Nucleosome of DNA. Computer artwork of a double loop of DNA (deoxyribonucleic acid) forming a unit called a nucleosome. A nucleosome is itself a unit of chromatin, the chromosomal material found in cell nuclei. DNA consists of a double helix formed from two twisting sugar-phosphate strands (solid loops). The stick structures are the nucleotide bases that pair up and join the two strands. The sequence (171 pairs here) of the four types of base forms the genetic code that controls the biochemistry of life. The DNA circles twice from upper centre. A nucleosome forms about a core of histone proteins (not seen), a part of the coiling necessary to compact DNA inside a cell nucleus.