Illustration of the structure of a nucleosome consisting of H2A (magenta), H2B (beige), H3 (grey) and H4 (red) histones and DNA (deoxyribonucleic acid). The view point is from the top through the superhelical axis. Nucleosomes are the fundamental structural unit of chromosomes in eukaryotic cell nuclei. Cells store chromosomal DNA as chromatin, a compact arrangement of nucleosomes. Each nucleosome consists of a core group of histone proteins (orange) wrapped in chromosomal DNA.