全文获取类型
收费全文 | 1856篇 |
免费 | 150篇 |
专业分类
2006篇 |
出版年
2024年 | 1篇 |
2023年 | 13篇 |
2022年 | 21篇 |
2021年 | 41篇 |
2020年 | 28篇 |
2019年 | 33篇 |
2018年 | 50篇 |
2017年 | 34篇 |
2016年 | 61篇 |
2015年 | 91篇 |
2014年 | 100篇 |
2013年 | 134篇 |
2012年 | 154篇 |
2011年 | 139篇 |
2010年 | 99篇 |
2009年 | 84篇 |
2008年 | 117篇 |
2007年 | 105篇 |
2006年 | 100篇 |
2005年 | 103篇 |
2004年 | 72篇 |
2003年 | 87篇 |
2002年 | 79篇 |
2001年 | 22篇 |
2000年 | 19篇 |
1999年 | 26篇 |
1998年 | 25篇 |
1997年 | 14篇 |
1996年 | 17篇 |
1995年 | 15篇 |
1994年 | 16篇 |
1993年 | 8篇 |
1992年 | 12篇 |
1991年 | 7篇 |
1990年 | 10篇 |
1989年 | 10篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 6篇 |
1984年 | 13篇 |
1983年 | 1篇 |
1982年 | 11篇 |
1981年 | 3篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有2006条查询结果,搜索用时 15 毫秒
71.
Tarabbo M Lapa D Castilletti C Tommaselli P Guarducci R Lucà G Emanuele A Zaccaria O La Gioia VF Girardi E Capobianchi MR Ippolito G 《PloS one》2011,6(1):e15933
Background
Clinical surveillance may have underestimated the real extent of the spread of the new strain of influenza A/H1N1, which surfaced in April 2009 originating the first influenza pandemic of the 21st century. Here we report a serological investigation on an influenza A/H1N1pdm outbreak in an Italian military ship while cruising in the Mediterranean Sea (May 24-September 6, 2009).Methods
The contemporary presence of HAI and CF antibodies was used to retrospectively estimate the extent of influenza A/H1N1pdm spread across the crew members (median age: 29 years).Findings
During the cruise, 2 crew members fulfilled the surveillance case definition for influenza, but only one was laboratory confirmed by influenza A/H1N1pdm-specific RT-PCR; 52 reported acute respiratory illness (ARI) episodes, and 183 reported no ARI episodes. Overall, among the 211 crew member for whom a valid serological result was available, 39.3% tested seropositive for influenza A/H1N1pdm. The proportion of seropositives was significantly associated with more crowded living quarters and tended to be higher in those aged <40 and in those reporting ARI or suspected/confirmed influenza A/H1N1pdm compared to the asymptomatic individuals. No association was found with previous seasonal influenza vaccination.Conclusions
These findings underline the risk for rapid spread of novel strains of influenza A in confined environment, such as military ships, where crowding, rigorous working environment, physiologic stress occur. The high proportion of asymptomatic infections in this ship-borne outbreak supports the concept that serological surveillance in such semi-closed communities is essential to appreciate the real extent of influenza A/H1N1pdm spread and can constitute, since the early stage of a pandemic, an useful model to predict the public health impact of pandemic influenza and to establish proportionate and effective countermeasures. 相似文献72.
Context
Levothyroxine monotherapy is the treatment of choice for hypothyroid patients because peripheral T4 to T3 conversion is believed to account for the overall tissue requirement for thyroid hormones. However, there are indirect evidences that this may not be the case in all patients.Objective
To evaluate in a large series of athyreotic patients whether levothyroxine monotherapy can normalize serum thyroid hormones and thyroid-pituitary feedback.Design
Retrospective study.Setting
Academic hospital.Patients
1,811 athyreotic patients with normal TSH levels under levothyroxine monotherapy and 3,875 euthyroid controls.Measurements
TSH, FT4 and FT3 concentrations by immunoassays.Results
FT4 levels were significantly higher and FT3 levels were significantly lower (p<0.001 in both cases) in levothyroxine-treated athyreotic patients than in matched euthyroid controls. Among the levothyroxine-treated patients 15.2% had lower serum FT3 and 7.2% had higher serum FT4 compared to euthyroid controls. A wide range of FT3/FT4 ratios indicated a major heterogeneity in the peripheral T3 production capacity in different individuals. The correlation between thyroid hormones and serum TSH levels indicated an abnormal feedback mechanism in levothyroxine-treated patients.Conclusions
Athyreotic patients have a highly heterogeneous T3 production capacity from orally administered levothyroxine. More than 20% of these patients, despite normal TSH levels, do not maintain FT3 or FT4 values in the reference range, reflecting the inadequacy of peripheral deiodination to compensate for the absent T3 secretion. The long-term effects of chronic tissue exposure to abnormal T3/T4 ratio are unknown but a sensitive marker of target organ response to thyroid hormones (serum TSH) suggests that this condition causes an abnormal pituitary response. A more physiological treatment than levothyroxine monotherapy may be required in some hypothyroid patients. 相似文献73.
Diego Micheletti Michela Troggio Andrey Zharkikh Fabrizio Costa Mickael Malnoy Riccardo Velasco Silvio Salvi 《Tree Genetics & Genomes》2011,7(4):857-868
Knowledge about the sequence-based genetic diversity of a crop species is important in order to develop highly informative
genotyping assays, which will eventually positively impact breeding practice. Diversity data were obtained from two pools
of 185 and 75 accessions each, representing most of the species belonging to the genus Malus, by re-sequencing 27 gene-specific amplicons and by screening 237 Malus × domestica SNPs using the multiplex genotyping technology SNPlex™. Nucleotide diversity and insertion/deletion rates in M. × domestica were estimated as π = 0.0037 and 1/333 bp, respectively. The SNP frequency was estimated as 0.0194 (1 SNP/52 bp) while within a single apple
cultivar an average of one SNP in every 455 bp was found. We also investigated transferability (T
SNP) of the heterozygous state of SNPs across the species M. × domestica and the genus Malus. Raw re-sequencing showed that 12–15% of M. × domestica SNPs are transferable to a second M. × domestica cultivar, however T
SNP rose to ∼41% with SNPs selected for high minor allele frequency. T
SNP of chosen SNPs averaged ∼27% in the two M. × domestica-related species, Malus sieversii and Malus sylvestris, but was much lower in more distantly related species. On the basis of T
SNP, simulations, and empirical results, we calculated that a close-design, multiplexed genotyping array with at least 2,000
SNPs is required for building a highly saturated linkage maps within any M. × domestica cross. The same array would gradually lose informativeness in increasingly phylogenetically distant Malus species. 相似文献
74.
Riccardo Aversano Francesco Di Dato Antonio Di Matteo Luigi Frusciante Domenico Carputo 《Plant biotechnology reports》2011,5(3):265-271
The cultivated potato as well as its tuber-bearing relatives are considered model plants for cell and tissue culture, and
therefore for exploiting the genetic variation induced by in vitro culture. The association between molecular stability and
tissue culture in different genetic backgrounds and ploidy levels has already been explored. However, it still remains to
be ascertained whether somaclonal variation differs between callus-derived chromosome-doubled and undoubled regenerants. Our
research aimed at investigating, through amplified fragment length polymorphism (AFLP) markers, the genetic changes in marker-banding
patterns of diploid and tetraploid regenerants obtained from one clone each of Solanum bulbocastanum Dunal and S. cardiophyllum Lindl (both 2n = 2x = 24) and tetraploids from cultivated S. tuberosum L. (2n = 4x = 48). Pairwise comparisons between the banding patterns of regenerants and parents allowed detecting considerable changes
associated to in vitro culture both at diploid and tetraploid level. The percentages of polymorphic bands between diploid
and tetraploid regenerants were, respectively, 57 and 69% in S. bulbocastanum and 58 and 63% in S. cardiophyllum. On average, the frequencies of lost parental fragments in regenerants were significantly higher than novel bands both in
S. bulbocastanum (48 vs. 22%) and S. tuberosum (36 vs. 18%) regenerants. By contrast, in S. cardiophyllum, a similar incidence of the two events was detected (32 vs. 29%). Our results revealed that structural changes after tissue
culture process strongly affected the genome of the species studied, but diploid and tetraploids regenerated plants responded
equally. 相似文献
75.
Gary B. Quistad Quyen Nguyen Paul Bernasconi Douglas J. Leisy 《Insect biochemistry and molecular biology》1994,24(10):955-961
The potency of venom from Bracon hebetor against lepidopterous larvae has been known for over 40 years, but previous attempts to purify and characterize individual protein toxins have been largely unsuccessful. Three protein toxins were purified from venom of this small parasitic wasp and the amino acid sequences of 22–31 consecutive residues at the amino-terminus were determined. These relatively large toxins (apparent molecular mass 73 kDa) were labile under many isolation techniques, but anion-exchange chromatography allowed purification with retention of biological activity. Two purified toxins were quite insecticidal (LD50 < 0.3μg/g) when injected into six species of lepidopterous larvae. On a molar basis, one toxin (Brh-I) has the highest known biocidal activity against Heliothis virescens (LD50 = 2 pmol/g). 相似文献
76.
77.
Flavio De Maio Giuseppe Maulucci Mariachiara Minerva Saber Anoosheh Ivana Palucci Raffaella Iantomasi Valentina Palmieri Serena Camassa Michela Sali Maurizio Sanguinetti Wilbert Bitter Riccardo Manganelli Marco De Spirito Giovanni Delogu 《PloS one》2014,9(11)
PE_PGRS proteins are unique to the Mycobacterium tuberculosis complex and a number of other pathogenic mycobacteria. PE_PGRS30, which is required for the full virulence of M. tuberculosis (Mtb), has three main domains, i.e. an N-terminal PE domain, repetitive PGRS domain and the unique C-terminal domain. To investigate the role of these domains, we expressed a GFP-tagged PE_PGRS30 protein and a series of its functional deletion mutants in different mycobacterial species (Mtb, Mycobacterium bovis BCG and Mycobacterium smegmatis) and analysed protein localization by confocal microscopy. We show that PE_PGRS30 localizes at the mycobacterial cell poles in Mtb and M. bovis BCG but not in M. smegmatis and that the PGRS domain of the protein strongly contributes to protein cellular localization in Mtb. Immunofluorescence studies further showed that the unique C-terminal domain of PE_PGRS30 is not available on the surface, except when the PGRS domain is missing. Immunoblot demonstrated that the PGRS domain is required to maintain the protein strongly associated with the non-soluble cellular fraction. These results suggest that the repetitive GGA-GGN repeats of the PGRS domain contain specific sequences that contribute to protein cellular localization and that polar localization might be a key step in the PE_PGRS30-dependent virulence mechanism. 相似文献
78.
Tommaso Chiti Elisa Grieco Lucia Perugini Ana Rey Riccardo Valentini 《Plant and Soil》2014,375(1-2):47-59
Background and aims
In the Jomoro district in Ghana, tree plantations were the first cause of deforestation in the past, drastically reducing the area occupied by primary forests. The aim of this study was to quantify soil organic carbon (SOC) losses due to a change in land use from primary forest to tree plantations (cocoa, coconut, rubber, oil palm) on the different substrates of the district. Secondary forests and mixed plantations were also included in the study.Methods
Soils were sampled at different depths up to 100 cm along a series of chronosequences in each of the three substrates (Granite, Lower Birrimian and Tertiary Sands) present in the area.Results
The highest SOC losses in the 0–30 cm layer were caused by the conversion of primary forests to tree plantations: cocoa ?61 % of the original SOC stock, coconut ?55 %, rubber ?35 % and oil palm 28 %, while mixed plantations and secondary forests showed a loss of 23 % and 21 % of the original SOC stock, respectively. C losses were less apparent from the entire profile (to a depth of 100 cm).Conclusions
All conversions to tree plantations caused substantial SOC losses, comparable to the conversion of forests to agricultural systems. Secondary forests and mixed plantations were the only sustainable land uses that restricted SOC losses considerably. 相似文献79.
The Contribution of Agriculture,Forestry and other Land Use activities to Global Warming, 1990–2012 下载免费PDF全文
Francesco N. Tubiello Mirella Salvatore Alessandro F. Ferrara Jo House Sandro Federici Simone Rossi Riccardo Biancalani Rocio D. Condor Golec Heather Jacobs Alessandro Flammini Paolo Prosperi Paola Cardenas‐Galindo Josef Schmidhuber Maria J. Sanz Sanchez Nalin Srivastava Pete Smith 《Global Change Biology》2015,21(7):2655-2660
We refine the information available through the IPCC AR5 with regard to recent trends in global GHG emissions from agriculture, forestry and other land uses (AFOLU), including global emission updates to 2012. Using all three available AFOLU datasets employed for analysis in the IPCC AR5, rather than just one as done in the IPCC AR5 WGIII Summary for Policy Makers, our analyses point to a down‐revision of global AFOLU shares of total anthropogenic emissions, while providing important additional information on subsectoral trends. Our findings confirm that the share of AFOLU emissions to the anthropogenic total declined over time. They indicate a decadal average of 28.7 ± 1.5% in the 1990s and 23.6 ± 2.1% in the 2000s and an annual value of 21.2 ± 1.5% in 2010. The IPCC AR5 had indicated a 24% share in 2010. In contrast to previous decades, when emissions from land use (land use, land use change and forestry, including deforestation) were significantly larger than those from agriculture (crop and livestock production), in 2010 agriculture was the larger component, contributing 11.2 ± 0.4% of total GHG emissions, compared to 10.0 ± 1.2% of the land use sector. Deforestation was responsible for only 8% of total anthropogenic emissions in 2010, compared to 12% in the 1990s. Since 2010, the last year assessed by the IPCC AR5, new FAO estimates indicate that land use emissions have remained stable, at about 4.8 Gt CO2 eq yr?1 in 2012. Emissions minus removals have also remained stable, at 3.2 Gt CO2 eq yr?1 in 2012. By contrast, agriculture emissions have continued to grow, at roughly 1% annually, and remained larger than the land use sector, reaching 5.4 Gt CO2 eq yr?1 in 2012. These results are useful to further inform the current climate policy debate on land use, suggesting that more efforts and resources should be directed to further explore options for mitigation in agriculture, much in line with the large efforts devoted to REDD+ in the past decade. 相似文献
80.
Goremykin VV Lockhart PJ Viola R Velasco R 《The Plant journal : for cell and molecular biology》2012,71(4):615-626
Mitochondrial genomes of spermatophytes are the largest of all organellar genomes. Their large size has been attributed to various factors; however, the relative contribution of these factors to mitochondrial DNA (mtDNA) expansion remains undetermined. We estimated their relative contribution in Malus domestica (apple). The mitochondrial genome of apple has a size of 396 947 bp and a one to nine ratio of coding to non-coding DNA, close to the corresponding average values for angiosperms. We determined that 71.5% of the apple mtDNA sequence was highly similar to sequences of its nuclear DNA. Using nuclear gene exons, nuclear transposable elements and chloroplast DNA as markers of promiscuous DNA content in mtDNA, we estimated that approximately 20% of the apple mtDNA consisted of DNA sequences imported from other cell compartments, mostly from the nucleus. Similar marker-based estimates of promiscuous DNA content in the mitochondrial genomes of other species ranged between 21.2 and 25.3% of the total mtDNA length for grape, between 23.1 and 38.6% for rice, and between 47.1 and 78.4% for maize. All these estimates are conservative, because they underestimate the import of non-functional DNA. We propose that the import of promiscuous DNA is a core mechanism for mtDNA size expansion in seed plants. In apple, maize and grape this mechanism contributed far more to genome expansion than did homologous recombination. In rice the estimated contribution of both mechanisms was found to be similar. 相似文献