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The chromosomes are thread-like structures, which become visible under a light microscope, during cell division. The term chromosome was proposed by W. Waldeyer. In higher organisms, the well organised nucleus contains a definite number of chromosomes of definite size and shape.  The chromosome shape is usually observable at metaphase and anaphase. During this phase the position of primary constriction or centromere is clearly visible.  


Based on the position of centromere, chromosomes are called:  

i. Telocentric
- with terminal centromere
ii. Acrocentric
- terminal centromere is capped by a telomere
iii. Submetacentric
- when centromere is sub terminal in position and
iv. Metacentric - having median centromere

Besides centromere, secondary constriction can also be observed in some chromosomes, in the distal region of an arm. This pinches off a small fragment called satellite.  The satellite remains attached to the rest of the body of chromosomes by a thread of chromatin.  The secondary constrictions are always constant in markers. The chromosomes having a satellite or marker chromosomes are called SAT chromosomes. Detailed study of chromosome morphology using light microscope revealed a coiled filament throughout the length of a chromosome.  This is called chromonema, which forms the gene bearing portion of the chromosomes.  It is a structure of subchromatid nature and there can be more than one chromonemata in a chromatid which is a half chromosome.  But electron microscope studies have revealed each chromatid to consist of a single chromatin fibre of ~30 nm diameter. In turn, each chromatin fibre contains a single DNA double helix.

Chromosomes are considered as hereditary vehicles as they store the necessary code in the DNA which is to be transmitted to the next generation. 


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