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

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

RNR1 RNA, ribosomal 1, also known as RNR1, 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, the ...

RNR2 RNA, ribosomal 2, also known as RNR2, is a human gene coding for ribosomal RNA. Genes for ribosomal RNA are clustered on the short arms of chromosomes 13 ( RNR1), 14 (RNR2), 15 ( RNR3), 20 (RNR4), Chromosome 21 (human), 21 ( RNR5). The gene for ...

RNR3 RNA, ribosomal 3, also known as RNR3, is a human gene. It is a minor isoform of large subunit of ribonucleotide-diphosphate reductase; the RNR complex catalyzes rate-limiting step in dNTP synthesis, regulated by DNA replication and DNA damage ch ...

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, the ...

, 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 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, hence th ...

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 ribosomal ...

. 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 concentr ...

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 a ...

s and telomeres. 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 (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 contain ...

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

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Barbara McClintock Barbara McClintock (June 16, 1902 – September 2, 1992) was an American scientist and cytogenetics, cytogeneticist who was awarded the 1983 Nobel Prize in Physiology or Medicine. McClintock received her PhD in botany from Cornell University in ...

first described the "nucleolar-organizing body" in '' Zea mays'' in 1934. In

karyotype A karyotype is the general appearance of the complete set of metaphase chromosomes in the cells of a species or in an individual organism, mainly including their sizes, numbers, and shapes. Karyotyping is the process by which a karyotype is disce ...

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 An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...

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, sirtuin-7 and other proteins. UBF has been identified as a mitotic "bookmark" of expressed rDNA, which allows it to resume transcription quickly after mitosis. The distal flanking junction (DJ) of the NORs has been shown to associate with the periphery of nucleoli. rDNA operons 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

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Molecular biology

See also

References