Life depends on the ability of cells to store, retrieve, and translate the genetic instructions required to make and maintain a living organism. This chapter describes the structure of DNA. It considers how genes and other important segments of DNA are arranged in the single, long DNA molecule that forms the core of each chromosome in the cell. The chapter discusses how eukaryotic cells fold the long DNA molecules into compact chromosomes inside these nucleus. Each chromosome consists of a single, enormously long, linear DNA molecule associated with proteins that fold and pack the fine thread of DNA into a more compact structure. The structure of the nucleosome core particle was determined after first isolating nucleosomes by treating chromatin in its unfolded, "beads on a string" form with enzymes called nucleases, which break down DNA by cutting the phosphodiester bonds between nucleotides.