Polytene chromosome

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Polytene chromosomes in a salivary gland cell of Chironomus sp.
Prepared polytene chromosomes from Drosophila
Chironomus riparius Meigen 1804, synonymous C. thummi Kieffer 1911: Polytene set of chromosomes (1 n = 4) from a strongly endoreplicated cell nucleus of a larval salivary gland. L = left chromosome arm, R = right arm, N = nucleolus, BR = Balbiani ring. See Keyl & Keyl 1959

A polytene chromosome is a chromosome that contains many parallel, individual DNA molecules ( chromatids ) each with identical gene sequences. Polytene chromosomes are usually much larger than "normal" mitotic chromosomes and can be seen in the light microscope even at moderate magnification. They are therefore also called giant chromosomes, a name that was sometimes used for the differently structured lampbrush chromosomes .

Prerequisite for Polytänie is endoreplication . In this process, the chromosomal DNA multiplies over several rounds within a cell nucleus ; therefore there is no cell division .

morphology

The chromatids of a polytene chromosome are closely and precisely aligned with one another with the corresponding DNA sequences. More densely packed areas of the Euchromatin appear as "bands", separated by "inter-bands". Heterochromatin forms even denser areas . Chromosome maps were created at an early stage on the basis of the band pattern that characterizes each individual polytene chromosome. In those organisms that form giant chromosomes, the homologous chromosomes (the corresponding chromosomes obtained from mother and father) are often already assigned to one another in pairs in mitotic cells. This circumstance should contribute to the close pairing of the maternal with the paternal chromatids of the giant chromosomes. Polytene sets with fully or partially paired homologues show the haploid chromosome number (1 n).

Polytenization can have ten or more rounds of replication (endocycles), creating “cables” with 2048 or more chromatids. As a chromatid contains a single DNA double helix , a set of giant chromosomes has a DNA content of 2048 C after 10 complete endocycles . The value "C" stands for the species-specific genome size .

Some heterochromatic areas do not participate (or only partially) in the polytainisation, first described in Drosophila virilis . This so-called under - replication can be determined by comparison with mitotic metaphases.

Gene activity

During the transcription activity there is a loosening (decondensation), whereby a band appears brighter in the light microscope. Such a region is known as a puff . Particularly large poufs are called Balbiani rings after their discoverer Édouard-Gérard Balbiani (1823–1899) . The transcription on puffs and Balbiani rings in Chironomus tentans was impressively displayed with a high-resolution scanning electron microscope. Those DNA sequences from which the ribosomal RNA is read constitutionally form the nucleolus , which is always the largest puff in a cell nucleus.

Occurrence

Particularly large polytene chromosomes are found in the salivary glands of the larvae of some insect species such as the mosquito ( Chironomus sp. ), Of which Balbiani first described it in 1881. At the Hamburg Botanical Institute, Emil Heitz and Hans Bauer succeeded in proving in 1933 with the garden hair mosquito: The huge "nuclear loops" in the oversized cell nuclei are actually 1 n = 5 chromosomes.

The best known are the 1 n = 4 giant chromosomes of the fruit fly Drosophila melanogaster . In Drosophila species and other two-winged animals , polytene chromosomes are also found in the cell nuclei of other tissues (e.g. bristle-forming cells, Malpighi vascular cells, nutrient cells of the ovary). In silk glands of insect larvae shortly before and during pupation one finds also polytene chromosomes. The genes that code for the silk proteins can be identified there as Balbiani rings because of the very high rate of transcription. Giant chromosomes also occur in springtails (Collembola), in ciliate animals (Ciliophora) and in some plants in the suspensor cells. In humans, trophoblast nuclei with polytene chromosomes allow implantation in the uterus at the beginning of a pregnancy.

See also

literature

  • Dietrich Ribbert: The polytane chromosomes of the bristle formation cells of Calliphora erythrocephala . In: Chromosoma . tape 21 , no. 3 . Springer, 1967, p. 296-344 .
  • Wolfgang Beermann : giant chromosomes. Springer, Vienna 1962, DNB 450303888 .
  • Igor F Zhimulev, Elena S Belyaeva, VF Semeshin, DE Koryakov, SA Demakov, OV Demakova, GV Pokholkova, EN Andreyeva: Polytene chromosomes: 70 years of genetic research. In: Int Rev Cytol 241, 2004: 203-275. Abstract & Outline.
  • Igor F Zhimulev, Dmitry E Koryakov: Polytene chromosomes. In: Wiley Online. 2009, doi : 10.1002 / 9780470015902.a0001183.pub2 .
  • The variability of the chromosomes. In: Jochen Graw: Genetics. 5th edition. Springer, Dordrecht / Heidelberg 2010, ISBN 978-3-642-04998-9 , p. 249 ff.
  • BM Stormo, DT Fox: Polyteny: Still a Giant Player in Chromosome Research. In: Chromosome research: an international journal on the molecular, supramolecular and evolutionary aspects of chromosome biology. Volume 25, number 3–4, 10 2017, pp. 201–214, doi : 10.1007 / s10577-017-9562-z , PMID 28779272 , PMC 5768140 (free full text) (review).
  • TD Kolesnikova, FP Goncharov, IF Zhimulev: Similarity in replication timing between polytene and diploid cells is associated with the organization of the Drosophila genome. In: PLOS ONE . Volume 13, number 4, 2018, p. E0195207, doi : 10.1371 / journal.pone.0195207 , PMID 29659604 , PMC 5902040 (free full text).
  • Tatyana D Kolesnikova: Banding pattern of polytene chromosomes as a representation of universal principles of chromatin organization into topological domains. In: Biochemistry (Moscow) 83, 4, 2018: 338-349. PDF.

Individual evidence

  1. ↑ Hans-Guenther Keyl, Ilse Keyl: The cytological diagnosis of the Chironomiden. I: Determination table for the genus Chironomus based on the salivary gland chromosomes. In: Archives for Hydrobiology 56, 1959, pp. 43–57.
  2. ↑ Thomas D. Pollard , William C. Earnshaw: Cell Biology . 2nd Edition. Spektrum Akademischer Verlag, Berlin, Heidelberg 2007, ISBN 978-3-8274-1861-6 , pp. 221 .
  3. ^ Rüdiger Wehner, Walter Gehring , Alfred Kühn : Zoologie . 24th edition. Georg Thieme, Stuttgart 2007, ISBN 978-3-13-772724-8 , p. 41 (online at Google Books ).
  4. ^ Emil Heitz : About total and partial somatic heteropycnosis in Drosophila funebris. In: Journal for Cell Research and Microscopic Anatomy 19/1933, pp. 720–742.
  5. ↑ Theophilus Painter : A new method for the study of chromosome aberrations and the plotting of chromosome maps. In: Genetics 19/1934, pp. 175-188.
  6. ↑ Calvin Bridges : Salivary chromosome maps. With a key to the banding of the chromosomes of Drosophila melanogaster. In: Journal of Heredity 26/1935, pp. 60-64.
  7. ^ Emil Heitz, Hans Bauer : Evidence for the chromosomal nature of the nuclear loops in the tangled nuclei of Bibio hortulanus L. Cytological investigations on Diptera: I. In: Zeitschrift für Zellforschung und Mikoskopische Anatomie 17/1933, pp. 67-82.
  8. ^ Emil Heitz: About α- and β-heterochromatin as well as constancy and structure of the chromomers in Drosophila. In: Biologisches Zentralblatt 54/1934, pp. 588-609.
  9. ↑ Claus Pelling: Chromosomal synthesis of ribonucleic acid as shown by the incorporation of uridine labeled with tritium. In: Nature 184/1959, pp. 655-656.
  10. ^ Wolfgang Beermann: Control of differentiation at the chromosomal level. In: Journal of Experimental Zoology 157/1964, pp. 49-62.
  11. ^ Claus Pelling, Terrence D. Allen: Scanning electron microscopy of polytene chromosomes, I. In: Chromosome Research 1/1993, pp. 221-237.
  12. ↑ Wolfgang Beermann: The nucleolus as a vital component of the cell nucleus. In: Chromosoma 11/1960, pp. 263-296.
  13. ^ Emil Heitz, Hans Bauer: Evidence for the chromosomal nature of the nuclear loops in the tangled nuclei of Bibio hortulanus L. Cytological investigations on Diptera: I. In: Zeitschrift für Zellforschung und Mikoskopische Anatomie 17/1933, pp. 67-82.
  14. ↑ Michael Ashburner : Puffing patterns in Drosophila melanogaster and related species. In: Wolfgang Beermann (Ed.): Developmental studies on giant chromosomes. Springer, Berlin / Heidelberg 1972, pp. 101–151.
  15. ↑ Eugenia V Zybina, Tatiana G Zybina: Polytene chromosomes in mammalian cells. In: International Review of Cytology 165/1996, pp. 53-119.