1. The Structure of DNA
Read the title aloud to students.

2. Learning Objectives Identify the chemical components of DNA.
Discuss the experiments leading to the identification of DNA as the molecule that carries the genetic code.
Describe the steps, leading to the development of the double-helix model of DNA.
Click to reveal each of the learning objectives.
Read each objective aloud or have a volunteer do so.
Explain that scientists often work much like detectives, and that the structure of DNA was an important “case” for scientists. The evidence included Griffith and Avery’s work. Make sure students understand that, by the end of the presentation, they should be able to describe the pieces of the puzzle different scientists provided to the double-helix model of DNA structure.

3. Nucleotide Structure
DNA is made up of nucleotides joined into long strands or chains by covalent bonds.
Nucleic acids are made up of building blocks called nucleotides.
Phosphate
group
Remind students that DNA is one of the group of macromolecules called nucleic acids. Explain that nucleic acids are built from subunits called nucleotides. Each nucleotide is made up of three components: a base, a deoxyribose sugar, and a phosphate group.
Click to reveal the label for each component as you identify it.
Base
Deoxyribose
sugar

4. Nitrogenous Bases Adenine Guanine Cytosine Thymine
Explain that there are four different nitrogenous bases found in DNA nucleotides: adenine, guanine, cytosine, and thymine.
Ask: How do the structure of adenine and guanine differ from the structure of cytosine and thymine?
Answer: Adenine and thymine have two rings, and cytosine and thymine have only one.
Thymine

5. Nucleic Acid Structure
One nucleotide
Covalent bond
between nucleotides
Have a volunteer come to the board to draw a square around a single nucleotide.
Click to reveal the square around one nucleotide.
Have another volunteer come to the board to identify one of the covalent bonds between nucleotides.
Click to reveal a label for one of the covalent bonds between nucleotides.
Ask: By which nucleotide components do nucleotides attach to one another?
Answer: The sugar of one attaches to the phosphate group of the next.

6. Chargaff’s Rule [A] = [T] and [C] = [G]
Explain that Erwin Chargaff, in carrying out biochemical studies, had discovered that the percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA. The same thing is true for the other two nucleotides, guanine [G] and cytosine [C].

7. Franklin’s X-rays DNA is a helix. Likely two strands to the molecule
Nitrogenous bases near the center of the molecule
Review how in the 1950s Rosalind Franklin used a technique called X-ray diffraction to study the structure of DNA. She purified a large amount of DNA and then stretched the fibers to make them parallel. Then she aimed an X-ray beam at the sample and recorded the scattering pattern that was made on X-ray film.
Explain that the results of her work revealed some important clues to DNA structure: the X-shaped pattern in the DNA X-ray that the strands in DNA are twisted around each other like the coils of a spring, a shape known as a helix. The angle of the “X” suggested that there were two strands to the molecule. Other clues suggested that the nitrogenous bases of the nucleotides were on the inside of the molecule.
Click to reveal each of the bullet points as you discuss the results of Franklin’s work.

8. The Work of Watson and Crick
DNA is a double helix, in which two strands of nucleotide sequences are wound around each other.
Explain that James Watson and Francis Crick approached the problem of DNA structure by building three-dimensional models made of cardboard and wire that would be possible from a biochemical point of view based on the known properties of DNA. They twisted and stretched the models in various ways, but their best efforts did nothing to explain DNA’s properties. When they found Franklin’s work in 1953, they realized how they needed to fix their model.
Emphasize that the clues in Franklin’s X-ray pattern enabled Watson and Crick to build a model that explained the specific structure and properties of DNA. The twisted double-helix model explains Chargaff’s rule of base pairing and how the two strands of DNA are held together.
Show the class a physical model of a DNA molecule. Point out to students that a double helix looks like a twisted ladder.
Ask: If a twisted ladder is used as a model of DNA, which parts of a DNA molecule correspond to the sides of the ladder?
Answer: The phosphate group and the five-carbon sugar deoxyribose
Ask: Which parts of a DNA molecule correspond to the rungs of the ladder?
Answer: nitrogenous base pairs
If students need additional help in grasping the structure of DNA, draw a picture of a ladder on the board. Explain how the ladder can model the structure of DNA. Label the rungs of the ladder Nitrogenous Bases and the sides of the ladder Sugar and Phosphate Groups. Ask students to imagine what the ladder would look like if it were twisted. Then show them a physical model of DNA. Help them make the connection between the ladder drawing and the DNA model by pointing out the nitrogenous bases, phosphate groups, and sugar molecules.

9. The Double Helix: Antiparallel Strands
The two strands in a DNA molecule run in opposite directions.
Explain that one of the surprising aspects of the double-helix model is that the two strands of DNA run in opposite directions. This arrangement of strands is described as “antiparallel.” This arrangement enables the nitrogenous bases on both strands to come into contact at the center of the molecule.
Click to reveal the arrows pointing in different directions.

10. The Double Helix: Hydrogen Bonding
Hydrogen bonds
Explain that the two strands of DNA are held together by hydrogen bonds between the nitrogenous bases adenine and thymine and between guanine and cytosine.
Ask: By which part of the nucleotide do the two strands of a DNA molecule join?
Answer: at the bases
Click to highlight.
Ask: How does bonding between nucleotides within a strand differ from the bonding between nucleotides in paired strands?
Answer: The nucleotides within a strand are joined by covalent bonds. The nucleotides between strands are joined by hydrogen bonds.

11. The Double Helix: Base Pairing
The two strands of DNA are held together by hydrogen
bonds between the nitrogenous bases adenine and
and between guanine and
thymine
cytosine
Explain that Watson and Crick had realized that hydrogen bonds between the two strands would explain the tight fit, but that hydrogen bonds would form only between A and T and between C and G. This fit between bases is called base pairing.
Ask a volunteer to read the complete sentence, filling in the missing terms.
Click to reveal the correct terms.
Challenge students to synthesize the evidence for DNA structure.
Ask: What is the relationship between the base pairing rules and the evidence for Chargaff’s rule?
Answer: If A only pairs with T and C only pairs with G, then for each base of a particular kind, there would have to be one of its partner bases.