首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2861篇
  免费   133篇
  国内免费   3篇
  2023年   27篇
  2022年   51篇
  2021年   91篇
  2020年   50篇
  2019年   60篇
  2018年   82篇
  2017年   67篇
  2016年   99篇
  2015年   145篇
  2014年   171篇
  2013年   206篇
  2012年   265篇
  2011年   232篇
  2010年   154篇
  2009年   112篇
  2008年   162篇
  2007年   143篇
  2006年   113篇
  2005年   110篇
  2004年   110篇
  2003年   96篇
  2002年   73篇
  2001年   27篇
  2000年   22篇
  1999年   27篇
  1998年   19篇
  1997年   13篇
  1996年   7篇
  1995年   12篇
  1994年   5篇
  1993年   9篇
  1992年   14篇
  1991年   31篇
  1990年   14篇
  1989年   16篇
  1988年   16篇
  1987年   9篇
  1986年   11篇
  1985年   14篇
  1984年   17篇
  1983年   13篇
  1982年   7篇
  1981年   8篇
  1980年   11篇
  1979年   9篇
  1978年   8篇
  1977年   5篇
  1974年   4篇
  1973年   4篇
  1972年   9篇
排序方式: 共有2997条查询结果,搜索用时 343 毫秒
21.
Tilletia indica teliospores were studied by use of thin sections and freeze-etch replicas. Surfaces of these spores have rodlet patterns which differ from those previously reported for spores of other fungi. The rodlets on T. indica teliospores average 240 nm in length and are not grouped into fascicles.  相似文献   
22.
The gametocytocidal efficacy of artemisinin (qinghaosu) was evaluated and scanning electron microscopical evidence is given for gametocytocidal action of a single 5-10-mg/kg dose of artemisinin against simian malarial parasite (Plasmodium cynomolgi B). No sporontocidal effect of artemisinin was observed.  相似文献   
23.
A life-table was constructed for a little known aphidiid waspLysiphlebia mirzai, a parasitoid of cereal aphid,Rhopalosiphum maidis. The female parasitoid survived 6.4 ± 1.17 (SD) days and oviposited intensively 4.0 ± 0.47 days. The total fecundity rate, Rt, was 169.2 ± 6.94 mummies/female and net reproductive rate, Ro, was 92.70 female offspring/female. The intrinsic total fecundity rate, rt, and intrinsic rate of natural increase, rm, the finite rate of total fecundity, λt, and finite rate of increase, λm, was 0.27048, 0.24155, 1.31059 and 1.27322 respectively. The mean generation time (18.75 days) and doubling time of the population (2.87 days) was slightly higher than other aphidiids studied so far. The proportion of female progenies decreased significantly on the successive oviposition days.   相似文献   
24.
Cropping on jhum fallows in north-eartern India is predominantly done for one year in a jhum cycle. If second year cropping is done, expanse of the forest land required for slashing and burning could be reduced significantly. We tested this hypothesis in a young (6 yr) and an old (20 yr) jhum fallow. We also evaluated if the productivity during second year cropping could be alleviated by auxiliary measures such as tilling the soil or application of fertilizers (chemical or farm-yard manure or both in combination). The results demonstrate that the ecosystem productivity (total dry matter production) and economic yield (rice grain production) decline with shortening of jhum cycle. Second year cropping causes a further decline in ecosystem productivity in old jhum field, but not in young jhum field. Economic yield from second year cropping in its traditional form (without any fertilizer treatment) is not much lower than that in the first year, and can be improved further by manuring the soil. Tilling of soil improves neither ecosystem productivity nor economic yield. Different fertilization treatments respond differently; while inorganic manuring enhances ecosystem productivity, a combination of inorganic and organic manuring improves economic yield  相似文献   
25.
We have analyzed the tyrosinase (TYR) gene in 38 unrelated patients with oculocutaneous albinism (OCA), derived from several different ethnic groups of the diverse population of Israel. We detected TYR gene mutations in 23 of the 34 patients with apparent type I (i.e., tyrosinase-deficient) OCA and in none of the patients with other clinical forms of albinism. Among Moroccan Jews with type IA (i.e., tyrosinase-negative) OCA, we detected a highly predominant mutant allele containing a missense substitution, Gly47Asp (G47D). This mutation occurs on the same haplotype as in patients from the Canary Islands and Puerto Rico, suggesting that the G47D mutation in these ethnically distinct populations may stem from a common origin.  相似文献   
26.
Cycling of six mineral elements (N, P, K, Na, Ca and Mg) was studied in a humid subtropical grassland at Cherrapunji, north-eastern India during 1988-1989. Elemental concentrations in the shoot of four dominant grass species,viz., Arundinella khaseana, Chrysopogon gryllus, Eragrostiella leioptera andEulalia trispicata were very low, and none of the species appears suitable for fodder use. Among different vegetation compartments, live root was the largest reservoir of all the nutrients (except Ca) followed by live shoot, dead shoot, litter and dead root. For Ca, live shoot was the major storage compartment. The total annual uptake (kg ha-1) was 137.3, 10.4, 51.1, 5.5, 8.7 and 18.2 for N, P, K, Na, Ca and Mg, respectively. In an annual cycle 98% N, 77% P, 49% K, 109% Na, 87% Ca and 65% Mg returned to the soil through litter and belowground detritus. A major portion of N, P and Na was recycled through the belowground system, whereas nearly half of K, Ca and Mg was recycled through the shoot system. Precipitation acts as the source of N and P input, but at the same time causes loss of cations.  相似文献   
27.
Small nuclear ribonucleoprotein (snRNP) particles play an important role in the processing of pre-mRNA. snRNPs have been identified immunologically in a variety of cells, but none have ever been observed in prokaryotic systems. This report provides the first evidence for the presence of snRNP-like components in two types of prokaryotic cells: those of the cyanobacterium Synechococcus leopoliensis and those of the gram-positive eubacterium Bacillus subtilis. These components consist of snRNP-immunoreactive proteins and RNAs, including some with the snRNP-unique 5' m2,2,7G (m3G) cap. Immunoreactivity was determined by immunoprecipitation procedures, with either antinuclear-antibody-positive (RNP- and Sm-monospecific) patient sera or a m3G monoclonal antibody, with radiolabelled cell extracts that were preadsorbed with antinuclear-antibody-negative sera. S. leopoliensis immunoprecipitates showed the presence of high-molecular-mass proteins (14 to 70 kDa) and RNAs (138 to 243 nucleotides) that are analogous in size to proteins and RNAs found in human (HEp-2) cell immunoprecipitates but absent in Escherichia coli immunoprecipitates. Thin-layer chromatography of S. leopoliensis immunoprecipitates confirmed the presence of a capped nucleotide similar to a capped nucleotide in HEp-2 immunoprecipitates; no such nucleotide was observed in E. coli immunoprecipitates. Immunoreactive RNAs (117-170 nucleotides) were identified in a second eubacterium, B. subtilis, as well. This work suggests that snRNPs or their evolutionary predecessors predate the emergence of eukaryotic cells.  相似文献   
28.
In recent years, drug manufacturers and researchers have begun to consider the nanobiotechnology approach to improve the drug delivery system for tumour and cancer diseases. In this article, we review current strategies to improve tumour and cancer drug delivery, which mainly focuses on sustaining biocompatibility, biodistribution, and active targeting. The conventional therapy using cornerstone drugs such as fludarabine, cisplatin etoposide, and paclitaxel has its own challenges especially not being able to discriminate between tumour versus normal cells which eventually led to toxicity and side effects in the patients. In contrast to the conventional approach, nanoparticle-based drug delivery provides target-specific delivery and controlled release of the drug, which provides a better therapeutic window for treatment options by focusing on the eradication of diseased cells via active targeting and sparing normal cells via passive targeting. Additionally, treatment of tumours associated with the brain is hampered by the impermeability of the blood–brain barriers to the drugs, which eventually led to poor survival in the patients. Nanoparticle-based therapy offers superior delivery of drugs to the target by breaching the blood–brain barriers. Herein, we provide an overview of the properties of nanoparticles that are crucial for nanotechnology applications. We address the potential future applications of nanobiotechnology targeting specific or desired areas. In particular, the use of nanomaterials, biostructures, and drug delivery methods for the targeted treatment of tumours and cancer are explored.  相似文献   
29.
Cancer cell lines play a crucial role as invaluable models in cancer research, facilitating the examination of cancer progression as well as the advancement of diagnostics and treatments. While they may not perfectly replicate the original tumor, they generally exhibit similar characteristics. Low-passage cancer cell lines are generally preferred due to their closer resemblance to the original tumor, as long-term culturing can alter the genetic and molecular profiles of a cell line thereby highlighting the importance of monitoring the passage number (PN). Variations in proliferation, migration, gene expression, and drug sensitivity can be linked to PN differences. PN can also influence DNA methylation levels, metabolic profiles, and the expression of genes/or proteins in cancer cell lines. When conducting research on cancer cell lines, it is crucial for researchers to carefully select the appropriate PN to maintain consistency and reliability of results. Moreover, to ensure dependability and replicability, scientists ought to actively track the growth, migration, and gene/or protein profiles of cancer cell lines at specific PNs. This approach enables the identification of the most suitable range of PNs for experiments, guaranteeing consistent and precise results. Additionally, such efforts serve to minimize disparities and uphold the integrity of research. In this review, we have laid out recommendations for laboratories to overcome these PN discrepancies when working with cancer cell lines.  相似文献   
30.
Mn(II) and Co(II) complexes of methyl-(Z)−N′-carbamothioylcarbamohydrazonate Schiff base ligand were synthesized. The ligand and metal salts were taken in 2 : 1 stoichiometric ratio. All the synthesized complexes were characterized using elemental analysis, molar conductance, magnetic moment and various spectroscopic techniques (FT-IR, UV/VIS, EPR) techniques. Elemental and spectroscopic results verified bidentate donor nature of the ligand and octahedral geometry of all the complexes. The non-electrolytic nature of Mn(II) and Co(II) complexes were suggested by conductivity data analysis. In vitro antibacterial (E. coli and S. aureus) and antifungal (C. albicans and C. tropicalis) screening were achieved by employing agar well diffusion method which revealed better antimicrobial activity of Co(II) complexes than Mn(II) complexes. In silico SwissADME study predicted the drug-likeness probability of ligand and complexes. The interaction of two bacterial proteins (E. coli and S. aureus) with compounds was also analyzed using molecular docking study, which corroborate the in vitro analysis.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号