For the most part, a cell lives, grows and accomplishes its functions according to what kind of cell it is. This form of existence is called interphase. When the cell needs to divide, however, it enters a spectacular phase of existence called mitosis. The point of mitosis is to evenly divide the mother cell’s genetic material into two genetically identical daughter cells. The entire process can easily be seen under a microscope, and it is composed of four distinct stages. Along with interphase, these stages can be remembered by the acronym IPMAT.
Prophase -- the longest besides interphase -- begins as DNA threads in the nucleus condense into easily visible chromosomes. The chromosomes were actually replicated towards the end of interphase, and each copy is called a chromatid. A chromosome is x-shaped, with two chromatid copies joined together at a point called the centromere. At the same time, spindle fibers extend from the two centrioles towards the chromosomes. Later in prophase, the nuclear envelope breaks down, spilling the chromosomes into the cytoplasm. The spindle fibers attach to the centromeres and the two centrioles begin moving away from each other.
Metaphase is the easiest portion of mitosis to identify under a microscope. After the chromosomes are released from the nucleus by the breakdown of the nuclear membrane near the end of prophase, the centrioles finish the migration to opposite sides of the cell. The spindle fibers, which still attach the chromosomes to both centrioles, along with the pulling force of the two centrioles, align the chromosomes along the cell equator. This arrangement is called the metaphase plate.
The cell moves into anaphase the instant the chromatids are separated. The chromatids become V-shaped as they are pulled at the center by the centrioles to the opposite sides of the cell. At the same time, the cell begins to elongate in preparation for cell division. Cytokinesis -- the actual cell division – begins toward the end of the phase. Anaphase is rapid, lasting only a few minutes. While the beginning of the phase is easy to identify, the transition line between anaphase and telophase is somewhat blurry.
Telophase begins around the time the chromosomes stop moving after being separated in anaphase. The condensed genetic material begins to uncoil and extend and spindle fibers disappear. At the same time, two nuclear membranes form around the newly-migrated DNA, and for a short time the cell has two complete nuclei. Cytokinesis continues throughout the phase and is finished after telophase. Once division is complete, the cell immediately moves back into interphase.
- Nature Education: Mitosis and Cell Division
- University of Maryland: An Introduction to Mitosis
- Florida State University: Mitosis Photo Gallery
- Human Anatomy and Physiology; Elaine N. Marieb
- Comstock Images/Stockbyte/Getty Images