Zygote differentiating into somatic and germs cells

Voiceover: In the last
video, we saw an egg

from our mothers fuse with
a sperm from our fathers

to form a zygote that
would keep replicating

and turn into us, if this...

Let's say this was the
sperm from my father

that fuses with the egg from my mother,

and then this zygote will keep replicating

and eventually turn into Sal,

and make a video about fertilization.

So how does that actually happen?

So now that this zygote has the diploid

number of chromosomes, and once again

they're referring to that as 2N,

where N would be the haploid number,

2N would be the diploid number,

and in the case of human beings,

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

N is 23 and two times N of course

would be 46 chromosomes.

I have my full contingency
of chromosomes here.

I got a Y sex chromosome from my father,

X of course from my mother,

so I'm going to be a male.

And so what then happens?

Well now, through mitosis, this zygote

is going to keep replicating.

So it will, you know, after one,

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

of the mechanics of mitosis,

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

It is now two cells.

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

I'm not going to draw it at scale.

It's now two cells.

I want to make sure I have enough space

on my little chalkboard here.

It has two cells.

Instead of drawing all of the chromosomes,

let me just say that each of these,

in my nucleus, I still have 2N.

I have the diploid number.

So each of these two cells

that it has differentiated to

still have the full contingency.

That's what mitosis does.

It essentially replicates
the entire cells.

You have the same number of chromosomes.

And then this process is

just going to keep happening.

These two characters
are going to replicate,

are going to replicate, and so then

you're going to have, through mitosis,

and now you're going...

So this is another round
of mitosis right over here.

Mitsosis.

So they just keep duplicating themselves.

And each of these cells
have the full contingency.

2N, the diploid number of chromosomes

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

for a human being.

And then this process

is just going to keep happening.

So this process is
going to keep happening.

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

a lot of this has been going on.

So mitosis is just
going to keep happening.

And so eventually you're going to have

thousands of these cells,

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

millions and ten millions of them.

So let me draw them really
really really small.

There's a bunch of them there.

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

the diploid number of chromosomes.

They're going to have 46 chromosomes.

23 pair of homologous chromosomes.

So we now have a big ball of these here.

And these cells, some
of them are going to,

they're going to differentiate into me.

They're going to differentiate into

the different parts of my body.

So for example, these
cells right over here

might eventually,
they'll keep replicating,

but then it's them and their offspring

might eventually differentiate
into my brain cells.

These cells here will keep replicating,

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

or the things that they replicate into,

might differentiate into my heart.

These right over here might differentiate

into my lungs, and of course

all of these eventually will differentiate

into all the different, and
they and their offspring

will differentiate into all of the things

that make me me.

And so you have a lot
more of this mitosis.

You're eventually going
to have a human being.

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

Mitosis.

And now let me make an attempt

to draw a human being.

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

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

I have big eyebrows as well.

Still doesn't look like me, but anyway.

You get the point.

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

But this is obviously not my best

rendering of a human being.

But you get the general idea.

Nice broad shoulders, that's nice.

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

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

Anyway, you get the general idea.

My stomach isn't quite that flat,

but anyway, you get the general idea.

So this, it'll eventually differentiate

into a human being.

These cells here in pink
eventually differentiate

into the cells in the brain.

These cells here eventually differentiate

into the cells into the lungs,

and obviously at this scale,

the cells are way too small to even see.

These cells differentiate into the cells

of the heart.

Now, I want to draw an
important distinction here.

Because most of the cells that I've just

depicted here that are
just a product of mitosis,

these are your, I guess you could say

these are your body cells,

or these are your somatic cells.

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

in your heart, your lung, your brain,

these are somatic cells, or body cells.

Somatic cells.

And so you're probably wondering,

well how do I eventually get these

haploid number cells?

How do I eventually get,

if I'm talking about a male,

how do I eventually get
these haploid sex cells,

these gametes, these sperm cells?

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

get these ova, these egg cells

that have a haploid number?

And the way that happens
is some of these cells

up here are going to
differentiate into germ cells.

So they're going to
differentiate into germ cells.

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

when I say into germ cells,

they're going to differentiate
into your gonads.

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

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

The gonads are the testes.

And the germ cells in the gonads

or the cells that have differentiated into

being part of the testes and ovaries,

those germ cells.

So we differentiate
them from somatic cells.

So there are germ cells there.

Germ cells in your ovaries and testes.

They, through the process of meiosis,

they through the process of meiosis,

can produce the gametes.

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

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

But this is through
the process of meiosis.

Meiosis you're going to produce sperm

in the case of a man,

and you're going to produce ova

in the case of a female.

And this brings up a
really interesting thing,

because throughout biology
we talk about mutations

and natural selection and whatever else.

And it's important to
realize how mutations

may affect you and your offspring.

So if you have a mutation

in one of the somatic cells here,

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

or in the heart,

that may affect your ability to, you know,

especially if God forbid it's a really

dangerous thing like cancer,

and it happens when you're young,

before you've had a chance to reproduce

and you're not able to survive,

that might affect your
ability to reproduce.

But if this is happening
in a somatic cell,

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

of what you pass on.

The DNA make-up of what you pass on,

that's determined by what's going on

in the gametes.

So a mutation, if on the
way to differentiating

into gametes a mutation happens,

so if one of these mutate

and then keep replicating,

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

and they keep replicating and

they differentiate into the germ cells

the mutation is right over there,

then through meiosis that produces

some mutated sperm.

Then that would pass on to your, well,

that has a chance of
passing on to your children.

Because once again, it might not be

all of the sperm cells
that have that mutation.

It could be only a handful of the

two to three hundred
million of the sperm cells,

and so if that mutation
somehow makes it harder

for some of the sperm cells

to kind of function,
either fuse with an egg

or even potentailly
develop and kind of swim

through fluid, then it still might not be

the thing that makes it.

So mutations only affect your offspring

in the situation where those, the cells

in which they happen are
eventually differentiated

into things, into gametes that you will

pass on to your children.