8.4.3 Genetic fingerprinting

• Variable number tandem repeats (VNTRs)

  ● Repeating sequences of nucleotides/bases (eg. GATA) found within non-coding sections of DNA at

  many sites throughout the genome

  ● Allow production of a virtually unique genetic fingerprint:

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

  ○ Because an organism’s genome contains many VNTRs and lengths differ between individuals
• • The technique of genetic fingerprinting in analysing DNA fragments (VNTRs)

  Students should be able to explain the biological principles that underpin genetic fingerprinting techniques.

  1. Amplification
  2. ● Extract DNA from sample eg. blood / cheek cells
  3. ● Amplify by the polymerase chain reaction (PCR)
• 2. Digestion

  ● Use restriction endonucleases to cut DNA into smaller fragments

  ● Which cut at specific sites near VNTRs, leaving VNTRs intact
• 3. Separation

  ● Separate DNA/VNTRs by gel electrophoresis

  ● According to length → shorter VNTRs travel further
• 4. Hybridisation

  ● Transfer from gel to nylon membrane (Southern blotting)

  ● Treat DNA to separate strands / make single-stranded (eg. add alkaline)

  ● Apply DNA probes complementary to VNTRs with a radioactive / fluorescent label
• 5. Development ● Use autoradiography (place on X ray film) or use UV light to visualise banding patterns
• Principles of gel electrophoresis - separating nucleic acid (DNA/RNA) fragments (or proteins) by size

  1. Load DNA samples into wells in porous gel and

  cover in buffer solution (conducts electricity)

  2. Pass electrical current through → DNA is

  negatively charged so moves towards positive

  electrode

  3. Shorter DNA fragments travel faster so travel

  further, ie. separated according to size
• The use of genetic fingerprinting in determining genetic relationships and in determining the genetic variability within a population

  Determining genetic relationships

  ● More closely related → more similar VNTRs → more similarities in genetic fingerprints

  ● Paternity testing - father should share around 50% of VNTRs/bands and ones that don't come from mother must be from father
• Determining genetic variability within a population

  • Differences in VNTRs arise from mutations, 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
• 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 analyse DNA fragments

  • 5. Add labelled DNA probes which hybridise / bind with complementary VNTRs (& wash to remove
  • unbound probe)
  • 6. To show bound probe, expose membrane to UV light if a fluorescently labelled probe was used
  • OR use autoradiography (expose to X-ray film) if a radioactive probe was used