Immunity, infection and forensicsEdexcel A-Level Biology A: Topic test

1. 1
   A forensic team is called to a body found in an unheated outbuilding in January. The air temperature in the outbuilding is 4 °C and the body is fully clothed.

   (a)

   Which statement explains why rigor mortis (stiffening of the muscles) develops a few hours after death?

   [1 mark]

   • AThe nervous system continues to send impulses to the muscles
   • BATP is no longer made, so myosin heads cannot detach from actin and the muscle fibres stay contracted
   • CBacterial enzymes cross-link the muscle proteins
   • DThe muscle fibres run out of calcium ions, so the filaments lock together

   (b)

   Decomposition of this body is slower than it would be in the same outbuilding in July. What is the main reason?

   [1 mark]

   • ALow temperatures denature the enzymes of the decomposers
   • BThe cold air contains less oxygen, so decomposers cannot respire
   • CMicroorganisms and insects are completely absent in winter
   • DThe enzymes of decomposer microorganisms are less active at low temperatures

   (c)

   Explain how measuring the body temperature can be used to estimate the time of death.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A traveller drinks water contaminated with bacteria and develops severe diarrhoea. The bacteria enter the body through the gut, which has several defences against infection.

   (a)

   How does stomach acid help to protect the body against swallowed pathogens?

   [1 mark]

   • AThe low pH denatures the proteins of many pathogens and kills them
   • BIt stimulates infected cells to release interferon
   • CIt is the source of the lysozyme that digests bacterial cell walls
   • DIt causes the production of antibodies by plasma cells

   (b)

   What is the role of the normal gut flora in protecting against infection?

   [1 mark]

   • AThey secrete antibodies against the pathogens
   • BThey engulf pathogens by phagocytosis
   • CThey compete with pathogens for nutrients and space, and so reduce their growth
   • DThey produce interferon to protect uninfected cells

   (c)

   The traveller is later given a long course of a broad-spectrum antibiotic for a different infection and develops severe diarrhoea again. Explain why.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A forensic laboratory recovers skin cells left on a door handle. The sample contains only 6 copies of the short tandem repeat (STR) region of DNA that must be analysed, so the region is amplified using the polymerase chain reaction (PCR). In an ideal PCR the number of copies doubles in every cycle.

   (a)

   Calculate the number of copies of the STR region after 10 cycles of PCR, and suggest one reason why the actual number of copies would be lower than your answer.

   [3 marks]

   (b)

   Describe what happens in one cycle of PCR, and explain why Taq polymerase is used.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   During an outbreak of measles at a school, several unvaccinated children who had been exposed to the virus were given an injection of antibodies against the measles virus, and were then offered the measles vaccine. Measles is caused by a virus that infects cells of the respiratory tract.

   (a)

   Describe how the immune system of an unvaccinated child responds to a first infection with the measles virus.

   [6 marks]

   (b)

   Compare the protection given to the exposed children by the injection of antibodies with the protection given by the measles vaccine, and explain why the children were given both.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Staphylococcus aureus is a bacterium that causes skin and wound infections. HIV is a virus that infects helper T cells. Doctors need to control both pathogens.

   (a)

   Which structure is found in S. aureus but not in HIV particles?

   [1 mark]

   • AA lipid envelope
   • BA protein capsid
   • CRibosomes
   • DReverse transcriptase

   (b)

   Which observation is an example of a pathogen evading the host's immune system, supporting the idea of an evolutionary race between pathogens and hosts?

   [1 mark]

   • AA pathogen that frequently changes its surface antigens, so memory cells made in an earlier infection no longer recognise it
   • BMemory cells persist in the blood for many years
   • CA higher dose of antibiotic is more effective against bacteria
   • DSkin and mucus act as barriers to infection

   (c)

   A patient with a severely weakened immune system has a S. aureus wound infection. Explain why the doctor chooses a bactericidal antibiotic rather than a bacteriostatic antibiotic.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A dairy farmer disputes the paternity of a calf. DNA from the calf, its mother and two bulls, X and Y, was analysed by DNA profiling based on short tandem repeats (STRs). The calf's profile has six bands. Four of them match bands in the mother's profile, and the other two match bands in the profile of bull X but not those of bull Y.

   (a)

   Why do STR profiles usually differ between unrelated individuals?

   [1 mark]

   • AEach person has a different number of chromosomes
   • BSTR regions code for different proteins in different people
   • CSTR sequences mutate every time a cell divides
   • DThe number of repeats at each STR site varies between individuals, so the fragments differ in length

   (b)

   In gel electrophoresis of the DNA fragments, in which direction do the fragments move and which fragments travel furthest?

   [1 mark]

   • ATowards the positive electrode, with the largest fragments travelling furthest
   • BTowards the positive electrode, with the smallest fragments travelling furthest
   • CTowards the negative electrode, with the smallest fragments travelling furthest
   • DTowards the negative electrode, with the largest fragments travelling furthest

   (c)

   Explain how these results show that bull X could be the father of the calf but that bull Y cannot be.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A microbiologist examines a sample from a patient with a lung infection under an electron microscope. She sees rod-shaped bacteria and many smaller spherical virus particles. On a micrograph with a magnification of × 250 000, one virus particle measures 30 mm across.

   (a)

   Calculate the actual diameter of the virus particle in nanometres (nm). Show your working.

   [3 marks]

   (b)

   Describe two structural features that the bacteria have but the virus particles do not, and explain why an antibiotic that prevents the formation of bacterial cell walls would not stop the virus particles from multiplying.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A hospital ward experiences an outbreak of infections caused by a bacterium that is resistant to most antibiotics. Patients with weakened immune systems, including a group of patients infected with HIV, are most at risk.

   (a)

   Explain how antibiotic-resistant bacteria can become common on a hospital ward, and how hospital practice can reduce this.

   [6 marks]

   (b)

   Explain how infection with HIV weakens the body's defence against bacterial infections.

   [6 marks]

   Total for question 8: 12 marks

End of questions