The recent sequencing of the entire genomes of Mycoplasma genitalium and M. pneumoniae has attracted considerable attention to the molecular biology of mycoplasmas, the smallest self-replicating organisms. It appears that we are now much closer to the goal of defining, in molecular terms, the entire machinery of a self-replicating cell. Comparative genomics based on comparison of the genomic makeup of mycoplasmal genomes with those of other bacteria, has opened new ways of looking at the evolutionary history of the mycoplasmas. There is now solid genetic support for the hypothesis that mycoplasmas have evolved as a branch of gram-positive bacteria by a process of reductive evolution. During this process, the mycoplasmas lost considerable portions of their ancestors’ chromosomes but retained the genes essential for life. Thus, the mycoplasmal genomes carry a high percentage of conserved genes, greatly facilitating gene annotation. The significant genome compaction that occurred in mycoplasmas was made possible by adopting a parasitic mode of life. The supply of nutrients from their hosts apparently enabled mycoplasmas to lose, during evolution, the genes for many assimilative processes. During their evolution and adaptation to a parasitic mode of life, the mycoplasmas have developed various genetic systems providing a highly plastic set of variable surface proteins to evade the host immune system. The uniqueness of the mycoplasmal systems is manifested by the presence of highly mutable modules combined with an ability to expand the antigenic repertoire by generating structural alternatives, all compressed into limited genomic sequences. In the absence of a cell wall and a periplasmic space, the majority of surface variable antigens in mycoplasmas are lipoproteins. Apart from providing specific antimycoplasmal defense, the host immune system is also involved in the development of pathogenic lesions and exacerbation of mycoplasma induced diseases. Mycoplasmas are able to stimulate as well as suppress lymphocytes in a nonspecific, polyclonal manner, both in vitro and in vivo. As well as to affecting various subsets of lymphocytes, mycoplasmas and mycoplasma-derived cell components modulate the activities of monocytes/macrophages and NK cells and trigger the production of a wide variety of up-regulating and down-regulating cytokines and chemokines. Mycoplasma-mediated secretion of proinflammatory cytokines, such as tumor necrosis factor alpha, interleukin-1 (IL-1), and IL-6, by macrophages and of up-regulating cytokines by mitogenically stimulated lymphocytes plays a major role in mycoplasma-induced immune system modulation and inflammatory responses.

Molecular Biology and Pathogenicity of Mycoplasmas

Micro-inclusions in diamonds from Zaire and Botswana differ in composition from the more common large inclusions of the peridotitic or eclogitic assemblages. These sub-micrometre inclusions resemble potassic magmas in their composition, but are enriched in H_2O, CO^(2−)_3 and K_2O and depleted in MgO. This composition represents a volatile-rich fluid or melt from the upper mantle, which was trapped in the diamonds as they grew.

Mantle-derived fluids in diamond micro-inclusions

The formation of diamond

This paper reviews applications of cathodoluminescence scanning electron microscopy in the assessment of optical and electronic properties of semiconductors. The assessment includes, for example, information on band structure and impurity levels derived from spectroscopic cathodoluminescence, analysis of dopant concentrations at a level which is in some cases several orders of magnitude better than x‐ray microanalysis, and mapping of carrier lifetimes and defects. Recent advances in both the various cathodoluminescence techniques and the processes leading to electron‐beam‐induced luminescence in semiconductors are reviewed. Possible future trends are also discussed.

Cathodoluminescence scanning electron microscopy of semiconductors

We review some of the factors that influence the hardness of polycrystalline materials with grain sizes less than 1 µm. The fundamental physical mechanisms that govern the hardness of nanocrystalline materials are discussed. The recently proposed dislocation curvature model for grain size-dependent strengthening and the 60-year-old Hall–Petch relationship are compared. For grains less than 30 nm in size, there is evidence for a transition from dislocation-based plasticity to grain boundary sliding, rotation, or diffusion as the main mechanism responsible for hardness. The evidence surrounding the inverse Hall–Petch phenomenon is found to be inconclusive due to processing artefacts, grain growth effects, and errors associated with the conversion of hardness to yield strength in nanocrystalline materials.

/pdf/the-hall-petch-and-inverse-hall-petch-relations-and-the-4yzntaqsgw.pdf

The Hall–Petch and inverse Hall–Petch relations and the hardness of nanocrystalline metals

Three natural cubes, each of edge length about 1·8 mm. were examined by x-ray topography. Two possessed very similar internal structures, describable in terms of space-filling by fibrous or columnar single-crystal growth in directions, with branching and equal growth velocities in these symmetrically equivalent directions. On a microscopic scale this structure appears similar to that of diamond coat: the cubic habit results from the branching mode of growth when the final crystal size is large compared with the dimensions of its core or nucleus. The internal structure of the third cube was entirely different. The record of growth horizons and dislocation trajectories showed that during most stages of its growth it had developed with a combination of forms: normal {111} faces plus hummocky, non-crystallographic surfaces of mean orientation {100}. The latter had become dominant in the outer parts of the crystal but the final overall cubic shape had been accentuated by solution following growth.

On the internal structure of natural diamonds of cubic habit

On the origin of the rounded dodecahedral habit of natural diamond

Dislocation cathodoluminescence exhibiting certain peculiarities in anisotropy of polarization localized in sheets parallel to octahedral planes in a mosaic, semiconducting natural diamond is interpreted as a macroscopic manifestation of microscopic interactions between glide dislocations on these octahedral slip planes and the dislocations of the mosaic, whereby passage of fresh glide dislocations through low-angle boundaries and forest dislocations of the mosaic has produced jogs which have persisted since the deformation.

Cathodoluminescence evidence of dislocation interactions in diamond

LET us grant, in accordance with Galimov's proposals1, that cavitation can occur when flowing magma in a pipe encounters a constriction, and that in the ensuing collapse of a bubble very substantial transient dynamic pressures, of magnitude sufficient to be of thermodynamic importance for diamond synthesis, can be produced. In granting this, we overlook some quantitative details in his calculation, such as the apparent implication that the bubbles would contain gas at 10 or 20 kbar, and yet that their compression (by a factor of 64,000 in volume) can be calculated by ideal gas theory. He ignores the fundamental difference in rate control between martensitic conversion of crystals from one modification to another, which makes a product of the same chemical composition as the starting material, and other processes of crystal growth requiring a composition change. In the former class, to which production of diamond by the action of shock waves on graphite belongs, the limit on growth velocity is essentially the shock wave velocity. Thus, so far as that is concerned, quite a large diamond might be made within a few microseconds. Further characteristics of martensitic processes are, however, that the product takes the form of thin lenses, whereby the constraint by the matrix on shape change in the converting region is minimised and, second, that as a rule there is a multiplicity of orientations of the martensitic product in the parent crystal, so that a microcrystalline product results.

/pdf/cavitation-as-a-mechanism-for-the-synthesis-of-natural-haug7tuht9.pdf

A. R. Lang

Papers

On the internal structure of natural diamonds of cubic habit

On the origin of the rounded dodecahedral habit of natural diamond

Cathodoluminescence evidence of dislocation interactions in diamond

Cavitation as a Mechanism for the Synthesis of Natural Diamonds

The Properties of Natural and Synthetic Diamond