- [Voiceover] Let's take a look at the nucleus
of a cell that's just starting to undergo meiosis.
So we have these 46 chromosomes
and beforehand each chromosome probably
looked something like that.
That's the centromere in middle.
And then, as meiosis is beginning, they will duplicate into,
each chromosome will duplicate into something like that.
That's again that centromere in middle.
So we have these 23 purple chromosomes,
we're gonna say that these are the maternal chromosomes.
And then we're gonna say that the blue ones
are the paternal chromosomes.
They come in homologous pairs.
And the nucleus that we're looking at
must belong to the cell of a male.
because right over here you can see
that is the Y chromosome.
It's a bit smaller than most of the other chromosomes.
I'm gonna digress just for a moment,
to clarify a very common point of confusion.
And that is that if you look over here
at these two chromosomes,
well, this is considered one chromosome,
but so is this, called one chromosome.
And this can be confusing because
this is only one chromatid and this
is two chromatids.
So why are we calling them each one chromosome?
And the answer to that question is
because we count chromosomes by the number of centromeres.
So this single chromatid has one centromere,
but these two chromatids are attached
in the middle by one centromere.
So we also call that one chromosome.
Just keep that in mind, it's just a technical point
to know that even though they're different,
they're still both considered one chromosome.
And in this chromosome, the two chromatids
are duplicates of each other.
So it's just a copy of itself.
Anyway, back to our nucleus.
So we have these 46 chromosomes,
23 homologous pairs.
And they're not, the chromosomes are not
necessarily arranged in the way that I drew them.
I just drew it that way for the sake of organization.
But, we have these pairs and we're gonna focus on
one pair of homologous chromosomes.
But I want you to keep in mind throughout this video,
that whatever we're describing that's happening
to this pair of homologous chromosomes,
is also happening to the other 22 pairs
of homologous chromosomes.
It's just that it would be too hard to depict
in a video how that's happening,
but keep in mind it's not just happening
to the pair that we're talking about,
but what we're going to be describing is happening
to all of the pairs of chromosomes in the nucleus.
So here we have our pair of homologous chromosomes.
And during prophase one of meiosis,
the homologous chromosomes pair up with each other
and form a unit called
a tetrad.
And it's called a tetrad because, well,
tetra means four and this unit
has four chromatids, right.
One, two, three, and four.
And the process during which
the homologous chromosomes pair up
with each other is called synapsis.
So during synapsis, the homologous chromosomes
will get a little bit closer to each other.
Something like that.
And at a certain spot, they might
actually cross over or overlap.
So I'm gonna circle that spot.
And that's called the chiasma.
And in some cases, another thing happens.
This protein complex that resembles
something like a railroad track forms.
We'll see in a minute why.
And this is called the synaptonemal complex.
You can actually see the word synapse in there
because this happens during synapsis.
So we've formed the synaptonemal complex
and with the help of the synaptonemal complex,
these two chromatids, the ones that are crossing over,
will actually swap material downward of that point.
So we're gonna get something that looks like that.
Look at how the purple chromosome
now has some blue over there.
And look at how the blue chromosome
now has some purple over there.
And the way that happened was that
the DNA in the chromosome,
actually some bonds in that DNA broke
and the DNAs just kinda swapped places.
So what we just described,
this process by which the two chromosomes swap information
is called crossing over.
Or, another way to say this,
is genetic recombination.
And let's see why this is called genetic recombination.
So we're gonna fast forward to the end of meiosis
to where the chromosomes get split into two
and all the chromatids get separated
into different gametes.
And I want to pause and remind you that
everything we're describing that's happening
to this pair of chromosomes is also happening
to all the other 22 pairs of homologous chromosomes.
But anyway, so now let's put each one of the chromatids
in a different gamete.
And look at how we get four different gametes.
And we can call these two, gametes recombinant.
And we're calling them recombinant because
they have a combination of alleles that's new.
We haven't had this combination of alleles,
even in a parent.
And just to clarify things,
let's see what the gametes would look like
if crossing over did not happen.
So let's go back to our original chromosomes.
And let's split them.
And let's put them into four different gametes.
And we are going to get that.
And you can see that in this case,
we only have two different types of gametes.
So we can see how genetic recombination
increases genetic
variability.
Which is usually a good thing.