  We study the basepairing dynamics of DNA with repetitive sequences where
local strand slippage can create, annihilate, and move bulge loops. Using an
explicit theoretical model, we find a rich dynamical behavior as a function of
an applied shear force f: reptation-like dynamics at f=f_c with a rupture time
scaling as N^3 with its length N, drift-diffusion dynamics for f_c < f < f^*,
and a dynamical transition to an unraveling mode of strand separation at f =
f^*. We predict a viscoelastic behavior for periodic DNA with time and force
scales that can be programmed into its sequence.
