cell

the basic building block of all life and the smallest structural unit in the body. Human body cells vary in size and function and number several billion. Each cell consists of a cell body surrounded by a membrane. The cell body consists of a substance known as cytoplasm, containing various organelles and also a nucleus. The nucleus contains the CHROMOSOMES , composed of the genetic material, the DNA . Most human body cells contain 46 chromosomes (23 pairs), half being derived from the individual's father and half from the mother. Cells are able to make exact copies of themselves by a process known as MITOSIS , and a full complement of chromosomes is received by each daughter cell. However, the human sex cells (sperm and ova) differ in always containing half the number of chromosomes. At fertilization, a sperm and ovum combine and a complete set of chromosomes is received by the new embryo. See also MEIOSIS .