Nucleolus organizer region

The human chromosome 21 has a NOR near its tip (area 12)

In genetics, the nucleolus organizer region (NOR) is the section of the metaphase - chromosomes that contain the 45S rDNA , i.e. a part of the genes for the rRNA , from which the nucleolus or nucleoli are then formed in the interphase . It was first described by Barbara McClintock in 1934 .

The ribosomal DNA is usually tightly packed in the form of a secondary constriction, which separates terminal areas and forms a kind of chromosome accessory body ( satellite ). This was postulated by Chiarelli as early as 1971, since the constrictions in Old World monkeys are conserved and always occur together with NORs.

However, not all secondary chromosomal areas are NORs. The number of NORs in the metaphase and the number of nucleoli do not correlate either, since some NORs are not active or because they often fuse to form a nucleolus.

Human nucleolus organizer regions are only found on acrocentric chromosomes 13 , 14 , 15 , 21, and 22 .

Individual evidence

1. ↑ Barbara McClintock: The relation of a particular chromosomal element to the development of the nucleoli in Zea mays . In: Journal of Cell Research and Microscopic Anatomy . tape 21 , no. January 2 , 1934, p. 294-326 , doi : 10.1007 / BF00374060 .
2. ↑ AS Henderson, D. Warburton, KC Atwood: Localization of rDNA in the chromosome complement of the Rhesus (Macaca mulatta) . In: Chromosoma . tape 44 , no. 4 , December 1974, p. 367-370 , doi : 10.1007 / BF00284896 .
3. ↑ AB Chiarelli: Comparative cytogenetics in primates and its relevance in human cytogenetics. In: AB Chiarelli (Ed.): Comparative genetics in monkeys, apes and man. Academic Press, 1971, pp. 273-308.