Satellite chromosomes are chromosomes that contain secondary constructs that serve as identifying markers. Besides the centromere, one or more secondary constrictions can also be observed in some chromosomes at metaphase. This is the case with the acrocentric chromosomes 13, 14, 15, 21 and 22; chromosomes which contain a segment that is separated from the main body of the chromosome by such a secondary constriction. The secondary constrictions are always constant in their positions and hence can be used as markers to identify these particular chromosomes. The Y chromosome can also contain satellites, although these are thought to be translocations from autosomes. Chromosomes containing satellites are called SAT Chromosomes. The term is due to Sergei Navashin, in 1912.
The satellite at metaphase appears to be attached to the rest of the body of chromosomes by a thread of chromatin.
There are at least 4 SAT chromosomes in each diploid nucleus, and the constriction corresponds to a nucleolar organizer (NOR). 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.
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