VirusesEdexcel A-Level Biology B: Subtopic test

1. 1
   Human immunodeficiency virus (HIV) is an enveloped virus that infects helper T cells. Each virus particle contains two molecules of RNA and the enzyme reverse transcriptase. Zidovudine, an antiviral drug, inhibits reverse transcriptase. Penicillin, an antibiotic, has no effect on HIV.

   (a)

   Why is HIV described as a retrovirus?

   [1 mark]

   • AIt uses reverse transcriptase to make DNA from its RNA
   • BIt contains DNA that is transcribed into RNA before replication
   • CIt reproduces by binary fission inside its host
   • DIt injects its nucleic acid into bacteria

   (b)

   Which statement best explains why penicillin has no effect on HIV?

   [1 mark]

   • AHIV has a thick cell wall that penicillin cannot cross
   • BHIV has 70S ribosomes, which penicillin cannot affect
   • CHIV is not a cell, so it has no peptidoglycan wall for penicillin to target and no metabolism of its own
   • DHIV has an outer membrane that prevents penicillin entering

   (c)

   Explain how zidovudine reduces the replication of HIV.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Lambda phage is a virus that infects the bacterium Escherichia coli. After infecting a bacterium, the phage can either destroy the cell within about an hour, releasing around 100 new phages, or have its DNA inserted into the bacterial chromosome and copied each time the bacterium divides. After many generations, exposure of the infected bacteria to ultraviolet light causes many of them to burst and release new phages.

   (a)

   Which statement describes latency?

   [1 mark]

   • AThe viral genes are transcribed in large amounts and new virus particles are assembled
   • BThe viral DNA is incorporated into the host genome and replicated with it, without producing new virus particles
   • CThe virus particles stay outside the host cell and do not take part in reactions
   • DThe host cell bursts and releases virus particles

   (b)

   Which of these happens only in the lytic cycle, and not during latency?

   [1 mark]

   • APhage DNA is copied each time the bacterium divides
   • BPhage DNA is inserted into the host chromosome
   • CThe bacterium divides normally with no harm
   • DThe host cell bursts, releasing new phage particles

   (c)

   Suggest two advantages to the virus of latency.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The 2014 outbreak of Ebola virus disease in West Africa spread rapidly through Guinea, Sierra Leone and Liberia. Ebola is an RNA virus spread by contact with the body fluids of infected people, including people who have died of the disease. At that time no licensed vaccine or antiviral drug existed. The response included isolating patients, tracing their contacts, training health workers to wear protective clothing and organising safe burials with community leaders.

   (a)

   Explain why control of the outbreak focused on preventing the spread of the virus rather than treating infected people.

   [3 marks]

   (b)

   Suggest how each of the four measures described reduced the spread of Ebola.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A virology course compares four viruses: lambda phage (a head and tail virus that infects bacteria and has DNA), tobacco mosaic virus (a rod-shaped virus with RNA), Ebola virus (a filament-shaped, enveloped virus with RNA) and HIV (a spherical, enveloped RNA retrovirus). During the 2014 Ebola epidemic in West Africa, health officials debated whether to offer patients an experimental antiviral drug that had been tested only in animals.

   (a)

   Describe how viruses are classified, using the four viruses as examples, and explain why viruses are not considered to be living cells.

   [6 marks]

   (b)

   Evaluate the ethical implications of giving the experimental drug to Ebola patients.

   [6 marks]

   Total for question 4: 12 marks

End of questions