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Prokaryotic cells and viruses: less machinery, same problems questions

The structure of a prokaryotic cell, meaning murein wall, capsule, flagellum, plasmids, circular DNA and 70S ribosomes, compared with a eukaryotic cell; binary fission; and the structure of viruses as acellular particles, with HIV as the worked example.

5 original questions · 16 marks · the prokaryotic cells and viruses: less machinery, same problems notes · Cells, microscopy and biological organisation

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• 4 marks · Compare

  Compare the genetic material and the ribosomes of a bacterial cell with those of a eukaryotic cell.

  Mark scheme

  1. B1 a bacterium has one circular DNA molecule, whereas a eukaryote has several linear ones
  2. B1 bacterial DNA is not wound round histone proteins, whereas eukaryotic DNA is
  3. B1 bacterial DNA lies free in the cytoplasm in the nucleoid, whereas eukaryotic DNA is enclosed by a nuclear envelope
  4. B1 bacterial ribosomes are 70S and smaller, whereas the ribosomes in eukaryotic cytoplasm are 80S

• 4 marks · Explain

  A student writes that a virus is simply a very small cell. Explain why this statement is wrong, referring to the structure of a virus in your answer.

  Mark scheme

  1. B1 a virus is acellular: it has no cytoplasm and no cell-surface membrane of its own
  2. B1 it has no ribosomes, so it cannot make any protein for itself
  3. B1 it has no enzyme system for respiration, so it has no metabolism and cannot release energy from food
  4. B1 it consists only of DNA or RNA inside a protein capsid with attachment proteins outside, sometimes within a lipid envelope taken from a host cell

• 3 marks · Calculate

  A culture is started with 200 bacteria which divide by binary fission every 30 minutes. Calculate how many bacteria are present after 5 hours, assuming that none of them die.

  Mark scheme

  1. M1 number of divisions = 300 minutes divided by 30, giving 10 divisions
  2. M1 uses N = N₀ × 2ⁿ, so the calculation is 200 × 2¹⁰
  3. A1 an answer of 204 800 bacteria

• 3 marks · Suggest

  A drug is designed to bind to the glycoprotein attachment proteins on the surface of HIV. Suggest why this drug would prevent HIV entering helper T cells, and suggest why it would have no effect on the influenza virus.

• 2 marks · Name

  Name the substance from which a bacterial cell wall is built, and name the structure that carries genes such as antibiotic resistance and can be passed between bacteria.

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