DNA Profiling
The Human Genome
Genome: All of an organism's genetic material.
Exons: The 2% of DNA which is used for coding proteins.
Introns: The 98% of DNA which does not code for proteins.
Satellite DNA: A section of DNA within an intron, telomere or centromere in which
a section of DNA is repeated many times.
Minisatellites/VNTRs: A sequence of 20-30 base pairs which is repeated from 50-100s of times. (Variable Number Tandem Repeats).
Microsatellite/STRs: A sequence of 2-4 base pairs which is repeated from 5-15 times. (Short tandem repeats).
DNA Profiling: Producing an image of the patterns of an individuals DNA, used to identify individuals or family relationships.
- Satellites always occur in the same place on chromosomes, but vary in lengths due to different satellites being inherited from the parent.
- Only identical twins have identical satellites, but family relationships can be established by the similarities between DNA profiles.
- Sir Alec Jeffreys discovered the patterns in non-coding DNA in 1984.
Electrophoresis
- DNA fragments are put into wells in agarose gel strips. A 'ladder' of known fragments are often put into the first or last wells to help identify different satellites.
- When an electrical current passes through the electrophoresis plate, the negatively charged phosphate groups in sugar-phosphate backbone means the fragments move away from the negative cathode and towards the positive anode.
- The mesh-like structure of the gel provides resistance for the movement of DNA through the gel, meaning the fragments spread out with smaller fragments moving further through than the larger fragments.
- The electrical current must be switched off before the fragments reach the end of the gel in order to be able to take a reading.
- The gel is placed into an alkaline buffer solution to denature the DNA fragments and expose the bases.
- Southern blotting is then done, in which the strands are transferred to nitrocellulose paper or a nylon membrane by placing it over the top of the gel, and placing absorbent paper over the top of the membrane.
- This means the alkaline buffer is absorbed by capillary action, but the DNA fragments remain on the membrane.
- The fragments are then fixed in place using UV light or heating to 80 degrees, and can then be examined.
Polymerase Chain Reaction (PCR)
- PCR is the method by which DNA samples are replicated into large quantities for gel electrophoresis and comparison.
- DNA sample, free nucleotides (in the form of deoxynucleoside triphosphates), small primer DNA sequences and DNA taq polymerase are mixed in a vial and placed in a PCR machine/thermal cycler.
- Temperature in a PCR machine is strictly controlled and rapidly changes at programmed intervals.
- The process cycles to produce billions of copies of the original sample.
- Primers are required to anneal to the complementary strand to allow polymerisation, as the strands won't just being joining automatically without it.
Uses of DNA Profiling
- Used to identify criminals in forensic science by doing PCR and DNA profiling on traces of DNA found at crime scenes.
- DNA profile is compared to that of ones on a criminal DNA database.
- Mis-identification of criminals can lead to miscarriages of justice. Raymond Easton was in the advanced stages of Parkinson's but charged for a burglary 200 miles from his home because of the coincidental, yet incredibly unlikely, match of 6 gene loci to the actual burglar.
- DNA profiling can be used to prove relationships where paternity is uncertain, for immigration cases or to see which species and organism belongs to (I think the latter is what they test on the Jeremy Kyle show).
- Can be used to identify individuals at a risk of particular diseases, as there is a correlation between intron satellites and certain diseases.