  Biophysicists are modeling conformations of interphase chromosomes, often
basing the strengths of interactions between segments distant on the genetic
map on contact frequencies determined experimentally. Here, instead, we develop
a fitting-free, minimal model: bivalent red and green "transcription factors"
bind to cognate sites in runs of beads ("chromatin") to form molecular bridges
stabilizing loops. In the absence of additional explicit forces, molecular
dynamic simulations reveal that bound "factors' spontaneously cluster -- red
with red, green with green, but rarely red with green -- to give structures
reminiscent of transcription factories. Binding of just two transcription
factors (or proteins) to active and inactive regions of human chromosomes
yields rosettes, topological domains, and contact maps much like those seen
experimentally. This emergent "bridging-induced attraction" proves to be a
robust, simple, and generic force able to organize interphase chromosomes at
all scales.
