Genetic Fingerprinting

  • The genome of any organism contains many repetitive non-coding bases of DNA
    • Approximately 95% of human DNA does not code for any characteristics
    • Non-coding sequences – introns
  • For every individual the number and length of core sequences has a unique pattern
    • Except for identical twins
  • The more closely related two individuals are the more similar the core sequence will be

Stage 1: Extraction

  • Small samples (e.g. drop of blood, hair root etc.) is adequate
  • To extract the DNA, it is separated from the rest of the cell
  • As the amount of DNA is usually small the quantity must be increased through PCR

Stage 2: Digestion

  • DNA is cut into fragments using restriction endonucleases
  • Endonucleases are chosen for their ability to cut close to groups of core sequences

Stage 3: Separation

  • Fragments of DNA are next separated according to size by gel electrophoresis under the influence of an electrical voltage
  • Gel is then immersed in alkali in order to separate the double strands into single strands
  • Single strands are ten transferred onto a nylon membrane through Southern Blotting:
    1. Thin nylon membrane is laid over the gel
    2. Membrane is covered with several sheets of absorbent paper (which draws up the liquid contains the DNA by capillary action)
    3. This transfers the DNA fragments to the nylon membrane in precisely the same relative positions that they occupied on the gel
    4. DNA fragment are then fixed to the membrane using UV light

Stage 4: Hybridisation

  • DNA probes (i.e. radioactive/fluorescent) are now used to bind with the core sequences
  • Probes have bas sequences which are complementary to the core sequences and bind to them under specific conditions (temperature and pH)
  • Process is carried out with different probes with each binding to a different core sequence

Stage 5: Development

  • An x-ray film is placed over the nylon membrane
  • Film is exposed by radiation from the radioactive probes (or if fluorescent proves then the position is located visually)
  • As these points correspond to the position of the DNA fragments as separated during electrophoresis, a series of bars can be seen
  • The pattern of these bands is unique to every individual