The tissues
of multicellular organisms are genetically homogeneous but phenotypically
heterogeneous. This phenomenon is due to
the unusual variable expressivity of characters in the absence of genetic
heterogeneity. The variation in the
expressivity of a character must have been due to the conditioning imparted
during development and retained through mitosis. The stable phenotypic alterations resulted
due to conditioning without involving a change in DNA code of an allele is said
to be “epigenetics” as both alleles and the conditioning factors are together
heritable wherein no mutation is involved.
However, this phenomenon is unusually distinct from the maturation of
B-cells whose genome undergoes hyper-somatic mutations to specialize for the
secretion of a customized assembled immunoglobulin as per the prompting of
T-helper cell. At times, the extent to
which environmental factors condition epigenetic responses is somewhat labile
culminating in phenotypic mosaicism (variegation) among tissues and cells. Thus, the epigenetic factors determine the variable
expressivity of alleles and biased expressivity (imprinting) of either paternal
or maternal alleles in the progeny.
In 19th
Century, Casper Friedrich Wolff introduced the term “epigenesis” to comprehend
the mysterious workings of Nature that awakes the structures to form de novo from the apparent homogeneous (structureless)
mass of a zygote (9). The jargon of
epigenesis is further brought into limelight through one of the branches of
biology viz., Developmental Biology, which deals with the causative features of
embryonic development wherein it is elaborated as: the study of changes in gene
function due to combinatorial factors that are heritable among the cells of an
embryo without bringing alterations in the concerned gene sequence, which means
that the factors other than DNA supplement in the process of Heredity. Further, the heritable epigenetic changes are
called ‘epimutations’. B. McClintock
observed one of the first epimutations in maize through transposon activity
(10). Demethylation of a tandem repeat
in the promoter accompanied the ectopic expression of the late flowering gene, fwa in ddm1 plants and this resulted in
a dominant epimutation (10). Methylated
genes in Neurospora crassa behaved as
epimutant alleles wherein methylation – associated silencing of transcript
elongation were shown in Neurospsora
(10). The term epiallele also comes into vogue whose epigenetic factors are
stable at least through transgenerations.
For example, the transgene (Tg-13
HBV – E36-pas) when inherited paternally is unmethylated but maternal
transmission of the same transgene results in methylation and transcriptional
silencing (11). Thus, as envisaged by
Jenuwein and Allis (12), the phenomenon of epigenesis implies a fundamental
regulatory system beyond nucleotide sequence information of DNA emphasizing
that Mendel’s alleles are more than just a DNA moiety.
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