  In higher organisms, all cells share the same genome, but every cell
expresses only a limited and specific set of genes that defines the cell type.
During cell division, not only the genome, but also the cell type is inherited
by the daughter cells. This intriguing phenomenon is achieved by a variety of
processes that have been collectively termed epigenetics: the stable and
inheritable changes in gene expression patterns. This article reviews the
extremely rich and exquisitely multi-scale physical mechanisms that govern the
biological processes behind the initiation, spreading and inheritance of
epigenetic states. These include not only the changes in the molecular
properties associated with the chemical modifications of DNA and histone
proteins, such as methylation and acetylation, but also less conventional ones,
such as the physics that governs the three-dimensional organization of the
genome in cell nuclei. Strikingly, to achieve stability and heritability of
epigenetic states, cells take advantage of many different physical principles,
such as the universal behavior of polymers and copolymers, the general features
of non-equilibrium dynamical systems, and the electrostatic and mechanical
properties related to chemical modifications of DNA and histones. By putting
the complex biological literature under this new light, the emerging picture is
that a limited set of general physical rules play a key role in initiating,
shaping and transmitting this crucial "epigenetic landscape". This new
perspective not only allows to rationalize the normal cellular functions, but
also helps to understand the emergence of pathological states, in which the
epigenetic landscape becomes dysfunctional.
