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Electrophoresis

Gel electrophoresis is a method that separates macromolecules-either nucleic acids

or proteins-on the basis of size, electric charge, and other physical properties. Separation of

large (macro) molecules depends upon two forces: charge and mass. When a biological

sample, such as proteins or DNA, is mixed in a buffer solution and applied to a gel, these

two forces act together. The electrical current from one electrode repels the molecules

while the other electrode simultaneously attracts the molecules. The frictional force of the

gel material acts as a "molecular sieve," separating the molecules by size. During

electrophoresis, macromolecules are forced to move through the pores when the electrical

current is applied. Their rate of migration through the electric field depends on the strength

of the field, size and shape of the molecules, relative hydrophobicity of the samples, and on

the ionic strength and temperature of the buffer in which the molecules are moving. After

staining, the separated macromolecules in each lane can be seen in a series of bands spread

from one end of the gel to the other.

Proteins are amphoteric compounds; their net charge therefore is determined by the

pH of the medium in which they are suspended. In a solution with a pH above its

isoelectric point, a protein has a net negative charge and migrates towards the anode in an

electrical field. Below its isoelectric point, the protein is positively charged and migrates

towards the cathode. The net charge carried by a protein differs from protein to protein. At

a given pH therefore, and under non-denaturing conditions, the electrophoretic separation

of proteins is determined by both size and charge of the molecules.

Nucleic acids however, remain negative at any pH used for electrophoresis and in

addition carry a fixed negative charge per unit length of molecule, provided by the PO4

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group of each nucleotide of the nucleic acid. Electrophoretic separation of nucleic acids

therefore is strictly according to .size.

Support matrices: Generally the sample is run in a support matrix such as cellulose

acetate, starch gel, agarose or polyacrylamide gel. The most commonly used support

matrices, agarose and polyacrylamide, which provide a means of separating molecules by

size, in that they are porous gels. A porous gel may act as a sieve by retarding, or in some

cases completely obstructing, the movement of large macromolecules while allowing

smaller molecules to migrate freely. Agarose is used to separate larger macromolecules

such as nucleic acids, large proteins and protein complexes.

Polyacrylamide gel electrophoresis allows protein separation on non-reactive matrix

formed by acrylamide and N, Nmethylene bisacrylamide co-monomers that undergo free

radical mediated polymerization to form a meshwork of pores that satirically resist protein

migration. Polymerization is initiated by ammonium persulfate (radical source) and

catalyzed by TEMED (a free radical donor and acceptor). Polyacrylamide, which is easy to

handle and to make at higher concentrations, is used to separate most proteins and small

oligonucleotides that require a small gel pore size.