Acrocentric

An acrocentric chromosome is a chromosome in which the centromere is positioned near one end, producing a very short short arm (p) and a longer long arm (q). This arrangement gives the chromosome a distinctive shape with a secondary constriction and often visible satellites on the short arm. The short arm commonly contains repetitive DNA, including ribosomal RNA gene clusters organized as nucleolar organizer regions (NORs). The presence of satellites and NORs is a characteristic feature used to identify acrocentric chromosomes in cytogenetic analysis.

In humans, five autosomes are classed as acrocentric: chromosomes 13, 14, 15, 21, and 22. These chromosomes typically display a small p arm with satellite stalks and a comparatively longer q arm. The acrocentric designation is clinically relevant because acrocentric chromosomes commonly participate in Robertsonian translocations, where the long arms fuse at the centromere and may be inherited as a single chromosome. Balanced Robertsonian translocations involving these chromosomes can produce phenotypically normal carriers but increase the risk of unbalanced offspring. A well-known example is the translocation between chromosomes 14 and 21 (t(14;21)), which is associated with Down syndrome when unbalanced.

Beyond humans, the term acrocentric is used in karyotype descriptions across species that exhibit a similar chromosome morphology, reflecting evolutionary and structural variations in chromosome organization. The concept helps cytogeneticists interpret chromosome structure, NOR location, and potential chromosomal rearrangements during diagnostic and research settings.