Biology Reference
In-Depth Information
3.9 Euchromatin and Heterochromatin
Insect nuclear genomes have two types of chromatin during somatic inter-
phase:
euchromatin
and
heterochromatin. Euchromatin
is uncoiled dur-
ing interphase, presumably to allow for gene transcription. Euchromatin
is condensed during mitosis, with a maximal condensation at metaphase.
Euchromatin contains most of the protein-coding genes. In polytene salivary
gland chromosomes of
Drosophila
, the darkly staining segments are hetero-
chromatic (DNA that is condensed because it is not actively being transcribed)
and the intervening less well-stained regions are euchromatic (regions that are
genetically active and do not dye as well).
The term
heterochromatin
was coined originally to define the chromosome
regions that remain condensed during most of the cell cycle and have a coiling cycle
out of phase with the rest of the genome. Unlike euchromatin, heterochromatin
exhibits maximal condensation in nuclei during interphase. Heterochromatin repli-
cates late in the cell cycle, compared with euchromatin, and contains a considerable
amount of middle- and highly repetitive DNA (
Weiler and Wakimoto 1995
).
In many organisms, large regions of the chromosome near the centromeres
and the telomeres are heterochromatic, and these regions contain primarily
middle- and highly repetitive DNA that plays an essential role in centromere and
telomere function (
Henikoff 2000
). It is now thought that heterochromatin is
not a type of DNA sequence but rather a “chromatin state” (
Jenuwein and Allis
2001
). Potentially, all parts of the genome could enter this state. The repression
of transcription in heterochromatin seems to involve a set of proteins and RNA
molecules, although the details of how they function remain limited (
Hennig
1999, Leach et al. 2000, Redi et al. 2001
).
Heterochromatin serves an important role in chromosome mechanics
(
Wallrath 1998
). Without sufficient heterochromatin, chromosomes segregate
inappropriately to daughter cells during mitosis. Strangely, a few genes nor-
mally located in heterochromatic regions are active, but become silenced or
inactive if moved into euchromatic regions (
Eissenberg and Hilliker 2000
).
Heterochromatic regions in
D. melanogaster
can cause
position effect varie-
gation
by inactivating (silencing) euchromatic genes that have been moved to
regions adjacent to heterochromatin by chromosomal rearrangements (
Wallrath
1998
). A change in location of a gene within the nucleus significantly modi-
fies the amount of “gene silencing,” perhaps due to its location within the
nuclear compartment. Furthermore, foreign genes (transgenes) experimen-
tally inserted into an insect's genome can be silenced
because
they become
