全文获取类型
收费全文 | 7764篇 |
免费 | 661篇 |
国内免费 | 6篇 |
出版年
2023年 | 19篇 |
2022年 | 25篇 |
2021年 | 113篇 |
2020年 | 63篇 |
2019年 | 109篇 |
2018年 | 153篇 |
2017年 | 113篇 |
2016年 | 237篇 |
2015年 | 363篇 |
2014年 | 424篇 |
2013年 | 436篇 |
2012年 | 551篇 |
2011年 | 559篇 |
2010年 | 348篇 |
2009年 | 328篇 |
2008年 | 450篇 |
2007年 | 433篇 |
2006年 | 393篇 |
2005年 | 396篇 |
2004年 | 399篇 |
2003年 | 349篇 |
2002年 | 313篇 |
2001年 | 303篇 |
2000年 | 242篇 |
1999年 | 214篇 |
1998年 | 76篇 |
1997年 | 79篇 |
1996年 | 53篇 |
1995年 | 57篇 |
1994年 | 37篇 |
1993年 | 27篇 |
1992年 | 77篇 |
1991年 | 83篇 |
1990年 | 50篇 |
1989年 | 54篇 |
1988年 | 46篇 |
1987年 | 44篇 |
1986年 | 32篇 |
1985年 | 35篇 |
1984年 | 33篇 |
1982年 | 20篇 |
1981年 | 17篇 |
1979年 | 19篇 |
1978年 | 22篇 |
1977年 | 23篇 |
1976年 | 24篇 |
1974年 | 24篇 |
1973年 | 19篇 |
1971年 | 20篇 |
1970年 | 17篇 |
排序方式: 共有8431条查询结果,搜索用时 15 毫秒
991.
Background and Aims For rare endemics or endangered plantspecies that reproduce both sexually and vegetatively it iscritical to understand the extent of clonality because assessmentof clonal extent and distribution has important ecological andevolutionary consequences with conservation implications. Asurvey was undertaken to understand clonal effects on fine-scalegenetic structure (FSGS) in two populations (one from a disturbedand the other from an undisturbed locality) of Echinosophorakoreensis, an endangered small shrub belonging to a monotypicgenus in central Korea that reproduces both sexually and vegetativelyvia rhizomes. Methods Using inter-simple sequence repeats (ISSRs) asgenetic markers, the spatial distribution of individuals wasevaluated using Ripley's L(d)-statistics and quantified thespatial scale of clonal spread and spatial distribution of ISSRgenotypes using spatial autocorrelation analysis techniques(join-count statistics and kinship coefficient, Fij) for totalsamples and samples excluding clones. Key Results A high degree of differentiation betweenpopulations was observed (ST(g) = 0·184, P < 0·001).Ripley's L(d)-statistics revealed a near random distributionof individuals in a disturbed population, whereas significantaggregation of individuals was found in an undisturbed site.The join-count statistics revealed that most clones significantlyaggregate at 6-m interplant distance. The Sp statistic reflectingpatterns of correlograms revealed a strong pattern of FSGS forall four data sets (Sp = 0·0720·154), butthese patterns were not significantly different from each other.At small interplant distances (2 m), however, jackknifed 95% CIs revealed that the total samples exhibited significantlyhigher Fij values than the same samples excluding clones. Conclusion The strong FSGS from genets is consistentwith two biological and ecological traits of E. koreensis: bee-pollinationand limited seed dispersal. Furthermore, potential clone matesover repeated generations would contribute to the observed highFij values among genets at short distance. To ensure long-termex situ genetic variability of the endangered E. koreensis,individuals located at distances of 1012 m should becollected across entire populations of E. koreensis. 相似文献
992.
Krosky DJ Bianchet MA Seiple L Chung S Amzel LM Stivers JT 《Nucleic acids research》2006,34(20):5872-5879
Human nuclear uracil DNA glycosylase (UNG2) is a cellular DNA repair enzyme that is essential for a number of diverse biological phenomena ranging from antibody diversification to B-cell lymphomas and type-1 human immunodeficiency virus infectivity. During each of these processes, UNG2 recognizes uracilated DNA and excises the uracil base by flipping it into the enzyme active site. We have taken advantage of the extrahelical uracil recognition mechanism to build large small-molecule libraries in which uracil is tethered via flexible alkane linkers to a collection of secondary binding elements. This high-throughput synthesis and screening approach produced two novel uracil-tethered inhibitors of UNG2, the best of which was crystallized with the enzyme. Remarkably, this inhibitor mimics the crucial hydrogen bonding and electrostatic interactions previously observed in UNG2 complexes with damaged uracilated DNA. Thus, the environment of the binding site selects for library ligands that share these DNA features. This is a general approach to rapid discovery of inhibitors of enzymes that recognize extrahelical damaged bases. 相似文献
993.
We have reviewed the mutagenicity of benzidine analogues (including benzidine-based dyes), with a primary emphasis on evaluating results of the Salmonella/microsome mutagenicity assay. Many of these amines are mutagenic in tester strains TA98 and TA100 but require exogenous mammalian activation (S9) for activity. A few amines with halogen or nitro-groups in the structure are direct-acting mutagens. The addition of a sulfonic acid moiety to the molecule of benzidine reduced the mutagenicity of benzidine; whereas, methoxy, chloro, or methyl group additions did not. Complexation with a metal ion also decreased the mutagenicity. A substitution of an alkyl group on the ortho position next to an amine group also influenced the mutagenicity. Most carcinogenic benzidine analogues are mutagenic, and their metabolism to electrophiles that interact with DNA, leading to mutations, plays a central role in their carcinogenesis. 相似文献
994.
2- and 4-Aminobiphenyls induce oxidative DNA damage in human hepatoma (Hep G2) cells via different mechanisms 总被引:1,自引:0,他引:1
4-Aminobiphenyl (4-ABP) and its analogue, 2-aminobiphenyl (2-ABP), were examined for their ability to induce oxidative DNA damage in Hep G2 cells. Using the alkaline comet assay, we showed that 2-ABP and 4-ABP (25-200 microM) were able to induce the DNA damage in Hep G2 cells. With both compounds, formation of intracellular reactive oxygen species (ROS) was detected using flow cytometry analysis. Post-treatment of 2-ABP and 4-ABP-treated cells by endonuclease III (Endo III) or formamidopyrimidine-DNA glycosylase (Fpg) to determine the formation of oxidized pyrimidines or oxidized purines showed a significant increase of the extent of DNA migration. This indicated that oxidative DNA damage occurs in Hep G2 cells after exposure to 2-ABP and 4-ABP. This assumption was further substantiated by the fact that the spin traps, 5,5-dimethyl-pyrroline-N-oxide (DMPO) and N-tert-butyl-alpha-phenylnitrone (PBN), decreased DNA damage significantly. Furthermore, addition of the catalase (100 U/ml) caused a decrease in the DNA damage induced by 2-ABP or 4-ABP, indicating that H(2)O(2) is involved in ABP-induced DNA damage. Pre-incubation of the cells with the iron chelator desferrioxamine (DFO) (1mM) and with the copper chelator neocupronine (NC) (100 microM) also decreased DNA damage in cells treated with 200 microM 2-ABP or 200 microM 4-ABP, while the calcium chelator {1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester}(BAPTA/AM) (10 microM) decreased only DNA strand breaks in cells exposed to 4-ABP. This suggested that ions are involved in the formation of DNA strand breaks. Using RT-PCR and Western blotting, lower inhibition of the expression of the OGG1 gene and of the OGG1 protein was observed in cells treated with 4-ABP, and 2-ABP-treated cells showed a marked reduction in the expression of OGG1 gene and OGG1 protein. Taken together, our finding indicated the mechanisms of induced oxidative DNA damage in Hep G2 cell by 2-ABP and 4-ABP are different, although both tested compounds are isomers. 相似文献
995.
Xyloglucan endotransglucosylase/hydrolases (XTHs) are a group of the enzymes that are responsible for reorganization of the cellulose–xyloglucan framework by catalyzing cleavage and religation of the xyloglucan chains in the plant cell wall. In this study, we report the isolation and characterization of a XTH gene from a pistil cDNA library of Brassica campestris. Sequence analysis of the gene, designated BcXTH1, revealed that it is homologous to the XTH9 gene of Arabidopsis. The highly conserved domain (DEIDFEFLG) found among all XTHs was also present in BcXTH1 but with the two amino acid substitutions (NEFDFEFLG) also found in Arabidopsis XTH9. These results suggest that BcXTH1 is the B. campestris homologue of XTH9. Expression analysis of BcXTH1 revealed that it was expressed in most of the plant organs. In situ hybridization showed that the gene is highly expressed in the floral primodia, especially in the epidermal cell layer. Southern blot analysis indicated that the BcXTH1 gene exists as a multi-copy gene in the B. campestris genome. The function of the BcXTH1 gene was deduced from using an overexpression strategy in Arabidopsis. Interestingly, the transgenic plants showed a pronounced cell expansion phenotype. Immunoelectron microscopy shows that BcXTH1 is localized almost exclusively to the cell wall, supporting our conclusion that it participates in the regulation of cell expansion in B. campestris. 相似文献
996.
997.
998.
Summary. A study of male sterility over a period of three consecutive years on a conifer species endemic to Taiwan, Taiwania cryptomerioides Hayata (Taxodiaceae), was done for this article. With the aids of fluorescence and electron microscopic observations, the
ontogenic processes in the fertile and sterile microsporangia are compared, using samples collected from Chitou Experimental
Forest and Yeou-Shoei-Keng Clonal Orchard of the National Taiwan University, Nantou, Taiwan. The development of male strobili
occurred from August to the end of March. Microsporogenesis starts with the formation of the archesporium and ends with the
maturation of 2-celled pollen grains within the dehiscing microsporangium. Before meiosis, there was no significant difference
in ultrastructure between the fertile and sterile microsporangia. Asynchronous pollen development with various tetrad forms
may occur in the same microsporangium of either fertile or sterile strobili. However, a callose wall was observable in the
fertile dyad and tetrad, but not in the sterile one. After dissolution of the callose wall, the fertile microspores were released
into the locule, while some sterile microspores still retained as tetrads or dyads with intertwining of exine walls in the
proximal faces. As a result, there was no well developed lamellated endexine and no granulate ectexine or intine in the sterile
microspores. Eventually, the intracellular structures in sterile microspores were dramatically collapsed before anthesis.
The present study shows that the abortion in pollen development is possibly attributed to the absence of the callose wall.
The importance of this structure to the male sterility of T. cryptomerioides is discussed.
Correspondence and reprints: Department of Life Science, National Taiwan University, 106 Taipei, Taiwan. 相似文献
999.
Global survey of organ and organelle protein expression in mouse: combined proteomic and transcriptomic profiling 总被引:15,自引:0,他引:15
Kislinger T Cox B Kannan A Chung C Hu P Ignatchenko A Scott MS Gramolini AO Morris Q Hallett MT Rossant J Hughes TR Frey B Emili A 《Cell》2006,125(1):173-186
Organs and organelles represent core biological systems in mammals, but the diversity in protein composition remains unclear. Here, we combine subcellular fractionation with exhaustive tandem mass spectrometry-based shotgun sequencing to examine the protein content of four major organellar compartments (cytosol, membranes [microsomes], mitochondria, and nuclei) in six organs (brain, heart, kidney, liver, lung, and placenta) of the laboratory mouse, Mus musculus. Using rigorous statistical filtering and machine-learning methods, the subcellular localization of 3274 of the 4768 proteins identified was determined with high confidence, including 1503 previously uncharacterized factors, while tissue selectivity was evaluated by comparison to previously reported mRNA expression patterns. This molecular compendium, fully accessible via a searchable web-browser interface, serves as a reliable reference of the expressed tissue and organelle proteomes of a leading model mammal. 相似文献
1000.
Kim YG Lee CS Chung WJ Kim EM Shin DS Kim JH Lee YS Chung J Kim BG 《Biotechnology letters》2006,28(2):79-84
Lipopolysaccharide (LPS)-binding peptides were enriched by using epoxy beads as a novel support to immobilize LPS for a phage
displayed peptide library screening. The sequence of Phe-Ala-Pro-Trp (FAPW) was the most significant consensus motif of 10
selected clones, and Pro-Phe (PF) was the key dipeptide for binding at the apex of the loop to form a characteristic structure
of CXXPFXXXC. Moreover, AWLPWAK, one of the highly conserved heptamer peptides, could detect specifically Gram-negative bacteria
via a whole cell binding test at 106 cells ml−1.
Received 12 July 2005; Revisions requested 1 August 2005 and 26 September 2005; Revisions received 12 September 2005 and 25
October 2005; Accepted 1 November 2005 相似文献