Gel electrophoresis
A method to sort DNA pieces by size. DNA is negatively charged, so in an electric field it moves through a gel toward the positive end. Small pieces move faster and further.
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Analogy
Running through a crowded forest: small runners get through quicker.
In practice
Comparing the band patterns of DNA cut with restriction enzymes shows similar or different samples, as in DNA profiling.
Practice
Flashcards
- Which DNA fragments travel furthest in a gel?The smallest ones.
Interview questions
- Explain the three steps of a PCR cycle and why the technique is so useful.
- Denaturation separates strands
- Annealing lets primers bind
- Extension by heat-stable polymerase
- Number of copies doubles each cycle
- Makes tiny samples testable
Questions people ask
Why does DNA move through the gel?
The phosphate groups in DNA carry negative charge, so DNA is pulled toward the positive electrode when voltage is applied. The gel is a mesh that slows long pieces more than short ones. Because every base pair adds about the same charge, size alone sorts the fragments.
How are DNA bands made visible?
The gel is stained with a dye that sticks to DNA and glows under ultraviolet or blue light, or the DNA is labelled in advance with a fluorescent tag. A ladder of fragments of known sizes is run beside the samples so that the length of each band can be estimated.