A three-dimensional structural dissection of Drosophila polytene chromosomes

We have analyzed the three-dimensional structural details of Drosophila melanogaster polytene chromosome bands and interbands using three- dimensional light microscopy and a novel method of sample preparation that does not involve flattening or stretching the chromosomes. Bands have been visualized in unfixed chromosomes stained with the DNA specific dye 4,6-Diamidino-2-phenylindole (DAPI). Interbands have been visualized using fixed chromosomes that have been immunostained with an antibody to RNA polymerase II. Additionally, these structures have been analyzed using in situ hybridization with probes from specific genetic loci (Notch and white). Bands are seen to be composed of approximately 36 substructural features that measure 0.2-0.4 micron in diameter. We suggest that these substructural features are in fact longitudinal fibers made up of bundles of chromatids. Band shape can be a reproducible characteristic of a particular band and is dependent on the spatial relationship of these bundles, varying from bands with a uniform distribution of bundles to bands with a peripheral concentration of chromatin. Interbands are composed of bundles of chromatids of a similar size and number as those seen in the bands. The distribution of bundles is similar between a band and the neighboring interband, implying that there is a long range organization to the DNA that includes both the coding and the noncoding portions of genes. Finally, we note that the polytene chromosome has a circular shape when viewed in cross section, whether there are one or two homologs present.