Soft x-ray lithographic studies of interphase chromosomes

Molecular biological mechanisms of speciation

Growing recognition that much of the evolutionary history of eukaryotic genomes reflects the operation of turnover processes involving repetitive DNA sequences has led to the recent formulation of models describing speciation as a consequence of such turnover. These models are of three general kinds: those attributing hybrid infertility to the process of transposition, those attributing hybrid infertility to mispairing between chromosomes of divergent repetitive DNA composition, and those assuming that change in repetitive DNA's can reset coordinated gene regulation. These models are discussed with respect to the kinds of evidence needed for their corroboration and to their significance for questions related to macroevolutionary punctuated equilibria and genetic revolutions.

Microscopic imaging of cells

The microworld was revealed to investigators through a glass bead or a hanging water droplet long before optics was understood. The cellular structure of plants was well resolved by such simple magnifying glasses, van Leeuwenhoek, the Dutch merchant and amateur microscopist, was the first to report to the English Royal Society his observations of bacteria with his single-lens microscope in 1665.

Contact x-ray microscopy. A new technique for imaging cellular fine structure

Contact x-ray microscopy potentially allows living, wet cells to be visualized at a resolution of up to 100 A. Furthermore, differential absorption by specific elements permits the study of the distribution of those elements in biological specimens. In contact x-ray microscopy, soft x-rays (10 A to 100 A) pass through a biological sample and expose an underlying x-ray sensitive polymer (resist), producing an image that reflects the photon absorbance within the specimen. The high penetrating power of soft x-ray enables images to be obtained from specimens up to several microns thick. In this paper, the technique is described, some of the areas currently under study are considered, and biological examples of the use of contact x-ray microscopy are given.

Individual interphase chromosome domains revealed by in situ hybridization

The position and arrangement of individual chromosomes in interphase nuclei were examined in mouse-human cell hybrids by in situ hybridization of biotinylated human DNA probes. Intense and even labeling of human chromosomes with little background was observed when polyethylene glycol and Tween-20 were included in hybridization solutions. Human interphase chromosomes were separated from each other in the nucleus, and were confined to well localized domains. Hybrid cells with a single human chromosome showed a reproducible position of this chromosome in the nucleus. Some chromosomes appeared to have a characteristic folding pattern in interphase. Optical section as well as electron microscopy of labeled regions revealed the presence of 0.2 micron wide fibers in each interphase domain, as well as adjacent, locally extended 500 nm fibers. Such fibers are consistent with previously proposed structural models of interphase chromosomes.

Specific proteins associated with Creutzfeldt-Jakob disease and scrapie share antigenic and carbohydrate determinants

Small amounts of brain tissue (2 g) infected with Creutzfeldt-Jakob disease (CJD) can be fractionated by using a simple 1-day method that includes lysis with N-lauroylsarcosine. Unique fibrils have been identified previously in scrapie- and CJD-infected tissue. These fibrils were abundant in final fractions. Preparations from human CJD autopsy material and from experimental hamster and guinea pig CJD all displayed readily identifiable fibrils that were not seen in control preparations. Thus, these methods appear to be of value in biopsy diagnosis of suspected human cases of CJD. Lysis with N-lauroylsarcosine quantitatively solubilized infectivity from membrane-rich fractions. Significant infectivity was recovered in microfractionations. After proteinase K digestion, a diffuse band at 29 kDa was detectable on NaDodSO4/PAGE. This 29-kDa material was not present in uninfected control brain and was similar to that seen in scrapie. Protein blots of human, guinea pig, and hamster CJD fractions were tested with an antibody raised against a 29-kDa band from mouse scrapie; 29-kDa proteins were labeled in all CJD and scrapie fractions but not in controls. These results indicate that specific proteins in both these diseases share common antigenic determinants. Ricin and wheat germ agglutinin, but not concanavalin A, also labeled a portion of the 29-kDa band from hamster CJD and hamster scrapie fractions, but they did not label any bands in normal hamster fractions at the same gel protein loads. When proteinase K treatment was omitted, specific bands of approximately equal to 35 kDa were detected in CJD samples. These results are consistent with the idea that some CJD- and scrapie-specific proteins are glycoproteins or sialoglycoproteins that can reside in or possibly derive from cell membranes.

Flash X-ray microscopy with a gas jet plasma source

A novel flash X-ray source, the gas jet plasma source, has been used for contact X-ray microscopy. Using a wavelength range of 2-7 nm a resolution of the order of 30 nm can be obtained. The gas jet plasma source provides a new and unique tool which should allow future imaging of wet live cells.

High-resolution mapping of satellite DNA using biotin-labeled DNA probes

We have developed a novel method for high resolution mapping of specific DNA sequences after in situ hybridization. DNA probes, labeled with biotin-nucleotides in conventional nick-translation reactions, are hybridized to cytological preparations and detected with affinity-purified rabbit antibiotin antibodies followed by antibodies to rabbit IgG that are conjugated to fluorescent or enzymatic reagents. Using peroxidase labeled anti-rabbit IgG, we are able to detect and localize specific sequences at both the light and electron microscopic levels. Initial studies were done with repeated DNA sequences previously mapped by light microscope autoradiography to assess the fidelity and resolution of this method. An analysis using biotin-labeled mouse satellite DNA is presented here.