Polytene Chromosomes

Abstract

Polytene chromosomes are specific interphase chromosomes consisting of thousands of deoxyribonucleic acid (DNA) strands. For this reason they are very large and display a characteristic band–interband morphology. Polyteny arises in tissues, organs and at developmental stages when there is need for the rapid development of an organ at an unaltered high level of function. Organs containing cells with polytene chromosomes are, as a rule, involved in intense secretory functions accomplished during a short time against a background of rapid growth. Chromosome rearrangements and in situ hybridization on polytene chromosomes allow genes to be mapped to a resolution of a few tens of kilobases. Polytene chromosomes allow a specific narrow region to be dissected out with a micromanipulator and a library of DNA clones to be derived from the region.

Key concepts:

  • Polytene chromosomes are specific type of interphase chromosomes consisting of thousands of DNA strands.

  • Polytene chromosomes have been found in many tissues of the representatives of two orders of insects: Diptera and Collembola, in the macronuclear anlagen of Infusoria, in certain organs and tissues of mammals and also in the cells of the synergids, antipods and endosperm of angiospermous plants.

  • Polyteny arises in tissues, organs and at developmental stages when there is need for the rapid development of an organ at an unaltered high level of function. Organs containing cells with polytene chromosomes are, as a rule, involved in intense secretory functions accomplished during a short time against a background of rapid growth.

  • Along the linear axis polytene chromosome have variation in the concentration of the chromatin. Regions of high concentrations are known as chromomeres (bands), and regions with low concentrations are known as interchromomeres (interbands).

  • The pattern of bands and interbands in each polytene chromosome is specific for the species, and in general is characteristic of that particular chromosome in different tissues or at different developmental stages.

  • Polytene chromosomes are now considered to be very important objects for the analysis of numerous features of interphase chromosome organization and the genome as a whole.

Keywords: genes; puffs; bands; interbands; heterochromatin

Figure 1.

First drawings of polytene chromosome made by Balbiani in (1881) (a) and (1890) (b). (a) Salivary gland cells of Chironomus plumosus and (b) Macronucleus (anlagen?) of Loxophyllum meleagris.

Figure 2.

Drawing of a polytene chromosome set of Drosophila melanogaster. The chromosomes have been spread out by squashing them on a microscopic slide. Each parental chromosome is tightly paired with its homologue (somatic synapsis). There are regions where two homologous chromosomes are separated (asynapsis). All the chromosomes are linked together by the pericentromeric regions to create a single chromocentre. In the left lower corner mitotic chromosomes from ovarian tissue are shown at the same magnification. From Painter T (1934) Salivary chromosomes and the attack on the gene. Journal of Heredity25: 465–476.

Figure 3.

Arrangement and degree of conjugation of chromatids in classic polytene chromosomes (a), cryptic polyteny (b) and pompon‐like chromosomes (c). (a) Individual chromatids with chromomeres (shown as black rectangles) contact each other tightly, the chromomeres forming bands. (b) The chromatids contact each other only in some of the chromomeres, forming a broom‐like structure. (c) The conjugation of the chromatids is completely disturbed and a ‘pompon’ is formed. Open circles indicate the centromeric region.

Figure 4.

The polytene chromosome set of Chironomus thummi. The chromosomes of this species lie separately from each other; they do not have a common chromocentre. BR, Balbiani rings; NU, nucleolus; CEN, pericentromeric regions. Courtesy of LI Gunderina, unpublished.

Figure 5.

Electron microscopic view of part of the 3R chromosome of Drosophila melanogaster. Polytene chromosome bands and interbands are seen as black and white transverse stripes. Courtesy of VF Semeshin, unpublished.

Figure 6.

Electron microscopic view of ecdysone‐ and heat shock‐induced puffs in Drosophila melanogaster. Chromosome region 63A–E is shown before (b) and after (c) heat shock, before (b) and after (a) induction of ecdysone. Inactive genes located in compact chromatin form bands. The bands shown in (b) and (c) are activated after administration of an agent inducing gene activity: ecdysone for 63E2–3 or heat shock for 63B9. As a result, the material of the bands loosens, becomes decompacted and local swelling of the chromosome region occurs. Courtesy of VF Semeshin, unpublished.

Figure 7.

View of heterozygous deletion in Drosophila melanogaster polytene chromosomes, showing normal and deleted chromosome regions. Redrawn from Painter (1934).

Figure 8.

The first drawing of heterozygous inversion in the X‐chromosome of Drosophila melanogaster. Synapsis is complete except at the points where the chromosome fragment is inverted. From Painter (1934).

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Further Reading

Ashburner M (1970) Function and structure of polytene chromosomes during insect development. Advances in Insect Physiology 7: 1–95.

Ashburner M and Berendes HD (1978) Puffing of polytene chromosomes. In: Ashburner M and Wright TRF (eds) The Genetics and Biology of Drosophila, vol. 2b, pp. 316–395. London: Academic Press.

Berendes HD (1973) Synthetic activity of polytene chromosomes. International Review of Cytology 35: 61–116.

Richards G (1997) The ecdysone regulatory cascades in Drosophila. Advances in Developmental Biology 5: 81–135.

Russell S and Ashburner M (1996) Ecdysone‐regulated chromosome puffing in Drosophila melanogaster. In: Gilbert LI, Tata JR and Atkinson BG (eds) Metamorphosis: Postembryonic reprogramming of gene expression in amphibion and insect cells, pp. 109–144. London: Academic Press.

Sorsa V (1998) Polytene Chromosomes in Genetic Research. Chichester: Ellis Harwood.

Zhimulev IF (1996) Morphology and structure of polytene chromosomes. Advances in Genetics 34: 1–497.

Zhimulev IF (1998) Polytene chromosomes, heterochromatin and position effect variegation. Advances in Genetics 37: 1–566.

Zhimulev IF (1999) Genetic organization of polytene chromosomes. Advances in Genetics 39: 1–589.

Zhimulev IF and Belyaeva ES (2003) Intercalary heterochromatin and genetic silencing. BioEssays 25: 1040–1051.

Zhimulev IF, Belyaeva ES and Semeshin VF (1981) Informational content of polytene chromosome bands and puffs. CRC Critical Reviews in Biochemistry 11: 303–340.

Zhimulev IF, Belyaeva ES, Semeshin VF et al. (2004) Polytene chromosomes: 70 years of genetic research. International review of cytology 241: 203–275.

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Zhimulev, Igor F, and Koryakov, Dmitry E(Mar 2009) Polytene Chromosomes. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001183.pub2]