Electron microscopic analysis of the banding pattern in the salivary gland chromosomes of Drosophila melanogaster. Division 1 and 2 of X

Electron microscopic analysis of the banding pattern in the salivary gland chromosomes of Drosophila melanogaster. Divisions 11 through 20 of X

The banding pattern of the proximal half of the polytene salivary gland X chromosome of Drosophila melanogaster was studied using thin section electron microscopy. The bands were identified according to Bridges' revised light microscopic map. On Bridges' map, the divisions 11 to 20 contain 112 single bands and as many as 181 double bands. A majority of Bridges' single bands were identified in the thin sections. A total of 23 Bridges' single bands (and 4 bands of division 20) could not be found; in particular, bands were missing from the difficult regions 11DE, 15A, 15C and 16A. Electron microscopy showed the existence of 18 additional faint bands, 4 at the region 18D and 7 at 19EF. Bridges' faintest single bands and the new bands were best seen in formaldehyde fixed material. About 1/4 of Bridges' double bands were found to be made up of two separate bands each. The remaining Bridges' doublets include all kinds of bands: broad, narrow, dark, faint, puffed. Many of them look single in thin sections.

Electron microscopical analysis of Drosophila polytene chromosomes. V. Characteristics of structures formed by transposed DNA segments of mobile elements

An electron microscopical (EM) analysis was performed on regions of polytene chromosomes which contained DNA segments of different genetic composition, inserted by P element-mediated transformation into the Drosophila melanogaster genome. In seven of ten regions examined, containing insertions of the hsp28-ry, hsp70-Adh, ryhsp 70-beta-gal genes and of the ry gene tetramer, new bands appeared. Lack of new bands in three other strains is apparently connected with the fusion of the inserted material to preexisting bands. The new bands do not differ morphologically from the usual bands of polytene chromosomes, and their formation is likely due to predominant insertion of DNA segments into interbands. Among the constructs examined, the minimal length of a DNA segment which appears as a new band is about 5 kb; the DNA packing ratio in the new bands varies from 30 to 50. Activation of the inserted genes by heat shock has enabled us to observe the puffing characteristics of new bands. A sequence of some one kb forms a large interband, or micropuff; the puff size is correlated with the length of the genes being activated. If a DNA segment contains a single gene, then its activation causes the decompaction of the whole band; however, when a DNA segment consists of two genes and the promoter element of the activated gene is positioned in the middle of the sequence, the band splits and only part is decompacted and puffed. The DNA packing ratio in the puffs is 1.4-3.5. The subsequent deletion of the hsp70 promoter but retention of 23, 59, and 73 by from the transcription start points leads to failure of puff formation. In all the transformed sites an increase in the total length of the interbands adjacent to the insert as compared with the initial interband was observed. This increase appears to be due to decompaction of the P element DAN flanking the inserted segments. It is shown that a DNA segment, consisting of four tandemly repeated ry gene copies and interspersed by material which includes P DNA, forms a complex of loose chromatin in which, however, four bands can be resolved. We also observed a lengthening of interband regions containing only the P element sequence itself. Insertion of the complete 2.9 kb P element into the large single 10A1-2 bound of the X chromosome (an insertion in the region approximately 10 kb to the right of the v gene) causes splitting of the band into two parts and formation of a new interband.(ABSTRACT TRUNCATED AT 400 WORDS)

The ultrastructural morphology of native salivary gland chromosomes of Drosophila melanogaster: the band-interband question

Native salivary gland chromosomes of Drosophila melanogaster, isolated without exposure to acid fixatives, have been examined in regions 1A-3B, 15A-17B, 19B-20D and 71E-73A and reveal improved aspects of preservation at the ultrastructural level. Three main points emerge: fine bands are well preserved allowing detection of some not recorded in maps made on classical acid-fixed preparations. Structures with the morphology of putative nascent ribonucleoprotein (RNP) particles are apparent in puffs, diffuse bands and virtually all interbands observed. At this level the morphology of native chromosomes is consistent with the hypothesis that all decondensed regions are members of a continuum of transcriptionally active structures. This notion is relevant to data obtained from other approaches to the band-interband question. (iii) Although the chromosomes have not been exposed to 45% acetic acid, at least some of the dark bands represented by the Bridges as doublets in their classical maps contain vacuoles which include putative RNP particles.

Transmission electron microscopic study of polytene chromosome 2R from Drosophila melanogaster

A simple and rapid method for studying polytene chromosome squashes by transmission electron microscope (TEM) is described. This technique provides close correlation between the light microscopic image and the TEM image. Fine structures of the chromosomes are preserved. The band pattern of region 44 A to 50 F of the chromosome 2 R has been analyzed and compared with Bridges' map (1935) and Lefevre's photographic representation (1976).