全文获取类型
收费全文 | 1230篇 |
免费 | 67篇 |
国内免费 | 1篇 |
出版年
2023年 | 5篇 |
2022年 | 8篇 |
2021年 | 15篇 |
2020年 | 12篇 |
2019年 | 9篇 |
2018年 | 15篇 |
2017年 | 10篇 |
2016年 | 22篇 |
2015年 | 43篇 |
2014年 | 59篇 |
2013年 | 53篇 |
2012年 | 76篇 |
2011年 | 84篇 |
2010年 | 56篇 |
2009年 | 36篇 |
2008年 | 62篇 |
2007年 | 54篇 |
2006年 | 59篇 |
2005年 | 54篇 |
2004年 | 52篇 |
2003年 | 42篇 |
2002年 | 53篇 |
2001年 | 13篇 |
2000年 | 12篇 |
1999年 | 14篇 |
1998年 | 9篇 |
1997年 | 16篇 |
1996年 | 12篇 |
1995年 | 5篇 |
1994年 | 14篇 |
1993年 | 3篇 |
1992年 | 10篇 |
1991年 | 6篇 |
1990年 | 14篇 |
1989年 | 8篇 |
1988年 | 8篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 9篇 |
1984年 | 22篇 |
1983年 | 11篇 |
1982年 | 31篇 |
1981年 | 28篇 |
1980年 | 27篇 |
1979年 | 47篇 |
1978年 | 24篇 |
1977年 | 31篇 |
1976年 | 9篇 |
1973年 | 3篇 |
1971年 | 4篇 |
排序方式: 共有1298条查询结果,搜索用时 15 毫秒
71.
Distal side tryptophan and tyrosine have been shown to be essential in the catalase but not the peroxidase activity of bifunctional catalase-peroxidases (KatGs). Recently published crystal structures suggest that both residues could be part of a novel adduct including in addition a conserved methionine. A mass spectrometric analysis of the tryptic peptides from recombinant wild-type Synechocystis KatG and the variants Trp122Phe, Tyr249Phe and Met275Ile confirms that this novel adduct really exists in solution and thus may be common to all KatGs. Exchange of either Trp122 or Tyr249 prevents cross-linking, whereas exchange of Met275 still allowed bond formation between Trp122 and Tyr249. It is proposed that the covalent bond between Trp and Tyr may form before that between Tyr and Met. The findings are discussed with respect to the mechanism of cross-linking and its role in KatG catalysis. 相似文献
72.
Phytopharmaceuticals and dietary supplements containing valerian are used as mild sleep-inducing agents. An in vitro radioligand binding assay at A(1) and A(2A) adenosine receptors (ARs) was conducted with a fixed extract combination of valerian and hop (Ze 91019) to investigate a possible mechanism for the pharmacological activity of the extract. Component extracts of valerian and hop were also individually investigated. The fixed combination Ze 91019 as well as the valerian extracts therein exhibited selective affinity to A(1)ARs (K(i) = 0.15-0.37 mg/mL vs [(3)H]CCPA). The same extracts exhibited partial agonist activity at the A(1) adenosine receptor as indicated by a lower degree of stimulation of [35S]GTP gamma S binding in membrane preparations of CHO-hA(1) cells as compared to the full A(1) AR agonist N(6)-cyclopentyladenosine (CPA). In addition valerian extract inhibited cAMP accumulation in CHO-hA(1) cell membranes. The partial agonistic activity at A(1)ARs may thus play a role in the sleep inducing effect of Ze 91019 and the valerian extract therein. 相似文献
73.
The influenza A virus genome is composed of eight negative-sense RNA segments (called vRNAs), all of which must be packaged to produce an infectious virion. It is not clear whether individual vRNAs are packaged specifically or at random, however, and the total vRNA capacity of the virion is unknown. We have created modified forms of the viral nucleoprotein (NP), neuraminidase (NA), and nonstructural (NS) vRNAs that encode green or yellow fluorescent proteins and studied the efficiency with which these are packaged by using a plasmid-based influenza A virus assembly system. Packaging was assessed precisely and quantitatively by scoring transduction of the fluorescent markers in a single-round infectivity assay with a flow cytometer. We found that, under conditions in which virions are limiting, pairs of alternatively tagged vRNAs compete for packaging but do so in a nonspecific manner. Reporters representing different vRNAs were not packaged additively, as would be expected under specific packaging, but instead appeared to compete for a common niche in the virion. Moreover, 3 to 5% of transduction-competent viruses were found to incorporate two alternative reporters, regardless of whether those reporters represented the same or different vRNAs - a finding compatible with random, but not with specific, packaging. Probabilistic estimates suggest that in order to achieve this level of dual transduction by chance alone, each influenza A virus virion must package an average of 9 to 11 vRNAs. 相似文献
74.
Zikmundová M Drandarov K Hesse M Werner C 《Zeitschrift für Naturforschung. C, Journal of biosciences》2002,57(7-8):660-665
The biotransformation of the phytoanticipin HBOA and its major degradation metabolites 2-hydroxy-N-(2-hydroxyphenyl)acetamide (7) and N-(2-hydroxyphenyl)acetamide (8) by Chaetosphaeria sp., an endophytic fungus isolated from Aphelandra tetragona, was studied. Three new metabolites could be identified as 2-amino-7-hydroxy-3H-phenoxazin-3-one (12), 2-acetylamino-7-hydroxy-3H-phenoxazin-3-one (13) and 7-hydroxy-2-(2-hydroxyacetyl)-amino-3H-phenoxazin-3-one (14). Structure elucidation of 12 and 13 was performed by MS, 1H, 13C NMR and 2D NMR techniques and confirmed by chemical transformation. 相似文献
75.
Diversity and abundance of Crenarchaeota in terrestrial habitats studied by 16S RNA surveys and real time PCR 总被引:11,自引:0,他引:11
Ochsenreiter T Selezi D Quaiser A Bonch-Osmolovskaya L Schleper C 《Environmental microbiology》2003,5(9):787-797
Novel phylogenetic lineages of as yet uncultivated crenarchaeota have been frequently detected in low to moderate-temperature, marine and terrestrial environments. In order to gain a more comprehensive view on the distribution and diversity of Crenarchaeota in moderate habitats, we have studied 18 different terrestrial and freshwater samples by 16S rDNA-based phylogenetic surveys. In seven different soil samples of diverse geographic areas in Europe (forest, grassland, ruderal) and Asia (permafrost, ruderal) as well as in two microbial mats, we have consistently found one particular lineage of crenarchaeota. The diversity of Crenarchaeota in freshwater sediments was considerably higher with respresentative 16S rDNA sequences distributed over four different groups within the moderate crenarchaeota. Systematic analysis of a 16S rDNA universal library from a sandy ecosystem containing 800 clones exclusively revealed the presence of the soil-specific crenarchaeotal cluster. With primers specific for non-thermophilic crenarchaeota we established a rapid method to quantify archaeal 16S rDNA in real time PCR. The relative abundance of crenarchaeotal rDNA was 0.5-3% in the bulk soil sample and only 0.16% in the rhizosphere of the sandy ecosystem. A nearby agricultural setting yielded a relative abundance of 0.17% crenarchaeotal rDNA. In total our data suggest that soil crenarchaeota represent a stable and specific component of the microbiota in terrestrial habitats. 相似文献
76.
Brunner RM Sanftleben H Goldammer T Kühn C Weikard R Kata SR Womack JE Schwerin M 《Genomics》2003,81(3):270-278
We have applied a targeted physical mapping approach, based on the isolation of bovine region-specific large-insert clones using homologous human sequences and chromosome microdissection, to enhance the physical gene map of the telomeric region of BTA18 and to prove its evolutionary conservation. The latter is a prerequisite to exploit the dense human gene map for future positional cloning approaches. Partial sequencing and homology search were used to characterize 20 BACs targeted to the BTA18q2.4-q2.6 region. We used fluorescence in situ hybridization (FISH) to create physical maps of 11 BACs containing 15 gene loci; these BACs served as anchor loci. Using these approaches, 12 new gene loci (CKM, STK13, PSCD2, IRF3, VASP, ACTN4, ITPKC, CYP2B6, FOSB, DMPK, MIA, SIX5) were assigned on BTA18 in the bovine cytogenetic map. A resolved physical map of BTA18q2.4-q2.6 was developed, which encompasses 28 marker loci and a comparative cytogenetic map that contains 15 genes. The mapping results demonstrate the high evolutionary conservation between the telomeric region of BTA18q and HSA19q. 相似文献
77.
Chlorophyll fluorescence measurements have a wide range of applications from basic understanding of photosynthesis functioning
to plant environmental stress responses and direct assessments of plant health. The measured signal is the fluorescence intensity
(expressed in relative units) and the most meaningful data are derived from the time dependent increase in fluorescence intensity
achieved upon application of continuous bright light to a previously dark adapted sample. The fluorescence response changes
over time and is termed the Kautsky curve or chlorophyll fluorescence transient. Recently, Strasser and Strasser (1995) formulated
a group of fluorescence parameters, called the JIP-test, that quantify the stepwise flow of energy through Photosystem II,
using input data from the fluorescence transient. The purpose of this study was to establish relationships between the biochemical
reactions occurring in PS II and specific JIP-test parameters. This was approached using isolated systems that facilitated
the addition of modifying agents, a PS II electron transport inhibitor, an electron acceptor and an uncoupler, whose effects
on PS II activity are well documented in the literature. The alteration to PS II activity caused by each of these compounds
could then be monitored through the JIP-test parameters and compared and contrasted with the literature. The known alteration
in PS II activity of Chenopodium album atrazine resistant and sensitive biotypes was also used to gauge the effectiveness and sensitivity of the JIP-test. The information
gained from the in vitro study was successfully applied to an in situ study. This is the first in a series of four papers. It shows that the trapping parameters of the JIP-test were most affected
by illumination and that the reduction in trapping had a run-on effect to inhibit electron transport. When irradiance exposure
proceeded to photoinhibition, the electron transport probability parameter was greatly reduced and dissipation significantly
increased. These results illustrate the advantage of monitoring a number of fluorescence parameters over the use of just one,
which is often the case when the FV/FM ratio is used.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
78.
Effects of physioball and conventional floor exercises on early phase adaptations in back and abdominal core stability and balance in women 总被引:1,自引:0,他引:1
Cosio-Lima LM Reynolds KL Winter C Paolone V Jones MT 《Journal of strength and conditioning research / National Strength & Conditioning Association》2003,17(4):721-725
The purpose of this study was to compare the effects of 5 weeks of physioball core stability and balance exercises with conventional floor exercises in women. The experimental group (n = 15) performed curl-ups and back extensions on the physioball while the control group (n = 15) performed the same exercises on the floor. Baseline and post-training tests included electromyography (EMG) recordings of the rectus abdominus and erector spinae muscles; abdominal, back, and knee strength measurements with the Cybex Norm System; and 2 unilateral stance balance tests. The physioball group was found to have significantly greater mean change in EMG flexion and extension activity (p = 0.04 and p = 0.01, respectively) and greater balance scores (p < 0.01) than the floor exercise group. No significant changes (p > 0.05) were observed for heart rate or Cybex strength measurements. Early adaptations in a short-term core exercise program using the physioball resulted in greater gains in torso balance and EMG neuronal activity in previously untrained women when compared to performing exercises on the floor. 相似文献
79.
The histone gene H1t is expressed exclusively in pachytene spermatocytes of the testis. In this report we have eliminated the single copy H1t gene by homologous recombination from the mouse genome to analyse the function of the H1t protein during spermatogenesis. Mice homozygous for the mutated H1t gene locus developed normally and showed no anatomic abnormalities until the adult stage. In addition, H1t-deficient mice were fertile and reproduced as wild-type mice. The process of spermatogenesis and the testicular morphology remained unchanged in the absence of H1t. RNase protection analysis demonstrated that H1.1, H1.2 and H1.4 histone gene expression is enhanced during spermatogenesis in H1t-deficient mice. 相似文献
80.
Sister chromatid cohesion and recombination in meiosis 总被引:10,自引:0,他引:10
Sister chromatids are associated from their formation until their disjunction. Cohesion between sister chromatids is provided
by protein complexes, of which some components are conserved across the kingdoms and between the mitotic and meiotic cell
cycles. Sister chromatid cohesion is intimately linked to other aspects of chromosome behaviour and metabolism, in particular
chromosome condensation, recombination and segregation. Recombination, sister chromatid cohesion and the relation between
the two processes must be regulated differently in mitosis and meiosis. In meiosis, cohesion and recombination are modified
in such a way that reciprocal exchange and reductional segregation of homologous chromosomes are ensured.
Received: 11 October 1999; in revised form: 3 December 1999 / Accepted: 6 December 1999 相似文献