DNA tetranucleosome. Molecular model of four nucleosomes, or a tetranucleosome. Nucleosomes are the fundamental repeating unit used to package DNA (deoxyribonucleic acid) inside cell nuclei. DNA is the molecule that carries the genetic code that forms the basis of all life on Earth. DNA's double helix structure (light blue and light pink) is seen here tightly coiled round a core of histone proteins (multicoloured). Each set of two DNA loops around a histone core is known as a nucleosome.