Chromosomes first become visible during prophase in mitosis, and they are fully visible during metaphase and anaphase. They lose their visibility during telophase and are not visible during cytokinesis, which follows mitosis.
Together, mitosis and cytokinesis form the process of cell division. Mitosis duplicates the cell's DNA and splits the nucleus, and cytokinesis splits the cytoplasm and forms the physical structure of the new cell. Prophase, metaphase, anaphase and telophase are the four phases that make up mitosis.
Mitosis is a form of asexual reproduction. It produces a cell that is an exact copy of the parent cell. The equivalent process in sexual reproduction is called meiosis. Cells that divide through meiosis receive half of the chromosomes from the parent cell, and they gain the other half of their chromosomes when they combine with a cell from the other parent organism.
It is possible for errors to occur during mitosis; these errors leave the new cell with too few or too many chromosomes. These are particularly common in humans during early embryonic development. Errors in mitosis lead to birth defects when they occur in embryos. When these errors occur in later stages of human development or during adulthood, they lead to cancer.