Polytene chromosome
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To increase cell volume, some specialised 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 chromatids that remain synapsed together in a haploid number of chromosomes. They have characteristic light and dark banding patterns which can be used to identify chromosomal rearragements and deletions.
Chromosome puffs are diffuse uncoiled regions of the polytene chromosome that are sites of RNA transcription. A Balbiani ring is a large chromosome puff.
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.
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. They are known to occur in secretory tissues of other dipteran insects such as 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.
[edit] References
- Balbiani EG (1881). "Sur la structure du noyau des cellules salivaires chez les larves de Chironomus". Zool. Anz. 4: 637-641.
- Baudisch W (1977). "Balbiani ring pattern and biochemical activities in the salivary gland of Acricotopus lucidus (Chironomidae)". Results Probl Cell Differ 8: 197-212. PMID 335467.
- Daneholt B (1992). "The transcribed template and the transcription loop in Balbiani rings". Cell Biol Int Rep 16 (8): 709-15. PMID 1446347.
- Werle SF, E Klekowski and DG Smith (2004). "Inversion polymorphism in a Connecticut River Axarus species (Diptera: Chironomidae): biometric effects of a triple inversion heterozygote". Can. J. Zool. 82: 118-129.
