全文获取类型
收费全文 | 31375篇 |
免费 | 2357篇 |
国内免费 | 2313篇 |
专业分类
36045篇 |
出版年
2024年 | 66篇 |
2023年 | 438篇 |
2022年 | 1076篇 |
2021年 | 1761篇 |
2020年 | 1162篇 |
2019年 | 1563篇 |
2018年 | 1419篇 |
2017年 | 999篇 |
2016年 | 1433篇 |
2015年 | 1986篇 |
2014年 | 2391篇 |
2013年 | 2597篇 |
2012年 | 2843篇 |
2011年 | 2554篇 |
2010年 | 1500篇 |
2009年 | 1389篇 |
2008年 | 1637篇 |
2007年 | 1433篇 |
2006年 | 1175篇 |
2005年 | 916篇 |
2004年 | 758篇 |
2003年 | 720篇 |
2002年 | 543篇 |
2001年 | 485篇 |
2000年 | 461篇 |
1999年 | 431篇 |
1998年 | 267篇 |
1997年 | 254篇 |
1996年 | 253篇 |
1995年 | 230篇 |
1994年 | 218篇 |
1993年 | 150篇 |
1992年 | 199篇 |
1991年 | 179篇 |
1990年 | 126篇 |
1989年 | 98篇 |
1988年 | 81篇 |
1987年 | 69篇 |
1986年 | 39篇 |
1985年 | 44篇 |
1984年 | 24篇 |
1983年 | 31篇 |
1982年 | 16篇 |
1981年 | 18篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1965年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Zhao Yan-Xiu Philip J. C. Harris Yao Dun-Yi 《Plant Cell, Tissue and Organ Culture》1995,40(2):119-123
Protoplasts were isolated from cotyledons of Sesbania bispinosa (Jacq.) W.F. Wight. In a liquid-over-agar culture system with Murashige and Skoog (MS) medium supplemented with 1 mg l-1 2,4-dichlorophenoxyacetic acid (2,4-d, 2 mg l-1 benzyladenine (BA), 1 mg l-1 glutamine and 0.5 and formed callus. The first division occurred after 3–4 days. Callus formed from the protoplasts differentiated shoots by organogenesis on MS medium with 1 mg l-1 indolebutyric acid (IBA) and 1 mg l-1 BA. These shoots developed into complete plantlets when excised and cultured on MS medium with 0.5 mg l-1 IBA. 相似文献
62.
Proteus mirabilis amino acid deaminase: cloning, nucleotide sequence, and characterization of aad. 总被引:2,自引:0,他引:2 下载免费PDF全文
Proteus, Providencia, and Morganella species produce deaminases that generate alpha-keto acids from amino acids. The alpha-keto acid products are detected by the formation of colored iron complexes, raising the possibility that the enzyme functions to secure iron for these species, which do not produce traditional siderophores. A gene encoding an amino acid deaminase of uropathogenic Proteus mirabilis was identified by screening a genomic library hosted in Escherichia coli DH5 alpha for amino acid deaminase activity. The deaminase gene, localized on a cosmid clone by subcloning and Tn5::751 mutagenesis, was subjected to nucleotide sequencing. A single open reading frame, designated aad (amino acid deaminase), which appears to be both necessary and sufficient for deaminase activity, predicts a 473-amino-acid polypeptide (51,151 Da) encoded within an area mapped by transposon mutagenesis. The predicted amino acid sequence of Aad did not share significant amino acid sequence similarity with any other polypeptide in the PIR or SwissProt database. Amino acid deaminase activity in both P. mirabilis and E. coli transformed with aad-encoding plasmids was not affected by medium iron concentration or expression of genes in multicopy in fur, cya, or crp E. coli backgrounds. Enzyme expression was negatively affected by growth with glucose or glycerol as the sole carbon source but was not consistent with catabolite repression. 相似文献
63.
Sequence requirements for the termination of rolling-circle replication of plasmid pT181 总被引:3,自引:0,他引:3
Most small multicopy plasmids of Gram-positive bacteria and many in Gram-negative bacteria replicate by a rolling-circle (RC) mechanism. The replication initiator proteins encoded by the RC plasmids and single-stranded bacteriophages of Escherichia coli have origin-specific nicking-closing activities that are required for the initiation and termination of RC replication. We have investigated the sequence requirements for termination of RC replication of plasmid pT181. The initiator nick site is located in the loop of a hairpin region (IRII) within the pT181 origin of replication. By mutational analysis, we have found that several nucleotides within the stem of IRII which are critical for the initiation activity are dispensable for termination of replication. We also demonstrate that nucleotides in the right arm of IRII, but not the left arm, are absolutely required for termination of RC replication. We have also identified specific nucleotides in IRII that are critical for its termination activity. The sequence of the right arm of the hairpin must be located downstream of the initiator nick site for termination, suggesting that termination requires a specific orientation of the initiator protein at the origin. 相似文献
64.
There is an active process by which sex pheromone reserves of female cabbage looper moths, Trichoplusia ni, are transported to the gland's surface during the nocturnal period of calling. We hypothesized that this mobilization was controlled by a head factor, possibly related to the pheromone biosynthesis activating neuropeptides (PBAN) that in other species stimulate pheromone synthesis. We evaluated the impact of head extracts of T. ni on pheromone emission and glandular content of pheromone. During the photophase injected head extracts stimulated an increased pheromone emission rate in females, but glandular content of pheromone was not affected. Head extracts of H. virescens, a species with known PBAN activity, and synthetic PBAN stimulated an increased pheromone emission rate in T. ni. There was some specificity of the response of female T. ni to PBAN, in that several other unrelated polypeptides did not stimulate this type of response. Previously it had been determined that brain factors do not play a role in stimulating pheromone biosynthesis in T. ni. Our results indicate that there may be additional avenues by which PBAN or related neuropeptides control pheromone emission, including transport of pheromone reserves to the surface of the sex pheromone gland. 相似文献
65.
δ-Glutamate Receptors Are Differentially Distributed at Parallel and Climbing Fiber Synapses on Purkinje Cells 总被引:1,自引:0,他引:1
Hui-Min Zhao Robert J. Wenthold Ya-Xian Wang Ronald S. Petralia 《Journal of neurochemistry》1997,68(3):1041-1052
Abstract: Neurons containing multiple excitatory inputs may sort and target glutamate receptor subtypes to subsets of synapses. A good model for testing this hypothesis is the Purkinje cell, which expresses significant levels of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, kainate, N -methyl- d -aspartate, δ-, and metabotropic glutamate receptors. Purkinje cells receive two excitatory inputs, the parallel and climbing fibers; the combined effect of stimulation of these two inputs is to produce long-term depression of parallel fiber/Purkinje cell neurotransmission. Distribution of glutamate receptors in these two synapse populations in rat cerebella was studied using preembedding immunocytochemistry with antibodies to GluR1, GluR2/3, GluR5-7, NR1, δ1/2, and mGluR1α. Moderate/dense postsynaptic staining was most frequent in postsynaptic densities and spines of both parallel and climbing fiber synapses with mGluR1α antibody, was intermediate in frequency with GluR2/3 and GluR5-7 antibodies, and was least frequent with GluR1 and NR1 antibodies. The most striking finding was the absence of significant postsynaptic staining with δ1/2 antibody in climbing fiber synapses in adult animals, even though postsynaptic staining was prevalent in parallel fiber synapses with this antibody. In contrast to adults, moderate/dense postsynaptic immunolabeling of climbing fiber synapses with δ1/2 antibody was common in rats at 10 days postnatal. This study provides direct morphological evidence that δ-glutamate receptors are differentially targeted to synapse populations. Our results support previous suggestions that δ2 is involved in development of parallel and climbing fiber synapses and in long-term depression of parallel fiber/Purkinje synaptic responses in adults. 相似文献
66.
Physiologic and yield effects of mixtalol at various concentrations sprayed on rape at the anthesis stage were examined. Foliar sprays of 4 and 2 ppm mixtalol significantly increased the chlorophyll content of rape leaves and pods, reduced the accumulation of malondialdehyde and ethylene production, and delayed the degradation of superoxide dismutase and catalase activities of the rape plant. Mixtalol also increased root oxidizability. Meanwhile, the number of branches and pods per plant was increased, and a 10.7% and 8.2% increase of seed yield over the controls was observed with treatments of 4 and 2 ppm mixtalol, respectively. No significant effects from mixtalol were observed on the maturation of plants or on the seed oil content or the erucic acid and glucosinolate content. Total rape oil production increased with 4 and 2 ppm mixtalol significantly by 12.4% and 10.5%, respectively, over the controls.Abbreviations MTL
mixtalol
- MDA
malondialdehyde
- TBA
thiobarbituric acid
- SOD
Superoxide dismutase
- CAT
catalase
- TTC
tetrazolium 相似文献
67.
Rulin Zhao Bernadette Oreski J. William Lown 《Bioorganic & medicinal chemistry letters》1995,5(24):3063-3066
A series of new camptothecin derivatives bearing five-membered ring heterocycle containing substituents in the 10-position were synthesized and evaluated for in vitro cytotoxic activity. Camptothecin derivatives bearing a pyrrole or a thiophene ring were significantly more potent than camptothecin, however those bearing furan were less potent than camptothecin. 相似文献
68.
69.
70.
Grantley W. Lycett Ashton J. Delauney Wenming Zhao John A. Gatehouse Ronald R. D. Croy Donald Boulter 《Plant molecular biology》1984,3(2):91-96
The sequence of two cDNA clones coding for the whole of the -subunit and most of the -subunit of legumin are presented together with a considerable amount of protein sequence data to confirm the predicted amino acid sequence. A unique feature shown by these cDNAs is the presence of three 56 base pair tandem repeats in the region encoding the C terminal of the polypeptide. The tandem repeats are also exhibited in the predicted polypeptide sequence as three 18 amino acid repeats which contain extremely high proportions of polar, mainly acidic, residues. The new sequences are compared to the previously published sequence of some shorter legumin cDNAs (Nature 295: 76–79). In the region where the sequences overlap, the previous cDNAs differ from the new ones by only a few base substitutions but most of the repeated region is not present though the sequences on either side are. The possibility that the absence of the repeats may reflect the difference between two types of legumin gene, rather than an artefact of the cloning of the cDNAs, is discussed. 相似文献