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:
- Thin nylon membrane is laid over the gel
- Membrane is covered with several sheets of absorbent paper (which draws up the liquid contains the DNA by capillary action)
- This transfers the DNA fragments to the nylon membrane in precisely the same relative positions that they occupied on the gel
- 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