Nucleosome

nucleosomesnucleosome core particlenuclesomenucleosomalnucleosomal unitsnucleosome assembly
A nucleosome is a fundamental unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound in sequence around eight histone protein cores.wikipedia
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Histone

histoneshistone modificationhistone modifications
A nucleosome is a fundamental unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound in sequence around eight histone protein cores. The nucleosome core particle consists of approximately 146 base pairs (bp) of DNA wrapped in 1.67 left-handed superhelical turns around a histone octamer, consisting of 2 copies each of the core histones H2A, H2B, H3, and H4. Nucleosomes are thought to carry epigenetically inherited information in the form of covalent modifications of their core histones.
In biology, histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes.

Chromatin

chromatin structuresex chromatinnuclear chromatin
Nucleosomes form the fundamental repeating units of eukaryotic chromatin, which is used to pack the large eukaryotic genomes into the nucleus while still ensuring appropriate access to it (in mammalian cells approximately 2 m of linear DNA have to be packed into a nucleus of roughly 10 µm diameter).

Chromosome

chromosomeschromosomalChromosomal number
Nucleosomes are folded through a series of successively higher order structures to eventually form a chromosome; this both compacts DNA and creates an added layer of regulatory control, which ensures correct gene expression.
Chromatin fiber is made of nucleosomes (histone octamers with part of a DNA strand attached to and wrapped around it).

Histone H2B

H2Bhistones H2B
The nucleosome core particle consists of approximately 146 base pairs (bp) of DNA wrapped in 1.67 left-handed superhelical turns around a histone octamer, consisting of 2 copies each of the core histones H2A, H2B, H3, and H4. The nucleosome core particle (shown in the figure) consists of about 146 base pair of DNA wrapped in 1.67 left-handed superhelical turns around the histone octamer, consisting of 2 copies each of the core histones H2A, H2B, H3, and H4.
Featuring a main globular domain and long N-terminal and C-terminal tails, H2B is involved with the structure of the nucleosomes.

Histone H3

H3histone 3H3.3
The nucleosome core particle consists of approximately 146 base pairs (bp) of DNA wrapped in 1.67 left-handed superhelical turns around a histone octamer, consisting of 2 copies each of the core histones H2A, H2B, H3, and H4. The nucleosome core particle (shown in the figure) consists of about 146 base pair of DNA wrapped in 1.67 left-handed superhelical turns around the histone octamer, consisting of 2 copies each of the core histones H2A, H2B, H3, and H4.
Featuring a main globular domain and a long N-terminal tail, H3 is involved with the structure of the nucleosomes of the 'beads on a string' structure.

Regulation of gene expression

gene regulationregulationregulatory
Nucleosome positions in the genome are not random, and it is important to know where each nucleosome is located because this determines the accessibility of the DNA to regulatory proteins.
Octameric protein complexes called nucleosomes are responsible for the amount of supercoiling of DNA, and these complexes can be temporarily modified by processes such as phosphorylation or more permanently modified by processes such as methylation.

Histone H4

H4H4 histone
The nucleosome core particle consists of approximately 146 base pairs (bp) of DNA wrapped in 1.67 left-handed superhelical turns around a histone octamer, consisting of 2 copies each of the core histones H2A, H2B, H3, and H4. The nucleosome core particle (shown in the figure) consists of about 146 base pair of DNA wrapped in 1.67 left-handed superhelical turns around the histone octamer, consisting of 2 copies each of the core histones H2A, H2B, H3, and H4.
Featuring a main globular domain and a long N-terminal tail, H4 is involved with the structure of the nucleosome of the 'beads on a string' organization.

Cell nucleus

nucleusnucleinuclear
Nucleosomes form the fundamental repeating units of eukaryotic chromatin, which is used to pack the large eukaryotic genomes into the nucleus while still ensuring appropriate access to it (in mammalian cells approximately 2 m of linear DNA have to be packed into a nucleus of roughly 10 µm diameter).
Antibodies to certain types of chromatin organization, in particular, nucleosomes, have been associated with a number of autoimmune diseases, such as systemic lupus erythematosus.

Histone octamer

octamer of histone proteins
The nucleosome core particle (shown in the figure) consists of about 146 base pair of DNA wrapped in 1.67 left-handed superhelical turns around the histone octamer, consisting of 2 copies each of the core histones H2A, H2B, H3, and H4.
A histone octamer is the eight protein complex found at the center of a nucleosome core particle.

DNA

deoxyribonucleic aciddouble-stranded DNAdsDNA
A nucleosome is a fundamental unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound in sequence around eight histone protein cores. Nucleosomes form the fundamental repeating units of eukaryotic chromatin, which is used to pack the large eukaryotic genomes into the nucleus while still ensuring appropriate access to it (in mammalian cells approximately 2 m of linear DNA have to be packed into a nucleus of roughly 10 µm diameter). The nucleosome core particle consists of approximately 146 base pairs (bp) of DNA wrapped in 1.67 left-handed superhelical turns around a histone octamer, consisting of 2 copies each of the core histones H2A, H2B, H3, and H4.
The histones form a disk-shaped complex called a nucleosome, which contains two complete turns of double-stranded DNA wrapped around its surface.

Roger D. Kornberg

Roger KornbergRoger David KornbergKornberg, Roger
Nucleosomes were first observed as particles in the electron microscope by Don and Ada Olins in 1974, and their existence and structure (as histone octamers surrounded by approximately 200 base pairs of DNA) were proposed by Roger Kornberg.
While a postdoctoral fellow working with Aaron Klug and Francis Crick at the MRC in the 1970s, Kornberg discovered the nucleosome as the basic protein complex packaging chromosomal DNA in the nucleus of eukaryotic cells (chromosomal DNA is often termed "Chromatin" when it is bound to proteins in this manner, reflecting Walther Flemming's discovery that certain structures within the cell nucleus would absorb dyes and become visible under a microscope).

Linker DNA

internucleosomalinternucleosomal linker
Core particles are connected by stretches of linker DNA, which can be up to about 80 bp long.
Linker DNA is double-stranded DNA 38-53 bp long in between two nucleosome cores that, in association with histone H1, holds the cores together.

Apoptotic DNA fragmentation

autodestruction of DNAchromosomal DNA fragmentationnuclear fragmentation
Such digestion can occur also under natural conditions during apoptosis ("cell suicide" or programmed cell death), because autodestruction of DNA typically is its role.
CAD cleaves DNA at internucleosomal linker sites between nucleosomes, protein-containing structures that occur in chromatin at ~180-bp intervals.

Histone H1

H1H1 histoneH1.
A chain of nucleosomes can be arranged in a 30 nm fiber, a compacted structure with a packing ratio of ~50 and whose formation is dependent on the presence of the H1 histone.
Unlike the other histones, H1 does not make up the nucleosome "bead".

Euchromatin

euchromatic
Beyond this, the structure of chromatin is poorly understood, but it is classically suggested that the 30 nm fiber is arranged into loops along a central protein scaffold to form transcriptionally active euchromatin.
The structure of euchromatin is reminiscent of an unfolded set of beads along a string, wherein those beads represent nucleosomes.

Histone H2A

H2AH2A family
The nucleosome core particle consists of approximately 146 base pairs (bp) of DNA wrapped in 1.67 left-handed superhelical turns around a histone octamer, consisting of 2 copies each of the core histones H2A, H2B, H3, and H4. The nucleosome core particle (shown in the figure) consists of about 146 base pair of DNA wrapped in 1.67 left-handed superhelical turns around the histone octamer, consisting of 2 copies each of the core histones H2A, H2B, H3, and H4.

H2AFZ

H2A.ZH2AZ
H2A can be replaced by H2AZ (which leads to reduced nucleosome stability) or H2AX (which is associated with DNA repair and T cell differentiation), whereas the inactive X chromosomes in mammals are enriched in macroH2A.
Histones are basic nuclear proteins that are responsible for the nucleosome structure of the chromosomal fiber in eukaryotes.

H2AFX

H2AXgammaH2AXH2A.X
H2A can be replaced by H2AZ (which leads to reduced nucleosome stability) or H2AX (which is associated with DNA repair and T cell differentiation), whereas the inactive X chromosomes in mammals are enriched in macroH2A.
H2AX contributes to nucleosome-formation, chromatin-remodeling and DNA repair, and is also used in vitro as an assay for double-strand breaks in dsDNA.

Epigenetics

epigeneticepigeneticallyepigenetic regulation
Nucleosomes are thought to carry epigenetically inherited information in the form of covalent modifications of their core histones.
This accumulation, in turn, directs recruitment and activation of the chromatin remodeling protein ALC1 that can cause nucleosome remodeling.

CENPA

CENP-Acentromere protein A
H3 can be replaced by H3.3 (which correlates with activate genes and regulatory elements) and in centromeres H3 is replaced by CENPA.
CENPA is proposed to be a component of a modified nucleosome or nucleosome-like structure in which it replaces 1 or both copies of conventional histone H3 in the (H3-H4)2 tetrameric core of the nucleosome particle.

Transcription factor

transcription factorsgene transcription factortranscriptional factors
In addition, the removal of nucleosomes usually corresponded to transcriptional activation and the replacement of nucleosomes usually corresponded to transcriptional repression, presumably because transcription factor binding sites became more or less accessible, respectively.
In eukaryotes, DNA is organized with the help of histones into compact particles called nucleosomes, where sequences of about 147 DNA base pairs make ~1.65 turns around histone protein octamers.

Promoter (genetics)

promoterpromoterspromoter region
Studies in 2007 have catalogued nucleosome positions in yeast and shown that nucleosomes are depleted in promoter regions and origins of replication.
Divergent transcription could shift nucleosomes to upregulate transcription of one gene, or remove bound transcription factors to downregulate transcription of one gene.

Transcription (biology)

transcriptiontranscribedtranscriptional
About 80% of the yeast genome appears to be covered by nucleosomes and the pattern of nucleosome positioning clearly relates to DNA regions that regulate transcription, regions that are transcribed and regions that initiate DNA replication.
In eukaryotes, however, nucleosomes act as major barriers to transcribing polymerases during transcription elongation.

Cellular differentiation

differentiationcell differentiationdifferentiate
The maintenance of a repressed or activated status of a gene is often necessary for cellular differentiation.
In particular, it is important to know whether a nucleosome is covering a given genomic binding site or not.

X-inactivation

X chromosome inactivationX inactivationX-chromosome inactivation
H2A can be replaced by H2AZ (which leads to reduced nucleosome stability) or H2AX (which is associated with DNA repair and T cell differentiation), whereas the inactive X chromosomes in mammals are enriched in macroH2A.
Additionally, a histone variant called macroH2A (H2AFY) is exclusively found on nucleosomes along the Xi.