Illustration showing how DNA (deoxyribonucleic acid) is packaged within cells. Two DNA strands, consisting of a sugar-phosphate backbone attached to nucleotide bases, twist into a double helix (top left). The DNA double helix is tightly coiled around histone proteins (yellow) which are tightly packaged together in groups of eight to form nucleosomes. These are then coiled together and further packaged into chromosomes (far right). Each human cell contains 23 pairs of chromosomes, which contain all the body's genetic information.