2. Histones or histone proteins
These are highly alkaline proteins found in
eukaryotic cell nuclei that package and order
the DNA into structural units called
nucleosome.
Histones are a group of
basic protein that
associate with DNA and
help the DNA to condense
it into chromatin.
3. Some Histones function as spools for
the thread like DNA to wrap around.
Chromatin, under the microscope in its extended
form, hooks to beads on a string, called
nucleosome.
Nucleosome consist of
DNA wrapped around
eight histone protein
called histone octamer.
4. Classification
Histone proteins are of two types:
-core Histones
-linker Histones
core
# H2A
# H2B
# H3
# H4
Two of each of these core histone protein
assemble to form 1 octameric nucleosome core
particle and 147 base pairs of DNA wrap
around this core particle.
5. H2A is important for packaging DNA
into chromatin. Since H2A packages DNA
molecules into chromatin, the packaging
process will effect gene expression.H2A has
been correlated with DNA modification.
H2A plays a major role in determining the
overall structure of chromatin.
Inadvertently, H2A has been found to
regulate gene expression.
6. H2B is also involved with the structure of
the nucleosomes of the 'beads on a string' structure.
H3 is involved in the structure
of chromatin in eukaryotic cells. Featuring a main
globular domain and a long N-terminal tail.
H3 is an important protein in the emerging field
of epigenetics, where its sequence variants and
variable modification states are thought to play a
role in the dynamic and long term regulation of
genes.
7. Histone H4 is involved in the structure
of chromatin in eukaryotic cells.H4 is a
structural component of the nucleosome,
and is subject to covalent modification
,including acetylation and methylation,
which may alter expression of genes located
on DNA associated with its parent histone
octamer.
8. Linker histone includes-
H1
H5
>Histone H1 is one of the five
main histone protein families which are
components of chromatin in eukaryotic cells.
>Though highlyconserved, it is nevertheless the
most variable histone in sequence across species.
9. Unlike the other histones, H1 does not make
up the nucleosome "bead". Instead, it sits on top
of the structure, keeping in place the DNA that
has wrapped around the nucleosome. H1 is
present in half the amount of the other four
histones, which contribute two molecules to each
nucleosome bead.
H5 histones are individual proteins involve in
the packaging of specific region of DNA.
10. Packaging of histones
In the core of nucleosome, the two dimmers H2A
and H2B and two tetramers H3 and H4 are
involve and form the tertiary structure.
These structure proteins are
relatively similar in
structure.
Histones are highly
positively charge and have
lysine and argentine
residues.
11. Modifications in histones
Histone proteins are altered according to the
packaging in DNA .
There can be many modification that affect
how DNA is packaged.
There are three types of modifications-
>methylation (increases packaging)
>acetylation (decrease packaging)
>phosporylation (decrease packaging)
12. Normally histones are
positively charged but with
the modification of
methylation it becomes
hydrophobic which enable
the histone to more highly
pack.
Acetylation and
phosphorylation make the
histone more negative
which weakens the
packing ability of histones
due to repelling of
negative-negative charges.
13. FUNCTIONS
Compacting DNA strand
Histones act as spools around which DNA winds. This
enables the compaction necessary to fit the large genomes of
eukaryotes inside cell nuclei: the compacted molecule is
40,000 times shorter than an unpacked molecule.
Chromatin regulation
Histones undergo modifications that alter their interaction
with DNA and nuclear proteins. The H3 and H4 histones
have long tails protruding from the nucleosome, which can
be covalently modified at several places.
Histone modifications are also involved in control of
transcription.
Histone modifications act as Marking sites of DNA
damage.
