Figure 4. Chromosome formation by condensin loop extrusion.

(A) At the start of mitosis in prophase, before nuclear envelope breakdown, condensin II is able to start loading on chromatin, resulting in the formation of large loops. (B) After nuclear envelope break down in metaphase, condensin I gains access to DNA, and forms multiple loops nested within the larger condensin II loops. Some condensin I loops could take the form of Z-loops.