Stem cells and differentiationEdexcel International A Level Biology: Subtopic test

1. 1
   An embryologist at a fertility clinic is observing the early development of a human embryo created by in vitro fertilisation. Three days after fertilisation the embryo is a solid ball of cells. By day five it has developed into a hollow structure with a cluster of cells on one side.

   (a)

   What is the name of the solid ball of cells formed by the early cleavage divisions of the zygote?

   [1 mark]

   • ABlastocyst
   • BZygote
   • CPrimary oocyte
   • DMorula

   (b)

   Which part of the blastocyst is the source of pluripotent embryonic stem cells?

   [1 mark]

   • AThe outer layer of cells, which forms the placenta
   • BThe fluid-filled cavity
   • CThe inner cell mass
   • DThe zona pellucida

   (c)

   Explain why a single cell from the morula could develop into a complete organism, whereas a single cell from the inner cell mass of the blastocyst could not.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A pancreatic β cell and a skeletal muscle cell from the same person both contain the gene for insulin and the gene for myosin. The β cell makes large amounts of insulin and no myosin, whereas the muscle cell makes large amounts of myosin and no insulin.

   (a)

   What is the role of a transcription factor in differential gene expression?

   [1 mark]

   • AIt joins amino acids together at the ribosome
   • BIt binds to DNA near a gene to switch transcription of that gene on or off
   • CIt copies the DNA before cell division
   • DIt removes introns from the primary mRNA transcript

   (b)

   Which statement about mRNA in the two cells is correct?

   [1 mark]

   • AThe β cell contains insulin mRNA, but the muscle cell does not
   • BBoth cells contain insulin mRNA because both contain the insulin gene
   • CNeither cell contains mRNA, because mRNA is only made during cell division
   • DThe muscle cell contains insulin mRNA but cannot translate it

   (c)

   Explain why the muscle cell does not produce insulin even though its nucleus contains the insulin gene.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A single human gene contains four exons separated by introns. When the gene is transcribed, the primary mRNA transcript is processed before it leaves the nucleus. In different tissues the same gene is used to make different proteins.

   (a)

   Describe how the primary mRNA transcript is processed to give mature mRNA, and how this can produce different mature mRNAs from the same gene.

   [3 marks]

   (b)

   Exons 1 and 4 are always present in the mature mRNA, but exons 2 and 3 can each be either included or removed in a given tissue. Calculate the number of different mature mRNA molecules that could be produced from this gene, and explain how this allows one gene to code for more than one protein.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A research team is developing a treatment for type 1 diabetes, in which a patient's own insulin-secreting β cells in the pancreas have been destroyed. They plan to take pluripotent stem cells from the inner cell mass of blastocysts donated after IVF treatment, treat them so that they become β cells, and transplant these cells into patients.

   (a)

   Evaluate the use of embryonic stem cells to treat type 1 diabetes, from the point of view of society.

   [6 marks]

   (b)

   Explain how differential gene expression could turn the pluripotent stem cells into β cells, and why a β cell differs from other body cells of the patient even though they contain the same genes.

   [6 marks]

   Total for question 4: 12 marks

End of questions