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Mutation and cancer: what goes wrong, and where it goes wrong questions

Types of gene mutation and their consequences at protein level, why many substitutions are silent, the difference between a mutation in a gene and one in a control sequence, benign and malignant tumours, proto-oncogenes and tumour suppressor genes with promoter hypermethylation, oestrogen and breast cancer, and the therapeutic use of stem cells with the ethical arguments set out on both sides.

5 original questions · 17 marks · the mutation and cancer: what goes wrong, and where it goes wrong notes · Gene regulation, genomics and biotechnology

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• 4 marks · Explain

  Explain why the deletion of a single base near the start of a gene usually destroys the protein, while the deletion of three bases at the same point usually does not.

  Mark scheme

  1. B1 the code is non-overlapping and read in triplets from a fixed starting point
  2. B1 losing one base moves every base after it one place forward, so the reading frame is shifted and every triplet downstream is read from a new position
  3. B1 all the amino acids after the deletion are therefore potentially wrong, and a stop codon usually appears early by chance, so the polypeptide is shortened as well as scrambled and will not fold into anything
  4. B1 three is a multiple of three, so the frame is not shifted: one amino acid is removed and every triplet after it is read exactly as before

• 4 marks · Compare

  Compare a proto-oncogene with a tumour suppressor gene, referring to what the product of each normally does and to how many alleles must be affected before division is lost from control.

  Mark scheme

  1. B1 a proto-oncogene codes for a protein that stimulates cell division when a growth signal arrives, whereas a tumour suppressor gene codes for a protein that halts the cell cycle at a checkpoint or triggers apoptosis
  2. B1 a proto-oncogene does harm when a mutation makes it permanently active or present in too many copies, whereas a tumour suppressor gene does harm when its product is lost
  3. B1 one faulty allele is enough for an oncogene, because a permanently active protein acts whatever the other allele makes, whereas both alleles of a tumour suppressor gene must go, because one working copy still makes the protein
  4. B1 in both cases the outcome is the same: cells pass checkpoints they should not and divide repeatedly, so a mass of cells accumulates

• 4 marks · Evaluate

  A research group wishes to obtain stem cells from embryos left over from IVF treatment. Evaluate the case for allowing this work.

  Mark scheme

  1. B1 for: embryonic stem cells are pluripotent and can give rise to any cell type of the body, so they offer a range of treatment that multipotent adult stem cells cannot
  2. B1 for: the embryos are surplus from IVF and would otherwise be discarded, and many people hold that there is a moral difference between using such an embryo and creating one in order to destroy it
  3. B1 against: those who hold that a human embryo has the full moral status of a person from fertilisation onwards regard its destruction as the deliberate ending of a human life, which the good done with the cells does not license
  4. B1 judgement: British law takes a middle position, licensing such research through the Human Fertilisation and Embryology Authority up to fourteen days; induced pluripotent cells narrow the disagreement, since they are pluripotent and need no embryo, though they can carry mutations acquired in the adult cell and a tendency to form tumours

• 3 marks · Suggest

  A drug that inhibits the enzymes adding methyl groups to DNA is being trialled in patients whose tumours carry an intact but heavily methylated tumour suppressor gene. Suggest why the drug might restore control of division in those cells, and suggest one reason it may also harm healthy cells.

• 2 marks · Name

  Name the kind of substitution that produces a stop codon, and name the property of the genetic code that allows a different substitution to change no amino acid at all.

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