Virus Lytic Cycle Answer Key Vocabulary

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Virus Lytic Cycle Answer Key

Vocabulary: bacteriophage, capsid, host cell, lyse, lytic cycle, virus

Prior Knowledge Questions (Do these BEFORE using the Gizmo.)

[Note: The purpose of these questions is to activate prior knowledge and get students thinking. Students are not expected to know the answers to the Prior Knowledge Questions.]

  1. A computer virus is a program that can copy itself and infect a computer without the permission of the owner. How do you think a computer virus compares to a real virus?

Answers will vary. [A computer virus infects a computer, and a real virus infects an organism. During an infection, both types of viruses make copies of themselves so that more infections can occur. A computer virus often disrupts the functions of a computer, while a real virus destroys its host cell (and disrupts the functions of a multicellular organism).]

  1. Have you ever been infected with a virus, such as the cold virus or flu virus?

Answers will vary.

  1. If so, how did the virus affect you?

Answers will vary. [Viruses often cause fevers, rashes, congestion, and other symptoms.]

Gizmo Warm-up

A virus is a microscopic particle that can infect a cell. Viruses are primarily composed of a protein coat, called a capsid, and nucleic acid. In the Virus Lytic Cycle Gizmo™, you will learn how a virus infects a cell and uses the cell to produce more viruses.

  1. Viruses are extremely small. A typical virus is about 100 times smaller than a single cell, such as a bacterium. Label the virus and a bacterial cell in the image at right.

  1. Bacteriophages are viruses that infect bacteria. Based on the diagram at left, label the head, tail, tail fibers, and the strand of nucleic acid in the image at right.

Activity A:
Lytic cycle

Get the Gizmo ready:

  • If necessary, click Reset ().

Introduction: Unlike living organisms, viruses cannot reproduce on their own. Instead, viruses infect host cells, taking over the cell’s machinery to produce more viruses. This process is called the lytic cycle.
Question: What are the steps of the lytic cycle?

  1. Observe: Use the navigation arrows on the DESCRIPTION tab to read about the stages of the lytic cycle. Using your own words, summarize each step of the cycle.




A lytic bacteriophage virus attaches itself to a bacterial cell. The virus is a very simple structure, composed of a protein capsid, nucleic acid, and a tail section.


The virus injects its nucleic acid into the cell. The nucleic acid uses the cell’s ribosomes to make virus proteins. The proteins break up the cell’s DNA.


The virus capsid and tail, now useless, detach from the cell and disintegrate. Inside the cell, the viral nucleic acid takes over and directs the production of new virus proteins and nucleic acid.


The virus proteins and nucleic acid assemble into new viruses.


Special proteins produced by the viral nucleic acid cause the host bacterial cell to lyse (burst), destroying the cell in the process. The numerous new viruses are now free to infect other cells.

(Activity A continued on next page)

Activity A (continued from previous page)

  1. Analyze: The yellow ring inside the bacterial cell represents the bacterial DNA. Why does this structure disappear by step 3 of the lytic cycle?

In step 2, the viral nucleic acid takes over control of the cell by destroying the host cell’s DNA.

  1. Describe: How does a virus destroy the host cell’s DNA?

The viral nucleic acid uses the cell’s ribosomes to make proteins. These proteins break up the bacterial cell’s DNA.

  1. Describe: How are new viruses reproduced?

Viral nucleic acid takes control of a cell’s mechanisms, including the structures the cell uses to make proteins and copy nucleic acid. The viral nucleic acid uses these mechanisms to make new virus parts. The parts assemble into new viruses.

  1. Think and discuss: Why can’t a virus reproduce on its own?

Viruses do not have the structures needed to make proteins or make copies of nucleic acid.

  1. Justify: To lyse is to burst apart or explode. Why do you think a virus’s reproduction cycle is called the “lytic cycle”?

After a cell has been used to reproduce new viruses, a special viral protein causes the cell’s membrane to break open, or lyse. This process gives the lytic cycle its name.

Activity B:
Spread of infection

Get the Gizmo ready:

  • If necessary, click Reset.

Question: How does a viral infection spread?

  1. Predict: Suppose that a virus infects a small population of bacteria. Predict how the numbers of viruses, infected cells, and uninfected cells will change as the infection progresses. On the blanks below, write increase, decrease, or stay the same.

Viruses: Increase Infected cells: Increase

Uninfected cells: Decrease

  1. Observe: Click Play (), and watch the simulation. Describe what you see.

Sample answer: As the infection progresses, viruses spread to uninfected cells at an increasingly faster rate.

  1. Test: Click Reset, and Select the BAR CHART tab. Turn on Show numerical values. Click Play, and watch each bar as the simulation runs. What do you notice, and how does this compare to your predictions?

Sample answer: The change in the numbers of viruses, infected cells, and uninfected cells closely matched my predictions. Over time, the number of viruses increased, as did the number of infected cells. The number of uninfected cells decreased from 50 down to 0.

  1. Record data: Select the TABLE tab, and use the data to complete the second column of the table below. To complete each cell in the third column, subtract the previous time value from the current time value. For example, if it took 80 minutes to reach 40 cells and 100 minutes to reach 30 cells, then the time difference is 20 minutes.

Answers will vary. Sample answers given below.

Number of cells

Time (m)

Amount of time to decrease population by 10


0 m



79 m

79 m


110 m

31 m


128 m

18 m


145 m

17 m

(Activity B continued on next page)

Activity B (continued from previous page)

  1. Analyze: What trend do you see in the third column of your data table?

Sample answer: The time it took the viruses to destroy the first set of 10 cells was about twice as long as the time it took to destroy the next set of 10 cells. This trend continued for the next 20 cells, but the rate of cell destruction tapered off for the last set of 10 cells.

  1. Explain: How would you explain this trend?

The number of viruses increases with time, thus allowing the rate of infection to increase. However, the rate of infection tapers off when there are no more healthy cells to infect.

  1. Interpret: Select the GRAPH tab. Run the Gizmo again, and observe what happens in the SIMULATION pane when the graph shows a decrease in the viruses’ population size.

      1. Why does the number of viruses sometimes increase and sometimes decrease?

At some points during a viral infection, all of the viruses are inside host cells. During these times, it appears as though the virus population is zero.

      1. Sometimes when a virus enters a cell, it becomes dormant for a while. Why might this make it difficult for a doctor to diagnose a viral infection?

Until an infected cell lyses, it will appear to be a healthy, functioning cell. The person will have no symptoms until the viruses begin destroying his or her cells.

  1. Extend your thinking: AIDS is one disease caused by a virus infection. The virus attacks immune system cells known as T cells.

Based on your observations from the Gizmo, how would you explain the data shown on this graph?

Over time, the viruses infect healthy T cells and destroy them. More viruses are produced in the process. These viruses continue to infect the person’s T cells until the person has a T cell count close to zero. [Note: The initial rise in the T cell count is caused by the immune system’s attempt to fight the infection by producing an increased number of T cells. The viruses quickly overtake the new T cells, causing the subsequent fall in the T cell count.]

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