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In the cell that is in the process of division, each condensed chromosome appears as a pair of rods joined at one point, the centromere. These two chromosomal "halves", called sister chromatids, are identical and arise from the duplication of the original chromosomal filament that occurs at the interphase shortly before cell division begins.
During the process of cell division, the sister chromatids separate: each chromatid migrates to one of the forming daughter cells.
The centromere is located in a heterochromatic region, so a constriction containing the centromere is called a primary constriction, and any others that may exist are called secondary constrictions.
The parts of a chromosome separated by the centromere are called chromosomal arms. The size relationship between the chromosomal arms, determined by the position of the centromere, allows the chromosomes to be classified into four types:
- metacentric: have the centromere in the middle, forming two arms of the same size;
- subcentric: have a slightly displaced centromere of the middle region, forming two arms of unequal sizes;
- acrocentric: have the centromere very close to one end, forming a large arm and a very small arm;
- telocentric: have the centromere at one end, having only one arm.