Lesson Explainer: DNA Discovery and Structure Biology

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

Nucleic acids are a type of biochemical molecules that carry genetic information. In every living cell, nucleic acids are responsible for controlling the cell’s functions, which in turn control every characteristic of the organism: its appearance, its size, its nutritional requirements, and so on. When organisms reproduce, the information in the nucleic acids of the parent is passed on to the offspring.

There are two types of nucleic acids: DNA, or deoxyribonucleic acid, and RNA, or ribonucleic acid. These molecules are primarily found in the nucleus of every living cell. Usually, living organisms store their genetic information in DNA, while molecules of RNA help in the transfer and interpretation of this information.

Key Term: Nucleic Acids

DNA and RNA are nucleic acids. They are polymers made of nucleotide monomers. These macromolecules are adapted to store and transmit genetic information.

Definition: DNA (Deoxyribonucleic Acid)

DNA is the molecule that carries the genetic instructions for life. It is composed of two strands of nucleotides that coil around each other to form a double helix.

Definition: RNA (Ribonucleic Acid)

RNA is a single-stranded polynucleotide that helps to transfer genetic information and to interpret this information to synthesize proteins. In some viruses, RNA carries the genetic material instead of DNA.

In the 19th and 20th centuries, scientists began uncovering information about DNA, its structure, and its functions. These discoveries marked a turning point, forming the foundations for innovations in several fields of modern science. Today, we have a clear picture of what DNA looks like and how it functions as the “blueprint of life.”

DNA was first discovered in 1869 by Friedrich Miescher, who isolated the molecule and identified that it was acidic in nature. In the early 20th century, several groups of scientists began to conduct research to understand the functions of DNA, and by 1952, it was firmly established that DNA is the carrier of genetic material. Parallel to these studies, in the early 1950s, other scientists were working to propose a model explaining the structure of a DNA molecule. In 1953, James Watson and Francis Crick proposed that DNA is a double-stranded helix. They were able to arrive at this conclusion by building on the work of Rosalind Franklin and Maurice Wilkins.

Before we learn more about the contributions of these scientists in the discovery of the structure of DNA, let’s understand the structure ourselves!

To visualize what a molecule of DNA looks like, first picture a ladder. The ladder has two long, parallel spines, with rungs at regular intervals. Now, imagine that this ladder is twisted, as shown in Figure 1. This is the shape of a DNA molecule: two strands twisted around one another. This shape is called a double helix.

Key Term: Double Helix

A double helix is a twisted-ladder shape, specifically the shape of a molecule of DNA.

Each strand in this double helix is a polymer, which means that it is made of several smaller, repeating units called monomers. In nucleic acids, an individual monomer is called a nucleotide. The entire strand of DNA, therefore, is called a polynucleotide. Figure 2 shows where an individual nucleotide sits in a strand of DNA.

Key Term: Nucleotide

A nucleotide is a monomer of a nucleic acid polymer. Nucleotides consist of a pentose sugar, a phosphate group, and a nitrogen-containing base.

Key Term: Polynucleotide

Polynucleotides are the polymers that make up nucleic acids. They consist of many repeating subunits called nucleotides.

Each nucleotide has three components: a pentose sugar molecule, a phosphate group, and a nitrogen-containing base. We will take a look at each of these individual components and learn how they link together to form a nucleotide and eventually a polynucleotide chain.

“Pentose sugar” is another name for a sugar molecule made of five carbon atoms. In DNA, the pentose sugar is called deoxyribose. The five carbon atoms take the shape of a closed ring, as shown in Figure 3. Each carbon atom in this ring is numbered, starting on the first carbon with 1 (read as “1 prime”), moving clockwise from the oxygen atom in the ring, and ending with 5 (read as “5 prime”) as shown below.

Key Term: Pentose Sugar

A pentose sugar is a sugar molecule that contains five atoms of carbon. The pentose sugar in DNA is deoxyribose sugar, and the pentose sugar in RNA is ribose sugar.

As we have learned, there are two different kinds of nucleic acids: DNA and RNA. In RNA, the pentose sugar is called ribose. The structure of ribose differs from that of deoxyribose at just one point. At the 2 carbon of ribose, there is a hydroxyl group (OH), which is absent from the deoxyribose molecule, as you can see in Figure 3.

In nucleic acids, the 1 carbon atom of the pentose sugar is covalently bonded to a nitrogenous base, as you can see in Figure 4. A pentose sugar molecule bound to a nitrogenous base is called a nucleoside.

There are four different nitrogenous bases in DNA: adenine (A), guanine (G), thymine (T), and cytosine (C). In RNA, thymine is replaced with uracil (U). Of these, adenine and guanine are called purines, and they have two-ring structures. Thymine, cytosine, and uracil, on the other hand, have a single-ring structure and are called pyrimidines. These structures are shown in Figure 5.

The final component of a nucleotide is the phosphate group. In the pentose sugar molecule, every 5 carbon atom is covalently bound to a phosphate group, as shown in Figure 6. A nucleoside bound to a phosphate group forms a nucleotide. Millions of these nucleotides attach together to form a polynucleotide chain.

We now know what a single nucleotide looks like. But how are these individual units linked together to form a polynucleotide chain? Between each nucleotide and its neighbor, there is a bond called a phosphodiester bond. Let’s see how this bond is formed.

Definition: Phosphodiester Bond

A phosphodiester bond is the chemical bond that forms between a phosphate group and two sugar molecules.

Remember how each nucleotide has a phosphate group bound to the 5 carbon of its pentose sugar? This phosphate group forms a bond with the 3 hydroxyl group (OH) of each neighboring nucleotide, as shown in Figure 7. This bond is called the phosphodiester bond. “Di” means “two,” indicating that two ester bonds are formed here, as the phosphate group binds to one sugar molecule on either side.

When these bonds link several nucleotides together to form a polynucleotide chain, you can see that it appears as if the chain alternates between a sugar molecule and a phosphate group. This part of the polynucleotide chain is therefore called the sugar–phosphate backbone.

Example 1: Identifying the Components of DNA

A simple diagram of the structure of DNA is provided. What part of this structure is indicated by label X?

  1. Sugar–phosphate backbone
  2. Triglyceride chain
  3. Ribosomal subunit
  4. Nitrogenous bases
  5. Ribose sugar backbone

Answer

DNA is the molecule that carries genetic information in every living cell. A molecule of DNA consists of two polynucleotide chains made of repeating units called nucleotides.

A single nucleotide is made of a pentose sugar, a nitrogenous base, and a phosphate group. A pentose sugar is a sugar molecule made of five carbon atoms. In DNA, the pentose sugar is deoxyribose, and in RNA, the pentose sugar is ribose. In the diagram in the question, you can see the deoxyribose sugars represented as gray pentagons.

Each pentose sugar is attached to a nitrogenous base. In DNA, there are four different types of nitrogenous bases: adenine (A), guanine (G), thymine (T), and cytosine (C). In the figure, the nitrogenous bases are the pink, green, blue, and orange structures that you can see on the inside of the DNA molecule. A pentose sugar bound to a nitrogenous base is called a nucleoside.

Every pentose sugar is also attached to a phosphate group, which you can see represented as the yellow circles in the figure above. A nucleoside bound to a phosphate group is called a nucleotide.

Individual nucleotides link together to form long polynucleotide chains by forming bonds called phosphodiester bonds. A phosphodiester bond forms between the phosphate group of one nucleotide and the pentose sugar of the next nucleotide. This is how, on either side of the figure, individual nucleotides form long chains.

Now that we know about the components of a DNA molecule, let’s take a quick look at the options provided in the question. We can straightaway eliminate two of them—ribosomal subunit and triglyceride chain—because they are not part of the DNA structure. We can also eliminate ribose sugar backbone, because the structure we are looking at is DNA, and not RNA.

In the section labeled X in the figure, it appears as if the chain alternates between a gray pentose sugar and a yellow phosphate group. These structures are called sugar–phosphate backbones.

The part of DNA labeled X is therefore the sugar–phosphate backbone.

So far, we have understood how a single long, repeating polynucleotide chain is formed, and we know the components it is made of. But remember, a strand of DNA is a double helix, which means that it has not one but two polynucleotide chains, twisted around one another! How does one strand of DNA connect with the other to form this shape?

This is accomplished through the nitrogenous bases we learned about earlier. Every nitrogenous base on one strand of DNA binds to a nitrogenous base on the opposite strand. This is how the “rungs” in our twisted-ladder shape are formed!

When these nitrogenous bases bind, they do so in a special way. In DNA, adenine can only bind to thymine on the opposite strand, and guanine can only bind to cytosine. This rule is called “complementary base pairing,” and is one of the defining features of DNA. Adenine binds to thymine through two hydrogen bonds, while guanine binds to cytosine through three hydrogen bonds, as shown in Figure 8.

Key Term: Complementary Base Pairing

DNA bases can pair according to specific rules, where adenine (A) binds to thymine (T), while guanine (G) binds to cytosine (C). In RNA, uracil (U) is substituted for thymine (T). These rules of complementary base pairing are critical for DNA replication and transcription.

Example 2: Identifying Bonds in the DNA Structure

What type of bond forms between base pairs in DNA to hold the two strands together in a double helix?

  1. Hydrogen
  2. Ionic
  3. Glycosidic
  4. Phosphodiester
  5. Covalent

Answer

A molecule of DNA is a polymer made of repeating subunits called nucleotides. A single nucleotide has a pentose sugar, a phosphate group, and a nitrogenous base.

Several of these nucleotides link together to form long, polynucleotide chains, as shown in the figure below. The bonds that form between one nucleotide and the next to form this linear chain are called phosphodiester bonds.

If we look at the question, we can see that “phosphodiester bonds” is one of the options. We can eliminate this option, because as we now know, phosphodiester bonds link one nucleotide to the next to form the sugar–phosphate backbone, rather than linking two DNA strands together.

The nitrogenous bases, represented above in pink, green, blue, and orange, are responsible for holding the two strands together. There are four different kinds of nitrogenous bases in DNA: adenine (A), guanine (G), thymine (T), and cytosine (C). When two strands of DNA bind to one another, they follow complementary base pairing: adenine binds to thymine through two hydrogen bonds, and guanine binds to cytosine through three hydrogen bonds.

The type of bond that holds two DNA strands together, therefore, is a hydrogen bond.

Example 3: Identifying the Number of Hydrogen Bonds between Nitrogenous Bases

Between which DNA bases do 2 hydrogen bonds form?

  1. C and A
  2. C and T
  3. A and T
  4. C and G
  5. G and T

Answer

A molecule of DNA is a polymer made of repeating subunits called nucleotides. A single nucleotide has a pentose sugar, a phosphate group, and a nitrogenous base.

Several of these nucleotides link together to form two long, polynucleotide chains, as shown in the figure below.

The nitrogenous bases, represented above in pink, green, blue, and orange, are responsible for holding the two strands together. There are four different kinds of nitrogenous bases in DNA: adenine (A), guanine (G), thymine (T), and cytosine (C). When two strands of DNA bind to one another, they follow complementary base pairing: adenine can only bind to thymine, and guanine can only bind to cytosine.

Adenine binds to thymine through two hydrogen bonds, and guanine binds to cytosine through three hydrogen bonds, as shown in the figure below.

If we now take a look back at the question and the options provided, we can see that the correct answer is that two hydrogen bonds form between A and T.

Now that we know about all the different components of DNA and how they link to one another, we can put them together to form the full structure of a DNA molecule. This is shown in Figure 9.

Remember how every carbon atom in the sugar molecule is numbered? If you look closely at Figure 9, you might notice that on the left strand, there is a free phosphate group at the top and a free hydroxyl group (OH) at the bottom. The phosphate group at the top left is attached to the last sugar’s 5 carbon. We call this the 5 end of the DNA strand. At the bottom of this strand, the free hydroxyl group is attached to the last sugar’s 3 carbon, which is why we call this the 3 end of the DNA strand.

Now, take a look at the strand on the right side of Figure 9. Can you see where the 5 and 3 ends of this strand are? The 5 end is at the bottom, and the 3 end is at the top! When two strands of DNA bind to one another through complementary base pairing, they are antiparallel to one another. This means that the two DNA strands each have different ends (5 and 3) on the same side, as you can see in Figure 9. The antiparallel nature is another defining feature of DNA.

Now that we have been over the structure of DNA, you might be wondering how this complex molecule stores genetic information! The nitrogenous bases we have learned about play a crucial role here.

The information that a cell needs to perform its functions is coded into a strand of DNA. In a polynucleotide chain, the sequence of nitrogenous bases can be “read” from the 5 end of the strand to the 3 end. The order of the bases forms a genetic code that a cell can interpret, converting the code into specific proteins that perform specific functions. For example, a sequence of nitrogenous bases in DNA might form the code for a protein that controls the color of your eyes or whether your hair is curly or straight!

We have learned that DNA strands follow complementary base pairing. Let’s consider a quick example of a sequence of nitrogenous bases, from the 5 end to the 3 end, as shown in Figure 10.

So far in this explainer, we have learned about two important, defining features of DNA: complementary base pairing and the antiparallel nature. If we apply our knowledge of these features here, we should be able to figure out the complementary DNA sequence!

One defining feature is that the two strands of DNA are antiparallel to one another, meaning that they run in opposite directions. The information on the strand we have been given runs in the 53 direction. This means that the other strand must run from 3 to 5. We can add these labels to the diagram.

Now, let’s think about the sequence of bases on the complementary strand. Another defining feature of DNA is complementary base pairing: adenine pairs with thymine, and guanine pairs with cytosine. Using this information, we can fill in the sequence of bases on the complementary strand, to figure out the complementary sequence, as shown in Figure 11.

The opposite strand of DNA, therefore, carries a complementary version of the code carried in the first strand. This essentially means that a single DNA molecule carries two complementary copies of the same information!

Example 4: Inferring the Complementary Sequence of a DNA Strand

A strand of DNA has the sequence 53-ATATGCGC-

State the corresponding sequence on the complementary strand, reading from the 35 direction.

  1. CACATGTG
  2. TATACGCG
  3. ATATGCGC

Answer

DNA is made of two polynucleotide chains that run antiparallel to one another. There are four different kinds of nitrogenous bases in DNA: adenine (A), guanine (G), thymine (T), and cytosine (C). When two strands of DNA bind to one another, they follow complementary base pairing: adenine binds to thymine through two hydrogen bonds, and guanine binds to cytosine through three hydrogen bonds.

The sequence given in the question from 5 to 3 is 53-ATATGCGC-

Let’s apply the information we have learned to figure out the complementary sequence, from its 3 end to its 5 end. We can replace the bases as follows:

  • A T
  • G C
  • T A
  • C G

The sequence we now obtain, on the complementary DNA strand, is 35-TATACGCG-

The correct option is therefore 35-TATACGCG-

In a double-stranded molecule of DNA, since adenine on one strand can only bind to thymine, this means that the number of adenine bases in the strand must be equal to the number of thymine bases. In the same way, since guanine can only bind to cytosine, this means that the number of guanine bases must be equal to the number of cytosine bases. For example, if a DNA molecule has 20 adenine bases, it will also have 20 thymine bases. This is called Chargaff’s rules, and they are useful for the calculation of the percentages of certain bases in a molecule of DNA.

How To: Calculating Percent Composition Using Chargaff’s Rules

When two strands of DNA bind to one another, hydrogen bonds are formed between their nitrogenous bases. In DNA, adenine always pairs with thymine on the opposite strand, and guanine always binds with cytosine on the opposite strand. This is called “complementary base pairing.” This is illustrated in the diagram below.

Because of complementary base pairing, the number of adenine bases in a molecule of DNA must always be equal to the number of thymine bases. Similarly, the number of guanine bases must always be equal to the number of cytosine bases. This is called Chargaff’s rules.

Using Chargaff’s rules, we can calculate the percentages of the different bases found in a molecule of DNA. Let’s understand how to do this.

Let’s call the total number of bases Btotal and the number of bases for each type of nucleotide BA, BT, BC, and BG for the bases adenine, thymine, cytosine, and guanine.

Using Chargaff’s rules, we know that BBandBBATCG==.

Finally, BBBBBtotalATCG=+++.

Let’s apply this knowledge to an example.

In the diagram above, the expanded segment possesses 12 base pairs. Since there are 12 pairs, there are 24 total bases: Btotal=24.

Let’s say that we are told that 4 of these bases are cytosine: BC=4.

Since BC= BG, BG=4.

Next, we can figure out the number of bases remaining after we have excluded those that are cytosine and guanine: BBBBBtotalATCG=+++ or 24=++4+424=++8248=++8816=+.BBBBBBBBATATATAT

This set of calculations tells us that the number of bases remaining after we have excluded those that are cytosine and guanine is 16.

Since, according to Chargaff’s rules, BBAT=, we can conclude that half of the remaining 16 bases are adenine and half are thymine: 16=+=+=216=2162=228=.BBBBBBBBATAAAAAA

And, since BA = BT, BT=8.

We can check these values by comparing them to what we see in the diagram. Count the number of A, T, C, and G to check our work.

To calculate the percent, we will divide the number of a particular base by the total number of bases and multiply that value by 100%: %=×100%.BBBAATOTAL

If we complete this calculation for each of the bases, we get the information in the table below.

BaseNumberPercent
Adenine833.3%
Thymine833.3%
Cytosine416.7%
Guanine416.7%

Now that we are clear about the structure of DNA, let’s learn about the discovery of this structure and the scientists whose work contributed to this discovery.

In 1953, James Watson and Francis Crick famously proposed the double helix model of DNA, building on the research that other scientists had previously conducted. In 1962, along with Maurice Wilkins, they were awarded the Nobel Prize in Physiology or Medicine for their work, which was heralded as a landmark event in the history of science. However, one scientist’s important contributions were uncredited. Without the work done by Rosalind Franklin, the proposal of the double helix model of DNA would have been impossible.

In London, in 1951, Rosalind Franklin began investigating the structure of DNA, building on the studies previously conducted by Maurice Wilkins. She had a great deal of expertise in a technique called X-ray crystallography, which she applied to take photographs of isolated DNA crystals. Let’s quickly understand how this technique works.

When X-rays pass through crystalline structures, they diffract or split in several different directions. The resulting diffraction patterns can be captured as photographs. Scientists can then examine these diffraction patterns and infer the molecular structure of a crystal.

Key Term: X-ray Crystallography

X-ray crystallography is a scientific technique in which X-rays are passed through crystalline substances, and the resulting diffraction pattern is analyzed to infer the molecular structure of the crystal.

Rosalind Franklin and her team developed several photographs of the crystal structure of DNA. In 1952, the 51st photograph they took showed strong evidence that DNA molecules have a double helix shape. The image showed a molecule of DNA as observed from above. From this image, it was clear that a DNA molecule must contain two strands since the diameter of the molecule was too large to contain a single strand. It was also clear that the nitrogenous bases or “rungs” of the structure were on the inside of the molecule. This historical photograph has come to be known as Photo 51.

Without Rosalind Franklin’s knowledge or consent, Maurice Wilkins showed Photo 51 to James Watson, along with all the data from Franklin’s work. James Watson and his research partner, Francis Crick, then used this information to propose the double helix model of DNA. This model was published in 1953, with no mention of Rosalind Franklin’s contributions!

Sadly, Rosalind Franklin passed away from ovarian cancer in 1958. During her lifetime, she was not honored or recognized for her groundbreaking discovery. After her death, Watson, Crick, and Wilkins were awarded the Nobel Prize in 1962, but it was several more years until Franklin’s contributions were properly recognized.

The knowledge of the structure of DNA is crucial to modern science and healthcare. Innovations like genetic engineering, gene therapy, and several diagnostic techniques are only possible because scientists now fully understand the structure and functions of DNA. When we learn about DNA, therefore, it is important to also learn about the history behind its discovery and the valuable contributions made by every scientist who worked to provide the model of DNA that we learn about today!

Let’s summarize everything we have learnt about DNA, its structure, and its discovery.

Key Points

  • DNA is a nucleic acid that stores genetic information. A molecule of DNA is in the shape of a twisted ladder, also called a double helix.
  • Each strand of a DNA molecule is a polynucleotide chain made up of several repeating units called nucleotides.
  • Each nucleotide consists of a pentose sugar, a nitrogenous base, and a phosphate group.
  • Individual nucleotides are connected through phosphodiester bonds that form between the phosphate group of one nucleotide and the pentose sugar of the next nucleotide.
  • The two DNA strands are connected to each other through hydrogen bonds that form between the nitrogenous bases on each strand, according to the rules of complementary base pairing.
  • James Watson and Francis Crick proposed the double helix model of DNA in 1953 and were awarded the Nobel Prize, along with Maurice Wilkins, in 1962.
  • However, they were only able to develop this model using the information from Rosalind Franklin’s X-ray crystallography studies. In spite of this, Franklin’s work was not credited, nor was she recognized for the huge role she played in the discovery of the structure of DNA.

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