Binary fission and virus replicationAQA A-Level Biology: Subtopic test

1. 1
   A microbiologist inoculated a flask of sterile nutrient broth with a single Escherichia coli cell. Under these conditions each cell divides by binary fission once every 20 minutes and none of the cells die.

   (a)

   Assuming no cells die, how many cells will be present in the flask after 2 hours?

   [1 mark]

   • A12
   • B36
   • C64
   • D128

   (b)

   Which statement correctly describes each daughter cell produced by binary fission?

   [1 mark]

   • AIt has two copies of the circular DNA and the same number of plasmids as the parent
   • BIt has a single copy of the circular DNA and a variable number of copies of plasmids
   • COne daughter has the circular DNA and the other has only plasmids
   • DIt has half of the circular DNA and half of the plasmids

   (c)

   Explain why the daughter cells each contain exactly one copy of the circular DNA but may contain different numbers of plasmids.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A virologist is studying a bacteriophage, a virus that infects bacteria. When a phage attaches to a bacterium it injects its nucleic acid into the host cell and its protein coat stays outside.

   (a)

   Why do viruses not undergo cell division?

   [1 mark]

   • AThey are non-living, so they cannot divide themselves
   • BTheir nucleus is too small to divide
   • CThey are made only of protein and so cannot divide
   • DThey divide only by binary fission, which needs a host

   (b)

   After the phage nucleic acid has entered the bacterium, which of these makes new phage particles?

   [1 mark]

   • ARibosomes carried by the phage
   • BEnzymes carried in the phage protein coat that supply ATP
   • CThe empty protein coat acting as a template
   • DThe host cell's enzymes and ribosomes, using the phage nucleic acid

   (c)

   Explain why phages cannot replicate in sterile nutrient broth that contains no bacteria.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A hospital laboratory is investigating two patients: one has a bacterial infection and the other has a viral infection.

   (a)

   Describe how a virus replicates after it has attached to a host cell.

   [3 marks]

   (b)

   Compare how bacteria increase in number by binary fission with how a virus increases in number.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A cheese manufacturer adds a starter culture of Lactococcus bacteria to milk. The bacteria ferment the milk sugar and make the cheese. Some batches fail because the culture is infected by a bacteriophage.

   (a)

   Describe how the bacteria in the starter culture divide by binary fission and explain why the population increases rapidly under suitable conditions.

   [6 marks]

   (b)

   Explain how a bacteriophage can cause a batch to fail, and why the phage itself cannot be said to undergo cell division.

   [6 marks]

   Total for question 4: 12 marks

End of questions