The Nucleosome Core Particle

The nucleosome core particle consists of an octamer containing four kinds of histones encircled by a left-handed coil of DNA. Nucleosome core particles play an important role in DNA function by controlling DNA compaction and chromatin structure. 

This role is so essential that histones are some of the most highly conserved proteins in eukaryotes - from peas to cows and other organisms. For example, there are only two different amino acids out of 102 between the H4 histones of a pea plant and a cow. 

The four kinds of histones that make up the nucleosome core particle, H2A, H2B, H3, and H4, share some characteristics. First, they are each small - only containing up to 135 amino acids. 

Second, they share a common structural motif: the histone fold. The histone fold consists of three α-helices connected by two loops. 

When nucleosome core particles are assembled, histone folds bind to each other first in an interaction described as a ‘handshake,’ forming two H2A-H2B dimers and two H3-H4 dimers. 

After this, the H3-H4 dimers form a tetramer, which goes on to form the octamer of the nucleosome core particle with the H2A-H2B dimers. 

The structure of this histone octamer begets extensive interactions between the histones and the DNA wound around them. 

There are more than 100 hydrogen bonds between the histones and the DNA of the nucleosome core and many of these are between the amino acid backbones of the histones and the sugar-phosphate backbone of the DNA.

Finally, many of the amino acids in each core histone are lysine or arginine, which have positive charges that effectively neutralize the negatively charged DNA backbone.