Cantilevers and optical tweezers are widely used for micromanipulating cells or biomolecules for measuring their mechanical properties. However, they do not allow easy rotary motion and can sometimes damage the handled material. We present here a system of magnetic tweezers that overcomes those drawbacks while retaining most of the previous dynamometers properties. Electromagnets are coupled to a microscope-based particle tracking system through a digital feedback loop. Magnetic beads are first trapped in a potential well of stiffness approximately 10(-7) N/m. Thus, they can be manipulated in three dimensions at a speed of approximately 10 microm/s and rotated along the optical axis at a frequency of 10 Hz. In addition, our apparatus can work as a dynamometer relying on either usual calibration against the viscous drag or complete calibration using Brownian fluctuations. By stretching a DNA molecule between a magnetic particle and a glass surface, we applied and measured vertical forces ranging from 50 fN to 20 pN. Similarly, nearly horizontal forces up to 5 pN were obtained. From those experiments, we conclude that magnetic tweezers represent a low-cost and biocompatible setup that could become a suitable alternative to the other available micromanipulators.

The nematode Caenorhabditis elegans has been central to the understanding of metazoan biology. However, C. elegans is but one species among millions and the significance of this important model organism will only be fully revealed if it is placed in a rich evolutionary context. Global sampling efforts have led to the discovery of over 50 putative species from the genus Caenorhabditis , many of which await formal species description. Here, we present species descriptions for 10 new Caenorhabditis species. We also present draft genome sequences for nine of these new species, along with a transcriptome assembly for one. We exploit these whole‐genome data to reconstruct the Caenorhabditis phylogeny and use this phylogenetic tree to dissect the evolution of morphology in the genus. We reveal extensive variation in genome size and investigate the molecular processes that underlie this variation. We show unexpected complexity in the evolutionary history of key developmental pathway genes. These new species and the associated genomic resources will be essential in our attempts to understand the evolutionary origins of the C. elegans model.

We present an experimental study of the microfluidic electrophoresis of long DNA in self-assembling matrixes of magnetic bead columns. Results are presented for the rapid separation of lambda-phage, 2lambda-DNA, and bacteriophage T4 DNA, where separation resolutions greater than 2 between lambda and T4 are achieved in times as short as 150 s. The use of a computer-piloted flow control system and injection results in high reproducibility between separations. We compare the experimentally measured mobility and dispersion with an exactly solvable lattice Monte Carlo model. The theory predicts that the mean velocity scales linearly with the field, the band broadening scales with the inverse of the field, and the resolution is independent of the field for intermediate fields-all of which are in accord with the experimental results. Moreover, reasonable quantitative agreement is achieved for band broadening for longer DNA (2lambda and T4) when the average postengagement time is measured experimentally. This work demonstrates the possibility of achieving fast microfluidic separation of large DNA on a routine basis.

We describe a method of controlled evaporation on a textured substrate for self-assembling and shaping gold-nanorod-based materials. Tridimensional wall features are formed over areas as large as several square millimeters. Furthermore, analyses by small-angle X-ray scattering and scanning electron microscopy techniques demonstrate that colloids are locally ordered as a smectic B phase. Such crystallization is in fact possible because we could finely adjust the nanoparticle charge, knowledge that additionally enables tuning the lattice parameters. In the future, the type of ordered self-assemblies of gold nanorods we have prepared could be used for amplifying optical signals.

Decisions on the fate of cells and their functions are dictated by the spatiotemporal dynamics of molecular signalling networks. However, techniques to examine the dynamics of these intracellular processes remain limited. Here, we show that magnetic nanoparticles conjugated with key regulatory proteins can artificially control, in time and space, the Ran/RCC1 signalling pathway that regulates the cell cytoskeleton. In the presence of a magnetic field, RanGTP proteins conjugated to superparamagnetic nanoparticles can induce microtubule fibres to assemble into asymmetric arrays of polarized fibres in Xenopus laevis egg extracts. The orientation of the fibres is dictated by the direction of the magnetic force. When we locally concentrated nanoparticles conjugated with the upstream guanine nucleotide exchange factor RCC1, the assembly of microtubule fibres could be induced over a greater range of distances than RanGTP particles. The method shows how bioactive nanoparticles can be used to engineer signalling networks and spatial self-organization inside a cell environment.

The designing of DNA intercalating drugs with high DNA affinity in the series of ellipticine has led to a new antitumoral agent, 9-hydroxyellipticine, which has a high DNA affinity, a high activity on L 1210 mice leukemia, and a lack of toxicity at therapeutic dose. The possible correlations among chemical structure, DNA reactivity, and pharmacological activity of DNA intercalating drugs are discussed.Except for the hormonal products, which can prevent specific cells from proliferating, and drugs acting on the immunological system, most of the presently used anticancerous agents are cytotoxic compounds acting preferentially on tumor cells. Presently used antitumoral agents are already highly specific. For instance, it has been shown in an appropriate experimental model that a cancerous cell could be about a million times more sensitive to bis-chlorethylnitrosourea than a normal cell (1). Further progress in specificity seems, therefore, very difficult, especially since the basis at the molecular level of this specific toxicity is poorly understood. A rigorous approach in the design of new anticancerous compounds seems, therefore, an almost insuperable task. Nevertheless, the mechanism of the cytotoxicity property itself can in some cases be understood; that is mainly true for drugs acting on DNA structure or metabolism, among which are most of the anticancerous agents. If cytotoxic compounds could be rationally designed, one could hope to have better chance of finding among them anticancerous agents, since at the same time one could study the specificity of their action and try to understand it.Among products susceptible to such an approach are DNA intercalating drugs, because DNA intercalation is a rather well-understood mechanism and because it is almost the only case where the structure of what is thought to be the pharmacological receptor is known at atomic resolution (2). On the other hand, there belong already to this class of compounds some of the most powerful anticancerous agents, such as actinomycin D, daunomycin, and adriamycin. Our reasoning was, therefore, very simple. If DNA is the real receptor of these drugs and if we are able to design DNA intercalating compounds with the highest possible affinity for DNA, we would have a much better chance of finding active anticancerous drugs among these compounds. If this reasoning is correct, for this class of compounds high DNA reactivity is a condition necessary for anticancerous activity but not a condition that is sufficient; high DNA reactivity is necessary for conferring a potential cytotoxicity but is insufficient for conferring a specificity directed towards the cancerous cells.With these assumptions, the approach to be taken is clear. First, DNA intercalating drugs with increasing DNA affinities must be synthesized. Second, the correlation between the DNA reactivity and the pharmacological activity must be studied. The ability of drugs to intercalate in DNA is conditioned by their stereochemical parameters, such as size, shape, and planarity...

Molecular beacons (MBs) are fluorescent nucleic acid probes with a hairpin-shaped structure in which the 5' and 3' ends are self-complementary. Due to a change in their emissive properties upon recognition with complementary sequences, MBs allow the diagnosis of single-stranded DNA or RNA with high mismatch discrimination, in vitro and in vivo. Whereas the stems of MB hairpins usually rely on the formation of a Watson-Crick duplex, we demonstrate in this report that the preceding structure can be replaced by a G-quadruplex motif (G4). Intramolecular quadruplexes may still be formed with a central loop composed of 12 to 21 bases, therefore extending the sequence repertoire of quadruplex formation. G4-MB can efficiently be used for oligonucleotide discrimination: in the presence of a complementary sequence, the central loop hybridizes and forms a duplex that causes opening of the quadruplex stem. The corresponding G4-MB unfolding can be detected by a change in its fluorescence emission. We discuss the thermodynamic and kinetic opportunities that are provided by using G4-MB instead of traditional MB. In particular, the intrinsic feature of the quadruplex motif facilitates the design of functional molecular beacons by independently varying the concentration of monovalent or divalent cations in the medium.

To facilitate thermal imaging, particularly in microdevices, one has to favor molecular thermometers in which the response is independent of the probe concentration and of the observation setup imperfections. Hence, this paper introduces two temperature fluorescent probes for ratiometric dual-emission-wavelength measurements in aqueous solutions. They are based on a nonathermal chemical reaction, either a conformational transition or a protonation, that induces a modification of their emission spectra as the temperature changes. Relying on both a straightforward theoretical analysis and thorough photophysical, thermodynamic, and kinetic investigations, we demonstrate how the flexible design of these two thermometers can be optimized to face applications with various requirements in terms of operating temperature and wavelength ranges as well as temporal resolution. For instance, the present molecules, which can be used between 5 and 35 degrees C, provide a relative sensitivity up to approximately 9 x 10(-2) K(-1) and milli- to microsecond response times. Finally, we utilize a two-color molecular beacon, a probe belonging to the first series of thermometers, to image temperature profiles in a microfluidic cell heated by a resistive strip. The ratiometric analysis of the fluorescence emission at two different wavelengths is performed on a widely available dual-view microscope, illustrating both the simplicity and reliability of the thermal mapping protocol.