Polytene Chromosomes

	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 Heredity 25: 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 Courtesy of LI Gunderina, unpublished.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 Courtesy of VF Semeshin, unpublished.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 Courtesy of VF Semeshin, unpublished.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).