- [Voiceover] So we're gonna talk a little bit about
DNA regulation.
And this is the general idea that if you look
at a organism's genome,
that not all of the genes are being
transcribed and translated at the same time.
It could actually depend on the type of
cell that that DNA is inside of,
or it could depend on the environment for that organism.
So for example, if you look at,
say, a multicellular organism,
this, maybe this is, and these are
oversimplifications right over here,
maybe this is type of immune cell,
immune cell,
and let's say that this over here is a muscle cell,
and they're not necessarily, or not likely
to be these perfect circles,
but this is just for the idea.
And they're going to have the exact same DNA.
So the DNA in both of these is the same.
So DNA is
the same
inside, and these are going to be,
these are Eukaryotes,
so I'll draw the nuclear membrane there,
same DNA.
But they have very different roles inside of this organism.
So it doesn't make sense, in fact,
in order for them to even have different structures,
they're gonna have to produce different proteins.
They're gonna have different enzyme proteins
inside of their cytoplasm.
And so DNA regulation, one way to think about it is,
well if they have the exact same genome,
how do they regulate which of those genes
are being transcribed and then translated
and which ones aren't?
And even if you think about a unicellular organism.
Right here we have a bacterium.
And so it's just one cell, but even it
will not want to transcribe and translate
all of its genes at the same time.
For example, this over here,
so this is its, this is the bacterial chromosome.
This right over here might be a gene
involved in the digestion of a certain type of food,
if that food is present.
This type of,
and actually it could even be several genes
that are involved in that type of food,
and we will actually talk, go into a little bit more detail
about when you have several genes that are related
and they tend to be transcribed all at once
or not transcribed all at once.
So maybe that's related to
digesting or consuming some type of food.
Maybe you have some genes over here
that are related to some type of stress mechanism.
Maybe it needs to go into hibernation some time.
And so if it's not under stress,
it does not have to express these genes,
but if it is under stress, it does have to express these.
Likewise, if that thing that needs to digest is around,
it needs to transcribe these.
If it's not around, it does not need to transcribe it.
So that's how DNA regulation works,
whether you're talking about a Eukaryote
or a Prokaryotic organism.
And so what we're gonna do in this video is focus
a little bit more, or a lot more,
on the Prokaryote side.
Especially, we're gonna talk about this bacterium.
When we talked about
transcription in general,
in several videos ago and in several videos,
we talked about the idea of a promoter.
That you have a gene that is
a sequence of DNA that's part of the broader chromosome,
and we said, "okay, that RNA polymerase
needs to attach some place,"
so that RNA polymerase needs to attach some place,
and we called that place that the RNA polymerase attaches,
we call that the promoter, and then the polymerase
will transcribe the gene.
And when we first talked about the idea of a promoter,
we said, and this is generally true in Eukaryotes,
that each promoter is associated with a gene,
or each gene has a promoter.
But when we're talking about Prokaryotes,
and in this case we're talking about this bacterium,
it's actually typical to have
multiple genes grouped together that have one promoter,
so this promoter here, and a promoter is actually called a
regulatory DNA sequence.
Let me write this down.
So the promoter, so that's this part right over here,
that's the sequence.
That is a
regulatory,
regulatory
DNA
sequence.
Well that's what the RNA polymerase,
which I drew as this big blob,
it's protein here, this big blob,
will attach to, and then it will begin to transcribe
all of these genes as a bundle.
And when you have a promoter associated with multiple genes,
that combination of the promoter
and the genes, and once again when I'm talking about
the promoters of the genes I'm talking about
sequences of DNA,
that combination is called an operon.
This is called an opeon.
It's a combination of that regulatory DNA sequence
which says, "Hey, RNA polymerase, bind here
"so you can start transcribing,"
and the genes that it, essentially,
promotes the transcription of.
And then of course, that transcription process
takes that genetic information in DNA,
transcribes it into messenger RNA,
which can then go with the ribosomes,
and we have the whole translation process
and this should all be review,
to produce the actual proteins that have functions
within, or even potentially, outside of the cell.
And so what we're gonna dig a little bit deeper in
is what can enhance this process,
make this happen more frequently,
or things that might inhibit this process in some way.
So as I mentioned before, this is just what I had
just drawn, we have our big RNA polymerase blob,
and this is an oversimplification for what it looks like,
attaching to the regulatory DNA sequence,
which we call the promoter,
and then it will do the transcription
which will produce mRNA,
which encodes the information in those genes.
But what if we're in an environment where we don't want to
transcribe this particular operon,
this particular,
or maybe I should say particular series of genes?
Well then we might,
something in our environment might
allow repressors to take action.
So what are we talking about a repressor?
Well a repressor,
a repressor right over here,
you see it attaching to a sequence of DNA
after the promoter, and so it blocks,
it blocks the RNA polymerase
from being able to do the transcription.
And so this right over here, this is a protein
that is called the repressor, it's literally
repressing the transcription,
and the regulatory DNA sequence where it attaches,
that is called the operator.
So once again, promoter was a regulatory sequence
where the RNA polymerase can attach,
and then operator is a regulatory sequence
where a repressor can attach and keep that RNA polymerase
from actually being able to perform
the actual transcription.
And so this keeps the gene from,
continuing to
transcribe and then translate
these actual genes.
And you might even have extra mechanisms,
and you can even think of them as feedback mechanisms
or ways to understand the environment
where the repressor,
this protein can only do its job,
so let's say that's its repressor,
where it can only do its job if it has other molecules
that attach to it.
So maybe this one can only do its job
if it has another molecule attached to it,
and in that case, these smaller molecules,
these are called corepressors,
co, co
repressor.
And we'll go into more detail when we talk about things
like the trp operon of
how tryptophan, an amino acid,
can actually act as a corepressor.
Now over here, we have the other way around
where we want even more transcription,
and in that case we would have something called,
we would have an activator.
And this,
let me shade it in,
this DNA right over here, this would be
the regulatory sequence where the activator binds,
and so this would be positive feedback.
When you have more activators, you're gonna get
more transcription, while this would be,
and actually I shouldn't even call it feedback
because that implies that somehow
these products produce the activator,
or this products produce the repressor,
but that's not the necessarily the case.
It could be, you could imagine that case,
but it's not necessarily the case.
I should just say that this is repressing
and this is activating.
It's going to make more of the transcription
actually happen.
And just as we could have corepressors,
small molecules that you could think of
as activating the repressor,
you can also have small molecules that can turn
the activator on,
and these small molecules that turn the activator on,
these are called inducers.
So this right over here,
these are inducers.
So this protein right here couldn't activate
that operon, but now that you have these inducers,
and we'll study that a little bit more when we
think about the lac operon,
this could be a small sugar of some kind,
well then it can turn on the activation.
So this right over here is called
an inducer.
So that's just a high-level overview of
DNA regulation.
As you can imagine, this can get very, very interesting
and complex, where you have your repressors
and corepressors and activators and inducers
that might be dependent on the environment
that the cell is in,
what's going on in its broader ecosystem.
There's all sorts of feedback and feedforward
loops that might be going on.
And that's why the study, we can have the sequence,
and in fact we do sequence entire genomes,
but even once you have the sequence,
it's incredibly complex to understand
all of these loops of these feedback and feedforward loops
to understand how these things actually interact
with each other.