  Condensation of hundreds of mega-base-pair-long human chromosomes in a small
nuclear volume is a spectacular phenomenon. This process is driven by the
formation of chromosome loops. ATP consuming motor, condensin, interacts with
chromatin segments to actively extrude loops. Motivated by real-time imaging of
loop extrusion (LE) and measurements using magnetic tweezer experiments, we
created an analytically solvable model, predicting the LE velocity and step
size distribution as a function of external load. The theory fits the
experimental data quantitatively, and suggests that condensin must undergo a
large conformational change, induced by ATP binding, bringing distant parts of
the motor to proximity. Simulations using a simple model confirm that the motor
transitions between an open to closed state in order to extrude loops by a
scrunching mechanism, similar to that proposed in DNA bubble formation during
bacterial transcription. Changes in the orientation of the motor domains are
transmitted over $\sim$ 50 nm, connecting the motor head and the hinge, thus
providing an allosteric basis for LE.
