Question

Why second constriction point is also known as known as Nuclear organiser region(NOR)?

Answer 1

Secondary constriction can be found at a chromosome in addition to primary constriction/centromere. Chromosomes 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 indicate sites of nucleolus formation and are called "Nucleolar Organizing Regions." The formations of nucleolus takes place around the NOR region. The secondary constriction also contains the genes for rRNA synthesis.