1. Characteristics of Life
Reproduce
Genetic code
Homeostasis
Grow and develop
Metabolism
Evolve
Respond to their environment
Tell students that the characteristics of life can be organized into a list of eight characteristics.
Click to reveal the first three terms.
Genetic code: Living things are based on a universal genetic language. This language is in the form of a molecular code.
Grow and develop: Living things grow and develop.
Respond to their environment: Living things respond to their environment. External factors such as changes in light or temperature stimulate a response in living things.
Click to reveal the remaining five terms.
Reproduce: Living things reproduce. Tell students there are two methods to reproduce: sexual reproduction (between a male and female) and asexual reproduction (a single organism reproduces on its own).
Homeostasis: Living things maintain a stable internal environment. Tell students the term homeostasis describes the state of stable maintenance by an organism. Example of homeostasis in a person: maintaining a stable internal temperature.
Metabolism: Living things obtain and use material and energy. Tell students a living thing is basically a chemical being that uses matter and energy to run a myriad of chemical reactions, constantly building and breaking down molecules and capturing and releasing energy. The summation of this activity is called metabolism.
Evolve: Taken as a group, living things evolve or change over time.
Made of cells: Living things are made of cells. Tell students many organisms consist of only one cell, while others, like people, consist of trillions of cells.
Made of cells

2. Viruses https://www.youtube.com/watch?v=7KXHwhTghWI
Read the lesson title aloud to students.

3. Virus Structure and Composition
Capsid: protein coat surrounding a virus
Core: Nucleic Acid- either DNA or RNA
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.
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.
Click to focus on the influenza virus.
Point out to students that this virus looks nothing like the other two.
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.
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 The virus injects DNA into a bacterium.
Viral genes are transcribed by the host cell.
The bacterium makes new viral proteins and nucleic acids.
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.
The proteins and nucleic acids assemble into new viruses.

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.

9. 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.

10. 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