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Exam questions · Biology · Health and Infectious Disease

The Life Cycle of a Virus

  • 6 exam questions
  • 21 marks
  • 9 quick checks
  1. 1 Name [2 marks]

    Name the two parts of a virus.

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    Model answer

    Genetic material (DNA or RNA) and a protein coat (capsid).

    Mark scheme

    • Genetic material — 1 mark
    • Protein coat — 1 mark
  2. 2 Describe [3 marks]

    Describe what happens when a virus infects a host cell by the lytic pathway.

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    Model answer

    The virus attaches to the host cell and its genetic material enters. The host cell then makes copies of the viral genetic material and proteins, which are assembled into new viruses. The cell then bursts, which is lysis, and releases the new viruses.

    Mark scheme

    • Attaches and enters — 1 mark
    • Cell makes copies and assembles viruses — 1 mark
    • Cell bursts and releases viruses — 1 mark
  3. 3 Describe [3 marks]

    Describe the lysogenic pathway of a virus.

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    Model answer

    The viral DNA is joined into the host cell's chromosome. When the host cell divides, the viral DNA is copied with the host's DNA, so it is passed on to the daughter cells. No new viruses are made until a trigger makes the viral DNA start the lytic pathway.

    Mark scheme

    • Viral DNA joins the host chromosome — 1 mark
    • Copied when the host divides — 1 mark
    • Trigger starts the lytic pathway — 1 mark
  4. 4 Calculate [3 marks]

    One virus infects a cell and the cell releases 200 new viruses. Each of these infects a new cell, which also releases 200 viruses. Calculate the number of viruses after the second round of infection.

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    Model answer

    After the first round there are 200 viruses. After the second round there are 200 × 200 = 40 000 viruses.

    Mark scheme

    • 200 after the first round — 1 mark
    • 200 × 200 — 1 mark
    • 40 000 — 1 mark
  5. 5 Explain [4 marks]

    Explain why a person infected with a virus that uses the lysogenic pathway may have no symptoms for many years.

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    Model answer

    In the lysogenic pathway the viral DNA is joined into the host chromosome, and no new viruses are made, so the cells are not destroyed. The virus is copied with the host's DNA when the cells divide, but without symptoms. Symptoms only appear when a trigger makes the virus enter the lytic pathway, and produce new viruses that destroy cells.

    Mark scheme

    • Viral DNA joined to host chromosome — 1 mark
    • No new viruses made — 1 mark
    • Cells not destroyed — 1 mark
    • Symptoms start after a trigger / lytic pathway — 1 mark
  6. 6 Evaluate [6 marks]

    Compare the lytic and lysogenic pathways, and explain why viral diseases such as HIV are difficult to cure.

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    Model answer

    In the lytic pathway the virus makes copies of itself at once, and the host cell is destroyed when it bursts and releases the new viruses. In the lysogenic pathway the viral DNA is joined into the host's chromosome, and is copied when the cell divides, with no new viruses made and no damage to the cell. This can be hidden for years until a trigger starts the lytic pathway. HIV is hard to cure because the viral DNA is hidden in the cells in this way and makes no proteins, so the immune system cannot recognise it, and drugs cannot target it. Viruses are also inside host cells, so it is difficult to attack them without harming the cell, and antibiotics do not work.

    Mark scheme

    • Lytic: new viruses at once, cell destroyed — 1 mark
    • Lysogenic: viral DNA joins host DNA — 1 mark
    • Copied when the cell divides, no new viruses — 1 mark
    • Hidden until a trigger — 1 mark
    • Hidden virus is not detected or targeted — 1 mark
    • Inside host cells / antibiotics do not work — 1 mark

Quick check

  1. 1

    What is the protein coat of a virus called?

    1. AThe cell wall
    2. BThe capsid
    3. CThe membrane
    4. DThe cytoplasm
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    B: The capsid

    The capsid is the protein coat that surrounds the genetic material.

  2. 2

    Where can a virus reproduce?

    1. AIn water
    2. BIn the air
    3. CInside a host cell
    4. DOn its own
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    C: Inside a host cell

    A virus can only reproduce inside a host cell.

  3. 3

    What happens at the end of the lytic pathway?

    1. AThe host cell bursts, releasing new viruses
    2. BThe virus becomes dormant
    3. CThe host divides
    4. DThe virus joins the chromosome
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    A: The host cell bursts, releasing new viruses

    The host cell bursts, which is lysis, releasing new viruses.

  4. 4

    In the lysogenic pathway, what happens to the viral DNA?

    1. AIt is destroyed
    2. BIt makes new viruses at once
    3. CIt stays outside the cell
    4. DIt joins the host's chromosome
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    D: It joins the host's chromosome

    It is joined into the host's chromosome and copied each time the host divides.

  5. 5

    What is a provirus?

    1. AA new virus particle
    2. BViral DNA in the host's chromosome
    3. CA type of antibiotic
    4. DA protein coat
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    B: Viral DNA in the host's chromosome

    A provirus is viral DNA that has been joined into the host cell's chromosome.

  6. 6

    Why do antibiotics not work on viruses?

    1. AViruses are too big
    2. BAntibiotics kill host cells
    3. CViruses do not have the bacterial structures that antibiotics attack
    4. DViruses are alive
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    C: Viruses do not have the bacterial structures that antibiotics attack

    Antibiotics target structures in bacteria, and viruses do not have them.

  7. 7

    Why is a virus in the lysogenic state difficult for the immune system to find?

    1. AIt makes no viral proteins
    2. BIt is outside the body
    3. CIt is in the blood
    4. DIt is too large
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    A: It makes no viral proteins

    It is not making viral proteins, so there is nothing for the immune system to recognise.

  8. 8

    Which of these can trigger the lysogenic pathway to become lytic?

    1. AEating
    2. BSleeping
    3. CTaking antibiotics
    4. DStress or damage to the cell
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    D: Stress or damage to the cell

    Stress or damage can trigger the viral DNA to start making new viruses.

  9. 9

    Each infected cell releases 10 viruses, and each virus infects a new cell. How many viruses after 4 rounds, from one virus?

    1. A1000
    2. B10 000
    3. C100 000
    4. D40
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    B: 10 000

    Each round multiplies by 10, so \(10^4 = 10\,000\).