Electron microscopical analysis of Drosophila polytene chromosomes

An electron microscopic (EM) analysis was performed on regions of Drosophila melanogaster polytene chromosomes that contain inserted DNA segments of 19 and 8 kb. These segments had been inserted by P-elementmediated transformation. The 19 kb segment includes both the Drosophila hsp70 gene fused to the Escherichia coli -galactosidase gene and the rosy gene (Lis et al. 1983). This insert generates a new moderate-size band at the 9D4-9E1-2 region in polytene chromosomes. Upon heat shock, a puff originates from a portion of the new band. The 8 kb segment includes the Sgs7 and Sgs3 genes (Richards et al. 1983). This insert generates very diffuse thin bands that decondense at the stage of activation of the Sgs genes to produce wide interbands or small puffs. In all of the above cases, the insertion appears to occur at interband regions, and the genetically complex DNA segments that are inserted generate only a single detectable band.