Video Transcript
In this video, we will learn about
DNA. We’ll discuss its structure and
learn about nucleotides, the sugar–phosphate backbone, and the complementary bases
that hold the two strands of DNA together. We’ll also learn about the
orientation of these strands. And finally, we’ll go over how we
can extract DNA from organic material, such as fruit.
DNA is an incredibly important
molecule. It’s inside nearly every cell of
our body and contains the information that makes us who we are. In fact, if you were to take all
the DNA out of a single cell and stretch it out, it would be about two meters
long. That’s probably a bit taller than
you are, but maybe not so much. This incredible molecule contains
the information that gives us all of our characteristics. For example, it controls our eye
color, whether or not we have a lot of hair or just a little, and can make us tall
or short. All of these characteristics are
coded by DNA. And in this lesson, we’ll learn
more about DNA structure, what it looks like, and what it’s made up of.
To visualize what a molecule of DNA
looks like, first picture a ladder. It has two parallel side rails and
rungs or steps at regular intervals. Now, imagine twisting the
ladder. This is the shape of a DNA
molecule. There are two strands of DNA
twisted around one another. This shape is called a double
helix. Now let’s zoom in and take a closer
look at DNA’s chemical structure. So here we can see the two strands
of DNA. This strand on the left is
indicated here, while this strand on the right is indicated here. Each strand of DNA is a polymer
that’s made up of many repeating subunits called nucleotides. You can see one of these
nucleotides circled here.
In this diagram, there’s a total of
six nucleotides, and each nucleotide is made up of three distinct parts. The first part is a phosphate
group. The second part is a pentose sugar,
which is a sugar molecule that contains five atoms of carbon. In DNA, this pentose sugar is
called deoxyribose. In fact, this is why we call it
DNA, or deoxyribonucleic acid. The “deoxyribo” part refers to the
deoxyribose sugar in DNA, while nucleic acid refers to how DNA is a nucleic acid,
which is a polymer of nucleotides.
The third part of a nucleotide is
the nitrogenous base. There’s actually four different
nitrogenous bases in DNA: guanine, or G for short, represented here in orange;
cytosine, represented in blue; adenine, represented in green; and thymine,
represented in pink. You can see these different
nitrogenous bases represented in both diagrams. These nucleotides can differ from
one another depending on what kind of nitrogenous base they have. So that’s the basic structure of a
nucleotide.
You might be wondering how
nucleotides combine with one another. They are joined by the phosphate
group and the two adjacent carbons in the deoxyribose sugar. This is called a phosphodiester
bond. These repeating phosphodiester
bonds attach one nucleotide to the next and form the backbone of DNA. This backbone is called the
sugar–phosphate backbone, and it’s represented in black in this diagram and also
black in this diagram. So there’s two sugar–phosphate
backbones in DNA because there’s two strands of DNA in the double helix. So how does one strand of DNA
combine with the other strand to form this double helix structure?
This has to do with these
nitrogenous bases and how they bond to one another. Every nitrogenous base on one
strand of DNA bonds to a nitrogenous base on the opposite strand. And this is how these rungs in this
twisted ladder shape are formed. When these nitrogenous bases bond,
they do so in a special way. In DNA, adenine can only bond to
thymine and cytosine can only bond to guanine. These are called the rules of
complementary base pairing. This is accomplished through
forming hydrogen bonds. And between cytosine and guanine,
three hydrogen bonds can form, while in adenine and thymine, two hydrogen bonds
form. These hydrogen bonds aren’t very
strong on their own. But collectively, over a large
molecule of DNA, this can be very strong and is what holds the two strands
together.
Now let’s talk about directionality
of DNA. You probably noticed that the
deoxyribose sugar of one strand seems to be pointing in the opposite direction of
the other. That’s because these two strands
are going in opposite directions. The way we talk about direction in
DNA has to do with how the carbons in the deoxyribose are numbered. This carbon here is called one
prime. This is two prime, three prime,
four prime, and five prime. This atom here is actually an
oxygen atom and not our carbon.
When we talk about direction in
DNA, we’re interested in the five prime and three prime positions. This is because this is where the
phosphodiester bond is formed. And when DNA is synthesized and new
phosphodiester bonds are formed, new nucleotides are always added in the five prime
to three prime direction. So this strand is pointing in this
direction because that’s the direction that this strand would grow during DNA
synthesis. This is just a convention that we
use in biology to describe DNA’s direction. So, when we read a sequence of DNA,
we read it in the five prime to three prime direction.
Let’s label this strand as the five
prime to three prime strand because it’s pointing in that direction. And if we look at the diagram on
the left and follow this strand, we can see this strand labeled here. Now let’s label the carbons on the
opposing strand. Five prime to three prime in this
strand is pointing in the opposite direction of the first strand. So let’s label this strand as the
three prime to five prime strand because that’s the direction it’s pointing in. And we can see this in the diagram
on the left here. Because these two strands are going
in opposite directions, this means that these strands are antiparallel, meaning they
run parallel to each other but in opposite directions.
Now let’s try reading the sequence
of these nitrogenous bases on these strands. So here we can see that the
sequence is CAC. And let’s do it on the diagram on
the left also. So we have CAC. And then the strand twists around
to TGC. Now let’s write this sequence up
here. We’ll also indicate that it’s in
the five prime to three prime direction. Now, what do we do when we want to
figure out the sequence of the opposing strand?
Remember that these nitrogenous
bases can form complementary base pairs that follow certain rules. So, whenever there’s a C in the
sequence, we’ll know that it base-pairs with G. So, if you go up here to this
sequence, we can fill it in, and the opposite is true too. So, if there’s a G, we know that it
will pair with C. And if we have an A, we know that
it will pair with T, so we can fill that in here, and T will pair with A. We can also double-check our work
right here. So we have GTGACG, which matches
our sequence.
Sometimes when we talk about the
sequence of nitrogenous bases, we call them base pairs because they pair
together. So, in this sequence, we have one,
two, three, four, five, six base pairs. In humans, most cells have over six
billion base pairs worth of DNA in a single cell. That’s why it’s two meters in
length when stretched out because there’s so much of it.
Many DNA sequences provide the
instructions for making proteins, which give us our unique characteristics. We call these DNA sequences
genes. If we represent a segment of our
double-stranded DNA as a line like this, we can see genes throughout our DNA
indicated here in blue. Some have a very long DNA sequence,
and some are very short. These different genes can code for
all of our different characteristics, like our eye color, our hair color, or for how
tall we might be. We have over 20,000 genes in our
DNA, and together they make us who we are. We have learned so much about the
structure of DNA, but how can we extract it so we can see it with our own eyes?
We can do this with different
organic materials, like fruits or vegetables. In this example, we’ll be using
strawberries. We’ll need a few more supplies:
dishwashing liquid, salt. We’ll need some beakers, but you
can use a cup. We’ll need some filter paper, but
you can use a coffee filter, and ethanol, or you can use rubbing alcohol. So, first, you take a couple of
strawberries and smash them up really good in a beaker or a cup. Then, take another beaker and add
100 milliliters of water, then 10 milliliters of dishwashing liquid, and half a
teaspoon of salt. Now, mix it all up, and this is
your extraction buffer. Now, add the extraction buffer to
your smashed strawberries. Then, stir it all up for one minute
and, if you can, leave it in a warm water bath for 10 minutes. The mixture might turn a bit pink
from the strawberries. The dishwashing liquid and salt
help to break down the cells of the strawberries so our DNA is now dissolved in this
solution.
Next, let’s filter this solution to
get rid of the strawberry bits and seeds. Now that we have our strawberry DNA
solution, we need to precipitate the DNA out from this solution. You can do this right here in this
beaker, or you can do it in a test tube if you have one. So you can add 10 milliliters of
this DNA solution to a test tube. Then, carefully add an equal volume
of cold ethanol so it forms a layer on top. Then, wait about five minutes, and
a stringy substance should form in this top layer. This is the DNA.
Now that we’ve covered DNA
structure and how to extract it, let’s try out a practice question.
A section of DNA contains the order
of bases ATGCTTAA. What would the complementary
sequence of bases be? (A) ATCCAATT, (B) TACCAATT, (C)
TACGAATT, (D) TACGGATT, or (E) TTCGAATT.
DNA is a nucleic acid that stores
the genetic information needed for life. It’s responsible for all of our
different characteristics, such as the color of our eyes or how tall we are. It’s made up of two strands of DNA
that are twisted around each other to make a double-helix shape as shown here.
Let’s untwist this DNA helix shape
so we can talk more about its components. The black line is the backbone of
these DNA strands and is called the sugar–phosphate backbone. It’s made up of phosphate groups
and deoxyribose sugars. And in between these two strands
are these different-colored boxes. These are called nitrogenous bases,
or just bases for short. There are four different types of
nitrogenous bases: guanine, or G for short, represented in orange; cytosine,
represented in blue; adenine, represented in green; and thymine, represented in
pink. These individual bases often come
in pairs as you may have noticed.
You can see all the guanines
indicated here, which are always paired with a cytosine, indicated here, whereas
adenine always pairs with thymine. This isn’t a coincidence. These specific bases pair with one
another because they have an affinity for each other. This is due to hydrogen bonding
between these bases. We call two bases that pair
together complementary. And DNA bases can pair according to
certain rules, where G always pairs with C and A always pairs with T. So, when we have a sequence of
these bases, like the one that’s given in the question, all we have to do is match
these up. So guanine will pair with cytosine,
and cytosine will pair with guanine. Adenine will pair with thymine, and
thymine will pair with adenine, which now gives us our complementary sequence of
bases. Therefore, the correct answer is
TACGAATT.
Now let’s go over the key points
that we covered in this video. DNA is a polymer made up of
repeating monomer subunits called nucleotides. In DNA, a nucleotide is made up of
a deoxyribose sugar, a phosphate group, and a nitrogenous base. DNA is made up of two strands of
DNA that form a double helix with a sugar–phosphate backbone. This shape is maintained by
complementary base pairs, where A binds to T and G binds to C. DNA is read in the five prime to
three prime direction. You can extract DNA from organic
material, such as fruits and vegetables.
