1,490 research outputs found
Good guide, bad guide:spacer sequence-dependent cleavage efficiency of Cas12a
Genome editing has recently made a revolutionary development with the introduction of the CRISPRâCas technology. The programmable CRISPR-associated Cas9 and Cas12a nucleases generate specific dsDNA breaks in the genome, after which host DNA-repair mechanisms can be manipulated to implement the desired editing. Despite this spectacular progress, the efficiency of Cas9/Cas12a-based engineering can still be improved. Here, we address the variation in guide-dependent efficiency of Cas12a, and set out to reveal the molecular basis of this phenomenon. We established a sensitive and robust in vivo targeting assay based on loss of a target plasmid encoding the red fluorescent protein (mRFP). Our results suggest that folding of both the precursor guide (pre-crRNA) and the mature guide (crRNA) have a major influence on Cas12a activity. Especially, base pairing of the direct repeat, other than with itself, was found to be detrimental to the activity of Cas12a. Furthermore, we describe different approaches to minimize base-pairing interactions between the direct repeat and the variable part of the guide. We show that design of the 3âČ end of the guide, which is not involved in target strand base pairing, may result in substantial improvement of the guide's targeting potential and hence of its genome editing efficiency
Timing of Optical Maturation of Recently Exposed Material on Ceres
On Ceres, multispectral imaging data from the Dawn spacecraft show a distinct bluish characteristic for recently exposed material from the subsurface in, for example, crater ejecta. Ejecta blankets of presumably old craters show a more reddish spectrum. We selected areas in which fresh material from the Cerean subsurface was exposed at a specific time in the past, and no later geologic process is expected to have changed its surface composition or its cratering record. For each area, we determined two color ratios and the crater retention age. The measured color ratios show an exponential diminishment of the bluish characteristic over time. Although the cause of the color change remains uncertain, the time-dependent change in spectral properties is evident, which could help identify the process
The formation and evolution of bright spots on Ceres
The otherwise homogeneous surface of Ceres is dotted with hundreds of anomalously bright, predominantly carbonate-bearing areas, termed "faculae," with Bond albedos ranging from âŒ0.02 to >0.5. Here, we classify and map faculae globally to characterize their geological setting, assess potential mechanisms for their formation and destruction, and gain insight into the processes affecting the Ceres surface and near-surface. Faculae were found to occur in four distinct geological settings, associated predominantly with impact craters: (1) crater pits, peaks, or floor fractures (floor faculae), (2) crater rims or walls (rim/wall faculae), (3) bright ejecta blankets, and (4) the mountain Ahuna Mons. Floor faculae were identified in eight large, deep, and geologically young (asteroid-derived model (ADM) ages of <420âŻÂ±âŻ60 Ma) craters: Occator, Haulani, Dantu, Ikapati, Urvara, Gaue, Ernutet, and Azacca. The geometry and geomorphic features of the eight craters with floor faculae are consistent with facula formation via impact-induced heating and upwelling of volatile-rich materials, upwelling/excavation of heterogeneously distributed subsurface brines or their precipitation products, or a combination of both processes. Rim/wall faculae and bright ejecta occur in and around hundreds of relatively young craters of all sizes, and the geometry of exposures is consistent with facula formation via the excavation of subsurface bright material, possibly from floor faculae that were previously emplaced and buried. A negative correlation between rim/wall facula albedo and crater age indicates that faculae darken over time. Models using the Ceres crater production function suggest initial production or exposure of faculae by large impacts, subsequent dissemination of facula materials to form additional small faculae, and then burial by impact-induced lateral mixing, which destroys faculae over timescales of less than 1.25âŻGyr. Cumulatively, these models and the observation of faculae limited to geologically young craters indicate relatively modern formation or exposure of faculae, indicating that Ceres' surface remains active and that the near surface may support brines in the present day
Ceres' spectral link to carbonaceous chondrites - Analysis of the dark background materials
Ceresâ surface has commonly been linked with carbonaceous chondrites (CCs) by groundâbased telescopic observations, because of its low albedo, flat to redâsloped spectra in the visible and nearâinfrared (VIS/NIR) wavelength region, and the absence of distinct absorption bands, though no currently known meteorites provide complete spectral matches to Ceres. Spatially resolved data of the Dawn Framing Camera (FC) reveal a generally dark surface covered with bright spots exhibiting reflectance values several times higher than Ceresâ background. In this work, we investigated FC data from High Altitude Mapping Orbit (HAMO) and Ceres eXtended Juling (CXJ) orbit (~140 m/pixel) for global spectral variations. We found that the cerean surface mainly differs by spectral slope over the whole FC wavelength region (0.4â1.0 ÎŒm). Areas exhibiting slopes <â10% ÎŒmâ1 constitute only ~3% of the cerean surface and mainly occur in the bright material in and around young craters, whereas slopes â„â10% ÎŒmâ1 occur on more than 90% of the cerean surface; the latter being denoted as Ceresâ background material in this work. FC and Visible and Infrared Spectrometer (VIR) spectra of this background material were compared to the suite of CCs spectrally investigated so far regarding their VIS/NIR region and 2.7 ÎŒm absorption, as well as their reflectance at 0.653 ÎŒm. This resulted in a good match to heated CI Ivuna (heated to 200â300 °C) and a better match for CM1 meteorites, especially Moapa Valley. This possibly indicates that the alteration of CM2 to CM1 took place on Ceres
Geologic mapping of the Urvara and Yalode Quadrangles of Ceres
We conducted geologic mapping of the Urvara (Ac-13) and Yalode (Ac-14) Quadrangles (21â66°S, 180â360°E) of the dwarf planet Ceres utilizing morphologic, topographic, and compositional information acquired by NASA's Dawn mission. The geologic characteristics of the two large impact basins Urvara (170âŻkm diameter) and Yalode (260âŻkm diameter) and their surroundings were investigated using Dawn Framing Camera datasets, including Survey (415âŻm/pixel), HAMO (140âŻm/pixel), and LAMO (35âŻm/pixel) images and mosaics, color and color ratio images, and DTMs derived from stereo-photogrammetry. Geologic mapping demonstrates that impact cratering has dominated the geologic history of the Urvara and Yalode Quadrangles, with early cratered terrain formation followed by formation of the large basins and widespread emplacement of basin-related smooth material. Impact craters display a wide range of preservation states from nearly completely buried/degraded forms to more recent pristine craters with terraced inner walls and lobate ejecta deposits. Cross-cutting relationships and morphologic signatures show that the Urvara impact followed the Yalode impact, consistent with ages derived from crater size-frequency distributions (580â±â40âŻMa for Yalode and 550â±â50âŻMa for Urvara). Observed differences in basin materials and rim morphology suggest heterogeneities in the substrate excavated by impact. Smooth deposits that cover large areas of the quadrangles, including the basin floors, rims, and exterior zones, are interpreted to be dominated by Urvara ejecta but Yalode ejecta and localized ice-rich flow material may be minor components. Geologic mapping results and simulations of ejecta emplacement suggest that Urvara and Yalode ejecta deposits extend for large distances (more than two crater diameters from the basin centers) and may serve as important stratigraphic markers for the geologic record of Ceres
Compositional mapping of Vesta quadrangle V-23
Since the arrival of the Dawn space-craft [1] at Vesta the Visible and InfraRed Imaging Spectrometer (VIR) [2] has acquired hyperspectral images of Vestaâs surface in the wavelength range from 0.25 to 5.1ÎŒm. As part of the analysis of Vestaâs sur-face composition, a series of four quadrangle maps following the scheme of [3] have been produced show-ing the results derived from the spectroscopic analysis of the VIR data. We present the results of the spectro-scopic analysis achieved for the quadrangle V-23, which covers Vestaâs surface between 57°N - 57°S and 180° - 360°
First mineralogical maps of 4 Vesta
Before Dawn arrived at 4 Vesta only very low spatial resolution (~50 km) albedo and color maps were available from HST data. Also ground-based color and spectroscopic data were utilized as a first attempt to map Vestaâs mineralogical diversity [1-4]. The VIR spectrometer [5] onboard Dawn has ac-quired hyperspectral data while the FC camera [6] ob-tained multi-color data of the Vestan surface at very high spatial resolutions, allowing us to map complex geologic, morphologic units and features. We here re-port about the results obtained from a preliminary global mineralogical map of Vesta, based on data from the Survey orbit. This map is part of an iterative map-ping effort; the map is refined with each improvement in resolution
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