Today's Technologies: Mitosis

Significance

The importance of mitosis is the maintenance of the chromosomal set; each cell formed receives chromosomes that are alike in composition and equal in number to the chromosomes of the parent cell. Transcription is generally believed to cease during mitosis, but epigenetic mechanisms such as bookmarking function during this stage of the cell cycle to ensure that the "memory" of which genes were active prior to entry into mitosis are transmitted to the daughter cells.^[17]

Consequences of errors

Although errors in mitosis are rare, the process may go wrong, especially during early cellular divisions in the zygote. Mitotic errors can be especially dangerous to the organism because future offspring from this parent cell will carry the same disorder.

In non-disjunction, a chromosome may fail to separate during anaphase. One daughter cell will receive both sister chromosomes and the other will receive none. This results in the former cell having three chromosomes coding for the same thing (two sisters and a homologue), a condition known as trisomy, and the latter cell having only one chromosome (the homologous chromosome), a condition known as monosomy. These cells are considered aneuploidic cells and these abnormal cells can cause cancer.^[18]

Mitosis is a traumatic process. The cell goes through dramatic changes in ultrastructure, its organelles disintegrate and reform in a matter of hours, and chromosomes are jostled constantly by probing microtubules. Occasionally, chromosomes may become damaged. An arm of the chromosome may be broken and the fragment lost, causing deletion. The fragment may incorrectly reattach to another, non-homologous chromosome, causing translocation. It may reattach to the original chromosome, but in reverse orientation, causing inversion. Or, it may be treated erroneously as a separate chromosome, causing chromosomal duplication. The effect of these genetic abnormalities depend on the specific nature of the error. It may range from no noticeable effect, cancer induction, or organism death.

Endomitosis

Endomitosis is a variant of mitosis without nuclear or cellular division, resulting in cells with many copies of the same chromosome occupying a single nucleus. This process may also be referred to as endoreduplication and the cells as endoploid.^[2] An example of endomitosis would be what occurs in megakaryocytes to generate platelets within its cytoplasm.^[19]

Timeline in pictures

Real mitotic cells can be visualized through the microscope by staining them with fluorescent antibodies and dyes. These light micrographs are included below.

Early prophase: Nonkinetochore microtubules, shown as green strands, have established a matrix around the degrading nucleus, in blue. The green nodules are the centrosomes.

Early prometaphase: The nuclear membrane has just degraded, allowing the microtubules to quickly interact with the kinetochores on the chromosomes, which have just condensed.

Late metaphase: The centrosomes have moved to the poles of the cell and have established the mitotic spindle. The chromosomes, in light blue, have all assembled at the metaphase plate, except for one.