Structural and Functional Aspects of Nucleolar Organizer Regions (NORs) of Human Chromosomes

Abstract

Publisher Summary The nucleolar organizer regions (NORs) have been demonstrated to be the morphological sites around which the nucleoli develop at the end of mitosis. This chapter describes the structural and functional aspects of NORs of human chromosomes and discusses the clinical significance of these regions. The chromosomal associations resulted from NORs play an important role in at least three types of chromosomal disorders, and the most frequent of them is the meiotic nondisjunction causing trisomic condition. The degree of NOR activity is represented by different types of nucleoli, such as small ring-shaped nucleoli with low activity and compact nucleoli with full activity. The human acrocentric chromosomes because of the presence of NORs on short arms have a tendency to remain associated with each other during the cell division, which have deleterious consequences through meiotic nondisjunction. However, the biological and clinical implications of NOR size heteromorphisms of human acrocentrics are poorly understood.