Biology Reference
In-Depth Information
multipleways (
Kouzarides, 2007
). Variation inwhat is referred toas the primary
structure of the chromatin can be thus achieved as a result of incorporation
of an increasing number of histone variants and their accompanying post-
translational modifications (
Luger, Dechassa, & Tremethick, 2012
). This var-
iation has the potential to move the equilibrium to different chromatin states
and therefore to impact on chromatin function. Globally, the chromatin is
organized in two main functional and structural types or states: euchromatin
and heterochromatin (
Grewal & Elgin, 2007
). The former is considered to
be an open structure favorable for transcription and is gene rich, whereas the
latter is considered to be in a closed structure that tends to be refractory for tran-
scription and is gene poor. The heterochromatin can be further subdivided into
two different types, facultative and constitutive. Facultative heterochromatin
is characterized mainly by high levels of trimethylation of the lysine 27 of
histone H3 (H3K27me3), a modification which is established by the poly-
comb repressive complex 2 (PRC2) andwhich alsoplays a role in the repression
of developmental genes (
Cao et al., 2002
). However, the constitutive hetero-
chromatin is characterized by strong enrichment of H3K9me3, H4K20me3,
and H3K64me3, as well as of high levels of DNA methylation (
Daujat
et al., 2009; Peters et al., 2001; Schotta et al., 2004
). The constitutive hetero-
chromatin assembles mainly on centromeric, pericentromeric, and telomeric
regions that are known to harbor repeated sequences such as the major and
minor satellites.Constitutiveheterochromatin is alsopresent at imprintedgenes
in an allele-specific fashion and is considered to be a heritable trait that can
bepassedon todaughter cells andmaintained (
Regha et al., 2007
). Several ques-
tions arise on how heterochromatin is established at these specific genomic
regions after fertilization and naturally on how they are maintained and prop-
agated through the cell cycle. Another key question that has so far been under-
investigated is whether there is any role for heterochromatin as such, in
regulating embryonic development and the restriction of cell fate and plasticity.
Indeed, as we propose below, the earliest stages of mammalian embryogenesis
are characterized by a lack of a “conventional” heterochromatin.
The period that follows fertilization is particularly interesting in terms of
chromatin remodeling due mainly to the genome-wide epigenetic repro-
gramming that the parental genomes are subject to (reviewed in
Burton &
Torres-Padilla, 2010
). Erasure of most of the epigenetic information carried
by the two highly differentiated gametes is thought to be necessary to restore
developmental plasticity in the newly formed organism. The formation of
the newly fertilized zygote constitutes therefore the climax of totipotency
because of the resulting zygote's inherent ability to produce all cell types in a
