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Centromeres

Centromere and band locations are important to understand when working with fluorescence in situ hybridization (FISH) probes. Centromere location is characteristic of each chromosome and within a metaphase spread, the chromosomes are distinguishable based on their centromere location and length of the p and q arms.
Centromeres can be located near the center of the two arms, slightly off center, or toward the end of one arm. When the centromere is located where the two arms are roughly equal in length, it is metacentric. Chromosomes 1-3 are easily recognizable because of their large size and metacentric centromeres. Likewise, chromosomes 16-18 are easily recognizable because of their small size and metacentric centromeres. Chromosomes 19-20 are short chromosomes with metacentric centromeres.
When the centromere is located where the two arms are NOT equal in length, it is submetacentric. Chromosomes 4-5 are large chromosomes with submetacentric centromeres. Chromosomes 6-12 and X are medium-sized chromosomes with submetacentric chromosomes.
When the centromere is located close to one end of the chromosome arm, it is acrocentric.
A major band is numbered in sequence with the first band being the closest to the centromere.