Chromosome morphology at the light microscopic level

Chromosomes are typically visualized at mitotic metaphase, when the DNA has replicated and chromosomal proteins are doubled, but have not yet separated to the opposite poles. Chromosomes in this state appear as two chromatids (sister chromatids) each comprising a single DNA double helix, attached to a single centromere. Chromosomes are classified according to the relative position of the centromere., which divides the chromatid into short (p) and long (q) arms. Basic patterns are metacentric (centromere in the middle), acrocentric (centromere nearer to the end than the middle), and telocentric (centromere all the way at one end.). Chromosomes in which the centromere is closer to the middle than the end are described as submetacentric. All chromatids are capped with special DNA sequences called telomeres. Careful examination of telocentric chromosomes show they have telomeric sequences as well as (very) short DNA sequences. However, the convention remains to count the number of arms in a karyotype (the Fundamental Number, or NF) as two for each metacentric or acrocentric chromosome, and one for each telocentric.

Figure © 2012 TA Brown, Introduction to Genetics (1st ed.); additional text © 2015 by Steven M. Carr