全文获取类型
收费全文 | 6315篇 |
免费 | 444篇 |
专业分类
6759篇 |
出版年
2024年 | 9篇 |
2023年 | 39篇 |
2022年 | 68篇 |
2021年 | 199篇 |
2020年 | 112篇 |
2019年 | 150篇 |
2018年 | 177篇 |
2017年 | 141篇 |
2016年 | 254篇 |
2015年 | 367篇 |
2014年 | 393篇 |
2013年 | 481篇 |
2012年 | 553篇 |
2011年 | 505篇 |
2010年 | 337篇 |
2009年 | 318篇 |
2008年 | 379篇 |
2007年 | 395篇 |
2006年 | 297篇 |
2005年 | 264篇 |
2004年 | 277篇 |
2003年 | 235篇 |
2002年 | 241篇 |
2001年 | 74篇 |
2000年 | 40篇 |
1999年 | 54篇 |
1998年 | 70篇 |
1997年 | 55篇 |
1996年 | 26篇 |
1995年 | 29篇 |
1994年 | 36篇 |
1993年 | 20篇 |
1992年 | 25篇 |
1991年 | 19篇 |
1990年 | 15篇 |
1989年 | 10篇 |
1988年 | 12篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 9篇 |
1982年 | 11篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1974年 | 3篇 |
1973年 | 4篇 |
排序方式: 共有6759条查询结果,搜索用时 15 毫秒
151.
152.
Interference between male and female functions within a monoecious plant may hinder crossing and decrease seed set. We assessed the probability of self-pollination and the effect of self-pollination on cross-pollination for two self-incompatible species: Nothofagus obliqua and N. nervosa. The probability of self-pollination was studied by tracking the phenologies of staminate and pistillate flowers, including an analysis of stigmatic receptivity. Pure and mixed pollinations were performed in order to evaluate the effect of self-pollination upon cross-pollination. Phenological observations suggest that self-pollination is highly likely in both species. Compared to pure cross-pollination, the application of self-pollination prior to cross-pollination resulted in lower numbers of germinated pollen grains for both species, and also in a lower production of viable seeds in N. nervosa. The low proportion of viable seeds often observed in natural populations of N. obliqua and N. nervosa may be related to self-pollination. 相似文献
153.
Vanina A. Campo Anne-Marie Patenaude Svenja Kaden Lori Horb Daniel Firka Josef Jiricny Javier M. Di Noia 《Nucleic acids research》2013,41(5):3032-3046
The mammalian antibody repertoire is shaped by somatic hypermutation (SHM) and class switch recombination (CSR) of the immunoglobulin (Ig) loci of B lymphocytes. SHM and CSR are triggered by non-canonical, error-prone processing of G/U mismatches generated by activation-induced deaminase (AID). In birds, AID does not trigger SHM, but it triggers Ig gene conversion (GC), a ‘homeologous’ recombination process involving the Ig variable region and proximal pseudogenes. Because recombination fidelity is controlled by the mismatch repair (MMR) system, we investigated whether MMR affects GC in the chicken B cell line DT40. We show here that Msh6−/− and Pms2−/− DT40 cells display cell cycle defects, including genomic re-replication. However, although IgVλ GC tracts in MMR-deficient cells were slightly longer than in normal cells, Ig GC frequency, donor choice or the number of mutations per sequence remained unaltered. The finding that the avian MMR system, unlike that of mammals, does not seem to contribute towards the processing of G/U mismatches in vitro could explain why MMR is unable to initiate Ig GC in this species, despite initiating SHM and CSR in mammalian cells. Moreover, as MMR does not counteract or govern Ig GC, we report a rare example of ‘homeologous’ recombination insensitive to MMR. 相似文献
154.
Carmen García-Ibarbia Jesús Delgado-Calle Iñigo Casafont Javier Velasco Jana Arozamena María I. Pérez-Núñez María A. Alonso María T. Berciano Fernando Ortiz José L. Pérez-Castrillón Agustín F. Fernández Mario F. Fraga María T. Zarrabeitia José A. Riancho 《Gene》2013
We reported previously that the expression of Wnt-related genes is lower in osteoporotic hip fractures than in osteoarthritis. We aimed to confirm those results by analyzing β-catenin levels and explored potential genetic and epigenetic mechanisms involved. 相似文献
155.
Guillermo Jiménez Mercedes Urdiain Ana Cifuentes Aránzazu López-López Anicet R. Blanch Javier Tamames Peter Kämpfer Anne-Brit Kolstø Daniel Ramón Juan F. Martínez Francisco M. Codoñer Ramon Rosselló-Móra 《Systematic and applied microbiology》2013
Strain BCT-7112T was isolated in 1966 in Japan from a survey designed to obtain naturally occurring microorganisms as pure cultures in the laboratory for use as probiotics in animal nutrition. This strain, which was primarily identified as Bacillus cereus var toyoi, has been in use for more than 30 years as the active ingredient of the preparation TOYOCERIN®, an additive for use in animal nutrition (e.g. swine, poultry, cattle, rabbits and aquaculture). Despite the fact that the strain was initially classified as B. cereus, it showed significant genomic differences from the type strains of the B. cereus group that were large enough (ANI values below 92%) to allow it to be considered as a different species within the group. The polyphasic taxonomic study presented here provides sufficient discriminative parameters to classify BCT-7112T as a new species for which the name Bacillus toyonensis sp. nov. is proposed, with BCT-7112T (=CECT 876T; =NCIMB 14858T) being designated as the type strain. In addition, a pairwise comparison between the available genomes of the whole B. cereus group by means of average nucleotide identity (ANI) calculations indicated that besides the eight classified species (including B. toyonensis), additional genomospecies could be detected, and most of them also had ANI values below 94%. ANI values were on the borderline of a species definition only in the cases of representatives of B. cereus versus B. thuringiensis, and B. mycoides and B. weihenstephanensis. 相似文献
156.
Sofía Fernández‐González Iván De la Hera Antón Pérez‐Rodríguez Javier Pérez‐Tris 《Oikos》2013,122(8):1227-1237
The diversity of symbionts (commensals, mutualists or parasites) that share the same host species may depend on opportunities and constraints on host exploitation associated with host phenotype or environment. Various host traits may differently influence host accessibility and within‐host population growth of each symbiont species, or they may determine the outcome of within‐host interactions among coexisting species. In turn, phenotypic diversity of a host species may promote divergent exploitation strategies among its symbiotic organisms. We studied the distribution of two feather mite species, Proctophyllodes sylviae and Trouessartia bifurcata, among blackcaps Sylvia atricapilla wintering in southern Spain during six winters. The host population included migratory and sedentary individuals, which were unequally distributed between two habitat types (forests and shrublands). Visual mite counts showed that both mite species often coexisted on sedentary blackcaps, but were seldom found together on migratory blackcaps. Regardless of host habitat, Proctophyllodes were highly abundant and Trouessartia were scarce on migratory blackcaps, but the abundance of both mite species converged in intermediate levels on sedentary blackcaps. Coexistence may come at a cost for Proctophyllodes, whose load decreased when Trouessartia was present on the host (the opposite was not true). Proctophyllodes load was positively correlated with host wing length (wings were longer in migratory blackcaps), while Trouessartia load was positively correlated to uropygial gland size (sedentary blackcaps had bigger glands), which might render migratory and sedentary blackcaps better hosts for Proctophyllodes and Trouessartia, respectively. Our results draw a complex scenario for mite co‐existence in the same host species, where different mite species apparently take advantage of, or are constrained by, divergent host phenotypic traits. This expands our understanding of bird–mite interactions, which are usually viewed as less dynamic in relation to variation in host phenotype, and emphasizes the role of host phenotypic divergence in the diversification of symbiotic organisms. 相似文献
157.
Dania O. Govea-Alonso Néstor Rubio-Infante Ana Lilia García-Hernández Javier T. Varona-Santos Schuyler S. Korban Leticia Moreno-Fierros Sergio Rosales-Mendoza 《Planta》2013,238(4):785-792
Elicitation of broad humoral immune responses is a critical factor in the development of effective HIV vaccines. In an effort to develop low-cost candidate vaccines based on multiepitopic recombinant proteins, this study has been undertaken to assess and characterize the immunogenic properties of a lettuce-derived C4(V3)6 multiepitopic protein. This protein consists of V3 loops corresponding to five different HIV isolates, including MN, IIIB, RF, CC, and RU. In this study, both Escherichia coli and lettuce-derived C4(V3)6 have elicited local and systemic immune responses when orally administered to BALB/c mice. More importantly, lettuce-derived C4(V3)6 has shown a higher immunogenic potential than that of E. coli-derived C4(V3)6. Moreover, when reactivity of sera from mice immunized with C4(V3)6 are compared with those elicited by a chimeric protein carrying a single V3 sequence, broader responses have been observed. The lettuce-derived C4(V3)6 has elicited antibodies with positive reactivity against V3 loops from isolates MN, RF, and CC. In addition, splenocyte proliferation assays indicate that significant T-helper responses are induced by the C4(V3)6 immunogen. Taken together, these findings account for the observed elicitation of broader humoral responses by the C4(V3)6 multiepitopic protein. Moreover, they provide further validation for the production of multiepitopic vaccines in plant cells as this serves not only as a low-cost expression system, but also as an effective delivery vehicle for orally administered immunogens. 相似文献
158.
Sphingolipids play an important role in signal transduction pathways that regulate physiological functions and stress responses in eukaryotes. In plants, recent evidence suggests that their metabolic precursors, the long-chain bases (LCBs) act as bioactive molecules in the immune response. Interestingly, the virulence of two unrelated necrotrophic fungi, Fusarium verticillioides and Alternaria alternata, which are pathogens of maize and tomato plants, respectively, depends on the production of sphinganine-analog mycotoxins (SAMs). These metabolites inhibit de novo synthesis of sphingolipids in their hosts causing accumulation of LCBs, which are key regulators of programmed cell death. Therefore, to gain more insight into the role of sphingolipids in plant immunity against SAM-producing necrotrophic fungi, we disrupted sphingolipid metabolism in Nicotiana benthamiana through virus-induced gene silencing (VIGS) of the serine palmitoyltransfersase (SPT). This enzyme catalyzes the first reaction in LCB synthesis. VIGS of SPT profoundly affected N. benthamiana development as well as LCB composition of sphingolipids. While total levels of phytosphingosine decreased, sphinganine and sphingosine levels increased in SPT-silenced plants, compared with control plants. Plant immunity was also affected as silenced plants accumulated salicylic acid (SA), constitutively expressed the SA-inducible NbPR-1 gene and showed increased susceptibility to the necrotroph A. alternata f. sp. lycopersici. In contrast, expression of NbPR-2 and NbPR-3 genes was delayed in silenced plants upon fungal infection. Our results strongly suggest that LCBs modulate the SA-dependent responses and provide a working model of the potential role of SAMs from necrotrophic fungi to disrupt the plant host response to foster colonization. 相似文献
159.
Estela D'Abril Ruíz-Leyja Rafael Villalobos-Molina Juan Javier López-Guerrero Itzell Alejandrina Gallardo-Ortíz Samuel Enoch Estrada-Soto Maximiliano Ibarra-Barajas 《Life sciences》2013
Aims
Hypertension is associated with the impairment of renal cyclooxygenase (COX) activity, which regulates vascular tone, salt and water balance and renin release. We aimed to evaluate the functional role of COX isoforms in kidneys isolated from spontaneously hypertensive rats (SHR) after α1-adrenoceptor (α1-AR) stimulation.Main methods
Male six-month-old SHR and normotensive Wistar-Kyoto rats (WKY) were used. The kidneys were isolated to measure perfusion pressure and COX-1- or COX-2-derived prostanoids in response to α1-AR activation.Key findings
The basal perfusion pressure was higher in SHR kidneys compared with WKY kidneys (95 ± 11 vs. 68 ± 6 mm Hg, P < 0.05). Phenylephrine induced a greater vasopressor response in SHR kidneys (EC50 of 1.89 ± 0.58 nmol) than WKY kidneys (EC50 of 3.30 ± 0.54 nmol, P < 0.05 vs. SHR). COX-1 inhibition decreased the α1-AR-induced vasoconstrictor response in WKY but did not affect SHR response, while COX-2 inhibition diminished the response in SHR. Both basal prostacyclin (PGI2) and thromboxane A2 (TxA2) values were higher in SHR kidney perfusates (P < 0.05) and were reduced by COX-1 and COX-2 inhibitors in both strains. Furthermore, phenylephrine increased PGI2 through COX-2 in WKY and through COX-1 in SHR, but the agonist did not significantly modify TxA2 in both strains.Significance
The data suggest that COX-1contributes to vasoconstrictor effects in WKY kidneys and that COX-2 has the same effect in SHR kidneys. The results also suggest that basal release of COX-2-derived vasoconstrictor prostanoids is involved in renal vascular hypersensitivity in SHR. 相似文献160.
Javier Echevarría 《Lethaia: An International Journal of Palaeontology and Stratigraphy》2013,46(4):518-539
Trigoniide clams were conspicuous components of bivalve faunas in shallow seas during most of the Mesozoic. Morphological trends in different features of shell shape and ornamentation of three related species of myophorelloids from the Agrio Formation (Early Cretaceous of Neuquén Basin, Argentina) that inhabited environments of different hydrodynamic energy conditions are explored in this study by means of a detailed morphometric analysis. Four hydrodynamic energy categories were established for depositional environments, from most (1), to least (4), energetic. General shell shape was characterized by measurements of height, length and width. New quantitative methods were developed to evaluate the distribution of costae along the marginal carina and the divergence of costae along the flank. Morphometric variables were detrended with respect to size, to avoid ontogenetic variability in further analyses. Relative width of the shell increases as environments become higher in hydrodynamic energy; this unusual trend may be interpreted as an anchoring/stabilizing strategy developed by the lineage in shallow marine environments. Trends in ornamentation correspond to more densely distributed costae in environments of higher hydrodynamic energy, a pattern that improves substrate penetration. 相似文献