question_answer

The common point of attachment of all the arms of polytene chromosomes, is known as:

A)  chromomere   

B)  centromere

C)  chromocentre  

D)  centrosome

Correct Answer: C

Solution :

Normally chromosomes are not visible during interphase, but polytene chromosomes are exceptional in the sense that these are visible during interphase. Balbiani first observed these chromosomes in the salivary glands of midge Chironomous in 1881. Thats why these are called salivary gland chromosomes. These polytene chromosomes are considered as somatic giant chromosomes and after their discover in salivary glands of larva, these chromosomes are also reported from gut epithelium, Malpighian tubules and fat bodies of some other dipterans also, e.g., Drosophila, Chironomous, Sciara, Rhyncosciara, etc. These chromosomes are very long (upto 200 times their size during mitotic metaphase in Drosophild) and very thick. Polytene chromosomes contain several dark stained region called bands, separated by lighter one or less stained areas, i.e., Inter bands. The bands are visible even without staining and after staining these become very sharp and clear. In D. melanogastor, the polytene chromosomes radiate as 5 long and 1 short arm from a deeply staining and more or less amorphous structure  called chromocentre. This chromocentre is formed by the fusion of centromeric region of all the chromosomes and in males, entire Y chromosome as shown below.