Cell Cycle And Cell Division

Polytene chromosomes, first reported by Balbiani .in 1881 in salivary glands of Chironomus and other insects, are popularly called salivary chromosomes. They can reach a length of 2000 ?m and may contain 1000- 16000 times DNA, as compared to ordinary somatic -chrornosomes.: They. are formed by. somatic pairing, between homologous chromosomes, ~ and repeated replication or endomitosis of their chromonemata. They bear conspicuous swellings called chromosome puffs, larger swellings are called Balbiani rings. In the puff region, DNA strands uncoil, become active and produce mRNA. For synthesis of mRNA; the puffs are concentrated with RNA polymerases.