Hi. It's Mr. Andersen and welcome to Biology Essentials video number 24. This
is on regulation of timing and coordination in development. And so it's really about development.
Figuring out how we go from that first zygote or that first fertilized egg to an embryo
and then eventually to a baby and eventually to you. And the magic of that we're starting
to kind of unlock. In other words how do we go from this one cell with its DNA to all
the various cells inside this baby that do different things even though they have the
same exact DNA inside them. And so it's really about new discoveries. Pretty exciting. And
so we start though by talking about seeds and how seeds germinate. Seeds can remain
dormant for a long period of time. But eventually they come to life. And so how do we go from
those cells inside that really small embryophyte inside that small seed to the seeds of a tree.
How do we go from an acorn to a tree? That's what development is. All those specific stages.
And so the first step is cellular differentiation. In other words each of the cells have to figure
out what kind of cell they are going to become. They're going to express specific proteins.
And those proteins as a result of which ones they express determine what cell it's going
to be. Cells eventually become embryos. And cells inside the embryo are going to induce
other cells to become cells like them. And we've been able to show through genetic transplantation
and the use of transcription factors how that actually works. Cell death, we're starting
to find, is just as important as cell growth. Example I'll talk about is how the fingers
and toes form and the importance of micro RNA in regulating that. And then finally we're
going to talk about the body genes. The homeotic genes and how they place different body parts
where they should go. And the study of mutants has unlocked a lot of that. And so let's start
with seed germination. And so if you take a sunflower seed and plant it, it probably
won't grow. And the reason why is that there's too much salt on it. But if you take a normal
sunflower seed and plant it, it will grow into sunflower. And in order to do that you
have to give it two things. First thing you have to do is you have to increase the temperature.
You have to have a perfect kind of a window for the temperature of the seed. But you know
this if you grow plants. The other thing that you have to do to activate a seed is you have
to add water to it. But if you do those two things, if you have the correct temperature
and the correct water, the seed is going to germinate. And so what we're going to get
is a beautiful sunflower eventually. But the cells here and the root are going to be different
than the cells here in the leaves and the cells here in the shoot. And so how cells
determine that is going to be development. It's the stages by which those cells eventually
go and become an organism. And so first thing I want to talk about is cell differentiation.
Cell differentiation is how cells become the cell that they're going to be. Or the cell
that they're going to actually act as. And so these are colonies of stem cells. Human
stem cells. And they can just kind of grow them on this lattice of these fibroblasts
of mice. But those cells inside us will eventually specialize. They'll become a cell, for example
these are nerve cells, and once you become that cell you can never go back to being a
stem cell again. In other words once you've made that choice to become a cell you can't
go back. Now we can kind of do that in the laboratory, but in life you don't really do
that with a few exceptions for sure. And so how do cells become cells? Well they have
tissue specific proteins. In other words if this is a stem cell and this is really simple.
It's just got one chromosome pair. And this is inside its nucleus. But how do we go from
that first cell to a red blood cell for example or a neural cell or this might be just a run
of the mill skin cell for example. How do we become that? Well the DNA in each of these
cells is identical. And so how could you form all these myriad of cells if the DNA is exactly
the same? Well what we've discovered is there are going to be cells in here or genes inside
here, let me label these ones red. And again this is really simple. And so when a cell
decides it's going to be a red blood cell, what we think happens is that all of the parts
of the DNA that don't express or don't express proteins to make red blood cell, all of that
DNA is going to wad up or crumple up. And so all of this DNA isn't used. It's wadded
up. And the only DNA that can actually make proteins are going to be the genes for a red
blood cell. And so even though the DNA in this red blood cell and this neural cell are
exactly the same, all the neural or the neurons, genes, aren't going to be expressed. I could
give a different color for them. There might be like a these blue genes that just make
the neuron. But since they're wadded up they're not going to be expressed. However in a neural
cell all the red blood cells would be wadded up. Those genes. And then the neural cells
are actually going to express. And so when cells make this choice as to what cell they're
eventually going to become, we call that cell differentiation. And that happens almost from
day one. Cells start to figure out what cells are going to be. Let's take a look at that
and how that might work. And so that one fertilized egg, that one zygote is going to copy itself
through mitosis and make a bunch of cells. And so now we call those stem cells. These
cells can become any cell that they want to. And in us what it will do is it will actually
form a sphere, a blastula and then it folds in on itself and causes forms something called
a gastrula. And the part on the folded inside becomes your digestive parts. And the outside
eventually becomes your nerves. And the middle part is going to become like the bones and
the muscle. And so how do cells determine what cell they're going to be? Well what we're
starting to figure out is there are transcription factors. In other words cells are going to
give off proteins and those proteins are going to determine what type of cell you're going
to become. And so I made it really simple. We call these the top and the bottom transcription
factors. In other words this cell right here, just as a result of being on the top of these
stem cells is going to express this transcription factor. We'll call that the top transcription
factor. What's that going to do? Well it's going to bind with the DNA inside the cell
and it's going to cause it to activate the specific proteins for a top kind of a cell.
It's also going to give off more transcription factors to the cells adjacent to it. And so
if I animate that here for a second. So what we'll have is we'll have the transcription
factor, or excuse me the transcription factor right here produced by this cell. It's a yellow
cell we'll say. And that's going to create transcription factors that are going to move
to adjacent cells. And those adjacent cells now are going to express more transcription
factors so they're going to determine what cells they should become. And so these cells
know that, hey, we're on the top. These ones are on the bottom. These ones are on the middle.
And so there's this cascade of these transcription factors that kind of play out how we go from
that first stem cell to the actual embryo. Let me give you an example of one. All of
us are born female. But there's one specific gene. It's called the SRY gene. Stands for
the sex determining region Y gene, because it's found on the Y chromosome. And what happens
is in males there is an X chromosome and a Y chromosome, but this one gene will be given
off and it will flood to adjacent cells which will also give off the SRY gene. And then
you become a boy. And if there is no SRY gene then you become a girl. And so you could,
for example, be XY and you become a boy. But you could also be XXY and you'd be a boy.
Or you could be XXXXY, we'll talk more about those later and you'd still become a boy.
Because you have that SRY gene. Likewise if you're XX you don't have the SRY gene. You
don't become a boy. A cool study was done to show the importance of induction. In other
words this idea that these cells can induce the cells adjacent to it or the tissues adjacent
to it to become something. These are going to be these cave fish that are found in Mexico.
And there was a study done where they took the lens, because some of these fish started
living in caves and lost the eye. We call these the eyeless cave fish. And what they
did is they took similar fish that are found above cave level and they actually have eyes.
They just pulled the lens out of this fish and they put it in the lens of one of these
cave fishes. And it developed a normal eye. They then took the lens of this cave fish,
put it in the embryo of a normal fish and nothing happened. And so there was something
about the cells inside that lens that were actually triggering the formation of all the
other parts of an eye. The retina. The optic nerve. All of that functioned correctly through
this induction. And so this is called a transplantation. And it's one of those studies that showed
the importance of where you are or location location location. Now not only differentiation
but cell death is super important as well. So these, all the parts of this embryo of
a human are going to develop through differentiation and induction, but you'll notice that right
here between the fingers and between the toes, the cells are actually dying. And that process
is called apoptosis is actually really really important. And so not only the growth of cells
but the death of cells is important in forming a new organism. A great example has been studied
in the drosophila. Drosophila, this would be an embryo of drosophila and this would
be an adult. And this will eventually become this. But apoptosis or cell death is super
important in that regulation. And so there are a number of different genes that scientists
identify. And I love the names here. They're the hid, the grim and the reaper genes. And
these three genes in drosophila cause the cells to die. In other words they make proteins
that cause cells to die. Another important thing that I should talk about is microRNA.
MicroRNA is going to be a single strand of RNA. It's found in all organisms. And what
it does is it actually disrupts RNA. So it disrupts RNA so it can't actually make proteins.
And so what scientists found is that if we disrupted the RNA in drosophila, in other
words we got rid of the microRNA, the hid, the grim and the reaper genes went crazy.
And they took this embryo and turned it from something that was eventually going to turn
into a fly and turned it into something that was nothing that died. In other words, these
death genes go crazy unless we have the micro RNA actually regulating them. When we put
the RNA back in, they're tempering these death genes and then we get development. And so
again development is growth of cells but it's also death of cells. And the microRNA we're
finding is really important in regulating this timing of the development. And the last
thing I want to talk about are homeotic genes. Homeotic genes are found in us. They're found
in a fruit fly. They're found in a mouse. But we've studied them in fruit flies first.
And what we find is that there is a series of genes in the chromosome of a fruit fly
that tell the fruit fly where to put the body parts. Now these genes actually lead to a
number of other genes. There's a cascade of these genes causing all these other things
that build an eye or build an antennae or build a leg. But these ones are super crucial
for where it goes. That's what homeotic genes do. An important set of those are called the
HOX genes. That's what I have pictured here. And so where did we learn about homeotic genes?
Well we learned about them from mutants. And so there is a mutation called the ultrabithorax.
Remember a head, thorax and abdomen of a fly or an insect. What the ultrabithorax genes
does is it actually causes a duplication of this thorax. And so a fruit fly normally looks
like this. But if you have a mutation or change in this UBX gene so it can't function, what
you get is not one thorax with two wings but you actually get two thoraxes. Or if there
is a mutation in this antennaepedia gene what you'll get is one of the legs of the fruit
fly will actually grow out of the head of the fruit fly. And so that's kind of science
fiction stuff. It's creepy. But what it tells us is that inside fruit flies and there by
inside us there are a series of genes that tell us where the body plan and where the
parts go. And so as an embryo starts to develop into a fetus and eventually into you, homeotic
genes are super important in kind of laying down the landscape of where things go. And
so that's development. Those are some of the cutting edge discoveries that we've made.
And I hope that's helpful.