  A theory of the unzipping of double-stranded (ds) DNA is presented, and is
compared to recent micromanipulation experiments. It is shown that the
interactions which stabilize the double helix and the elastic rigidity of
single strands (ss) simply determine the sequence dependent =12 pN force
threshold for DNA strand separation. Using a semi-microscopic model of the
binding between nucleotide strands, we show that the greater rigidity of the
strands when formed into dsDNA, relative to that of isolated strands, gives
rise to a potential barrier to unzipping. The effects of this barrier are
derived analytically. The force to keep the extremities of the molecule at a
fixed distance, the kinetic rates for strand unpairing at fixed applied force,
and the rupture force as a function of loading rate are calculated. The
dependence of the kinetics and of the rupture force on molecule length is also
analyzed.
