Egg, sperm, and fertilization | Behavior | MCAT | Khan Academy
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Egg, sperm, and fertilization | Behavior | MCAT | Khan Academy

– [Voiceover] Organisms
that reproduce sexually have got to get their
genes together somehow. To do this, they package
their genetic material into specialized cells called sex cells. This is the sperm cell. It’s the sex cell of the male. The sole purpose of the sperm, the entire reason for its existence, is to transfer the male’s genetic material into the female sex cell or the egg. So the sperm cell is packed with features that allow it to fulfill its job. It’s basically a little torpedo. You can see, just like a torpedo, it has a pointed head which allows it to travel in the forward direction. At the back end it has the tail, and the tail is just a flagellum, and as the flagellum spins it
acts like a little propeller. And then it has a middle section. Now within that middle section, wrapped around the base
of the flagellar tail, are all of these little
organelles called mitochondria. You can see I’m drawing these mitochondria wrapped around the flagellar base here. And mitochondria are organelles that are responsible for
giving energy to a cell. And the sperm cell has
a whole bunch packed right into the base of the flagellum here. Probably 75 to 100, and they’re
quite large mitochondria. In fact, these mitochondria
are often fused together to create big organelles. And the reason is because
in order to propel the sperm torpedo towards the egg, it needs a lot of energy. And that all comes from
these mitochondria. Now the payload of the
torpedo is here in the head, and that’s the genetic material,
our DNA within the nucleus. I’ll draw it kind of all coiled back here. And here’s the nuclear envelope here. And just like any regular torpedo, the sperm torpedo has a
warhead right in the front. And that warhead is a
little collection of enzymes called the acrosome. And the acrosome is gonna be important later on for fertilization. But that’s pretty much it. There’s a tail at the back
to provide propulsion, some mitochondria in the middle section to give energy to the tail, a head which contains
the nuclear material, and the acrosome. This is a pretty bare bones cell. It’s designed to move fast
and to get to the egg. There are no bells and whistles here. Now that’s in contrast to the egg cell. Now the first thing you’ll notice is that the egg cell is round, unlike the torpedo
shape of the sperm cell. This is not a cell that’s
made for active mobility. The second thing is that
the egg cell is huge compared to the sperm cell. It’s so big, in fact,
that it’s almost visible. In fact, sometimes it is
visible to the human eye. Now compared to the sperm
cell, which I’ll draw in here, the egg cell is about
10,000 times more massive. And similar to the sperm cell, the egg cell has its
share of genetic material ready to combine during fertilization. You can see it here within the nucleus. And so you may have noticed this thick outer coating on the egg cell here, that’s a very important structure
called the zona pellucida. And the zona pellucida is a
thick layer of glycoproteins that sit on the outside of the egg. And glycoproteins are basically a protein, I’ll draw a protein here in green, with a whole bunch of
branching sugar chains that are coming off of them. And so what this looks like
is basically a little tree or a long branching
thing that’s growing out of the edge of the egg cell. And there’s a whole bunch of them, and they form this very thick
kind of protective layer that the sperm has to get through. And the edge of the egg cell
is the actual plasma membrane. And once the sperm can deliver its genetic material beyond that, fertilization has occurred. Now there’s a whole
bunch of other structures within the egg cytoplasm as well. And remember this thing is huge. And I’m gonna draw in a few here in green. Now what I’m drawing in are
actually more mitochondria. Now remember the egg cell had
75 to 100 big mitochondria right at the base of the flagellum to provide energy to drive locomotion. Well, the egg cell has mitochondria too. It’s got a lot of other
different organelles as well. But the egg cell is so large, it’s got somewhere between 100 and 200
thousand mitochondria present. So keep those mitochondria
in mind, we’ll talk about them a little bit
later in the next section. So now that you’ve met the
two major players here, the sperm and the egg,
or the male and female sex cells respectively, we can talk about what happens when they meet. And that process is called fertilization. So we’ll go ahead and
label our egg down here. And we’ll put a quick
label on this as well. This is the zona pellucida
that we talked about earlier. So we’ll draw the sperm here
coming in to meet the egg. We’ll draw its tail. We’ll draw its middle section here. And we’ll draw its torpedo-like head here. Get rid of all this zona pellucida glycoprotein in the middle here. Now here are the mitochondria
in the middle section here, and we have the genetic material payload of our sperm torpedo here in the back, and our acrosome here in the front. Now the first thing that
happens during fertilization is that the sperm comes into contact with the zona pellucida. And the zona pellucida actually binds to the outside of the sperm, and that’s called sperm binding. And it’s step number one. Now what happens when the sperm gets bound to the zona pellucida is
that that sets up a reaction called the acrosomal reaction. So step number two is called
the acrosome reaction. And that little warhead
tip of the sperm torpedo gets released, and so we have
all of the acrosomal enzymes that were sitting in the head
that just kind of leak out into that zona pellucida. And as those enzymes leak
out, they actually start to digest away the zona pellucida. Here you can see I’m kind of eating away here at those glycoprotein, and that allows the sperm
head to dive in deeper towards our plasma membrane. Now as the sperm gets closer
to the plasma membrane of the egg, and it comes
in contact right here, it starts up a process of binding. The two touch and they come together. And as they start together,
it causes another reaction, and that third reaction is
called cortical reaction. And what I haven’t drawn here is another structure in the egg, and those structures are right underneath the plasma membrane, and they just sit there waiting. And they wait and their entire job is to wait for a sperm to bind. And as soon as a single sperm binds, they get ejected out into
that zone pellucida as well. Just like the acrosomal enzymes, these enzymes that are contained within the cortical granules also start eating away at the zona pellucida. These enzymes eat away, and
they dissolve and chew up these glycoproteins, but
specifically they chew up the glycoprotein that
allows sperm to bind. So at this point we have a
single sperm that’s bound, set off the cortical reaction, and these cortical granules are released that chew up all the other places that more sperm can bind. So as our other sperm
torpedoes are coming in, they’re just bouncing off. They hit the glycoproteins,
but the one that they need to bind to isn’t there because
it’s all been chewed up and degraded by these cortical granules. So that’s actually called
a block to polyspermy. Now that’s a very important concept. Polyspermy is a term that
just means multiple sperm. And what we don’t want is
for more than one sperm to inject its nuclear material,
its DNA into this egg. What you’d end up with is an egg with a single contribution from mom and then one or two or
three or 100 contributions or genetic material from dad. And that would never
work, you’d end up with all sorts of problems as
the egg started to divide. Occasionally, that does happen, and it can result in a zygote that fails. But for the most part, as
these cortical granules dissolve away all the
sperm binding glycoproteins of the zona pellucida, other
sperm just can’t get in and they bounce off as they arrive. So now we have a sperm that’s made its way all the way to the plasma
membrane of our egg cell. It’s started to bind
to the plasma membrane, the acrosome is gone,
I’ll erase that here. It’s done its job. The cortical granules have been released, and they’re preventing
other sperm from getting in. And we start to actually
fuse our plasma membranes of our sperm cell and our egg cell. And that allows for this
entire structure to come in. All of the genetic
material here within the, all of the genetic
material within the nucleus of the sperm cell can start to come out and get released here into the egg. And once we have fusion
of genetic material, that is fertilization. So just to recap, we’ll go back to look at our close-up of our sperm. We can see that it’s a
very mobile structure evolved basically to get genetic material from the male to the female egg cell. It’s got a tail that propulses it, it’s got mitochondria that feed it energy, it’s got a head with a
payload of nuclear material and an acrosome warhead on the tip. The egg cell is a giant
cell by comparison. It’s got a specialized
layer of glycoproteins on the outside that have a
bunch of specialized features and then a bunch of cytoplasm,
including mitochondria. And then the process of
egg meeting sperm itself is called fertilization. Sperm binds to the zona
pellucida, the glycoproteins, you have an acrosomal reaction, and then a cortical reaction prevents more than one sperm getting in. And then material, the genetic material of the sperm is transferred. Now you’ll notice here that I drew the genetic material from
the nucleus coming in. Now some of you may be wondering, “Well don’t mitochondria have
genetic material as well?” Well, that’s true. Mitochondria
do have mitochondrial DNA. And potentially, some
of these mitochondria can get sucked in during that genetic transfer process as well. But remember, our egg cell had 100 to 200 thousand mitochondrial copies, and our sperm cell only had 75 to 100. Now there’s a little bit of debate, but in the end the male contributes essentially no mitochondria to the zygote that’s formed after
the egg and sperm fuse. Now, it could be that
some of those mitochondria actually do make their way
in and then are degraded. We’re not really sure. But given just the numbers, statistically with one
to two hundred thousand versus only 75 to 100, nearly
all of the genetic material from the mitochondria is gonna
be from the mother anyway.

About James Carlton

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34 thoughts on “Egg, sperm, and fertilization | Behavior | MCAT | Khan Academy

  1. the zona pellucida you labeled is corona radiata. zona pellucida is only the thin membrane you draw below the corona radiata.

  2. just to reiterate Priscilla Low – the zona pellucida is a thin membrane below the corona radiata – please  could you amend this on the video with an annotation thank you!

  3. I must say not the quality expected from Khan academic…. really poor stuff….

    what about the proximal and distal centriole of the sperm what about the corona radiata cells…..

    pathetic stuff…..

  4. Start video at 4:46 if you only need the detailed info about the actual fertilization process… Only if you already know the in's and out's of the sperm and the egg!

  5. awesome…can u provide more detailed videos on cleavage and gastrulation of c.elegans, zebra fish,xenopus and chick..and organogenesis of chick embryo….?

  6. Hi there all the information you provided is great but I would like to know about basal body temperature what role play the temperature during the 28th day cycle when rise up and when and why goes down. Can you pleas school me on that topic thank you

  7. Is anyone bothered by the military metaphors? Take another look or read about the unnecessary militarization of cell life

  8. at about 4:07 there is a little verbal mistake where mentioning how many mitochondria the sperm cell consists of (70-100), the author says "egg cell". I don't believe it'll trip anyone up too badly but it could be something to note

  9. This video was bad, didn't mention the ZP3 protein, what about the acrosomal apparatus? Depolarization of the oocyte? Hardening of the zone pellucida? Release of Calcium ions? The terminology is not in depth enough for the MCAT.

  10. This is not very accurate. For example, completely forgot to mention the corona radiata. The acrosome is more like a football helmet which surrounds the sperm plasma membrane and head. Also forgot to mention that none of this can happen without the sperm going through the process of capacitation. Again, I wouldn't study this if you're in graduate school.

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