1. Identifying the Substance of the Gene
Read the lesson title aloud to students.

2. Learning Objectives
Identify the clues that bacterial transformation yielded about the gene.
Explain the role bacterial viruses played in identifying genetic material.
Describe the role of DNA in heredity.
Click to reveal each learning objective.
Read the objectives aloud or have a volunteer do so.

3. Griffith’s Experiments 1928
S strain:
heat killed
Mixture:
R, dead S
Point out and describe what happens in each vertical panel of the diagram. Click to reveal the descriptive text as you discuss.
Ask: In this experiment, which strain of bacteria caused disease?
Answer: the S strain
Ask: What happened when heat-killed S strain was injected into a mouse?
Answer: It no longer caused disease.
Ask: What happened when the heat-killed S strain was mixed with the harmless R-strain bacteria?
Answer: The mouse got sick.
Tell students the word transformation means “change.”
Ask: Why did Griffith test to see whether the bacteria recovered from the sick mice in his last experiment would produce smooth or rough colonies in a Petri dish?
Answer: to determine whether the substance transferred from the heat-killed bacteria to the R strain was heritable
Ask: Why is the word transformation a good description of what happened in Griffith’s experiment?
Sample answer: One strain of bacteria transformed, or changed, into another.
Ask: Why did Griffith conclude that the transforming factor had to be a gene?
Answer: The factor was inherited by offspring of the transformed bacteria.
Mouse
lives.
Mouse dies
of pneumonia.
Live S strain

4. Griffith’s Experiments 1928
Griffith proved that one type of bacteria was transformed into a different one.
Transformation: one type of bacteria is permanently changed into a different type.

5. Avery’s Experiments 1944 How did his team do that?
DNA
Scientists discovered that the nucleic acid stores and transmits from one generation of bacteria to the next.
How did his team do that?
Avery destroyed everything but DNA in bacteria and the bacteria still transformed.
Then, Avery destroyed only the DNA and transformation did not occur… proving that DNA is the molecule for inheritance.
genetic information
Discuss with students the experiments of Avery and his team.
Ask: What was the manipulated, or independent, variable in the experiment?
Answer: The type of enzyme used to treat the extract from heat-killed bacteria
Make sure they realize that only one enzyme was used in each experiment.
Ask: What was the responding, or dependent, variable in this experiment?
Answer: Whether transformation occurred
Ask: What could Avery conclude from the results of these experiments?
Answer: DNA stores and transmits genetic information.
Emphasize that, by observing bacterial transformation, Avery and other scientists discovered that the nucleic acid DNA stores and transmits genetic information from one generation of bacteria to the next.
Ask for a student to read the statement on the screen, filling in the missing blanks.
Click to reveal the complete statement.

6. Hershey-Chase Experiment 1952
Their team confirmed that DNA was genetic material.
They worked with viruses.
How did their team do it?

7. Bacterial Viruses Bacteriophage: a kind of virus that infects bacteria
DNA
head
Explain that the work by Avery and his team was confirmed in the 1950s by a research team that was studying viruses.
Introduce the parts of a bacteriophage virus. Describe how a virus infects a bacterium: When a bacteriophage infects a bacterium, it attaches to the surface of the bacterial cell and injects its genetic information into it. The viral genes act to produce many new bacteriophages, which gradually destroy the bacterium. When the cell splits open, hundreds of new viruses burst out.
tail sheath
tail fiber

8. Hershey-Chase Experiment 1952
Bacteriophage with
phosphorus-32 in DNA
Phage infects
bacterium.
Radioactivity
inside bacterium
Explain how Alfred Hershey and Martha Chase used different radioactive markers to label the DNA and proteins of bacteriophages. The bacteriophages injected only DNA, not proteins, into bacterial cells.
Walk students through the steps of the Hershey-Chase experiment. Click to reveal parts of the diagram as you discuss each one.
Begin a discussion on the Hershey-Chase experiment.
Ask: What happens when a bacteriophage infects a bacterium?
Answer: The bacteriophage injects its genetic material into the bacterium.
Ask: What did the bacteriophage used by Hershey and Chase consist of?
Answer: protein and DNA
Ask: What question did Hershey and Chase seek to answer with their experiment?
Answer: Does DNA or protein transmit genetic information?
Challenge students to analyze the Hershey-Chase experiment.
Ask: What are the independent and dependent variables in the Hershey-Chase experiment, and what are some possible control variables?
Answer: The independent variable is the substance that was labeled DNA or protein. The dependent variable is the presence of radioactivity in the infected cell. An example of a control variable is the amount of time Hershey and Chase waited for the viruses to infect the bacteria.
Emphasize that Hershey and Chase’s experiment with bacteriophages confirmed Avery’s results, convincing many scientists that DNA was the genetic material found in genes—not just in viruses and bacteria, but in all living cells.
Bacteriophage with
sulfur-35 in
protein coat
Phage infects
bacterium.
No radioactivity
inside bacterium

9. The Role of DNA
Have students examine the diagram, which shows the three main roles of DNA.
Ask: How does DNA store information?
Sample Answer: in its molecular structure
Ask: Why is it important that DNA can be accurately copied?
Answer: so that each daughter cell receives a complete and correct copy of the genetic material during cell division
Explain that DNA contains coded instructions for a cell to carry out important biological processes, such as how to move or transport ions. DNA is also copied and passed along to the next generation. These three tasks—storing, copying, and transmitting information—are also the three main functions of DNA.

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

11. Learning Objectives Identify the chemical components of DNA.
Describe the clues that helped scientists determine the structure of DNA.
Explain what the double-helix model shows about 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.

12. 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.
What are the three main parts of a nucleotide?
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 5-carbon sugar, and a phosphate group.
Click to reveal the label for each component as you identify it.
Base
5-carbon sugar

13. Nitrogenous Bases (can you name them?)
Adenine
Guanine
Cytosine
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

14. 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.

15. 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].

16. Chargaff’s Rule
If there is 40% Adenine in a cell, then what is the percentage of Thymine?
40%
If there is 30% adenine in a cell, then what is the percentage of guanine?
20%

17. Franklin’s X-rays (what did they prove?)
1952
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.

18. The Work of Watson and Crick 1953
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.

19. 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.

20. 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.

21. 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.

22. DNA Replication
Read the title aloud to students.

23. Learning Objectives Explain the role of DNA polymerase in copying DNA.
Compare DNA replication in prokaryotic cells and in eukaryotic cells.
Click to reveal each of the learning objectives in turn.
Read the objectives aloud or have a volunteer do so.
Ask: What are some ways to make a copy of a page of the text?
Sample Answers: by hand, by using a copier
Ask: Why is it important to make an exact copy?
Answer: so the information doesn’t change
Explain that cells copy DNA in a process called DNA replication. Then lead a discussion in which you challenge students to apply this same thinking about copying pages to the process of DNA replication. Before a cell divides, its DNA must first be copied. Ask them to consider the following: How might the double-helix structure of DNA make that possible? What might happen if one of the nucleotides were damaged or chemically altered just before the copying process? How might this affect the DNA inherited by each daughter cell after cell division?

24. The Code
The specific order of nitrogen bases located on a specific region of a chromosome is a gene. (20,000-25,000)
The order of base pairs is a code for a specific form of a gene.
That gene code instructs the cell to synthesize certain proteins.
Those proteins give organisms their expressed physical traits.

25. DNA Replication Duplicating DNA
Before a cell divides, it duplicates its DNA in a process called replication.
Replication ensures that each resulting cell will have a complete set of DNA.

26. Review of DNA Structure
nitrogenous
bases
sugar-phosphate
backbone
Ask for volunteers to identify the different features of the DNA diagram.
Click to reveal the correct answers.
Ask: How are the two strands joined together?
Answer: by hydrogen bonding at paired bases
double helix

27. Copying DNA replication fork DNA polymerase Direction Direction
of replication
Direction
of replication
Walk students through the diagram to describe the process of DNA replication. Emphasize that DNA polymerase joins nucleotides to synthesize a new, complementary strand of DNA. Also emphasize the role of DNA polymerase: Besides producing the sugar-phosphate bonds that join nucleotides together, DNA polymerase also “proofreads” each new DNA strand so that each molecule is a near-perfect copy of the original.
Ask: How is DNA unzipped at the replication forks?
Answer: Hydrogen bonds are broken.
One replication fork is labeled. Ask for a volunteer to go to the screen to point out the other replication fork.
Click to reveal the correct answer.
Ask students how the base-pairing rules play a role in building a strand of DNA that is complementary to the original, template strand.
One DNA polymerase molecule is labeled. Ask for a volunteer to go to the screen to point out another region where nucleotides are being added to build a new strand.
Click to reveal the label.
Ask: What are the two roles of DNA polymerase in replication?
Answer: DNA polymerase joins individual nucleotides to produce a new strand of DNA and proofreads the new strand.
new nucleotides
being added

28. DNA Replication The blue strand represents the DNA strand. original
The orange strand represents the
original
new
Ask: Is a new strand identical to its original template strand?
Answer: No, they are complementary, not identical.
Ask for a volunteer to read the statements, filling in the appropriate terms.
Click to reveal the correct answers.

29. Telomeres Telomeres: the tips of eukaryotic chromosomes
The enzyme telomerase adds short, repeated DNA sequences to telomeres as the chromosomes are replicated.
Telomeres
Explain that the tips, or telomeres, of a DNA molecule are difficult to replicate. Cells use a special enzyme called telomerase to add short repeated DNA sequences to telomeres as chromosomes are replicated.
Tell students that, in the micrograph, the red structures are human chromosomes and the telomeres are stained white, and appear in photograph as yellowish orange.

30. What are the similarities of differences of DNA replication in prokaryotes and eukaryotes?
same process, parts, and result.
Differences:
Prokaryotes start at 1 part and replication occurs in both directions until the entire single chromosome is copied.
Eukaryotes have multiple sections replicating simultaneously until that whole chromosome is replicated.

31. Prokaryotic DNA Replication
new DNA
replication fork
replication fork
Remind students that, unlike eukaryotic DNA, the DNA of prokaryotic organisms exists as a single loop. Explain that replication in most prokaryotic cells begins at a single starting point and proceeds in two directions until the entire chromosome is copied.
Have volunteers go to the board to label the parts of the diagram based on what they have already learned about DNA replication. Use the labels: replication fork (twice), new DNA, and unreplicated DNA.
Click to reveal each correct term.
unreplicated DNA

32. Eukaryotic DNA Replication
Unreplicated DNA
Replication
forks
Point out that the process students looked at earlier in the presentation was for replication eukaryotic DNA replication. Remind students that eukaryotic chromosomes are linear.
Ask for a volunteer to point out the replication forks in the diagram.
Click to reveal the labels.
Ask: Where is the origin of replication?
Answer: in the center of each newly forming strand
Emphasize that in eukaryotic cells replication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied.
New DNA