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Isochromosome
An isochromosome is an abnormal chromosome produced by either transverse splitting of the centromere that both arms are derived from same side of the centromere or duplication of one arm followed by loss of the other.



The chromosome arm is already copied during S phase of the cell cycle. During mitosis (or meiosis I or II), the sister chromatid sets line up along the midline in metaphase. The affected chromosome simply lines up at a right angle to its normal position, and as anaphase begins, the centromere is divided in the opposite plane from all the other chromosomes. This leaves the two long arms together and the two short arms together. The two new mirror-image chromosomes are pulled into opposite daughter cells. This produces two cells, each lacking one arm (e.g. the short arm) and containing an extra arm (e.g. the long arm) of the affected chromatid (or vice versa). It can also be formed by exchange involving one arm of a chromosome and its homologue (or sister chromatid) at the proximal edge of the arm, adjacent to the centromere. As a result, both arms are of equal length and possess identical genes. Each arm has identical genes arranged in the same order counting away from the centromere.

An isochromosome is named as combination of chromosome number and arm notation of chromosome (p or q). For example, isochromosome 12p refers to an isochromosome with two short arms (p) of chromosome 12.

Medical researches
First used in 1940, isochromosomes were studied by many medical researches due to its close relatedness with tumors. In tumor researches, isochromosome 17q is highly studied for cancer research since it plays major role in tumor development and progression. Also in disease studies, for example, isochromosome in X chromosome is studied since 20% of all cases of Turner syndrome result from having isochromosome in X chromosome in females. Listed below are some abnormalities associated with isochromosomes:


 * Patients with Pallister-Killian mosaic syndrome have extra isochromsome 12p.


 * Patients with isochromosome 18p syndrome have extra isochromosome 18p.


 * Patients with splenic lymphoma have isochromosome 17q.


 * Patients with Turner syndrome have isochromosome Xq.