Besides the centromere, one or more secondary constrictions can also be observed in some chromosomes at metaphase. The secondary constrictions are always constant in their positions and hence can be used as markers that identify particular chromosomes. A satellite chromosome or SAT chromosome has a chromosome segment that is separated from the main body of the chromosome by such a secondary constriction. 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 2 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.
In humans, the chromosomes number 13, 14, 15, 21 and 22 are examples of SAT chromosomes.