Julia just mentioned that a few of you had commented,
when we were talking about the genetic code, that some of you
thought the fact that it was degenerate, it had some redundancy
in it, like multiple codons or threonine, that that was kind of
cool, and some of you thought it was sort of a waste and would have maybe
designed the thing differently. That's, you know, part of when you
study biology you don't get to design it from first principles.
You found out what happened during evolution and what got selected for.
And once it gets selected for then that gets sort of fixed in nature.
If there were four nucleotides then you could have one,
two and three-letter words. And it's going to be a three-letter
word to have at least 20 then you've got some degeneracy or redundancy,
but that's not necessarily a bad thing. And, in fact,
if you go into the evolution of the code more deeply,
people are beginning to suspect it evolved from a simpler one.
And there actually are some relationships between some of the
codons that go back to the similarities, the chemical
similarities between the amino acids. And it also allows some things for
some cells, for example, if they want proteins to be present
at very low levels they will use a codon that has just a very low level
of the corresponding tRNA. And if they want to make a lot of
the protein they'll use a tRNA that it makes in abundance.
And so it's sort of another way of controlling levels of proteins.
There are a lot of different subtleties in here.
And also in biology redundancy is not necessarily a bad thing.
It's just like on a space flight, if something goes wrong and if
there's some kind of redundant function then you've got some
backups, too. OK. Well, in any case,
today is a pretty interesting first part of the lecture.
I've heard a few people express the view that why can't I just teach
what's in the textbook and get on with it?
And I think this part, for those of you who are following,
really trying to understand what I'm trying to do with this course,
I hope this will help you to see this. Because what I've talked
about, this thing that Crick called the "central dogma" which was the
direction of information flow in biology which was from DNA
to RNA to proteins. And I'll just remind you,
although proteins do many things they are, for example,
enzymes that are biological catalysts. And it was pretty
well-established, even by the time I was an undergrad
that this was the way information flow went in biology and this was
how it worked. And there were various statements
in the literature that what was true for E. coli was true
for an elephant. And it is still true today in a
broad sense that, as I've tried to emphasize
throughout the course, when you get down to a cellular
molecule level there's an awful lot in common and things look much more
alike than different compared to what we see at a more macroscopic
scale. However, that doesn't mean that all the
details are the same. And maybe you could begin to get a
glimmering of that when I told you that although the genetic code is
virtually universal. That almost every organism,
with only a couple very tiny exceptions, uses exactly the same
genetic code to have nucleotides correspond to three-letter words in
the nucleic acid alphabet correspond to particular amino acids in a
protein. But the other languages that are written in there such as
the sequence to start transcribing a gene, making an RNA copy are stopped.
Those are different between different organisms. Yeah?
Glycolysis enzymes are amazingly
similar. They are very clearly, they arose once, and they have
stayed right through evolution. You could, in principle, sometimes
in evolution you get something that creates a function and something
that starts out, and then like what they call
convergent evolution you end up with two things that came from a
different evolutionary origin but have learned to do,
let's say, catalyze the same biochemical reaction or something.
Glycolysis came once. But if you were to look inside E.
coli or yeast, let's say E. coli and look at how those enzymes are
regulated, the thing that says this is the start of a gene,
start making the RNA, it would look totally different than if you looked
in a mouse because the language, the promoter does not have the same
sequence in an E. coli and in a human or in a mouse.
And I'll tell you more about that today. But there were --
I want to just now tell you sort of three things that were sort of
exceptions to this general way of thinking. Every one of them
generated a Nobel Prize. And this is a fun lecture for me to
give because the individuals involved in all of these things had
a very, very close association with MIT. And when I told you when Crick
called this a central dogma he meant a hypothesis, or at least an idea
for which there was not reasonable evidence.
And he learned later it was something a true believer cannot
doubt. And once this gets established it does get in the
textbook and it does get in your thinking. And so information goes
down this way. But there were a few oddities.
I mean there were some viruses that had RNA inside them.
They didn't have DNA. So how where these handled?
Well, there turned out to be two classes of RNA virus.
One that was studied quite heavily called, it's a plant virus called
the tobacco mosaic virus. And it had a coat. And then it had
in it a piece of RNA. Now, you can see if that virus were
to inject RNA in the cell it could encode proteins.
But that genetic material has to be copied. And the RNA was copied --
-- by an RNA dependent
RNA polymerase. And so it's sort of just like the
RNA polymerase before, except instead of using DNA as its
template it can use RNA. So that sort of somehow would be a
little loop in here about RNA being able to copy itself that hadn't been
anticipated. And although this is an important virus in the plant
industry, for plants and agriculture, it's not so important for humans.
But there's another class of RNA viruses that are very important.
And these are called retroviruses. And the reason these are so
important is that the HIV-1 virus that's associated with AIDS is such
a retrovirus. It's a virus that has a coat and it has an RNA that's it
genetic material. And the person who worked out how
this goes was a person at MIT, Dave Baltimore. He was a colleague
of mine here for many years. He was the person who founded the
White Head Institute and got that up and going. And he then finally,
to move up one more administrative challenge, went to Caltech
to be president. And that's where he is today.
And David was working on this problem trying to figure out how
these retroviruses work. And they're important. Not only
the HIV-1 virus, but there are certain viruses that
are associated with cancer. In general, what they do is they've
picked up what's called an oncogene which is sort of often a mutated
version of one of your normal genes.
And if that virus gets inside one of your cells and brings in this
mutated gene it's sort of kind of the same consequence as mutating one
of your own genes along that progression of cancer.
So it can kind of, say, bring in a cell that screws up the
control on when cells are supposed to replicate and stop dividing and
so on. So David started to work on these, and what he discovered was
that these viruses encoded, they had information encoding
proteins. And one of the proteins encoded in
their RNA is an enzyme de-characterized which is given the
name "reverse transcriptase". And what this can do is take an RNA
template and make the corresponding complimentary DNA strand in this way.
So that if we took the -- We'll just take this RNA out of the
virus. What this virus encodes then is an enzyme that's able to take
this RNA and make the corresponding DNA copy. So there's the original
RNA that was in the virus. There is the RNA that it started
out. And so what is happening, if you will in that case, is the
information is flowing in the other direction.
That was a marvelous discovery.
And it was discovered by someone who wasn't willing just to take what
was in the textbooks but was trying to figure out what could possibly be
going on here. Now, the way these viruses work
then, once they've done this it's not so bad because they've got their
information now in the form of DNA. So this strand of DNA can be made
into a double-stranded DNA by just using the kinds of enzymes that
we've already talked about. A DNA dependent DNA polymerase will
be able to copy the other thing. And now you've got a DNA copy of
the information that used to be in the virus. But what happens to that
is that you have a piece of the host DNA.
And this viral DNA then inserts into it, so you end up with this
situation where you have DNA from the host, and this
is the virus DNA. So this is the DNA that encodes the
information needed for the virus. And if this was our DNA then it
would be inserted that way. And there are just a handful of
health messages I've tried to drive home in this thing.
I mentioned smoking the other day. If you smoke --
If you stop smoking you basically, well, let me try another way. The
risk of smoking is about equal to the sum of everything else you can
possibly do in your life that will affect your chances of getting
cancer, leaving aside what you inherited from mom and dad.
The one single thing to not do if you want to avoid cancer,
or to help loved ones who smoke avoid cancer, is just don't smoke,
or if you do smoke, stop. You freeze the risk of whatever
increased risk you've got, and then just live with that,
but it doesn't keep getting worse with time. The other one is
practice safe sex, and this is why. HIV-1 is a
retrovirus. If you get infected with it, it makes a DNA copy of the
RNA, it makes the other strand of the DNA, and it sticks itself in.
So what you've got is your DNA here, your DNA there.
And HIV-1 is a permanent traveling companion for the rest of your life.
There's no way of getting that out of there right now.
All the systems for dealing with AIDS are just managing the infection.
So when someone is HIV-1 positive, they've got those viral genes now
permanently integrated into their DNA.
So it's extremely important that you be aware of that,
or if you know people who don't appreciate this because they haven't
got so much of a biology background that you help them understand that.
OK. So I just wanted to show you, I found one other picture last night.
And this is you see all these old scientists, right?
Of course, David didn't look like this when he was doing his work.
In fact, I think he's fairly cleaned up here.
I found this one in the Cold Spring Harbor archives last night.
I've seen pictures of him looking considerably more shaggy and perhaps
disreputable and stuff. But anyway, when David was making
all these discoveries he was still quite a young man.
I believe he got his Nobel Prize when he was still in his thirties.
And so many of these discoveries are made by people that are not all
that much older than you. But, again, it's trying to
understand why we know what we know and then trying to fit
other things into it. Now, the next thing I want to tell
you about that has some of this same character, I've sort of told you
that you have a piece of DNA. Let's say there's a gene here and
this is the coding region, and then we make a mRNA copy,
and then we use the genetic code and we make the protein.
And so if we sequence the DNA and find the beginning of this protein
we can read along using that genetic code and away it should go.
And that was beautifully worked out, understood, just like I sort of
finished up telling you the other day. So Phil Sharp who got a Nobel
Prize for this work and his colleague in the Biology
Department. He's in the Cancer Center just
across the street from the building I'm in. That was the cancer center
that Salvador Laurier, who Jim Watson trained with,
had founded. And Phil was studying this process. It was before we
could sequence DNA. It was in the mid '70s.
And he was working with the tools we had then trying to map the
relationship of an RNA to a gene that was on a virus.
It was a DNA virus, not an RNA virus, so don't get
yourself mixed up with that. But what he had was basically a
fragment of DNA that he knew encoded the gene. So he knew somewhere on
this piece of DNA there was a gene somewhere in here,
and he had isolated the mRNA. And one way you could map,
physically see the relationship of an RNA and a DNA would be to take,
let's just take away one of these strands.
So we have the complimentary strand of the DNA to the RNA.
And if we mix them together and let them slowly cool down they will form
hydrogen bonds. They'll form a DNA-RNA hybrid just
the same way two strands of DNA come on. And so if the gene was a little
shorter than the piece of DNA then you might have expected to see
something that looked like this. And the way you'd see this,
if you looked in an electron microscope --
-- perhaps it would look sort of
like this. You cannot actually see the two strands,
but you'd see a thick part. That would be the RNA duplex.
So this would be just DNA. And the thick part is RNA base
paired with a single strand of DNA. You got it? That's what textbooks
said you should have seen. And so this is more. This is data
from Phil's paper describing this. And let me focus on this one in
particular. That's what he actually saw.
You guys got any idea what's going
on? Why don't you take a minute, find somebody who's near you and see
if you can come up with any ideas. Here's the hybrid. Forget about
this little bit at the 3 prime end. That's not a worry. Here is the
thing. And this, I think, is a piece of single
stranded DNA sticking out the end. But it looks a bit more complicated.
Any ideas? Most people put this data in their drawers.
Phil didn't. Phil and his colleagues didn't.
What they realized was, I'm going to try and redraw this
just very slightly to help you see what's going on.
What they were seeing was something that looked rather like what they
were expecting. They were seeing a region of hybrid
DNA and they were seeing a region of single-stranded DNA like this,
but what it looked like was there were little loops of single-stranded
DNA sticking out. And what Phil had discovered was a
phenomenon we now know as RNA splicing.
And here's what goes on.
In bacteria, with very few exceptions, you can look at the DNA,
you can find the open reading frame and you can just read off the
sequence of the protein. You find the ATG, AUG, methionine
codon, and then it keeps going no stops, and finally you come to a
stop codon and you see there is the protein. So the coding information
is essentially continuous in almost all bacterial genes.
And there's a few, some genes like that in eukaryotes,
but many eukaryotic genes are constructed, it's almost as if you
took the gene you'd find in a bacterium and then you'd cut it in a
bunch of places and stuck extra DNA in between all of the pieces.
So you'd get something like this where there's,
in the DNA there'd be coding information.
And then non-coding information and another block of coding information.
And then a block of non-coding and say another one of coding
information. So this is a double-stranded DNA.
And what happens then when the cell makes RNA is the whole thing gets
copied into what's known now as a pre-messenger RNA.
And so there's a bit of coding stuff here, there's a bit of coding stuff
here, and there's some more coding stuff there. But what the cell has
is sort of like your unedited footage from your family summer
vacating when you were running the video camera.
And maybe you don't want to show everybody ever second of video that
you took during the thing. So what you do, you get in there
and you edit it. In the old days you used to have to
take the film and splice it. And now you can all do it with
iMovie or something like that. But what you do is take the pieces
of information you want, and this is what the cell is doing.
It takes this part of the RNA. And this part of the RNA,
and joins it together, and then this part. And when it's done it has the
mRNA that now looks like the kind of mRNA that you would find in a
bacterium where you can find the start codon.
And then you could read in three-letter words all the way
through to the end of the protein. So, in essence, what Phil found was
that in many organisms at least there's another step in here where
we get RNA splicing. And only after that you get down to
proteins. What was quite remarkable about this
result and why I'm kind of hammering on it a little bit is this is the
data that's out of Phil's paper. You can look it up on the Internet.
Type in Phil Sharp 1977 and you'll find this original paper with that
figure in it. The moment Phil realized what he was and talked
about it at a meeting, a whole lot of people suddenly sort
of almost simultaneously discovered RNA splicing because they opened
their drawers and there were all these uninterpretable electron
micrographs they had. And they were in very short order
able to save it in the system. The same thing was going on, but it
was just confusing, it didn't fit, and to some extent
most people's minds were set by this paradigm, this central dogma as
something that a true believer cannot doubt. And you had to have a
flexible enough mind to be able to see that.
And so this is an important piece of biology that hadn't been anticipated.
And it can be quite remarkable. I'm just going to give you a couple
of extreme examples. Well, not even extreme examples.
But just show you how much non-coding information there can be.
Factor 8 is a protein that plays a part in blood clotting.
And the gene is 200 kilobase pairs. And the pre-mRNA is just a direct
copy, so it's 200 kilobases. It's just a single strand so it's
not a base pair. And the actually spliced mRNA when
it's done is 10 kilobases. So that means that only 5% of the
gene is coding information and 95% of that information gets thrown away
when the RNA gets spliced. And even a more extreme example is a
protein called dystrophin. This is what's affected in a human
genetic disease known as Duchenne muscular dystrophy.
In this case, the gene is two mega
base pairs. So of course then the pre-mRNA is also two mega bases but
the pre-RNA is 16 kilobases. So in this case less than 1% of the
gene has coding information for making a protein.
There are a lot of interesting reasons as to why it would be like
this. One this, things can evolve more rapidly
sometimes because you have parts of proteins that are sort of like
modules and evolution can probably connect them.
I fact, it also provides ways of regulating because we now know there
are alternative ways of splicing RNA. So you can take one RNA and then
splice it in different ways in different cells and end up
generating different proteins that were all encoded by one particular
gene. And so it gives cells different kinds of regulatory
strategies they can use. Now, the third sort of thing that
came out that falls in this same kind of thing of people having their
minds open and not fixed by the current understanding or bounded by
the current understanding is the discovery that RNA can act as an
enzyme. And I've already talked to you about that and I've told it was
ribozyme, but it was discovered by Tom Cech. Tom is currently
president of the Howard Hughes Medical Institute,
but he did his post-doctoral work at MIT with Mary Lou Pardue.
I've been a post-doc at Berkeley when he was just finishing his
graduate work, and I met him out there.
And then he came to MIT to do his post-doc. And a year later I got a
job so I'd become friends there and became friends when we started here.
So I had a pretty close link to this particular story.
Here's a picture of Tom together with Phil. That's actually my wife
right there who was in this picture. But Tom actually looks much more
like that. He's very colorful,
very fun, a very interesting person. But anyway, when Tom left MIT to
take a faculty position at Bolder he was interested in trying to
understand the biochemistry of RNA splicing. And so he went --
He did what a good scientist will do. They'll try and find an
experimental system where the question they want to address is
simple enough you can actually get an answer. There's a kind of way of
doing science where you pick a system that's too complicated and
you never actually get an answer. It sounds very important because
you're working on something that's important but you cannot,
you don't have the tools you need to get to the answer.
So Tom wanted to work on the biochemistry of RNA splicing because
that had just been discovered. And so he went to a small little
tiny organism called tetrahymena. And the reason he looked at that
was because it had a ribosomal RNA, so it was an RNA that was made in
great abundance within the organism. And it only had one of these
non-coding regions. I'll tell you the words for these
coding and non-coding. To me they're non-intuitive,
but I guess you should know them.
The coding region is called, the part that codes is called an
exon and the non-coding part is called an intron.
So, anyway, Tom worked on this organism because the pre-mRNA
was basically this. Or the pre-RNA before the splicing And whenever I got out to Bolder we'd try and get in a squash game.
looked like this. This was going to give this like
that. He could get large quantities of this RNA, so he was all set to
make extracts of the cells of this organism and then start cooking up here. And he went off to, I guess it was Denmark to learn how
this RNA substrate with all sorts of cell extracts. And so I first heard about this, Tom was working on this when he was
And then his plan was to purify the enzymes that did the RNA splicing.
to grow this organism. Then they were back and he was off
at Bolder. And we used to play squash all the time.
So I was out there at a meeting and we were sitting around in the locker
room. And I said so how's the splicing biochemistry projecting
going? Tom says, well, it's going OK,
I guess. There's only one little problem, he says.
The controls are splicing. Now, what he meant was if you were
trying to add cell extract and get this thing to go what you would
start out with is the RNA in a tube basically.
And that would be your control. And then you'd start adding stuff
to it and start looking for splicing. And what Tom was finding was that
if you just took this RNA and let it sit in a test tube that the splicing
happened without him putting anything in. And here he was
already to find all the enzymes, the proteins that did it. And Tom
did an absolutely gorgeous piece of science to prove that what was
happening was the RNA was catalyzing its own splicing.
And he had to work very, very hard to prove that it wasn't a
contaminating protein. Remember we had this sort of
discussion? We were talking about is DNA the genetic material and how
would we know that it wasn't just a little tiny bit of something else in
our DNA perhaps that was doing it. Tom had to go through pretty much a
similar exercise, but this was one of these key
insights that lead to the proof that RNA could function as a catalyst,
what we now know as a ribozyme. And I've shown you now we now sort
of accept that the actual ribosome itself is a ribozyme and that the
formation of the peptide bond, the thing that's the heart of all
proteins is made by a ribozyme, not catalyzed by ribosomes and not
by a protein. OK. So the next topic that I want to
try on which sort of we've already set up from this is that if the
information is all in DNA to begin with then if you make an RNA copy
you're only taking a segment of that information at a time.
And that gives the cells a lot of possibilities for regulating how
they respond to the environment or just controlling what genes are
expressed. And there are basically two kinds of strategies that are
involved in these regulatory decisions. They can either be --
Can either be reversible changes.
For example, a bacterium and a food source. If you're a bacterium and
you've got enzymes that let you eat a hundred different kinds of food
and you're in an environment where there's only one of them there,
you're really wasting energy if you make the proteins to make the other
99. So you might guess that somehow evolution has selected four systems
that have learned how to turn on and off the things they need to eat
certain food sources depending on whether the food source
is available. We only carry umbrellas when it
rains. If you had to carry an umbrella and a snowsuit and a
surfboard, everything all the time, it would slow you down in evolution.
So the other type, which we've talked about as well when we talked
about starting as a single cell and going up to the 14 cells that make
us up, then many of those changes, as those cells go along and
progressively more specialized need to be irreversible.
And this is particularly important
in development. We don't want a cell in our retina
suddenly deciding it should be part of a heart and start to make a heart
in the middle of your eye or something like that.
So things in development tend to be once you're off you're off or once
you're on you're on or something. And just to give you another little
look at that picture I've shown you before of the nematode.
And at the time, the first time I showed you this,
I was just trying to emphasize that we could take the gene encoding
green fluorescent protein and put it in anything and it would go green.
In this case, Barbara Meyer who is at Berkeley now but used to be my
office-mate at MIT for many years, what she's done is she's taken that
green fluorescent protein, the gene for that, and she's put it
under the control of a regulatory system, a gene that is made to be
expressed in the esophagus of the worm.
And so even though that gene is present in all the cells of that
organism, it's under the control of a system that usually permits the
genes to be made that are needed for making esophagus but not in other
parts of the body. So you probably didn't pick that
part up now but sort of take another look at that same thing and see
something different. So how do we learn about gene
regulation? The key work, like so many of these
things, started kind of inauspiciously,
if you will. There were two French scientists, Jacques Monod,
who is a biochemist, Francois Jacob who was a geneticist.
And they were working on the metabolism of lactose by E. coli.
Lactose is galactose, beta 1,4 glucose. And you don't
have to know exactly the structure. You can just remember there were a
lot of different hydroxyls, and that was one particular linkage.
And there's an enzyme that cleaves this into galactose and glucose.
And this can go right into glycolysis and make energy
for the organism. And the galactose undergoes a couple
of different transformations, and it can get in there as well.
But in order to get at the energy that's in those carbohydrates,
this linkage has to be broken. And it was broken by an enzyme called
beta-galactosidase. That's a protein that's able to
catalyze the cleavage of those two sugars.
That's what Jacques Monod and Francois Jacob were studying.
They were helped out in this exercise. I guess part of the
reason they got going on this was people had noticed for many years
that if you grew E. coli in glucose there was no
beta-gal. I'm going to abbreviate this as
beta-gal just so I won't have to keep writing the same thing.
But if they grew E. coli in lactose beta-gal was present.
And they had to be able to assay for this enzyme. And they used --
There were standard types of
biochemical assays you could use. But some chemists that helped
design a very cleaver kind of substrate that helped them,
that could be used in these kinds of studies, and I'll show you one of
them. What this enzyme really looks at is it looks at, let's
see, galactose. What it sees is sort of the
galactose side of this linkage, and then it reaches in and catalyzes
the cleavage of what's joined to it. And it turns out not to be specific
for whether glucose is on the other side. It can accept substrates that
have other things as well. So some chemists made some variants
like this. This is a compound that's commonly known as X-gal.
If you talk to it in the lab it's got a longer chemical name.
But what happens if beta-galactosidase is there,
it's able to cleave this substrate so you get galactose,
which is colorless. But if you get just X, this is colored,
but up here this original material is also colorless.
So this is very convenient because if you use a substrate such as this
you could put the cells on a plate with this indicator.
And if they are colored, and the color is blue, you'd know
they were making beta-galactosidase. And if you don't see a color, you
know they're not. There are a variety of ways of
assaying for this enzyme. With that I'm just trying to give
you a little bit of flavor of one of the different ways that they could
assay for it. Now, one of the issues was it looked as
though E. coli didn't have any beta-galactosidase activity if
lactose was absent when growing a glucose.
And they made it if lactose was present. Well,
that would be kind of what you would expect evolution would have figured
out how to do, only make the enzyme for
metabolizing lactose if the lactose is present, but they had to figure
out what the molecular basis of this was. And one of the possibilities
was that the protein was made that it was all sort of unfolded,
and when the substrate came in then it folded all around it and then it
could cleave it. Or another possibility,
which would be the kind we're talking about now,
is the protein is not made until the lactose is present,
and then it makes it new. So they had to figure out,
between these two, which of these two was true. When you see the
lactose present, is it just beta-galactosidase is
already made but it's inactive, or is it being made de novo when you
add the lactose? So what they did was they grew cells
in glucose plus radioactive C14-leucine for a long time.
So all the proteins --
-- were radioactive. And once they got, that's going for
a long time. So every protein being made is radioactive.
Then they add excess unlabeled leucine. So this means that from
now on any new proteins that are made will not be radioactive because
you're just going to swamp out any radioactive stuff with this.
And they added glucose, excuse me, now they added lactose through the
cells. And then they isolated the beta-gal
enzyme. It was actually pretty easy to do. It's a huge enzyme and it's
a tetramer. So very large. Even in those days it was fairly
easy to isolate this enzyme. And then they looked to see is it
radioactive? If it's radioactive it was there all along and it's
refolded to become the active enzyme.
Or if it had been only after lactose then it would be made de novo in
response to it. And what they found was that it was
non-radioactive.
Which implied that it was made after you added the lactose.
So they knew then that they were studying a system in which a protein
was only made after the cells had experienced a particular growth
substrate. And so a lot of work went into figuring out how this
system worked. Let's see. We're a little short on
time. So I'll tell you what I'll do.
I'll tell you, I'll just put out quickly the mechanics of what they
saw, and we'll start in on the regulation on how this works.
And some of you may be able to figure it out.
What we now know is that the gene that encodes beta-galactosidase is
in a stretch of DNA that's pretty interesting. It's got three genes.
It's the gene lacZ. This is the gene for beta-galactosidase.
And another gene called lacY and lacZ. There's a promoter.
That's a start signal for transcription.
Remember that? So there's a sequence here that
says start transcription. Down here is a terminator.
Another word written in the nucleic acid alphabet that means stop making
mRNA. And there is one long mRNA, as you can see, that has the
peculiarity of encoding three different genes.
So if you have more than one gene in a single message then that's
called an operon.
You've got one mRNA. But, in any case, so whenever
beta-galactosidase was being made then RNA has to start being made
here, goes to there. And we won't worry about the
functions of these other two genes. But, as you might guess from the way
evolution has selected for it, they have related activities to what
beta-galactosidase does. And for bacteria it's a very
efficient way to control the expression of a bunch of genes at
once. Then there was another gene up here known as lacI that had a
promoter and a terminator, and it made an mRNA as well.
And that mRNA encoded a protein that's known as the lac repressor.
And what that lac repressor does, it's a protein that has the ability
to recognize a very, very specific DNA sequence and bind
there. And I'm just going to kind of blow up this part of the thing.
So what we have here is the, this is the promoter here. And it
happens that the binding sequence --
-- for lac repressor overlaps with
the promoter. Weird, right? Maybe not.
So I'll tell you, well, you can think about this over
the weekend, if you haven't run into this system before.
So this gene gets made all the time. So this protein gets made all the
time. What does that protein do if it's just like this?
Its job in life is to look for this sequence and bind to it.
If it binds to it, it covers up the promoter.
And the beta-galactosidase gene is not expressed because the cell
cannot make mRNA. So this may seem a little obscure,
but there's something very important here. Now the conditionality on
whether this gene is expressed or not is controlled by a protein,
right? It's controlled by this lac repressor. If it's on there the
gene will be made. And if it's off the gene now you
can make it. There's a promoter and the RNA
polymerase will see it. And so you've learned something
about proteins. They can bind various things.
And so what lac repressor has, it's got a little binding site that
lactose is able to bind to and change the confirmation of the lac
repressor. So why don't you take those pieces of information and see
if you can figure out how the circuitry goes.
Yeah? Did I do something wrong? Sorry. Oh, sorry. Excuse me. Yes,
Z-Y-A. Excuse me. OK? We'll walk through that on
Monday, but focus on the fact that if the repressor is there and
lactose isn't, it binds to this sequence.
The repressor is made all the time, but this repressor is something that
can tell you whether lactose is there or not. So you can put the
circuit together, OK?