Separation of DNA fragments by electrophoresis through an agarose gel. An electric current is passed through the gel and separates the DNA fragments according to size. The mixture of fragments is applied to a well at the top of the gel before the current is started. The smaller fragments travel further and reach the bottom of the gel while the larger ones remain towards the origin.

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Credit: Separation of DNA fragments by electrophoresis through an agarose gel. An electric current is passed through the gel and separates the DNA fragments according to size. The mixture of fragments is applied to a well at the top of the gel before the current is started. The smaller fragments travel further and reach the bottom of the gel while the larger ones remain towards the origin. Guy Tear. Source: Wellcome Collection.