Hi. It's Mr. Andersen and in this video I am going to talk about the operon.
The operon was discovered in the 1960s by three French scientists. Jacob, Lwoff and
Monod. That's really bad French. But they got to coin the term operon. And basically
it means to operate. And so it's mostly found in prokaryotic cells. Mostly found in bacteria.
There's a few eukaryotic operons but in general it's just going to be found in bacteria. And
so it led to other terms you maybe never heard of before. Like regulons and modulons and
stimulons. Stimulons are groups of operons operating together from one stimuli. And so
it just means to operate. That's what an operon means. And so the one that they discovered
was the lac Operon. They were studying this in e. coli. And so the lac Operon is designed
for e. coli to breakdown lactose. So lactose is going to be a disaccharide. And remember
if you're a bacteria living in our gut, you're eating whatever we're eating. And so as the
material moves down through our digestive system, they have to be ready to quickly shift,
depending on what the food is. If it's sugar they have to be able to breakdown the sugar.
If it's proteins, they have to be able to break it down into amino acids. And if it's
a disaccharide in this case, they have to move it into the cell and then break it down
and chemically digest it. And so they use a lac Operon to do that. Now this is a picture
of "Prog". But that's not how you spell Prague. But it is the way that I remember the parts
of an operon. You've got a promoter, a repressor, an operator and then genes. And so if you
can remember "Prog", it's going to get you half of the way there to remembering how an
operon works. And so the neat thing about bacteria is that if they have a number of
different genes that are used to achieve a certain task, they'll put them right next
to each other. And so as the RNA polymerase moves down the DNA and makes all of the messenger
RNA and makes all of the proteins, you're just ready to do whatever the job is. And
so the genes are going to be located right next to each other. In this case, in the lac
Operon to deal with lactose. What else do we have? Well we've got the promoter. The
promoter's going to be the region of the DNA where the RNA polymerase can grab on to the
DNA. We also have an operator. And so think of that like an off switch that's going to
turn on or off the operon. And then the lat thing we have is a repressor. And so this
is a lac Operon. It's Prog. It's promoter, repressor, operator and then all of the gene.
And so what are we trying to breakdown? We're trying to breakdown lactose, or deal with
lactose. Remember lactose looks like this. It's a disaccharide. But in this little model
right here, we're going to represent it as a little pentagon. A yellow little object
that look like this. And so all of a sudden a lot of lactose is present. So what is the
lac Operon going to do? Well it's going to deal with that lactose. And so all of this
lactose is moving around remember, but eventually one of the molecules is going to bump into
the repressor. As it bumps into the repressor, it's going to change the shape of that repressor.
And so a quick digression, everything, promoter, operator and genes are DNA. So they're going
to be part of the DNA of the bacteria. But the repressor is a little bit different. It's
a protein. It's a protein that came from a different part of the DNA. There's going to
be a regulatory sequence either upstream or downstream of the operon that creates the
repressor. It plugs in nicely to the operator but once the lactose fits inside it, it's
going to change the shape of that repressor. What does that do? Well it opens up a region
where that promoter can allow the RNA polymerase to get on. And then that RNA polymerase is
just going to drive right down the DNA. It's going to make a bunch of RNA. It's going to
make a bunch of those proteins. What are the proteins designed to deal with? Lactose. And
so they're going to breakdown the lactose. Now all the lactose is gone. What's going
to happen to that repressor? It's going to return to its original shape. And that shape
is going to fit into the operator. And so if there's no lactose present, we would say
the operator is in the off position because the repressor is activated. It's activated.
It's sitting inside the operator. It's physically breaking the movement of that RNA polymerase.
The RNA polymerase can't move down and code for those genes because the repressor is sitting
in it's way. What happens next? Well lactose shows up again. Lactose would move into the
repressor, free it up and then it's going to be able to make those genes. And I've got
a simulation at the end of this video that shows you in a little more detail how that
takes place. Before we get there, I want to talk about the trp Operon. The trp Operon
does essentially the opposite of that. So the trp Operon is evolved in bacteria to deal
with tryptophan. Or more specifically to deal with the absence of tryptophan. So tryptophan
is going to be an amino acid. And it's required to make proteins. It's one of those 20 essential
amino acids. And so the trp Operon basically is designed to make tryptophan if it's not
present. And so we're going to get tryptophan in, you know in poultry, in milk for that
matter. There's going to be high levels of tryptophan. But if a bacteria does't have
tryptophan, there's a number of different genes that are required to make it. And so
they can make their own. And so we've got an operon called the trp Operon. How does
that work? Well again we've got "Prog". We've got our promoter, our repressor, our operator
and then our gene. But if we have tryptophan present, so tryptophan's going to be these
yellow hexagons. If we have tryptophan, tryptophan's going to fit inside the repressor and it's
going to change its shape so that it fits in the operator. And so in this case, if we
have a bunch of tryptophan present, then we don't want to make tryptophan. And so the
repressor is going to set that operator to the off position. What happens if all of a
sudden the tryptophan goes away? If there's no tryptophan in the diet? Well the bacteria
is not out of luck because it changes now the shape of that repressor. And so once it's
changed the shape of the repressor, RNA polymerase can grab on, drive down that operon, make
all of those genes and those genes can be used to create more tryptophan. And so it's
a great kind of a feedback loop. So in a lac Operon we're going to turn it on if we want
to breakdown lactose. In a trp Operon, what we're going to do is create tryptophan if
we don't have it or if it's not present. And I really didn't' understand how operons worked
until I stopped thinking like a scientist and really started thinking like an engineer.
We've got a problem that we have to solve, and one of the great ways that we can do that
is using an operon. And so I'm going to show you a quick simulation of how this works.
It's a phet simulation. Here's a website. I'll put a link to the simulation down in
the video description down below. But it's a great way, and I've used it with my students,
for them to wrap their head around how an operon works. And so we're looking at a lac
Operon. And to make it simple, we're only dealing with one gene. On of those three genes
that are found. And so if we were to look at the simulation, you could try to figure
out what everything is. But it's easier if I show you the legend and show you what everything
is. And so we've got RNA polymerase kind of bouncing around here. Again, they'd be moving
around. There's molecular motion. They'd just be randomly bouncing around. But this is one
of the players. We've got the RNA polymerase right here. We could look down below and I
could ask you, now where is the operon? Is this the operon here? Or is this the operon?
Well the right answer is this. This is going to be the the operon here because it's got
a promoter. So let's put that promoter out here. It's got an operator, an on-off switch.
And then it's going to have a gene. In this case the lacZ gene. So what's this up here?
Well this is not an operon. But this is going to be another, what we call a regulatory sequence.
And so this one down here is going to make an enzyme that's going to breakdown lactose.
But what's this RNA polymerase up here going to make? Well it's coding, it's making a little
bit of messenger RNA. Well what it's eventually going to make, is it's going to make a repressor.
Okay so now we've got the fourth part of our operator. We've got the promoter right here.
We've got the repressor. We've got the operator. And now we've got the gene. But you can see
that the RNA polymerase can't make that gene because the repressor is blocking its way.
And so it's in an off position at this point. And it should be. In other words we shouldn't
start making those genes until lactose is present. And so let's add a bunch of lactose.
So I'm going to add a bunch of lactose here. And let's watch what happens. Well if we add
a bunch of lactose, and we could speed up the simulation a little bit, what's going
to happen is the repressor, and I could pause it right here. That repressor is now having
the lactose bind to it. And it's changing it's shape. Or it's changing it's confirmation.
Once the lactose, and you can see it's happened in two cases, once the lactose binds to the
repressor, it can't bind to the operator anymore. And so if we play the video, what's going
to happen. Well now RNA polymerase can drive down the operator, or excuse me down the operon.
It can produce a protein. In this case that's going to be a lacZ protein. What's it going
to do? It's going to digest and breakdown all of the lactose that's present. And you
can see that it's gobbling up all of those lactose. The only lactose left is those that
are bound to the repressor itself. What happens eventually? All of the lactose is going to
disappear. In other words we will have broken it down. What happens now? Well that repressor
is going to bind to the operator again. And now we can't make that protein to break it.
And so it's a wonderful feedback loop. It's a feedback loop that deals with a bunch of
lactose present. Because e. coli would be silly to make all of the proteins to break
it down until we've got a bunch of lactose present. And so that's an operon. What is
it again? It's all of the genes and then a way to control those genes in one tidy little
package. And I hope that was helpful.