Question

Nucleolar organizer is a
A. Primary constriction
B. Secondary constriction
C. Tertiary constriction
D. Centriole

Answer 1

Hint: A few chromosomes may have extra constrictions termed as non staining secondary constrictions or NOR (nucleolar organiser) near the ends.

Complete answer: Option A: A chromosome consists of two identical halves, the chromatids which are held together at one point called centromere. The centromere appears as an arrow region called the primary constriction of the chromosome. Primary constriction present between long and short arms.
Option B: Nucleolar organizer is a narrow constriction in the short arm of a few chromosomes. The nucleolar organizer also is known as secondary constriction.
Option C: In the chromosome, there is no tertiary constriction.
Option D: Two cylindrical structures present in a centromere called centrioles, which are surrounded by a cloud of amorphous pericentriolar material called controllers or kinoplasm. These two centrioles called diplosomes are localized in the centrosome at right angles to each other. A centriole possesses a whorl of nine evenly spaced peripheral fibrils of tubulin. It is absent in the center. Therefore, the arrangement is called a 9+0.
So, the correct option is (B).
Additional Information:
Based on the location of the centromere, chromosomes can be organized into four types:
1. Metacentric chromosome: The centromere is present at the center. They appear V-shaped.
2. Sub-metacentric chromosome: The centromere is located slightly away from the center of a chromosome or nearer to one end of the chromosome.
3. Acrocentric chromosome: The centromere is located very close to one end of the chromosome.
4. Telocentric chromosome: The centromere is located at the terminal end of the chromosome and thus appears to have a single arm.

Note: The part of the chromosome beyond the secondary constriction is called a satellite. A chromosome having a satellite is called an SAT-chromosome and these are considered marker chromosomes.