All revision notes topics

Genome sequencing, personalised medicine and GMOsEdexcel A-Level Biology A: Revision notes

Section 1

Genome sequencing and personalised medicine

Genome sequencing determines the order of all the bases in an organism's DNA. Projects such as the Human Genome Project have produced reference genomes, and modern methods now sequence an individual's genome quickly and cheaply.

Personalised medicine uses a patient's genotype to choose the drug and dose most likely to work and least likely to cause harm. Examples:

  • Warfarin dose depends on alleles of genes for the enzymes that break it down.
  • Genotype of a cancer can be used to select a targeted drug.
  • Knowing disease risk alleles allows early screening and lifestyle advice.
Key termsgenome sequencingpersonalised medicinegenotype

Section 2

Social, moral and ethical issues

Genetic data raises issues:

  • Privacy and confidentiality: insurers or employers could discriminate if they gain access.
  • Psychological harm: knowing you carry an allele for a serious, untreatable disease can cause anxiety.
  • Consent: results also affect relatives, and children cannot give informed consent.
  • Unexpected findings unrelated to the original test.
  • Cost and inequality: not everyone can afford testing.

A good answer weighs benefits against risks and suggests safeguards such as consent, counselling and laws on data protection.

Key termsinformed consent
Exam tip

Always give an issue and its consequence, e.g. 'insurers could access data, so people may be refused cover'.

Section 3

Making drugs with genetically modified organisms

A genetically modified organism (GMO) has had a gene from another species inserted. To make human insulin in bacteria:

  1. The gene is made as a DNA copy from mRNA using reverse transcriptase, so it has no introns (bacteria cannot remove them).
  2. A restriction endonuclease cuts the plasmid (vector) and the gene has matching sticky ends.
  3. DNA ligase joins the gene into the plasmid.
  4. The recombinant plasmid is taken up by bacteria, which are grown in fermenters.

Other GM sources: animals (e.g. goats make antithrombin in milk), plants (e.g. tobacco making antibodies). Animals and plants can make complex proteins that need modification that bacteria cannot do.

Key termsrestriction endonucleaseDNA ligaseplasmidreverse transcriptase
Common mistake

Ligase joins DNA. Restriction enzymes cut it. Do not swap them.

Section 4

Advantages of GM drugs

  • Product is identical to the human protein, so less chance of an immune reaction (e.g. human insulin versus pig insulin).
  • Large amounts can be made cheaply and reliably, as bacteria grow quickly.
  • Avoids disease transmission from animal or human tissue, e.g. contamination of growth hormone taken from cadavers.
  • Fewer ethical objections to slaughter for tissue.
  • Plants and animals can make complex proteins in large quantities.
Key termsrecombinant protein

Section 5

Risks and benefits of GMOs

Benefits: drugs and vaccines, higher crop yield, pest resistance (e.g. Bt crops), added nutrients, less pesticide use.

Risks and concerns:

  • Gene flow to wild relatives or non-GM crops via pollen.
  • Harm to non-target organisms and biodiversity.
  • Resistance (e.g. herbicide resistance spreading to weeds).
  • Possible allergens or unknown long-term effects.
  • Welfare and ethics of modifying animals.
  • Patents may make seed or drugs expensive and increase dependence on companies.

Containment, regulation and testing reduce the risks.

Key termsgene flow

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Genome sequencing, personalised medicine and GMOs

  1. Until the 1980s, insulin for people with diabetes was extracted from the pancreases of pigs and cattle. Today most insulin is made by genetically modified bacteria. The human insulin gene is copied from mRNA and inserted into a plasmid, which is taken up by the bacteria and grown in large fermenters.
    Explain two advantages of using genetically modified bacteria to make human insulin rather than extracting it from pigs.2 marks
  2. Warfarin is a drug that prevents blood clots. The dose that is safe varies between patients because of differences in the alleles of genes coding for enzymes that break down the drug. A hospital sequences the genomes of its patients before prescribing, and stores the results on a secure database.
    Suggest how sequencing a patient's genome could reduce the chance of an adverse reaction to warfarin.2 marks
  3. A company has genetically modified tobacco plants so that their leaves make a human antibody that is used as a drug. The antibody gene was inserted into the chromosomes of the plant cells. The plants are currently grown in sealed greenhouses, and the company is considering growing them in open fields instead.
    Suggest three advantages of producing the antibody in plants, compared with producing it in animals.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).