- Let's talk about how DNA is replicated,
and in order to understand that
it's important to review the structure of DNA.
So, let's take a piece of DNA that we have right over here,
and let's just unwind the two strands,
and then let's focus on a very, very small area
in this double strand DNA,
and that's the diagram that you're seeing in the middle.
So, the functional unit that makes up DNA
is known as a nucleotide,
and a nucleotide is made up of three components.
The first is a sugar,
or more specifically, deoxyribose,
and deoxyribose is derivative of ribose, another sugar.
The one difference being
that right over here there is an H instead of an OH,
but in the central diagram I actually omitted
all the OHs and Hs just for clarities sake,
and in the corner of the molecule
that blue O represents an Oxygen,
and I'm just mentioning that because typically
a point would represent the Carbon,
but in our case it's just an Oxygen.
We label the Carbons: this Carbon is 1', this Carbon is 2',
this Carbon is 3', this Carbon is 4',
and this Carbon is 5',
and you may have noticed that the sugars in the two chains
are actually oriented in opposite directions.
So, if you look at the sugar right here, for example,
the oxygen is pointing upwards,
but in the corresponding nucleotide on the other chain,
it's pointing downwards.
So the two chains are running in opposite directions,
or another way to say this is
they are anti-parallel to each other.
Okay, the next component of a nucleotide
is a phosphate group,
and these are the purple P that you see over here,
and they connect the 3' Carbon and the 5' Carbon
of two nucleotides,
and the last component of a nucleotide is a Nitrogen base.
These are our A's, our T's, our C's and our G's.
So, for example, here's a Nitrogen base,
and here's another Nitrogen base.
So, this would be one nucleotide.
This, right over here, would be another nucleotide.
Notice each nucleotide has one sugar unit, one Nitrogen base
and the Phosphate group attached to the 3' Carbon.
So, what happens when DNA is ready to get replicated?
Let's see.
Let's take our DNA chain,
and let's just orient it horizontally,
and an enzyme will come along.
This enzyme is DNA helicase,
and DNA helicase effectively acts like a zipper.
It's going to unzip or unwind our double strand of DNA,
and then another enzyme is going to come along,
this enzyme is DNA Polymerase,
and DNA Polymerase is going to start synthesizing
new strands of DNA
that are complimentary to the old strands.
So, for example, on the bottom it's going to synthesize
a strand and it's going to put an A over here,
and a C over here,
and a T over here,
and then in the top strand
it's going to put a T over here,
a G over here,
and an A over here,
and actually, come to think of it,
I really should have drawn the A first, then the G,
and then the T.
You'll see in a few moments why,
but anyway, what's the end result of all this?
Let's skip a few steps, and let's see
what happens at the end of this entire procedure.
So, we're going to end up with two new pairs of DNA,
and in each pair there's going to be one strand
that's from the original DNA that we copied,
and then there's going to be one newly synthesized strand.
So, we call the old strand a parental strand,
and we call the newly synthesized strand
that DNA Polymerase just put together, the daughter strand,
and this idea, this concept of the fact
that each new pair has one strand that's from the old DNA,
and one strand that's new is called
semi-conservative replication.
Let's talk about DNA Polymerase.
DNA Polymerase works in very specific ways.
So, let's pretend that DNA Polymerase wanted to
elongate this chain here on the left.
So, DNA Polymerase has the ability to add nucleotides
to a 3' Carbon,
so it would add a nucleotide right here,
but it cannot add a nucleotide to a 5' Carbon.
So, if DNA Polymerase was elongating this chain,
it would be doing so in this direction.
Let's look at the chain on the right.
Again, DNA Polymerase can add a nucleotide
to this 3' Carbon,
but it can absolutely not add a nucleotide
to this 5' Carbon.
So, if DNA Polymerase were elongating
this chain here on the right,
it would be doing so in this direction.
So, the very, very important take-home message is
that DNA is synthesized
in the 5' 3' direction.