Within the nucleus, DNA (deoxyribonucleic acid)—the chemical that carries the genetic instructions for “making” living organisms—organizes with proteins into a fibrous material called chromatin. As a cell prepares to divide or reproduce, the thin chromatin fibers condense, becoming thick enough to be seen as separate structures— what we know as chromosomes. (For more about DNA and chromosomes, see the chapter “DNA, RNA, Chromosomes, and Genes.”)