HAZEL SIVE: Good.
From your practice,
you should now
be familiar with the
following DNA rules.
You should understand
base and be
able to put in the opposite
complementary strand
relative to a piece of DNA.
You should understand
polarity that we
talked about before,
the 5 prime and the 3
prime ends of nucleic acid.
And you should understand the
concept of antiparallel strands
where one strand goes 5 prime
to 3 prime and the other 3 prime
to 5 prime.
This will set you up
for the next thing
that we're going to talk about,
which is DNA replication.
What do we mean by
DNA replication?
We mean that DNA is produced
by copying a DNA template.
DNA replication produces
DNA from a DNA template.
This is a process
that takes place
in the nucleus of the cell.
And it is the mechanism
by which DNA, the genes,
make more of themselves before
they undergo cell division,
mitosis or meiosis.
What do we mean by
DNA replication?
Let us start by looking at a
piece of double-stranded DNA.
And I'm not going to put
in the exact nucleotides.
I'm just going to put some
dots and things to represent
the nucleotides.
So here is one strand.
And here is the
antiparallel strand
with the complementary bases.
DNA is replicated firstly
by separating the strands.
So a process of
strand separation
leads to two strands,
the top one 5 prime plus
the bottom one that
goes 3 prime to 5 prime.
These two separated strands now
serve as templates, something
to copy in order
to make more DNA.
So each strand is a
template that is then copied
to make the complementary DNA.
And the outcome will
be that this top strand
5 prime to 3 prime will be
filled in with its complement,
the matching other strand.
And the bottom template
that went 3 prime to 5 prime
will be filled in with its
matching complementary strand.
So out of this one DNA
template or this one
double-stranded
piece of DNA, there
are two single-stranded
templates
that then lead to two
new DNA molecules that
are identical to the one
that you started with.
So you'll have doubled
the amount of DNA
and made identical new molecules
starting with this parental DNA
template.
This is DNA replication.
It is how your genes are
replicated from cell division
to cell division from
parent to offspring.
The process is absolutely
dependent on base pairing.
Without base pairing,
you wouldn't know
which the right strand was.
It would be a mess.
But because of base pairing,
because of complementarity,
you can take the two
strands, fill them in,
and come up with DNA that
is identical to the parent.
So here is your parent DNA.
And here is your new DNA.
And they are identical.
Very profound.
Let's take a look at
a couple of slides.
So here is a double-stranded
DNA template.
I put in just some
of the nucleotides.
The strands separate.
DNA replication occurs
from each 3 prime end.
Now, there's a little wrinkle
here that you need to know.
DNA replication
doesn't exactly occur
the way I drew it on the board.
It occurs with a running start.
It needs a little,
tiny piece of DNA
called a primer that allows
the DNA then to elongate
from that 3 prime end.
So there's something called
an RNA primer which matches
a little bit of the DNA.
And that then gets elongated.
What is really,
really crucial here
is that using the rules
of complementarity
and adding on to
the 3 prime end,
we make two identical
DNA molecules that
are identical to the parent.
And I see here on the board I
must add a 3 prime end there.
If we look at a representation
of the double helix,
this is what it looks
like more in the cell.
The DNA has the
spiral helical form.
It unwinds so that you get
two single strands in part
of the helix, and then
those single strands
get filled in with new DNA.
There's some various nuances
that we're not discussing here.
I threw out the notion
of a primer to you.
All that means is that there's
a 3 prime end you can add on to.
But the notion is as we
have put it on the board--
single strands filled in
with the complementary bases.
And voila, you have
DNA replication.
Take a look at the next
assignment, the next class
exercise, and see if
you can replicate DNA.
