The part of chromosome beyond the nuclear organiser is very short and appears like sphere (satellite). The Satellite Chromosome is the chromosome which has a bulge on the telomeric end and have deficiency of DNA in the nuclear organiser region. There are at least 2 SAT chromosomes in each diploid nucleus. They play a vital role in the formation of the nucleolus after division is completed.
It shows repetitive sequences of genes.
Besides centromere, secondary constriction can also be observed in some chromosomes, which if present in the distal region of the arm, would pinch off a small fragment called Satellite.
The satellite remains attached to the rest of the body of chromosomes by a thread of chromatin. The secondary constrictions are always constant in their positions and hence can be used as markers. The chromosomes having a satellite are marker chromosomes and are also called SAT chromosomes.
Secondary constriction is seen at the chromosome in addition to primary constriction/centromere. Secondary constrictions are useful in identifying a chromosome from a set. These constrictions indicate sites of nucleolus formation and so they are called "Nucleolar Organizing Region. The formations of nucleolus takes place around the NOR region. The secondary constriction also contains the genes for rRNA synthesis (18 S, 5.8 S, 28 S)
NOR occurs in SAT (satellite chromosome) chromosomes (13,14,15,21,22) in human.
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