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.[1]
Polytene chromosomes were first reported by
They are produced when repeated rounds of DNA replication without cell division forms a giant chromosome. Thus polytene chromosomes form when multiple rounds of replication produce many sister chromatids which stay fused together.
Polytene chromosomes, at
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
The interbands are involved in the interaction with the active chromatin proteins, nucleosome remodeling, and origin recognition complexes. Their primary functions are: to act as binding sites for RNA pol II, to initiate replication and, to start nucleosome remodeling of short fragments of DNA.[6]
Structure
In insects, polytene chromosomes are commonly found in the salivary glands; they are also referred to as "salivary gland chromosomes". The large size of the chromosome is due to the presence of many longitudinal strands called
The bands of polytene chromosomes become enlarged at certain times to form swellings called puffs. The formation of puffs is called puffing. In the regions of puffs, the chromonemata uncoil and open out to form many loops. The puffing is caused by the uncoiling of individual chromomeres in a band. The puffs indicate the site of active genes where mRNA synthesis takes place.
In protozoans, there is no transcription, since the puff consists only of DNA.[2]
History
Polytene chromosomes were originally observed in the larval salivary glands of Chironomus midges by Édouard-Gérard Balbiani in 1881.[8] Balbiani described the chromosomal puffs among the tangled thread inside the nucleus, and named it "permanent spireme". In 1890, he observed similar spireme in a ciliated protozoan Loxophyllum meleagris.[1] The existence of such spireme in Drosophila melanogaster was reported by Bulgarian geneticist Dontcho Kostoff in 1930. Kostoff predicted that the discs (bands) which he observed were "the actual packets in which inherited characters are passed from generation to generation."[9]
The hereditary nature of these structures was not confirmed until they were studied in
It seems that we can regard these chromosomes as corresponding with paired pachytene chromosomes at meiosis in which the intercalary parts between chromomeres have been stretched and separated into smaller units, and in which, instead of two threads lying side by side, we have 16 or even more. Hence they are "polytene" rather than pachytene; I do not, however, propose to use this term; I shall refer to them as "multiple threads."[15]
Occurrence
Polytene chromosomes are present in secretory tissues of
In plants, they are found in only a few species, and are restricted to ovary and immature seed tissues such as in Phaseolus coccineus and P. vulgaris (Nagl, 1981), and the anther tapetum of Vigna unguiculata and of some Phaseolus species.[16]
Polytene chromosomes are also used to identify the species of chironomid larvae that are notoriously difficult to identify. Each morphologically distinct group of larvae consists of a number of morphologically identical (sibling) species that can only be identified by rearing adult males or by cytogenetic analysis of the polytene chromosomes of the larvae. Karyotypes are used to confirm the presence of specific species and to study genetic diversity in species with a wide range of genetic variation.[17][18]
References
- ^ ISBN 978-0-470-01617-6.
- ^ ISBN 978-0-470-69522-7.
- .
- ISBN 978-0-07-352526-6.
- PMID 29606905.
- S2CID 32071983.
- PMID 15548421.
- ^ Balbiani EG (1881). "Sur la structure du noyau des cellules salivaires chez les larves de Chironomus". Zool. Anz. 4: 637–641.
- .
- PMID 21014277.
- ISBN 978-0-203-37529-7.
- PMID 17801695.
- PMID 8536991.
- PMID 16577650.
- .
- .
- ^ Int Panis L, Kiknadze I, Bervoets L, Aimanova A (1994). "Karyological identification of some species of the genus Chironomus Meigen, 1803 from Belgium". Bull. Ann. Soc. R. Ent. Belg. 130: 135–142.
- ^ Кикнадзе ИИ; Михайлова П; Истомина АГ; Голыгина ВВ; Инт Панис Л; Крастанов Б (2006). "Хромосомный полиморфизм и дивергенция популяций у Chironomus nuditarsis Keyl (Diptera, Chironomidae)". Tsitologia. 48: 595–609.
Further reading
- Baudisch W (1977). "Balbiani Ring Pattern and Biochemical Activities in the Salivary Gland of Acricotopus lucidus (Chironomidae)". Biochemical Differentiation in Insect Glands. Results and Problems in Cell Differentiation. Vol. 8. pp. 197–212. )
- Bridges CB (1935). "Salivary chromosome maps with a key to the banding of the chromosomes of Drosophila melanogaster". Journal of Heredity. 26: 60–64. .
- Daneholt B (1992). "The transcribed template and the transcription loop in Balbiani rings". Cell Biol Int Rep. 16 (8): 709–715. PMID 1446347.
- Werle SF, E Klekowski & 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. S2CID 16451289.
- Pavan, C.; Breuer, M. E. (1952). "Polytene chromosomes in different tissues of Rhynchosciara". Journal of Heredity. 63: 151–157. .
- Pavan, C. (1967). "Chromosomal changes induced by infective agents Triangle". Sandoz J. Med. Sci. 8 (2): 42–48. PMID 5602878.
- Pavan, C.; Biesele, J.; Riess, R. W.; Wertz, A. V. (1971). "XIII. Changes in the ultrastructure of Rhynchosciara cells infected by Microsporidia". Studies in Genetics. VI: 7103.
- Pavan, C.; Da Cunha, A. B.; Morsoletto, C. (1971). "Virus-chromosome relationships in cells of Rhynchosciara (Diptera, Sciaridae)". Caryologia. 24 (3): 371–389. .
External links
- [1] (87A&C Heat-shock puffs)
- [2] (High resolution spreads)
- Phaseolus Polytene chromosomes (Plants)