Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 70

Figure 2.4 The crystal structure of a short DNA helix, which is methylated on both strands
at the center cytosine.
Source : From Christoph Bock (Max Planck Institute for Informatics), 2006. Available from:
http://en.wikipedia.org/wiki/File:DNA_methylation.jpg
The epigenetic information contained in methylated/demethylated DNA and his-
tone marks have control over the expression of genetic information and epigenetic
changes in DNA and histones are considered to represent an “epigenetic inheritance
system” ( Shea, 2007 ). But, if by the term system , we commonly mean the entirety of
the parts that function as a whole to perform a particular function, then neither DNA
methylation/demethylation nor histone acetylation/deacetylation and so on represent
“systems.” DNA methylation and chromatin remodeling are no more than epige-
netically changed structures. No parts of an epigenetic inheritance system are vis-
ible in DNA methylation and histone marks, and the following discussion is aimed at
revealing the parts of the epigenetic inheritance system, if at all.
If there is an “epigenetic inheritance system,” then one must identify interacting parts
of the system and possible flow of the epigenetic information into the system. This may
also show whether these epigenetic information-carrying structures are genuine sources
of the information they contain or they are just media or channels through which the
epigenetic information is transmitted to, and embodied into, biological structures.
The simplest approach to this problem is to proceed from the knowns, which
are DNA methylation and histone acetylation, to the unknowns, i.e., to the inducers
of methylation and acetylation in the DNA and chromatin histones. The fact these
epigenetic changes in DNA and chromatin histones are nonrandom and generally
adaptive implies that the epigenetic information is invested in the right place in the
genome or chromatin. Any inquisitive mind would ask: how does the organism man-
age to induce epigenetic changes exactly to the right nitrogen bases and histone mol-
ecules out of the millions of such bases and histone molecules during the processes
of gametogenesis, zygote formation, and individual development. We need to know
how these epigenetic structures come into being and what determines their occur-
rence at exactly the sites where they are needed. Where does the information for pro-
ducing these adaptive changes in the structure of the genetic and nongenetic material
come from?