Mechanical response of plectonemic DNA: an analytical solution

We consider an elastic rod model for twisted DNA in the plectonemic regime. The molecule is treated as an impenetrable tube with an effective, adjustable radius. The model is solved analytically and we derive formulas for the contact pressure, twisting moment and geometrical parameters of the supercoiled region. We apply our model to magnetic tweezer experiments of a DNA molecule subjected to a tensile force and a torque, and extract mechanical and geometrical quantities from the linear part of the experimental response curve. These reconstructed values are derived in a self-contained manner, and are found to be consistent with those available in the literature.Comment: 14 pages, 4 figure

The EDELWEISS Experiment : Status and Outlook

The EDELWEISS Dark Matter search uses low-temperature Ge detectors with heat and ionisation read- out to identify nuclear recoils induced by elastic collisions with WIMPs from the galactic halo. Results from the operation of 70 g and 320 g Ge detectors in the low-background environment of the Modane Underground Laboratory (LSM) are presented.Comment: International Conference on Dark Matter in Astro and Particle Physics (Dark 2000), Heidelberg, Germany, 10-16 Jul 2000, v3 minor revision

Dark Matter Search in the Edelweiss Experiment

Preliminary results obtained with 320g bolometers with simultaneous ionization and heat measurements are described. After a few weeks of data taking, data accumulated with one of these detectors are beginning to exclude the upper part of the DAMA region. Prospects for the present run and the second stage of the experiment, EDELWEISS-II, using an innovative reversed cryostat allowing data taking with 100 detectors, are briefly described.Comment: IDM 2000, 3rd International Workshop on the Identification of Dark Matter, York (GB), 18-22/09/2000, v2.0 minor modification

First Results of the EDELWEISS WIMP Search using a 320 g Heat-and-Ionization Ge Detector

The EDELWEISS collaboration has performed a direct search for WIMP dark matter using a 320 g heat-and-ionization cryogenic Ge detector operated in a low-background environment in the Laboratoire Souterrain de Modane. No nuclear recoils are observed in the fiducial volume in the 30-200 keV energy range during an effective exposure of 4.53 kg.days. Limits for the cross-section for the spin-independent interaction of WIMPs and nucleons are set in the framework of the Minimal Supersymmetric Standard Model (MSSM). The central value of the signal reported by the experiment DAMA is excluded at 90% CL.Comment: 14 pages, Latex, 4 figures. Submitted to Phys. Lett.

Event categories in the EDELWEISS WIMP search experiment

Four categories of events have been identified in the EDELWEISS-I dark matter experiment using germanium cryogenic detectors measuring simultaneously charge and heat signals. These categories of events are interpreted as electron and nuclear interactions occurring in the volume of the detector, and electron and nuclear interactions occurring close to the surface of the detectors(10-20 mu-m of the surface). We discuss the hypothesis that low energy surface nuclear recoils,which seem to have been unnoticed by previous WIMP searches, may provide an interpretation of the anomalous events recorded by the UKDMC and Saclay NaI experiments. The present analysis points to the necessity of taking into account surface nuclear and electron recoil interactions for a reliable estimate of background rejection factors.Comment: 11 pages, submitted to Phys. Lett.

Background discrimination capabilities of a heat and ionization germanium cryogenic detector

The discrimination capabilities of a 70 g heat and ionization Ge bolometer are studied. This first prototype has been used by the EDELWEISS Dark Matter experiment, installed in the Laboratoire Souterrain de Modane, for direct detection of WIMPs. Gamma and neutron calibrations demonstrate that this type of detector is able to reject more than 99.6% of the background while retaining 95% of the signal, provided that the background events distribution is not biased towards the surface of the Ge crystal. However, the 1.17 kg.day of data taken in a relatively important radioactive environment show an extra population slightly overlapping the signal. This background is likely due to interactions of low energy photons or electrons near the surface of the crystal, and is somewhat reduced by applying a higher charge-collecting inverse bias voltage (-6 V instead of -2 V) to the Ge diode. Despite this contamination, more than 98% of the background can be rejected while retaining 50% of the signal. This yields a conservative upper limit of 0.7 event.day^{-1}.kg^{-1}.keV^{-1}_{recoil} at 90% confidence level in the 15-45 keV recoil energy interval; the present sensitivity appears to be limited by the fast ambient neutrons. Upgrades in progress on the installation are summarized.Comment: Submitted to Astroparticle Physics, 14 page

Status of the EDELWEISS Experiment

The status of the EDELWEISS experiment (underground dark matter search with heat-ionisation bolometers) is reviewed. Auspicious results achieved with a prototype 70 g Ge heat-ionisation detector under a 2 V reverse bias tension are discussed. Based on gamma and neutron calibrations, a best-case rejection factor, over the 15-45 keV range, of 99.7 % for gammas, with an acceptance of 94 % for neutrons, is presented first. Some operational results of physical interest obtained under poor low radioactivity conditions follow. They include a raw event rate of around 30 events/day/kg/keV over the same energy range, and, after rejection of part of the background, lead to a conservative upper limit on the signal of approximately 1.6 events/day/kg/keV at a 90 % confidence level. Performance degrading surface effects of the detector are speculated upon; and planned upgrades are summarized.Comment: 5 pages, 4 eps figures, LaTeX requires espcrc2.sty; Proceedings of TAUP97, Gran Sasso, Italy, September 7-11, 199

A novel large-volume Spherical Detector with Proportional Amplification read-out

A new type of radiation detector based on a spherical geometry is presented. The detector consists of a large spherical gas volume with a central electrode forming a radial electric field. Charges deposited in the conversion volume drift to the central sensor where they are amplified and collected. We introduce a small spherical sensor located at the center acting as a proportional amplification structure. It allows high gas gains to be reached and operates in a wide range of gas pressures. Signal development and the absolute amplitude of the response are consistent with predictions. Sub-keV energy threshold with good energy resolution is achieved. This new concept has been proven to operate in a simple and robust way and allows reading large volumes with a single read-out channel. The detector performance presently achieved is already close to fulfill the demands of many challenging projects from low energy neutrino physics to dark matter detection with applications in neutron, alpha and gamma spectroscopy.Comment: 13 pages, 13 figure

Adaptation to DNA damage checkpoint in senescent telomerase-negative cells promotes genome instability.

In cells lacking telomerase, telomeres gradually shorten during each cell division to reach a critically short length, permanently activate the DNA damage checkpoint, and trigger replicative senescence. The increase in genome instability that occurs as a consequence may contribute to the early steps of tumorigenesis. However, because of the low frequency of mutations and the heterogeneity of telomere-induced senescence, the timing and mechanisms of genome instability increase remain elusive. Here, to capture early mutation events during replicative senescence, we used a combined microfluidic-based approach and live-cell imaging in yeast. We analyzed DNA damage checkpoint activation in consecutive cell divisions of individual cell lineages in telomerase-negative yeast cells and observed that prolonged checkpoint arrests occurred frequently in telomerase-negative lineages. Cells relied on the adaptation to the DNA damage pathway to bypass the prolonged checkpoint arrests, allowing further cell divisions despite the presence of unrepaired DNA damage. We demonstrate that the adaptation pathway is a major contributor to the genome instability induced during replicative senescence. Therefore, adaptation plays a critical role in shaping the dynamics of genome instability during replicative senescence

DNA cruciform arms nucleate through a correlated but non-synchronous cooperative mechanism

Inverted repeat (IR) sequences in DNA can form non-canonical cruciform structures to relieve torsional stress. We use Monte Carlo simulations of a recently developed coarse-grained model of DNA to demonstrate that the nucleation of a cruciform can proceed through a cooperative mechanism. Firstly, a twist-induced denaturation bubble must diffuse so that its midpoint is near the centre of symmetry of the IR sequence. Secondly, bubble fluctuations must be large enough to allow one of the arms to form a small number of hairpin bonds. Once the first arm is partially formed, the second arm can rapidly grow to a similar size. Because bubbles can twist back on themselves, they need considerably fewer bases to resolve torsional stress than the final cruciform state does. The initially stabilised cruciform therefore continues to grow, which typically proceeds synchronously, reminiscent of the S-type mechanism of cruciform formation. By using umbrella sampling techniques we calculate, for different temperatures and superhelical densities, the free energy as a function of the number of bonds in each cruciform along the correlated but non-synchronous nucleation pathways we observed in direct simulations.Comment: 12 pages main paper + 11 pages supplementary dat