The chromosome besides having the primary constriction or the centromere possesses second­ary constriction at any point of the chromosome.
Constant in their position and extent, these constrictions are useful in identifying particular chromosomes in a set.
The chromosome region distal to the secondary constriction i.e., the region between the secondary constriction and the nearest telomere is known as satellite.
Therefore, chromosomes having secondary constrictions are called satellite chromo­somes or satchromosomes.
Nucleolus is always associated with the secondary constriction of satchromosomes. There­fore, secondary constrictions are also called nucleolus organiser region (NOR) and satchromo­somes are often referred to as nucleolus organiser chromosomes.
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