Histone

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In biology, histones are highly alkaline proteins found in eukaryotic cell nuclei, which package and order the DNA into structural units called nucleosomes.[1][2] They are the chief protein components of chromatin, act as spools around which DNA winds, and play a role in gene regulation. Without histones, the unwound DNA in chromosomes would be very long (a length to width ratio of more than 10 million to one in human DNA). For example, each human cell has about 1.8 meters of DNA, but wound on the histones it has about 90 millimeters of chromatin, which, when duplicated and condensed during mitosis, result in about 120 micrometers of chromosomes.[3]

Contents

Classes

Histones "are highly conserved and can be grouped into five major classes: H1/H5, H2A, H2B, H3, and H4".[2][4][5] These are organised into two super-classes as follows:

  • core histones – H2A, H2B, H3 and H4
  • linker histones – H1 and H5

Two of each of the core histones assemble to form one octameric nucleosome core particle by wrapping 147 base pairs of DNA around the protein spool in 1.65 left-handed super-helical turn.[6] The linker histone H1 binds the nucleosome and the entry and exit sites of the DNA, thus locking the DNA into place[7] and allowing the formation of higher order structure. The most basic such formation is the 10 nm fiber or beads on a string conformation. This involves the wrapping of DNA around nucleosomes with approximately 50 base pairs of DNA separating each pair of nucleosomes (also referred to as linker DNA). The assembled histones and DNA is called chromatin. Higher-order structures include the 30 nm fiber (forming an irregular zigzag) and 100 nm fiber, these being the structures found in normal cells. During mitosis and meiosis, the condensed chromosomes are assembled through interactions between nucleosomes and other regulatory proteins.

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