1. Viruses EQ: Are viruses considered living? Why or why not?
Read the lesson title aloud to students.

2. Virus Reproduction A virus is nonliving.
Viruses can reproduce only by infecting living cells.
Explain that American biochemist Wendell Stanley is the scientist who isolated crystals of tobacco mosaic virus. He knew that living organisms do not crystallize, so he inferred that viruses were not truly alive.
This is still recognized as a valid conclusion today.
Tell students: A virus is a nonliving particle made of proteins, nucleic acids, and sometimes lipids.
Ask: How do viruses reproduce?
Answer: Viruses can reproduce only by infecting living cells.
Click to reveal the answer.

3. Virus Structure and Composition
Viruses have a very simple construction.
Capsid: protein coat surrounding a virus
Surface proteins: help virus bind to the cell
Genetic material: DNA or RNA
Bacteriophage: virus that infects a bacterial cell
Explain to students that viruses can differ widely in size and structure. The simplest viruses contain only a few genes. The most complex may have hundreds of genes.
Click to focus on the T4 bacteriophage.
Point out the head, the tail sheath, and the tail fiber.
Ask: What is the magnification necessary to see a T4 bacteriophage with a transmission electron microscope (TEM)?
Answer: 60,000x
Click to focus on the tobacco mosaic virus.
Point out that this virus looks similar to the tail sheath of the T4 bacteriophage, but that it does not have the head and the tail fiber of the bacteriophage. Also, emphasize the difference in size.
Ask: What is the magnification necessary to see a tobacco mosaic virus with a transmission electron microscope?
Answer: 400,000x
Make sure that students are aware of the order of magnitude difference between this and the T4 bacteriophage.
Click to focus on the influenza virus.
Point out to students that this virus looks nothing like the other two.
Review the parts shown and labeled and tell students to notice the magnification necessary for this virus to be visible is only 21,000x.
Ask: What kind of nucleic acid does each virus type have?
Answer: The T4 bacteriophage has DNA; the tobacco mosaic virus and influenza virus have RNA.

4. Viral Infections
Viruses use their genetic information to reproduce inside living cells.
Two types of infection
Lytic – quick cell death
Lysogenic – delayed cell death
Lysogenic infection
Lytic infection
Ask: What happens after a virus infects a cell?
Answer: Inside living cells, viruses use their genetic information to reproduce.
Click to reveal this answer.
Explain that some viruses replicate immediately. This is called a lytic infection.
Click to reveal the lytic infection label.
Other viruses initially persist in an inactive state within the host. This is called a lysogenic infection.
Click to reveal the lysogenic infection label.

5. Lytic Infections 1. The virus injects DNA into a bacterium.
2. Viral genes are transcribed by the host cell.
3. The bacterium makes new viral proteins and nucleic acid.
5. Viral enzymes lyse the bacterium’s cell wall. The new viruses escape.
Tell students: A virus enters a bacterial cell, makes copies of itself, and causes the cell to burst, or lyse.
Step students through the typical process of a lytic infection.
Click to highlight the first step.
Tell students: The DNA core inside a protein capsid binds to the surface of a host cell. The virus injects its DNA into the cell, and the cell then begins to make messenger RNA (mRNA) from the viral genes.
Click to highlight the second step.
Tell students: The viral mRNA is translated into viral proteins that act like a molecular wrecking crew, chopping up the cell’s DNA.
Click to highlight the third step.
Tell students: Under the control of viral genes, the host cell now makes thousands of copies of viral nucleic acid and capsid proteins, enabling the virus to reproduce.
Click to highlight the fourth step.
Tell students: The viral DNA is assembled into new virus particles.
Click to highlight the final step.
Tell students: Before long, the infected cell lyses, releasing hundreds of virus particles that may go on to infect other cells.
Ask: In a lytic infection, how does the virus make copies of itself?
Answer: by inserting its genetic information into a cell, which directs the cell to make and assemble new viral parts
Ask: Why can a lytic virus remain in a particular host cell only for a limited time?
Answer: It eventually destroys the host cell by causing it to burst.
4. The proteins and nucleic acids assemble into new viruses.

6. Lytic Infections Analogy
A lytic virus is similar to the Wild West of the American frontier.
Use the analogy of a lytic virus being like an outlaw in the Wild West. The virus makes certain demands on the host.
Tell students: First, the outlaw eliminates the town’s existing authority.
Click to reveal the first step.
Explain that this is like the host cell’s DNA being chopped up.
Tell students: Next, the outlaw demands to be outfitted with new equipment from the local townspeople.
Click to reveal the second step.
Explain that this is like the virus using the host cell to make viral DNA and viral proteins.
Tell students: Finally, the outlaw forms a gang and leaves the town to attack new communities.
Click to highlight the third step.
Explain that this is like when the host cell bursts and releases hundreds of virus particles.
To make sure that students grasp both the analogy and the way in which a lytic virus works, ask the following questions.
Ask: In this analogy, what represents the host cell?
Answer: the town
Ask: Think about how viruses enter host cells. Extend the analogy to describe the outlaw’s entrance into town.
Answer: Answers should describe the outlaw being sneaky or using trickery to gain entrance to the town.
Ask: Think about how viruses exit host cells. Extend the analogy to describe the condition of the town after the outlaw leaves.
Answer: Answers will vary, but should describe the outlaw destroying the town or leaving it in disarray as he leaves.
The host cell’s DNA is chopped up.
Virus uses host cell to make viral DNA and viral proteins.
The host cell bursts, releasing hundreds of virus particles.

7. Lysogenic Infections Prophage
The prophage may replicate with the bacterium for many generations.
The viral DNA inserts itself into the bacterial chromosome.
Prophage
The virus injects DNA into the bacterium.
The prophage can exit the bacterial chromosome and enter a lytic cycle.
Step students through the typical process of a lysogenic infection.
Click to highlight the first step.
Tell students: Viral nucleic acid is inserted into the host cell’s DNA, where it is replicated along with the host DNA without damaging the host.
Explain to students that the virus’ DNA becomes embedded in the host’s DNA and is called a prophage.
Click to highlight this step and to point out the prophage.
Tell students: The prophage may remain part of the DNA of the host cell for many generations. It is replicated along with the host DNA without damaging the host. Viral DNA multiplies as the host cells reproduce. In this way, each generation of daughter cells derived from the original host cell is infected.
Click to highlight this step.
Explain that influences from the environment—including radiation, heat, and certain chemicals—trigger the prophage to become active. It then removes itself from the host cell DNA and reproduces by forming new virus particles. The lysogenic infection, at this point, becomes an active lytic infection.
Work with students to extend the analogy of the outlaw in the Wild West to a lysogenic infection.
Ask: How would you modify the story of the outlaw in the Wild West to make it analogous to a lysogenic cycle?
Answer: A correct response will include an analogy for the prophage being a part of the bacterium for an extended period of time.
Sample answer: The outlaw pretends to be looking for work, takes a job at a nearby ranch, and hides out there for a time.

8. An RNA Virus: The Common Cold
Once the cold virus has penetrated the host’s cells, it uses the host’s cellular machinery to replicate itself.
The copies are translated by the host into new viral parts.
The virus makes many copies of its RNA.
Tell students: About 70 percent of viruses contain RNA rather than DNA. In humans, RNA viruses cause a wide range of infections, from relatively mild colds to influenza and AIDS. Certain kinds of cancer also begin with an infection by viral RNA.
Ask: What happens when you get a cold?
Answer: Cold viruses attack with a very simple, fast-acting infection.
Explain to students that a capsid settles on a cell, typically in the host’s nose, and is brought inside, where a viral protein makes many new copies of the viral RNA.
Click to reveal this step.
Tell students: The host cell’s ribosomes mistake the viral RNA for the host’s own mRNA and translate it into capsids and other viral proteins.
Explain that the new capsids assemble around the viral RNA copies, and within eight hours, the host cell releases hundreds of new virus particles to infect other cells.
Cytoplasm
The parts assemble into new viruses and burst from the host cell.

9. An RNA Virus: HIV
HIV makes a DNA copy of itself that inserts into the host’s DNA. There, it may remain inactive for many cell cycles.
A DNA copy of the viral RNA is made.
The parts assemble into new viruses and burst from the host cell.
DNA
The copy is inserted into the host’s genome.
Explain to students that the deadly disease called acquired immune deficiency syndrome (AIDS) is caused by an RNA virus called human immunodeficiency virus (HIV). HIV belongs to a group of RNA viruses that are called retroviruses. The genetic information of a retrovirus is copied from RNA to DNA instead of from DNA to RNA.
Step students through the process of an HIV infection.
Tell students: When a retrovirus infects a cell, it makes a DNA copy of its RNA.
Click to reveal this step.
Tell students: Then, the viral DNA is inserted into the host cell.
Explain that retroviral infections are similar to lysogenic infections of bacteria. Much like a prophage in a bacterial host, the viral DNA may remain inactive for many cell cycles before making new virus particles and damaging the cells of the host’s immune system. Eventually, the DNA is transcribed and new viral parts are made.
Click to highlight this step.
Tell students: Once activated, the viruses leave the cell and begin to destroy the very system of the body that would normally fight infection.
Tell students: The word retro means “backward.”
Ask: Why is the word retrovirus used to describe HIV?
Answer: The genetic information of a retrovirus like HIV is copied from RNA to DNA instead of from DNA to RNA.
Ask: If the virus that causes the common cold is also an RNA virus, why is it not called a retrovirus?
Answer: In a cell infected with the common cold virus, RNA is not copied to make DNA. Instead, the viral RNA is translated by the cell’s ribosomes to produce viral proteins.
It is later transcribed and translated into new viral parts.
Cytoplasm

10. Viruses and Cells
Remind students that viruses must infect living cells to grow and reproduce, taking advantage of the nutrients and cellular machinery of their hosts. This means that all viruses are parasites.
Explain that, despite the fact that they are not alive, viruses have many of the characteristics of living things. After infecting living cells, viruses can reproduce, regulate gene expression, and even evolve.
Direct students to compare the principal differences between cells and viruses that are shown in the table.
Ask: Based on this information, would you classify viruses as living or nonliving? Explain.
Sample answer: I would classify viruses as nonliving, because they cannot grow, develop, or obtain energy, and because they cannot reproduce independently.

11. Summary of Viruses Viruses reproduce by infecting living cells.
Some viruses replicate immediately; others initially persist in an inactive state within the host.
Lysogenic infection
Lead students in a short discussion of viruses. Review with students the key points of this lesson.
Remind them that viruses can reproduce only by infecting living cells.
Click to reveal this bullet point.
Tell students: Inside living cells, viruses use their genetic information to reproduce. Some replicate immediately. Others remain inactive for a period of time within the host.
Click to reveal this bullet point and the image of lysogenic and lytic infection cycles.
Ask for a volunteer to properly label the virus infection cycles.
Click to reveal the correct answers.
Lytic infection