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).