- [Voiceover] If you were to
take a look at a chromosome you would see see that it is made up of this very densely packed
(mumbling) known as chromatin. And then if you were to
further break down chromatin you would see that it's made
up of tremendous amount of DNA wrapped around these
proteins known as histones. And DNA stores our genetic information. We get it from our
parents and we pass it on to our children and DNA
basically determines the identity of all living organisms. And just some interesting facts about DNA. If you were to take the
DNA that was contained in one human cell and stretch it out, it would measure about two meters or approximately six feel long. So, that is a lot of DNA to pack into a cell that's relatively so tiny. And how's that done? Well, with the help of those
proteins I mentioned histones, they help to wrap DNA in a very tightly coiled and very dense fashion. Just another interesting fact: If you were to take all the
DNA found in one human's body and line it up together it would measure, brace yourself for a very large number, it would measure one
hundred trillion meters. That is a huge number. In fact, something that long
can go around the equator of the Earth two and a half million times. So, we hold in our cells a tremendous, tremendous amount of DNA. But anyway, let's talk about the structure of this super, super important molecule that basically determines the identity of all living organisms. So, DNA's made up of three components. The first is a sugar known as deoxyribose. And it's deoxyribose because
there is a sugar Ribose that has an oxygen right over here but deoxyribose doesn't have that oxygen. So, this molecule's deoxyribose and the carbons in
deoxyribose are labeled. This carbon is labeled one prime, prime's first of that little
apostrophe after the number. Two prime, three prime. This carbon is four prime and this carbon is five prime. And in case you're wondering
why we need those primes, like, why can't we just
leave all the carbons? carbon one, two, three, four, five. So, the answer to that question is that we're trying to differentiate between the carbons in this molecule. I'll explain to you in a
minute what this molecule is. But, we're trying to differentiate between the carbons in this molecule and the carbons in the deoxyribose. So, for some reason, the carbons in this molecule took precedence and the carbons there are labeled one, two, three, four, five, etc. And so the carbons in deoxyribose are labeled one prime, two
prime, three prime, etc. But anyway, that takes care of deoxyribose and then the next molecule
in DNA is a nitrogen base. And the nitrogen base you're looking at here's actually adenine. We've heard of the molecule
ATP, adenosine triphosphate, and that also has adenine in it. But anyway, there are actually four different nitrogen bases
that you can find in DNA. So, I'm gonna pause for a second
from what we're looking at and we're gonna take a look
at those four nitrogen bases. This one here is adenine. That's the base that we
just saw a moment ago. And then right next to
it looking very similar is another nitrogen base guanine. And you can see that adenine and guanine are both double ring structures. On the left you can see they have a ring with six sides to it, and then attached on the right they have a ring with five sides to it. And, well, these are all
called nitrogen bases 'cause they have couple nitrogens in them. And adenine and guanine
are known as purines. So, the double ring bases
are known as purines and I always have this
hint to help me remember. So, it's really an exstrinsic hint because it has nothing
to do with the material but it always helped me. So, when something is pure it glows, so purines always glow. That was my hint and then
I would always remember that A stands for adenine and
G always stands for guanine. So, if it helps you then use that. Then we have these other two bases. This one here is thymine. And then right next to
it we have something that also looks similar to it, cytosine. And you can see thymine and cytosine are single ring structures. They only have one ring with six sides and they're known as pyrimidines. So, again, the purines
are adenine and guanine and the pyrimidines are
thymine and cytosine. And the purines and
pyrimidines will always pair up with each other in this fashion. Adenine always pairs up with thymine and guanine always pairs up with cytosine, unless, of course, there's a problem. And I wanna just, let's just take a look at how these molecules
pair up with each other. So, let's look at this diagram. So, the bonds that hold the nitrogen bases together are hydrogen bonds. So, it's hydrogen bonding
that puts them together and let's just remind ourselves, a hydrogen bonding
takes place in molecules that have a hydrogen attached to one of three very electronegative atoms: fluorine, or oxygen, or nitrogen. Let me remind you, electronegative means that they like to hog electrons. They pull electrons towards themselves. And what's going to happen
in molecules like this is that since fluorine, or oxygen,
or nitrogen hog electrons they are going to get a slightly, or maybe more than slightly, negative charge which
leaves the hydrogens kind of bereft of electron density and gives them a positive charge. And then the molecules
will orient themselves in a way where the
positive and negative sides are attracted and attached to each other. So, let's actually take a look at what I just explains in the molecules. So, let's look at thymine and adenine. So, we have this oxygen
over here which is going to be somewhat negative
because it's pulling electrons away from that carbon and
for in this double bond, and then these hydrogens are going to be somewhat positive
because the nitrogen near them is pulling electrons away. And so they form this
hydrogen bond right over here. Then we have another hydrogen bond between this positive hydrogen. Remember, it's positive
because the nitrogen here is very electronegative and
hogs all the electrons. And then we have this
negative nitrogen because it hogs electrons from
the carbons around it. So, between thymine and adenine, we're going to have two hydrogen bonds. But if you look at cytosine and guanine, there're actually three
hydrogen bonds between them. So, we can see that cytosine and guanine are attached to each other a little bit more strongly than thymine and adenine and well, what would the
implications of this be? So, what do we have? Two pairs of DNA. We're gonna soon see DNAs
at double stranded molecule where the nitrogen bases
pair up with each other, something like this. And I'm gonna label this DNA
set A and this I'll label B. And let's say I tell you that in A we have a very high number of As and Ts, so, let's say most of these are As and Ts, so, I'm just gonna, I don't know, put an A here and put a, well, let's make that a little bit clearer. So, an A and then there's
gonna be a T here, and let's say that most of
this DNA looks like that. And let's say that B has a very, very high number of Cs and Gs. So, here's a C and here's a G, and let's say that most of
the DNA looks like that. So, B has a lot of Cs and Gs. So, which DNA do you think
it's gonna be harder to break? And by break, I mean
basically break the bonds between the nitrogen bases just like that and make two separate strand, and that's actually called denaturization. So, to denature DNA means to kind of split it down the middle,
break the nitrogen base bonds, and have two strands instead of one. So, again, which of
these DNAs do you think it's going to be harder
to denature, A or B? Well, we just explained
that between Cs and Gs, between cytosines and guanines, there are three hydrogen bonds. So, it would be harder to break down B because it has more Cs and Gs. And so, one way to denature DNA
is to raise the temperature. That's one way to break down DNA. So, breaking down DNA B is going to take a higher temperature than breaking down DNA A. That's just one example of
why this fact would matter. This fact thymine and adenine
have two hydrogen bonds and cytosine and guanine have three. Anyway, now that we've
discussed the nitrogen bases that make up DNA let's go
back to actually putting our DNA together and the
various components in it. So, again, we said the first
component in DNA deoxyribose. The second thing we
discussed just now were the nitrogens bases and now the
third component in DNA is going to be a phosphate group. And actually, what I
drew was a triphosphate. It's three phosphates together and I drew it as a triphosphate because we start off with a triphosphate but eventually two of the
phosphates get lopped off and we're gonna be left with
only one phosphate group. So, we're gonna pause out
and in part two of this topic we're gonna pick up on this and see how we put together
all of these components to make the DNA that we have in our cells.