Light micrograph of onion (Allium cepa) meristematic cells during mitosis (nuclear division), the formation of two daughter nuclei from one parent nucleus. During mitosis sister chromatids (dark orange) are separated and pulled to opposite poles of the cell. The nucleolus, which is responsible for producing components of ribosomes, the cell's protein-manufacturing organelles, divides and moves with the chromatids. In telophase of mitosis the nuclear envelope reforms around the chromatids and nucleoli. Silver staining has been used to highlight the nucleolus.