<P> Nucleic acid electrophoresis is an analytical technique used to separate DNA or RNA fragments by size and reactivity . Nucleic acid molecules which are to be analyzed are set upon a viscous medium, the gel, where an electric field induces the nucleic acids (which are negatively charged due to their sugar - phosphate backbone) to migrate toward the anode (which is positively charged because this is an electrolytic rather than galvanic cell). The separation of these fragments is accomplished by exploiting the mobilities with which different sized molecules are able to pass through the gel . Longer molecules migrate more slowly because they experience more resistance within the gel . Because the size of the molecule affects its mobility, smaller fragments end up nearer to the anode than longer ones in a given period . After some time, the voltage is removed and the fragmentation gradient is analyzed . For larger separations between similar sized fragments, either the voltage or run time can be increased . Extended runs across a low voltage gel yield the most accurate resolution . Voltage is, however, not the sole factor in determining electrophoresis of nucleic acids . </P> <P> The nucleic acid to be separated can be prepared in several ways before separation by electrophoresis . In the case of large DNA molecules, the DNA is frequently cut into smaller fragments using a DNA restriction endonuclease (or restriction enzyme). In other instances, such as PCR amplified samples, enzymes present in the sample that might affect the separation of the molecules are removed through various means before analysis . Once the nucleic acid is properly prepared, the samples of the nucleic acid solution are placed in the wells of the gel and a voltage is applied across the gel for a specified amount of time . </P>

Why do longer dna molecules move more slowly than shorter ones during electrophoresis