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.

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.

ClassificationHistone proteins are of two types:-core Histones-linker Histones

core# H2A# H2B# H3# H4

Two of each of these core histone proteinassemble to form 1 octameric nucleosome coreparticle and 147 base pairs of DNA wraparound this core particle.

H2A is important for packaging DNAinto chromatin. Since H2A packages DNAmolecules into chromatin, the packagingprocess will effect gene expression.H2A hasbeen correlated with DNA modification.H2A plays a major role in determining theoverall structure of chromatin.Inadvertently, H2A has been found toregulate gene expression.

H2B is also involved with the structure ofthe nucleosomes of the 'beads on a string' structure.

H3 is involved in the structureof chromatin in eukaryotic cells. Featuring a mainglobular domain and a long N-terminal tail.

H3 is an important protein in the emerging fieldof epigenetics, where its sequence variants andvariable modification states are thought to play arole in the dynamic and long term regulation ofgenes.

Histone H4 is involved in the structureof chromatin in eukaryotic cells.H4 is astructural component of the nucleosome,and is subject to covalent modification,including acetylation and methylation,which may alter expression of genes locatedon DNA associated with its parent histoneoctamer.

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.

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.

Packaging of histonesIn the core of nucleosome, the two dimmers H2Aand H2B and two tetramers H3 and H4 areinvolve and form the tertiary structure.

These structure proteins are

relatively similar in

structure.

Histones are highly

positively charge and have

lysine and argentine

residues.

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)

Normally histones arepositively charged but withthe modification ofmethylation it becomeshydrophobic which enablethe histone to more highlypack.

Acetylation andphosphorylation make thehistone more negativewhich weakens thepacking ability of histonesdue to repelling ofnegative-negative charges.

FUNCTIONSCompacting DNA strandHistones act as spools around which DNA winds. Thisenables the compaction necessary to fit the large genomes ofeukaryotes inside cell nuclei: the compacted molecule is40,000 times shorter than an unpacked molecule.

Chromatin regulationHistones undergo modifications that alter their interactionwith DNA and nuclear proteins. The H3 and H4 histoneshave long tails protruding from the nucleosome, which canbe covalently modified at several places.

Histone modifications are also involved in control oftranscription.Histone modifications act as Marking sites of DNAdamage.

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