The morphology of normal and denatured polytene chromosomes from Drosophila melanogaster

Normal and denatured polytene chromosomes from Drosophila were compared by light, transmission (TEM) and scanning (SEM) electron microscopy, under different conditions of pretreatment, fixation and specimen preparation. Apart from variations in contrast, acid-alcohol squashes of normal chromosomes were quite comparable in terms of resolution and image quality, regardless of mode of visualization, i.e. stained, unstained, phase contrast, etc. These chromosomes do not appear markedly different from the unfixed or "native" chromosomes isolated by other workers. Thus at the light microscope level at least, any soluble components extracted by acid-alcohol fixation seem to play minor structural roles only. SEM examination of similar squash preparations provided useful topographic views of selected chromosome regions, particularly in favourable spreads where metal coating and tilt angles were optimized. Information on the internal organization of polytene chromosomes was sought by subjecting isolated preparations to alkali-urea denaturation. Under these conditions, chromosomes gradually extend laterally to give rise to lampbrush-like structures, remarkably similar to the meiotic chromosomes of amphibian oocytes. This effect is at least partially reversed by return of alkali-urea treated preparations to physiological Ringer solution, where chromosomes contract again and near-normal banding patterns are re-established. It is suggested that apart from bulk differences due to polyteny, the basic structure of polytene chromosomes and true lampbrush chromosomes may be rather similar.