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

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So let's start with the germ cell.

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

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it can either go to mitosis
to produce other germ cells

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or it can undergo meiosis
in order to produce gametes.

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So this is a germ cell right over here.

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Let me draw the nuclear membrane.

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Let me draw the nucleus
larger because that's where

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we care a lot about the chromosomes in it.

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And let me draw a centrosome

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which will play a role later on.

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I wanna do that in ...

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

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Each centromosome has
two centrioles in it.

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I just wanna clarify
some of the terminology.

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And in the mitosis videos, I focused on

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

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

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that had one homologous
pair, that had one chromosome

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from each of its parents.

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

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not human beings, that would have

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23 pairs or 46 chromosomes.

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I'm gonna focus on a species that has,

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that's diploid number is four.

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

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And let me do that.

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I'll do that in this orange color.

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Now, I'll do that in the chromatin,

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I'll kind of depict the chromatin state,

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it's kind of unwound.

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So maybe it has a long one from the father

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and it has a short one from the father.

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And then it has homologous
chromosomes from the mother.

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So it would have the
long one from the mother

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and it would have the short one

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from the mother just like that.

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And obviously this is
a huge simplification

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but hopefully this
discuss the point across.

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So here, it has a diploid
number of chromosomes.

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So this is, let me write this down.

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This is diploid

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number is equal to,

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we have four chromosomes.

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And then this thing, this germ cell.

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Let me write this down.

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This is a germ cell right over here.

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It will go through interphase.

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So let me draw that.

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So it will go through interphase, in which

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

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And so, let me draw that.

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So after it goes through interface,

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I wanna use my space
carefully because I have a lot

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of steps to go through.

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After it goes through
interface, I am going to have

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in my nucleus here,

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my DNA will have replicated.

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So this long chromosome from my father,

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now all the DNA will have replicated so

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it may look something like that.

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And it's attached

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at a centromere,

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All these centro words,

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at a centromere right here.

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

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the chromatin state.

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It's actually all spread out.

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

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as these X's in a simple microscope.

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So it's just replicated.

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And after replicating, it
is still one chromosome.

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It has twice the genetic material

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but it is still one chromosome.

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That one chromosome is now made up of

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two sister chromatids.

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we talked a lot about
that in the mitosis video,

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but it doesn't hurt to reinforce

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because it can get a little bit confusing.

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And then you have that shorter
chromosome from the father

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and then that also replicates
into two sister chromatids

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attached at a centromere.

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So these are still two
chromosomes from the father.

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

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the same information,

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just duplicate versions
of that same information.

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

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You had that long
chromosome from the mother,

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homologous to this right over here.

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It's going to replicate.

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

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And then you have a short
strand from the mother

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that was homologous to
this one from your father.

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And that's also gonna replicate.

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And so, it's like that.

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And at the end of interface,

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it would actually all be spread out.

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

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clearly discernible X's.

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I drew them a little
bit that way, otherwise,

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because you would have trouble
seeing how that replicated.

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And we also have replicated our centrosome

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as we've gone through interface.

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Now, we are ready.

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

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But as I said, the focus of this video

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

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So the first phase,

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so the first several
phases we call meiosis I.

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And the beginning of
meiosis I is prophase I.

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So let's see what happens in prophase I.

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So prophase I.

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

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So prophase I.

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A couple of things happen.

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The nuclear membrane begins to dissolve.

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This is very similar to prophase

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when we're looking at mitosis.

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So the nuclear envelope
begins to dissolve.

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These things start to
maybe migrate a little bit.

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So these characters are trying
to go at different ends.

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And the DNA starts to
bunch up into kind of

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its condensed form.

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So now I can draw it.

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So now I can start to draw it as proper.

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So this is the one from the father

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right over here.

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And this is the one from the mother.

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

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because something very interesting happens

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especially in meiosis.

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So it's the mother right over here.

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Let me see.

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Let's now do the centromere in blue now.

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That's the centromere.

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Now this is the shorter
ones from the father.

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These are the shorter
ones from the mother.

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

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just to show that they don't have to,

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the ones from the father aren't always

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on the left hand side.

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So this is the shorter
one from the father.

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They couldn't be all on the left hand side

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but doesn't this all they have to be.

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And this is the shorter
one from the mother.

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And I will draw this overlapping
although they could have.

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Shorter one from the mother.

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

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that's a homologous pair over there.

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Now, the DNA has been replicated so

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in each of the chromosomes
in a homologous pair,

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you have two sister chromatids.

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And so, in this entire homologous pair,

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you have four chromatids.

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And so, this is sometimes called a tetrad.

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So let me just give
ourselves some terminology.

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So this right over here is called a tetrad

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or often called a tetrad.

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Now, the reason why I
drew this overlapping

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is when we are in prophase I in meiosis I.

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Let me label this.

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This is prophase I.

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You can get some genetic recombination,

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some homologous recombination.

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Once again, this is homologous pair.

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One chromosome from the father

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that I've gotten from the father.

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

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and one from the mother.

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And they're homologous.

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They might contain different base pairs,

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

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

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

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personality.

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Nothing is that simple in how tall you get

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

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an idea of how it is.

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And the reason why I
overlapped them like this

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is to show how the
recombination can occur.

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So actually, let me zoom in.

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So this is the one from the father.

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Once again, it's on the condensed form.

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This is one chromosome made
up of two sister chromatids

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right over here.

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And I drew the centromere,

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not to be confused with centrosomes.

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

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And then, I will draw the homologous

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chromosome from the mother.

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So the homologous
chromosome from the mother

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just like that.

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Homologous chromosome from the mother.

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And the recombination can occur at a point

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right over here.

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So after you're done
with the recombination,

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this side might look
something more like this.

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So let me draw it like this.

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So, they essentially break up

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and they swap those little sections.

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There's one way to think about it.

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

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It might code for similar genes.

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But now it contains the
mother's genetic information.

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And then this one over here

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will now have the piece.

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And you could say even
homologous piece from the father.

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Let me do these two centromeres.

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And this is really interesting.

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All the time, there
couldn't be recombination

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

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nonsense code and DNA.

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It might lead to a nonfunctional organism.

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But this happens fairly
common in the meiosis

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

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We've talked about sexual
reproduction before.

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And sexual reproduction
introduces variation

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into a population.

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And this, obviously, when different sperms

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find different eggs that
introduces variation.

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But then, even amongst homologous pairs

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you can actually have exchange
between this chromosome.

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And that's interesting
because as we mentioned,

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each of these chromosomes,

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they code for a bunch of different genes.

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And a gene is kinda
looking code for a specific

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or a set of proteins.

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So this right over here,

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

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over simplification.

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Maybe right over here
you coded for eye color

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

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And then you got that from your dad.

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And here, it helped code for eye color.

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And you got that from your mom.

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Your mom might have trended
you towards a lighter eye color

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and your dad might have trended you

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towards a darker eye color.

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

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this gene, and then the one

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or they've both the same gene.

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They're just different allele.

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They're coding for different
variance of that gene.

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And then the allele from
your dad is over here.

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And once again, some people
get confused with genes

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and chromosomes and all of these.

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Each of these chromosomes
contain a bunch of genes.

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These are very long DNA molecules.

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This code for a bunch of different genes.

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

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a particular protein.

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So that's what happens in prophase I.

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In prophase I, you have this condensation

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of your chromosomes, of
your homologous pairs.

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You can have this recombination.

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And it's really interesting,
this recombination

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doesn't tend to happen
at just random points

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that would kind of break
the genetic information.

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It tends to happen at fairly clean points.

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And the places where this
breakup is happening,

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these are called the plural,

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

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

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Sounds like it could be a horror movie.

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So, chiasma.

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Chiasma.

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And the fact that they happen,

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

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kind of the beauty of
the universe or at least

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of biology is that through

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billions of years of evolution,
these things have kind of

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optimized for more variation and to happen

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in fairly clean ways.

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So I'm gonna leave this video right there.

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I know I just got to prophase I.

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But this was a really,
really important idea of

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this homologous recombination
or this chromosomal crossover

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that we see right over here.

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And then from there, we can continue

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through the rest of meiosis
I and then meiosis II.