  Cytoplasmic dynein exhibits a directional processive movement on microtubule
filaments and is known to move in steps of varying length based on the number
of ATP molecules bound to it and the load that it carries. It is experimentally
observed that dynein takes occasional backward steps and the frequency of such
backward steps increases as the load approaches the stall force. Using a
stochastic process model, we investigate the bidirectional movement of single
head of a dynein motor. The probability for backward step is implemented based
on Crook's fluctuation theorem of non-equilibrium statistical mechanics. We
find that the movement of dynein motor is characterized with negative velocity
implying backward motion beyond stall force. We observe that the motor moves
backward for super stall forces by hydrolyzing the ATP exactly the same way as
it does while moving forward for sub stall forces.
