DNA compaction and histones. Illustration of stages in the compaction of DNA (deoxyribonucleic acid). The DNA strand coils around proteins called histones (pink, red; lower right). This forms circular structures called nucleosomes (four shown here). These nucleosomes form the 10 nanometre 'beads-on-a-string' structure (centre), followed by the more compact coiling seen in the '30 nanometre fibre' structure (top left). The more loose packaging shown here is typical of DNA coding genes that are actively transcribed ('turned on'), and are found associated with RNA polymerases. This illustration includes marker chemicals such as methylcytosine.