Acrocentric chromosome

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An acrocentric chromosome is one in which the centromere is located very near to one of the ends of the chromosome, thus making the short arm of the chromosome negligible. The "acro-" in acrocentric refers to the Greek word for "end." There are five acrocentric chromosomes in the human genome: 13, 14, 15, 21 and 22. These five chromosomes are the site of genes encoding rRNAs. In contrast, all chromosomes in the mouse genome are acrocentric with the centromere at the "top" of each chromosome (Mouse Genome Sequencing Consortium, Initial Sequencing and Comparative Analysis of the Mouse Genome, Nature, 2002)

Each chromosome has two arms, labeled p (for petite, or short) and q (for long). If both arms are equal in length, the chromosome is said to be metacentric. If arms' lengths are unequal, the chromosome is said to be submetacentric, and if p arm is so short that is hard to observe, but still present, then the chromosome is acrocentric. A telocentric chromosome's centromere is located at the terminal end of the chromosome. Telomeres may extend from both ends of the chromosome. All mice chromosomes are telocentric; humans do not possess any telocentric chromosomes.