[Instructor] Hello, and welcome
to the first lecture in Module 2.
This lecture is called, "The Cell Perspective".
And what I'd like to do here is to get you to start thinking
about biology from this more of a cell-based
or a microscopic perspective,
because looking at things from a cell's perspective,
we can start to better understand the role
that genetics is playing in normal function and pathologies.
The human body is made up of between 50
and 100 trillion cells, and we have
about 200 different kinds, different types of cells,
that have completely different functions.
And I'm sure you're quite familiar with some of these,
probably some more than others.
For example, red blood cells have a very specific
and different function from kidney cells,
from neurons in the brain, from cardiomyocytes in the heart,
which have to beat at a particular rhythm,
and in sync with their neighboring cells
in order for the heart to function properly.
So, as you can appreciate, they're just a huge variety
and spectrum of different cells.
And with those different cell types,
you have different functions that they're performing.
And each is very important and necessary for the organism,
as a whole, to function properly.
But it's important to keep in mind
that despite all the variety of different cell types
and the trillions of cells that we have,
each one of them, with a few exception,
contains your full genome.
So each of these cells, even though they're all acting
in very different ways,
they each have the same genetic material.
The one exception here that I'm noting,
and I'll note this a couple of times,
that I'd like to mention is mature erythrocytes
or red blood cells, as they have been enucleated,
which means they no longer contain a nucleus,
and they no longer contain nuclear DNA,
or your genome is no longer contained in those cells.
The genome is housed in the cell's nucleus.
So if we're looking at a schematic drawing of a cell
or kinda like a generic cell and all of its components,
it's not really important that you know the different names
of the components that are all listed here,
I'm just showing you this to give you a perspective
of where in the cell you would find it.
The nucleus, I mean, it depends upon the cell,
but very often found sort of in the middle of the cell
and it has its own membrane to protect it.
Contained within the nucleus is all of the genetic material.
One exception to this, and this will be a recurring theme
where I keep saying the one exception to this
because one thing that I'm sure
you're probably familiar with
but is absolutely the case in really any study of biology is
that for every rule there is an exception
and that is going to keep coming up.
So I apologize for that.
I'd like to be able to make blanket statements
'cause I know that makes it easier.
And I'll try to do that when possible,
but I will pretty much always have
to make some kind of disclaimer of,
well, there may be a few exceptions.
And in this case, so the genome is housed
in the cell nucleus, yes, a very small amount
of specific genetic material related to metabolism
and specifically oxidative phosphorylation is housed
in the mitochondria of cells.
And you can kinda see this little,
these little guys here,
these little yellow jelly bean things
with the folds in there and their membranes here
are depicting mitochondria, they contain a handful of genes
which are specifically related to mitochondria function.
You can say, as a general rule though,
that 99.999% of your genetic material is actually contained
within the cell's nucleus.
So let's take a a zoomed in view here, again.
This what looks like a big hairball
that you might find in your shower is DNA.
So these are strands of DNA
and we'll talk about in a lot more detail
what that means shortly.
Just know that this is where it's located.
It's protected from the rest of the cell
through a nuclear membrane.
So this is keeping it, your DNA,
your genetic material is very precious
and needs to be protected.
So the cell protects it by keeping it in the nucleus.
All information from your DNA basically will be copied
and sent out through the nucleus
into what's called the cytoplasm of the cell,
which is basically all of the space outside of the nucleus
that's not within one of these other structures.
So the cytoplasm, that's literally just
like, the goo within a cell
that is not what are called organelles
or these little structures
that have other specific functions.
So again, you do not need to know what is a golgi
or a lysosome or endoplasmic reticulum necessarily.
If you're interested, I'm more than happy
to talk with you about it.
But as it stands, really all it's important here is
to know that there's a nucleus,
it's contained within a cell,
and within the nucleus is housed your genetic information.
All cells came from one fertilized egg.
Here, this is showing a scanning electron micrograph
of an egg being fertilized by lots
of very excited go-getter sperm.
One of those lucky guys will make it into the egg
and will fertilize the egg
and will deposit its genetic information.
And from that, it'll form a zygote.
So a fertilized egg will have the full compliment
of DNA that that individual will ever have.
So you start out from day one, really
at the moment of conception is the point
at which all genetic information
that you will really ever have is contained within you.
Now, that's not to say your genetic information
can't change slightly over time,
and it certainly does, a very small percentage
of it will mutate or change over time,
but the vast, vast majority of it is going to be identical
to what was present at the time of conception.
So of course this is a single cell.
It then goes on to divide, and I'm sure you're very familiar
with this process, it divides
into multiple cells after that.
And the process of cell division is called mitosis.
And we'll talk about that in more detail
in a subsequent module.
But for now it's just suffice it to say the cell divides
and as it divides, it also duplicates its DNA exactly,
again, within 99.999%, exactly the same
as what it originally started with.
So one cell becomes two, two become four, four become eight,
and so on and so on.
And this is during very early stages of development.
So each cell in the human body contains
the same DNA originally found
in the fertilized egg at conception.
So here are a few exceptions that, again, I have to note.
One is mature erythrocytes,
which again, do not contain any nuclear genetic material
because they do not contain a nucleus,
and egg and sperm, which only contain half
of the DNA that you started with
at the time of conception.
Because when an egg and sperm fuse, you don't want
to have twice as much DNA as you would want in the end.
We'll talk about this again in another module in the future
when we talk about the process of meiosis.
So each cell in your body has the same DNA, roughly,
but each cell has very different function.
So how does this happen?
We're not going to spend a lot of time on this,
but cell differentiation is the process
in which cells take on unique characteristics and functions.
All starting though from that original same cell,
divide, divide, divide, become a blastocyst stage.
And then you have gastrulation, and I imagine you've come
across this in other coursework that you've had.
In the gastrula stage of development
it's still very early on, very early zygote stage,
there are different layers.
And each of these layers will become different types
of, can become different types of cells depending
upon signaling events around them.
Now again, they still contain the same DNA,
but the way that the cell goes
about basically becoming different from another cell
with both of them having the same DNA, is
that one cell will start to express,
which means turning on or turning off,
or it can also mean slightly turning up
or turning down the the level
to which the gene is expressed.
So while all cells possess the same genes,
some turn off or turn on subsets of genes
and this is called gene expression,
which enables only the necessary functions.
So in the case of, let's take for example a neuron
in the brain, you do not want a neuron
in the brain secreting insulin.
There's no reason for it.
It would be detrimental potentially
to the surrounding tissue
and it would be confusing to the body.
So instead in, for example, the pancreatic cells
where you would have expression of insulin,
you would not have expression
of say, a neurotransmitter receptor,
but in the neuron you would have expression
of a neurotransmitter receptor gene,
but you would not have expression of the insulin gene.
So you can imagine this multiplied by thousands
and thousands when we're talking about different genes
and different combinations of genes.
This allows for all of the unique characteristics
that we see in different types of cells.
So here, I'm just giving you a few images
of different types of cells,
and I'm sure you've seen many of these,
if you've done any kind of mic,
if you've done some microscopy work or pathology work.
So here, just to give you kind of a gamut
of different types of cells and morphologies,
dermal fibroblasts so found in the skin
which provide elasticity and strength to the skin.
So they form these sheets.
And as you can see the cell or maybe you,
hopefully you can see the cells are kind of outlined here.
These are individual cells that I'm sort of drawing
around with the arrow.
This would be one cell, this would be another cell.
And they form these sheets
so they're actually connected to one another.
And they need to do that so that the skin can provide
the barrier that's necessary.
Compare that with a neuron where you have, you know,
very different morphology.
This is the center of the cell
and it sends out all of these processes
and connects with cells in its surrounding environment,
but in a very different way.
And as you can see, it looks quite different.
T cells on the other hand, which are, of course,
immune cells and they float around in the body,
they're not adhered to any particular substrate
like a dermal fibroblast would be,
which basically stay put in the skin.
T cells are floating around in the body
and clearly have very different morphology, as well.
And it's necessary for their function.
Compare that with erythrocytes, mature erythrocytes
in which I wanted to show here just an image
of some erythrocytes.
You can see they have, as I'm sure you're familiar,
they have that characteristic
round dented shape in the center.
And part of the reason why it's dented is
because the nucleus is gone.
The nucleus is normally sitting kind of in the middle
and would sort of press the cell membrane out.
And it also allows for greater surface area
for oxygen to bind.
So, you know, each of these has its own function,
has its own characteristics,
yet they all have the same genetic material.
So let's summarize what we've talked about here.
The human body is made up of 50 to 100 trillion cells.
The cells contain their genome in the nucleus,
so that's where it's housed.
A fertilized egg contains the complete genome,
which is replicated each time the cell divides
and cell division is called mitosis.
All cells in a person's body originated
from the same fertilized egg, thus have the same genome
because with each cell division, the cell replicates
its genome and passes that on to all subsequent cells.
Even though all cells in a person's body have
the same genetic sequence,
cells take on very different characteristics
by expressing genes in different ways.
So turning some genes off or on,
or even just up or down slightly
can make a big difference in terms
of the functionality of the cell.
We'll talk more about gene expression,
what that means and some
of the details of that in module three.
So what's next in this module?
The next lecture video I'd like you to watch is entitled
The Structure of DNA.
And then in that one, we're going to start
to talk a little bit about the actual physical structure
of DNA, how it's housed in the nucleus.
The final lecture for this module is The Central Dogma,
which almost sounds more philosophical than scientific,
but this was such an enormous advancement
in the understanding of not just genetics
but molecular biology as a whole
that it is, I think, rightly still called
The Central Dogma, which is we'll use
to understand what exactly does DNA do.
So we'll talk about that
in the third lecture in this module.
So until then, I look forward to speaking
with you on the Structure of DNA.
Thanks very much.