1. 1
   A student examines root tip cells from an onion under a microscope after treating them with a stain that binds to chromosomes. In one field of view, she counts 200 cells and finds that 14 of them show visibly condensed, thread-like chromosomes rather than a diffuse nucleus. She wants to use this sample to investigate the stages of the cell cycle and calculate how many cells are actively dividing.

   (a)

   Which stage of the cell cycle are these 14 cells most likely in?

   [1 mark]

   • AInterphase
   • BProphase
   • CCytokinesis
   • DA resting, non-dividing state

   (b)

   The student notices that some cells in the sample have chromosomes lined up in a single row across the middle of the cell, with spindle fibres attached to each chromosome from opposite poles. What is this stage called, and what happens immediately afterwards?

   [1 mark]

   • AMetaphase; the sister chromatids are then pulled apart to opposite poles
   • BAnaphase; the chromosomes then condense further
   • CTelophase; the nuclear envelope then breaks down
   • DInterphase; the DNA is then replicated

   (c)

   Explain why it is important that the two daughter cells produced by mitosis in this onion root tip contain identical sets of chromosomes to the parent cell.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A patient has a skin wound that is healing over a period of several days. New skin cells are produced at the wound edge to replace damaged tissue. However, a biopsy taken from a different patch of skin on the same patient reveals a group of cells that are dividing uncontrollably and forming a lump, which doctors suspect may be a tumour.

   (a)

   Which process is directly responsible for producing these new skin cells?

   [1 mark]

   • AMeiosis
   • BDiffusion
   • CFertilisation
   • DMitosis

   (b)

   The biopsy from the second patch of skin shows cells dividing uncontrollably and forming a lump. Which statement best describes what has most likely happened to cause this?

   [1 mark]

   • AThe cells have undergone meiosis instead of mitosis
   • BThe cells have stopped dividing completely and are dying
   • CChanges in the cells have led to uncontrolled cell division, forming a tumour
   • DThe cells have differentiated into a new specialised cell type

   (c)

   Compare the role of mitosis in healing the wound with the role of mitosis in the formation of the tumour.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A researcher is studying a population of 500 cells taken from a growing plant shoot tip. Over a 24-hour period, she observes that the total number of cells increases to 1000, with each original cell dividing exactly once by mitosis. She also notices that many of the newly formed cells begin to change shape and take on different functions, such as forming long, narrow cells suited to transporting water.

   (a)

   State the number of chromosomes present in each daughter cell compared with each original parent cell, and describe how the DNA in the daughter cells compares to the DNA in the parent cell.

   [3 marks]

   (b)

   Explain why some of the newly formed plant cells change shape and take on different functions, such as forming long, narrow water-transporting cells, even though they were all produced from the same parent cells by mitosis.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A patient with a severe spinal cord injury is told that embryonic stem cells might one day be used to help repair damaged nerve tissue, but that this treatment is not yet widely available and remains controversial. Doctors explain that the decision to use such treatments involves weighing scientific potential against medical risk and ethical debate.

   (a)

   Analyse why embryonic stem cells, rather than the patient's own adult stem cells or meristem cells from a plant, are being considered for this type of medical treatment, referring to the properties that make each type of cell more or less suitable.

   [6 marks]

   (b)

   Evaluate the risks and benefits of using embryonic stem cells to treat this patient's spinal cord injury, and use your evaluation to justify whether such treatment should be pursued despite it not yet being widely available.

   [6 marks]

   Total for question 4: 12 marks

End of questions