Chromatin fibre. Computer artwork of strands of DNA (deoxyribonucleic acid, green) coiled around histone cores (multicoloured) to form a chromatin fibre. The coiled ribbon structures in the cores represent the molecular structure of the histone proteins. DNA, the molecule that carries the genetic code that forms the basis of all life on Earth, is found in cell nuclei. It is a long molecule and has to be tightly coiled to fit inside the nucleus. Each set of two DNA loops around a histone core is known as a nucleosome. Further compacting and packaging of these units (as seen here) forms chromatin. The chromatin is then packaged with more proteins (not seen) to form the cell's chromosomes.