DNA is the genetic material in most of the organisms.
This DNA with histone and non-histone protein constitutes the chromatin. The
chromatin organized into chromosome during cell division and is composed of two
chromatids.
Histones- Histones are main structural proteins found in
eukaryotic cells. these are low molecular weight proteins with high proportion
of positively charged amino acids (basic amino acids, arginine and lysine). The
positive charge helps histones to bind to DNA and play a crucial part in
packing long DNA molecules.
There are five different types of histones fall into
two categories-
1.
Nucleosomal
histones- There are
eight molecules of four types of small proteins. These are responsible for
coiling of DNA into nucleosome. These are (H2A)2, (H2B)2,
(H3)2 and (H4)2 . Each of them is formed of
about 102- 135 amino acids. These form the inner core of nucleosome.
Nucleosomal histones are very similar in different species. These are present
in equimolar amounts, two of each type being present every 200 base pairs, thus
forming a histones octamer or core particle.
2.
H1
histones- These are large (of
about 200 amino acids) and tissue specific. They are present one per 200 base
pairs. These are loosely associated with DNA H1 histones are
responsible for packing of nucleosomes into 30 nm fibre.
Functions of Histones-
Histones
of eukaryotic chromosome serve two functions:
1.
These
either serve as structural elements and help in coiling and packing of long DNA
molecule,
2.
These
cover or repress specific segments of DNA, so that these segments are unable to
transcribe. Their transcription is possible only by the dissolution of histones
in response to certain molecular signals.
Nucleosomes-
Nucleosomes are the fundamental packing units of
chromatin and give chromatin the “beads on a string” appearance. Each
nucleosome is disc-shaped about 11 nm in diameter. It consists of core particle
and a smaller spacer or linear DNA.
a)
Core
particle: The core
particle consists of the octamer of histones, having two copies each of H2A,
H2B, H3 and H4. It is about 11 nm in diameter and 6 nm in height. A strand of
DNA having 146 base pairs is tightly wrapped around this core forming two
circles (73 nucleotides per turn).
b)
Spacer
DNA or linker DNA: It
is a small segment of DNA having just four base pairs. One unit of histone H1
is associated with it. There is considerable variation in the length of spacer
region in different species varying from base to about 80 base pairs in
sea-urchin sperm.
Nucleosome Packing or Formation of Chromatin Fibre
Nucleosomes are packed upon one about one another to
form a chromatin fibre or nucleoprotein fibre with a diameter of about 30 nm.
It appears to have a beaded appearance. The beads are nucleosomes. These are
connected by linker DNA.
The
11 nm nucleoprotein in fibre represents the first level organization of
chromatin and is seen in interphase nucleus. The thick chromatin fibre of 30 nm
arises by spiral coiling of thin 11 nm chromatin fibre. It has a solenoid type
of ultrastructure and has 6-7 nucleosomes per turn.
This
type of packing requires one molecules of histone H1 per nucleosome. H1 gives
polarity to chromatin fibre. The 30 nm fibre is called a solenoid. Inside the
cells this solenoid is further folded and packed into the supersolenoid of 300
nm diameter. In mitotic chromosomes, the supersolenoid is further condensed to
produce the final shape and diameter of metaphase or anaphase chromosome. In
metaphase chromosome, the two daughter DNA molecules are separately folded to
produce two sister chromosomes called sister chromatids, held together by a
sibgle centromere.
