Figure 1.

Cartoon representation of the loop extrusion model. Extruders (a) and (b) have already produced large loops; (c) and (d) are only starting. Extruders (e–g) are presently disconnected from the fiber; they diffuse around, and will initiate new loops once they absorb somewhere on the fiber. Extruder (a) is already approaching the (mirror symmetric) stop signs, while (b) and (d) have a long way to go. (Black arrows) Directions in which fiber is pushed by every extruder. Two large loops are shown to collide in three dimensions. To see this figure in color, go online.