The Stages of Mitosis


The cell is the building block of life: from unicellular bacteria, to the largest of organisms with trillions of cells. The basic method of propagation of life (in the case of unicellular organisms) and growth (in the case of multicellular organisms) from a 1-cell embryo in all eukaryotes is essentially the same: they all undergo mitosis. Mitosis is the process of cell division in which cells segregate their replicated DNA equally and exactly into two daughter cells. After the discovery of cells at the advent of light microscopy, it was not long before the general details of mitosis were first observed. In 1879, Flemming proposed the term mitosis to describe the manner in which a cell multiplied both its cytoplasm and its nucleus, as he had seen under the microscope.
In 1911, Edmund Wilson published a description of the different stages of mitosis. In the prophases (now shortened to simply "prophase"), the nuclear envelope breaks down, and the chromosomes (so named for their dark appearance under the microscope) become highly condensed and take on a rod-like appearance. In fact, Flemming predicted that cells have a characteristic number of chromosomes, and that after mitosis, that number was preserved. Each chromosome in a mitotic cell has been replicated, and identical copies of the chromosome (sister chromatids) are held together by proteins called cohesins.
These replicated chromosomes also contain a pair of sister kinetochores, one on each sister chromatid. A kinetochore is a large proteinaceous machine that assembles onto centromeric DNA, and is responsible for moving the sister chromatids to opposite sides of the cell at the end of mitosis. They accomplish this by attaching to the mitotic spindle. The mitotic spindle, so named for its resemblance to a part of a spinning wheel, is a football-shaped structure comprised of two centrosomes, or spindle poles, microtubule fibers, kinetochores, and the chromosomes attached to the kinetochores. Before nuclear envelope breakdown (NEB), the two spindle poles are very close together, and it is during prophase that they begin to move apart from each other, and the true mitotic spindle takes shape. It is also during prophase that initial kinetochore-microtubule attachments are formed.
During prometaphase, the spindle poles have fully separated, and mature kinetochore-microtubule attachments are forming. The end result of the kinetochore-microtubule attachments will be to align all of the chromosomes at the center of the cell (the metaphase plate), and prometaphase is the stage just before this alignment. During prometaphase, any problems with the spindle or with microtubule-kinetochore attachments are corrected.
Once all chromosomes are correctly attached and ready to be segregated, the cell is said to be in metaphase. Metaphase cells have all of their chromosomes tightly aligned at the metaphase plate, and once there, the regulatory checkpoint that monitors the spindle assembly is satisfied, and the cell is allowed to proceed to the later stages of mitosis.
In anaphase, the cohesins that hold sister chromatids together are degraded, and the sister chromatids are pulled by their kinetochore attachments to the spindle towards opposite poles. There are two phases of anaphase: A and B. In anaphase A, the distance between spindle poles remains the same, and it is the shrinking of kinetochore microtubules that pulls the chromosomes apart. In anaphase B, the pole-to-pole distance becomes greater, pulling the chromosomes even further apart.
After anaphase, cytokinesis occurs, and the one original cell is split into two. Telophase is the final stage of mitosis, during which the chromosomes begin to decondense, and a new nuclear envelope forms around the chromatin. Now, each daughter cell has one spindle pole, or centrosome, and exactly one copy of each maternal and paternal chromosome, just as when it began.