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排序方式: 共有6326条查询结果,搜索用时 15 毫秒
931.
Thomas Wernberg Mads S. Thomsen Sean D. Connell Bayden D. Russell Jonathan M. Waters Giuseppe C. Zuccarello Gerald T. Kraft Craig Sanderson John A. West Carlos F. D. Gurgel 《PloS one》2013,8(11)
Explaining spatial patterns of biological organisation remains a central challenge for biogeographic studies. In marine systems, large-scale ocean currents can modify broad-scale biological patterns by simultaneously connecting environmental (e.g. temperature, salinity and nutrients) and biological (e.g. amounts and types of dispersed propagules) properties of adjacent and distant regions. For example, steep environmental gradients and highly variable, disrupted flow should lead to heterogeneity in regional communities and high species turnover. In this study, we investigated the possible imprint of the Leeuwin (LC) and East Australia (EAC) Currents on seaweed communities across ~7,000 km of coastline in temperate Australia. These currents flow poleward along the west and east coasts of Australia, respectively, but have markedly different characteristics. We tested the hypothesis that, regional seaweed communities show serial change in the direction of current flow and that, because the LC is characterised by a weaker temperature gradient and more un-interrupted along-shore flow compared to the EAC, then coasts influenced by the LC have less variable seaweed communities and lower species turnover across regions than the EAC. This hypothesis was supported. We suggest that this pattern is likely caused by a combination of seaweed temperature tolerances and current-driven dispersal. In conclusion, our findings support the idea that the characteristics of continental-scale currents can influence regional community organisation, and that the coupling of ocean currents and marine biological structure is a general feature that transcends taxa and spatial scales. 相似文献
932.
Gerald Garry 《BMJ (Clinical research ed.)》1905,2(2333):683-685
933.
934.
Individual Variations in Serum Melatonin Levels through Time: Implications for Epidemiologic Studies
Leticia M. Nogueira Joshua N. Sampson Lisa W. Chu Kai Yu Gerald Andriole Timothy Church Frank Z. Stanczyk Jill Koshiol Ann W. Hsing 《PloS one》2013,8(12)
Melatonin, a marker for the circadian rhythm with serum levels peaking between 2AM and 5AM, is hypothesized to possess anti-cancer properties, making it a mechanistic candidate for the probable carcinogenic effect of circadian rhythm disruption. In order to weigh epidemiologic evidence on the association of melatonin with cancer, we must first understand the laboratory and biological sources of variability in melatonin levels measured in samples. Participants for this methodological study were men enrolled in the Prostate Lung Colorectal and Ovarian Cancer Screening Trial (PLCO). We measured serum melatonin levels over a five year period in 97 individuals to test if melatonin levels are steady over time. The Pearson correlation coefficient between two measures separated by 1 year was 0.87, while the correlation between two measures separated by 5 years was to 0.70. In an additional cross-sectional study of 292 individuals, we used Analysis of Variance to identify differences in melatonin levels between different lifestyle and environmental characteristics. Serum melatonin levels were slightly higher in samples collected from 130 individuals during the winter, (6.36±0.59 pg/ml) than in samples collected from 119 individuals during the summer (4.83±0.62 pg/ml). Serum melatonin levels were lowest in current smokers (3.02±1.25 pg/ml, p = 0.007) compared to never (6.66±0.66 pg/ml) and former (5.59±0.50 pg/ml) smokers whereas BMI did not significantly affect serum melatonin levels in this study. In conclusion, the high 5 year correlation of melatonin levels implies that single measurements may be used to detect population level associations between melatonin and risk of cancer. Furthermore, our results reiterate the need to record season of sample collection, and individual characteristics in order to maximize study power and prevent confounding. 相似文献
935.
David C. Nickle Gerald H. Learn Matthew W. Rain James I. Mullins John E. Mittler 《Journal of molecular evolution》2002,54(1):134-137
Studies of ancient DNA have attracted considerable attention in scientific journals and the popular press. Several of the
more extreme claims for ancient DNA have been questioned on biochemical grounds (i.e., DNA surviving longer than expected)
and evolutionary grounds (i.e., nucleotide substitution patterns not matching theoretical expectations for ancient DNA). A
recent letter to Nature from Vreeland et al. (2000), however, tops all others with respect to age and condition of the specimen. These researchers
extracted and cultured a bacterium from an inclusion body from what they claim is a 250 million-year (Myr)-old salt crystal.
If substantiated, this observation could fundamentally alter views about bacterial physiology, ecology and evolution. Here
we report on molecular evolutionary analyses of the 16S rDNA from this specimen. We find that 2-9-3 differs from a modern
halophile, Salibacillus marismortui, by just 3 unambiguous bp in 16S rDNA, versus the ∼59 bp that would be expected if these bacteria evolved at the same rate
as other bacteria. We show, using a Poisson distribution, that unless it can be shown that S. marismortui evolves 5 to 10 times more slowly than other bacteria for which 16S rDNA substitution rates have been established, Vreeland
et al.'s claim would be rejected at the 0.05 level. Also, a molecular clock test and a relative rates test fail to substantiate
Vreeland et al.'s claim that strain 2-9-3 is a 250-Myr-old bacterium. The report of Vreeland et al. thus falls into a long
series of suspect ancient DNA studies.
Received: 12 April 2001 / Accepted: 9 June 2001 相似文献
936.
937.
938.
Mary Holland Gerald Rhodes Mark DalleAve Donald Wiesler Milos Novotny 《Life sciences》1983,32(7):787-794
Qualitative and quantitative differences in the urinary excretion of volatile and acidic metabolites in germfree and conventional rats were examined by capillary gas chromatography and gas chromatography/mass spectrometry. A number of carbonyl compounds, including several short-chain aliphatic ketones and acetophenone, were higher in the conventional urines, while many heterocyclic compounds (furan derivatives, benzothiazole and others) were lower. Both qualitative and quantitative differences were observed in the urinary excretion of acidic metabolites. Three meta-hydroxy phenolic acids appeared only in the conventional rat urines, while levels of many other aromatic and aliphatic acids were also higher. 相似文献
939.
940.
AU-rich element RNA-binding protein 1 (AUF1) regulates the stability and/or translational efficiency of diverse mRNA targets, including many encoding products controlling the cell cycle, apoptosis, and inflammation by associating with AU-rich elements residing in their 3′-untranslated regions. Previous biochemical studies showed that optimal AUF1 binding requires 33–34 nucleotides with a strong preference for U-rich RNA despite observations that few AUF1-associated cellular mRNAs contain such extended U-rich domains. Using the smallest AUF1 isoform (p37AUF1) as a model, we employed fluorescence anisotropy-based approaches to define thermodynamic parameters describing AUF1 ribonucleoprotein (RNP) complex formation across a panel of RNA substrates. These data demonstrated that 15 nucleotides of AU-rich sequence were sufficient to nucleate high affinity p37AUF1 RNP complexes within a larger RNA context. In particular, p37AUF1 binding to short AU-rich RNA targets was significantly stabilized by interactions with a 3′-purine residue and largely base-independent but non-ionic contacts 5′ of the AU-rich site. RNP stabilization by the upstream RNA domain was associated with an enhanced negative change in heat capacity consistent with conformational changes in protein and/or RNA components, and fluorescence resonance energy transfer-based assays demonstrated that these contacts were required for p37AUF1 to remodel local RNA structure. Finally, reporter mRNAs containing minimal high affinity p37AUF1 target sequences associated with AUF1 and were destabilized in a p37AUF1-dependent manner in cells. These findings provide a mechanistic explanation for the diverse population of AUF1 target mRNAs but also suggest how AUF1 binding could regulate protein and/or microRNA binding events at adjacent sites. 相似文献