Cell division. Confocal light micrograph of a HeLa cell during cytokinesis (cell division). Cytokinesis occurs after nuclear division (mitosis), which produces two daughter nuclei (yellow) from one parent nuclei. A contractile ring made of actin filaments (green) forms beneath the cell's plasma membrane. This contracts, creating a furrow in the cytoplasm. The furrow deepens, separating the cytoplasm and cell organelles (such as mitochondria, red), until the two new cells are formed. HeLa cells are a continuously cultured cell line of human cancer cells. They are immortal and so thrive in the laboratory. HeLa cells are widely used in biological and medical research.