  A DNA-protein complex modelled by a semiflexible chain and an attractive
spherical core is studied in the situation when an external stretching force is
acting on one end monomer of the chain while the other end monomer is kept
fixed in space. Without stretching force, the chain is wrapped around the core.
By applying an external stretching force, unwrapping of the complex is induced.
We study the statics and the dynamics of the unwrapping process by computer
simulation and simple phenomenological theory. We find two different scenarios
depending on the chain stiffness: For a flexible chain, the extension of the
complex scales linearly with the external force applied. The sphere-chain
complex is disordered, i.e. there is no clear winding of the chain around the
sphere. For a stiff chain, on the other hand, the complex structure is ordered,
which is reminiscent to nucleosome. There is a clear winding number and the
unwrapping process under external stretching is discontinuous with jumps of the
distance-force curve. This is associated to discrete unwinding processes of the
complex. Our predictions are of relevance for experiments, which measure
force-extension curves of DNA-protein complexes, such as nucleosome, using
optical tweezers.
