1. 1
   A student examines a human skin cell that is about to divide by mitosis as part of normal tissue repair. Before division, the cell grows, and its sub-cellular structures and DNA are copied. The student is asked to answer several questions about this stage and the outcome of division.

   (a)

   A student examines a description of a human skin cell about to divide. The cell's nucleus contains 46 chromosomes arranged in 23 pairs, and DNA replication has just been completed in preparation for mitosis. Which statement correctly describes the chromosomes at this point?

   [1 mark]

   • AEach chromosome now consists of two identical copies joined together
   • BThe number of chromosomes has halved to 23
   • CEach chromosome has lost one of its paired copies
   • DThe chromosomes have combined into a single loop of DNA

   (b)

   Which of the following correctly lists, in order, the three overall stages of the cell cycle that this skin cell will pass through?

   [1 mark]

   • AMitosis, growth, division of cytoplasm and cell membranes
   • BGrowth, mitosis, division of cytoplasm and cell membranes
   • CDivision of cytoplasm and cell membranes, growth, mitosis
   • DGrowth, division of cytoplasm and cell membranes, mitosis

   (c)

   State two ways in which the two daughter cells produced by this mitotic division are similar to each other and to the original skin cell.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Doctors are investigating whether stem cells taken from a three-day-old human embryo could be used to treat a patient with type 1 diabetes by growing replacement insulin-producing cells. The embryo cells are undifferentiated and able to become almost any type of human cell.

   (a)

   A three-day-old human embryo consists of a small ball of cells, each of which is undifferentiated and capable of giving rise to almost any type of human cell. What term describes these embryonic cells?

   [1 mark]

   • AGamete cells
   • BMeristem cells
   • CStem cells
   • DDifferentiated cells

   (b)

   Which of the following is a genuine risk associated with using embryonic stem cells to treat this patient, rather than a benefit?

   [1 mark]

   • AProduction of new insulin-secreting cells
   • BRestoration of normal blood glucose control
   • CReduced need for daily insulin injections
   • DPossible transfer of viral infection from the donor tissue

   (c)

   Give one ethical or religious objection that some people raise against using embryonic stem cells in this way, and suggest an alternative source of stem cells that could avoid this objection.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A rare species of orchid produces very few seeds naturally, so conservationists take small samples of meristem tissue from the tips of surviving plants and grow them in sterile nutrient jelly to produce many new, genetically identical orchids, protecting the species from extinction.

   (a)

   Explain why cells taken from the meristem, rather than from a mature leaf, are used for this cloning process.

   [3 marks]

   (b)

   The conservationists claim that this cloning method could also benefit farmers growing food crops. Explain how cloning crop plants from meristem stem cells could be used to produce large numbers of plants with a desirable feature such as disease resistance, and give one possible limitation of relying on this method to grow a whole crop.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A patient with type 1 diabetes is offered a choice between two possible future treatments: one using stem cells taken from a donated human embryo, and one using stem cells grown from the patient's own bone marrow, to produce replacement insulin-producing cells. A separate skin biopsy from the same patient reveals a group of cells dividing uncontrollably and forming a lump.

   (a)

   Analyse the differences between embryonic and adult stem cells that are relevant to this choice, and evaluate the risks and potential benefits of each option for treating this patient's diabetes.

   [6 marks]

   (b)

   A separate biopsy taken from an unrelated patch of skin on this same patient reveals a group of cells that are dividing uncontrollably and forming a lump, which doctors suspect is a tumour. Explain what has most likely gone wrong with cell division in this tumour, and compare this uncontrolled division with the way mitosis normally occurs during the healthy growth and repair of body tissue.

   [6 marks]

   Total for question 4: 12 marks

End of questions