Also known as deoxyribonucleic acid, is a thin, chainlike molecule found in every living cell on earth. It directs the formation, growth, and reproduction of cells and organisms. Short sections of DNA called genes determine heredity— that is, the passing on of characteristics—in living things. DNA is found mainly within a cell's nucleus, in threadlike structures called chromosomes. DNA even occurs in bacterial cells, which do not have a nucleus, and in some viruses.