] Nucleolus organizer regions (NORs) are chromosome, chromosomal regions crucial for the formation of the

nucleolus The nucleolus (, plural: nucleoli ) is the largest structure in the nucleus of eukaryotic cells. It is best known as the site of ribosome biogenesis, which is the synthesis of ribosomes. The nucleolus also participates in the formation of ...

. In humans, the NORs are located on the short arms of the

acrocentric The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers ...

chromosomes 13, 14, 15, 21 and 22, the genes

RNR1 RNA, ribosomal 1, also known as RNR1, is a human gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can ha ...

, RNR2, RNR3,

RNR4 RNA, ribosomal 4, also known as RNR4, is a human gene. References Further reading * Nucleolus organizer region ] Nucleolus organizer regions (NORs) are chromosome, chromosomal regions crucial for the formation of the nucleolus. In humans, th ...

, and

RNR5 RNA, ribosomal 5, also known as RNR5, is a human gene. Genes for ribosomal RNA are clustered on the short arms of chromosomes 13, 14, 15, 20, 21. The gene for RNR5 exists in multiple copies on chromosome 22. Each gene cluster contains 30–4 ...

respectively. These regions code for 5.8S, 18S, and

28S 28S ribosomal RNA is the structural ribosomal RNA (rRNA) for the LSU rRNA, large subunit (LSU) of eukaryotic cytoplasmic ribosomes, and thus one of the basic components of all eukaryotic cells. It has a size of 25S in plants and 28S in mammals, ...

ribosomal RNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosom ...

. The NORs are "sandwiched" between the repetitive,

heterochromatic Heterochromia is a variation in coloration. The term is most often used to describe color differences of the iris, but can also be applied to color variation of hair or skin. Heterochromia is determined by the production, delivery, and concentra ...

DNA sequences of the

centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers ...

s and

telomere A telomere (; ) is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes. Although there are different architectures, telomeres, in a broad sense, are a widespread genetic feature mos ...

s. The exact sequence of these regions is not included in the human reference genome as of 2016 or the GRCh38.p10 released January 6, 2017. On 28 February 2019, GRCh38.p13 was released, which added the NOR sequences for the short arms of chromosomes 13, 14, 15, 21, and 22. However, it is known that NORs contain tandem copies of

ribosomal DNA Ribosomal DNA (rDNA) is a DNA sequence that codes for ribosomal RNA. These sequences regulate transcription initiation and amplification, and contain both transcribed and non-transcribed spacer segments. In the human genome there are 5 chromo ...

(rDNA) genes. Some sequences of flanking sequences proximal and distal to NORs have been reported. The NORs of a

loris Loris is the common name for the strepsirrhine mammals of the subfamily Lorinae (sometimes spelled Lorisinae) in the family Lorisidae. ''Loris'' is one genus in this subfamily and includes the slender lorises, ''Nycticebus'' is the genus conta ...

have been reported to be highly variable. There are also DNA sequences related to rDNA that are on other chromosomes and may be involved in nucleoli formation. NORAgc

Visualization

] Barbara McClintock first described the "nucleolar-organizing body" in '' Zea mays'' in 1934. In karyotype analysis, a

silver stain In pathology, silver staining is the use of silver to selectively alter the appearance of a target in microscopy of histological sections; in temperature gradient gel electrophoresis; and in polyacrylamide gels. In traditional stained glass, silv ...

can be used to identify the NOR. NORs can also be seen in nucleoli using silver stain, and that is being used to investigate cancerous changes. NORs can also be seen using antibodies directed against the protein UBF, which binds to NOR DNA.

Molecular biology

In addition to UBF, NORs also bind to

ATRX Transcriptional regulator ATRX also known as ATP-dependent helicase ATRX, X-linked helicase II, or X-linked nuclear protein (XNP) is a protein that in humans is encoded by the ''ATRX'' gene. Function Transcriptional regulator ATRX contains an ...

protein,

treacle Treacle () is any uncrystallised syrup made during the refining of sugar.Oxford Dictionary The most common forms of treacle are golden syrup, a pale variety, and a darker variety known as black treacle, similar to molasses. Black treacle has ...

, sirtuin-7 and other proteins. UBF has been identified as a mitotic "bookmark" of expressed rDNA, which allows it to resume

transcription Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including: Genetics * Transcription (biology), the copying of DNA into RNA, the fir ...

quickly after mitosis. The distal flanking junction (DJ) of the NORs has been shown to associate with the periphery of nucleoli. rDNA

operon In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo splic ...

s in ''

Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Esc ...

'' have been found to cluster near each other, similar to a eukaryotic nucleolus.

References

{{DEFAULTSORT:Nucleolus Organizer Region Cell anatomy Molecular genetics Nuclear substructures

Visualization

Molecular biology

See also

References