In this video, we’ll learn about
the structure of DNA, from the molecules it’s made of to the bonds that hold it
together. We’ll also find out how the work of
scientists in the mid-1900s has informed our current understanding of DNA. So let’s unwind the double helix
and see what’s inside.
Nucleic acids are a type of
biochemical molecule that carry genetic information. They’re found in almost every
living cell and are responsible for controlling cell functions, which in turn
control every characteristic of the organism, from their appearance to their size
and even their nutritional requirements. When organisms reproduce, the
information in the nucleic acids of each parent is passed on to the offspring.
There are two types of nucleic
acid: DNA, which stands for deoxyribonucleic acid, and RNA, which stands for
ribonucleic acid. These molecules are predominantly
found in the nucleus of every living cell. Living organisms usually store
their genetic information in DNA, while molecules of RNA help in the transfer and
interpretation of this information.
Although the molecule we now know
as DNA was first discovered back in the 1800s, it was not until the mid-1900s that
its structure and its role in genetic inheritance were uncovered. These discoveries marked a turning
point, laying the foundations for innovations in many fields of modern science. Today, we have a clear idea of what
DNA looks like and how it functions as the so-called blueprint of life.
First, let’s consider the simplest
building blocks of DNA. DNA is made of many
nucleotides. Each nucleotide has three
components. The first is deoxyribose sugar,
represented here as a pentagon. The reason we show it as a pentagon
is because deoxyribose is a pentose sugar, which means it contains five carbon atoms
in its atomic structure, as you can see here. We give each of these carbons a
number and with an apostrophe refer to them as prime. So, for example, we would refer to
this as the five prime carbon atom.
The second component of a
nucleotide is a nitrogen-containing, or nitrogenous, base, which is covalently
bonded to the deoxyribose. While all DNA nucleotides contain
the same deoxyribose sugar, they each have one of four possible different
nitrogenous bases. These are guanine, represented by
the letter G and shown here in orange; adenine, represented by the letter A and
shown here in green; cytosine, represented by the letter C and shown here in blue;
and thymine, represented by, you guessed it, the letter T and shown here in
pink. Guanine and adenine are known as
purines because they have a large two-ring chemical structure. And cytosine and thymine are known
as pyrimidines because they have a smaller single-ring chemical structure.
The third and final component of a
nucleotide is a phosphate group. The phosphate group is important
because it’s what allows one nucleotide to bond with an adjacent nucleotide. And it does this by forming two
ester bonds between the phosphate group and one carbon atom on each side. These are collectively known as a
phosphodiester bond. Phosphodiester bonding between
millions of nucleotides leads to the formation of a polynucleotide chain. Because these bonds form between
sugars and phosphate groups, we say that the polynucleotide has a sugar–phosphate
How do polynucleotides become fully
formed DNA molecules? If we zoom in on one end of the
polynucleotide, we can see that it has a free phosphate group, which is not bound to
a second deoxyribose sugar. Remember how we said that every
carbon atom in the sugar molecule is numbered. Well, this free phosphate group is
bound to the five prime carbon, so we say that this is the five prime end of the
polynucleotide. If we now zoom in on the opposite
end, we can see that it has a free OH group, which is bound to the three prime
carbon. So this is known as the three prime
end of the polynucleotide.
DNA is not often found as a single
polynucleotide. It’s usually double stranded. In other words, it’s usually made
of two polynucleotides joined together. So how does this happen? A second polynucleotide chain is
assembled, which is then joined to the first via hydrogen bonds between the
nitrogenous bases. These hydrogen bonds don’t form
randomly however. Thymine only forms bonds with
adenine, and cytosine only forms bonds with guanine. This phenomenon is known as
complementary base pairing. And it means that the two strands
that make up a DNA molecule are complementary to one another.
Another feature of these two
strands is that they’re antiparallel. This means they’re oriented in
opposite directions. So the five prime end of one strand
lines up with the three prime end of the other strand, and vice versa. The structure now looks quite a bit
like a ladder, with the hydrogen bonds between the bases acting like the rungs. To form the final DNA structure,
this ladder-like molecule twists in a clockwise direction. This final shape is known as the
double helix, and it’s the form in which DNA is most commonly found.
We can use this knowledge of DNA
structure to calculate the percentage composition of DNA. Due to complementary base pairing,
a DNA molecule must always have the same number of guanine bases, shown here in
orange, as cytosine bases, shown here in blue. For example, in the piece of DNA
shown here, there are seven of each. For the same reason, there must
always be the same number of adenine bases, shown here in green, as thymine bases,
shown here in pink, in this case six of each. This was discovered by an American
scientist called Chargaff in the late 1940s before the structure of DNA had even
been explained. Chargaff’s rule, as it became
known, can be used to calculate the percentage of each base in the DNA of a
particular organism. This is also known as percentage
Now let’s see how DNA sequences are
interpreted by scientists. You can think of DNA as being a bit
like an instruction book for everything that happens inside a cell, with the
nitrogenous bases G, A, C, and T acting as a sort of genetic alphabet. The order or sequence of these
bases is therefore really important in determining what proteins will eventually be
made. Scientists have been studying DNA
sequences since the 1970s in order to get a better understanding of how genes
function and how we can use them to our advantage, for example, to develop
personalized medical treatments.
The problem when investigating
these sequences is that DNA is not always found in the same orientation. For example, the length of DNA
shown here could be found this way round or this way round. And depending on which direction a
scientist analyzed it in, reading it left to right would give them a totally
different idea about what genetic information it contained. To avoid this problem, DNA is
always read in the five prime to three prime direction. Therefore, regardless of their
orientation, DNA sequences will always be interpreted in the same way.
Now let’s travel back in time to
see how the structure of DNA was first discovered. In 1951, an English scientist
called Rosalind Franklin began investigating the structure of DNA. She had a great deal of experience
with the laboratory technique called X-ray crystallography. This involves passing X-rays
through crystalline structures and observing how they scatter or diffract. By capturing these diffraction
patterns as photographs and analyzing them, scientists can identify the molecular
structure of a particular crystal.
In Franklin’s experiment, she
applied the technique to isolated crystals of DNA. Along with her team, she developed
many photographs of the crystal structure of DNA. This photograph in particular,
taken in 1952, provided the most conclusive evidence for the structure of DNA. Firstly, it was determined from the
X-shaped diffraction pattern that DNA has a helical shape. Secondly, using information about
the density of Franklin’s original DNA samples, it was concluded that DNA is made of
two strands. Thirdly, it was deduced that
nitrogenous bases face inwards towards the center of the helix, whereas the
phosphate groups face outwards.
This image was the 51st photograph
taken by Franklin and her team and has therefore more recently become known as Photo
51 in recognition of its significance. In January 1953, Franklin’s
colleague Maurice Wilkins showed Photo 51 to James Watson. Watson was another scientist who,
along with his research partner Francis Crick, was also trying to establish the
structure of DNA. Watson and Crick used the
information from Photo 51 to propose a three-dimensional double-helix model for the
structure of DNA, which they published in 1953 without mentioning Franklin’s
contributions at all.
In 1962, Wilkins, Watson, and Crick
were awarded the Nobel Prize in physiology or medicine for their discoveries, by
which time Franklin had sadly passed away from ovarian cancer. It was not until 1968 when Watson
published a personal account of their work that Rosalind Franklin’s significant
contributions to the discovery of the structure of DNA were properly recognized. The fact that the double-helix
structure of DNA is still the model we use today shows just how groundbreaking and
influential the work of Franklin and her colleagues was.
Now we’ve learnt all about DNA,
let’s have a go at a practice question.
What type of bond forms between
base pairs in DNA to hold the two strands together in a double helix? (A) Glycosidic, (B) hydrogen, (C)
ionic, (D) covalent, or (E) phosphodiester.
This diagram represents the
double-helix structure of DNA. If we zoom in on a small section of
this DNA molecule, we can see that it’s made of individual units that we call
nucleotides. Each nucleotide contains a
base. There are four different possible
bases, as represented by the four different colors you see here. A special type of covalent bond
called a phosphodiester bond forms between nucleotides. These phosphodiester bonds form the
two strands that make up the DNA molecule. But this question is asking us what
bonds form between base pairs to hold the two strands together. So we can rule out (E) because
phosphodiester is not the correct answer.
The bonds that form between base
pairs are hydrogen bonds. As you can see on this diagram, two
hydrogen bonds form between thymine and adenine, which are the names of these two
bases, and three hydrogen bonds form between guanine and cytosine, which are the
names of the other two bases. We have therefore determined that
the type of bond that forms between base pairs in DNA to hold the two strands
together in a double helix is (B) hydrogen.
Let’s summarize what we’ve learnt
in this video by reviewing the key points. DNA is a nucleic acid that stores
genetic information. It is made of individual units
called nucleotides, which join together to form polynucleotide chains. Two polynucleotide chains join
together via hydrogen bonds between bases to form DNA. Guanine only forms bonds with
cytosine, and adenine only forms bonds with thymine. DNA is usually found as a
double-helix-shaped molecule, and it’s read in the five prime to three prime
direction. Finally, the double-helix model of
DNA was established in the 1950s by Rosalind Franklin and her colleagues.