Akrosentrik

Akrosentrik, or acrocentric, describes a chromosome in which the centromere is located near one end, producing a very short p arm and a long q arm. The p arm is typically small and rich in repetitive DNA, including satellite sequences, and often carries the nucleolar organizer region (NOR) that contains ribosomal RNA gene repeats. Most genetic material is located on the q arm, with the kilobase content of the p arm being comparatively limited.

In humans, five chromosomes are acrocentric: 13, 14, 15, 21 and 22. The short arms of these chromosomes contain NORs and satellite DNA; the centromere position creates the characteristic appearance of a small stalk and a relatively long arm. The p arms are frequently rearranged or lost in chromosomal events without necessarily causing phenotypic abnormalities, provided there is no imbalance in essential genes.

A notable feature of akrosentrik chromosomes is their involvement in Robertsonian translocations, where two acrocentric chromosomes fuse near their centromeres to form a single chromosome with two long arms and a reduced short arm. Such translocations can be balanced, yielding a normal phenotype, or unbalanced, leading to developmental disorders or miscarriages. The classic example is a translocation involving chromosomes 14 and 21, which can contribute to Down syndrome in offspring when unbalanced.

Across species, the number and identity of acrocentric chromosomes vary, but their defining morphology aids karyotyping and cytogenetic comparisons. Identification relies on banding patterns and the presence of small p arms with satellite sequences and NORs.