Fig. 6. Schematic model of how transcription and replication control cohesin loop extrusion boundaries.

Loop-extruding cohesin complexes (green open circles) are blocked by head-on collisions with transcribing RNA polymerases in convergently oriented genes (yellow arrows), which thereby constitute loop extrusion boundaries. Stalled replication forks (red arrows) also create chromosome interaction boundaries by blocking loop-extruding cohesin complexes. The resulting positioning of cohesin complexes in the vicinity of the polymerases might underlie cohesin’s functions in transcription regulation and fork stability. Black dashed arrows indicate the direction of cohesin movement along chromosomes during extrusion.