Gel Electrophoresis in Forensics (Edexcel International A Level Biology)

• Gel electrophoresis is a technique used widely in the analysis of DNA, RNA and proteins
  • DNA fragments are created, e.g. using enzymes known as restriction endonucleases that cut DNA at specific restriction sites
  • The resulting fragments are inserted into a well at the end of a piece of agar gel, before a current is passed through the gel
• Molecules move through the agar due to the difference in charge across the gel
  • Positively charged molecules will move towards the cathode (negative pole) while negatively charged molecules will move towards the anode (positive pole)
  • DNA is negatively charged due to the phosphate groups and so when placed in an electric field the molecules move towards the anode
• The molecules are separated according to their size / mass 
  • Different sized molecules move through the gel at different rates
  • The tiny pores in the gel allow smaller molecules to move quickly, whereas larger molecules move more slowly

• The process of gel electrophoresis involves the following stages
  • An agarose gel plate is created and wells are cut into the gel at one end
  • The gel is submerged in a tank containing electrolyte solution; this is a salt solution that conducts electricity
  • The DNA samples are transferred into the wells using a micropipette, ensuring that a sample of DNA standard is loaded into the first well
    • The purpose of the standard is to produce a set of known results with which to compare any new results 
  • The negative electrode is connected to the end of the plate with the wells and the positive anode is connected at the far end
    • The DNA fragments move towards the anode due to the attraction between the negatively charged phosphates of DNA and the anode
    • The smaller mass / shorter pieces of DNA fragments move faster and therefore further from the wells than the larger fragments
  • Probes are then added, after which an X-ray image is taken or UV-light is shone onto the paper producing a pattern of bands which can be compared to the control, or standard, fragments of DNA
    • Probes are single-stranded DNA sequences that are complementary to the regions of interest; they can be
      • A radioactive label which causes the probes to emit radiation that makes the X-ray film go dark, creating a pattern of dark bands
      • A fluorescent dye which fluoresces when exposed to UV light, creating a pattern of coloured bands

Gel electrophoresis can be used in DNA profiling

Analysing the results of gel electrophoresis

• Gel electrophoresis produces a pattern of bands on the gel that represent DNA fragments of different length
  • The fragments were produced after PCR by cutting the DNA samples into pieces using restriction endonuclease enzymes
  • Restriction endonucleases cut DNA at specific locations in the DNA base sequence, so will always cut in between sections of repeated bases known as variable number tandem repeats (VNTRs)
    • VNTRs are known as micro- or mini-satellites depending on the number of repeats that occur; micro-satellites have fewer repeats than mini-satellites
  • Different people have different numbers of repeats in their VNTR regions, so the fragments will differ in length depending on whether there are few or many repeats
• Different individuals will have different lengths of DNA fragments, so a different pattern of banding will form on each profile
• Every banding pattern will be unique to an individual, so comparisons of DNA from crime scenes with that of suspects is a reliable way of finding out who was present at a crime scene