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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!