The secondary constrictions are nucleolar organizer regions and must contain rDNA tandem repeats. Further information Chromosome size and centromere position are, in many species, insufficient to unambiguously identify each of the chromosomes. Centromeres are not clearly recognizable on all chromosomes. No substructure beyond secondary constrictions were visible in chromosome arms with the early staining regimes. Electron micrographs reveal substructure.
Chromosomes also condense in preparation for cell division .
The chromosomes at metaphase are particularly distinctive. An arrangement of metaphase chromosomes according to size and type is called a karyotype .