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[Instructor] Hello, and welcome

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to the second lecture in module three.

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This lecture is going to
be on DNA replication,

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and this lecture is going
to be a bit shorter,

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and it's really just to help prepare you

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to understand mitosis and meiosis.

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But given that this is a
bit of a different topic,

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I wanted to cover it separately.

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So how do new cells get their DNA?

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And so, as we've talked about before,

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you get your complete genome
from the moment of conception

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when the sperm fuses with the egg

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and you have your full compliments
of 46 chromosomes total,

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23 pairs,

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one of each that came from your mother

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and one of each that
came from your father.

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The chromosomes that come from your mother

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are called your maternal chromosomes.

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And the chromosomes that
come from your father

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are called paternal chromosomes.

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We start out as one of these
single fertilized eggs.

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That single egg is a single cell.

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It will divide many
times during development

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to form a fully developed human

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made of trillions of cells.

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As a result, the complete
genomes, all 46 chromosomes,

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is replicated prior to each cell division

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using each of the two
strands of DNA as a template

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for a new complementary strand.

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And we'll take a look at
what that actually looks like

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in the next slide.

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But basically, it's a way
to create an identical copy

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of each chromosome.

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It is critical that each
of these replications

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are highly accurate,

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as all 6 billion bases
are in the right order

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every single time.

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That's a tall order,

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and the cell actually does a
phenomenally good job of this.

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And I'll give you some
statistics on that in a moment.

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So what is the mechanism of replication?

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Again, remember we start
with double stranded DNA,

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and what happens is, is you can see here,

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the double stranded DNA,
which is base paired

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between the two complimentary strands

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actually starts to unzip or unwind.

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And as that occurs, we
start to have formation

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of a new complimentary strand

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to each one of those original strands.

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And these new complimentary
strands, or also,

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will base pair with each
of those other strands.

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So as a result, in the end,

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you have two identical double
stranded copies of DNA.

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Now it's important, as I
mentioned, it's important

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to do this in as error-free
a way as possible.

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And when we talk about the error rate,

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we're talking about
really the fidelity of,

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you know, how well the
copies are being duplicated

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and creating identical copies

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with each round of replication.

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But errors do happen.

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About one in 10,000 base
pairs has a mismatch occur,

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a G or a C with an A or a T,

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as opposed to just A and T together

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or G and C together.

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But the cells come up
with a very clever way

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of dealing with this.

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It has mechanisms called proofreading,

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where a protein called DNA polymerase,

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will actually review
the newly formed strand

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for these mismatches

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or the places where G is
binding to a T or an A

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or where C is binding to a T or an A.

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And the DNA polymerase,

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when it finds one of these mismatches,

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will actually replace one of the two bases

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with its matching base.

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With proofreading, DNA
replication errors occur only

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in one in every one billion,
billion with a B, bases.

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And the entire six billion
bases are replicated

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in a matter of hours.

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So can you try to imagine
replicating six billion bases

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in six hours and only
getting two wrong? (chuckles)

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Our cells are really, really good.

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I mean that is a
phenomenal rate of fidelity

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and rate of speed for this
whole process to occur.

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And it happens, it
happens many, many times

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as the person develops, as
cells are needed to divide.

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Each and every time a cell divides,

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you need to have the DNA replicated

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so that the two new cells that form

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from this one original cell
each can have a full copy

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of the genome.

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So let's take a quick
peek at proofreading.

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So as I mentioned, it's
basically when you have in place,

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in this case, it's looking at
a G is being paired with a T.

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So this should have been, right,

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what base should this
have been instead of a G?

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It should have been an
A. How do we know that?

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Well, we know that because,

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on the complementary strand, there is a T.

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So we know in order for
its base pair properly,

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it would have to be an A.

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The DNA polymerase will
recognize this mismatch

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and remove the mismatching base.

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So it'll remove the G in this case,

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and it will replace it with
the proper base of an A.

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So the DNA polymerase
actually scans the entire

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newly formed strands of DNA

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and will replace and repair
as many of these as it can.

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Instead of having an
error one in 10,000 bases,

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the error rate declines to
one in one billion bases,

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which is a huge improvement.

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So having proper DNA replication
proofreading apparatus

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in place is really important

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for maintaining the
fidelity of the the DNA

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with every cell division.

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So let's take a quick peek at
what this actually looks like.

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So on the left here, what we're looking at

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is the original double
stranded DNA molecule,

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looking at it like, again,

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the two complimentary
strands base paired together,

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sort of like the rungs on the ladder.

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It starts to unzip, and as it does,

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new strands begin forming

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on each of the two complimentary strands.

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And in the end, you have
two identical strands

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to one another,

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two identical double
stranded DNA molecules.

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And each of these has a new
complementary strand to it.

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So now when the cell divides,

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one of the cells can
get one of these copies

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and the other cell can get the other copy.

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And in the end, you have two cells

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that have identical DNA
to the original cell.

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What does this actually
look like in a karyotype?

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Just want to show you here.

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So, because it can get kind of confusing,

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because here we're talking
about duplicating chromosomes,

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as opposed to homologous chromosomes.

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What were homologous chromosomes?

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Those were the copies of chromosomes

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where you received one from the mother

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and one from your father.

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What we're talking about here is actually

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creating an identical
copy of each one of those.

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So for Chromosome 1, for example,

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after DNA replication has occurred,

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you can see there are two copies

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of each one of these two chromosomes.

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So what we would say is
these two chromosomes

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are homologous to one another

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because one came originally
from the individual's mother,

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one came from the individual's father.

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And then we would also, to look,

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we would also look at the
duplicated chromosomes

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and what these are
called, sister chromatids.

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Sister chromatids.

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That just means identical copies

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of the same chromosome.

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Not homologous.

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Remember homologous, there
are slight differences.

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While they're both still Chromosome 1,

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they have slight differences

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because you received one from your mother

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and one from your father,

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and those are going to
be a little bit different

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from one another.

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Sister chromatids, on the other hand,

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are identical copies to one another.

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So the only differences
between sister chromatids,

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so the only differences
between, say, this DNA molecule

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and this DNA molecule
would be any of those very,

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very rare errors that
occurred during replication.

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So, unlikely to even be a
single error that's occurred

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in this one chromosome.

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Let's pull this all together
for a quick summary.

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So prior to cell division,
the genome is replicated

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to have one genome for
each of the two cells

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that will result from cell division.

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Each strand of double
stranded DNA molecules serves

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as a template for a new
complementary strand to form.

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Errors in replication are rare

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and are generally
repaired with proofreading

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for base mismatches.

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However, a few errors do
occur with each replication.

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The end result of DNA replication

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is identical sister chromatids

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making up each of the 46 chromosomes.

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So each of those 46 chromosomes
has been duplicated,

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so they each have a sister chromatid.

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And during the process,
which we will talk about next

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in the process of mitosis,
where cells will divide,

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each of these cells that forms
from the process of division

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will receive one of
those sister chromatids.

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So what's next in this module?

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Well, I kind of just gave it
away a little bit. (chuckles)

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What we're going to be talking about next

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is mitosis and meiosis

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and start to better understand
the process of cell division.

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So with that, I will say goodbye

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and I will speak with
you in the next lecture.