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polytene chromosome
Polytene chromosomes are over-sized chromosomes which have developed from standard chromosomes and are commonly found in the salivary glands of ''Drosophila melanogaster.'' Specialized cells undergo repeated rounds of DNA replication without cell division (endomitosis), to increase cell volume, forming a giant polytene chromosome. Polytene chromosomes form when multiple rounds of replication produce many sister chromatids that remain fused together.
== Function ==
In addition to increasing the volume of the cells' 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 endoreduplication, to produce large amounts of glue before pupation. Another example, within the organism itself is the tandem duplication of various polytene bands located near the centromere of the X chromosome which results in the Bar phenotype of kidney-shaped eyes.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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