Non-specific (entropic) forces as major determinants of the structure of mammalian chromosomes.

Four specific forces (H-bonds, van der Waals forces, hydrophobic and charge interactions) shape the structure of proteins, and many biologists assume they will determine the shape of all structures in the cell. However, as the mass and contour length of a human chromosome are ~7 orders of magnitude larger than those of a typical protein, additional forces can become significant. We review evidence that additional non-specific (entropic) forces are major determinants of chromosomal shape and position. They are sufficient to drive the segregation (de-mixing) of newly replicated DNA to the poles of bacterial cells, while an entropic centrifuge can both form human chromosomes into territories and position them appropriately in nuclei; more locally, a depletion attraction can loop bacterial and human genomes.