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.
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We will see you next week.