- Reproduction! Always a popular topic, and
one that I don't mind saying that I am personally interested in. The kind of reproduction
that we're most familiar with is of course 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, 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? Ah, 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, different? Sure, we both wear glasses, and we both kind of look
like a tall Doctor 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. The answer to these questions, and a lot of other life's
mysteries, is meiosis. (Crash Course intro song plays) In the last episode, we talked
about how most of your cells, your body or somatic cells grow themselves through the process of mitosis. Mitosis replicates a cell with the 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 grow up 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 the 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 kind 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 the stages again, and they have the same
names, conveniently, except with a two 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. 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 wanna go
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 babymakers. Just like with mitosis,
there's a spell between rounds of cell division where
the cell is gearing up for the next big split, and
this is called interphase, when all the key players
are replicated themselves. 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 back. A similar process takes
place with the centrosomes, a 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. And that brings us to the first
round of meiosis prophase I. This is nearly the same as in mitosis, the centrosomes start heading
to their corners of the cell, unspooling the microtubules,
and the DNA clumps up with some proteins in the chromosomes. Each single chromosome is
linked to its duplicate copy to make an X-shaped double chromosome. And keep this in mind, once
attached, each single chromosome is called a chromatid,
one on each side of the X. Each double chromosome,
has two 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 homologue, 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 four chromatids. Now watch, one chromatid from each X, gets tangled up with the
other X, that's crossover. And while they're all tangled up, 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. This is important, what
just happened here, creating new gene combinations
on a single chromosome. It's 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, and this is a
major source of that variation. What's more, since all
of the four chromatids have swapped some DNA segment 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, and that's why all eggs
produced by the same woman have a slightly different genetic code. Same for sperm and men. And that's why my brother
John and I look different, even though we're 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, your X chromosomes. Since they're the same, they can 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 kind of 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 don't match up,
and because the XY pairs on these chromosomes will split later into single chromatids, half
of the four resulting sperm 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, and 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 reforms
a nucleoli form within them, the chromosomes fray
out back into chromatid. A crease forms between the
two new cells called cleavage, and then the two new nuclei
move apart from each other, the cells separate in a
process called cytokinesis, literally again cell movement. And that is the end of round one. We now have two haploid cells, each with 23 double
chromosomes that are new, unique combinations of the
original chromosome pairs. And these new cells, the
chromosomes are still duplicated and still 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 chromatid, the crease forming cleavage
in the final separation of cytokinesis then mark
the end of telophase II. From one original cell with
46 original chromosomes, we now have four new cells with
23 single chromosomes each. If these are sperm, all
four of the 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,
then 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 ol' fat remaining
cell becomes the egg, with more of the nutrients
and cytoplasm and organelles that it will take to make a new embryo. The other three 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, the 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't say
that you know more about the miracle of reproduction-- it's not actually a miracle, it's science!