Chromosomes contain one or more other sites of contraction other than the centromere. This is called secondary shrinkage. A secondary contraction called SAT (Sine Acid Thymonucleic) forms the nucleolus. Each diploid chromosome has a pair of secondary chromosomes. It is called Nucleolar Organizer Region-NOR. Human chromosomes 13, 14, 15, 21 and 22 have a total of five pairs of NORs.