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In human karyotype, how many pairs of the chromosome have secondary constriction and satellite?
A. 3
B. 4
C. 2
D. 5

Last updated date: 20th Jun 2024
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Hint: In identifying a chromosome from a set, secondary constrictions are useful. A knob-like structure at the top, called a satellite chromosome (SAT chromosome), is formed due to secondary constriction. DNA is called rDNA in a secondary constriction which forms rRNA. SAT chromosomes (13,14,15,21,22) contain NORs.

Complete answer:
In both chromosomes, primary constriction is present. But in all chromosomes, secondary constriction is not present. Secondary constriction is also known as the area of the nucleolar organiser since NOR is involved in the nucleolus creation.
In addition to primary constriction / centromere, secondary constriction may be observed on a chromosome. During Anaphase, chromosomes can bend only at the site of primary constriction. In identifying a chromosome from a set, secondary constrictions are useful. In a cell at anaphase, there are either 0, 1 , 2, 3, or 4 secondary constriction sites. "Many parts of these constrictions imply the development of nucleolus sites and are known as" nucleolar organising areas. The nucleolus is the nucleus connected to the NOR zone of the secondary constriction field. The number of NORs in humans is equal to the number of nucleoli. The number of NORs in humans is 10. NOR occurs (13,14,15,21,22) in SAT (satellite chromosome) chromosomes.

Hence, option D: 5 is the correct answer.

Note: Each cell usually contains 23 pairs of chromosomes in human beings, for a total of 46. In both males and females, 22 of these pairs, called autosomes, look the same. The 23rd pair varies between males and females, the sex chromosomes.