The nucleus of a cell contains genetic material called chromosomes; the nuclear membrane, called the nuclear envelope, which controls the cell's growth and reproduction; and a liquid called nucleoplasm. The nucleus manages the cell's genetic material and controls the activities of the cell.
The chromosomes in the nucleus contain DNA, or nucleic acid, which holds heredity information and instructions for cell growth, development and reproduction. Ribonucleic acid, also known as RNA, is combined with proteins to create dense structures called ribosomes, which are also contained within the nucleus. Ribosomes basically organize and manage inner-cell proteins. The nucleoskeleton is a part of the nucleus that adds mechanical support to the overall cell.
The nuclear membrane, which consists of an inner membrane and an outer membrane, completely separates the nucleus from the rest of the cell. The membrane controls molecules that come and go from the cell. The membrane is impermeable to large molecules, unless the larger molecules pass through one of the membrane's pores and are transported to the cell's nucleus by specific proteins. The pores are also necessary for the transfer of important smaller molecules back and forth between the inner cell and other cells. In mammals, the nucleus composes approximately 10 percent of the cell.