Biology · Cell: The Unit of Life · NEET
Every chromosome has one main narrow point called the primary constriction (the centromere). Some chromosomes have a SECOND narrow point at a fixed place. This second narrow point is the secondary constriction. NCERT says it is 'non-staining', which means it does not take up stain and looks like a pale gap on the chromosome.
When a chromosome has a secondary constriction, it cuts off a small piece near the tip. This small round piece that lies beyond the secondary constriction is called the satellite. Think of the secondary constriction as a thin thread and the satellite as a tiny ball hanging from it.
The primary constriction is the centromere. Every chromosome has one, and the spindle fibres attach here during cell division. The secondary constriction is an EXTRA narrow point, present only in some chromosomes, and it produces the satellite. So: primary = centromere (always present, holds spindle); secondary = makes the satellite (only in some chromosomes).
The centromere is the PRIMARY constriction. This is a very common NEET trap. Do not mix it up. The secondary constriction is a separate narrow point that gives the satellite. NEET 2024 tested exactly this by pairing 'Satellite' with 'Chromosome'.
When we stain chromosomes to see them under a microscope, most parts colour darkly. The secondary constriction stays pale and does not take the stain. Because it stays light, it looks like a thin gap, and the satellite beyond it stands out as a separate small body. This is why NCERT uses the exact word 'non-staining'.
No. This is an easy mix-up. A satellite chromosome (also called SAT-chromosome) is a chromosome that carries a satellite body beyond its secondary constriction, which you learn in the Cell chapter. Satellite DNA is a type of repetitive DNA sequence, which you learn later in Molecular Basis of Inheritance. Same word, totally different topic. For this concept, only the chromosome body matters.
Match List I with List II: List I: A. Axoneme B. Cartwheel pattern C. Crista D. Satellite List II: I. Centriole II. Cilia and flagella III. Chromosome IV. Mitochondria Choose the correct answer:
Match List-I with List-II: List I: (a) Cristae (b) Thylakoid (c) Centromere (d) Cisternae List II: (i) Primary constriction in chromosome (ii) Disc-shaped sacs in Golgi apparatus (iii) Infoldings in mitochondria (iv) Flattened membranous sacs in stroma of plastids Choose the correct answer:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A satellite is the small round piece of a chromosome that lies beyond the secondary constriction. Chromosomes carrying it are called satellite chromosomes or SAT-chromosomes.
The primary constriction is the centromere, present in every chromosome, where spindle fibres attach. The secondary constriction is an extra narrow point, found only in some chromosomes, that produces the satellite.
No. NCERT clearly states the secondary constriction is non-staining, meaning it stays pale and does not colour, so it looks like a thin gap on the chromosome.
No. The centromere is the primary constriction. Treating it as a secondary constriction is a common NEET mistake.
