Some theoretical aspects of osmium tetroxide fixation with special reference to the metaphase chromosomes of cell cultures. (17/157)

Fixation of cell cultures with 1 per cent OsO(4) at constant pH and tonicity but variable cationic valence and dielectric constant causes profound changes in metaphase chromosomes. It is possible to make them disappear, flocculate, or show little change from the living cell in the phase contrast microscope. Conventional fixation for the electron microscope causes almost complete disappearance of metaphase chromosomes in phase contrast. Reasons for this behavior are discussed. It is postulated that intermolecular distances and consequently internal structure in chromosomes are governed by the same forces that govern these distances in colloidal sols.  (+info)

Chemostratigraphic evidence of Deccan volcanism from the marine osmium isotope record. (18/157)

Continental flood basalt (CFB) volcanism is hypothesized to have played a causative role in global climate change and mass extinctions. Uncertainties associated with radiometric dating preclude a clear chronological assessment of the environmental consequences of CFB volcanism. Our results document a 25% decline in the marine 187Os/188Os record that predates the Cretaceous-Tertiary boundary (KTB) and coincides with late Maastrichtian warming. We argue that this decline provides a chemostratigraphic marker of Deccan volcanism and thus constitutes compelling evidence that the main environmental consequence of Deccan volcanism was a transient global warming event of 3 degrees to 5 degrees C that is fully resolved from the KTB mass extinction.  (+info)

Markers for detergent-resistant lipid rafts occupy distinct and dynamic domains in native membranes. (19/157)

Lipid rafts isolated by detergent extraction and sucrose gradient fractionation from mast cells are enriched for the glycosylphosphatidylinositol-linked protein Thy-1, the ganglioside GM1, palmitoylated LAT, and cross-linked IgE receptors, FcepsilonRI. This study addresses the relationship of fractionation data to the organization of raft markers in native membranes. Immunogold labeling and electron microscopy shows there is little or no colocalization of the raft markers Thy-1, GM1, and LAT with each other or with FcepsilonRI on native membrane sheets prepared from unstimulated cells. External cross-linking of Thy-1 promotes coclustering of Thy-1 with LAT, but not with GM1. Thy-1 and LAT clusters occur on membrane regions without distinctive features. In contrast, external cross-linking of FcepsilonRI and GM1 causes their redistribution to electron-dense membrane patches independently of each other and of Thy-1. The distinctive patches that accumulate cross-linked FcepsilonRI and GM1 also accumulate osmium, a stain for unsaturated lipids, and are sites for coated vesicle budding. Electron microscopy reveals a more complex and dynamic topographical organization of membrane microdomains than is predicted by biochemical analysis of detergent-resistant membranes.  (+info)

Endonuclease from Escherichia coli that acts specifically upon duplex DNA damaged by ultraviolet light, osmium tetroxide, acid, or x-rays. (20/157)

An endonuclease which is active upon DNA exposed to ultraviolet light at a photoproduct other than thymine dimers has been extensively purified from Escherichia coli. The small (2.7 S) enzyme is active in the presence of EDTA, has a neutral pH optimum, and is inhibited by tRNA and 1 M NaCl. It has no detectable exonuclease, DNA-N-glycosidase, or ribonuclease activities. The enzyme also nicks duplex DNA exposed to OsO4, x-rays, or acid, but it does not act upon undamaged DNA or irradiated single-stranded DNA. The majority of sites of action in DNA exposed to ultraviolet light or OsO4 appear to be alkali-stable, but those in DNA exposed to x-rays or acid are not. The incisions created by the endonuclease contain 5'-phosphate termini. The enzyme is possibly the same as E. coli endonuclease III described by Radman (Radman, M. (1976) J. Biol. Chem. 251, 1438-1445), but it is distinguishable from the other endodeoxyribonucleases described from that organism.  (+info)

Two populations or granular vesicles in constricted post-ganglionic sympathetic nerves. (21/157)

1. A comparative study has been made of the effects of different fixatives on the ultrastructural appearance of granular vesicles accumulated against a constriction in cat hypogastric nerves. 2. After fixation in either osmium tetroxide or in glutaraldehyde followed by post-osmication accumulations of large granular vesicles (65-90 nm in diameter) were observed in profiles of swollen axons. 3. Fixation in acrolein and sodium dichromate revealed a second population of small granular vesicles (mostly 30-50 nm in diameter) in addition to the large vesicles seen after the other fixatives. 4. In reserpinized cats the small granular vesicles were absent. The large granular vesicles were much less numerous, irrespective of the fixative used. 5. It is suggested that either the small vesicles occur in normal noradrenergic axons but are only revealed by catecholamine-sensitive fixatives or they occur as a result of constricting the nerve trunk. Evidence for and against these possibilities is discussed.  (+info)

Development of bipyridine-modified nucleobase for methylcytosine-selective crosslink reaction. (22/157)

Recently, we have reported a novel epigenotyping method utilizing methylcytosine (M)-selective modification through osmium oxidation at a specific site of a long sequence using the formation of a bulge structure by hybridization with a guide DNA. In the key step of this chemical modification, the coordination of bipyridine ligand to osmium tetroxide accelerated the formation of a stable complex (M-Os-ligand). Herein, we report the development of novel capture oligodeoxy-nucleotides (ODNs) containing a bipyridine-modified nucleobase for M-selective interstrand crosslinking through cyclic osmate formation. The crosslink formation resulted in a drastic increase in the melting temperature (T(m)) of the duplex.  (+info)

Fluorescence quenching by methylcytosine-metal complexation. (23/157)

We report the control of fluorescence emission from the fluorophore fixed on DNA using the methylcytosine-selective addition of an osmium(VI)-bipyridine complex. We synthesized the DNA modified by a microenvironment-sensitive fluorophore, 2-dimethylamino-6-acylnaphthalene. The fluorescence from the fluorophore tethered to a probe DNA was effectively quenched by the addition of the osmium(VIII)-bipyridine to the methylcytosine which is located at the immediate neighborhood of the fluorophore. The discrimination of cytosine methylation status at the mutation hot spot in p53 gene was also executed using a well-designed fluorescent DNA probe.  (+info)

Development of electrochemical detection for methylcytosine and its application. (24/157)

We report electrochemical detection of methylcytosine. We developed a bipyridine ligand possessing an amino linker. This ligand was used for methylcytosine-selective osmium oxidation and subsequent redox labeling. The DNA labeled methylcytosine selectively with anthraquinone showed the current signal through hybridization with a DNA probe fixed on a gold surface.  (+info)