全文获取类型
收费全文 | 4163篇 |
免费 | 263篇 |
专业分类
4426篇 |
出版年
2023年 | 19篇 |
2022年 | 43篇 |
2021年 | 66篇 |
2020年 | 45篇 |
2019年 | 71篇 |
2018年 | 95篇 |
2017年 | 70篇 |
2016年 | 97篇 |
2015年 | 170篇 |
2014年 | 182篇 |
2013年 | 309篇 |
2012年 | 312篇 |
2011年 | 320篇 |
2010年 | 205篇 |
2009年 | 158篇 |
2008年 | 267篇 |
2007年 | 286篇 |
2006年 | 225篇 |
2005年 | 184篇 |
2004年 | 184篇 |
2003年 | 190篇 |
2002年 | 200篇 |
2001年 | 37篇 |
2000年 | 24篇 |
1999年 | 37篇 |
1998年 | 41篇 |
1997年 | 54篇 |
1996年 | 27篇 |
1995年 | 37篇 |
1994年 | 36篇 |
1993年 | 32篇 |
1992年 | 32篇 |
1991年 | 30篇 |
1990年 | 29篇 |
1989年 | 15篇 |
1988年 | 16篇 |
1987年 | 20篇 |
1986年 | 16篇 |
1985年 | 17篇 |
1984年 | 23篇 |
1983年 | 19篇 |
1982年 | 18篇 |
1981年 | 23篇 |
1980年 | 16篇 |
1979年 | 13篇 |
1978年 | 12篇 |
1977年 | 10篇 |
1976年 | 13篇 |
1975年 | 11篇 |
1974年 | 10篇 |
排序方式: 共有4426条查询结果,搜索用时 0 毫秒
1.
Gian Maria Rossolini Patrizia Muscas Alessandra Chiesurin Giuseppe Satta 《FEMS microbiology letters》1994,119(3):321-328
Abstract Analysis of the Salmonella chromosomal region located upstream of the fimA gene (coding for the major type 1 fimbrial subunit) showed a close linkage of this gene to the folD gene (coding for the enzyme 5,10-methylenetetrahydrofolate dehydrogenase/5, 10-methenyltetrahydrofolate cyclohydrolase), indicating that the fim gene cluster of Salmonella , unlike that of Escherichia coli , has no regulatory genes located upstream of fimA and apparently terminates with this gene. The respective locations of the fim and folD genes in the E. coli and Salmonella genetic maps suggests that the fimA-folD intergenic region of Salmonella encompasses a junctional site of a genetic rearrangement that probably originated from the different chromosomal location of the fim genes in these species. 相似文献
2.
3.
4.
5.
Internal Representations of Temporal Statistics and Feedback Calibrate Motor-Sensory Interval Timing
Humans have been shown to adapt to the temporal statistics of timing tasks so as to optimize the accuracy of their responses, in agreement with the predictions of Bayesian integration. This suggests that they build an internal representation of both the experimentally imposed distribution of time intervals (the prior) and of the error (the loss function). The responses of a Bayesian ideal observer depend crucially on these internal representations, which have only been previously studied for simple distributions. To study the nature of these representations we asked subjects to reproduce time intervals drawn from underlying temporal distributions of varying complexity, from uniform to highly skewed or bimodal while also varying the error mapping that determined the performance feedback. Interval reproduction times were affected by both the distribution and feedback, in good agreement with a performance-optimizing Bayesian observer and actor model. Bayesian model comparison highlighted that subjects were integrating the provided feedback and represented the experimental distribution with a smoothed approximation. A nonparametric reconstruction of the subjective priors from the data shows that they are generally in agreement with the true distributions up to third-order moments, but with systematically heavier tails. In particular, higher-order statistical features (kurtosis, multimodality) seem much harder to acquire. Our findings suggest that humans have only minor constraints on learning lower-order statistical properties of unimodal (including peaked and skewed) distributions of time intervals under the guidance of corrective feedback, and that their behavior is well explained by Bayesian decision theory. 相似文献
6.
Sequence complexity and diversity of polyadenylated RNA molecules transcribed in human myeloid cells
7.
8.
Alberto Buson Alessandro Negro Luigi Grassato Massimo Tagliaro Marina Basaglia Claudio Grandi Angelo Fontana Marco P. Nuti 《FEMS microbiology letters》1996,145(1):55-62
Abstract A clone positive for d-carbamoylase activity (2.7 kb Hin dIII- Bam H1 DNA fragment) was obtained by screening a genomic library of Agrobacterium radiobacter in Escherichia coli . This DNA fragment contains an open reading frame of 912 bp which is predicted to encode a peptide of 304 amino acids with a calculated molecular mass of 34247 Da. The d-carbamoylase gene. named cauA , was placed under the control of T7 RNA-dependent promoter and expressed in E. coli BL21 (DE3). After induction with isopropyl-thio-β-d-galactopyranoside, the synthesis of d-carbamoylase in E. coli reached about 40% of the total protein. The expressed protein was shown to possess a molecular mass, on SDS-PAGE, of 36 kDa and showed an enhanced allowed us to establish that a Pro14 →Leu14 exchange leads to an inactive enzyme species, while a Cys279 →Ser279 exchange did not impair the functional properties of the enxyme. 相似文献
9.
This paper investigates the kinetic parameters of Na+–K+-ATPase in glial, neuronal, and synaptosomal enriched fractions isolated from rabbit cerebral cortex. Under normal conditions, kinetic parameters-Vmax and K
0.5
K+
-of Na+–K+-ATPase are the same in the three fractions, suggesting that this enzyme behaves as the same molecular entity. Following a cryogenic lesion, the alterations of these parameters appear to be different in the different fractions. These data suggest that the same enzyme exhibits various responses when exposed to the same pathological event. The dissimilar lipid composition of the Na+–K+-ATPase environment, and/or different adaptative responses to abnormal ion concentrations in glial, neuronal, and synaptosomal fractions could account for these different responses. 相似文献
10.
L De Gara F Tommasi R Liso O Arrigoni 《Bollettino della Società italiana di biologia sperimentale》1989,65(10):959-965
The capacity of ascorbic acid biosynthesis in potato tuber tissue is closely correlated with the ascorbic acid content of the cells: the lower the endogenous content of ascorbic acid, the greater its biosynthesis. At the highest level of ascorbic acid found in the cells, the biosynthetic capacity is virtually zero. In these conditions, adding glucose (the first precursor of ascorbic acid) has no effect whatsoever, whereas adding galactono-gamma-lactone (the last precursor) induces a high rate of ascorbic acid synthesis. It is suggested that AA biosynthesis is subject to a regulatory mechanism "in vivo" which controls an initial step in the biosynthetic pathway. The last step in this pathway, catalyzed by galactone oxidase, is never blocked and, moreover, its activity is greater than that of the preceding steps. 相似文献