= Gel electrophoresis
{wiki}
Separation of charged <molecules> through a gel in an <electric field>. Denaturing conditions suppress folded structures, helping compare <RNA> lengths; matching migration alone does not establish an identical <nucleotide sequence> or identical <covalent bonds>. In the <ribozyme> experiments, labelled <RNA> was detected after separation by exposing photographic film.
In <DNA> <agarose gel electrophoresis>, the backbone <phosphate> groups carry negative <electric charge> under the usual mildly alkaline <buffer solution> conditions. <DNA> therefore moves toward the positive <anode>, away from the negative <cathode>.<Agarose Gel Electrophoresis by Addgene>[Introduction; Loading Samples and Running an Agarose Gel, step 6: DNA migration toward the positive electrode]{ref}
The <electric field> points the other way: the <electric force> $\mathbf F=q\mathbf E$ reverses direction when $q<0$. The apparatus is driven by an external source of <electric potential difference>; the positive <anode> convention here belongs to that operating mode. Direction follows <electric charge>, so it is not a rule that all <molecules> move toward the same <electrode>.
The useful result is a spatial comparison: a mixture becomes separated <gel bands>, which can be compared with a <DNA ladder> or recovered for subsequent analysis. <Agarose gel electrophoresis> of linear double-stranded <DNA>, <gel electrophoresis> of denatured <RNA>, and <SDS-PAGE> of <proteins> use different conditions to make migration informative about chain size. An <electric field> supplies motion; the gel supplies a porous obstacle course.
\Image[/gel-electrophoresis-polarity-and-agarose-sieving.png]
{title=<DNA> migrates toward the positive <anode>, opposite to the <electric field>; the <agarose> mesh separates linear <DNA> fragments. Conceptual apparatus and mesh, not measured trajectories or a drawing to scale}
Back to article page
