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Gel Electrophoresis

Biology: Cambridge International A Level, topic 19.1.3

Gel Electrophoresis

Gel electrophoresis is the technique used to separate nucleic acid fragments or proteins from each other, in order to form a genetic fingerprint. The steps involved in this technique are:

1) Amplification

1) Amplification

  • The DNA sample is extracted from the individual. This is done by taking a swab inside someone's mouth or taking a blood sample.
  • The DNA sample is amplified many times using PCR (polymerase chain reaction).
  • PCR generates many copies of the same sample.
2) Labelling

2) Labelling

  • The DNA fragments produced from PCR are labelled using a fluorescent label.
  • The label allows the DNA fragments to be identified when exposed to UV light.
3) Inserting the DNA

3) Inserting the DNA

  • The many DNA fragments are inserted into a well in a gel.
  • The gel is covered in a buffer solution that conducts electricity with a positively charged electrode at one end of the gel and a negatively charged electrode at the other end.
  • DNA is inserted at the negative end of the gel.
4) Movement of DNA

4) Movement of DNA

  • DNA is negatively charged so when an electric current is passed through the gel, the DNA will move away from the negative electrode towards the positive electrode.
  • Smaller DNA fragments will move through the gel more quickly and travel further than larger fragments.
5) Ladder of DNA

5) Ladder of DNA

  • The electric current is removed after approximately 10 minutes.
  • The DNA fragments that are different lengths in a sample will have moved differing distances up the gel.
  • The presence of DNA fragments in the gel form bands of DNA.
  • The different bands of DNA in an individual sample produce a 'ladder' of DNA.
6) Identifying DNA fragments

6) Identifying DNA fragments

  • The different DNA fragments in a sample can be identified by exposing the gel to UV light.
  • The DNA fragments are fluorescently labelled so UV light shows the bands of DNA present.
7) Genetic fingerprinting

7) Genetic fingerprinting

  • The lengths of DNA fragments are determined by the number of repeats in a VNTR (variable number tandem repeat), which is a section of DNA.
  • The number of repeats varies between individuals.
  • This means the DNA fragments in an individual will move different distances and the ladder of DNA for every individual will be unique.
  • The ladder of DNA in an individual is considered their genetic fingerprint.

Test yourself on Gel Electrophoresis

  • Stages in Gel Electrophoresis

  • In which direction does DNA move in gel electrophoresis?

  • Which of the following statements about gel electrophoresis are true?

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