Secondary constriction is seen at the chromosome in addition to primary constriction/centromere. Chromosome can bend only at the site of primary constriction during Anaphase. Secondary constrictions are useful in identifying a chromosome from a set. There are either 0, 1, 2, 3, or 4 secondary constriction sites in a cell at anaphase. Some parts of these constrictions indicates sites of nucleolus formation and so they are called "Nucleolar Organizing Region." (Anaphase is part of cell division) The formations of nucleolus takes place around the NOR region. The secondary constriction also contains the genes for rRNA synthesis (18 S, 5.8 S, 28 S)
NOR occurs in SAT (satellite chromosome) chromosomes (13,14,15,21,22)
References
Secondary constriction Wikipedia(Text) CC BY-SA