Isochromosome
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An isochromosome is a chromosome that has lost one of its arms and replaced it with an exact copy of the other arm. This is sometimes seen in some girls with Turner syndrome or in tumor cells.
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).
