Secondary constriction

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)

External links

• "Karyomorphological Studies of Species in Taxodiaceae " - METLA
• "Physical locations of 5S and 18S-25S rDNA in Asian and American diploid Hordeum species with the I genome" - Heredity
• "Evolutionary aspects of the ZZ/ZW sex chromosome system in the Characidae fish, genus Triportheus. A monophyletic state and NOR location on the W chromosome" - Heredity
• "Evolutionary implications of permanent odd polyploidy in the stable sexual, pentaploid of Rosa canina L" - Heredity
• "Molecular characterization of the secondary constriction region (qh) of human chromosome 9 with pericentric inversion" - Journal of Cell Science