DNA nucleosome. Molecular model of a nucleosome, 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 (green and red) is seen here tightly coiled round a core of histone proteins (centre, multicoloured). Each set of two DNA loops around a histone core is known as a nucleosome.