全文获取类型
收费全文 | 134894篇 |
免费 | 4683篇 |
国内免费 | 893篇 |
专业分类
140470篇 |
出版年
2022年 | 374篇 |
2021年 | 728篇 |
2020年 | 509篇 |
2019年 | 595篇 |
2018年 | 12398篇 |
2017年 | 11191篇 |
2016年 | 8463篇 |
2015年 | 2411篇 |
2014年 | 2292篇 |
2013年 | 2828篇 |
2012年 | 7251篇 |
2011年 | 15659篇 |
2010年 | 13814篇 |
2009年 | 9879篇 |
2008年 | 12020篇 |
2007年 | 13609篇 |
2006年 | 2585篇 |
2005年 | 2567篇 |
2004年 | 3052篇 |
2003年 | 2885篇 |
2002年 | 2606篇 |
2001年 | 940篇 |
2000年 | 805篇 |
1999年 | 684篇 |
1998年 | 530篇 |
1997年 | 418篇 |
1996年 | 397篇 |
1995年 | 387篇 |
1994年 | 350篇 |
1993年 | 366篇 |
1992年 | 457篇 |
1991年 | 417篇 |
1990年 | 368篇 |
1989年 | 388篇 |
1988年 | 404篇 |
1987年 | 356篇 |
1986年 | 277篇 |
1985年 | 313篇 |
1984年 | 321篇 |
1983年 | 290篇 |
1982年 | 269篇 |
1981年 | 242篇 |
1980年 | 215篇 |
1979年 | 220篇 |
1978年 | 186篇 |
1977年 | 187篇 |
1976年 | 185篇 |
1975年 | 210篇 |
1972年 | 388篇 |
1971年 | 398篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Maureen P. Martin Anita Harding Robert Chadwick Mel Kronick Michael Cullen Ling Lin Emmanuel Mignot M. Carrington 《Immunogenetics》1997,47(2):131-138
The human genome contains a large number of interspersed microsatellite repeats which exhibit a high degree of polymorphism
and are inherited in a Mendelian fashion, making them extremely useful genetic markers. Several microsatellites have been
described in the HLA region, but allele nomenclature, a set of broadly distributed controls, and typing methods have not been standardized, which
has resulted in discrepant microsatellite data between laboratories. In this report we present a detailed protocol for genotyping
microsatellites using a semi-automated fluorescence-based method. Twelve microsatellites within or near the major histocompatibility
complex (MHC) were typed in the 10th International Histocompatibility Workshop homozygous typing cell lines (HTCs) and alleles
were designated based on size. All loci were sequenced in two HTCs providing some information on the level of complexity of
the repeat sequence. A comparison of allele size obtained by genotyping versus that obtained by direct sequencing showed minor
discrepancies in some cases, but these were not unexpected given the technical differences in the methodologies. Fluorescence-based
typing of microsatellites in the MHC described herein is highly efficient, accurate, and reproducible, and will allow comparison
of results between laboratories.
Received: 10 May 1997 / Revised: 1 August 1997 相似文献
992.
Kazuki Fujita Takashi Kunito Hitoshi Moro Hideshige Toda Shigeto Otsuka Kazunari Nagaoka 《Biogeochemistry》2017,132(3):325-342
According to the resource allocation model for extracellular enzyme synthesis, microorganisms should preferentially allocate their resources to phosphorus (P)-acquiring enzyme synthesis when P availability is low in soils. However, the validity of this model across different soil types and soils differing in their microbial community composition has not been well demonstrated. Here we investigated whether the resource allocation model for phosphatase synthesis is applicable across different soil types (Andosols, Acrisols, Cambisols, and Fluvisols) and land uses (arable and forest), and we examined which soil test P and/or P fraction microorganisms responded to when investing their resources in phosphatase synthesis in the soils. The ratio of alkaline phosphatase (ALP) to β-d-glucosidase (BG) activities in the arable soils and the ratio of acid phosphatase (ACP) to BG activities in the forest soils were significantly negatively related with the available inorganic P concentration. We also observed significant effects of available inorganic P, pH, soil types, and land uses on the (ACP + ALP)/BG ratio when the data for the arable and forest soils were combined and used in a stepwise multiple regression analysis. These results suggest that microbial resource allocation for phosphatase synthesis is primarily controlled by available inorganic P concentration and soil pH, but the effects of soil types and land uses are also significant. 相似文献
993.
Petra van der Lelij Krystyna H. Chrzanowska Barbara C. Godthelp Martin A. Rooimans Anneke B. Oostra Markus Stumm Ma?gorzata Z. Zdzienicka Hans Joenje Johan P. de Winter 《American journal of human genetics》2010,86(2):262-1182
The iron-sulfur-containing DNA helicases XPD, FANCJ, DDX11, and RTEL represent a small subclass of superfamily 2 helicases. XPD and FANCJ have been connected to the genetic instability syndromes xeroderma pigmentosum and Fanconi anemia. Here, we report a human individual with biallelic mutations in DDX11. Defective DDX11 is associated with a unique cellular phenotype in which features of Fanconi anemia (drug-induced chromosomal breakage) and Roberts syndrome (sister chromatid cohesion defects) coexist. The DDX11-deficient patient represents another cohesinopathy, besides Cornelia de Lange syndrome and Roberts syndrome, and shows that DDX11 functions at the interface between DNA repair and sister chromatid cohesion. 相似文献
994.
Cheryl Baldwin Nana Wilberforce Amit Kapur 《The International Journal of Life Cycle Assessment》2011,16(1):40-49
Purpose
There is no clear guidance for responsible food service operations to reduce their environmental footprint, so the efforts put forth by a restaurant may not have the environmental impact intended. As a result, Green Seal conducted life cycle assessment research on restaurants and food service operations to define priorities for environmental improvement. This information was then used to develop a sustainability standard and certification (i.e., ecolabel) program. 相似文献995.
Manco G Carrea G Giosuè E Ottolina G Adamo G Rossi M 《Extremophiles : life under extreme conditions》2002,6(4):325-331
The esterase genes est2 from Alicyclobacillus acidocaldarius and AF1716 from Archaeoglobus fulgidus were subjected to error-prone PCR in an effort to increase the low enantioselectivity of the corresponding enzymes EST2 and AFEST, respectively. The model substrate ( RS)- p-nitrophenyl-2-chloropropionate was chosen to produce ( S)-2-chloropropionic acid, an important intermediate in the synthesis of some optically pure compounds, such as the herbicide mecoprop. In the case of EST2, a single mutant, Leu212Pro, was obtained showing a slightly enhanced preference toward the ( S) substrate; in the case of AFEST, a double mutant, Leu101Ile/Asp117Gly, was obtained showing an increased preference in the opposite direction. The 3-D structures of the EST2 and AFEST enzymes were analyzed by molecular modeling to determine the effects of the mutations. Mutations were positioned differently in the structures, but in both cases caused small modifications around the active site and in the oxyanion loop. 相似文献
996.
997.
Rasoul Ghasemi Hoorieh Share Roza Sharifi Robert S. Boyd Nishanta Rajakaruna 《Ecological Research》2018,33(4):737-747
The importance of calcium in nickel tolerance was studied in the nickel hyperaccumulator plant Alyssum inflatum by gene transformation of CAX1, a vacuolar membrane transporter that reduces cytosolic calcium. CAX1 from Arabidopsis thaliana with a CaMV35S promoter accompanying a kanamycin resistance gene was transferred into A. inflatum using Agrobacterium tumefaciens. Transformed calli were sub-cultured three times on kanamycin-rich media and transformation was confirmed by PCR using a specific primer for CAX1. At least 10 callus lines were used as a pool of transformed material. Both transformed and untransformed calli were treated with varying concentrations of either calcium (1–15 mM) or nickel (0–500 µM) to compare their responses to those ions. Increased external calcium generally led to increased callus biomass, however, the increase was greater for untransformed callus. Further, increased external calcium led to increased callus calcium concentrations. Transformed callus was less nickel tolerant than untransformed callus: under increasing nickel concentrations callus relative growth rate was significantly less for transformed callus. Transformed callus also contained significantly less nickel than untransformed callus when exposed to the highest external nickel concentration (200 µM). We suggest that transformation with CAX1 decreased cytosolic calcium and resulted in decreased nickel tolerance. This in turn suggests that, at low cytosolic calcium concentrations, other nickel tolerance mechanisms (e.g., complexation and vacuolar sequestration) are insufficient for nickel tolerance. We propose that high cytosolic calcium is an important mechanism that results in nickel tolerance by nickel hyperaccumulator plants. 相似文献
998.
Marc-Frédérick Blanchet Karine St-Onge Véronique Lisi Julie Robitaille Sylvie Hamel Fran?ois Major 《Nucleic acids research》2014,42(17):11261-11271
Anti-infection drugs target vital functions of infectious agents, including their ribosome and other essential non-coding RNAs. One of the reasons infectious agents become resistant to drugs is due to mutations that eliminate drug-binding affinity while maintaining vital elements. Identifying these elements is based on the determination of viable and lethal mutants and associated structures. However, determining the structure of enough mutants at high resolution is not always possible. Here, we introduce a new computational method, MC-3DQSAR, to determine the vital elements of target RNA structure from mutagenesis and available high-resolution data. We applied the method to further characterize the structural determinants of the bacterial 23S ribosomal RNA sarcin–ricin loop (SRL), as well as those of the lead-activated and hammerhead ribozymes. The method was accurate in confirming experimentally determined essential structural elements and predicting the viability of new SRL variants, which were either observed in bacteria or validated in bacterial growth assays. Our results indicate that MC-3DQSAR could be used systematically to evaluate the drug-target potentials of any RNA sites using current high-resolution structural data. 相似文献
999.
Amira Sfaxi-Bousbih Abdelilah Chaoui Ezzedine El Ferjani 《Biological trace element research》2010,137(1):110-116
Seeds of bean (Phaseolus vulgaris L.) were germinated by soaking in distilled water or copper chloride solution. The relationships among copper excess treatment,
germination rate, dry weight, sugar contents, and carbohydrase activities in cotyledon were investigated. Heavy metal stress
provoked a diminution in germination rate and biomass mobilization, as compared with the control. A drastic disorder in soluble
sugars export, especially glucose and fructose liberation, was also imposed after exposure to excess copper. This restricted
the starch and sucrose breakdown in reserve tissue, as evidenced by the inhibition in the activities of α-amylase and invertase
isoenzymes (soluble acid, soluble neutral, cell wall-bound acid). 相似文献
1000.
Jakub Horák Christopher Grefen Kenneth W Berendzen Achim Hahn York-Dieter Stierhof Bettina Stadelhofer Mark Stahl Csaba Koncz Klaus Harter 《BMC plant biology》2008,8(1):77