Polytene chromosome

Polytene chromosomes are large chromosomes which have thousands of DNA strands. They provide a high level of function in certain tissues such as salivary glands of insects.

Polytene chromosomes were first reported by E.G.Balbiani in 1881. Polytene chromosomes are found in dipteran flies: the best understood are those of Drosophila, Chironomus and Rhynchosciara. They are present in another group of arthropods of the class Collembola, a protozoan group Ciliophora, mammalian trophoblasts and antipodal, and suspensor cells in plants. In insects, they are commonly found in the salivary glands when the cells are enlarged but not divided.

They are produced when repeated rounds of DNA replication without cell division occur. Thus polytene chromosomes form when multiple rounds of replication produce many sister chromatidswhich stay fused together.

Polytene chromosomes, at interphase, are seen to have patterns of distinct thick and thin bands. These patterns were originally used to help map chromosomes, identify small chromosome mutations, and in taxonomic identification. They are now used to study the function of genes in transcription.