Your question implies that you already have a
pretty good idea of the difference between
heterochromatin and euchromatin in terms of
structure and function.
In the past few years, the
highly organized, compact heterochromatin has
moved from being viewed as the packaging or
encapsulating of "dead" (unused) DNA to a more
important role as creating different types of
compact DNA with distinct features and functions.
One of the most intriguing roles of
heterochromatin might be to protect the genome
from being overcome by "parasitic" mobile elements
of DNA. But we are still left with your
Let's consider an
experiment: If you take a gene that is normally
expressed in euchromatin and you place it in a
region of heterochromatin, it ceases to be
expressed. The gene is said to be
difference in gene expression is an example of
something called a "positional effect;"
the activity of a gene depends on its position
along a chromosome. Also, the ends of
(the telomeres) as well as the
discrete functional regions of the chromosomes,
even though they do not encode transcribed genes.
Typically, these regions are highly organized as
heterochromatin. Now let's consider what
heterochromatin is -- I referred to is as "highly
organized" but what does that mean?
heterochromatin is physically "organized" on
platforms of proteins that recognize and bind the
DNA, creating a compact architecture.
- consider all of this together. First, why do
only certain regions bind the proteins? Why not
all along the chromosome?
There must be
some sort of "address" or "indicator" along the
DNA that mediates the binding of the proteins. In
fact, I used the word "recognition" in the
scenario. It is thought that specific, "hallmark"
sequences of DNA designate a region for
heterochromatin. It turns out that there are some
general rules for predicting this, though the
rules are not absolute. In general, euchromatin
tends to be GC rich while heterochromatin tends to
be AT rich. And telomeres and centromeres, which
have distinguishing features in terms of sequence,
clearly are distinct from the rest of the
An interesting aspect of this is
that heterochromatin can be "dynamic" --
during development or differing from cell to cell.
Another intriguing area of investigation is how
the heterochromatin is perpetuated from mother
cell to daughter cell or even from gamete to
embryo. It is a bit too complicated to get into
via email, but there is lots out there on this
really "hot" topic in molecular and cellular
biology. Dive into it if you are interested! And
keep asking questions.
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