Stem cells and cell specialisationEdexcel A-Level Biology A: Subtopic test

1. 1
   A human zygote divides by mitosis. Cells taken from the embryo at the four-cell stage can each develop into a complete, separate individual, which is how identical twins and triplets can arise. A few days later the embryo is a hollow ball called a blastocyst, and cells from its inner cell mass can form almost any tissue in the body but cannot form the placenta.

   (a)

   Which term describes a cell that can develop into any type of cell, including placental cells, and into a complete organism?

   [1 mark]

   • ATotipotent
   • BPluripotent
   • CSpecialised
   • DSomatic

   (b)

   How are the cells of the inner cell mass of the blastocyst best described?

   [1 mark]

   • ATotipotent, because they can form the placenta
   • BSpecialised, because they have a fixed function
   • CPluripotent, because they can form almost any cell type but not a whole organism
   • DSomatic cells that have lost genes during development

   (c)

   State what is meant by a stem cell and explain how a stem cell becomes a specialised cell.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A medical team proposes treating patients with macular degeneration, a disease that destroys retinal cells and causes blindness, using retinal cells grown from human embryonic stem cells. The stem cells would be taken from surplus embryos created during IVF treatment that would otherwise be discarded.

   (a)

   Which is the main ethical objection that some people raise to this proposed treatment?

   [1 mark]

   • AThe stem cells stop dividing after one cell division
   • BObtaining embryonic stem cells involves destroying human embryos, which some people believe have a right to life
   • CThe stem cells are always taken from the patient, so tissue is wasted
   • DThe use of stem cells is banned in all countries

   (b)

   The team considers using the patient's own adult stem cells instead. What is the main advantage of this?

   [1 mark]

   • AAdult stem cells are totipotent so can form any tissue
   • BAdult stem cells always divide faster than embryonic stem cells
   • CAdult stem cells are available in unlimited numbers from any tissue
   • DThe cells are genetically identical to the patient, so there is less risk of immune rejection

   (c)

   Suggest why decisions about stem cell therapies are not made by scientists alone.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The bacterium Escherichia coli can use lactose as a respiratory substrate. In the lac operon, the regulatory gene lacI codes for a repressor protein. The operon also contains a promoter, an operator, and structural genes including lacZ, which codes for the enzyme beta-galactosidase that hydrolyses lactose. The repressor protein can bind to the operator, and lactose can bind to the repressor protein.

   (a)

   Explain why beta-galactosidase is made by E. coli only when lactose is present.

   [3 marks]

   (b)

   A mutant strain of E. coli has a mutation in lacI so that its repressor protein cannot bind to the operator. Predict the effect on beta-galactosidase production when there is no lactose, and explain why this strain would be at a disadvantage compared with the normal strain.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Every body cell in a person, other than the gametes, contains the same genome. Yet pancreatic beta cells make insulin while the precursors of red blood cells make haemoglobin, and in plants the xylem vessels develop thick, lignified walls while neighbouring parenchyma cells do not. Researchers hope to treat type 1 diabetes, in which the beta cells of the pancreas are destroyed, by using stem cells to replace them.

   (a)

   Explain how cells that contain the same genome become specialised to carry out different functions in animals and in plants.

   [6 marks]

   (b)

   Evaluate the use of embryonic stem cells to treat type 1 diabetes. Include a justified conclusion.

   [6 marks]

   Total for question 4: 12 marks

End of questions