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. It is a long molecule and has to be tightly coiled to fit inside the nucleus. DNA's double helix structure is shown here as a coiled spiral. The DNA is coiled round a core of histone proteins (multicoloured ribbons). The ribbons represent the molecular structure of the histone proteins. Each set of two DNA loops around a histone core is known as a nucleosome. Further compacting and packaging (not seen here) form the denser forms of chromatin, and eventually the cell's chromosomes.