How DNA is replicated 1


– 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.

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