Secondary constriction

Secondary constrictions are useful in identifying a chromosome from a set.

Some parts of these constrictions indicate sites of nucleolus formation and are called "nucleolar organizing regions" (NORs).

The nucleolus in the nucleus remains associated with the NOR of the secondary constriction area.

The formations of nucleoli takes place around the NOR region.

DNA in a secondary constriction which forms rRNA is called rDNA.

Schematic karyogram of a human. Each row is vertically aligned at centromere level. The top of chromosomes 13, 14, 15, 21, & 22 are satellites, with a secondary constriction between the satellite and the centromere.