At its simplest and for low voltages then if you increase the voltage you will increase the speed at which dna travels through a gel. The speed depends on the size of the dna, the charge on the dna,the type and concentration of buffer and the concentration of the agarose or PAG in the gel. If the voltage gets too high then heating occurs and gels can melt and dna conformations change and dna separation gets very messy. There is no simple answer to this question but in 1-2% agarose and tris buffer a voltage of about 5V/cm of gel separates dna fragments in the range of 50-10,000 bases effectively
Ladders are a number of proprietary DNA plasmids digested with appropriate restriction enzymes to yield multiple bands suitable for use as molecular weight standards for gel electrophoresis specifically created for quantitative estimation of DNA size/mass in gels.
A molecule with a net charge will migrate in an electric field. This phenomenon, termed electrophoresis, offers a powerful means of separating macromolecules, such as proteins, DNA and RNA. The velocity of migration (v) of a molecule in an electric field depends on the electric field strength (E) and on the electrophoretic mobility (μ) of the molecule,
Equation 1: v = μE
The electrophoretic mobility is a parameter unique for each molecule and each medium. Electrophoretic separation is nearly always carried out in gels due to their ability to serve as a molecular sieve that enhances separation by modifying μ. Molecules that are small compared to the pores in the gel readily move through the gel, whereas molecules much larger than the pores are almost immobile. Intermediate- size molecules move through the gel with various degrees of facility.
In the case of gel electrophoresis, the following relation is found:
Equation 2: log μ = log μ0 − Krτ
where μ0 is the free electrophoretic mobility of the molecule (mobility in a non-sieving medium), Kr the retardation coefficient and τ the concentration of the gel. μ0 is dependant on the mass-to-charge ratio of the molecule, whereas Kr is related to the propriety of the gel, the size and the shape of the migrating molecule.
Hope these helps!
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