Mitosis, Meiosis and Sexual Reproduction

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Kind: captions
Language: en

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I touched on this a little bit
in the video on how variation

00:00:03.560 --> 00:00:05.970
can be introduced into a
population, but I think it's

00:00:05.970 --> 00:00:08.750
fairly common knowledge that all
of us-- when I talk about

00:00:08.750 --> 00:00:11.360
us I'm talking about human
beings, and frankly, most

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eukaryotic organisms-- we're
the product of sexual

00:00:14.150 --> 00:00:15.360
reproduction.

00:00:15.360 --> 00:00:20.440
So if this is the first cell
that had the potential to

00:00:20.440 --> 00:00:24.260
become Sal, we know that this
first cell-- let me say this

00:00:24.260 --> 00:00:26.190
is the nucleus of that first
cell so I can draw the whole

00:00:26.190 --> 00:00:28.350
cell and all that, but let's
just focus on the nucleus.

00:00:28.350 --> 00:00:30.820
It has 23 chromosomes.

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Well, let me put it this way.

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It has 46 chromosomes, 23 from
my father and 23 from my

00:00:36.290 --> 00:00:41.010
mother, so that's 1, 2 3, 4, 3
5, 6, 7, 8, 9, 10, 11, 12, 13,

00:00:41.010 --> 00:00:45.040
14, 15, 16, 17, 18, 19, 20,
21, 22, 23 from my father.

00:00:45.040 --> 00:00:47.000
And then let's say that last
one actually helps to

00:00:47.000 --> 00:00:49.460
determine my gender, or it fully
determines my gender.

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That's my Y chromosome.

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And let's say I had 23
homologous chromosomes, or one

00:00:56.080 --> 00:00:59.120
chromosome that kind of was
the homologue for each of

00:00:59.120 --> 00:01:03.730
these, but I have 23 of them
from my mother, so 1, 2, 3, 4,

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5-- oh, you get the idea.

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I can just draw a bunch of
them, and then have the X

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chromosome that is essentially
one of the gender-determining

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chromosomes from my mother.

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And we learned before that
each of these pairs are

00:01:18.550 --> 00:01:21.820
homologous chromosomes, that
they essentially code for the

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same gene, one from my father
and one from my mother.

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Now, that first cell that had
the potential to become me, it

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was a product of fertilization,
of an egg from

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my mother-- so an egg
from my mother.

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I'll just draw the whole
egg like that.

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I'll just focus on the DNA from
now, so my mother's DNA,

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it had 23 chromosomes.

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So it didn't have pairs,
and this is key.

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So there's 1, 2, 3, 4, 5, 6, 7,
8, 9, 10, 11, 12, 13, 14,

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15, 16, 17, 18, 19, 20, 21,
22, and then 23 was the X

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chromosome.

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And so it's a combination from
my mother, so this is from my

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mother, and a sperm
from my father.

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Let me do that here.

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And I'll draw the sperm much
larger than it is normally

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relatively to the egg.

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This is kind of the nucleus of
the egg, but let's say that

00:02:21.840 --> 00:02:27.030
this is the sperm, and it has a
tail that helps it swim, and

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it has 23 chromosomes.

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So 1, 2, 3, 4, 5, 6, 7, 8, 9,
10, 11, 12, 13, 14, 15, 16,

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17, 18, 19, 20, 21, 22, and then
it has that Y chromosome.

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Let me do that Y chromosome
in a separate color.

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Just as an aside, this
unification, this

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fertilization that occurred from
this sperm cell to this

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egg cell, so it essentially
penetrates into this egg cell

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and it creates this zygote,
which is a fertilized egg cell

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from my mother, and this
contains all DNA from both my

00:03:03.240 --> 00:03:04.070
father and my mother.

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So this very first cell that
was created from this

00:03:05.940 --> 00:03:10.700
fertilized egg, this
is called a zygote.

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It's a product of

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fertilization between two gametes.

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So that's a gamete and
this is a gamete.

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Both a sperm cell or an
egg cell, they're

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both examples of gametes.

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Now, the whole reason why I'm
doing this is I want to

00:03:33.340 --> 00:03:36.150
introduce you-- and I already
introduced this notion to you

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when we talked about the
variation of population, that,

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look, this has my full
chromosome complement.

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It has 23 pairs, and each pair
is a pair of homologous

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chromosomes.

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They essentially code for the
same things, one from my

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mother, one from my father,
and that is 46 individual

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chromosomes, 23 from my mother,
23 from my father.

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These gametes, they each have
only 23 chromosomes, or half

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the number of a full
complement.

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Now, everything that I'm talking
about here, the number

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46, or 23 pairs, or 23
individual chromosomes, this

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is unique to human beings.

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If I talked about other species,
they might have 10

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chromosomes or they might
have 5 chromosomes.

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But one thing that is universal
for all sexually

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reproducing organisms is that
gametes have half the number

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of chromosomes as the zygote, or
you can kind of view it as

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the organism itself, the way we

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conventionally think about it.

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So when people talk about half
the number of chromosomees,

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they say it has a
haploid number.

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And that literally just means
half the number of

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chromosomes.

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It's very easy to memorize,
because haploid starts with

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the same two letters as half.

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Haploid number for humans
is 23 chromosomes.

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And so, you say, oh, if you say
this is a haploid number,

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what do you call it when you
have the full complement of

00:05:08.970 --> 00:05:09.840
chromosomes?

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Well, that's called the
diploid number.

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And I remember that because
di- often is a prefix

00:05:19.270 --> 00:05:24.230
associated with having two of
something, and so you have

00:05:24.230 --> 00:05:25.480
twice the number
of chromosomes.

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So this is haploid, this is
diploid number, and this is

00:05:29.820 --> 00:05:32.310
for humans, right?

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For an organism where the
diploid number is N, and

00:05:36.360 --> 00:05:38.840
you'll sometimes see this
notation, so I want to make

00:05:38.840 --> 00:05:41.890
sure you're comfortable with it,
there's some organism, or

00:05:41.890 --> 00:05:42.800
actually any organism.

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If the diploid number is 2N,
then the haploid number is

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going to be half of
that, or just N.

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Now, in the case of humans, the
diploid number is 46, so N

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is equal to 23.

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So a fertilized egg or even just
a regular somatic cell or

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a body cell will have a diploid
number of chromosome,

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while a sex cell, and I'll be
a little bit clearer about

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that in a second, will have a
haploid number of chromosomes.

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So gametes, which are either a
sperm or an egg, those are

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both examples of gametes, they
have half the number, they

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merge, and then you get a
zygote, which is that very

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first cell that had the
potential to turn into me,

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that has a diploid number
of chromosomes.

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And I actually want to do a
little bit of a side here,

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because it's fascinating.

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We talk about natural selection,
and we even wonder

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today to what degree is it
occurring, because our

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society, it's not as tough of an
environment as the natural

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world would be where we're being
stalked by predators and

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we have to live out in
the wild and find

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food and all of that.

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But even this process of
fertilization is an incredibly

00:06:51.360 --> 00:06:57.780
competitive process, because
this sperm that happened to be

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the one that kind of won the
race from my father to

00:07:02.090 --> 00:07:05.480
fertilize my mother's egg, it
was actually the first of

00:07:05.480 --> 00:07:09.540
roughly 200 million other
sperms. There were 200

00:07:09.540 --> 00:07:10.740
million, roughly.

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There could've been 200
to 300 million other

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sperms in that race.

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From the moment we're born,
we're already the product of

00:07:20.800 --> 00:07:24.730
an intense competition amongst
these male-- I guess we could

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call them male gametes, or
amongst these sperm cells.

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Some of them might have had
weird mutations, that they

00:07:30.480 --> 00:07:32.630
didn't know which direction to
swim, they happened to go in

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the wrong direction, maybe some
of them had weird tails

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that didn't allow them to swim
as fast, so you're already on

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some level selecting for
fitness within this

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environment.

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So if you had some weird
mutations from the get go in

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some of these sperm cells, it
would have been less likely,

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especially if they affected
their ability to kind of swim,

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it would've been less likely
that they would have been the

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ones to win this race.

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So already, you are the product
of a race of 280

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million organisms, if you
consider each of these sperm

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cells an organism, and you are
the product of that winning

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combination.

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So, you know, sometimes we
feel lost on this planet.

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We're one of 6 billion people
and all that, or just a

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number, but we already are the
product of a pretty intense

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accomplishment.

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But now with some of this
vocabulary thrown out of the

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way, let's talk a little bit
about zygotes and how do

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zygotes turn into people, and
then how do those people

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essentially produce gametes,
which then can fertilize other

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people's gametes to
form more zygotes.

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So the general idea: So that
very first cell that was

00:08:41.440 --> 00:08:45.410
essentially my mom's egg
fertilized by a sperm cell

00:08:45.410 --> 00:08:50.500
from my father, that was a
zygote, and as soon as it's

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successfully fertilized, it has
2N, or it has the diploid

00:08:55.400 --> 00:08:58.430
number of chromosomes in the
case of humans, which I

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believe I am one of them.

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I have 46 chromosomes.

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And then this cell right here
begins to split and divide

00:09:06.920 --> 00:09:08.300
over and over and over again.

00:09:08.300 --> 00:09:10.610
We'll do a whole series of
videos on the actual mechanics

00:09:10.610 --> 00:09:16.260
of that, but it splits by a
mechanism called mitosis.

00:09:16.260 --> 00:09:19.730
And mitosis literally is just
a cell splitting to form

00:09:19.730 --> 00:09:21.370
copies of itself.

00:09:21.370 --> 00:09:25.710
So it just starts splitting into
two more cells that are--

00:09:25.710 --> 00:09:27.910
and actually, let me do it this
way, just because the

00:09:27.910 --> 00:09:32.450
actual way it works is right
when a cell is split, the

00:09:32.450 --> 00:09:35.650
cells that it splits into aren't
that much larger than

00:09:35.650 --> 00:09:36.670
the original one.

00:09:36.670 --> 00:09:40.375
But now each of these have 2N
chromosomes, or 46 in the case

00:09:40.375 --> 00:09:42.850
of humans, and you keep
splitting, and it happens over

00:09:42.850 --> 00:09:44.020
and over and over again.

00:09:44.020 --> 00:09:48.810
So eventually-- well, let
me just do it this way.

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This keeps splitting, and then
you have-- and I'll go into

00:09:53.150 --> 00:09:56.430
the words for some of these
initial collections of cell,

00:09:56.430 --> 00:09:58.440
but I won't go into
that right now.

00:09:58.440 --> 00:10:02.350
2N, all of these are original
copies from a genetic point of

00:10:02.350 --> 00:10:04.220
view of that original cell.

00:10:04.220 --> 00:10:07.860
And then eventually, they start
to really-- I start to

00:10:07.860 --> 00:10:10.270
have tons of them.

00:10:10.270 --> 00:10:15.670
There's just a gazillion of them
that are all duplicates

00:10:15.670 --> 00:10:19.240
of the cell, and they all
contain the 2N number of

00:10:19.240 --> 00:10:21.900
chromosomes, the diploid
number of chromosomes.

00:10:21.900 --> 00:10:25.900
They all contain all of my
genetic material, but based on

00:10:25.900 --> 00:10:28.320
how they relate to each other
and what they see around them,

00:10:28.320 --> 00:10:31.200
they start differentiating.

00:10:31.200 --> 00:10:33.110
So all of these have 2N number,

00:10:33.110 --> 00:10:33.900
so they're all diploid.

00:10:33.900 --> 00:10:36.830
And mitosis-- this is the
process the whole time-- is

00:10:36.830 --> 00:10:40.200
these divide one cell into two
cells and those two cells into

00:10:40.200 --> 00:10:41.610
four cells and keep going.

00:10:41.610 --> 00:10:43.610
And then these begin
to differentiate.

00:10:43.610 --> 00:10:46.840
Maybe these cells eventually
differentiate into things

00:10:46.840 --> 00:10:48.660
that'll turn to my brain.

00:10:48.660 --> 00:10:51.430
These cells right here
differentiate into things

00:10:51.430 --> 00:10:53.440
that'll turn into my heart.

00:10:53.440 --> 00:10:56.410
These cells here differentiate
into things that will turn

00:10:56.410 --> 00:10:59.570
into my lungs and so
forth and so on.

00:10:59.570 --> 00:11:03.540
And eventually, you
get a human being.

00:11:03.540 --> 00:11:04.690
But it doesn't have to
be a human being.

00:11:04.690 --> 00:11:07.450
It could be whatever species we
happen to be talking about.

00:11:07.450 --> 00:11:10.630
So let me draw the
human being.

00:11:10.630 --> 00:11:14.820
So I'll draw my best shot at an
outline of a human being.

00:11:17.860 --> 00:11:20.660
Now, we're talking about
gazillions of cells.

00:11:20.660 --> 00:11:25.670
You have your human being, and
I'll just draw a very simple

00:11:25.670 --> 00:11:28.170
diagram, outline of
a human being.

00:11:32.080 --> 00:11:34.120
When I was in high school, I was
a class artist, so I don't

00:11:34.120 --> 00:11:36.020
want to make this representative
of my true

00:11:36.020 --> 00:11:37.220
artistic ability.

00:11:37.220 --> 00:11:43.210
I'm doing this here just to
kind of give you an idea.

00:11:43.210 --> 00:11:46.595
But anyway, eventually, you keep
dividing these cells and

00:11:46.595 --> 00:11:48.040
you end up with a human being,
and this human being, you

00:11:48.040 --> 00:11:51.570
know, you wouldn't even notice
the cells on this scale.

00:11:51.570 --> 00:11:55.380
Now, most of these cells of this
human being, if this is

00:11:55.380 --> 00:11:59.430
me or you, these are all the
product of mitosis that

00:11:59.430 --> 00:12:02.170
started off with that zygote,
and it just kept dividing and

00:12:02.170 --> 00:12:05.020
dividing and dividing
into mitosis.

00:12:05.020 --> 00:12:06.450
But it differentiated.

00:12:06.450 --> 00:12:09.470
I said some of them will
turn into brain cells.

00:12:09.470 --> 00:12:11.750
Some of them will turn
into heart cells.

00:12:11.750 --> 00:12:13.650
The whole process of
differentiation is actually

00:12:13.650 --> 00:12:15.710
fascinating, and we'll talk a
lot more about that when we

00:12:15.710 --> 00:12:20.230
talk about stem cells, embryonic
stem cells, and

00:12:20.230 --> 00:12:22.960
maybe we'll even talk about
the debate of it.

00:12:22.960 --> 00:12:25.170
But the question is,
well, how do I then

00:12:25.170 --> 00:12:26.460
produce those gametes?

00:12:26.460 --> 00:12:29.240
How do I produce those things
that eventually, if I'm going

00:12:29.240 --> 00:12:32.710
to reproduce, turn into
these kind of

00:12:32.710 --> 00:12:34.350
haploid number of cells?

00:12:34.350 --> 00:12:36.690
And that's what happens
in your sexual organs.

00:12:36.690 --> 00:12:41.310
So in a male, you have some germ
cells, so some of these

00:12:41.310 --> 00:12:43.980
cells turn into germ cells.

00:12:43.980 --> 00:12:47.640
And the germ cells exist
as part of your

00:12:47.640 --> 00:12:49.210
reproductive organs.

00:12:49.210 --> 00:12:50.990
So let's say those are
the germ cells.

00:12:50.990 --> 00:12:54.450
In a male, they would be
part of the gonads, so

00:12:54.450 --> 00:12:56.040
they would be there.

00:12:56.040 --> 00:13:00.840
In a female, they would be
involved in the ovaries.

00:13:00.840 --> 00:13:04.270
And these germ cells, they're
the product of mitosis.

00:13:04.270 --> 00:13:06.720
So let me draw a germ cell.

00:13:06.720 --> 00:13:16.690
So a germ cell is the product of
mitosis, so it still has 2N

00:13:16.690 --> 00:13:22.980
number of chromosomes, so it
still is a diploid cell or has

00:13:22.980 --> 00:13:23.990
a diploid number.

00:13:23.990 --> 00:13:26.960
But what's special about a
germ cell is it has the

00:13:26.960 --> 00:13:32.300
potential, one, it can either
continue to do mitosis and

00:13:32.300 --> 00:13:35.690
produce more germ cells that
are identical to it, so it

00:13:35.690 --> 00:13:39.880
could produce two germ cells
that are identical to it, or

00:13:39.880 --> 00:13:41.400
it can undergo meiosis.

00:13:48.420 --> 00:13:52.330
And meiosis is essentially what
a germ cell undergoes to

00:13:52.330 --> 00:13:53.940
produce gametes.

00:13:53.940 --> 00:13:56.840
And so if this germ cell
undergoes meiosis, and I'll do

00:13:56.840 --> 00:14:00.470
a whole video on the mechanics
of it, instead of two cells,

00:14:00.470 --> 00:14:05.810
it'll actually produce four
cells that each have half the

00:14:05.810 --> 00:14:12.460
number of chromosomes in them,
so these cells are haploid.

00:14:12.460 --> 00:14:15.930
In the case of a male, these
would be sperm cells.

00:14:15.930 --> 00:14:17.690
This would be sperm.

00:14:17.690 --> 00:14:24.560
In the case of a female, these
would be ova, sperm or ova,

00:14:24.560 --> 00:14:26.030
and these are the gametes.

00:14:26.030 --> 00:14:29.160
So it's an interesting thing to
talk about, because in the

00:14:29.160 --> 00:14:32.090
last several videos, I talked a
lot about mutations and what

00:14:32.090 --> 00:14:35.250
does that do to a species, but
think about what happens.

00:14:35.250 --> 00:14:39.010
If I have a mutation in some
cell here, some somatic cell,

00:14:39.010 --> 00:14:45.290
some body cell, somatic cell,
will that mutation or can that

00:14:45.290 --> 00:14:48.490
mutation in any way affect
what's going to

00:14:48.490 --> 00:14:49.750
happen to my kids?

00:14:49.750 --> 00:14:52.160
Will that mutation be carried
on to my kids?

00:14:52.160 --> 00:14:53.000
Well, no.

00:14:53.000 --> 00:14:56.760
Because in no way will what goes
on in this cell affect

00:14:56.760 --> 00:15:00.660
what I actually pass on
eventually in the sperm cells.

00:15:00.660 --> 00:15:01.850
It'll just be a random
mutation.

00:15:01.850 --> 00:15:04.570
It could affect my ability
to reproduce.

00:15:04.570 --> 00:15:09.760
For example, it could be-- God
forbid, it could be some type

00:15:09.760 --> 00:15:12.990
of cancer or something that,
especially if you contract it

00:15:12.990 --> 00:15:16.080
at a young age, it might be some
type of terminal form so

00:15:16.080 --> 00:15:18.960
that might affect your ability
to reproduce, but it will not

00:15:18.960 --> 00:15:24.120
affect the actual DNA that you
pass on to your offspring.

00:15:24.120 --> 00:15:27.720
So if you have some really bad
mutation here, it could affect

00:15:27.720 --> 00:15:31.310
how you live or it could turn
cancerous and start

00:15:31.310 --> 00:15:35.040
reproducing, but it will not
affect what you pass on to

00:15:35.040 --> 00:15:38.280
your children.

00:15:38.280 --> 00:15:41.640
The traits that will be passed
on or the changes that will be

00:15:41.640 --> 00:15:45.200
passed on are those that occur
in the germ cells.

00:15:45.200 --> 00:15:49.000
So if you have mutations in your
germ cells, or during the

00:15:49.000 --> 00:15:53.270
process of meiosis, you have
essentially recombination of

00:15:53.270 --> 00:15:56.130
DNA because of crossovers, and
we saw that in the variation

00:15:56.130 --> 00:15:59.510
video, then that will introduce
new forms or new

00:15:59.510 --> 00:16:03.350
variants inside that could be
passed on to your children.

00:16:03.350 --> 00:16:04.970
And I really want to make that
point there, because we talk

00:16:04.970 --> 00:16:07.520
about mutations, but there's
different types of mutations.

00:16:07.520 --> 00:16:09.580
There's some mutations that
won't be passed on to your

00:16:09.580 --> 00:16:12.030
children, and those are the
ones that occur in your

00:16:12.030 --> 00:16:13.120
somatic cells.

00:16:13.120 --> 00:16:15.170
Maybe some of them do nothing
so then it really doesn't

00:16:15.170 --> 00:16:18.180
affect your overall function,
but in the mutations that

00:16:18.180 --> 00:16:24.350
either occur in your germ cells
or the recombination or

00:16:24.350 --> 00:16:26.830
the variation that is introduced
during meiosis,

00:16:26.830 --> 00:16:28.880
that will be passed on
to your children.

00:16:28.880 --> 00:16:30.290
But even there I want
to be careful.

00:16:30.290 --> 00:16:33.700
Because remember, this is
a severe competition.

00:16:33.700 --> 00:16:37.270
So out of all of the-- let's say
there's 280 million sperm

00:16:37.270 --> 00:16:42.520
cells that at one time are being
competitive for an egg,

00:16:42.520 --> 00:16:46.710
it's possible that some of
them have mutations.

00:16:46.710 --> 00:16:49.360
In order for one of those
mutations-- let me do the

00:16:49.360 --> 00:16:51.140
mutations in different colors.

00:16:51.140 --> 00:16:52.120
That's a purple mutation.

00:16:52.120 --> 00:16:54.340
That's a blue mutation.

00:16:54.340 --> 00:16:57.400
But in order for that mutation
to truly be passed on to my

00:16:57.400 --> 00:17:01.530
offspring, the sperm containing
the mutation is the

00:17:01.530 --> 00:17:03.430
one that has to win the race.

00:17:03.430 --> 00:17:06.540
So already you have a selection
going on at kind of

00:17:06.540 --> 00:17:11.240
this sexual reproduction level
where you're selecting for

00:17:11.240 --> 00:17:13.810
things that are at least good
enough-- I mean, to some

00:17:13.810 --> 00:17:16.910
degree, the sperm has to be
good enough to win this is

00:17:16.910 --> 00:17:19.930
incredibly, incredibly
competitive race.

00:17:19.930 --> 00:17:22.810
So that mutation that somehow
made the sperm deformed or

00:17:22.810 --> 00:17:25.200
didn't allow it to swim or made
it behave in some weird

00:17:25.200 --> 00:17:28.670
way, it's very unlikely that
that mutation would go on to

00:17:28.670 --> 00:17:31.820
be the one or that cell would go
on to be the one that would

00:17:31.820 --> 00:17:34.120
successfully fertilize an egg.

00:17:34.120 --> 00:17:36.820
So anyway, I wanted to introduce
you to these ideas.

00:17:36.820 --> 00:17:38.580
The main idea is really
some of the

00:17:38.580 --> 00:17:40.090
vocabulary: haploid, diploid.

00:17:40.090 --> 00:17:42.105
It's very confusing when you
first learn it, but it

00:17:42.105 --> 00:17:48.950
literally just means half the
normal group of chromosomes.

00:17:48.950 --> 00:17:51.050
And in the case of humans,
that would be 23.

00:17:51.050 --> 00:17:55.360
And the cells that have a
haploid number of chromosomes

00:17:55.360 --> 00:18:00.130
are our gametes, which are sperm
cells for men, and ova,

00:18:00.130 --> 00:18:01.840
or egg cells for women.

00:18:01.840 --> 00:18:04.490
But everything else in our
body, all of our somatic

00:18:04.490 --> 00:18:07.510
cells, are diploid, which means
that the full complement

00:18:07.510 --> 00:18:10.300
of chromosomes, they all
have a copy of our DNA.

00:18:10.300 --> 00:18:12.460
And that's why DNA testing is so
interesting because you can

00:18:12.460 --> 00:18:14.850
get any cell from someone
anywhere, and you have their

00:18:14.850 --> 00:18:16.410
full complement of DNA.

00:18:16.410 --> 00:18:19.120
You have all of the information
that describes

00:18:19.120 --> 00:18:20.860
them genetically.

00:18:20.860 --> 00:18:23.530
Anyway, see you in
the next video.


Chromosomes, Chromatids, Chromatin, etc.

WEBVTT
Kind: captions
Language: en

00:00:00.550 --> 00:00:04.100
Before I dive into the mechanics
of how cells divide,

00:00:04.100 --> 00:00:06.810
I think it could be useful to
talk a little bit about a lot

00:00:06.810 --> 00:00:09.730
of the vocabulary that
surrounds DNA.

00:00:09.730 --> 00:00:12.300
There's a lot of words and some
of them kind of sound

00:00:12.300 --> 00:00:14.520
like each other, but they
can be very confusing.

00:00:14.520 --> 00:00:16.600
So the first few I'd like to
talk about is just about how

00:00:16.600 --> 00:00:21.240
DNA either generates more DNA,
makes copies of itself, or how

00:00:21.240 --> 00:00:24.580
it essentially makes proteins,
and we've talked about this in

00:00:24.580 --> 00:00:27.220
the DNA video.

00:00:27.220 --> 00:00:29.340
So let's say I have a little--
I'm just going to draw a small

00:00:29.340 --> 00:00:30.705
section of DNA.

00:00:30.705 --> 00:00:37.740
I have an A, a G, a T, let's say
I have two T's and then I

00:00:37.740 --> 00:00:38.930
have two C's.

00:00:38.930 --> 00:00:40.000
Just some small section.

00:00:40.000 --> 00:00:41.400
It keeps going.

00:00:41.400 --> 00:00:43.010
And, of course, it's
a double helix.

00:00:43.010 --> 00:00:45.440
It has its corresponding
bases.

00:00:45.440 --> 00:00:47.780
Let me do that in this color.

00:00:47.780 --> 00:00:53.390
So A corresponds to T, G with
C, it forms hydrogen bonds

00:00:53.390 --> 00:01:01.320
with C, T with A, T with
A, C with G, C with G.

00:01:01.320 --> 00:01:03.280
And then, of course,
it just keeps

00:01:03.280 --> 00:01:05.480
going on in that direction.

00:01:05.480 --> 00:01:07.770
So there's a couple of different
processes that this

00:01:07.770 --> 00:01:09.310
DNA has to do.

00:01:09.310 --> 00:01:12.440
One is when you're just dealing
with your body cells

00:01:12.440 --> 00:01:16.630
and you need to make more
versions of your skin cells,

00:01:16.630 --> 00:01:21.550
your DNA has to copy itself,
and this process is called

00:01:21.550 --> 00:01:22.510
replication.

00:01:22.510 --> 00:01:23.920
You're replicating the DNA.

00:01:23.920 --> 00:01:26.190
So let me do replication.

00:01:26.190 --> 00:01:28.660
So how can this DNA
copy itself?

00:01:28.660 --> 00:01:30.750
And this is one of the beautiful
things about how DNA

00:01:30.750 --> 00:01:32.320
is structured.

00:01:32.320 --> 00:01:34.770
Replication.

00:01:34.770 --> 00:01:37.120
So I'm doing a gross
oversimplification, but the

00:01:37.120 --> 00:01:39.600
idea is these two strands
separate, and it doesn't

00:01:39.600 --> 00:01:40.240
happen on its own.

00:01:40.240 --> 00:01:43.510
It's facilitated by a bunch of
proteins and enzymes, but I'll

00:01:43.510 --> 00:01:45.520
talk about the details of the

00:01:45.520 --> 00:01:48.610
microbiology in a future video.

00:01:48.610 --> 00:01:53.744
So these guys separate
from each other.

00:01:53.744 --> 00:01:57.280
Let me put it up here.

00:01:57.280 --> 00:01:58.590
They separate from each other.

00:01:58.590 --> 00:02:02.190
Let me take the other guy.

00:02:02.190 --> 00:02:03.710
Too big.

00:02:03.710 --> 00:02:07.430
That guy looks something
like that.

00:02:07.430 --> 00:02:11.050
They separate from each other,
and then once they've

00:02:11.050 --> 00:02:12.680
separated from each other,
what could happen?

00:02:12.680 --> 00:02:16.950
Let me delete some of that
stuff over here.

00:02:16.950 --> 00:02:18.900
Delete that stuff right there.

00:02:18.900 --> 00:02:20.600
So you have this double helix.

00:02:20.600 --> 00:02:21.410
They were all connected.

00:02:21.410 --> 00:02:22.230
They're base pairs.

00:02:22.230 --> 00:02:23.980
Now, they separate
from each other.

00:02:23.980 --> 00:02:26.010
Now once they separate, what
can each of these do?

00:02:26.010 --> 00:02:28.500
They can now become the template
for each other.

00:02:32.620 --> 00:02:35.410
If this guy is sitting by
himself, now all of a sudden,

00:02:35.410 --> 00:02:40.750
a thymine base might come and
join right here, so these

00:02:40.750 --> 00:02:42.110
nucleotides will start
lining up.

00:02:42.110 --> 00:02:46.640
So you'll have a thymine and a
cytosine, and then an adenine,

00:02:46.640 --> 00:02:49.750
adenine, guanine, guanine,
and it'll keep happening.

00:02:49.750 --> 00:02:53.835
And then on this other part,
this other green strand that

00:02:53.835 --> 00:02:56.210
was formerly attached to this
blue strand, the same thing

00:02:56.210 --> 00:02:56.970
will happen.

00:02:56.970 --> 00:03:01.670
You have an adenine, a guanine,
thymine, thymine,

00:03:01.670 --> 00:03:02.770
cytosine, cytosine.

00:03:02.770 --> 00:03:03.550
So what just happened?

00:03:03.550 --> 00:03:06.460
By separating and then just
attracting their complementary

00:03:06.460 --> 00:03:10.670
bases, we just duplicated
this molecule, right?

00:03:10.670 --> 00:03:13.280
We'll do the microbiology of it
in the future, but this is

00:03:13.280 --> 00:03:14.030
just to get the idea.

00:03:14.030 --> 00:03:17.370
This is how the DNA makes
copies of itself.

00:03:17.370 --> 00:03:20.190
And especially when we talk
about mitosis and meiosis, I

00:03:20.190 --> 00:03:22.030
might say, oh, this is the stage
where the replication

00:03:22.030 --> 00:03:23.280
has occurred.

00:03:23.280 --> 00:03:26.460
Now, the other thing that
you'll hear a lot, and I

00:03:26.460 --> 00:03:32.900
talked about this in the DNA
video, is transcription.

00:03:32.900 --> 00:03:36.000
In the DNA video, I didn't focus
much on how does DNA

00:03:36.000 --> 00:03:38.730
duplicate itself, but one of
the beautiful things about

00:03:38.730 --> 00:03:41.420
this double helix design is
it really is that easy to

00:03:41.420 --> 00:03:42.160
duplicate itself.

00:03:42.160 --> 00:03:46.670
You just split the two strips,
the two helices, and then they

00:03:46.670 --> 00:03:48.760
essentially become a template
for the other one, and then

00:03:48.760 --> 00:03:49.410
you have a duplicate.

00:03:49.410 --> 00:03:53.200
Now, transcription is what needs
to occur for this DNA

00:03:53.200 --> 00:03:56.380
eventually to turn into
proteins, but transcription is

00:03:56.380 --> 00:03:57.870
the intermediate step.

00:03:57.870 --> 00:04:05.280
It's the step where you
go from DNA to mRNA.

00:04:05.280 --> 00:04:08.930
And then that mRNA leaves the
nucleus of the cell and goes

00:04:08.930 --> 00:04:12.810
out to the ribosomes, and I'll
talk about that in a second.

00:04:12.810 --> 00:04:13.810
So we can do the same thing.

00:04:13.810 --> 00:04:15.760
So this guy, once again
during transcription,

00:04:15.760 --> 00:04:18.120
will also split apart.

00:04:18.120 --> 00:04:24.970
So that was one split there and
then the other split is

00:04:24.970 --> 00:04:26.220
right there.

00:04:31.270 --> 00:04:34.240
And actually, maybe it makes
more sense just to do one-half

00:04:34.240 --> 00:04:36.880
of it, so let me delete that.

00:04:41.450 --> 00:04:43.730
Let's say that we're just going
to transcribe the green

00:04:43.730 --> 00:04:46.010
side right here.

00:04:46.010 --> 00:04:49.695
Let me erase all this stuff
right-- nope, wrong color.

00:04:49.695 --> 00:04:52.530
Let me erase this stuff
right here.

00:04:52.530 --> 00:04:59.860
Now, what happens is instead
of having deoxyribonucleic

00:04:59.860 --> 00:05:05.030
acid nucleotides pair up with
this DNA strand, you have

00:05:05.030 --> 00:05:07.950
ribonucleic acid, or RNA
pair up with this.

00:05:07.950 --> 00:05:11.620
And I'll do RNA in magneta.

00:05:11.620 --> 00:05:14.130
So the RNA will pair
up with it.

00:05:14.130 --> 00:05:19.480
And so thymine on the DNA side
will pair up with adenine.

00:05:19.480 --> 00:05:22.810
Guanine, now, when we talk about
RNA, instead of thymine,

00:05:22.810 --> 00:05:27.160
we have uracil, uracil,
cytosine, cytosine, and it

00:05:27.160 --> 00:05:28.080
just keeps going.

00:05:28.080 --> 00:05:29.330
This is mRNA.

00:05:33.210 --> 00:05:35.070
Now, this separates.

00:05:35.070 --> 00:05:38.950
That mRNA separates, and
it leaves the nucleus.

00:05:38.950 --> 00:05:43.490
It leaves the nucleus, and then
you have translation.

00:05:49.750 --> 00:05:53.790
That is going from the mRNA to--
you remember in the DNA

00:05:53.790 --> 00:05:56.280
video, I had the little tRNA.

00:05:56.280 --> 00:06:01.650
The transfer RNA were kind of
the trucks that drove up the

00:06:01.650 --> 00:06:05.100
amino acids to the mRNA, and
this all occurs inside these

00:06:05.100 --> 00:06:07.150
parts of the cell called
the ribosome.

00:06:07.150 --> 00:06:09.030
But the translation is
essentially going from the

00:06:09.030 --> 00:06:16.710
mRNA to the proteins, and we
saw how that happened.

00:06:16.710 --> 00:06:20.560
You have this guy-- let
me make a copy here.

00:06:20.560 --> 00:06:22.880
Let me actually copy
the whole thing.

00:06:22.880 --> 00:06:32.980
This guy separates, leaves the
nucleus, and then you had

00:06:32.980 --> 00:06:37.060
those little tRNA trucks that
essentially drive up.

00:06:37.060 --> 00:06:38.820
So maybe I have some tRNA.

00:06:38.820 --> 00:06:41.420
Let's see, adenine, adenine,
guanine, and guanine.

00:06:41.420 --> 00:06:43.430
This is tRNA.

00:06:43.430 --> 00:06:44.230
That's a codon.

00:06:44.230 --> 00:06:46.650
A codon has three base pairs,
and attached to it, it has

00:06:46.650 --> 00:06:48.540
some amino acid.

00:06:48.540 --> 00:06:50.750
And then you have some
other piece of tRNA.

00:06:50.750 --> 00:06:54.890
Let's say it's a uracil,
cytosine, adenine.

00:06:54.890 --> 00:06:57.305
And attached to that, it has
a different amino acid.

00:07:00.850 --> 00:07:03.530
Then the amino acids attach to
each other, and then they form

00:07:03.530 --> 00:07:09.365
this long chain of amino acids,
which is a protein, and

00:07:09.365 --> 00:07:12.840
the proteins form these weird
and complicated shapes.

00:07:12.840 --> 00:07:17.080
So just to kind of make sure you
understand, so if we start

00:07:17.080 --> 00:07:23.510
with DNA, and we're essentially
making copies of

00:07:23.510 --> 00:07:29.790
DNA, this is replication.

00:07:29.790 --> 00:07:33.430
You're replicating the DNA.

00:07:33.430 --> 00:07:39.270
Now, if you're starting with DNA
and you are creating mRNA

00:07:39.270 --> 00:07:42.006
from the DNA template, this
is transcription.

00:07:48.050 --> 00:07:51.160
You are transcribing the
information from one form to

00:07:51.160 --> 00:07:53.830
another: transcription.

00:07:53.830 --> 00:07:57.520
Now, when the mRNA leaves the
nucleus of the cell, and I've

00:07:57.520 --> 00:08:00.200
talked-- well, let me just draw
a cell just to hit the

00:08:00.200 --> 00:08:04.100
point home, if this is a whole
cell, and we'll do the

00:08:04.100 --> 00:08:05.090
structure of a cell
in the future.

00:08:05.090 --> 00:08:06.870
If that's the whole cell, the
nucleus is the center.

00:08:06.870 --> 00:08:10.380
That's where all the DNA is
sitting in there, and all of

00:08:10.380 --> 00:08:13.500
the replication and the
transcription occurs in here,

00:08:13.500 --> 00:08:16.310
but then the mRNA leaves the
cell, and then inside the

00:08:16.310 --> 00:08:19.270
ribosomes, which we'll talk
about more in the future, you

00:08:19.270 --> 00:08:22.470
have translation occur and
the proteins get formed.

00:08:22.470 --> 00:08:29.430
So mRNA to protein
is translation.

00:08:29.430 --> 00:08:33.230
You're translating from the
genetic code, so to speak, to

00:08:33.230 --> 00:08:34.809
the protein code.

00:08:34.809 --> 00:08:36.059
So this is translation.

00:08:38.330 --> 00:08:43.030
So these are just good words to
make sure you get clear and

00:08:43.030 --> 00:08:45.290
make sure you're using the right
word when you're talking

00:08:45.290 --> 00:08:46.660
about the different processes.

00:08:46.660 --> 00:08:51.560
Now, the other part of the
vocabulary of DNA, which, when

00:08:51.560 --> 00:08:54.250
I first learned it, I found
tremendously confusing, are

00:08:54.250 --> 00:08:56.600
the words chromosome.

00:08:56.600 --> 00:08:58.120
I'll write them down here
because you can already

00:08:58.120 --> 00:09:04.100
appreciate how confusing
they are: chromosome,

00:09:04.100 --> 00:09:09.590
chromatin and chromatid.

00:09:14.130 --> 00:09:17.210
So a chromosome, we already
talked about.

00:09:17.210 --> 00:09:18.760
You can have DNA.

00:09:18.760 --> 00:09:20.880
You can have a strand of DNA.

00:09:20.880 --> 00:09:23.530
That's a double helix.

00:09:23.530 --> 00:09:27.470
This strand, if I were to zoom
in, is actually two different

00:09:27.470 --> 00:09:29.910
helices, and, of course,
they have their base

00:09:29.910 --> 00:09:32.140
pairs joined up.

00:09:32.140 --> 00:09:35.730
I'll just draw some base pairs
joined up like that.

00:09:35.730 --> 00:09:38.470
So I want to be clear, when I
draw this little green line

00:09:38.470 --> 00:09:40.630
here, it's actually
a double helix.

00:09:40.630 --> 00:09:45.340
Now, that double helix gets
wrapped around proteins that

00:09:45.340 --> 00:09:46.590
are called histones.

00:09:51.290 --> 00:09:53.660
So let's say it gets wrapped
like there, and it gets

00:09:53.660 --> 00:09:56.000
wrapped around like that, and
it gets wrapped around like

00:09:56.000 --> 00:09:59.480
that, and you have here these
things called histones, which

00:09:59.480 --> 00:10:00.730
are these proteins.

00:10:04.900 --> 00:10:12.890
Now, this structure, when you
talk about the DNA in

00:10:12.890 --> 00:10:16.040
combination with the proteins
that kind of give it structure

00:10:16.040 --> 00:10:18.060
and then these proteins are
actually wrapped around more

00:10:18.060 --> 00:10:20.680
and more, and eventually,
depending on what stage we are

00:10:20.680 --> 00:10:22.660
in the cell's life, you have
different structures.

00:10:22.660 --> 00:10:26.440
But when you talk about the
nucleic acid, which is the

00:10:26.440 --> 00:10:29.420
DNA, and you combine that with
the proteins, you're talking

00:10:29.420 --> 00:10:30.670
about the chromatin.

00:10:32.980 --> 00:10:39.100
So this is DNA plus-- you can
view it as structural proteins

00:10:39.100 --> 00:10:41.170
that give the DNA its shape.

00:10:44.100 --> 00:10:47.640
And the idea, chromatin was
first used-- because when

00:10:47.640 --> 00:10:50.460
people look at a cell, every
time I've drawn these cell

00:10:50.460 --> 00:10:54.620
nucleuses so far, I've drawn
these very well defined-- I'll

00:10:54.620 --> 00:10:55.150
use the word.

00:10:55.150 --> 00:10:57.120
So let's say this is
a cell's nucleus.

00:10:57.120 --> 00:11:01.060
I've been drawing very
well-defined structures here.

00:11:01.060 --> 00:11:03.840
So that's one, and then this
could be another one, maybe

00:11:03.840 --> 00:11:07.420
it's shorter, and then it has
its homologous chromosome.

00:11:07.420 --> 00:11:10.410
So I've been drawing these
chromosomes, right?

00:11:10.410 --> 00:11:13.790
And each of these chromosomes
I did in the last video are

00:11:13.790 --> 00:11:18.270
essentially these long
structures of DNA, long chains

00:11:18.270 --> 00:11:21.010
of DNA kind of wrapped tightly
around each other.

00:11:21.010 --> 00:11:23.790
So when I drew it like that, if
we zoomed in, you'd see one

00:11:23.790 --> 00:11:25.830
strand and it's really
just wrapped

00:11:25.830 --> 00:11:28.680
around itself like this.

00:11:28.680 --> 00:11:32.170
And then its homologous
chromosome-- and remember, in

00:11:32.170 --> 00:11:34.500
the variation video, I talked
about the homologous

00:11:34.500 --> 00:11:36.960
chromosome that essentially
codes for the same genes but

00:11:36.960 --> 00:11:39.420
has a different version.

00:11:39.420 --> 00:11:42.640
If the blue came from the dad,
the red came from the mom, but

00:11:42.640 --> 00:11:46.130
it's coding for essentially
the same genes.

00:11:46.130 --> 00:11:52.360
So when we talk about this one
chain, let's say this one

00:11:52.360 --> 00:11:57.290
chain that I got from my dad of
DNA in this structure, we

00:11:57.290 --> 00:11:58.660
refer to that as a chromosome.

00:12:01.680 --> 00:12:05.400
Now, if we refer generally-- and
I want to be clear here.

00:12:05.400 --> 00:12:08.950
DNA only takes this shape at
certain stages of its life

00:12:08.950 --> 00:12:14.900
when it's actually replicating
itself-- not when it's

00:12:14.900 --> 00:12:17.500
replicating.

00:12:17.500 --> 00:12:19.940
Before the cell can divide,
DNA takes this very

00:12:19.940 --> 00:12:21.430
well-defined shape.

00:12:21.430 --> 00:12:24.240
Most of the cell's life, when
the DNA is actually doing its

00:12:24.240 --> 00:12:26.700
work, when it's actually
creating proteins or proteins

00:12:26.700 --> 00:12:30.080
are being essentially
transcribed and translated

00:12:30.080 --> 00:12:32.380
from the DNA, the DNA isn't
all bundled up like this.

00:12:32.380 --> 00:12:34.630
Because if it was bundled up
like, it would be very hard

00:12:34.630 --> 00:12:42.310
for the replication and the
transcription machinery to get

00:12:42.310 --> 00:12:44.710
onto the DNA and make the
proteins and do whatever else.

00:12:44.710 --> 00:12:48.610
Normally, DNA-- let me draw
that same nucleus.

00:12:48.610 --> 00:12:53.780
Normally, you can't even see
it with a normal light

00:12:53.780 --> 00:12:54.240
microscope.

00:12:54.240 --> 00:12:57.690
It's so thin that the DNA strand
is just completely

00:12:57.690 --> 00:12:59.980
separated around the cell.

00:12:59.980 --> 00:13:02.160
I'm drawing it here so you can
try to-- maybe the other one

00:13:02.160 --> 00:13:05.560
is like this, right?

00:13:05.560 --> 00:13:08.880
And then you have that shorter
strand that's like this.

00:13:08.880 --> 00:13:10.560
And so you can't even see it.

00:13:10.560 --> 00:13:12.500
It's not in this well-defined
structure.

00:13:12.500 --> 00:13:14.400
This is the way it
normally is.

00:13:14.400 --> 00:13:16.660
And they have the other short
strand that's like that.

00:13:16.660 --> 00:13:21.550
So you would just see this
kind of big mess of a

00:13:21.550 --> 00:13:24.760
combination of DNA and proteins,
and this is what

00:13:24.760 --> 00:13:26.410
people essentially refer
to as chromatin.

00:13:29.950 --> 00:13:32.540
So the words can be very
ambiguous and very confusing,

00:13:32.540 --> 00:13:34.980
but the general usage is when
you're talking about the

00:13:34.980 --> 00:13:39.580
well-defined one chain of DNA
in this kind of well-defined

00:13:39.580 --> 00:13:42.260
structure, that is
a chromosome.

00:13:42.260 --> 00:13:46.770
Chromatin can either refer to
kind of the structure of the

00:13:46.770 --> 00:13:50.730
chromosome, the combination of
the DNA and the proteins that

00:13:50.730 --> 00:13:54.450
give the structure, or it can
refer to this whole mess of

00:13:54.450 --> 00:13:59.530
multiple chromosomes of which
you have all of this DNA from

00:13:59.530 --> 00:14:01.100
multiple chromosomes
and all the

00:14:01.100 --> 00:14:03.240
proteins all jumbled together.

00:14:03.240 --> 00:14:04.520
So I just want to
make that clear.

00:14:04.520 --> 00:14:05.990
Now, then the next word
is, well, what is

00:14:05.990 --> 00:14:08.290
this chromatid thing?

00:14:08.290 --> 00:14:10.310
What is this chromatid thing?

00:14:10.310 --> 00:14:12.050
Actually, just in case I didn't,
I don't remember if I

00:14:12.050 --> 00:14:12.550
labeled these.

00:14:12.550 --> 00:14:16.210
These proteins that give
structure to the chromatin or

00:14:16.210 --> 00:14:18.890
that make up the chromatin or
that give structure to the

00:14:18.890 --> 00:14:23.540
chromosome, they're
called histones.

00:14:23.540 --> 00:14:25.440
And there are multiple types
that give structure at

00:14:25.440 --> 00:14:28.310
different levels, and we'll
do that in more detail.

00:14:28.310 --> 00:14:30.850
So what's a chromatid?

00:14:30.850 --> 00:14:34.780
When DNA replicates-- so
let's say that was

00:14:34.780 --> 00:14:37.790
my DNA before, right?

00:14:37.790 --> 00:14:40.400
When it's just in its normal
state, I have one version from

00:14:40.400 --> 00:14:42.620
my dad, one version
from my mom.

00:14:42.620 --> 00:14:44.270
Now, let's say it replicates.

00:14:44.270 --> 00:14:48.680
So my version from my dad,
at first it's like this.

00:14:48.680 --> 00:14:50.240
It's a big strand of DNA.

00:14:50.240 --> 00:14:53.560
It creates another version of
itself that is identical, if

00:14:53.560 --> 00:14:57.850
the machinery worked properly,
and so that identical piece

00:14:57.850 --> 00:14:59.590
will look like this.

00:14:59.590 --> 00:15:01.470
And they actually are initially

00:15:01.470 --> 00:15:03.150
attached to each other.

00:15:03.150 --> 00:15:05.000
They're attached to each other
at a point called the

00:15:05.000 --> 00:15:06.250
centromere.

00:15:08.755 --> 00:15:10.850
Now, even though I
have two strands

00:15:10.850 --> 00:15:12.310
here, they're now attached.

00:15:12.310 --> 00:15:14.860
When I have these two strands
that contain the exact-- so I

00:15:14.860 --> 00:15:20.420
have this strand right here, and
then I have-- well, let me

00:15:20.420 --> 00:15:21.180
actually draw it a
different way.

00:15:21.180 --> 00:15:23.090
I could draw it multiple
different ways.

00:15:23.090 --> 00:15:27.240
I could say this is one strand
here and then I have another

00:15:27.240 --> 00:15:32.000
strand here.

00:15:32.000 --> 00:15:34.180
Now, I have two copies.

00:15:34.180 --> 00:15:36.120
They're coding for the
exact same DNA.

00:15:36.120 --> 00:15:37.330
They're identical.

00:15:37.330 --> 00:15:39.002
I still call this
a chromosome.

00:15:45.760 --> 00:15:48.470
This whole thing is still called
a chromosome, but now

00:15:48.470 --> 00:15:52.400
each individual copy is
called a chromatid.

00:15:52.400 --> 00:15:57.660
So that's one chromatid and
this is another chromatid.

00:15:57.660 --> 00:15:59.950
Sometimes they'll call them
sister chromatids.

00:15:59.950 --> 00:16:01.740
Maybe they should call them twin
chromatids because they

00:16:01.740 --> 00:16:05.580
have the same genetic
information.

00:16:05.580 --> 00:16:08.490
So this chromosome has
two chromatids.

00:16:08.490 --> 00:16:12.750
Now, before the replication
occurred or the DNA duplicated

00:16:12.750 --> 00:16:16.540
itself, you could say that this
chromosome right here,

00:16:16.540 --> 00:16:18.570
this chromosome like a father,
has one chromatid.

00:16:18.570 --> 00:16:20.740
You could call it a chromatid,
although that tends to not be

00:16:20.740 --> 00:16:21.610
the convention.

00:16:21.610 --> 00:16:25.100
People start talking about
chromatids once you have two

00:16:25.100 --> 00:16:26.780
of them in a chromosome.

00:16:26.780 --> 00:16:29.760
And we'll learn in mitosis and
meiosis, these two chromatids

00:16:29.760 --> 00:16:34.710
separate, and once they
separate, that same strand of

00:16:34.710 --> 00:16:38.570
DNA that you once called a
chromatid, you now call them

00:16:38.570 --> 00:16:40.380
individually chromosomes.

00:16:40.380 --> 00:16:43.600
So that's one of them, and then
you have another one that

00:16:43.600 --> 00:16:46.370
maybe gets separated
in this direction.

00:16:46.370 --> 00:16:48.710
Let me circle that one
with the green.

00:16:48.710 --> 00:16:51.190
So this one might move away like
that, and the one that I

00:16:51.190 --> 00:16:53.510
circled in the orange might
move away like this.

00:16:53.510 --> 00:16:55.820
Now, once they separate and
they're no longer connected by

00:16:55.820 --> 00:16:59.290
the centromere, now what we
originally called as one

00:16:59.290 --> 00:17:02.670
chromosome with two chromatids,
you will now refer

00:17:02.670 --> 00:17:04.819
to as two separate
chromosomes.

00:17:04.819 --> 00:17:07.950
Or you could say now you have
two separate chromosomes, each

00:17:07.950 --> 00:17:10.790
made up of one chromatid.

00:17:10.790 --> 00:17:13.530
So hopefully, that clears up a
little bit some of this jargon

00:17:13.530 --> 00:17:14.240
around DNA.

00:17:14.240 --> 00:17:16.130
I always found it
quite confusing.

00:17:16.130 --> 00:17:18.359
But it'll be a useful tool
when we start going into

00:17:18.359 --> 00:17:20.270
mitosis and meiosis, and
I start saying, oh, the

00:17:20.270 --> 00:17:21.369
chromosomes become chromatids.

00:17:21.369 --> 00:17:23.970
And you'll say, like, wait, how
did one chromosome become

00:17:23.970 --> 00:17:24.859
two chromosomes?

00:17:24.859 --> 00:17:27.280
And how did a chromatid
become a chromosome?

00:17:27.280 --> 00:17:29.470
And it all just revolves
around the vocabulary.

00:17:29.470 --> 00:17:32.730
I would have picked different
vocabulary than calling this a

00:17:32.730 --> 00:17:34.730
chromosome and calling each
of these individually

00:17:34.730 --> 00:17:36.880
chromosomes, but that's
the way we have

00:17:36.880 --> 00:17:37.920
decided to name them.

00:17:37.920 --> 00:17:39.520
Actually, just in case you're
curious, you're probably

00:17:39.520 --> 00:17:41.750
thinking, where does this
word chromo come?

00:17:41.750 --> 00:17:43.510
I don't know if you know
old Kodak film was

00:17:43.510 --> 00:17:44.550
called chromo color.

00:17:44.550 --> 00:17:47.460
And chromo essentially
relates to color.

00:17:47.460 --> 00:17:49.910
I think it comes from the Greek
word actually for color.

00:17:49.910 --> 00:17:52.830
It got that word because when
people first started looking

00:17:52.830 --> 00:17:56.000
in the nucleus of a cell, they
would apply dye, and these

00:17:56.000 --> 00:17:58.480
things that we call chromosomes
would take up the

00:17:58.480 --> 00:18:02.020
dye so that we could see it well
with a light microscope.

00:18:02.020 --> 00:18:04.540
And some comes from soma for
body, so you could kind of

00:18:04.540 --> 00:18:06.560
view it as colored body,
so that's why

00:18:06.560 --> 00:18:07.770
they call it a chromsome.

00:18:07.770 --> 00:18:12.920
So chromatin also will take up--
well, I won't go into all

00:18:12.920 --> 00:18:13.530
of that as well.

00:18:13.530 --> 00:18:15.080
But hopefully, that clears
a little bit this whole

00:18:15.080 --> 00:18:19.480
chromatid, chromosome, chromatin
debate, and we're

00:18:19.480 --> 00:18:21.790
well equipped now to study
mitosis and meiosis.


Variation in a Species

WEBVTT
Kind: captions
Language: en

00:00:00.440 --> 00:00:03.570
The whole process of natural
selection is to some degree

00:00:03.570 --> 00:00:06.690
dependent on the idea of
variation, that within any

00:00:06.690 --> 00:00:11.450
population of a species, you
have some genetic variation.

00:00:11.450 --> 00:00:15.630
So, for example, let's say I
have a bunch of-- well, this

00:00:15.630 --> 00:00:19.500
is a circle species, and one guy
is that color, and then I

00:00:19.500 --> 00:00:22.960
got a bunch more, maybe some
are that color-- oh, that's

00:00:22.960 --> 00:00:26.080
the same color-- that one, and
that one, and that one.

00:00:26.080 --> 00:00:28.630
And for whatever reason,
sometimes there are no

00:00:28.630 --> 00:00:32.030
environmental factors that will
predispose one of these

00:00:32.030 --> 00:00:34.800
guys to be able to survive and
reproduce over the other, but

00:00:34.800 --> 00:00:36.530
every now and then, there might
be some environmental

00:00:36.530 --> 00:00:40.620
factor, and it makes maybe, all
of a sudden, this guy more

00:00:40.620 --> 00:00:42.090
fit to reproduce.

00:00:42.090 --> 00:00:45.170
And so for whatever reason, this
guy is able to reproduce

00:00:45.170 --> 00:00:49.820
more frequently and these
guys less frequently.

00:00:49.820 --> 00:00:53.140
And some of them get killed,
or whatever, or eaten by

00:00:53.140 --> 00:00:56.460
birds, or whatever, or they're
just not able to reproduce for

00:00:56.460 --> 00:00:59.070
whatever reason, and then maybe
these guys are something

00:00:59.070 --> 00:00:59.860
in between.

00:00:59.860 --> 00:01:03.960
So over time, the frequency of
the different traits you see

00:01:03.960 --> 00:01:05.400
in this population
will change.

00:01:05.400 --> 00:01:07.710
And if they are drastic enough,
maybe these guys start

00:01:07.710 --> 00:01:10.910
becoming dominant and start not
liking these guys, because

00:01:10.910 --> 00:01:12.330
they're so different
or whatever else.

00:01:12.330 --> 00:01:14.110
We could see a lot of
different reasons.

00:01:14.110 --> 00:01:16.460
This could eventually turn
into a different species.

00:01:16.460 --> 00:01:21.760
Now, the obvious question is
what leads to this variation?

00:01:21.760 --> 00:01:27.520
In a population what leads to
this-- in fact, even in our

00:01:27.520 --> 00:01:30.710
population, what leads to one
person having dirty blonde

00:01:30.710 --> 00:01:33.490
hair, one person having brown
hair, one person having black

00:01:33.490 --> 00:01:38.630
hair, and we have the spectrum
of skin complexions and

00:01:38.630 --> 00:01:40.590
heights is pretty
much infinite.

00:01:40.590 --> 00:01:41.900
What causes that?

00:01:41.900 --> 00:01:44.930
And then one thing that I kind
of point to, we talked about

00:01:44.930 --> 00:01:47.500
this a little bit in
the DNA video, is

00:01:47.500 --> 00:01:49.440
this notion of mutations.

00:01:49.440 --> 00:01:52.310
DNA, we learned, is just a
sequence of these bases.

00:01:52.310 --> 00:01:56.430
So adenine, guanine, let's say
I've got some thymine going.

00:01:56.430 --> 00:01:58.990
I have some more adenine,
some cytosine.

00:01:58.990 --> 00:02:01.780
And that these code, if you have
enough of these in a row,

00:02:01.780 --> 00:02:04.030
maybe you have a few hundred
or a few thousand of these,

00:02:04.030 --> 00:02:06.720
these code for proteins or
they code for things that

00:02:06.720 --> 00:02:10.020
control other proteins,
but maybe you have a

00:02:10.020 --> 00:02:11.770
change in one of them.

00:02:11.770 --> 00:02:14.330
Maybe this cytosine for whatever
reason becomes a

00:02:14.330 --> 00:02:18.270
guanine randomly, or maybe these
got deleted, and that

00:02:18.270 --> 00:02:19.820
would change the DNA.

00:02:19.820 --> 00:02:23.150
But you could imagine, if I
went to someone's computer

00:02:23.150 --> 00:02:26.220
code and just randomly started
changing letters and randomly

00:02:26.220 --> 00:02:29.480
started inserting letters
without really knowing what

00:02:29.480 --> 00:02:31.790
I'm doing, most of the time,
I'm going to break the

00:02:31.790 --> 00:02:33.060
computer program.

00:02:33.060 --> 00:02:35.800
Most of the time, the great
majority of the time, this is

00:02:35.800 --> 00:02:37.050
going to go nowhere.

00:02:39.150 --> 00:02:41.390
It'll either do nothing, for
example, if I go into

00:02:41.390 --> 00:02:43.240
someone's computer program and
if I just add a couple of

00:02:43.240 --> 00:02:45.190
spaces or something, that might
not change the computer

00:02:45.190 --> 00:02:47.200
program, but if I start getting
rid of semicolons and

00:02:47.200 --> 00:02:49.610
start changing numbers and all
that, it'll probably make the

00:02:49.610 --> 00:02:50.840
computer program break.

00:02:50.840 --> 00:02:53.050
So it'll either do nothing or
it'll actually kill the

00:02:53.050 --> 00:02:55.600
organisms most of the time.

00:02:55.600 --> 00:02:59.730
Mutations: sometimes, they might
make the actual cell

00:02:59.730 --> 00:03:03.060
kind of run amok, and we'll do a
whole maybe series of videos

00:03:03.060 --> 00:03:06.920
on cancer, and that itself
obviously would hurt the

00:03:06.920 --> 00:03:09.840
organism as well as a whole,
although if it occurs after

00:03:09.840 --> 00:03:12.500
the organism has reproduced, it
might not be something that

00:03:12.500 --> 00:03:14.750
selects against the organism
and it also

00:03:14.750 --> 00:03:15.930
wouldn't be passed on.

00:03:15.930 --> 00:03:17.790
But anyway, I won't go
detailed into that.

00:03:17.790 --> 00:03:21.470
But the whole point is that
mutations don't seem to be a

00:03:21.470 --> 00:03:23.470
satisfying source
of variation.

00:03:23.470 --> 00:03:26.350
They could be a source or kind
of contribute on the margin,

00:03:26.350 --> 00:03:29.910
but there must be something more
profound than mutations

00:03:29.910 --> 00:03:32.970
that's creating the diversity
even within, or maybe I should

00:03:32.970 --> 00:03:36.280
call it the variation, even
within a population.

00:03:36.280 --> 00:03:38.800
And the answer here is
really it's kind of

00:03:38.800 --> 00:03:39.710
right in front of us.

00:03:39.710 --> 00:03:43.300
It really addresses kind of one
of the most fundamental

00:03:43.300 --> 00:03:46.260
things about biology, and it's
so fundamental that a lot of

00:03:46.260 --> 00:03:49.790
people never even question
why it is the way it is.

00:03:49.790 --> 00:03:51.910
And that is sexual
reproduction.

00:03:59.460 --> 00:04:02.670
And when I mean sexual
reproduction, it's this notion

00:04:02.670 --> 00:04:07.870
that you have, and pretty much
if you look at all organisms

00:04:07.870 --> 00:04:10.680
that have nucleuses-- and we
call those eukaroytes.

00:04:10.680 --> 00:04:14.780
Maybe I'll do a whole video on
eukaryotes versus prokaryotes,

00:04:14.780 --> 00:04:18.050
but it's the notion that if you
look universally all the

00:04:18.050 --> 00:04:20.190
way from plants-- not
universally, but if you look

00:04:20.190 --> 00:04:23.640
at cells that have nucleuses,
they almost universally have

00:04:23.640 --> 00:04:29.640
this phenomenon that you have
males and you have females.

00:04:29.640 --> 00:04:33.510
In some organisms, an organism
can be both a male and a

00:04:33.510 --> 00:04:39.920
female, but the common idea
here is that all organisms

00:04:39.920 --> 00:04:42.530
kind of produce versions of
their genetic material that

00:04:42.530 --> 00:04:44.960
mix with other organisms'
version of

00:04:44.960 --> 00:04:46.140
their genetic material.

00:04:46.140 --> 00:04:49.410
If mutations were the only
source of variation, then I

00:04:49.410 --> 00:04:53.900
could just bud off other Sals.

00:04:53.900 --> 00:04:58.640
Maybe just other Sals would just
bud off from me, and then

00:04:58.640 --> 00:05:01.550
randomly one Sal might be a
little bit different and

00:05:01.550 --> 00:05:02.320
whatever else.

00:05:02.320 --> 00:05:04.630
But that would, as we already
talked about, most of the

00:05:04.630 --> 00:05:06.730
time, we would have very little
change, very little

00:05:06.730 --> 00:05:09.500
variation, and whatever
variation does occur because

00:05:09.500 --> 00:05:14.160
of any kind of noise being
introduced into this kind of

00:05:14.160 --> 00:05:17.650
budding process where I just
replicate myself identically,

00:05:17.650 --> 00:05:19.100
most of the time it'll
be negative.

00:05:19.100 --> 00:05:21.790
Most of the time, it'll
break the organism.

00:05:21.790 --> 00:05:24.660
Now, when you have sexual
reproduction, what happens?

00:05:24.660 --> 00:05:27.380
Well, you keep mixing and
matching every possible

00:05:27.380 --> 00:05:35.750
combination of DNA in a kind
of species pool of DNA.

00:05:35.750 --> 00:05:39.510
So let me make this a little
bit more concrete for you.

00:05:39.510 --> 00:05:42.780
So let me erase this horrible
drawing I just did.

00:05:42.780 --> 00:05:47.300
So we all have-- let me stick
to humans because

00:05:47.300 --> 00:05:48.550
that's what we are.

00:05:50.880 --> 00:05:56.700
We have 23 pairs of chromosomes,
and in each pair,

00:05:56.700 --> 00:06:00.210
we have one chromosome from our
mother and one chromosome

00:06:00.210 --> 00:06:01.550
from our father.

00:06:01.550 --> 00:06:02.470
So let me draw that.

00:06:02.470 --> 00:06:04.890
So I'll draw my father's
chromosomes in blue.

00:06:04.890 --> 00:06:15.270
So I have 1, 2, 3, 4, 5, 6, 7,
8, 9, 10, 11, 12, 13, 14, 15--

00:06:15.270 --> 00:06:17.280
I'm running out of space.

00:06:17.280 --> 00:06:25.410
Let me do more here-- 16, 17,
18, 19, 20, 21, 22, and then

00:06:25.410 --> 00:06:26.770
I'll throw another one
here that looks

00:06:26.770 --> 00:06:27.970
a little bit different.

00:06:27.970 --> 00:06:30.660
I'll throw one here that looks
like a Y, and we'll talk more

00:06:30.660 --> 00:06:32.340
about the X's and the
Y chromosomes.

00:06:32.340 --> 00:06:35.870
Then I have 23 chromosomes
from my mother.

00:06:35.870 --> 00:06:39.470
And not to be stereotypical,
but maybe I'll do that in a

00:06:39.470 --> 00:06:41.220
more feminine color.

00:06:41.220 --> 00:06:44.360
Let's see, so I have 23
chromosomes from my mother.

00:06:44.360 --> 00:06:56.480
1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
11, 12, 13, 14, 15, 16, 17,

00:06:56.480 --> 00:07:02.740
18, 19, 20, 21, 22, 23.

00:07:02.740 --> 00:07:03.730
So what's going on here?

00:07:03.730 --> 00:07:07.870
I have 23 from my mother.

00:07:07.870 --> 00:07:14.620
I have 23 from my father.

00:07:14.620 --> 00:07:18.070
Now, each of these chromosomes,
and I made them

00:07:18.070 --> 00:07:19.320
right next to each other.

00:07:19.320 --> 00:07:24.870
So let me zoom in on
one pair of these.

00:07:24.870 --> 00:07:28.390
So let's say we look at
chromosome number-- I'll just

00:07:28.390 --> 00:07:31.340
call this chromosome number 3.

00:07:31.340 --> 00:07:33.150
So let me zoom in on chromosome
number 3.

00:07:33.150 --> 00:07:36.330
I have one from my mother
right here.

00:07:36.330 --> 00:07:37.650
Actually, maybe I'll
do it this way.

00:07:37.650 --> 00:07:43.230
Remember, chromosome is just a
big-- if you take the DNA, the

00:07:43.230 --> 00:07:45.930
DNA just keeps wrapping around,
and it actually wraps

00:07:45.930 --> 00:07:48.100
around all these proteins, and
it creates this structure, but

00:07:48.100 --> 00:07:50.430
it's just a big-- when you see
it like that, you're like, oh,

00:07:50.430 --> 00:07:53.860
maybe the DNA-- no, but this
could have millions of base

00:07:53.860 --> 00:07:56.010
pair, so maybe it'll look
something like that.

00:07:56.010 --> 00:08:01.100
It's a densely wrapped version
of-- well, it's a long string

00:08:01.100 --> 00:08:04.230
of DNA, and when it's normally
drawn like this, which is not

00:08:04.230 --> 00:08:06.640
always the way it is, and we'll
talk more about that,

00:08:06.640 --> 00:08:08.240
they draw it as densely
packed like that.

00:08:08.240 --> 00:08:10.170
So let's say that's
from my mother and

00:08:10.170 --> 00:08:11.990
that's from my father.

00:08:11.990 --> 00:08:21.120
Now, let's call this
chromosome 3.

00:08:21.120 --> 00:08:22.680
They're both chromosome 3.

00:08:22.680 --> 00:08:26.270
And what the idea here is that
I'm getting different traits

00:08:26.270 --> 00:08:28.410
from my father and
from my mother.

00:08:28.410 --> 00:08:30.990
And I'm doing a gross
oversimplification here, but

00:08:30.990 --> 00:08:32.620
this is really just to
give you the idea of

00:08:32.620 --> 00:08:35.070
what's going on.

00:08:35.070 --> 00:08:37.820
This chromosome 3, maybe
it contains this

00:08:37.820 --> 00:08:39.460
trait for hair color.

00:08:39.460 --> 00:08:45.930
And maybe my father had-- and
I'll use my actual example.

00:08:45.930 --> 00:08:48.470
My father had very
straight hair.

00:08:48.470 --> 00:08:53.530
So someplace on this chromosome,
there's a gene for

00:08:53.530 --> 00:08:54.470
hair straightness.

00:08:54.470 --> 00:08:56.210
Let's say it's a little
thing right there.

00:08:56.210 --> 00:08:58.610
And remember, that gene could
be thousands of base pairs,

00:08:58.610 --> 00:09:00.540
but let's say this is
hair straightness.

00:09:00.540 --> 00:09:05.360
So my father's version of that
gene, he had the allele for

00:09:05.360 --> 00:09:05.800
straightness.

00:09:05.800 --> 00:09:10.230
And remember, an allele is just
a version of a gene, so

00:09:10.230 --> 00:09:13.800
I'll call it the allele straight
for straight hair.

00:09:16.580 --> 00:09:21.770
Now, this other chromosome that
my mother gave me, this

00:09:21.770 --> 00:09:24.150
essentially, and there are
exceptions, but for the most

00:09:24.150 --> 00:09:26.750
part, it codes for the same
genes, and that's why I put

00:09:26.750 --> 00:09:27.860
them next to each other.

00:09:27.860 --> 00:09:31.210
So this will also have the gene
for hair straightness or

00:09:31.210 --> 00:09:33.520
curlyness, but my mom
does happen to

00:09:33.520 --> 00:09:35.600
actually have curly hair.

00:09:35.600 --> 00:09:40.250
So she has the gene right
there for curly hair.

00:09:40.250 --> 00:09:43.700
The version of the gene
here is allele curly.

00:09:43.700 --> 00:09:46.610
The gene just says, look, this
is the gene for whether or not

00:09:46.610 --> 00:09:47.600
your hair is curly.

00:09:47.600 --> 00:09:50.500
Each version of the gene
is called an allele.

00:09:50.500 --> 00:09:52.640
Allele curly.

00:09:52.640 --> 00:09:56.690
Now, when I got both of these in
my body or in my cells, and

00:09:56.690 --> 00:10:00.040
this is in every cell of my
body, every cell of my body

00:10:00.040 --> 00:10:04.080
except for, and we'll talk in
a few seconds about my germ

00:10:04.080 --> 00:10:07.330
cells, but every cells other
than the ones that I use for

00:10:07.330 --> 00:10:12.060
reproduction have this complete
set of chromosomes in

00:10:12.060 --> 00:10:13.330
it, which I find amazing.

00:10:13.330 --> 00:10:17.690
But only certain chromosomes--
for example, these genes will

00:10:17.690 --> 00:10:20.360
be completely useless in my
fingernails, because all of a

00:10:20.360 --> 00:10:22.760
sudden, the straight and the
curly don't matter that much.

00:10:22.760 --> 00:10:23.880
And I'm simplifying.

00:10:23.880 --> 00:10:25.980
Maybe they will on some
other dimension.

00:10:25.980 --> 00:10:27.700
But let's say for simplicity,
they won't

00:10:27.700 --> 00:10:28.850
matter in certain places.

00:10:28.850 --> 00:10:32.240
So certain genes are expressed
in certain parts of the body,

00:10:32.240 --> 00:10:35.600
but every one of your body
cells, and we call those

00:10:35.600 --> 00:10:38.980
somatic cells, and we'll
separate those from the sex

00:10:38.980 --> 00:10:45.050
sells or the germs that we'll
talk about later.

00:10:45.050 --> 00:10:48.060
So this is my body cells.

00:10:48.060 --> 00:10:50.220
So this is the great majority
of your cells, and this is

00:10:50.220 --> 00:10:52.030
opposed to your germ cells.

00:10:52.030 --> 00:10:54.080
And the germ cells-- I'll just
write it here, just so you get

00:10:54.080 --> 00:10:58.950
a clear-- for a male, that's
the sperm cells, and for

00:10:58.950 --> 00:11:01.210
female that's the egg
cells, or the ova.

00:11:04.550 --> 00:11:09.310
But most of my cells have a
complete collection of these,

00:11:09.310 --> 00:11:13.370
and what I want to give you
the idea is that for every

00:11:13.370 --> 00:11:16.200
trait, I essentially have two
versions: one from my mother

00:11:16.200 --> 00:11:17.630
and one from my father.

00:11:17.630 --> 00:11:21.065
Now, these right here are called
homologous chromosomes.

00:11:25.780 --> 00:11:32.840
What that means is every time
you see this prefix homologous

00:11:32.840 --> 00:11:35.850
or if you see like Homo sapiens
or even the word

00:11:35.850 --> 00:11:41.110
homosexual or homogeneous,
it means same, right?

00:11:41.110 --> 00:11:42.640
You see that all the time.

00:11:42.640 --> 00:11:46.990
So homologous means that they're
almost the same.

00:11:46.990 --> 00:11:52.230
They're coding for the most part
the same set of genes,

00:11:52.230 --> 00:11:55.100
but they're not identical.

00:11:55.100 --> 00:11:58.950
They actually might code for
slightly different versions of

00:11:58.950 --> 00:11:59.660
the same gene.

00:11:59.660 --> 00:12:02.920
So depending on what versions
I get, what is actually

00:12:02.920 --> 00:12:06.120
expressed for me, so my
genotype-- let me introduce

00:12:06.120 --> 00:12:08.940
another word, and
I'm overwhelming

00:12:08.940 --> 00:12:10.490
you with words here.

00:12:10.490 --> 00:12:17.630
So my genotype is exactly what
alleles I have, what versions

00:12:17.630 --> 00:12:18.170
of the gene.

00:12:18.170 --> 00:12:21.210
So I got like the fifth version
of the curly allele.

00:12:21.210 --> 00:12:23.850
There could be multiple versions
of the curly allele

00:12:23.850 --> 00:12:25.490
in our gene pool.

00:12:25.490 --> 00:12:27.820
And maybe I got some version
of the straight allele.

00:12:27.820 --> 00:12:29.980
That is my genotype.

00:12:29.980 --> 00:12:36.170
My phenotype is what my hair
really looks like.

00:12:36.170 --> 00:12:39.720
So, for example, two people
could have different genotypes

00:12:39.720 --> 00:12:42.880
with the same-- they might
code for hair that looks

00:12:42.880 --> 00:12:44.560
pretty much the same,
so it might have a

00:12:44.560 --> 00:12:47.870
very similar phenotype.

00:12:47.870 --> 00:12:48.980
So one phenotype can be

00:12:48.980 --> 00:12:50.640
represented by multiple genotypes.

00:12:50.640 --> 00:12:53.690
So that's just one thing to
think about, and we'll talk a

00:12:53.690 --> 00:12:55.780
lot about that in the future,
but I just wanted to introduce

00:12:55.780 --> 00:12:57.380
you to that there.

00:12:57.380 --> 00:13:01.200
Now, I entered this whole
discussion because I wanted to

00:13:01.200 --> 00:13:02.430
talk about variation.

00:13:02.430 --> 00:13:04.570
So how does variation happen?

00:13:04.570 --> 00:13:09.130
Well, what's going to happen
when I-- well, let

00:13:09.130 --> 00:13:09.640
me put it this way.

00:13:09.640 --> 00:13:11.290
What's going to happen
when I reproduce?

00:13:11.290 --> 00:13:13.990
And I have. I have a son.

00:13:13.990 --> 00:13:18.160
Well, my contribution to my son
is going to be a random

00:13:18.160 --> 00:13:22.530
collection of half
of these genes.

00:13:22.530 --> 00:13:25.830
For each homologous pair, I'm
either going to contribute the

00:13:25.830 --> 00:13:28.740
one that I got from my mother or
the one that I got from my

00:13:28.740 --> 00:13:30.250
father, right?

00:13:30.250 --> 00:13:37.360
So let's say that the sperm cell
that went on to fertilize

00:13:37.360 --> 00:13:43.440
my wife's egg, let's say it
happened to have that one,

00:13:43.440 --> 00:13:45.400
that one, or I could
just pick one from

00:13:45.400 --> 00:13:46.830
each of these 23 sets.

00:13:46.830 --> 00:13:49.020
And you say, well, how many
combinations are there?

00:13:49.020 --> 00:13:56.240
Well, for every set, I could
pick one of the two homologous

00:13:56.240 --> 00:13:59.550
chromosomes, and I'm going
to do that 23 times.

00:13:59.550 --> 00:14:02.360
2 times 2 times 2, so that's
2 to the twenty third.

00:14:02.360 --> 00:14:09.220
So 2 to the 23 different
versions that I can contribute

00:14:09.220 --> 00:14:12.800
to any son or daughter that I
might have. We'll talk about

00:14:12.800 --> 00:14:16.310
how that happens when we talk
about meiosis or mitosis, that

00:14:16.310 --> 00:14:20.660
when I generate my sperm cells,
sperm cells essentially

00:14:20.660 --> 00:14:26.590
takes one-- instead of having
23 pairs of chromosomes in

00:14:26.590 --> 00:14:29.540
sperm, you only have
23 chromosomes.

00:14:29.540 --> 00:14:32.760
So, for example, I'll take one
from each of those, and

00:14:32.760 --> 00:14:35.030
through the process of meiosis,
which we'll go into,

00:14:35.030 --> 00:14:37.200
I'll generate a bunch
of sperm cells.

00:14:37.200 --> 00:14:44.740
And each sperm cell will have
one from each of these pairs,

00:14:44.740 --> 00:14:46.440
one version from each
of those pairs.

00:14:46.440 --> 00:14:51.980
So maybe for this chromosome I
get it from my dad, from the

00:14:51.980 --> 00:14:54.370
next chromosome, I get
it from my mom.

00:14:54.370 --> 00:14:56.840
Then I donate a couple more
from-- I should've drawn them

00:14:56.840 --> 00:14:57.820
next to each other.

00:14:57.820 --> 00:14:59.580
I donate a couple more
from my mom.

00:14:59.580 --> 00:15:03.540
Then for chromosome number 5,
it comes from my dad, and so

00:15:03.540 --> 00:15:04.200
on and so forth.

00:15:04.200 --> 00:15:06.200
But there's 2 to the
twenty-third combinations

00:15:06.200 --> 00:15:08.240
here, because there
are 23 pairs that

00:15:08.240 --> 00:15:09.290
I'm collecting from.

00:15:09.290 --> 00:15:13.530
Now, my wife's egg is going to
have the same situation.

00:15:13.530 --> 00:15:17.600
There are 2 to the 23 different
combinations of DNA

00:15:17.600 --> 00:15:20.730
that she can contribute just
based on which of the

00:15:20.730 --> 00:15:24.590
homologous pairs she
will contribute.

00:15:24.590 --> 00:15:28.390
So the possible combinations
that just one couple can

00:15:28.390 --> 00:15:34.990
produce, and I'm using my life
as an example, but this

00:15:34.990 --> 00:15:35.980
applies to everything.

00:15:35.980 --> 00:15:39.430
This applies to every species
that experiences sexual

00:15:39.430 --> 00:15:41.190
reproduction.

00:15:41.190 --> 00:15:46.280
So if I can give 2 to
twenty-third combinations of

00:15:46.280 --> 00:15:50.620
DNA and my wife can give 2 to
the 23 combinations of DNA,

00:15:50.620 --> 00:15:55.240
then we can produce 2 to the
forty-sixth combinations.

00:15:55.240 --> 00:15:58.220
Now, just to give an idea of how
large of a number this is,

00:15:58.220 --> 00:16:02.560
this is roughly 12,000 times the
number of human beings on

00:16:02.560 --> 00:16:03.490
the planet today.

00:16:03.490 --> 00:16:07.210
So there's a huge amount of
variation that even one couple

00:16:07.210 --> 00:16:08.270
can produce.

00:16:08.270 --> 00:16:12.260
And if you thought that even
that isn't enough, it turns

00:16:12.260 --> 00:16:15.560
out that amongst these
homologous pairs, and we'll

00:16:15.560 --> 00:16:18.510
talk about when this happens in
meiosis, you can actually

00:16:18.510 --> 00:16:20.630
have DNA recombination.

00:16:20.630 --> 00:16:24.450
And all that means is when these
homologous pairs during

00:16:24.450 --> 00:16:27.730
meiosis line up near each other,
you can have this thing

00:16:27.730 --> 00:16:32.340
called crossover, where all of
this DNA here crosses over and

00:16:32.340 --> 00:16:34.430
touches over here, and all
this DNA crosses over and

00:16:34.430 --> 00:16:35.200
touches over there.

00:16:35.200 --> 00:16:38.550
So all of this goes there and
all of this goes there.

00:16:38.550 --> 00:16:44.010
What you end up with after the
crossover is that one DNA, the

00:16:44.010 --> 00:16:46.980
one that came from my mom, or
that I thought came from my

00:16:46.980 --> 00:16:49.980
mom, now has a chunk that came
from my dad, and the chunk

00:16:49.980 --> 00:16:55.150
that came from my dad, now has a
chunk that came from my mom.

00:16:55.150 --> 00:16:56.700
Let me do that in
the right color.

00:16:56.700 --> 00:16:59.110
It came from my mom like that.

00:16:59.110 --> 00:17:01.980
And so that even increases the
amount of variety even more.

00:17:01.980 --> 00:17:04.329
So you can almost now, instead
of talking about the different

00:17:04.329 --> 00:17:06.140
chromosomes that you're
contributing where the

00:17:06.140 --> 00:17:09.359
chromosomes are each of these
collections of DNA, you're now

00:17:09.359 --> 00:17:11.579
talking about-- you can almost
go to the different

00:17:11.579 --> 00:17:13.700
combinations at the gene level,
and now you can think

00:17:13.700 --> 00:17:17.410
about it in almost infinite
form of variation.

00:17:17.410 --> 00:17:20.910
You can think about all of the
variation that might emerge

00:17:20.910 --> 00:17:23.720
when you start mixing and
matching different versions of

00:17:23.720 --> 00:17:25.560
the same gene in a population.

00:17:25.560 --> 00:17:27.150
And you don't just
look at one gene.

00:17:27.150 --> 00:17:31.350
I mean, the reality is that
genes by themselves very

00:17:31.350 --> 00:17:35.290
seldom code for a specific-- you
can very seldom look for

00:17:35.290 --> 00:17:38.330
one gene and say, oh, that is
brown hair, or look for one

00:17:38.330 --> 00:17:41.330
gene and say, oh, that's
intelligence, or that is how

00:17:41.330 --> 00:17:42.550
likable someone is.

00:17:42.550 --> 00:17:45.020
It's usually a whole set of
genes interacting in an

00:17:45.020 --> 00:17:46.840
incredibly complicated way.

00:17:46.840 --> 00:17:50.430
You know, hair might be coded
for by this whole set of genes

00:17:50.430 --> 00:17:52.720
on multiple chromosomes and
this might be coded for a

00:17:52.720 --> 00:17:54.880
whole set of genes on multiple
chromosomes.

00:17:54.880 --> 00:17:56.630
And so then you can start
thinking about all of the

00:17:56.630 --> 00:17:57.690
different combinations.

00:17:57.690 --> 00:17:59.500
And then all of a sudden, maybe
some combination that

00:17:59.500 --> 00:18:02.190
never existed before all of a
sudden emerges, and that's

00:18:02.190 --> 00:18:03.450
very successful.

00:18:03.450 --> 00:18:06.020
But I'll leave you to think
about it because maybe that

00:18:06.020 --> 00:18:08.770
combination might be passed on,
or it may not be passed on

00:18:08.770 --> 00:18:10.580
because of this recombination.

00:18:10.580 --> 00:18:12.480
But we'll talk more about
that in the future.

00:18:12.480 --> 00:18:15.720
But I wanted to introduce this
idea of sexual reproduction to

00:18:15.720 --> 00:18:21.690
you, because this really is the
main source of variation

00:18:21.690 --> 00:18:24.080
within a population.

00:18:24.080 --> 00:18:26.730
To me, it's kind of a
philosophical idea, because we

00:18:26.730 --> 00:18:31.580
almost take the idea of having
males and females for granted

00:18:31.580 --> 00:18:34.080
because it's this
universal idea.

00:18:34.080 --> 00:18:36.400
But I did a little reading on
it, and it turns out that this

00:18:36.400 --> 00:18:40.310
actually only emerged about 1.4
billion years ago, that

00:18:40.310 --> 00:18:43.820
this is almost a useful trait,
because once you introduce

00:18:43.820 --> 00:18:48.360
this level of variation, the
natural selection can start--

00:18:48.360 --> 00:18:50.300
you can kind of say that when
you have this more powerful

00:18:50.300 --> 00:18:53.910
form of variation than just pure
mutations, and maybe you

00:18:53.910 --> 00:18:56.540
might have some primitive form
of crossover before, but now

00:18:56.540 --> 00:18:59.220
that you have this sexual
reproduction and you have this

00:18:59.220 --> 00:19:01.980
variation, natural selection
can occur in a

00:19:01.980 --> 00:19:03.320
more efficient way.

00:19:03.320 --> 00:19:06.510
So that species that were able
to reproduce and essentially

00:19:06.510 --> 00:19:09.650
recombine their DNA and mix and
match it in this way were

00:19:09.650 --> 00:19:13.060
able to produce more variety and
were able to essentially

00:19:13.060 --> 00:19:15.120
be selected for their
environment in a more

00:19:15.120 --> 00:19:17.740
efficient way so they started
to essentially outnumber the

00:19:17.740 --> 00:19:20.050
ones that couldn't, so it
became a kind of very

00:19:20.050 --> 00:19:21.720
universal trait.

00:19:21.720 --> 00:19:24.760
But you could have imagined a
world, and there are science

00:19:24.760 --> 00:19:27.180
fiction books written about
this, where you have three

00:19:27.180 --> 00:19:30.160
genders, where you have gender
one, two, three.

00:19:30.160 --> 00:19:31.410
You could have 10 genders.

00:19:31.410 --> 00:19:36.040
It just happens to be that on
Earth, this notion of having

00:19:36.040 --> 00:19:39.750
two genders turned out to be a
very efficient and stable way

00:19:39.750 --> 00:19:42.640
of introducing variation
into a population.

00:19:42.640 --> 00:19:44.070
So, hopefully, you found
that interesting.

00:19:44.070 --> 00:19:46.140
In the next video, I'll go more
into the detail of how

00:19:46.140 --> 00:19:49.790
exactly meiosis and
mitosis works.