Confocal light micrograph of a cultured human colon cancer cell undergoing mitosis (nuclear division). Mitosis is the formation of two daughter nuclei from one parent nucleus. Fluorescent markers have been used to highlight DNA (deoxyribonucleic acid, blue) and alpha tubulin (green), a component of microtubules. Centrosomes, which are made from microtubules, appear as bright red dots. The cell at centre is in prophase, the first stage of mitosis. The cell at centre has four centrosomes rather than the usual two. This will lead to missegregation when the chromosomes are separated into their two identical chromatids.