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the resulting complex signal cascades. In other words, how could a gene
expression
be initiated, passed with the DNA through mito-
sis, and then maintained using only the great molecular complexity of
TF regulation system that is based on tenuous protein
memory
-
protein and
protein
DNA interactions and not on stable covalent associations. It
would be nearly impossible to maintain the high stability and reproduc-
ibility that are needed to faithfully produce a precise body form and
myriad of specialized cell functions using only the TF system. Certainly,
the hierarchical TF control system is required and critical to multi-
cellular pattern formation and function and body plan execution. How-
ever, other particular aspects of development, especially cell fate and
competence, also strongly suggested that something in addition to these
tenuous interactions and their dependence on initiating signals in the TF
system could alone establish the cell memory that was observed in
multicellular development.
Cell competence is a developmental state that allows cells to exhibit
several possible differentiated fates. Once the fate of a cell is determined,
it becomes highly stable and is not able to form other cell types until
undergoing the rare event of dedifferentiation (Narayanaswamy 1977;
Gra
-
et al. 2011). This concept has been strongly supported by analyses
of numerous mutants and cell and tissue transplantation experiments in
Drosophila (Hadorn 1968; Hackett et al. 1987; Williamson and Lehmann
1996). Cell competence and fate is an ongoing process that can be
in
uenced by the cellular environment (Henshaw et al. 1982; Cronk
2001; Gilbert 2003), which was elegantly demonstrated also in trans -
determination experiments and mutation analyses (Hadorn 1968; Meins
and Foster 1986; Williamson and Lehmann 1996; Klebes et al. 2005; Lee
et al. 2005; Gehring et al. 2009). The understanding of genetic
was seen to be largely restricted to a short-term process since the
specialized characteristics that somatic cells acquire during develop-
ment do not pass through the meiotic cycle in sexual reproduction
because only germ cells (undifferentiated) or dedifferentiated cells
can undergo meiosis (Williamson and Lehmann 1996; Grafi
memory
et al. 2011).
Eventually, investigations began again to point to chromatin as an
important part of genetic memory. A crucial step in the revival of this
view arose from elegant mutation experiments that are possible in yeast
because here chromatin histone genes are not composed of multi-loci
families (Clark-Adams et al. 1988). Although DNA methylation was well
known and associated with transposon silencing (McClintock 1965), the
first eukaryotic DNA methyltransferase enzyme activity was reported
only in 1988 (Bestor 1988, 2000) starting the revolution of our knowledge
of the biochemistry of epigenetics (Cheng 1995; Finnegan and Kovac
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