Meiosis: Where the Sex Starts – Crash Course Biology #13

Reproduction! Always a popular topic,
and one that I don't mind saying that I'm personally
interested in. The kind of reproduction that
we're most familiar with of course is sexual reproduction,
where sperm meets egg, they share genetic information,
and then that fertilized egg splits in half, and then those
halves split in half, and so on and so on and so on,
to make a living thing with trillions of cells that
all do specialized things. And if you're not suitably
impressed by the fact that we all come from one single cell
and then we become THIS: then I don't- just- I don't
know how to impress you! But riddle me this my friends:
If sexual reproduction begins with sex cells,
the sperm and the egg Where do the sperm
and egg come from? Oh dude So how do sex cells form so that
they each have only half of the genetic information that
the resulting offspring will end up with? And for that matter, why aren't
all of our sex cells the same? Like, why are my brother John
and I are different? Sure we both wear glasses and
we both kind of look like a tall Dr.

Who, but, you know,
we have different color hair and different noses and I'm way
better at Assassin's Creed than he is. So why aren't we identical?
As far as we know, we both came from the same two people with
the same two sets of DNA, right? The answer to these questions,
and a lot of other of life's mysteries, is meiosis.

In the last episode we talked
about how most of your cells your body, or somatic, cells clone themselves through
the process of mitosis. Mitosis replicates a cell with a
complete set of 46 chromosomes into two daughter cells that are
each identical to each other. But of course, even though the
vast majority of your cells can clone themselves,
you cannot clone yourself, and for good reason.
Actually, reasons. If mitosis were the only kind of
cell division we were capable of, that would mean: A) you would be a clone of one
of your parents, which would be, awkward to say the least,
or possibly B) half of your cells would be
clones from your mom, and half would be clones from your dad.
And you would look REALLY weird.

But that's not how we roll;
we do things a better way, where all of your body cells
contain the same mix of DNA 46 chromosomes grouped into
23 pairs, one in each pair from your mom,
and one from your dad. Those pairs of chromosomes
are pretty similar, but they're not identical.
They contain versions of the same genes, or alleles, in the
same spot for any given trait. Since they're so similar,
we call the pairs homologous chromosome pairs. Homologous is a word that comes
up a lot in genetics.

It just means that two things have
the same (homo) relation (logos) even if they are
a little bit different. However there are some very
special cells that you have that have only one half of that amount,
23 chromosomes. Those are sperm and egg cells.
These are haploid cells they have half of a full set of
chromosomes. And they need each other to combine
to make the complete 46. Creating those kinds of cells
requires a process that's very similar to
mitosis but with a totally different outcome: meiosis. That's when a specialized diploid
cell splits in half twice, producing four separate cells,
each of which is genetically distinct from the others. Meiosis is a lot like mitosis,
except, twice. It goes through the
same stages as mitosis: prophase, metaphase,
anaphase, and telophase. But then it goes through another
round of those stages again and they have the same names,
conveniently, except with a 'II' after them, they're like sequels. And just as with the
Final Destination movies, the sequels have pretty much the
same plot, just some new actors.

So the raw materials for this
process are in your ovaries or your testes, depending on…
you know… you know what it depends on. They're diploid cells called
either primary oocytes or primary spermatocytes, depending on what
kind of gamete they make. Men produce sperm, you may
have heard, and they produce it throughout their adult lives,
whereas women are born with a certain amount of eggs
that they'll release over many years after puberty. Here you might want to watch the
previous episode about mitosis again because that's where we
go into detail about each stage of the process. Once you're done with that,
we can start making some baby-makers! Now, just like with mitosis,
there's a spell between rounds of cell division where the cell is
gearing up for the next big split. This is called interphase,
when all the key players are replicating themselves. The long strings of DNA in the
nucleus begin to duplicate, leaving two copies of every strand.

To jog your memory about how DNA
does this, we did a whole episode on it you can watch it
and come on back. A similar process takes place
with the centrosomes, the set of protein cylinders next to the
nucleus that will regulate how all of the materials will be
moved around, along these ropey proteins called microtubules. That brings us to the first
round of meiosis, prophase I. This is nearly the
same as in mitosis as the centrosomes start heading
to their corners of the cell, unspooling the microtubules,
and the DNA clumps up with some proteins into chromosomes. Each single chromosome is linked
to its duplicate copy to make an X-shaped double chromosome.
Now keep this in mind: once attached, each single
chromosome is called a chromatid, one on each side of the X.

Each double chromosome
has 2 chromatids. Here, meiosis prophase I
includes two additional and VERY IMPORTANT steps: crossover and
homologous recombination. Remember that the point here
is to end up with four sex cells that each have just one single
chromosome from each of the homologous pairs.
But unlike in mitosis, where all the copies
end up the same, here every copy is going to
be different from the rest. Each double chromosome lines up
next to its homolog, so there's your mother's version lined up
right next to your father's version of the same chromosome.

Now If you look, you'll see that
these two double chromosomes, each with two chromatids,
add up to 4 chromatids. Now watch: One chromatid from
each X gets tangled up with the other X.
That's crossover. And while they're tangled,
they trade sections of DNA. That's the recombination. The sections that they're trading
are from the same location on each chromosome, so one is giving up
its genetic code for, like, hair color or body odor, and in
return it's getting the other chromosome's genes for that trait. Now this is important.
What just happened here, creating new gene combinations
on a single chromosome, is the whole point of
reproducing this way. Life might be a lot less stressful
if we could just clone ourselves, but then we'd also clone all our
bad gene combinations, and we wouldn't be able to change
and adapt to our environment. Remember that one of the pillars
of natural selection is variation.

This is a major source
of that variation. What's more, since all of the
four chromatids have swapped some DNA segments at random,
that means that all four chromatids are now different.
Later on in the process, each chromatid will end up
in a separate sex cell. This is why all eggs produced
by the same woman have a slightly different genetic code;
same for sperm in men. And that's why my brother
John and I look different, even though we're both made
from the same two sets of DNA. Because of the luck
of the genetic draw that happens in recombination,
I got this mane of luscious hair, and John was stuck
with his trash, brown puff. And don't forget about my mad
Assassin's Creed skills. But then of course,
there is that one pair of chromosomes that doesn't
always go through the crossover or recombination:
that's the 23rd pair.

And those are your sex chromosomes.
If you're female, you have two matched, beautiful, fully capable
chromosomes there, X chromosomes. Since they're the same,
they do the whole crossover and recombination thing.
But, if you, like me, are a male, you get one of those X chromosomes
and another from your dad that's kinda ugly and short
and runted and doesn't have a lot of genetic information on it.
During prophase, the X wants nothing to do with the little Y,
because they're not homologous, so they just don't match up. And because the X-Y pairs on
these chromosomes will split later on into single chromatids,
half of the four resulting sperm cells will be X
(leading to female offspring), and half will be Y
(leading to male offspring).

Now what comes next is another
kind of amazing feat of alignment. This is Metaphase I, and in
mitosis you might recall that all of the chromosomes lined up
in a single row, powered by motor proteins, and were then
pulled in half. But Not here. In meiosis, each chromosome lines
up next to its homologous pair-partner that it's already
swapped a few genes with. Now the homologous pairs get
pulled apart and migrate to either end of the cell.
That's Anaphase I.

The final phase of the first round,
Telophase I, rolls out in pretty much the same
way as mitosis. The nuclear membrane re-forms,
and nucleoli form within them. The chromosomes fray out
back into chromatin. Then a crease forms between
the two new cells, called cleavage,
and as the two new nuclei move apart from each other,
the cells separate in a process called cytokinesis
literally again "cell movement." That's the end of round 1.
We now have two haploid cells, each with 23 double chromosomes
that are new, unique combinations of the original chromosome pairs.
In these new cells, the chromosomes are still
duplicated and connected at the centromeres
they still look like X's. But remember the aim is
to end up with four cells, so it's time for those sequels.
Here the process is exactly the same as mitosis,
except that the aim here isn't to duplicate the double
chromosomes, but instead to pull them apart into separate
single-strand chromosomes. Because of this, there's no
DNA replication involved in Prophase II; instead the DNA
just clumps up again into chromosomes, and the
infrastructure for moving them, the microtubules, are
put back in place.

In metaphase II, the chromosomes
are moved into alignment into the middle of the cell.
And in anaphase II, the chromatids are pulled apart
into separate, single chromosomes. The chromosomes uncoil into
chromatin, and the crease-forming cleavage and the final
separation of cytokinesis then mark the end of telophase II. From one original cell with
46 chromosomes, we now have four new cells with 23
single chromosomes each. If these are sperm,
all four resulting cells are the same size,
but they each have slightly different
genetic information. And half will be for making girls,
and half will be for making boys. But if this is the
egg-making process, then it goes a little bit
differently here and the result is only one egg. To rewind a little,
during telophase I, more of the inner goodness
of a cell(the cytoplasm, the organelles, heads into one
of the cells that gets split off than to the other one. In telophase II, when it's time
to split again the same thing happens, with more stuff going
into one of the cells than the other.

This big old fat,
remaining cell becomes the egg, with more of the nutrients,
cytoplasm, and organelles that it will take to
make a new embryo. The other 3 cells that were
produced, the little ones, are called polar bodies,
and they're totally useless in people, though they are
useful in plants. In plants, those polar bodies
actually also get fertilized too, and they become the endosperm,
that's the starchy-proteiny stuff that we grind into wheat
or pop into popcorn. And it's basically the nutrients
that feed the plant embryo seed. And that's all there is to it. I know, you probably were really
excited when I started talking about reproduction but then I
rambled on for a long time about haploid and diploid cells.

But now you can say you know more
about the miracle of reproduction. It's not actually a miracle.
It's science! Thank you for joining us here
on Crash Course Biology/Meiosis. If you want to re-watch anything
you can do that now. Just click the annotations that
are on top of the words next to me. Just do it.

Otherwise, congratulations on
learning stuff and being smarter. Thanks to everybody who
helped make this video. If you have any questions,
please ask them down in the comments below or on
Facebook or Twitter. We will see you next week..

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