Nucleolus organizer region

The nucleolus organizer region (NOR) or nucleolar organizer is a chromosomal region around which the nucleolus forms. This region is the particular part of a chromosome that is associated with a nucleolus after the nucleus divides. The region contains several tandem copies of ribosomal DNA genes. In humans, the NOR contains genes for 5.8S, 18S, and 28S rRNA clustered on the short arms of chromosomes 13, 14, 15, 21 and 22 (the acrocentric chromosomes).

Nucleolus organizer regions (NORs) are head-to-tail arrays of genes encoding the precursor of the three largest ribosomal RNAs (18S, 5.8S and 25S in plants). NORs include active rRNA genes, which give rise to secondary constrictions of metaphase chromosomes, and silent rRNA genes, which are often highly compacted in dense heterochromatin. At metaphase, a proteinaceous remnant of the nucleolus often remains associated with the secondary constriction. Each rRNA gene at a NOR is nearly identical in sequence, although variation in size due to differences in the number of repeated DNA elements in the intergenic spacer region is common.

In the polytene chromosomes of Phaseolus coccineus cv complete chromosome complements there were always 6 chromosomes with a terminal Nucleolus organizing region (NOR). In most cases part of the NOR was decondensed, and several of these decondensed areas formed together a big collecting nucleolus (Ger. Sammelnukleolus). This collecting nucleolus was easily visible in phase contrast even without pretreatment due to its large size and particular structure. The shape of the collecting nucleolus ranged from nearly circular to irregular. Instead of just one collecting nucleolus of 6 NOR there were also several smaller collecting nucleoli made up from the NORs of only 2 to 5 nucleolus organizing chromosomes.

Evaluation of nucleolar organizer region-associated proteins in breast malignancy Nucleolus organizer regions (NORs) have been identified by means of an argyrophilic technique (Ag-NOR) in routinely processed, formalin-fixed paraffin sections of breast lesions. This method reveals NORs as black dots in the nuclei of cells, by virtue of the argyrophilia of NOR-associated proteins. The number of Ag-NORs has been thought to be related to cellular activation and has recently been applied to non-Hodgkin’s lymphomas and melanocytic skin lesions. It was found in the present study that the total number of Ag-NORs in malignant breast lesions significantly exceeded those of normal breast and benign lesions. The number of clumps of Ag-NORs, however, were not useful discriminators. Neither numbers of total Ag-NORs nor of clumps of Ag-NORs correlate with mitotic counts and it may be that their numbers relate to ploidy. It is suggested that the Ag-NOR technique will find increasing application as an adjunct to diagnostic histopathology.

In karyotype analysis, a silver stain can be used to identify the NOR. Silver nitrate inserts into the NOR-associated protein in the stalks and satellites, staining the proteins dark black. The amount of stain deposited and the number of NORs differs among the population, although the cell should normally have a maximum of 10 NORs per cell.

Discovery

Barbara McClintock first described the "nucleolar-organizing body" in Zea mays in 1934.

See also

• G banding