WEBVTT

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Voiceover: In the last
video, we saw an egg

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from our mothers fuse with
a sperm from our fathers

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to form a zygote that
would keep replicating

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and turn into us, if this...

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Let's say this was the
sperm from my father

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that fuses with the egg from my mother,

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and then this zygote will keep replicating

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and eventually turn into Sal,

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and make a video about fertilization.

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So how does that actually happen?

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So now that this zygote has the diploid

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number of chromosomes, and once again

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they're referring to that as 2N,

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where N would be the haploid number,

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2N would be the diploid number,

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and in the case of human beings,

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N is 23, so in the case of human beings,

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N is 23 and two times N of course

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would be 46 chromosomes.

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I have my full contingency
of chromosomes here.

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I got a Y sex chromosome from my father,

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X of course from my mother,

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so I'm going to be a male.

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And so what then happens?

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Well now, through mitosis, this zygote

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is going to keep replicating.

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So it will, you know, after one,

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so after one, and we're
going to go into the details

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of the mechanics of mitosis,

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but after one round of
mitosis, it is now two cells.

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It is now two cells.

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And I'm going to draw it, once again,

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I'm not going to draw it at scale.

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It's now two cells.

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I want to make sure I have enough space

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on my little chalkboard here.

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It has two cells.

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Instead of drawing all of the chromosomes,

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let me just say that each of these,

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in my nucleus, I still have 2N.

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I have the diploid number.

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So each of these two cells

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that it has differentiated to

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still have the full contingency.

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That's what mitosis does.

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It essentially replicates
the entire cells.

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You have the same number of chromosomes.

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And then this process is

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just going to keep happening.

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These two characters
are going to replicate,

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are going to replicate, and so then

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you're going to have, through mitosis,

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and now you're going...

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So this is another round
of mitosis right over here.

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Mitsosis.

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So they just keep duplicating themselves.

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And each of these cells
have the full contingency.

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2N, the diploid number of chromosomes

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for, well, in this case
it's going to be 46

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for a human being.

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And then this process

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is just going to keep happening.

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So this process is
going to keep happening.

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I'll do dot dot dot to
show that, you know,

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a lot of this has been going on.

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So mitosis is just
going to keep happening.

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And so eventually you're going to have

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thousands of these cells,

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and eventually as we'll
see, you're going to have

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millions and ten millions of them.

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So let me draw them really
really really small.

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There's a bunch of them there.

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And each of them, each
of them are going to have

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the diploid number of chromosomes.

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They're going to have 46 chromosomes.

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23 pair of homologous chromosomes.

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So we now have a big ball of these here.

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And these cells, some
of them are going to,

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they're going to differentiate into me.

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They're going to differentiate into

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the different parts of my body.

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So for example, these
cells right over here

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might eventually,
they'll keep replicating,

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but then it's them and their offspring

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might eventually differentiate
into my brain cells.

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These cells here will keep replicating,

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and them and their offspring,
I guess you could say,

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or the things that they replicate into,

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might differentiate into my heart.

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These right over here might differentiate

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into my lungs, and of course

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all of these eventually will differentiate

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into all the different, and
they and their offspring

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will differentiate into all of the things

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that make me me.

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And so you have a lot
more of this mitosis.

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You're eventually going
to have a human being.

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So let me just say this
is more mitosis going on.

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Mitosis.

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And now let me make an attempt

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to draw a human being.

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That doesn't really look like me, but...

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Well, I have a lot of
hair, so that's my...

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I have big eyebrows as well.

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Still doesn't look like me, but anyway.

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You get the point.

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And so, and I'll try
to draw fairly quickly.

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But this is obviously not my best

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rendering of a human being.

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But you get the general idea.

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Nice broad shoulders, that's nice.

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All right, so let me, let me...

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I'm focusing too much on
drawing this human being.

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Anyway, you get the general idea.

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My stomach isn't quite that flat,

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but anyway, you get the general idea.

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So this, it'll eventually differentiate

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into a human being.

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These cells here in pink
eventually differentiate

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into the cells in the brain.

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These cells here eventually differentiate

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into the cells into the lungs,

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and obviously at this scale,

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the cells are way too small to even see.

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These cells differentiate into the cells

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of the heart.

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Now, I want to draw an
important distinction here.

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Because most of the cells that I've just

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depicted here that are
just a product of mitosis,

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these are your, I guess you could say

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these are your body cells,

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or these are your somatic cells.

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So these, all of these
cells that I'm pointing out

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in your heart, your lung, your brain,

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these are somatic cells, or body cells.

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Somatic cells.

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And so you're probably wondering,

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well how do I eventually get these

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haploid number cells?

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How do I eventually get,

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if I'm talking about a male,

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how do I eventually get
these haploid sex cells,

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these gametes, these sperm cells?

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I'm talking about a
female, how do I eventually

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get these ova, these egg cells

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that have a haploid number?

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And the way that happens
is some of these cells

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up here are going to
differentiate into germ cells.

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So they're going to
differentiate into germ cells.

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In the case of, and they're
going to differentiate

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when I say into germ cells,

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they're going to differentiate
into your gonads.

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In the case of a female,
the gonads are the ovaries.

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And in the case of a male,
the gonads are the testes.

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The gonads are the testes.

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And the germ cells in the gonads

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or the cells that have differentiated into

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being part of the testes and ovaries,

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those germ cells.

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So we differentiate
them from somatic cells.

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So there are germ cells there.

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Germ cells in your ovaries and testes.

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They, through the process of meiosis,

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they through the process of meiosis,

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can produce the gametes.

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So if you're female you're
going to produce eggs.

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If you're male you're
going to produce sperm.

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But this is through
the process of meiosis.

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Meiosis you're going to produce sperm

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in the case of a man,

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and you're going to produce ova

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in the case of a female.

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And this brings up a
really interesting thing,

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because throughout biology
we talk about mutations

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and natural selection and whatever else.

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And it's important to
realize how mutations

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may affect you and your offspring.

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So if you have a mutation

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in one of the somatic cells here,

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let's say in a skin cell, or in you brain,

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or in the heart,

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that may affect your ability to, you know,

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especially if God forbid it's a really

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dangerous thing like cancer,

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and it happens when you're young,

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before you've had a chance to reproduce

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and you're not able to survive,

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that might affect your
ability to reproduce.

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But if this is happening
in a somatic cell,

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it's not going to affect the DNA make-up

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of what you pass on.

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The DNA make-up of what you pass on,

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that's determined by what's going on

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in the gametes.

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So a mutation, if on the
way to differentiating

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into gametes a mutation happens,

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so if one of these mutate

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and then keep replicating,

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so let's say there's a mutation here,

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and they keep replicating and

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they differentiate into the germ cells

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the mutation is right over there,

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then through meiosis that produces

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some mutated sperm.

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Then that would pass on to your, well,

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that has a chance of
passing on to your children.

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Because once again, it might not be

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all of the sperm cells
that have that mutation.

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It could be only a handful of the

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two to three hundred
million of the sperm cells,

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and so if that mutation
somehow makes it harder

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for some of the sperm cells

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to kind of function,
either fuse with an egg

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or even potentailly
develop and kind of swim

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through fluid, then it still might not be

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the thing that makes it.

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So mutations only affect your offspring

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in the situation where those, the cells

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in which they happen are
eventually differentiated

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into things, into gametes that you will

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pass on to your children.