A new mechanism for accessing damaged DNA

UV light damages the DNA of skin cells, which can lead to skin cancer. But this process is counteracted by DNA repair machinery, acting as a molecular sunscreen. It has been unclear, however, how repair proteins work on DNA ...

How DNA is read and copied

Two scientists at the Johns Hopkins University School of Medicine have unraveled aspects of how DNA organizes and preserves genetic information. Newly published research by Cynthia Wolberger, Ph.D., and James Berger, Ph.D., ...

Bending DNA costs less energy than assumed

The way DNA folds largely determines which genes are read out. John van Noort and his group have quantified how easily rolled-up DNA parts stack. This costs less energy than previously assumed. Publication in Biophysical ...

Genome-wide rules of nucleosome phasing in drosophila

LMU researchers have, for the first time, systematically determined the positioning of the packing units of the fruit fly genome and discovered a new protein that defines their relationship to the DNA sequence.

Slip-sliding away…

In the cell nucleus, the genomic DNA is packaged into a tightly condensed form, which is referred to as chromatin. The basic unit of chromatin organization is the nucleosome, a DNA-protein complex consisting of a defined ...

Finding the proteins that unpack DNA

A new method allows researchers to systematically identify specialized proteins that unpack DNA inside the nucleus of a cell, making the usually dense DNA more accessible for gene expression and other functions. The method, ...

Molecular biologists compared human and yeast FACT

A protein complex called facilitates chromatin transcription (FACT) plays a role in DNA packing within a nucleus, as well as in oncogenesis. A team of scientists from MSU, working in cooperation with foreign colleagues, have ...

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Nucleosome

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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