In biology, a nucleus is a large, membrane-bounded organelle that contains genetic material in the form of multiple linear DNA molecules. It is found only in eukaryotic cells and is the defining characteristic feature of eukaryotic cells. The nucleus is completely bound by membranes and is engirdled by a structure referred to as the nuclear envelope. The membrane distinguishes the cytoplasm from the contents of the nucleus, and the cell’s chromosomes are also confined within it. The nucleus controls and regulates the activities of the cell, such as growth and metabolism, and carries the genes, structures that contain the hereditary information. In nuclear physics, the atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom. The nucleus of an atom consists of neutrons and protons, which in turn are the manifestation of more elementary particles, called quarks, that are held in association by the nuclear strong force in certain stable combinations of hadrons, called baryons.