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 nondisjunction, 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 containing the same genes (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 aneuploid, a condition often associated with cancer. Occasionally when cells experience nondisjunction, they fail to complete cell division and retain both nuclei in one cell, resulting in binucleated cells.
Mitosis is a demanding process for the cell, which 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 depends on the specific nature of the error.
When mutations occur in the genes that control the timing and number of mitotic cell cycles, cells may lose control of nuclear replication and cellular division. This can result in abnormal cell growth and the synthesis of excess tissue in a single organ. The excess tissue is a cancerous cell mass known as a tumor. Tumors that remain in their original location are called benign tumors and may not be harmful if they do not grow to excessive sizes. Tumors that leave their original location and invade other cells there are called malignant tumors. The migrating tumor cells may lodge in other parts of the body and form new tumors in a process known as metastasis.
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