idea here is,
just as we have shown,
the messenger RNA
5-prime to 3-prime.
The tRNA is the little
anticodons indicated
that have their
5-prime to 3-prime,
their anti-parallel bonding
to particular codons
on the messenger RNA,
and are also carrying
the corresponding amino acids.
Coming in sequentially into the
ribosome as the amino acids are
brought next to each other
by virtue of the tRNAs,
they form peptide
bonds between them.
And voila, you get
the protein chain
that is busy growing from
amino to carboxyl end.
I want you now to go
to the assignment that
has to do with translation and
get some good practice on how
you go from RNA to protein
and from gene to protein.
