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To increase cell volume, some specialized cells undergo repeated rounds of DNA replication without cell division (endomitosis), forming a giant polytene chromosome. Polytene chromosomes form when multiple rounds of replication produce many homologous chromatids that remain synapsed together. In addition to increasing the volume of the cell's nuclei and causing cell expansion, polytene cells may also have a metabolic advantage as multiple copies of genes permits a high level of gene expression. In Drosophila melanogaster, for example, the chromosomes of the larval salivary glands undergo many rounds of endoreplication, to produce large amounts of glue before pupation.
Additional recommended knowledge
Polytene chromosomes have characteristic light and dark banding patterns which can be used to identify chromosomal rearragements and deletions. Dark banding frequently corresponds to inactive chromatin, while light banding is usually found at areas with higher transcriptional activity. The banding patterns of the polytene chromosomes of Drosophila melanogaster were sketched in 1935 by Calvin B. Bridges, in such detail that his maps are still widely used today. The banding patterns of the chromosomes are especially helpful in research, as they provide an excellent visualization of transcriptionally active chromatin and general chromatin structure.
Polytene chromosomes were originally observed in the larval salivary glands of Chironomus midges by Balbiani in 1881, but the hereditary nature of these structures was not confirmed until they were studied in Drosophila melanogaster in the early 1930s by Emil Heitz and Hans Bauer. They are known to occur in secretory tissues of other dipteran insects such as the Malpighian tubules of Sciara and also in protists, plants, mammals, or in cells from other insects. Some of the largest polytene chromosomes described thus far (see scale bar in figure below) occur in larval salivary gland cells of the Chironomid genus Axarus.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Polytene_chromosome". A list of authors is available in Wikipedia.|