What are secondary constrictions and satellite chromosomes?
GeneralClass 12AllAnswered 27 Mar 2026
Answer
Secondary Constrictions:
- Non-centromeric constrictions appearing as narrow regions on certain chromosomes
- Always at constant, specific positions
- Contain Nucleolar Organizer Region (NOR) – genes encoding ribosomal RNA
- Site of nucleolus formation during interphase
Satellite Chromosomes (SAT chromosomes):
- Chromosomes possessing secondary constrictions near terminal ends
- Satellite: Small, spherical, chromatin body beyond the secondary constriction
- Connected to main chromosome by thin chromatin thread
- Terminology: "SAT" from Latin sine acido thymonucleinico (without thymonucleic acid – histological observation)
Example: In humans, chromosomes 13, 14, 15, 21, 22 bear NORs and can form satellites.
Function: NORs on multiple SAT chromosomes coalesce to form the nucleolus, coordinating ribosome biogenesis.
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