Kinds of Macromolecules in Gel Electrophoresis

• DNA is a macromolecule often analyzed with gel electrophoresis. human gene image by Keith Frith from Fotolia.com

  Gel electrophoresis is a technique for separating biological macromolecules on the basis of their size. Solutions containing the macromolecules are loaded into wells in a slab of agarose or polyacrylamide gel; an electric field created by electrodes at either end causes the macromolecules to migrate through the gel. Smaller molecules will migrate more quickly, so the molecules will become sorted by size. There are three kinds of molecules typically studied in gel electrophoresis.

DNA

• Deoxyribonucleic acid (DNA) is a molecule that contains coded genetic instructions and is found in all known forms of life, with the exception of some viruses that use RNA instead. Molecules of DNA are polymers composed of chains of subunits called nucleotides. A nucleotide is made of a sugar and phosphate group attached to a base. There are four different bases found in DNA nucleotides, commonly abbreviated A, T, G, and C. Most current DNA sequencing techniques involve gel electrophoresis, as does DNA fingerprinting in forensics.

Protein

• Proteins are biological macromolecules formed from chains of amino acids. The order of the bases in a gene in DNA codes for the order of the amino acids in a given protein. Proteins are the molecular workhorses of the cell and perform a wide variety of different functions, ranging from transporting oxygen in your bloodstream to generating the force that causes your muscles to contract. Scientists often use gel electrophoresis to separate proteins based on their size or on the pH at which they have no net charge. One common gel electrophoresis-based technique to determine whether a sample contains protein is called Western blotting.

RNA

• Like DNA, RNA is a nucleic acid, a polymer composed of a chain of nucleotides. The nucleotides in RNA, however, contain a ribose sugar rather than a deoxyribose sugar, and the base T is replaced with the base U. Genes in DNA are transcribed to make RNA molecules that a cell will use to make a protein. Scientists can use gel electrophoresis to study RNA molecules extracted from cells and thereby determine whether a given gene is expressed in that cell. One such technique is called Southern blotting.