Mitosis is one of the two main types of cell division, the other being meiosis. Mitosis is the process by which the diploid nucleus (having two sets of homologous chromosomes) of a somatic cell divides to produce two daughter nuclei, both of which are still diploid.


mitosis. Image: US National Library of Medicine
Mitosis. Credit: NIH


To more easily describe this process, imagine a cell with only one chromosome. Before a cell enters mitosis, it is said to be in interphase – the state of a eukaryotic cell when not undergoing division. Every time a cell divides, it must first replicate all of its DNA. Because chromosomes are simply DNA wrapped around protein, the cell replicates its chromosomes also. These two chromosomes, positioned side by side, are called sister chromatids and are identical copies of one another. Before the cell can divide, it must separate these sister chromatids from one another. To do this, the chromosomes have to condense. This stage of mitosis is called prophase. Next, the nuclear envelope breaks down, and a large protein network, called the spindle, attaches to each sister chromatid. The chromosomes are now aligned perpendicular to the spindle in a process called metaphase. Next, "molecular motors" pull the chromosomes away from the metaphase plate to the spindle poles of the cell. This is called anaphase. Once this process is completed, the cells divide, the nuclear envelope reforms, and the chromosomes relax and decondense during telophase. The cell can now replicate its DNA again during interphase and go through mitosis once more.