Biology Reference
In-Depth Information
chromatin state. An important time point to consider is the first DNA
replication phase ( Fig. 9.2 ), which starts around 5-6 h after fertilization
and is completed before 9 h in the mouse, with strain-to-strain variability.
Any histone or DNA modification preceding the first S-phase is replication-
independent and likely to involve active enzymatic mechanisms; changes
occurring at replication are likely to be passive, resulting from the synthesis
of new unmethylated DNA strands and the incorporation of new
replication-dependent histones.
Fertilization
Implantation
Birth
Adulthood
1st S-phase
Gamete-inherited histone modifications
Somatic histone variant and modification acquisition
Gamete-inherited DNA methylation: gDMRs and other sequences
Multi step restriction to ICRs
Somatically acquired parental histone and DNA modifications: somDMRs
Paternal XCI in females
Restricted to
extra embryonic tissues
Figure 9.2 Dynamics of epigenetic distinction between parental alleles during mouse
development. Red and blue colors represent the maternally inherited and paternally
inherited genome, respectively. Most gamete-inherited epigenetic marks are erased
after fertilization, during preimplantation development, with the exception of some
parent-specific DNA methylation that is maintained throughout life on ICRs. Gamete-
inherited histone modifications are mostly replaced by somatic histones of maternal
origin at the time of protamine to histone exchange on the paternal genome and mas-
sively on the two parental genomes of the one-cell zygote by replication-dependent
incorporation at the first S-phase. Gametic DNA methylation is lost in part on the pater-
nal genome before the first S-phase by an active mechanism and then progressively on
the two parental genomes by a passive mechanism of replication-dependent dilution
during preimplantation development. Only ICRs maintain gamete and parent-specific
DNA methylation patterns throughout life. At implantation, new loci acquire parent-
specific histone and DNA modifications, under the control of ICRs, the somatic DMRs
(somDMRs). Finally, paternal XCI occurs in female preimplantation embryos but is reset
at implantation to allow random XCI, except in extraembryonic tissues where paternal
XCI continues.
Search WWH ::




Custom Search