Polytene chromosome or salivary gland chromosome are the giant chromosomes first observed by E.G. Balbiani (1881) from the salivary galnd cells are much larger insect. These chromosomes are much larger in size, than the normal somatic chromosomes which consistts of a bundle of chromonemal fibrils which arise by a series of 10 consecutive duplications of initial chromonemata that increase the DNA content about 1,000 times the DNA content of somatic cells.
There are alternative light abd dark regions called Euchromantin and heterochromatin regions are found.
The chromonemantin of polytenic chromosomes give out many series of loops laterally forming rings or bands which called as Balbiani rings.
At this region the chromatins are rich in DNA and RNA.