Amino acids were most likely available on the primitive Earth, produced in the primitive atmosphere or in hydrothermal vents. Import of extraterrestrial amino acids may have represented the major supply, as suggested by micrometeorite collections and simulation experiments in space and in the laboratory. Selective condensation of amino acids in water has been achieved via N-carboxy anydrides. Homochiral peptides with an alternating sequence of hydrophobic and hydrophilic amino acids adopt stereoselective and thermostable beta-pleated sheet structures. Some of the homochiral beta-sheets strongly accelerate the hydrolysis of oligoribonucleotides. The beta-sheet-forming peptides have also been shown to protect their amino acids from racemization. Even if peptides are not able to self-replicate, i.e., to replicate a complete sequence from the mixture of amino acids, the accumulation of chemically active peptides on the primitive Earth appears plausible via thermostable and stereoselective beta-sheets made of alternating sequences. 相似文献
This study investigates how epithelial cells moving together function to coordinate their collective movement to repair a wound. Using a lens ex vivo mock cataract surgery model we show that region-specific reorganization of cell–cell junctions, cytoskeletal networks and myosin function along apical and basal domains of an epithelium mediates the process of collective migration. An apical junctional complex composed of N-cadherin/ZO-1/myosin II linked to a cortical actin cytoskeleton network maintains integrity of the tissue during the healing process. These cells’ basal domains often preceded their apical domains in the direction of movement, where an atypical N-cadherin/ZO-1 junction, linked to an actin stress fiber network rich in phosphomyosin, was prominent in cryptic lamellipodia. These junctions joined the protruding forward-moving lamellipodia to the back end of the cell moving directly in front of it. These were the only junctions detected in cryptic lamellipodia of lens epithelia migrating in response to wounding that could transmit the protrusive forces that drive collective movement. Both integrity of the epithelium and ability to effectively heal the wound was found to depend on myosin mechanical cues. 相似文献
Despite its central role in the protein folding process, the specific mechanism(s) behind β-sheet formation has yet to be determined. For example, whether the nucleation of β-sheets, often containing strands separated in sequence by many residues, is local or not remains hotly debated. Here, we investigate the initial nucleation step of β-sheet formation by performing an analysis of the smallest β-sheets in a non-redundant dataset on the grounds that the smallest sheets, having undergone little growth after nucleation, will be enriched for nucleating characteristics. We find that the residue propensities are similar for small and large β-sheets as are their interstrand pairing preferences, suggesting that nucleation is not primarily driven by specific residues or interacting pairs. Instead, an examination of the structural environments of the two-stranded sheets shows that virtually all of them are contained in single, compact structural modules, or when multiple modules are present, one or both of the chain termini are involved. We, therefore, find that β-nucleation is a local phenomenon resulting either from sequential or topological proximity. We propose that β-nucleation is a result of two opposite factors; that is, the relative rigidity of an associated folding module that holds two stretches of coil close together in topology coupled with sufficient chain flexibility that enables the stretches of coil to bring their backbones in close proximity. Our findings lend support to the hydrophobic zipper model of protein folding (Dill, K. A., Fiebig, K. M., and Chan, H. S. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 1942–1946). Implications for protein folding are discussed. 相似文献
We present the first long‐term, highly resolved prokaryotic cell concentration record obtained from a polar ice core. This record, obtained from the West Antarctic Ice Sheet (WAIS) Divide (WD) ice core, spanned from the Last Glacial Maximum (LGM) to the early Holocene (EH) and showed distinct fluctuations in prokaryotic cell concentration coincident with major climatic states. The time series also revealed a ~1,500‐year periodicity with greater amplitude during the Last Deglaciation (LDG). Higher prokaryotic cell concentration and lower variability occurred during the LGM and EH than during the LDG. A sevenfold decrease in prokaryotic cell concentration coincided with the LGM/LDG transition and the global 19 ka meltwater pulse. Statistical models revealed significant relationships between the prokaryotic cell record and tracers of both marine (sea‐salt sodium [ssNa]) and burning emissions (black carbon [BC]). Collectively, these models, together with visual observations and methanosulfidic acid (MSA) measurements, indicated that the temporal variability in concentration of airborne prokaryotic cells reflected changes in marine/sea‐ice regional environments of the WAIS. Our data revealed that variations in source and transport were the most likely processes producing the significant temporal variations in WD prokaryotic cell concentrations. This record provided strong evidence that airborne prokaryotic cell deposition differed during the LGM, LDG, and EH, and that these changes in cell densities could be explained by different environmental conditions during each of these climatic periods. Our observations provide the first ice‐core time series evidence for a prokaryotic response to long‐term climatic and environmental processes. 相似文献
Background: Heathlands are relatively abundant in the landscape of the western Mediterranean region, especially in the Strait of Gibraltar region, where it is locally known as herriza. They are associated with a mild Mediterranean climate regime and with acid, nutrient-poor soils. They harbour a high plant diversity, often viewed as a consequence of the transition between European Atlantic heathland and Mediterranean sclerophyllous shrubland floras.
Aims: To determine whether species-rich Mediterranean heathlands, including the herriza, constitute distinct heathland formations rather than transitional vegetation units between Atlantic heathlands and Mediterranean garrigue shrublands.
Methods: We quantified species richness, endemism and analysed the β-diversity of the woody component of Mediterranean heathland communities throughout its geographic range, with special emphasis on the Strait of Gibraltar region.
Results: Mediterranean heathlands, including the herriza, are not transitional communities between Atlantic heathlands and Mediterranean shrublands. Woody species richness and, particularly, endemic richness was the highest in the herriza.
Conclusions: The high biodiversity values of the herriza are a likely consequence of the ecological singularity of the Strait of Gibraltar region and its known role as a glacial refugium. Despite its treeless feature, the herriza deserves special recognition and protection from both in its European and North African extension. 相似文献
Single-molecule detection and tracking is important for observing biomolecule interactions in the microenvironment. Here we report selective plane illumination microscopy (SPIM) with single-molecule detection in living organisms, which enables fast imaging and single-molecule tracking and optical penetration beyond 300 μm. We detected single nanocrystals in Drosophila larvae and zebrafish embryo. We also report our first tracking of single quantum dots during zebrafish development, which displays a transition from flow to confined motion prior to the blastula stage. The new SPIM setup represents a new technique, which enables fast single-molecule imaging and tracking in living systems. 相似文献
An ellipsoidal chamber which is composed of a very thin organic layer and emplaced in an enlargement continuing upwards into a vertical burrow is found in thin sections of a fluvial channel‐fill sandstone bed of the Lower Cretaceous Jinju Formation in South Korea. Micromorphological analysis demonstrates that the ellipsoidal chamber is attributable to Fictovichnus. It can be interpreted as a free‐standing wall of a wasp cocoon attributable to Fictovichnus sciuttoi or Fictovichnus aragon, or an organic lining of chafer beetle or weevil pupation chamber attributable to Fictovichnus gobiensis. The analysis further shows that the vertical burrow and enlargement appear to reflect chafer and weevil larval behaviour, but the arrangement of the chamber within the enlargement appears to be the same as a wasp cocoon inside a cell. Such uncertainties of ichnotaxonomic assignment and putative producer suggest that the Jinju specimen may reflect an aberrant pupation behaviour of coleopterans or hymenopterans. This Early Cretaceous specimen may be the earliest record of an insect pupation structure probably associated with a burrow. Its atypical morphologies and very rare occurrence in a channel sandstone bed without other evidence of pedogenesis suggest that the Jinju specimen was constructed in incipiently and briefly exposed sediment, unlike typical insect nesting structures constructed in soil. 相似文献
The fossil plants studied in this paper were collected by the Chinese Antarctic Expeditions (1986—1987 and 1987—1988) from the locality Fossil Hill on the Fildes Peninsula southwest of the King George Island of the South Shetland Islands. 相似文献