Bacteria, viruses and infectionEdexcel International A Level Biology: Subtopic test

1. 1
   A microbiologist is comparing the structure of three infectious agents: the bacterium Escherichia coli, tobacco mosaic virus (TMV) and human immunodeficiency virus (HIV). TMV is a rod-shaped virus with a single strand of RNA surrounded by a protein capsid. HIV has two strands of RNA inside a capsid, which is surrounded by a lipid envelope containing glycoproteins.

   (a)

   Which feature is present in E. coli but absent from both TMV and HIV?

   [1 mark]

   • ARibosomes
   • BProtein
   • CNucleic acid
   • DA capsid

   (b)

   Which statement correctly compares TMV with HIV?

   [1 mark]

   • ATMV has a DNA genome and HIV has an RNA genome
   • BHIV has a lipid envelope derived from a host cell membrane, but TMV has no envelope
   • CBoth have a lipid envelope
   • DOnly TMV has a protein capsid

   (c)

   Compare the structure of E. coli with the structure of TMV, giving two differences.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Bacteriophage lambda is a virus with a head, containing double-stranded DNA, and a tail with tail fibres. It infects the bacterium E. coli. In some infected bacteria the viral DNA is inserted into the bacterial chromosome and is copied each time the bacterium divides, without new phages being made. When these bacteria are later exposed to ultraviolet light, they burst open and release hundreds of new phages.

   (a)

   Which term describes the state of the phage DNA when it has been inserted into the bacterial chromosome and is copied without new phages being made?

   [1 mark]

   • ALytic phase
   • BCapsid assembly
   • CLatency
   • DAdsorption

   (b)

   What is the function of the tail fibres of the phage?

   [1 mark]

   • ATo digest the bacterial chromosome
   • BTo make new capsid proteins
   • CTo inject the phage RNA into the bacterium
   • DTo attach to specific receptors on the bacterial surface

   (c)

   Explain why exposing the bacteria to ultraviolet light leads to the release of new phages.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A man has had a persistent cough, with blood in his sputum, together with weight loss, fever and night sweats for six weeks. A sample of his sputum grew Mycobacterium tuberculosis, a bacterium with a waxy cell wall that contains mycolic acid. The bacteria were inhaled in droplets and reached the alveoli of the lungs, where they were engulfed by macrophages.

   (a)

   Explain how Mycobacterium tuberculosis infects cells in the lungs and avoids destruction.

   [3 marks]

   (b)

   Explain how the infection has caused this man's symptoms, and why a person who also has HIV is more likely to develop active tuberculosis.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A patient has been infected with HIV for eight years. At first he had only a mild illness, then few symptoms for several years. Recently his helper T cell count has fallen sharply and he has developed pneumonia and tuberculosis. HIV is an enveloped RNA virus whose envelope contains glycoproteins that bind to CD4 receptors on helper T cells, and it carries the enzyme reverse transcriptase.

   (a)

   Describe how HIV enters a helper T cell and replicates inside it.

   [6 marks]

   (b)

   Explain how HIV infection leads to the development of AIDS, and compare the way latency occurs in HIV with the lytic cycle of the lambda phage.

   [6 marks]

   Total for question 4: 12 marks

End of questions