[Instructor] Hello, and welcome
to the second lecture in module three.
This lecture is going to be on DNA replication,
and this lecture is going to be a bit shorter,
and it's really just to help prepare you
to understand mitosis and meiosis.
But given that this is a bit of a different topic,
I wanted to cover it separately.
So how do new cells get their DNA?
And so, as we've talked about before,
you get your complete genome from the moment of conception
when the sperm fuses with the egg
and you have your full compliments of 46 chromosomes total,
23 pairs,
one of each that came from your mother
and one of each that came from your father.
The chromosomes that come from your mother
are called your maternal chromosomes.
And the chromosomes that come from your father
are called paternal chromosomes.
We start out as one of these single fertilized eggs.
That single egg is a single cell.
It will divide many times during development
to form a fully developed human
made of trillions of cells.
As a result, the complete genomes, all 46 chromosomes,
is replicated prior to each cell division
using each of the two strands of DNA as a template
for a new complementary strand.
And we'll take a look at what that actually looks like
in the next slide.
But basically, it's a way to create an identical copy
of each chromosome.
It is critical that each of these replications
are highly accurate,
as all 6 billion bases are in the right order
every single time.
That's a tall order,
and the cell actually does a phenomenally good job of this.
And I'll give you some statistics on that in a moment.
So what is the mechanism of replication?
Again, remember we start with double stranded DNA,
and what happens is, is you can see here,
the double stranded DNA, which is base paired
between the two complimentary strands
actually starts to unzip or unwind.
And as that occurs, we start to have formation
of a new complimentary strand
to each one of those original strands.
And these new complimentary strands, or also,
will base pair with each of those other strands.
So as a result, in the end,
you have two identical double stranded copies of DNA.
Now it's important, as I mentioned, it's important
to do this in as error-free a way as possible.
And when we talk about the error rate,
we're talking about really the fidelity of,
you know, how well the copies are being duplicated
and creating identical copies
with each round of replication.
But errors do happen.
About one in 10,000 base pairs has a mismatch occur,
a G or a C with an A or a T,
as opposed to just A and T together
or G and C together.
But the cells come up with a very clever way
of dealing with this.
It has mechanisms called proofreading,
where a protein called DNA polymerase,
will actually review the newly formed strand
for these mismatches
or the places where G is binding to a T or an A
or where C is binding to a T or an A.
And the DNA polymerase,
when it finds one of these mismatches,
will actually replace one of the two bases
with its matching base.
With proofreading, DNA replication errors occur only
in one in every one billion, billion with a B, bases.
And the entire six billion bases are replicated
in a matter of hours.
So can you try to imagine replicating six billion bases
in six hours and only getting two wrong? (chuckles)
Our cells are really, really good.
I mean that is a phenomenal rate of fidelity
and rate of speed for this whole process to occur.
And it happens, it happens many, many times
as the person develops, as cells are needed to divide.
Each and every time a cell divides,
you need to have the DNA replicated
so that the two new cells that form
from this one original cell each can have a full copy
of the genome.
So let's take a quick peek at proofreading.
So as I mentioned, it's basically when you have in place,
in this case, it's looking at a G is being paired with a T.
So this should have been, right,
what base should this have been instead of a G?
It should have been an A. How do we know that?
Well, we know that because,
on the complementary strand, there is a T.
So we know in order for its base pair properly,
it would have to be an A.
The DNA polymerase will recognize this mismatch
and remove the mismatching base.
So it'll remove the G in this case,
and it will replace it with the proper base of an A.
So the DNA polymerase actually scans the entire
newly formed strands of DNA
and will replace and repair as many of these as it can.
Instead of having an error one in 10,000 bases,
the error rate declines to one in one billion bases,
which is a huge improvement.
So having proper DNA replication proofreading apparatus
in place is really important
for maintaining the fidelity of the the DNA
with every cell division.
So let's take a quick peek at what this actually looks like.
So on the left here, what we're looking at
is the original double stranded DNA molecule,
looking at it like, again,
the two complimentary strands base paired together,
sort of like the rungs on the ladder.
It starts to unzip, and as it does,
new strands begin forming
on each of the two complimentary strands.
And in the end, you have two identical strands
to one another,
two identical double stranded DNA molecules.
And each of these has a new complementary strand to it.
So now when the cell divides,
one of the cells can get one of these copies
and the other cell can get the other copy.
And in the end, you have two cells
that have identical DNA to the original cell.
What does this actually look like in a karyotype?
Just want to show you here.
So, because it can get kind of confusing,
because here we're talking about duplicating chromosomes,
as opposed to homologous chromosomes.
What were homologous chromosomes?
Those were the copies of chromosomes
where you received one from the mother
and one from your father.
What we're talking about here is actually
creating an identical copy of each one of those.
So for Chromosome 1, for example,
after DNA replication has occurred,
you can see there are two copies
of each one of these two chromosomes.
So what we would say is these two chromosomes
are homologous to one another
because one came originally from the individual's mother,
one came from the individual's father.
And then we would also, to look,
we would also look at the duplicated chromosomes
and what these are called, sister chromatids.
Sister chromatids.
That just means identical copies
of the same chromosome.
Not homologous.
Remember homologous, there are slight differences.
While they're both still Chromosome 1,
they have slight differences
because you received one from your mother
and one from your father,
and those are going to be a little bit different
from one another.
Sister chromatids, on the other hand,
are identical copies to one another.
So the only differences between sister chromatids,
so the only differences between, say, this DNA molecule
and this DNA molecule would be any of those very,
very rare errors that occurred during replication.
So, unlikely to even be a single error that's occurred
in this one chromosome.
Let's pull this all together for a quick summary.
So prior to cell division, the genome is replicated
to have one genome for each of the two cells
that will result from cell division.
Each strand of double stranded DNA molecules serves
as a template for a new complementary strand to form.
Errors in replication are rare
and are generally repaired with proofreading
for base mismatches.
However, a few errors do occur with each replication.
The end result of DNA replication
is identical sister chromatids
making up each of the 46 chromosomes.
So each of those 46 chromosomes has been duplicated,
so they each have a sister chromatid.
And during the process, which we will talk about next
in the process of mitosis, where cells will divide,
each of these cells that forms from the process of division
will receive one of those sister chromatids.
So what's next in this module?
Well, I kind of just gave it away a little bit. (chuckles)
What we're going to be talking about next
is mitosis and meiosis
and start to better understand the process of cell division.
So with that, I will say goodbye
and I will speak with you in the next lecture.