DNA nucleosome. Computer model based on X-ray diffraction data of the structure of a nucleosome. A nucleosome is a complex of DNA (deoxyribonucleic acid, red, orange and yellow) and proteins called histones (blue and green helices). The nucleosomes form insbeads on a stringins of DNA, which are coiled in turn to form a fibre of chromatin, the form in which DNA is stored inside cell nuclei. The structure of chromatin allows huge lengths of DNA to be stored in a very small volume. During DNA replication or the expression of genes in the DNA, the chromatin unwinds into a form that allows regulatory proteins and RNA complexes access to the genes.