From Wikipedia,
the free encyclopedia.
Chromatin is the
substance of a
chromosome and consists of a
complex of
DNA and
protein in
eukaryotic
cells. It can be made visible
by staining, hence its name, which
literally means coloured
material. The
nucleic acids are generally in
the form of double-stranded DNA.
The major
proteins involved in chromatin
are
histone
proteins. DNA is packaged into
chromatin both to constrain the
size of the molecule and to allow
the cell to control
expression of the chromatin
packaged genes. Changes in
chromatin structure are affected
mainly by
methylation and
acylation of the
nucleous proteins. Chromatin
structure is also of importance in
DNA replication.
There are three major levels of
chromatin organization:
-
nucleosome - "beads on a
string"
- 30 nm condensed chromatin
fiber
-
chromosome
Sperm cell chromatin is an
exception to the above. During
spermiogenesis, the
spermatid's chromatin is
remodelled into a more tightly
packaged, compact structure. This
process is associated with the
cessation of
transcription and involves
nuclear protein exchange. The
histones are mostly displaced, and
replaced by
protamines, small,
arginine-rich proteins.
Figure 2: Different
levels of DNA condensation.
(1) Single DNA strand. (2)
Chromatin strand (
DNA
with
histones). (3)
Condensed chromatin during
interphase with
centromere. (4)
Condensed chromatin during
prophase. (Two copies of
the DNA molecule are now
present) (5) Chromosome
during
metaphase.
Two distinct types of chromatin
can be distinguished:
See also
Reference
