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Lesson Video: DNA Discovery and Structure Biology

In this video, we will learn how to describe the structure of DNA and outline contributions made to its discovery.

12:42

Video Transcript

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 backbone.

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 composition.

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.

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