Gel electrophoresis is a standard molecular biological technique for
separating nucleic acids and proteins by size. DNA, RNA or
protein samples are placed in a special gel and subjected to an
electric field. Negatively-charged nucleic acid fragments or proteins
move toward the positive electrode while positively-charged ones move
toward the negative electrode. Those fragments or molecules that are
smallest travel farthest from their original location in the gel, while
the largest ones remain closest to the origin. After the samples have
been separated, the resulting bands are stained with dyes such as
ethidium bromide or methylene blue so that they can be seen.
Fragments of DNA from 200 to 50,000 base pair (bp) can be separated
using standard electrophoresis techniques. DNA strands have a
phosphate-sugar backbone with two negatively-charged phosphate
compounds per base pair.
The molecular weight of an average base pair is 635 daltons (a dalton
is the mass of
a single hydrogen atom, or a proton). The molecular weight of DNA
in a haploid nucleus is 1.9x10 12
daltons. Typical DNA analysis using gel electrophoresis takes 30
to 60
minutes. With this information, what is the net force on a DNA
fragment as it migrates toward the positive electrode?