3.3. Mitosis

Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membraneinto two cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle—the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell. This accounts for approximately 10% of the cell cycle.

Mitosis occurs only in eukaryotic cells and the process varies in different species. For example, animals undergo an “open” mitosis, where the nuclear envelope breaks down before the chromosomes separate, while fungi such as Aspergillus nidulans andSaccharomyces cerevisiae (yeast) undergo a “closed” mitosis, where chromosomes divide within an intact cell nucleus. Prokaryotic cells, which lack a nucleus, divide by a process called binary fission



The DNA in the nucleus has already been duplicated in the previous stage of cell division, so by the time prophase starts, the nucleus contains two complete identical sets of DNA. As prophase begins, the chromosomes, which are normally spread throughout the nucleus, begin to condense into an X shape, held together in the middle with a specific sequence of DNA called a centromere. Each half of the X is one replicated half of DNA. Once they coil together into the X, they’re called mitotic chromosomes. Towards the end of prophase, the material enclosing the nucleus and the cytoskeleton disappear, except in the case of some fungi, algae, and similar organisms, in which the process happens entirely inside the nuclear membrane. This is called closed mitosis.

Once the material enclosing the nucleus dissolves, or, in the case of closed mitosis, after the DNA forms into Xs, structures called centrioles move to opposite ends of the cell and help make a spindle apparatus of microtubules, which is essentially like ropes running across the cell. The chromosomes also develop structures in the middle called kinectochores, which are later used to hook onto the microtubules.


As prophase finishes and metaphase begins, the rope-like microtubules connect to the kinectochores on each side of the chromosome, so that they can later pull them apart. The chromosomes align themselves with the spindle apparatus, which is spread around the cell like the vertical lines on a globe. The soon to be divided chromosomes are symmetrically positioned on the metaphase plate, which is essentially the equator of the parent cell. At the end of metaphase, each chromosome has microtubules connected to both of its halves, and they are lined up in a straight line along the equator of the cell.


Once the chromosomes get lined up properly, the spindle apparatus immediately pulls the two identical DNA halves apart from one another and moves them to opposite sides of the cell. These two sets of chromosomes will develop into the nuclei of two daughter cells which are perfectly identical to each other and the parent cell.


After the chromosomes arrive at the ends of the cell, they start to uncoil and spread out again, as they were before they formed into Xs. This is basically the opposite of the beginning of prophase. While this happens, the spindle apparatus is broken down. After that, the nuclear membrane, which encases the nucleus, forms again around the chromosomes, unless it never dissolved in the first place, as in closed mitosis. Although this is the last phase, cell division is not complete until cytokinesis happens.


Cytokinesis is the next stage of cellular development, and is similar to mitosis, except it involves the other parts of the cell instead of the nucleus. During this phase, the equator-like metaphasal plate of the cell pinches together, separating the cell into two new cells. Once this is completed, there are two functioning, identical cells.

Mitosis animation I (MC Graw – Hill)

Mitosis animation II (Mc Graw – Hill)

Mitosis animation (Learn genetics – University of Utah)


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