Describe the composition and structure of the nucleosome. 10
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A nucleosome is a basic unit of DNApackaging in eukaryotes, consisting of a segment of DNA wound in sequence around eight[1] histone protein cores.[2] This structure is often compared to thread wrapped around a spool.[3]
Nucleosomes form the fundamental repeating units of eukaryotic chromatin,[4] 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). 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. Nucleosomes are thought to carry epigenetically inherited information in the form of covalent modifications of their core histones. Nucleosomes were observed as particles in the electron microscope by Don and Ada Olins in 1974,[5] and their existence and structure (as histone octamers surrounded by approximately 200 base pairs of DNA) were proposed by Roger Kornberg.[6][7] The role of the nucleosome as a general gene repressor was demonstrated by Lorch et al. in vitro,[8] and by Han and Grunstein in vivo in 1987 and 1988, respectively.[9]
The crystal structure of the nucleosome core particle. Histones H2A, H2B, H3 and H4 are coloured, DNA is gray. (PDB: 1EQZ[10][11])
The nucleosome core particle consists of approximately 146 base pairs (bp) of DNA[12]wrapped in 1.67 left-handed superhelical turnsaround a histone octamer consisting of 2 copies each of the core histones H2A, H2B, H3, and H4.[13] Core particles are connected by stretches of "linker DNA", which can be up to about 80 bp long. Technically, a nucleosome is defined as the core particle plus one of these linker regions; however the word is often synonymous with the core particle.[14] Genome-wide nucleosome positioning maps are now available for many model organisms including mouse liver and brain.[15]
Linker histones such as H1 and its isoforms are involved in chromatin compaction and sit at the base of the nucleosome near the DNA entry and exit binding to the linker region of the DNA.[16] Non-condensed nucleosomes without the linker histone resemble "beads on a string of DNA" under an electron microscope.[17]
In contrast to most eukaryotic cells, mature sperm cells largely use protamines to package their genomic DNA, most likely to achieve an even higher packaging ratio.[18]Histone equivalents and a simplified chromatin structure have also been found in Archea,[19] suggesting that eukaryotes are not the only organisms that use nucleosomes.
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Nucleosomes form the fundamental repeating units of eukaryotic chromatin,[4] 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). 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. Nucleosomes are thought to carry epigenetically inherited information in the form of covalent modifications of their core histones. Nucleosomes were observed as particles in the electron microscope by Don and Ada Olins in 1974,[5] and their existence and structure (as histone octamers surrounded by approximately 200 base pairs of DNA) were proposed by Roger Kornberg.[6][7] The role of the nucleosome as a general gene repressor was demonstrated by Lorch et al. in vitro,[8] and by Han and Grunstein in vivo in 1987 and 1988, respectively.[9]
The crystal structure of the nucleosome core particle. Histones H2A, H2B, H3 and H4 are coloured, DNA is gray. (PDB: 1EQZ[10][11])
The nucleosome core particle consists of approximately 146 base pairs (bp) of DNA[12]wrapped in 1.67 left-handed superhelical turnsaround a histone octamer consisting of 2 copies each of the core histones H2A, H2B, H3, and H4.[13] Core particles are connected by stretches of "linker DNA", which can be up to about 80 bp long. Technically, a nucleosome is defined as the core particle plus one of these linker regions; however the word is often synonymous with the core particle.[14] Genome-wide nucleosome positioning maps are now available for many model organisms including mouse liver and brain.[15]
Linker histones such as H1 and its isoforms are involved in chromatin compaction and sit at the base of the nucleosome near the DNA entry and exit binding to the linker region of the DNA.[16] Non-condensed nucleosomes without the linker histone resemble "beads on a string of DNA" under an electron microscope.[17]
In contrast to most eukaryotic cells, mature sperm cells largely use protamines to package their genomic DNA, most likely to achieve an even higher packaging ratio.[18]Histone equivalents and a simplified chromatin structure have also been found in Archea,[19] suggesting that eukaryotes are not the only organisms that use nucleosomes.
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●Structure of nucleosome
The smallest DNA bundle is called a nucleosome and it is made of DNA and protein. The protein portion is made of small units called histones. The core particle is made of four types of histones (H2A, H2B, H3, and H4). DNA wraps around the core particle.
●Composition of nucleosome
The nucleosome is the fundamental subunit of chromatin. Each nucleosome is composed ofa little less than two turns of DNA wrapped around a set of eight proteins called histones, which are known as a histone octamer. Each histone octamer is composed of two copies each of the histone proteins H2A, H2B, H3, and H4.
The smallest DNA bundle is called a nucleosome and it is made of DNA and protein. The protein portion is made of small units called histones. The core particle is made of four types of histones (H2A, H2B, H3, and H4). DNA wraps around the core particle.
●Composition of nucleosome
The nucleosome is the fundamental subunit of chromatin. Each nucleosome is composed ofa little less than two turns of DNA wrapped around a set of eight proteins called histones, which are known as a histone octamer. Each histone octamer is composed of two copies each of the histone proteins H2A, H2B, H3, and H4.
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