Nucleolus organizer region

As Nukleolusorganisatorregion (NOR ) is called in the genetics section of metaphase chromosomes containing the 45S rDNA, ie a portion of the genes for rRNA localized, are then formed from the in interphase nucleolus or nucleoli. The ribosomal DNA is usually tightly packed in the form of a secondary constriction ( constriction ), which is separated and a type of terminal regions of chromosomes Beikörper (satellite) is formed. This was back in 1971 postulated by Chiarelli, because the constrictions are conserved in Old World monkeys and always were to appear together with NORs.

However, not all secondary constricted chromosomes spaces NORs. The number of NORs in the metaphase and the number of nucleoli do not correlate, as some NORs are not active or because they often merge into one nucleolus.

Nukleolusorganisatorregionen of people are only on the acrocentric chromosomes 13, 14, 15, 21 and 22