Chromosomal crossover in Meiosis I

- Let's now jump into understanding
meiosis in some depth.

So let's start with the germ cell.

As we mentioned already,
a germ cell is a cell that

it can either go to mitosis
to produce other germ cells

or it can undergo meiosis
in order to produce gametes.

So this is a germ cell right over here.

Let me draw the nuclear membrane.

Let me draw the nucleus
larger because that's where

we care a lot about the chromosomes in it.

And let me draw a centrosome

which will play a role later on.

I wanna do that in ...

Let's see, I'll do that
in this blue color.

Each centromosome has
two centrioles in it.

I just wanna clarify
some of the terminology.

And in the mitosis videos, I focused on

cells of an organism, I
just kind of made it up,

that had two chromosomes, that
had a diploid number of two

that had one homologous
pair, that had one chromosome

from each of its parents.

For this video, I'm
gonna focus on a species,

not human beings, that would have

23 pairs or 46 chromosomes.

I'm gonna focus on a species that has,

that's diploid number is four.

And so, let's say it has two
chromosomes from the father.

And let me do that.

I'll do that in this orange color.

Now, I'll do that in the chromatin,

I'll kind of depict the chromatin state,

it's kind of unwound.

So maybe it has a long one from the father

and it has a short one from the father.

And then it has homologous
chromosomes from the mother.

So it would have the
long one from the mother

and it would have the short one

from the mother just like that.

And obviously this is
a huge simplification

but hopefully this
discuss the point across.

So here, it has a diploid
number of chromosomes.

So this is, let me write this down.

This is diploid

number is equal to,

we have four chromosomes.

And then this thing, this germ cell.

Let me write this down.

This is a germ cell right over here.

It will go through interphase.

So let me draw that.

So it will go through interphase, in which

it grows and it can replicate
its DNA and its centrosome.

And so, let me draw that.

So after it goes through interface,

I wanna use my space
carefully because I have a lot

of steps to go through.

After it goes through
interface, I am going to have

in my nucleus here,

my DNA will have replicated.

So this long chromosome from my father,

now all the DNA will have replicated so

it may look something like that.

And it's attached

at a centromere,

All these centro words,

at a centromere right here.

But I'm still trying to draw it in kind of

the chromatin state.

It's actually all spread out.

It's not bunched up so that
you can see it very clearly

as these X's in a simple microscope.

So it's just replicated.

And after replicating, it
is still one chromosome.

It has twice the genetic material

but it is still one chromosome.

That one chromosome is now made up of

two sister chromatids.

we talked a lot about
that in the mitosis video,

but it doesn't hurt to reinforce

because it can get a little bit confusing.

And then you have that shorter
chromosome from the father

and then that also replicates
into two sister chromatids

attached at a centromere.

So these are still two
chromosomes from the father.

It has twice the amount
of DNA but it's containing

the same information,

just duplicate versions
of that same information.

And the same thing's gonna
happen from the mother.

You had that long
chromosome from the mother,

homologous to this right over here.

It's going to replicate.

So it's now going to be
two sister chromatids.

And then you have a short
strand from the mother

that was homologous to
this one from your father.

And that's also gonna replicate.

And so, it's like that.

And at the end of interface,

it would actually all be spread out.

Once again, it won't be
bunched up into these

clearly discernible X's.

I drew them a little
bit that way, otherwise,

because you would have trouble
seeing how that replicated.

And we also have replicated our centrosome

as we've gone through interface.

Now, we are ready.

In fact, now we are ready for
either mitosis or meiosis.

But as I said, the focus of this video

is going to be meiosis
so let's do some meiosis.

So the first phase,

so the first several
phases we call meiosis I.

And the beginning of
meiosis I is prophase I.

So let's see what happens in prophase I.

So prophase I.

And so, let me draw the
cell right over here.

So prophase I.

A couple of things happen.

The nuclear membrane begins to dissolve.

This is very similar to prophase

when we're looking at mitosis.

So the nuclear envelope
begins to dissolve.

These things start to
maybe migrate a little bit.

So these characters are trying
to go at different ends.

And the DNA starts to
bunch up into kind of

its condensed form.

So now I can draw it.

So now I can start to draw it as proper.

So this is the one from the father

right over here.

And this is the one from the mother.

And I'm drawing, I'm
overlapping on purpose

because something very interesting happens

especially in meiosis.

So it's the mother right over here.

Let me see.

Let's now do the centromere in blue now.

That's the centromere.

Now this is the shorter
ones from the father.

These are the shorter
ones from the mother.

And actually, let me just do
draw them on opposite sides

just to show that they don't have to,

the ones from the father aren't always

on the left hand side.

So this is the shorter
one from the father.

They couldn't be all on the left hand side

but doesn't this all they have to be.

And this is the shorter
one from the mother.

And I will draw this overlapping
although they could have.

Shorter one from the mother.

And once again, each of these,
this is a homologous pair,

that's a homologous pair over there.

Now, the DNA has been replicated so

in each of the chromosomes
in a homologous pair,

you have two sister chromatids.

And so, in this entire homologous pair,

you have four chromatids.

And so, this is sometimes called a tetrad.

So let me just give
ourselves some terminology.

So this right over here is called a tetrad

or often called a tetrad.

Now, the reason why I
drew this overlapping

is when we are in prophase I in meiosis I.

Let me label this.

This is prophase I.

You can get some genetic recombination,

some homologous recombination.

Once again, this is homologous pair.

One chromosome from the father

that I've gotten from the father.

The species or the cell got
it from its father's cell

and one from the mother.

And they're homologous.

They might contain different base pairs,

different actual DNA, but
they code for the same genes.

Over simplification, but in a
similar place on each of these

it might code for eye
color or I don't know,

personality.

Nothing is that simple in how tall you get

and it's not that simple
in DNA but just to give you

an idea of how it is.

And the reason why I
overlapped them like this

is to show how the
recombination can occur.

So actually, let me zoom in.

So this is the one from the father.

Once again, it's on the condensed form.

This is one chromosome made
up of two sister chromatids

right over here.

And I drew the centromere,

not to be confused with centrosomes.

That's where they are, those
sister chromatids are attached.

And then, I will draw the homologous

chromosome from the mother.

So the homologous
chromosome from the mother

just like that.

Homologous chromosome from the mother.

And the recombination can occur at a point

right over here.

So after you're done
with the recombination,

this side might look
something more like this.

So let me draw it like this.

So, they essentially break up

and they swap those little sections.

There's one way to think about it.

So this one, we'll now have a
little piece from the mother.

It might code for similar genes.

But now it contains the
mother's genetic information.

And then this one over here

will now have the piece.

And you could say even
homologous piece from the father.

Let me do these two centromeres.

And this is really interesting.

All the time, there
couldn't be recombination

and often times it can lead to
kind of non-optimal things,

nonsense code and DNA.

It might lead to a nonfunctional organism.

But this happens fairly
common in the meiosis

and it's a way, once again,
to get more variation.

We've talked about sexual
reproduction before.

And sexual reproduction
introduces variation

into a population.

And this, obviously, when different sperms

find different eggs that
introduces variation.

But then, even amongst homologous pairs

you can actually have exchange
between this chromosome.

And that's interesting
because as we mentioned,

each of these chromosomes,

they code for a bunch of different genes.

And a gene is kinda
looking code for a specific

or a set of proteins.

So this right over here,

and this is what I'm about
to say is gonna be huge

over simplification.

Maybe right over here
you coded for eye color

or it was related to, or it
helps code for eye color.

And then you got that from your dad.

And here, it helped code for eye color.

And you got that from your mom.

Your mom might have trended
you towards a lighter eye color

and your dad might have trended you

towards a darker eye color.

But now, the one from your
mom is on this chromosome,

this gene, and then the one

or they've both the same gene.

They're just different allele.

They're coding for different
variance of that gene.

And then the allele from
your dad is over here.

And once again, some people
get confused with genes

and chromosomes and all of these.

Each of these chromosomes
contain a bunch of genes.

These are very long DNA molecules.

This code for a bunch of different genes.

So gene will be a little section
of here that could code for

a particular protein.

So that's what happens in prophase I.

In prophase I, you have this condensation

of your chromosomes, of
your homologous pairs.

You can have this recombination.

And it's really interesting,
this recombination

doesn't tend to happen
at just random points

that would kind of break
the genetic information.

It tends to happen at fairly clean points.

And the places where this
breakup is happening,

these are called the plural,

if you just talk about
one point, it's a chiasma,

or if you're talking about
the plural, it's chiasmata.

Sounds like it could be a horror movie.

So, chiasma.

Chiasma.

And the fact that they happen,

they tend to happen fairly
cleanly, this is once again,

kind of the beauty of
the universe or at least

of biology is that through

billions of years of evolution,
these things have kind of

optimized for more variation and to happen

in fairly clean ways.

So I'm gonna leave this video right there.

I know I just got to prophase I.

But this was a really,
really important idea of

this homologous recombination
or this chromosomal crossover

that we see right over here.

And then from there, we can continue

through the rest of meiosis
I and then meiosis II.