全文获取类型
收费全文 | 1252篇 |
免费 | 108篇 |
国内免费 | 81篇 |
专业分类
1441篇 |
出版年
2024年 | 2篇 |
2023年 | 14篇 |
2022年 | 42篇 |
2021年 | 65篇 |
2020年 | 28篇 |
2019年 | 50篇 |
2018年 | 55篇 |
2017年 | 29篇 |
2016年 | 47篇 |
2015年 | 71篇 |
2014年 | 97篇 |
2013年 | 94篇 |
2012年 | 83篇 |
2011年 | 96篇 |
2010年 | 64篇 |
2009年 | 73篇 |
2008年 | 68篇 |
2007年 | 48篇 |
2006年 | 59篇 |
2005年 | 46篇 |
2004年 | 29篇 |
2003年 | 36篇 |
2002年 | 38篇 |
2001年 | 25篇 |
2000年 | 23篇 |
1999年 | 26篇 |
1998年 | 12篇 |
1997年 | 8篇 |
1996年 | 7篇 |
1995年 | 9篇 |
1994年 | 4篇 |
1993年 | 8篇 |
1992年 | 11篇 |
1991年 | 7篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1981年 | 2篇 |
1977年 | 2篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1969年 | 2篇 |
1964年 | 2篇 |
1960年 | 1篇 |
排序方式: 共有1441条查询结果,搜索用时 15 毫秒
971.
Aprivesa unimaculatasp. n. (Hemiptera: Fulgoromorpha: Ricaniidae) is described and illustrated from Coorg, south India. This represents the first record of the genus Aprivesa Melichar from India and the fourth known species of Aprivesa. The new taxon greatly extends the range of the genus Aprivesa, which was previously known as an endemic Australian genus. A checklist of all known species of the Ricaniidae from India and keys to all the known genera of the Ricaniidae from India and all species in the genus are provided. 相似文献
972.
973.
974.
Twenty-one novel polymorphic microsatellite loci were isolated from the Chinese soft-shelled turtle, Pelodiscus sinensis (Trionychidae). This is a commonly consumed Asian species, especially in China, where it is reared in large numbers on farms. We screened 34 unrelated individuals from Wuhu of Anhui province in China and detected high levels of polymorphism for all 21 loci, with the number of alleles/locus ranging from 5 to 23 (mean 15.67). The expected and observed heterozygosities ranged from 0.421 to 0.946 and from 0.324 to 0.941, respectively. Hence, these microsatellites could facilitate studies on genetic diversity and population structure and marker-assisted breeding of this vulnerable species. 相似文献
975.
Isolation and biochemical characterization of two lipases from a metagenomic library of China Holstein cow rumen 总被引:1,自引:0,他引:1
Kailang Liu Dengpan Bu Chris McSweeney Dan Li 《Biochemical and biophysical research communications》2009,385(4):605-611
Two novel lipase genes RlipE1 and RlipE2 which encoded 361- and 265-amino acid peptides, respectively, were recovered from a metagenomic library of the rumen microbiota of Chinese Holstein cows. A BLAST search revealed a high similarity (90%) between RlipE2 and a carboxylesterase from Thermosinus carboxydivorans Nor1, while there was a low similarity (below 50%) between RlipE1 and other lipases. Phylogenetic analysis indicated that RlipE2 clustered with the lipolytic enzymes from family V while RlipE1 clustered with six other putative bacterial lipases which might constitute a new subfamily. The recombinant lipases were thermally unstable and retained 60% activity over a pH range of 6.5-8.5. Substrate specificity assay indicated that both enzymes had higher hydrolytic activity toward laurate (C12), palmitate (C16) and stearate (C18). The novel phylogenetic affiliation and high specificity of both enzymes for long-chain fatty acid make them interesting targets for manipulation of rumen lipid metabolism. 相似文献
976.
Uniform Action Potential Repolarization within the Sarcolemma of In Situ Ventricular Cardiomyocytes 总被引:1,自引:0,他引:1
Previous studies have speculated, based on indirect evidence, that the action potential at the transverse (t)-tubules is longer than at the surface membrane in mammalian ventricular cardiomyocytes. To date, no technique has enabled recording of electrical activity selectively at the t-tubules to directly examine this hypothesis. We used confocal line-scan imaging in conjunction with the fast response voltage-sensitive dyes ANNINE-6 and ANNINE-6plus to resolve action potential-related changes in fractional dye fluorescence (ΔF/F) at the t-tubule and surface membranes of in situ mouse ventricular cardiomyocytes. Peak ΔF/F during action potential phase 0 depolarization averaged −21% for both dyes. The shape and time course of optical action potentials measured with the water-soluble ANNINE-6plus were indistinguishable from those of action potentials recorded with intracellular microelectrodes in the absence of the dye. In contrast, optical action potentials measured with the water-insoluble ANNINE-6 were significantly prolonged compared to the electrical recordings obtained from dye-free hearts, suggesting electrophysiological effects of ANNINE-6 and/or its solvents. With either dye, the kinetics of action potential-dependent changes in ΔF/F during repolarization were found to be similar at the t-tubular and surface membranes. This study provides what to our knowledge are the first direct measurements of t-tubule electrical activity in ventricular cardiomyocytes, which support the concept that action potential duration is uniform throughout the sarcolemma of individual cells. 相似文献
977.
Scaffolding proteins are molecular switches that control diverse signaling events. The scaffolding protein Na+/H+ exchanger regulatory factor 1 (NHERF1) assembles macromolecular signaling complexes and regulates the macromolecular assembly, localization, and intracellular trafficking of a number of membrane ion transport proteins, receptors, and adhesion/antiadhesion proteins. NHERF1 begins with two modular protein-protein interaction domains—PDZ1 and PDZ2—and ends with a C-terminal (CT) domain. This CT domain binds to ezrin, which, in turn, interacts with cytosekeletal actin. Remarkably, ezrin binding to NHERF1 increases the binding capabilities of both PDZ domains. Here, we use deuterium labeling and contrast variation neutron-scattering experiments to determine the conformational changes in NHERF1 when it forms a complex with ezrin. Upon binding to ezrin, NHERF1 undergoes significant conformational changes in the region linking PDZ2 and its CT ezrin-binding domain, as well as in the region linking PDZ1 and PDZ2, involving very long range interactions over 120 Å. The results provide a structural explanation, at mesoscopic scales, of the allosteric control of NHERF1 by ezrin as it assembles protein complexes. Because of the essential roles of NHERF1 and ezrin in intracellular trafficking in epithelial cells, we hypothesize that this long-range allosteric regulation of NHERF1 by ezrin enables the membrane-cytoskeleton to assemble protein complexes that control cross-talk and regulate the strength and duration of signaling. 相似文献
978.
Rebecca A. Chanoux Bu Yin Karen A. Urtishak Amma Asare Craig H. Bassing Eric J. Brown 《The Journal of biological chemistry》2009,284(9):5994-6003
Chromosomal abnormalities are frequently caused by problems encountered
during DNA replication. Although the ATR-Chk1 pathway has previously been
implicated in preventing the collapse of stalled replication forks into
double-strand breaks (DSB), the importance of the response to fork collapse in
ATR-deficient cells has not been well characterized. Herein, we demonstrate
that, upon stalled replication, ATR deficiency leads to the phosphorylation of
H2AX by ATM and DNA-PKcs and to the focal accumulation of Rad51, a marker of
homologous recombination and fork restart. Because H2AX has been shown to play
a facilitative role in homologous recombination, we hypothesized that H2AX
participates in Rad51-mediated suppression of DSBs generated in the absence of
ATR. Consistent with this model, increased Rad51 focal accumulation in
ATR-deficient cells is largely dependent on H2AX, and dual deficiencies in ATR
and H2AX lead to synergistic increases in chromatid breaks and translocations.
Importantly, the ATM and DNA-PK phosphorylation site on H2AX
(Ser139) is required for genome stabilization in the absence of
ATR; therefore, phosphorylation of H2AX by ATM and DNA-PKcs plays a pivotal
role in suppressing DSBs during DNA synthesis in instances of ATR pathway
failure. These results imply that ATR-dependent fork stabilization and
H2AX/ATM/DNA-PKcs-dependent restart pathways cooperatively suppress
double-strand breaks as a layered response network when replication
stalls.Genome maintenance prevents mutations that lead to cancer and age-related
diseases. A major challenge in preserving genome integrity occurs in the
simple act of DNA replication, in which failures at numerous levels can occur.
Besides the mis-incorporation of nucleotides, it is during this phase of the
cell cycle that the relatively stable double-stranded nature of DNA is
temporarily suspended at the replication fork, a structure that is susceptible
to collapse into
DSBs.2 Replication
fork stability is maintained by a variety of mechanisms, including activation
of the ATR-dependent checkpoint pathway.The ATR pathway is activated upon the generation and recognition of
extended stretches of single-stranded DNA at stalled replication forks
(1-4).
Genome maintenance functions for ATR and orthologs in yeast were first
indicated by increased chromatid breaks in ATR-/- cultured cells
(5) and by the
“cut” phenotype observed in Mec1 (Saccharomyces
cerevisiae) and Rad3 (Schizosaccharomyces pombe) mutants
(6-9).
Importantly, subsequent studies in S. cerevisiae demonstrated that
mutation of Mec1 or the downstream checkpoint kinase Rad53 led to increased
chromosome breaks at regions of the genome that are inherently difficult to
replicate (10), and a
decreased ability to reinitiate replication fork progression following DNA
damage or deoxyribonucleotide depletion
(11-14).In vertebrates, similar replication fork stabilizing functions have been
demonstrated for ATR and the downstream protein kinase Chk1
(15-20).
Several possible mechanisms have been put forward to explain how ATR-Chk1 and
orthologous pathways in yeast maintain replication fork stability, including
maintenance of replicative polymerases (α, δ, and ε) at forks
(17,
21), regulation of branch
migrating helicases, such as Blm
(22-25),
and regulation of homologous recombination, either positively or negatively
(26-29).Consistent with the role of the ATR-dependent checkpoint in replication
fork stability, common fragile sites, located in late-replicating regions of
the genome, are significantly more unstable (5-10-fold) in the absence of ATR
or Chk1 (19,
20). Because these sites are
favored regions of instability in oncogene-transformed cells and preneoplastic
lesions (30,
31), it is possible that the
increased tumor incidence observed in ATR haploinsufficient mice
(5,
32) may be related to subtle
increases in genomic instability. Together, these studies indicate that
maintenance of replication fork stability may contribute to tumor
suppression.It is important to note that prevention of fork collapse represents an
early response to problems occurring during DNA replication. In the event of
fork collapse into DSBs, homologous recombination (HR) has also been
demonstrated to play a key role in genome stability during S phase by
catalyzing recombination between sister chromatids as a means to re-establish
replication forks (33).
Importantly, a facilitator of homologous recombination, H2AX, has been shown
to be phosphorylated under conditions that cause replication fork collapse
(18,
34).Phosphorylation of H2AX occurs predominantly upon DSB formation
(34-38)
and has been reported to require ATM, DNA-PKcs, or ATR, depending on the
context
(37-42).
Although H2AX is not essential for HR, studies have demonstrated that H2AX
mutation leads to deficiencies in HR
(43,
44), and suppresses events
associated with homologous recombination, such as the focal accumulation of
Rad51, BRCA1, BRCA2, ubiquitinated-FANCD2, and Ubc13-mediated chromatin
ubiquitination (43,
45-51).
Therefore, through its contribution to HR, it is possible that H2AX plays an
important role in replication fork stability as part of a salvage pathway to
reinitiate replication following collapse.If ATR prevents the collapse of stalled replication forks into DSBs, and
H2AX facilitates HR-mediated restart, the combined deficiency in ATR and H2AX
would be expected to dramatically enhance the accumulation of DSBs upon
replication fork stalling. Herein, we utilize both partial and complete
elimination of ATR and H2AX to demonstrate that these genes work cooperatively
in non-redundant pathways to suppress DSBs during S phase. As discussed, these
studies imply that the various components of replication fork protection and
regeneration cooperate to maintain replication fork stability. Given the large
number of genes involved in each of these processes, it is possible that
combined deficiencies in these pathways may be relatively frequent in humans
and may synergistically influence the onset of age-related diseases and
cancer. 相似文献
979.
Brain amyloid-β (Aβ) peptide accumulation and aggregation are critical events in the pathogenesis of Alzheimer disease. Increasing evidence has demonstrated that LRP1 is involved in Alzheimer disease pathogenesis. The physiological ligands of LRP1, including apoE, play significant roles in the cellular clearance of Aβ. The receptor-associated protein (RAP) is a specialized chaperone for members of the low density lipoprotein receptor family. RAP shares structural and receptor-binding properties with apoE. Here, we show that RAP binds to both Aβ40 and Aβ42 in a concentration-dependent manner and forms complexes with them. Fluorescence-activated cell sorter analysis showed that RAP significantly enhances the cellular internalization of Aβ in different cell types, including brain vascular smooth muscle, neuroblastoma, glioblastoma, and Chinese hamster ovary cells. This effect of RAP was confirmed by fluorescence microscopy and enzyme-linked immunosorbent assay. RAP binds to both LRP1 and heparin; however, the ability of RAP to enhance Aβ cellular uptake was blocked by heparin and heparinase treatment but not by LRP1 deficiency. Furthermore, the effects of RAP were significantly decreased in heparan sulfate proteoglycan-deficient Chinese hamster ovary cells. Our findings reveal that RAP is a novel Aβ-binding protein that promotes cellular internalization of Aβ. 相似文献
980.