• Gel electrophoresis is a technique commonly used in laboratories to separate charged molecules like DNA, RNA and proteins according to their size.
• Charged molecules move through a gel when an electric current is passed across it.
• An electric current is applied across the gel so that one end of the gel has a positive charge and the other end has a negative charge.
• The movement of charged molecules is called migration. Molecules migrate towards the opposite charge. A molecule with a negative charge will therefore be pulled towards the positive end (opposites attract!).
• The gel consists of a permeable matrix, a bit like a sieve, through which molecules can travel when an electric current is passed across it.
• Smaller molecules migrate through the gel more quickly and therefore travel further than larger fragments that migrate more slowly and therefore will travel a shorter distance. As a result the molecules are separated by size.

Gel electrophoresis and DNA

• Electrophoresis enables you to distinguish DNA fragments of different lengths.
• DNA is negatively charged, therefore, when an electric current is applied to the gel, DNA will migrate towards the positively charged electrode.
• Shorter strands of DNA move more quickly through the gel than longer strands resulting in the fragments being arranged in order of size.
• The use of dyes, fluorescent tags or radioactive labels enables the DNA on the gel to be seen after they have been separated. They will appear as bands on the gel.
• A DNA marker with fragments of known lengths is usually run through the gel at the same time as the samples.
• By comparing the bands of the DNA samples with those from the DNA marker, you can work out the approximate length of the DNA fragments in the samples.

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