We present quantitative results for the drift velocity of a polymer in a gel
if a force (e.g. through an electric or magnetic field) acts on a tag, attached
to one of its ends. This is done by introducing a modification of the
Rubinstein-Duke model for electrophoresis of DNA. We analyze this modified
model with exact and Monte Carlo calculations. Tagged magnetophoresis does not
show band collapse, a phenomenon that limits the applicability of traditional
electrophoresis to short polymers.Comment: 10 pages revtex, 3 PostScript figure
