全文获取类型
收费全文 | 12608篇 |
免费 | 1018篇 |
国内免费 | 9篇 |
专业分类
13635篇 |
出版年
2023年 | 51篇 |
2022年 | 99篇 |
2021年 | 233篇 |
2020年 | 134篇 |
2019年 | 201篇 |
2018年 | 238篇 |
2017年 | 218篇 |
2016年 | 313篇 |
2015年 | 607篇 |
2014年 | 636篇 |
2013年 | 771篇 |
2012年 | 1052篇 |
2011年 | 1013篇 |
2010年 | 634篇 |
2009年 | 580篇 |
2008年 | 818篇 |
2007年 | 841篇 |
2006年 | 721篇 |
2005年 | 717篇 |
2004年 | 679篇 |
2003年 | 630篇 |
2002年 | 635篇 |
2001年 | 116篇 |
2000年 | 75篇 |
1999年 | 127篇 |
1998年 | 153篇 |
1997年 | 125篇 |
1996年 | 113篇 |
1995年 | 115篇 |
1994年 | 98篇 |
1993年 | 75篇 |
1992年 | 84篇 |
1991年 | 54篇 |
1990年 | 58篇 |
1989年 | 60篇 |
1988年 | 33篇 |
1987年 | 36篇 |
1986年 | 39篇 |
1985年 | 38篇 |
1984年 | 48篇 |
1983年 | 34篇 |
1982年 | 59篇 |
1981年 | 39篇 |
1980年 | 43篇 |
1979年 | 27篇 |
1978年 | 31篇 |
1977年 | 32篇 |
1976年 | 18篇 |
1975年 | 17篇 |
1973年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
Marc Klein 《Invertebrate neuroscience : IN》1995,1(1):15-24
Up and down-regulation of calcium and potassium conductances are associated with several forms of short-term synaptic modulation.
Detailed investigation of synaptic plasticity in the marine gastropodAplysia, and in other mollusks, indicates that synaptic transmission can be influenced in a number of ways by modulatory neurotransmitters
acting through several second-messenger cascades. Modulation at the synapse itself occurs by means of the regulation of calcium
current as well as through effects on processes directly involved in transmitter mobilization and exocytosis. Modulation of
potassium current plays a major role in controlling neuronal excitability and may contribute to a lesser extent to the regulation
of transmitter release through actions on the resting potential and on action potential configuration. 相似文献
62.
Giuseppe Rotondo Marc Gillespie David Frendewey 《Molecular genetics and genomics : MGG》1995,247(6):698-708
The Schizosaccharomyces pombe temperature-sensitive mutant snm1 maintains reduced steady-state quantities of the spliceosomal small nuclear RNAs (snRNAs) and the RNA subunit of the tRNA processing enzyme RNase P. We report here the isolation of the pac1 + gene as a multi-copy suppressor of snm1. The pac1 + gene was previously identified as a suppressor of the ran1 mutant and by its ability to cause sterility when overexpressed. The pac1 + gene encodes a double-strand-specific ribonuclease that is similar to RNase III, an RNA processing and turnover enzyme in Escherichia coli. To investigate the essential structural features of the Pac1 RNase, we altered the pac1 + gene by deletion and point mutation and tested the mutant constructs for their ability to complement the snm1 and ran1 mutants and to cause sterility. These experiments identified four essential amino acids in the Pac1 sequence: glycine 178, glutamic acid 251, and valines 346 and 347. These amino acids are conserved in all RNase III-like proteins. The glycine and glutamic acid residues were previously identified as essential for E. coli RNase III activity. The valines are conserved in an element found in a family of double-stranded RNA binding proteins. Our results support the hypothesis that the Pac1 RNase is an RNase III homolog and suggest a role for the Pac1 RNase in snRNA metabolism. 相似文献
63.
Plasmid transfer to indigenous marine bacterial populations by natural transformation 总被引:3,自引:0,他引:3
Abstract Horizontal gene transfer among microbial populations has been assumed to occur in the environment, yet direct observations of this phenomenon are rare or limited to observations where the mechanism(s) could not be explicitly determined. Here we demonstrate the transfer of exogenous plasmid DNA to members of indigenous marine bacterial populations by natural transformation, the first report of this process for any natural microbial community. Ten percent of marine bacterial isolates examined were transformed by plasmid DNA while 14% were transformed by chromosomal DNA. Transformation of mixed marine microbial assemblages was observed in 5 of 14 experiments. In every case, acquisition of the plasmid by members of the indigenous flora was accompanied by modification (probably from genetic rearrangement or methylation) that altered its restriction enzyme digestion pattern. Estimation of transformation rates in estuarine environments based upon the distribution of competency and transformation frequencies in isolates and mixed populations ranged from 5 × 10−4 to 1.5 transformants/1 day. Extrapolation of these rates to ecosystem scales suggests that natural transformation may be an important mechanism for plasmid transfer among marine bacterial communities. 相似文献
64.
Margaret E. Halloran Marc Bekoff 《Ethology : formerly Zeitschrift fur Tierpsychologie》1994,97(3):236-248
Nesting behaviour of Abert squirrels (Sciurus aberti), including site selection and use, was studied in the foothills of the Rocky Mountains in Boulder County, Colorado. Only females were observed building nests, although both males and females maintained nests once they were built. Communal nesting by Abert squirrels was rare, but the majority of observed nest sharings involved unrelated male and female pairs. A total of 14 variables were used to evaluate the nests (n = 49) inhabited by Abert squirrels from May 1988 to Jun. 1991. All nests were located in Ponderosa pine (Pinus ponderosa) trees. The majority of nests were constructed of twigs and located in the upper one-third of the canopy, near the trunk, on the south-east side of the tree. Trees with nests were predominantly located in closed stands. Nest trees, when compared with unused control trees that were equally accessible to squirrels, were significantly different from control trees in five of nine variables. Nest tree crowns intertwined with a larger number of adjacent tree crowns than did control tree crowns. Nest trees were also significantly taller than control trees, but subdominant to adjacent trees within a stand. Seasonal and diurnal patterns of nest use indicate that Abert squirrels do not choose nest locations on the south-east sides of trees to facilitate behavioural thermoregulation. Rather, Abert squirrels select nest site locations to (1) maximize accessibility and (2) maximize structural stability which may provide protection from wind and rain. 相似文献
65.
66.
67.
Inheritance of Strain Instability (Sectoring) in the Commercial Button Mushroom, Agaricus bisporus 总被引:1,自引:1,他引:0 下载免费PDF全文
Aimin Li Marc Begin Karl Kokurewicz Christine Bowden Paul A. Horgen 《Applied microbiology》1994,60(7):2384-2388
The button mushroom, Agaricus bisporus, is a commercially important cultivated filamentous fungus. During the last decade, the button mushroom industry has depended mainly on two strains (or derivatives of these two strains). Using one of these highly successful strains (strain U1) we examined the phenomenon of strain instability, specifically, the production of irreversible sectors. Three “stromatal” and three “fluffy” sectors were compared with a healthy type U1 strain and with a wild-collected isolate. Compost colonization and fruit body morphology were examined. The main objective of this study, however, was to examine the meiotic stability of the sectored phenotype. Single basidiospores were isolated and subjected to a grain bioassay in which the ability to produce sectors was measured. Our results were as follows: (i) basidiospore cultures obtained from a wild-collected isolate showed no tendency to produce sectors; (ii) approximately 5% of the basidiospore cultures obtained from healthy type U1 strains produced irreversible sectors in the grain bioassay; (iii) the five primary sectors examined produced basidiospore cultures, half of which produced normal-looking growth in the grain bioassay and half of which produced some degree of sectoring; and (iv) the one sectored isolate that represented the F2 generation gave ratios similar to the 1:1 ratio observed for the F1 cultures. 相似文献
68.
Marc Yudkoff Yevgeny Daikhin Zhi-Ping Lin Liana Nissim Janet Stern David Pleasure Itzhak Nissim 《Journal of neurochemistry》1994,62(3):1192-1202
Abstract: The aim was to study the extent to which leu-cine furnishes α-NH2 groups for glutamate synthesis via branched-chain amino acid aminotransferase. The transfer of N from leucine to glutamate was determined by incubating astrocytes in a medium containing [15N]leucine and 15 unlabeled amino acids; isotopic abundance was measured with gas chromatography-mass spectrometry. The ratio of labeling in both [15N]glutamate/[15N]leucine and [2-15N]glutamine/[15N]leucine suggested that at least one-fifth of all glutamate N had been derived from leucine nitrogen. At the same time, enrichment in [15N]leucine declined, reflecting dilution of the 16N label by the unlabeled amino acids that were in the medium. Isotopic abundance in [16N]-isoleucine increased very quickly, suggesting the rapidity of transamination between these amino acids. The appearance of 15N in valine was more gradual. Measurement of branched-chain amino acid transaminase showed that the reaction from leucine to glutamate was approximately six times more active than from glutamate to leucine (8.72 vs. 1.46 nmol/min/mg of protein). However, when the medium was supplemented with α-ketoisocaproate (1 mM), the ketoacid of leucine, the reaction readily ran in the “reverse” direction and intraastrocytic [glutamate] was reduced by ~50% in only 5 min. Extracellular concentrations of α-ketoisocaproate as low as 0.05 mM significantly lowered intracellular [glutamate]. The relative efficiency of branched-chain amino acid transamination was studied by incubating astrocytes with 15 unlabeled amino acids (0.1 mM each) and [15N]glutamate. After 45 min, the most highly labeled amino acid was [15N]alanine, which was closely followed by [15N]leucine and [15N]isoleucine. Relatively little 15N was detected in any other amino acids, except for [15N]serine. The transamination of leucine was ~17 times greater than the rate of [1-14C]leucine oxidation. These data indicate that leucine is a major source of glutamate nitrogen. Conversely, reamination of a-ketoisocaproate, the ketoacid of leucine, affords a mechanism for the temporary “buffering” of intracellular glutamate. 相似文献
69.
The medical textbooks in our university library present ‘principles’ as the basis which underlies medical ‘practice’. In this article it is argued that this helps different medical logics to co-exist. The example analyzed is that of anemia in the Netherlands. Currently this is defined pathophysiologically, statistically and clinically. These three definitions are intertwined with different strategies for the creation of normal hemoglobin levels and the detection of patients with anemia. The discrepancies between them, however, do not lead to the controversies that might be expected by those who believe in consistency. Instead, the rhetoric of principles-and-practice helps to bring about peaceful co-existence. 相似文献
70.