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Nucleosomes are thought to carry epigenetically inherited information in the form of covalent modifications of their core histones. 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. [4]
The prototypical examples are nucleosomes, complexes in which genomic DNA is wrapped around clusters of eight histone proteins in eukaryotic cell nuclei to form chromatin. Protamines replace histones during spermatogenesis.
Histone tails and their function in chromatin formation. Histones undergo posttranslational modifications that alter their interaction with DNA and nuclear proteins. The H3 and H4 histones have long tails protruding from the nucleosome, which can be covalently modified at several places.
It has been found that the SWI/SNF complex (in yeast) is capable of altering the position of nucleosomes along DNA. [8] [9] These alterations are classified in three different ways, and they are seen as the processes of sliding nucleosomes, ejecting nucleosomes, and ejecting only certain components of the nucleosome. [5]
Histones are proteins that package DNA into nucleosomes. [1] Histones are responsible for maintaining the shape and structure of a nucleosome. One chromatin molecule is composed of at least one of each core histones per 100 base pairs of DNA. [2] There are five families of histones known to date; these histones are termed H1/H5, H2A, H2B, H3 ...
Cumulative evidence suggests that such code is written by specific enzymes which can (for example) methylate or acetylate DNA ('writers'), removed by other enzymes having demethylase or deacetylase activity ('erasers'), and finally readily identified by proteins ('readers') that are recruited to such histone modifications and bind via specific ...
This is because the histone tail domains are involved in interactions between nucleosomes. The linker histone, or H1 protein, is also involved maintaining nucleosome structure. The H1 protein has the special role of ensuring that DNA stays tightly wound. [4] Modifications to histone proteins and their DNA are classified as quaternary structure.
[14] [15] These proteins are important in bending arrays of nucleosomes and arranging them into the larger structures that form chromosomes. [16] Recently FK506 binding protein 25 (FBP25) was also shown to non-specifically bind to DNA which helps in DNA repair.