Enthalpy

WEBVTT
Kind: captions
Language: en

00:00:00.650 --> 00:00:03.460
Let me draw a good
old PV diagram.

00:00:03.460 --> 00:00:08.510
That's my pressure axis,
this is my volume axis.

00:00:08.510 --> 00:00:12.360
Just like that, I have
pressure and volume.

00:00:12.360 --> 00:00:15.650
I showed several videos ago that
if we start at some state

00:00:15.650 --> 00:00:19.560
here in the PV diagram, right
there, and that I change the

00:00:19.560 --> 00:00:22.090
pressure and the volume to get
to another state, and I do it

00:00:22.090 --> 00:00:25.770
in a quasistatic way, so
essentially I'm always close

00:00:25.770 --> 00:00:29.170
to equilibrium, so my state
variables are always defined,

00:00:29.170 --> 00:00:31.640
I could have some path that
takes me to some other state

00:00:31.640 --> 00:00:32.950
right there.

00:00:32.950 --> 00:00:34.600
And this is my path.

00:00:34.600 --> 00:00:38.020
I'm going from this state
to that state.

00:00:38.020 --> 00:00:42.230
And we showed that if I just did
this, the work done by the

00:00:42.230 --> 00:00:44.460
system is the area
under this curve.

00:00:44.460 --> 00:00:47.730
And then if I were to move back
to the previous state,

00:00:47.730 --> 00:00:51.480
and then if I were, you know, by
some path, just some random

00:00:51.480 --> 00:00:54.830
path that I happened to be
drawing, the work done to the

00:00:54.830 --> 00:00:57.600
system would be the area under
this light blue curve.

00:00:57.600 --> 00:01:01.860
So the net work done by the
system ended up being the area

00:01:01.860 --> 00:01:04.090
inside of this path.

00:01:04.090 --> 00:01:06.850
So this is-- let me do it
in different color.

00:01:06.850 --> 00:01:11.700
The net work done would be the
area inside of this path when

00:01:11.700 --> 00:01:14.390
I go in this clockwise
type of direction.

00:01:14.390 --> 00:01:26.770
So this is the net work
done by the system.

00:01:26.770 --> 00:01:33.360
And now, we also know that if
we're at some point on this PV

00:01:33.360 --> 00:01:36.260
diagram, that our state is the
same as it was before.

00:01:36.260 --> 00:01:39.150
So if we go all the way here,
and then go all the way back,

00:01:39.150 --> 00:01:41.300
all of our state variables
will not have changed.

00:01:41.300 --> 00:01:43.580
Our pressure is the same
as it was before.

00:01:43.580 --> 00:01:45.430
Our volume was the same as it
was before because we went all

00:01:45.430 --> 00:01:47.880
the way back to that same
point on the PV diagram.

00:01:47.880 --> 00:01:50.660
And our internal energy is also
the same point as it was

00:01:50.660 --> 00:01:53.130
before, so our change in
internal energy over this

00:01:53.130 --> 00:01:55.290
path, you're going to have a
different internal energy here

00:01:55.290 --> 00:01:57.350
than you had here, but when you
go around the circle and

00:01:57.350 --> 00:01:58.840
you get back, your change
in internal

00:01:58.840 --> 00:02:01.420
energy is equal to zero.

00:02:01.420 --> 00:02:07.865
And we know that our change in
internal energy is defined as,

00:02:07.865 --> 00:02:11.450
and this is from the first law
of thermodynamics, the heat

00:02:11.450 --> 00:02:18.250
added to the system minus the
work done by the system.

00:02:18.250 --> 00:02:22.650
Now, if we go on a closed loop
on our PV diagram, then what's

00:02:22.650 --> 00:02:23.810
our change in internal energy?

00:02:23.810 --> 00:02:24.970
It's zero.

00:02:24.970 --> 00:02:29.150
So we get zero change in
internal energy, because we're

00:02:29.150 --> 00:02:33.330
at the same state is equal to
the heat applied to the system

00:02:33.330 --> 00:02:37.930
minus the work done or-- and
I've done this little exercise

00:02:37.930 --> 00:02:38.810
multiple times.

00:02:38.810 --> 00:02:41.020
I think is probably the fourth
or fifth time I'm doing it.

00:02:41.020 --> 00:02:43.970
We get that the heat added to
the system, if we just add w

00:02:43.970 --> 00:02:47.170
to both sides, is equal to the
work done by the system.

00:02:47.170 --> 00:02:52.430
So this area inside of this
path, I already said, it's the

00:02:52.430 --> 00:02:55.360
work done by the system, and
if you don't remember even

00:02:55.360 --> 00:02:58.310
where that came from, it was,
remember, pressure times

00:02:58.310 --> 00:03:01.155
volume times change in volume is
a little incremental change

00:03:01.155 --> 00:03:02.800
in work, and that's why it
relates to the area.

00:03:02.800 --> 00:03:04.070
But we've done that
multiple times.

00:03:04.070 --> 00:03:06.180
I won't go there just yet.

00:03:06.180 --> 00:03:09.150
So if you have any area here,
some heat was added to the

00:03:09.150 --> 00:03:11.360
system, some net heat, right?

00:03:11.360 --> 00:03:14.380
Some heat was added here, and
some heat was probably taken

00:03:14.380 --> 00:03:16.780
out here, but you have
some net heat that's

00:03:16.780 --> 00:03:18.260
added to the system.

00:03:18.260 --> 00:03:24.330
And I use that argument
to say why heat isn't

00:03:24.330 --> 00:03:25.410
a good state variable.

00:03:25.410 --> 00:03:27.410
Because-- and I had a whole
video on this-- that if I

00:03:27.410 --> 00:03:33.350
define some state variable,
let's just say, heat content.

00:03:33.350 --> 00:03:35.200
Let's say I want to
define some state

00:03:35.200 --> 00:03:36.900
variable heat content.

00:03:36.900 --> 00:03:41.990
And I would say that the change
in heat content would,

00:03:41.990 --> 00:03:44.350
of course, be equal to
the change in heat.

00:03:44.350 --> 00:03:45.100
That's what I'm defining.

00:03:45.100 --> 00:03:46.970
If I'm adding heat to
the system, my heat

00:03:46.970 --> 00:03:48.550
content should go up.

00:03:48.550 --> 00:03:52.230
But the problem with that heat
content state variable was

00:03:52.230 --> 00:03:58.630
that, let's say over here, I say
that the heat content is

00:03:58.630 --> 00:04:00.270
equal to 5.

00:04:00.270 --> 00:04:03.560
Now, I just showed you that if
we go on some path here and we

00:04:03.560 --> 00:04:08.280
come back, and there's some area
in this little path that

00:04:08.280 --> 00:04:10.700
I took, that some
heat was added.

00:04:10.700 --> 00:04:15.490
So let's say that this area
right here, so this is q is

00:04:15.490 --> 00:04:17.439
equal to the work done by
the system, let's say

00:04:17.439 --> 00:04:19.200
it's equal to 2.

00:04:19.200 --> 00:04:22.470
So every time, if I start at
heat content is equal to 5,

00:04:22.470 --> 00:04:24.850
that's just an arbitrary number,
and I were to do this

00:04:24.850 --> 00:04:26.720
entire path, when I go back,
the heat content

00:04:26.720 --> 00:04:27.930
would have to be 7.

00:04:27.930 --> 00:04:30.030
And then when I go back and do
it again, my heat content

00:04:30.030 --> 00:04:31.250
would have to be 9.

00:04:31.250 --> 00:04:33.150
And it would have to increment
by 2 every time I do this

00:04:33.150 --> 00:04:33.860
exact path.

00:04:33.860 --> 00:04:36.590
It would have to increment by
the amount of area that this

00:04:36.590 --> 00:04:38.390
path goes around.

00:04:38.390 --> 00:04:40.420
So heat content can't be a state
variable, because it's

00:04:40.420 --> 00:04:42.180
dependent on how
you got there.

00:04:42.180 --> 00:04:44.170
A state variable-- and
remember this.

00:04:44.170 --> 00:04:46.750
In order to be a state variable,
if you're at this

00:04:46.750 --> 00:04:49.340
point, you have to have
the same value.

00:04:49.340 --> 00:04:52.190
If your internal energy was 10
here, when you do the path and

00:04:52.190 --> 00:04:54.630
you come back, your internal
energy will be 10 again.

00:04:54.630 --> 00:04:57.380
That's why internal energy is
a valid state variable.

00:04:57.380 --> 00:04:59.520
It's dependent only
on your state.

00:04:59.520 --> 00:05:03.010
If your entropy was 50 here,
when you soon. go back and you

00:05:03.010 --> 00:05:04.890
do all sorts of crazy things,
and you come back to this

00:05:04.890 --> 00:05:07.040
point, your entropy
is once again 50.

00:05:07.040 --> 00:05:12.030
If your pressure here is 5
atmospheres, when you come

00:05:12.030 --> 00:05:14.170
back here, your pressure
will be 5 atmospheres.

00:05:14.170 --> 00:05:17.060
Your state variable cannot
change based on

00:05:17.060 --> 00:05:18.050
what path you took.

00:05:18.050 --> 00:05:20.420
If you're at a certain state,
that's all that matters to the

00:05:20.420 --> 00:05:21.030
state variable.

00:05:21.030 --> 00:05:24.090
Now this heat content didn't
work, and that's why we

00:05:24.090 --> 00:05:26.590
actually led into some videos
where I divided by t and we

00:05:26.590 --> 00:05:29.260
got entropy, which was an
interesting variation.

00:05:29.260 --> 00:05:30.790
But that's still
not satisfying.

00:05:30.790 --> 00:05:33.950
What if we really wanted to
develop something that could

00:05:33.950 --> 00:05:37.670
in some way be a state variable,
but at the same time

00:05:37.670 --> 00:05:39.020
measure heat?

00:05:39.020 --> 00:05:41.090
So obviously, we're going to
have to make some compromises,

00:05:41.090 --> 00:05:44.170
because if we just do a very
arbitrary kind of heat content

00:05:44.170 --> 00:05:46.440
variable, then every time
you go around this,

00:05:46.440 --> 00:05:47.440
it's going to change.

00:05:47.440 --> 00:05:51.370
That's not a valid
state variable.

00:05:51.370 --> 00:05:53.780
So let's see if we
can make up one.

00:05:53.780 --> 00:05:55.950
So let's just make
up a definition.

00:05:55.950 --> 00:05:58.930
Let's call my new thing that
I'm going to try to maybe

00:05:58.930 --> 00:06:02.350
approximate heat, let's call it
h, and just as a little bit

00:06:02.350 --> 00:06:03.870
of a preview, we're going
to call it enthalpy.

00:06:07.830 --> 00:06:08.980
And let's just define it.

00:06:08.980 --> 00:06:10.730
I'm just playing around.

00:06:10.730 --> 00:06:15.430
Let's just define it as the
internal energy plus my

00:06:15.430 --> 00:06:16.883
pressure times my volume.

00:06:20.880 --> 00:06:25.620
So then what would my change
in enthalpy be?

00:06:25.620 --> 00:06:28.660
So my change in enthalpy will
be, of course, the change of

00:06:28.660 --> 00:06:29.340
these things.

00:06:29.340 --> 00:06:32.520
But I could just say, that's
my change in my internal

00:06:32.520 --> 00:06:39.130
energy plus my change in
pressure times volume.

00:06:39.130 --> 00:06:40.300
Now, this is interesting.

00:06:40.300 --> 00:06:41.840
And I want to make
a point here.

00:06:41.840 --> 00:06:44.230
This, by definition, is a
valid state variable.

00:06:44.230 --> 00:06:45.220
Why is it?

00:06:45.220 --> 00:06:49.320
Because it's the addition of
other state variables, right?

00:06:49.320 --> 00:06:52.870
At any point in my PV diagram,
and it's also true if I did

00:06:52.870 --> 00:06:56.030
diagrams that were entropy in
temperature or anything that

00:06:56.030 --> 00:07:00.220
dealt with state variables, at
any point on my diagram, u is

00:07:00.220 --> 00:07:02.800
going to be the same, no matter
how I got there. p is

00:07:02.800 --> 00:07:04.200
by definition going
to be the same.

00:07:04.200 --> 00:07:05.540
That's why it's at that point.

00:07:05.540 --> 00:07:07.510
v is definitely going to
be the same point.

00:07:07.510 --> 00:07:10.940
So if I just add them up, this
is a valid state variable

00:07:10.940 --> 00:07:12.480
because it's just the
sum of a bunch of

00:07:12.480 --> 00:07:14.595
other valid state variables.

00:07:14.595 --> 00:07:17.780
So let's see if we can somehow
relate this thing that we've

00:07:17.780 --> 00:07:19.930
already established as a
valid state variable.

00:07:19.930 --> 00:07:23.550
From the get-go, from our
definition, this works because

00:07:23.550 --> 00:07:24.470
it's just the sum of

00:07:24.470 --> 00:07:26.030
completely valid state variables.

00:07:26.030 --> 00:07:28.820
So let's see if we can relate
this somehow to heat.

00:07:28.820 --> 00:07:30.320
So we know what delta u is.

00:07:33.070 --> 00:07:35.790
If we're dealing with all of
the internal energy or the

00:07:35.790 --> 00:07:37.810
change in internal energy, and
I'm not going to deal with all

00:07:37.810 --> 00:07:40.590
the other chemical potentials
and all of that, it's equal to

00:07:40.590 --> 00:07:48.770
the heat applied to the system
minus the work done by the

00:07:48.770 --> 00:07:50.040
system, right?

00:07:50.040 --> 00:07:51.380
Let me put everything
else there.

00:07:51.380 --> 00:07:55.480
The change in enthalpy is equal
to the heat applied

00:07:55.480 --> 00:07:57.740
minus the work done-- that's
just the change in internal

00:07:57.740 --> 00:08:00.520
energy-- plus delta PV.

00:08:00.520 --> 00:08:03.910
This is just from the definition
of my enthalpy.

00:08:03.910 --> 00:08:06.090
Now this is starting to
look interesting.

00:08:06.090 --> 00:08:11.570
What's the work done
by a system?

00:08:11.570 --> 00:08:15.020
So I could write change in h,
or enthalpy, is equal to the

00:08:15.020 --> 00:08:20.140
heat applied to the system
minus-- what's the work done

00:08:20.140 --> 00:08:20.740
by a system?

00:08:20.740 --> 00:08:25.280
If I have some system here, it's
got some piston on it,

00:08:25.280 --> 00:08:27.450
you know, if we're doing it in
a quasistatic, I have those

00:08:27.450 --> 00:08:29.310
classic pebbles that
I've talked

00:08:29.310 --> 00:08:31.450
about in multiple videos.

00:08:31.450 --> 00:08:36.220
When I apply heat or let's say
I remove some of these

00:08:36.220 --> 00:08:39.340
pebbles, so I'm at a different
equilibrium, but what's

00:08:39.340 --> 00:08:40.150
actually happening?

00:08:40.150 --> 00:08:41.200
When is the work being done?

00:08:41.200 --> 00:08:45.030
You have some pressure being
applied up here, and this

00:08:45.030 --> 00:08:48.370
piston is going to be moving up,
and your volume is going

00:08:48.370 --> 00:08:48.880
to increase.

00:08:48.880 --> 00:08:52.710
And we showed multiple videos
ago that the work done by the

00:08:52.710 --> 00:08:55.160
system can be, and you can
kind of view this as the

00:08:55.160 --> 00:09:01.560
volume expansion work, it's
equal to pressure

00:09:01.560 --> 00:09:02.975
times change in volume.

00:09:05.600 --> 00:09:07.180
And let's add the other part.

00:09:07.180 --> 00:09:09.475
So this was our change
in internal energy.

00:09:09.475 --> 00:09:12.110
And I had several videos
where I show this.

00:09:12.110 --> 00:09:13.750
And now let me add the other
part of the equation.

00:09:13.750 --> 00:09:15.690
So our enthalpy, our change
in enthalpy, can

00:09:15.690 --> 00:09:17.390
be defined by this.

00:09:17.390 --> 00:09:17.570
Now.

00:09:17.570 --> 00:09:20.780
Something interesting
is going on.

00:09:20.780 --> 00:09:23.780
I said that I wanted to define
something, because I wanted to

00:09:23.780 --> 00:09:26.000
somehow measure heat content.

00:09:26.000 --> 00:09:30.270
My change in enthalpy will be
equal to the heat added to the

00:09:30.270 --> 00:09:34.940
system, if these last two
terms cancel out.

00:09:34.940 --> 00:09:38.510
If I can somehow get these last
two terms to cancel out,

00:09:38.510 --> 00:09:41.160
then my change in enthalpy
will be equal to this, if

00:09:41.160 --> 00:09:43.360
somehow these are equal
to each other.

00:09:43.360 --> 00:09:47.800
So under what conditions are
these equal to each other?

00:09:47.800 --> 00:09:52.060
Or another way, under what
conditions is delta pressure

00:09:52.060 --> 00:09:57.010
times volume equal to pressure
times delta volume?

00:09:57.010 --> 00:09:58.360
When does this happen?

00:09:58.360 --> 00:09:59.560
When can I make this
statement?

00:09:59.560 --> 00:10:01.670
Because if I can make this
statement, then these two

00:10:01.670 --> 00:10:04.285
terms are equivalent right here,
and then you my change

00:10:04.285 --> 00:10:07.580
in enthalpy will be equal
to the heat added.

00:10:07.580 --> 00:10:10.790
Well, the only way I can make
this statement is if pressure

00:10:10.790 --> 00:10:12.040
is constant.

00:10:20.280 --> 00:10:21.100
Now why is that?

00:10:21.100 --> 00:10:23.040
Let's just think about
it mathematically.

00:10:23.040 --> 00:10:26.740
If this is a constant, then if
I just change-- you know, if

00:10:26.740 --> 00:10:30.130
this is just 5, 5 times a change
in something is the

00:10:30.130 --> 00:10:33.930
same thing as the change in 5
times that thing, so it just

00:10:33.930 --> 00:10:36.360
mathematically works out.

00:10:36.360 --> 00:10:40.830
Or if you view it another way,
if this is a constant, you can

00:10:40.830 --> 00:10:44.660
just factor it out, right?

00:10:44.660 --> 00:10:54.050
Well, if I said, the change in
5x, that would be equal to 5

00:10:54.050 --> 00:10:58.900
times x final minus
5 times x initial.

00:10:58.900 --> 00:11:01.740
And you could say, well, that's
just equal to 5 times x

00:11:01.740 --> 00:11:03.950
final minus x initial.

00:11:03.950 --> 00:11:06.710
Well, that's just equal to
5 times the change in x.

00:11:06.710 --> 00:11:09.180
It's kind of almost too obvious
for me to explain.

00:11:09.180 --> 00:11:11.270
I think sometimes when you
overexplain things, it might

00:11:11.270 --> 00:11:12.480
become more confusing.

00:11:12.480 --> 00:11:16.050
So this applies-- and the 5 I'm
just doing as the analogy

00:11:16.050 --> 00:11:16.810
for a constant.

00:11:16.810 --> 00:11:20.830
So if pressure is constant, then
this equation is true.

00:11:20.830 --> 00:11:26.690
So if pressure is constant-- so
this is a key assumption--

00:11:26.690 --> 00:11:31.700
then if heat is being applied
in a constant pressure

00:11:31.700 --> 00:11:34.410
system-- so we could
write it this way.

00:11:34.410 --> 00:11:36.780
I'll write it multiple times,
because this is key.

00:11:36.780 --> 00:11:45.450
If pressure is constant, then
our definition, our little

00:11:45.450 --> 00:11:47.330
thing we made up, this enthalpy
thing, which we

00:11:47.330 --> 00:11:52.900
defined as internal energy plus
pressure and volume, then

00:11:52.900 --> 00:11:56.070
in a constant pressure system,
our change in enthalpy we just

00:11:56.070 --> 00:12:01.320
showed is equal to the heat
added to the system because

00:12:01.320 --> 00:12:04.390
all of these two things become
equivalent under constant

00:12:04.390 --> 00:12:05.360
pressure, so I should
write that.

00:12:05.360 --> 00:12:07.270
This is only true when
heat is added in a

00:12:07.270 --> 00:12:09.220
constant pressure system.

00:12:09.220 --> 00:12:11.325
So how does this gel
with what we did up

00:12:11.325 --> 00:12:13.270
here on our PV diagram?

00:12:13.270 --> 00:12:15.990
What's happening in a constant
pressure system?

00:12:15.990 --> 00:12:18.780
Let me draw our PV diagram.

00:12:18.780 --> 00:12:20.920
That's P, that's V.

00:12:26.350 --> 00:12:28.210
So what's happening in
constant pressure?

00:12:28.210 --> 00:12:30.440
We're at some pressure
right there.

00:12:30.440 --> 00:12:32.160
So if we're under constant
pressure, that means we can

00:12:32.160 --> 00:12:34.900
only move along this line.

00:12:34.900 --> 00:12:39.395
So we could go from here to
there and back to there, or we

00:12:39.395 --> 00:12:41.160
could go from there to
there, back to there.

00:12:41.160 --> 00:12:44.280
So we could go there, all the
way there, and then go back.

00:12:44.280 --> 00:12:46.040
But what do we see about this?

00:12:46.040 --> 00:12:48.850
Is there any area
in this curve?

00:12:48.850 --> 00:12:50.540
I mean, there is no curve to
speak of, because we're

00:12:50.540 --> 00:12:51.680
staying in a constant
pressure.

00:12:51.680 --> 00:12:55.000
We've kind of squeezed
out this diagram.

00:12:55.000 --> 00:12:58.170
We've made the forward path and
the return path the same

00:12:58.170 --> 00:12:59.590
exact path.

00:12:59.590 --> 00:13:04.420
So because of this, you don't
have that state problem

00:13:04.420 --> 00:13:08.490
because no net heat is being
added to the system when you

00:13:08.490 --> 00:13:11.710
go from this point all the way
to this point and then back to

00:13:11.710 --> 00:13:12.790
this point.

00:13:12.790 --> 00:13:14.840
So because of that, you can
kind of see visually that

00:13:14.840 --> 00:13:17.090
enthalpy in a constant pressure,
when you're not

00:13:17.090 --> 00:13:19.540
moving up and down in pressure,
is the same thing as

00:13:19.540 --> 00:13:20.400
heat added.

00:13:20.400 --> 00:13:21.780
So you might say, hey Sal,
this was a bit of a

00:13:21.780 --> 00:13:24.610
compromise, constant pressure,
you know, that's a big

00:13:24.610 --> 00:13:25.490
assumption to make.

00:13:25.490 --> 00:13:27.370
Why is this useful at all?

00:13:27.370 --> 00:13:30.530
Well, it's useful, because
most chemical reactions,

00:13:30.530 --> 00:13:33.530
especially ones that occur in an
open beaker, or that might

00:13:33.530 --> 00:13:36.755
occur at sea level, and that
should be a big clue, they

00:13:36.755 --> 00:13:38.250
occur at constant pressure.

00:13:38.250 --> 00:13:41.070
You know, if I'm sitting at
the beach, and I have my

00:13:41.070 --> 00:13:45.290
chemistry set, and I have some
beaker of something, and I'm

00:13:45.290 --> 00:13:48.790
throwing other stuff into it,
and I'm looking for a reaction

00:13:48.790 --> 00:13:50.450
or something, it's a constant
pressure system.

00:13:50.450 --> 00:13:52.070
This is going to be atmospheric
pressure.

00:13:52.070 --> 00:13:52.830
1 atmosphere.

00:13:52.830 --> 00:13:54.310
I'm sitting at sea level.

00:13:54.310 --> 00:14:00.110
So this is actually a very
useful concept for everyday

00:14:00.110 --> 00:14:00.910
chemical expressions.

00:14:00.910 --> 00:14:02.980
It might not be so useful for
engines, because engines

00:14:02.980 --> 00:14:05.710
always have pressure changing,
but it's very useful for

00:14:05.710 --> 00:14:10.130
actual chemistry, for actually
dealing with what's going to

00:14:10.130 --> 00:14:13.910
happen to a reaction at
a constant pressure.

00:14:13.910 --> 00:14:16.520
So what we're going to see is
that this enthalpy, you can

00:14:16.520 --> 00:14:18.960
kind of view it as
the heat content

00:14:18.960 --> 00:14:20.450
when pressure is constant.

00:14:20.450 --> 00:14:22.080
In fact, it is the heat content

00:14:22.080 --> 00:14:24.200
when pressure is constant.

00:14:24.200 --> 00:14:27.960
So somehow-- well, not somehow,
I showed you how-- we

00:14:27.960 --> 00:14:29.930
were able to make this
definition, which by

00:14:29.930 --> 00:14:32.350
definition was a state variable,
because it was the

00:14:32.350 --> 00:14:35.710
sum of other state variables,
and if we just make that one

00:14:35.710 --> 00:14:39.270
assumption of constant pressure,
it all of a sudden

00:14:39.270 --> 00:14:43.420
reduces to the heat content
of that system.

00:14:43.420 --> 00:14:46.290
So we'll talk more in the future
of measuring enthalpy,

00:14:46.290 --> 00:14:49.310
but you just have to say, if
pressure is constant, enthalpy

00:14:49.310 --> 00:14:52.280
is the same thing as-- and it's
really only useful when

00:14:52.280 --> 00:14:53.380
we're dealing with a
constant pressure.

00:14:53.380 --> 00:14:56.340
But if we have a pressure
constant, enthalpy can be

00:14:56.340 --> 00:14:57.655
imagined as heat content.

00:14:57.655 --> 00:15:00.900
And it's very useful for
understanding whether chemical

00:15:00.900 --> 00:15:03.980
reactions need heat to occur or
whether they release heat,

00:15:03.980 --> 00:15:05.200
so on and so forth.

00:15:05.200 --> 00:15:06.450
See


Phases of Meiosis

WEBVTT
Kind: captions
Language: en

00:00:00.570 --> 00:00:02.070
We've gone over the
general idea

00:00:02.070 --> 00:00:03.610
behind mitosis and meiosis.

00:00:03.610 --> 00:00:04.820
It's a good idea in
this video to go a

00:00:04.820 --> 00:00:05.730
little bit more in detail.

00:00:05.730 --> 00:00:08.230
I've already done a video on
mitosis, and in this one,

00:00:08.230 --> 00:00:11.130
we'll go into the details
of meiosis.

00:00:11.130 --> 00:00:17.710
Just as a review, mitosis, you
start with a diploid cell, and

00:00:17.710 --> 00:00:20.270
you end up with two
diploid cells.

00:00:20.270 --> 00:00:23.050
Essentially, it just
duplicates itself.

00:00:23.050 --> 00:00:25.540
And formally, mitosis is really
the process of the

00:00:25.540 --> 00:00:27.810
duplication of the nucleus, but
it normally ends up with

00:00:27.810 --> 00:00:29.350
two entire cells.

00:00:29.350 --> 00:00:32.119
Cytokinesis takes place.

00:00:32.119 --> 00:00:33.880
So this is mitosis.

00:00:33.880 --> 00:00:36.020
We have a video on it where we
go into the phases of it:

00:00:36.020 --> 00:00:39.290
prophase, metaphase, anaphase
and telophase.

00:00:45.050 --> 00:00:49.800
Mitosis occurs in pretty much
all of our somatic cells as

00:00:49.800 --> 00:00:56.810
our skin cells replicate, and
our hair cells and all the

00:00:56.810 --> 00:00:59.080
tissue in our body as it
duplicates itself, it goes

00:00:59.080 --> 00:01:00.180
through mitosis.

00:01:00.180 --> 00:01:02.970
Meiosis occurs in the germ
cells and it's used

00:01:02.970 --> 00:01:06.960
essentially to produce gametes
to facilitate sexual

00:01:06.960 --> 00:01:07.630
reproduction.

00:01:07.630 --> 00:01:13.020
So if I start off with a diploid
cell, and that's my

00:01:13.020 --> 00:01:15.560
diploid cell right there, this
would be a germ cell.

00:01:15.560 --> 00:01:17.710
It's not just any cell
in the body.

00:01:17.710 --> 00:01:19.670
It's a germ cell.

00:01:19.670 --> 00:01:22.870
It could undergo mitosis to
produce more germ cells, but

00:01:22.870 --> 00:01:24.780
we'll talk about how it
produces the gametes.

00:01:24.780 --> 00:01:28.420
It actually goes under
two rounds.

00:01:28.420 --> 00:01:30.960
They're combined, called
meiosis, but the first round

00:01:30.960 --> 00:01:35.670
you could call it meiosis
1, so I'll call that M1.

00:01:35.670 --> 00:01:37.840
I'm not talking about the
money supply here.

00:01:37.840 --> 00:01:41.170
And in the first round of
meiosis, this diploid cell

00:01:41.170 --> 00:01:45.270
essentially splits into
two haploid cells.

00:01:45.270 --> 00:01:49.190
So if you started off with 43
chromosomes, formally have 23

00:01:49.190 --> 00:01:52.110
chromosomes in each one, or you
can almost view it if you

00:01:52.110 --> 00:01:56.240
have 23 pairs here, each have
two chromosomes, those pairs

00:01:56.240 --> 00:01:58.930
get split into this stage.

00:01:58.930 --> 00:02:06.850
And then in meiosis 2, these
things get split in a

00:02:06.850 --> 00:02:11.050
mechanism very similar
to mitosis.

00:02:11.050 --> 00:02:12.010
We'll see that when we actually

00:02:12.010 --> 00:02:12.900
go through the phases.

00:02:12.900 --> 00:02:16.160
In fact, the prophase,
metaphase, anaphase, telophase

00:02:16.160 --> 00:02:19.795
also exist in each of these
phases of meiosis.

00:02:19.795 --> 00:02:22.100
So let me just draw
the end product.

00:02:22.100 --> 00:02:27.490
The end product is you have four
cells and each of them

00:02:27.490 --> 00:02:28.740
are haploid.

00:02:32.220 --> 00:02:35.520
And you could already see, this
process right here, you

00:02:35.520 --> 00:02:39.260
essentially split up your
chromosomes, because you end

00:02:39.260 --> 00:02:41.790
up with half in each one, but
here, you start with N and you

00:02:41.790 --> 00:02:44.180
end up with two chromosomes that
each have N, so it's very

00:02:44.180 --> 00:02:44.760
similar to this.

00:02:44.760 --> 00:02:46.380
You preserve the number
of chromosomes.

00:02:46.380 --> 00:02:49.390
So let's delve into the details
of how it all happens.

00:02:49.390 --> 00:02:53.290
So all cells spend most of
their time in interphase.

00:02:53.290 --> 00:02:57.910
Interphase is just a time when
the cell is living and

00:02:57.910 --> 00:03:00.770
transcribing and doing
what it needs to do.

00:03:00.770 --> 00:03:05.030
But just like in mitosis, one
key thing does happen during

00:03:05.030 --> 00:03:07.980
the interphase, and actually,
it happens during the same

00:03:07.980 --> 00:03:10.390
thing, the S phase of
the interphase.

00:03:10.390 --> 00:03:15.370
So if that's my cell, that's
my nucleus right here.

00:03:15.370 --> 00:03:17.580
And I'm going to draw it as
chromosomes, but you have to

00:03:17.580 --> 00:03:22.430
remember that when we're outside
of mitosis or meiosis

00:03:22.430 --> 00:03:26.340
formally, the chromosomes are
all unwound, and they exist as

00:03:26.340 --> 00:03:28.440
chromatin, which we've
talked about before.

00:03:28.440 --> 00:03:31.200
It's kind of the unwound
state of the DNA.

00:03:31.200 --> 00:03:32.950
But I'm going to draw them wound
up because I need to

00:03:32.950 --> 00:03:35.050
show you that they replicate.

00:03:35.050 --> 00:03:36.750
Now, I'm going to be a
little careful here.

00:03:36.750 --> 00:03:40.290
In the mitosis video, I just
had two chromosomes.

00:03:40.290 --> 00:03:42.350
They replicated and then
they split apart.

00:03:42.350 --> 00:03:44.920
When we talk about meiosis, we
have to be careful to show the

00:03:44.920 --> 00:03:46.670
homologous pairs.

00:03:46.670 --> 00:03:49.100
So let's say that I have
two homologous pairs.

00:03:49.100 --> 00:03:54.100
So let's say I have-- let me do
it in appropriate colors.

00:03:54.100 --> 00:03:56.800
So this is the one I
got from my dad.

00:03:56.800 --> 00:03:58.660
This is the one I
got from my mom.

00:03:58.660 --> 00:04:00.470
They're homologous.

00:04:00.470 --> 00:04:02.140
And let's say that I have
another one that

00:04:02.140 --> 00:04:03.390
I got from my dad.

00:04:06.610 --> 00:04:08.770
Let me do it in blue.

00:04:08.770 --> 00:04:10.625
Actually, maybe I should do all
the ones from my dad in

00:04:10.625 --> 00:04:11.900
this color.

00:04:11.900 --> 00:04:13.570
Maybe it's a little
bit longer.

00:04:13.570 --> 00:04:14.660
You get the idea.

00:04:14.660 --> 00:04:17.269
And then a homologous one for
my mom that's also a little

00:04:17.269 --> 00:04:18.589
bit longer.

00:04:18.589 --> 00:04:22.019
Now, during the S phase of the
interphase-- and this is just

00:04:22.019 --> 00:04:25.700
like what happens in mitosis,
so you can almost view it as

00:04:25.700 --> 00:04:27.490
it always happens during
interphase.

00:04:27.490 --> 00:04:29.810
It doesn't happen in either
meiosis or mitosis.

00:04:29.810 --> 00:04:32.650
You have replication
of your DNA.

00:04:32.650 --> 00:04:34.760
So each of these from the
homologous pair-- and

00:04:34.760 --> 00:04:37.270
remember, homologous pairs
mean that they're not

00:04:37.270 --> 00:04:39.900
identical chromosomes,
but they do code

00:04:39.900 --> 00:04:41.040
for the same genes.

00:04:41.040 --> 00:04:43.750
They might have different
versions or different alleles

00:04:43.750 --> 00:04:46.430
for a gene or for a certain
trait, but they code

00:04:46.430 --> 00:04:48.500
essentially for the same
kind of stuff.

00:04:48.500 --> 00:04:51.870
Now, replication of these, so
each of these chromosomes in

00:04:51.870 --> 00:04:54.580
this pair replicate.

00:04:54.580 --> 00:04:59.690
So that one from my dad
replicates like this, it

00:04:59.690 --> 00:05:02.690
replicates and it's connected
by a centromere, and the one

00:05:02.690 --> 00:05:08.710
from my mom replicates like
that, and it's connected by a

00:05:08.710 --> 00:05:11.010
centromere like that, and then
the other one does as well.

00:05:11.010 --> 00:05:12.010
That's the shorter one.

00:05:12.010 --> 00:05:14.260
Oh, that's the longer
one, actually.

00:05:14.260 --> 00:05:15.110
That's the longer one.

00:05:15.110 --> 00:05:17.220
I should be a little bit more
explicit in which one's

00:05:17.220 --> 00:05:18.180
shorter and longer.

00:05:18.180 --> 00:05:19.980
The one from my mom does
the same thing.

00:05:19.980 --> 00:05:21.720
This is in the S phase
of interphase.

00:05:21.720 --> 00:05:25.830
We haven't entered the actual
cell division yet.

00:05:25.830 --> 00:05:28.320
And the same thing is true-- and
this is kind of a little

00:05:28.320 --> 00:05:30.830
bit of a sideshow-- of
the centrosomes.

00:05:30.830 --> 00:05:33.380
And we saw in the mitosis video
that these are involved

00:05:33.380 --> 00:05:37.230
in kind of eventually creating
the microtubule structure in

00:05:37.230 --> 00:05:40.560
pulling everything apart, but
you'll have one centrosome

00:05:40.560 --> 00:05:43.450
that's hanging out here, and
then it facilitates its own

00:05:43.450 --> 00:05:46.140
replication, so then you
have two centrosomes.

00:05:46.140 --> 00:05:50.380
So this is all occurring
in the interphase, and

00:05:50.380 --> 00:05:52.490
particularly in the S part
of the interphase,

00:05:52.490 --> 00:05:54.950
not the growth part.

00:05:54.950 --> 00:05:59.040
But once that's happens, we're
ready-- in fact, we're ready

00:05:59.040 --> 00:06:02.405
for either mitosis or meiosis,
but we're going

00:06:02.405 --> 00:06:03.280
to do meiosis now.

00:06:03.280 --> 00:06:04.990
This is a germ cell.

00:06:04.990 --> 00:06:09.120
So what happens is we enter
into prophase I.

00:06:09.120 --> 00:06:11.360
So if you remember, in my-- let
me write this down because

00:06:11.360 --> 00:06:12.550
I think it's important.

00:06:12.550 --> 00:06:22.930
In mitosis you have prophase,
metaphase,

00:06:22.930 --> 00:06:26.720
anaphase and telophase.

00:06:26.720 --> 00:06:28.420
I won't keep writing
phase down.

00:06:28.420 --> 00:06:29.940
PMAT.

00:06:29.940 --> 00:06:33.420
In meiosis, you experience these
in each stage, so you

00:06:33.420 --> 00:06:38.320
have to prophase I, followed
by metaphase I, followed by

00:06:38.320 --> 00:06:41.010
anaphase I, followed
by telophase I.

00:06:41.010 --> 00:06:43.720
Then after you've done meiosis
1, then it all happens again.

00:06:43.720 --> 00:06:47.790
You have prophase II, followed
by metaphase II, followed by

00:06:47.790 --> 00:06:50.490
anaphase II, followed
by telophase.

00:06:50.490 --> 00:06:53.050
So if you really just want to
memorize the names, which you

00:06:53.050 --> 00:06:55.670
unfortunately have to do in
this, especially if you're

00:06:55.670 --> 00:06:57.830
going to get tested on it,
although it's not that

00:06:57.830 --> 00:06:59.990
important to kind of understand
the concept of

00:06:59.990 --> 00:07:02.410
what's happening, you just have
to remember prophase,

00:07:02.410 --> 00:07:04.270
metaphase, anaphase, telophase,
and it'll really

00:07:04.270 --> 00:07:05.150
cover everything.

00:07:05.150 --> 00:07:09.480
You just after memorize in
meiosis, it's happening twice.

00:07:09.480 --> 00:07:11.370
And what's happening is a little
bit different, and

00:07:11.370 --> 00:07:12.930
that's what I really want
to focus on here.

00:07:12.930 --> 00:07:17.260
So let's enter prophase
I of meiosis I.

00:07:17.260 --> 00:07:23.590
So let me call this
prophase I.

00:07:23.590 --> 00:07:25.210
So what's going to happen?

00:07:25.210 --> 00:07:29.250
So just like in prophase and
mitosis, a couple of things

00:07:29.250 --> 00:07:29.830
start happening.

00:07:29.830 --> 00:07:35.725
Your nuclear envelope
starts disappearing.

00:07:42.680 --> 00:07:45.180
The centromeres-- sorry,
not centromeres.

00:07:45.180 --> 00:07:45.750
I'm getting confused now.

00:07:45.750 --> 00:07:46.420
The centrosomes.

00:07:46.420 --> 00:07:49.750
The centromeres are these things
connecting these sister

00:07:49.750 --> 00:07:51.140
chromatids.

00:07:51.140 --> 00:07:55.680
The centrosomes start
facilitating the development

00:07:55.680 --> 00:07:58.490
of the spindles, and they start
pushing apart a little

00:07:58.490 --> 00:08:02.210
bit from the spindles.

00:08:02.210 --> 00:08:05.830
They start pushing apart and
going to opposite sides of the

00:08:05.830 --> 00:08:06.610
chromosomes.

00:08:06.610 --> 00:08:09.280
And this is the really important
thing in prophase I.

00:08:09.280 --> 00:08:10.630
And actually, I'll
make this point.

00:08:10.630 --> 00:08:13.860
Remember, in interphase, even
though I drew it this way,

00:08:13.860 --> 00:08:16.290
they don't exist in this state,
the actual chromosomes.

00:08:16.290 --> 00:08:18.030
They exist more in a
chromatin state.

00:08:18.030 --> 00:08:23.300
So if I were to really draw it,
it would look like this.

00:08:23.300 --> 00:08:27.800
The chromosomes, it would all be
all over the place, and it

00:08:27.800 --> 00:08:29.560
actually would be very difficult
to actually see it

00:08:29.560 --> 00:08:29.990
in a microscope.

00:08:29.990 --> 00:08:34.919
It would just be a big mess of
proteins and of histones,

00:08:34.919 --> 00:08:37.650
which are proteins, and
the actual DNA.

00:08:37.650 --> 00:08:40.140
And that's what's actually
referred to as the chromatin.

00:08:40.140 --> 00:08:44.300
Now, in prophase, that starts to
form into the chromosomes.

00:08:44.300 --> 00:08:46.170
It starts to have a little bit
of structure, and this is

00:08:46.170 --> 00:08:47.710
completely analogous
to what happens

00:08:47.710 --> 00:08:50.420
in prophase in mitosis.

00:08:50.420 --> 00:08:53.000
Now, the one interesting thing
that happens is that the

00:08:53.000 --> 00:08:55.320
homologous pairs line up.

00:08:55.320 --> 00:08:57.420
And actually, I drew it like
that over here and maybe I

00:08:57.420 --> 00:08:59.310
should just cut and paste it.

00:08:59.310 --> 00:09:00.910
Let me just do that.

00:09:00.910 --> 00:09:10.320
If I just cut and paste that,
although I said that the

00:09:10.320 --> 00:09:14.455
nucleus is disappearing, so let
me get rid of the nucleus.

00:09:14.455 --> 00:09:15.350
I already said that.

00:09:15.350 --> 00:09:18.580
The nucleus is slowly
disassembling.

00:09:18.580 --> 00:09:22.120
The proteins are coming apart
during this prophase I.

00:09:22.120 --> 00:09:24.230
I won't draw the whole cell,
because what's interesting

00:09:24.230 --> 00:09:28.730
here is happening at the
nuclear, or what once was the

00:09:28.730 --> 00:09:30.320
nucleus level.

00:09:30.320 --> 00:09:34.070
So the interesting thing here
that's different from mitosis

00:09:34.070 --> 00:09:37.180
is that the homologous pairs
line up next to each other.

00:09:37.180 --> 00:09:41.520
Not only do they line up, but
they can actually share-- they

00:09:41.520 --> 00:09:43.880
can actually have genetic
recombination.

00:09:43.880 --> 00:09:47.700
So you have these points where
analogous-- or I guess you

00:09:47.700 --> 00:09:52.960
could say homologous-- points
on two of these chromosomes

00:09:52.960 --> 00:09:54.240
will cross over each other.

00:09:54.240 --> 00:09:55.660
So let me draw that in detail.

00:09:55.660 --> 00:09:59.610
So let me just focus on maybe
these two right here.

00:09:59.610 --> 00:10:05.950
So I have one chromosome from
my dad, and it's made up of

00:10:05.950 --> 00:10:08.400
two chromatids, so it's already
replicated, but we

00:10:08.400 --> 00:10:10.870
only consider it one chromosome,
and then I have

00:10:10.870 --> 00:10:16.040
one from my mom in green.

00:10:16.040 --> 00:10:18.920
I'm going to draw
it like that.

00:10:18.920 --> 00:10:23.370
One from my mom in green, and
it also has two chromatids.

00:10:23.370 --> 00:10:27.570
Sometimes this is called a
tetrad because it has four

00:10:27.570 --> 00:10:32.130
chromatids in it, but it's
in a pair of homologous

00:10:32.130 --> 00:10:33.500
chromosomes.

00:10:33.500 --> 00:10:34.750
These are the centromeres,
of course.

00:10:34.750 --> 00:10:38.730
What happens is you have
crossing over, and it's a

00:10:38.730 --> 00:10:42.770
surprisingly organized
process.

00:10:42.770 --> 00:10:44.520
When I say organized,
it crosses over at

00:10:44.520 --> 00:10:45.450
a homologous point.

00:10:45.450 --> 00:10:49.450
It crosses over at a point
where, for the most part,

00:10:49.450 --> 00:10:52.350
you're exchanging
similar genes.

00:10:52.350 --> 00:10:54.690
It's not like one is getting two
versions of a gene and the

00:10:54.690 --> 00:10:56.500
other is getting two versions
of another gene.

00:10:56.500 --> 00:10:59.850
You're changing in a way that
both chromosomes are still

00:10:59.850 --> 00:11:01.610
coding for the different genes,
but they're getting

00:11:01.610 --> 00:11:05.670
different versions of those
genes or different alleles,

00:11:05.670 --> 00:11:07.240
which are just versions
of those genes.

00:11:07.240 --> 00:11:12.020
So once this is done, the ones
from my father are now not

00:11:12.020 --> 00:11:13.880
completely from my father,
so it might look

00:11:13.880 --> 00:11:15.140
something like this.

00:11:15.140 --> 00:11:18.460
Let me see, it'll
look like this.

00:11:18.460 --> 00:11:20.860
The one from my father now has
this little bit from my

00:11:20.860 --> 00:11:25.480
mother, and the one from my--
oh, no, my mother's chromosome

00:11:25.480 --> 00:11:29.000
is green-- a little bit from my
mother, and the one from my

00:11:29.000 --> 00:11:33.360
mother has a little bit
from my father.

00:11:33.360 --> 00:11:36.560
And this is really amazing
because it shows you that this

00:11:36.560 --> 00:11:40.230
is so favorable for creating
variation in a population that

00:11:40.230 --> 00:11:44.230
it has really become a formal
part of the meiosis process.

00:11:44.230 --> 00:11:45.500
It happens so frequently.

00:11:45.500 --> 00:11:48.120
This isn't just some random
fluke, and it happens in a

00:11:48.120 --> 00:11:49.490
reasonably organized way.

00:11:49.490 --> 00:11:53.840
It actually happens at a point
where it doesn't kind of

00:11:53.840 --> 00:11:56.120
create junk genes.

00:11:56.120 --> 00:11:59.670
Because you can imagine, this
cut-off point, which is called

00:11:59.670 --> 00:12:02.690
a chiasma, it could have
happened in the middle of some

00:12:02.690 --> 00:12:05.270
gene, and it could have created
some random noise, and

00:12:05.270 --> 00:12:07.980
it could have broken down some
protein development in the

00:12:07.980 --> 00:12:09.010
future or who knows what.

00:12:09.010 --> 00:12:09.965
But it doesn't happen
that way.

00:12:09.965 --> 00:12:12.360
It happens in a reasonably
organized way, which kind of

00:12:12.360 --> 00:12:15.710
speaks to the idea that it's
part of the process.

00:12:15.710 --> 00:12:17.970
So prophase in I, you also
have this happening.

00:12:17.970 --> 00:12:21.210
So once that happens you could
have this guy's got a little

00:12:21.210 --> 00:12:26.810
bit of that chromatid and then
this guy's got a little bit of

00:12:26.810 --> 00:12:28.350
that chromatid.

00:12:28.350 --> 00:12:30.700
So all of this stuff happens
in prophase I.

00:12:30.700 --> 00:12:32.550
You have this crossing over.

00:12:32.550 --> 00:12:36.890
The nuclear envelope starts to
disassemble, and then all of

00:12:36.890 --> 00:12:40.460
these guys align and the
chromatin starts forming into

00:12:40.460 --> 00:12:42.990
these more tightly wound
structures of chromosomes.

00:12:42.990 --> 00:12:45.840
And really, that's all-- when
we talk about even mitosis,

00:12:45.840 --> 00:12:48.000
that's where a lot of the action
really took place.

00:12:48.000 --> 00:12:51.020
Once that happens, then we're
ready to enter into the

00:12:51.020 --> 00:12:54.920
metaphase I, so let's go
down to metaphase I.

00:12:54.920 --> 00:13:00.100
In metaphase I-- let me just
copy and paste what I've

00:13:00.100 --> 00:13:03.710
already done-- the nuclear
envelope is now gone.

00:13:09.740 --> 00:13:12.820
The centrosomes have
gone to opposite

00:13:12.820 --> 00:13:17.130
sides of the cell itself.

00:13:17.130 --> 00:13:20.010
Maybe I should draw the
entire cell now

00:13:20.010 --> 00:13:22.030
that there's no nucleus.

00:13:22.030 --> 00:13:24.090
Let me erase the nucleus
a little bit

00:13:24.090 --> 00:13:26.990
better than I've done.

00:13:26.990 --> 00:13:30.070
Let me erase all of that.

00:13:30.070 --> 00:13:34.460
And, of course, we have the
spindles fibers that have been

00:13:34.460 --> 00:13:39.750
generated by now with the
help of the centrosomes.

00:13:39.750 --> 00:13:41.940
And some of them, as we learned,
this is exactly what

00:13:41.940 --> 00:13:44.310
happened in mitosis.

00:13:44.310 --> 00:13:46.290
They attach to the kinetochores,
which are

00:13:46.290 --> 00:13:52.550
attached to the centromeres
of these chromosomes.

00:13:52.550 --> 00:13:55.970
Now, what's interesting here is
that they each attach-- so

00:13:55.970 --> 00:13:59.510
this guy's going to attach to--
and actually, let me do

00:13:59.510 --> 00:14:00.950
something interesting here.

00:14:00.950 --> 00:14:02.770
Instead of doing it this way,
because I want to show that

00:14:02.770 --> 00:14:05.070
all my dad's chromosomes don't
go to one side and all my

00:14:05.070 --> 00:14:06.760
mom's chromosomes don't
go to the other side.

00:14:06.760 --> 00:14:10.000
So instead of drawing these two
guys like this, let me see

00:14:10.000 --> 00:14:12.310
if I can flip them.

00:14:12.310 --> 00:14:12.880
Let me see.

00:14:12.880 --> 00:14:15.150
Let me just flip them
the other way.

00:14:15.150 --> 00:14:17.560
Whether or not which direction
they're flipped is completely

00:14:17.560 --> 00:14:20.500
random, and that's what
adds to the variation.

00:14:20.500 --> 00:14:23.550
As we said before, sexual
reproduction is key to

00:14:23.550 --> 00:14:25.610
introducing variation
into a population.

00:14:25.610 --> 00:14:28.205
So that's the mom's and
that's the dad's.

00:14:28.205 --> 00:14:28.790
They don't have to.

00:14:28.790 --> 00:14:30.700
All of the ones from my dad
might have ended up on one

00:14:30.700 --> 00:14:33.300
side and all of them from my mom
might end up on one side,

00:14:33.300 --> 00:14:35.490
although when you're talking
about 23 pairs, the

00:14:35.490 --> 00:14:38.720
probability becomes
a lot, lot lower.

00:14:38.720 --> 00:14:41.560
So this is one from my dad.

00:14:41.560 --> 00:14:44.930
Of course, it has some
centromeres.

00:14:44.930 --> 00:14:46.790
Let me draw that there.

00:14:46.790 --> 00:14:50.160
And so these microspindles,
some of them attach to

00:14:50.160 --> 00:14:52.480
kinetochores, which are these
protein structures on the

00:14:52.480 --> 00:14:54.560
centromeres.

00:14:54.560 --> 00:14:57.100
And this is just
like metaphase.

00:14:57.100 --> 00:15:00.050
It's very similar to metaphase
in mitosis.

00:15:00.050 --> 00:15:05.160
This is called metaphase I,
and everything aligns.

00:15:05.160 --> 00:15:08.410
Now we're going to
enter anaphase I.

00:15:08.410 --> 00:15:13.050
Now, anaphase I is interesting,
because remember,

00:15:13.050 --> 00:15:16.710
in mitosis in anaphase, the
actual chromatids, the sister

00:15:16.710 --> 00:15:18.880
chromatids separated
from each other.

00:15:18.880 --> 00:15:22.990
That's not the case in anaphase
I here in meiosis.

00:15:22.990 --> 00:15:28.020
So when we enter anaphase I, you
have just the homologous

00:15:28.020 --> 00:15:30.840
pairs separate, so the
chromatids stay with their

00:15:30.840 --> 00:15:32.630
sister chromatids.

00:15:32.630 --> 00:15:37.130
So on this side, you'll have
these to go there.

00:15:37.130 --> 00:15:42.620
While I have the green out, let
me see if I can draw this

00:15:42.620 --> 00:15:45.070
respectably.

00:15:45.070 --> 00:15:46.780
I have the purple.

00:15:46.780 --> 00:15:49.780
It's a little bit shorter
version here.

00:15:49.780 --> 00:15:53.480
He's got a little bit of
a stub of green there.

00:15:53.480 --> 00:15:56.180
This guy's got little stub
of purple there.

00:15:56.180 --> 00:16:00.000
And then they have this longer
purple chromosome here.

00:16:00.000 --> 00:16:01.510
This is anaphase I.

00:16:01.510 --> 00:16:03.290
They're being pulled apart,
but they're being pulled

00:16:03.290 --> 00:16:07.250
apart-- the homologous pair is
being pulled apart, not the

00:16:07.250 --> 00:16:10.610
actual chromosomes, not
the chromatids.

00:16:10.610 --> 00:16:13.230
So let me just draw this.

00:16:13.230 --> 00:16:14.900
So then you have your
microtubules.

00:16:14.900 --> 00:16:16.060
Some are connected to
these kinetochores.

00:16:16.060 --> 00:16:17.310
You have your centromeres.

00:16:19.780 --> 00:16:21.640
Of course, all of this is
occurring within the cell and

00:16:21.640 --> 00:16:23.570
these are getting
pulled apart.

00:16:23.570 --> 00:16:28.090
So it's analogous to anaphase
in mitosis, but the key

00:16:28.090 --> 00:16:31.830
difference is you're pulling
apart homologous pairs.

00:16:31.830 --> 00:16:34.070
You're not actually splitting
the chromosomes into their

00:16:34.070 --> 00:16:37.240
constituent chromatids,
and that's key.

00:16:37.240 --> 00:16:40.300
And if you forget that, you can
review the mitosis video.

00:16:40.300 --> 00:16:41.840
So this is anaphase I.

00:16:48.400 --> 00:16:51.060
And then as you could imagine,
telophase I is essentially

00:16:51.060 --> 00:16:57.990
once these guys are at their
respective ends of the cell--

00:16:57.990 --> 00:17:01.190
it's getting tiring redrawing
all of these, but I guess it

00:17:01.190 --> 00:17:05.050
gives you time to let
it all sink in.

00:17:05.050 --> 00:17:09.240
So these guys are now at the
left end of the cell and these

00:17:09.240 --> 00:17:15.490
guys are now at the right
end of the cell.

00:17:15.490 --> 00:17:18.099
Now, the microtubules start
to disassemble.

00:17:18.099 --> 00:17:21.109
So maybe they're there a
little bit, but they're

00:17:21.109 --> 00:17:21.800
disassembling.

00:17:21.800 --> 00:17:24.170
You still have your centromeres
here and they're

00:17:24.170 --> 00:17:25.380
at opposite poles.

00:17:25.380 --> 00:17:27.339
And to some degree, in the
early part of telophase,

00:17:27.339 --> 00:17:29.870
they're still pushing the cell
apart, and at the same time,

00:17:29.870 --> 00:17:32.510
you have cytokinesis
happening.

00:17:32.510 --> 00:17:38.030
So by the end of telophase I,
you have the actual cytoplasm

00:17:38.030 --> 00:17:42.480
splitting during telophase right
there, and the nuclear

00:17:42.480 --> 00:17:44.810
envelope is forming.

00:17:44.810 --> 00:17:46.640
You can almost view it as the
opposite of prophase.

00:17:50.020 --> 00:17:52.300
The nuclear envelope is forming,
and by the end of

00:17:52.300 --> 00:17:55.530
telophase I, it will have
completely divided.

00:17:55.530 --> 00:17:58.030
So this is telophase I.

00:17:58.030 --> 00:18:02.710
Now, notice: we started off with
a diploid cell, right?

00:18:02.710 --> 00:18:07.610
It had two pairs of homologous
chromosome, but it had four

00:18:07.610 --> 00:18:09.310
chromosomes.

00:18:09.310 --> 00:18:13.330
Now, each cell only has
two chromosomes.

00:18:13.330 --> 00:18:18.790
Essentially, each cell got one
of the pair of each of those

00:18:18.790 --> 00:18:20.960
homologous pairs, but it was
done randomly, and that's

00:18:20.960 --> 00:18:23.690
where a lot of the variation
is introduced.

00:18:23.690 --> 00:18:26.680
Now, once we're at this stage,
each of these cells now

00:18:26.680 --> 00:18:29.320
undergo meiosis II,
which is actually

00:18:29.320 --> 00:18:32.540
very similar to mitosis.

00:18:32.540 --> 00:18:34.550
And sometimes, there's actually
an in-between stage

00:18:34.550 --> 00:18:37.500
called interphase II, where
the cell kind of rests and

00:18:37.500 --> 00:18:40.790
whatever else, and actually the
centromeres now have to

00:18:40.790 --> 00:18:41.570
duplicate again.

00:18:41.570 --> 00:18:44.610
So these two cells-- I've drawn
them separately-- let's

00:18:44.610 --> 00:18:46.100
see what happens next.

00:18:46.100 --> 00:18:48.420
So let's say that the
centromere-- actually, I

00:18:48.420 --> 00:18:49.820
shouldn't have drawn the
centromere inside the

00:18:49.820 --> 00:18:50.740
nucleus like that.

00:18:50.740 --> 00:18:55.360
The centromere's going to be
outside the nucleus, outside

00:18:55.360 --> 00:19:00.880
of the newly formed nucleus
there and there.

00:19:00.880 --> 00:19:04.320
And then it'll actually
replicate itself at

00:19:04.320 --> 00:19:06.450
this point as well.

00:19:06.450 --> 00:19:07.700
So now we have two cells.

00:19:10.470 --> 00:19:17.900
Let me just cut and paste what
I have. I have this one, this

00:19:17.900 --> 00:19:20.380
chromosome right here.

00:19:20.380 --> 00:19:22.650
It's got this little green stub
there and then I have

00:19:22.650 --> 00:19:25.630
this longer fully green
chromosome there.

00:19:25.630 --> 00:19:32.360
Now, this guy, he's got this
little purple stub here.

00:19:32.360 --> 00:19:35.770
Let me draw this whole purple
chromosome there.

00:19:35.770 --> 00:19:39.510
Then this guy has one chromatid
like that and one

00:19:39.510 --> 00:19:41.370
chromatid like that.

00:19:41.370 --> 00:19:47.060
Now, when we enter prophase
II, what do you

00:19:47.060 --> 00:19:48.610
think is going to happen?

00:19:48.610 --> 00:19:53.830
Well, just like before, you
have your nuclear envelope

00:19:53.830 --> 00:19:56.090
that formed in telophase I.

00:19:56.090 --> 00:19:57.830
It's kind of a temporary
thing.

00:19:57.830 --> 00:19:59.470
It starts to disintegrate
again.

00:20:06.330 --> 00:20:10.520
And then you have your
centromeres.

00:20:10.520 --> 00:20:13.550
They start pushing apart so
now I had two centromeres.

00:20:13.550 --> 00:20:17.290
They replicated, and now they
start pushing apart while they

00:20:17.290 --> 00:20:19.260
generate their little
spindles.

00:20:19.260 --> 00:20:20.840
They push apart in opposite
directions.

00:20:20.840 --> 00:20:23.530
Now, this is happening in
two cells, of course.

00:20:23.530 --> 00:20:26.290
They go in opposite directions
while they generate their

00:20:26.290 --> 00:20:27.020
spindle fibers.

00:20:27.020 --> 00:20:29.310
And let me make it very clear
that this is two cells we're

00:20:29.310 --> 00:20:30.680
talking about.

00:20:30.680 --> 00:20:35.180
That's one of them and that's
the second of them.

00:20:35.180 --> 00:20:42.860
Now it's going to enter
metaphase II, which is

00:20:42.860 --> 00:20:45.920
analogous to metaphase I, or
metaphase in mitosis, where

00:20:45.920 --> 00:20:48.050
the chromosomes get lined up.

00:20:48.050 --> 00:20:49.050
Let me draw it this way.

00:20:49.050 --> 00:20:52.480
So now the centromeres, they've
migrated to the two

00:20:52.480 --> 00:20:54.810
poles of the cell.

00:20:54.810 --> 00:20:57.770
So those are my centromeres.

00:20:57.770 --> 00:21:00.085
I have all of my spindles
fibers.

00:21:05.230 --> 00:21:06.630
Oh, sorry, did I call
those centromeres?

00:21:06.630 --> 00:21:07.880
The centrosomes.

00:21:10.190 --> 00:21:11.655
I don't know how long I've been
calling them centromeres.

00:21:11.655 --> 00:21:14.830
These are centrosomes, and my
brain keeps confusing them.

00:21:14.830 --> 00:21:17.360
The centromeres, and maybe
this'll help you not do what I

00:21:17.360 --> 00:21:20.520
just did, the centromeres are
the things that are connecting

00:21:20.520 --> 00:21:22.090
the two sister chromatids.

00:21:22.090 --> 00:21:23.070
Those are centromeres.

00:21:23.070 --> 00:21:26.810
Centrosomes are the things that
are pushing back the--

00:21:26.810 --> 00:21:29.450
that generate the
spindle fibers.

00:21:29.450 --> 00:21:31.500
The chromosomes line up
during metaphase.

00:21:31.500 --> 00:21:34.200
Metaphase always involves the
lining up of chromosomes so

00:21:34.200 --> 00:21:35.790
that one-- let me
just draw it.

00:21:35.790 --> 00:21:38.790
So I have that and that.

00:21:38.790 --> 00:21:41.820
This one's got a purple
guy, too.

00:21:41.820 --> 00:21:44.920
This guy's got a purple guy,
a long purple guy, and then

00:21:44.920 --> 00:21:46.880
there's a little stub
for the other guy.

00:21:46.880 --> 00:21:51.410
This guy's got a long green
guy and this guy's got a

00:21:51.410 --> 00:21:56.540
little green stub, and then this
is the short green guy

00:21:56.540 --> 00:21:57.220
right there.

00:21:57.220 --> 00:21:58.650
And, of course, they're
being aligned.

00:21:58.650 --> 00:22:01.270
Some of these spindle fibers
have been attached to the

00:22:01.270 --> 00:22:03.360
centromeres or the kinetochores
that are on the

00:22:03.360 --> 00:22:08.460
centromeres that connect these
two chromatids, these sister

00:22:08.460 --> 00:22:09.650
chromatids.

00:22:09.650 --> 00:22:11.820
And, of course, we don't have
a nuclear membrane anymore,

00:22:11.820 --> 00:22:14.830
and these are, of course,
two separate cells.

00:22:14.830 --> 00:22:17.740
And then you can guess what
happens in anaphase II.

00:22:17.740 --> 00:22:19.970
It's just like anaphase
in mitosis.

00:22:24.210 --> 00:22:27.340
These things get pulled apart
by the kinetochore

00:22:27.340 --> 00:22:30.190
microtubules, while the other
microtubules keep growing and

00:22:30.190 --> 00:22:33.140
push and these two things
further apart.

00:22:33.140 --> 00:22:34.480
So let me show that.

00:22:34.480 --> 00:22:36.530
And they the key here: this
is the difference between

00:22:36.530 --> 00:22:37.970
anaphase II and anaphase I.

00:22:37.970 --> 00:22:42.960
Anaphase I, the homologous pairs
were broken up, but the

00:22:42.960 --> 00:22:45.290
chromosomes themselves
were not.

00:22:45.290 --> 00:22:48.120
Now, in anaphase II, we don't
have homologous pairs.

00:22:48.120 --> 00:22:51.280
We just have chromatid pairs,
sister chromatids.

00:22:51.280 --> 00:22:54.910
Now, those are separated,
which is very similar to

00:22:54.910 --> 00:22:57.080
anaphase in mitosis.

00:22:57.080 --> 00:23:00.200
So now, this guy gets pulled in
that direction so it look

00:23:00.200 --> 00:23:01.940
something like this.

00:23:01.940 --> 00:23:04.830
The drawing here is the hardest
part of this video.

00:23:04.830 --> 00:23:06.200
So that guy gets pulled there.

00:23:06.200 --> 00:23:08.570
That guy's getting pulled
in that direction.

00:23:08.570 --> 00:23:10.840
He's got that little
green stub on him.

00:23:10.840 --> 00:23:13.470
And then you have one green
guy getting pulled in that

00:23:13.470 --> 00:23:16.210
direction with the longer
chromosome.

00:23:16.210 --> 00:23:18.220
And then one of the other longer
is getting pulled in

00:23:18.220 --> 00:23:21.190
that direction, and it's all
by these microtubules

00:23:21.190 --> 00:23:25.670
connected at the kinetochore
structures by a centrosome as

00:23:25.670 --> 00:23:28.850
kind of the coordinating body.

00:23:28.850 --> 00:23:30.350
It's all being pulled apart.

00:23:30.350 --> 00:23:32.850
Anaphase has always involved
the pulling apart of the

00:23:32.850 --> 00:23:36.440
chromosomes or pulling
apart of something.

00:23:36.440 --> 00:23:37.300
Let me put it that way.

00:23:37.300 --> 00:23:41.110
And it's happening on this
side of the cell as well.

00:23:41.110 --> 00:23:43.830
Of course, this is
all one cell.

00:23:43.830 --> 00:23:49.480
And just like in mitosis, as
soon as the sister chromatids

00:23:49.480 --> 00:23:52.450
are split apart, they are now
referred to as chromosomes, or

00:23:52.450 --> 00:23:53.910
sister chromosomes.

00:23:53.910 --> 00:23:55.530
And, of course, this
is happening twice.

00:23:55.530 --> 00:23:59.690
This is also happening
in the other cell.

00:23:59.690 --> 00:24:01.010
The other cell's a little
bit cleaner.

00:24:01.010 --> 00:24:03.730
It didn't have that
crossover occur.

00:24:03.730 --> 00:24:08.900
So you have the longer
purple one.

00:24:08.900 --> 00:24:12.600
He gets split up into two
chromatids, which we are now

00:24:12.600 --> 00:24:14.820
calling chromosomes, or
sister chromosomes.

00:24:14.820 --> 00:24:19.530
And then this guy up here, he
gets split up into this short

00:24:19.530 --> 00:24:23.270
green, and then there's a-- let
me do it this way-- this

00:24:23.270 --> 00:24:24.820
short green, and he's
got a little purple

00:24:24.820 --> 00:24:26.490
stub on it right there.

00:24:26.490 --> 00:24:29.090
And, of course, it's all being
pulled away by the same idea,

00:24:29.090 --> 00:24:30.885
by the centrosomes.

00:24:30.885 --> 00:24:33.700
I want to make sure I
get that word right.

00:24:33.700 --> 00:24:36.320
I'm afraid whether I used
centromeres for the whole

00:24:36.320 --> 00:24:39.110
first part of the video, but
hopefully, my confusion will

00:24:39.110 --> 00:24:42.540
help you from getting confused
because you'll realize that

00:24:42.540 --> 00:24:44.590
it's a pitfall to fall into.

00:24:44.590 --> 00:24:45.380
So that's anaphase.

00:24:45.380 --> 00:24:46.790
Everything is getting
pulled apart.

00:24:46.790 --> 00:24:49.810
And then you can imagine
what telophase II is.

00:24:49.810 --> 00:24:51.040
In fact, I won't
even redraw it.

00:24:51.040 --> 00:24:55.010
Telophase II, these things get
pulled apart even more, so

00:24:55.010 --> 00:24:59.170
this is telophase II.

00:24:59.170 --> 00:25:00.420
They get pulled apart
even more.

00:25:00.420 --> 00:25:02.180
The cell elongates.

00:25:02.180 --> 00:25:09.180
You start having this cleavage
occur right here.

00:25:09.180 --> 00:25:11.350
So at the same time that in
telophase II these get pulled

00:25:11.350 --> 00:25:13.560
part, you have the
cytokinesis.

00:25:13.560 --> 00:25:15.440
The tubules start disintegrating
and then you

00:25:15.440 --> 00:25:19.880
have a nucleus that forms
around these.

00:25:19.880 --> 00:25:22.720
So what is the end result
of all of these?

00:25:22.720 --> 00:25:29.280
Well, that guy's going to turn
into a nucleus that has this

00:25:29.280 --> 00:25:33.590
purple dude with a little green
stub, and then a long

00:25:33.590 --> 00:25:38.460
green guy, and then he's got
his nuclear membrane.

00:25:38.460 --> 00:25:41.620
And, of course, there's the
entire cytoplasm in the rest

00:25:41.620 --> 00:25:43.300
of the cell.

00:25:43.300 --> 00:25:46.280
The other person that was his
kind of partner in this

00:25:46.280 --> 00:25:50.180
meiosis II, he's going
to have a short

00:25:50.180 --> 00:25:55.980
purple and a long green.

00:25:55.980 --> 00:26:00.610
He has a nuclear membrane,
and, of course, it has

00:26:00.610 --> 00:26:03.340
cytoplasm around it.

00:26:03.340 --> 00:26:04.730
And then on this side, you have

00:26:04.730 --> 00:26:05.955
something similar happening.

00:26:05.955 --> 00:26:10.300
You see this first guy, this
first one right here has two

00:26:10.300 --> 00:26:11.840
long purple ones.

00:26:11.840 --> 00:26:13.080
They get separated.

00:26:13.080 --> 00:26:15.880
So let me see, you have one long
purple in that cell and

00:26:15.880 --> 00:26:19.670
you have another long
purple in this cell.

00:26:19.670 --> 00:26:22.780
In that top one, you have a
short green one, and in this

00:26:22.780 --> 00:26:25.260
bottom, you have a short green
one that had got a little bit

00:26:25.260 --> 00:26:28.960
of one of my dad's-- a
homologous part of one of my

00:26:28.960 --> 00:26:30.520
dad's chromosomes on it.

00:26:30.520 --> 00:26:33.740
And, of course, these also
have nuclear membranes,

00:26:33.740 --> 00:26:37.630
nuclear membranes, and, of
course, it has a cytoplasm in

00:26:37.630 --> 00:26:39.900
the rest of the cell, which
we'll learn more about all

00:26:39.900 --> 00:26:41.110
those other things.

00:26:41.110 --> 00:26:44.900
So what we see here is that we
went from a diploid starting

00:26:44.900 --> 00:26:46.530
way-- where did we start?

00:26:46.530 --> 00:26:50.890
We started up here with a
diploid germ cell, and we went

00:26:50.890 --> 00:26:52.820
through two stages
of division.

00:26:52.820 --> 00:26:56.220
The first stage split up
homologous pairs, but it

00:26:56.220 --> 00:26:58.590
started over with that crossing
over, that genetic

00:26:58.590 --> 00:27:01.380
combination, which is a key
feature of meiosis, which adds

00:27:01.380 --> 00:27:05.620
a lot a variation to a species
or to a gene pool.

00:27:05.620 --> 00:27:08.080
And then the second phase
separated the sister

00:27:08.080 --> 00:27:11.950
chromatids, just like what
happens in mitosis.

00:27:11.950 --> 00:27:15.980
And we end up with four haploid
cells because they

00:27:15.980 --> 00:27:18.750
have half the contingency of
chromosomes, and these are

00:27:18.750 --> 00:27:20.000
called gametes.


Phases of Mitosis

WEBVTT
Kind: captions
Language: en

00:00:00.500 --> 00:00:03.940
In its most popular sense,
when people talk about

00:00:03.940 --> 00:00:08.310
mitosis, they're referring to
a cell, a diploid cell.

00:00:08.310 --> 00:00:10.670
So diploid just means it has
its full complement of

00:00:10.670 --> 00:00:13.780
chromosomes, so it has
2N chromosomes.

00:00:13.780 --> 00:00:15.950
So that's the nucleus.

00:00:15.950 --> 00:00:17.410
This is the whole cell.

00:00:17.410 --> 00:00:19.960
And so most people are saying,
look, the cell itself

00:00:19.960 --> 00:00:23.590
replicates into two diploid
cells, so it turns into two

00:00:23.590 --> 00:00:29.710
cells, each that have a full
complement of chromosomes, 2N

00:00:29.710 --> 00:00:30.640
chromosomes.

00:00:30.640 --> 00:00:33.400
And so when people say a cell
has experienced mitosis, they

00:00:33.400 --> 00:00:34.580
normally mean this.

00:00:34.580 --> 00:00:37.850
But I want to make one slight
clarification, that formally,

00:00:37.850 --> 00:00:42.120
mitosis only refers to the
process of the replication of

00:00:42.120 --> 00:00:45.350
the genetic material
and the nucleus.

00:00:45.350 --> 00:00:50.180
So, for example, if I were to
draw this-- let me draw the

00:00:50.180 --> 00:00:54.300
cell-- and it has now two
nucleuses, each with the

00:00:54.300 --> 00:00:57.770
diploid number of chromosomes,
this cell

00:00:57.770 --> 00:00:59.040
has experienced mitosis.

00:01:01.870 --> 00:01:05.030
It has not experienced
cytokinesis, which we will

00:01:05.030 --> 00:01:07.730
talk about in a few moments, but
that's the process of the

00:01:07.730 --> 00:01:12.430
actual cytoplasm of the
cell being split into

00:01:12.430 --> 00:01:13.490
two different cells.

00:01:13.490 --> 00:01:17.160
And just as a clarity, the
cytoplasm is all the stuff

00:01:17.160 --> 00:01:21.630
outside of the nucleus.

00:01:21.630 --> 00:01:23.590
So I'll talk about that in a
second, but just know in

00:01:23.590 --> 00:01:25.680
everyday usage, this is normally
the case when people

00:01:25.680 --> 00:01:26.670
talk about mitosis.

00:01:26.670 --> 00:01:28.860
But if you've got a teacher
that likes to get you on a

00:01:28.860 --> 00:01:31.730
technicality, this is
technically what mitosis is.

00:01:31.730 --> 00:01:35.030
It's the splitting of the
nucleus or the replication of

00:01:35.030 --> 00:01:37.460
the nucleus into two
separate nucleuses.

00:01:37.460 --> 00:01:42.820
That's normally accompanied
by cytokinesis where the

00:01:42.820 --> 00:01:45.930
cytoplasms of the cells
actually separate.

00:01:45.930 --> 00:01:51.250
Now, with that said, let's go
into the mechanics of mitosis.

00:01:51.250 --> 00:01:55.150
So the first steps that are
really necessary for mitosis

00:01:55.150 --> 00:01:58.920
actually occur outside of
mitosis when the cell is just

00:01:58.920 --> 00:02:02.883
doing its day-to-day life, and
that's during the interphase.

00:02:07.570 --> 00:02:10.449
And the interphase, literally
it's not a phase of mitosis.

00:02:10.449 --> 00:02:14.810
It's literally when the
cell is just living.

00:02:14.810 --> 00:02:17.730
Let's say we have
some new cell.

00:02:17.730 --> 00:02:20.460
Let me do it in green.

00:02:20.460 --> 00:02:22.430
That's a new cell here.

00:02:22.430 --> 00:02:24.780
Maybe this is its nucleus.

00:02:24.780 --> 00:02:29.630
It's got 2N chromosomes,
and then it grows.

00:02:29.630 --> 00:02:32.790
It brings in nutrients from
the outside and builds

00:02:32.790 --> 00:02:37.660
proteins and does whatever,
and so it grows a bit.

00:02:37.660 --> 00:02:42.610
It's obviously got its full
chromosomal complement still.

00:02:42.610 --> 00:02:46.080
And then at some point during
this life cycle, and I'll

00:02:46.080 --> 00:02:49.220
label these actually, so this
phase in interphase, and this

00:02:49.220 --> 00:02:52.460
might not even be covered in
some biology classes, but they

00:02:52.460 --> 00:02:53.070
give it a label.

00:02:53.070 --> 00:02:56.650
They call it G1, which
is really just

00:02:56.650 --> 00:02:58.380
when the cell is growing.

00:02:58.380 --> 00:03:00.840
It's just growing, accumulating
materials and

00:03:00.840 --> 00:03:05.990
building itself out, and then
it actually replicates its

00:03:05.990 --> 00:03:06.770
chromosomes.

00:03:06.770 --> 00:03:09.760
So you still have a diploid
number of chromosomes.

00:03:09.760 --> 00:03:11.390
So let me zoom in.

00:03:11.390 --> 00:03:12.160
So let me draw this.

00:03:12.160 --> 00:03:16.490
This is called the S phase of
interphase, so this is S.

00:03:16.490 --> 00:03:19.190
And S is where you have
replication of the actual

00:03:19.190 --> 00:03:19.430
chromosomes.

00:03:19.430 --> 00:03:22.340
Once again, we're not
even in mitosis yet.

00:03:22.340 --> 00:03:28.910
So S, you have replication
of your chromosomes.

00:03:28.910 --> 00:03:34.770
So if I were to zoom in on the
nucleus during the S phase, if

00:03:34.770 --> 00:03:38.230
I were to start off-- let me
just start with some organism

00:03:38.230 --> 00:03:40.940
that has two chromosomes.

00:03:40.940 --> 00:03:45.210
So let's say that at the
beginning of S phase, and I'll

00:03:45.210 --> 00:03:48.060
draw things as chromosomes just
to make it clear that

00:03:48.060 --> 00:03:49.300
things are being replicated.

00:03:49.300 --> 00:03:54.010
So let me say it has this
chromosome right here and then

00:03:54.010 --> 00:03:57.400
let's say it has this chromosome
right here.

00:03:57.400 --> 00:04:01.180
As it goes through S phase,
these chromosomes replicate.

00:04:01.180 --> 00:04:02.430
And I'm just drawing
the nucleus here.

00:04:02.430 --> 00:04:05.740
I've zoomed in on just this part
right here, where N is 1,

00:04:05.740 --> 00:04:10.050
where our full diploid
complement is two chomosomes.

00:04:10.050 --> 00:04:16.010
During S phase, our chromosomes
will replicate and

00:04:16.010 --> 00:04:21.269
will have-- so that green one
will completely replicate and

00:04:21.269 --> 00:04:24.220
generate a copy of itself, and
we've learned this a little

00:04:24.220 --> 00:04:26.380
bit, they're connected
at the centromere.

00:04:26.380 --> 00:04:30.360
Now, each of those copies are
called chromatids, and that

00:04:30.360 --> 00:04:33.350
magenta one will do
the same thing.

00:04:33.350 --> 00:04:38.260
Even though we have two
chromatids, one for each

00:04:38.260 --> 00:04:41.170
chromosome, now we have four
chromatids, two for each

00:04:41.170 --> 00:04:45.120
chromosome, we still say we
only have two chromosomes.

00:04:45.120 --> 00:04:47.000
That's its centromere
right there.

00:04:47.000 --> 00:04:52.410
This occurs in the S phase, and
then the cell will just

00:04:52.410 --> 00:04:53.710
continue to grow more.

00:04:53.710 --> 00:04:59.160
So the cell was already big--
I'll focus on the cell again.

00:04:59.160 --> 00:05:02.720
The cell was already big
and it gets bigger.

00:05:02.720 --> 00:05:05.930
It gets bigger, and that's
during the G2 phase, so it's

00:05:05.930 --> 00:05:09.380
just growing more.

00:05:09.380 --> 00:05:12.400
Now, there's another little part
of the cell we haven't

00:05:12.400 --> 00:05:13.900
even talked about yet,
but I'll talk

00:05:13.900 --> 00:05:14.800
about it a little bit.

00:05:14.800 --> 00:05:17.580
It's not super-duper important,
but it's the idea

00:05:17.580 --> 00:05:19.010
of these centrosomes.

00:05:19.010 --> 00:05:21.530
These are going to be very
important later on when the

00:05:21.530 --> 00:05:25.170
cell is actually dividing,
and those also duplicate.

00:05:25.170 --> 00:05:27.270
So let's say I have a little
centrosome here.

00:05:29.860 --> 00:05:31.940
It has centrioles inside it.

00:05:31.940 --> 00:05:33.820
You don't have to worry too much
about that, but they're

00:05:33.820 --> 00:05:36.370
these little cylindrical-looking
things.

00:05:36.370 --> 00:05:38.560
But I just want to-- so you
don't get confused if you see

00:05:38.560 --> 00:05:41.710
the word centriole and
centrosomes, not to be

00:05:41.710 --> 00:05:44.690
confused with centromeres, which
are these little points

00:05:44.690 --> 00:05:46.760
where the two chromatids
attach.

00:05:46.760 --> 00:05:51.640
Unfortunately, they named many
things in this process very

00:05:51.640 --> 00:05:53.520
similarly, or a lot
of the parts

00:05:53.520 --> 00:05:55.590
of a cell very similarly.

00:05:55.590 --> 00:05:57.320
But you have these things called
centrosomes that are

00:05:57.320 --> 00:05:59.720
going to enter the picture very
soon, that are sitting

00:05:59.720 --> 00:06:05.260
outside of the nucleus, and
they also replicate.

00:06:05.260 --> 00:06:08.280
They also replicate during
the interphase.

00:06:08.280 --> 00:06:11.100
So you had one before, now
you have two of them.

00:06:11.100 --> 00:06:13.530
And, of course, they each have
their two little centrioles

00:06:13.530 --> 00:06:15.930
inside, but we're not going
to focus too much

00:06:15.930 --> 00:06:17.020
on those just yet.

00:06:17.020 --> 00:06:20.460
So that's what happened
in the interphase.

00:06:20.460 --> 00:06:23.280
This is most of the cell's life,
and it's kind of growing

00:06:23.280 --> 00:06:24.420
and doing what it wants.

00:06:24.420 --> 00:06:26.200
Actually, I'll make a
slight point here.

00:06:26.200 --> 00:06:29.310
When I drew the DNA here, I
drew them as chromosomes.

00:06:29.310 --> 00:06:32.140
But the reality is when we're
sitting in the interphase,

00:06:32.140 --> 00:06:34.760
this is not what the DNA would
actually look like.

00:06:34.760 --> 00:06:37.930
The DNA, if I were to actually
draw this, it's in its

00:06:37.930 --> 00:06:40.580
chromatin form.

00:06:40.580 --> 00:06:43.670
It's not all tightly wound
like I drew it here.

00:06:43.670 --> 00:06:46.050
I drew it tightly wound so that
you can see that it got

00:06:46.050 --> 00:06:51.920
replicated, but the reality is
that that green chromosome

00:06:51.920 --> 00:06:53.980
would actually be all unwound,
and if you were looking in a

00:06:53.980 --> 00:06:56.330
microscope, you would even
have trouble seeing it.

00:06:56.330 --> 00:06:59.470
This is its chromatin form.

00:06:59.470 --> 00:07:02.370
We'll talk a little bit about
where it actually organizes

00:07:02.370 --> 00:07:06.050
itself back into a chromosome,
but in its chromatin form,

00:07:06.050 --> 00:07:09.270
it's just a bunch of DNA and
proteins that the DNA is

00:07:09.270 --> 00:07:11.610
wrapped around a little bit,
so you might have some

00:07:11.610 --> 00:07:14.990
proteins here that the DNA is
wrapped around a little bit.

00:07:14.990 --> 00:07:16.560
But if you're looking at it in
a microscope, it just looks

00:07:16.560 --> 00:07:19.210
like a big blur of
DNA and proteins.

00:07:19.210 --> 00:07:22.090
Same thing for the
magenta molecule.

00:07:22.090 --> 00:07:24.010
Really, for DNA to
do anything, it

00:07:24.010 --> 00:07:25.080
has to be like this.

00:07:25.080 --> 00:07:29.150
It has to be open to its
environment in order for the

00:07:29.150 --> 00:07:33.960
mRNA and the different types of
helper proteins to really

00:07:33.960 --> 00:07:34.790
be able to function with it.

00:07:34.790 --> 00:07:37.260
And even for it to be able to
replicate, it has to be

00:07:37.260 --> 00:07:39.360
unwound like this in order
for it to function.

00:07:39.360 --> 00:07:41.600
It only gets tightly wound
like this later on.

00:07:41.600 --> 00:07:44.630
I just drew it like this, so
really it had one green one,

00:07:44.630 --> 00:07:47.080
and it's going to replicate to
form another green one, and

00:07:47.080 --> 00:07:49.010
they're going to be attached
at some point.

00:07:49.010 --> 00:07:51.260
That magenta one is going to
replicate to form another

00:07:51.260 --> 00:07:53.440
magenta one, and they'll be
attached at some point, but

00:07:53.440 --> 00:07:54.680
it's not going to be clear.

00:07:54.680 --> 00:07:57.060
I just drew it this way to show
that it really happened.

00:07:57.060 --> 00:07:58.320
This is the reality.

00:07:58.320 --> 00:07:59.870
It's in its chromatin form.

00:08:03.070 --> 00:08:06.930
Now, we're ready for mitosis.

00:08:06.930 --> 00:08:09.060
So the first stage
of mitosis is

00:08:09.060 --> 00:08:12.420
essentially-- let me draw this.

00:08:12.420 --> 00:08:17.200
So I'll draw the
cell in green.

00:08:17.200 --> 00:08:20.790
I'm going to draw the nucleus a
lot bigger than it normally

00:08:20.790 --> 00:08:23.470
is relative to the cell just
because, at least right now, a

00:08:23.470 --> 00:08:25.820
lot of the action is going
in the nucleus.

00:08:25.820 --> 00:08:28.890
So the first stage of mitosis
is the prophase.

00:08:37.559 --> 00:08:40.809
These are somewhat arbitrary
names that were assigned.

00:08:40.809 --> 00:08:41.890
People looked in a microscope.

00:08:41.890 --> 00:08:45.380
Oh, that's a certain type of
step that we always see when a

00:08:45.380 --> 00:08:48.100
nucleus is dividing so we'll
call this the prophase.

00:08:48.100 --> 00:08:55.210
What happens in the prophase is
that the actual chromatin

00:08:55.210 --> 00:08:58.050
starts actually turning into
this type of form.

00:08:58.050 --> 00:09:02.330
So as I just said, when we're
in the interphase, the DNA's

00:09:02.330 --> 00:09:04.750
in this form where it's all
separated and unwound.

00:09:04.750 --> 00:09:08.220
It actually starts to wind
together, so this is where

00:09:08.220 --> 00:09:09.940
you'll actually have-- and
remember, it's already

00:09:09.940 --> 00:09:10.480
replicated.

00:09:10.480 --> 00:09:13.990
The replication happened
before mitosis begins.

00:09:13.990 --> 00:09:16.840
So I had that one chromosome
there, and then I

00:09:16.840 --> 00:09:18.400
have another one here.

00:09:18.400 --> 00:09:21.740
It has two sister chromatids
that we'll see

00:09:21.740 --> 00:09:24.270
soon get pulled apart.

00:09:24.270 --> 00:09:30.000
Now, during prophase, you
also start to have these

00:09:30.000 --> 00:09:35.060
centromeres appear that I
was touching on before.

00:09:35.060 --> 00:09:40.380
These guys over here, they
start to facilitate the

00:09:40.380 --> 00:09:44.060
generation of what you call
microtubules, and you can kind

00:09:44.060 --> 00:09:46.770
of view these as these sticks or
these ropes that are going

00:09:46.770 --> 00:09:50.890
to be key in moving things
around as we divide the cell.

00:09:50.890 --> 00:09:52.160
All of this is pretty amazing.

00:09:52.160 --> 00:09:54.630
I mean, you think of a cell, you
think of something that's

00:09:54.630 --> 00:09:56.090
inherently pretty simple.

00:09:56.090 --> 00:10:01.940
It's the most basic living
structure in us or in life.

00:10:01.940 --> 00:10:05.750
But even here, you have these
complex mechanics going on,

00:10:05.750 --> 00:10:07.360
and a lot of it still
isn't understood.

00:10:07.360 --> 00:10:09.820
I mean, we can observe it, but
we really don't know what's

00:10:09.820 --> 00:10:14.140
happening at the atomic level
or at the protein level that

00:10:14.140 --> 00:10:17.720
allows these things to move
around in such a nicely

00:10:17.720 --> 00:10:18.760
choreographed way.

00:10:18.760 --> 00:10:21.480
It's still an area
of research.

00:10:21.480 --> 00:10:23.520
Some of this is understood,
some of it isn't.

00:10:23.520 --> 00:10:26.910
But you have these two
centrosomes, and they

00:10:26.910 --> 00:10:30.970
facilitate the development of
these microtubules, which are

00:10:30.970 --> 00:10:32.740
literally like these little
microstructures.

00:10:32.740 --> 00:10:40.310
You can view them as tubes
or as some type of rope.

00:10:40.310 --> 00:10:44.230
Now as prophase progresses, it
eventually gets to the point

00:10:44.230 --> 00:10:45.720
where-- let me do it.

00:10:45.720 --> 00:10:47.955
I don't want this word
replication written here.

00:10:47.955 --> 00:10:49.030
It makes it confusing.

00:10:49.030 --> 00:10:49.950
Let me delete that.

00:10:49.950 --> 00:10:51.775
Let me get rid of this
replication.

00:10:54.460 --> 00:10:58.690
So as prophase progresses, the
nuclear envelope actually

00:10:58.690 --> 00:10:59.430
disappears.

00:10:59.430 --> 00:11:01.435
So let me redraw this.

00:11:01.435 --> 00:11:03.855
Let me copy and paste what
I've done before.

00:11:06.820 --> 00:11:08.536
Put it there.

00:11:08.536 --> 00:11:15.760
So as prophase progresses-- the
nuclear envelope actually

00:11:15.760 --> 00:11:18.790
starts to disassemble.

00:11:18.790 --> 00:11:24.400
So this starts to actually
dissolve and disassemble, and

00:11:24.400 --> 00:11:29.330
then these things start to grow
and attach themselves to

00:11:29.330 --> 00:11:29.840
the centromere.

00:11:29.840 --> 00:11:31.290
So actually, let me do that.

00:11:31.290 --> 00:11:34.230
So this is all during
prophase.

00:11:37.790 --> 00:11:40.430
Since all of this happens during
prophase, this latter

00:11:40.430 --> 00:11:43.390
part of prophase, sometimes
they'll call it late prophase,

00:11:43.390 --> 00:11:44.820
sometimes it'll be called
prometaphase.

00:11:52.070 --> 00:11:54.140
Sometimes it's considered-- I
don't think there's a hyphen

00:11:54.140 --> 00:11:55.390
really there.

00:11:57.745 --> 00:12:00.890
So sometimes it's actually
considered a separate phase of

00:12:00.890 --> 00:12:02.700
mitosis, although when I learned
it in school, they

00:12:02.700 --> 00:12:04.180
didn't bother with
prometaphase.

00:12:04.180 --> 00:12:06.620
They just called it
all prophase.

00:12:06.620 --> 00:12:09.620
But by the end of prophase,
or actually by the end of

00:12:09.620 --> 00:12:13.220
prometaphase, depending on how
you want to view it, the whole

00:12:13.220 --> 00:12:15.510
situation is going to look
something like this.

00:12:15.510 --> 00:12:17.390
You have your overall cell.

00:12:17.390 --> 00:12:20.460
The nuclear envelope has
disassembled, so to some

00:12:20.460 --> 00:12:21.980
degree, it doesn't
exist anymore.

00:12:21.980 --> 00:12:24.380
Although the proteins that
formed it are still there and

00:12:24.380 --> 00:12:26.350
they're going to be
used later on.

00:12:26.350 --> 00:12:29.840
And you have your two
chromosomes in this case.

00:12:29.840 --> 00:12:32.800
In a human's case, you would
have 46 of them.

00:12:32.800 --> 00:12:35.320
You have your two chomosomes,
each made with sister

00:12:35.320 --> 00:12:41.050
chromatids, each made with
two sister chromatids.

00:12:41.050 --> 00:12:42.190
Two chromosomes.

00:12:42.190 --> 00:12:46.900
They, of course, have their
centromeres right there, and

00:12:46.900 --> 00:12:55.160
then these centrosomes will
have migrated roughly on

00:12:55.160 --> 00:12:59.930
opposite sides of what
was once the nucleus.

00:12:59.930 --> 00:13:03.370
And these things have kind
of spread apart, these

00:13:03.370 --> 00:13:06.740
microtubules, so they're doing
two functions, really.

00:13:06.740 --> 00:13:08.270
At this point, they're
kind of pushing these

00:13:08.270 --> 00:13:10.470
two centrosomes apart.

00:13:10.470 --> 00:13:12.370
So you have all of these things,
and they're connecting

00:13:12.370 --> 00:13:13.880
the-- you know, some of them
are coming from this

00:13:13.880 --> 00:13:15.760
centrosome, some are coming from
this centrosome, some are

00:13:15.760 --> 00:13:17.120
connecting the two.

00:13:17.120 --> 00:13:20.900
And then some of these
microtubules, these tubes or

00:13:20.900 --> 00:13:23.210
these ropes, however you want
to view them, attach

00:13:23.210 --> 00:13:31.340
themselves to the centromeres of
the actual chromosomes, and

00:13:31.340 --> 00:13:34.610
the protein structure that they
attach them to is called

00:13:34.610 --> 00:13:36.570
the kinetochore.

00:13:36.570 --> 00:13:39.160
So there's the kinetochore
there, and that may or may not

00:13:39.160 --> 00:13:41.040
be-- kinetochore.

00:13:41.040 --> 00:13:42.430
It's a protein structure.

00:13:42.430 --> 00:13:43.940
It's actually fascinating.

00:13:43.940 --> 00:13:46.980
It's still an open area of
research on how exactly the

00:13:46.980 --> 00:13:49.350
microtubule attaches to the
kinetochore, and as we'll see

00:13:49.350 --> 00:13:53.740
in a second, it's at the
kinetochore that the

00:13:53.740 --> 00:13:58.400
microtubules essentially start
to pull at the two separate

00:13:58.400 --> 00:14:02.670
sister chromatids and actually
pull them apart.

00:14:02.670 --> 00:14:03.530
And it's actually
not understood

00:14:03.530 --> 00:14:04.720
exactly how that works.

00:14:04.720 --> 00:14:09.080
It's just been observed that
this actually happens.

00:14:09.080 --> 00:14:14.020
Once prophase is done,
essentially the cells then

00:14:14.020 --> 00:14:17.190
just make sure that the
chromosomes are well aligned.

00:14:17.190 --> 00:14:19.270
I kind of drew them well aligned
here, but that just

00:14:19.270 --> 00:14:22.760
kind of formally occurs
during metaphase,

00:14:22.760 --> 00:14:24.090
which is the next phase.

00:14:24.090 --> 00:14:26.040
The first one was prophase.

00:14:26.040 --> 00:14:29.280
Now we're in metaphase, and
metaphase really is just an

00:14:29.280 --> 00:14:32.680
aligning of the chromosomes, so
all of the chromosomes are

00:14:32.680 --> 00:14:35.300
going to be aligned at the
center of the cell.

00:14:35.300 --> 00:14:42.430
So I have my magenta one here,
I have my magenta one here,

00:14:42.430 --> 00:14:45.660
and I have my other one here,
my green one there, and, of

00:14:45.660 --> 00:14:49.960
course, you have your
centrosomes, the microspindles

00:14:49.960 --> 00:14:51.570
that are coming off of them.

00:14:51.570 --> 00:14:54.250
Some of them are kinetochore
microspindles that are

00:14:54.250 --> 00:14:59.200
actually attaching to the
centromeres of the actual

00:14:59.200 --> 00:15:00.470
chromosomes.

00:15:00.470 --> 00:15:01.910
It's very confusing, right?

00:15:01.910 --> 00:15:05.850
The centrosomes are these
structures that help direct

00:15:05.850 --> 00:15:08.070
what happens to these
microtubules.

00:15:08.070 --> 00:15:11.880
Centrioles are these little
structures, these little

00:15:11.880 --> 00:15:14.850
can-shaped structures inside
the centrosomes, and the

00:15:14.850 --> 00:15:19.050
centromere are the center
points where the two

00:15:19.050 --> 00:15:22.170
chromatids attached to each
other within a chromosome.

00:15:22.170 --> 00:15:25.850
So this is one sister chromatid,
that's another

00:15:25.850 --> 00:15:28.250
sister chromatid, and they
attach at the centromere.

00:15:28.250 --> 00:15:30.180
But this is metaphase.

00:15:30.180 --> 00:15:33.100
It's fairly easy.

00:15:33.100 --> 00:15:36.310
Metaphase, you just have this
aligning of the cells, and

00:15:36.310 --> 00:15:38.220
there's actually some theories,
how does the cell

00:15:38.220 --> 00:15:39.770
know to progress past
this point?

00:15:39.770 --> 00:15:40.740
How does it know
that everything

00:15:40.740 --> 00:15:42.240
is aligned and attached?

00:15:42.240 --> 00:15:46.000
And then there are some theories
that there's actually

00:15:46.000 --> 00:15:48.970
some signaling mechanism that
if one of these kinetochore

00:15:48.970 --> 00:15:52.110
proteins isn't properly attached
to one of these

00:15:52.110 --> 00:15:56.130
ropes, that somehow a signal
is sent that mitosis should

00:15:56.130 --> 00:15:56.950
not continue.

00:15:56.950 --> 00:15:59.200
So this is a very intricate
process.

00:15:59.200 --> 00:16:01.540
You can imagine if you have 46
chromosomes and you have all

00:16:01.540 --> 00:16:05.630
of this stuff going on in the
cell, and it's not like

00:16:05.630 --> 00:16:08.080
there's some individual
pushing stuff, or some

00:16:08.080 --> 00:16:08.800
computer here.

00:16:08.800 --> 00:16:14.360
It's really directed
by chemistry and by

00:16:14.360 --> 00:16:15.910
thermodynamic processes.

00:16:15.910 --> 00:16:23.130
But just by the intricacy or
the elegance of how these

00:16:23.130 --> 00:16:26.690
things are, it happens
spontaneously with all of the

00:16:26.690 --> 00:16:29.780
proper checks and balances, so
that most of the time, nothing

00:16:29.780 --> 00:16:32.210
bad happens, which is
all quite amazing.

00:16:32.210 --> 00:16:36.270
So after metaphase, now we're
ready to pull the stuff apart,

00:16:36.270 --> 00:16:37.520
and that's anaphase.

00:16:42.570 --> 00:16:45.760
So in anaphase-- let
me write that down.

00:16:45.760 --> 00:16:48.250
I've changed the color
of my cell.

00:16:48.250 --> 00:16:50.280
These guys get pulled apart.

00:16:50.280 --> 00:16:53.490
And as soon as they get pulled
apart-- so let's see, this

00:16:53.490 --> 00:16:54.520
guy's getting pulled.

00:16:54.520 --> 00:16:57.150
Let me do it in green.

00:16:57.150 --> 00:17:00.540
So one of the sister-- nope,
that's not green.

00:17:00.540 --> 00:17:03.410
One of the sister chromatids is
pulling in that direction.

00:17:03.410 --> 00:17:05.660
One is getting pulled
in that direction.

00:17:05.660 --> 00:17:08.520
And then the same is true
for the magenta ones.

00:17:08.520 --> 00:17:10.000
Pulled in that direction,
and one is getting

00:17:10.000 --> 00:17:11.780
pulled in that direction.

00:17:11.780 --> 00:17:15.630
And, of course, you have your
centrosomes here and then

00:17:15.630 --> 00:17:19.040
they're connected to the
kinetochores that are right

00:17:19.040 --> 00:17:21.119
there and that's where
they're pulling.

00:17:21.119 --> 00:17:23.890
There's also a whole microtubule
structure that

00:17:23.890 --> 00:17:25.650
isn't connected to the actual
chromosomes, but they're

00:17:25.650 --> 00:17:29.260
helping to actually push apart
these two centrosomes so that

00:17:29.260 --> 00:17:32.570
everything is going to opposite
sides of the cell.

00:17:32.570 --> 00:17:37.960
And so as soon as these two
chromatids are separated, and

00:17:37.960 --> 00:17:40.140
I touched on this a little bit
before when we talked about

00:17:40.140 --> 00:17:44.120
the vocabulary of DNA, then as
soon as that happens, these

00:17:44.120 --> 00:17:47.030
are each referred to
as chromosomes.

00:17:47.030 --> 00:17:49.960
So now you can say that
the cell has what it

00:17:49.960 --> 00:17:50.750
used to have here.

00:17:50.750 --> 00:17:51.770
It has two chromosomes.

00:17:51.770 --> 00:17:53.900
It now has four chromosomes.

00:17:53.900 --> 00:17:56.210
Because as soon as a chromatid
is no longer connected to its

00:17:56.210 --> 00:17:59.590
sister chromatid, they're then
considered sister chromosomes,

00:17:59.590 --> 00:18:01.100
which is just a naming
convention.

00:18:01.100 --> 00:18:02.740
I mean, they were there before,
they were there after.

00:18:02.740 --> 00:18:04.510
They were just attached
before.

00:18:04.510 --> 00:18:06.390
Now they're not attached, so
you kind of consider them

00:18:06.390 --> 00:18:08.930
their own individual entity.

00:18:08.930 --> 00:18:10.590
And then we're almost done.

00:18:10.590 --> 00:18:11.960
The last stage is telophase.

00:18:16.350 --> 00:18:19.900
I'm going to draw the cell a
little bit different here

00:18:19.900 --> 00:18:22.650
because something is happening
simultaneously with telophase

00:18:22.650 --> 00:18:24.720
most of the time.

00:18:24.720 --> 00:18:26.910
So telophase, and actually I'll

00:18:26.910 --> 00:18:28.520
rotate the cell 90 degrees.

00:18:28.520 --> 00:18:30.800
Let's say that this was
one centromere.

00:18:30.800 --> 00:18:32.930
This is the other centromere.

00:18:32.930 --> 00:18:34.640
So at this point,
it's essentially

00:18:34.640 --> 00:18:37.670
pulled the DNA to itself.

00:18:37.670 --> 00:18:43.000
So this guy has pulled one copy
of that chromosome and

00:18:43.000 --> 00:18:45.130
one copy of this chromosome.

00:18:45.130 --> 00:18:46.730
That guy's done the
same up here.

00:18:46.730 --> 00:18:50.370
He's pulled over one copy of
each-- oh, I used a different

00:18:50.370 --> 00:18:54.060
color-- one copy of each
chromosome to himself.

00:18:54.060 --> 00:18:57.190
Let me draw that right
there like that.

00:18:57.190 --> 00:19:01.200
And now the nuclear membranes
start forming around each of

00:19:01.200 --> 00:19:01.980
these two ends.

00:19:01.980 --> 00:19:04.660
So now you start having a
nuclear membrane form around

00:19:04.660 --> 00:19:06.760
each of these two ends.

00:19:06.760 --> 00:19:09.310
And so by the end of the
telophase-- that's what we're

00:19:09.310 --> 00:19:13.880
in, the telophase-- we will
have completed mitosis.

00:19:13.880 --> 00:19:17.110
We will have completely
replicated our two original

00:19:17.110 --> 00:19:21.300
nucleuses and all of the genetic
content inside of it.

00:19:21.300 --> 00:19:24.430
Now, at the same time telophase
is happening, you

00:19:24.430 --> 00:19:27.440
also normally have this
cytokinesis, where this

00:19:27.440 --> 00:19:31.260
cleavage furrow forms, where
essentially-- during

00:19:31.260 --> 00:19:33.570
telophase, these things are
getting pushed further and

00:19:33.570 --> 00:19:37.720
further apart by those
microtubules so that they're

00:19:37.720 --> 00:19:42.166
already at the ends of the cell,
of the cytoplasm of the

00:19:42.166 --> 00:19:45.270
cell, and you can almost view
them as pushing on the sides

00:19:45.270 --> 00:19:46.860
to elongate the cell.

00:19:46.860 --> 00:19:49.440
As that is happening, you have
this furrow forming, this

00:19:49.440 --> 00:19:51.310
little indentation.

00:19:51.310 --> 00:19:54.520
By the end of telophase in
mitosis, you also have this

00:19:54.520 --> 00:19:58.340
process of cytokinesis, where
this cleavage furrow forms and

00:19:58.340 --> 00:20:01.900
deepens, deepens, deepens
until the cytoplasm is

00:20:01.900 --> 00:20:04.730
actually split into two
separate cells.

00:20:04.730 --> 00:20:08.560
So this is cytokinesis, which
is formally not a part of

00:20:08.560 --> 00:20:13.450
mitosis, but it normally occurs
with the telophase, so

00:20:13.450 --> 00:20:16.950
right at the end of mitosis,
you do normally have two

00:20:16.950 --> 00:20:18.770
complete identical cells.

00:20:18.770 --> 00:20:22.800
Once you have each of these
two cells, then they, each

00:20:22.800 --> 00:20:25.140
individually, enter their
own interphase.

00:20:25.140 --> 00:20:27.840
Or they each individually, if
we look at just this one, he

00:20:27.840 --> 00:20:30.780
will then be in his G1 phase.

00:20:30.780 --> 00:20:34.650
At some point, these two things
are going to replicate,

00:20:34.650 --> 00:20:36.500
and that's the S phase, and you
go to the G2 phase, and

00:20:36.500 --> 00:20:41.280
then this guy will experience
mitosis all over again.