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DNMT1, considered to be the primary if not sole maintenance methyl-
transferase, in ES cells reduces the overall level of genomic methylation
and changes the patterning of CG methylation throughout the genome
(
Lei et al., 1996
). The residual methylation is most likely due to continuing
de novo
methyltransferase activity.
DNMT1, like the
A. thaliana
CpG maintenance methyltransferase
MET1, is expressed in mammals at virtually all times, yet there are significant
fluctuations in genomic methylation during the mammalian life cycle
(
Reinhart and Chaillet, 2005
). This stands in contrast to plant species, where
genomic methylation changes, but the changes are less dynamic (overall
smaller changes in the percentage of methylated CpG dinucleotides),
suggesting that there is less overall regulation of genomic methylation pat-
terns in plants than in animals (
Zhang et al., 2010
). This may seem paradox-
ical, given that plants have more DNA cytosine methyltransferase enzymes
and more target sequence types that can be methylated.
Changes in imprinted gene methylation and different categories of non-
imprinted genomic methylation during the reproductive cycle of the mouse
are shown in
Fig. 1.7
. Oscillations in genomic methylation during this cycle
are invariant. That is, changes are the same for every cycle, and the absolute
Figure 1.7 Genomic methylation during the mammalian life cycle. The differentiation
of germ cells into mature gametes (gametogenesis) and the differentiation of cells of
the early postimplantation embryo represent the two primary stages of acquisition
of new methylation marks via the action of de novo methyltransferases. These stages
are interspersed during the reproductive cycle with loss of genomic methylation in
primordial germ cells (earliest fetal cells allocated to the germ lineage) and loss of geno-
mic methylation (with the notable maintenance of imprinted DMD methylation) during
preimplantation development.
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