全文获取类型
收费全文 | 164845篇 |
免费 | 5787篇 |
国内免费 | 5535篇 |
专业分类
176167篇 |
出版年
2024年 | 154篇 |
2023年 | 1035篇 |
2022年 | 2392篇 |
2021年 | 4076篇 |
2020年 | 2619篇 |
2019年 | 3232篇 |
2018年 | 14131篇 |
2017年 | 12237篇 |
2016年 | 10217篇 |
2015年 | 5359篇 |
2014年 | 5820篇 |
2013年 | 6267篇 |
2012年 | 10966篇 |
2011年 | 18328篇 |
2010年 | 15171篇 |
2009年 | 11203篇 |
2008年 | 13229篇 |
2007年 | 14370篇 |
2006年 | 3150篇 |
2005年 | 2866篇 |
2004年 | 2896篇 |
2003年 | 2646篇 |
2002年 | 2161篇 |
2001年 | 1483篇 |
2000年 | 1375篇 |
1999年 | 1145篇 |
1998年 | 668篇 |
1997年 | 677篇 |
1996年 | 675篇 |
1995年 | 616篇 |
1994年 | 549篇 |
1993年 | 405篇 |
1992年 | 588篇 |
1991年 | 472篇 |
1990年 | 413篇 |
1989年 | 289篇 |
1988年 | 261篇 |
1987年 | 247篇 |
1986年 | 167篇 |
1985年 | 194篇 |
1984年 | 116篇 |
1983年 | 134篇 |
1982年 | 73篇 |
1981年 | 58篇 |
1980年 | 38篇 |
1979年 | 61篇 |
1974年 | 38篇 |
1973年 | 34篇 |
1972年 | 276篇 |
1971年 | 297篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
摘要:【目的】本研究通过百日咳杆菌黏附素(PRN)基因的分段克隆表达及其在BALB/c小鼠的主动和被动免疫保护试验筛选PRN中的保护性抗原肽。【方法和结果】利用大肠杆菌进行PRN的完整蛋白、N端和C端多肽及其RI和RII区域多肽(双拷贝)的表达,命名为GST-PRN、GST-PN、GST-PC、GST-2PRI和GST-2PRII。Western blot检测证实5种表达产物均具有良好的反应原性。在主动免疫保护试验中,5种表达产物均能诱导小鼠产生较高的PRN抗体水平;当使用3 LD50的支气管败血波氏杆菌 相似文献
912.
Khaled M. Abdel Rahman Marek Barta Ľudovít Cagáň 《Central European Journal of Biology》2010,5(4):472-480
We tested the combined effect of the fungus Beauveria bassiana and the microsporidium Nosema pyrausta on the European corn borer larvae, Ostrinia nubilalis, in the laboratory. The first instar of O. nubilalis larvae was the most sensitive to the B. bassiana infection followed by the fifth, second, third, and fourth instar (LC50s were 4.91, 6.67, 7.13, 9.15, and 6.51 × 105 conidia/ml for the first to fifth instars, respectively). Mortality of each instar increases positively with concentration
of conidia. When B. bassiana and N. pyrausta were used in combination, mortality increased significantly in all instars. Relative to the B. bassiana treatment alone, the B. bassiana + N. pyrausta treatment decreased the LC50s by 42.16%, 37.63%, 21.60%, 27.11%, and 33.95% for the first to fifth instars, respectively. The combined effects of the
two pathogens were mostly additive. However, at the two highest concentrations the pathogens interacted synergistically in
the first and second instar. Individuals that survived the B. bassiana and B. bassiana + N. pyrausta treatments and developed into adults had significantly shorter lifespans and females oviposited fewer eggs than non-exposed
insects. The effects on the longevity and the egg production were most pronounced at high concentration of B. bassiana conidia. 相似文献
913.
The G-protein coupled receptor CCR5 functions pathologically as the primary co-receptor for macrophage tropic (R5) strains of HIV-1. The interactions responsible for co-receptor activity are unknown. Molecular-dynamics simulations of the extracellular and adjacent transmembrane domains of CCR5 were performed with explicit solvation utilizing a rhodopsin-based homology model. The functional unit of co-receptor binding was constructed via docking and molecular-dynamics simulation of CCR5 and the variable 3 loop of gp120, which is a dominant determinant of co-receptor utilization. The variable 3 loop was demonstrated to interact primarily with the amino terminus and the second extracellular loop of CCR5, providing novel structural information regarding the co-receptor-binding site. Alanine mutants that alter chemokine binding and co-receptor activity were examined. Molecular-dynamics simulations with and without the variable 3 loop of gp120 were able to rationalize the activities of these mutants successfully, providing support for the proposed model. Based on these results, the global complex of CCR5, gp120 including the V3 loop and CD4, was investigated. The utilization of computational analysis, in combination with molecular biological data, provides a powerful approach for understanding the use of CCR5 as a co-receptor by HIV-1. 相似文献
914.
Rodrigo J. Mercader Nathan W. Siegert Andrew M. Liebhold Deborah G. McCullough 《Population Ecology》2011,53(2):271-285
Management programs for invasive species are often developed at a regional or national level, but physical intervention generally
takes place over relatively small areas occupied by newly founded, isolated populations. The ability to predict how local
habitat variation affects the expansion of such newly founded populations is essential for efficiently targeting resources
to slow the spread of an invasive species. We assembled a coupled map lattice model that simulates the local spread of newly
founded colonies of the emerald ash borer (Agrilus planipennis Fairmaire), a devastating forest insect pest of ash (Fraxinus spp.) trees. Using this model, we investigated the spread of A. planipennis in environments with different Fraxinus spp. distributions, and explored the consequences of ovipositional foraging behavior on the local spread of A. planipennis. Simulations indicate that increased larval density, resulting from lower host tree density or higher initial population
sizes, can increase the spread rate during the first few years after colonization by increasing a density-dependent developmental
rate and via host resource depletion. Both the radial spread rate and population size were greatly influenced by ovipositional
foraging behavior. Two known behaviors of ovipositing A. planipennis females, attraction towards areas with high ash tree density and attraction to stressed trees, had opposing effects on spread.
Results from this model illustrate the significant influence of resource distribution and foraging behavior on localized spread,
and the importance of these factors when formulating strategies to monitor and manage invasive pests. 相似文献
915.
916.
917.
Natácia Evangelista de Lima Alexandre Assis Carvalho Alan William Meerow Maura Helena Manfrin 《Organisms Diversity & Evolution》2018,18(2):151-161
The genus Acrocomia, popularly known as macaw palm or macaúba, occurs in savanna areas and open forests of tropical America, with distribution from Central to southern South America. They are important oleaginous palm trees, due to their role in ecosystems and local economies and their potential for biofuel production and vegetable oil. Although the taxonomy of the genus is not resolved because of observed phenotypic diversity in A. aculeata (Jacq.) Lodd. ex Mart., there are several conflicting treatments. Some authors recognize three caulescent spp. occurring in South America: A. aculeata, A. intumescens Drude, and A. totai Mart, although a new one was described recently—Acrocomia corumbaensis. Because some Latin American governments want to expand production of macaw palm in their territory as raw material for agro-energy, several groups have been encouraged to study this genus, focusing on the production of biodiesel, seed germination, phenotypic aspects, and genetic diversity. The goal of this review is to compile key information available in the literature and herbarium data, focusing on South American populations of the genus. 相似文献
918.
Genetic variation and population structure in Portunus trituberculatus along the coast of China were revealed according to 617 bp of mitochondrial DNA control region. 90 polymorphic sites defined
53 distinct haplotypes, showing a moderately high diversity among 72 individuals sampled from eight localities. Neighbor-joining
tree, statistics analyses of gene flow and genetic differentiation index indicated two populations from Beihai and Laizhou
had differentiated. The population from Yingkou, Dandong, Laizhou and Beihai had smaller genetic diversity compared to that
from Ningbo, Lianyungang, Qingdao and Japan according to the genetic distance. And mantel test showed significant positive
correlation between genetic distance and geographic distance for P.
trituberculatus. TCS parsimony network suggested that all the animals sampled were probably the result of recent divergence from a common
ancestral haplotype but for Laizhou population. Moreover, the haplotype distribution appeared to correlate with a recent colonization
followed by localized genetic differentiation. Mismatch distribution results suggested that Ningbo, Yingkou, Qingdao, Lianyungang
and Japan populations, particularly Dandong population had experienced a sudden demographic or spatial expansion. The Pleistocene
glaciations might contribute to this process. 相似文献
919.
Santos ME das C L E Silva F Gomes KB Fernandes AP Freitas FR Faria MC Mota AP Carvalho MG 《Molecular biology reports》2011,38(5):3361-3366
Peripheral arterial disease (PAD) is an atherosclerotic disturbance characterized by a progressive obstruction of lower limb
arteries. Many risk factors associated with PAD development have being reported in the literature. The present study aimed
to investigate whether mutations in the methylenetetrahydrofolate reductase (MTHFR) or in the cystathionine beta synthase
(CBS) genes are associated with higher levels of homocysteine and the risk of PAD in patients from Brazil. This study analyzed
39 patients with PAD and 32 without PAD in whom risk factors and C677T mutations in the MTHFR gene and both 844ins68 and T833C
mutations in the CBS gene were investigated. Although higher levels of homocysteine could be observed in patients with PAD
compared to controls, no association between the increase of homocysteine and the frequency of C677T, 844ins68, and T833C
mutations could be observed. The results suggest that these mutations do not appear to be related to either homocysteine levels
or the development of the disease. However, hyperhomocysteinemia and smoking are important factors in PAD development. 相似文献
920.
The aim of this study was to investigate whether a moderate‐intensity static magnetic field (SMF) can enhance the killing effect of adriamycin (ADM) on K562 cells, and to explore the effects of SMF combined with ADM on K562 cells. We analyzed the metabolic activity of cells, cell cycle distribution, DNA damage, change in cell ultrastructure, and P‐glycoprotein (P‐gp) expression after K562 cells were exposed continuously to a uniform 8.8 mT SMF for 12 h, with or without ADM. Our results showed that the SMF combined with ADM (25 ng/ml) significantly inhibited the metabolic activity of K562 cells (P < 0.05), while neither ADM nor the SMF alone affected the metabolic activity of these cells. Cell ultrastructure was altered in the SMF + ADM group. For example, cell membrane was depressed, some protuberances were observable, and vacuoles in the cytoplasm became larger. Cells were arrested at the G2/M phase and DNA damage increased after cells were treated with the SMF plus ADM. ADM also induced the P‐gp expression. In contrast, in the SMF group and SMF + ADM group, the P‐gp expression was decreased compared with the ADM group. Taken together, our results showed that the 8.8 mT SMF enhanced the cytotoxity potency of ADM on K562 cells, and the decrease in P‐gp expression may be one reason underlying this effect. Bioelectromagnetics 32:191–199, 2011. © 2010 Wiley‐Liss, Inc. 相似文献