Figure 3: Two regimes of loop extrusion produce different conformations, consistent with interphase and mitosis.

A. Activity of a loop extruder: the complex loads, extrudes some amount of time after which it may dissociate, or is actively unloaded. B. During interphase (in vertebrates), cohesin is the main loop extrusion complex. It has a short residence time, generating a low density of transient loops, and the chromosomes appear diffuse in shape. Cohesin can be blocked by CTCF-bound sites, generating enrichment of positioned loops at these elements. C. During mitosis, condensins are the main loop extrusion complexes. Condensin II has a long residence time, generating stable arrays of consecutive loops that lead to compaction into the rod-shaped mitotic chromosomes. Condensin is not blocked by CTCF, and the loop array is not positioned at reproducible loci in the cell population. C. In bacteria, repeated loading of loop extruding complexes at defined loading sites can lead to juxtaposition of the chromosome arms, sequences on either side of the loading site.