全文获取类型
收费全文 | 882篇 |
免费 | 114篇 |
专业分类
996篇 |
出版年
2022年 | 5篇 |
2021年 | 7篇 |
2020年 | 8篇 |
2019年 | 6篇 |
2018年 | 12篇 |
2017年 | 14篇 |
2016年 | 16篇 |
2015年 | 30篇 |
2014年 | 39篇 |
2013年 | 30篇 |
2012年 | 50篇 |
2011年 | 38篇 |
2010年 | 25篇 |
2009年 | 35篇 |
2008年 | 40篇 |
2007年 | 43篇 |
2006年 | 58篇 |
2005年 | 33篇 |
2004年 | 35篇 |
2003年 | 32篇 |
2002年 | 31篇 |
2001年 | 43篇 |
2000年 | 38篇 |
1999年 | 35篇 |
1998年 | 25篇 |
1997年 | 9篇 |
1996年 | 13篇 |
1995年 | 11篇 |
1994年 | 12篇 |
1993年 | 11篇 |
1992年 | 20篇 |
1991年 | 21篇 |
1990年 | 16篇 |
1989年 | 14篇 |
1988年 | 9篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1985年 | 7篇 |
1984年 | 11篇 |
1983年 | 7篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 8篇 |
1978年 | 8篇 |
1977年 | 11篇 |
1976年 | 5篇 |
1975年 | 6篇 |
1973年 | 5篇 |
1968年 | 4篇 |
排序方式: 共有996条查询结果,搜索用时 0 毫秒
61.
Linder JU Hoffmann T Kurz U Schultz JE 《The Journal of biological chemistry》2000,275(15):11235-11240
Paramecium has a 280-kDa guanylyl cyclase. The N terminus resembles a P-type ATPase, and the C terminus is a guanylyl cyclase with the membrane topology of canonical mammalian adenylyl cyclases, yet with the cytosolic loops, C1 and C2, inverted compared with the mammalian order. We expressed in Escherichia coli the cytoplasmic domains of the protozoan guanylyl cyclase, independently and linked by a peptide, as soluble proteins. The His(6)-tagged proteins were enriched by affinity chromatography and analyzed by immunoblotting. Guanylyl cyclase activity was reconstituted upon mixing of the recombinant C1a- and C2-positioned domains and in a linked C1a-C2 construct. Adenylyl cyclase activity was minimal. The nucleotide substrate specificity was switched from GTP to ATP upon mutation of the substrate defining amino acids Glu(1681) and Ser(1748) in the C1-positioned domain to the adenylyl cyclase specific amino acids Lys and Asp. Using the C2 domains of mammalian adenylyl cyclases type II or IX and the C2-positioned domain from the Paramecium guanylyl cyclase we reconstituted a soluble, all C2 adenylyl cyclase. All enzymes containing protozoan domains were not affected by Galpha(s)/GTP or forskolin, and P site inhibitors were only slightly effective. 相似文献
62.
Viktorsson K Heiden T Molin M Akusjärvi G Linder S Shoshan MC 《Apoptosis : an international journal on programmed cell death》2000,5(4):355-367
Mutationally activated Ras is involved in tumor progression and likely also in drug resistance. Using survival, viability and apoptosis assays, we have here compared the cisplatin sensitivities of FR3T3 rat fibroblasts and a 12V-H-ras transformed subline (Ras2:3). Around 24 h after cisplatin treatment Ras2:3 cells showed higher apoptosis levels and lower viability than FR3T3. This increased sensitivity correlated with weaker cisplatin-induced activation of Jun N-terminal kinase (JNK). In contrast to apoptosis assays, colony formation assays showed that Ras2:3 were more resistant to cisplatin than were FR3T3. This was partly due to the increased cisplatin sensitivity of FR3T3 seeded at low densities, as required in colony formation assays. In addition, Ras2:3 cisplatin survivors had a higher relative proliferative capacity. Cell cycle analyses showed that FR3T3 cells initially responded with a dose-dependent G2 arrest, while Ras2:3 accumulated in S-phase. Experiments with an anti-apoptotic mutant of MEKK1 suggested that the apoptotic response of Ras2:3 cells is not specific to the S-phase fraction. In summary, the cisplatin response of ras-transformed fibroblasts is distinct from that of parental cells, in that they show increased apoptosis, a different cell cycle response and increased post-treatment proliferative capacity. The results illustrate the need to carefully consider methods and protocols for in vitro studies on chemotherapy sensitivity. 相似文献
63.
RGS4 binds to membranes through an amphipathic alpha -helix 总被引:1,自引:0,他引:1
Bernstein LS Grillo AA Loranger SS Linder ME 《The Journal of biological chemistry》2000,275(24):18520-18526
RGS4, a mammalian GTPase-activating protein for G protein alpha subunits, requires its N-terminal 33 amino acids for plasma membrane localization and biological activity (Srinivasa, S. P., Bernstein, L. S., Blumer, K. J., and Linder, M. E. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 5584-5589). In this study, we tested the hypothesis that the N-terminal domain mediates membrane binding by forming an amphipathic alpha-helix. RGS4 bound to liposomes containing anionic phospholipids in a manner dependent on the first 33 amino acids. Circular dichroism spectroscopy of a peptide corresponding to amino acids 1-31 of RGS4 revealed that the peptide adopted an alpha-helical conformation in the presence of anionic phospholipids. Point mutations that either neutralized positive charges on the hydrophilic face or substituted polar residues on the hydrophobic face of the model helix disrupted plasma membrane targeting and biological activity of RGS4 expressed in yeast. Recombinant mutant proteins were active as GTPase-activating proteins in solution but exhibited diminished binding to anionic liposomes. Peptides corresponding to mutants with the most pronounced phenotypes were also defective in forming an alpha-helix as measured by circular dichroism spectroscopy. These results support a model for direct interaction of RGS4 with membranes through hydrophobic and electrostatic interactions of an N-terminal alpha-helix. 相似文献
64.
As part of a large project to determine rooting depth and resource uptake on the Edwards Plateau of central Texas, we developed a DNA-based technique that allows the below-ground parts of all plants to be identified to the level of genus and usually to species. Identification is achieved by comparing DNA sequences of the internal transcribed spacer (ITS) region of the 18S-26S nuclear ribosomal DNA repeat, derived from below-ground plant material, with a reference ITS region database for plants at a site. The method works throughout plants because the plant ITS region can be PCR amplified using a set of universal primers. Congeneric species can usually be identified because the ITS region evolves relatively rapidly. In our study, all roots were easily identified to the level of genus; most congeneric species were identified solely by ITS sequence differences but some required a combination of ITS sequence data and above-ground surveys of species at a site. In addition to showing the feasibility and efficacy of our technique, we compare it with another DNA-based technique used to identify below-ground plant parts. Finally, we also describe a DNA extraction and purification technique that reliably provides high-quality DNA of sufficient quantity from roots so that PCR can be readily accomplished. Our technique should allow the below-ground parts of plants in any system to be identified and thereby open new possibilities for the study of below-ground plant communities. 相似文献
65.
Development of an anti-vitellin ELISA for the assessment of reproduction in the ridgeback shrimp, Sicyonia ingentis 总被引:1,自引:0,他引:1
Tsukimura B Bender JS Linder CJ 《Comparative biochemistry and physiology. Part A, Molecular & integrative physiology》2000,127(2):215-224
To investigate the reproductive regulation of the ridgeback shrimp, Sicyonia ingentis, vitellin (Vn) synthesis was studied. Using gel filtration chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, Vn was found to have a molecular mass of 322 kDa and to be composed of three subunits of 182, 91 and 85 kDa. Purified Vn was used to prepare anti-Vn antiserum, which was used to develop an enzyme-linked immunosorbent assay (ELISA) with a dynamic range of 0.3-300 ng. The ELISA was used to measure hemolymph levels of yolk proteins. The mean hemolymph concentrations in fresh caught animals ranged from 256 (+/-36.6 S.E.M.) to 1073 (+/-87.6 S.E.M.) mg/ml in stage 2 and 4 animals, respectively. The ELISA was also used to determine the effects of steroid hormone injections in adult non-reproductive female shrimp. One milligram injections of progesterone, 17alpha-hydroxyprogesterone or 17beta-estradiol were administered for three consecutive days to individual females. There were no changes in hemolymph vitellogenin levels during the successive 7-day period following the first injection of any steroid. 相似文献
66.
Past changes in the taxonomy of Chaetobromus , a Cape grass with considerable fodder potential, reflect a history of instability due to inadequate past sampling and intergradation among taxa in most characters. This biosystematic study differs from earlier investigations by its much more intensive approach to sampling, taking into consideration inter- and intrapopulational variation in 75 anatomical, morphological and cytological characters, across a total of 169 samples. Both phenetic methods and population aggregation analysis revealed the existence of three major groups, approximating three formerly described taxa and reflecting divergent ecological strategies in Chaetobromus . However, continuity among groups in most characters and a lack of clear-cut diagnostic field characters argues against their recognition at species level. Thus, Chaetobromus is here described as monotypic, the type species, C. involucratus , comprising three subspecies C. involucratus ssp. involucratus, C. involucratus ssp. sericeus and C. involucratus ssp. dregeanus . 相似文献
67.
Spectrometric studies on stability of tenuazonic acid (TeA) solution in organic solvents 总被引:1,自引:0,他引:1
The stability of tenuazonic acid solution at different temperatures and storage times was studied using methanol, methanol-water
(8:2 v/v), benzene and benzene-acetonitrile (98:2 v/v) as solvents. Solutions were analysed by a spectrometric method TeA
U.V.-spectrum was recorded. Results indicated that the optimum temperature for long-time storage period of tenuazonic acid
solution in any solvent assayed is -20°C. Benzene and benzene-acetonitrile (98:2 v/v) could be advised to make tenuazonic
acid solution which will be stored less than 2 months at 4°C. Methanol and methanolwater (8:2 v/v) are not recommended because
a low stability of TeA solution in this solvents. 相似文献
68.
Boland JL Corbeij HA Van Der Giessen W Seabra-Gomes R Suryapranata H Wijns W Hanet C Suttorp MJ Buller C Bonnier JJ Colombo A Van Birgelen C Pieper M Mangioni JA Londero H Carere RG Hamm CW Bonan R Bartorelli A Kyriakides ZS Chauhan A Rothman M Grinfeld L Oosterwijk C Serruys PW Cumberland DC 《International journal of cardiovascular interventions》2000,3(4):215-225
AIMS: The BiodivYsio trade mark stent (Biocompatibles Ltd, Farnham, UK) is coated with a phosphorylcholine (PC)-containing copolymer to confer biocompatibility. The SOPHOS (Study Of PHosphorylcholine coating On Stents) study was designed to assess the safety and efficacy of this novel coronary stent and by indirect comparison to indicate equivalence with other formal stent studies. METHODS AND RESULTS: Patients with angina and a single short (#x2A7F;12 mm) de novo lesion in a native coronary artery of >/=2.75 mm diameter were included. A total of 425 patients were allocated in 24 centers. Clinical data were collected at one-, six- and nine-month follow-up. Angiography was performed before and after the stent implantation. In addition, in the first 200 patients (SOPHOS A) angiography was routinely performed at six months. The following 225 patients (SOPHOS B) were merely followed up clinically. The primary end-point of the study, the six-month MACE-rate (MACE = Major Adverse Cardiac Events) was 13.4% (two cardiac death; five Q-wave/nine non-Q-wave myocardial infarctions (MI); nine CABG and 32 target lesion revascularization (TLR), which is similar to the calculated 15% MACE-rate in comparable reference studies. Secondary end-points included among others restenosis at six months in the SOPHOS A population. The target vessel diameter was 2.98 +/- 0.48 mm. Minimal lumen diameter pre/post procedure and at follow-up was 1.00 +/- 0.32, 2.69 +/- 0.37, 1.91 +/- 0.71 mm, respectively. The binary restenosis rate (>/=50% diameter stenosis at follow-up) was 17.7%. CONCLUSION: The coronary BiodivYsio stent is safe and effective as a primary device for the treatment of native coronary artery lesions in patients with stable or unstable angina pectoris. Clinical and angiographic results are in the statistical range of equivalence with comparable studies with other current stents. 相似文献
69.
Trans-splicing is an unusual process in which two separate RNA strands are spliced together to yield a mature mRNA. We present a novel computational approach which has an overall accuracy of 82% and can predict 92% of known trans-splicing sites. We have applied our method to chromosomes 1 and 3 of Leishmania major, with high-confidence predictions for 85% and 88% of annotated genes respectively. We suggest some extensions of our method to other systems. 相似文献
70.
This investigation examined the exposure of Egyptian infants to Aflatoxin M1 (AfM1) and of lactating mothers to Aflatoxin B1, using AfM1 in human milk as a biomarker for exposure to AfB1. The presence of ochratoxin A (OA) in human milk was also investigated to determine the levels of infants exposure to OA
from human milk. The results indicated that AfM1 was found in 66 (55 %) of 120 human milk samples with a mean of 0.3 ± 0.53 ng/mL (range 0.02 to 2.09 ng/mL). OA was found
in 43 (35.8 %) of 120 human milk samples with a mean of 21.1 ± 13.7 ng/mL (range 5.07 to 45.01 ng/mL), which will cause a
daily intake of OA from human milk exceeding the suggested tolerable dose of 5 ng/kg-1 of OA body weight. On the other side AfM1 was found in 25 % of blood samples (5 out of 20 samples), at a mean of 1.18 ng/mL, but it was detected only in one urine
sample (1 out of 20 samples). OA was detected only in 2 out of 13 blood samples (15.4 %) with an average 3.67 ng/mL. Whereas
OA was not detected in all analyzed urine samples. 相似文献