Human karyotype with bands and sub-bands

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, such as in the chromosomes 13, 14, 15, 21, & 22. The

Y chromosome The Y chromosome is one of two sex chromosomes in therian mammals and other organisms. Along with the X chromosome, it is part of the XY sex-determination system, in which the Y is the sex-determining chromosome because the presence of the ...

can also contain satellites, although these are thought to be translocations from autosomes. The secondary constriction always keeps its position, so it can be used as markers to identify specific chromosomes. The name derives from the small chromosomal segment behind the secondary constriction, called a satellite, named by

Sergei Navashin Sergei Gavrilovich Navashin (; 14 December 1857 – 10 December 1930) was a Russian Empire The Russian Empire was an empire that spanned most of northern Eurasia from its establishment in November 1721 until the proclamation of the Russia ...

, in 1912. 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. The satellite at metaphase appears to be attached to the

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

s by a thread of

chromatin Chromatin is a complex of DNA and protein found in eukaryote, eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important r ...

. SAT-chromosomes whose secondary constriction is associated with the formation of the

nucleolus The nucleolus (; : nucleoli ) is the largest structure in the cell nucleus, nucleus of eukaryote, eukaryotic cell (biology), cells. It is best known as the site of ribosome biogenesis. The nucleolus also participates in the formation of signa ...

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 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 ...

required by ribosomes. The appearance of secondary constrictions at NORs is thought to be due to rRNA transcription and/or structural features of the

impeding chromosome

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor ...

References

Chromosomes {{Genetics-stub