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Sodium-dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) is an analytical technique widely used to study proteins.
A mixture of proteins is exposed to SDS which is an anionic detergent. In the presence of the detergent, proteins are denatured and lose their secondary and tertiary structures. They look like ropes the length of which depends on the number of amino acids of the primary structure of the proteins.
SDS also loads all the proteins with negative charges and, when submitted to an electrical field, the charged proteins migrate to the anode.
In conclusion the proteins present in the mixture only diverge with respect to their molecular weight, not to their charge.
The gel used to perform the electrophoresis is made of long chains of polyacrylamide which have been cross-linked by bisacrylamide. The ratio between bisacrylamide and acrylamide used determines the size of the pores of the gel. During the synthesis of the gel, this ratio is not constant but increases, which explains why the pores of the gel become narrower. The gel has been formed in such a way that during their migration towards the anode, the proteins have to pass through the denser part of the gel and, finally are stopped by pores narrower than their size. The bands seen after staining of the gels correspond to population of proteins with similar molecular weight.
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