Mitosis


Mitosis is the process of asexual cell division in eukaryotic organisms. It is one of the phases of the cell cycle, occurring after interphase, and is often referred to as the mitotic (M) phase.

The DNA in the cell's chromosomes is replicated during the S phase of interphase. This causes each chromosome to have two sister chromatids attached at the centromere of the chromosome, which can later be separated into daughter cells. In mitosis, the sister chromatids are split between daughter nuclei - and ultimately daughter cells. Mitosis is usually followed by cytokinesis, the process in which cytoplasm, organelles and membranes are divided with the daughter nuclei to generate two complete daughter cells. However, cytokinesis may not be immediate; in fungi and slime moulds, for example, many daughter nuclei are held in a single cell (multinucleate cells) without division of the cytoplasm or organelles into complete daughter cells.

Unlike in meiosis, the products of mitosis remain diploid (or whatever the ploidy of the somatic cells is) and all of the products are genetically identical; mitosis generates clones.

Errors in chromosome segregation or damage to DNA during the mitotic phase results in a phenomenon called mitotic catastrophe, where the cell is subjected to programmed suicide, or apoptosis. Where mitotic catastrophe fails, there is a risk of aneuploidy and possibly cancer in the daughter cells.

Mitosis is divided into multiple phases. These phases are as follows:

  1. Preprophase (exclusive to plant cells): Because plant cells cannot migrate in the same way as, for example, animal cells, the limits and planes of cell division must be established in advance of mitosis to ensure correct cell wall placement. A transient microtubule structure, called the preprophase band, establishes the precise location of cell wall position and the plane of division before the cell enters prophase. In highly vacuolated cells, the formation of a phragmosome allows the nucleus to be placed at the centre of the cell, ready for division.
  2. Prophase: Chromosomes condense from the loosely-bound chromatin that previously existed in the nucleus, and become visible under the microscope.
  3. Prometaphase: The nuclear membrane disintegrates and kinetochore microtubules, originating from microtubule-organising structures called centrosomes, invade the nucleus, hooking on to kinetochores, which are protein-based structures attached to each of the two sister chromatids of the chromosome. The non-kinetochore microtubules from one centrosome attach to non-kinetochore microtubules of another centrosome, generating a long track between the two centrosomes called a spindle fibre.
  4. Metaphase: The chromosomes, with their kinetochores now attached to the kinetochore microtubules, align along an imaginary equatorial line that is equidistant between the poles of the cell called a metaphase plate. There is thought to be a signal at this stage called the spindle checkpoint that ensures that all kinetochores are attached to kinetochore microtubules before proceeding to the next phase
  5. Anaphase: The proteins holding together the two sister chromatids are cleaved, such that they are now independent sister chromosomes. The kinetochore microtubules depolymerise, causing them to contract and pull the sister chromosomes apart. Non-kinetochore microtubules, which attach the two centrosomes, polymerise and elongate, causing the centrosomes and the now-attached sister chromosomes to move completely apart to opposite poles of the cell. Anaphase can be sub-divided into early anaphase, which was the cleavage of the sister chromatids into sister chromosomes, and late anaphase, which is the elongation of non-kinetochore microtubules to completely separate the centrosomes and the populations of sister chromosomes
  6. Telophase: nuclear membranes reform around the two populations of sister chromosomes, using fragments of the disintegrated parent nucleus, to form complete daughter nuclei. The chromosomes 'un-condense' back into the loosely-bound chromatin form they were in before prophase and prometaphase. Although mitosis is complete, cell division continues with cytokinesis
  7. Cytokinesis: In animal cells, a contractile ring develops where the metaphase plate was, causing the cytoplasm to 'pinch' off and surround the two daughter nuclei. Cell membranes (and walls) develop between the regions of cytoplasm to create completed daughter cells. This marks the end of M phase and the cells return to the interphase in the cell cycle.