Gel electrophoresis
Gel electrophoresis is a technique used to separate large molecules, like DNA fragments according to their size.
An electric current
Passes through a gel
Where the separation
Takes place.
Molecules can be identified and
Isolated.
sample wells
O electrode
The gel is made from tangled polysaccharide polymer molecules immersed in a saline solution that conducts electrical charge.
Small DNA fragments pass more easily through the gel so travel further than large fragments.
Gel
Electrophoresis
Electricity moves DNA molecules.
The samples are loaded into rectangular wells in the gel at the negative electrode (usually shown at the top as gravity was used before electricity).
The DNA fragments are attracted to the positive electrode ...
... at the bottom of the gel.
Molecule size in kilobases
DNA ladder
TwO DNA
A DNA ladder is made using a mixture of DNA fragments of known sizes.
samples
Kilobases
10.0
DNA profiles
In the DNA of two unrelated people there will be many similarities and also some places where the DNA varies a lot.
Forensic scientists have developed PR primers which copy fragments of human
DNA from parts of the human genome which are often different.
To reduce the chance of a false match it is best to have several fragments.
The USA use a 20-fragment system, and some other countries use 17 or 18.
The sample shown with 5 fragments is obviously a simplification.
DNA profiles
The photograph of the electrophoresis gel shown here has many more DNA bands in each sample.
The samples used would have been prepared using restriction enzymes to cut
DNA, and multiple primers in the PCR which has amplified the DNA fragments.
How many DNA fragments are there?
In the gel shown DNA fragments that are 10000 bases long hardly move any distance.
DNA fragments with just 100 bases (0.1 kilobases) move to the bottom.
By comparing the position of bands in the gel the size of fragments in a sample can be estimated.
Key points
The basis of separation of DNA fragments in gel electrophoresis:
* A gel is set up in a rectangular box, with electrodes at each end.
* A positive electrode is put at the end of the gel furthest from the wells where the samples of DNA are loaded.
* A saline solution is poured onto the gel covering it and filling the wells.
* The samples are loaded into 'wells' in the gel just in front of the negative electrode.
* The electricity is turned on
* The negatively charged phosphates of the DNA fragments are arr to the +ve electrode.
* The smallest DNA fragments will travel farthest, while the longest fragments will remain closest to the origin.
Extension idea