WEBVTT

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- [Voiceover] Organisms
that reproduce sexually

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have got to get their
genes together somehow.

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To do this, they package
their genetic material

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into specialized cells called sex cells.

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This is the sperm cell.

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It's the sex cell of the male.

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The sole purpose of the sperm,

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the entire reason for its existence,

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is to transfer the male's genetic material

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into the female sex cell or the egg.

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So the sperm cell is packed with features

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that allow it to fulfill its job.

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It's basically a little torpedo.

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You can see, just like a torpedo,

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it has a pointed head which allows it

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to travel in the forward direction.

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At the back end it has the tail,

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and the tail is just a flagellum,

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and as the flagellum spins it
acts like a little propeller.

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And then it has a middle section.

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Now within that middle section,

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wrapped around the base
of the flagellar tail,

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are all of these little
organelles called mitochondria.

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You can see I'm drawing these mitochondria

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wrapped around the flagellar base here.

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And mitochondria are organelles

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that are responsible for
giving energy to a cell.

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And the sperm cell has
a whole bunch packed

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right into the base of the flagellum here.

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Probably 75 to 100, and they're
quite large mitochondria.

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In fact, these mitochondria
are often fused together

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to create big organelles.

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And the reason is because
in order to propel

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the sperm torpedo towards the egg,

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it needs a lot of energy.

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And that all comes from
these mitochondria.

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Now the payload of the
torpedo is here in the head,

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and that's the genetic material,
our DNA within the nucleus.

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I'll draw it kind of all coiled back here.

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And here's the nuclear envelope here.

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And just like any regular torpedo,

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the sperm torpedo has a
warhead right in the front.

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And that warhead is a
little collection of enzymes

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called the acrosome.

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And the acrosome is gonna be important

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later on for fertilization.

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But that's pretty much it.

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There's a tail at the back
to provide propulsion,

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some mitochondria in the middle section

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to give energy to the tail,

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a head which contains
the nuclear material,

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and the acrosome.

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This is a pretty bare bones cell.

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It's designed to move fast
and to get to the egg.

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There are no bells and whistles here.

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Now that's in contrast to the egg cell.

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Now the first thing you'll notice

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is that the egg cell is round,

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unlike the torpedo
shape of the sperm cell.

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This is not a cell that's
made for active mobility.

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The second thing is that
the egg cell is huge

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compared to the sperm cell.

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It's so big, in fact,
that it's almost visible.

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In fact, sometimes it is
visible to the human eye.

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Now compared to the sperm
cell, which I'll draw in here,

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the egg cell is about
10,000 times more massive.

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And similar to the sperm cell,

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the egg cell has its
share of genetic material

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ready to combine during fertilization.

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You can see it here within the nucleus.

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And so you may have noticed this

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thick outer coating on the egg cell here,

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that's a very important structure
called the zona pellucida.

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And the zona pellucida is a
thick layer of glycoproteins

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that sit on the outside of the egg.

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And glycoproteins are basically a protein,

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I'll draw a protein here in green,

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with a whole bunch of
branching sugar chains

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that are coming off of them.

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And so what this looks like
is basically a little tree

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or a long branching
thing that's growing out

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of the edge of the egg cell.

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And there's a whole bunch of them,

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and they form this very thick
kind of protective layer

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that the sperm has to get through.

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And the edge of the egg cell
is the actual plasma membrane.

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And once the sperm can deliver

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its genetic material beyond that,

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fertilization has occurred.

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Now there's a whole
bunch of other structures

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within the egg cytoplasm as well.

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And remember this thing is huge.

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And I'm gonna draw in a few here in green.

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Now what I'm drawing in are
actually more mitochondria.

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Now remember the egg cell had
75 to 100 big mitochondria

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right at the base of the flagellum

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to provide energy to drive locomotion.

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Well, the egg cell has mitochondria too.

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It's got a lot of other
different organelles as well.

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But the egg cell is so large, it's got

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somewhere between 100 and 200
thousand mitochondria present.

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So keep those mitochondria
in mind, we'll talk

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about them a little bit
later in the next section.

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So now that you've met the
two major players here,

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the sperm and the egg,
or the male and female

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sex cells respectively, we can talk

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about what happens when they meet.

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And that process is called fertilization.

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So we'll go ahead and
label our egg down here.

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And we'll put a quick
label on this as well.

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This is the zona pellucida
that we talked about earlier.

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So we'll draw the sperm here
coming in to meet the egg.

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We'll draw its tail.

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We'll draw its middle section here.

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And we'll draw its torpedo-like head here.

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Get rid of all this zona pellucida

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glycoprotein in the middle here.

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Now here are the mitochondria
in the middle section here,

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and we have the genetic material payload

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of our sperm torpedo here in the back,

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and our acrosome here in the front.

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Now the first thing that
happens during fertilization

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is that the sperm comes into contact

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with the zona pellucida.

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And the zona pellucida actually binds

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to the outside of the sperm,

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and that's called sperm binding.

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And it's step number one.

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Now what happens when the sperm gets bound

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to the zona pellucida is
that that sets up a reaction

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called the acrosomal reaction.

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So step number two is called
the acrosome reaction.

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And that little warhead
tip of the sperm torpedo

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gets released, and so we have
all of the acrosomal enzymes

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that were sitting in the head
that just kind of leak out

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into that zona pellucida.

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And as those enzymes leak
out, they actually start

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to digest away the zona pellucida.

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Here you can see I'm kind of eating away

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here at those glycoprotein,

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and that allows the sperm
head to dive in deeper

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towards our plasma membrane.

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Now as the sperm gets closer
to the plasma membrane

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of the egg, and it comes
in contact right here,

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it starts up a process of binding.

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The two touch and they come together.

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And as they start together,
it causes another reaction,

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and that third reaction is
called cortical reaction.

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And what I haven't drawn here

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is another structure in the egg,

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and those structures are right

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underneath the plasma membrane,

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and they just sit there waiting.

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And they wait and their entire job

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is to wait for a sperm to bind.

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And as soon as a single sperm binds,

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they get ejected out into
that zone pellucida as well.

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Just like the acrosomal enzymes,

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these enzymes that are contained

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within the cortical granules also start

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eating away at the zona pellucida.

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These enzymes eat away, and
they dissolve and chew up

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these glycoproteins, but
specifically they chew up

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the glycoprotein that
allows sperm to bind.

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So at this point we have a
single sperm that's bound,

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set off the cortical reaction,

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and these cortical granules are released

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that chew up all the other places

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that more sperm can bind.

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So as our other sperm
torpedoes are coming in,

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they're just bouncing off.

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They hit the glycoproteins,
but the one that they need

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to bind to isn't there because
it's all been chewed up

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and degraded by these cortical granules.

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So that's actually called
a block to polyspermy.

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Now that's a very important concept.

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Polyspermy is a term that
just means multiple sperm.

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And what we don't want is
for more than one sperm

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to inject its nuclear material,
its DNA into this egg.

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What you'd end up with is an egg

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with a single contribution from mom

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and then one or two or
three or 100 contributions

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or genetic material from dad.

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And that would never
work, you'd end up with

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all sorts of problems as
the egg started to divide.

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Occasionally, that does happen,

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and it can result in a zygote that fails.

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But for the most part, as
these cortical granules

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dissolve away all the
sperm binding glycoproteins

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of the zona pellucida, other
sperm just can't get in

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and they bounce off as they arrive.

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So now we have a sperm that's made its way

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all the way to the plasma
membrane of our egg cell.

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It's started to bind
to the plasma membrane,

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the acrosome is gone,
I'll erase that here.

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It's done its job.

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The cortical granules have been released,

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and they're preventing
other sperm from getting in.

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And we start to actually
fuse our plasma membranes

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of our sperm cell and our egg cell.

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And that allows for this
entire structure to come in.

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All of the genetic
material here within the,

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all of the genetic
material within the nucleus

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of the sperm cell can start to come out

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and get released here into the egg.

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And once we have fusion
of genetic material,

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that is fertilization.

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So just to recap, we'll go back to look

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at our close-up of our sperm.

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We can see that it's a
very mobile structure

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evolved basically to get genetic material

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from the male to the female egg cell.

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It's got a tail that propulses it,

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it's got mitochondria that feed it energy,

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it's got a head with a
payload of nuclear material

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and an acrosome warhead on the tip.

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The egg cell is a giant
cell by comparison.

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It's got a specialized
layer of glycoproteins

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on the outside that have a
bunch of specialized features

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and then a bunch of cytoplasm,
including mitochondria.

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And then the process of
egg meeting sperm itself

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is called fertilization.

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Sperm binds to the zona
pellucida, the glycoproteins,

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you have an acrosomal reaction,

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and then a cortical reaction prevents

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more than one sperm getting in.

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And then material, the genetic material

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of the sperm is transferred.

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Now you'll notice here that I drew

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the genetic material from
the nucleus coming in.

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Now some of you may be wondering,

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"Well don't mitochondria have
genetic material as well?"

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Well, that's true. Mitochondria
do have mitochondrial DNA.

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And potentially, some
of these mitochondria

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can get sucked in during that

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genetic transfer process as well.

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But remember, our egg cell had

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100 to 200 thousand mitochondrial copies,

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and our sperm cell only had 75 to 100.

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Now there's a little bit of debate,

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but in the end the male contributes

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essentially no mitochondria to the zygote

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that's formed after
the egg and sperm fuse.

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Now, it could be that
some of those mitochondria

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actually do make their way
in and then are degraded.

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We're not really sure.

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But given just the numbers,

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statistically with one
to two hundred thousand

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versus only 75 to 100, nearly
all of the genetic material

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from the mitochondria is gonna
be from the mother anyway.