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The title of this talk
is Heterochromatin,
Epigenetics and Gene Expression.
I'm Joel Eissenberg.
I'm a professor in the
Department of Biochemistry
and Molecular Biology at Saint
Louis University School of Medicine.
My goal in this talk is to
survey our understanding
of heterochromatin
and its connection
to the idea of epigenetic
control of gene expression.
Terms like heterochromatin
and epigenetics
are frequently used to
disguise, rather than explicate,
our understanding of
how cells organize
and use genetic information.
In this presentation,
I'll define these terms
and discuss epigenetics in
the context of heterochromatin
and the evidence that it can
influence gene expression.
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The term heterochromatin
was coined by Emil Heitz
to refer to the material in the
eukaryotic nucleus that fails
to decondense after
telophase in the cell cycle.
On the left in this slide is an
image showing the chromosomes
of a cell at or near telophase
stained with a fluorescent dye
that labels DNA.
The bright fluorescent staining, is
coextensive with the chromosomes.
In contrast, the interphase nucleus
on the right is filled with DNA,
but only certain regions stain
brightly, the heterochromatin.
The condensed state
of heterochromatin
concentrates the DNA, making these
regions stand out on the background
of the rest of the DNA
fluorescents in the nucleus.
This property distinguishes
it from the remaining
so-called euchromatin, or
true chromatin, that undergoes
cyclic condensation
and decondensation.
In contrast, heterochromatin
exhibits heterocyclic behavior,
hence the term heterochromatin.
Thus, the term
heterochromatin was originally
coined to describe a
cytological phenomenon, not
a genetic or biochemical phenomenon.
Heitz ultimately showed that most
or all eukaryotic chromosomes
are differentiated
along their lengths
by zones of euchromatin, which he
recognized as relatively gene rich,
and heterochromatin, which he
recognized as relatively gene poor.
More recently, the
term heterochromatin
has been used more promiscuously
to describe any form of chromatin
associated with
transcriptional silencing
and/or chromatin enriched for
certain biochemical markers,
such as cytosine methylation, or
certain histone modifications.
In this presentation, I'll
stick to examples that
are consistent with the
original definition of the word.
