In its most popular sense,
when people talk about mitosis, they're referring to
a cell, a diploid cell. So diploid just means it has
its full complement of chromosomes, so it has
2N chromosomes. So that's the nucleus. This is the whole cell. And so most people are saying,
look, the cell itself replicates into two diploid
cells, so it turns into two cells, each that have a full
complement of chromosomes, 2N chromosomes. And so when people say a cell
has experienced mitosis, they normally mean this. But I want to make one slight
clarification, that formally, mitosis only refers to the
process of the replication of the genetic material
and the nucleus. So, for example, if I were to
draw this-- let me draw the cell-- and it has now two
nucleuses, each with the diploid number of chromosomes,
this cell has experienced mitosis. It has not experienced
cytokinesis, which we will talk about in a few moments, but
that's the process of the actual cytoplasm of the
cell being split into two different cells. And just as a clarity, the
cytoplasm is all the stuff outside of the nucleus. So I'll talk about that in a
second, but just know in everyday usage, this is normally
the case when people talk about mitosis. But if you've got a teacher
that likes to get you on a technicality, this is
technically what mitosis is. It's the splitting of the
nucleus or the replication of the nucleus into two
separate nucleuses. That's normally accompanied
by cytokinesis where the cytoplasms of the cells
actually separate. Now, with that said, let's go
into the mechanics of mitosis. So the first steps that are
really necessary for mitosis actually occur outside of
mitosis when the cell is just doing its day-to-day life, and
that's during the interphase. And the interphase, literally
it's not a phase of mitosis. It's literally when the
cell is just living. Let's say we have
some new cell. Let me do it in green. That's a new cell here. Maybe this is its nucleus. It's got 2N chromosomes,
and then it grows. It brings in nutrients from
the outside and builds proteins and does whatever,
and so it grows a bit. It's obviously got its full
chromosomal complement still. And then at some point during
this life cycle, and I'll label these actually, so this
phase in interphase, and this might not even be covered in
some biology classes, but they give it a label. They call it G1, which
is really just when the cell is growing. It's just growing, accumulating
materials and building itself out, and then
it actually replicates its chromosomes. So you still have a diploid
number of chromosomes. So let me zoom in. So let me draw this. This is called the S phase of
interphase, so this is S. And S is where you have
replication of the actual chromosomes. Once again, we're not
even in mitosis yet. So S, you have replication
of your chromosomes. So if I were to zoom in on the
nucleus during the S phase, if I were to start off-- let me
just start with some organism that has two chromosomes. So let's say that at the
beginning of S phase, and I'll draw things as chromosomes just
to make it clear that things are being replicated. So let me say it has this
chromosome right here and then let's say it has this chromosome
right here. As it goes through S phase,
these chromosomes replicate. And I'm just drawing
the nucleus here. I've zoomed in on just this part
right here, where N is 1, where our full diploid
complement is two chomosomes. During S phase, our chromosomes
will replicate and will have-- so that green one
will completely replicate and generate a copy of itself, and
we've learned this a little bit, they're connected
at the centromere. Now, each of those copies are
called chromatids, and that magenta one will do
the same thing. Even though we have two
chromatids, one for each chromosome, now we have four
chromatids, two for each chromosome, we still say we
only have two chromosomes. That's its centromere
right there. This occurs in the S phase, and
then the cell will just continue to grow more. So the cell was already big--
I'll focus on the cell again. The cell was already big
and it gets bigger. It gets bigger, and that's
during the G2 phase, so it's just growing more. Now, there's another little part
of the cell we haven't even talked about yet,
but I'll talk about it a little bit. It's not super-duper important,
but it's the idea of these centrosomes. These are going to be very
important later on when the cell is actually dividing,
and those also duplicate. So let's say I have a little
centrosome here. It has centrioles inside it. You don't have to worry too much
about that, but they're these little cylindrical-looking
things. But I just want to-- so you
don't get confused if you see the word centriole and
centrosomes, not to be confused with centromeres, which
are these little points where the two chromatids
attach. Unfortunately, they named many
things in this process very similarly, or a lot
of the parts of a cell very similarly. But you have these things called
centrosomes that are going to enter the picture very
soon, that are sitting outside of the nucleus, and
they also replicate. They also replicate during
the interphase. So you had one before, now
you have two of them. And, of course, they each have
their two little centrioles inside, but we're not going
to focus too much on those just yet. So that's what happened
in the interphase. This is most of the cell's life,
and it's kind of growing and doing what it wants. Actually, I'll make a
slight point here. When I drew the DNA here, I
drew them as chromosomes. But the reality is when we're
sitting in the interphase, this is not what the DNA would
actually look like. The DNA, if I were to actually
draw this, it's in its chromatin form. It's not all tightly wound
like I drew it here. I drew it tightly wound so that
you can see that it got replicated, but the reality is
that that green chromosome would actually be all unwound,
and if you were looking in a microscope, you would even
have trouble seeing it. This is its chromatin form. We'll talk a little bit about
where it actually organizes itself back into a chromosome,
but in its chromatin form, it's just a bunch of DNA and
proteins that the DNA is wrapped around a little bit,
so you might have some proteins here that the DNA is
wrapped around a little bit. But if you're looking at it in
a microscope, it just looks like a big blur of
DNA and proteins. Same thing for the
magenta molecule. Really, for DNA to
do anything, it has to be like this. It has to be open to its
environment in order for the mRNA and the different types of
helper proteins to really be able to function with it. And even for it to be able to
replicate, it has to be unwound like this in order
for it to function. It only gets tightly wound
like this later on. I just drew it like this, so
really it had one green one, and it's going to replicate to
form another green one, and they're going to be attached
at some point. That magenta one is going to
replicate to form another magenta one, and they'll be
attached at some point, but it's not going to be clear. I just drew it this way to show
that it really happened. This is the reality. It's in its chromatin form. Now, we're ready for mitosis. So the first stage
of mitosis is essentially-- let me draw this. So I'll draw the
cell in green. I'm going to draw the nucleus a
lot bigger than it normally is relative to the cell just
because, at least right now, a lot of the action is going
in the nucleus. So the first stage of mitosis
is the prophase. These are somewhat arbitrary
names that were assigned. People looked in a microscope. Oh, that's a certain type of
step that we always see when a nucleus is dividing so we'll
call this the prophase. What happens in the prophase is
that the actual chromatin starts actually turning into
this type of form. So as I just said, when we're
in the interphase, the DNA's in this form where it's all
separated and unwound. It actually starts to wind
together, so this is where you'll actually have-- and
remember, it's already replicated. The replication happened
before mitosis begins. So I had that one chromosome
there, and then I have another one here. It has two sister chromatids
that we'll see soon get pulled apart. Now, during prophase, you
also start to have these centromeres appear that I
was touching on before. These guys over here, they
start to facilitate the generation of what you call
microtubules, and you can kind of view these as these sticks or
these ropes that are going to be key in moving things
around as we divide the cell. All of this is pretty amazing. I mean, you think of a cell, you
think of something that's inherently pretty simple. It's the most basic living
structure in us or in life. But even here, you have these
complex mechanics going on, and a lot of it still
isn't understood. I mean, we can observe it, but
we really don't know what's happening at the atomic level
or at the protein level that allows these things to move
around in such a nicely choreographed way. It's still an area
of research. Some of this is understood,
some of it isn't. But you have these two
centrosomes, and they facilitate the development of
these microtubules, which are literally like these little
microstructures. You can view them as tubes
or as some type of rope. Now as prophase progresses, it
eventually gets to the point where-- let me do it. I don't want this word
replication written here. It makes it confusing. Let me delete that. Let me get rid of this
replication. So as prophase progresses, the
nuclear envelope actually disappears. So let me redraw this. Let me copy and paste what
I've done before. Put it there. So as prophase progresses-- the
nuclear envelope actually starts to disassemble. So this starts to actually
dissolve and disassemble, and then these things start to grow
and attach themselves to the centromere. So actually, let me do that. So this is all during
prophase. Since all of this happens during
prophase, this latter part of prophase, sometimes
they'll call it late prophase, sometimes it'll be called
prometaphase. Sometimes it's considered-- I
don't think there's a hyphen really there. So sometimes it's actually
considered a separate phase of mitosis, although when I learned
it in school, they didn't bother with
prometaphase. They just called it
all prophase. But by the end of prophase,
or actually by the end of prometaphase, depending on how
you want to view it, the whole situation is going to look
something like this. You have your overall cell. The nuclear envelope has
disassembled, so to some degree, it doesn't
exist anymore. Although the proteins that
formed it are still there and they're going to be
used later on. And you have your two
chromosomes in this case. In a human's case, you would
have 46 of them. You have your two chomosomes,
each made with sister chromatids, each made with
two sister chromatids. Two chromosomes. They, of course, have their
centromeres right there, and then these centrosomes will
have migrated roughly on opposite sides of what
was once the nucleus. And these things have kind
of spread apart, these microtubules, so they're doing
two functions, really. At this point, they're
kind of pushing these two centrosomes apart. So you have all of these things,
and they're connecting the-- you know, some of them
are coming from this centrosome, some are coming from
this centrosome, some are connecting the two. And then some of these
microtubules, these tubes or these ropes, however you want
to view them, attach themselves to the centromeres of
the actual chromosomes, and the protein structure that they
attach them to is called the kinetochore. So there's the kinetochore
there, and that may or may not be-- kinetochore. It's a protein structure. It's actually fascinating. It's still an open area of
research on how exactly the microtubule attaches to the
kinetochore, and as we'll see in a second, it's at the
kinetochore that the microtubules essentially start
to pull at the two separate sister chromatids and actually
pull them apart. And it's actually
not understood exactly how that works. It's just been observed that
this actually happens. Once prophase is done,
essentially the cells then just make sure that the
chromosomes are well aligned. I kind of drew them well aligned
here, but that just kind of formally occurs
during metaphase, which is the next phase. The first one was prophase. Now we're in metaphase, and
metaphase really is just an aligning of the chromosomes, so
all of the chromosomes are going to be aligned at the
center of the cell. So I have my magenta one here,
I have my magenta one here, and I have my other one here,
my green one there, and, of course, you have your
centrosomes, the microspindles that are coming off of them. Some of them are kinetochore
microspindles that are actually attaching to the
centromeres of the actual chromosomes. It's very confusing, right? The centrosomes are these
structures that help direct what happens to these
microtubules. Centrioles are these little
structures, these little can-shaped structures inside
the centrosomes, and the centromere are the center
points where the two chromatids attached to each
other within a chromosome. So this is one sister chromatid,
that's another sister chromatid, and they
attach at the centromere. But this is metaphase. It's fairly easy. Metaphase, you just have this
aligning of the cells, and there's actually some theories,
how does the cell know to progress past
this point? How does it know
that everything is aligned and attached? And then there are some theories
that there's actually some signaling mechanism that
if one of these kinetochore proteins isn't properly attached
to one of these ropes, that somehow a signal
is sent that mitosis should not continue. So this is a very intricate
process. You can imagine if you have 46
chromosomes and you have all of this stuff going on in the
cell, and it's not like there's some individual
pushing stuff, or some computer here. It's really directed
by chemistry and by thermodynamic processes. But just by the intricacy or
the elegance of how these things are, it happens
spontaneously with all of the proper checks and balances, so
that most of the time, nothing bad happens, which is
all quite amazing. So after metaphase, now we're
ready to pull the stuff apart, and that's anaphase. So in anaphase-- let
me write that down. I've changed the color
of my cell. These guys get pulled apart. And as soon as they get pulled
apart-- so let's see, this guy's getting pulled. Let me do it in green. So one of the sister-- nope,
that's not green. One of the sister chromatids is
pulling in that direction. One is getting pulled
in that direction. And then the same is true
for the magenta ones. Pulled in that direction,
and one is getting pulled in that direction. And, of course, you have your
centrosomes here and then they're connected to the
kinetochores that are right there and that's where
they're pulling. There's also a whole microtubule
structure that isn't connected to the actual
chromosomes, but they're helping to actually push apart
these two centrosomes so that everything is going to opposite
sides of the cell. And so as soon as these two
chromatids are separated, and I touched on this a little bit
before when we talked about the vocabulary of DNA, then as
soon as that happens, these are each referred to
as chromosomes. So now you can say that
the cell has what it used to have here. It has two chromosomes. It now has four chromosomes. Because as soon as a chromatid
is no longer connected to its sister chromatid, they're then
considered sister chromosomes, which is just a naming
convention. I mean, they were there before,
they were there after. They were just attached
before. Now they're not attached, so
you kind of consider them their own individual entity. And then we're almost done. The last stage is telophase. I'm going to draw the cell a
little bit different here because something is happening
simultaneously with telophase most of the time. So telophase, and actually I'll rotate the cell 90 degrees. Let's say that this was
one centromere. This is the other centromere. So at this point,
it's essentially pulled the DNA to itself. So this guy has pulled one copy
of that chromosome and one copy of this chromosome. That guy's done the
same up here. He's pulled over one copy of
each-- oh, I used a different color-- one copy of each
chromosome to himself. Let me draw that right
there like that. And now the nuclear membranes
start forming around each of these two ends. So now you start having a
nuclear membrane form around each of these two ends. And so by the end of the
telophase-- that's what we're in, the telophase-- we will
have completed mitosis. We will have completely
replicated our two original nucleuses and all of the genetic
content inside of it. Now, at the same time telophase
is happening, you also normally have this
cytokinesis, where this cleavage furrow forms, where
essentially-- during telophase, these things are
getting pushed further and further apart by those
microtubules so that they're already at the ends of the cell,
of the cytoplasm of the cell, and you can almost view
them as pushing on the sides to elongate the cell. As that is happening, you have
this furrow forming, this little indentation. By the end of telophase in
mitosis, you also have this process of cytokinesis, where
this cleavage furrow forms and deepens, deepens, deepens
until the cytoplasm is actually split into two
separate cells. So this is cytokinesis, which
is formally not a part of mitosis, but it normally occurs
with the telophase, so right at the end of mitosis,
you do normally have two complete identical cells. Once you have each of these
two cells, then they, each individually, enter their
own interphase. Or they each individually, if
we look at just this one, he will then be in his G1 phase. At some point, these two things
are going to replicate, and that's the S phase, and you
go to the G2 phase, and then this guy will experience
mitosis all over again.
