Let's talk a little bit about the structure of the cell.
I've done a lot of videos where we
deal with things that go on inside of them,
but I haven't done one where we just
talk about the entire structure of them.
So a good place to start is-- let me just draw the membrane.
And the cellular membrane is a good place to start,
because this is what separates the cell
from the outside world.
And to a large degree, it kind of defines the cell.
It defines it as this very, very small compartment.
That's where the word cell came from.
So let me label that.
Cellular membrane.
And all cells have a cellular membrane.
Now, if we think about maybe the most important thing that
defines a cell-- you've probably seen in the DNA videos,
and we'll talk about translation and transcription and all that.
But what defines what a living organism is is its DNA,
so all cells have DNA inside of it.
And I won't go into the detail of how
DNA defines what an organism is.
I've done that in some detail on the DNA videos,
but all cells have DNA here.
This is more of kind of an anatomy of a cell
video than necessarily the function.
But we'll go into the function, because we
need to know what these different parts do.
So right here, this is the DNA.
And it's in its chromatin form here.
There's also little proteins here, not in all organisms.
But we're going to stick to eukaryotes,
and I'll talk a little bit about the difference
between eukaryotes and prokaryotes in a second.
But we have DNA.
As I've drawn this cell right now,
this is pretty much any cell, any cell in any animal or plant
or whatever kingdom could look like this.
I haven't drawn a lot of the details.
I've just drawn the DNA and the cellular membrane.
Now, here is kind of the first major division in the living
world, or at least from our point of view,
or it seemed obvious, is that some cells have
a membrane around the DNA.
So they'll have a membrane around the DNA that
separates the DNA and the chromatin and everything that
makes up the stuff within the DNA,
separates that from the rest of the cell.
And this is called a nucleus.
And I said that's a major division,
because when some people looked at some cells
and they saw a nucleus and other cells
and they didn't see a nucleus, they say, hey,
this is a good way to classify organisms.
So they called the things that had nucleuses eukaryotes.
These have a nucleus.
So as I've drawn this cell right here, it is a eukaryote.
Now, if you do not have a nucleus,
you are dealing with a prokaryote.
No nucleus.
And examples of prokaryotes, the two big groups of them,
are bacteria and archaea.
And archaea are really interesting.
We know very, very little about them.
They were originally thought to be types of bacteria,
but now people are realizing that they're
this whole completely other group.
And we've actually observed a very small subset of them,
so it's a very fascinating group.
And actually it turns out that evolutionarily speaking,
you shouldn't make this division first.
And it actually makes more sense to divide things
into eukaryotes.
I'll just write that.
Euk, bacteria, and archaea.
You don't want to do this division first.
There are actually three separate groups
that you want to start off with, archaea.
We'll do that, we'll talk more about this in future videos.
But if you want to say who was a nucleus, well,
eukaryotes have a nucleus by definition.
Who does not have a nucleus?
Well, the bacteria and the archaea
do not have nuclei, just like that.
But I'm going to focus on eukaryotes, because they
tend to be a little bit more complex.
They tend to be larger.
And most of what we talk about, at least in the videos so far,
are dealing with eukaryotes.
Eukaryotes include plants, animals.
We're animals.
At least I am.
Animals and fungi.
And there are other groups within eukaryotes,
but these are the ones that we normally
deal with in our everyday world.
Let's go back to looking at the anatomy itself.
So we have our DNA.
We know that it gets transcribed into mRNA.
That mRNA leaves the nucleus, and it
gets translated into proteins at the ribosomes.
So the ribosomes are these little complexes
that could be floating all over the cell,
and we'll see in a second that they can also
be attached to these other membrane structures.
So this is a ribosome.
And if all this talk of DNA transcription
into mRNA and mRNA leaving the nucleus
and traveling to the ribosome to be translated into proteins
makes no sense to you, there are several videos
where I go into that in detail.
But what I want to do is just focus on
all of the different parts, kind of give
a big picture of things.
So ribosomes are where mRNA that gets
transcribed inside the nucleus from DNA
where it gets translated into proteins.
So you can kind of view them as a place
where information actually turns into proteins which
then can be used anywhere else in the cell.
Now, one interesting-- and these ribosomes,
they're made up of proteins.
And actually, they're made up of RNA.
And so one question is, well, where
are the pieces of the ribosomes made?
Well, some of them are made by proteins,
which might be made in other ribosomes.
But some of it, the mRNA, ribosomes, you can kind of
view them as this big mass if you were to look detailed.
There's some protein there.
And I'm not drawing it that realistically,
but then you have some mRNA tied in with the protein.
And the mRNA isn't used for actual information purposes
like it normally is when it goes from DNA to the ribosome.
Within the ribosome itself, ribosomal RNA
is actually used as part of the structure.
It actually helps the ribosome function as a ribosome,
so it's actually part of the ribosome.
And all of that gets built in a part of the nucleus
called the nucleolus or the nucleole.
Let me write that down.
So this right here, this is interesting right here.
This is the nucleolus or the nucleole.
And it isn't a separate organelle,
and it's not separated by a membrane.
But it shows up in a microscope.
So when people first saw that, hey, there's like a bundle
there.
That must be like the core of the nucleus or something.
But it turns out what it is, it's
the densely packed-- you got DNA and RNA.
And that's actually where the ribosomal RNA, the stuff that
makes up the ribosomes, is actually produced.
But it's so dense that it shows up on a microscope,
and that's why people decided to name it something different.
But it is not membrane bound.
It's not an organelle within an organelle.
It's just densely packed proteins and ribosomal RNA.
It's where ribosomal RNA gets produced.