全文获取类型
收费全文 | 301篇 |
免费 | 23篇 |
国内免费 | 1篇 |
专业分类
325篇 |
出版年
2024年 | 2篇 |
2023年 | 8篇 |
2022年 | 8篇 |
2021年 | 3篇 |
2020年 | 10篇 |
2019年 | 12篇 |
2018年 | 7篇 |
2017年 | 9篇 |
2016年 | 11篇 |
2015年 | 15篇 |
2014年 | 21篇 |
2013年 | 28篇 |
2012年 | 26篇 |
2011年 | 20篇 |
2010年 | 11篇 |
2009年 | 10篇 |
2008年 | 13篇 |
2007年 | 7篇 |
2006年 | 11篇 |
2005年 | 5篇 |
2004年 | 7篇 |
2003年 | 12篇 |
2002年 | 3篇 |
2001年 | 8篇 |
2000年 | 4篇 |
1999年 | 1篇 |
1998年 | 4篇 |
1997年 | 1篇 |
1996年 | 5篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1993年 | 5篇 |
1992年 | 4篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1984年 | 2篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1975年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有325条查询结果,搜索用时 0 毫秒
81.
Technological advances have led to the development of powerful yet portable tablet computers whose touch-screen resolutions now permit the presentation of targets small enough to test the limits of normal visual acuity. Such devices have become ubiquitous in daily life and are moving into the clinical space. However, in order to produce clinically valid tests, it is important to identify the limits imposed by the screen characteristics, such as resolution, brightness uniformity, contrast linearity and the effect of viewing angle. Previously we have conducted such tests on the iPad 3. Here we extend our investigations to 2 other devices and outline a protocol for calibrating such screens, using standardised methods to measure the gamma function, warm up time, screen uniformity and the effects of viewing angle and screen reflections. We demonstrate that all three devices manifest typical gamma functions for voltage and luminance with warm up times of approximately 15 minutes. However, there were differences in homogeneity and reflectance among the displays. We suggest practical means to optimise quality of display for vision testing including screen calibration. 相似文献
82.
Rumana Aslam M.Y.K. Ansari Sana Choudhary Towseef Mohsin Bhat Nusrat Jahan 《Saudi Journal of Biological Sciences》2014,21(5):465-472
The present study was designed to investigate the effects of cadmium (Cd) on biochemical, physiological and cytological parameters of Capsicum annuum L. treated with five different concentrations (20, 40, 60, 80 and 100 ppm) of the metal. Shoot–root length, pigment and protein content showed a continuous decrease with increasing Cd concentrations and the maximal decline was observed at the higher concentration. Proline content was found to be increased upto 60 ppm while at higher concentrations it gradually decreased. MDA content and chromosomal aberrations increased as the concentration increased. Additionally Random amplified polymorphic DNA (RAPD) technique was used for the detection of genotoxicity induced by Cd. A total of 184 bands (62 polymorphic and 122 monomorphic) were generated in 5 different concentrations with 10 primers where primer OPA-02 generated the highest percentage of polymorphism (52.63%). Dendrogram showed that control, R1 and R2 showed similar cluster and R4 and R5 grouped with R3 into one cluster, which showed that plants from higher doses showed much difference than the plants selected at mild doses which resemble control at the DNA level. This investigation showed that RAPD marker is a useful tool for evaluation of genetic diversity and relationship among different metal concentrations. 相似文献
83.
84.
Mahmoud F. Seleiman Muhammad Talha Aslam Bushra Ahmed Alhammad Muhammad Umair Hassan Rizwan Maqbool Muhammad Umer Chattha Imran Khan Harun Ireri Gitari Omer S. Uslu Rana Roy Martin Leonardo Battaglia 《Phyton》2022,91(4):667-694
Salinity stress is a major threat to global food production and its intensity is continuously increasing because of
anthropogenic activities. Wheat is a staple food and a source of carbohydrates and calories for the majority of
people across the globe. However, wheat productivity is adversely affected by salt stress, which is associated with
a reduction in germination, growth, altered reproductive behavior and enzymatic activity, disrupted photosynthesis, hormonal imbalance, oxidative stress, and yield reductions. Thus, a better understanding of wheat (plant)
behavior to salinity stress has essential implications to devise counter and alleviation measures to cope with salt
stress. Different approaches including the selection of suitable cultivars, conventional breeding, and molecular
techniques can be used for facing salt stress tolerance. However, these techniques are tedious, costly, and
labor-intensive. Management practices are still helpful to improve the wheat performance under salinity stress.
Use of arbuscular mycorrhizal fungi, plant growth-promoting rhizobacteria, and exogenous application of phytohormones, seed priming, and nutrient management are important tools to improve wheat performance under
salinity stress. In this paper, we discussed the effect of salinity stress on the wheat crop, possible mechanisms to
deal with salinity stress, and management options to improve wheat performance under salinity conditions. 相似文献
85.
A facile method was used for the synthesis of peanut-shaped very emissive NaGdF4:Yb, Er upconversion nanospheres (UCNSs) at lower temperatures with uniform size distribution. Crystallographic structure, phase purity, morphology, thermal robustness, biocompatibility, colloidal stability, surface chemistry, optical properties, and luminesce properties were explored by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), zeta potential, thermogravimetric/differential thermal analysis (TGA/DTA), Fourier-transform infrared (FTIR), ultraviolet (UV)-visible and photoluminescence spectroscopic tools. XRD pattern verified the construction of a single-phase, highly-crystalline NaGdF4 phase with a hexagonal structure. Peanut-shaped morphology of the sample was obtained from SEM micrographs which were validated from high-resolution TEM images, to have an equatorial diameter of 170 to 200 nm and a length of 220 to 230 nm, with irregular size, monodispersed, porous structure, and rough surface of the particles. The positive zeta potential value exhibited good biocompatibility along with high colloidal stability as observed from the absorption spectrum. The prepared UCNSs revealed high dispersibility, irregular size peanut-shaped morphology, rough surface, good colloidal stability, and excellent biocompatibility in aqueous media. A hexagonal phase NaGdF4 doped with ytterbium (Yb) and erbium (Er) UCNSs revealed the characteristics of highly dominant emissions located at 520–525, 538–550, and 659–668 nm corresponding to the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transition of Er3+ ions, respectively, as a result of energy transfer from sensitizer Yb3+ ion to emitter Er3+ ion. 相似文献
86.
Petukhov M Lebedev D Shalguev V Islamov A Kuklin A Lanzov V Isaev-Ivanov V 《Proteins》2006,65(2):296-304
RecA protein is a central enzyme in homologous DNA recombination, repair and other forms of DNA metabolism in bacteria. It functions as a flexible helix-shaped filament bound on stretched single-stranded or double-stranded DNA in the presence of ATP. In this work, we present an atomic level model for conformational transitions of the RecA filament. The model describes small movements of the RecA N-terminal domain due to coordinated rotation of main chain dihedral angles of two amino acid residues (Psi/Lys23 and Phi/Gly24), while maintaining unchanged the RecA intersubunit interface. The model is able to reproduce a wide range of observed helix pitches in transitions between compressed and stretched conformations of the RecA filament. Predictions of the model are in agreement with Small Angle Neutron Scattering (SANS) measurements of the filament helix pitch in RecA::ADP-AlF(4) complex at various salt concentrations. 相似文献
87.
Biology Bulletin - As part of comprehensive study of lichen diversity of northern areas of Pakistan using molecular and morphological approaches, we found four species of the family Graphidaceae.... 相似文献
88.
Aslam M Guthridge JM Hack BK Quigg RJ Holers VM Perkins SJ 《Journal of molecular biology》2003,329(3):525-550
Complement receptor-related gene/protein y (Crry) is a cell membrane-bound regulator of complement activation found in mouse and rat. Crry contains only short complement/consensus repeat (SCR) domains. X-ray and neutron scattering was performed on recombinant rat Crry containing the first five SCR domains (rCrry) and mouse Crry with five SCR domains conjugated to the Fc fragment of mouse IgG1 (mCrry-Ig) in order to determine their solution structures at medium resolution. The radius of gyration R(G) of rCrry was determined to be 4.9-5.0 nm, and the R(G) of the cross-section was 1.2-1.5 nm as determined by X-ray and neutron scattering. The R(G) of mCrry-Ig was 6.6-6.7 nm, and the R(G) of the cross-section were 2.3-2.4 nm and 1.3 nm. The maximum dimension of rCrry was 18 nm and that for mCrry-Ig was 26 nm. The neutron data indicated that rCrry and mCrry-Ig have molecular mass values of 45,000 Da and 140,000 Da, respectively, in agreement with their sequences, and sedimentation equilibrium data supported these determinations. Time-derivative velocity experiments gave sedimentation coefficients of 2.4S for rCrry and 5.4S for mCrry-Ig. A medium-resolution model of rCrry was determined using homology models that were constructed for the first five SCR domains of Crry from known crystal and NMR structures, and linked by randomly generated linker peptide conformations. These trial-and-error calculations revealed a small family of extended rCrry structures that best accounted for the scattering and ultracentrifugation data. These were shorter than the most extended rCrry models as the result of minor bends in the inter-SCR orientations. The mCrry-Ig solution data were modelled starting from a fixed structure for rCrry and the crystal structure of mouse IgG1, and was based on conformational searches of the hinge peptide joining the mCrry and Fc fragments. The best-fit models showed that the two mCrry antennae in mCrry-Ig were extended from the Fc fragment. No preferred orientation of the antennae was identified, and this indicated that the accessibility of the antennae for the molecular targets C4b and C3b was not affected by the covalent link to Fc. A structural comparison between Crry and complement receptor type 1 indicated that the domain arrangement of Crry SCR 1-3 is as extended as that of the CR1 SCR 15-17 NMR structure. 相似文献
89.
Wistuba J Schrod A Greve B Hodges JK Aslam H Weinbauer GF Luetjens CM 《Biology of reproduction》2003,69(2):582-591
The succession in time and space of specific germ cell associations, denoted as spermatogenic stages, is a typical feature of mammalian spermatogenesis. The arrangement of these stages is either single stage (one spermatogenic stage per tubular cross-section) or multistage (more than one spermatogenic stage per tubular cross-section). It has been proposed that the single-stage versus multistage arrangement is related to spermatogenic efficiency and that the multistage arrangement is typical for hominids. In the present work, the arrangement of spermatogenic stages and the spermatogenic efficiency of 17 primate species, comprising Strepsirrhini (Prosimians: Lemuriformes, Lorisiformes), Platyrrhini (New World primates), Catarrhini (Old World primates), and Hominoidea (great apes and humans), were analyzed comparatively by quantitative histological and flow cytometric means. We found a predominant single-stage tubular organization in the Strepsirrhini, indicating that the single-stage form represents the ancestral state. The highest degree of multistage complexity was found in Hominoidea (except orangutan) and in Platyrrhini, but not in Catarrhini. Hence, no direct relationship between single-stage/multistage tubular topography and phylogeny could be established across primates. In fact, the tubule arrangement seen in Platyrrhini and Catarrhini primates is the reverse of what might be expected from phylogeny. Interestingly, spermatogenic efficiency was similar in all species. We found no correlation between single-stage/multistage arrangement and spermatogenic efficiency or mating system. We speculate that the presence of a single-stage/multistage organization might simply reflect germ cell clonal size. Our findings further indicate that sperm competition in primates is not reflected at the level of testicular function. 相似文献
90.
Multiple levels of Notch signal regulation (review) 总被引:6,自引:0,他引:6
Baron M Aslam H Flasza M Fostier M Higgs JE Mazaleyrat SL Wilkin MB 《Molecular membrane biology》2002,19(1):27-38
Notch is a vitally important signalling receptor controlling cell fate determination and pattern formation in numerous ways during development of both invertebrate and vertebrate species. An intriguing pathway for the Notch signal has emerged where, after ligand-dependent proteolysis, an intracellular fragment of the receptor itself translocates to the nucleus to regulate gene expression. The nuclear activity of the Notch intracellular domain is linked to complexes regulating chromatin organization through histone deacetylation and acetylation. To allow the Notch signal to be deployed in numerous contexts, many different mechanisms have evolved to regulate the level, duration and spatial distribution of Notch activity. Regulation occurs at multiple levels including patterns of ligand and receptor expression, Notch-ligand interactions, trafficking of the receptor and ligands, and covalent modifications including glycosylation, phosphorylation and ubiquitination. Several Notch regulatory proteins have conserved domains that link them to the ubiquitination pathway, and ubiquitination of the Notch intracellular domain has recently been linked to its degradation. Different proteolytically derived isoforms of Notch have also been identified that may be involved in alternative Notch-dependent signals or regulatory mechanisms, and differences between the four mammalian Notch homologues are beginning to be appreciated. 相似文献