Gel electrophoresis is a laboratory technique that separates DNA, RNA, and proteins by their movement through a porous gel under an electric field. The gel acts as a molecular sieve, while differences in size, electrical charge, and shape influence migration. Separated components appear as bands or spots after detection. The technique supports both analytical measurements and the recovery of selected molecules for further study. (genome.gov)
Physical principles
Electrophoresis is the movement of charged particles through a medium under an electric field. Negatively charged molecules migrate toward the positive electrode, or anode; positively charged molecules migrate toward the negative electrode, or cathode. The porous gel restricts movement, generally slowing larger molecules more strongly than smaller ones under comparable conditions. Consequently, separation reflects both electrical mobility and interactions with the gel network. (genome.gov)
A buffer solution supplies conducting ions and helps maintain a controlled pH. Buffer composition, gel concentration, field strength, and run duration affect resolution. Higher gel concentrations usually improve separation of smaller nucleic acid fragments, whereas lower concentrations accommodate larger fragments. Excessive electrical power can cause heating and distorted migration rather than simply accelerating a useful separation. (bio-rad.com)
Gel materials
Agarose and polyacrylamide are the principal gel materials. Agarose forms a hydrated network when a heated solution cools and is commonly used for routine nucleic acid separation. Its concentration controls the useful fragment-size range. Agarose gels are typically cast as horizontal slabs containing wells into which samples are loaded. (thermofisher.com)
Polyacrylamide is a cross-linked polymer with a pore structure controlled by its formulation. Polyacrylamide gel electrophoresis, abbreviated PAGE, is extensively used for proteins and also for relatively short nucleic acids requiring high resolution. Protein PAGE commonly uses vertical slabs held between plates. Gradient gels contain increasing polymer concentrations along the migration path, extending the range of molecular sizes resolved within one gel. (thermofisher.com)
Nucleic acid separation
DNA and RNA possess negatively charged phosphate backbones and ordinarily migrate toward the anode. For linear DNA fragments of comparable structure, migration through a suitable gel depends predominantly on length, with shorter fragments generally moving farther during a fixed run. A molecular-size ladder containing fragments of known length provides a reference for estimating sample fragment sizes. (thermofisher.com)
Molecular conformation complicates this relationship. A plasmid may occur as supercoiled, nicked circular, or linear DNA, and these forms can migrate differently despite containing the same number of base pairs. A linear DNA ladder therefore does not necessarily give an accurate size estimate for an uncut plasmid. Denaturing conditions can reduce the influence of secondary structure when separating single-stranded nucleic acids. (thermofisher.com)
Pulsed-field gel electrophoresis separates very large DNA molecules by periodically changing the field direction. Molecules reorient at different rates, allowing separation of fragments that conventional, fixed-direction electrophoresis resolves poorly. This approach can resolve DNA extending into the megabase range. (bio-rad.com)
Protein separation
In SDS-PAGE, sodium dodecyl sulfate denatures proteins and binds to their polypeptide chains, producing an approximately uniform charge-to-mass ratio. Separation therefore depends mainly on molecular size. Reducing agents may additionally break disulfide bonds, allowing disulfide-linked chains to separate. Nonreducing SDS-PAGE omits this reduction but remains a denaturing method. Migration relative to protein standards gives an apparent molecular mass rather than an exact measurement. (thermofisher.com)
Native PAGE avoids the strongly denaturing conditions of SDS-PAGE and can retain protein complexes and some biological activities. Migration depends jointly on charge, size, and shape, so band position cannot ordinarily be interpreted as molecular mass alone. The method can examine associations between protein subunits or preserve samples for subsequent activity measurements. (thermofisher.com)
Two-dimensional gel electrophoresis combines two separation principles. Typically, isoelectric focusing first separates proteins along a pH gradient according to their isoelectric points, where their net charge is zero. SDS-PAGE then separates them by apparent molecular mass in a perpendicular direction. The resulting spot pattern provides a means of resolving complex mixtures in proteomics. (bio-rad.com)
Detection and applications
A typical workflow includes gel preparation, sample and ladder preparation, electrophoresis, visualization, and image documentation. Loading solutions often contain a density-increasing component and a tracking dye; the dye indicates run progress but does not itself identify the sample molecules. Nucleic acid stains are commonly detected using ultraviolet or blue-light illumination, depending on the dye. (thermofisher.com)
Proteins can be visualized with Coomassie dyes, silver staining, or fluorescent stains. These methods differ in sensitivity, quantitative range, and compatibility with subsequent analysis. Band intensity can estimate relative abundance within a validated detection range, but staining and imaging conditions influence the measurement. (thermofisher.cn)
Applications include checking PCR products, examining nucleic acid fragmentation, isolating selected DNA fragments, and evaluating protein preparations. Separated proteins may be transferred to a membrane for Western blotting, or excised and processed for identification by mass spectrometry. (thermofisher.com)
Interpretation and limitations
A band is not necessarily a single molecular species: different molecules can share similar mobility. Conversely, one DNA species can produce several bands through different conformations. Smearing or distorted bands may arise from sample overloading, excessive salt, unsuitable buffers, or inappropriate running conditions. Results therefore depend on the separation system and reference standards, not solely on the visible number or positions of bands. (thermofisher.com)