Gel Electrophoresis in Forensics (Edexcel International A Level Biology)

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Gel Electrophoresis in Forensics

  • 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

Exam Tip

Don't get too bogged down in the details regarding restriction enzymes and VNTRs, but you should be able to explain how gel electrophoresis works to separate DNA fragments of different length.

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Author: Marlene

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.