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Control of gene expression: which genes a cell reads questions

Transcription factors and how they change the rate of transcription of a specific gene, the lac operon as the prokaryotic worked example, oestrogen and its receptor as the eukaryotic one, epigenetic control by DNA methylation and histone acetylation with a careful account of what is and is not inherited, and RNA interference by small interfering RNA.

5 original questions · 16 marks · the control of gene expression: which genes a cell reads notes · Gene regulation, genomics and biotechnology

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• 4 marks · Describe

  Describe how the lac operon is switched on when lactose enters an Escherichia coli cell, naming the parts of the operon involved.

  Mark scheme

  1. B1 the regulatory gene lacI is transcribed steadily whatever else is happening, and its product is the lac repressor, which binds to the operator
  2. B1 with the repressor on the operator, RNA polymerase cannot move on from the promoter, so the structural genes are transcribed only at a very low background rate
  3. B1 lactose binds to the repressor's other binding site and changes the protein's tertiary structure, so the site that fitted the operator no longer fits it
  4. B1 the repressor leaves the operator, so RNA polymerase transcribes lacZ, lacY and lacA as one mRNA and the enzymes are made

• 4 marks · Explain

  Explain why oestrogen can act on a gene inside the nucleus while a peptide hormone such as insulin cannot.

  Mark scheme

  1. B1 oestrogen is a steroid and therefore lipid-soluble
  2. B1 it diffuses straight through the phospholipid bilayer without a channel or a carrier, so it reaches the inside of the cell and then of the nucleus
  3. B1 there it binds an oestrogen receptor, changing the receptor's tertiary structure and exposing its DNA-binding site, so the complex binds the oestrogen response element and transcription of the target gene increases
  4. B1 insulin is a peptide and is not lipid-soluble, so it binds a receptor on the outside of the membrane and its message is carried inwards by a second messenger instead

• 3 marks · Explain

  Explain how small interfering RNA stops a protein being produced from a gene that has already been transcribed.

  Mark scheme

  1. B1 double-stranded RNA in the cytoplasm is cut into fragments about twenty-one nucleotides long, and one strand of each fragment is loaded into a protein complex
  2. B1 the complex uses that single strand as a search template and pairs with any mRNA whose bases are complementary to it
  3. B1 the mRNA is cut and broken down, so it is never translated and no polypeptide is made, even though transcription of the gene did happen

• 3 marks · Suggest

  A drug occupies the hormone-binding site of the oestrogen receptor without changing the receptor's shape. Suggest the effect of this drug on transcription of an oestrogen-responsive gene, and suggest why it affects breast tissue far more than liver tissue.

• 2 marks · State

  State the effect that increased methylation of a promoter has on transcription of the gene, and state the effect of decreased acetylation of the histones around it.

Practise control of gene expression: which genes a cell reads one question at a time

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