8.4.3 Genetic fingerprinting

• What are variable number tandem repeats (VNTRs)?

  ● Repeating sequences of nucleotides / bases (eg. GATA)

  ● Found within non-coding sections of DNA at many sites throughout an organism’s genome
• Why are VNTRs useful in genetic fingerprinting?

  ● Probability of two individuals having the same VNTRs is very low

  ● As an organism’s genome contains many VNTRs and lengths at each loci differ between individuals
• Explain how genetic fingerprinting can be used to analyse DNA fragments

  1. Extract DNA from sample + amplify by PCR

  2. Cut DNA at recognition sites (either side) using restriction enzymes

  3. Separate VNTR fragments according to length using gel electrophoresis

  4. Transfer to a nylon membrane + treat to form single strands with exposed bases

  5. Add labelled DNA probes which bind with complementary VNTRs (wash to remove unbound probe)

  6. To show bound probe, expose to UV light if fluorescently labelled probe used OR use autoradiography (expose to X-ray film) if radioactive probe used
• Compare and contrast genetic fingerprinting with genetic screening

  ● Both use PCR to amplify DNA sample

  ● Both use electrophoresis to separate DNA fragments

  ● Both use labelled DNA probes to visualise specific DNA fragments

  ● Genetic fingerprinting analyses VNTRs whereas genetic screening analyses specific alleles of a gene
• Explain how genetic fingerprinting can be used to determine genetic

  relationships

  ● More closely related organisms have more similar VNTRs, so more similarities in genetic fingerprints

  ● Paternity testing - father should share around 50% of VNTRs / bands with child (due to inheritance)
• Explain how genetic fingerprinting can be used to determine genetic

  variability within a population

  Differences in VNTRs arise from mutations, so more differences show greater diversity within a population
• Explain the use of genetic fingerprinting in the fields of forensic science,

  medical diagnosis, animal and plant breeding

  Forensic science

  ● Compare genetic fingerprint of suspects to genetic fingerprint of DNA at crime scene

  ● If many bands match, the suspect was likely present at the crime scene

  Medical diagnosis

  ● Some VNTR patterns are associated with an increased risk of certain genetic

  disorders eg. Huntington’s

  Animal and plant breeding

  ● Shows how closely related 2 individuals are, so that inbreeding can be avoided

  ● Breed pairs with dissimilar genetic fingerprints
• suggest 3 features of the structure of different proteins that enable them to be separated by gel electrophoresis

  • length of polypeptide
  • charge
  • R groups
• describe how scientists would use a radioactively labelled DNA probe to show the leaves of a plant contained a specific gene

  • extract DNA and add restriction endonucleases
  • separate DNA fragments using electrophoresis
  • treat DNA to form single strands
  • DNA probe will bind
  • use autoradiography to show bound DNA probe