The process by which a cell divides into two or more cells. Among prokaryotes, cell division occurs by simple fission. Among eukaryotes, the cell nucleus divides first, and then a new cell membrane is formed between the nuclei to form the new cell. Cell division is used as a means of reproduction in organisms that reproduce asexually, as by fission or spore formation, and sexually reproducing organisms form gametes through cell division. Cell division is also the source of tissue growth and repair in multicellular organisms. The two types of cell division in eukaryotic organisms are mitosis and meiosis.