全文获取类型
收费全文 | 10596篇 |
免费 | 1573篇 |
国内免费 | 1549篇 |
专业分类
13718篇 |
出版年
2024年 | 63篇 |
2023年 | 344篇 |
2022年 | 316篇 |
2021年 | 413篇 |
2020年 | 509篇 |
2019年 | 555篇 |
2018年 | 482篇 |
2017年 | 529篇 |
2016年 | 533篇 |
2015年 | 512篇 |
2014年 | 603篇 |
2013年 | 761篇 |
2012年 | 472篇 |
2011年 | 492篇 |
2010年 | 462篇 |
2009年 | 591篇 |
2008年 | 571篇 |
2007年 | 662篇 |
2006年 | 522篇 |
2005年 | 500篇 |
2004年 | 395篇 |
2003年 | 355篇 |
2002年 | 330篇 |
2001年 | 250篇 |
2000年 | 217篇 |
1999年 | 193篇 |
1998年 | 184篇 |
1997年 | 152篇 |
1996年 | 165篇 |
1995年 | 142篇 |
1994年 | 126篇 |
1993年 | 140篇 |
1992年 | 120篇 |
1991年 | 92篇 |
1990年 | 73篇 |
1989年 | 84篇 |
1988年 | 64篇 |
1987年 | 49篇 |
1986年 | 67篇 |
1985年 | 88篇 |
1984年 | 85篇 |
1983年 | 67篇 |
1982年 | 89篇 |
1981年 | 77篇 |
1980年 | 61篇 |
1979年 | 56篇 |
1978年 | 24篇 |
1977年 | 16篇 |
1975年 | 12篇 |
1973年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Genetic analysis of sterile mutants in the dpy-5 unc-13 (I) genomic region of Caenorhabditis elegans
Essential genes were identified in the 1.5-map unit dpy-5 unc-13 region of chromosome I in the Caenorhabditis elegans genome by rescuing lethal mutations using the duplication sDp2. In this paper, we report the mapping and complementation testing of lethal mutations, 45 of which identify 18 new, essential
genes. This analysis brings the number of essential genes defined by the sDp2 rescue of lethal mutants to 97; 64 of these map between dpy-5 and unc-13. 61% of these essential genes are identified by more than one allele. Positioning of the mutations was done using the breakpoints
of six duplications. The mutant phenotypes of 14 loci essential for fertility were characterized by Nomarski microscopy and
DAPI staining. None of the mutants were rescued by wild-type male sperm. The cytological data showed that four genes produced
mutants with defects in gonadogenesis, let-395, let-603, let-605 and let-610. Mutations in seven genes, let-355, let-367, let-384, let-513, let-544, let-545 and let-606, affected germ cell proliferation or gametogenesis. Mutants for the remaining three genes, let-370, let-599 and let-604, produced eggs that failed to develop or hatch, thereby acting as maternal effect lethals. We observed a nonrandom distribution
of arrest phenotypes with regard to map position.
Received: 8 May 1996 / Accepted : 27 January 1997 相似文献
2.
《Journal of molecular biology》2021,433(15):167097
DNA glycosylases remove damaged or modified nucleobases by cleaving the N-glycosyl bond and the correct nucleotide is restored through subsequent base excision repair. In addition to excising threatening lesions, DNA glycosylases contribute to epigenetic regulation by mediating DNA demethylation and perform other important functions. However, the catalytic mechanism remains poorly defined for many glycosylases, including MBD4 (methyl-CpG binding domain IV), a member of the helix-hairpin-helix (HhH) superfamily. MBD4 excises thymine from G·T mispairs, suppressing mutations caused by deamination of 5-methylcytosine, and it removes uracil and modified uracils (e.g., 5-hydroxymethyluracil) mispaired with guanine. To investigate the mechanism of MBD4 we solved high-resolution structures of enzyme-DNA complexes at three stages of catalysis. Using a non-cleavable substrate analog, 2′-deoxy-pseudouridine, we determined the first structure of an enzyme-substrate complex for wild-type MBD4, which confirms interactions that mediate lesion recognition and suggests that a catalytic Asp, highly conserved in HhH enzymes, binds the putative nucleophilic water molecule and stabilizes the transition state. Observation that mutating the Asp (to Gly) reduces activity by 2700-fold indicates an important role in catalysis, but probably not one as the nucleophile in a double-displacement reaction, as previously suggested. Consistent with direct-displacement hydrolysis, a structure of the enzyme-product complex indicates a reaction leading to inversion of configuration. A structure with DNA containing 1-azadeoxyribose models a potential oxacarbenium-ion intermediate and suggests the Asp could facilitate migration of the electrophile towards the nucleophilic water. Finally, the structures provide detailed snapshots of the HhH motif, informing how these ubiquitous metal-binding elements mediate DNA binding. 相似文献
3.
4.
J. Kelley Bentley Peter W. Reed 《Biochimica et Biophysica Acta (BBA)/General Subjects》1981,678(2):238-244
All of the common cytochalasins activate superoxide anion release and exocytosis of β-N-acetylglucosaminidase and lysozyme from guinea-pig polymorphonuclear leukocytes (neutrophils) incubated in a buffered sucrose medium. Half-maximal activation of both processes is produced by approx. 2 μM cytochalasin A, C >μM cytochalasin B ? 4–5 μM cytochalasin D, E. While maximal rates of O2? release and extents of exocytosis require extracellular calcium (1–2 mM), replacing sucrose with monovalent cation chlorides is inhibitory to neutrophil activation by cytochalasins. Na+, K+ or choline inhibited either cytochalasin B- or E-stimulated O2? production with IC50 values of 5–10 mM and inhibition occurs whether Cl?, NO3? or SCN? is the anion added with Na+ or K+. Release of β-N-acetylglucosaminidase in control or cytochalasin B-stimulated cells is inhibited by NaCl (IC50 ≈ 10 mM), while cytochalasin E-stimulated exocytosis is reduced less and K+ or choline chloride are ineffective in inhibiting either cytochalasin B- or E-stimulated exocytosis. Release of β-glucuronidase, myeloperoxidase or acid phosphatase from neutrophils incubated in buffered sucrose is not stimulated by cytochalasin B. Stimulation of either O2? or β-N-acetylglucosaminidase release by low concentrations of cytochalasin A is followed by inhibition of each at higher concentrations. It appears that all cytochalasins can activate both NAD(P)H oxidase and selective degranulation of neutrophils incubated in salt-restricted media and that differential inhibition of these two processes by monovalent cations and/or anions is produced at some step(s) subsequent to cytochalasin interaction with the cell. 相似文献
5.
FRANÇOIS MUNOZ THIERRY PAILLER INGRID KOTTKE CÉDRIC GONNEAU MARC‐ANDRÉ SELOSSE 《Molecular ecology》2012,21(20):5098-5109
Characterizing the architecture of bipartite networks is increasingly used as a framework to study biotic interactions within their ecological context and to assess the extent to which evolutionary constraint shape them. Orchid mycorrhizal symbioses are particularly interesting as they are viewed as more beneficial for plants than for fungi, a situation expected to result in an asymmetry of biological constraint. This study addressed the architecture and phylogenetic constraint in these associations in tropical context. We identified a bipartite network including 73 orchid species and 95 taxonomic units of mycorrhizal fungi across the natural habitats of Reunion Island. Unlike some recent evidence for nestedness in mycorrhizal symbioses, we found a highly modular architecture that largely reflected an ecological barrier between epiphytic and terrestrial subnetworks. By testing for phylogenetic signal, the overall signal was stronger for both partners in the epiphytic subnetwork. Moreover, in the subnetwork of epiphytic angraecoid orchids, the signal in orchid phylogeny was stronger than the signal in fungal phylogeny. Epiphytic associations are therefore more conservative and may co‐evolve more than terrestrial ones. We suggest that such tighter phylogenetic specialization may have been driven by stressful life conditions in the epiphytic niches. In addition to paralleling recent insights into mycorrhizal networks, this study furthermore provides support for epiphytism as a major factor affecting ecological assemblage and evolutionary constraint in tropical mycorrhizal symbioses. 相似文献
6.
Peter Shaw 《Restoration Ecology》2009,17(1):68-77
Plots of fresh pulverized fuel ash (PFA, an industrial waste) were inoculated with soils from existing PFA sites and fertilizers in a factorial design, then left unmanaged for 12 years during which time the floral development and soil chemistry were monitored annually. For the first 3 years, the site supported a sparse mix of chenopods (including the scarce Chenopodium glaucum ) and halophytes. As salinity declined, ruderals, legumes, and grasses plus the fire-site moss Funaria hygrometrica colonized, followed by Festuca arundinacea grassland (NVC community MG12) and Hippophae rhamnoides scrub. Dactylorhiza incarnata (orchidacea) appeared after 7 years, but only in plots that had received soil from existing orchid colonies. Four years later, a larger second generation of Dactylorhiza appeared, but only in the central zone of the site where vegetation was thinnest. By year 12, the site was dominated by coarse grasses and scrub, with early successional species persisting only in the sparsely vegetated center, where nitrate levels were lowest. This edge effect is interpreted as centripetal encroachment, a process of potentially wider concern for the conservation of low-fertility habitat patches. Overall, seed bank inoculation seems to have introduced few but desirable species ( D. incarnata , Pyrola rotundifolia , some halophytes, and annuals), whereas initial application of organic fertilizer had long-lasting (≥10 years) effects on cover and soil composition. 相似文献
7.
Big, beautiful organisms are useful for biological education, increasing evolution literacy, and biodiversity conservation. But if educators gloss over the ubiquity of streamlined and miniaturized organisms, they unwittingly leave students and the public vulnerable to the idea that the primary evolutionary plot of every metazoan lineage is “progressive” and "favors" complexity. We show that simple, small, and intriguingly repulsive invertebrate animals provide a counterpoint to misconceptions about evolution. Our examples can be immediately deployed in biology courses and outreach. This context emphasizes that chordates are not the pinnacle of evolution. Rather, in the evolution of animals, miniaturization, trait loss, and lack of perfection are at least as frequent as their opposites. Teaching about invertebrate animals in a “tree thinking” context uproots evolution misconceptions (for students and the public alike), provides a mental scaffold for understanding all animals, and helps to cultivate future ambassadors and experts on these little‐known, weird, and fascinating taxa. 相似文献
8.
Bruno Jirgensons 《Journal of Protein Chemistry》1982,1(1):71-84
The factors determining the onset and extent of reconstructive denaturation of proteins were considered by comparing circular dichroism (CD) data of seven proteins and previously published findings. The effects of sodium dodecyl sulfate (SDS) on the conformation of the following proteins were tested: lysozyme, the mitogens fromPhytolacca americana (fractions Pa2 and Pa4), lectin fromWistaria floribunda, ovine lutropin, a Bence Jones protein, and histone H2B. While the helix content of lysozyme was raised by SDS slightly, in the Bence Jones protein andW. floribunda lectin it increased from near zero to about 25–30%. In histone H2B the helix content was raised by SDS even to about 48%. However, no clear indication of helix formation could be observed in the mitogens and lutropin, even at low pH or 2.0–2.5. The tertiary structure of the proteins was perturbed by SDS. It was concluded that the reorganization of secondary structure of the proteins was favored by the following factors: (1) presence of helicogenic amino acid sequences in the protein, (2) availability of positively charged sites of the basic amino acids for interactions with the dodecyl ion, (3) absence of a large surplus of negatively charged sites on the surface of protein, and (4) absence of extensive disulfide cross-linking within the macromolecule. Both hydrophobic and electrostatic interactions occur in reconstructive denaturation, and the newly formed helices are stabilized by hydrophobic shielding by the alkyl chains of the alkyl sulfate. 相似文献
9.
Jerome Goddard 《Journal of vector ecology》2009,34(2):238-242
Rickettsia parkeri, a member of the spotted fever group rickettsias, was first described in 1939 and was thought to be non‐pathogenic until recently, when it was found to cause a spotted fever‐like illness in humans and areas of necrosis (eschars) at the sites of tick bites. Accordingly, there is currently much interest in this emerging pathogen. In this study, all published articles concerning R. parkeri were reviewed and analyzed for evidence of relatedness among this agent and other spotted fever group (SFG) rickettsiae which also produce similar clinical syndromes and/or eschars, including R. conorii, R. africae, and R. sibirica. A synthesis of the historical (antigenic) and recent (molecular) data supporting a phylogenetic sub‐grouping of these SFG organisms is presented and comments are offered about the taxonomy of rickettsial organisms in general, and R. parkeri in particular. 相似文献
10.
1. Larval success was compared when one, two, or three egg clutches were laid in kumquat fruits (≈ 10 ml in volume) either successively on the same day or at the rate of one clutch per day. 2. Increased clutch density was associated with a significant decrease in larval survival rate and non‐significant decreases in larval growth rate and pupal mass. 3. Larval and pupal parameters showed significantly larger variance when clutches were laid on successive days than on the same day, suggesting a competitive advantage for older larvae over younger larvae. 4. The results suggest that, in small fruit, reduced fitness due to larval competition may act against possible fitness benefits due to social facilitation among adult females, hence reducing the likelihood of non‐linear population dynamics caused by processes such as the Allee effect. 相似文献