Nucleosome core particles are composed of two units each of H2A, H2B, H3, and H4 histone proteins, forming an octamer. These small, positively charged histone proteins are highly conserved in eukaryotes. For example, only two of the 102 amino acids in the H4 histones of a pea plant and a cow are different. Each of these four histones contains around 135 amino acids and shares a common structural histone fold motif, making up the nucleosome core particle. During assembly, histone folds bind to each other in an interaction described as a 'handshake,' forming two H2A-H2B dimers and two H3-H4 dimers. The H3-H4 dimers tetramerize with the H2A-H2B dimers, forming the octamer core. One point seven turns of DNA wrap around the octamer which compresses the long DNA molecule to fit inside the nucleus.