Chromatin compaction from interphase to mitosis is still not fully understood. In the last decade loop extrusion performed by condensins has become the main hypothetical mechanism to explain the formation of the highly compacted cylindrical chromosomes visible during mitosis. However, it is now evident that SMC complexes, such as condensins, not only load onto chromatin to extrude loops, but they also create bridges between distant loci. In this talk I will explain an alternative model we developed, where the compaction is driven by the interplay between two types of condensin-like proteins: one stabilising loops and the other binding multivalently to chromatin to form bridges.
Characteristics of mitotic chromosomes observed by experiments arise naturally in our simulations, which then complement models based solely on loop extrusion.