(Science: cell biology) giant chromosomes produced by the successive replication of homologous pairs of chromosomes, joined together (synapsed) without chromosome separation or nuclear division. They thus consist of many up to 1000) identical chromosomes (strictly chromatids) running parallel and in strict register. The chromosomes remain visible during interphase and are found in some ciliates, ovule cells in angiosperms and in larval dipteran tissue. The best known polytene chromosomes are those of the salivary gland of the larvae of drosophila melanogaster which appear as a series of dense bands interspersed by light interbands, in a pattern characteristic for each chromosome. The bands, of which there are about 5,000 in drosophila melanogaster, contain most of the dNA (ca 95%) of the chromosomes and each band roughly represents one gene. The banding pattern of polytene chromosomes provides a visible map to compare with the linkage map determined by genetic studies. Some segments of polytene chromosome show chromosome puffs, areas of high transcription.
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... is used for many reasons: short generation time and the existence of polytene chromosomes in its salivary glands are the two most important. Those chromosomes are giant: ...
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You're correct, polytene chromosmes are the result of several rounds of DNA replication without cell division...in ... ( initiation and termination of trasncription) in organisms containg these 'giant' chromosome. Hope this helps! - KIm
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