全文获取类型
收费全文 | 8072篇 |
免费 | 501篇 |
出版年
2023年 | 63篇 |
2022年 | 127篇 |
2021年 | 285篇 |
2020年 | 237篇 |
2019年 | 249篇 |
2018年 | 282篇 |
2017年 | 268篇 |
2016年 | 385篇 |
2015年 | 495篇 |
2014年 | 531篇 |
2013年 | 609篇 |
2012年 | 666篇 |
2011年 | 556篇 |
2010年 | 367篇 |
2009年 | 337篇 |
2008年 | 464篇 |
2007年 | 377篇 |
2006年 | 350篇 |
2005年 | 291篇 |
2004年 | 299篇 |
2003年 | 216篇 |
2002年 | 199篇 |
2001年 | 109篇 |
2000年 | 103篇 |
1999年 | 87篇 |
1998年 | 60篇 |
1997年 | 36篇 |
1996年 | 36篇 |
1995年 | 44篇 |
1994年 | 18篇 |
1993年 | 16篇 |
1992年 | 39篇 |
1991年 | 21篇 |
1990年 | 20篇 |
1989年 | 22篇 |
1988年 | 32篇 |
1987年 | 25篇 |
1986年 | 20篇 |
1985年 | 22篇 |
1984年 | 24篇 |
1983年 | 17篇 |
1982年 | 22篇 |
1981年 | 10篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1976年 | 9篇 |
1975年 | 10篇 |
1974年 | 10篇 |
1972年 | 10篇 |
1971年 | 10篇 |
排序方式: 共有8573条查询结果,搜索用时 15 毫秒
171.
Jorge Assis Nelson Castilho Coelho Filipe Alberto Myriam Valero Pete Raimondi Dan Reed Ester Alvares Serr?o 《PloS one》2013,8(7)
The genetic consequences of living on the edge of distributional ranges have been
the subject of a largely unresolved debate. Populations occurring along
persistent low latitude ranges (rear-edge) are expected to retain high and
unique genetic diversity. In contrast, currently less favourable environmental
conditions limiting population size at such range-edges may have caused genetic
erosion that prevails over past historical effects, with potential consequences
on reducing future adaptive capacity. The present study provides an empirical
test of whether population declines towards a peripheral range might be
reflected on decreasing diversity and increasing population isolation and
differentiation. We compare population genetic differentiation and diversity
with trends in abundance along a latitudinal gradient towards the peripheral
distribution range of
Saccorhiza
polyschides
, a large brown
seaweed that is the main structural species of kelp forests in SW Europe.
Signatures of recent bottleneck events were also evaluated to determine whether
the recently recorded distributional shifts had a negative influence on
effective population size. Our findings show decreasing population density and
increasing spatial fragmentation and local extinctions towards the southern
edge. Genetic data revealed two well supported groups with a central contact
zone. As predicted, higher differentiation and signs of bottlenecks were found
at the southern edge region. However, a decrease in genetic diversity associated
with this pattern was not verified. Surprisingly, genetic diversity increased
towards the edge despite bottlenecks and much lower densities, suggesting that
extinctions and recolonizations have not strongly reduced diversity or that
diversity might have been even higher there in the past, a process of shifting
genetic baselines. 相似文献
172.
Olga Martinho Filipe Pinto Sara Granja Vera Miranda-Gon?alves Marise A. R. Moreira Luis F. J. Ribeiro Celso di Loreto Marsha R. Rosner Adhemar Longatto-Filho Rui Manuel Reis 《PloS one》2013,8(3)
Cervical cancer is one of the most common cancers in women worldwide, being high-risk group the HPV infected, the leading etiological factor. The raf kinase inhibitory protein (RKIP) has been associated with tumor progression and metastasis in several human neoplasms, however its role on cervical cancer is unclear. In the present study, 259 uterine cervix tissues, including cervicitis, cervical intraepithelial lesions and carcinomas, were analyzed for RKIP expression by immunohistochemistry. We found that RKIP expression was significantly decreased during malignant progression, being highly expressed in non-neoplastic tissues (54% of the samples; 73/135), and expressed at low levels in the cervix invasive carcinomas (∼15% (19/124). Following in vitro downregulation of RKIP, we observed a viability and proliferative advantage of RKIP-inhibited cells over time, which was associated with an altered cell cycle distribution and higher colony number in a colony formation assay. An in vitro wound healing assay showed that RKIP abrogation is associated with increased migratory capability. RKIP downregulation was also associated with an increased vascularization of the tumors in vivo using a CAM assay. Furthermore, RKIP inhibition induced cervical cancer cells apoptotic resistance to cisplatin treatment. In conclusion, we described that RKIP protein is significantly depleted during the malignant progression of cervical tumors. Despite the lack of association with patient clinical outcome, we demonstrate, in vitro and in vivo, that loss of RKIP expression can be one of the factors that are behind the aggressiveness, malignant progression and chemotherapy resistance of cervical cancer. 相似文献
173.
Background
A previous study using an intercross between the inbred rat strains Lewis (LEW) and Spontaneously Hypertensive Rats (SHR) identified a locus on chromosome 4, named Anxrr16, influencing an experimental index of anxiety and showing a transgressive effect, with alleles from the LEW strain (more anxious) decreasing rather than increasing anxiety.Objective
To confirm the location and isolate the effect of a rat genome region named Anxrr16 through a planned genomic recombination strategy, where the target locus in SHR rats was replaced with LEW genetic material.Methods
A new congenic strain, named SHR.LEW-Anxrr16 (SLA16), was developed from a cross between LEW (donor) and SHR (receptor) rats and then evaluated in several anxiety-related tests. The activity and attention levels of the new strain were also evaluated, since hyperactivity was observed during its construction and because SHR is a model of attention deficit hyperactivity disorder.Results
Significant effects of Anxrr16 were found for open field central locomotion, as well as for other indices of anxiety from the light/dark box, triple test and T-maze. In all cases, the low-anxiety levels of SHR rats were further reduced by the insertion of LEW alleles. Differences in locomotor activity were found only in unfamiliar (hence stressful) environments and no genetic effects were observed in indices of attention.Conclusion
The SLA16 strain can help in the identification of the molecular pathways involved in experimental anxiety and it demonstrates how apparently extreme phenotypes sometimes hide major opposite-acting genes. 相似文献174.
Iracema Andrade Nascimento Sheyla Santa Izabel Marques Iago Teles Dominguez Cabanelas Solange Andrade Pereira Janice Isabel Druzian Carolina Oliveira de Souza Daniele Vital Vich Gilson Correia de Carvalho Maurício Andrade Nascimento 《Bioenergy Research》2013,6(1):1-13
The viability of algae-based biodiesel industry depends on the selection of adequate strains in regard to profitable yields and oil quality. This work aimed to bioprospecting and screening 12 microalgae strains by applying, as selective criteria, the volumetric lipid productivity and the fatty acid profiles, used for estimating the biodiesel fuel properties. Volumetric lipid productivity varied among strains from 22.61 to 204.91 mg l?1 day?1. The highest lipid yields were observed for Chlorella (204.91 mg l?1 day1) and Botryococcus strains (112.43 and 98.00 mg l?1 day?1 for Botryococcus braunii and Botryococcus terribilis, respectively). Cluster and principal components analysis analysis applied to fatty acid methyl esters (FAME) profiles discriminated three different microalgae groups according to their potential for biodiesel production. Kirchneriella lunaris, Ankistrodesmus fusiformis, Chlamydocapsa bacillus, and Ankistrodesmus falcatus showed the highest levels of polyunsaturated FAME, which incurs in the production of biodiesels with the lowest (42.47–50.52) cetane number (CN), the highest (101.33–136.97) iodine values (IV), and the lowest oxidation stability. The higher levels of saturated FAME in the oils of Chlamydomonas sp. and Scenedesmus obliquus indicated them as source of biodiesel with higher oxidation stability, higher CN (63.63–64.94), and lower IV (27.34–35.28). The third group, except for the Trebouxyophyceae strains that appeared in isolation, are composed by microalgae that generate biodiesel of intermediate values for CN, IV, and oxidation stability, related to their levels of saturated and monosaturated lipids. Thus, in this research, FAME profiling suggested that the best approach for generating a microalgae-biodiesel of top quality is by mixing the oils of distinct cell cultures. 相似文献
175.
Suzan Kelly V. Bertolucci Ana Bárbara D. Pereira José Eduardo B. P. Pinto Alaíde B. Oliveira Fernão C. Braga 《化学与生物多样性》2013,10(2):288-295
Coumarin ( 1 ) and kaurane‐type diterpenes are considered the bioactive constituents of Mikania glomerata and M. laevigata, used in Brazil to treat respiratory affective disorders. The seasonal variation of 1 , ortho‐coumaric acid ( 2 ), benzoylgrandifloric acid ( 3 ), cinnamoylgrandifloric acid ( 4 ), and kaurenoic acid ( 5 ) in leaves of both species, cultivated in full sunlight and under shade levels of 40 and 80%, was quantified by HPLC. Compound 2 was detected solely in M. laevigata in concentrations below the limit of quantification. Coumarin was not found in M. glomerata, whereas its concentration reached 0.94±0.24% (w/w) in M. laevigata farmed in summer under 80% shading. Both Mikania species produced higher amounts of kaurane diterpenes when cultivated in plenty of sunlight. Hence, maximum contents of 1 are reached in M. laevigata cultivated under high shading, but with reduced concentrations of 3 – 5 . Conversely, M. glomerata should be cultivated under full sunlight and harvested in winter for highest concentrations of kaurane‐type diterpenes. 相似文献
176.
Central giant cell lesion of the jaws: study of CCND1 gene amplification and p16INK4a protein levels
Renato Luiz Maia Nogueira Mário Henrique Girão Faria Rafael Lima Verde Osterne Roberta Barroso Cavalcante Ronaldo Albuquerque Ribeiro Cassiano Francisco Weege Nonaka Silvia Helena Barem Rabenhorst 《Journal of molecular histology》2013,44(5):527-534
Central giant cell lesions (CGCLs) are uncommon benign jaw lesions with uncertain etiology and a variable clinical behavior. In neoplasms, alterations in molecules involved in the G1/S checkpoint are frequently found. Loss of p16INK4a expression or overexpression of cyclin D1 may stimulate cell proliferation. The purpose of this study was to analyze CCND1 gene amplification and the expression of p16INK4a in CGCLs. Structural analysis of the CCND1 was performed using chromogenic in situ hybridization. Immmunohistochemistry was used to identify p16INK4a protein levels. Statistical analysis correlated the two biomarkers with clinical behavior and between each other. Twenty-four lesions were included, being 11 aggressive and 13 non-aggressive. Moderate/high-level CCND1 amplification was found in 12 lesions. Also, immunoreactivity for p16INK4a was present in 12 cases, mainly in mononuclear cells. There was a significantly higher level of p16INK4a expression in mononuclear cells of non-aggressive lesions and lesions with moderate/high-level CCND1 amplification in mononuclear cells. It could be speculated that some CGCLs may develop as a true benign neoplasm. The higher expression of p16INK4a in non-aggressive lesions and in cases with moderate/high-level CCND1 amplification may show that these molecules have a role in CGCLs. 相似文献
177.
178.
Germana Bueno Dias Valdirene Moreira Gomes Umberto Zottich Pereira Suzanna F. Ferreira Ribeiro André O. Carvalho Rosana Rodrigues Olga L. Tavares Machado Kátia Valevski Sales Fernandes André Teixeira S. Ferreira Jonas Perales Maura Da Cunha 《The protein journal》2013,32(1):15-26
Capsicum species belong to the Solanaceae family and have great social, economic and agronomical significance. The present research presents data on the isolation and characterization of Capsicum chinense Jacq. peptides which were scrutinized in relation to their toxicity towards a diverse set of yeast species. The protein extract was separated with C18 reverse-phase chromatography in high performance liquid chromatography, resulting in three different peptide enriched fractions (PEFs) termed PEF1, PEF2 and PEF3. Tricine-SDS-PAGE of the PEF2 revealed peptides with molecular masses of approximately 5.0 and 8.5 kDa. These PEFs also exhibited strong antifungal activity against different yeasts. In the presence of the PEF2, Candida tropicalis exhibited morphological changes, including cellular agglomeration and formation of pseudohyphae. Determined N-terminal sequences of PEF2 and PEF3 were proven to be highly homologous to serine proteinase inhibitors, when analysed by comparative database sequence tools. For this reason were performed protease inhibitory activity assay. The PEFs displayed high inhibitory activity against trypsin and low inhibitory activity against chymotrypsin. PEF2 and PEF3 were considerably unsusceptible to a broad interval of pH and temperatures. Due to the myriad of application of Proteinase inhibitors (PIs) in fields ranging from plant protection against pathogens and pests to medicine such as in cancer and virus replication inhibition, the discovery of new PIs with new properties are of great interest. 相似文献
179.
Naomi Kato Simas Elisangela da Costa Lima Dellamora Jan Schripsema Celso Luiz Salgueiro Lage Alfredo Martins de Oliveira Filho Ludger Wessjohann Andrea Porzel Ricardo Machado Kuster 《Phytochemistry letters》2013,6(1):67-72
Ethanol extract obtained from dried leaves of Acmella oleracea afforded after a liquid/liquid partition procedure a larvicidal hexane fraction (LC50 = 145.6 ppm) and a non larvicidal dichloromethane one. From the inactive fraction, three amides were identified, two new structures, named deca-6,9-dihydroxy-(2E,7E)-dienoic acid isobutylamide (1), deca-8,9-dihydroxy-(2E,6Z)-dienoic acid isobutylamide (2) and the known nona-2,3-dihydroxy-6,8-diynoic acid 2-phenylethylamide (3). Bioassay-guided chromatographic fractionation of the hexane partition led to the identification of an amide mixture, nona-(2Z)-en-6,8-diynoic acid 2-phenylethylamide (4) and deca-(2Z)-en-6,8-diynoic acid 2-phenylethlylamide (5). This mixture was active against Aedes aegypti larvae at LC50 = 7.6 ppm. Low toxicity of crude extracts and derived fractions on Artemia salina nauplies showed the possibility of using them to control the A. aegypti mosquito larvae. This is the first report on larvicidal activity of acetylenic 2-phenylethylamides and their identification in A. oleracea leaves. 相似文献
180.