Download Epigenetics: We often discuss genes as if their presence in our cells

Epigenetics: We often discuss genes as if their presence in our cells assures that they will be expressed.
This is not true. The expression of genes is highly regulated. That is to say that our genes can be turned on
or off, and this can be good (most of the time) or bad (sometimes). We understand the mechanisms in
some cases, but not others. Section 14.3 in our text (genomic imprinting) is but one example. Sections
10.7-10.11 deal with the processes involved with differentiation of cells as we develop from a single,
undifferentiated cell (zygote) to 100 trillion-cell being that you see in the mirror. The difference in your
brain, muscle, liver, and skin cells is not the DNA/genes in their nuclei, but rather, these cells are different
because of the genes that are being expressed in them. Genes are turned on an off in ways that we often
don't understand. Sometimes this is a part of natural development and sometimes it is a result of external
factors such as toxins, diet, stress, etc... Furthermore, some genes can be turned off (silenced) for several
generations, and then turned on again. How?
"Upstream" of all of our genes is a DNA sequence that we call the PROMOTER region. Chemical signals
in this region - most commonly the addition of a methyl group (methylation) - can essentially instruct the
RNA polymerase that would otherwise transcribe a gene to skip the gene with the methylated promoter...
the gene is turned off. Another mechanism of gene regulation involves those histone proteins that wrap
DNA tightly around them. When DNA is tightly wrapped around histones, it is not accessible to RNA
polymerase and the genes are turned off. Histones can be modified by external factors as mentioned above
and this affects coiling and thus the regulation of genes. Another mechanism of silencing genes is found in
inactive regions of one of the X chromosomes in females. Inactivity of one X chomosome results from vast
regions of DNA being condensed around histones, and this can be seen in stained preparations. The
condensed areas of one X chromosome are inaccessible for transcription/translation. These regions are
called Barr Bodies. As evidenced by males of our species, we really only need one X chromosome. Why
we don't see something like Barr Bodies in other chromosomes is a mystery. Since the human genome has
been sequenced (10 years), scientists have been frantically trying to make sense of what they found.
Among the findings presented in the video "Ghost in Your Genes" is the evidence that the same gene/allele
can have a different phenotypic effect if inherited from the mother or the father. The take home message of
all this (to me) is that we are accumulating data faster than we can process it, and we realize now that we
don't understand many of the very complex processes occurring in our cells.
Recent studies have shown that the regulation of gene expression is itself a heritable trait. As strange as
this may seem, the evidence is undeniable. Even identical twins diverge genetically beginning with their
intrauterine development. This is how identical twins end up with different fingerprints. We now talk
about "DNA Fingerprinting" as the ultimate test of indentification. However, a genetic test would show
identical twins to be the same person. Their literal fingerprints would show otherwise. If the patterns in
the epidermis of fingertips is different, then why not other genetically-controlled traits?