Factors affecting gene expressionEdexcel A-Level Biology B: Subtopic test

1. 1
   A researcher studies a gene in liver cells that is transcribed only when a protein, factor F, enters the nucleus and binds to a DNA sequence next to the gene. In muscle cells factor F is absent and the gene is not transcribed, although muscle cells contain the same gene.

   (a)

   Which of the following best describes factor F?

   [1 mark]

   • AAn enzyme that joins exons together
   • BA tRNA molecule that carries amino acids
   • CA protein that binds to DNA to regulate transcription
   • DA non-coding RNA that modifies histones

   (b)

   Which statement best explains why the gene is not transcribed in muscle cells?

   [1 mark]

   • AWithout factor F, RNA polymerase is not helped to bind to the gene and begin transcription
   • BThe gene has been deleted from the DNA of muscle cells
   • CMuscle cell ribosomes are unable to translate the mRNA
   • DThe gene has been spliced out of the muscle cell genome

   (c)

   Explain how factor F can increase the rate of transcription of the gene in liver cells.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The gene for a muscle protein is transcribed in two different tissues. In both tissues the primary transcript has six exons. In tissue 1 the mature mRNA contains exons 1 to 6. In tissue 2 the mature mRNA contains exons 1, 2, 4, 5 and 6, and the protein it codes for is shorter and has a different function.

   (a)

   Which parts of a primary transcript are always removed during splicing?

   [1 mark]

   • AExons
   • BCodons
   • CPromoters
   • DIntrons

   (b)

   Which statement best explains the difference between the mRNA molecules in the two tissues?

   [1 mark]

   • AExon 3 has been deleted from the DNA in tissue 2
   • BExon 3 has been spliced out of the primary transcript in tissue 2
   • CRibosomes in tissue 2 skip exon 3 during translation
   • DA different DNA polymerase copies the gene in tissue 2

   (c)

   Explain why the two mRNA molecules lead to proteins with different functions.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Cells from a tumour show that the promoter region of a tumour suppressor gene has many methyl groups attached to its cytosine bases, and the histones around the gene have had their acetyl groups removed. The tumour suppressor gene is not expressed in these cells.

   (a)

   Explain how these modifications prevent expression of the tumour suppressor gene.

   [3 marks]

   (b)

   Drugs that remove methylation or prevent histone deacetylation are being tested as cancer treatments. Suggest how such a drug could help to treat this tumour and why it might cause side effects.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A motor neurone and a heart muscle cell are taken from the same person, so both contain the same genome. Gene G is transcribed in the heart muscle cell but not in the neurone. Another gene, H, is transcribed in both cell types, but the two cells produce different proteins from it.

   (a)

   Explain how epigenetic modification could silence gene G in the neurone, and why epigenetic modification is important in cell differentiation.

   [6 marks]

   (b)

   Explain how gene H can produce different proteins in the neurone and in the heart muscle cell.

   [6 marks]

   Total for question 4: 12 marks

End of questions