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Genomes, screening and gene therapy: what a sequence is good for questions

DNA sequencing and what a genome project actually produces, DNA probes and microarrays, genetic screening and the role of genetic counselling, DNA profiling using variable number tandem repeats and how a match probability should be interpreted, and somatic against germ line gene therapy with the arguments on each side.

5 original questions · 16 marks · the genomes, screening and gene therapy: what a sequence is good for notes · Gene regulation, genomics and biotechnology

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• 4 marks · Explain

  Explain why the sequenced genome of a bacterium gives its proteome almost directly, while the sequenced genome of a human does not.

  Mark scheme

  1. B1 bacterial genes are continuous, with no introns, so one gene gives one polypeptide and the coding sequence can be read straight off the DNA
  2. B1 human genes contain introns, which have to be identified and removed before the coding sequence is known
  3. B1 alternative splicing lets one human gene give several different polypeptides
  4. B1 large stretches of the human genome are regulatory rather than coding, and post-translational modification changes the polypeptide again after translation

• 4 marks · Compare

  Compare somatic gene therapy with germ line gene therapy, referring to which cells are altered, what is inherited, and how each stands in law.

  Mark scheme

  1. B1 somatic therapy alters body cells such as bone marrow, retina or airway epithelium, whereas germ line therapy alters a gamete, a zygote or an early embryo
  2. B1 a somatic change is not inherited, because the cells that become sperm or eggs are untouched, whereas a germ line change is present in every cell of the resulting person, including those cells, and passes to every descendant
  3. B1 somatic treatment often has to be repeated, because cells that are not stem cells are replaced within weeks, whereas a germ line change is permanent and cannot be withdrawn from the family line
  4. B1 somatic therapy is licensed for several conditions, whereas germ line therapy in a pregnancy is prohibited by law in every country that has legislated on the question

• 3 marks · Explain

  Explain how a labelled DNA probe is used to find out whether a person carries a particular allele.

  Mark scheme

  1. B1 the probe is a short single strand of DNA whose base sequence is complementary to the sequence being looked for, carrying a radioactive or a fluorescent label
  2. B1 the sample DNA is made single-stranded by heating and the probe is added, and hybridisation occurs wherever the probe meets a complementary sequence, held by hydrogen bonds between complementary bases
  3. B1 unbound probe is washed away, and the label is then detected, on photographic film or under ultraviolet light, only where the probe has bound

• 3 marks · Explain

  A profile from a crime scene matches a suspect, and the court is told that the probability of such a match in an unrelated person chosen at random is one in a billion. A lawyer says there is therefore a one in a billion chance that the suspect is innocent. Explain why that statement is wrong.

• 2 marks · State

  State what a genome project produces, and state what annotation adds to that product afterwards.

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