Satellite chromosomes, also known as SAT chromosomes, are a unique type of chromosomes that feature secondary structures for identification purposes. They are typically found in acrocentric chromosomes and are characterized by the presence of one or more secondary constrictions during metaphase. In humans, these satellite chromosomes are commonly associated with the short arm of acrocentric chromosomes like 13, 14, 15, 21, and 22. Additionally, the Y chromosome may also contain satellite regions, although these are believed to result from translocations from autosomes. These secondary constrictions maintain a consistent position, making them valuable markers for distinguishing specific chromosomes.
The term "satellite" originates from the small chromosomal segment located behind the secondary constriction, which was named by Sergei Navashin in 1912. Later, Heitz in 1931 described the secondary constriction as the "SAT state" (Sine Acido Thymonucleinico), indicating lack of Thymonucleic acid. Over time, "SAT-chromosome" became a synonym and abbreviation for satellite chromosome.
During metaphase, the satellite region appears to be connected to the main chromosomes by a thread of chromatin. Some SAT-chromosomes with secondary constrictions associated with nucleolus formation are known as nucleolar SAT-chromosomes. Each diploid nucleus typically contains at least four SAT chromosomes, and the constriction corresponds to a nucleolar organizer (NOR), a region that houses multiple copies of the 18S and 28S ribosomal genes responsible for synthesizing ribosomal RNA essential for ribosomes. The presence of secondary constrictions at NORs is believed to result from rRNA transcription and structural characteristics of the nucleolus that affect chromosome condensation.
