全文获取类型
收费全文 | 5021篇 |
免费 | 370篇 |
专业分类
5391篇 |
出版年
2023年 | 16篇 |
2022年 | 38篇 |
2021年 | 93篇 |
2020年 | 41篇 |
2019年 | 56篇 |
2018年 | 78篇 |
2017年 | 62篇 |
2016年 | 135篇 |
2015年 | 249篇 |
2014年 | 260篇 |
2013年 | 319篇 |
2012年 | 372篇 |
2011年 | 340篇 |
2010年 | 233篇 |
2009年 | 195篇 |
2008年 | 279篇 |
2007年 | 274篇 |
2006年 | 252篇 |
2005年 | 267篇 |
2004年 | 263篇 |
2003年 | 219篇 |
2002年 | 202篇 |
2001年 | 50篇 |
2000年 | 38篇 |
1999年 | 49篇 |
1998年 | 67篇 |
1997年 | 54篇 |
1996年 | 49篇 |
1995年 | 56篇 |
1994年 | 53篇 |
1993年 | 47篇 |
1992年 | 33篇 |
1991年 | 36篇 |
1990年 | 26篇 |
1989年 | 30篇 |
1988年 | 35篇 |
1987年 | 32篇 |
1986年 | 25篇 |
1985年 | 24篇 |
1984年 | 30篇 |
1983年 | 21篇 |
1982年 | 17篇 |
1981年 | 24篇 |
1980年 | 19篇 |
1979年 | 25篇 |
1977年 | 28篇 |
1976年 | 20篇 |
1971年 | 14篇 |
1959年 | 14篇 |
1954年 | 11篇 |
排序方式: 共有5391条查询结果,搜索用时 15 毫秒
61.
62.
Kevin Karbstein Elisabeth Rahmsdorf Salvatore Tomasello Ladislav Hoda
Elvira Hrandl 《Ecology and evolution》2020,10(24):14435
The larger distribution area of asexuals compared with their sexual relatives in geographical parthenogenesis (GP) scenarios has been widely attributed to the advantages of uniparental reproduction and polyploidy. However, potential disadvantages of sexuals due to their breeding system have received little attention so far. Here, we study the breeding system of five narrowly distributed sexual lineages of Ranunculus notabilis s.l. (R. auricomus complex) and its effects on outcrossing, inbreeding, female fitness, and heterozygosity. We performed selfing and intra‐ and interlineage crossings by bagging 481 flowers (59 garden individuals) followed by germination experiments. We compared seed set and germination rates, and related them to genetic distance and genome‐wide heterozygosity (thousands of RADseq loci). Selfings (2.5%) unveiled a significantly lower seed set compared with intra‐ (69.0%) and interlineage crossings (69.5%). Seed set of intra‐ (65%) compared to interpopulation crossings (78%) was significantly lower. In contrast, all treatments showed comparable germination rates (32%–43%). Generalized linear regressions between seed set and genetic distance revealed positive relationships in general and between lineages, and a negative one within lineages. Seed set was the main decisive factor for female fitness. Germination rates were not related to genetic distance at any level, but were positively associated with heterozygosity in interlineage crossings. Experiments confirmed full crossability and predominant outcrossing among sexual R. notabilis s.l. lineages. However, up to 5% (outliers 15%–31%) of seeds were formed by selfing, probably due to semi‐self‐compatibility in a multi‐locus gametophytic SI system. Less seed set in intrapopulation crossings, and higher seed set and germination rates from crossings of genetically more distant and heterozygous lineages (interlineage) indicate negative inbreeding and positive outbreeding effects. In GP scenarios, sexual species with small and/or isolated populations can suffer from decreased female fitness due to their breeding system. This factor, among others, probably limits range expansion of sexuals. 相似文献
63.
Long-term observational studies in a number of animal species suggest that exchange patterns of social acts depend on long-term
emotional bonds. Therefore, it is expected that the frequency of prosocial behavior will depend on the strength of such a
bond. In this study we tested whether variation in relationship quality among unrelated individuals, i.e., “friends” and “nonfriends,”
is predictive of the prosocial behavior of long-tailed macaques in two experiments. First, we related relationship quality
to prosociality in a dyadic prosociality test, and second, we gave subjects the choice to give to either a friend or a nonfriend
in a triadic choice test. We show that prosocial behavior of long-tailed macaques in the dyadic test is not related to relationship
quality. When given the choice to give to either a friend or a nonfriend in the triadic test, there is a minor indication
that long-tailed macaques show a preference to give to their friends, yet this indication is neither significant nor consistent.
In contrast, subordinate long-tailed macaques make a more “competitive” choice and avoid giving to the individual closest
in rank. Therefore, in the short-term situation of experimental tests, prosocial behavior of long-tailed macaques seems unaffected
by the relationship quality of the dyad/triad tested, and the relative dominance position of these dyads/triads seems to have
a much stronger effect on their prosocial behavior. 相似文献
64.
Laminin alpha5 is necessary for submandibular gland epithelial morphogenesis and influences FGFR expression through beta1 integrin signaling 总被引:3,自引:0,他引:3
Rebustini IT Patel VN Stewart JS Layvey A Georges-Labouesse E Miner JH Hoffman MP 《Developmental biology》2007,308(1):15-29
Laminin alpha chains have unique spatiotemporal expression patterns during development and defining their function is necessary to understand the regulation of epithelial morphogenesis. We investigated the function of laminin alpha5 in mouse submandibular glands (SMGs). Lama5(-/-) SMGs have a striking phenotype: epithelial clefting is delayed, although proliferation occurs; there is decreased FGFR1b and FGFR2b, but no difference in Lama1 expression; later in development, epithelial cell organization and lumen formation are disrupted. In wild-type SMGs alpha5 and alpha1 are present in epithelial clefts but as branching begins alpha5 expression increases while alpha1 decreases. Lama5 siRNA decreased branching, p42 MAPK phosphorylation, and FGFR expression, and branching was rescued by FGF10. FGFR siRNA decreased Lama5 suggesting that FGFR signaling provides positive feedback for Lama5 expression. Anti-beta1 integrin antibodies decreased FGFR and Lama5 expression, suggesting that beta1 integrin signaling provides positive feedback for Lama5 and FGFR expression. Interestingly, the Itga3(-/-):Itga6(-/-) SMGs have a similar phenotype to Lama5(-/-). Our findings suggest that laminin alpha5 controls SMG epithelial morphogenesis through beta1 integrin signaling by regulating FGFR expression, which also reciprocally regulates the expression of Lama5. These data link changes in basement membrane composition during branching morphogenesis with FGFR expression and signaling. 相似文献
65.
Proteomic and functional analysis of Argonaute-containing mRNA-protein complexes in human cells 总被引:5,自引:0,他引:5 下载免费PDF全文
Höck J Weinmann L Ender C Rüdel S Kremmer E Raabe M Urlaub H Meister G 《EMBO reports》2007,8(11):1052-1060
Members of the Argonaute (Ago) protein family associate with small RNAs and have important roles in RNA silencing. Here, we analysed Ago1- and Ago2-containing protein complexes in human cells. Separation of Ago-associated messenger ribonucleoproteins (mRNPs) showed that Ago1 and Ago2 reside in three complexes with distinct Dicer and RNA-induced silencing complex activities. A comprehensive proteomic analysis of Ago-containing mRNPs identified a large number of proteins involved in RNA metabolism. By using co-immunoprecipitation experiments followed by RNase treatment, we biochemically mapped interactions within Ago mRNPs. Using reporter assays and knockdown experiments, we showed that the putative RNA-binding protein RBM4 is required for microRNA-guided gene regulation. 相似文献
66.
Budde BS Binner P Waldmüller S Höhne W Blankenfeldt W Hassfeld S Brömsen J Dermintzoglou A Wieczorek M May E Kirst E Selignow C Rackebrandt K Müller M Goody RS Vosberg HP Nürnberg P Scheffold T 《PloS one》2007,2(12):e1362
Noncompaction of the ventricular myocardium (NVM) is the morphological hallmark of a rare familial or sporadic unclassified heart disease of heterogeneous origin. NVM results presumably from a congenital developmental error and has been traced back to single point mutations in various genes. The objective of this study was to determine the underlying genetic defect in a large German family suffering from NVM. Twenty four family members were clinically assessed using advanced imaging techniques. For molecular characterization, a genome-wide linkage analysis was undertaken and the disease locus was mapped to chromosome 14ptel-14q12. Subsequently, two genes of the disease interval, MYH6 and MYH7 (encoding the alpha- and beta-myosin heavy chain, respectively) were sequenced, leading to the identification of a previously unknown de novo missense mutation, c.842G>C, in the gene MYH7. The mutation affects a highly conserved amino acid in the myosin subfragment-1 (R281T). In silico simulations suggest that the mutation R281T prevents the formation of a salt bridge between residues R281 and D325, thereby destabilizing the myosin head. The mutation was exclusively present in morphologically affected family members. A few members of the family displayed NVM in combination with other heart defects, such as dislocation of the tricuspid valve (Ebstein's anomaly, EA) and atrial septal defect (ASD). A high degree of clinical variability was observed, ranging from the absence of symptoms in childhood to cardiac death in the third decade of life. The data presented in this report provide first evidence that a mutation in a sarcomeric protein can cause noncompaction of the ventricular myocardium. 相似文献
67.
Polycyclic aromatic hydrocarbons (PAHs) are a large class of organic chemicals typically found as mixtures in the aquatic environment from natural, petrogenic, and pyrogenic sources. People can be exposed to PAHs through ingestion or dermal contact with contaminated sediments or through ingestion of finfish and shellfish exposed to contaminated sediments. Although more than 100 PAHs have been identified, human exposure and risk are commonly evaluated for 18 individual PAHs. Other PAHs, such as alkylated PAHs, likely contribute to biological activity of environmental PAH mixtures; however, insufficient toxicity data are available to quantify their potential risk. This article presents an initial evaluation of the potential for human health risk from exposure to alkylated PAHs in sediment and fish. Individual alkylated PAHs have been observed to have potentially mutagenic, tumor-promoting, or carcinogenic activity. However, except for 1-and 2-methylnaphthalene, insufficient toxicity data are available to quantify toxicity or cancer risk from exposure to individual alkylated PAHs or mixtures of alkylated PAHs. This article describes a proposed strategy to better understand the potential human health risk from exposure to alkylated PAHs. Implementation of this strategy will contribute to evaluations of human exposure to complex PAH mixtures in the environment. 相似文献
68.
Effects of Moxonidine and Low‐Calorie Diet: Cardiometabolic Benefits from Combination of Both Therapies 下载免费PDF全文
69.
Sánchez-Soriano N Bottenberg W Fiala A Haessler U Kerassoviti A Knust E Löhr R Prokop A 《Developmental biology》2005,288(1):126-138
Dendrites represent arborising neurites in both vertebrates and invertebrates. However, in vertebrates, dendrites develop on neuronal cell bodies, whereas in higher invertebrates, they arise from very different neuronal structures, the primary neurites, which also form the axons. Is this anatomical difference paralleled by principal developmental and/or physiological differences? We address this question by focussing on one cellular model, motorneurons of Drosophila and characterise the compartmentalisation of these cells. We find that motorneuronal dendrites of Drosophila share with typical vertebrate dendrites that they lack presynaptic but harbour postsynaptic proteins, display calcium elevation upon excitation, have distinct cytoskeletal features, develop later than axons and are preceded by restricted localisation of Par6-complex proteins. Furthermore, we demonstrate in situ and culture that Drosophila dendrites can be shifted from the primary neurite to their soma, i.e. into vertebrate-like positions. Integrating these different lines of argumentation, we propose that dendrites in vertebrates and higher invertebrates have a common origin, and differences in dendrite location can be explained through translocation of neuronal cell bodies introduced during the evolutionary process by which arthropods and vertebrates diverged from a common urbilaterian ancestor. Implications of these findings for studies of dendrite development, neuronal polarity, transport and evolution are discussed. 相似文献
70.
Marshall JC Arévalo E Benavides E Sites JL Sites JW 《Evolution; international journal of organic evolution》2006,60(5):1050-1065
Species form the fundamental units of analysis in many areas of biology and, therefore, rigorous delimitation of this unit is important to a broad array of researchers. Recently, many new empirical methods have been proposed to delimit species in nature, and a large literature exists on the theoretical merit and superiority of each method. However, few empirical studies actually compare the results of these methods applied in the same study system. We used a large allozyme and chromosome dataset to apply a number of genetic-distance, character-based, and tree-based methods to a well-studied, data-rich system: the Sceloporus grammicus lizard complex of central Mexico. We hypothesized species boundaries under a general lineage or evolutionary species conceptual framework in an a priori fashion using mapped restriction-site data (mitochondrial DNA and nuclear rDNA), allozymes, and morphology. We then compared the ability of different methods to recover the "hypothesized evolutionary species" (HES). Highton's genetic-distance method and a tree-based method consistently recovered all four HES, although sometimes with weak support. With two exceptions, other methods recovered the same HES, but additional groups were weakly delimited and nested within the HES. Given the apparent recent divergence of some of the chromosome races and distinct populations in this complex, these are encouraging results. We emphasize the value of specifying testable criteria as clearly as possible and testing these with methods that make use of different properties of a single dataset. 相似文献