Polytene Chromosome, Chironomuslarva, Salivary Gland Chromosome, Salivary Gland Cells, Drosophila Melanogaster

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Polytene chromosomes

Polytene chromosome have the following salient features:

1. It was discovered by Balbiani in 1881.

2. It is found in the salivary gland cells of Chironomuslarva. Hence it is also called salivary gland chromosome.

3. It is a giant chromosome. It is larger in size. For example, in Drosophila melanogaster it is 1000 times larger than somatic chromosomes.

4. The larger size of the chromosomes is due to the presence of many longitudinal strands called chromonemata. Hence they are also called Polytene chromosomes (many standard).

5. The many strands of the giant chromosomes are due to repeated division of the chromosome without the cytoplasmic division. This type division is called endomitosis.

6. The Polytene chromosome contains two types of transverse bands, namely dark bands and inter bands. The dark bands are darkly stained and the inter bands are lightly stained with nuclear stains. The dark bands contain more DNA and less RNA. The inter bands contain more RNA and less DNA.

Polytene Chromosomes

Polytene Chromosome

7. The bands of Polytene chromosomes become enlarged at certain times to form swellings called puffs or Balbiani rings. The formation of puffs is called puffing.

In the regions of puffs the chromonemata uncoil and open out from many loops. Thus puffing is caused by the uncoiling of individual chromomeres in a band. The puffs indicate the site active genes when mRNA synthesis takes place.

Polytene Chromosomes

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