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121.
Seedling regeneration on forest floors is often impaired by competition with established plants. In some lowland temperate
rain forests, tree fern trunks provide safe sites on which tree species establish, and grow large enough to take root in the
ground and persist. Here we explore the competitive and facilitative effects of two tree fern species, Cyathea
smithii and Dicksonia
squarrosa, on the epiphytic regeneration of tree species in nutrient-rich alluvial forests in New Zealand. The difficulties that seedlings
have in establishing on vertical tree fern trunks were indicated by the following observations. First, seedling abundance
was greatest on the oldest sections of tree fern trunks, near the base, suggesting that trunks gradually recruited more and
more seedlings over time, but many sections of trunk were devoid of seedlings, indicating the difficulty of establishment
on a vertical surface. Second, most seedlings were from small-seeded species, presumably because smaller seeds can easily
lodge on tree fern trunks. Deer browsing damage was observed on 73% of epiphytic seedlings growing within 2 m of the ground,
whereas few seedlings above that height were browsed. This suggests that tree ferns provide refugia from introduced deer,
and may slow the decline in population size of deer-preferred species. We reasoned that tree ferns would compete with epiphytic
seedlings for light, because below the tree fern canopy photosynthetically active radiation (PAR) was about 1% of above-canopy
PAR. Frond removal almost tripled %PAR on the forest floor, leading to a significant increase in the height growth rate (HGR)
of seedlings planted on the forest floor, but having no effects on the HGRs of epiphytic seedlings. Our study shows evidence
of direct facilitative interactions by tree ferns during seedling establishment in plant communities associated with nutrient-rich
soils. 相似文献
122.
Ruiz-Herrera A Robinson TJ 《BioEssays : news and reviews in molecular, cellular and developmental biology》2008,30(11-12):1126-1137
In this review, we focus on the evolutionary and biomedical aspects of the architecture of human chromosome 3 (HSA3) by analyzing chromosomal regions that have been conserved during the evolutionary process, compared to those that have been involved in the genomic restructuring of different placental lineages. Given that the organization of human chromosome 3 is derived when compared to the ancestral primate karyotype, and is an autosome that is commonly implicated in human tumour formation, we examined the patterns of change and the genomic consequences that have resulted from its complex evolutionary history. The data show four discrete chromosomal regions that are frequently implicated in chromosomal rearrangements (3p25, 3p22, 3p12 and 3q21). These are rich in repetitive elements and are commonly implicated in structural rearrangements that underpin human genomic disorders and neoplasias. Additional Supporting Information may be found in the online version of this article. 相似文献
123.
Recent experimental advances have shown that enzymes are flexible molecules, and point to a direct link between dynamics and
catalysis. Movements span a wide time range, from nano- to milli-seconds. In this paper we introduce two aspects of enzyme
flexibility that are treated with two appropriate techniques. First, transition path sampling is used to obtain an unbiased
picture of the transition state ensemble in chorismate mutase, as well as its local flexibility and the energy flow during
the chemical step. Second, we consider the binding and release of substrates in L-rhamnulose-1-phosphate aldolase. We have calculated the normal modes of the enzyme with the elastic network model. The lowest
frequency modes generate active site deformations that change the coordination number of the catalytic zinc ion. The coordination
lability of zinc allows the binding and release of substrates. Substitution of zinc by magnesium blocks the exchange of ligands.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
124.
Challenges to human embryonic stem cell patents 总被引:1,自引:0,他引:1
The patenting of human embryonic stem (hES) cells has produced one of the most unusual and fraught situations in the history of science, ethics, and law. This Commentary examines legal and moral challenges to three foundational patents held by the Wisconsin Alumni Research Foundation (WARF). We conclude that, in the United States, technical challenges may, paradoxically, produce a stronger patent position for WARF. In the European Union, moral challenges mean confusion for member states. We demonstrate that hES cell intellectual property will be guided and bound by a welter of moral, technical, and legal inputs, with discrete national and jurisdictional dimensions. 相似文献
125.
Sotillo E Garriga J Kurimchak A Graña X 《The Journal of biological chemistry》2008,283(17):11280-11292
Cyclin E overexpression is observed in multiple human tumors and linked to poor prognosis. We have previously shown that ectopic expression of cyclin E is sufficient to induce mitogen-independent cell cycle entry in a variety of tumor/immortal cell lines. Here we have investigated the rate-limiting step leading to cell cycle entry in quiescent normal human fibroblasts (NHF) ectopically expressing cyclin E. We found that in serum-starved NHF, cyclin E forms inactive complexes with CDK2 and fails to induce DNA synthesis. Coexpression of SV40 small t antigen (st), but not other tested oncogenes, efficiently induces mitogen-independent CDK2 phosphorylation on Thr-160, CDK2 activation, and DNA synthesis. Additionally, in contact-inhibited NHF ectopically expressing cyclin E, st induces cell cycle entry, continued proliferation, and foci formation. Coexpression of cyclin E and st also bypasses G(0)/G(1) arrests induced by CDK inhibitors. Although CDK2 is dispensable for G(0)/G(1) cell cycle entry and normal proliferation in mammals, CDK2 activity is an essential rate-limiting step in NHF with deregulated cyclin E expression and altered PP2A activity, which endows primary cells with transformed features. Consequently, CDK2 could be targeted therapeutically in tumors that involve these alterations. These data also suggest that alterations prior to cyclin E deregulation facilitate proliferation of tumor cells by bypassing mitogenic requirements and negative regulation by adjacent cells. 相似文献
126.
Navarro A Ordóñez C Martínez E Pérez C Astudillo A Tolivia J 《Histology and histopathology》2008,23(8):995-1001
Apolipoprotein D (apo D), a lipocalin transporter of small hydrophobic molecules could play an important role in several neurodegenerative diseases. However, its role in those diseases remains unclear. There has been reported increments of apo D in relation with different neuropathologic diseases. Recently, we reported the absence of apo D in neurons of substantia nigra which can contribute to the lability of neurons to oxidative damage. In order to determine the relationship between apo D expression and neuronal death, we studied the expression of apo D in various regions of human brains from patients without any neurological or psychological disorders, in relation with the neuronal damage revealed by Fluoro-Jade B staining. The absence of expression for apo D in injured neurons and the negative staining for Fluoro-Jade B of neurons that express apo D was observed in all sections studied. These findings are in accordance with the role possibly played by apo D in the neuroprotection of the nervous system. 相似文献
127.
A new method for the identification of Enterococcus species has been developed. It combines PCR amplification of sodA gene and 16S-23S intergenic spacer region with restriction enzyme digestion followed by a melting curve analysis of the restriction fragments (MCARF). All strains analyzed were correctly identified by MCARF. This method was proved to be a reliable enterococcal identification tool. 相似文献
128.
129.
Aurora M. Nedelcu 《Journal of molecular evolution》2001,53(6):670-679
This study provides a phylogenetic/comparative approach to deciphering the processes underlying the evolution of plastid
rRNA genes in genomes under relaxed functional constraints. Nonphotosynthetic green algal taxa that belong to two distinct
classes, Chlorophyceae (Polytoma) and Trebouxiophyceae (Prototheca), were investigated. Similar to the situation described previously for plastid 16S rRNA genes in nonphotosynthetic land plants,
nucleotide substitution levels, extent of structural variations, and percentage AT values are increased in nonphotosynthetic
green algae compared to their closest photosynthetic relatives. However, the mutational processes appear to be different in
many respects. First, with the increase in AT content, more transversions are noted in Polytoma and holoparasite angiosperms, while more transitions characterize the evolution of the 16S rDNA sequences in Prototheca. Second, although structural variations do accumulate in both Polytoma and Prototheca (as well as holoparasitic plastid 16S rRNAs), insertions as large as 1.6 kb characterize the plastid 16S rRNA genes in the
former, whereas significantly smaller indels (not exceeding 24 bp) seem to be more prevalent in the latter group. The differences
in evolutionary rates and patterns within and between lineages might be due to mutations in replication/repair-related genes;
slipped-strand mispairing is likely the mechanism responsible for the expansion of insertions in Polytoma plastid 16S rRNA genes.
Received: 29 December 2000 / Accepted: 18 May 2001 相似文献
130.
Miguel Angel Pujana Monica Gratacós Jordi Corral Isabel Banchs Aurora Sánchez David Genís Carlos Cervera Víctor Volpini X. Estivill 《Human genetics》1997,101(1):18-21
Genetic anticipation – increasing severity and a decrease in the age of onset with successive generations of a pedigree –
is clearly present in autosomal dominant cerebellar ataxia (ADCA). Anticipation is correlated with expansion of the CAG/CTG
repeat sequence to sizes above those in the normal range through the generations of a pedigree. Genetic heterogeneity has
been demonstrated for ADCA, with four cloned genes (SCA1, SCA2, SCA3/MJD, and SCA6) and three mapped loci (SCA4, SCA5 and
SCA7). Another related dominant ataxia, dentatorubral-pallidoluysian atrophy (DRPLA), presents anticipation with CAG/CTG repeat
expansions. We had previously analysed ADCA patients who had not shown repeat expansions in cloned genes for CAG/CTG repeat
expansions by the repeat expansion detection method (RED) and had detected expansions of between 48 and 88 units in 17 unrelated
familial cases. We present here an analysis of 13 genes and expressed sequence tags (ESTs) containing 10 or more CAG/ CTG
repeat sequences selected from public databases in the 17 unrelated ADCA patients. Of the 13 selected genes and ESTs, 9 were
found to be polymorphic with heterozygosities ranging between 0.09 and 0.80 and 2 to 17 alleles. In ADCA patients none of
the loci showed expansions above the normal range of the CAG/CTG repeat sequences, excluding them as the mutation causing
ADCA.
Received: 28 May 1997 / Accepted: 30 June 1997 相似文献