L4 - Nucleus : Structure & Function - The Fundamental Unit of Life, Class 9, Science Video Lecture

- [Voiceover] Here's a
diagram of a eukaryotic cell.
Let's look a little bit
closer at the structure
of one of the most important organelles
in the eukaryotic cell, the nucleus.
So here we have a
depiction of the nucleus.
And the most important
function of the nucleus
is to contain the genetic
material of the cell.
But what is the structure?
Let's look at the outside
of the nucleus first.
The nucleus is surrounded
by two separate membranes,
an outer membrane and an inner membrane.
Let's look at this structure up close.
So, we have an outer membrane,
and we have an inner membrane.
These two membranes separate
the nucleus from the cytoplasm,
which is the liquid-filled
space that makes up
the majority of the cell, and all of
the non-compartmentalized
parts of the cell.
Essentially, the parts
of the cell that aren't
enclosed within an organelle.
On the inside of the nucleus is
what's called the nucleoplasm.
In the nucleoplasm is the
fluid inside the nucleus.
And so these two, the
cytoplasm and nucleoplasm,
aren't continuous.
However, a lot of the
times, the stuff needs to be
transported between the
nucleoplasm and the cytoplasm.
So think the interior
of the nucleus is where
MRNA is produced, but MRNA needs to get
into the cytoplasm where it
can be translated into protein.
And also, proteins in the
cytoplasm need to be able to get
into the nucleus, such as polymerases,
which convert DNA into messenger RNA.
So this transport of things
like MRNA out of the nucleus,
and protein into the
nucleus, happens at a special
complex called a nuclear pore.
And so there's another
nuclear pore over here
in my blown-up diagram of the
outer and inner membranes.
What's interesting about the nuclear pore,
is that it spans both membranes,
the outer membrane and the inner membrane,
so that compounds in the
cytoplasm can be transported
into the nucleus, and
compounds in the nucleus
can be transported out.
But now, the cell has
gone to a lot of trouble
to create these two lipid membranes
to isolate the nucleus
from the rest of the cell.
So the nuclear pore is
actually very selective
in what can be transported inside and out.
And so the nuclear pore
actually recognizes
special signals on different proteins,
and only with the
presence of these signals,
can proteins be transported
into and out of the nucleus
through the nuclear pore.
So the nucleus' primary
function is to contain
the cell's DNA, or chromosomes.
But when you look at the nucleus up close,
you see there is a very
densely compacted area.
And this area is called the nucleolus.
Just as the nucleus is kind
of the center of the cell,
the nucleolus is the
center of the nucleus.
And now, but why is the
nucleolus so much more
compact and dense than
the rest of the nucleus?
Well, that's because the nucleolus
has a very important function.
And that is, the site
of ribosome assembly.
So the nucleolus is densely
packed with regions of DNA
that produce not messenger
RNA, but ribosomal RNA.
Which is the RNA that makes up
the majority of the ribosome.
And it is at the site
of the nucleolus where
this ribosomal RNA is assembled with
the proteins that also
make up the ribosome,
into a fully-formed ribosome.
And the ribosomes then, can be trafficked
through the nuclear pore,
out into the cytoplasm.
The ribosomes also have proteins in them,
and these proteins are
produced in the cytoplasm,
and are trafficked back into the nucleus
where they can be assembled, and then make
the return trip back into the cytoplasm.
So there's lots of stuff
moving back and forth
in the nuclear pores.
But it's very important for
the compartmentalization
of the nucleus from the rest of the cell.
To move into our next discussion,
the nucleus is intimately associated with
another organelle, the
endoplasmic reticulum,
which you can see in
the faded green outline.
An interesting topological
feature of the nucleus,
is that the outer membrane,
remember which we've drawn
up in the top left, is actually continuous
with the membranes that make
up the endoplasmic reticulum.
So I'll just kinda draw that.
This outer membrane just turns around
and actually becomes the membranes
of the endoplasmic reticulum.
And I won't draw it all in.
So the interior space of
the endoplasmic reticulum,
is continuous with the interior space
of between the outer and
inner nuclear membranes.
And one final term used
to describe the nucleus,
is the nuclear envelope.
And the nuclear envelope just refers to
the combination of the
inner and outer membranes
along with the nuclear pores.
So these three structures
together comprise
the nuclear envelope,
which encloses the nucleus.