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Chromosome Puffs

The bands of polytene chromosomes become enlarged at certain times to form swellings which have called chromosome puffs or Balbiani rings. Beerman and Bahr (1954) have studied the fine structure of the puffs represent regions where the tightly coiled chromosomal fibers open out to from many loops. Thus puffing is due to unfolding or uncoiling of individual chromomeres in a band. The puffs are active genes and represent sites of RNA synthesis.

Experimental evidence for mRNA synthesis at puffs. There are several evidences to show that puffing patterns along the chromosomes represent gene activity. (1) The puffs contain large amount of RNA. The unpuffed bands contain chiefly DNA and histone (protein). When chromosomes are stained with tolouidine blue, RNA is stained reddish violet and DNA blue. (2) Pelling injected radioactive uridine into Chironnomous larvae which were then killed and processed for autoradiography. Only the puffs were labeled with balck dots, indicating RNA formation, sometimes as quickly as two minutes afterwards. The rest of the chromosome and cytoplasm showed very little activity. (3) Treatment with the antibiotic actinomysin D, which is an inhibitor of RNA synthesis, stops RNA formation (4) When radioactive leucine or amino acid is injected the proteins take up no radioactive material. This leads to the conclusion that the proteins take up no radioactive material. This leads to the conclusion that the proteins in the puff are not made in the chromosome but elsewhere. This is supported by the fact that inhibitors of protein synthesis like chloramphenicol and puromycin have no effect on puffing.

mRNA synthesis. Analysis has revealed that the RNA in the puff is messenger RNA (mRNA). It has been found that the puffs are active in mRNA synthesis but not in protein synthesis. The mRNA is synthesized in the puffs from a DNA template. It moves to the cytoplasm where it associates with the ribosomes and acts as a template for the synthesis of special proteins. The mRNA of one puff varies from the mRNA made in another.

Conditions of puffing. Clever has studied changes in puffing patterns during metamorphosis in Chironomous. Many types of puffing patterns have been observed. (1) Some bands do not puff at all. (2) Others undergo puffing but there is no connection between puffing and moulting. (3) Puffs are present during the intermoult stage but become larger during metamorphosis. Thus here there is intensification of an activity already present. (4) Puffing begins at the commencement of moulting. since putting represents gene activity the conclusion is that different genes become active at different stages.

Differential gene activity. Beermann (1961) has described differential gene activity in Chironomous. In C. pallidivittatus four cells near the duct of the salivary glands produce a secretion (protein). A group of bands at the tip of chromosome IV undergo puffing, showing that these bands control the secretion.

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