Stem cells and cell specialisationEdexcel A-Level Biology A: Mind map

Exam questions on Stem cells and cell specialisation

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.
   State what is meant by a stem cell and explain how a stem cell becomes a specialised cell.2 marks
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.
   Suggest why decisions about stem cell therapies are not made by scientists alone.2 marks
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.
   Explain why beta-galactosidase is made by E. coli only when lactose is present.3 marks