Figure 3. Loop extrusion as a topologically associating domain generating machine.

( A) The chromatin fiber extrudes over time through an extruding factor (possibly cohesin; yellow cylinders). ( B) A boundary element (possibly CTCF, green cube) can block loop extrusion when the CTCF-binding element is in the proper orientation. It has been proposed that CTCF can block extrusion by one of the cohesin extruding motors while the second motor will be unobstructed and continue to extrude the loop ¹⁰². ( C) Regulatory elements may come into close molecular contact by the process of loop extrusion. These interactions will occur only within a topologically associating domain and in the presence of extrusion blocking elements.