Medical Genetics B the nucleosome The fundamental structural unit of chromatin is an assemblage, called the nucleosome composed of five types of histones(designated H1 H2A, H2B, H3, and H4) and DNAMedical Genetics B. the nucleosome The fundamental structural unit of chromatin is an assemblage, called the nucleosome, composed of five types of histones (designated H1, H2A, H2B, H3, and H4) and DNA