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• Gel electrophoresis


  A laboratory technique used to separate and identify segments of DNA
• DNA Fingerprint/DNA Profile

  • Analysis of the pattern of DNA fragments enables us to discriminate between suspects accused of a crime or potential fathers in a paternity suit
• How Gel Electrophoresis is Done

  1. Segments of DNA are isolated or cut from chromosomes using restriction enzymes

  2. DNA fragments are applied to a thin layer of agarose gel

  3. An electric field is applied, making one end of the gel positive and the other end negative

  4. DNA segments remain in their final positions after the electrical field is shut off

  5. DNA segments are identified by their reaction with radioactive chemicals
• Restriction enzymes


  Bacterial proteins that can cut both strands of the DNA molecule at a specific nucleotide sequence
• Restriction enzymes and their cutting patterns

  • EcoR1 (GAATTC/CTTAAG)
  • BamHI (GGATCC/CCTAGG)
  • HindIII (AAGCTT/TTCGAA)
  • KpnI (GGTACC/CCATGG)
• DNA Fingerprints are highly specific, the chances of two people having exactly the same DNA profile is 30,000 million to 1 (except for identical twins)
• Biological materials used for DNA fingerprinting

  • Blood
  • Hair
  • Saliva
  • Semen
  • Body tissue cells
• Example of DNA fingerprinting in a murder case

  • Forensics team retrieved a blood sample from the crime scene and compared the DNA fingerprint to several suspects
• Preparation of gel

  1. 1% w/v agar gel is prepared using 100ml of working buffer and 1g agar powder

  2. Solution is heated in microwave to dissolve agar, then allowed to cool to 60°C

  3. Sealed casting tray is poured with cooled agar solution

  4. Combs are removed once gel has set

  5. Gel is positioned in electrophoresis chamber
• Dyes used in gel electrophoresis

  • Bromophenol Blue, Methyl Green, Orange G, Indigo Carmine, Rose Bengal, Tartrazine, Xylene Cyanol
  • Dyes are made up to 0.2% w/v in 20% glycerol/distilled water
  • Ladder can be prepared by mixing dyes for reference
  • Most dyes are negatively charged like DNA, except Methyl Green which is positively charged
• Loading the gel

  1. Gel is placed in electrophoresis chamber with wells closer to negative terminal

  2. Chamber is flooded with working buffer solution

  3. 10μL of sample is transferred into wells using a micropipette
• Running the gel

  1. Power supply is connected, set to 100V, and turned on

  2. Entire run takes approximately 30-40 minutes

  3. Progress is monitored by observing position of dyes, power is turned off once sufficiently separated

  4. Photo of gel is taken shortly after run is complete
• Never remove the lid from the gel tank while the unit is still connected to the power supply
• Interpreting results of gel electrophoresis involves analyzing the pattern of DNA bands