Gene Regulation and the Order of the Operon


Captioning is on! Click the CC button at bottom right to turn off. Follow the amoebas on Twitter (@amoebasisters) and Facebook! Have you ever thought about how disastrous
it could be if the cells in your eyes started producing the same hydrochloric acid that
is made by your stomach cells? Your stomach cells produce HCL to help break down food,
but you definitely don’t want that in your eyes. Thank goodness that doesn’t happen!
But it’s surprising—because both your eye cells and stomach cells contain all of
your DNA. All of your DNA is found in your body cells, but see—the portions that are
used need to be regulated somehow. Otherwise we could end up with something ridiculous
like…eye cells producing digestive enzymes. And that wouldn’t just be a waste of resources—that
would actually be very difficult to explain to your friends.You want some genes to be
regulated. Controlled. Remember that genes are made up of DNA. DNA is used to give instructions
for the production of proteins in the process of protein synthesis. But an important concept
is that there needs to be a method of determining which genes will be turned on and which genes
will be turned off. This is called gene regulation. There are many ways that genes are regulated.
In your human body cells, you can have proteins that can bind to certain gene regions to increase
the rate of transcription for the transcription enzyme RNA polymerase. Or you can have proteins
decrease transcription to the point that it may not be transcribed at all. That is a form
of gene regulation. Your eye cells don’t use the portion of DNA that codes for making
HCL like your stomach cells do, because there is regulation like this in all of your cells
to determine which portions of DNA is used. But we want to shift gears now to talk about
a very interesting way of regulating genes that can sometimes be challenging to visualize.
A way that has not been found in humans, but instead is found in prokaryotes—-with a
few eukaryote exceptions. It’s called an operon. An operon is a fancy way of regulating
genes and it usually is made up of a few genes that involve enzymes. Remember that enzymes
are proteins with the ability to break down or build up the substances that they act on.
Let’s talk about some key players in an operon so we can see some gene regulation.
First, RNA polymerase. It’s a builder- a builder enzyme actually because RNA polymerase
is an enzyme. Remember that many things in biology that end in that –ase are enzymes.
RNA polymerase is needed in order to start transcription. Remember that transcription
and translation are steps in protein synthesis. Protein synthesis which means to make proteins—enzymes
in this case. The thing about RNA polymerase though—it gets a little confusing for RNA
polymerase without somewhere to bind. If you watched our DNA replication video, you learned
about DNA polymerase and how it needs to have a primer to know where to start. Well, RNA
Polymerase needs a promoter. A promoter is a sequence of DNA where RNA polymerase can
bind to. So you would think that’s it—you get RNA polymerase attached to a promoter
and boom! You make your mRNA which eventually will be used to make a protein right? But
there’s this other sequence of DNA called an operator. The operator is a part of the
DNA where something called a repressor can bind. The big bad repressor, if bound to the
operator, blocks RNA polymerase. Poor RNA polymerase cannot move forward and no mRNA
can be made. Therefore, no proteins. So take a look at our setup here. This is an example
called a Lac Operon. Notice there is a promoter region of the DNA, the operator region of
the DNA, and these are three genes {have labeled lacZ, lacY, and lacA) that code for enzymes
that help in the process of breaking down lactose. Lactose is a sugar. If lactose sugar
is around, bacteria want these enzymes to be made so they can use them to break down
the lactose sugar. Then they can metabolize it! Fed bacteria are happy bacteria. Here’s
the repressor. There’s actually a gene here on the operon that codes for producing the
repressor. See this gene that we call “I”? It has its own promoter. This codes for the
production of the repressor. So why do we need this repressor? Well, it’s wasteful
to make things that you don’t need. If there’s no lactose, it wouldn’t make sense to start
making enzymes that work together to break down lactose. It would be a waste—the enzymes
would just sit there. So if lactose is not present, then the repressor binds to the operator.
This blocks RNA polymerase. mRNA cannot be made. And therefore the proteins—enzymes
in this case—cannot be made. But if lactose is around in the environment, something pretty
cool happens. The lactose—remember, that’s the sugar, binds to the repressor. This changes
the repressor’s conformation. Try as it might—-the repressor can’t bind to the
operator. RNA polymerase finds its promoter, binds, and transcribes to make mRNA from the
genes on the operon. That mRNA will be used to make enzymes to break down that lactose
sugar. Bacteria like to eat so…that makes them pretty happy. We have to say that we
think it is pretty impressive to think about all the gene regulation that goes on in cells—and
if you find it fascinating—know that there are careers that focus on gene regulation.
By understanding how genes can be turned on and off, we can also gain a better understanding
of treating a variety of diseases that have gene influences in the human body. Well that’s
it for the amoeba sisters and we remind you to stay curious!

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