Agriculture Reference
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the idea that the genomic DNA sequence encodes all essential informa-
tion to produce an organism from a fertilized egg, which can include
more than 200 distinguishable cell types (Allis et al. 2007), certain
problems persisted. If development was the result of DNA sequence
changes, or of loss of unnecessary DNA sequences, as more specialized
cells are produced, they would have to be changed back or somehow
recovered in order to recycle the developmental process during gamete
formation and fertilization. Alternatively, germ cells, originally thought
to be a
organism, would have to be maintained in an
undeveloped constant lineage or germ line to preserve the original
form and amount of hereditary material to restart the life cycle. The
germ line or, germplasm theory of August Weismann (Weismann 1893),
maintained that only the germ cells contained a full complement of the
hereditary material (genome) because part of this material was actually
lost or discarded as development progressed. In a fascinating twist of
history, we now know that in ciliates, as much as 90% of the genome is
actually eliminated from developing somatic cells. The legacy of this
process is also preserved in higher order taxa such as mammals by the
Variable, Diversity, and Joining V(D)J recombination/elimination pro-
cess in the adaptive immune system (Mostoslavsky et al. 2001; Bassing
and Alt 2002; Bossen et al. 2012).
“
preformed
”
C. Gene Expression and Phenotype: The Mystery of Genetic Memory
Early evidence did not support a scenario where genotypes (DNA
sequence) had the sequence plasticity to control development. Even-
tually, with DNA sequencing and other technologies, it was obvious
that different cell tissue and organ types clearly maintained essentially
the entire original DNA sequence, butthatexpression(mRNA)patterns
differed greatly between cell types. Thus, changes in phenotype during
development, the ontogeny process, appeared to be based on diffe-
rential gene expression in different cells that shared the same DNA
sequence. These cell-speci
c gene expression programs had to be
stable through mitosis to be able to build multicellular tissues and
organs, but they were not manifested in germ cells and, therefore, did
not pass through meiosis. How could gene expression be controlled in
this way? Interestingly, this search began with the chromosomal and
chromatin structure withinwhich the DNA resided (Muller 1930; Brink
1973). Chromatin became a primary possible controlling mechanism
because just activating the set of genes needed for one cell type was not
suf
cient to explain development, since genes producing other cell
types would also need to be maintained in an inactivated or silenced
