text: Core histones are four proteins called H2A, H2B, H3 and H4 and they are all found in equal parts in the cell.  All four of the core histone amino acid sequences contain between 20 and 24% of lysine and arginine and the size or the protein ranges between 11400 and 15400 daltons, making them relatively small, yet highly positively charged proteins.  High content of positively charged amino acids allow them to closely associate with negatively charged DNA.  Heterodimers, or histone-only intermediates are formed from histone-fold domains.  The formation of histone only-intermediates proceeds when core histones are paired into the interlocked crescent shape quasi-symmetric heterodimer.  Each histone fold domain is composed of 3 α-helix regions that are separated by disordered loops.  The histone fold domain is responsible for formation of head-to-tail heterodimers of two histones: H2A-H2B and H3-H4.  However, H3 and H4 histones first form a heterodimer and then in turn the heterodimer dimerizes to form a tetramer (H3-H4).  The heterodimer formation is based on the interaction of hydrophobic amino acid residue interactions between the two proteins.
  Quasi symmetry allows the heterodimer to be superimposed on itself by a 180 degree rotation around this symmetry axis.  As a result of the rotation, two ends of histones involved in DNA binding of the crescent shape H3-H4 are equivalent, yet they organize different stretches of DNA.  The H2A-H2B dimer also folds similarly.  The (H3-H4) tetramer is wrapped with DNA around it as a first step of nucleosome formation.  Then two H2A-H2B dimers are connected to the DNA-(H3-H4) complex to form a nucleosome.
  Each of the four core histones, in addition to their histone-fold domains, also contain flexible, unstructured extensions called histone “tails”.  Treatment of nucleosomes with protease trypsin indicates that after histone tails are removed, DNA is able to stay tightly bound to the nucleosome.  Histone tails are subject to a wide array of modifications which includes phosphorylation, acetylation, and methylation of serine, lysine and arginine residues.