B1 Intermediate 1231 Folder Collection
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Hi. It's Mr. Andersen. Today I want to talk about DNA. Before we get into
transcription, translation, mitosis, meiosis, Mendelian genetics, all of these things are
based on DNA. And so we should have a real good understanding of what DNA is. Now this
animation here is just gorgeous as DNA spins and it gets this reputation for being a kind
of this mysterious magical kind of a molecule. But it's actually pretty simple compared to
proteins. It's pretty straight forward and so you should understand the parts of it.
And so basically the building blocks of DNA are nucleotides. And so just like carbohydrates
are made of sugars and protein is made of amino acids, the building blocks of nucleic
acids, both DNA & RNA, are going to be nucleotides. And so here's one nucleotide right here. A
simpler one if we were to look at basically building blocks of RNA, you've seen something
that looks like a nucleotide before, because if you take the adenine and a ribosugar and
add three phosphates to it then you have ATP. So you've seen something that looks a lot
like this. But a nucleotide has three parts to it. So part one is going to be this purply
part. And that's going to be what's called a nitrogenous base. And so in DNA there are
four types of nitrogenous bases. This one pictured here is called guanine, but there
are three other types in DNA. So you've got guanine. You've also have cytosine, thymine
and adenine. So when you see these letters in DNA, what they're really referring to is
this nitrogenous base. And the reason it's called a nitrogenous base, just look at all
the nitrogen that we have inside there. So that's a nitrogenous base. They come in two
different types. Some of the nitrogenous bases like this one are called purines. Purines
are going to have two of these carbon chains. The purines are going to be guanine. So that
would be one purine. And then the other one's going to be adenine. We'll also have some
that are a little simpler. They're not quite as big and those are going to be called pyrimidines.
Pyrimidines. Pyrimidines, the ones in DNA are going to be cytosine and thymine. And
I'll show you those in just a second. So the first part of a nucleotide is going to be
the nitrogenous base. The second part is going to be this sugar right here. And in DNA that's
called deoxyribose sugar. Now in RNA that's going to be a ribosugar, but in DNA it's going
to be a deoxyribo sugar. And the reason why it that if you were to look right here in
a ribosugar found in RNA there'd be a hydroxyl group coming off of it. But in DNA you're
missing that oxygen and so we call it de- or missing oxyribo, so deoxyribose sugar.
So that'd be the sugar right here. And then the third thing that's going to be found is
a phosphate group. And so a phosphate group, we're familiar with that from ATP, that's
going to be a phosphate group right here. And so those are the building blocks of nucleotides.
So let me get my writing out of the way and we'll go to the next slide. And so now this
is maybe a little bit more familiar when you're looking at the structure of DNA. And so this
right here, this nucleotide that we just kind of went over the parts of it, that nucleotide
is going to be found right here. Now one more thing I should mention about nucleotides is
that if you look up here on DNA there's a 3 prime end to DNA and then there's a 5 prime
end. And so what does that mean? Well that's going to refer to the sugar itself. And so
the sugar itself is going to have, since carbon is so ubiquitous or it's found everywhere
in organic material, we don't even draw the symbol on here. So there would be a carbon
right here, a carbon right here, a carbon here, a carbon here and then a carbon here.
And so we simply know number of those carbons. And so this carbon right here is called the
1 prime carbon. This one is called the 2 prime carbon. This one right would be the 3 prime
carbon. This'd be the 4 prime carbon. And then this would be the 5 prime carbon right
here. And so the 3 prime carbon is going to be coming off this side and 5 prime coming
off the other side and so basically when you're looking at DNA this right here would be the
3 prime carbon and this down here would be the 5 prime carbon. And so when you hear DNA
and the idea that DNA flows from 3 to 5 prime, well if you look at all of these deoxyribose
sugar with the oxygen on the top, they're gong to flow in this direction, from 3 prime
to 5 prime. But if you look at the other side of the DNA, it's going to go in the opposite
direction and so it's going to go into 3 prime to 5 prime. So DNA is said to be anti-parallel.
So it's going to flow in one direction on one side and it's going to flow in the opposite
direction. They're parallel to each other but they're flowing in opposite directions.
And so if we were to look across from the 3 prime end here, we're going to have a 5
prime end over here. Now why is that important? Well, when you're building DNA, when we're
going to add another nucleotide on. So here's one nucleotide. Here's another nucleotide,
when we're going to add another nucleotide we can only add it on the 3 prime end. And
so I could add a new nucleotide on this side, but I can't add it to the 5 prime end. And
so when we get to DNA replication, how DNA copies itself, that's going to be really,
really important. We could add one down here to this 3 prime end because this is going
to be our other nucleotide right here, so we could add one here, but we can't add it
to the 5 prime end. Okay. So enough with the 3 prime and the 5 prime. Let's talk about
some other of the large parts of DNA. If we look at the backbones, so the backbone is
going to be on this side, and on this side. And it kind of, DNA, looks like a ladder.
In other words if this is the backbone, then the rungs of the ladder are going to be the
nitrogenous bases that go right down the middle. But if we look at the backbone itself, it's
simply a deoxyribo sugar, a phosphate, a deoxyribose, phosphate, deoxyribose, phosphate, deoxyribose,
phosphate, deoxyribose. So the back is going to be the same with every nucleotide. And
if we look on the other side, the other side it's going to be phosphate, deoxyribose, it's
going to be the same thing on the other side. So the backbone is going to be relatively
boring. That's not too exciting. But if we look to the inside, that's where we're going
to have these purines and the pyrimidines, these nitrogenous bases. And if you look right
here, the adenine, which again is a purine, so it's got these two rings, that's going
to be connected using these hydrogen bonds to the pyrimidine on the other side. And if
we have a purine on this side, we're going to have a pyrimidine on the other side. And
there's base pairing. In other words, adenine is always going to bond to thymine and if
we look down here, guanine is going to bond to cytosine. Or this would be a cytosine and
a guanine. And this is going to be an adenine and a thymine. Now what do I mean by they
bond to each other? Well the bonds are going to be right here in the middle. So if you
look at DNA, the structure of DNA, everyone of these atoms is going to be connected to
every other atom by a covalent bond. Except if we look right down here in the middle.
If we look right down the middle, these are actually hydrogen bonds. Remember hydrogen
bonds are very weak, so these are relatively weak bonds that go right down the middle.
Why is that important? Well if I pull DNA in either direction like this, the DNA will
unzip in the middle and we're just breaking those hydrogen bonds. If we let go of it,
it'll just go right back together again because those hydrogen bonds are going to form. And
just like adenine is covalently bond to the nucleotide below it, it's hydrogen bond between
the two. And so that'll be super important in like when we're doing DNA replication or
when we're making messenger RNA. These hydrogen bonds, super easy to break. And so that would
be the gross anatomy of deoxyribonucleic acid. Those are going to be the parts. What's important
is that we can store information in here. So when we talk about transcription and translation,
every three letters here are going to code for one amino acid. And so if we were to break
down the word deoxyribonucleic acid, you should be able to understand where that name comes
from. And so in the other words the deoxyribo part comes from this. It comes from the deoxyribose
sugar. The nucleic part comes from the idea that it's found inside a cell. The DNA is
going to be found inside the nucleus of the cell. And then the acid part actually comes
from the phosphate group. Phosphates are going to donate hydrogen and so that's going to
make it acidic which will become important in just a second. So here's our DNA. It's
just repeated nucleotides over and over and over. But we know the DNA looks like this
and so DNA is going to have this three dimensional shape. And scientists think that RNA was the
first genetic material on our planet and then DNA is kind of an upgrade to that. And so
if we see this is our DNA, it's essentially that same ladder, but that ladder has been
twisted into a helix. And the reason why it's not really drawn here is that there are also
going to by hydrogen bonds that are holding each of these, so there are going to by hydrogen
bonds here and hydrogen bonds here, and so basically what that does is it gives it this
three dimensional shape. And that makes it really, really stable. And so DNA has some
advantages over RNA. Number 1 it has a more stable three dimensional shape, but the other
nice part is that we could have a mutation on one side of the DNA and since mutations
are passed from generation to generation to generation, if it's in DNA then we have a
back-up copy on the other side, so if there's a mistake here we can actually look at the
DNA that's on the other side and then enzymes can actually cut this out and replace it on
the other side. And so that's DNA. Those are the major parts of DNA and it's really not
that complex. If you understand breaking it down to a nucleotide and the idea that it's
simply just a ladder. A ladder of information and that information essentially tells a cell
how to make a protein. And so that's DNA and I hope that's helpful.
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What is DNA?

1231 Folder Collection
Cheng-Hong Liu published on November 22, 2014
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