全文获取类型
收费全文 | 714篇 |
免费 | 35篇 |
专业分类
749篇 |
出版年
2023年 | 3篇 |
2022年 | 3篇 |
2021年 | 6篇 |
2020年 | 3篇 |
2019年 | 5篇 |
2018年 | 3篇 |
2017年 | 5篇 |
2016年 | 18篇 |
2015年 | 30篇 |
2014年 | 20篇 |
2013年 | 22篇 |
2012年 | 36篇 |
2011年 | 44篇 |
2010年 | 25篇 |
2009年 | 17篇 |
2008年 | 40篇 |
2007年 | 34篇 |
2006年 | 37篇 |
2005年 | 36篇 |
2004年 | 45篇 |
2003年 | 41篇 |
2002年 | 38篇 |
2001年 | 10篇 |
2000年 | 11篇 |
1999年 | 10篇 |
1998年 | 13篇 |
1997年 | 8篇 |
1996年 | 9篇 |
1995年 | 18篇 |
1994年 | 8篇 |
1993年 | 8篇 |
1992年 | 16篇 |
1991年 | 6篇 |
1990年 | 14篇 |
1989年 | 9篇 |
1988年 | 4篇 |
1987年 | 8篇 |
1985年 | 3篇 |
1984年 | 11篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1978年 | 3篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1975年 | 7篇 |
1974年 | 5篇 |
1973年 | 3篇 |
1972年 | 4篇 |
排序方式: 共有749条查询结果,搜索用时 0 毫秒
51.
Sexually dimorphic mate selection strategies were examined in 200 university students reporting their willingness to engage in casual sexual encounters with hypothetical individuals of the opposite sex. Using a questionnaire format, the possibility of forming a long-term relationship was manipulated, while risk of disease, pregnancy, and detection was eliminated across all conditions. In addition, potential partners varied in level of attractiveness, and in personality and behavioral characteristics. As expected, men reported a greater anticipated willingness to engage in sexual intercourse across all conditions compared with women. The possibility of forming a long-term relationship elevated women’s, but not men’s, willingness for sexual intercourse. While a potential partner’s attractiveness had a significant positive overall effect on responses, reducing their relative attractiveness had a greater negative impact on men’s responses. Reference to the parental qualities of a potential partner significantly increased women’s, but not men’s, anticipated willingness for sexual intercourse. Describing a hypothetical partner as non-aggressive (safe) marginally increased women’s willingness (p<.09) and did not affect men’s responses. The wording of items relevant to this condition may have resulted in the potential partner sounding "wimpy" rather than nonaggressive, and this may have reduced the expected effect of this manipulation. The possibility that women may trade off personality and behavioral characteristics with attractiveness to a greater degree than men when assessing potential mates is considered. 相似文献
52.
Marwan Osman Dima El Safadi Sadia Benamrouz Karine Guyot Eduardo Dei-Cas El Moukhtar Aliouat Colette Creusy Hassan Mallat Monzer Hamze Fouad Dabboussi Eric Viscogliosi Gabriela Certad 《PloS one》2015,10(5)
Cryptosporidium spp. represent a major public health problem worldwide and infect the gastrointestinal tract of both immunocompetent and immunocompromised persons. The prevalence of these parasites varies by geographic region, and no data are currently available in Lebanon. To promote an understanding of the epidemiology of cryptosporidiosisin this country, the main aim of this study was to determine the prevalence Cryptosporidium in symptomatic hospitalized patients, and to analyze the genetic diversity of the corresponding isolates. Fecal specimens were collected in four hospitals in North Lebanon from 163 patients (77 males and 86 females, ranging in age from 1 to 88 years, with a mean age of 22 years) presenting gastrointestinal disorders during the period July to December 2013. The overall prevalence of Cryptosporidium spp. infection obtained by modified Ziehl-Neelsen staining and/or nested PCR was 11%, and children <5 years old showed a higher rate of Cryptosporidium spp. The PCR products of the 15 positive samples were successfully sequenced. Among them, 10 isolates (66.7%) were identified as C. hominis, while the remaining 5 (33.3%) were identified as C. parvum. After analysis of the gp60 locus, C. hominis IdA19, a rare subtype, was found to be predominant. Two C. parvum subtypes were found: IIaA15G1R1 and IIaA15G2R1. The molecular characterization of Cryptosporidium isolates is an important step in improving our understanding of the epidemiology and transmission of the infection. 相似文献
53.
Joy Perrier Patrice Clochon Fran?oise Bertran Colette Couque Jan Bulla Pierre Denise Marie-Laure Bocca 《PloS one》2015,10(1)
Objective
To assess the specific prefrontal activity in comparison to those in the other main cortical areas in primary insomnia patients and in good sleepers.Methods
Fourteen primary insomnia patients and 11 good sleepers were included in the analysis. Participants completed one night of polysomnography in the sleep lab. Power spectra were calculated during the NREM (Non-rapid eyes movements) and the REM (Rapid eyes movements) sleep periods at prefrontal, occipital, temporal and central electrode positions.Results
During the NREM sleep, the power spectra did not differ between groups in the prefrontal cortex; while primary insomnia patients exhibited a higher beta power spectrum and a lower delta power spectrum compared to good sleepers in other areas. During the REM sleep, the beta1 power spectrum was lower in the prefrontal cortex in primary insomnia patients compared to good sleepers; while no significant difference between groups was obtained for the other areas.Conclusions
The present study shows a specific prefrontal sleep pattern during the whole sleep period. In addition, we suggest that primary insomnia patients displayed a dysfunction in the reactivation of the limbic system during the REM sleep and we give additional arguments in favor of a sleep-protection mechanism displayed by primary insomnia patients. 相似文献54.
55.
Ariella Goldblatt Zachary Kwena Maureen Lahiff Kawango Agot Alexandra Minnis Ndola Prata Jessica Lin Elizabeth A. Bukusi Colette L. Auerswald 《PloS one》2015,10(10)
Introduction
Despite their perceived vulnerability to HIV, East African street youth have been neglected in HIV prevention research. We examined HIV seroprevalence and correlates of HIV infection in a sample of male street youth in Kisumu, Kenya.Methods
We enrolled a street-recruited sample of 13–21 year old street youth. Participants completed a survey followed by voluntary HIV counseling and testing. Survey items included demographics, homelessness history, survival activities, sexual behavior and substance use. We examined the relationship between predictor variables, markers of coercion and marginalization and HIV.Results
The sample included 296 males. Survival activities included garbage picking (55%), helping market vendors (55%), begging (17%), and working as porters (46%) or domestic workers (4%). Forty-nine percent of participants reported at least weekly use of alcohol and 32% marijuana. Forty-six percent of participants reported lifetime inhalation of glue and 8% fuel. Seventy-nine percent of participants reported lifetime vaginal sex, 6% reported lifetime insertive anal sex and 8% reported lifetime receptive anal sex. Twelve (4.1%; 95% CI: 2.3–7.0) participants tested positive for HIV. Of those, all had been on the street for at least one year and all had engaged in vaginal sex. Occupations placing youth at particular risk of coercion by adults, including helping market vendors (prevalence ratio (PR) = 8.8; 95% CI: 1.2–67.5) and working as domestic workers (PR = 4.6; 95% CI: 1.1–19.0), were associated with HIV infection. Both insertive anal sex (PR = 10.2; 95% CI: 3.6–29.4) and receptive anal sex (PR = 3.9; 95% CI: 1.1–13.4) were associated with HIV infection. Drug use, begging, and garbage picking were not associated with HIV infection.Conclusions
Although HIV prevalence in our sample of street youth is comparable to that of similarly-aged male youth in Nyanza Province, our findings highlight behavioral factors associated with HIV infection that offer opportunities for targeted prevention among street youth in East Africa. 相似文献56.
Michael J. Lawes Diana O. Fisher Chris N. Johnson Simon P. Blomberg Anke S. K. Frank Susanne A. Fritz Hamish McCallum Jeremy VanDerWal Brett N. Abbott Sarah Legge Mike Letnic Colette R. Thomas Nikki Thurgate Alaric Fisher Iain J. Gordon Alex Kutt 《PloS one》2015,10(6)
Australia has experienced dramatic declines and extinctions of its native rodent species over the last 200 years, particularly in southern Australia. In the tropical savanna of northern Australia significant declines have occurred only in recent decades. The later onset of these declines suggests that the causes may differ from earlier declines in the south. We examine potential regional effects (northern versus southern Australia) on biological and ecological correlates of range decline in Australian rodents. We demonstrate that rodent declines have been greater in the south than in the tropical north, are strongly influenced by phylogeny, and are consistently greater for species inhabiting relatively open or sparsely vegetated habitat. Unlike in marsupials, where some species have much larger body size than rodents, body mass was not an important predictor of decline in rodents. All Australian rodent species are within the prey-size range of cats (throughout the continent) and red foxes (in the south). Contrary to the hypothesis that mammal declines are related directly to ecosystem productivity (annual rainfall), our results are consistent with the hypothesis that disturbances such as fire and grazing, which occur in non-rainforest habitats and remove cover used by rodents for shelter, nesting and foraging, increase predation risk. We agree with calls to introduce conservation management that limits the size and intensity of fires, increases fire patchiness and reduces grazing impacts at ecological scales appropriate for rodents. Controlling feral predators, even creating predator-free reserves in relatively sparsely-vegetated habitats, is urgently required to ensure the survival of rodent species, particularly in northern Australia where declines are not yet as severe as those in the south. 相似文献
57.
58.
Fabien Labroussaa Nathalie Arricau-Bouvery Marie-Pierre Dubrana Colette Saillard 《Applied and environmental microbiology》2010,76(6):1879-1886
Transmission of the phytopathogenic mollicutes, spiroplasmas, and phytoplasmas by their insect vectors mainly depends on their ability to pass through gut cells, to multiply in various tissues, and to traverse the salivary gland cells. The passage of these different barriers suggests molecular interactions between the plant mollicute and the insect vector that regulate transmission. In the present study, we focused on the interaction between Spiroplasma citri and its leafhopper vector, Circulifer haematoceps. An in vitro protein overlay assay identified five significant binding activities between S. citri proteins and insect host proteins from salivary glands. One insect protein involved in one binding activity was identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS) as actin. Confocal microscopy observations of infected salivary glands revealed that spiroplasmas colocated with the host actin filaments. An S. citri actin-binding protein of 44 kDa was isolated by affinity chromatography and identified by LC-MS/MS as phosphoglycerate kinase (PGK). To investigate the role of the PGK-actin interaction, we performed competitive binding and internalization assays on leafhopper cultured cell lines (Ciha-1) in which His6-tagged PGK from S. citri or purified PGK from Saccharomyces cerevisiae was added prior to the addition of S. citri inoculum. The results suggested that exogenous PGK has no effect on spiroplasmal attachment to leafhopper cell surfaces but inhibits S. citri internalization, demonstrating that the process leading to internalization of S. citri in eukaryotic cells requires the presence of PGK. PGK, regardless of origin, reduced the entry of spiroplasmas into Ciha-1 cells in a dose-dependent manner.Phloem-feeding leafhoppers transmit plant pathogenic mollicutes, spiroplasmas, and phytoplasmas from plant to plant in a persistent propagative manner (26, 43). These phytopathogenic mollicutes are restricted to phloem and to certain vector tissues; thus, their vectors are phloem sap-feeding specialists. After being ingested from plant phloem by their insect vectors, they traverse the insect gut wall, move into the hemolymph, where they multiply, and invade the salivary glands (20, 33, 34, 36). During their movements in the insect vector until its transmission to a new host plant, spiroplasmas and phytoplasmas must traverse two major physical barriers, namely, the insect intestine and the salivary gland (35, 53). Until now, little was known about the molecular and cellular interactions contributing to the crossing of these physical barriers. Several lines of evidence suggest that host-pathogen interactions could be a prerequisite for invasion and colonization of insect vector organs (2, 48, 53). For human and animal pathogenic mollicutes, it is well established that successful colonization of the host cells requires adhesion as the first step. This event is mediated by surface proteins, and among these proteins adhesins play an important role (8, 44). Recently, it was reported that an antigenic membrane protein (Amp) of onion yellow phytoplasma interacts with the insect microfilament complex and that interaction plays an important role in determining the insect vector specificity (48). Several other immunodominant membrane proteins from various phytoplasmas have been mentioned in the literature as candidates for involvement in host-phytoplasma interactions (29, 30).Spiroplasma citri, the first phytopathogenic mollicute available in culture (45), has emerged as an outstanding model for studying spiroplasma interactions with its two hosts: the periwinkle plant and the insect vector Circulifer haematoceps. Following observations of membrane-bound cytoplasmic vesicles of midgut epithelium and salivary gland cells, S. citri was hypothesized to cross these physical barriers by receptor-mediated cell endocytosis (3, 33, 39). Several S. citri protein candidates have been identified as involved in transmission and, for a few of them, in an interaction with leafhopper vector proteins. Spiralin, the most abundant membrane protein, was suspected to be involved in the transmission for two reasons: (i) a S. citri spiralinless mutant was less effective in its transmissibility (19); (ii) spiralin acted in vitro as a lectin able to bind to glycoproteins of insect vectors and therefore might function as a ligand able to interact with leafhopper receptors (32). In addition, the ability of S. citri to be transmitted by C. haematoceps is clearly affected by disruption of a gene predicted to encode a lipoprotein with homology to a solute-binding protein of an ABC transporter (14). The proteome of nontransmissible S. citri strains specifically lacks adhesion-related proteins (ScARPs) and the membrane-associated protein P32 present in the proteome of transmissible strains (12, 13, 31). These proteins are encoded by plasmids pSci1 to -6 (46), which are present only in transmissible strains, and ScARPs share strong similarities with the adhesion-related protein SARP1 of S. citri strain BR3, in which the presence has been correlated to the ability for the spiroplasma to adhere to insect cells in vitro (9, 55). The specific interactions of S. citri with eukaryotic cells remain to be elucidated, but a combination of the effects of several proteins or a complex would be necessary to explain the invasion of a variety of host cell types by S. citri (33).Nevertheless, in the last sequence of events involved in insect vector transmission, the first contact and recognition for the efficient penetration of the salivary gland cells represents an essential step. In the present study, confocal images of infected salivary glands show the localization of S. citri cells along the actin filaments. We report the results of the first attempt to decipher the role of the spiroplasma''s phosphoglycerate kinase (PGK) in the internalization of S. citri in its insect vector''s cells. 相似文献
59.
Svenja Beckmann Christin Buro Colette Dissous J?rg Hirzmann Christoph G. Grevelding 《PLoS pathogens》2010,6(2)
The signal transduction protein SmTK4 from Schistosoma mansoni belongs to the family of Syk kinases. In vertebrates, Syk kinases are known to play specialized roles in signaling pathways in cells of the hematopoietic system. Although Syk kinases were identified in some invertebrates, their role in this group of animals has not yet been elucidated. Since SmTK4 is the first Syk kinase from a parasitic helminth, shown to be predominantly expressed in the testes and ovary of adult worms, we investigated its function. To unravel signaling cascades in which SmTK4 is involved, yeast two-/three-hybrid library screenings were performed with either the tandem SH2-domain, or with the linker region including the tyrosine kinase domain of SmTK4. Besides the Src kinase SmTK3 we identified a new Src kinase (SmTK6) acting upstream of SmTK4 and a MAPK-activating protein, as well as mapmodulin acting downstream. Their identities and colocalization studies pointed to a role of SmTK4 in a signaling cascade regulating the proliferation and/or differentiation of cells in the gonads of schistosomes. To confirm this decisive role we performed biochemical and molecular approaches to knock down SmTK4 combined with a novel protocol for confocal laser scanning microscopy for morphological analyses. Using the Syk kinase-specific inhibitor Piceatannol or by RNAi treatment of adult schistosomes in vitro, corresponding phenotypes were detected in the testes and ovary. In the Xenopus oocyte system it was finally confirmed that Piceatannol suppressed the activity of the catalytic kinase domain of SmTK4. Our findings demonstrate a pivotal role of SmTK4 in gametogenesis, a new function for Syk kinases in eukaryotes. 相似文献
60.
Sauvage E Duez C Herman R Kerff F Petrella S Anderson JW Adediran SA Pratt RF Frère JM Charlier P 《Journal of molecular biology》2007,371(2):528-539
The genome of Bacillus subtilis encodes 16 penicillin-binding proteins (PBPs) involved in the synthesis and/or remodelling of the peptidoglycan during the complex life cycle of this sporulating Gram-positive rod-shaped bacterium. PBP4a (encoded by the dacC gene) is a low-molecular mass PBP clearly exhibiting in vitro DD-carboxypeptidase activity. We have solved the crystal structure of this protein alone and in complex with a peptide (D-alpha-aminopymelyl-epsilon-D-alanyl-D-alanine) that mimics the C-terminal end of the Bacillus peptidoglycan stem peptide. PBP4a is composed of three domains: the penicillin-binding domain with a fold similar to the class A beta-lactamase structure and two domains inserted between the conserved motifs 1 and 2 characteristic of the penicillin-recognizing enzymes. The soaking of PBP4a in a solution of D-alpha-aminopymelyl-epsilon-D-alanyl-D-alanine resulted in an adduct between PBP4a and a D-alpha-aminopimelyl-epsilon-D-alanine dipeptide and an unbound D-alanine, i.e. the products of acylation of PBP4a by D-alpha-aminopymelyl-epsilon-D-alanyl-D-alanine with the release of a D-alanine. The adduct also reveals a binding pocket specific to the diaminopimelic acid, the third residue of the peptidoglycan stem pentapeptide of B. subtilis. This pocket is specific for this class of PBPs. 相似文献