Satellite chromosome

Not to be confused with Satellite DNA.

Satellite chromosomes or SAT-chromosomes are chromosomes that contain secondary constructs that serve as identification. They are observed in acrocentric chromosomes. In addition to the centromere, one or more secondary constrictions can be observed in some chromosomes at metaphase. In humans they are usually associated with the short arm of an acrocentric chromosome,^[1] such as in the chromosomes 13, 14, 15, 21, & 22.^[2][3] The Y chromosome can also contain satellites, although these are thought to be translocations from autosomes.^[4] The secondary constriction always keeps its position, so it can be used as markers to identify specific chromosomes.

Schematic karyogram of a human. Each row is vertically aligned at centromere level. The top of chromosomes 13, 14, 15, 21, & 22 are satellites, with a secondary constriction between the satellite and the centromere.

Further information: Karyotype

The name derives from the small chromosomal segment behind the secondary constriction, called a satellite, named by Sergei Navashin, in 1912.^[5] Later, Heitz (1931) qualified the secondary constriction as the SAT state (Sine Acido Thymonucleinico, which means "without thymonucleic acid"), because it didn't stain with the Feulgen reaction. With time, the term "SAT-chromosome" became an abbreviation for satellite chromosome.^[6][7][8]

The satellite at metaphase appears to be attached to the chromosomes by a thread of chromatin.

SAT-chromosomes whose secondary constriction is associated with the formation of the nucleolus are referred to as nucleolar SAT-chromosomes. There are at least 4 SAT-chromosomes in each diploid nucleus, and the constriction corresponds to a nucleolar organizer (NOR), a region containing multiple copies of the 18S and 28S ribosomal genes that synthesize ribosomal RNA required by ribosomes. The appearance of secondary constrictions at NORs is thought to be due to rRNA transcription and/or structural features of the nucleolus impeding chromosome condensation.^[9]

References

1. "chromosome satellite. (n.d.)". The Free Dictionary. Medical Dictionary for the Health Professions and Nursing. 2012. Retrieved 26 September 2020.
2. Sullivan, G.J.; Bridger, J.M.; Cuthbert, A.P.; Newbold, R.F.; Bickmore, W.A.; McStay, B. (2001), "Human acrocentric chromosomes with transcriptionally silent nucleolar organizer regions associate with nucleoli", The EMBO Journal, 20 (11): 2867–2877, doi:10.1093/emboj/20.11.2867, PMC 125486, PMID 11387219
3. Nussbaum, Robert L.; McInnes, Roderick R.; Willard, Huntington F.; Hamosh, Ada (21 May 2015). Thompson & Thompson genetics in medicine (8th ed.). Philadelphia, PA. ISBN 9781437706963. OCLC 908336124.{{cite book}}: CS1 maint: location missing publisher (link)
4. Schmid, M.; Haaf, T.; Solleder, E.; Schempp, W.; Leipoldt, M.; Heilbronner, H. (1984). "Satellited Y chromosomes: structure, origin, and clinical significance". Human Genetics. 67 (1): 72–85. doi:10.1007/bf00270562. ISSN 0340-6717. PMID 6745929. S2CID 25550661.
5. Rieger, R.; Michaelis, A.; Green, M.M. (1968). A glossary of genetics and cytogenetics: Classical and molecular. New York: Springer-Verlag. ISBN 9780387076683.
6. Jeon, Kwang W. (2004). International review of cytology : a survey of cell biology. Academic Press. ISBN 0-12-364639-1. OCLC 648269868.
7. Battaglia, E.. (1999). The chromosome satellite (Navashin's "sputnik" or satelles): A terminological comment. Acta biologica Cracoviensia. Series botanica. 41. 15-18.
8. Berger, C. A. (1940-10-25). "SAT-Chromosomes". Science. 92 (2391): 380–381. doi:10.1126/science.92.2391.380.b. ISSN 0036-8075. PMID 17734582. S2CID 239873007.
9. Pikaard, C.S. (2000), "The epigenetics of nucleolar dominance", Trends in Genetics, 16 (11): 495–500, doi:10.1016/S0168-9525(00)02113-2, PMID 11074291