Histones are one major example of these associated proteins, and are separated into six classes: H1, H2A, H2B, H3, H4, and H5. Two each of histones H2A, H2B, H3, and H4 assemble into an eight-subunit nucleosome and wrap 146 or 147 DNA base pairs of the simple, double helix structure around the complex.
These proteins enable not only efficient packing of over 12 billion nucleotides in human DNA, but also selective unwinding of these chromosomes to expose genes for DNA replication, DNA to RNA transcription and mRNA processing.
Histone Structure. The histone contains a core of histone molecules, including pairs of H2A, H2B, H3, and H4, wrapped by double-helix DNA and held together by histone H1. See text for further details. DNA, deoxyribonucleic acid. [Reproduced with permission from Mescher AL: Junqueira's Basic Histology Text and Atlas, 12th edition, McGraw-Hill, 2010.]
This allows the very long DNA molecules to fit into the cell nucleus.
Negatively charged DNA loops twice around positively charged histone octamer (2 sets of H2A, H2B, H3 and H4) to form nucleosome "bead" like "beads on a string." Octamer subunits consists primarily of lysine and arginine amino acids (positive charged) H1 ties nucleosome beads together in a string. H1 is the only histone that is not in the nucleosome core.
For DNA to function, it is combined with proteins and organized into a precise, compact structure, a dense string-like fiber called chromatin.
Histones
Histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes.They are the chief protein components of chromatic, acting 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 1 in human DNA). For example, each human cell has about 1.8 meters of DNA, but wound on the histones it has about 90 micrometers (0.09 mm) of chromatin, which, when duplicated and condensed during mitosis, result in about 120 micrometers of chromosomes.
