Editing S phase (section)

== Reestablishment of chromatin domains ==
Immediately after division, each daughter chromatid only possesses half the epigenetic modifications present in the paternal chromatid.<ref name=":8" /> The cell must use this partial set of instructions to re-establish functional chromatin domains before entering mitosis.

For large genomic regions, inheritance of old H3-H4 nucleosomes is sufficient for accurate re-establishment of chromatin domains.<ref name=":8" /> Polycomb Repressive Complex 2 ([[PRC2]]) and several other histone-modifying complexes can "copy" modifications present on old histones onto new histones.<ref name=":8" /> This process amplifies epigenetic marks and counters the dilutive effect of nucleosome duplication.<ref name=":8" />

However, for small domains approaching the size of individual genes, old nucleosomes are spread too thinly for accurate propagation of histone modifications.<ref name=":8" /> In these regions, chromatin structure is probably controlled by incorporation of histone variants during nucleosome reassembly.<ref name=":8" /> The close correlation seen between H3.3/H2A.Z and transcriptionally active regions lends support to this proposed mechanism.<ref name=":8" /> Unfortunately, a causal relationship has yet to be proven.<ref name=":8" />