  At the onset of X Chromosomes Inactivation, the vital process whereby female
mammal cells equalize X products with respect to males, the X chromosomes are
colocalized along their Xic (X-Inactivation Center) regions. The mechanism
inducing recognition and pairing of the X's remains, though, elusive. Starting
from recent discoveries on the molecular factors and on the DNA sequences (the
so-called ``pairing sites'') involved, we dissect the mechanical basis of Xic
colocalization by using a Statistical Physics model. We show that soluble DNA
specific binding molecules, as those experimentally identified, can be indeed
sufficient to induce the spontaneous colocalization of the homologous
chromosomes, but only when their concentration, or chemical affinity, rises
above a threshold value, as a consequence of a thermodynamic phase transition.
We derive the likelihood of pairing and its probability distribution.
Chromosome dynamics has two stages: an initial independent Brownian diffusion
followed, after a characteristic time scale, by recognition and pairing.
Finally, we investigate the effects of DNA deletion/insertions in the region of
pairing sites and compare model predictions to available experimental data.
