A glimpse into the hexasome: 40 years on

In 1983, scientists discovered hexasomes—a unique molecular structure that helps cells package their DNA. Now, a study conducted by the Eustermann group at EMBL Heidelberg has shed light on how DNA packaging into hexasomes ...

How does an aging-associated enzyme access our genetic material?

New research provides insight into how an enzyme that helps regulate aging and other metabolic processes accesses our genetic material to modulate gene expression within the cell. A team led by Penn State researchers have ...

Euchromatin is not really open in living cells, shows study

DNA and associated proteins in active regions of the genome are condensed but behave like a viscous liquid at the molecular level. This finding greatly increases our understanding of the physical nature of expressed genome ...

Biomolecular sliding at the nanoscale

In organisms whose cells have a nucleus, like plants and animals, the basic packaging units of DNA are the so-called nucleosomes. A nucleosome consists of a segment of DNA wound around eight proteins known as histones. Gene ...

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Nucleosomes are the basic unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound around a histone protein core. This structure is often compared to thread wrapped around a spool.

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). 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. The nucleosome hypothesis was proposed by Don and Ada Olins in 1974 and Roger Kornberg.

The nucleosome core particle consists of approximately 147 base pairs 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. 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.

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. Non-condensed nucleosomes without the linker histone resemble "beads on a string of DNA" under an electron microscope.

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. Histone equivalents and a simplified chromatin structure have also been found in Archea, proving that eukaryotes are not the only organisms that use nucleosomes.

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