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An ultimate theory on Epigenetics in Mammalian Reproduction with Emphasis on Human Reproduction

Kaur K

Introduction:
Epigenetic mechanisms play a fundamental role in controlled development and gene expression in
different types of cells of an organism, carrying the same genomic DNA sequence. These mechanisms
control differences in the gene expression that are mitotically heritable although not altering the primary
DNA sequence [1]. A large number of proteins write, read or erase particular epigenetic modifications
and thus define where and when the transcriptional machinery can access the primary DNA sequence to
drive normal growth and differentiation in the developing embryo along with the fetus. Different type of
epigenetic marks work in concert to drive appropriate gene expression. These are DNA methylation at
CpG dinucleotides, covalent modifications of histone proteins, noncoding RNA’s (ncRNA) along with
other complementary mechanisms contributing to higher order chromatin organization, within the cell
nucleus. There are two special examples e.g., chromosome inactivation and genome imprinting, which
explains how 2016 Vol. 2 No. 4: 20 Journal of Clinical Epigenetics ISSN 2472-1158 2 This article is
available in: http://www.clinical-epigenetics.imedpub.com/ important are the epigenetic mechanisms in
regulating correct patterns of gene expression during early development chromosome inactivation
basically is an example of dosage compensation in females leading to monoallelic expression of a huge
number of X linked genes in female. Genome imprinting is a process in which special genes carrying
epigenetic marks from parents of origin have the capacity for getting monoallelic parent of origin specific
cell types at specific times of development. In germ cells in development as well as in embryo, there is
genome wide reprogramming which is responsible for erasure as well as reestablishing of the correct
epigenetic patterns. In contrast to these naturally occurring processes, the processes used in induced
pluripotent stem cells from somatic cells are quite different [2], reviewed by Huang et al[3]. Changes in
epigenetics can occur by different mechanisms and lead to infertility and imprinting disorders. Genetic as
well as environmental factors impact genetic marks, which develop phenotypic differences varying from
normal variation to human disease [4]. Both environmental factors e.g., starvation as well as artificial
reproductive technologies (ART) have been shown to affect the epigenome of the embryo e.g., of the
epigenetic changes which are associated with maternal starvation in fetal life can remain throughout
adulthood, contributing to late onset disorders e.g., CVS disorders and type 2 diabetes mellitus [5-9].